Combined Coal Gasification and Methane Reforming for Production of

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Energy & Fuels 2005, 19, 512-516

Combined Coal Gasification and Methane Reforming for Production of Syngas in a Fluidized-Bed Reactor Jinhu Wu,* Yitian Fang, and Yang Wang Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan, 030001, People Republic of China

Dong-ke Zhang Centre for Fuels and Energy, Curtin University of Technology, G.P.O. Box U1987, Perth, WA 6845, Australia Received June 22, 2004. Revised Manuscript Received November 21, 2004

The concept of combining coal gasification and methane reforming for syngas production in a fluidized-bed reactor, as a means for utilization of both coal and coal-bed methane, was proposed and evaluated experimentally. Preliminary experiments were performed in a laboratory-scale fluidized-bed reactor operating at ∼1000 °C, using two Chinese coals: a bituminous coal (Coal A) and an anthracite (Coal B). To demonstrate combined coal gasification and methane reforming, the reactor was fed with 1.5-2.5 kg/h coal, 0.4-0.5 Nm3/h methane, 1.45-2.0 Nm3/h oxygen, 0.4 Nm3/h air, and 2.0-2.4 kg/h steam. Up to 90% methane conversion and 70% carbon conversion were achieved, and the resulting syngas had a composition of 35%-39% H2, 31%-34% CO, and 20%-22% CO2, balanced by N2. For comparison, the results of the gasification of Coal A and Coal B without added methane were also reported. These results proved the feasibility of such an integrated coal gasification and methane reforming process. Further kinetic studies and process development opportunities were also identified.

1. Introduction The production of syngas (H2 and CO), using fossil fuels as the primary energy sources, is the first step of synthesis of many chemicals. Coal gasification and natural gas reforming are two dominant means for the production of syngas and form the backbone of the modern chemical industries. The downstream synthesis of different chemicals requires syngas with different H2/ CO ratios. The H2/CO ratio of a syngas is usually dictated by the H/C ratio of the primary feedstock and the reaction routes of the syngas production. For example, steam gasification of coal will produce a syngas with a H2/CO ratio closer to 1, whereas steam reforming of methane will produce a syngas with a H2/CO ratio closer to 3. The production of syngas from methane can be realized by the following three reactions:

CH4 + H2O ) CO + 3H2 (∆H°298K ) 205.9 kJ/mol) (1) CH4 + CO2 ) 2CO + 2H2 (∆H°298K ) 247.1 kJ/mol) (2) CH4 + 0.5O2 ) CO + 2H2 (∆H°298K ) -35.9 kJ/mol) (3) * Author to whom correspondence should be addressed. Telephone: +86-351-4068345. Fax: +86-351-4048313. E-mail address: wujh@ sxicc.ac.cn.

which are also known as steam reforming, CO2 reforming, and partial oxidation reforming, respectively. Different combinations of these three basic reforming reactions are possible to achieve different objectives. Until now, steam reforming of natural gas is the most mature and widely used technology for methane-based syngas production.1-3 However, this process has several inherent drawbacks. As shown by reaction 1, the H2/ CO ratio in the syngas is theoretically 3, which is usually too high, in comparison with what is required by many downstream synthesis processes. To avoid carbon deposition on the catalyst, excess steam (more than the stoichiometric requirement) must be used.2,4 This causes higher operation costs. In addition, the reactor, which is of the high-temperature tubular heatexchanger type, is often inefficient and very expensive. Recently, much attention has been given to CO2 reforming,5 because it has the potential advantages of lower theoretical H2/CO ratios and reuse of CO2, as suggested by reaction 2 (mentioned previously). However, a critical difficulty of this process is its greater potential for carbon deposition, which rapidly deactivates the catalyst.2-8 (1) Cheng, W.-H.; Kung, H. H. In Methanol Production and Use; Cheng, W.-H., Kung, H. H., Eds.; Marcel Dekker: New York, 1994; pp 1-18. (2) LeBlanc, J. R.; Schneider, R. V.; Strait, R. B. In Methanol Production and Use; Cheng, W.-H., Kung, H. H., Eds.; Marcel Dekker: New York, 1994; pp 51-131. (3) Sdris, S. M.; Pruden, B. B.; Lim, C. J.; Grace, J. R. Can. J. Chem. Eng. 1996, 74, 177-186. (4) Armor, A. N. Appl. Catal., A 1999, 176, 159-176. (5) Qin, D.; Lapszewicz, J. Catal. Today 1994, 21, 551-560.

