Master´s thesis
We offer master´s theses about challenges of chemical and technical character withinpower and heat production.
As thesis worker, you will come in direct contact with issues of industrial relevance.
We represent an international research group (about 30 people) with cooperation withseveral international companies such as Valmet, Andritz, MetsäFibre, Fortum, UPM,International Paper and others.
Duration: 6 months
For more information contact prof. Leena Hupa, [email protected] or Patrik Yrjas,[email protected]. We are both located in Axelia II on the 4th floor in rooms B 417 (Leena)and B 416 (Patrik).
Combustion of solid fuels – an
introduction to ashes and related
problems
Patrik Yrjas
2
Contents
Combustion technologies => fluidised beds
Ash formation and why is it worth studying?
Fly ash
How can it be so difficult?
Co-firing
What can we do?
Research methods
Empirical
Theoretical
3
Combustion technologies
Pulverized fuel combustionng
– mainly coal
Grate firing – today mainly
waste and biomass in smaller
scale
Fluidised beds – suitable for
several different types of fuels
4Sumitomo SHI FW
Fluidised beds Important in Finland, three world wide manufacturers
located here
Sumitomo SH FW (originates from Ahlström)
Andritz
Valmet
Proven technology; export
Suitable for both domestic and other CO2-neutral fuel
mixes
5
0
20
40
60
80
Nu
mb
er
of
co-f
irin
g in
itia
tive
s
Unknown
Grate
CFB
BFB
PF
(> 50 MWel)
IEA-Bioenergy, 2009
CYMIC multi-fuel boiler
TSE, Naantali, FinlandSteam 164 bar
555 C
390 MWth
(142 MWe, 244 MWheat)
Fuels: Wood biomass,
agrofuel, peat,
coal, SRF
Start-up: Fall 2017
Courtesy of Valmet Technologies Oy https://www.youtube.com/watch?v=8rzhZQ0nDhs
Ash formation - simplified
7
Fuel
(wood, coal, etc.)
Ash elements
(SiO2, Al2O3, Fe2O3, CaO, K2O, S, Cl, etc.)
Flue gases
(CO2, N2, O2, H2O, SO2, NO, HCl, etc.)
Air
(O2, N2)
Heat, Q
Ash formation – fly ash
Physical
transport
Pyrolysis Char combustion and
fragmentation
Evaporation
Homogeneous
nucleation
Coagulation
Heterogeneous
condensation
Fly ash
0.1 -1 µm
Fly ash
1 - 100 µm
Included
minerals
Excluded
minerals
Mineral
coalescens och
fragmentation
Why is ash then so important....?
9
70-90% of the ash becomes fly ash in FB
Ash is the most common reason for un-planned
shut-downs due to:
deposits on superheaters
corrosion (mainly due to alkali chlorides)
if ash melts, even if only partly, then always a risk for heavy
corrosion
bed-agglomeration
… which are dependent on:
ash amount and ash type (fuel, additives)
process conditions (temp., air staging, etc. osv)
Challenges in biomass combustion
Superheater corrosion is the major single reason for
efficiency limitations and operational problems
Bubbling fluidising
bed (BFB)
Valmet Technologies
Secondary air
Bottom-
ash
Transportation,
transformation,
reactions
Ash elements
released in bed
Deposits
on super-
heaters
Separation
of fly ash
Fuels
Additive 1
e.g. limestone
Additive 2
e.g. (NH4)2SO4
Primary air
Tertiary air
70-90%
Factors affecting corrosion
Material temperature
Steel composition
Deposit composition (connected to
ash but not directly the same)
Gas composition
Typical temperatures
Wall tubes and drum
Temperature of supersaturated steam
(depends on pressure) ~ 300°C
Material temperature at walls +50°C
Superheaters
Steam ~ 400°C → 550°C (…600°C?)
