8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 111
Effects of operating conditions and fuel properties on emission performance and
combustion ef 1047297ciency of a swirling 1047298uidized-bed combustor 1047297red with
a biomass fuel
Vladimir I Kuprianov a Rachadaporn Kaewklum b Songpol Chakritthakul a
a School of Manufacturing Systems and Mechanical Engineering Sirindhorn International Institute of Technology Thammasat University PO Box 22 Thammasat Rangsit Post Of 1047297ce
Pathum Thani 12121 Thailandb Department of Mechanical Engineering Faculty of Engineering Burapha University 169 Long-Hard Bangsaen Road Chonburi 20131 Thailand
a r t i c l e i n f o
Article history
Received 29 October 2009
Received in revised form
10 April 2010
Accepted 15 May 2010
Available online 12 June 2010
Keywords
Swirling 1047298uidized bed
Rice husk
Temperature
Gas concentrations
Combustion ef 1047297ciency
a b s t r a c t
This work reports an experimental study on 1047297ring 80 kgh rice husk in a swirling 1047298uidized-bed
combustor (SFBC) using an annular air distributor as the swirl generator Two NO x emission control
techniques were investigated in this work (1) air staging of the combustion process and (2) 1047297ring rice
husk as moisturized fuel In the 1047297rst test series for the air-staged combustion CO NO and C xH y emissions
and combustion ef 1047297ciency were determined for burning ldquoas-receivedrdquo rice husk at 1047297xed excess air of 40
while secondary-to-primary air ratio (SAPA) was ranged from 026 to 075 The effects of SAPA on CO
and NO emissions from the combustor were found to be quite weak whereas C xH y emissions exhibited
an apparent in1047298uence of air staging In the second test series rice husks with the fuel-moisture content
of 84 to 35 were 1047297red at excess air varied from 20 to 80 while the 1047298ow rate of secondary air was
1047297xed Radial and axial temperature and gas concentration (O2 CO NO) pro1047297les in the reactor as well as
CO and NO emissions are discussed for the selected operating conditions The temperature and gas
concentration pro1047297les for variable fuel quality exhibited signi1047297cant effects of both fuel-moisture and
excess air As revealed by experimental results the emission of NO from this SFBC can be substantiallyreduced through moisturizing rice husk while CO is effectively mitigated by injection of secondary air
into the bed splash zone resulting in a rather low emission of CO and high (over 99) combustion
ef 1047297ciency of the combustor for the ranges of operating conditions and fuel properties
2010 Elsevier Ltd All rights reserved
1 Introduction
For many years rice husk hasbeen an importantenergy resource
in most Asian countries The 1047298uidized-bed combustion technology
with its apparent economical and environmental bene1047297ts is proven
to be the most effective technology for energy production from this
agricultural residue However the combustion of rice husk gener-ally characterized by elevated fuel-N and fuel-ash contents is
accompanied by substantial NO x and CO emissions the rate of those
depends on fuel properties as well as on the design features and
operating conditions of a combustion system used [1e4]
For typical ranges of the bed temperature and excess air in
a 1047298uidized-bed combustion system (combustor or boiler furnace)
burning biomass NO x are known to originate mainly from fuel-N
via homogeneous oxidation of the dominant nitrogenous volatile
species NH3 and HCN to fuel-NO since the contributions of
thermal-NO and prompt-NO are insigni1047297cant [35] The relevant
studies reveal that NO x emissions from the reactor are in a quasi-
linear correlation with fuel-N [467] With rising of the bed
temperature NO x emissions are weakly increased or stay constant
[689] but exhibit a substantial increase with rising excess air inboth conventional and air-staged combustion systems [16e12]
However some studies report insigni1047297cant effects of the air staging
on NO x emissions when burning biomass [18912]
The emission of CO from a biomass-fuelled system is affected by
several operating factors excess air combustion temperature resi-
dence time of reactants fuel-ash content and particle size and also
fueleair mixing conditions During the combustion formation of CO
is known to include several sources CO released with fuel volatiles
oxidation of volatile hydrocarbons by oxygen as well as oxidation of
char-C by O2 H2O and CO2 on the char surface [36] As any other
unburned pollutant CO is effectively controlled (reduced) by Corresponding author Tel thorn66 2 986 9009x2208 fax thorn66 2 986 9112
E-mail address ivlaanovsiittuacth (VI Kuprianov)
Contents lists available at ScienceDirect
Energy
j o u r n a l h o m e p a g e w w w e l s e v i e r c o m l o c a t e e n e r g y
0360-5442$ e see front matter 2010 Elsevier Ltd All rights reserved
doi101016jenergy201005026
Energy 36 (2011) 2038e2048
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 211
increasing excess air andor combustion temperature both
enhancing therateof COoxidation to CO2 [18911] Some reductionin
COcan be achieved when1047297ring biomass fuels withlowerash content
[1113] The bubbling 1047298uidization mode seems to be one of the effec-
tive regimes for operating the 1047298uidized-bed combustion systems
[131013] as ensuringthe high intensive mixingof fuel particles and
air in the bed region (promoting CO reduction) the latter being
signi1047297cantly affected by the air distributor design [14] However an
in1047298uence of the air staging on CO emission during the 1047298uidized-bed
combustion of biomass is reported to be rather weak [15]
A large number of research studies have addressed emission
characteristics and combustion ef 1047297ciency for 1047297ring rice husk in
various laboratory-scale 1047298uidized-bed combustion techniques
such as bubbling 1047298uidized-bed vortexing 1047298uidized-bed and circu-
lating 1047298uidized-bed combustors [9e12] Due to moderate bed
temperatures (normally not higher than 850 C) NO x emissions
from the combustors are generally below 180 ppm (on 6 O2 dry
gas basis) while CO emission is found to be elevated up to
800 ppm Combustion ef 1047297ciency of these devices operated at
optimal conditions is reported to be within 96e98 Experimental
results revealed minor effects of the air staging on these emissions
as well as on combustion ef 1047297ciency of the vortexing and circulating
1047298uidized-bed combustors 1047297ring rice husk [912]Recently two novel combustion techniques ensuring fuel
oxidation in a strongly swirled 1047298ow a vortex combustor and
a cyclonic 1047298uidized-bed combustor have been developed and
tested for 1047297ring rice husk [1617] Under optimal operating condi-
tions high (over 99) combustion ef 1047297ciency can be achieved in
these pilot reactors while controlling CO emission below 400 ppm
However NO x emissions from the combustors are reported to be
elevated up to 300 ppm for the vortex combustor [16] or rather
high 350e425 ppm for the cyclonic 1047298uidized-bed combustor [17]
Such substantial NO x emissions are mainly caused by (i) elevated
excess air required for sustaining the strongly swirled gasesolid
1047298ow and (ii) high-temperature conditions in these rice husk-fuel-
led combustors operated with a signi1047297cant heat release rate per
unit volume Effects of the air staging on both emissions andcombustion ef 1047297ciency of the vortex and cyclonic 1047298uidized-bed
combustors are reported to be rather weak It should be noted that
elevated excess air basically leads to lower thermal ef 1047297ciency of
a power plant (or any other energy conversion units) using these
devices mainly due to an increase in the heat loss with waste gas
(affected by a signi1047297cant volume of excessive air) [18]
Kaewklum and Kuprianov [19] have recently reported a pio-
neering study on a laboratory-scale swirling 1047298uidized-bed
combustor (SFBC) 1047297ring rice husk In this innovative combustion
technique a swirling 1047298uidized bed is generated due to the special
design of a primary air distributor used in this combustion tech-
nique as the swirl generator Unlike in the vortexing 1047298uidized-bed
combustor secondaryair in this SFBC is injected into the bed splash
zone ie at a relatively low level above the (primary) air distrib-utor The tangential injection of secondary air sustains the rota-
tional gasesolid 1047298ow in the combustor At optimal excess air
40e60 the burning of rice husk in the SFBC is characterized by
high about 995 combustion ef 1047297ciency while CO and NO emis-
sions can be limited within 150e300 ppm and 170e210 ppm
respectively However no effects of the air staging on emission
performance and combustion ef 1047297ciency of the SFBC have been
addressed in this pioneering study
As can be generally concluded from the literature review
compared to the conventional (ie non-swirling) 1047298uidized-bed
combustors the combustion techniques with a rotational gasesolid
1047298ow ensure higher combustion ef 1047297ciency at minimized CO emis-
sion accompanied however by moderate (for the SFBC) or
elevated (for the vortex combustor) or high (for the cyclonic
1047298uidized-bed combustor) NO x emissions The NO x control in these
high ef 1047297ciency devices 1047297ring rice husk is therefore an issue of
paramount importance
Burning biomass in the form of moisturized fuel which prior to
the combustion can be prepared by adding water to ldquoas-receivedrdquo
fuel is proven to be an effective least-cost NO x emission control
technique as reportedin studies on1047297ring of wood sawdust and rice
husk in a conventional 1047298uidized-bed combustor with a cone-shape
bed [420] Moreover a substantial reduction in the bed tempera-
ture occurring with increasing fuel moisture provides more
favorable operating conditions for preventing undesirable ash-
related problems in the 1047298uidized-bed combustor (eg bed
agglomeration and wall slagging) particularly when 1047297ring high-
alkali biomass fuels [3] Howeverwhen using this conical 1047298uidized-
bed combustor the reduction of NO x emissions has been accom-
panied by a noticeable increase in CO emission and corresponding
deterioration of the combustion ef 1047297ciency Thus selection of the
most appropriate fuel-moisture content should be considered
along with optimization of air supply to the combustion system
This study was aimed at determining the technical feasibility of
an effective control of NO emission during the combustion of rice
husk in the SFBC through air staging of the combustion and fuel
moisturizing Detail analysis of the formation and decomposition of major gaseous pollutants (CO and NO) at different locations in this
reactor for variable operating conditions and fuel properties were
the focus of this work Optimization of the fuel-moisture content
and air supply for minimizing CO and NO emissions from this SFBC
1047297ring moisturized rice husk was also among the main objectives of
this study
2 Materials and methods
21 Experimental facilities
Fig1 depicts the general view of the experimental setup and the
schematic diagram of the SFBC It can be seen in Fig 1a that the
system included the combustor with a start-up burner a cyclonea fuel screw feeder and a blower Additionally Fig 1b provides the
design and geometrical details of the SFBC which was made of 45-
mm-thick steel sheet and covered internally with the 50-mm-thick
refractory
The combustor consisted of a conical (bottom) part1047297lled in with
lsquoroundrsquo quartz sand (with the particle sphericity of 086 and density
of 2650 kgm3) used as the inert bed material and a cylindrical
(upper) part The particle (sieve) size of 05e06 mm and static bed
height of 30 cm were selected to be the main characteristics of the
bed material as those ensuring the stable swirling 1047298uidized-bed
regime Under ldquocoldrdquo operating conditions the minimum 1047298uidiza-
tion velocity of the air-sand bed with these characteristics was
about 08 ms while the minimum velocity of the fully swirling
1047298uidized-bed mode was 13 ms [21]The annular spiral air distributor at the combustor bottom was
made up of eleven blades 1047297xed at an angle of 14 to the horizontal
and served as the swirl generator of primary air (PA) the latter
being supplied by the 25-hp blower The distributor had an annular
air exit with 01 m inner and 025 m outer diameters The distance
between two neighbor blades was variable (in a linear relationship
with radius) thus forming a trapezoidal cross-sectional area of
0012 m2 (total) for the air1047298ow between the blades To stabilize the
swirl motion of the gasesolid bed a steel cone was 1047297xed on the top
of the air distributor as shown in Fig 1b
Primary air was supplied to the air distributor by the blower
through an air pipe of a 01-m inner diameter as shown in Fig 1a
The 1047298owrate of primary air was controlled using a butter1047298y valve
arranged on the air pipe downstream from the blower The
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2039
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 311
relationship between the actual air1047298ow rate and valve opening was
developed using a measuring system ldquoTesto-454rdquo (Testo Germany)
with a hot-wire probe The measurement uncertainty in the 1047298ow-
rate of primary air was about 3 as estimated in Appendix A
The screw-type feeder delivered the fuel over the bed at a 06 mlever above the air distributor A three-phase inverter was used to
control the fuel feed rate via changing rotation speed of the screw
feeder As established by repeated calibrations the fuel feed rate
was in a quasi-linear correlation with the rotational speed (rpm) of
the feeder For the fuel feed rates of 30e100 kgh the measurement
accuracy of the fuel feed rate was within the range of 3 to 5
when varying the fuel-moisture content from 84 (in ldquoas-receivedrdquo
fuel) to 40
During the combustor start-up a diesel-1047297red burner from Riello
Burners Co (model ldquoPress G24rdquo) was used to preheat the sand to
the speci1047297ed temperature (of about 700 C) The burner air was
tangentially injected into a splash bed zone at a 05 m level through
the burner inclined at a 30 angle to the horizontal Upon
attaining required bed temperature the burner was turned offwhereas desired fuel supply was ensured by the screw feeder
However the burner fan remained to operate delivering secondary
air (SA) to the SFBC during the combustion tests with the aim to
mitigate CO in the bed splash zone and also to protect the burner
head against overheating and impacts from solids The 1047298owrate of
secondary air was controlled by changing an opening of the burner
fan The measurement uncertainty in the 1047298owrate of secondary air
(estimated by the same method as for the primary air) was found to
be about 4
A ldquoTesto-350XL rdquo gas analyzer (Testo Germany) was used to
measure the temperature and gas concentrations (O2 CO NO and
C xH y) along radial and axial directions in the combustor as well as
at the exit of the ash-collecting cyclone The measurement accu-
racies were 05 for the temperature 5 for CO and C xH y
ranged from 100 to 2000 ppm 10 for CO and C xH y higher than
2000 ppm 5 for NO and 02 vol for O2 Besides Chro-
meleAlumel thermocouples were 1047297xed at different levels in the
reactor for (i) monitoring the temperatures during the combustor
start-up (with the accuracy 1) and (ii) obtaining the axialtemperature pro1047297les For the particular test run the excess air ratio
was quanti1047297ed by Ref [22] using the O2 CO and C xH y concentra-
tions at the cyclone exit with an uncertainty of 2 Afterwards
corresponding percentages of total air (TA) and excess air (EA) were
calculated for each trial
Fly ash was sampled from the ash collector (see Fig 1a) to
quantify the content of unburned carbon in the ash required for
predicting the associated heat loss (as discussed below)
22 The fuels
In order to approach the work objectives two series of experi-
mental tests for (i) variable air staging and (ii) variable fuel prop-erties were carried out in this experimental study Table 1 shows
major fuel properties the ultimate and proximate analyses as well
as the lower heating value (LHV) of rice husk used in the tests at
different secondary-to-primary air ratios (SAPA)
Fig 1 (a) Experimental setup and (b) the laboratory-scale swirling 1047298uidized-bed combustor (SFBC)
Table 1
Ultimate and proximate analyses and lower heating value of rice husk 1047297red in the
SFBC during the experimental tests for variable air staging (W frac14 fuel-moisture
A frac14 fuel-ash VM frac14 volatile matter FC frac14 1047297xed carbon LHV frac14 lower heating value)
Ultimate analysis (wt on ldquoas-
receivedrdquo basis)
Proximate analysis (wt on ldquoas-
receivedrdquo basis)
C H O N S W A VM FC LHV (kJkg)
4220 458 2784 025 003 92 159 574 155 13600
VI Kuprianov et al Energy 36 (2011) 2038e 20482040
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 411
The ultimate analysis and LHV of rice husks1047297red in the test runs
for variable fuel quality are shown in Table 2 Since the variation in
the fuel-moisture content affected all other fuel properties the fuel
ultimate analysis and LHV of moisturized rice husks are provided in
Table 2 on ldquoas-1047297redrdquo basis For the particular fuel option the
constituents of the fuel analysis were calculated using the fuel
properties of ldquoas-receivedrdquo fuel (see Table 2 Option 1) accounting
for the actual fuel-moisture content Afterwards LHV for the
moisturized fuels was determined by Ref [22] using corresponding
fuel ultimate analyses from Table 2
The rice husks used in the two experimental series were quite
similar by their chemical and physical properties on ldquoas-receivedrdquo
basis It can be seen in Tables 1 and 2 that the sulfur content in the
rice husks was quite low For this reason SO 2 was not addressed in
this study The dimensions of ldquoas-receivedrdquo rice husk particles were
about 2 mm wide 05 mm thick and 10 mm long while the particle
density was about 1000 kgm3
23 Experimental planning
231 Tests for variable air staging
During this test series rice husk was burned at the 1047297xed fuel
feed rate 80 kgh and excess air of 40 for four values of SAPA
026 040 056 and 075 In each trial (ie for the particular oper-
ating conditions) CO NO and C xH y emissions were determined
together with the O2 concentration at the cyclone exit The main
goal of this test series was to determine the value (or range) of SA
PA ensuring the minimum of these emissions which could be taken
into consideration in the detailed study (test series) below
232 Tests for variable fuel moisture
Fuel moisture (W) and excess air (EA) were chosen as inde-
pendent variables in this test series while the fuel feed rate in all
trials was adjusted at nearly the same value about 80 kgh as in
previous test series Secondary air was supplied to the SFBC at
a minimum 1047298ow rate Q ba frac14 0024 Nm3s required for reliable
cooling of the start-up burner during the combustion testsThis test series included two groups of trials The1047297rst group was
devoted to the behavior of temperature and gas (O2 CO NO)
concentrations in radial and axial directions in the combustor In
this detailed study 1047297ve rice husks (in Table 2 Options 1e5) were
burned at a similar EA value of about 40 The radial temperature
and gas concentration pro1047297les were obtained for 1047297ve levels ( Z )
above the datum Z frac14 0 (see Fig 1b) However the axial pro1047297les
were plotted using the variables measured at eight levels along the
combustor centerline
In the second group of trials CO and NO emissions from the
combustor were quanti1047297ed for all (six) rice husks in Table 2 which
were burned in the SFBC at the excess air values of about 20 40
60 and 80 Using the emission magnitudes optimal values
(ranges) of both EA and fuel-moisture content were determined
using a cost-based approach as discussed below
For the two test series the heat losses with unburned carbon
quc and owing to incomplete combustion qic were quanti1047297ed
together with combustion ef 1047297ciency by using models provided in
Appendix B Note that the effects of C xH y were taken into account
when determining qic for the tests at variable air staging However
these effects were neglected in the qic for the second test series
when the emission of hydrocarbons was at a rather low level in all
trials
24 A model for optimizing excess air and fuel-moisture content
In this work a cost-based approach [23] was applied to deter-
mine the optimal values (ranges) of excess air and fuel-moisture
content leading to the minimized emission (or ldquoexternalrdquo) costs of
1047297ring rice husk in this SFBC Ignoring the effects of SO2 (as negli-
gible) and CO2 (due to the relatively low speci1047297c ldquoexternalrdquo cost of
carbon dioxide) the corresponding objective function used for the
optimization can be represented as
J ec frac14 Min
P NO x_mNO x
thorn P CO _mCO
(1)
where _mNO x and _mCO are emission rates (calculated by Ref [23])
and P NO x and P CO are speci1047297c ldquoexternalrdquo costs of NO x (as NO2) and
CO respectively
It can be concluded from analysis of the objective function that
with the above assumptions the optimal values of excess air and
fuel-moisture content are solely dependent on the cost ratio
P NO x=P CO while the emission costs are apparently affected by all the
variables in Eq (1)
In every country the (average) emission externalities are
strongly affected by the economic structure and activities Studies
on the externalities of heat and electricity generation reveal
therefore a signi1047297cant diversity of the speci1047297c ldquoexternalrdquo costs For
the EU-27 countries the speci1047297c cost of NO x emissions is reportedto be about 7000 Vt (including impacts on human health
ecosystems crops and materials) [24] whereas for Asian countries
this index seems to be substantially (or signi1047297cantly) lower
[25e27] For instance for neighbor China P NO x frac14 2438 US$=t
(including only dominant costs ie those related to the health
damage and climate change) [25] Unlike for NO x limited data on
the externalities by CO is available in literature As revealed by
some relevant studies P CO rises as P NO x increases However the
ratio of P NO x to P CO (ie P NO x
=P CO) is reported to be within certain
limits ranging basically from 5 to 8 [28e30]
Taking the above into consideration it was decided to consider
two options in this optimization study using (1)
P NO x frac14 3000 US$=t and P CO frac14 600 US$t (ie for P NO x
=P CO frac14 5)
and (2) P NO x frac14 3000 US$=t and P CO frac14 400 US$t (ie forP NO x=P CO frac14 75)
3 Results and discussion
31 Emission and combustion characteristics of SFBC for variable
air staging
311 Emissions
Fig 2 shows the CO NO and C xH y emissions (on 6 O2 dry gas
basis) from the combustor 1047297ring 80 kgh rice husk at different SA
PA ratios when excess air was adjusted at about 40
Asseen in Fig 2a the CO emissionwas weakly in1047298uenced by the
air staging factor With increasing SAPA this emission somewhat
increased from about 360 to 450 ppm staying nevertheless at
Table 2
Ultimate analysis and LHV of rice husks 1047297red in the experimental tests for variable
fuel moisture (W)
Property Fuel sample (Option)
1 2 3 4 5 6
Ultimate analysis (wta)
C 4050 3758 3537 3316 3095 2874
H 407 377 355 333 311 289
O 2869 2663 2506 2349 2193 2036
N 031 029 027 025 024 022
S 003 003 003 002 002 002
W 84 150 200 250 300 350
A 180 1670 1572 1474 1376 1277
LHV (kJkg) 14620 13390 12460 11530 10600 9670
a On ldquoas-receivedrdquo basis for Option 1 on ldquoas-1047297redrdquo basis for Options 2e6
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2041
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 511
a rather low level basically due to the secondary air injection into
the bed splash zone As can be concluded based on the results from
Ref [12] and present study excess air (or percentage of total air) is
an important factor in controlling the CO emission in this SFBC
whereas SAPA shows quite weak effects
With increasing SAPA within the selected range 026e075 the
NO emission exhibited a slight reduction from about 150 to140 ppm (see Fig 2a) mainly due to the diminishing of the bed
temperature and reduction of the O2 concentration in the bed the
latter being occurred because of the lowering of PA Thus the air
staging does not seem to be an effective measure to control the NO
emission in this combustor 1047297ring rice husk
It can be seen in Fig 2b that at relatively small proportions of
secondary air the C xH y emissions were at a quite low level
However at SAPA gt 04 these emissions showed a signi1047297cant
increase from 120 to 1400 ppm which can be explained by the
sub-stoichiometric conditions in the bed region Under such
conditions more volatiles were carried over from the combustor
bottom causing the above increase in the C xH y emissions Thus
primary air should be supplied to the SFBC at a 1047298ow rate ensuring
the oxidation conditions in the bed ie with some excess withregard to the theoretical (stoichiometric) air For instance at
EA frac14 40 the combustor should be operated at 026 lt SAPA lt 04
(see Fig 2b) for avoiding high C xH y emissions from this SFBC 1047297ring
rice husk As can be generally concluded the lower limit of SAPA is
speci1047297ed with the aim to provide the reliable coolingof the start-up
burner whereas the upper one is selected taking into account that
EA should be somewhat greater than SA
312 Heat losses and combustion ef 1047297ciency
The analyses of 1047298y ashes for unburned carbon for this test series
indicated the high rate of fuel burnout in this conical SFBC
Depending on SAPA the unburned carbon content in the 1047298y ashes
varied from 081 to 24 the minimum value being found at the
highest SAPA ratio
Table 3 shows the heat losses with unburned carbon (quc) and
owing to incomplete combustion (qic) together with the combus-
tion ef 1047297ciency (all as the percentage of LHV) for 1047297ring 80 kgh rice
husk at excess air of about 40 for different values of SAPA An
increase in SAPA led to a noticeable reduction in the heat loss with
unburned carbon basically due to the higher rate of fuel burnout
which waslikelycaused by an increase in the residence time of char
particles in the combustor However the exponential rise of qic can
be explained by the above behavior of CO and C xH y emissions
Due to the opposite trends exhibited by the heat losses the
combustion ef 1047297ciency was found to have a maximum 991 for1047297ring rice husk at SAPA of 04e06 and EA of about 40
32 Emission and combustion characteristics of SFBC for variable
fuel moisture
As revealed by the experimental results the temperature and
gas concentrations (O2 CO and NO) in this conical SFBC were rep-
resented by three-dimensional patterns (1047297elds) showing the
effects of combustor hydrodynamics fuel quality and operating
conditions on the radial and axial pro1047297les of the temperature and
chemical species Note that at a given excess air SA raised with
increasing fuel moisture because of the reduction in the theoretical
air while the 1047298owrate of secondary air was 1047297xed at the above
constant value changing from 29 (for ldquoas-receivedrdquo fuel) to 41(for rice husk with the highest moisture content) which resulted in
corresponding diminishing of PA Due to the reduction in the
combustion temperature and also in the theoretical air and PA
the residence time of char particles in the bottomregion of the SFBC
was substantially greater when burning rice husks with higher
moisture content leading to the higher rates of devolatilization and
burnout of fuel particles in this region and thus affecting signi1047297-
cantly the behavior of all variables in the reactor
At EA frac14 40 or higher oxidizing conditions were basically
provided in the combustor bottom which justi1047297ed the ignorance of
C xH y emissions in this test series
321 Radial and axial temperature and gas concentration pro 1047297les
in the SFBC Fig 3 shows the radial temperature and O2 concentration
pro1047297les at 1047297ve levels ( Z ) above the air distributor in the SFBC 1047297ring
80 kgh rice husk at excess air of about 40 for variable fuel prop-
erties (Options 1e5 in Table 2) As seen in Fig 3 the variables
exhibitedquitesimilar behaviorsat different levels ( Z ) above the air
distributor The radial temperature pro1047297les were found to be rather
uniform indicating the highly intensive heat-and-mass transfer
along the radius With increasing fuel moisture (at a 1047297xed excess air
level) the temperature at all points in the combustor volume was
found to be reduced (despite the above increase in the residence
time) because of the apparent in1047298uence of the latent heat of water
evaporation Similar results are reported in some studies on
conventional 1047298uidized-bed and 1047297xed-bed combustion systems
1047297ring biomass fuels with variable fuel moisture [4203132]
Fig 2 Effects of secondary-to-primary air ratio (SAPA) on the (a) CO NO and (b) C xH y emissions from the SFBC 1047297ring rice husk at excess air of 40
Table 3
Heat losses and combustion ef 1047297ciency of the SFBC 1047297ring 80 kgh rice husk at excess
air of 40 for variable secondary-to-primary air ratio
SAPA quc () qic () Combustion ef 1047297ciency ()
026 094 024 988
040 051 035 991
056 047 043 991
075 031 212 976
VI Kuprianov et al Energy 36 (2011) 2038e 20482042
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 611
Z = 267 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r
e p m e T
Z = 267 m
0
5
10
15
20
00 02 04 06 08 10
rR
O 2
) l o v ( n o i t a r t
n e c n o c
Z = 217 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r e p m e T
Z = 217 m0
5
10
15
20
00 02 04 06 08 10
rR
O 2
) l o v ( n o i t a r t n e c n o c
Z = 155 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r e p m e T
Z = 155 m
0
5
10
15
20
00 02 04 06 08 10
rR
O 2
) l o v ( n o i t a r t n e c n o c
Z = 101 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r e p m e T
Z = 101 m
0
5
10
15
20
00 02 04 06 08 10
rR
O 2
) l o v ( n o i t a r t n e c n o c
Z = 047 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r e p m e T
Z = 047 m
0
5
10
15
20
00 02 04 06 08 10
rR
O 2
)
l o v ( n o i t a r t n e c n o c
a b
Fig 3 Radial (a) temperature and (b) O2 concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-)
W frac14 84 () W frac1415 () W frac14 20 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2043
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 711
Implementation of air staging seems to have a signi1047297cant impact
on the behavior of gaseous species in both radial and axial direc-
tions Due to the tangential injection of secondary air into the bed
splash zone the radial O2 concentration pro1047297les were characterized
by a positive radial gradient (the most signi1047297cant being observed at
Z frac14 101 m) which resulted in the higher O2 concentration near the
combustor wall than that at the centerline (ie at r R frac14 0)
However the radial gradient of O2 was gradually attenuated along
the bed height Note that the injection of secondary air affected the
radial O2 concentration pro1047297les not only in upper regions of the
reactor but also at levels below the injection point (in Fig 3b see
the pro1047297les at Z frac14 047 m) since the secondary air was injected at
the negative angle In the meantime with increasing the fuel-
moisture content the O2 concentration at all the points across the
combustor was found to be reduced and this reduction was caused
by some physical and chemical factors as addressed below in the
discussion of axial O2 concentration pro1047297les
The axial temperature and O2 concentration pro1047297les in the SFBC
are shown in Fig 4 for the same fuel options and operating
conditions as in Fig 3 At 1047297xed excess air a positive axial temper-
ature gradient was found to occur in the lower part of the reactor
for all the fuels (see Fig 4a) likely due to the diminishing of heat
release in the dense bed (caused by air staging) and (ii) in1047298uence of secondary air injected into the bed splash zone at the ambient
temperature However with higher fuel moisture (ie with dete-
riorating fuel quality) the temperature attained its maximum at
lower levels (Z) above the air distributor which can be explained by
the effects of the residence time The maximum temperature for
burning ldquoas-receivedrdquo rice husk (with W frac14 84) was rather high
about 980 C however it was reduced to 850e860 C when
increasing the fuel-moisture content to 25e30
However with raising fuel moisture the rate of oxygen
consumption in the bottom region of the SFBC ( Z lt 08 m) was
apparently higher which despite the above reduction in the bed
temperature resulted in the lower O2 concentration at all locations
in this region (as seen in Fig 4b) This phenomenon can be
explained by the greater residence time (leading to a higher yield of CO and other combustibles with fuel volatiles) and also greater
contribution of the ldquowetrdquo oxidation of char-C by OH radicals both
leading to higher rates of CO formation and consequently O2
consumption in this region The next region (08 lt Z lt 10 m) was
characterized by a noticeable regaining (rise) of O2 as the response
to secondary air injection However in the freeboard of the
combustor ( Z gt 10 m) the O2 concentration was diminished along
the reactor height at a rather low rate and this reduction was
accompanied by the gradual converging of the axial pro1047297les as all
the tests in Fig 4 for variable fuel moisture were conducted at
(nearly) the same EA
Fig 5 shows the radial CO and NO concentration pro1047297les at
different levels above the air distributor for the same fuels and
operating conditions as in Fig 3 Both CO (Fig 5a) and NO (Fig 5b)
were signi1047297cantly affected by fuel moisture and showed negative
gradients along the radius (different numerically) at all the levels in
the SFBC For the 1047297xed fuel-moisture content due to the effects of
secondary air the CO concentration in the peripheral zone across
the combustor was much lower compared to that at the centerline
thus forming the above radial gradient of CO However the NO
concentration varied weakly along the radius except at Z frac14 047 m
The occurrence of the NO maximum at the centerline indicated
higher rates of both fuel devolatilization and oxidation in the
central zone of the reactor (compared to those at the combustor
wall) despite the uniformity of the temperature and the opposite
trends of the O2 concentration pro1047297les across the SFBC (see Fig 3b)
In the freeboardof the reactor the radial CO and NO gradients were
found to be gradually attenuated with higher Z
Fig 6 depicts the axial CO and NO concentration pro1047297les in this
combustor As seen in Fig 6a the pro1047297les exhibited four sequent
regions along the combustor height With increasing fuel moisture
the CO concentration in the 1047297rst region (0lt Z lt 08 m) was
apparently higher at all the levels above the air distributor
particularly at the centerline (see Fig 5a) mainly due to (i) longerresidence time (leading to greater yield of CO with volatiles) (ii)
lower PA (reducing the rate of CO oxidation) (iii) higher contri-
bution of ldquowetrdquo oxidation of char-C by OH radicals and (iv) lower
bed temperature causing an increase in the COCO2 ratio in the
products of fuel-char oxidation [33]
In the second region (08 lt Z lt 10 m) the CO concentration
along the combustor axis was found to be drastically reduced
mainly due to the effects of secondaryair the greater rate of the CO
reduction being observed at a higher level of fuel moisture (ie at
higher SA) In the third region (10 lt Z lt 18 m) the CO concentra-
tion regained substantial values along the centerline mainly due to
oxidation of unburned hydrocarbons and fuel-C carried over from
the bed region to CO However in the fourth (upper) region the CO
concentration was found to be gradually reduced along thecombustor height likely via homogeneous reaction of CO with
residual O2 and OH [333]
Like for CO four speci1047297c regions can be distinguished in the
axial NO concentration pro1047297les as can be seen in Fig 6b In the 1047297rst
region (0 lt Z lt 08 m) NO was mainly formed from NH3 in volatiles
(a major precursor of NO in biomass combustion) via the fuel-NO
formation mechanism [343435] With increasing fuel moisture
despite the reduction in bed temperature the NO concentration at
the reactor centerline showed a trend to increase at any given Z
mainly due to (i) greater residence time promoting a higher yield
of nitrogenous species with fuel volatiles and (ii) enhanced
a b
500
600
700
800
900
1000
1100
0 1 2 3
Height above air distributor (m)
) C deg ( e r u t a r e p m e T
0
5
10
15
20
0 1 2 3Height above air distributor (m)
O 2
) l o v ( n o i t a r t n e c n o c
Fig 4 Axial (a) temperature and (b) O2 concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415
(
) Wfrac14
20 (6
) Wfrac14
25 and (A
) Wfrac14
30
VI Kuprianov et al Energy 36 (2011) 2038e 20482044
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 811
Z = 267 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 267 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t
n e c n o c O N
Z = 217 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 217 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O N
Z = 155 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 155 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0rR
) m
p p ( n o i t a r t n e c n o c O N
Z = 101 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 101 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O N
Z = 047 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
)
m
p p ( n o i t a r t n e c n o c
O C
Z = 047 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O N
ab
Fig 5 Radial (a) CO and (b) NO concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 ()
W frac14 15 () W frac1420 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2045
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 911
oxidation of NH3 by OH [36] In the second region (08 lt Z lt 10 m)
the chemical reactions responsible for NO decomposition such as
catalytic reduction of NO by CO on the char surface [3] as well as
reaction of NO with NH3 at oxygen de1047297ciency [34] were predom-
inant This resulted in a signi1047297cant reduction of the NO concen-
tration along the combustor height and the NO reduction was more
apparent when burning rice husks with higher moisture contentsHowever the injection of secondary air promoted a slight increase
in NO in the third region (10 lt Z lt 16 m) of the reactor as a result
of oxidation of the nitrogenous species carried over from the
combustor bottom In the fourth (upper) region the pro1047297les
exhibited some diminishing of the NO concentration along the
reactor height because of the catalytic reduction of NO by CO
occurred however at a rather low rate
322 Emissions
Fig 7 depicts the CO and NO emissions (on 6 O 2 dry gas basis)
from the combustor 1047297ring 80 kgh rice husk at variable EA for the
whole range of fuel moisture (Options 1e6 in Table 2) It can be
concluded from analysis of data in Fig 7a that at EA greater than
40 the CO emission from the SFBC can be controlled ata comparatively low level below 350 ppm regardless of the fuel
quality On the contrary at excess air lower than 40 the CO
emission exhibited quite strong effects of both fuel quality and EA
Note that at EA frac14 20 the CO emission was extremely high
3000e7000 ppm for the whole range of fuel moisture However
with increasing the fuel-moisture content from 84 (in ldquoas-
receivedrdquo rice husk) to 25 the CO emission at this lowest EA
exhibited some reduction roughly from 4000 to 3000 ppm basi-
cally caused by (i) the higher rate of chemical reaction between CO
and OH [333] and (ii) higher rate of CO decomposition in the
freeboard (due to enhanced SA and greater residence time) Similar
trend is reported in some studies on effects of fuel moisture on the
CO emission from the 1047297xed-bed combustion systems [3132]
However with further increase in the fuel-moisture content (from
25 to 35) the CO emission from the SFBC was found to rise from
about 3000 to 7000 ppm likely due to the signi1047297cant contribution
of ldquowetrdquo oxidation of char-C occurred at the lowered combustion
temperatures This trend is in concordance with the behavior of theCO emission during the conventional 1047298uidized-bed combustion of
some biomass fuels with variable moisture [420]
It can be seen in Fig 7b that at EA frac14 40 (the ldquocriticalrdquo value for
the CO emission) the NO emission diminished from about 170 to
130 ppm (or by some 25) when changing the fuel-moisture
content from 84 to 25 A further increase in fuel moisture up to
30e35 resulted in some more emission reduction Note that this
positive result was accompanied by deterioration of combustion
stability (likely caused by inconsistency in fuel properties) which
showed itself by the noticeable time-domain 1047298uctuations of the