10.1021/ef049853t CCC: $30.25 © 2005 American Chemical Society Published on Web 02/05/2005

Production of Syngas in a Fluidized-Bed Reactor

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Figure 1. Flow sheet of the laboratory-scale fluidized-bed coal and methane co-conversion system. Legend is as follows: 1, gas cylinders; 2, gas rotameters; 3, distilled water tank; 4, rotameter for water flow rate; 5, steam generator and mixer; 6, coal hopper; 7, screw feeder for metering the coal feed rate; 8, pushing feeder; 9, motor; 10, the reactor; 11, gas distributor; 12, ash discharging valve; 13, first ash hopper; 14, second ash hopper; 15, cyclone; 16, first fly ash hopper; 17, second fly ash hopper; 18, gas cleaner/ cooler; 19, liquid and gas separator; 20, driving belt; and 21, air compressor.

The CH4 partial oxidation reforming, as shown by reaction 3, can theoretically yield a syngas with a H2/ CO ratio of 2, which is suitable for many downstream synthesis processes. There are two ways to reform CH4 to syngas via partial oxidation, namely, noncatalytic and catalytic partial oxidation. The noncatalytic process operates under the conditions of 30-100 atm and ∼1573 K.4 The high operating temperatures mean high operation costs. In contrast, the catalytic process can operate at relative lower temperatures. Because of efficiency and economics, it has been considered to be the most promising CH4 reforming process for the future. However, a truly catalytic partial oxidation process is now still limited to the laboratory stages, largely because of the lack of durable catalysts.1,2,6,9 In this paper, we introduce the concept of combined coal gasification and coal-bed methane reforming in a fluidized-bed reactor as an alternative path for syngas production. Based on the features of natural gas reforming and steam gasification of coal, it was suspected that the partially reacted coal char and/or the coal ash might exert a catalytic effect on the partial oxidation of the gas. This may be best effected in a fluidized-bed reactor operating at ∼1000 °C, in which the bed of char undergoes gasification while it catalyzes the methanereforming reactions. The 1000 °C operating temperature is common for fluidized-bed coal gasification.10 This (6) Richardson, J. T. Principles of Catalyst Development; Plenum Press: New York, 1989. (7) Xu, G.; Shi, K.; Gao, Y.; Xu, H.; Wei, Y. J. Mol. Catal. A: Chem. 1999, 147, 47-54. (8) Tang, S. B. Catal. Today 1995, 24, 253-255. (9) Zhu, J. A Feasibility Study of Methane Reforming by Partial Oxidation, Ph.D. Thesis, Curtin University of Technology, Perth, Australia, 2001. (10) Kimura, T.; Kojima, T. Chem. Eng. Sci. 1992, 47, 2529-2534.

technology, if developed, may provide an effective means to utilize both coal and coal-bed methane with a significant environmental advantage. This paper reports preliminary experiments performed on a laboratoryscale fluidized-bed reactor and discusses the feasibility of the combined coal gasification and methane reforming. 2. Experimental Section The combined coal gasification and methane reforming experiments were conducted on an autothermal fluidized-bed reactor, as schematically shown in Figure 1. The reactor is made of a stainless-steel alloy (25Cr20Ni), with a inner diameter (ID) of 145 mm and height of 1500 mm. The gas distributor of the fluidized bed is a perforated stainless steel with 138 holes, each 1.5 mm in diameter. The system includes a coal feeding device, gas supply and distribution, the fluidizedbed reactor, gas cooling and dust removal, flow measurement and control equipment. The coal feeding system involves two screw feeders; the first one measures the solid flow rate, whereas the second pushes the coal particles into the reactor. Industrial-grade oxygen, steam, nitrogen, and methane (used to simulate coal-bed methane) are metered and supplied to the reactor through both a gas distributor and the ash discharge pipe. Steam is generated by injecting and evaporating water in an electrically heated tube furnace. Constant steam flow rates can be maintained by controlling the water injection rate with a rotameter. The ash is discharged from the bottom of the reactor. By opening the rotary valve, the ash is discharged continually into an ash hopper. The exit gas stream leaves the reactor from the top, which first enters a cyclone to remove the fine particulates and then passes through a water scrubber, where the remaining ash particles are captured and the gas is cooled at the same time. The reactor system is equipped with necessary instruments for temperature, pressure, and flow rate monitoring and control.