Material
Radiation area+ 50°C
Convective area + 20-35°C
Economizers
Pre-heating of water max ~300°C
Air-pre-heaters
Pre heating of air (~120°C ±)
(Frandsen, doctoral thesis, DTU, 2011, CB 2008 - Hupa, Chirone et al., 2006)
Examples: deposits, corrosion and
an agglomerate due to unsuitable fuel mixes
Increased steam/material-temperatures
give higher efficiency but increases the
risk for corrosion
15
Net
ele
ctr
ic e
ffic
ien
cy (
%)
50 %
45 %
40 %
35 %
Coal
Biomass
Waste
DOUBLE
REHEAT
SINGLE
REHEAT
30 %
25 %
20 %
NO
REHEAT
15 %
10 %
350 400 450 500 550 600 650
Steam temperature (°C)
Vainikka, P., doctoral thesis, 2011
adopted from Dr.-Ing. Klaus Hein
Rule of thumb: 10°C increase gives about +2% in electrical efficiency
Standardised fuel characterisation
and prediction of ash behaviour
16
Analyses
• Moisture – and ash amount, volatiles, C,H,O,N,S
• Ash forming elements; Si, Al, Fe, Ti, Ca, Mg, Mn, P, Na, K, S, Cl
• Thermodynamic evaluations/calculations
• Trace elements; As, Hg, Cd, Pb, Cr, Zn, Cu, Ni, etc
• Different indices and empirical experiences
• Ash sintering test (ASTM, DIN)
IDT HT FTheight = 1/3 of height at HTheight = 1/2 of basetop disappeares
Standardised fuel characterisation
och ash behaviour prediction
Fuel
sample
Standardised analyses
Ash composition
A number of different indices
Standardised ash melting temp.
(ASTM, DIN)
Bed
agglomeration
(FB-boilers)
Emissions
Deposits
& corrosion
• originally developed for coal
fuels (not mixes)
17
Why is it so difficult then?
New, more demanding and cheaper fuels
Co-firing (fuel mixes) – positive and negative synergy effects
Laboratory ash versus ”real” ash from fuel mixes
Demands for higher efficiencies (less fuel = lower CO2/produced
electricity) - corrosion
Need for:
Prediction of the ash composition and its behaviour, deposit
formation and agglomeration tendencies from fuel analyses and
characterisation methods without full-scale tests (and a minimum
of laboratory tests).
18
Sustainable fuels, examples
Resources:
Biomass: wood, sawdust, forest residue, bark, (black
liquor)
Other biomass: agricultural residues (agro-fuels), energy
crops
Other sources: demolition wood, municipal waste,
industrial waste, sludges
100
90
80
70
60
50
40
30
20
10
0 100
90
80
70
60
50
40
30
20
10
0
0 10 20 30 40 50 60 70 80 90 100
CaO+MgO
K2O+Na2OSiO2
Kol
Main ash forming elements
ÅA fuel database 20
Coal
100
90
80
70
60
50
40
30
20
10
0 100
90
80
70
60
50
40
30
20
10
00 10 20 30 40 50 60 70 80 90 100
CaO+Mg
K2O+Na2O SiO2
KolTorv
ÅA fuel database 21
Coal
Peat
Main ash forming elements
ÅA fuel database 22
100
90
80
70
60
50
40
30
20
10
0 100
90
80
70
60
50
40
30
20
10
0
0 10 20 30 40 50 60 70 80 90 100
CaO+MgO
K2O+Na2O SiO2
KolTorv
TräGrotBarkAvfallsträ
Coal
Peat
Wood
Forest residue
Bark
Waste wood
Main ash forming elements
ÅA fuel database 23
100
90
80
70
60
50
40
30
20
10
0 100
90
80
70
60
50
40
30
20
10
0
0 10 20 30 40 50 60 70 80 90 100
CaO+MgO
K2O+Na2O SiO2
KolTorv
TräGrotBarkAvfallsträ
Jordbruksrester
Coal
Peat
Wood
Forest residue
Bark
Waste wood
Agricultural waste
Main ash forming elements
