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Hydrogen production by coal gasification in supercritical water with a fluidized bed reactor
international journal of hydrogen energy 35 (2010) 7151–7160
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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
article info
abstract
Article history:
The technology of supercritical water gasification of coal can converse coal to hydrogen-
Received 16 November 2009
rich gaseous products effectively and cleanly. However, the slugging problem in the
Received in revised form
tubular reactor is the bottleneck of the development of continuous large-scale hydrogen
21 January 2010
production from coal. The reaction of coal gasification in supercritical water was analyzed
Accepted 22 January 2010
from the point of view of thermodynamics. A chemical equilibrium model based on Gibbs
Available online 4 March 2010
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
Keywords:
gasification system with a fluidized bed reactor was applied to investigate the gasification
Coal
of coal in supercritical water. 24 wt% coal-water-slurry was continuously transported and
Supercritical water
stably gasified without plugging problems; a hydrogen yield of 32.26 mol/kg was obtained
Gasification
and the hydrogen fraction was 69.78%. The effects of operational parameters upon the
Hydrogen production
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 society for more than a century [1]. However, as a solid fuel coal is difficult to handle and NOx and SOx are produced during the burning process [2]. With a strong demand for an affordable 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 supercritical 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 hydrogenrich 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). 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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HE
Nomenclature GE CE
gasification efficiency, mass of gaseous product/ mass of dry matter in the water-coal slurry carbon gasification efficiency, mass of carbon element in gaseous product/mass of carbon in dry matter in the water-coal slurry
gasified in supercritical water in an autoclave and highheating-value gas was produced. No significant char was found [16]. Lin proposed a novel HyPr-RING method to produce hydrogen from lignite, subbituminous, and bituminous 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 molecule gases, which led to the decrease of the residual char. Vostrikov found that the coal gasification reaction was a weakly endothermic process and experimentally investigated combustion of single coal particles in H2O/O2 supercritical 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-efficient and environmentally clean working media of steam–gas power plants [15,20,21]. Yamaguchi [22] investigated the noncatalytic 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 supercritical water. The slurry of 16 wt% coal þ 1.5 wt% CMC (sodium carboxymethyl cellulose) was successfully transported into the reactor and continuously gasified in supercritical water in the system. However, plugging problem inhibited 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
YH2 CgE
hydrogen gasification efficiency, mass of hydrogen gas/the mass of hydrogen in dry matter in the water-coal slurry yield of hydrogen, the mass of certain gas product/ the mass of dry matter in feedstock cold gas efficiency, chemical energy content in the product gas/the chemical energy in the fuel (based on the lower-heating-value)
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 gasification 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]. It is demonstrated that 24 wt% coal-water-slurry was 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 composition of the mineral constitutes the process of coal gasification 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
CO(g) þ H2O(g) / CO2(g) þ H2(g)
CO(g) þ 3H2(g) / CH4(g) þ H2O(g)
DH ¼ 41 kJ/mol
DH ¼ 206 kJ/mol
(1)
(2)
(3)
Chemical equilibrium model based on Gibbs free energy minimization was adopted to analyze the gaseous product
Table 1 – Analysis data of the Shenmu coal. Elemental analysis (wt%) Species Shenmu coal a Difference.
C 69.63
H 3.75
N 0.80
S 0.41
Proximate analysis (wt%) Oa 12.25
M 5.31
A 7.85
V 30.92
Qb, ad FC 55.92
(MJ/kg) 27.826
international journal of hydrogen energy 35 (2010) 7151–7160
0 .0 1 0
CO2
60
CH4
0 .0 0 8
H2 CO
0 .0 0 6 30 0 .0 0 4
0 .0 0 2
0 0
G a s Y ie ld ( m o l/k g )
b
5 10 15 C o n c e n tra tio n (w t% )
20
160 H2
120
CO CH4
80
CO2
40
C O fr a c tio n ( % )
H 2 ,C H 4 ,C O 2 f r a c t io n ( % )
a
7153
yields and their fraction. When the equation of the conservation 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 calculation, 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 and reaction (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.
0 0
5 10 15 C o n c e n tra tio n (w t% )
20
Fig. 1 – Effect of concentration upon gasification equilibrium products from coal: (a) Gas Fraction; (b) Gas Yield. (Temperature, 500 8C; Pressure, 25 MPa).
