Hydrogen Production by Coal Gasification in Supercritical Water With a Fluidized Bed Reactor
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Transcript of Hydrogen Production by Coal Gasification in Supercritical Water With a Fluidized Bed Reactor
8/10/2019 Hydrogen Production by Coal Gasification in Supercritical Water With a Fluidized Bed Reactor
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Hydrogen production by coal gasification in supercritical
water with a fluidized bed reactor
Hui Jin, Youjun Lu, Bo Liao, Liejin Guo*, Ximin Zhang
State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China
a r t i c l e i n f o
Article history:Received 16 November 2009
Received in revised form
21 January 2010
Accepted 22 January 2010
Available online 4 March 2010
Keywords:
Coal
Supercritical water
Gasification
Hydrogen production
a b s t r a c t
The technology of supercritical water gasification of coal can converse coal to hydrogen-rich gaseous products effectively and cleanly. However, the slugging problem in the
tubular reactor is the bottleneck of the development of continuous large-scale hydrogen
production from coal. The reaction of coal gasification in supercritical water was analyzed
from the point of view of thermodynamics. A chemical equilibrium model based on Gibbs
free energy minimization was adopted to predict the yield of gaseous products and their
fractions. The gasification reaction was calculated to be complete. A supercritical water
gasification system with a fluidized bed reactor was applied to investigate the gasification
of coal in supercritical water. 24 wt% coal-water-slurry was continuously transported and
stably gasified without plugging problems; a hydrogen yield of 32.26 mol/kg was obtained
and the hydrogen fraction was 69.78%. The effects of operational parameters upon the
gasification characteristics were investigated. The recycle of the liquid residual from the
gasification system was also studied.
ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1. Introduction
Coal is an important fossil fuel due to the abundant deposit
and distribution all over the world and has been a vital part of
our societyformorethana century [1]. However, as a solid fuel
coalis difficult to handle and NOx and SOx are produced during
the burning process [2]. With a strong demand for an afford-
able energy supply and the urgent need for the pollutant
emission control, the clean and efficient utilization of coal
presents a challenge to the current global R&D efforts [3].
Supercritical water has special physical and chemical
properties, and it has high diffusion rates, low viscosity, and
is miscible with light gases, hydrocarbons and aromatics.
Various organic reactions such as hydrolysis usually proceed
without catalysts, so supercritical water is an excellent
medium for homogeneous, fast, and efficient reactions [4–6].
Therefore, scientists focus on dealing with coal in super-
critical water in different methods, such as hydrolysis,
pyrolysis, desulfurization, liquefaction and extraction and
gasification [7–14]. Here, supercritical water gasification of
coal is a newly developed technology for clean and effective
conversion of coal. It can converse coal to hydrogen-rich
gaseous products, and hydrogen is considered to be an
ideal energy carrier. It is reported that coal gasification in
supercritical water has higher energetic efficiency than
pulverized coal power plants and pressurized fluidized bed
power plant [2]. Moreover, relatively low temperature of SCW
(supercritical water) conversion impedes formation of NOx
and SOx, and closeness of this system excludes emissions of
fine ashes, and the main reaction in the system excluded
steam reforming, water–gas shift reaction and methanation
reactions to realize the conversion from coal to hydrogen-
rich gas [14,15].
The advantages of the technology of coal gasification in
supercritical water have attracted great interest of research
recently. Modell firstly reported that bituminous coal was
* Corresponding author. Tel.: þ86 29 82663895; fax: þ86 29 82669033.E-mail address: [email protected] (L. Guo).
A v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
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 / h e
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 7 1 5 1 – 7 1 6 0
0360-3199/$ – see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijhydene.2010.01.099
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gasified in supercritical water in an autoclave and high-
heating-value gas was produced. No significant char was
found [16]. Lin proposed a novel HyPr-RING method to
produce hydrogen from lignite, subbituminous, and bitumi-
nous coal. Ca(OH)2 was used as a catalyst and a absorbent of
CO2. The hydrogen fraction in gaseous product was as high as
over 80% without chlorine or sulfur gases. However, eutectic
melt of Ca(OH)2 /CaCO3 was found in this operating condition
and this eutectic melt caused the growth of large particles of solid materials. This may cause plugging problems that
hindered the continuous operation [17–19]. Gasification of
low-rank coals in supercritical water was carried out in an
autoclave by Wang [13]. It was found that the presence of
Ca(OH)2 facilitated the extraction of volatile matter from coal
and the decomposition of the volatile matter to small mole-
cule gases, which led to the decrease of the residual char.