Ash chemistry of different fuel types
Coal => silicate based ash chemistry, Na, S and
Ca (in FBC when used for desulphurization)
Biomass => Ca, K, Na, S, and Cl (+Si in some
cases)
Agrofuels => Si, Ca, K, P, S and Cl
Waste fuels => .......... + Zn and Pb (+Br)
Synergy effects between fuels in fuel mixes
Physical effects - sand blasting, dilution
Chemical effects - reactions
25
Depositio
n r
ate
, g/m
2h
Fuel A, weight-%
negative synergy
positive synergy
Exemple on physical synergy effect
rice husk and barkTest rig at the University of Toronto
26
100% bark 36% bark + 64% rice husk
100% rice husk 64% rice husk, 36% bark,100% bark
deposit = 13 mg deposit = 10 mgdeposit = 195 mg
Rate of build-up with different rice
husk/bark mixes
(prof. H.Tran, Toronto, Canada)
0
50
100
150
200
250
0 16 36 60 84 100
På
sla
g, m
g
Bark, vikt-%
Deposit,
mg
Bark, weight-%
Example on chemical synergy effects
bark + sludgeTest rig 20 kW, BFB,
VTT, Jyväskylä
29
To stack
Sampling port
Sampling port
Sampling port
Gas cooling
Bagfilter
Gas probe
Observationport
Cyclone
Gas sample
Temperature control
Tertiary air optional
Tertiary air optional
Tertiary air (preheated)
Fuel container 2Fuel container 1
Secondary air(preheated)
Nitrogen
Air
Additivecontainer
Primary gas heating
Heating zone 2/Cooling zone 2
Heating zone 3
Heating zone 4
Heating zone 1/Cooling zone 1
BEDmade of quarz
PC control and data logging system
Obervation port
Obervation port
Obervation port
Obervation port/Deposit probe
Deposit probe
Sludge has high ash and high contents
of sulphur and alumina silicates.
Bark Slam
Aska, 550oC, vikt-% 3.30 55.60
C vikt-% 51.40 23.00
H 5.90 3.40
N 0.40 2.60
S 0.036 1.19
Cl 0.021 0.076
LHV, MJ/kg d.s. 19.27 9.14
LHV, MJ/kg a.r. 19.03 8.98
SiO2 vikt-% 7.71 25.74
Al2O3 1.75 7.20
Fe2O3 1.03 32.40
TiO2 0.07 0.70
MnO 2.08 0.07
CaO 40.11 6.72
MgO 4.09 1.64
P2O5 3.58 15.37
Na2O 1.00 1.08
K2O 6.83 1.45
SUM, % 68.25 92.37
Element i askan som oxider
HCl and SO2 emissions and Cl in deposits
30
0
10
20
30
40
50
60
70
80
90
100
100% Bark 2%
Sewage
sludge
4%
Sewage
sludge
6%
Sewage
sludge
8%
Sewage
sludge
Gaseo
us S
O2 a
nd
HC
l em
issio
ns,
mg
/MJ (
LH
V d
.s.)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Avera
ge C
l in
dep
osit
s (
valu
es f
rom
win
d, 50 d
eg
., a
nd
lee),
wt-
%
HCl
SO2
Cl
Al2O3∙2SiO2(s) + 2KCl(g) + H2O(g) => K2O∙Al2O3∙2SiO2(s) + 2HCl(g)
2KCl(g) + SO2(g) + ½ O2(g) + H2O(g) => K2SO4(s)+ HCl(g)
Yrjas et al., 2009
How predict behaviour of fuel mixes?
Bed
agglomeration
Standardised fuel analysis
? ? ?
Fuel
2
Emissions
Deposits
& corrosion
Fuel
3
Fuel
1
?
?
?
31
Chemical fractionation
32
Today we have about 250 fuels in the ÅA database
(fractionation data, C, H, O, N, S, ash, and heat values)
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77 81
mg
/kg
Rest Fraction
Leached in HCl
Leached in Acetate
Leached in H2O
Coal Peat Wood Bark For.Res. Agr.Res.
K
33
0
500
1000
1500
2000
2500
3000
3500
4000
4500
1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77 81
mg/k
g
Rest Fraction
Leached in HCl
Leached in Acetate
Leached in H2O
Coal Peat Wood Bark For.Res. Agr.Res.
