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.




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.

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 7153



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).




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

)


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).




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.




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

)


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 ).




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

)


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 ).




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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Hydrogen Production by Coal Gasification in Supercritical Water With a Fluidized Bed Reactor