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Table 1. Coal Analysis Data

Table 2. Typical Results of Coal Gasification When Coal A is Used

Value analysis

A

Proximate Analysis (As-Received) moisture (wt %) 2.52 ash (wt %) 10.14 volatile matter, VM (wt %) 24.43 fixed carbon, FC (wt %) 62.91 low heating value (MJ/kg) 29.09 C (wt %) H (wt %) O (wt %) N (wt %) S (wt %)

Ultimate Analysis (As-Received) 69.94 3.85 12.73 0.36 0.46

Ash Characteristics deformation temp. (°C) 1160 sintering temp. (°C) 1210 fusion temp. (°C) 1300

Exp. A-1

Exp. A-2

Exp. A-3

coal feed rate (kg/h) N2 flow rate (Nm3/h) O2 flow rate (Nm3/h) O2/coal ratio (Nm3/kg) steam flow rate (kg/h) steam/coal ratio (kg/kg) temp. (°C)

property/characteristic

3.06 0.20 1.50 0.49 2.4 0.78 960

3.06 0.20 1.50 0.49 2.4 0.78 940

3.06 0.20 1.50 0.49 2.4 0.78 951

gas production rate (Nm3/kg)

1.84

1.89

1.79

gas composition CO2 (dry basis, vol %) H2 (dry basis, vol %) N2 (dry basis, vol %) CO (dry basis, vol %) CH4 (dry basis, vol %)

20.58 36.67 3.90 37.14 1.71

20.60 37.67 3.81 37.43 0.48

20.64 37.21 4.01 36.48 1.66

B 2.06 27.82 8.93 61.19 22.84 61.49 2.83 3.45 0.89 1.46 1500 >1500 >1500

The reaction temperature is controlled manually by adjusting feed gas composition and flow rates and coal feed rate. Two Chinese coals were selected for the experiments. Coal A is a bituminous coal, and Coal B is an anthracite. The coals were provided by two coal producers in China’s Shaanxi and Shanxi provinces, respectively; identical samples can be obtained by contacting the authors. The proximate and ultimate analyses of these two coals are given in Table 1. The raw coal is crushed to a size fraction of 0-1 mm with a jaw crusher. After air-drying in an electrically heated oven at ∼200 °C, the coal was placed in a hopper that was located above the reactor. During an experiment, the reactor is preheated by electrical heating to 800 °C under the protection of N2. Meanwhile, the steam evaporator is preheated to 600 °C. After the desired reactor temperature is reached, the protective N2 flow is switched to air and steam flow is activated. In the meantime, the coal feeding system is commenced. By adjusting the flow rates of coal, air, and steam, the temperature of the reactor is increased gradually. The bed material inventory is determined by measuring the decrease in bed pressure, using a U-tube, and it can be controlled by adjusting the rate of coal feed and of ash discharge. When the reactor temperature reaches ∼900 °C and the material inventory in the bed reaches ∼1.0 kg, the electrical heating is switched off. The system is kept at a steady temperature for ∼30 min and then oxygen feed is switched on, to replace the air. After the steam/oxygen-blown coal gasification has operated in the steady state for ∼1-2 h, CH4 is added into the system through the ash discharging pipe at the bottom of the reactor. To keep the temperature constant and steady, the operation parameters must be readjusted. After the entire system becomes steady again, operating parameters and experimental data are collected. In the reactor, the dry coal reacts with methane, oxygen, and steam, producing CO, H2, and CO2. In the reactor, some of the oxygen and steam enters the reaction zone through the inverse-cone-shaped gas distributor, to maintain fluidization, and the remaining oxygen and steam, as well as methane, enter the reactor through the ash discharging pipe. A small amount of N2 (0.016-0.30 m3/h) is fed to the reactor, also through the ash discharging pipe, to adjust the temperature and to prevent ash sintering and slagging. The composition of the exit gas stream is analyzed using a gas chromatograph. The experiments are repeated several times for each of the two coals under the same conditions.