Vostrikov found that the coal gasification reaction was
a weakly endothermic process and experimentally investi-
gated combustion of single coal particles in H2O/O2 super-
critical fluid in the semi-batch reactor. It is proposed that coal
gasification and oxidation in supercritical H2O/O2 fluid
together offer the possibility of generating energy-efficientand environmentally clean working media of steam–gas
power plants [15,20,21]. Yamaguchi [22] investigated the non-
catalytic gasification characteristics of Victorian brown coal in
supercritical water by with quartz batch reactors. Various
operating parameters were selected to investigate their effect
on the gasification behavior. The measured data showed
a large deviation from the equilibrium level maybe due to the
heat and mass transfer in batch reactor. Li [23] developed
a continuous pipe flow system for coal gasification in super-
critical water. The slurry of 16 wt% coal þ 1.5 wt% CMC
(sodium carboxymethyl cellulose) was successfully trans-
ported into the reactor and continuously gasified in super-
critical water in the system. However, plugging probleminhibited further increase of the coal slurry concentration.
Due to the complex structure of coal and the plugging
problems existing in the process of gasification process, the
technology of coal gasification in supercritical required to be
improved. We proposed an approach to overcome such
disadvantages. Theoretically, a thermodynamic model based
on the chemical equilibrium [24] was applied to predict the
product of coal gasification. Experimentally, a novel gasifica-
tion system for coal gasification in supercritical water with
a fluidized bed reactor was adopted to achieve continuous
gasification. The fluidized bed reactor was proved to increase
the heating rate, enhance the mass/heat transfer rates in the
reactor and increase the gasification efficiency [25]. I t i sdemonstratedthat 24 wt% coal-water-slurrywas continuously
transported and stably gasified without blockage problems.
The influences of the operational effects were experimentally
investigated to obtain the optimal reaction condition.
2. Thermodynamics analysis
Coal is a complicated mixture and has different structure and
composition of the organic matter. The fraction and compo-
sition of the mineral constitutes the process of coal gasifica-
tion in supercritical water is very complicated. It is commonly
proposed [26,27] that the reaction process mainly includes
three reactions: steam reforming (1) (coal is considered to be
pure carbon in this equation), water–gas shift reaction (2), and
methanation reaction (3). An equilibrium calculation is
necessary to predict the product composition and weather
coal can be gasified completely.
C(s) þ H2O(g)/ CO(g) þ H2(g) DH ¼ 132 kJ/mol (1)
CO(g) þ H2O(g)/ CO2(g) þ H2(g) DH ¼ 41 kJ/mol (2)
CO(g) þ 3H2(g)/ CH4(g) þ H2O(g) DH ¼ 206 kJ/mol (3)
Chemical equilibrium model based on Gibbs free energy
minimization was adopted to analyze the gaseous product
Nomenclature
GE gasification efficiency, mass of gaseous product/
mass of dry matter in the water-coal slurry
CE carbon gasification efficiency, mass of carbon
element in gaseous product/mass of carbon in dry
matter in the water-coal slurry
HE hydrogen gasification efficiency, mass of
hydrogen gas/the mass of hydrogen in dry matter
in the water-coal slurry
YH2 yield of hydrogen, the mass of certain gas product/
the mass of dry matter in feedstock
CgE coldgas efficiency,chemical energycontent inthe
product gas/the chemical energy in the fuel (basedon the lower-heating-value)
Table 1 – Analysis data of the Shenmu coal.
Elemental analysis (wt%) Proximate analysis (wt%) Qb, ad
Species C H N S Oa M A V FC (MJ/kg)
Shenmu coal 69.63 3.75 0.80 0.41 12.25 5.31 7.85 30.92 55.92 27.826
a Difference.