temperature and gaseous species particularly in the vicinity of the
fuel injection As revealed by experimental data from this study
through moisturizing of ldquoas-receivedrdquo rice husk the NO emission
from the SFBC can be substantially reduced However thisachievement was accompanied by the conventional effects of EA
leading to the increase in the NO emission with higher EA [16e12]
323 Optimal excess air and fuel-moisture content
Fig 8 shows the emission costs US$ per 1 ton of fuel for 1047297ring
rice husk in the SFBC at variable fuel moisture and excess air pre-
dicted using the above CO and NO emissions and the speci1047297c
ldquoexternalrdquo costs P NO x frac14 3000 US$=t and P CO frac14 600 US$t (ie for
P NO x=P CO frac14 5) At low values of EA the contribution of CO to the
emission costs was predominant whereas the effects of NO were
substantial at higher EA values It can be seen in Fig 8 that the
Fig 6 Axial (a) CO and (b) NO concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415 ()
W frac1420 (6) W frac1425 and (A) W frac1430
Fig 7 Effects of the fuel-moisture content and excess air on the (a) CO and (b) NO emissions from the SFBC 1047297ring 80 kghr rice husk (-) W frac1484 () W frac1415 () W frac1420 (6)
Wfrac14
25 (A
) Wfrac14
30 and (A
) Wfrac14
35
VI Kuprianov et al Energy 36 (2011) 2038e 20482046
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 1011
emission costs were at the minimal value when 1047297ring rice husk
with the fuel-moisture content of 25e
35 at excess air of 40e
50
Switching P NO x=P CO from 5 to 75 (keeping however P NO x
at the
above value) led in effect to nearly the same optimal ranges of fuel
moisture and excess air quanti1047297ed however at different magni-
tudes of the emission costs Thus the optimized variables are not
sensitive to the speci1047297c ldquoexternalrdquo costs P NO x and P CO within their
actual ranges
324 Combustion heat losses and ef 1047297ciency
Table 4 shows the heat losses with unburned carbon quc and
owing to incomplete combustion qic together with the combustion
ef 1047297ciency of the SFBC actual EA values and SAPA ratios for some
selected fuel options
The heat loss with unburned carbon exhibited a rather weak
correlation with fuel moisture showing however the substantialin1047298uence of excess air Unlike quc the heat loss owing to incomplete
combustion was strongly affected by both fuel moisture and excess
air and it was characterized by quite small magnitudes at EAgt 40
Thus an increase in excess air basically resulted in the improve-
ment of the combustion ef 1047297ciency of the SFBC For the fuel range
the highest combustion ef 1047297ciency 992e997 was obtained at
excess air of 40e80 Numerous studies on 1047297ring various biomass
fuels in conventional 1047298uidized-bed combustion systems report
similar trends [1210e1337]
Based on the analysis of both emission characteristics and
combustion ef 1047297ciency and also taking into consideration an
important issue of the combustion stability it can be generally
concluded that the best performance of this SFBC is achievable
when 1047297ring moisturized rice husk with the moisture content of
about 25 at excess air of 40e50 Under these conditions the
major gaseous emissions from the conical SFBC can be controlled
below 350 ppm for CO and within 130e140 ppm for NO (both on
6 O2 dry gas basis) while the combustion ef 1047297ciency is ensured at
a rather high value about 995
4 Conclusions
Combustion and emission characteristics have been experi-
mentally studied on a SFBC 1047297ring 80 kgh rice husk Two NO x
emission control techniques have been investigated in this work
(1) air staging of the combustion process and (2) 1047297ring rice husk as
moisturized fuel
In the test series for variable air staging the combustorhas been
tested with the aim to investigate the behavior of CO NO and C xH yemissions as well as combustion ef 1047297ciency for burning ldquoas-
receivedrdquo rice husk at a 1047297xed excess air value (of about 40) for
variable secondary-to-primary air ratio (SAPA) With increasing
SAPA from 026 to 075 the CO emission from the SFBC ranges from
about 360 to 450 ppm (on 6 O2 dry gas basis) while the NO
emission reduces at a quite low rate from about 150 to 140 ppm
However with higher SAPA C xH y emissions increase from 120 to
1400 ppm the dramatic rise being observed at SAPA gt 04 Thus
the air staging has minor effects on the CO and NO emissions To
avoid elevated C xH y emissions primary air should be supplied to
the combustor at the amount greater than the stoichiometric air
affected by the fuel properties
During the trialsfor variable fuel quality ricehusks with different
fuel moisture contents (of 84e35) have been burned at different
excess air values rangedfrom about 20 to 80 The analysis of radial
and axial CO and NO concentration pro1047297les has shown the occur-
rence of four speci1047297c regions along the combustor height charac-
terized by different rates of formation and decomposition of CO and
NO The highest rates of CO and NO decomposition are found to
occur in the bed splash zone whereas the top region in the
combustor is characterized by small axial gradients of these species
Through moisturizing rice husk the NO emission from this
SFBC can be substantially reduced while the CO emission is
effectively controlled by the secondary air injection into the bed
splash zone The best combustion and emission performance of
the SFBC is achievable when burning moisturized rice husk with
the moisture content of about 25 at excess air of 40e50 For
these optimal operating conditions the CO emission is expected
to be below 350 ppm ensuring high combustion ef 1047297ciency (about
995 maximum) while the NO emission may range from 130 to
140 ppm With increasing the fuel-moisture content to higher
values eg 30e35 the NO emission from the SFBC can be
secured even at lower values below 110 ppm However elevated
fuel moisture may likely result in deterioration of combustion
stability
Acknowledgements
The authors would like to acknowledge the 1047297nancial support
from the Thailand Research Fund (contract No BRG 50800011) The
authors also sincerely thank Dr Kasama Sirisomboon and Mr
Porametr Arromdee for their effective help in experimental tests
Fig 8 Effects of the fuel-moisture content and excess air on the emission costs for
1047297ring rice husk in the SFBC
Table 4
Heat losses and combustion ef 1047297ciency of the SFBC 1047297ring 80 kgh rice husk for
variable fuel-moisture content and excess air
W () EA () SAPA quc () qic () Combustion
ef 1047297ciency ()
84 17 033 074 204 972
41 026 049 015 994
64 022 041 007 995
76 019 049 008 994
15 17 036 046 173 978
36 030 041 040 992
63 024 050 008 994
89 021 036 005 996
25 18 042 047 134 982
41 033 035 013 995
63 027 029 003 997
83 024 024 003 997
35 16 055 048 242 971
39 042 033 012 995
65 033 026 003 997
84 029 034 005 996
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2047
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 1111
Appendix A Measurement uncertainty in the 1047298owrate of
primary air
The fractional uncertainty in the 1047298owrate of primary air was
estimated by Ref [38]
sPA frac14 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
4s2D thorn s2
V thorn s2R thorn s2
P
q (A1)
where sD is the error in physical measurement of inner diameter of the primary air pipe sV is the measurement error of the velocity
(dependent mainlyon the type of a selected probe) sR is therelative
error associated with imperfect1047297xing of the probe in air1047298ow and sP
is the uncertainty in the velocity pro1047297le across the pipe
AssumingsD frac14 1 sV frac14 15 sR frac14 1 and sP frac14 15(as advised in
Ref [38]) the uncertainty in the 1047298owrate of primary air was
calculated using Eq (A1) to be about 3
Appendix B Determining heat losses and combustion
ef 1047297ciency
In this work the combustion ef 1047297ciency of the SFBC 1047297ring rice
husk was calculated using the the heat-loss method [18]
For this combustor with no ashremoval through thebottompartthe heat loss with unburned carbon quc () was determined by
quc frac14 32866
LHV
C fa
100 C fa
A (B1)
where C fa is the unburned carbon content in the1047298y ash wt and A
is the fuel-ash content wt on ldquoas-receivedrdquo basis
The heat loss owing to incomplete combustion qic () was
quanti1047297ed based on theconcentrations (vol)of COand C xH y (asCH4)
measured in the 1047298ue gas at the cyclone outlet at actual excess air
qic frac14 eth1264 CO thorn 3582 CH4THORN V dgeth100 qucTHORN
LHV (B2)
where V dg is the volume of dry 1047298ue gas at the cyclone outlet Nm3
kg calculated by Refs [1822] using the fuel ultimate analysis onldquoas-receivedrdquo basis and excess air ratio at this point
The combustion ef 1047297ciency of the 1047298uidized-bed combustor hc
() was then determined by
hc frac14 100 ethquc thorn qicTHORN (B3)
References
[1] Natarajan E Nordin A Rao AN Overview of combustion and gasi1047297cation of rice husk in 1047298uidized bed reactors Biomass Bioenergy 199814533e46
[2] Bhattacharya SC Abdul Salam P Sharma M Emissions from biomass energyuse in some selected Asian countries Energy 200025169e88
[3] Werther J Saenger M Hartge EU Ogada T Siagi Z Combustion of agriculturalresidues Prog Energy Combust Sci 2000261e27
[4] Janvijitsakul K Kuprianov VI Similarity and modeling of axial CO and NO
concentration pro1047297les in a 1047298uidized-bed combustor (co-)1047297ring biomass fuelsFuel 2008871574e84
[5] Demirbas A Combustion characteristics of different biomass fuels ProgEnergy Combust Sci 200430219e30
[6] Leckner B Karlsson M Gaseous emissions form circulating 1047298uidized bedcombustion of wood Biomass Bioenergy 19934379e89
[7] Spliethoff H Hein KRG Effect of co-combustion of biomass on emissions inpulverized fuel furnaces Fuel Process Technol 199854189e205
[8] Lyngfelt A Leckner B Combustion of wood-chips in circulating 1047298uidized bedboilers e NO and CO emissions as functions of temperature and air-stagingFuel 1999781065e72
[9] Chyang CS Wu KT Lin CS Emission of nitrogen oxides in a vortexing 1047298uidizedbed combustor Fuel 200786234e43
[10] Armesto E Bahillo A Cabanillas A Veijonen K Otero J Combustion behavior of rice husk in a bubbling 1047298uidised bed Biomass Bioenergy 200223171e9
[11] Kuprianov VI Janvijitsakul K Permchart W Co-1047297ring of sugar canebagasse with rice husk in a conical 1047298uidized-bed combustor Fuel 200685434e42
[12] Fang M Yang L Chen G Shi Z Luo Z Cen K Experimental study on rice huskcombustion in a circulating 1047298uidized bed Fuel Process Technol2004851273e82
[13] Permchart W Kouprianov VI Emission performance and combustion ef 1047297-ciency of a conical 1047298uidized-bed combustor 1047297ring various biomass fuelsBioresour Technol 20049283e91
[14] Sirisomboon K Kuprianov VI Arromdee P Effects of design features oncombustion ef 1047297ciency and emission performance of a biomass-fuelled 1047298uid-ized-bed combustor Chem Eng Process Process Intens 201049270e7
[15] Leckner B Amand LE Lucke K Werther J Gaseous emissions from co-combustion of sewage sludge and coalwood in a 1047298uidized bed Fuel200483477e86
[16] Eaimsa-ard S Kaewkohkiet Y Thianpong C Promvonge P Combustionbehavior in a dual-staging vortex rice husk combustor with snail entry IntCommun Heat Mass Transfer 2008351134e40
[17] Madhiyanon T Lapirattanakun A Sathitruangsak P Soponronnarit S A novelcyclonic 1047298uidized-bed combustor (J-FBC) combustion and thermal ef 1047297-ciency temperature distribution combustion intensity and emission of pollutants Combust Flame 2006146232e45
[18] Basu P Cen KF Jestin L Boilers and burners New York Springer 2000[19] Kaewklum R Kuprianov VI Experimental studies on a novel swirling 1047298uid-
ized-bed combustor using an annular spiral air distributor Fuel20108943
e52
[20] Kouprianov VI Permchart W Emission from a conical FBC 1047297red witha biomass fuel Appl Energy 200374383e92
[21] Kaewklum R Kuprianov VI Douglas PL Hydrodynamics of airesand 1047298ow ina conical swirling 1047298uidized bed a comparative study between tangential andaxial air entries Energy Convers Manage 2009502999e3006
[22] Bezgreshnov AN Lipov YM Shleipher BM Computations of steam boilersMoscow Energoatomizdat 1991 [in Russian]
[23] Kuprianov VI Application of a cost-based method of excess air optimizationfor the improvement of thermal ef 1047297ciency and environmental performance of steam boilers Renew Sustain Energy Rev 20059474e98
[24] Streimikiene D Roos I Rekis J External cost of electricity generation in BalticStates Renew Sustain Energy Rev 200913863e70
[25] Zhang Q Weili T Yumei W Yingxu C External costs from electricity gener-ation of China up to 2030 in energy and abatement scenarios Energy Pol2007354295e304
[26] Hainoun A Almoustafa A Aldin MS Estimating the health damage costs of Syrian electricity generation system using impact pathway approach Energy
201035628e
38[27] Nguyen KQ Internalizing externalities into capacity expansion planning thecase of electricity in Vietnam Energy 200833740e6
[28] Kitou E Horvath A External air pollution costs of telework Int J Life CycleAssess 200813155e65
[29] Wei X Zhang L Zhou H Evaluating the environmental value of pollutants inChina power industry In Proceedings of the international conference onenergy and the environment Shanghai China 2003
[30] Salisdisouk N The concept of integrated resource planning In Proceeding of the workshop on electric power quality safety and ef 1047297ciency of its useselectric power system management Pathum Thani Thailand 1994
[31] Yang YB Shari1047297 VN Swithenbank J Effect of air 1047298ow rate and fuel moisture onthe burning behaviours of biomass and simulated municipal solid wastes inpacked beds Fuel 2004831553e62
[32] Zhao W Li Z Zhao G Zhang F Zhu Q Effect of air preheating and fuel moistureon combustion characteristics of corn straw in a 1047297xed bed Energy ConversManage 2008493560e5
[33] Tillman DA Rossi AJ Kitto WD Wood combustion New York AcademicPress 1981
[34] Winter F Wartha C Hofbauer H NO and N2O formation during thecombustion of wood straw malt waste and peat Bioresour Technol19997039e49
[35] Sun Z Jin M Zhang M Liu R Zhang Y Experimental studies on cotton stalkcombustion in a 1047298uidized bed Energy 2008331224e32
[36] Smart JP Robert PA De Soete GG The formation of nitrous oxide in a large-scale pulverized-coal 1047298ames J Inst Energy 199063131e5
[37] Abu-Qudais M Fluidized-bed combustion for energy production from olivecake Energy 199621173e8
[38] Trembovlya VI Finger ED Avdeeva AA Thermo-technical tests of boiler unitsMoscow Energoatomizdat 1991 [in Russian]
VI Kuprianov et al Energy 36 (2011) 2038e 20482048
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 211
increasing excess air andor combustion temperature both
enhancing therateof COoxidation to CO2 [18911] Some reductionin
COcan be achieved when1047297ring biomass fuels withlowerash content
[1113] The bubbling 1047298uidization mode seems to be one of the effec-
tive regimes for operating the 1047298uidized-bed combustion systems
[131013] as ensuringthe high intensive mixingof fuel particles and
air in the bed region (promoting CO reduction) the latter being
signi1047297cantly affected by the air distributor design [14] However an
in1047298uence of the air staging on CO emission during the 1047298uidized-bed
combustion of biomass is reported to be rather weak [15]
A large number of research studies have addressed emission
characteristics and combustion ef 1047297ciency for 1047297ring rice husk in
various laboratory-scale 1047298uidized-bed combustion techniques
such as bubbling 1047298uidized-bed vortexing 1047298uidized-bed and circu-
lating 1047298uidized-bed combustors [9e12] Due to moderate bed
temperatures (normally not higher than 850 C) NO x emissions
from the combustors are generally below 180 ppm (on 6 O2 dry
gas basis) while CO emission is found to be elevated up to
800 ppm Combustion ef 1047297ciency of these devices operated at
optimal conditions is reported to be within 96e98 Experimental
results revealed minor effects of the air staging on these emissions
as well as on combustion ef 1047297ciency of the vortexing and circulating
1047298uidized-bed combustors 1047297ring rice husk [912]Recently two novel combustion techniques ensuring fuel
oxidation in a strongly swirled 1047298ow a vortex combustor and
a cyclonic 1047298uidized-bed combustor have been developed and
tested for 1047297ring rice husk [1617] Under optimal operating condi-
tions high (over 99) combustion ef 1047297ciency can be achieved in
these pilot reactors while controlling CO emission below 400 ppm
However NO x emissions from the combustors are reported to be
elevated up to 300 ppm for the vortex combustor [16] or rather
high 350e425 ppm for the cyclonic 1047298uidized-bed combustor [17]
Such substantial NO x emissions are mainly caused by (i) elevated
excess air required for sustaining the strongly swirled gasesolid
1047298ow and (ii) high-temperature conditions in these rice husk-fuel-
led combustors operated with a signi1047297cant heat release rate per
unit volume Effects of the air staging on both emissions andcombustion ef 1047297ciency of the vortex and cyclonic 1047298uidized-bed
combustors are reported to be rather weak It should be noted that
elevated excess air basically leads to lower thermal ef 1047297ciency of
a power plant (or any other energy conversion units) using these
devices mainly due to an increase in the heat loss with waste gas
(affected by a signi1047297cant volume of excessive air) [18]
Kaewklum and Kuprianov [19] have recently reported a pio-
neering study on a laboratory-scale swirling 1047298uidized-bed
combustor (SFBC) 1047297ring rice husk In this innovative combustion
technique a swirling 1047298uidized bed is generated due to the special
design of a primary air distributor used in this combustion tech-
nique as the swirl generator Unlike in the vortexing 1047298uidized-bed
combustor secondaryair in this SFBC is injected into the bed splash
zone ie at a relatively low level above the (primary) air distrib-utor The tangential injection of secondary air sustains the rota-
tional gasesolid 1047298ow in the combustor At optimal excess air
40e60 the burning of rice husk in the SFBC is characterized by
high about 995 combustion ef 1047297ciency while CO and NO emis-
sions can be limited within 150e300 ppm and 170e210 ppm
respectively However no effects of the air staging on emission
performance and combustion ef 1047297ciency of the SFBC have been
addressed in this pioneering study
As can be generally concluded from the literature review
compared to the conventional (ie non-swirling) 1047298uidized-bed
combustors the combustion techniques with a rotational gasesolid
1047298ow ensure higher combustion ef 1047297ciency at minimized CO emis-
sion accompanied however by moderate (for the SFBC) or
elevated (for the vortex combustor) or high (for the cyclonic
1047298uidized-bed combustor) NO x emissions The NO x control in these
high ef 1047297ciency devices 1047297ring rice husk is therefore an issue of
paramount importance
Burning biomass in the form of moisturized fuel which prior to
the combustion can be prepared by adding water to ldquoas-receivedrdquo
fuel is proven to be an effective least-cost NO x emission control
technique as reportedin studies on1047297ring of wood sawdust and rice
husk in a conventional 1047298uidized-bed combustor with a cone-shape
bed [420] Moreover a substantial reduction in the bed tempera-
ture occurring with increasing fuel moisture provides more
favorable operating conditions for preventing undesirable ash-
related problems in the 1047298uidized-bed combustor (eg bed
agglomeration and wall slagging) particularly when 1047297ring high-
alkali biomass fuels [3] Howeverwhen using this conical 1047298uidized-
bed combustor the reduction of NO x emissions has been accom-
panied by a noticeable increase in CO emission and corresponding
deterioration of the combustion ef 1047297ciency Thus selection of the
most appropriate fuel-moisture content should be considered
along with optimization of air supply to the combustion system
This study was aimed at determining the technical feasibility of
an effective control of NO emission during the combustion of rice
husk in the SFBC through air staging of the combustion and fuel
moisturizing Detail analysis of the formation and decomposition of major gaseous pollutants (CO and NO) at different locations in this
reactor for variable operating conditions and fuel properties were
the focus of this work Optimization of the fuel-moisture content
and air supply for minimizing CO and NO emissions from this SFBC
1047297ring moisturized rice husk was also among the main objectives of
this study
2 Materials and methods
21 Experimental facilities
Fig1 depicts the general view of the experimental setup and the
schematic diagram of the SFBC It can be seen in Fig 1a that the
system included the combustor with a start-up burner a cyclonea fuel screw feeder and a blower Additionally Fig 1b provides the
design and geometrical details of the SFBC which was made of 45-
mm-thick steel sheet and covered internally with the 50-mm-thick
refractory
The combustor consisted of a conical (bottom) part1047297lled in with
lsquoroundrsquo quartz sand (with the particle sphericity of 086 and density
of 2650 kgm3) used as the inert bed material and a cylindrical
(upper) part The particle (sieve) size of 05e06 mm and static bed
height of 30 cm were selected to be the main characteristics of the
bed material as those ensuring the stable swirling 1047298uidized-bed
regime Under ldquocoldrdquo operating conditions the minimum 1047298uidiza-
tion velocity of the air-sand bed with these characteristics was
about 08 ms while the minimum velocity of the fully swirling
1047298uidized-bed mode was 13 ms [21]The annular spiral air distributor at the combustor bottom was
made up of eleven blades 1047297xed at an angle of 14 to the horizontal
and served as the swirl generator of primary air (PA) the latter
being supplied by the 25-hp blower The distributor had an annular
air exit with 01 m inner and 025 m outer diameters The distance
between two neighbor blades was variable (in a linear relationship
with radius) thus forming a trapezoidal cross-sectional area of
0012 m2 (total) for the air1047298ow between the blades To stabilize the
swirl motion of the gasesolid bed a steel cone was 1047297xed on the top
of the air distributor as shown in Fig 1b
Primary air was supplied to the air distributor by the blower
through an air pipe of a 01-m inner diameter as shown in Fig 1a
The 1047298owrate of primary air was controlled using a butter1047298y valve
arranged on the air pipe downstream from the blower The
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2039
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 311
relationship between the actual air1047298ow rate and valve opening was
developed using a measuring system ldquoTesto-454rdquo (Testo Germany)
with a hot-wire probe The measurement uncertainty in the 1047298ow-
rate of primary air was about 3 as estimated in Appendix A
The screw-type feeder delivered the fuel over the bed at a 06 mlever above the air distributor A three-phase inverter was used to
control the fuel feed rate via changing rotation speed of the screw
feeder As established by repeated calibrations the fuel feed rate
was in a quasi-linear correlation with the rotational speed (rpm) of
the feeder For the fuel feed rates of 30e100 kgh the measurement
accuracy of the fuel feed rate was within the range of 3 to 5
when varying the fuel-moisture content from 84 (in ldquoas-receivedrdquo
fuel) to 40
During the combustor start-up a diesel-1047297red burner from Riello
Burners Co (model ldquoPress G24rdquo) was used to preheat the sand to
the speci1047297ed temperature (of about 700 C) The burner air was
tangentially injected into a splash bed zone at a 05 m level through
the burner inclined at a 30 angle to the horizontal Upon
attaining required bed temperature the burner was turned offwhereas desired fuel supply was ensured by the screw feeder
However the burner fan remained to operate delivering secondary
air (SA) to the SFBC during the combustion tests with the aim to
mitigate CO in the bed splash zone and also to protect the burner
head against overheating and impacts from solids The 1047298owrate of
secondary air was controlled by changing an opening of the burner
fan The measurement uncertainty in the 1047298owrate of secondary air
(estimated by the same method as for the primary air) was found to
be about 4
A ldquoTesto-350XL rdquo gas analyzer (Testo Germany) was used to
measure the temperature and gas concentrations (O2 CO NO and
C xH y) along radial and axial directions in the combustor as well as
at the exit of the ash-collecting cyclone The measurement accu-
racies were 05 for the temperature 5 for CO and C xH y
ranged from 100 to 2000 ppm 10 for CO and C xH y higher than
2000 ppm 5 for NO and 02 vol for O2 Besides Chro-
meleAlumel thermocouples were 1047297xed at different levels in the
reactor for (i) monitoring the temperatures during the combustor
start-up (with the accuracy 1) and (ii) obtaining the axialtemperature pro1047297les For the particular test run the excess air ratio
was quanti1047297ed by Ref [22] using the O2 CO and C xH y concentra-
tions at the cyclone exit with an uncertainty of 2 Afterwards
corresponding percentages of total air (TA) and excess air (EA) were
calculated for each trial
Fly ash was sampled from the ash collector (see Fig 1a) to
quantify the content of unburned carbon in the ash required for
predicting the associated heat loss (as discussed below)
22 The fuels
In order to approach the work objectives two series of experi-
mental tests for (i) variable air staging and (ii) variable fuel prop-erties were carried out in this experimental study Table 1 shows
major fuel properties the ultimate and proximate analyses as well
as the lower heating value (LHV) of rice husk used in the tests at
different secondary-to-primary air ratios (SAPA)
Fig 1 (a) Experimental setup and (b) the laboratory-scale swirling 1047298uidized-bed combustor (SFBC)
Table 1
Ultimate and proximate analyses and lower heating value of rice husk 1047297red in the
SFBC during the experimental tests for variable air staging (W frac14 fuel-moisture
A frac14 fuel-ash VM frac14 volatile matter FC frac14 1047297xed carbon LHV frac14 lower heating value)
Ultimate analysis (wt on ldquoas-
receivedrdquo basis)
Proximate analysis (wt on ldquoas-
receivedrdquo basis)
C H O N S W A VM FC LHV (kJkg)
4220 458 2784 025 003 92 159 574 155 13600
VI Kuprianov et al Energy 36 (2011) 2038e 20482040
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 411
The ultimate analysis and LHV of rice husks1047297red in the test runs
for variable fuel quality are shown in Table 2 Since the variation in
the fuel-moisture content affected all other fuel properties the fuel
ultimate analysis and LHV of moisturized rice husks are provided in
Table 2 on ldquoas-1047297redrdquo basis For the particular fuel option the
constituents of the fuel analysis were calculated using the fuel
properties of ldquoas-receivedrdquo fuel (see Table 2 Option 1) accounting
for the actual fuel-moisture content Afterwards LHV for the
moisturized fuels was determined by Ref [22] using corresponding
fuel ultimate analyses from Table 2
The rice husks used in the two experimental series were quite
similar by their chemical and physical properties on ldquoas-receivedrdquo
basis It can be seen in Tables 1 and 2 that the sulfur content in the
rice husks was quite low For this reason SO 2 was not addressed in
this study The dimensions of ldquoas-receivedrdquo rice husk particles were
about 2 mm wide 05 mm thick and 10 mm long while the particle
density was about 1000 kgm3
23 Experimental planning
231 Tests for variable air staging
During this test series rice husk was burned at the 1047297xed fuel
feed rate 80 kgh and excess air of 40 for four values of SAPA
026 040 056 and 075 In each trial (ie for the particular oper-
ating conditions) CO NO and C xH y emissions were determined
together with the O2 concentration at the cyclone exit The main
goal of this test series was to determine the value (or range) of SA
PA ensuring the minimum of these emissions which could be taken
into consideration in the detailed study (test series) below
232 Tests for variable fuel moisture
Fuel moisture (W) and excess air (EA) were chosen as inde-
pendent variables in this test series while the fuel feed rate in all
trials was adjusted at nearly the same value about 80 kgh as in
previous test series Secondary air was supplied to the SFBC at
a minimum 1047298ow rate Q ba frac14 0024 Nm3s required for reliable
cooling of the start-up burner during the combustion testsThis test series included two groups of trials The1047297rst group was
devoted to the behavior of temperature and gas (O2 CO NO)
concentrations in radial and axial directions in the combustor In
this detailed study 1047297ve rice husks (in Table 2 Options 1e5) were
burned at a similar EA value of about 40 The radial temperature
and gas concentration pro1047297les were obtained for 1047297ve levels ( Z )
above the datum Z frac14 0 (see Fig 1b) However the axial pro1047297les
were plotted using the variables measured at eight levels along the
combustor centerline
In the second group of trials CO and NO emissions from the
combustor were quanti1047297ed for all (six) rice husks in Table 2 which
were burned in the SFBC at the excess air values of about 20 40
60 and 80 Using the emission magnitudes optimal values
(ranges) of both EA and fuel-moisture content were determined
using a cost-based approach as discussed below
For the two test series the heat losses with unburned carbon
quc and owing to incomplete combustion qic were quanti1047297ed
together with combustion ef 1047297ciency by using models provided in
Appendix B Note that the effects of C xH y were taken into account
when determining qic for the tests at variable air staging However
these effects were neglected in the qic for the second test series
when the emission of hydrocarbons was at a rather low level in all
trials
24 A model for optimizing excess air and fuel-moisture content
In this work a cost-based approach [23] was applied to deter-
mine the optimal values (ranges) of excess air and fuel-moisture
content leading to the minimized emission (or ldquoexternalrdquo) costs of
1047297ring rice husk in this SFBC Ignoring the effects of SO2 (as negli-
gible) and CO2 (due to the relatively low speci1047297c ldquoexternalrdquo cost of
carbon dioxide) the corresponding objective function used for the
optimization can be represented as
J ec frac14 Min
P NO x_mNO x
thorn P CO _mCO
(1)
where _mNO x and _mCO are emission rates (calculated by Ref [23])
and P NO x and P CO are speci1047297c ldquoexternalrdquo costs of NO x (as NO2) and
CO respectively
It can be concluded from analysis of the objective function that
with the above assumptions the optimal values of excess air and
fuel-moisture content are solely dependent on the cost ratio
P NO x=P CO while the emission costs are apparently affected by all the
variables in Eq (1)
In every country the (average) emission externalities are
strongly affected by the economic structure and activities Studies
on the externalities of heat and electricity generation reveal
therefore a signi1047297cant diversity of the speci1047297c ldquoexternalrdquo costs For
the EU-27 countries the speci1047297c cost of NO x emissions is reportedto be about 7000 Vt (including impacts on human health
ecosystems crops and materials) [24] whereas for Asian countries
this index seems to be substantially (or signi1047297cantly) lower
[25e27] For instance for neighbor China P NO x frac14 2438 US$=t
(including only dominant costs ie those related to the health
damage and climate change) [25] Unlike for NO x limited data on
the externalities by CO is available in literature As revealed by
some relevant studies P CO rises as P NO x increases However the
ratio of P NO x to P CO (ie P NO x
=P CO) is reported to be within certain
limits ranging basically from 5 to 8 [28e30]
Taking the above into consideration it was decided to consider
two options in this optimization study using (1)
P NO x frac14 3000 US$=t and P CO frac14 600 US$t (ie for P NO x
=P CO frac14 5)
and (2) P NO x frac14 3000 US$=t and P CO frac14 400 US$t (ie forP NO x=P CO frac14 75)
3 Results and discussion
31 Emission and combustion characteristics of SFBC for variable
air staging
311 Emissions
Fig 2 shows the CO NO and C xH y emissions (on 6 O2 dry gas
basis) from the combustor 1047297ring 80 kgh rice husk at different SA
PA ratios when excess air was adjusted at about 40
Asseen in Fig 2a the CO emissionwas weakly in1047298uenced by the
air staging factor With increasing SAPA this emission somewhat
increased from about 360 to 450 ppm staying nevertheless at
Table 2
Ultimate analysis and LHV of rice husks 1047297red in the experimental tests for variable
fuel moisture (W)
Property Fuel sample (Option)
1 2 3 4 5 6
Ultimate analysis (wta)
C 4050 3758 3537 3316 3095 2874
H 407 377 355 333 311 289
O 2869 2663 2506 2349 2193 2036
N 031 029 027 025 024 022
S 003 003 003 002 002 002
W 84 150 200 250 300 350
A 180 1670 1572 1474 1376 1277
LHV (kJkg) 14620 13390 12460 11530 10600 9670
a On ldquoas-receivedrdquo basis for Option 1 on ldquoas-1047297redrdquo basis for Options 2e6
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2041
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 511
a rather low level basically due to the secondary air injection into
the bed splash zone As can be concluded based on the results from
Ref [12] and present study excess air (or percentage of total air) is
an important factor in controlling the CO emission in this SFBC
whereas SAPA shows quite weak effects
With increasing SAPA within the selected range 026e075 the
NO emission exhibited a slight reduction from about 150 to140 ppm (see Fig 2a) mainly due to the diminishing of the bed
temperature and reduction of the O2 concentration in the bed the
latter being occurred because of the lowering of PA Thus the air
staging does not seem to be an effective measure to control the NO
emission in this combustor 1047297ring rice husk
It can be seen in Fig 2b that at relatively small proportions of
secondary air the C xH y emissions were at a quite low level
However at SAPA gt 04 these emissions showed a signi1047297cant
increase from 120 to 1400 ppm which can be explained by the
sub-stoichiometric conditions in the bed region Under such
conditions more volatiles were carried over from the combustor
bottom causing the above increase in the C xH y emissions Thus
primary air should be supplied to the SFBC at a 1047298ow rate ensuring
the oxidation conditions in the bed ie with some excess withregard to the theoretical (stoichiometric) air For instance at
EA frac14 40 the combustor should be operated at 026 lt SAPA lt 04
(see Fig 2b) for avoiding high C xH y emissions from this SFBC 1047297ring
rice husk As can be generally concluded the lower limit of SAPA is
speci1047297ed with the aim to provide the reliable coolingof the start-up
burner whereas the upper one is selected taking into account that
EA should be somewhat greater than SA
312 Heat losses and combustion ef 1047297ciency
The analyses of 1047298y ashes for unburned carbon for this test series
indicated the high rate of fuel burnout in this conical SFBC
Depending on SAPA the unburned carbon content in the 1047298y ashes
varied from 081 to 24 the minimum value being found at the
highest SAPA ratio
Table 3 shows the heat losses with unburned carbon (quc) and
owing to incomplete combustion (qic) together with the combus-
tion ef 1047297ciency (all as the percentage of LHV) for 1047297ring 80 kgh rice
husk at excess air of about 40 for different values of SAPA An
increase in SAPA led to a noticeable reduction in the heat loss with
unburned carbon basically due to the higher rate of fuel burnout
which waslikelycaused by an increase in the residence time of char
particles in the combustor However the exponential rise of qic can
be explained by the above behavior of CO and C xH y emissions
Due to the opposite trends exhibited by the heat losses the
combustion ef 1047297ciency was found to have a maximum 991 for1047297ring rice husk at SAPA of 04e06 and EA of about 40
32 Emission and combustion characteristics of SFBC for variable
fuel moisture
As revealed by the experimental results the temperature and
gas concentrations (O2 CO and NO) in this conical SFBC were rep-
resented by three-dimensional patterns (1047297elds) showing the
effects of combustor hydrodynamics fuel quality and operating
conditions on the radial and axial pro1047297les of the temperature and
chemical species Note that at a given excess air SA raised with
increasing fuel moisture because of the reduction in the theoretical
air while the 1047298owrate of secondary air was 1047297xed at the above
constant value changing from 29 (for ldquoas-receivedrdquo fuel) to 41(for rice husk with the highest moisture content) which resulted in
corresponding diminishing of PA Due to the reduction in the
combustion temperature and also in the theoretical air and PA
the residence time of char particles in the bottomregion of the SFBC
was substantially greater when burning rice husks with higher
moisture content leading to the higher rates of devolatilization and
burnout of fuel particles in this region and thus affecting signi1047297-
cantly the behavior of all variables in the reactor
At EA frac14 40 or higher oxidizing conditions were basically
provided in the combustor bottom which justi1047297ed the ignorance of
C xH y emissions in this test series
321 Radial and axial temperature and gas concentration pro 1047297les
in the SFBC Fig 3 shows the radial temperature and O2 concentration
pro1047297les at 1047297ve levels ( Z ) above the air distributor in the SFBC 1047297ring
80 kgh rice husk at excess air of about 40 for variable fuel prop-
erties (Options 1e5 in Table 2) As seen in Fig 3 the variables
exhibitedquitesimilar behaviorsat different levels ( Z ) above the air
distributor The radial temperature pro1047297les were found to be rather
uniform indicating the highly intensive heat-and-mass transfer
along the radius With increasing fuel moisture (at a 1047297xed excess air
level) the temperature at all points in the combustor volume was
found to be reduced (despite the above increase in the residence
time) because of the apparent in1047298uence of the latent heat of water
evaporation Similar results are reported in some studies on
conventional 1047298uidized-bed and 1047297xed-bed combustion systems
1047297ring biomass fuels with variable fuel moisture [4203132]
Fig 2 Effects of secondary-to-primary air ratio (SAPA) on the (a) CO NO and (b) C xH y emissions from the SFBC 1047297ring rice husk at excess air of 40
Table 3
Heat losses and combustion ef 1047297ciency of the SFBC 1047297ring 80 kgh rice husk at excess
air of 40 for variable secondary-to-primary air ratio
SAPA quc () qic () Combustion ef 1047297ciency ()
026 094 024 988
040 051 035 991
056 047 043 991
075 031 212 976
VI Kuprianov et al Energy 36 (2011) 2038e 20482042
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 611
Z = 267 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r
e p m e T
Z = 267 m
0
5
10
15
20
00 02 04 06 08 10
rR
O 2
) l o v ( n o i t a r t
n e c n o c
Z = 217 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r e p m e T
Z = 217 m0
5
10
15
20
00 02 04 06 08 10
rR
O 2
) l o v ( n o i t a r t n e c n o c
Z = 155 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r e p m e T
Z = 155 m
0
5
10
15
20
00 02 04 06 08 10
rR
O 2
) l o v ( n o i t a r t n e c n o c
Z = 101 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r e p m e T
Z = 101 m
0
5
10
15
20
00 02 04 06 08 10
rR
O 2
) l o v ( n o i t a r t n e c n o c
Z = 047 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r e p m e T
Z = 047 m
0
5
10
15
20
00 02 04 06 08 10
rR
O 2
)
l o v ( n o i t a r t n e c n o c
a b
Fig 3 Radial (a) temperature and (b) O2 concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-)
W frac14 84 () W frac1415 () W frac14 20 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2043
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 711
Implementation of air staging seems to have a signi1047297cant impact
on the behavior of gaseous species in both radial and axial direc-
tions Due to the tangential injection of secondary air into the bed
splash zone the radial O2 concentration pro1047297les were characterized
by a positive radial gradient (the most signi1047297cant being observed at
Z frac14 101 m) which resulted in the higher O2 concentration near the
combustor wall than that at the centerline (ie at r R frac14 0)
However the radial gradient of O2 was gradually attenuated along
the bed height Note that the injection of secondary air affected the
radial O2 concentration pro1047297les not only in upper regions of the
reactor but also at levels below the injection point (in Fig 3b see
the pro1047297les at Z frac14 047 m) since the secondary air was injected at