3.
Apparatus and experimental procedures
The experimental study was performed in a coal gasification system with a fluidized bed reactor and the schematic diagram of system is shown in Fig. 2. The reactor is constructed 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
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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H2
a 100
CO CH4
25
YH 2
60 20 40 15
20
Y H 2 ( m o l/k g )
G a s F ra c tio n (% )
C2
CO C H4
80 60 40
10
0
520
540
560
0 23
580
25
27
P re s s u re (M P a )
o
T( C)
b
C2 YH 2
20
0
C O2
20
Y H 2( m o l/k g )
G a s F r a c tio n ( % )
CO2
80
H2
a 100
b
450
HE CgE
210
GE TOC
360
80 270
40 540
o
560
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).
70 40
0 23
580
T( C)
80
0
180 520
140
T O C (p p m )
HE GE CgE TOC
H E ,C g E ,G E ( % )
120 T O C (p p m )
H E ,G E ,C g E (% )
120
25 P re s s u re (M P a )
27
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).
was 10 ml/min. The total carbon contents of the liquid phase were determined using Elemental High TOCII. 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 distributor. 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 literature [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 and separated 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 suspending 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 Chemical 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
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) were applied 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. It can be seen 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
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international journal of hydrogen energy 35 (2010) 7151–7160
H2
a 100
CO C H4
25
80
20
60
15
40
10
20
5
30
20
40 20
10
0
60 90 120 150 180 F lo w ra te o f p re h e a te d w a te r (g /m in )
b
240
40
H E ,C g E ,G E ( % )
400
160
80 80
T O C (p p m )
GE TOC
T O C (p p m )
600
80
180
HE C gE GE TOC
120
160 H E ,C g E ,G E (% )
150
F lo w ra te o f p re h e a te d w a te r(g /m in )
800
120
C2
0 120
HE C gE
C O2 Y H2
0
b
CO C H4
Y H 2 ( m o l/k g )
60
YH 2
G a s F r a c tio n ( % )
C2
Y H 2 ( m o l/k g )
G a s F r a c tio n (% )
C O2
80
H2
a 100
40
200 60
90
120
150
120
F lo w ra te o f p re h e a te d w a te r (g /m in )
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% coal D 2 wt% CMC D 1 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 theoretical analysis, there is the occurrence of two competing reaction pathways in supercritical water: ionic pathway preferred at higher pressures and/or lower temperatures and free radical degradation reaction pathways preferred at lower pressures and/or higher temperatures. It is commonly acknowledged that hydrogen is produced in the free racial pathways [32], so high temperature favors hydrogen production 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 gasification 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
0
0
180
150
180
F lo w ra te o f p re h e a te d w a te r(g /m in ) 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).
reaction. It is likely that it did not play a decisive role. Consequently, 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 gasification characteristics in supercritical water within the experimental region investigated. The influences of pressure upon the gasification characteristics 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
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a 100
H2
30
CO C H4 C O2 C2 Y H2
20 60 40 10 20 0
Y H 2 ( m o l/k g )
G a s F r a tio n ( % )
80
0 4
8
12
16
20
24
C o n c e n tra tio n (w t% )
600
120
HE CgE GE TOC
400
80 200
T O C (p p m )
H E ,C g E ,G E (% )
b 160
40 0 4
8
12
16
20
24
C o n c e n tra tio n (w t% )
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.
the combination of the multi-mechanism mentioned above, pressure had no significant effect upon gasification characteristics of coal in supercritical water seen from the experimental results.
4.3.
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 gasification process. (3) Different fluidizing velocity leads to different coal slurry concentration 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 investigated 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 via saturation of free radicals. It means that more intense fluidization 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.
Effect of fluidizing velocity (I) 4.5.
In the fluidized bed reactor, the flow of water preheated to a certain temperature flows through the distributor, and its velocity 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 fluidization state. Simultaneously, and better heat/mass transfer
Effect of concentration
Fig. 7 showed that when the concentration of the slurry equaled 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
international journal of hydrogen energy 35 (2010) 7151–7160
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a flo w to ta liz e r o f g a s yie ld 37600
flo w to ta liz e r o f c o a l s lu rry 2090
37400 2080
2070
37200
2060 1 3 :0 0
1 3 :1 0
1 3 :2 0
1 3 :3 0
1 3 :4 0
flo w to ta liz e r o f c o a l s lu r r y ( g )
flo w to t a liz e r o f g a s y ie ld ( L )
2100
37000 1 3 :5 0
T im e (H H :M M )
b
100
G a s F r a c tio n ( % )
80 C2 CO2
60
CH4 CO H2
40
20
0 1
2
3
4 O rd e r(1 )
5
6
7
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).