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yields and their fraction. When the equation of the conser-
vation of matters is satisfied, the expression of Gibbs free
energy obtains its minimum value when a multicomponent
system reaches chemical equilibrium [25,28]. In the calcula-
tion, the molecular formula of coal is assumed to be
CH0.647O0.132 according to the elemental analysis shown in
Table 1. The calculated properties of the coke as solid residual
of gasification were taken as graphite [29].Fig. 1 shows the gas yields and fraction from different
concentration of coal in supercritical water. The amount of
coke can be a neglect compared with other species. It means
that the coal gasification in supercritical water is complete
according to the thermodynamic modeling. It can be seen that
when the concentration of coal is low, the gaseous product
fraction order is H2 > CO2 > CH4 > CO. The yield of carbon
monoxide is very low and the fraction of carbon monoxide is
below 0.01%. This implies that the reaction (2) is nearly
complete. When the concentration of coal is high, reaction (1)
in inhibited andreaction (3) is promoted due to the insufficient
supply of water. It results in the hydrogen fraction decrease
and the methane fraction increase with the increasing of concentration of coal. The above analysis agrees well with the
calculation results.
3. Apparatus and experimental procedures
The experimental study was performed in a coal gasifica-
tion system with a fluidized bed reactor and the schematic
diagram of system is shown in Fig. 2. The reactor is con-
structed of 316 stainless steel. The bed diameter and the
freeboard diameter are 30 mm and 40 mm respectively,
and the total length is 915 mm. The distributor is located
0 5 10 15 20
0
30
60
a
b
C O2
C H4
H2
C O
Concentrat ion(wt%)
H 2
H C ,
4
O
C ,
2
)
% ( n o i t c a r f
0.002
0.004
0.006
0.008
0.010
)
% ( n
oi t c ar f
O C
0 5 10 15 2 0
0
4 0
8 0
12 0
16 0
) g k / l o m ( d l e i Y s a G
Concentrat ion(wt%)
H2
C O
C H4
C O2
Fig. 1 – Effect of concentration upon gasification
equilibrium products from coal: (a) Gas Fraction; (b) Gas
Yield. (Temperature, 500 8C; Pressure, 25 MPa).
Fig. 2 – Scheme of system for hydrogen production from coal in supercritical water with a fluidized bed reactor: 1 feedstock
tank; 2,3 feeder; 4 fluidization bed reactor; 5 heat exchanger; 6 pre-heater; 7 cooler; 8,9,10 back-pressure regulator; 11 high
pressure separator; 12 low pressure separator; 13,14 wet test meter; 15,16,17,18 high pressure metering pump; 19,20,21,22
mass flow meter; 23 water tank.
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in the bottom of the reactor, and water preheated to the
desired temperature flows through the distributor at the
bottom to form a fluidization state. Coal slurry flows into
the reactor from the feedstock entrance above the distrib-
utor. A metal foam filter is installed at the exit of the
reactor in order to prevent the bed material escaping from
the reactor. Detailed description was reported in the liter-
ature [25].
The bituminous coal was produced from Shenmu,Shaanxi,
China, and the elemental analysis and proximate results can
be seen in Table 1. Coal was pulverized into particles andseparated by sieve of 100 mesh, 140 mesh, and 200 mesh.
Particles <74 mm, <105 mm, and <149 mm were obtained. The
water-coal slurry was homemade, with 2 wt% CMC as sus-
pending agent to generate uniform slurry and with 1 wt%
K2CO3 as catalyst. Sodium carboxymethyl cellulose (CMC) and
Anhydrous potassium carbonate (K2CO3) were purchased
from Shanghai Shanpu chemical Co. Ltd. and Tianjin Chem-
ical Reagent, respectively.
The molar fraction of the gaseous product was analyzed by
HP6890 gas chromatograph. It is equipped with thermal
conductivity detector and capillary column C-2000 that was
purchased from Lanzhou Institute of Chemical Physics in
China. High purity He was used as carrier gas and the flow rate
was 10 ml/min. The total carbon contents of the liquid phase
were determined using Elemental High TOCII.
4. Result and discussion
The effects of temperature, pressure, fluidizing velocity,
concentration of coal slurry, and coal particle diameter upon
the coal gasification characteristics were investigated. GE
(gasification efficiency), HE (hydrogen gasification efficiency),
CE (carbon gasification efficiency), CgE (cold gas efficiency),
YH2 (hydrogen yield) and TOC (total organic carbon) wereapplied to evaluate the gasification characteristics of coal, and
their definition can be seen in the nomenclature.