P
34
Coal
35
0
5000
10000
15000
20000
25000
30000
35000
Si Al Fe Ti Mn Ca Mg P Na K S Cl
mg/k
g
161 Rest Fraction
161 Leached in HCl
161 Leached in Acetate
161 Leached in H2O
Bark
36
0
2000
4000
6000
8000
10000
12000
Si Al Fe Ti Mn Ca Mg P Na K S Cl
mg/k
g
67 Rest fraction, analysed
67 Leached in HCl
67 Leached in Acetate
67 Leached in H2O
Prediction of ash behaviour in fuel mixes
Fuel
2
Chemical
fractionation
&
lab. methods
(SEM, others)
Reactive
Inert
Chemical
thermo-
dynamics
CompositionFuel
1
Chemical
fractionation
&
lab. methods
(SEM, others)
Ash compounds
Melting behaviour
%-melt
+ Practical experience
Knowledge and solutions
37
40th Anniversary International Recovery Boiler Conference, Porvoo, Finland, May 12-14, 2004 - Mikko Hupa
Fraction of Molten Phase vs. Temperature
0
20
40
60
80
100
500 600 700 800 900
Temperature [°C]
Pe
rce
nta
ge
Liq
uid
Ph
as
e [
wt
%]
T0
T100
Challenging fuel mixes
Sumitomo SHI FW´s fuel system:
Courtesy of Sumitomo SHI FW
Challenging fuel mixesValmet´s fuel system:
Empirical use of fractionation results
combined with deposit measurements Full-scale deposits measurements
SEM/EDX analyses of Cl in deposits
Based on the well known reactions between S and K
which reduce the formation of chlorides, while forming
sulphates and releasing Cl to the flue gas as HCl:
2KCl/NaCl + SO2 +H2O +1/2O2=> K2SO4/Na2SO4+ HCl
• Short term, in-situ deposit sampling
• Surface temperature regulated probe
60 – 200 cm
41
Common to predict the risk for Cl in deposits with the S/Cl ratio in the fuel.
Not always valid for biofuels, due to varying amounts of active sulphur binding species, e.g Ca-compounds present in the fuel:
CaO +SO2 +1/2 O2=> CaSO4
ref. Yrjas et al. 18th FBC, 2005
Co-firing of peat, coal, bark, wood chips,
and forest residue in a large scale CFB
42
Co-firing of peat, coal, bark, wood chips,
and forest residue in a large scale CFB
S/(Ca+K2+Na2)reactive
molar ratio calculated
separately for every
fuel mixture and
plotted against the
amount of chlorine in
the deposits.
0.0
0.5
1.0
1.5
2.0
2.5
0.00 0.20 0.40 0.60 0.80 1.00
S/(Ca+K2+Na2)reactive, molar ratio
Ch
lori
ne
in
de
po
sit
s, w
t-%
350°C, 50 deg. side
350°C, lee side
350°C, wind side
Yrjas et al.18th FBC43
(Ca+Na2+K2)/S molar ratio as a
funct. of Cl in deposits and SO2
0.0
0.5
1.0
1.5
2.0
2.5
0 1 2 3 4 5 6 7 8
(Ca+Na2+K2)/S
Cl in
dep
osit
s (
350C
, 50
deg
.)
0
20
40
60
80
100
120
140
160
SO
2 p
pm
, 6%
O2
ClSO2
EU-Biomax project
Agglomeration very hard to predict – mainly
based on experience and laboratory tests
45
Bed height: 5 cm
Five thermocouples
Pressure drop measured
with a mikromanometer
Instrumental and
flue gas outlet
Combustion
chamber
Fuel
feeding
Tube furnace
Pre-heater
Bed net
(105 µm)
Air feed
Experiments with SiO2 and addition of KCl
46
Defluidisation after 12 g
(50 mg/min) or 3.1 weight-%
of the bed weight
Agglomerates of SiO2 and KCl
Scanning electron microscope/energy
dispersive x-ray analyzer (SEM/EDXA)
SEM; a method for high resolution
imaging of surfaces
EDXA; a method for elemental
analysis
KCl K2CO3
Sevonius et al., 2014
Tricks to decrease corrosion risks
Co-firing with sulphur and/or aluminasilicate rich
fuels
Use of additives containing sulphur
Fuel pre-treatments (heat, washing....)
Use right materials at each boiler location
Improved/better materials (improvement vs. price)
Design
QuizWhich boiler is a dedicated waste firing boiler?
Energy statistics app
App:
https://webstore.iea.org/key-world-energy-
statistics-2019
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