the negative angle In the meantime with increasing the fuel-
moisture content the O2 concentration at all the points across the
combustor was found to be reduced and this reduction was caused
by some physical and chemical factors as addressed below in the
discussion of axial O2 concentration pro1047297les
The axial temperature and O2 concentration pro1047297les in the SFBC
are shown in Fig 4 for the same fuel options and operating
conditions as in Fig 3 At 1047297xed excess air a positive axial temper-
ature gradient was found to occur in the lower part of the reactor
for all the fuels (see Fig 4a) likely due to the diminishing of heat
release in the dense bed (caused by air staging) and (ii) in1047298uence of secondary air injected into the bed splash zone at the ambient
temperature However with higher fuel moisture (ie with dete-
riorating fuel quality) the temperature attained its maximum at
lower levels (Z) above the air distributor which can be explained by
the effects of the residence time The maximum temperature for
burning ldquoas-receivedrdquo rice husk (with W frac14 84) was rather high
about 980 C however it was reduced to 850e860 C when
increasing the fuel-moisture content to 25e30
However with raising fuel moisture the rate of oxygen
consumption in the bottom region of the SFBC ( Z lt 08 m) was
apparently higher which despite the above reduction in the bed
temperature resulted in the lower O2 concentration at all locations
in this region (as seen in Fig 4b) This phenomenon can be
explained by the greater residence time (leading to a higher yield of CO and other combustibles with fuel volatiles) and also greater
contribution of the ldquowetrdquo oxidation of char-C by OH radicals both
leading to higher rates of CO formation and consequently O2
consumption in this region The next region (08 lt Z lt 10 m) was
characterized by a noticeable regaining (rise) of O2 as the response
to secondary air injection However in the freeboard of the
combustor ( Z gt 10 m) the O2 concentration was diminished along
the reactor height at a rather low rate and this reduction was
accompanied by the gradual converging of the axial pro1047297les as all
the tests in Fig 4 for variable fuel moisture were conducted at
(nearly) the same EA
Fig 5 shows the radial CO and NO concentration pro1047297les at
different levels above the air distributor for the same fuels and
operating conditions as in Fig 3 Both CO (Fig 5a) and NO (Fig 5b)
were signi1047297cantly affected by fuel moisture and showed negative
gradients along the radius (different numerically) at all the levels in
the SFBC For the 1047297xed fuel-moisture content due to the effects of
secondary air the CO concentration in the peripheral zone across
the combustor was much lower compared to that at the centerline
thus forming the above radial gradient of CO However the NO
concentration varied weakly along the radius except at Z frac14 047 m
The occurrence of the NO maximum at the centerline indicated
higher rates of both fuel devolatilization and oxidation in the
central zone of the reactor (compared to those at the combustor
wall) despite the uniformity of the temperature and the opposite
trends of the O2 concentration pro1047297les across the SFBC (see Fig 3b)
In the freeboardof the reactor the radial CO and NO gradients were
found to be gradually attenuated with higher Z
Fig 6 depicts the axial CO and NO concentration pro1047297les in this
combustor As seen in Fig 6a the pro1047297les exhibited four sequent
regions along the combustor height With increasing fuel moisture
the CO concentration in the 1047297rst region (0lt Z lt 08 m) was
apparently higher at all the levels above the air distributor
particularly at the centerline (see Fig 5a) mainly due to (i) longerresidence time (leading to greater yield of CO with volatiles) (ii)
lower PA (reducing the rate of CO oxidation) (iii) higher contri-
bution of ldquowetrdquo oxidation of char-C by OH radicals and (iv) lower
bed temperature causing an increase in the COCO2 ratio in the
products of fuel-char oxidation [33]
In the second region (08 lt Z lt 10 m) the CO concentration
along the combustor axis was found to be drastically reduced
mainly due to the effects of secondaryair the greater rate of the CO
reduction being observed at a higher level of fuel moisture (ie at
higher SA) In the third region (10 lt Z lt 18 m) the CO concentra-
tion regained substantial values along the centerline mainly due to
oxidation of unburned hydrocarbons and fuel-C carried over from
the bed region to CO However in the fourth (upper) region the CO
concentration was found to be gradually reduced along thecombustor height likely via homogeneous reaction of CO with
residual O2 and OH [333]
Like for CO four speci1047297c regions can be distinguished in the
axial NO concentration pro1047297les as can be seen in Fig 6b In the 1047297rst
region (0 lt Z lt 08 m) NO was mainly formed from NH3 in volatiles
(a major precursor of NO in biomass combustion) via the fuel-NO
formation mechanism [343435] With increasing fuel moisture
despite the reduction in bed temperature the NO concentration at
the reactor centerline showed a trend to increase at any given Z
mainly due to (i) greater residence time promoting a higher yield
of nitrogenous species with fuel volatiles and (ii) enhanced
a b
500
600
700
800
900
1000
1100
0 1 2 3
Height above air distributor (m)
) C deg ( e r u t a r e p m e T
0
5
10
15
20
0 1 2 3Height above air distributor (m)
O 2
) l o v ( n o i t a r t n e c n o c
Fig 4 Axial (a) temperature and (b) O2 concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415
(
) Wfrac14
20 (6
) Wfrac14
25 and (A
) Wfrac14
30
VI Kuprianov et al Energy 36 (2011) 2038e 20482044
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 811
Z = 267 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 267 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t
n e c n o c O N
Z = 217 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 217 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O N
Z = 155 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 155 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0rR
) m
p p ( n o i t a r t n e c n o c O N
Z = 101 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 101 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O N
Z = 047 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
)
m
p p ( n o i t a r t n e c n o c
O C
Z = 047 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O N
ab
Fig 5 Radial (a) CO and (b) NO concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 ()
W frac14 15 () W frac1420 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2045
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 911
oxidation of NH3 by OH [36] In the second region (08 lt Z lt 10 m)
the chemical reactions responsible for NO decomposition such as
catalytic reduction of NO by CO on the char surface [3] as well as
reaction of NO with NH3 at oxygen de1047297ciency [34] were predom-
inant This resulted in a signi1047297cant reduction of the NO concen-
tration along the combustor height and the NO reduction was more
apparent when burning rice husks with higher moisture contentsHowever the injection of secondary air promoted a slight increase
in NO in the third region (10 lt Z lt 16 m) of the reactor as a result
of oxidation of the nitrogenous species carried over from the
combustor bottom In the fourth (upper) region the pro1047297les
exhibited some diminishing of the NO concentration along the
reactor height because of the catalytic reduction of NO by CO
occurred however at a rather low rate
322 Emissions
Fig 7 depicts the CO and NO emissions (on 6 O 2 dry gas basis)
from the combustor 1047297ring 80 kgh rice husk at variable EA for the
whole range of fuel moisture (Options 1e6 in Table 2) It can be
concluded from analysis of data in Fig 7a that at EA greater than
40 the CO emission from the SFBC can be controlled ata comparatively low level below 350 ppm regardless of the fuel
quality On the contrary at excess air lower than 40 the CO
emission exhibited quite strong effects of both fuel quality and EA
Note that at EA frac14 20 the CO emission was extremely high
3000e7000 ppm for the whole range of fuel moisture However
with increasing the fuel-moisture content from 84 (in ldquoas-
receivedrdquo rice husk) to 25 the CO emission at this lowest EA
exhibited some reduction roughly from 4000 to 3000 ppm basi-
cally caused by (i) the higher rate of chemical reaction between CO
and OH [333] and (ii) higher rate of CO decomposition in the
freeboard (due to enhanced SA and greater residence time) Similar
trend is reported in some studies on effects of fuel moisture on the
CO emission from the 1047297xed-bed combustion systems [3132]
However with further increase in the fuel-moisture content (from
25 to 35) the CO emission from the SFBC was found to rise from
about 3000 to 7000 ppm likely due to the signi1047297cant contribution
of ldquowetrdquo oxidation of char-C occurred at the lowered combustion
temperatures This trend is in concordance with the behavior of theCO emission during the conventional 1047298uidized-bed combustion of
some biomass fuels with variable moisture [420]
It can be seen in Fig 7b that at EA frac14 40 (the ldquocriticalrdquo value for
the CO emission) the NO emission diminished from about 170 to
130 ppm (or by some 25) when changing the fuel-moisture
content from 84 to 25 A further increase in fuel moisture up to
30e35 resulted in some more emission reduction Note that this
positive result was accompanied by deterioration of combustion
stability (likely caused by inconsistency in fuel properties) which
showed itself by the noticeable time-domain 1047298uctuations of the
temperature and gaseous species particularly in the vicinity of the
fuel injection As revealed by experimental data from this study
through moisturizing of ldquoas-receivedrdquo rice husk the NO emission
from the SFBC can be substantially reduced However thisachievement was accompanied by the conventional effects of EA
leading to the increase in the NO emission with higher EA [16e12]
323 Optimal excess air and fuel-moisture content
Fig 8 shows the emission costs US$ per 1 ton of fuel for 1047297ring
rice husk in the SFBC at variable fuel moisture and excess air pre-
dicted using the above CO and NO emissions and the speci1047297c
ldquoexternalrdquo costs P NO x frac14 3000 US$=t and P CO frac14 600 US$t (ie for
P NO x=P CO frac14 5) At low values of EA the contribution of CO to the
emission costs was predominant whereas the effects of NO were
substantial at higher EA values It can be seen in Fig 8 that the
Fig 6 Axial (a) CO and (b) NO concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415 ()
W frac1420 (6) W frac1425 and (A) W frac1430
Fig 7 Effects of the fuel-moisture content and excess air on the (a) CO and (b) NO emissions from the SFBC 1047297ring 80 kghr rice husk (-) W frac1484 () W frac1415 () W frac1420 (6)
Wfrac14
25 (A
) Wfrac14
30 and (A
) Wfrac14
35
VI Kuprianov et al Energy 36 (2011) 2038e 20482046
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 1011
emission costs were at the minimal value when 1047297ring rice husk
with the fuel-moisture content of 25e
35 at excess air of 40e
50
Switching P NO x=P CO from 5 to 75 (keeping however P NO x
at the
above value) led in effect to nearly the same optimal ranges of fuel
moisture and excess air quanti1047297ed however at different magni-
tudes of the emission costs Thus the optimized variables are not
sensitive to the speci1047297c ldquoexternalrdquo costs P NO x and P CO within their
actual ranges
324 Combustion heat losses and ef 1047297ciency
Table 4 shows the heat losses with unburned carbon quc and
owing to incomplete combustion qic together with the combustion
ef 1047297ciency of the SFBC actual EA values and SAPA ratios for some
selected fuel options
The heat loss with unburned carbon exhibited a rather weak
correlation with fuel moisture showing however the substantialin1047298uence of excess air Unlike quc the heat loss owing to incomplete
combustion was strongly affected by both fuel moisture and excess
air and it was characterized by quite small magnitudes at EAgt 40
Thus an increase in excess air basically resulted in the improve-
ment of the combustion ef 1047297ciency of the SFBC For the fuel range
the highest combustion ef 1047297ciency 992e997 was obtained at
excess air of 40e80 Numerous studies on 1047297ring various biomass
fuels in conventional 1047298uidized-bed combustion systems report
similar trends [1210e1337]
Based on the analysis of both emission characteristics and
combustion ef 1047297ciency and also taking into consideration an
important issue of the combustion stability it can be generally
concluded that the best performance of this SFBC is achievable
when 1047297ring moisturized rice husk with the moisture content of
about 25 at excess air of 40e50 Under these conditions the
major gaseous emissions from the conical SFBC can be controlled
below 350 ppm for CO and within 130e140 ppm for NO (both on
6 O2 dry gas basis) while the combustion ef 1047297ciency is ensured at
a rather high value about 995
4 Conclusions
Combustion and emission characteristics have been experi-
mentally studied on a SFBC 1047297ring 80 kgh rice husk Two NO x
emission control techniques have been investigated in this work
(1) air staging of the combustion process and (2) 1047297ring rice husk as
moisturized fuel
In the test series for variable air staging the combustorhas been
tested with the aim to investigate the behavior of CO NO and C xH yemissions as well as combustion ef 1047297ciency for burning ldquoas-
receivedrdquo rice husk at a 1047297xed excess air value (of about 40) for
variable secondary-to-primary air ratio (SAPA) With increasing
SAPA from 026 to 075 the CO emission from the SFBC ranges from
about 360 to 450 ppm (on 6 O2 dry gas basis) while the NO
emission reduces at a quite low rate from about 150 to 140 ppm
However with higher SAPA C xH y emissions increase from 120 to
1400 ppm the dramatic rise being observed at SAPA gt 04 Thus
the air staging has minor effects on the CO and NO emissions To
avoid elevated C xH y emissions primary air should be supplied to
the combustor at the amount greater than the stoichiometric air
affected by the fuel properties
During the trialsfor variable fuel quality ricehusks with different
fuel moisture contents (of 84e35) have been burned at different
excess air values rangedfrom about 20 to 80 The analysis of radial
and axial CO and NO concentration pro1047297les has shown the occur-
rence of four speci1047297c regions along the combustor height charac-
terized by different rates of formation and decomposition of CO and
NO The highest rates of CO and NO decomposition are found to
occur in the bed splash zone whereas the top region in the
combustor is characterized by small axial gradients of these species
Through moisturizing rice husk the NO emission from this
SFBC can be substantially reduced while the CO emission is
effectively controlled by the secondary air injection into the bed
splash zone The best combustion and emission performance of
the SFBC is achievable when burning moisturized rice husk with
the moisture content of about 25 at excess air of 40e50 For
these optimal operating conditions the CO emission is expected
to be below 350 ppm ensuring high combustion ef 1047297ciency (about
995 maximum) while the NO emission may range from 130 to
140 ppm With increasing the fuel-moisture content to higher
values eg 30e35 the NO emission from the SFBC can be
secured even at lower values below 110 ppm However elevated
fuel moisture may likely result in deterioration of combustion
stability
Acknowledgements
The authors would like to acknowledge the 1047297nancial support
from the Thailand Research Fund (contract No BRG 50800011) The
authors also sincerely thank Dr Kasama Sirisomboon and Mr
Porametr Arromdee for their effective help in experimental tests
Fig 8 Effects of the fuel-moisture content and excess air on the emission costs for
1047297ring rice husk in the SFBC
Table 4
Heat losses and combustion ef 1047297ciency of the SFBC 1047297ring 80 kgh rice husk for
variable fuel-moisture content and excess air
W () EA () SAPA quc () qic () Combustion
ef 1047297ciency ()
84 17 033 074 204 972
41 026 049 015 994
64 022 041 007 995
76 019 049 008 994
15 17 036 046 173 978
36 030 041 040 992
63 024 050 008 994
89 021 036 005 996
25 18 042 047 134 982
41 033 035 013 995
63 027 029 003 997
83 024 024 003 997
35 16 055 048 242 971
39 042 033 012 995
65 033 026 003 997
84 029 034 005 996
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2047
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 1111
Appendix A Measurement uncertainty in the 1047298owrate of
primary air
The fractional uncertainty in the 1047298owrate of primary air was
estimated by Ref [38]
sPA frac14 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
4s2D thorn s2
V thorn s2R thorn s2
P
q (A1)
where sD is the error in physical measurement of inner diameter of the primary air pipe sV is the measurement error of the velocity
(dependent mainlyon the type of a selected probe) sR is therelative
error associated with imperfect1047297xing of the probe in air1047298ow and sP
is the uncertainty in the velocity pro1047297le across the pipe
AssumingsD frac14 1 sV frac14 15 sR frac14 1 and sP frac14 15(as advised in
Ref [38]) the uncertainty in the 1047298owrate of primary air was
calculated using Eq (A1) to be about 3
Appendix B Determining heat losses and combustion
ef 1047297ciency
In this work the combustion ef 1047297ciency of the SFBC 1047297ring rice
husk was calculated using the the heat-loss method [18]
For this combustor with no ashremoval through thebottompartthe heat loss with unburned carbon quc () was determined by
quc frac14 32866
LHV
C fa
100 C fa
A (B1)
where C fa is the unburned carbon content in the1047298y ash wt and A
is the fuel-ash content wt on ldquoas-receivedrdquo basis
The heat loss owing to incomplete combustion qic () was
quanti1047297ed based on theconcentrations (vol)of COand C xH y (asCH4)
measured in the 1047298ue gas at the cyclone outlet at actual excess air
qic frac14 eth1264 CO thorn 3582 CH4THORN V dgeth100 qucTHORN
LHV (B2)
where V dg is the volume of dry 1047298ue gas at the cyclone outlet Nm3
kg calculated by Refs [1822] using the fuel ultimate analysis onldquoas-receivedrdquo basis and excess air ratio at this point
The combustion ef 1047297ciency of the 1047298uidized-bed combustor hc
() was then determined by
hc frac14 100 ethquc thorn qicTHORN (B3)
References
[1] Natarajan E Nordin A Rao AN Overview of combustion and gasi1047297cation of rice husk in 1047298uidized bed reactors Biomass Bioenergy 199814533e46
[2] Bhattacharya SC Abdul Salam P Sharma M Emissions from biomass energyuse in some selected Asian countries Energy 200025169e88
[3] Werther J Saenger M Hartge EU Ogada T Siagi Z Combustion of agriculturalresidues Prog Energy Combust Sci 2000261e27
[4] Janvijitsakul K Kuprianov VI Similarity and modeling of axial CO and NO
concentration pro1047297les in a 1047298uidized-bed combustor (co-)1047297ring biomass fuelsFuel 2008871574e84
[5] Demirbas A Combustion characteristics of different biomass fuels ProgEnergy Combust Sci 200430219e30
[6] Leckner B Karlsson M Gaseous emissions form circulating 1047298uidized bedcombustion of wood Biomass Bioenergy 19934379e89
[7] Spliethoff H Hein KRG Effect of co-combustion of biomass on emissions inpulverized fuel furnaces Fuel Process Technol 199854189e205
[8] Lyngfelt A Leckner B Combustion of wood-chips in circulating 1047298uidized bedboilers e NO and CO emissions as functions of temperature and air-stagingFuel 1999781065e72
[9] Chyang CS Wu KT Lin CS Emission of nitrogen oxides in a vortexing 1047298uidizedbed combustor Fuel 200786234e43
[10] Armesto E Bahillo A Cabanillas A Veijonen K Otero J Combustion behavior of rice husk in a bubbling 1047298uidised bed Biomass Bioenergy 200223171e9
[11] Kuprianov VI Janvijitsakul K Permchart W Co-1047297ring of sugar canebagasse with rice husk in a conical 1047298uidized-bed combustor Fuel 200685434e42
[12] Fang M Yang L Chen G Shi Z Luo Z Cen K Experimental study on rice huskcombustion in a circulating 1047298uidized bed Fuel Process Technol2004851273e82
[13] Permchart W Kouprianov VI Emission performance and combustion ef 1047297-ciency of a conical 1047298uidized-bed combustor 1047297ring various biomass fuelsBioresour Technol 20049283e91
[14] Sirisomboon K Kuprianov VI Arromdee P Effects of design features oncombustion ef 1047297ciency and emission performance of a biomass-fuelled 1047298uid-ized-bed combustor Chem Eng Process Process Intens 201049270e7
[15] Leckner B Amand LE Lucke K Werther J Gaseous emissions from co-combustion of sewage sludge and coalwood in a 1047298uidized bed Fuel200483477e86
[16] Eaimsa-ard S Kaewkohkiet Y Thianpong C Promvonge P Combustionbehavior in a dual-staging vortex rice husk combustor with snail entry IntCommun Heat Mass Transfer 2008351134e40
[17] Madhiyanon T Lapirattanakun A Sathitruangsak P Soponronnarit S A novelcyclonic 1047298uidized-bed combustor (J-FBC) combustion and thermal ef 1047297-ciency temperature distribution combustion intensity and emission of pollutants Combust Flame 2006146232e45
[18] Basu P Cen KF Jestin L Boilers and burners New York Springer 2000[19] Kaewklum R Kuprianov VI Experimental studies on a novel swirling 1047298uid-
ized-bed combustor using an annular spiral air distributor Fuel20108943
e52
[20] Kouprianov VI Permchart W Emission from a conical FBC 1047297red witha biomass fuel Appl Energy 200374383e92
[21] Kaewklum R Kuprianov VI Douglas PL Hydrodynamics of airesand 1047298ow ina conical swirling 1047298uidized bed a comparative study between tangential andaxial air entries Energy Convers Manage 2009502999e3006
[22] Bezgreshnov AN Lipov YM Shleipher BM Computations of steam boilersMoscow Energoatomizdat 1991 [in Russian]
[23] Kuprianov VI Application of a cost-based method of excess air optimizationfor the improvement of thermal ef 1047297ciency and environmental performance of steam boilers Renew Sustain Energy Rev 20059474e98
[24] Streimikiene D Roos I Rekis J External cost of electricity generation in BalticStates Renew Sustain Energy Rev 200913863e70
[25] Zhang Q Weili T Yumei W Yingxu C External costs from electricity gener-ation of China up to 2030 in energy and abatement scenarios Energy Pol2007354295e304
[26] Hainoun A Almoustafa A Aldin MS Estimating the health damage costs of Syrian electricity generation system using impact pathway approach Energy
201035628e
38[27] Nguyen KQ Internalizing externalities into capacity expansion planning thecase of electricity in Vietnam Energy 200833740e6
[28] Kitou E Horvath A External air pollution costs of telework Int J Life CycleAssess 200813155e65
[29] Wei X Zhang L Zhou H Evaluating the environmental value of pollutants inChina power industry In Proceedings of the international conference onenergy and the environment Shanghai China 2003
[30] Salisdisouk N The concept of integrated resource planning In Proceeding of the workshop on electric power quality safety and ef 1047297ciency of its useselectric power system management Pathum Thani Thailand 1994
[31] Yang YB Shari1047297 VN Swithenbank J Effect of air 1047298ow rate and fuel moisture onthe burning behaviours of biomass and simulated municipal solid wastes inpacked beds Fuel 2004831553e62
[32] Zhao W Li Z Zhao G Zhang F Zhu Q Effect of air preheating and fuel moistureon combustion characteristics of corn straw in a 1047297xed bed Energy ConversManage 2008493560e5
[33] Tillman DA Rossi AJ Kitto WD Wood combustion New York AcademicPress 1981
[34] Winter F Wartha C Hofbauer H NO and N2O formation during thecombustion of wood straw malt waste and peat Bioresour Technol19997039e49
[35] Sun Z Jin M Zhang M Liu R Zhang Y Experimental studies on cotton stalkcombustion in a 1047298uidized bed Energy 2008331224e32
[36] Smart JP Robert PA De Soete GG The formation of nitrous oxide in a large-scale pulverized-coal 1047298ames J Inst Energy 199063131e5
[37] Abu-Qudais M Fluidized-bed combustion for energy production from olivecake Energy 199621173e8
[38] Trembovlya VI Finger ED Avdeeva AA Thermo-technical tests of boiler unitsMoscow Energoatomizdat 1991 [in Russian]
VI Kuprianov et al Energy 36 (2011) 2038e 20482048
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 311
relationship between the actual air1047298ow rate and valve opening was
developed using a measuring system ldquoTesto-454rdquo (Testo Germany)
with a hot-wire probe The measurement uncertainty in the 1047298ow-
rate of primary air was about 3 as estimated in Appendix A
The screw-type feeder delivered the fuel over the bed at a 06 mlever above the air distributor A three-phase inverter was used to
control the fuel feed rate via changing rotation speed of the screw
feeder As established by repeated calibrations the fuel feed rate
was in a quasi-linear correlation with the rotational speed (rpm) of
the feeder For the fuel feed rates of 30e100 kgh the measurement
accuracy of the fuel feed rate was within the range of 3 to 5
when varying the fuel-moisture content from 84 (in ldquoas-receivedrdquo
fuel) to 40
During the combustor start-up a diesel-1047297red burner from Riello
Burners Co (model ldquoPress G24rdquo) was used to preheat the sand to
the speci1047297ed temperature (of about 700 C) The burner air was
tangentially injected into a splash bed zone at a 05 m level through
the burner inclined at a 30 angle to the horizontal Upon
attaining required bed temperature the burner was turned offwhereas desired fuel supply was ensured by the screw feeder
However the burner fan remained to operate delivering secondary
air (SA) to the SFBC during the combustion tests with the aim to
mitigate CO in the bed splash zone and also to protect the burner
head against overheating and impacts from solids The 1047298owrate of
secondary air was controlled by changing an opening of the burner
fan The measurement uncertainty in the 1047298owrate of secondary air
(estimated by the same method as for the primary air) was found to
be about 4
A ldquoTesto-350XL rdquo gas analyzer (Testo Germany) was used to
measure the temperature and gas concentrations (O2 CO NO and
C xH y) along radial and axial directions in the combustor as well as
at the exit of the ash-collecting cyclone The measurement accu-
racies were 05 for the temperature 5 for CO and C xH y
ranged from 100 to 2000 ppm 10 for CO and C xH y higher than
2000 ppm 5 for NO and 02 vol for O2 Besides Chro-
meleAlumel thermocouples were 1047297xed at different levels in the
reactor for (i) monitoring the temperatures during the combustor
start-up (with the accuracy 1) and (ii) obtaining the axialtemperature pro1047297les For the particular test run the excess air ratio
was quanti1047297ed by Ref [22] using the O2 CO and C xH y concentra-
tions at the cyclone exit with an uncertainty of 2 Afterwards
corresponding percentages of total air (TA) and excess air (EA) were
calculated for each trial
Fly ash was sampled from the ash collector (see Fig 1a) to
quantify the content of unburned carbon in the ash required for
predicting the associated heat loss (as discussed below)
22 The fuels
In order to approach the work objectives two series of experi-
mental tests for (i) variable air staging and (ii) variable fuel prop-erties were carried out in this experimental study Table 1 shows
major fuel properties the ultimate and proximate analyses as well
as the lower heating value (LHV) of rice husk used in the tests at
different secondary-to-primary air ratios (SAPA)
Fig 1 (a) Experimental setup and (b) the laboratory-scale swirling 1047298uidized-bed combustor (SFBC)
Table 1
Ultimate and proximate analyses and lower heating value of rice husk 1047297red in the
SFBC during the experimental tests for variable air staging (W frac14 fuel-moisture
A frac14 fuel-ash VM frac14 volatile matter FC frac14 1047297xed carbon LHV frac14 lower heating value)
Ultimate analysis (wt on ldquoas-
receivedrdquo basis)
Proximate analysis (wt on ldquoas-
receivedrdquo basis)
C H O N S W A VM FC LHV (kJkg)
4220 458 2784 025 003 92 159 574 155 13600
VI Kuprianov et al Energy 36 (2011) 2038e 20482040
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 411
The ultimate analysis and LHV of rice husks1047297red in the test runs
for variable fuel quality are shown in Table 2 Since the variation in
the fuel-moisture content affected all other fuel properties the fuel
ultimate analysis and LHV of moisturized rice husks are provided in
Table 2 on ldquoas-1047297redrdquo basis For the particular fuel option the
constituents of the fuel analysis were calculated using the fuel
properties of ldquoas-receivedrdquo fuel (see Table 2 Option 1) accounting
for the actual fuel-moisture content Afterwards LHV for the
moisturized fuels was determined by Ref [22] using corresponding
fuel ultimate analyses from Table 2
The rice husks used in the two experimental series were quite
similar by their chemical and physical properties on ldquoas-receivedrdquo
basis It can be seen in Tables 1 and 2 that the sulfur content in the
rice husks was quite low For this reason SO 2 was not addressed in
this study The dimensions of ldquoas-receivedrdquo rice husk particles were
about 2 mm wide 05 mm thick and 10 mm long while the particle
density was about 1000 kgm3
23 Experimental planning
231 Tests for variable air staging
During this test series rice husk was burned at the 1047297xed fuel
feed rate 80 kgh and excess air of 40 for four values of SAPA
026 040 056 and 075 In each trial (ie for the particular oper-
ating conditions) CO NO and C xH y emissions were determined
together with the O2 concentration at the cyclone exit The main
goal of this test series was to determine the value (or range) of SA
PA ensuring the minimum of these emissions which could be taken
into consideration in the detailed study (test series) below
232 Tests for variable fuel moisture
Fuel moisture (W) and excess air (EA) were chosen as inde-
pendent variables in this test series while the fuel feed rate in all
trials was adjusted at nearly the same value about 80 kgh as in
previous test series Secondary air was supplied to the SFBC at
a minimum 1047298ow rate Q ba frac14 0024 Nm3s required for reliable
cooling of the start-up burner during the combustion testsThis test series included two groups of trials The1047297rst group was
devoted to the behavior of temperature and gas (O2 CO NO)
concentrations in radial and axial directions in the combustor In
this detailed study 1047297ve rice husks (in Table 2 Options 1e5) were
burned at a similar EA value of about 40 The radial temperature
and gas concentration pro1047297les were obtained for 1047297ve levels ( Z )
above the datum Z frac14 0 (see Fig 1b) However the axial pro1047297les
were plotted using the variables measured at eight levels along the
combustor centerline
In the second group of trials CO and NO emissions from the
combustor were quanti1047297ed for all (six) rice husks in Table 2 which
were burned in the SFBC at the excess air values of about 20 40
60 and 80 Using the emission magnitudes optimal values
(ranges) of both EA and fuel-moisture content were determined
using a cost-based approach as discussed below
For the two test series the heat losses with unburned carbon
quc and owing to incomplete combustion qic were quanti1047297ed
together with combustion ef 1047297ciency by using models provided in
Appendix B Note that the effects of C xH y were taken into account
when determining qic for the tests at variable air staging However
these effects were neglected in the qic for the second test series
when the emission of hydrocarbons was at a rather low level in all
trials
24 A model for optimizing excess air and fuel-moisture content
In this work a cost-based approach [23] was applied to deter-
mine the optimal values (ranges) of excess air and fuel-moisture
content leading to the minimized emission (or ldquoexternalrdquo) costs of
1047297ring rice husk in this SFBC Ignoring the effects of SO2 (as negli-
gible) and CO2 (due to the relatively low speci1047297c ldquoexternalrdquo cost of
carbon dioxide) the corresponding objective function used for the
optimization can be represented as
J ec frac14 Min
P NO x_mNO x
thorn P CO _mCO
(1)
where _mNO x and _mCO are emission rates (calculated by Ref [23])
and P NO x and P CO are speci1047297c ldquoexternalrdquo costs of NO x (as NO2) and
CO respectively
It can be concluded from analysis of the objective function that
with the above assumptions the optimal values of excess air and
fuel-moisture content are solely dependent on the cost ratio
P NO x=P CO while the emission costs are apparently affected by all the
variables in Eq (1)
In every country the (average) emission externalities are
strongly affected by the economic structure and activities Studies
on the externalities of heat and electricity generation reveal
therefore a signi1047297cant diversity of the speci1047297c ldquoexternalrdquo costs For
the EU-27 countries the speci1047297c cost of NO x emissions is reportedto be about 7000 Vt (including impacts on human health
ecosystems crops and materials) [24] whereas for Asian countries
this index seems to be substantially (or signi1047297cantly) lower
[25e27] For instance for neighbor China P NO x frac14 2438 US$=t
(including only dominant costs ie those related to the health
damage and climate change) [25] Unlike for NO x limited data on
the externalities by CO is available in literature As revealed by
some relevant studies P CO rises as P NO x increases However the
ratio of P NO x to P CO (ie P NO x
=P CO) is reported to be within certain
limits ranging basically from 5 to 8 [28e30]
Taking the above into consideration it was decided to consider
two options in this optimization study using (1)
P NO x frac14 3000 US$=t and P CO frac14 600 US$t (ie for P NO x
=P CO frac14 5)
and (2) P NO x frac14 3000 US$=t and P CO frac14 400 US$t (ie forP NO x=P CO frac14 75)
3 Results and discussion
31 Emission and combustion characteristics of SFBC for variable
air staging
311 Emissions
Fig 2 shows the CO NO and C xH y emissions (on 6 O2 dry gas
basis) from the combustor 1047297ring 80 kgh rice husk at different SA
PA ratios when excess air was adjusted at about 40
Asseen in Fig 2a the CO emissionwas weakly in1047298uenced by the
air staging factor With increasing SAPA this emission somewhat
increased from about 360 to 450 ppm staying nevertheless at
Table 2
Ultimate analysis and LHV of rice husks 1047297red in the experimental tests for variable
fuel moisture (W)
Property Fuel sample (Option)
1 2 3 4 5 6
Ultimate analysis (wta)
C 4050 3758 3537 3316 3095 2874
H 407 377 355 333 311 289
O 2869 2663 2506 2349 2193 2036
N 031 029 027 025 024 022
S 003 003 003 002 002 002
W 84 150 200 250 300 350
A 180 1670 1572 1474 1376 1277
LHV (kJkg) 14620 13390 12460 11530 10600 9670
a On ldquoas-receivedrdquo basis for Option 1 on ldquoas-1047297redrdquo basis for Options 2e6
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2041
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 511
a rather low level basically due to the secondary air injection into
the bed splash zone As can be concluded based on the results from
Ref [12] and present study excess air (or percentage of total air) is
an important factor in controlling the CO emission in this SFBC
whereas SAPA shows quite weak effects
With increasing SAPA within the selected range 026e075 the
NO emission exhibited a slight reduction from about 150 to140 ppm (see Fig 2a) mainly due to the diminishing of the bed
temperature and reduction of the O2 concentration in the bed the
latter being occurred because of the lowering of PA Thus the air
staging does not seem to be an effective measure to control the NO
emission in this combustor 1047297ring rice husk
It can be seen in Fig 2b that at relatively small proportions of
secondary air the C xH y emissions were at a quite low level
However at SAPA gt 04 these emissions showed a signi1047297cant
increase from 120 to 1400 ppm which can be explained by the
sub-stoichiometric conditions in the bed region Under such
conditions more volatiles were carried over from the combustor
bottom causing the above increase in the C xH y emissions Thus
primary air should be supplied to the SFBC at a 1047298ow rate ensuring
the oxidation conditions in the bed ie with some excess withregard to the theoretical (stoichiometric) air For instance at
EA frac14 40 the combustor should be operated at 026 lt SAPA lt 04
(see Fig 2b) for avoiding high C xH y emissions from this SFBC 1047297ring
rice husk As can be generally concluded the lower limit of SAPA is
speci1047297ed with the aim to provide the reliable coolingof the start-up
burner whereas the upper one is selected taking into account that
EA should be somewhat greater than SA
312 Heat losses and combustion ef 1047297ciency
The analyses of 1047298y ashes for unburned carbon for this test series
indicated the high rate of fuel burnout in this conical SFBC
Depending on SAPA the unburned carbon content in the 1047298y ashes
varied from 081 to 24 the minimum value being found at the
highest SAPA ratio
Table 3 shows the heat losses with unburned carbon (quc) and
owing to incomplete combustion (qic) together with the combus-
tion ef 1047297ciency (all as the percentage of LHV) for 1047297ring 80 kgh rice
husk at excess air of about 40 for different values of SAPA An
increase in SAPA led to a noticeable reduction in the heat loss with
unburned carbon basically due to the higher rate of fuel burnout
which waslikelycaused by an increase in the residence time of char
particles in the combustor However the exponential rise of qic can
be explained by the above behavior of CO and C xH y emissions
Due to the opposite trends exhibited by the heat losses the
combustion ef 1047297ciency was found to have a maximum 991 for1047297ring rice husk at SAPA of 04e06 and EA of about 40
32 Emission and combustion characteristics of SFBC for variable
fuel moisture
As revealed by the experimental results the temperature and
gas concentrations (O2 CO and NO) in this conical SFBC were rep-
resented by three-dimensional patterns (1047297elds) showing the
effects of combustor hydrodynamics fuel quality and operating
conditions on the radial and axial pro1047297les of the temperature and
chemical species Note that at a given excess air SA raised with
increasing fuel moisture because of the reduction in the theoretical
air while the 1047298owrate of secondary air was 1047297xed at the above
constant value changing from 29 (for ldquoas-receivedrdquo fuel) to 41(for rice husk with the highest moisture content) which resulted in
corresponding diminishing of PA Due to the reduction in the
combustion temperature and also in the theoretical air and PA
the residence time of char particles in the bottomregion of the SFBC
was substantially greater when burning rice husks with higher
moisture content leading to the higher rates of devolatilization and
burnout of fuel particles in this region and thus affecting signi1047297-
cantly the behavior of all variables in the reactor
At EA frac14 40 or higher oxidizing conditions were basically
provided in the combustor bottom which justi1047297ed the ignorance of
C xH y emissions in this test series
321 Radial and axial temperature and gas concentration pro 1047297les
in the SFBC Fig 3 shows the radial temperature and O2 concentration
pro1047297les at 1047297ve levels ( Z ) above the air distributor in the SFBC 1047297ring
80 kgh rice husk at excess air of about 40 for variable fuel prop-
erties (Options 1e5 in Table 2) As seen in Fig 3 the variables
exhibitedquitesimilar behaviorsat different levels ( Z ) above the air
distributor The radial temperature pro1047297les were found to be rather
uniform indicating the highly intensive heat-and-mass transfer
along the radius With increasing fuel moisture (at a 1047297xed excess air
level) the temperature at all points in the combustor volume was
found to be reduced (despite the above increase in the residence
time) because of the apparent in1047298uence of the latent heat of water
evaporation Similar results are reported in some studies on
conventional 1047298uidized-bed and 1047297xed-bed combustion systems
1047297ring biomass fuels with variable fuel moisture [4203132]
Fig 2 Effects of secondary-to-primary air ratio (SAPA) on the (a) CO NO and (b) C xH y emissions from the SFBC 1047297ring rice husk at excess air of 40
Table 3
Heat losses and combustion ef 1047297ciency of the SFBC 1047297ring 80 kgh rice husk at excess
air of 40 for variable secondary-to-primary air ratio
SAPA quc () qic () Combustion ef 1047297ciency ()
026 094 024 988
040 051 035 991
056 047 043 991
075 031 212 976
VI Kuprianov et al Energy 36 (2011) 2038e 20482042
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 611
Z = 267 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r
e p m e T
Z = 267 m
0
5
10
15
20
00 02 04 06 08 10
rR
O 2
) l o v ( n o i t a r t
n e c n o c
Z = 217 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r e p m e T
Z = 217 m0
5
10
15
20
00 02 04 06 08 10
rR
O 2
) l o v ( n o i t a r t n e c n o c