4.1. Effect of temperature
The effect of temperature is shown in Fig. 3. Itcan beseen that
the temperature has a significant effect on coal gasification in
supercritical water. As the temperature of reaction fluid
increased from 520 to 580 C, the fraction of hydrogen
increased from 53.59% to 61.65%, while the fraction of
methane decreased from 6.97% to 4.86%, because the higher
temperatures drove the methane steam reforming reaction to
increase hydrogen yields at the expense of methane [30]. The
23 25 270
20
40
60
80
100a
b
)
% ( n o i t c a r F
s a G
HY
2
) gk / l om
(
Pressure(MPa)
H2
C O
C H4
C O2
C2
YH2
0
10
20
23 25 270
40
80
120
H E
C gE
G E
T OC
Pressure(MPa)
)
% ( E G , E
g C , E H
0
70
140
210
) m p p ( C OT
Fig. 4 – Effect of pressure upon gasification characteristic of
coal (a): Gas Fraction and YH2; (b) HE, GE, CgE and TOC.
(580 8C, water flow rate 120 g/min, slurry flow rate
12 g/min, 6 wt% coal D2 wt% CMCD1 wt% K2CO3, coal
particle<105 mm).
520 540 560 5800
20
40
60
80
100a
b
)
% ( n o i t c a r F s a G
HY
2
) gk / l o
m (
T(oC )
H2
C O
C H4
C O2
C2
YH2
15
20
25
520 54 0 560 580
40
80
120
T(oC )
H E
G E
C gE
T OC
)
% ( E g C , E G , E H
180
270
360
450
)
m p p
( C OT
Fig. 3 – Effect of temperature upon gasification
characteristic of coal (a): Gas Fraction and YH2; (b) HE, GE,
CgE and TOC. (25 MPa, water flow rate 120 g/min, slurry
flow rate 12 g/min, 6 wt% coal D 2 wt% CMCD1 wt% K2CO3
coal particle<105 mm).
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yield of hydrogen increased from 12.28 mol to 22.75 mol/kg of
coal. GE increased from 44.32% to 60.71%.It is obtained that HE
was more than unity. It was proven that hydrogen element in
water was released to gaseous products. The yield of carbon
monoxide was almost negligible, which agrees with the
thermodynamics analysis. Potassium carbonate is proven to
be an effective catalyst to decrease the yield of carbon
monoxide to produce hydrogen [31].
From the theoreticalanalysis, there is theoccurrenceof two
competing reaction pathways in supercritical water: ionicpathway preferred at higher pressures and/or lower tempera-
tures and free radical degradation reaction pathways preferred
at lower pressuresand/or highertemperatures. It is commonly
acknowledged that hydrogen is produced in the free racial
pathways [32], so high temperature favors hydrogen produc-
tion reaction. According to the thermodynamic calculation,
there is deviation between experimental data and equilibrium
state, so high temperature accelerates the reaction velocity to
equilibrium state. Therefore, high temperature favors gasifi-
cation reaction and improves the gasification efficiency.
However, higher temperature means lower density of water
when the pressure is kept constant and lower density of water
inhibits the extraction reaction of volatile and hydrolysis
reaction. It is likely that it did not play a decisive role. Conse-
quently, high temperature favors gasification reaction.
4.2. Effect of pressure
23 MPa, 25 MPa and 27 MPa were selected to investigate the
effect of pressure. The experimental results are shown in
Fig. 4. The hydrogen fraction peaked when the pressure was
25 MPa, but the peak was not obvious. Generally speaking,
pressure had no significant effect upon coal gasificationcharacteristics in supercritical water within the experimental
region investigated.
The influences of pressure upon the gasification charac-
teristics are complicated. As mentioned in Section 4.1, high
pressure favors ionic reaction pathway, which inhibits gas
production reactions. In addition, higher pressure leads to
higher water density and higher ionic product, so hydrolysis
reactions, the extraction of volatile component from coal and
pyrolysis reactions are promoted and a higher coal conversion
could probably be obtained [9,33]. Therefore, higher pressure
favors gasification process. Higher pressure is not favorable
for gas formation according to the Le Chatelier’s principle
because the volume expansion during the gasification. Due to
60 90 120 150 1800
20
40
60
80
100a
b
)
% ( n o i t c
a r F s a G
HY
2
) gk / l om (
Flow rate of preheated water (g/min)
H2
C O
C H4
C O2
C2
Y H2
10
20
30
60 90 120 150 180
40
80
120
160
Flow rate of preheated w ater (g/min)
H E
C gE
G E
T O C
)
% ( E
G , E g C , E H
20 0
40 0
60 0
80 0
) m p p ( C OT
Fig. 5 – Effect of flow rate of preheated water upon
gasification characteristic of coal (a): Gas Fraction and YH2;
(b) HE, GE, CgE and TOC. (580 8C, 25 MPa, flow rate of
slurry[ 12 g/min, 6 wt% coalD 2 wt% CMCD 1 wt% K2CO3
coal particle<
105 m
m).