3. Results and Discussion First, the gasification of the bituminous coal (Coal A) was investigated. Table 2 summarizes the typical results. The reactor was fed with ∼3.0 kg/h coal, 1.5

gas heating valuea (MJ/Nm3)

10.0

9.7

10.0

carbon conversion (%)

84

85

81

a

Lower heating value, LHV.

Table 3. Typical Results of Combined Coal Gasification and Methane Reforming When Coal A is Used Exp. A-1

Exp. A-2

Exp. A-3

coal feed rate (kg/h) CH4 flow rate (Nm3/h) CH4/coal ratio (Nm3/kg) N2 flow rate (Nm3/h) O2 flow rate (Nm3/h) O2/coal ratio (Nm3/kg) steam flow rate (kg/h) steam/coal ratio (kg/kg) temp. (°C)

property/characteristic

1.54 0.5 0.3 0.30 1.65 1.07 2.4 1.56 1000

1.58 0.5 0.3 0.30 1.55 0.98 2.4 1.52 1007

1.58 0.5 0.3 0.31 1.45 0.92 2.4 1.52 1021


2.82

2.62

2.73

gas compositions CO2 (dry basis, vol %) H2 (dry basis, vol %) N2 (dry basis, vol %) CO (dry basis, vol %) CH4 (dry basis, vol %)

20.78 37.43 7.15 32.52 2.12

20.74 37.89 7.48 32.09 1.79

20.70 39.18 7.42 31.73 0.96


9.7

9.6

9.4

CH4 conversion (%)

78.4

82.5

90.3

average carbon conversion (%) a

68


Nm3/h O2, 0.2 Nm3/h N2, and 2.4 kg/h steam. Based on the unburnt carbon in the bottom ash and fly ash, the carbon conversion was 80.6%-84.7%, and the gas production rate was 1.79-1.89 Nm3/kg coal. The product gas composition was 36.67%-37.21% H2, 36.48%37.14% CO, and ∼20.6% CO2. The combined reforming of pure CH4 and gasification of Coal A was then studied. Table 3 summarizes the typical results. The reactor was fed with ∼1.5 kg/h coal, 0.5 Nm3/h methane, 1.45-1.65 Nm3/h O2, 0.3 Nm3/h N2, and 2.4 kg/h steam. Varying the O2 feed rate was necessary, to maintain the desired operating temperature. Based on the concentration of the unreacted CH4 in the product gas, the CH4 conversion was 78.4%90.3%. The gas production rate was 2.62-2.82 Nm3/kg coal. The product gas composition was 37.43%-39.18% H2, 31.73%-32.52% CO, ∼20.4% CO2, and 0.96%2.12% CH4. The carbon conversion was ∼68%, which could be further improved by adjusting the operation parameters. These results prove that (i) a fairly good CH4 conversion can be achieved without a catalyst and (ii) syngas of favorable quality can be produced.

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Table 4. Typical Results of Coal Gasification When Coal B is Used Exp. B-1 property/characteristic

Exp. B-2

B-1a

B-1b

B-2a

B-2b

coal feed rate (kg/h) air flow rate (Nm3/h) O2 flow rate (Nm3/h) steam flow rate (kg/h) steam/coal ratio (kg/kg) temp. (°C)