Z = 155 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r e p m e T
Z = 155 m
0
5
10
15
20
00 02 04 06 08 10
rR
O 2
) l o v ( n o i t a r t n e c n o c
Z = 101 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r e p m e T
Z = 101 m
0
5
10
15
20
00 02 04 06 08 10
rR
O 2
) l o v ( n o i t a r t n e c n o c
Z = 047 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r e p m e T
Z = 047 m
0
5
10
15
20
00 02 04 06 08 10
rR
O 2
)
l o v ( n o i t a r t n e c n o c
a b
Fig 3 Radial (a) temperature and (b) O2 concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-)
W frac14 84 () W frac1415 () W frac14 20 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2043
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 711
Implementation of air staging seems to have a signi1047297cant impact
on the behavior of gaseous species in both radial and axial direc-
tions Due to the tangential injection of secondary air into the bed
splash zone the radial O2 concentration pro1047297les were characterized
by a positive radial gradient (the most signi1047297cant being observed at
Z frac14 101 m) which resulted in the higher O2 concentration near the
combustor wall than that at the centerline (ie at r R frac14 0)
However the radial gradient of O2 was gradually attenuated along
the bed height Note that the injection of secondary air affected the
radial O2 concentration pro1047297les not only in upper regions of the
reactor but also at levels below the injection point (in Fig 3b see
the pro1047297les at Z frac14 047 m) since the secondary air was injected at
the negative angle In the meantime with increasing the fuel-
moisture content the O2 concentration at all the points across the
combustor was found to be reduced and this reduction was caused
by some physical and chemical factors as addressed below in the
discussion of axial O2 concentration pro1047297les
The axial temperature and O2 concentration pro1047297les in the SFBC
are shown in Fig 4 for the same fuel options and operating
conditions as in Fig 3 At 1047297xed excess air a positive axial temper-
ature gradient was found to occur in the lower part of the reactor
for all the fuels (see Fig 4a) likely due to the diminishing of heat
release in the dense bed (caused by air staging) and (ii) in1047298uence of secondary air injected into the bed splash zone at the ambient
temperature However with higher fuel moisture (ie with dete-
riorating fuel quality) the temperature attained its maximum at
lower levels (Z) above the air distributor which can be explained by
the effects of the residence time The maximum temperature for
burning ldquoas-receivedrdquo rice husk (with W frac14 84) was rather high
about 980 C however it was reduced to 850e860 C when
increasing the fuel-moisture content to 25e30
However with raising fuel moisture the rate of oxygen
consumption in the bottom region of the SFBC ( Z lt 08 m) was
apparently higher which despite the above reduction in the bed
temperature resulted in the lower O2 concentration at all locations
in this region (as seen in Fig 4b) This phenomenon can be
explained by the greater residence time (leading to a higher yield of CO and other combustibles with fuel volatiles) and also greater
contribution of the ldquowetrdquo oxidation of char-C by OH radicals both
leading to higher rates of CO formation and consequently O2
consumption in this region The next region (08 lt Z lt 10 m) was
characterized by a noticeable regaining (rise) of O2 as the response
to secondary air injection However in the freeboard of the
combustor ( Z gt 10 m) the O2 concentration was diminished along
the reactor height at a rather low rate and this reduction was
accompanied by the gradual converging of the axial pro1047297les as all
the tests in Fig 4 for variable fuel moisture were conducted at
(nearly) the same EA
Fig 5 shows the radial CO and NO concentration pro1047297les at
different levels above the air distributor for the same fuels and
operating conditions as in Fig 3 Both CO (Fig 5a) and NO (Fig 5b)
were signi1047297cantly affected by fuel moisture and showed negative
gradients along the radius (different numerically) at all the levels in
the SFBC For the 1047297xed fuel-moisture content due to the effects of
secondary air the CO concentration in the peripheral zone across
the combustor was much lower compared to that at the centerline
thus forming the above radial gradient of CO However the NO
concentration varied weakly along the radius except at Z frac14 047 m
The occurrence of the NO maximum at the centerline indicated
higher rates of both fuel devolatilization and oxidation in the
central zone of the reactor (compared to those at the combustor
wall) despite the uniformity of the temperature and the opposite
trends of the O2 concentration pro1047297les across the SFBC (see Fig 3b)
In the freeboardof the reactor the radial CO and NO gradients were
found to be gradually attenuated with higher Z
Fig 6 depicts the axial CO and NO concentration pro1047297les in this
combustor As seen in Fig 6a the pro1047297les exhibited four sequent
regions along the combustor height With increasing fuel moisture
the CO concentration in the 1047297rst region (0lt Z lt 08 m) was
apparently higher at all the levels above the air distributor
particularly at the centerline (see Fig 5a) mainly due to (i) longerresidence time (leading to greater yield of CO with volatiles) (ii)
lower PA (reducing the rate of CO oxidation) (iii) higher contri-
bution of ldquowetrdquo oxidation of char-C by OH radicals and (iv) lower
bed temperature causing an increase in the COCO2 ratio in the
products of fuel-char oxidation [33]
In the second region (08 lt Z lt 10 m) the CO concentration
along the combustor axis was found to be drastically reduced
mainly due to the effects of secondaryair the greater rate of the CO
reduction being observed at a higher level of fuel moisture (ie at
higher SA) In the third region (10 lt Z lt 18 m) the CO concentra-
tion regained substantial values along the centerline mainly due to
oxidation of unburned hydrocarbons and fuel-C carried over from
the bed region to CO However in the fourth (upper) region the CO
concentration was found to be gradually reduced along thecombustor height likely via homogeneous reaction of CO with
residual O2 and OH [333]
Like for CO four speci1047297c regions can be distinguished in the
axial NO concentration pro1047297les as can be seen in Fig 6b In the 1047297rst
region (0 lt Z lt 08 m) NO was mainly formed from NH3 in volatiles
(a major precursor of NO in biomass combustion) via the fuel-NO
formation mechanism [343435] With increasing fuel moisture
despite the reduction in bed temperature the NO concentration at
the reactor centerline showed a trend to increase at any given Z
mainly due to (i) greater residence time promoting a higher yield
of nitrogenous species with fuel volatiles and (ii) enhanced
a b
500
600
700
800
900
1000
1100
0 1 2 3
Height above air distributor (m)
) C deg ( e r u t a r e p m e T
0
5
10
15
20
0 1 2 3Height above air distributor (m)
O 2
) l o v ( n o i t a r t n e c n o c
Fig 4 Axial (a) temperature and (b) O2 concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415
(
) Wfrac14
20 (6
) Wfrac14
25 and (A
) Wfrac14
30
VI Kuprianov et al Energy 36 (2011) 2038e 20482044
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 811
Z = 267 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 267 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t
n e c n o c O N
Z = 217 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 217 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O N
Z = 155 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 155 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0rR
) m
p p ( n o i t a r t n e c n o c O N
Z = 101 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 101 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O N
Z = 047 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
)
m
p p ( n o i t a r t n e c n o c
O C
Z = 047 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O N
ab
Fig 5 Radial (a) CO and (b) NO concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 ()
W frac14 15 () W frac1420 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2045
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 911
oxidation of NH3 by OH [36] In the second region (08 lt Z lt 10 m)
the chemical reactions responsible for NO decomposition such as
catalytic reduction of NO by CO on the char surface [3] as well as
reaction of NO with NH3 at oxygen de1047297ciency [34] were predom-
inant This resulted in a signi1047297cant reduction of the NO concen-
tration along the combustor height and the NO reduction was more
apparent when burning rice husks with higher moisture contentsHowever the injection of secondary air promoted a slight increase
in NO in the third region (10 lt Z lt 16 m) of the reactor as a result
of oxidation of the nitrogenous species carried over from the
combustor bottom In the fourth (upper) region the pro1047297les
exhibited some diminishing of the NO concentration along the
reactor height because of the catalytic reduction of NO by CO
occurred however at a rather low rate
322 Emissions
Fig 7 depicts the CO and NO emissions (on 6 O 2 dry gas basis)
from the combustor 1047297ring 80 kgh rice husk at variable EA for the
whole range of fuel moisture (Options 1e6 in Table 2) It can be
concluded from analysis of data in Fig 7a that at EA greater than
40 the CO emission from the SFBC can be controlled ata comparatively low level below 350 ppm regardless of the fuel
quality On the contrary at excess air lower than 40 the CO
emission exhibited quite strong effects of both fuel quality and EA
Note that at EA frac14 20 the CO emission was extremely high
3000e7000 ppm for the whole range of fuel moisture However
with increasing the fuel-moisture content from 84 (in ldquoas-
receivedrdquo rice husk) to 25 the CO emission at this lowest EA
exhibited some reduction roughly from 4000 to 3000 ppm basi-
cally caused by (i) the higher rate of chemical reaction between CO
and OH [333] and (ii) higher rate of CO decomposition in the
freeboard (due to enhanced SA and greater residence time) Similar
trend is reported in some studies on effects of fuel moisture on the
CO emission from the 1047297xed-bed combustion systems [3132]
However with further increase in the fuel-moisture content (from
25 to 35) the CO emission from the SFBC was found to rise from
about 3000 to 7000 ppm likely due to the signi1047297cant contribution
of ldquowetrdquo oxidation of char-C occurred at the lowered combustion
temperatures This trend is in concordance with the behavior of theCO emission during the conventional 1047298uidized-bed combustion of
some biomass fuels with variable moisture [420]
It can be seen in Fig 7b that at EA frac14 40 (the ldquocriticalrdquo value for
the CO emission) the NO emission diminished from about 170 to
130 ppm (or by some 25) when changing the fuel-moisture
content from 84 to 25 A further increase in fuel moisture up to
30e35 resulted in some more emission reduction Note that this
positive result was accompanied by deterioration of combustion
stability (likely caused by inconsistency in fuel properties) which
showed itself by the noticeable time-domain 1047298uctuations of the
temperature and gaseous species particularly in the vicinity of the
fuel injection As revealed by experimental data from this study
through moisturizing of ldquoas-receivedrdquo rice husk the NO emission
from the SFBC can be substantially reduced However thisachievement was accompanied by the conventional effects of EA
leading to the increase in the NO emission with higher EA [16e12]
323 Optimal excess air and fuel-moisture content
Fig 8 shows the emission costs US$ per 1 ton of fuel for 1047297ring
rice husk in the SFBC at variable fuel moisture and excess air pre-
dicted using the above CO and NO emissions and the speci1047297c
ldquoexternalrdquo costs P NO x frac14 3000 US$=t and P CO frac14 600 US$t (ie for
P NO x=P CO frac14 5) At low values of EA the contribution of CO to the
emission costs was predominant whereas the effects of NO were
substantial at higher EA values It can be seen in Fig 8 that the
Fig 6 Axial (a) CO and (b) NO concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415 ()
W frac1420 (6) W frac1425 and (A) W frac1430
Fig 7 Effects of the fuel-moisture content and excess air on the (a) CO and (b) NO emissions from the SFBC 1047297ring 80 kghr rice husk (-) W frac1484 () W frac1415 () W frac1420 (6)
Wfrac14
25 (A
) Wfrac14
30 and (A
) Wfrac14
35
VI Kuprianov et al Energy 36 (2011) 2038e 20482046
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 1011
emission costs were at the minimal value when 1047297ring rice husk
with the fuel-moisture content of 25e
35 at excess air of 40e
50
Switching P NO x=P CO from 5 to 75 (keeping however P NO x
at the
above value) led in effect to nearly the same optimal ranges of fuel
moisture and excess air quanti1047297ed however at different magni-
tudes of the emission costs Thus the optimized variables are not
sensitive to the speci1047297c ldquoexternalrdquo costs P NO x and P CO within their
actual ranges
324 Combustion heat losses and ef 1047297ciency
Table 4 shows the heat losses with unburned carbon quc and
owing to incomplete combustion qic together with the combustion
ef 1047297ciency of the SFBC actual EA values and SAPA ratios for some
selected fuel options
The heat loss with unburned carbon exhibited a rather weak
correlation with fuel moisture showing however the substantialin1047298uence of excess air Unlike quc the heat loss owing to incomplete
combustion was strongly affected by both fuel moisture and excess
air and it was characterized by quite small magnitudes at EAgt 40
Thus an increase in excess air basically resulted in the improve-
ment of the combustion ef 1047297ciency of the SFBC For the fuel range
the highest combustion ef 1047297ciency 992e997 was obtained at
excess air of 40e80 Numerous studies on 1047297ring various biomass
fuels in conventional 1047298uidized-bed combustion systems report
similar trends [1210e1337]
Based on the analysis of both emission characteristics and
combustion ef 1047297ciency and also taking into consideration an
important issue of the combustion stability it can be generally
concluded that the best performance of this SFBC is achievable
when 1047297ring moisturized rice husk with the moisture content of
about 25 at excess air of 40e50 Under these conditions the
major gaseous emissions from the conical SFBC can be controlled
below 350 ppm for CO and within 130e140 ppm for NO (both on
6 O2 dry gas basis) while the combustion ef 1047297ciency is ensured at
a rather high value about 995
4 Conclusions
Combustion and emission characteristics have been experi-
mentally studied on a SFBC 1047297ring 80 kgh rice husk Two NO x
emission control techniques have been investigated in this work
(1) air staging of the combustion process and (2) 1047297ring rice husk as
moisturized fuel
In the test series for variable air staging the combustorhas been
tested with the aim to investigate the behavior of CO NO and C xH yemissions as well as combustion ef 1047297ciency for burning ldquoas-
receivedrdquo rice husk at a 1047297xed excess air value (of about 40) for
variable secondary-to-primary air ratio (SAPA) With increasing
SAPA from 026 to 075 the CO emission from the SFBC ranges from
about 360 to 450 ppm (on 6 O2 dry gas basis) while the NO
emission reduces at a quite low rate from about 150 to 140 ppm
However with higher SAPA C xH y emissions increase from 120 to
1400 ppm the dramatic rise being observed at SAPA gt 04 Thus
the air staging has minor effects on the CO and NO emissions To
avoid elevated C xH y emissions primary air should be supplied to
the combustor at the amount greater than the stoichiometric air
affected by the fuel properties
During the trialsfor variable fuel quality ricehusks with different
fuel moisture contents (of 84e35) have been burned at different
excess air values rangedfrom about 20 to 80 The analysis of radial
and axial CO and NO concentration pro1047297les has shown the occur-
rence of four speci1047297c regions along the combustor height charac-
terized by different rates of formation and decomposition of CO and
NO The highest rates of CO and NO decomposition are found to
occur in the bed splash zone whereas the top region in the
combustor is characterized by small axial gradients of these species
Through moisturizing rice husk the NO emission from this
SFBC can be substantially reduced while the CO emission is
effectively controlled by the secondary air injection into the bed
splash zone The best combustion and emission performance of
the SFBC is achievable when burning moisturized rice husk with
the moisture content of about 25 at excess air of 40e50 For
these optimal operating conditions the CO emission is expected
to be below 350 ppm ensuring high combustion ef 1047297ciency (about
995 maximum) while the NO emission may range from 130 to
140 ppm With increasing the fuel-moisture content to higher
values eg 30e35 the NO emission from the SFBC can be
secured even at lower values below 110 ppm However elevated
fuel moisture may likely result in deterioration of combustion
stability
Acknowledgements
The authors would like to acknowledge the 1047297nancial support
from the Thailand Research Fund (contract No BRG 50800011) The
authors also sincerely thank Dr Kasama Sirisomboon and Mr
Porametr Arromdee for their effective help in experimental tests
Fig 8 Effects of the fuel-moisture content and excess air on the emission costs for
1047297ring rice husk in the SFBC
Table 4
Heat losses and combustion ef 1047297ciency of the SFBC 1047297ring 80 kgh rice husk for
variable fuel-moisture content and excess air
W () EA () SAPA quc () qic () Combustion
ef 1047297ciency ()
84 17 033 074 204 972
41 026 049 015 994
64 022 041 007 995
76 019 049 008 994
15 17 036 046 173 978
36 030 041 040 992
63 024 050 008 994
89 021 036 005 996
25 18 042 047 134 982
41 033 035 013 995
63 027 029 003 997
83 024 024 003 997
35 16 055 048 242 971
39 042 033 012 995
65 033 026 003 997
84 029 034 005 996
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2047
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 1111
Appendix A Measurement uncertainty in the 1047298owrate of
primary air
The fractional uncertainty in the 1047298owrate of primary air was
estimated by Ref [38]
sPA frac14 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
4s2D thorn s2
V thorn s2R thorn s2
P
q (A1)
where sD is the error in physical measurement of inner diameter of the primary air pipe sV is the measurement error of the velocity
(dependent mainlyon the type of a selected probe) sR is therelative
error associated with imperfect1047297xing of the probe in air1047298ow and sP
is the uncertainty in the velocity pro1047297le across the pipe
AssumingsD frac14 1 sV frac14 15 sR frac14 1 and sP frac14 15(as advised in
Ref [38]) the uncertainty in the 1047298owrate of primary air was
calculated using Eq (A1) to be about 3
Appendix B Determining heat losses and combustion
ef 1047297ciency
In this work the combustion ef 1047297ciency of the SFBC 1047297ring rice
husk was calculated using the the heat-loss method [18]
For this combustor with no ashremoval through thebottompartthe heat loss with unburned carbon quc () was determined by
quc frac14 32866
LHV
C fa
100 C fa
A (B1)
where C fa is the unburned carbon content in the1047298y ash wt and A
is the fuel-ash content wt on ldquoas-receivedrdquo basis
The heat loss owing to incomplete combustion qic () was
quanti1047297ed based on theconcentrations (vol)of COand C xH y (asCH4)
measured in the 1047298ue gas at the cyclone outlet at actual excess air
qic frac14 eth1264 CO thorn 3582 CH4THORN V dgeth100 qucTHORN
LHV (B2)
where V dg is the volume of dry 1047298ue gas at the cyclone outlet Nm3
kg calculated by Refs [1822] using the fuel ultimate analysis onldquoas-receivedrdquo basis and excess air ratio at this point
The combustion ef 1047297ciency of the 1047298uidized-bed combustor hc
() was then determined by
hc frac14 100 ethquc thorn qicTHORN (B3)
References
[1] Natarajan E Nordin A Rao AN Overview of combustion and gasi1047297cation of rice husk in 1047298uidized bed reactors Biomass Bioenergy 199814533e46
[2] Bhattacharya SC Abdul Salam P Sharma M Emissions from biomass energyuse in some selected Asian countries Energy 200025169e88
[3] Werther J Saenger M Hartge EU Ogada T Siagi Z Combustion of agriculturalresidues Prog Energy Combust Sci 2000261e27
[4] Janvijitsakul K Kuprianov VI Similarity and modeling of axial CO and NO
concentration pro1047297les in a 1047298uidized-bed combustor (co-)1047297ring biomass fuelsFuel 2008871574e84
[5] Demirbas A Combustion characteristics of different biomass fuels ProgEnergy Combust Sci 200430219e30
[6] Leckner B Karlsson M Gaseous emissions form circulating 1047298uidized bedcombustion of wood Biomass Bioenergy 19934379e89
[7] Spliethoff H Hein KRG Effect of co-combustion of biomass on emissions inpulverized fuel furnaces Fuel Process Technol 199854189e205
[8] Lyngfelt A Leckner B Combustion of wood-chips in circulating 1047298uidized bedboilers e NO and CO emissions as functions of temperature and air-stagingFuel 1999781065e72
[9] Chyang CS Wu KT Lin CS Emission of nitrogen oxides in a vortexing 1047298uidizedbed combustor Fuel 200786234e43
[10] Armesto E Bahillo A Cabanillas A Veijonen K Otero J Combustion behavior of rice husk in a bubbling 1047298uidised bed Biomass Bioenergy 200223171e9
[11] Kuprianov VI Janvijitsakul K Permchart W Co-1047297ring of sugar canebagasse with rice husk in a conical 1047298uidized-bed combustor Fuel 200685434e42
[12] Fang M Yang L Chen G Shi Z Luo Z Cen K Experimental study on rice huskcombustion in a circulating 1047298uidized bed Fuel Process Technol2004851273e82
[13] Permchart W Kouprianov VI Emission performance and combustion ef 1047297-ciency of a conical 1047298uidized-bed combustor 1047297ring various biomass fuelsBioresour Technol 20049283e91
[14] Sirisomboon K Kuprianov VI Arromdee P Effects of design features oncombustion ef 1047297ciency and emission performance of a biomass-fuelled 1047298uid-ized-bed combustor Chem Eng Process Process Intens 201049270e7
[15] Leckner B Amand LE Lucke K Werther J Gaseous emissions from co-combustion of sewage sludge and coalwood in a 1047298uidized bed Fuel200483477e86
[16] Eaimsa-ard S Kaewkohkiet Y Thianpong C Promvonge P Combustionbehavior in a dual-staging vortex rice husk combustor with snail entry IntCommun Heat Mass Transfer 2008351134e40
[17] Madhiyanon T Lapirattanakun A Sathitruangsak P Soponronnarit S A novelcyclonic 1047298uidized-bed combustor (J-FBC) combustion and thermal ef 1047297-ciency temperature distribution combustion intensity and emission of pollutants Combust Flame 2006146232e45
[18] Basu P Cen KF Jestin L Boilers and burners New York Springer 2000[19] Kaewklum R Kuprianov VI Experimental studies on a novel swirling 1047298uid-
ized-bed combustor using an annular spiral air distributor Fuel20108943
e52
[20] Kouprianov VI Permchart W Emission from a conical FBC 1047297red witha biomass fuel Appl Energy 200374383e92
[21] Kaewklum R Kuprianov VI Douglas PL Hydrodynamics of airesand 1047298ow ina conical swirling 1047298uidized bed a comparative study between tangential andaxial air entries Energy Convers Manage 2009502999e3006
[22] Bezgreshnov AN Lipov YM Shleipher BM Computations of steam boilersMoscow Energoatomizdat 1991 [in Russian]
[23] Kuprianov VI Application of a cost-based method of excess air optimizationfor the improvement of thermal ef 1047297ciency and environmental performance of steam boilers Renew Sustain Energy Rev 20059474e98
[24] Streimikiene D Roos I Rekis J External cost of electricity generation in BalticStates Renew Sustain Energy Rev 200913863e70
[25] Zhang Q Weili T Yumei W Yingxu C External costs from electricity gener-ation of China up to 2030 in energy and abatement scenarios Energy Pol2007354295e304
[26] Hainoun A Almoustafa A Aldin MS Estimating the health damage costs of Syrian electricity generation system using impact pathway approach Energy
201035628e
38[27] Nguyen KQ Internalizing externalities into capacity expansion planning thecase of electricity in Vietnam Energy 200833740e6
[28] Kitou E Horvath A External air pollution costs of telework Int J Life CycleAssess 200813155e65
[29] Wei X Zhang L Zhou H Evaluating the environmental value of pollutants inChina power industry In Proceedings of the international conference onenergy and the environment Shanghai China 2003
[30] Salisdisouk N The concept of integrated resource planning In Proceeding of the workshop on electric power quality safety and ef 1047297ciency of its useselectric power system management Pathum Thani Thailand 1994
[31] Yang YB Shari1047297 VN Swithenbank J Effect of air 1047298ow rate and fuel moisture onthe burning behaviours of biomass and simulated municipal solid wastes inpacked beds Fuel 2004831553e62
[32] Zhao W Li Z Zhao G Zhang F Zhu Q Effect of air preheating and fuel moistureon combustion characteristics of corn straw in a 1047297xed bed Energy ConversManage 2008493560e5
[33] Tillman DA Rossi AJ Kitto WD Wood combustion New York AcademicPress 1981
[34] Winter F Wartha C Hofbauer H NO and N2O formation during thecombustion of wood straw malt waste and peat Bioresour Technol19997039e49
[35] Sun Z Jin M Zhang M Liu R Zhang Y Experimental studies on cotton stalkcombustion in a 1047298uidized bed Energy 2008331224e32
[36] Smart JP Robert PA De Soete GG The formation of nitrous oxide in a large-scale pulverized-coal 1047298ames J Inst Energy 199063131e5
[37] Abu-Qudais M Fluidized-bed combustion for energy production from olivecake Energy 199621173e8
[38] Trembovlya VI Finger ED Avdeeva AA Thermo-technical tests of boiler unitsMoscow Energoatomizdat 1991 [in Russian]
VI Kuprianov et al Energy 36 (2011) 2038e 20482048
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 411
The ultimate analysis and LHV of rice husks1047297red in the test runs
for variable fuel quality are shown in Table 2 Since the variation in
the fuel-moisture content affected all other fuel properties the fuel
ultimate analysis and LHV of moisturized rice husks are provided in
Table 2 on ldquoas-1047297redrdquo basis For the particular fuel option the
constituents of the fuel analysis were calculated using the fuel
properties of ldquoas-receivedrdquo fuel (see Table 2 Option 1) accounting
for the actual fuel-moisture content Afterwards LHV for the
moisturized fuels was determined by Ref [22] using corresponding
fuel ultimate analyses from Table 2
The rice husks used in the two experimental series were quite
similar by their chemical and physical properties on ldquoas-receivedrdquo
basis It can be seen in Tables 1 and 2 that the sulfur content in the
rice husks was quite low For this reason SO 2 was not addressed in
this study The dimensions of ldquoas-receivedrdquo rice husk particles were
about 2 mm wide 05 mm thick and 10 mm long while the particle
density was about 1000 kgm3
23 Experimental planning
231 Tests for variable air staging
During this test series rice husk was burned at the 1047297xed fuel
feed rate 80 kgh and excess air of 40 for four values of SAPA
026 040 056 and 075 In each trial (ie for the particular oper-
ating conditions) CO NO and C xH y emissions were determined
together with the O2 concentration at the cyclone exit The main
goal of this test series was to determine the value (or range) of SA
PA ensuring the minimum of these emissions which could be taken
into consideration in the detailed study (test series) below
232 Tests for variable fuel moisture
Fuel moisture (W) and excess air (EA) were chosen as inde-
pendent variables in this test series while the fuel feed rate in all
trials was adjusted at nearly the same value about 80 kgh as in
previous test series Secondary air was supplied to the SFBC at
a minimum 1047298ow rate Q ba frac14 0024 Nm3s required for reliable
cooling of the start-up burner during the combustion testsThis test series included two groups of trials The1047297rst group was
devoted to the behavior of temperature and gas (O2 CO NO)
concentrations in radial and axial directions in the combustor In
this detailed study 1047297ve rice husks (in Table 2 Options 1e5) were
burned at a similar EA value of about 40 The radial temperature
and gas concentration pro1047297les were obtained for 1047297ve levels ( Z )
above the datum Z frac14 0 (see Fig 1b) However the axial pro1047297les
were plotted using the variables measured at eight levels along the
combustor centerline
In the second group of trials CO and NO emissions from the
combustor were quanti1047297ed for all (six) rice husks in Table 2 which
were burned in the SFBC at the excess air values of about 20 40
60 and 80 Using the emission magnitudes optimal values
(ranges) of both EA and fuel-moisture content were determined
using a cost-based approach as discussed below
For the two test series the heat losses with unburned carbon
quc and owing to incomplete combustion qic were quanti1047297ed
together with combustion ef 1047297ciency by using models provided in
Appendix B Note that the effects of C xH y were taken into account
when determining qic for the tests at variable air staging However
these effects were neglected in the qic for the second test series
when the emission of hydrocarbons was at a rather low level in all
trials
24 A model for optimizing excess air and fuel-moisture content
In this work a cost-based approach [23] was applied to deter-
mine the optimal values (ranges) of excess air and fuel-moisture
content leading to the minimized emission (or ldquoexternalrdquo) costs of
1047297ring rice husk in this SFBC Ignoring the effects of SO2 (as negli-
gible) and CO2 (due to the relatively low speci1047297c ldquoexternalrdquo cost of
carbon dioxide) the corresponding objective function used for the
optimization can be represented as
J ec frac14 Min
P NO x_mNO x
thorn P CO _mCO
(1)
where _mNO x and _mCO are emission rates (calculated by Ref [23])
and P NO x and P CO are speci1047297c ldquoexternalrdquo costs of NO x (as NO2) and
CO respectively
It can be concluded from analysis of the objective function that
with the above assumptions the optimal values of excess air and
fuel-moisture content are solely dependent on the cost ratio
P NO x=P CO while the emission costs are apparently affected by all the
variables in Eq (1)
In every country the (average) emission externalities are
strongly affected by the economic structure and activities Studies
on the externalities of heat and electricity generation reveal
therefore a signi1047297cant diversity of the speci1047297c ldquoexternalrdquo costs For
the EU-27 countries the speci1047297c cost of NO x emissions is reportedto be about 7000 Vt (including impacts on human health
ecosystems crops and materials) [24] whereas for Asian countries
this index seems to be substantially (or signi1047297cantly) lower
[25e27] For instance for neighbor China P NO x frac14 2438 US$=t
(including only dominant costs ie those related to the health
damage and climate change) [25] Unlike for NO x limited data on
the externalities by CO is available in literature As revealed by
some relevant studies P CO rises as P NO x increases However the
ratio of P NO x to P CO (ie P NO x
=P CO) is reported to be within certain
limits ranging basically from 5 to 8 [28e30]
Taking the above into consideration it was decided to consider
two options in this optimization study using (1)
P NO x frac14 3000 US$=t and P CO frac14 600 US$t (ie for P NO x
=P CO frac14 5)
and (2) P NO x frac14 3000 US$=t and P CO frac14 400 US$t (ie forP NO x=P CO frac14 75)
3 Results and discussion
31 Emission and combustion characteristics of SFBC for variable
air staging
311 Emissions
Fig 2 shows the CO NO and C xH y emissions (on 6 O2 dry gas
basis) from the combustor 1047297ring 80 kgh rice husk at different SA
PA ratios when excess air was adjusted at about 40
Asseen in Fig 2a the CO emissionwas weakly in1047298uenced by the
air staging factor With increasing SAPA this emission somewhat
increased from about 360 to 450 ppm staying nevertheless at
Table 2
Ultimate analysis and LHV of rice husks 1047297red in the experimental tests for variable
fuel moisture (W)
Property Fuel sample (Option)
1 2 3 4 5 6
Ultimate analysis (wta)
C 4050 3758 3537 3316 3095 2874
H 407 377 355 333 311 289
O 2869 2663 2506 2349 2193 2036
N 031 029 027 025 024 022
S 003 003 003 002 002 002
W 84 150 200 250 300 350
A 180 1670 1572 1474 1376 1277
LHV (kJkg) 14620 13390 12460 11530 10600 9670
a On ldquoas-receivedrdquo basis for Option 1 on ldquoas-1047297redrdquo basis for Options 2e6
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2041
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 511
a rather low level basically due to the secondary air injection into
the bed splash zone As can be concluded based on the results from
Ref [12] and present study excess air (or percentage of total air) is
an important factor in controlling the CO emission in this SFBC
whereas SAPA shows quite weak effects
With increasing SAPA within the selected range 026e075 the
NO emission exhibited a slight reduction from about 150 to140 ppm (see Fig 2a) mainly due to the diminishing of the bed
temperature and reduction of the O2 concentration in the bed the
latter being occurred because of the lowering of PA Thus the air
staging does not seem to be an effective measure to control the NO
emission in this combustor 1047297ring rice husk
It can be seen in Fig 2b that at relatively small proportions of
secondary air the C xH y emissions were at a quite low level
However at SAPA gt 04 these emissions showed a signi1047297cant
increase from 120 to 1400 ppm which can be explained by the
sub-stoichiometric conditions in the bed region Under such
conditions more volatiles were carried over from the combustor
bottom causing the above increase in the C xH y emissions Thus
primary air should be supplied to the SFBC at a 1047298ow rate ensuring
the oxidation conditions in the bed ie with some excess withregard to the theoretical (stoichiometric) air For instance at
EA frac14 40 the combustor should be operated at 026 lt SAPA lt 04
(see Fig 2b) for avoiding high C xH y emissions from this SFBC 1047297ring
rice husk As can be generally concluded the lower limit of SAPA is
speci1047297ed with the aim to provide the reliable coolingof the start-up
burner whereas the upper one is selected taking into account that
EA should be somewhat greater than SA
312 Heat losses and combustion ef 1047297ciency
The analyses of 1047298y ashes for unburned carbon for this test series
indicated the high rate of fuel burnout in this conical SFBC
Depending on SAPA the unburned carbon content in the 1047298y ashes
varied from 081 to 24 the minimum value being found at the
highest SAPA ratio
Table 3 shows the heat losses with unburned carbon (quc) and
owing to incomplete combustion (qic) together with the combus-
tion ef 1047297ciency (all as the percentage of LHV) for 1047297ring 80 kgh rice
husk at excess air of about 40 for different values of SAPA An
increase in SAPA led to a noticeable reduction in the heat loss with
unburned carbon basically due to the higher rate of fuel burnout
which waslikelycaused by an increase in the residence time of char
particles in the combustor However the exponential rise of qic can
be explained by the above behavior of CO and C xH y emissions
Due to the opposite trends exhibited by the heat losses the
combustion ef 1047297ciency was found to have a maximum 991 for1047297ring rice husk at SAPA of 04e06 and EA of about 40
32 Emission and combustion characteristics of SFBC for variable
fuel moisture
As revealed by the experimental results the temperature and
gas concentrations (O2 CO and NO) in this conical SFBC were rep-
resented by three-dimensional patterns (1047297elds) showing the
effects of combustor hydrodynamics fuel quality and operating
conditions on the radial and axial pro1047297les of the temperature and
chemical species Note that at a given excess air SA raised with
increasing fuel moisture because of the reduction in the theoretical
air while the 1047298owrate of secondary air was 1047297xed at the above
constant value changing from 29 (for ldquoas-receivedrdquo fuel) to 41(for rice husk with the highest moisture content) which resulted in
corresponding diminishing of PA Due to the reduction in the
combustion temperature and also in the theoretical air and PA
the residence time of char particles in the bottomregion of the SFBC
was substantially greater when burning rice husks with higher
moisture content leading to the higher rates of devolatilization and
burnout of fuel particles in this region and thus affecting signi1047297-
cantly the behavior of all variables in the reactor
At EA frac14 40 or higher oxidizing conditions were basically
provided in the combustor bottom which justi1047297ed the ignorance of
C xH y emissions in this test series
321 Radial and axial temperature and gas concentration pro 1047297les
in the SFBC Fig 3 shows the radial temperature and O2 concentration
pro1047297les at 1047297ve levels ( Z ) above the air distributor in the SFBC 1047297ring
80 kgh rice husk at excess air of about 40 for variable fuel prop-
erties (Options 1e5 in Table 2) As seen in Fig 3 the variables
exhibitedquitesimilar behaviorsat different levels ( Z ) above the air
distributor The radial temperature pro1047297les were found to be rather
uniform indicating the highly intensive heat-and-mass transfer
along the radius With increasing fuel moisture (at a 1047297xed excess air
level) the temperature at all points in the combustor volume was
found to be reduced (despite the above increase in the residence
time) because of the apparent in1047298uence of the latent heat of water
evaporation Similar results are reported in some studies on
conventional 1047298uidized-bed and 1047297xed-bed combustion systems
1047297ring biomass fuels with variable fuel moisture [4203132]
Fig 2 Effects of secondary-to-primary air ratio (SAPA) on the (a) CO NO and (b) C xH y emissions from the SFBC 1047297ring rice husk at excess air of 40
Table 3
Heat losses and combustion ef 1047297ciency of the SFBC 1047297ring 80 kgh rice husk at excess
air of 40 for variable secondary-to-primary air ratio
SAPA quc () qic () Combustion ef 1047297ciency ()
026 094 024 988
040 051 035 991
056 047 043 991
075 031 212 976
VI Kuprianov et al Energy 36 (2011) 2038e 20482042
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 611
Z = 267 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r
e p m e T
Z = 267 m
0
5
10
15
20
00 02 04 06 08 10
rR
O 2
) l o v ( n o i t a r t
n e c n o c
Z = 217 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r e p m e T
Z = 217 m0
5
10
15
20
00 02 04 06 08 10
rR
O 2
) l o v ( n o i t a r t n e c n o c
Z = 155 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r e p m e T
Z = 155 m
0
5
10
15
20
00 02 04 06 08 10
rR
O 2
) l o v ( n o i t a r t n e c n o c
Z = 101 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r e p m e T
Z = 101 m
0
5
10
15
20
00 02 04 06 08 10
rR
O 2
) l o v ( n o i t a r t n e c n o c
Z = 047 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r e p m e T
Z = 047 m
0
5
10
15
20
00 02 04 06 08 10
rR
O 2
)
l o v ( n o i t a r t n e c n o c
a b
Fig 3 Radial (a) temperature and (b) O2 concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-)
W frac14 84 () W frac1415 () W frac14 20 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2043
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 711
Implementation of air staging seems to have a signi1047297cant impact
on the behavior of gaseous species in both radial and axial direc-
tions Due to the tangential injection of secondary air into the bed
splash zone the radial O2 concentration pro1047297les were characterized
by a positive radial gradient (the most signi1047297cant being observed at
Z frac14 101 m) which resulted in the higher O2 concentration near the
combustor wall than that at the centerline (ie at r R frac14 0)
However the radial gradient of O2 was gradually attenuated along
the bed height Note that the injection of secondary air affected the
radial O2 concentration pro1047297les not only in upper regions of the
reactor but also at levels below the injection point (in Fig 3b see
the pro1047297les at Z frac14 047 m) since the secondary air was injected at
the negative angle In the meantime with increasing the fuel-
moisture content the O2 concentration at all the points across the
combustor was found to be reduced and this reduction was caused
by some physical and chemical factors as addressed below in the
discussion of axial O2 concentration pro1047297les
The axial temperature and O2 concentration pro1047297les in the SFBC
are shown in Fig 4 for the same fuel options and operating
conditions as in Fig 3 At 1047297xed excess air a positive axial temper-
ature gradient was found to occur in the lower part of the reactor
for all the fuels (see Fig 4a) likely due to the diminishing of heat
release in the dense bed (caused by air staging) and (ii) in1047298uence of secondary air injected into the bed splash zone at the ambient