120 150 1800
20
40
60
80
100a
b
)
% ( n o i t c a r F s a G
HY
2
) gk / l o
m (
Flow rate of preheated water(g/min)
H2
C O
C H4
C O2
C2
YH2
0
5
10
15
20
25
120 150 1800
40
80
120
Flow rate of preheated w ater(g/min)
H E
C gE
G E
T O C )
% ( E
G , E g C , E H
0
80
160
240
) m p p (
C OT
Fig. 6 – Effect of flow rate of preheated water upon
gasification characteristic of coal (a): Gas Fraction and YH2;
(b) HE, GE, CgE and TOC (580 8C, 25 MPa, flow rate of
slurry:flow rate of water [ 1:10 6 wt% coal D 2 wt%
CMC D 1 wt %K2CO3, coal particle<105 mm).
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the combination of the multi-mechanism mentioned above,
pressure had no significant effect upon gasification charac-
teristics of coal in supercritical water seen from the experi-
mental results.
4.3. Effect of fluidizing velocity (I)
In the fluidized bed reactor, the flow of water preheated to
a certain temperature flows through the distributor, and itsvelocity is called fluidizing velocity. The effect of fluidizing
velocity or the flow rate the preheated water was investigated
from two aspects: (I) the feeding velocity of coal slurry is kept
constant; (II) the ratio of the feeding velocity of the coal slurry
to the fluidizing velocity is kept constant. In this section,
situation (I) is discussed first.
From the mechanism analysis, fluidizing velocity mainly
affects gasification characteristics in at least three ways: (1)
According to the calculations reported by Matsumura [34], the
fluidizing velocity investigated is kept above the minimum
fluidization velocity and below the terminal velocity of coal
particle. Higher fluidizing velocity led to more intense fluid-
ization state. Simultaneously, and better heat/mass transfer
rate, which favored hydrogen production and gasification
process was obtained [35]. (2) High fluidizing velocity means
shorter reactor residence time of the liquid intermediate
product which might cause the incompleteness of the gasifi-
cation process. (3) Different fluidizing velocity leads to
different coal slurryconcentration in the fluidized bed reactor.
When the feeding velocity of slurry is kept constant, higher
fluidizing velocity means lower concentration in the reactor,which favors the hydrogen production.
From the experimental result in Fig. 5, when the flow rate
of water increased from 60 to 150 g/min, the fraction of
hydrogen increased from 48.96% to 69.78%. The yield of
hydrogen increased from 7.80 mol to 32.26 mol/kg coal. HE
increased from 53.35% to 177.76%. The experimental result
with the HE more than 100% was obtained because the
hydrogen atoms in water was released to produce hydrogen
[30], e.g. as reaction (1).
4.4. Effect of fluidizing velocity (II)
In order to keep the concentration constant, we also investi-gated the effect of fluidizing velocity on the coal gasification
when the ratio of the feeding velocity of the coal slurry to the
fluidizing velocity was kept constant. Fig. 6 showed that as
the fluidizing velocity increased from 120 g/min to 180 g/min,
the hydrogen fraction decreased from 61.65% to 56.09%, yield
of hydrogen decreased from 22.74 g/min to 15.91 g/min and
TOC showed in the liquid residual increased from 195.7 ppm
to 217.2 ppm.
In the fluidized bed reactor, the irregular movement of bed
material causes the back-mixing of reactant and product.
According to Kruse’s research work [36,37], the back-mixing
active hydrogen present in all steps of degradation reaction
may lead to an inhibition of the unwanted polymerization viasaturation of free radicals. It means that more intense fluid-
ization state caused by higher fluidizing velocity may lead to
the production of small intermediates and inhibition of coke
or tar, so as to favors complete gasification reaction. However,
higher flow rate decreases the resident time of the liquid
residual and high-molecular-weight compounds may
decompose insufficiently [38]. Within the investigation of the
operating parameters, higher fluidizing velocity has negative
effect the hydrogen production reaction.