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 flow regulation 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 gasification 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 fluidization 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
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CO C H4
G a s F r a c tio n ( % )
C2 Y H2
60
15
40
10
20
5
Y H 2(m o l/k g )
<149
<105
b
HE CgE GE TOC
120
2nd
40
20
H2
80 70 40
0
C2
0
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.
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 watersoluble 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
2nd 1st
200
200
100
100
0
0
<105 <74 C o a l D ia m e te r( µ m )
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% coal D 2 wt% CMC D 1 wt% K2CO3).
300
Y H 2 ( m o l/ k g )
140
CO CH4 CO2 G a s o u s s p e c ie s
b 300
210
T O C (p p m )
H E ,C g E ,G E (% )
1st
60
<74
C o a l D ia m e te r( µ m )
<149
80
0
0
0
(H E ,C E ,G E ,C g E ) %
G a s F r a c tio n ( % )
C O2
20
80
4.7.
a
H2
a 100
HE
CE
GE
CgE
YH2
G a s ific a tio n C h a ra c te ris tic s 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).
each feedstock, so the solubility of CO2 appeared to be higher in the liquid residual due to dilution of the feedstock. Therefore, 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 measured to 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 effective and clean way for hydrogen production from coal: (1) Thermodynamically, a multiphase model of coal gasification 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
international journal of hydrogen energy 35 (2010) 7151–7160
Multiphase Flow in power engineering) was adopted to converse coal slurry to hydrogen-rich gas. A high concentration with 24 wt% coal-water-slurry was successfully gasified. (3) Effects of temperature, pressure, flow rate, concentration and diameter on gasification characteristics were experimentally investigated. Higher temperature favored hydrogen production and gasification reaction. Pressure had 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.
references
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Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
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
article info
abstract
Article history:
The technology of supercritical water gasification of coal can converse coal to hydrogen-
Received 16 November 2009
rich gaseous products effectively and cleanly. However, the slugging problem in the
Received in revised form
tubular reactor is the bottleneck of the development of continuous large-scale hydrogen
21 January 2010
production from coal. The reaction of coal gasification in supercritical water was analyzed
Accepted 22 January 2010
from the point of view of thermodynamics. A chemical equilibrium model based on Gibbs
Available online 4 March 2010
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
Keywords:
gasification system with a fluidized bed reactor was applied to investigate the gasification
Coal
of coal in supercritical water. 24 wt% coal-water-slurry was continuously transported and
Supercritical water
stably gasified without plugging problems; a hydrogen yield of 32.26 mol/kg was obtained
Gasification
and the hydrogen fraction was 69.78%. The effects of operational parameters upon the
Hydrogen production
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 society for more than a century [1]. However, as a solid fuel coal is difficult to handle and NOx and SOx are produced during the burning process [2]. With a strong demand for an affordable 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 supercritical 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 hydrogenrich 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). 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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HE
Nomenclature GE CE
gasification efficiency, mass of gaseous product/ mass of dry matter in the water-coal slurry carbon gasification efficiency, mass of carbon element in gaseous product/mass of carbon in dry matter in the water-coal slurry
gasified in supercritical water in an autoclave and highheating-value gas was produced. No significant char was found [16]. Lin proposed a novel HyPr-RING method to produce hydrogen from lignite, subbituminous, and bituminous 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 molecule gases, which led to the decrease of the residual char. Vostrikov found that the coal gasification reaction was a weakly endothermic process and experimentally investigated combustion of single coal particles in H2O/O2 supercritical 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-efficient and environmentally clean working media of steam–gas power plants [15,20,21]. Yamaguchi [22] investigated the noncatalytic 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 supercritical water. The slurry of 16 wt% coal þ 1.5 wt% CMC (sodium carboxymethyl cellulose) was successfully transported into the reactor and continuously gasified in supercritical water in the system. However, plugging problem inhibited 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
YH2 CgE
hydrogen gasification efficiency, mass of hydrogen gas/the mass of hydrogen in dry matter in the water-coal slurry yield of hydrogen, the mass of certain gas product/ the mass of dry matter in feedstock cold gas efficiency, chemical energy content in the product gas/the chemical energy in the fuel (based on the lower-heating-value)
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 gasification 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]. It is demonstrated that 24 wt% coal-water-slurry was 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 composition of the mineral constitutes the process of coal gasification 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
CO(g) þ H2O(g) / CO2(g) þ H2(g)
CO(g) þ 3H2(g) / CH4(g) þ H2O(g)
DH ¼ 41 kJ/mol
DH ¼ 206 kJ/mol
(1)
(2)
(3)
Chemical equilibrium model based on Gibbs free energy minimization was adopted to analyze the gaseous product
Table 1 – Analysis data of the Shenmu coal. Elemental analysis (wt%) Species Shenmu coal a Difference.