temperature However with higher fuel moisture (ie with dete-
riorating fuel quality) the temperature attained its maximum at
lower levels (Z) above the air distributor which can be explained by
the effects of the residence time The maximum temperature for
burning ldquoas-receivedrdquo rice husk (with W frac14 84) was rather high
about 980 C however it was reduced to 850e860 C when
increasing the fuel-moisture content to 25e30
However with raising fuel moisture the rate of oxygen
consumption in the bottom region of the SFBC ( Z lt 08 m) was
apparently higher which despite the above reduction in the bed
temperature resulted in the lower O2 concentration at all locations
in this region (as seen in Fig 4b) This phenomenon can be
explained by the greater residence time (leading to a higher yield of CO and other combustibles with fuel volatiles) and also greater
contribution of the ldquowetrdquo oxidation of char-C by OH radicals both
leading to higher rates of CO formation and consequently O2
consumption in this region The next region (08 lt Z lt 10 m) was
characterized by a noticeable regaining (rise) of O2 as the response
to secondary air injection However in the freeboard of the
combustor ( Z gt 10 m) the O2 concentration was diminished along
the reactor height at a rather low rate and this reduction was
accompanied by the gradual converging of the axial pro1047297les as all
the tests in Fig 4 for variable fuel moisture were conducted at
(nearly) the same EA
Fig 5 shows the radial CO and NO concentration pro1047297les at
different levels above the air distributor for the same fuels and
operating conditions as in Fig 3 Both CO (Fig 5a) and NO (Fig 5b)
were signi1047297cantly affected by fuel moisture and showed negative
gradients along the radius (different numerically) at all the levels in
the SFBC For the 1047297xed fuel-moisture content due to the effects of
secondary air the CO concentration in the peripheral zone across
the combustor was much lower compared to that at the centerline
thus forming the above radial gradient of CO However the NO
concentration varied weakly along the radius except at Z frac14 047 m
The occurrence of the NO maximum at the centerline indicated
higher rates of both fuel devolatilization and oxidation in the
central zone of the reactor (compared to those at the combustor
wall) despite the uniformity of the temperature and the opposite
trends of the O2 concentration pro1047297les across the SFBC (see Fig 3b)
In the freeboardof the reactor the radial CO and NO gradients were
found to be gradually attenuated with higher Z
Fig 6 depicts the axial CO and NO concentration pro1047297les in this
combustor As seen in Fig 6a the pro1047297les exhibited four sequent
regions along the combustor height With increasing fuel moisture
the CO concentration in the 1047297rst region (0lt Z lt 08 m) was
apparently higher at all the levels above the air distributor
particularly at the centerline (see Fig 5a) mainly due to (i) longerresidence time (leading to greater yield of CO with volatiles) (ii)
lower PA (reducing the rate of CO oxidation) (iii) higher contri-
bution of ldquowetrdquo oxidation of char-C by OH radicals and (iv) lower
bed temperature causing an increase in the COCO2 ratio in the
products of fuel-char oxidation [33]
In the second region (08 lt Z lt 10 m) the CO concentration
along the combustor axis was found to be drastically reduced
mainly due to the effects of secondaryair the greater rate of the CO
reduction being observed at a higher level of fuel moisture (ie at
higher SA) In the third region (10 lt Z lt 18 m) the CO concentra-
tion regained substantial values along the centerline mainly due to
oxidation of unburned hydrocarbons and fuel-C carried over from
the bed region to CO However in the fourth (upper) region the CO
concentration was found to be gradually reduced along thecombustor height likely via homogeneous reaction of CO with
residual O2 and OH [333]
Like for CO four speci1047297c regions can be distinguished in the
axial NO concentration pro1047297les as can be seen in Fig 6b In the 1047297rst
region (0 lt Z lt 08 m) NO was mainly formed from NH3 in volatiles
(a major precursor of NO in biomass combustion) via the fuel-NO
formation mechanism [343435] With increasing fuel moisture
despite the reduction in bed temperature the NO concentration at
the reactor centerline showed a trend to increase at any given Z
mainly due to (i) greater residence time promoting a higher yield
of nitrogenous species with fuel volatiles and (ii) enhanced
a b
500
600
700
800
900
1000
1100
0 1 2 3
Height above air distributor (m)
) C deg ( e r u t a r e p m e T
0
5
10
15
20
0 1 2 3Height above air distributor (m)
O 2
) l o v ( n o i t a r t n e c n o c
Fig 4 Axial (a) temperature and (b) O2 concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415
(
) Wfrac14
20 (6
) Wfrac14
25 and (A
) Wfrac14
30
VI Kuprianov et al Energy 36 (2011) 2038e 20482044
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 811
Z = 267 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 267 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t
n e c n o c O N
Z = 217 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 217 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O N
Z = 155 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 155 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0rR
) m
p p ( n o i t a r t n e c n o c O N
Z = 101 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 101 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O N
Z = 047 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
)
m
p p ( n o i t a r t n e c n o c
O C
Z = 047 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O N
ab
Fig 5 Radial (a) CO and (b) NO concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 ()
W frac14 15 () W frac1420 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2045
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 911
oxidation of NH3 by OH [36] In the second region (08 lt Z lt 10 m)
the chemical reactions responsible for NO decomposition such as
catalytic reduction of NO by CO on the char surface [3] as well as
reaction of NO with NH3 at oxygen de1047297ciency [34] were predom-
inant This resulted in a signi1047297cant reduction of the NO concen-
tration along the combustor height and the NO reduction was more
apparent when burning rice husks with higher moisture contentsHowever the injection of secondary air promoted a slight increase
in NO in the third region (10 lt Z lt 16 m) of the reactor as a result
of oxidation of the nitrogenous species carried over from the
combustor bottom In the fourth (upper) region the pro1047297les
exhibited some diminishing of the NO concentration along the
reactor height because of the catalytic reduction of NO by CO
occurred however at a rather low rate
322 Emissions
Fig 7 depicts the CO and NO emissions (on 6 O 2 dry gas basis)
from the combustor 1047297ring 80 kgh rice husk at variable EA for the
whole range of fuel moisture (Options 1e6 in Table 2) It can be
concluded from analysis of data in Fig 7a that at EA greater than
40 the CO emission from the SFBC can be controlled ata comparatively low level below 350 ppm regardless of the fuel
quality On the contrary at excess air lower than 40 the CO
emission exhibited quite strong effects of both fuel quality and EA
Note that at EA frac14 20 the CO emission was extremely high
3000e7000 ppm for the whole range of fuel moisture However
with increasing the fuel-moisture content from 84 (in ldquoas-
receivedrdquo rice husk) to 25 the CO emission at this lowest EA
exhibited some reduction roughly from 4000 to 3000 ppm basi-
cally caused by (i) the higher rate of chemical reaction between CO
and OH [333] and (ii) higher rate of CO decomposition in the
freeboard (due to enhanced SA and greater residence time) Similar
trend is reported in some studies on effects of fuel moisture on the
CO emission from the 1047297xed-bed combustion systems [3132]
However with further increase in the fuel-moisture content (from
25 to 35) the CO emission from the SFBC was found to rise from
about 3000 to 7000 ppm likely due to the signi1047297cant contribution
of ldquowetrdquo oxidation of char-C occurred at the lowered combustion
temperatures This trend is in concordance with the behavior of theCO emission during the conventional 1047298uidized-bed combustion of
some biomass fuels with variable moisture [420]
It can be seen in Fig 7b that at EA frac14 40 (the ldquocriticalrdquo value for
the CO emission) the NO emission diminished from about 170 to
130 ppm (or by some 25) when changing the fuel-moisture
content from 84 to 25 A further increase in fuel moisture up to
30e35 resulted in some more emission reduction Note that this
positive result was accompanied by deterioration of combustion
stability (likely caused by inconsistency in fuel properties) which
showed itself by the noticeable time-domain 1047298uctuations of the
temperature and gaseous species particularly in the vicinity of the
fuel injection As revealed by experimental data from this study
through moisturizing of ldquoas-receivedrdquo rice husk the NO emission
from the SFBC can be substantially reduced However thisachievement was accompanied by the conventional effects of EA
leading to the increase in the NO emission with higher EA [16e12]
323 Optimal excess air and fuel-moisture content
Fig 8 shows the emission costs US$ per 1 ton of fuel for 1047297ring
rice husk in the SFBC at variable fuel moisture and excess air pre-
dicted using the above CO and NO emissions and the speci1047297c
ldquoexternalrdquo costs P NO x frac14 3000 US$=t and P CO frac14 600 US$t (ie for
P NO x=P CO frac14 5) At low values of EA the contribution of CO to the
emission costs was predominant whereas the effects of NO were
substantial at higher EA values It can be seen in Fig 8 that the
Fig 6 Axial (a) CO and (b) NO concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415 ()
W frac1420 (6) W frac1425 and (A) W frac1430
Fig 7 Effects of the fuel-moisture content and excess air on the (a) CO and (b) NO emissions from the SFBC 1047297ring 80 kghr rice husk (-) W frac1484 () W frac1415 () W frac1420 (6)
Wfrac14
25 (A
) Wfrac14
30 and (A
) Wfrac14
35
VI Kuprianov et al Energy 36 (2011) 2038e 20482046
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 1011
emission costs were at the minimal value when 1047297ring rice husk
with the fuel-moisture content of 25e
35 at excess air of 40e
50
Switching P NO x=P CO from 5 to 75 (keeping however P NO x
at the
above value) led in effect to nearly the same optimal ranges of fuel
moisture and excess air quanti1047297ed however at different magni-
tudes of the emission costs Thus the optimized variables are not
sensitive to the speci1047297c ldquoexternalrdquo costs P NO x and P CO within their
actual ranges
324 Combustion heat losses and ef 1047297ciency
Table 4 shows the heat losses with unburned carbon quc and
owing to incomplete combustion qic together with the combustion
ef 1047297ciency of the SFBC actual EA values and SAPA ratios for some
selected fuel options
The heat loss with unburned carbon exhibited a rather weak
correlation with fuel moisture showing however the substantialin1047298uence of excess air Unlike quc the heat loss owing to incomplete
combustion was strongly affected by both fuel moisture and excess
air and it was characterized by quite small magnitudes at EAgt 40
Thus an increase in excess air basically resulted in the improve-
ment of the combustion ef 1047297ciency of the SFBC For the fuel range
the highest combustion ef 1047297ciency 992e997 was obtained at
excess air of 40e80 Numerous studies on 1047297ring various biomass
fuels in conventional 1047298uidized-bed combustion systems report
similar trends [1210e1337]
Based on the analysis of both emission characteristics and
combustion ef 1047297ciency and also taking into consideration an
important issue of the combustion stability it can be generally
concluded that the best performance of this SFBC is achievable
when 1047297ring moisturized rice husk with the moisture content of
about 25 at excess air of 40e50 Under these conditions the
major gaseous emissions from the conical SFBC can be controlled
below 350 ppm for CO and within 130e140 ppm for NO (both on
6 O2 dry gas basis) while the combustion ef 1047297ciency is ensured at
a rather high value about 995
4 Conclusions
Combustion and emission characteristics have been experi-
mentally studied on a SFBC 1047297ring 80 kgh rice husk Two NO x
emission control techniques have been investigated in this work
(1) air staging of the combustion process and (2) 1047297ring rice husk as
moisturized fuel
In the test series for variable air staging the combustorhas been
tested with the aim to investigate the behavior of CO NO and C xH yemissions as well as combustion ef 1047297ciency for burning ldquoas-
receivedrdquo rice husk at a 1047297xed excess air value (of about 40) for
variable secondary-to-primary air ratio (SAPA) With increasing
SAPA from 026 to 075 the CO emission from the SFBC ranges from
about 360 to 450 ppm (on 6 O2 dry gas basis) while the NO
emission reduces at a quite low rate from about 150 to 140 ppm
However with higher SAPA C xH y emissions increase from 120 to
1400 ppm the dramatic rise being observed at SAPA gt 04 Thus
the air staging has minor effects on the CO and NO emissions To
avoid elevated C xH y emissions primary air should be supplied to
the combustor at the amount greater than the stoichiometric air
affected by the fuel properties
During the trialsfor variable fuel quality ricehusks with different
fuel moisture contents (of 84e35) have been burned at different
excess air values rangedfrom about 20 to 80 The analysis of radial
and axial CO and NO concentration pro1047297les has shown the occur-
rence of four speci1047297c regions along the combustor height charac-
terized by different rates of formation and decomposition of CO and
NO The highest rates of CO and NO decomposition are found to
occur in the bed splash zone whereas the top region in the
combustor is characterized by small axial gradients of these species
Through moisturizing rice husk the NO emission from this
SFBC can be substantially reduced while the CO emission is
effectively controlled by the secondary air injection into the bed
splash zone The best combustion and emission performance of
the SFBC is achievable when burning moisturized rice husk with
the moisture content of about 25 at excess air of 40e50 For
these optimal operating conditions the CO emission is expected
to be below 350 ppm ensuring high combustion ef 1047297ciency (about
995 maximum) while the NO emission may range from 130 to
140 ppm With increasing the fuel-moisture content to higher
values eg 30e35 the NO emission from the SFBC can be
secured even at lower values below 110 ppm However elevated
fuel moisture may likely result in deterioration of combustion
stability
Acknowledgements
The authors would like to acknowledge the 1047297nancial support
from the Thailand Research Fund (contract No BRG 50800011) The
authors also sincerely thank Dr Kasama Sirisomboon and Mr
Porametr Arromdee for their effective help in experimental tests
Fig 8 Effects of the fuel-moisture content and excess air on the emission costs for
1047297ring rice husk in the SFBC
Table 4
Heat losses and combustion ef 1047297ciency of the SFBC 1047297ring 80 kgh rice husk for
variable fuel-moisture content and excess air
W () EA () SAPA quc () qic () Combustion
ef 1047297ciency ()
84 17 033 074 204 972
41 026 049 015 994
64 022 041 007 995
76 019 049 008 994
15 17 036 046 173 978
36 030 041 040 992
63 024 050 008 994
89 021 036 005 996
25 18 042 047 134 982
41 033 035 013 995
63 027 029 003 997
83 024 024 003 997
35 16 055 048 242 971
39 042 033 012 995
65 033 026 003 997
84 029 034 005 996
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2047
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 1111
Appendix A Measurement uncertainty in the 1047298owrate of
primary air
The fractional uncertainty in the 1047298owrate of primary air was
estimated by Ref [38]
sPA frac14 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
4s2D thorn s2
V thorn s2R thorn s2
P
q (A1)
where sD is the error in physical measurement of inner diameter of the primary air pipe sV is the measurement error of the velocity
(dependent mainlyon the type of a selected probe) sR is therelative
error associated with imperfect1047297xing of the probe in air1047298ow and sP
is the uncertainty in the velocity pro1047297le across the pipe
AssumingsD frac14 1 sV frac14 15 sR frac14 1 and sP frac14 15(as advised in
Ref [38]) the uncertainty in the 1047298owrate of primary air was
calculated using Eq (A1) to be about 3
Appendix B Determining heat losses and combustion
ef 1047297ciency
In this work the combustion ef 1047297ciency of the SFBC 1047297ring rice
husk was calculated using the the heat-loss method [18]
For this combustor with no ashremoval through thebottompartthe heat loss with unburned carbon quc () was determined by
quc frac14 32866
LHV
C fa
100 C fa
A (B1)
where C fa is the unburned carbon content in the1047298y ash wt and A
is the fuel-ash content wt on ldquoas-receivedrdquo basis
The heat loss owing to incomplete combustion qic () was
quanti1047297ed based on theconcentrations (vol)of COand C xH y (asCH4)
measured in the 1047298ue gas at the cyclone outlet at actual excess air
qic frac14 eth1264 CO thorn 3582 CH4THORN V dgeth100 qucTHORN
LHV (B2)
where V dg is the volume of dry 1047298ue gas at the cyclone outlet Nm3
kg calculated by Refs [1822] using the fuel ultimate analysis onldquoas-receivedrdquo basis and excess air ratio at this point
The combustion ef 1047297ciency of the 1047298uidized-bed combustor hc
() was then determined by
hc frac14 100 ethquc thorn qicTHORN (B3)
References
[1] Natarajan E Nordin A Rao AN Overview of combustion and gasi1047297cation of rice husk in 1047298uidized bed reactors Biomass Bioenergy 199814533e46
[2] Bhattacharya SC Abdul Salam P Sharma M Emissions from biomass energyuse in some selected Asian countries Energy 200025169e88
[3] Werther J Saenger M Hartge EU Ogada T Siagi Z Combustion of agriculturalresidues Prog Energy Combust Sci 2000261e27
[4] Janvijitsakul K Kuprianov VI Similarity and modeling of axial CO and NO
concentration pro1047297les in a 1047298uidized-bed combustor (co-)1047297ring biomass fuelsFuel 2008871574e84
[5] Demirbas A Combustion characteristics of different biomass fuels ProgEnergy Combust Sci 200430219e30
[6] Leckner B Karlsson M Gaseous emissions form circulating 1047298uidized bedcombustion of wood Biomass Bioenergy 19934379e89
[7] Spliethoff H Hein KRG Effect of co-combustion of biomass on emissions inpulverized fuel furnaces Fuel Process Technol 199854189e205
[8] Lyngfelt A Leckner B Combustion of wood-chips in circulating 1047298uidized bedboilers e NO and CO emissions as functions of temperature and air-stagingFuel 1999781065e72
[9] Chyang CS Wu KT Lin CS Emission of nitrogen oxides in a vortexing 1047298uidizedbed combustor Fuel 200786234e43
[10] Armesto E Bahillo A Cabanillas A Veijonen K Otero J Combustion behavior of rice husk in a bubbling 1047298uidised bed Biomass Bioenergy 200223171e9
[11] Kuprianov VI Janvijitsakul K Permchart W Co-1047297ring of sugar canebagasse with rice husk in a conical 1047298uidized-bed combustor Fuel 200685434e42
[12] Fang M Yang L Chen G Shi Z Luo Z Cen K Experimental study on rice huskcombustion in a circulating 1047298uidized bed Fuel Process Technol2004851273e82
[13] Permchart W Kouprianov VI Emission performance and combustion ef 1047297-ciency of a conical 1047298uidized-bed combustor 1047297ring various biomass fuelsBioresour Technol 20049283e91
[14] Sirisomboon K Kuprianov VI Arromdee P Effects of design features oncombustion ef 1047297ciency and emission performance of a biomass-fuelled 1047298uid-ized-bed combustor Chem Eng Process Process Intens 201049270e7
[15] Leckner B Amand LE Lucke K Werther J Gaseous emissions from co-combustion of sewage sludge and coalwood in a 1047298uidized bed Fuel200483477e86
[16] Eaimsa-ard S Kaewkohkiet Y Thianpong C Promvonge P Combustionbehavior in a dual-staging vortex rice husk combustor with snail entry IntCommun Heat Mass Transfer 2008351134e40
[17] Madhiyanon T Lapirattanakun A Sathitruangsak P Soponronnarit S A novelcyclonic 1047298uidized-bed combustor (J-FBC) combustion and thermal ef 1047297-ciency temperature distribution combustion intensity and emission of pollutants Combust Flame 2006146232e45
[18] Basu P Cen KF Jestin L Boilers and burners New York Springer 2000[19] Kaewklum R Kuprianov VI Experimental studies on a novel swirling 1047298uid-
ized-bed combustor using an annular spiral air distributor Fuel20108943
e52
[20] Kouprianov VI Permchart W Emission from a conical FBC 1047297red witha biomass fuel Appl Energy 200374383e92
[21] Kaewklum R Kuprianov VI Douglas PL Hydrodynamics of airesand 1047298ow ina conical swirling 1047298uidized bed a comparative study between tangential andaxial air entries Energy Convers Manage 2009502999e3006
[22] Bezgreshnov AN Lipov YM Shleipher BM Computations of steam boilersMoscow Energoatomizdat 1991 [in Russian]
[23] Kuprianov VI Application of a cost-based method of excess air optimizationfor the improvement of thermal ef 1047297ciency and environmental performance of steam boilers Renew Sustain Energy Rev 20059474e98
[24] Streimikiene D Roos I Rekis J External cost of electricity generation in BalticStates Renew Sustain Energy Rev 200913863e70
[25] Zhang Q Weili T Yumei W Yingxu C External costs from electricity gener-ation of China up to 2030 in energy and abatement scenarios Energy Pol2007354295e304
[26] Hainoun A Almoustafa A Aldin MS Estimating the health damage costs of Syrian electricity generation system using impact pathway approach Energy
201035628e
38[27] Nguyen KQ Internalizing externalities into capacity expansion planning thecase of electricity in Vietnam Energy 200833740e6
[28] Kitou E Horvath A External air pollution costs of telework Int J Life CycleAssess 200813155e65
[29] Wei X Zhang L Zhou H Evaluating the environmental value of pollutants inChina power industry In Proceedings of the international conference onenergy and the environment Shanghai China 2003
[30] Salisdisouk N The concept of integrated resource planning In Proceeding of the workshop on electric power quality safety and ef 1047297ciency of its useselectric power system management Pathum Thani Thailand 1994
[31] Yang YB Shari1047297 VN Swithenbank J Effect of air 1047298ow rate and fuel moisture onthe burning behaviours of biomass and simulated municipal solid wastes inpacked beds Fuel 2004831553e62
[32] Zhao W Li Z Zhao G Zhang F Zhu Q Effect of air preheating and fuel moistureon combustion characteristics of corn straw in a 1047297xed bed Energy ConversManage 2008493560e5
[33] Tillman DA Rossi AJ Kitto WD Wood combustion New York AcademicPress 1981
[34] Winter F Wartha C Hofbauer H NO and N2O formation during thecombustion of wood straw malt waste and peat Bioresour Technol19997039e49
[35] Sun Z Jin M Zhang M Liu R Zhang Y Experimental studies on cotton stalkcombustion in a 1047298uidized bed Energy 2008331224e32
[36] Smart JP Robert PA De Soete GG The formation of nitrous oxide in a large-scale pulverized-coal 1047298ames J Inst Energy 199063131e5
[37] Abu-Qudais M Fluidized-bed combustion for energy production from olivecake Energy 199621173e8
[38] Trembovlya VI Finger ED Avdeeva AA Thermo-technical tests of boiler unitsMoscow Energoatomizdat 1991 [in Russian]
VI Kuprianov et al Energy 36 (2011) 2038e 20482048
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 511
a rather low level basically due to the secondary air injection into
the bed splash zone As can be concluded based on the results from
Ref [12] and present study excess air (or percentage of total air) is
an important factor in controlling the CO emission in this SFBC
whereas SAPA shows quite weak effects
With increasing SAPA within the selected range 026e075 the
NO emission exhibited a slight reduction from about 150 to140 ppm (see Fig 2a) mainly due to the diminishing of the bed
temperature and reduction of the O2 concentration in the bed the
latter being occurred because of the lowering of PA Thus the air
staging does not seem to be an effective measure to control the NO
emission in this combustor 1047297ring rice husk
It can be seen in Fig 2b that at relatively small proportions of
secondary air the C xH y emissions were at a quite low level
However at SAPA gt 04 these emissions showed a signi1047297cant
increase from 120 to 1400 ppm which can be explained by the
sub-stoichiometric conditions in the bed region Under such
conditions more volatiles were carried over from the combustor
bottom causing the above increase in the C xH y emissions Thus
primary air should be supplied to the SFBC at a 1047298ow rate ensuring
the oxidation conditions in the bed ie with some excess withregard to the theoretical (stoichiometric) air For instance at
EA frac14 40 the combustor should be operated at 026 lt SAPA lt 04
(see Fig 2b) for avoiding high C xH y emissions from this SFBC 1047297ring
rice husk As can be generally concluded the lower limit of SAPA is
speci1047297ed with the aim to provide the reliable coolingof the start-up
burner whereas the upper one is selected taking into account that
EA should be somewhat greater than SA
312 Heat losses and combustion ef 1047297ciency
The analyses of 1047298y ashes for unburned carbon for this test series
indicated the high rate of fuel burnout in this conical SFBC
Depending on SAPA the unburned carbon content in the 1047298y ashes
varied from 081 to 24 the minimum value being found at the
highest SAPA ratio
Table 3 shows the heat losses with unburned carbon (quc) and
owing to incomplete combustion (qic) together with the combus-
tion ef 1047297ciency (all as the percentage of LHV) for 1047297ring 80 kgh rice
husk at excess air of about 40 for different values of SAPA An
increase in SAPA led to a noticeable reduction in the heat loss with
unburned carbon basically due to the higher rate of fuel burnout
which waslikelycaused by an increase in the residence time of char
particles in the combustor However the exponential rise of qic can
be explained by the above behavior of CO and C xH y emissions
Due to the opposite trends exhibited by the heat losses the
combustion ef 1047297ciency was found to have a maximum 991 for1047297ring rice husk at SAPA of 04e06 and EA of about 40
32 Emission and combustion characteristics of SFBC for variable
fuel moisture
As revealed by the experimental results the temperature and
gas concentrations (O2 CO and NO) in this conical SFBC were rep-
resented by three-dimensional patterns (1047297elds) showing the
effects of combustor hydrodynamics fuel quality and operating
conditions on the radial and axial pro1047297les of the temperature and
chemical species Note that at a given excess air SA raised with
increasing fuel moisture because of the reduction in the theoretical
air while the 1047298owrate of secondary air was 1047297xed at the above
constant value changing from 29 (for ldquoas-receivedrdquo fuel) to 41(for rice husk with the highest moisture content) which resulted in
corresponding diminishing of PA Due to the reduction in the
combustion temperature and also in the theoretical air and PA
the residence time of char particles in the bottomregion of the SFBC
was substantially greater when burning rice husks with higher
moisture content leading to the higher rates of devolatilization and
burnout of fuel particles in this region and thus affecting signi1047297-
cantly the behavior of all variables in the reactor
At EA frac14 40 or higher oxidizing conditions were basically
provided in the combustor bottom which justi1047297ed the ignorance of
C xH y emissions in this test series
321 Radial and axial temperature and gas concentration pro 1047297les
in the SFBC Fig 3 shows the radial temperature and O2 concentration
pro1047297les at 1047297ve levels ( Z ) above the air distributor in the SFBC 1047297ring
80 kgh rice husk at excess air of about 40 for variable fuel prop-
erties (Options 1e5 in Table 2) As seen in Fig 3 the variables
exhibitedquitesimilar behaviorsat different levels ( Z ) above the air
distributor The radial temperature pro1047297les were found to be rather
uniform indicating the highly intensive heat-and-mass transfer
along the radius With increasing fuel moisture (at a 1047297xed excess air
level) the temperature at all points in the combustor volume was
found to be reduced (despite the above increase in the residence
time) because of the apparent in1047298uence of the latent heat of water
evaporation Similar results are reported in some studies on
conventional 1047298uidized-bed and 1047297xed-bed combustion systems
1047297ring biomass fuels with variable fuel moisture [4203132]
Fig 2 Effects of secondary-to-primary air ratio (SAPA) on the (a) CO NO and (b) C xH y emissions from the SFBC 1047297ring rice husk at excess air of 40
Table 3
Heat losses and combustion ef 1047297ciency of the SFBC 1047297ring 80 kgh rice husk at excess
air of 40 for variable secondary-to-primary air ratio
SAPA quc () qic () Combustion ef 1047297ciency ()
026 094 024 988
040 051 035 991
056 047 043 991
075 031 212 976
VI Kuprianov et al Energy 36 (2011) 2038e 20482042
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 611
Z = 267 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r
e p m e T
Z = 267 m
0
5
10
15
20
00 02 04 06 08 10
rR
O 2
) l o v ( n o i t a r t
n e c n o c
Z = 217 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r e p m e T
Z = 217 m0
5
10
15
20
00 02 04 06 08 10
rR
O 2
) l o v ( n o i t a r t n e c n o c
Z = 155 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r e p m e T
Z = 155 m
0
5
10
15
20
00 02 04 06 08 10
rR
O 2
) l o v ( n o i t a r t n e c n o c
Z = 101 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r e p m e T
Z = 101 m
0
5
10
15
20
00 02 04 06 08 10
rR
O 2
) l o v ( n o i t a r t n e c n o c
Z = 047 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r e p m e T
Z = 047 m
0
5
10
15
20
00 02 04 06 08 10
rR
O 2
)
l o v ( n o i t a r t n e c n o c
a b
Fig 3 Radial (a) temperature and (b) O2 concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-)
W frac14 84 () W frac1415 () W frac14 20 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2043
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 711
Implementation of air staging seems to have a signi1047297cant impact
on the behavior of gaseous species in both radial and axial direc-
tions Due to the tangential injection of secondary air into the bed
splash zone the radial O2 concentration pro1047297les were characterized
by a positive radial gradient (the most signi1047297cant being observed at
Z frac14 101 m) which resulted in the higher O2 concentration near the
combustor wall than that at the centerline (ie at r R frac14 0)
However the radial gradient of O2 was gradually attenuated along
the bed height Note that the injection of secondary air affected the
radial O2 concentration pro1047297les not only in upper regions of the
reactor but also at levels below the injection point (in Fig 3b see
the pro1047297les at Z frac14 047 m) since the secondary air was injected at
the negative angle In the meantime with increasing the fuel-
moisture content the O2 concentration at all the points across the
combustor was found to be reduced and this reduction was caused
by some physical and chemical factors as addressed below in the
discussion of axial O2 concentration pro1047297les
The axial temperature and O2 concentration pro1047297les in the SFBC
are shown in Fig 4 for the same fuel options and operating
conditions as in Fig 3 At 1047297xed excess air a positive axial temper-
ature gradient was found to occur in the lower part of the reactor
for all the fuels (see Fig 4a) likely due to the diminishing of heat
release in the dense bed (caused by air staging) and (ii) in1047298uence of secondary air injected into the bed splash zone at the ambient
temperature However with higher fuel moisture (ie with dete-
riorating fuel quality) the temperature attained its maximum at
lower levels (Z) above the air distributor which can be explained by
the effects of the residence time The maximum temperature for
burning ldquoas-receivedrdquo rice husk (with W frac14 84) was rather high
about 980 C however it was reduced to 850e860 C when
increasing the fuel-moisture content to 25e30
However with raising fuel moisture the rate of oxygen
consumption in the bottom region of the SFBC ( Z lt 08 m) was
apparently higher which despite the above reduction in the bed
temperature resulted in the lower O2 concentration at all locations
in this region (as seen in Fig 4b) This phenomenon can be
explained by the greater residence time (leading to a higher yield of CO and other combustibles with fuel volatiles) and also greater
contribution of the ldquowetrdquo oxidation of char-C by OH radicals both
leading to higher rates of CO formation and consequently O2
consumption in this region The next region (08 lt Z lt 10 m) was
characterized by a noticeable regaining (rise) of O2 as the response
to secondary air injection However in the freeboard of the
combustor ( Z gt 10 m) the O2 concentration was diminished along
the reactor height at a rather low rate and this reduction was
accompanied by the gradual converging of the axial pro1047297les as all
the tests in Fig 4 for variable fuel moisture were conducted at
(nearly) the same EA
Fig 5 shows the radial CO and NO concentration pro1047297les at
different levels above the air distributor for the same fuels and
operating conditions as in Fig 3 Both CO (Fig 5a) and NO (Fig 5b)
were signi1047297cantly affected by fuel moisture and showed negative
gradients along the radius (different numerically) at all the levels in
the SFBC For the 1047297xed fuel-moisture content due to the effects of
secondary air the CO concentration in the peripheral zone across
the combustor was much lower compared to that at the centerline
thus forming the above radial gradient of CO However the NO
concentration varied weakly along the radius except at Z frac14 047 m
The occurrence of the NO maximum at the centerline indicated
higher rates of both fuel devolatilization and oxidation in the
central zone of the reactor (compared to those at the combustor
wall) despite the uniformity of the temperature and the opposite
trends of the O2 concentration pro1047297les across the SFBC (see Fig 3b)
In the freeboardof the reactor the radial CO and NO gradients were
found to be gradually attenuated with higher Z
Fig 6 depicts the axial CO and NO concentration pro1047297les in this
combustor As seen in Fig 6a the pro1047297les exhibited four sequent
regions along the combustor height With increasing fuel moisture
the CO concentration in the 1047297rst region (0lt Z lt 08 m) was
apparently higher at all the levels above the air distributor
particularly at the centerline (see Fig 5a) mainly due to (i) longerresidence time (leading to greater yield of CO with volatiles) (ii)
lower PA (reducing the rate of CO oxidation) (iii) higher contri-
bution of ldquowetrdquo oxidation of char-C by OH radicals and (iv) lower
bed temperature causing an increase in the COCO2 ratio in the
products of fuel-char oxidation [33]
In the second region (08 lt Z lt 10 m) the CO concentration
along the combustor axis was found to be drastically reduced
mainly due to the effects of secondaryair the greater rate of the CO
reduction being observed at a higher level of fuel moisture (ie at
higher SA) In the third region (10 lt Z lt 18 m) the CO concentra-
tion regained substantial values along the centerline mainly due to
oxidation of unburned hydrocarbons and fuel-C carried over from
the bed region to CO However in the fourth (upper) region the CO
concentration was found to be gradually reduced along thecombustor height likely via homogeneous reaction of CO with
residual O2 and OH [333]
Like for CO four speci1047297c regions can be distinguished in the
axial NO concentration pro1047297les as can be seen in Fig 6b In the 1047297rst
region (0 lt Z lt 08 m) NO was mainly formed from NH3 in volatiles
(a major precursor of NO in biomass combustion) via the fuel-NO
formation mechanism [343435] With increasing fuel moisture
despite the reduction in bed temperature the NO concentration at
the reactor centerline showed a trend to increase at any given Z
mainly due to (i) greater residence time promoting a higher yield
of nitrogenous species with fuel volatiles and (ii) enhanced
a b
500
600
700
800
900
1000
1100
0 1 2 3
Height above air distributor (m)
) C deg ( e r u t a r e p m e T
0
5
10
15
20
0 1 2 3Height above air distributor (m)
O 2
) l o v ( n o i t a r t n e c n o c
Fig 4 Axial (a) temperature and (b) O2 concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415
(
) Wfrac14
20 (6
) Wfrac14
25 and (A
) Wfrac14
30
VI Kuprianov et al Energy 36 (2011) 2038e 20482044
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 811
Z = 267 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 267 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t
n e c n o c O N
Z = 217 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 217 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O N
Z = 155 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 155 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0rR
) m
p p ( n o i t a r t n e c n o c O N
Z = 101 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 101 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O N
Z = 047 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
)
m
p p ( n o i t a r t n e c n o c
O C
Z = 047 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O N
ab
Fig 5 Radial (a) CO and (b) NO concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 ()
W frac14 15 () W frac1420 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2045
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 911
oxidation of NH3 by OH [36] In the second region (08 lt Z lt 10 m)
the chemical reactions responsible for NO decomposition such as
catalytic reduction of NO by CO on the char surface [3] as well as
reaction of NO with NH3 at oxygen de1047297ciency [34] were predom-
inant This resulted in a signi1047297cant reduction of the NO concen-
tration along the combustor height and the NO reduction was more
apparent when burning rice husks with higher moisture contentsHowever the injection of secondary air promoted a slight increase
in NO in the third region (10 lt Z lt 16 m) of the reactor as a result
of oxidation of the nitrogenous species carried over from the
combustor bottom In the fourth (upper) region the pro1047297les
exhibited some diminishing of the NO concentration along the
reactor height because of the catalytic reduction of NO by CO
occurred however at a rather low rate
322 Emissions
Fig 7 depicts the CO and NO emissions (on 6 O 2 dry gas basis)
from the combustor 1047297ring 80 kgh rice husk at variable EA for the
whole range of fuel moisture (Options 1e6 in Table 2) It can be
concluded from analysis of data in Fig 7a that at EA greater than
40 the CO emission from the SFBC can be controlled ata comparatively low level below 350 ppm regardless of the fuel
quality On the contrary at excess air lower than 40 the CO
emission exhibited quite strong effects of both fuel quality and EA
Note that at EA frac14 20 the CO emission was extremely high
3000e7000 ppm for the whole range of fuel moisture However
with increasing the fuel-moisture content from 84 (in ldquoas-
receivedrdquo rice husk) to 25 the CO emission at this lowest EA
exhibited some reduction roughly from 4000 to 3000 ppm basi-
cally caused by (i) the higher rate of chemical reaction between CO
and OH [333] and (ii) higher rate of CO decomposition in the
freeboard (due to enhanced SA and greater residence time) Similar
trend is reported in some studies on effects of fuel moisture on the
CO emission from the 1047297xed-bed combustion systems [3132]
However with further increase in the fuel-moisture content (from
25 to 35) the CO emission from the SFBC was found to rise from
about 3000 to 7000 ppm likely due to the signi1047297cant contribution
of ldquowetrdquo oxidation of char-C occurred at the lowered combustion
temperatures This trend is in concordance with the behavior of theCO emission during the conventional 1047298uidized-bed combustion of
some biomass fuels with variable moisture [420]
It can be seen in Fig 7b that at EA frac14 40 (the ldquocriticalrdquo value for
the CO emission) the NO emission diminished from about 170 to
130 ppm (or by some 25) when changing the fuel-moisture
content from 84 to 25 A further increase in fuel moisture up to
30e35 resulted in some more emission reduction Note that this
positive result was accompanied by deterioration of combustion
stability (likely caused by inconsistency in fuel properties) which
showed itself by the noticeable time-domain 1047298uctuations of the
temperature and gaseous species particularly in the vicinity of the
fuel injection As revealed by experimental data from this study
through moisturizing of ldquoas-receivedrdquo rice husk the NO emission