4.5. Effect of concentration
Fig. 7 showed that when the concentration of the slurryequaled 4 wt%, the hydrogen fraction and the gasification
efficiency were 63.02% and 70.12% respectively. The hydrogen
gasification efficiency was 145.04%. If the concentration of
coal slurry increases, the hydrogen fraction decreases while
the methane fraction increases. It can be seen that the
competition of hydrogen element between H2 and CH4, which
is not only similar to the regulation obtained by Antal [30] but
also consists with the thermodynamics analysis. As the
concentration increased, the gasification efficiency decreased.
The slurry of 22 wt% coal and 2 wt% CMC could be
continuously gasified in the fluidized bed reactor without
plugging problems. Take the case in 15th April 2009 as
example, the gasification system operated stably and the flow
H2
C O
C H4
C O2
C2
Y H2
4 8 12 16 20 240
20
40
60
80
100a
b
HY
2
) gk / l om (
)
% ( n o i t a r
F s a G
Concentrat ion(wt%)
0
10
20
30
4 8 12 16 20 24
40
80
120
160
Concentrat ion(wt%)
H E
C gE
G E
T OC
)
% ( E G ,
E g C , E H
0
200
400
600
) m p p
( C OT
Fig. 7 – Effect of concentration* upon gasification
characteristic of coal (a): Gas Fraction and YH2 (b) HE, GE,
CgE and TOC (580 8C, 25 MPa, water flow rate 120 g/min,
slurry flow rate 12 g/min coal particle<105 mm). * The
slurry concentration in this paper contained 2wt% CMC
and the amount of K2CO3 was not included.
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of coal slurry was pumped into the system in 12:25 after
35 min, we started to record the flow totalizer of gas yield and
flow totalizer of coal slurry as Fig. 8(a). It can be seen that from
13:00 to 13:50, the line of totalizer of gas yield is almost linear,
in another word, the gas yield was stable. Without the flowregulation of the plunger metering pump, the flow totalizer of
coal slurry was almost linear, which means that the system
pressure was stable. Seven Air bags of gaseous products were
collected. Their gas fraction can be seen in Fig. 8(b) and it can
be seen that the gas fraction didn’t have much deviation.
What’s more, it was not observed that the pressure drop
between the reactor exist and the pump outlet increased
during the operational process of the experiment. All the
above phenomena show that 24 wt% coal-water-slurry was
continuously transported and stably gasified without plugging
problems. On average, the hydrogen fraction and the gasifi-
cation efficiency were 52.15% and 29.56%, respectively. High
concentration means high handling capacity but high
concentration usually leads to incomplete gasification and
plugging problem [30]. So it is meaningful to find out the
optimal concentration of coal slurry.
4.6. Effect of diameter of coal particles
In the experiment investment, certain size range of the coal
particle is selected to ensure that the superficial velocity is
between the minimum fluidization velocity and the terminal
velocity. Smaller coal particle means more intense fluidiza-
tion state in the fluidized bed reactor. Moreover, smaller coal
particle is gasified completely more easily.
It is surprising to see in Fig. 9(a) that the coal particle size
had no significant effect on the gaseous product concentration
as seen in Fig. 9(a), while Fig. 9(b) shows that the smaller coal
particle favored gasification reaction. When the particle size
was <149 mm, the hydrogen yield and gasification efficiency
were 17.01 mol/kg and 47.43%, respectively. As the particle
13:00 13:10 13:20 13:30 13:40 13:50
2060
a
b
2070
2080
2090
2100
) g ( y r r u l s l
a o c f o r e z i l a t o t
w o l f
) L ( d l e i y s a g
f o r e z i l a t o t
w o l f
T ime(HH:MM)
f low tota l izer of gas yie ld
f low totalizer of coal slurry
37000
37200
37400
37600
1 2 3 4 5 6 7
0
20
40
60
80
100
)
% ( n o i t c a r F s a G
Order(1 )
C2
C O2
C H4
C O
H2
Fig. 8 – Effect of operation time upon gasification characteristic of coal (a) The flow tantalizer of gas yield and coal slurry in
different time (b) The gas fraction in different gas bag (580 8C, 25 MPa, water flow rate 120 g/min, slurry flow rate 12 g/min
coal particle<105 mm, 6 wt% coal D 2 wt% CMCD1 wt% K2CO3 ).
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size decreased to <105 mm, the amplitude of hydrogen yield
and gasification efficiency compared with <149 mm were
33.73% and 28.00%, respectively. When the particle size
decreased to <74 mm, the amplitude of hydrogen yield and
gasification efficiency compared with <105 mm were 0.74%
and 5.80% respectively. It is suggested that when the coal
particle was <105 mm, further grind of coal was not necessary.