C 69.63
H 3.75
N 0.80
S 0.41
Proximate analysis (wt%) Oa 12.25
M 5.31
A 7.85
V 30.92
Qb, ad FC 55.92
(MJ/kg) 27.826
international journal of hydrogen energy 35 (2010) 7151–7160
0 .0 1 0
CO2
60
CH4
0 .0 0 8
H2 CO
0 .0 0 6 30 0 .0 0 4
0 .0 0 2
0 0
G a s Y ie ld ( m o l/k g )
b
5 10 15 C o n c e n tra tio n (w t% )
20
160 H2
120
CO CH4
80
CO2
40
C O fr a c tio n ( % )
H 2 ,C H 4 ,C O 2 f r a c t io n ( % )
a
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yields and their fraction. When the equation of the conservation 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 calculation, 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 and reaction (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.
0 0
5 10 15 C o n c e n tra tio n (w t% )
20
Fig. 1 – Effect of concentration upon gasification equilibrium products from coal: (a) Gas Fraction; (b) Gas Yield. (Temperature, 500 8C; Pressure, 25 MPa).
3.
Apparatus and experimental procedures
The experimental study was performed in a coal gasification system with a fluidized bed reactor and the schematic diagram of system is shown in Fig. 2. The reactor is constructed 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
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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H2
a 100
CO CH4
25
YH 2
60 20 40 15
20
Y H 2 ( m o l/k g )
G a s F ra c tio n (% )
C2
CO C H4
80 60 40
10
0
520
540
560
0 23
580
25
27
P re s s u re (M P a )
o
T( C)
b
C2 YH 2
20
0
C O2
20
Y H 2( m o l/k g )
G a s F r a c tio n ( % )
CO2
80
H2
a 100
b
450
HE CgE
210
GE TOC
360
80 270
40 540
o
560
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).
70 40
0 23
580
T( C)
80
0
180 520
140
T O C (p p m )
HE GE CgE TOC
H E ,C g E ,G E ( % )
120 T O C (p p m )
H E ,G E ,C g E (% )
120
25 P re s s u re (M P a )
27
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).
was 10 ml/min. The total carbon contents of the liquid phase were determined using Elemental High TOCII. 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 distributor. 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 literature [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 and separated 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 suspending 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 Chemical 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
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) were applied 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. It can be seen 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
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international journal of hydrogen energy 35 (2010) 7151–7160
H2
a 100
CO C H4
25
80
20
60
15
40
10
20
5
30
20
40 20
10
0
60 90 120 150 180 F lo w ra te o f p re h e a te d w a te r (g /m in )
b
240
40
H E ,C g E ,G E ( % )
400
160
80 80
T O C (p p m )
GE TOC
T O C (p p m )
600
80
180
HE C gE GE TOC
120
160 H E ,C g E ,G E (% )
150
F lo w ra te o f p re h e a te d w a te r(g /m in )
800
120
C2
0 120
HE C gE
C O2 Y H2
0
b
CO C H4
Y H 2 ( m o l/k g )
60
YH 2
G a s F r a c tio n ( % )
C2
Y H 2 ( m o l/k g )
G a s F r a c tio n (% )
C O2
80
H2
a 100
40
200 60
90
120
150
120
F lo w ra te o f p re h e a te d w a te r (g /m in )
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% coal D 2 wt% CMC D 1 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 theoretical analysis, there is the occurrence of two competing reaction pathways in supercritical water: ionic pathway preferred at higher pressures and/or lower temperatures and free radical degradation reaction pathways preferred at lower pressures and/or higher temperatures. It is commonly acknowledged that hydrogen is produced in the free racial pathways [32], so high temperature favors hydrogen production 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 gasification 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
0
0
180
150
180
F lo w ra te o f p re h e a te d w a te r(g /m in ) 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).