from the SFBC can be substantially reduced However thisachievement was accompanied by the conventional effects of EA
leading to the increase in the NO emission with higher EA [16e12]
323 Optimal excess air and fuel-moisture content
Fig 8 shows the emission costs US$ per 1 ton of fuel for 1047297ring
rice husk in the SFBC at variable fuel moisture and excess air pre-
dicted using the above CO and NO emissions and the speci1047297c
ldquoexternalrdquo costs P NO x frac14 3000 US$=t and P CO frac14 600 US$t (ie for
P NO x=P CO frac14 5) At low values of EA the contribution of CO to the
emission costs was predominant whereas the effects of NO were
substantial at higher EA values It can be seen in Fig 8 that the
Fig 6 Axial (a) CO and (b) NO concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415 ()
W frac1420 (6) W frac1425 and (A) W frac1430
Fig 7 Effects of the fuel-moisture content and excess air on the (a) CO and (b) NO emissions from the SFBC 1047297ring 80 kghr rice husk (-) W frac1484 () W frac1415 () W frac1420 (6)
Wfrac14
25 (A
) Wfrac14
30 and (A
) Wfrac14
35
VI Kuprianov et al Energy 36 (2011) 2038e 20482046
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 1011
emission costs were at the minimal value when 1047297ring rice husk
with the fuel-moisture content of 25e
35 at excess air of 40e
50
Switching P NO x=P CO from 5 to 75 (keeping however P NO x
at the
above value) led in effect to nearly the same optimal ranges of fuel
moisture and excess air quanti1047297ed however at different magni-
tudes of the emission costs Thus the optimized variables are not
sensitive to the speci1047297c ldquoexternalrdquo costs P NO x and P CO within their
actual ranges
324 Combustion heat losses and ef 1047297ciency
Table 4 shows the heat losses with unburned carbon quc and
owing to incomplete combustion qic together with the combustion
ef 1047297ciency of the SFBC actual EA values and SAPA ratios for some
selected fuel options
The heat loss with unburned carbon exhibited a rather weak
correlation with fuel moisture showing however the substantialin1047298uence of excess air Unlike quc the heat loss owing to incomplete
combustion was strongly affected by both fuel moisture and excess
air and it was characterized by quite small magnitudes at EAgt 40
Thus an increase in excess air basically resulted in the improve-
ment of the combustion ef 1047297ciency of the SFBC For the fuel range
the highest combustion ef 1047297ciency 992e997 was obtained at
excess air of 40e80 Numerous studies on 1047297ring various biomass
fuels in conventional 1047298uidized-bed combustion systems report
similar trends [1210e1337]
Based on the analysis of both emission characteristics and
combustion ef 1047297ciency and also taking into consideration an
important issue of the combustion stability it can be generally
concluded that the best performance of this SFBC is achievable
when 1047297ring moisturized rice husk with the moisture content of
about 25 at excess air of 40e50 Under these conditions the
major gaseous emissions from the conical SFBC can be controlled
below 350 ppm for CO and within 130e140 ppm for NO (both on
6 O2 dry gas basis) while the combustion ef 1047297ciency is ensured at
a rather high value about 995
4 Conclusions
Combustion and emission characteristics have been experi-
mentally studied on a SFBC 1047297ring 80 kgh rice husk Two NO x
emission control techniques have been investigated in this work
(1) air staging of the combustion process and (2) 1047297ring rice husk as
moisturized fuel
In the test series for variable air staging the combustorhas been
tested with the aim to investigate the behavior of CO NO and C xH yemissions as well as combustion ef 1047297ciency for burning ldquoas-
receivedrdquo rice husk at a 1047297xed excess air value (of about 40) for
variable secondary-to-primary air ratio (SAPA) With increasing
SAPA from 026 to 075 the CO emission from the SFBC ranges from
about 360 to 450 ppm (on 6 O2 dry gas basis) while the NO
emission reduces at a quite low rate from about 150 to 140 ppm
However with higher SAPA C xH y emissions increase from 120 to
1400 ppm the dramatic rise being observed at SAPA gt 04 Thus
the air staging has minor effects on the CO and NO emissions To
avoid elevated C xH y emissions primary air should be supplied to
the combustor at the amount greater than the stoichiometric air
affected by the fuel properties
During the trialsfor variable fuel quality ricehusks with different
fuel moisture contents (of 84e35) have been burned at different
excess air values rangedfrom about 20 to 80 The analysis of radial
and axial CO and NO concentration pro1047297les has shown the occur-
rence of four speci1047297c regions along the combustor height charac-
terized by different rates of formation and decomposition of CO and
NO The highest rates of CO and NO decomposition are found to
occur in the bed splash zone whereas the top region in the
combustor is characterized by small axial gradients of these species
Through moisturizing rice husk the NO emission from this
SFBC can be substantially reduced while the CO emission is
effectively controlled by the secondary air injection into the bed
splash zone The best combustion and emission performance of
the SFBC is achievable when burning moisturized rice husk with
the moisture content of about 25 at excess air of 40e50 For
these optimal operating conditions the CO emission is expected
to be below 350 ppm ensuring high combustion ef 1047297ciency (about
995 maximum) while the NO emission may range from 130 to
140 ppm With increasing the fuel-moisture content to higher
values eg 30e35 the NO emission from the SFBC can be
secured even at lower values below 110 ppm However elevated
fuel moisture may likely result in deterioration of combustion
stability
Acknowledgements
The authors would like to acknowledge the 1047297nancial support
from the Thailand Research Fund (contract No BRG 50800011) The
authors also sincerely thank Dr Kasama Sirisomboon and Mr
Porametr Arromdee for their effective help in experimental tests
Fig 8 Effects of the fuel-moisture content and excess air on the emission costs for
1047297ring rice husk in the SFBC
Table 4
Heat losses and combustion ef 1047297ciency of the SFBC 1047297ring 80 kgh rice husk for
variable fuel-moisture content and excess air
W () EA () SAPA quc () qic () Combustion
ef 1047297ciency ()
84 17 033 074 204 972
41 026 049 015 994
64 022 041 007 995
76 019 049 008 994
15 17 036 046 173 978
36 030 041 040 992
63 024 050 008 994
89 021 036 005 996
25 18 042 047 134 982
41 033 035 013 995
63 027 029 003 997
83 024 024 003 997
35 16 055 048 242 971
39 042 033 012 995
65 033 026 003 997
84 029 034 005 996
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2047
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 1111
Appendix A Measurement uncertainty in the 1047298owrate of
primary air
The fractional uncertainty in the 1047298owrate of primary air was
estimated by Ref [38]
sPA frac14 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
4s2D thorn s2
V thorn s2R thorn s2
P
q (A1)
where sD is the error in physical measurement of inner diameter of the primary air pipe sV is the measurement error of the velocity
(dependent mainlyon the type of a selected probe) sR is therelative
error associated with imperfect1047297xing of the probe in air1047298ow and sP
is the uncertainty in the velocity pro1047297le across the pipe
AssumingsD frac14 1 sV frac14 15 sR frac14 1 and sP frac14 15(as advised in
Ref [38]) the uncertainty in the 1047298owrate of primary air was
calculated using Eq (A1) to be about 3
Appendix B Determining heat losses and combustion
ef 1047297ciency
In this work the combustion ef 1047297ciency of the SFBC 1047297ring rice
husk was calculated using the the heat-loss method [18]
For this combustor with no ashremoval through thebottompartthe heat loss with unburned carbon quc () was determined by
quc frac14 32866
LHV
C fa
100 C fa
A (B1)
where C fa is the unburned carbon content in the1047298y ash wt and A
is the fuel-ash content wt on ldquoas-receivedrdquo basis
The heat loss owing to incomplete combustion qic () was
quanti1047297ed based on theconcentrations (vol)of COand C xH y (asCH4)
measured in the 1047298ue gas at the cyclone outlet at actual excess air
qic frac14 eth1264 CO thorn 3582 CH4THORN V dgeth100 qucTHORN
LHV (B2)
where V dg is the volume of dry 1047298ue gas at the cyclone outlet Nm3
kg calculated by Refs [1822] using the fuel ultimate analysis onldquoas-receivedrdquo basis and excess air ratio at this point
The combustion ef 1047297ciency of the 1047298uidized-bed combustor hc
() was then determined by
hc frac14 100 ethquc thorn qicTHORN (B3)
References
[1] Natarajan E Nordin A Rao AN Overview of combustion and gasi1047297cation of rice husk in 1047298uidized bed reactors Biomass Bioenergy 199814533e46
[2] Bhattacharya SC Abdul Salam P Sharma M Emissions from biomass energyuse in some selected Asian countries Energy 200025169e88
[3] Werther J Saenger M Hartge EU Ogada T Siagi Z Combustion of agriculturalresidues Prog Energy Combust Sci 2000261e27
[4] Janvijitsakul K Kuprianov VI Similarity and modeling of axial CO and NO
concentration pro1047297les in a 1047298uidized-bed combustor (co-)1047297ring biomass fuelsFuel 2008871574e84
[5] Demirbas A Combustion characteristics of different biomass fuels ProgEnergy Combust Sci 200430219e30
[6] Leckner B Karlsson M Gaseous emissions form circulating 1047298uidized bedcombustion of wood Biomass Bioenergy 19934379e89
[7] Spliethoff H Hein KRG Effect of co-combustion of biomass on emissions inpulverized fuel furnaces Fuel Process Technol 199854189e205
[8] Lyngfelt A Leckner B Combustion of wood-chips in circulating 1047298uidized bedboilers e NO and CO emissions as functions of temperature and air-stagingFuel 1999781065e72
[9] Chyang CS Wu KT Lin CS Emission of nitrogen oxides in a vortexing 1047298uidizedbed combustor Fuel 200786234e43
[10] Armesto E Bahillo A Cabanillas A Veijonen K Otero J Combustion behavior of rice husk in a bubbling 1047298uidised bed Biomass Bioenergy 200223171e9
[11] Kuprianov VI Janvijitsakul K Permchart W Co-1047297ring of sugar canebagasse with rice husk in a conical 1047298uidized-bed combustor Fuel 200685434e42
[12] Fang M Yang L Chen G Shi Z Luo Z Cen K Experimental study on rice huskcombustion in a circulating 1047298uidized bed Fuel Process Technol2004851273e82
[13] Permchart W Kouprianov VI Emission performance and combustion ef 1047297-ciency of a conical 1047298uidized-bed combustor 1047297ring various biomass fuelsBioresour Technol 20049283e91
[14] Sirisomboon K Kuprianov VI Arromdee P Effects of design features oncombustion ef 1047297ciency and emission performance of a biomass-fuelled 1047298uid-ized-bed combustor Chem Eng Process Process Intens 201049270e7
[15] Leckner B Amand LE Lucke K Werther J Gaseous emissions from co-combustion of sewage sludge and coalwood in a 1047298uidized bed Fuel200483477e86
[16] Eaimsa-ard S Kaewkohkiet Y Thianpong C Promvonge P Combustionbehavior in a dual-staging vortex rice husk combustor with snail entry IntCommun Heat Mass Transfer 2008351134e40
[17] Madhiyanon T Lapirattanakun A Sathitruangsak P Soponronnarit S A novelcyclonic 1047298uidized-bed combustor (J-FBC) combustion and thermal ef 1047297-ciency temperature distribution combustion intensity and emission of pollutants Combust Flame 2006146232e45
[18] Basu P Cen KF Jestin L Boilers and burners New York Springer 2000[19] Kaewklum R Kuprianov VI Experimental studies on a novel swirling 1047298uid-
ized-bed combustor using an annular spiral air distributor Fuel20108943
e52
[20] Kouprianov VI Permchart W Emission from a conical FBC 1047297red witha biomass fuel Appl Energy 200374383e92
[21] Kaewklum R Kuprianov VI Douglas PL Hydrodynamics of airesand 1047298ow ina conical swirling 1047298uidized bed a comparative study between tangential andaxial air entries Energy Convers Manage 2009502999e3006
[22] Bezgreshnov AN Lipov YM Shleipher BM Computations of steam boilersMoscow Energoatomizdat 1991 [in Russian]
[23] Kuprianov VI Application of a cost-based method of excess air optimizationfor the improvement of thermal ef 1047297ciency and environmental performance of steam boilers Renew Sustain Energy Rev 20059474e98
[24] Streimikiene D Roos I Rekis J External cost of electricity generation in BalticStates Renew Sustain Energy Rev 200913863e70
[25] Zhang Q Weili T Yumei W Yingxu C External costs from electricity gener-ation of China up to 2030 in energy and abatement scenarios Energy Pol2007354295e304
[26] Hainoun A Almoustafa A Aldin MS Estimating the health damage costs of Syrian electricity generation system using impact pathway approach Energy
201035628e
38[27] Nguyen KQ Internalizing externalities into capacity expansion planning thecase of electricity in Vietnam Energy 200833740e6
[28] Kitou E Horvath A External air pollution costs of telework Int J Life CycleAssess 200813155e65
[29] Wei X Zhang L Zhou H Evaluating the environmental value of pollutants inChina power industry In Proceedings of the international conference onenergy and the environment Shanghai China 2003
[30] Salisdisouk N The concept of integrated resource planning In Proceeding of the workshop on electric power quality safety and ef 1047297ciency of its useselectric power system management Pathum Thani Thailand 1994
[31] Yang YB Shari1047297 VN Swithenbank J Effect of air 1047298ow rate and fuel moisture onthe burning behaviours of biomass and simulated municipal solid wastes inpacked beds Fuel 2004831553e62
[32] Zhao W Li Z Zhao G Zhang F Zhu Q Effect of air preheating and fuel moistureon combustion characteristics of corn straw in a 1047297xed bed Energy ConversManage 2008493560e5
[33] Tillman DA Rossi AJ Kitto WD Wood combustion New York AcademicPress 1981
[34] Winter F Wartha C Hofbauer H NO and N2O formation during thecombustion of wood straw malt waste and peat Bioresour Technol19997039e49
[35] Sun Z Jin M Zhang M Liu R Zhang Y Experimental studies on cotton stalkcombustion in a 1047298uidized bed Energy 2008331224e32
[36] Smart JP Robert PA De Soete GG The formation of nitrous oxide in a large-scale pulverized-coal 1047298ames J Inst Energy 199063131e5
[37] Abu-Qudais M Fluidized-bed combustion for energy production from olivecake Energy 199621173e8
[38] Trembovlya VI Finger ED Avdeeva AA Thermo-technical tests of boiler unitsMoscow Energoatomizdat 1991 [in Russian]
VI Kuprianov et al Energy 36 (2011) 2038e 20482048
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 611
Z = 267 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r
e p m e T
Z = 267 m
0
5
10
15
20
00 02 04 06 08 10
rR
O 2
) l o v ( n o i t a r t
n e c n o c
Z = 217 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r e p m e T
Z = 217 m0
5
10
15
20
00 02 04 06 08 10
rR
O 2
) l o v ( n o i t a r t n e c n o c
Z = 155 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r e p m e T
Z = 155 m
0
5
10
15
20
00 02 04 06 08 10
rR
O 2
) l o v ( n o i t a r t n e c n o c
Z = 101 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r e p m e T
Z = 101 m
0
5
10
15
20
00 02 04 06 08 10
rR
O 2
) l o v ( n o i t a r t n e c n o c
Z = 047 m
700
800
900
1000
1100
00 02 04 06 08 10
rR
) C deg ( e r u t a r e p m e T
Z = 047 m
0
5
10
15
20
00 02 04 06 08 10
rR
O 2
)
l o v ( n o i t a r t n e c n o c
a b
Fig 3 Radial (a) temperature and (b) O2 concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-)
W frac14 84 () W frac1415 () W frac14 20 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2043
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 711
Implementation of air staging seems to have a signi1047297cant impact
on the behavior of gaseous species in both radial and axial direc-
tions Due to the tangential injection of secondary air into the bed
splash zone the radial O2 concentration pro1047297les were characterized
by a positive radial gradient (the most signi1047297cant being observed at
Z frac14 101 m) which resulted in the higher O2 concentration near the
combustor wall than that at the centerline (ie at r R frac14 0)
However the radial gradient of O2 was gradually attenuated along
the bed height Note that the injection of secondary air affected the
radial O2 concentration pro1047297les not only in upper regions of the
reactor but also at levels below the injection point (in Fig 3b see
the pro1047297les at Z frac14 047 m) since the secondary air was injected at
the negative angle In the meantime with increasing the fuel-
moisture content the O2 concentration at all the points across the
combustor was found to be reduced and this reduction was caused
by some physical and chemical factors as addressed below in the
discussion of axial O2 concentration pro1047297les
The axial temperature and O2 concentration pro1047297les in the SFBC
are shown in Fig 4 for the same fuel options and operating
conditions as in Fig 3 At 1047297xed excess air a positive axial temper-
ature gradient was found to occur in the lower part of the reactor
for all the fuels (see Fig 4a) likely due to the diminishing of heat
release in the dense bed (caused by air staging) and (ii) in1047298uence of secondary air injected into the bed splash zone at the ambient
temperature However with higher fuel moisture (ie with dete-
riorating fuel quality) the temperature attained its maximum at
lower levels (Z) above the air distributor which can be explained by
the effects of the residence time The maximum temperature for
burning ldquoas-receivedrdquo rice husk (with W frac14 84) was rather high
about 980 C however it was reduced to 850e860 C when
increasing the fuel-moisture content to 25e30
However with raising fuel moisture the rate of oxygen
consumption in the bottom region of the SFBC ( Z lt 08 m) was
apparently higher which despite the above reduction in the bed
temperature resulted in the lower O2 concentration at all locations
in this region (as seen in Fig 4b) This phenomenon can be
explained by the greater residence time (leading to a higher yield of CO and other combustibles with fuel volatiles) and also greater
contribution of the ldquowetrdquo oxidation of char-C by OH radicals both
leading to higher rates of CO formation and consequently O2
consumption in this region The next region (08 lt Z lt 10 m) was
characterized by a noticeable regaining (rise) of O2 as the response
to secondary air injection However in the freeboard of the
combustor ( Z gt 10 m) the O2 concentration was diminished along
the reactor height at a rather low rate and this reduction was
accompanied by the gradual converging of the axial pro1047297les as all
the tests in Fig 4 for variable fuel moisture were conducted at
(nearly) the same EA
Fig 5 shows the radial CO and NO concentration pro1047297les at
different levels above the air distributor for the same fuels and
operating conditions as in Fig 3 Both CO (Fig 5a) and NO (Fig 5b)
were signi1047297cantly affected by fuel moisture and showed negative
gradients along the radius (different numerically) at all the levels in
the SFBC For the 1047297xed fuel-moisture content due to the effects of
secondary air the CO concentration in the peripheral zone across
the combustor was much lower compared to that at the centerline
thus forming the above radial gradient of CO However the NO
concentration varied weakly along the radius except at Z frac14 047 m
The occurrence of the NO maximum at the centerline indicated
higher rates of both fuel devolatilization and oxidation in the
central zone of the reactor (compared to those at the combustor
wall) despite the uniformity of the temperature and the opposite
trends of the O2 concentration pro1047297les across the SFBC (see Fig 3b)
In the freeboardof the reactor the radial CO and NO gradients were
found to be gradually attenuated with higher Z
Fig 6 depicts the axial CO and NO concentration pro1047297les in this
combustor As seen in Fig 6a the pro1047297les exhibited four sequent
regions along the combustor height With increasing fuel moisture
the CO concentration in the 1047297rst region (0lt Z lt 08 m) was
apparently higher at all the levels above the air distributor
particularly at the centerline (see Fig 5a) mainly due to (i) longerresidence time (leading to greater yield of CO with volatiles) (ii)
lower PA (reducing the rate of CO oxidation) (iii) higher contri-
bution of ldquowetrdquo oxidation of char-C by OH radicals and (iv) lower
bed temperature causing an increase in the COCO2 ratio in the
products of fuel-char oxidation [33]
In the second region (08 lt Z lt 10 m) the CO concentration
along the combustor axis was found to be drastically reduced
mainly due to the effects of secondaryair the greater rate of the CO
reduction being observed at a higher level of fuel moisture (ie at
higher SA) In the third region (10 lt Z lt 18 m) the CO concentra-
tion regained substantial values along the centerline mainly due to
oxidation of unburned hydrocarbons and fuel-C carried over from
the bed region to CO However in the fourth (upper) region the CO
concentration was found to be gradually reduced along thecombustor height likely via homogeneous reaction of CO with
residual O2 and OH [333]
Like for CO four speci1047297c regions can be distinguished in the
axial NO concentration pro1047297les as can be seen in Fig 6b In the 1047297rst
region (0 lt Z lt 08 m) NO was mainly formed from NH3 in volatiles
(a major precursor of NO in biomass combustion) via the fuel-NO
formation mechanism [343435] With increasing fuel moisture
despite the reduction in bed temperature the NO concentration at
the reactor centerline showed a trend to increase at any given Z
mainly due to (i) greater residence time promoting a higher yield
of nitrogenous species with fuel volatiles and (ii) enhanced
a b
500
600
700
800
900
1000
1100
0 1 2 3
Height above air distributor (m)
) C deg ( e r u t a r e p m e T
0
5
10
15
20
0 1 2 3Height above air distributor (m)
O 2
) l o v ( n o i t a r t n e c n o c
Fig 4 Axial (a) temperature and (b) O2 concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415
(
) Wfrac14
20 (6
) Wfrac14
25 and (A
) Wfrac14
30
VI Kuprianov et al Energy 36 (2011) 2038e 20482044
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 811
Z = 267 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 267 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t
n e c n o c O N
Z = 217 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 217 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O N
Z = 155 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 155 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0rR
) m
p p ( n o i t a r t n e c n o c O N
Z = 101 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 101 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O N
Z = 047 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
)
m
p p ( n o i t a r t n e c n o c
O C
Z = 047 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O N
ab
Fig 5 Radial (a) CO and (b) NO concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 ()
W frac14 15 () W frac1420 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2045
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 911
oxidation of NH3 by OH [36] In the second region (08 lt Z lt 10 m)
the chemical reactions responsible for NO decomposition such as
catalytic reduction of NO by CO on the char surface [3] as well as
reaction of NO with NH3 at oxygen de1047297ciency [34] were predom-
inant This resulted in a signi1047297cant reduction of the NO concen-
tration along the combustor height and the NO reduction was more
apparent when burning rice husks with higher moisture contentsHowever the injection of secondary air promoted a slight increase
in NO in the third region (10 lt Z lt 16 m) of the reactor as a result
of oxidation of the nitrogenous species carried over from the
combustor bottom In the fourth (upper) region the pro1047297les
exhibited some diminishing of the NO concentration along the
reactor height because of the catalytic reduction of NO by CO
occurred however at a rather low rate
322 Emissions
Fig 7 depicts the CO and NO emissions (on 6 O 2 dry gas basis)
from the combustor 1047297ring 80 kgh rice husk at variable EA for the
whole range of fuel moisture (Options 1e6 in Table 2) It can be
concluded from analysis of data in Fig 7a that at EA greater than
40 the CO emission from the SFBC can be controlled ata comparatively low level below 350 ppm regardless of the fuel
quality On the contrary at excess air lower than 40 the CO
emission exhibited quite strong effects of both fuel quality and EA
Note that at EA frac14 20 the CO emission was extremely high
3000e7000 ppm for the whole range of fuel moisture However
with increasing the fuel-moisture content from 84 (in ldquoas-
receivedrdquo rice husk) to 25 the CO emission at this lowest EA
exhibited some reduction roughly from 4000 to 3000 ppm basi-
cally caused by (i) the higher rate of chemical reaction between CO
and OH [333] and (ii) higher rate of CO decomposition in the
freeboard (due to enhanced SA and greater residence time) Similar
trend is reported in some studies on effects of fuel moisture on the
CO emission from the 1047297xed-bed combustion systems [3132]
However with further increase in the fuel-moisture content (from
25 to 35) the CO emission from the SFBC was found to rise from
about 3000 to 7000 ppm likely due to the signi1047297cant contribution
of ldquowetrdquo oxidation of char-C occurred at the lowered combustion
temperatures This trend is in concordance with the behavior of theCO emission during the conventional 1047298uidized-bed combustion of
some biomass fuels with variable moisture [420]
It can be seen in Fig 7b that at EA frac14 40 (the ldquocriticalrdquo value for
the CO emission) the NO emission diminished from about 170 to
130 ppm (or by some 25) when changing the fuel-moisture
content from 84 to 25 A further increase in fuel moisture up to
30e35 resulted in some more emission reduction Note that this
positive result was accompanied by deterioration of combustion
stability (likely caused by inconsistency in fuel properties) which
showed itself by the noticeable time-domain 1047298uctuations of the
temperature and gaseous species particularly in the vicinity of the
fuel injection As revealed by experimental data from this study
through moisturizing of ldquoas-receivedrdquo rice husk the NO emission
from the SFBC can be substantially reduced However thisachievement was accompanied by the conventional effects of EA
leading to the increase in the NO emission with higher EA [16e12]
323 Optimal excess air and fuel-moisture content
Fig 8 shows the emission costs US$ per 1 ton of fuel for 1047297ring
rice husk in the SFBC at variable fuel moisture and excess air pre-
dicted using the above CO and NO emissions and the speci1047297c
ldquoexternalrdquo costs P NO x frac14 3000 US$=t and P CO frac14 600 US$t (ie for
P NO x=P CO frac14 5) At low values of EA the contribution of CO to the
emission costs was predominant whereas the effects of NO were
substantial at higher EA values It can be seen in Fig 8 that the
Fig 6 Axial (a) CO and (b) NO concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415 ()
W frac1420 (6) W frac1425 and (A) W frac1430
Fig 7 Effects of the fuel-moisture content and excess air on the (a) CO and (b) NO emissions from the SFBC 1047297ring 80 kghr rice husk (-) W frac1484 () W frac1415 () W frac1420 (6)
Wfrac14
25 (A
) Wfrac14
30 and (A
) Wfrac14
35
VI Kuprianov et al Energy 36 (2011) 2038e 20482046
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 1011
emission costs were at the minimal value when 1047297ring rice husk
with the fuel-moisture content of 25e
35 at excess air of 40e
50
Switching P NO x=P CO from 5 to 75 (keeping however P NO x
at the
above value) led in effect to nearly the same optimal ranges of fuel
moisture and excess air quanti1047297ed however at different magni-
tudes of the emission costs Thus the optimized variables are not
sensitive to the speci1047297c ldquoexternalrdquo costs P NO x and P CO within their
actual ranges
324 Combustion heat losses and ef 1047297ciency
Table 4 shows the heat losses with unburned carbon quc and
owing to incomplete combustion qic together with the combustion
ef 1047297ciency of the SFBC actual EA values and SAPA ratios for some
selected fuel options
The heat loss with unburned carbon exhibited a rather weak
correlation with fuel moisture showing however the substantialin1047298uence of excess air Unlike quc the heat loss owing to incomplete
combustion was strongly affected by both fuel moisture and excess
air and it was characterized by quite small magnitudes at EAgt 40
Thus an increase in excess air basically resulted in the improve-
ment of the combustion ef 1047297ciency of the SFBC For the fuel range
the highest combustion ef 1047297ciency 992e997 was obtained at
excess air of 40e80 Numerous studies on 1047297ring various biomass
fuels in conventional 1047298uidized-bed combustion systems report
similar trends [1210e1337]
Based on the analysis of both emission characteristics and
combustion ef 1047297ciency and also taking into consideration an
important issue of the combustion stability it can be generally
concluded that the best performance of this SFBC is achievable
when 1047297ring moisturized rice husk with the moisture content of
about 25 at excess air of 40e50 Under these conditions the
major gaseous emissions from the conical SFBC can be controlled
below 350 ppm for CO and within 130e140 ppm for NO (both on
6 O2 dry gas basis) while the combustion ef 1047297ciency is ensured at
a rather high value about 995
4 Conclusions
Combustion and emission characteristics have been experi-
mentally studied on a SFBC 1047297ring 80 kgh rice husk Two NO x
emission control techniques have been investigated in this work
(1) air staging of the combustion process and (2) 1047297ring rice husk as
moisturized fuel
In the test series for variable air staging the combustorhas been
tested with the aim to investigate the behavior of CO NO and C xH yemissions as well as combustion ef 1047297ciency for burning ldquoas-
receivedrdquo rice husk at a 1047297xed excess air value (of about 40) for
variable secondary-to-primary air ratio (SAPA) With increasing
SAPA from 026 to 075 the CO emission from the SFBC ranges from
about 360 to 450 ppm (on 6 O2 dry gas basis) while the NO
emission reduces at a quite low rate from about 150 to 140 ppm
However with higher SAPA C xH y emissions increase from 120 to
1400 ppm the dramatic rise being observed at SAPA gt 04 Thus
the air staging has minor effects on the CO and NO emissions To
avoid elevated C xH y emissions primary air should be supplied to
the combustor at the amount greater than the stoichiometric air
affected by the fuel properties
During the trialsfor variable fuel quality ricehusks with different
fuel moisture contents (of 84e35) have been burned at different
excess air values rangedfrom about 20 to 80 The analysis of radial
and axial CO and NO concentration pro1047297les has shown the occur-
rence of four speci1047297c regions along the combustor height charac-
terized by different rates of formation and decomposition of CO and
NO The highest rates of CO and NO decomposition are found to
occur in the bed splash zone whereas the top region in the
combustor is characterized by small axial gradients of these species
Through moisturizing rice husk the NO emission from this
SFBC can be substantially reduced while the CO emission is
effectively controlled by the secondary air injection into the bed
splash zone The best combustion and emission performance of
the SFBC is achievable when burning moisturized rice husk with
the moisture content of about 25 at excess air of 40e50 For
these optimal operating conditions the CO emission is expected
to be below 350 ppm ensuring high combustion ef 1047297ciency (about
995 maximum) while the NO emission may range from 130 to
140 ppm With increasing the fuel-moisture content to higher
values eg 30e35 the NO emission from the SFBC can be
secured even at lower values below 110 ppm However elevated
fuel moisture may likely result in deterioration of combustion
stability
Acknowledgements
The authors would like to acknowledge the 1047297nancial support
from the Thailand Research Fund (contract No BRG 50800011) The
authors also sincerely thank Dr Kasama Sirisomboon and Mr
Porametr Arromdee for their effective help in experimental tests
Fig 8 Effects of the fuel-moisture content and excess air on the emission costs for
1047297ring rice husk in the SFBC
Table 4
Heat losses and combustion ef 1047297ciency of the SFBC 1047297ring 80 kgh rice husk for
variable fuel-moisture content and excess air
W () EA () SAPA quc () qic () Combustion
ef 1047297ciency ()
84 17 033 074 204 972
41 026 049 015 994
64 022 041 007 995
76 019 049 008 994
15 17 036 046 173 978
36 030 041 040 992
63 024 050 008 994
89 021 036 005 996
25 18 042 047 134 982
41 033 035 013 995
63 027 029 003 997
83 024 024 003 997
35 16 055 048 242 971
39 042 033 012 995
65 033 026 003 997
84 029 034 005 996
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2047
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 1111
Appendix A Measurement uncertainty in the 1047298owrate of
primary air
The fractional uncertainty in the 1047298owrate of primary air was
estimated by Ref [38]
sPA frac14 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
4s2D thorn s2
V thorn s2R thorn s2
P
q (A1)
where sD is the error in physical measurement of inner diameter of the primary air pipe sV is the measurement error of the velocity
(dependent mainlyon the type of a selected probe) sR is therelative
error associated with imperfect1047297xing of the probe in air1047298ow and sP
is the uncertainty in the velocity pro1047297le across the pipe
AssumingsD frac14 1 sV frac14 15 sR frac14 1 and sP frac14 15(as advised in
Ref [38]) the uncertainty in the 1047298owrate of primary air was
calculated using Eq (A1) to be about 3
Appendix B Determining heat losses and combustion
ef 1047297ciency
In this work the combustion ef 1047297ciency of the SFBC 1047297ring rice
husk was calculated using the the heat-loss method [18]
For this combustor with no ashremoval through thebottompartthe heat loss with unburned carbon quc () was determined by
quc frac14 32866
LHV
C fa
100 C fa
A (B1)
where C fa is the unburned carbon content in the1047298y ash wt and A
is the fuel-ash content wt on ldquoas-receivedrdquo basis
The heat loss owing to incomplete combustion qic () was
quanti1047297ed based on theconcentrations (vol)of COand C xH y (asCH4)
measured in the 1047298ue gas at the cyclone outlet at actual excess air
qic frac14 eth1264 CO thorn 3582 CH4THORN V dgeth100 qucTHORN
LHV (B2)
where V dg is the volume of dry 1047298ue gas at the cyclone outlet Nm3
kg calculated by Refs [1822] using the fuel ultimate analysis onldquoas-receivedrdquo basis and excess air ratio at this point
The combustion ef 1047297ciency of the 1047298uidized-bed combustor hc
() was then determined by
hc frac14 100 ethquc thorn qicTHORN (B3)
References
[1] Natarajan E Nordin A Rao AN Overview of combustion and gasi1047297cation of rice husk in 1047298uidized bed reactors Biomass Bioenergy 199814533e46
[2] Bhattacharya SC Abdul Salam P Sharma M Emissions from biomass energyuse in some selected Asian countries Energy 200025169e88
[3] Werther J Saenger M Hartge EU Ogada T Siagi Z Combustion of agriculturalresidues Prog Energy Combust Sci 2000261e27
[4] Janvijitsakul K Kuprianov VI Similarity and modeling of axial CO and NO
concentration pro1047297les in a 1047298uidized-bed combustor (co-)1047297ring biomass fuelsFuel 2008871574e84
[5] Demirbas A Combustion characteristics of different biomass fuels ProgEnergy Combust Sci 200430219e30
[6] Leckner B Karlsson M Gaseous emissions form circulating 1047298uidized bedcombustion of wood Biomass Bioenergy 19934379e89
[7] Spliethoff H Hein KRG Effect of co-combustion of biomass on emissions inpulverized fuel furnaces Fuel Process Technol 199854189e205
[8] Lyngfelt A Leckner B Combustion of wood-chips in circulating 1047298uidized bedboilers e NO and CO emissions as functions of temperature and air-stagingFuel 1999781065e72
[9] Chyang CS Wu KT Lin CS Emission of nitrogen oxides in a vortexing 1047298uidizedbed combustor Fuel 200786234e43
[10] Armesto E Bahillo A Cabanillas A Veijonen K Otero J Combustion behavior of rice husk in a bubbling 1047298uidised bed Biomass Bioenergy 200223171e9
[11] Kuprianov VI Janvijitsakul K Permchart W Co-1047297ring of sugar canebagasse with rice husk in a conical 1047298uidized-bed combustor Fuel 200685434e42
[12] Fang M Yang L Chen G Shi Z Luo Z Cen K Experimental study on rice huskcombustion in a circulating 1047298uidized bed Fuel Process Technol2004851273e82
[13] Permchart W Kouprianov VI Emission performance and combustion ef 1047297-ciency of a conical 1047298uidized-bed combustor 1047297ring various biomass fuelsBioresour Technol 20049283e91
[14] Sirisomboon K Kuprianov VI Arromdee P Effects of design features oncombustion ef 1047297ciency and emission performance of a biomass-fuelled 1047298uid-ized-bed combustor Chem Eng Process Process Intens 201049270e7
[15] Leckner B Amand LE Lucke K Werther J Gaseous emissions from co-combustion of sewage sludge and coalwood in a 1047298uidized bed Fuel200483477e86
[16] Eaimsa-ard S Kaewkohkiet Y Thianpong C Promvonge P Combustionbehavior in a dual-staging vortex rice husk combustor with snail entry IntCommun Heat Mass Transfer 2008351134e40
[17] Madhiyanon T Lapirattanakun A Sathitruangsak P Soponronnarit S A novelcyclonic 1047298uidized-bed combustor (J-FBC) combustion and thermal ef 1047297-ciency temperature distribution combustion intensity and emission of pollutants Combust Flame 2006146232e45
[18] Basu P Cen KF Jestin L Boilers and burners New York Springer 2000[19] Kaewklum R Kuprianov VI Experimental studies on a novel swirling 1047298uid-
ized-bed combustor using an annular spiral air distributor Fuel20108943
e52
[20] Kouprianov VI Permchart W Emission from a conical FBC 1047297red witha biomass fuel Appl Energy 200374383e92
[21] Kaewklum R Kuprianov VI Douglas PL Hydrodynamics of airesand 1047298ow ina conical swirling 1047298uidized bed a comparative study between tangential andaxial air entries Energy Convers Manage 2009502999e3006
[22] Bezgreshnov AN Lipov YM Shleipher BM Computations of steam boilersMoscow Energoatomizdat 1991 [in Russian]
[23] Kuprianov VI Application of a cost-based method of excess air optimizationfor the improvement of thermal ef 1047297ciency and environmental performance of steam boilers Renew Sustain Energy Rev 20059474e98
[24] Streimikiene D Roos I Rekis J External cost of electricity generation in BalticStates Renew Sustain Energy Rev 200913863e70
[25] Zhang Q Weili T Yumei W Yingxu C External costs from electricity gener-ation of China up to 2030 in energy and abatement scenarios Energy Pol2007354295e304
[26] Hainoun A Almoustafa A Aldin MS Estimating the health damage costs of Syrian electricity generation system using impact pathway approach Energy
201035628e
38[27] Nguyen KQ Internalizing externalities into capacity expansion planning thecase of electricity in Vietnam Energy 200833740e6
[28] Kitou E Horvath A External air pollution costs of telework Int J Life CycleAssess 200813155e65
[29] Wei X Zhang L Zhou H Evaluating the environmental value of pollutants inChina power industry In Proceedings of the international conference onenergy and the environment Shanghai China 2003
[30] Salisdisouk N The concept of integrated resource planning In Proceeding of the workshop on electric power quality safety and ef 1047297ciency of its useselectric power system management Pathum Thani Thailand 1994
[31] Yang YB Shari1047297 VN Swithenbank J Effect of air 1047298ow rate and fuel moisture onthe burning behaviours of biomass and simulated municipal solid wastes inpacked beds Fuel 2004831553e62
[32] Zhao W Li Z Zhao G Zhang F Zhu Q Effect of air preheating and fuel moistureon combustion characteristics of corn straw in a 1047297xed bed Energy ConversManage 2008493560e5
[33] Tillman DA Rossi AJ Kitto WD Wood combustion New York AcademicPress 1981
[34] Winter F Wartha C Hofbauer H NO and N2O formation during thecombustion of wood straw malt waste and peat Bioresour Technol19997039e49
[35] Sun Z Jin M Zhang M Liu R Zhang Y Experimental studies on cotton stalkcombustion in a 1047298uidized bed Energy 2008331224e32
[36] Smart JP Robert PA De Soete GG The formation of nitrous oxide in a large-scale pulverized-coal 1047298ames J Inst Energy 199063131e5
[37] Abu-Qudais M Fluidized-bed combustion for energy production from olivecake Energy 199621173e8
[38] Trembovlya VI Finger ED Avdeeva AA Thermo-technical tests of boiler unitsMoscow Energoatomizdat 1991 [in Russian]
VI Kuprianov et al Energy 36 (2011) 2038e 20482048
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 711
Implementation of air staging seems to have a signi1047297cant impact
on the behavior of gaseous species in both radial and axial direc-
tions Due to the tangential injection of secondary air into the bed
splash zone the radial O2 concentration pro1047297les were characterized
by a positive radial gradient (the most signi1047297cant being observed at
Z frac14 101 m) which resulted in the higher O2 concentration near the