4.7. Recycle of the liquid residual
The gasification characteristics of the liquid residual under
the reaction condition (580 C, 25 MPa, 120 g/min flow rate of preheated water, 12 g/min flow rate of coal slurry 6 wt%
coal þ 2 wt% CMC) was also studied. The liquid residual was
collected after the back-pressure regulator and recycled to the
continuous system without separation of oil and water-
soluble components. The study of the component is not
within the scope of this paper and its main component is
speculated to be phenolic compounds and aldehydes [4,39].
The TOC level of water-soluble components was measured to
be 195.7 ppm.
The gasification result of the residual compared with its
original slurry can be seen in Fig. 10(a). Gasification of the
residual liquid can obtain a gas with higher hydrogen fraction
(77.72%). It is likely that K2CO3 amount was kept at 1 wt% of
each feedstock, so the solubility of CO2 appeared to be higher
in the liquid residual due to dilution of the feedstock. There-
fore, the CO2 fraction was lower and facilitated the reaction (2)
to produce hydrogen.
Extra gasification efficiency, cold gas efficiency and
hydrogen yield were obtained in Fig. 10(b). Cold gas efficiency
and hydrogen yield increased from 45.78% to 22.75 mol/kg to
93.28% and 47.47 mol/kg. Meanwhile, the TOC level in the
final product was measuredto be 13.8 ppm. It is suggested that
the cycling of the liquid residual increases the gasification
efficiency.
5. Conclusions
Gasification in supercritical water was proven to be an effec-
tive and clean way for hydrogen production from coal:
(1) Thermodynamically, a multiphase model of coal gasifica-
tion was established based on the Gibbs free energy
minimum to predict the gas yield and its composition. The
production of coke appeared to be negligible and the
feasibility of complete gasification was confirmed.
(2) A novel coal supercritical water gasification system with
a fluidized bed reactor in SLMFL (State key Laboratory of
<149 <105 <740
20
40
60
80
100a
b
)
% ( n o i t c
a r F s a G
HY
2
) gk / l om (
Coal Diameter(µm )
H2
C O
C H4
C O2
C2
Y H2
0
5
10
15
20
<149 <105 <740
40
80
120
Coal Diame ter(µm )
H E
C gE
G E
T OC
)
% ( E G ,
E g C , E H
0
70
140
210
) m p p (
C OT
Fig. 9 – Effect of coal diameter upon gasification
characteristic of coal (a): Gas Fraction and YH2(b) HE, GE,
CgE and TOC (580 8C, 25 MPa, water flow rate 120 g/min,
slurry flow rate 12 g/min, 6 wt% coalD 2 wt% CMCD 1 wt%
K2CO3 ).
H 2 CO C H4 CO 2 C20
20
40
60
80a
b
)
% ( n o i t c a r F s a G
Gasous spec ies
1 st
2 nd
HE CE G E C gE YH 20
100
200
300
HY
2
) gk / l om (
% ) E g C , E
G , E C , E H (
2 nd
1 s t
Gasification Characteristics
0
100
200
300
Fig. 10 – Recycle of the gasification liquid residual (a): Gas
Fraction (b) HE, GE, CgE, CE and YH2 (580 8C, 25 MPa, water
flow rate 120 g/min, slurry flow rate 12 g/min, coal particle
<105 mm, 6 wt% coal D 2 wt% CMC D 1 wt% K2CO3 ).
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Multiphase Flow in power engineering) was adopted to
converse coal slurry to hydrogen-rich gas. A high concen-
tration with 24 wt% coal-water-slurry was successfully
gasified.
(3) Effects of temperature, pressure, flow rate, concentration
and diameter on gasification characteristics were experi-
mentally investigated. Higher temperature favored
hydrogen production and gasification reaction. Pressurehad little significant effect on gasification characteristics.
When the coal particle of coal is <105 mm, further grind is
not necessary.
(4) The liquid residual was recycled for coal gasification and
produced gases with the hydrogen fraction of 77.72%. The
TOC of the liquid residual from the recycled gasification
was 13.8 ppm. It is suggested that a system with the
recycling liquid residual may increase the gasification
efficiency and further decrease the TOC level.
Acknowledgements
This work is currently supported by the National Key Project
for Basic Research of China through Contract No.
2009CB220000 and the National High Technology Research
and Development Program of China (863 Program) through
contract No. 2007AA05Z147.
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