reaction. It is likely that it did not play a decisive role. Consequently, 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 gasification characteristics in supercritical water within the experimental region investigated. The influences of pressure upon the gasification characteristics 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
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international journal of hydrogen energy 35 (2010) 7151–7160
a 100
H2
30
CO C H4 C O2 C2 Y H2
20 60 40 10 20 0
Y H 2 ( m o l/k g )
G a s F r a tio n ( % )
80
0 4
8
12
16
20
24
C o n c e n tra tio n (w t% )
600
120
HE CgE GE TOC
400
80 200
T O C (p p m )
H E ,C g E ,G E (% )
b 160
40 0 4
8
12
16
20
24
C o n c e n tra tio n (w t% )
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.
the combination of the multi-mechanism mentioned above, pressure had no significant effect upon gasification characteristics of coal in supercritical water seen from the experimental results.
4.3.
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 gasification process. (3) Different fluidizing velocity leads to different coal slurry concentration 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 investigated 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 via saturation of free radicals. It means that more intense fluidization 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.
Effect of fluidizing velocity (I) 4.5.
In the fluidized bed reactor, the flow of water preheated to a certain temperature flows through the distributor, and its velocity 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 fluidization state. Simultaneously, and better heat/mass transfer
Effect of concentration
Fig. 7 showed that when the concentration of the slurry equaled 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
international journal of hydrogen energy 35 (2010) 7151–7160
7157
a flo w to ta liz e r o f g a s yie ld 37600
flo w to ta liz e r o f c o a l s lu rry 2090
37400 2080
2070
37200
2060 1 3 :0 0
1 3 :1 0
1 3 :2 0
1 3 :3 0
1 3 :4 0
flo w to ta liz e r o f c o a l s lu r r y ( g )
flo w to t a liz e r o f g a s y ie ld ( L )
2100
37000 1 3 :5 0
T im e (H H :M M )
b
100
G a s F r a c tio n ( % )
80 C2 CO2
60
CH4 CO H2
40
20
0 1
2
3
4 O rd e r(1 )
5
6
7
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).
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 flow regulation 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 gasification 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 fluidization 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
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CO C H4
G a s F r a c tio n ( % )
C2 Y H2
60
15
40
10
20
5
Y H 2(m o l/k g )
<149
<105
b
HE CgE GE TOC
120
2nd
40
20
H2
80 70 40
0
C2
0
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.
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 watersoluble 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
2nd 1st
200
200
100
100
0
0
<105 <74 C o a l D ia m e te r( µ m )
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% coal D 2 wt% CMC D 1 wt% K2CO3).
300
Y H 2 ( m o l/ k g )
140
CO CH4 CO2 G a s o u s s p e c ie s
b 300
210
T O C (p p m )
H E ,C g E ,G E (% )
1st
60
<74
C o a l D ia m e te r( µ m )
<149
80
0
0
0
(H E ,C E ,G E ,C g E ) %
G a s F r a c tio n ( % )
C O2
20
80
4.7.
a
H2
a 100
HE
CE
GE
CgE
YH2
G a s ific a tio n C h a ra c te ris tic s 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).
each feedstock, so the solubility of CO2 appeared to be higher in the liquid residual due to dilution of the feedstock. Therefore, 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 measured to 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 effective and clean way for hydrogen production from coal: (1) Thermodynamically, a multiphase model of coal gasification 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
international journal of hydrogen energy 35 (2010) 7151–7160
Multiphase Flow in power engineering) was adopted to converse coal slurry to hydrogen-rich gas. A high concentration with 24 wt% coal-water-slurry was successfully gasified. (3) Effects of temperature, pressure, flow rate, concentration and diameter on gasification characteristics were experimentally investigated. Higher temperature favored hydrogen production and gasification reaction. Pressure had 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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