combustor wall than that at the centerline (ie at r R frac14 0)
However the radial gradient of O2 was gradually attenuated along
the bed height Note that the injection of secondary air affected the
radial O2 concentration pro1047297les not only in upper regions of the
reactor but also at levels below the injection point (in Fig 3b see
the pro1047297les at Z frac14 047 m) since the secondary air was injected at
the negative angle In the meantime with increasing the fuel-
moisture content the O2 concentration at all the points across the
combustor was found to be reduced and this reduction was caused
by some physical and chemical factors as addressed below in the
discussion of axial O2 concentration pro1047297les
The axial temperature and O2 concentration pro1047297les in the SFBC
are shown in Fig 4 for the same fuel options and operating
conditions as in Fig 3 At 1047297xed excess air a positive axial temper-
ature gradient was found to occur in the lower part of the reactor
for all the fuels (see Fig 4a) likely due to the diminishing of heat
release in the dense bed (caused by air staging) and (ii) in1047298uence of secondary air injected into the bed splash zone at the ambient
temperature However with higher fuel moisture (ie with dete-
riorating fuel quality) the temperature attained its maximum at
lower levels (Z) above the air distributor which can be explained by
the effects of the residence time The maximum temperature for
burning ldquoas-receivedrdquo rice husk (with W frac14 84) was rather high
about 980 C however it was reduced to 850e860 C when
increasing the fuel-moisture content to 25e30
However with raising fuel moisture the rate of oxygen
consumption in the bottom region of the SFBC ( Z lt 08 m) was
apparently higher which despite the above reduction in the bed
temperature resulted in the lower O2 concentration at all locations
in this region (as seen in Fig 4b) This phenomenon can be
explained by the greater residence time (leading to a higher yield of CO and other combustibles with fuel volatiles) and also greater
contribution of the ldquowetrdquo oxidation of char-C by OH radicals both
leading to higher rates of CO formation and consequently O2
consumption in this region The next region (08 lt Z lt 10 m) was
characterized by a noticeable regaining (rise) of O2 as the response
to secondary air injection However in the freeboard of the
combustor ( Z gt 10 m) the O2 concentration was diminished along
the reactor height at a rather low rate and this reduction was
accompanied by the gradual converging of the axial pro1047297les as all
the tests in Fig 4 for variable fuel moisture were conducted at
(nearly) the same EA
Fig 5 shows the radial CO and NO concentration pro1047297les at
different levels above the air distributor for the same fuels and
operating conditions as in Fig 3 Both CO (Fig 5a) and NO (Fig 5b)
were signi1047297cantly affected by fuel moisture and showed negative
gradients along the radius (different numerically) at all the levels in
the SFBC For the 1047297xed fuel-moisture content due to the effects of
secondary air the CO concentration in the peripheral zone across
the combustor was much lower compared to that at the centerline
thus forming the above radial gradient of CO However the NO
concentration varied weakly along the radius except at Z frac14 047 m
The occurrence of the NO maximum at the centerline indicated
higher rates of both fuel devolatilization and oxidation in the
central zone of the reactor (compared to those at the combustor
wall) despite the uniformity of the temperature and the opposite
trends of the O2 concentration pro1047297les across the SFBC (see Fig 3b)
In the freeboardof the reactor the radial CO and NO gradients were
found to be gradually attenuated with higher Z
Fig 6 depicts the axial CO and NO concentration pro1047297les in this
combustor As seen in Fig 6a the pro1047297les exhibited four sequent
regions along the combustor height With increasing fuel moisture
the CO concentration in the 1047297rst region (0lt Z lt 08 m) was
apparently higher at all the levels above the air distributor
particularly at the centerline (see Fig 5a) mainly due to (i) longerresidence time (leading to greater yield of CO with volatiles) (ii)
lower PA (reducing the rate of CO oxidation) (iii) higher contri-
bution of ldquowetrdquo oxidation of char-C by OH radicals and (iv) lower
bed temperature causing an increase in the COCO2 ratio in the
products of fuel-char oxidation [33]
In the second region (08 lt Z lt 10 m) the CO concentration
along the combustor axis was found to be drastically reduced
mainly due to the effects of secondaryair the greater rate of the CO
reduction being observed at a higher level of fuel moisture (ie at
higher SA) In the third region (10 lt Z lt 18 m) the CO concentra-
tion regained substantial values along the centerline mainly due to
oxidation of unburned hydrocarbons and fuel-C carried over from
the bed region to CO However in the fourth (upper) region the CO
concentration was found to be gradually reduced along thecombustor height likely via homogeneous reaction of CO with
residual O2 and OH [333]
Like for CO four speci1047297c regions can be distinguished in the
axial NO concentration pro1047297les as can be seen in Fig 6b In the 1047297rst
region (0 lt Z lt 08 m) NO was mainly formed from NH3 in volatiles
(a major precursor of NO in biomass combustion) via the fuel-NO
formation mechanism [343435] With increasing fuel moisture
despite the reduction in bed temperature the NO concentration at
the reactor centerline showed a trend to increase at any given Z
mainly due to (i) greater residence time promoting a higher yield
of nitrogenous species with fuel volatiles and (ii) enhanced
a b
500
600
700
800
900
1000
1100
0 1 2 3
Height above air distributor (m)
) C deg ( e r u t a r e p m e T
0
5
10
15
20
0 1 2 3Height above air distributor (m)
O 2
) l o v ( n o i t a r t n e c n o c
Fig 4 Axial (a) temperature and (b) O2 concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415
(
) Wfrac14
20 (6
) Wfrac14
25 and (A
) Wfrac14
30
VI Kuprianov et al Energy 36 (2011) 2038e 20482044
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 811
Z = 267 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 267 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t
n e c n o c O N
Z = 217 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 217 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O N
Z = 155 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 155 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0rR
) m
p p ( n o i t a r t n e c n o c O N
Z = 101 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 101 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O N
Z = 047 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
)
m
p p ( n o i t a r t n e c n o c
O C
Z = 047 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O N
ab
Fig 5 Radial (a) CO and (b) NO concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 ()
W frac14 15 () W frac1420 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2045
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 911
oxidation of NH3 by OH [36] In the second region (08 lt Z lt 10 m)
the chemical reactions responsible for NO decomposition such as
catalytic reduction of NO by CO on the char surface [3] as well as
reaction of NO with NH3 at oxygen de1047297ciency [34] were predom-
inant This resulted in a signi1047297cant reduction of the NO concen-
tration along the combustor height and the NO reduction was more
apparent when burning rice husks with higher moisture contentsHowever the injection of secondary air promoted a slight increase
in NO in the third region (10 lt Z lt 16 m) of the reactor as a result
of oxidation of the nitrogenous species carried over from the
combustor bottom In the fourth (upper) region the pro1047297les
exhibited some diminishing of the NO concentration along the
reactor height because of the catalytic reduction of NO by CO
occurred however at a rather low rate
322 Emissions
Fig 7 depicts the CO and NO emissions (on 6 O 2 dry gas basis)
from the combustor 1047297ring 80 kgh rice husk at variable EA for the
whole range of fuel moisture (Options 1e6 in Table 2) It can be
concluded from analysis of data in Fig 7a that at EA greater than
40 the CO emission from the SFBC can be controlled ata comparatively low level below 350 ppm regardless of the fuel
quality On the contrary at excess air lower than 40 the CO
emission exhibited quite strong effects of both fuel quality and EA
Note that at EA frac14 20 the CO emission was extremely high
3000e7000 ppm for the whole range of fuel moisture However
with increasing the fuel-moisture content from 84 (in ldquoas-
receivedrdquo rice husk) to 25 the CO emission at this lowest EA
exhibited some reduction roughly from 4000 to 3000 ppm basi-
cally caused by (i) the higher rate of chemical reaction between CO
and OH [333] and (ii) higher rate of CO decomposition in the
freeboard (due to enhanced SA and greater residence time) Similar
trend is reported in some studies on effects of fuel moisture on the
CO emission from the 1047297xed-bed combustion systems [3132]
However with further increase in the fuel-moisture content (from
25 to 35) the CO emission from the SFBC was found to rise from
about 3000 to 7000 ppm likely due to the signi1047297cant contribution
of ldquowetrdquo oxidation of char-C occurred at the lowered combustion
temperatures This trend is in concordance with the behavior of theCO emission during the conventional 1047298uidized-bed combustion of
some biomass fuels with variable moisture [420]
It can be seen in Fig 7b that at EA frac14 40 (the ldquocriticalrdquo value for
the CO emission) the NO emission diminished from about 170 to
130 ppm (or by some 25) when changing the fuel-moisture
content from 84 to 25 A further increase in fuel moisture up to
30e35 resulted in some more emission reduction Note that this
positive result was accompanied by deterioration of combustion
stability (likely caused by inconsistency in fuel properties) which
showed itself by the noticeable time-domain 1047298uctuations of the
temperature and gaseous species particularly in the vicinity of the
fuel injection As revealed by experimental data from this study
through moisturizing of ldquoas-receivedrdquo rice husk the NO emission
from the SFBC can be substantially reduced However thisachievement was accompanied by the conventional effects of EA
leading to the increase in the NO emission with higher EA [16e12]
323 Optimal excess air and fuel-moisture content
Fig 8 shows the emission costs US$ per 1 ton of fuel for 1047297ring
rice husk in the SFBC at variable fuel moisture and excess air pre-
dicted using the above CO and NO emissions and the speci1047297c
ldquoexternalrdquo costs P NO x frac14 3000 US$=t and P CO frac14 600 US$t (ie for
P NO x=P CO frac14 5) At low values of EA the contribution of CO to the
emission costs was predominant whereas the effects of NO were
substantial at higher EA values It can be seen in Fig 8 that the
Fig 6 Axial (a) CO and (b) NO concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415 ()
W frac1420 (6) W frac1425 and (A) W frac1430
Fig 7 Effects of the fuel-moisture content and excess air on the (a) CO and (b) NO emissions from the SFBC 1047297ring 80 kghr rice husk (-) W frac1484 () W frac1415 () W frac1420 (6)
Wfrac14
25 (A
) Wfrac14
30 and (A
) Wfrac14
35
VI Kuprianov et al Energy 36 (2011) 2038e 20482046
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 1011
emission costs were at the minimal value when 1047297ring rice husk
with the fuel-moisture content of 25e
35 at excess air of 40e
50
Switching P NO x=P CO from 5 to 75 (keeping however P NO x
at the
above value) led in effect to nearly the same optimal ranges of fuel
moisture and excess air quanti1047297ed however at different magni-
tudes of the emission costs Thus the optimized variables are not
sensitive to the speci1047297c ldquoexternalrdquo costs P NO x and P CO within their
actual ranges
324 Combustion heat losses and ef 1047297ciency
Table 4 shows the heat losses with unburned carbon quc and
owing to incomplete combustion qic together with the combustion
ef 1047297ciency of the SFBC actual EA values and SAPA ratios for some
selected fuel options
The heat loss with unburned carbon exhibited a rather weak
correlation with fuel moisture showing however the substantialin1047298uence of excess air Unlike quc the heat loss owing to incomplete
combustion was strongly affected by both fuel moisture and excess
air and it was characterized by quite small magnitudes at EAgt 40
Thus an increase in excess air basically resulted in the improve-
ment of the combustion ef 1047297ciency of the SFBC For the fuel range
the highest combustion ef 1047297ciency 992e997 was obtained at
excess air of 40e80 Numerous studies on 1047297ring various biomass
fuels in conventional 1047298uidized-bed combustion systems report
similar trends [1210e1337]
Based on the analysis of both emission characteristics and
combustion ef 1047297ciency and also taking into consideration an
important issue of the combustion stability it can be generally
concluded that the best performance of this SFBC is achievable
when 1047297ring moisturized rice husk with the moisture content of
about 25 at excess air of 40e50 Under these conditions the
major gaseous emissions from the conical SFBC can be controlled
below 350 ppm for CO and within 130e140 ppm for NO (both on
6 O2 dry gas basis) while the combustion ef 1047297ciency is ensured at
a rather high value about 995
4 Conclusions
Combustion and emission characteristics have been experi-
mentally studied on a SFBC 1047297ring 80 kgh rice husk Two NO x
emission control techniques have been investigated in this work
(1) air staging of the combustion process and (2) 1047297ring rice husk as
moisturized fuel
In the test series for variable air staging the combustorhas been
tested with the aim to investigate the behavior of CO NO and C xH yemissions as well as combustion ef 1047297ciency for burning ldquoas-
receivedrdquo rice husk at a 1047297xed excess air value (of about 40) for
variable secondary-to-primary air ratio (SAPA) With increasing
SAPA from 026 to 075 the CO emission from the SFBC ranges from
about 360 to 450 ppm (on 6 O2 dry gas basis) while the NO
emission reduces at a quite low rate from about 150 to 140 ppm
However with higher SAPA C xH y emissions increase from 120 to
1400 ppm the dramatic rise being observed at SAPA gt 04 Thus
the air staging has minor effects on the CO and NO emissions To
avoid elevated C xH y emissions primary air should be supplied to
the combustor at the amount greater than the stoichiometric air
affected by the fuel properties
During the trialsfor variable fuel quality ricehusks with different
fuel moisture contents (of 84e35) have been burned at different
excess air values rangedfrom about 20 to 80 The analysis of radial
and axial CO and NO concentration pro1047297les has shown the occur-
rence of four speci1047297c regions along the combustor height charac-
terized by different rates of formation and decomposition of CO and
NO The highest rates of CO and NO decomposition are found to
occur in the bed splash zone whereas the top region in the
combustor is characterized by small axial gradients of these species
Through moisturizing rice husk the NO emission from this
SFBC can be substantially reduced while the CO emission is
effectively controlled by the secondary air injection into the bed
splash zone The best combustion and emission performance of
the SFBC is achievable when burning moisturized rice husk with
the moisture content of about 25 at excess air of 40e50 For
these optimal operating conditions the CO emission is expected
to be below 350 ppm ensuring high combustion ef 1047297ciency (about
995 maximum) while the NO emission may range from 130 to
140 ppm With increasing the fuel-moisture content to higher
values eg 30e35 the NO emission from the SFBC can be
secured even at lower values below 110 ppm However elevated
fuel moisture may likely result in deterioration of combustion
stability
Acknowledgements
The authors would like to acknowledge the 1047297nancial support
from the Thailand Research Fund (contract No BRG 50800011) The
authors also sincerely thank Dr Kasama Sirisomboon and Mr
Porametr Arromdee for their effective help in experimental tests
Fig 8 Effects of the fuel-moisture content and excess air on the emission costs for
1047297ring rice husk in the SFBC
Table 4
Heat losses and combustion ef 1047297ciency of the SFBC 1047297ring 80 kgh rice husk for
variable fuel-moisture content and excess air
W () EA () SAPA quc () qic () Combustion
ef 1047297ciency ()
84 17 033 074 204 972
41 026 049 015 994
64 022 041 007 995
76 019 049 008 994
15 17 036 046 173 978
36 030 041 040 992
63 024 050 008 994
89 021 036 005 996
25 18 042 047 134 982
41 033 035 013 995
63 027 029 003 997
83 024 024 003 997
35 16 055 048 242 971
39 042 033 012 995
65 033 026 003 997
84 029 034 005 996
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2047
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 1111
Appendix A Measurement uncertainty in the 1047298owrate of
primary air
The fractional uncertainty in the 1047298owrate of primary air was
estimated by Ref [38]
sPA frac14 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
4s2D thorn s2
V thorn s2R thorn s2
P
q (A1)
where sD is the error in physical measurement of inner diameter of the primary air pipe sV is the measurement error of the velocity
(dependent mainlyon the type of a selected probe) sR is therelative
error associated with imperfect1047297xing of the probe in air1047298ow and sP
is the uncertainty in the velocity pro1047297le across the pipe
AssumingsD frac14 1 sV frac14 15 sR frac14 1 and sP frac14 15(as advised in
Ref [38]) the uncertainty in the 1047298owrate of primary air was
calculated using Eq (A1) to be about 3
Appendix B Determining heat losses and combustion
ef 1047297ciency
In this work the combustion ef 1047297ciency of the SFBC 1047297ring rice
husk was calculated using the the heat-loss method [18]
For this combustor with no ashremoval through thebottompartthe heat loss with unburned carbon quc () was determined by
quc frac14 32866
LHV
C fa
100 C fa
A (B1)
where C fa is the unburned carbon content in the1047298y ash wt and A
is the fuel-ash content wt on ldquoas-receivedrdquo basis
The heat loss owing to incomplete combustion qic () was
quanti1047297ed based on theconcentrations (vol)of COand C xH y (asCH4)
measured in the 1047298ue gas at the cyclone outlet at actual excess air
qic frac14 eth1264 CO thorn 3582 CH4THORN V dgeth100 qucTHORN
LHV (B2)
where V dg is the volume of dry 1047298ue gas at the cyclone outlet Nm3
kg calculated by Refs [1822] using the fuel ultimate analysis onldquoas-receivedrdquo basis and excess air ratio at this point
The combustion ef 1047297ciency of the 1047298uidized-bed combustor hc
() was then determined by
hc frac14 100 ethquc thorn qicTHORN (B3)
References
[1] Natarajan E Nordin A Rao AN Overview of combustion and gasi1047297cation of rice husk in 1047298uidized bed reactors Biomass Bioenergy 199814533e46
[2] Bhattacharya SC Abdul Salam P Sharma M Emissions from biomass energyuse in some selected Asian countries Energy 200025169e88
[3] Werther J Saenger M Hartge EU Ogada T Siagi Z Combustion of agriculturalresidues Prog Energy Combust Sci 2000261e27
[4] Janvijitsakul K Kuprianov VI Similarity and modeling of axial CO and NO
concentration pro1047297les in a 1047298uidized-bed combustor (co-)1047297ring biomass fuelsFuel 2008871574e84
[5] Demirbas A Combustion characteristics of different biomass fuels ProgEnergy Combust Sci 200430219e30
[6] Leckner B Karlsson M Gaseous emissions form circulating 1047298uidized bedcombustion of wood Biomass Bioenergy 19934379e89
[7] Spliethoff H Hein KRG Effect of co-combustion of biomass on emissions inpulverized fuel furnaces Fuel Process Technol 199854189e205
[8] Lyngfelt A Leckner B Combustion of wood-chips in circulating 1047298uidized bedboilers e NO and CO emissions as functions of temperature and air-stagingFuel 1999781065e72
[9] Chyang CS Wu KT Lin CS Emission of nitrogen oxides in a vortexing 1047298uidizedbed combustor Fuel 200786234e43
[10] Armesto E Bahillo A Cabanillas A Veijonen K Otero J Combustion behavior of rice husk in a bubbling 1047298uidised bed Biomass Bioenergy 200223171e9
[11] Kuprianov VI Janvijitsakul K Permchart W Co-1047297ring of sugar canebagasse with rice husk in a conical 1047298uidized-bed combustor Fuel 200685434e42
[12] Fang M Yang L Chen G Shi Z Luo Z Cen K Experimental study on rice huskcombustion in a circulating 1047298uidized bed Fuel Process Technol2004851273e82
[13] Permchart W Kouprianov VI Emission performance and combustion ef 1047297-ciency of a conical 1047298uidized-bed combustor 1047297ring various biomass fuelsBioresour Technol 20049283e91
[14] Sirisomboon K Kuprianov VI Arromdee P Effects of design features oncombustion ef 1047297ciency and emission performance of a biomass-fuelled 1047298uid-ized-bed combustor Chem Eng Process Process Intens 201049270e7
[15] Leckner B Amand LE Lucke K Werther J Gaseous emissions from co-combustion of sewage sludge and coalwood in a 1047298uidized bed Fuel200483477e86
[16] Eaimsa-ard S Kaewkohkiet Y Thianpong C Promvonge P Combustionbehavior in a dual-staging vortex rice husk combustor with snail entry IntCommun Heat Mass Transfer 2008351134e40
[17] Madhiyanon T Lapirattanakun A Sathitruangsak P Soponronnarit S A novelcyclonic 1047298uidized-bed combustor (J-FBC) combustion and thermal ef 1047297-ciency temperature distribution combustion intensity and emission of pollutants Combust Flame 2006146232e45
[18] Basu P Cen KF Jestin L Boilers and burners New York Springer 2000[19] Kaewklum R Kuprianov VI Experimental studies on a novel swirling 1047298uid-
ized-bed combustor using an annular spiral air distributor Fuel20108943
e52
[20] Kouprianov VI Permchart W Emission from a conical FBC 1047297red witha biomass fuel Appl Energy 200374383e92
[21] Kaewklum R Kuprianov VI Douglas PL Hydrodynamics of airesand 1047298ow ina conical swirling 1047298uidized bed a comparative study between tangential andaxial air entries Energy Convers Manage 2009502999e3006
[22] Bezgreshnov AN Lipov YM Shleipher BM Computations of steam boilersMoscow Energoatomizdat 1991 [in Russian]
[23] Kuprianov VI Application of a cost-based method of excess air optimizationfor the improvement of thermal ef 1047297ciency and environmental performance of steam boilers Renew Sustain Energy Rev 20059474e98
[24] Streimikiene D Roos I Rekis J External cost of electricity generation in BalticStates Renew Sustain Energy Rev 200913863e70
[25] Zhang Q Weili T Yumei W Yingxu C External costs from electricity gener-ation of China up to 2030 in energy and abatement scenarios Energy Pol2007354295e304
[26] Hainoun A Almoustafa A Aldin MS Estimating the health damage costs of Syrian electricity generation system using impact pathway approach Energy
201035628e
38[27] Nguyen KQ Internalizing externalities into capacity expansion planning thecase of electricity in Vietnam Energy 200833740e6
[28] Kitou E Horvath A External air pollution costs of telework Int J Life CycleAssess 200813155e65
[29] Wei X Zhang L Zhou H Evaluating the environmental value of pollutants inChina power industry In Proceedings of the international conference onenergy and the environment Shanghai China 2003
[30] Salisdisouk N The concept of integrated resource planning In Proceeding of the workshop on electric power quality safety and ef 1047297ciency of its useselectric power system management Pathum Thani Thailand 1994
[31] Yang YB Shari1047297 VN Swithenbank J Effect of air 1047298ow rate and fuel moisture onthe burning behaviours of biomass and simulated municipal solid wastes inpacked beds Fuel 2004831553e62
[32] Zhao W Li Z Zhao G Zhang F Zhu Q Effect of air preheating and fuel moistureon combustion characteristics of corn straw in a 1047297xed bed Energy ConversManage 2008493560e5
[33] Tillman DA Rossi AJ Kitto WD Wood combustion New York AcademicPress 1981
[34] Winter F Wartha C Hofbauer H NO and N2O formation during thecombustion of wood straw malt waste and peat Bioresour Technol19997039e49
[35] Sun Z Jin M Zhang M Liu R Zhang Y Experimental studies on cotton stalkcombustion in a 1047298uidized bed Energy 2008331224e32
[36] Smart JP Robert PA De Soete GG The formation of nitrous oxide in a large-scale pulverized-coal 1047298ames J Inst Energy 199063131e5
[37] Abu-Qudais M Fluidized-bed combustion for energy production from olivecake Energy 199621173e8
[38] Trembovlya VI Finger ED Avdeeva AA Thermo-technical tests of boiler unitsMoscow Energoatomizdat 1991 [in Russian]
VI Kuprianov et al Energy 36 (2011) 2038e 20482048
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 811
Z = 267 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 267 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t
n e c n o c O N
Z = 217 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 217 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O N
Z = 155 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 155 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0rR
) m
p p ( n o i t a r t n e c n o c O N
Z = 101 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O C
Z = 101 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O N
Z = 047 m
0
5000
10000
15000
20000
25000
30000
0 0 0 2 0 4 0 6 0 8 1 0
rR
)
m
p p ( n o i t a r t n e c n o c
O C
Z = 047 m
0
100
200
300
400
500
0 0 0 2 0 4 0 6 0 8 1 0
rR
) m
p p ( n o i t a r t n e c n o c
O N
ab
Fig 5 Radial (a) CO and (b) NO concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 ()
W frac14 15 () W frac1420 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2045
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 911
oxidation of NH3 by OH [36] In the second region (08 lt Z lt 10 m)
the chemical reactions responsible for NO decomposition such as
catalytic reduction of NO by CO on the char surface [3] as well as
reaction of NO with NH3 at oxygen de1047297ciency [34] were predom-
inant This resulted in a signi1047297cant reduction of the NO concen-
tration along the combustor height and the NO reduction was more
apparent when burning rice husks with higher moisture contentsHowever the injection of secondary air promoted a slight increase
in NO in the third region (10 lt Z lt 16 m) of the reactor as a result
of oxidation of the nitrogenous species carried over from the
combustor bottom In the fourth (upper) region the pro1047297les
exhibited some diminishing of the NO concentration along the
reactor height because of the catalytic reduction of NO by CO
occurred however at a rather low rate
322 Emissions
Fig 7 depicts the CO and NO emissions (on 6 O 2 dry gas basis)
from the combustor 1047297ring 80 kgh rice husk at variable EA for the
whole range of fuel moisture (Options 1e6 in Table 2) It can be
concluded from analysis of data in Fig 7a that at EA greater than
40 the CO emission from the SFBC can be controlled ata comparatively low level below 350 ppm regardless of the fuel
quality On the contrary at excess air lower than 40 the CO
emission exhibited quite strong effects of both fuel quality and EA
Note that at EA frac14 20 the CO emission was extremely high
3000e7000 ppm for the whole range of fuel moisture However
with increasing the fuel-moisture content from 84 (in ldquoas-
receivedrdquo rice husk) to 25 the CO emission at this lowest EA
exhibited some reduction roughly from 4000 to 3000 ppm basi-
cally caused by (i) the higher rate of chemical reaction between CO
and OH [333] and (ii) higher rate of CO decomposition in the
freeboard (due to enhanced SA and greater residence time) Similar
trend is reported in some studies on effects of fuel moisture on the
CO emission from the 1047297xed-bed combustion systems [3132]
However with further increase in the fuel-moisture content (from
25 to 35) the CO emission from the SFBC was found to rise from
about 3000 to 7000 ppm likely due to the signi1047297cant contribution
of ldquowetrdquo oxidation of char-C occurred at the lowered combustion
temperatures This trend is in concordance with the behavior of theCO emission during the conventional 1047298uidized-bed combustion of
some biomass fuels with variable moisture [420]
It can be seen in Fig 7b that at EA frac14 40 (the ldquocriticalrdquo value for
the CO emission) the NO emission diminished from about 170 to
130 ppm (or by some 25) when changing the fuel-moisture
content from 84 to 25 A further increase in fuel moisture up to
30e35 resulted in some more emission reduction Note that this
positive result was accompanied by deterioration of combustion
stability (likely caused by inconsistency in fuel properties) which
showed itself by the noticeable time-domain 1047298uctuations of the
temperature and gaseous species particularly in the vicinity of the
fuel injection As revealed by experimental data from this study
through moisturizing of ldquoas-receivedrdquo rice husk the NO emission
from the SFBC can be substantially reduced However thisachievement was accompanied by the conventional effects of EA
leading to the increase in the NO emission with higher EA [16e12]
323 Optimal excess air and fuel-moisture content
Fig 8 shows the emission costs US$ per 1 ton of fuel for 1047297ring
rice husk in the SFBC at variable fuel moisture and excess air pre-
dicted using the above CO and NO emissions and the speci1047297c
ldquoexternalrdquo costs P NO x frac14 3000 US$=t and P CO frac14 600 US$t (ie for
P NO x=P CO frac14 5) At low values of EA the contribution of CO to the
emission costs was predominant whereas the effects of NO were
substantial at higher EA values It can be seen in Fig 8 that the
Fig 6 Axial (a) CO and (b) NO concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415 ()
W frac1420 (6) W frac1425 and (A) W frac1430
Fig 7 Effects of the fuel-moisture content and excess air on the (a) CO and (b) NO emissions from the SFBC 1047297ring 80 kghr rice husk (-) W frac1484 () W frac1415 () W frac1420 (6)
Wfrac14
25 (A
) Wfrac14
30 and (A
) Wfrac14
35
VI Kuprianov et al Energy 36 (2011) 2038e 20482046
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 1011
emission costs were at the minimal value when 1047297ring rice husk
with the fuel-moisture content of 25e
35 at excess air of 40e
50
Switching P NO x=P CO from 5 to 75 (keeping however P NO x
at the
above value) led in effect to nearly the same optimal ranges of fuel
moisture and excess air quanti1047297ed however at different magni-
tudes of the emission costs Thus the optimized variables are not
sensitive to the speci1047297c ldquoexternalrdquo costs P NO x and P CO within their
actual ranges
324 Combustion heat losses and ef 1047297ciency
Table 4 shows the heat losses with unburned carbon quc and
owing to incomplete combustion qic together with the combustion
ef 1047297ciency of the SFBC actual EA values and SAPA ratios for some
selected fuel options
The heat loss with unburned carbon exhibited a rather weak
correlation with fuel moisture showing however the substantialin1047298uence of excess air Unlike quc the heat loss owing to incomplete
combustion was strongly affected by both fuel moisture and excess
air and it was characterized by quite small magnitudes at EAgt 40
Thus an increase in excess air basically resulted in the improve-
ment of the combustion ef 1047297ciency of the SFBC For the fuel range
the highest combustion ef 1047297ciency 992e997 was obtained at
excess air of 40e80 Numerous studies on 1047297ring various biomass
fuels in conventional 1047298uidized-bed combustion systems report
similar trends [1210e1337]
Based on the analysis of both emission characteristics and
combustion ef 1047297ciency and also taking into consideration an
important issue of the combustion stability it can be generally
concluded that the best performance of this SFBC is achievable
when 1047297ring moisturized rice husk with the moisture content of
about 25 at excess air of 40e50 Under these conditions the
major gaseous emissions from the conical SFBC can be controlled
below 350 ppm for CO and within 130e140 ppm for NO (both on
6 O2 dry gas basis) while the combustion ef 1047297ciency is ensured at
a rather high value about 995
4 Conclusions
Combustion and emission characteristics have been experi-
mentally studied on a SFBC 1047297ring 80 kgh rice husk Two NO x
emission control techniques have been investigated in this work
(1) air staging of the combustion process and (2) 1047297ring rice husk as
moisturized fuel
In the test series for variable air staging the combustorhas been
tested with the aim to investigate the behavior of CO NO and C xH yemissions as well as combustion ef 1047297ciency for burning ldquoas-
receivedrdquo rice husk at a 1047297xed excess air value (of about 40) for
variable secondary-to-primary air ratio (SAPA) With increasing
SAPA from 026 to 075 the CO emission from the SFBC ranges from
about 360 to 450 ppm (on 6 O2 dry gas basis) while the NO
emission reduces at a quite low rate from about 150 to 140 ppm
However with higher SAPA C xH y emissions increase from 120 to
1400 ppm the dramatic rise being observed at SAPA gt 04 Thus
the air staging has minor effects on the CO and NO emissions To
avoid elevated C xH y emissions primary air should be supplied to
the combustor at the amount greater than the stoichiometric air
affected by the fuel properties
During the trialsfor variable fuel quality ricehusks with different
fuel moisture contents (of 84e35) have been burned at different
excess air values rangedfrom about 20 to 80 The analysis of radial
and axial CO and NO concentration pro1047297les has shown the occur-
rence of four speci1047297c regions along the combustor height charac-
terized by different rates of formation and decomposition of CO and
NO The highest rates of CO and NO decomposition are found to
occur in the bed splash zone whereas the top region in the
combustor is characterized by small axial gradients of these species
Through moisturizing rice husk the NO emission from this
SFBC can be substantially reduced while the CO emission is
effectively controlled by the secondary air injection into the bed
splash zone The best combustion and emission performance of
the SFBC is achievable when burning moisturized rice husk with
the moisture content of about 25 at excess air of 40e50 For
these optimal operating conditions the CO emission is expected
to be below 350 ppm ensuring high combustion ef 1047297ciency (about
995 maximum) while the NO emission may range from 130 to
140 ppm With increasing the fuel-moisture content to higher
values eg 30e35 the NO emission from the SFBC can be
secured even at lower values below 110 ppm However elevated
fuel moisture may likely result in deterioration of combustion
stability
Acknowledgements
The authors would like to acknowledge the 1047297nancial support
from the Thailand Research Fund (contract No BRG 50800011) The
authors also sincerely thank Dr Kasama Sirisomboon and Mr
Porametr Arromdee for their effective help in experimental tests
Fig 8 Effects of the fuel-moisture content and excess air on the emission costs for
1047297ring rice husk in the SFBC
Table 4
Heat losses and combustion ef 1047297ciency of the SFBC 1047297ring 80 kgh rice husk for
variable fuel-moisture content and excess air
W () EA () SAPA quc () qic () Combustion
ef 1047297ciency ()
84 17 033 074 204 972
41 026 049 015 994
64 022 041 007 995
76 019 049 008 994
15 17 036 046 173 978
36 030 041 040 992
63 024 050 008 994
89 021 036 005 996
25 18 042 047 134 982
41 033 035 013 995
63 027 029 003 997
83 024 024 003 997
35 16 055 048 242 971
39 042 033 012 995
65 033 026 003 997
84 029 034 005 996
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2047
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 1111
Appendix A Measurement uncertainty in the 1047298owrate of
primary air
The fractional uncertainty in the 1047298owrate of primary air was
estimated by Ref [38]
sPA frac14 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
4s2D thorn s2
V thorn s2R thorn s2
P
q (A1)
where sD is the error in physical measurement of inner diameter of the primary air pipe sV is the measurement error of the velocity
(dependent mainlyon the type of a selected probe) sR is therelative
error associated with imperfect1047297xing of the probe in air1047298ow and sP
is the uncertainty in the velocity pro1047297le across the pipe
AssumingsD frac14 1 sV frac14 15 sR frac14 1 and sP frac14 15(as advised in
Ref [38]) the uncertainty in the 1047298owrate of primary air was
calculated using Eq (A1) to be about 3
Appendix B Determining heat losses and combustion
ef 1047297ciency
In this work the combustion ef 1047297ciency of the SFBC 1047297ring rice
husk was calculated using the the heat-loss method [18]
For this combustor with no ashremoval through thebottompartthe heat loss with unburned carbon quc () was determined by
quc frac14 32866
LHV
C fa
100 C fa
A (B1)
where C fa is the unburned carbon content in the1047298y ash wt and A
is the fuel-ash content wt on ldquoas-receivedrdquo basis
The heat loss owing to incomplete combustion qic () was
quanti1047297ed based on theconcentrations (vol)of COand C xH y (asCH4)
measured in the 1047298ue gas at the cyclone outlet at actual excess air
qic frac14 eth1264 CO thorn 3582 CH4THORN V dgeth100 qucTHORN
LHV (B2)
where V dg is the volume of dry 1047298ue gas at the cyclone outlet Nm3
kg calculated by Refs [1822] using the fuel ultimate analysis onldquoas-receivedrdquo basis and excess air ratio at this point
The combustion ef 1047297ciency of the 1047298uidized-bed combustor hc
() was then determined by
hc frac14 100 ethquc thorn qicTHORN (B3)
References
[1] Natarajan E Nordin A Rao AN Overview of combustion and gasi1047297cation of rice husk in 1047298uidized bed reactors Biomass Bioenergy 199814533e46
[2] Bhattacharya SC Abdul Salam P Sharma M Emissions from biomass energyuse in some selected Asian countries Energy 200025169e88
[3] Werther J Saenger M Hartge EU Ogada T Siagi Z Combustion of agriculturalresidues Prog Energy Combust Sci 2000261e27
[4] Janvijitsakul K Kuprianov VI Similarity and modeling of axial CO and NO
concentration pro1047297les in a 1047298uidized-bed combustor (co-)1047297ring biomass fuelsFuel 2008871574e84
[5] Demirbas A Combustion characteristics of different biomass fuels ProgEnergy Combust Sci 200430219e30
[6] Leckner B Karlsson M Gaseous emissions form circulating 1047298uidized bedcombustion of wood Biomass Bioenergy 19934379e89
[7] Spliethoff H Hein KRG Effect of co-combustion of biomass on emissions inpulverized fuel furnaces Fuel Process Technol 199854189e205
[8] Lyngfelt A Leckner B Combustion of wood-chips in circulating 1047298uidized bedboilers e NO and CO emissions as functions of temperature and air-stagingFuel 1999781065e72
[9] Chyang CS Wu KT Lin CS Emission of nitrogen oxides in a vortexing 1047298uidizedbed combustor Fuel 200786234e43
[10] Armesto E Bahillo A Cabanillas A Veijonen K Otero J Combustion behavior of rice husk in a bubbling 1047298uidised bed Biomass Bioenergy 200223171e9
[11] Kuprianov VI Janvijitsakul K Permchart W Co-1047297ring of sugar canebagasse with rice husk in a conical 1047298uidized-bed combustor Fuel 200685434e42
[12] Fang M Yang L Chen G Shi Z Luo Z Cen K Experimental study on rice huskcombustion in a circulating 1047298uidized bed Fuel Process Technol2004851273e82
[13] Permchart W Kouprianov VI Emission performance and combustion ef 1047297-ciency of a conical 1047298uidized-bed combustor 1047297ring various biomass fuelsBioresour Technol 20049283e91
[14] Sirisomboon K Kuprianov VI Arromdee P Effects of design features oncombustion ef 1047297ciency and emission performance of a biomass-fuelled 1047298uid-ized-bed combustor Chem Eng Process Process Intens 201049270e7
[15] Leckner B Amand LE Lucke K Werther J Gaseous emissions from co-combustion of sewage sludge and coalwood in a 1047298uidized bed Fuel200483477e86
[16] Eaimsa-ard S Kaewkohkiet Y Thianpong C Promvonge P Combustionbehavior in a dual-staging vortex rice husk combustor with snail entry IntCommun Heat Mass Transfer 2008351134e40
[17] Madhiyanon T Lapirattanakun A Sathitruangsak P Soponronnarit S A novelcyclonic 1047298uidized-bed combustor (J-FBC) combustion and thermal ef 1047297-ciency temperature distribution combustion intensity and emission of pollutants Combust Flame 2006146232e45
[18] Basu P Cen KF Jestin L Boilers and burners New York Springer 2000[19] Kaewklum R Kuprianov VI Experimental studies on a novel swirling 1047298uid-
ized-bed combustor using an annular spiral air distributor Fuel20108943
e52
[20] Kouprianov VI Permchart W Emission from a conical FBC 1047297red witha biomass fuel Appl Energy 200374383e92
[21] Kaewklum R Kuprianov VI Douglas PL Hydrodynamics of airesand 1047298ow ina conical swirling 1047298uidized bed a comparative study between tangential andaxial air entries Energy Convers Manage 2009502999e3006
[22] Bezgreshnov AN Lipov YM Shleipher BM Computations of steam boilersMoscow Energoatomizdat 1991 [in Russian]
[23] Kuprianov VI Application of a cost-based method of excess air optimizationfor the improvement of thermal ef 1047297ciency and environmental performance of steam boilers Renew Sustain Energy Rev 20059474e98
[24] Streimikiene D Roos I Rekis J External cost of electricity generation in BalticStates Renew Sustain Energy Rev 200913863e70
[25] Zhang Q Weili T Yumei W Yingxu C External costs from electricity gener-ation of China up to 2030 in energy and abatement scenarios Energy Pol2007354295e304
[26] Hainoun A Almoustafa A Aldin MS Estimating the health damage costs of Syrian electricity generation system using impact pathway approach Energy
201035628e
38[27] Nguyen KQ Internalizing externalities into capacity expansion planning thecase of electricity in Vietnam Energy 200833740e6
[28] Kitou E Horvath A External air pollution costs of telework Int J Life CycleAssess 200813155e65
[29] Wei X Zhang L Zhou H Evaluating the environmental value of pollutants inChina power industry In Proceedings of the international conference onenergy and the environment Shanghai China 2003
[30] Salisdisouk N The concept of integrated resource planning In Proceeding of the workshop on electric power quality safety and ef 1047297ciency of its useselectric power system management Pathum Thani Thailand 1994
[31] Yang YB Shari1047297 VN Swithenbank J Effect of air 1047298ow rate and fuel moisture onthe burning behaviours of biomass and simulated municipal solid wastes inpacked beds Fuel 2004831553e62
[32] Zhao W Li Z Zhao G Zhang F Zhu Q Effect of air preheating and fuel moistureon combustion characteristics of corn straw in a 1047297xed bed Energy ConversManage 2008493560e5
[33] Tillman DA Rossi AJ Kitto WD Wood combustion New York AcademicPress 1981
[34] Winter F Wartha C Hofbauer H NO and N2O formation during thecombustion of wood straw malt waste and peat Bioresour Technol19997039e49
[35] Sun Z Jin M Zhang M Liu R Zhang Y Experimental studies on cotton stalkcombustion in a 1047298uidized bed Energy 2008331224e32
[36] Smart JP Robert PA De Soete GG The formation of nitrous oxide in a large-scale pulverized-coal 1047298ames J Inst Energy 199063131e5
[37] Abu-Qudais M Fluidized-bed combustion for energy production from olivecake Energy 199621173e8
[38] Trembovlya VI Finger ED Avdeeva AA Thermo-technical tests of boiler unitsMoscow Energoatomizdat 1991 [in Russian]
VI Kuprianov et al Energy 36 (2011) 2038e 20482048
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 911
oxidation of NH3 by OH [36] In the second region (08 lt Z lt 10 m)
the chemical reactions responsible for NO decomposition such as
catalytic reduction of NO by CO on the char surface [3] as well as
reaction of NO with NH3 at oxygen de1047297ciency [34] were predom-
inant This resulted in a signi1047297cant reduction of the NO concen-
tration along the combustor height and the NO reduction was more
apparent when burning rice husks with higher moisture contentsHowever the injection of secondary air promoted a slight increase
in NO in the third region (10 lt Z lt 16 m) of the reactor as a result
of oxidation of the nitrogenous species carried over from the
combustor bottom In the fourth (upper) region the pro1047297les
exhibited some diminishing of the NO concentration along the
reactor height because of the catalytic reduction of NO by CO
occurred however at a rather low rate
322 Emissions
Fig 7 depicts the CO and NO emissions (on 6 O 2 dry gas basis)
from the combustor 1047297ring 80 kgh rice husk at variable EA for the
whole range of fuel moisture (Options 1e6 in Table 2) It can be
concluded from analysis of data in Fig 7a that at EA greater than
40 the CO emission from the SFBC can be controlled ata comparatively low level below 350 ppm regardless of the fuel
quality On the contrary at excess air lower than 40 the CO
emission exhibited quite strong effects of both fuel quality and EA
Note that at EA frac14 20 the CO emission was extremely high
3000e7000 ppm for the whole range of fuel moisture However
with increasing the fuel-moisture content from 84 (in ldquoas-
receivedrdquo rice husk) to 25 the CO emission at this lowest EA
exhibited some reduction roughly from 4000 to 3000 ppm basi-
cally caused by (i) the higher rate of chemical reaction between CO
and OH [333] and (ii) higher rate of CO decomposition in the
freeboard (due to enhanced SA and greater residence time) Similar
trend is reported in some studies on effects of fuel moisture on the
CO emission from the 1047297xed-bed combustion systems [3132]
However with further increase in the fuel-moisture content (from
25 to 35) the CO emission from the SFBC was found to rise from
about 3000 to 7000 ppm likely due to the signi1047297cant contribution
of ldquowetrdquo oxidation of char-C occurred at the lowered combustion
temperatures This trend is in concordance with the behavior of theCO emission during the conventional 1047298uidized-bed combustion of
some biomass fuels with variable moisture [420]
It can be seen in Fig 7b that at EA frac14 40 (the ldquocriticalrdquo value for
the CO emission) the NO emission diminished from about 170 to
130 ppm (or by some 25) when changing the fuel-moisture
content from 84 to 25 A further increase in fuel moisture up to
30e35 resulted in some more emission reduction Note that this
positive result was accompanied by deterioration of combustion
stability (likely caused by inconsistency in fuel properties) which
showed itself by the noticeable time-domain 1047298uctuations of the
temperature and gaseous species particularly in the vicinity of the
fuel injection As revealed by experimental data from this study
through moisturizing of ldquoas-receivedrdquo rice husk the NO emission
from the SFBC can be substantially reduced However thisachievement was accompanied by the conventional effects of EA
leading to the increase in the NO emission with higher EA [16e12]
323 Optimal excess air and fuel-moisture content
Fig 8 shows the emission costs US$ per 1 ton of fuel for 1047297ring
rice husk in the SFBC at variable fuel moisture and excess air pre-
dicted using the above CO and NO emissions and the speci1047297c
ldquoexternalrdquo costs P NO x frac14 3000 US$=t and P CO frac14 600 US$t (ie for
P NO x=P CO frac14 5) At low values of EA the contribution of CO to the
emission costs was predominant whereas the effects of NO were
substantial at higher EA values It can be seen in Fig 8 that the
Fig 6 Axial (a) CO and (b) NO concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415 ()
W frac1420 (6) W frac1425 and (A) W frac1430
Fig 7 Effects of the fuel-moisture content and excess air on the (a) CO and (b) NO emissions from the SFBC 1047297ring 80 kghr rice husk (-) W frac1484 () W frac1415 () W frac1420 (6)
Wfrac14
25 (A
) Wfrac14
30 and (A
) Wfrac14
35
VI Kuprianov et al Energy 36 (2011) 2038e 20482046
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 1011
emission costs were at the minimal value when 1047297ring rice husk
with the fuel-moisture content of 25e
35 at excess air of 40e
50
Switching P NO x=P CO from 5 to 75 (keeping however P NO x
at the
above value) led in effect to nearly the same optimal ranges of fuel
moisture and excess air quanti1047297ed however at different magni-
tudes of the emission costs Thus the optimized variables are not
sensitive to the speci1047297c ldquoexternalrdquo costs P NO x and P CO within their
actual ranges
324 Combustion heat losses and ef 1047297ciency
Table 4 shows the heat losses with unburned carbon quc and
owing to incomplete combustion qic together with the combustion
ef 1047297ciency of the SFBC actual EA values and SAPA ratios for some
selected fuel options
The heat loss with unburned carbon exhibited a rather weak
correlation with fuel moisture showing however the substantialin1047298uence of excess air Unlike quc the heat loss owing to incomplete
combustion was strongly affected by both fuel moisture and excess
air and it was characterized by quite small magnitudes at EAgt 40
Thus an increase in excess air basically resulted in the improve-
ment of the combustion ef 1047297ciency of the SFBC For the fuel range
the highest combustion ef 1047297ciency 992e997 was obtained at
excess air of 40e80 Numerous studies on 1047297ring various biomass
fuels in conventional 1047298uidized-bed combustion systems report
similar trends [1210e1337]
Based on the analysis of both emission characteristics and
combustion ef 1047297ciency and also taking into consideration an
important issue of the combustion stability it can be generally
concluded that the best performance of this SFBC is achievable
when 1047297ring moisturized rice husk with the moisture content of
about 25 at excess air of 40e50 Under these conditions the
major gaseous emissions from the conical SFBC can be controlled
below 350 ppm for CO and within 130e140 ppm for NO (both on
6 O2 dry gas basis) while the combustion ef 1047297ciency is ensured at
a rather high value about 995
4 Conclusions
Combustion and emission characteristics have been experi-
mentally studied on a SFBC 1047297ring 80 kgh rice husk Two NO x
emission control techniques have been investigated in this work
(1) air staging of the combustion process and (2) 1047297ring rice husk as
moisturized fuel
In the test series for variable air staging the combustorhas been
tested with the aim to investigate the behavior of CO NO and C xH yemissions as well as combustion ef 1047297ciency for burning ldquoas-
receivedrdquo rice husk at a 1047297xed excess air value (of about 40) for
variable secondary-to-primary air ratio (SAPA) With increasing
SAPA from 026 to 075 the CO emission from the SFBC ranges from
about 360 to 450 ppm (on 6 O2 dry gas basis) while the NO
emission reduces at a quite low rate from about 150 to 140 ppm
However with higher SAPA C xH y emissions increase from 120 to
1400 ppm the dramatic rise being observed at SAPA gt 04 Thus
the air staging has minor effects on the CO and NO emissions To
avoid elevated C xH y emissions primary air should be supplied to
the combustor at the amount greater than the stoichiometric air
affected by the fuel properties
During the trialsfor variable fuel quality ricehusks with different
fuel moisture contents (of 84e35) have been burned at different
excess air values rangedfrom about 20 to 80 The analysis of radial
and axial CO and NO concentration pro1047297les has shown the occur-
rence of four speci1047297c regions along the combustor height charac-
terized by different rates of formation and decomposition of CO and
NO The highest rates of CO and NO decomposition are found to
occur in the bed splash zone whereas the top region in the
combustor is characterized by small axial gradients of these species
Through moisturizing rice husk the NO emission from this
SFBC can be substantially reduced while the CO emission is
effectively controlled by the secondary air injection into the bed
splash zone The best combustion and emission performance of
the SFBC is achievable when burning moisturized rice husk with
the moisture content of about 25 at excess air of 40e50 For
these optimal operating conditions the CO emission is expected
to be below 350 ppm ensuring high combustion ef 1047297ciency (about
995 maximum) while the NO emission may range from 130 to
140 ppm With increasing the fuel-moisture content to higher
values eg 30e35 the NO emission from the SFBC can be
secured even at lower values below 110 ppm However elevated
fuel moisture may likely result in deterioration of combustion
stability
Acknowledgements
The authors would like to acknowledge the 1047297nancial support
from the Thailand Research Fund (contract No BRG 50800011) The
authors also sincerely thank Dr Kasama Sirisomboon and Mr
Porametr Arromdee for their effective help in experimental tests
Fig 8 Effects of the fuel-moisture content and excess air on the emission costs for
1047297ring rice husk in the SFBC
Table 4
Heat losses and combustion ef 1047297ciency of the SFBC 1047297ring 80 kgh rice husk for
variable fuel-moisture content and excess air
W () EA () SAPA quc () qic () Combustion
ef 1047297ciency ()
84 17 033 074 204 972
41 026 049 015 994
64 022 041 007 995
76 019 049 008 994
15 17 036 046 173 978
36 030 041 040 992
63 024 050 008 994
89 021 036 005 996
25 18 042 047 134 982
41 033 035 013 995
63 027 029 003 997
83 024 024 003 997
35 16 055 048 242 971
39 042 033 012 995
65 033 026 003 997
84 029 034 005 996
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2047
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 1111
Appendix A Measurement uncertainty in the 1047298owrate of
primary air
The fractional uncertainty in the 1047298owrate of primary air was
estimated by Ref [38]
sPA frac14 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
4s2D thorn s2
V thorn s2R thorn s2
P
q (A1)
where sD is the error in physical measurement of inner diameter of the primary air pipe sV is the measurement error of the velocity
(dependent mainlyon the type of a selected probe) sR is therelative
error associated with imperfect1047297xing of the probe in air1047298ow and sP
is the uncertainty in the velocity pro1047297le across the pipe
AssumingsD frac14 1 sV frac14 15 sR frac14 1 and sP frac14 15(as advised in
Ref [38]) the uncertainty in the 1047298owrate of primary air was
calculated using Eq (A1) to be about 3
Appendix B Determining heat losses and combustion
ef 1047297ciency
In this work the combustion ef 1047297ciency of the SFBC 1047297ring rice
husk was calculated using the the heat-loss method [18]
For this combustor with no ashremoval through thebottompartthe heat loss with unburned carbon quc () was determined by
quc frac14 32866
LHV
C fa
100 C fa
A (B1)
where C fa is the unburned carbon content in the1047298y ash wt and A
is the fuel-ash content wt on ldquoas-receivedrdquo basis
The heat loss owing to incomplete combustion qic () was
quanti1047297ed based on theconcentrations (vol)of COand C xH y (asCH4)
measured in the 1047298ue gas at the cyclone outlet at actual excess air
qic frac14 eth1264 CO thorn 3582 CH4THORN V dgeth100 qucTHORN
LHV (B2)
where V dg is the volume of dry 1047298ue gas at the cyclone outlet Nm3
kg calculated by Refs [1822] using the fuel ultimate analysis onldquoas-receivedrdquo basis and excess air ratio at this point
The combustion ef 1047297ciency of the 1047298uidized-bed combustor hc
() was then determined by
hc frac14 100 ethquc thorn qicTHORN (B3)
References
[1] Natarajan E Nordin A Rao AN Overview of combustion and gasi1047297cation of rice husk in 1047298uidized bed reactors Biomass Bioenergy 199814533e46
[2] Bhattacharya SC Abdul Salam P Sharma M Emissions from biomass energyuse in some selected Asian countries Energy 200025169e88
[3] Werther J Saenger M Hartge EU Ogada T Siagi Z Combustion of agriculturalresidues Prog Energy Combust Sci 2000261e27
[4] Janvijitsakul K Kuprianov VI Similarity and modeling of axial CO and NO
concentration pro1047297les in a 1047298uidized-bed combustor (co-)1047297ring biomass fuelsFuel 2008871574e84
[5] Demirbas A Combustion characteristics of different biomass fuels ProgEnergy Combust Sci 200430219e30
[6] Leckner B Karlsson M Gaseous emissions form circulating 1047298uidized bedcombustion of wood Biomass Bioenergy 19934379e89
[7] Spliethoff H Hein KRG Effect of co-combustion of biomass on emissions inpulverized fuel furnaces Fuel Process Technol 199854189e205
[8] Lyngfelt A Leckner B Combustion of wood-chips in circulating 1047298uidized bedboilers e NO and CO emissions as functions of temperature and air-stagingFuel 1999781065e72
[9] Chyang CS Wu KT Lin CS Emission of nitrogen oxides in a vortexing 1047298uidizedbed combustor Fuel 200786234e43
[10] Armesto E Bahillo A Cabanillas A Veijonen K Otero J Combustion behavior of rice husk in a bubbling 1047298uidised bed Biomass Bioenergy 200223171e9
[11] Kuprianov VI Janvijitsakul K Permchart W Co-1047297ring of sugar canebagasse with rice husk in a conical 1047298uidized-bed combustor Fuel 200685434e42
[12] Fang M Yang L Chen G Shi Z Luo Z Cen K Experimental study on rice huskcombustion in a circulating 1047298uidized bed Fuel Process Technol2004851273e82
[13] Permchart W Kouprianov VI Emission performance and combustion ef 1047297-ciency of a conical 1047298uidized-bed combustor 1047297ring various biomass fuelsBioresour Technol 20049283e91
[14] Sirisomboon K Kuprianov VI Arromdee P Effects of design features oncombustion ef 1047297ciency and emission performance of a biomass-fuelled 1047298uid-ized-bed combustor Chem Eng Process Process Intens 201049270e7
[15] Leckner B Amand LE Lucke K Werther J Gaseous emissions from co-combustion of sewage sludge and coalwood in a 1047298uidized bed Fuel200483477e86
[16] Eaimsa-ard S Kaewkohkiet Y Thianpong C Promvonge P Combustionbehavior in a dual-staging vortex rice husk combustor with snail entry IntCommun Heat Mass Transfer 2008351134e40
[17] Madhiyanon T Lapirattanakun A Sathitruangsak P Soponronnarit S A novelcyclonic 1047298uidized-bed combustor (J-FBC) combustion and thermal ef 1047297-ciency temperature distribution combustion intensity and emission of pollutants Combust Flame 2006146232e45
[18] Basu P Cen KF Jestin L Boilers and burners New York Springer 2000[19] Kaewklum R Kuprianov VI Experimental studies on a novel swirling 1047298uid-
ized-bed combustor using an annular spiral air distributor Fuel20108943
e52
[20] Kouprianov VI Permchart W Emission from a conical FBC 1047297red witha biomass fuel Appl Energy 200374383e92
[21] Kaewklum R Kuprianov VI Douglas PL Hydrodynamics of airesand 1047298ow ina conical swirling 1047298uidized bed a comparative study between tangential andaxial air entries Energy Convers Manage 2009502999e3006
[22] Bezgreshnov AN Lipov YM Shleipher BM Computations of steam boilersMoscow Energoatomizdat 1991 [in Russian]
[23] Kuprianov VI Application of a cost-based method of excess air optimizationfor the improvement of thermal ef 1047297ciency and environmental performance of steam boilers Renew Sustain Energy Rev 20059474e98
[24] Streimikiene D Roos I Rekis J External cost of electricity generation in BalticStates Renew Sustain Energy Rev 200913863e70
[25] Zhang Q Weili T Yumei W Yingxu C External costs from electricity gener-ation of China up to 2030 in energy and abatement scenarios Energy Pol2007354295e304
[26] Hainoun A Almoustafa A Aldin MS Estimating the health damage costs of Syrian electricity generation system using impact pathway approach Energy
201035628e
38[27] Nguyen KQ Internalizing externalities into capacity expansion planning thecase of electricity in Vietnam Energy 200833740e6
[28] Kitou E Horvath A External air pollution costs of telework Int J Life CycleAssess 200813155e65
[29] Wei X Zhang L Zhou H Evaluating the environmental value of pollutants inChina power industry In Proceedings of the international conference onenergy and the environment Shanghai China 2003
[30] Salisdisouk N The concept of integrated resource planning In Proceeding of the workshop on electric power quality safety and ef 1047297ciency of its useselectric power system management Pathum Thani Thailand 1994
[31] Yang YB Shari1047297 VN Swithenbank J Effect of air 1047298ow rate and fuel moisture onthe burning behaviours of biomass and simulated municipal solid wastes inpacked beds Fuel 2004831553e62
[32] Zhao W Li Z Zhao G Zhang F Zhu Q Effect of air preheating and fuel moistureon combustion characteristics of corn straw in a 1047297xed bed Energy ConversManage 2008493560e5
[33] Tillman DA Rossi AJ Kitto WD Wood combustion New York AcademicPress 1981
[34] Winter F Wartha C Hofbauer H NO and N2O formation during thecombustion of wood straw malt waste and peat Bioresour Technol19997039e49
[35] Sun Z Jin M Zhang M Liu R Zhang Y Experimental studies on cotton stalkcombustion in a 1047298uidized bed Energy 2008331224e32
[36] Smart JP Robert PA De Soete GG The formation of nitrous oxide in a large-scale pulverized-coal 1047298ames J Inst Energy 199063131e5
[37] Abu-Qudais M Fluidized-bed combustion for energy production from olivecake Energy 199621173e8
[38] Trembovlya VI Finger ED Avdeeva AA Thermo-technical tests of boiler unitsMoscow Energoatomizdat 1991 [in Russian]
VI Kuprianov et al Energy 36 (2011) 2038e 20482048
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 1011
emission costs were at the minimal value when 1047297ring rice husk
with the fuel-moisture content of 25e
35 at excess air of 40e
50
Switching P NO x=P CO from 5 to 75 (keeping however P NO x
at the
above value) led in effect to nearly the same optimal ranges of fuel
moisture and excess air quanti1047297ed however at different magni-
tudes of the emission costs Thus the optimized variables are not
sensitive to the speci1047297c ldquoexternalrdquo costs P NO x and P CO within their
actual ranges
324 Combustion heat losses and ef 1047297ciency
Table 4 shows the heat losses with unburned carbon quc and
owing to incomplete combustion qic together with the combustion
ef 1047297ciency of the SFBC actual EA values and SAPA ratios for some
selected fuel options
The heat loss with unburned carbon exhibited a rather weak
correlation with fuel moisture showing however the substantialin1047298uence of excess air Unlike quc the heat loss owing to incomplete
combustion was strongly affected by both fuel moisture and excess
air and it was characterized by quite small magnitudes at EAgt 40
Thus an increase in excess air basically resulted in the improve-
ment of the combustion ef 1047297ciency of the SFBC For the fuel range
the highest combustion ef 1047297ciency 992e997 was obtained at
excess air of 40e80 Numerous studies on 1047297ring various biomass
fuels in conventional 1047298uidized-bed combustion systems report
similar trends [1210e1337]
Based on the analysis of both emission characteristics and
combustion ef 1047297ciency and also taking into consideration an
important issue of the combustion stability it can be generally
concluded that the best performance of this SFBC is achievable
when 1047297ring moisturized rice husk with the moisture content of
about 25 at excess air of 40e50 Under these conditions the
major gaseous emissions from the conical SFBC can be controlled
below 350 ppm for CO and within 130e140 ppm for NO (both on
6 O2 dry gas basis) while the combustion ef 1047297ciency is ensured at
a rather high value about 995
4 Conclusions
Combustion and emission characteristics have been experi-
mentally studied on a SFBC 1047297ring 80 kgh rice husk Two NO x
emission control techniques have been investigated in this work
(1) air staging of the combustion process and (2) 1047297ring rice husk as
moisturized fuel
In the test series for variable air staging the combustorhas been
tested with the aim to investigate the behavior of CO NO and C xH yemissions as well as combustion ef 1047297ciency for burning ldquoas-
receivedrdquo rice husk at a 1047297xed excess air value (of about 40) for
variable secondary-to-primary air ratio (SAPA) With increasing
SAPA from 026 to 075 the CO emission from the SFBC ranges from
about 360 to 450 ppm (on 6 O2 dry gas basis) while the NO
emission reduces at a quite low rate from about 150 to 140 ppm
However with higher SAPA C xH y emissions increase from 120 to
1400 ppm the dramatic rise being observed at SAPA gt 04 Thus
the air staging has minor effects on the CO and NO emissions To
avoid elevated C xH y emissions primary air should be supplied to
the combustor at the amount greater than the stoichiometric air
affected by the fuel properties
During the trialsfor variable fuel quality ricehusks with different
fuel moisture contents (of 84e35) have been burned at different
excess air values rangedfrom about 20 to 80 The analysis of radial
and axial CO and NO concentration pro1047297les has shown the occur-
rence of four speci1047297c regions along the combustor height charac-
terized by different rates of formation and decomposition of CO and
NO The highest rates of CO and NO decomposition are found to
occur in the bed splash zone whereas the top region in the
combustor is characterized by small axial gradients of these species
Through moisturizing rice husk the NO emission from this
SFBC can be substantially reduced while the CO emission is
effectively controlled by the secondary air injection into the bed
splash zone The best combustion and emission performance of
the SFBC is achievable when burning moisturized rice husk with
the moisture content of about 25 at excess air of 40e50 For
these optimal operating conditions the CO emission is expected
to be below 350 ppm ensuring high combustion ef 1047297ciency (about
995 maximum) while the NO emission may range from 130 to
140 ppm With increasing the fuel-moisture content to higher
values eg 30e35 the NO emission from the SFBC can be
secured even at lower values below 110 ppm However elevated
fuel moisture may likely result in deterioration of combustion
stability
Acknowledgements
The authors would like to acknowledge the 1047297nancial support
from the Thailand Research Fund (contract No BRG 50800011) The
authors also sincerely thank Dr Kasama Sirisomboon and Mr
Porametr Arromdee for their effective help in experimental tests
Fig 8 Effects of the fuel-moisture content and excess air on the emission costs for
1047297ring rice husk in the SFBC
Table 4
Heat losses and combustion ef 1047297ciency of the SFBC 1047297ring 80 kgh rice husk for
variable fuel-moisture content and excess air
W () EA () SAPA quc () qic () Combustion
ef 1047297ciency ()
84 17 033 074 204 972
41 026 049 015 994
64 022 041 007 995
76 019 049 008 994
15 17 036 046 173 978
36 030 041 040 992
63 024 050 008 994
89 021 036 005 996
25 18 042 047 134 982
41 033 035 013 995
63 027 029 003 997
83 024 024 003 997
35 16 055 048 242 971
39 042 033 012 995
65 033 026 003 997
84 029 034 005 996
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2047
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 1111
Appendix A Measurement uncertainty in the 1047298owrate of
primary air
The fractional uncertainty in the 1047298owrate of primary air was
estimated by Ref [38]
sPA frac14 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
4s2D thorn s2
V thorn s2R thorn s2
P
q (A1)
where sD is the error in physical measurement of inner diameter of the primary air pipe sV is the measurement error of the velocity
(dependent mainlyon the type of a selected probe) sR is therelative
error associated with imperfect1047297xing of the probe in air1047298ow and sP
is the uncertainty in the velocity pro1047297le across the pipe
AssumingsD frac14 1 sV frac14 15 sR frac14 1 and sP frac14 15(as advised in
Ref [38]) the uncertainty in the 1047298owrate of primary air was
calculated using Eq (A1) to be about 3
Appendix B Determining heat losses and combustion
ef 1047297ciency
In this work the combustion ef 1047297ciency of the SFBC 1047297ring rice
husk was calculated using the the heat-loss method [18]
For this combustor with no ashremoval through thebottompartthe heat loss with unburned carbon quc () was determined by
quc frac14 32866
LHV
C fa
100 C fa
A (B1)
where C fa is the unburned carbon content in the1047298y ash wt and A
is the fuel-ash content wt on ldquoas-receivedrdquo basis
The heat loss owing to incomplete combustion qic () was
quanti1047297ed based on theconcentrations (vol)of COand C xH y (asCH4)
measured in the 1047298ue gas at the cyclone outlet at actual excess air
qic frac14 eth1264 CO thorn 3582 CH4THORN V dgeth100 qucTHORN
LHV (B2)
where V dg is the volume of dry 1047298ue gas at the cyclone outlet Nm3
kg calculated by Refs [1822] using the fuel ultimate analysis onldquoas-receivedrdquo basis and excess air ratio at this point
The combustion ef 1047297ciency of the 1047298uidized-bed combustor hc
() was then determined by
hc frac14 100 ethquc thorn qicTHORN (B3)
References
[1] Natarajan E Nordin A Rao AN Overview of combustion and gasi1047297cation of rice husk in 1047298uidized bed reactors Biomass Bioenergy 199814533e46
[2] Bhattacharya SC Abdul Salam P Sharma M Emissions from biomass energyuse in some selected Asian countries Energy 200025169e88
[3] Werther J Saenger M Hartge EU Ogada T Siagi Z Combustion of agriculturalresidues Prog Energy Combust Sci 2000261e27
[4] Janvijitsakul K Kuprianov VI Similarity and modeling of axial CO and NO
concentration pro1047297les in a 1047298uidized-bed combustor (co-)1047297ring biomass fuelsFuel 2008871574e84
[5] Demirbas A Combustion characteristics of different biomass fuels ProgEnergy Combust Sci 200430219e30
[6] Leckner B Karlsson M Gaseous emissions form circulating 1047298uidized bedcombustion of wood Biomass Bioenergy 19934379e89
[7] Spliethoff H Hein KRG Effect of co-combustion of biomass on emissions inpulverized fuel furnaces Fuel Process Technol 199854189e205
[8] Lyngfelt A Leckner B Combustion of wood-chips in circulating 1047298uidized bedboilers e NO and CO emissions as functions of temperature and air-stagingFuel 1999781065e72
[9] Chyang CS Wu KT Lin CS Emission of nitrogen oxides in a vortexing 1047298uidizedbed combustor Fuel 200786234e43
[10] Armesto E Bahillo A Cabanillas A Veijonen K Otero J Combustion behavior of rice husk in a bubbling 1047298uidised bed Biomass Bioenergy 200223171e9
[11] Kuprianov VI Janvijitsakul K Permchart W Co-1047297ring of sugar canebagasse with rice husk in a conical 1047298uidized-bed combustor Fuel 200685434e42
[12] Fang M Yang L Chen G Shi Z Luo Z Cen K Experimental study on rice huskcombustion in a circulating 1047298uidized bed Fuel Process Technol2004851273e82
[13] Permchart W Kouprianov VI Emission performance and combustion ef 1047297-ciency of a conical 1047298uidized-bed combustor 1047297ring various biomass fuelsBioresour Technol 20049283e91
[14] Sirisomboon K Kuprianov VI Arromdee P Effects of design features oncombustion ef 1047297ciency and emission performance of a biomass-fuelled 1047298uid-ized-bed combustor Chem Eng Process Process Intens 201049270e7
[15] Leckner B Amand LE Lucke K Werther J Gaseous emissions from co-combustion of sewage sludge and coalwood in a 1047298uidized bed Fuel200483477e86
[16] Eaimsa-ard S Kaewkohkiet Y Thianpong C Promvonge P Combustionbehavior in a dual-staging vortex rice husk combustor with snail entry IntCommun Heat Mass Transfer 2008351134e40
[17] Madhiyanon T Lapirattanakun A Sathitruangsak P Soponronnarit S A novelcyclonic 1047298uidized-bed combustor (J-FBC) combustion and thermal ef 1047297-ciency temperature distribution combustion intensity and emission of pollutants Combust Flame 2006146232e45
[18] Basu P Cen KF Jestin L Boilers and burners New York Springer 2000[19] Kaewklum R Kuprianov VI Experimental studies on a novel swirling 1047298uid-
ized-bed combustor using an annular spiral air distributor Fuel20108943
e52
[20] Kouprianov VI Permchart W Emission from a conical FBC 1047297red witha biomass fuel Appl Energy 200374383e92
[21] Kaewklum R Kuprianov VI Douglas PL Hydrodynamics of airesand 1047298ow ina conical swirling 1047298uidized bed a comparative study between tangential andaxial air entries Energy Convers Manage 2009502999e3006
[22] Bezgreshnov AN Lipov YM Shleipher BM Computations of steam boilersMoscow Energoatomizdat 1991 [in Russian]
[23] Kuprianov VI Application of a cost-based method of excess air optimizationfor the improvement of thermal ef 1047297ciency and environmental performance of steam boilers Renew Sustain Energy Rev 20059474e98
[24] Streimikiene D Roos I Rekis J External cost of electricity generation in BalticStates Renew Sustain Energy Rev 200913863e70
[25] Zhang Q Weili T Yumei W Yingxu C External costs from electricity gener-ation of China up to 2030 in energy and abatement scenarios Energy Pol2007354295e304
[26] Hainoun A Almoustafa A Aldin MS Estimating the health damage costs of Syrian electricity generation system using impact pathway approach Energy
201035628e
38[27] Nguyen KQ Internalizing externalities into capacity expansion planning thecase of electricity in Vietnam Energy 200833740e6
[28] Kitou E Horvath A External air pollution costs of telework Int J Life CycleAssess 200813155e65
[29] Wei X Zhang L Zhou H Evaluating the environmental value of pollutants inChina power industry In Proceedings of the international conference onenergy and the environment Shanghai China 2003
[30] Salisdisouk N The concept of integrated resource planning In Proceeding of the workshop on electric power quality safety and ef 1047297ciency of its useselectric power system management Pathum Thani Thailand 1994
[31] Yang YB Shari1047297 VN Swithenbank J Effect of air 1047298ow rate and fuel moisture onthe burning behaviours of biomass and simulated municipal solid wastes inpacked beds Fuel 2004831553e62
[32] Zhao W Li Z Zhao G Zhang F Zhu Q Effect of air preheating and fuel moistureon combustion characteristics of corn straw in a 1047297xed bed Energy ConversManage 2008493560e5
[33] Tillman DA Rossi AJ Kitto WD Wood combustion New York AcademicPress 1981
[34] Winter F Wartha C Hofbauer H NO and N2O formation during thecombustion of wood straw malt waste and peat Bioresour Technol19997039e49
[35] Sun Z Jin M Zhang M Liu R Zhang Y Experimental studies on cotton stalkcombustion in a 1047298uidized bed Energy 2008331224e32
[36] Smart JP Robert PA De Soete GG The formation of nitrous oxide in a large-scale pulverized-coal 1047298ames J Inst Energy 199063131e5
[37] Abu-Qudais M Fluidized-bed combustion for energy production from olivecake Energy 199621173e8
[38] Trembovlya VI Finger ED Avdeeva AA Thermo-technical tests of boiler unitsMoscow Energoatomizdat 1991 [in Russian]
VI Kuprianov et al Energy 36 (2011) 2038e 20482048
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
httpslidepdfcomreaderfullkuprianov-2011-effects-of-operating-conditions-and-fuel-properties-on-emission 1111
Appendix A Measurement uncertainty in the 1047298owrate of
primary air
The fractional uncertainty in the 1047298owrate of primary air was
estimated by Ref [38]
sPA frac14 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
4s2D thorn s2
V thorn s2R thorn s2
P
q (A1)
where sD is the error in physical measurement of inner diameter of the primary air pipe sV is the measurement error of the velocity
(dependent mainlyon the type of a selected probe) sR is therelative
error associated with imperfect1047297xing of the probe in air1047298ow and sP
is the uncertainty in the velocity pro1047297le across the pipe
AssumingsD frac14 1 sV frac14 15 sR frac14 1 and sP frac14 15(as advised in
Ref [38]) the uncertainty in the 1047298owrate of primary air was
calculated using Eq (A1) to be about 3
Appendix B Determining heat losses and combustion
ef 1047297ciency
In this work the combustion ef 1047297ciency of the SFBC 1047297ring rice
husk was calculated using the the heat-loss method [18]
For this combustor with no ashremoval through thebottompartthe heat loss with unburned carbon quc () was determined by
quc frac14 32866
LHV
C fa
100 C fa
A (B1)
where C fa is the unburned carbon content in the1047298y ash wt and A
is the fuel-ash content wt on ldquoas-receivedrdquo basis
The heat loss owing to incomplete combustion qic () was
quanti1047297ed based on theconcentrations (vol)of COand C xH y (asCH4)
measured in the 1047298ue gas at the cyclone outlet at actual excess air
qic frac14 eth1264 CO thorn 3582 CH4THORN V dgeth100 qucTHORN
LHV (B2)
where V dg is the volume of dry 1047298ue gas at the cyclone outlet Nm3
kg calculated by Refs [1822] using the fuel ultimate analysis onldquoas-receivedrdquo basis and excess air ratio at this point
The combustion ef 1047297ciency of the 1047298uidized-bed combustor hc
() was then determined by
hc frac14 100 ethquc thorn qicTHORN (B3)
References
[1] Natarajan E Nordin A Rao AN Overview of combustion and gasi1047297cation of rice husk in 1047298uidized bed reactors Biomass Bioenergy 199814533e46
[2] Bhattacharya SC Abdul Salam P Sharma M Emissions from biomass energyuse in some selected Asian countries Energy 200025169e88
[3] Werther J Saenger M Hartge EU Ogada T Siagi Z Combustion of agriculturalresidues Prog Energy Combust Sci 2000261e27
[4] Janvijitsakul K Kuprianov VI Similarity and modeling of axial CO and NO
concentration pro1047297les in a 1047298uidized-bed combustor (co-)1047297ring biomass fuelsFuel 2008871574e84
[5] Demirbas A Combustion characteristics of different biomass fuels ProgEnergy Combust Sci 200430219e30
[6] Leckner B Karlsson M Gaseous emissions form circulating 1047298uidized bedcombustion of wood Biomass Bioenergy 19934379e89
[7] Spliethoff H Hein KRG Effect of co-combustion of biomass on emissions inpulverized fuel furnaces Fuel Process Technol 199854189e205
[8] Lyngfelt A Leckner B Combustion of wood-chips in circulating 1047298uidized bedboilers e NO and CO emissions as functions of temperature and air-stagingFuel 1999781065e72
[9] Chyang CS Wu KT Lin CS Emission of nitrogen oxides in a vortexing 1047298uidizedbed combustor Fuel 200786234e43
[10] Armesto E Bahillo A Cabanillas A Veijonen K Otero J Combustion behavior of rice husk in a bubbling 1047298uidised bed Biomass Bioenergy 200223171e9
[11] Kuprianov VI Janvijitsakul K Permchart W Co-1047297ring of sugar canebagasse with rice husk in a conical 1047298uidized-bed combustor Fuel 200685434e42
[12] Fang M Yang L Chen G Shi Z Luo Z Cen K Experimental study on rice huskcombustion in a circulating 1047298uidized bed Fuel Process Technol2004851273e82
[13] Permchart W Kouprianov VI Emission performance and combustion ef 1047297-ciency of a conical 1047298uidized-bed combustor 1047297ring various biomass fuelsBioresour Technol 20049283e91
[14] Sirisomboon K Kuprianov VI Arromdee P Effects of design features oncombustion ef 1047297ciency and emission performance of a biomass-fuelled 1047298uid-ized-bed combustor Chem Eng Process Process Intens 201049270e7
[15] Leckner B Amand LE Lucke K Werther J Gaseous emissions from co-combustion of sewage sludge and coalwood in a 1047298uidized bed Fuel200483477e86
[16] Eaimsa-ard S Kaewkohkiet Y Thianpong C Promvonge P Combustionbehavior in a dual-staging vortex rice husk combustor with snail entry IntCommun Heat Mass Transfer 2008351134e40
[17] Madhiyanon T Lapirattanakun A Sathitruangsak P Soponronnarit S A novelcyclonic 1047298uidized-bed combustor (J-FBC) combustion and thermal ef 1047297-ciency temperature distribution combustion intensity and emission of pollutants Combust Flame 2006146232e45
[18] Basu P Cen KF Jestin L Boilers and burners New York Springer 2000[19] Kaewklum R Kuprianov VI Experimental studies on a novel swirling 1047298uid-
ized-bed combustor using an annular spiral air distributor Fuel20108943
e52
[20] Kouprianov VI Permchart W Emission from a conical FBC 1047297red witha biomass fuel Appl Energy 200374383e92
[21] Kaewklum R Kuprianov VI Douglas PL Hydrodynamics of airesand 1047298ow ina conical swirling 1047298uidized bed a comparative study between tangential andaxial air entries Energy Convers Manage 2009502999e3006
[22] Bezgreshnov AN Lipov YM Shleipher BM Computations of steam boilersMoscow Energoatomizdat 1991 [in Russian]
[23] Kuprianov VI Application of a cost-based method of excess air optimizationfor the improvement of thermal ef 1047297ciency and environmental performance of steam boilers Renew Sustain Energy Rev 20059474e98
[24] Streimikiene D Roos I Rekis J External cost of electricity generation in BalticStates Renew Sustain Energy Rev 200913863e70
[25] Zhang Q Weili T Yumei W Yingxu C External costs from electricity gener-ation of China up to 2030 in energy and abatement scenarios Energy Pol2007354295e304
[26] Hainoun A Almoustafa A Aldin MS Estimating the health damage costs of Syrian electricity generation system using impact pathway approach Energy
201035628e
38[27] Nguyen KQ Internalizing externalities into capacity expansion planning thecase of electricity in Vietnam Energy 200833740e6
[28] Kitou E Horvath A External air pollution costs of telework Int J Life CycleAssess 200813155e65
[29] Wei X Zhang L Zhou H Evaluating the environmental value of pollutants inChina power industry In Proceedings of the international conference onenergy and the environment Shanghai China 2003
[30] Salisdisouk N The concept of integrated resource planning In Proceeding of the workshop on electric power quality safety and ef 1047297ciency of its useselectric power system management Pathum Thani Thailand 1994
[31] Yang YB Shari1047297 VN Swithenbank J Effect of air 1047298ow rate and fuel moisture onthe burning behaviours of biomass and simulated municipal solid wastes inpacked beds Fuel 2004831553e62
[32] Zhao W Li Z Zhao G Zhang F Zhu Q Effect of air preheating and fuel moistureon combustion characteristics of corn straw in a 1047297xed bed Energy ConversManage 2008493560e5
[33] Tillman DA Rossi AJ Kitto WD Wood combustion New York AcademicPress 1981
[34] Winter F Wartha C Hofbauer H NO and N2O formation during thecombustion of wood straw malt waste and peat Bioresour Technol19997039e49
[35] Sun Z Jin M Zhang M Liu R Zhang Y Experimental studies on cotton stalkcombustion in a 1047298uidized bed Energy 2008331224e32
[36] Smart JP Robert PA De Soete GG The formation of nitrous oxide in a large-scale pulverized-coal 1047298ames J Inst Energy 199063131e5
[37] Abu-Qudais M Fluidized-bed combustion for energy production from olivecake Energy 199621173e8
[38] Trembovlya VI Finger ED Avdeeva AA Thermo-technical tests of boiler unitsMoscow Energoatomizdat 1991 [in Russian]
VI Kuprianov et al Energy 36 (2011) 2038e 20482048
Top Related