3.0 0.4 1.75 1.8 0.6 970

3.0 0.4 1.75 1.8 0.6 970

3.0 0.4 1.75 1.8 0.6 971

3.0 0.4 1.75 1.8 0.6 970


1.81

1.80

1.64

1.80


19.53 36.62 6.51 36.49 0.85

20.54 36.22 6.52 35.8 0.92

21.69 35.29 7.18 34.92 0.92

21.82 35.79 6.55 34.84 1.00


9.6

9.5

9.5

average carbon conversion (%) a

85

9.3 82


Besides natural gas, there are also other gases that contain rich methane, such as coke oven gas, coal-bed methane, or coalmine drainage gas. Among these gases, the coalmine drainage gas can be approximately considered as a mixture of air and pure methane. The concentration of oxygen is ∼10%. It is quite dangerous to separate methane from coalmine drainage gas using the traditional pressure swing adsorption (PSA) process. Therefore, the co-conversion of coal and simulated coalmine drainage gas was also investigated. Table 4 presents the typical results of gasification with the anthracite (Coal B). The reactor was fed with 3.0 kg/h coal, 1.75 Nm3/h O2, 0.4 Nm3/h air, and 1.8 kg/h steam. The carbon conversion was 82%-85%, and the gas production rate was 1.64-1.81 Nm3/kg coal. The product gas composition was 35.29%-36.62% H2, 34.84%-36.49% CO, and 19.53%-21.82% CO2. Table 5 presents the typical results of the co-conversions of simulated coalmine drainage gas with Coal B. The reactor was fed with ∼2.5 kg/h coal, 0.4 Nm3/h methane, 2.0 Nm3/h O2, 0.4 Nm3/h air, and 2.0 kg/h steam. The CH4 conversion was 68.7%-78.2%, and the

gas production rate was 2.13-2.38 Nm3/kg coal. The product gas composition was 35.49%-39.01% H2, 32.48%-33.99% CO, 17.89%-22.39% CO2, and 2.3%3.0% CH4. The carbon conversion was ∼70%-71%, which could also be further improved. In this combined process, the feedstock of methane does not require special treatment, which is most important for the utilization of real coalmine drainage methane. This could enable effective utilization of coal-bed methane, drastically reducing the environmental impact of coal mining and enhancing the utilization of available resources. In addition, the integrated coal gasification and gas reforming offers a further advantage that the H2/CO ratio in the syngas produced can be adjusted by varying the ratio of coal/gas in the feed streams, to tailor the syngas to suit downstream processing requirements. Comparing the results shown in Tables 2 and 3, as well as those in Tables 4 and 5, one can see that the total gas production rate of this combined process is distinctly higher than that of coal gasification alone and the net production rate of H2 and CO contributed by CH4 conversion is higher than that of CO2. This proves that CH4 is actually reformed and is not simply combusted to provide the heat for coal/char gasification. In the present experiments, the flow rate of CH4 is limited, because the operation temperature must be kept steady after CH4 is added, so the contribution to the production of H2 and CO by CH4 reforming is not very obvious. To ascertain this processing concept, future efforts will be focused on reaction mechanisms and kinetics that are important for optimizing the reactor design and operating conditions, to further improve the conversion levels of carbon and CH4. Important fundamental issues include identification of the catalytic effect of char and/ or ash, determination of the complex heterogeneoushomogeneous reaction kinetics of gas reforming and oxidation, char gasification and combustion, and the water gas shift reaction, as well as their interactions. 4. Conclusions The preliminary experiments in the laboratory-scale fluidized-bed reactor proved the concept of combined

Table 5. Typical Results of Combined Coal Gasification and Methane Reforming When Coal B is Used Exp. B-1 property/characteristic

Exp. B-2

B-1a

B-1b

B-1c

B-2a

B-2b

B-2c

coal feed rate (kg/h) CH4 flow rate (Nm3/h) CH4/coal ratio (Nm3/kg) air flow rate (Nm3/h) O2 flow rate (Nm3/h) steam flow rate (kg/h) steam/coal ratio (kg/kg) temp. (°C)

2.5 0.4 0.16 0.4 2.0 2.0 0.8 968

2.5 0.4 0.16 0.4 2.0 2.0 0.8 971

2.5 0.4 0.16 0.4 2.0 2.0 0.8 968

2.5 0.4 0.16 0.4 2.0 2.0 0.8 966

2.5 0.4 0.16 0.4 2.0 2.0 0.8 970

2.5 0.4 0.16 0.4 2.0 2.0 0.8 970


2.22

2.2

2.13

2.2

2.38

2.38


22.39 35.49 6.33 33.46 2.34

17.89 39.01 6.52 33.63 2.95

21.56 36.27 6.59 32.96 2.62

20.48 36.78 6.37 33.99 2.38

21.16 36.61 5.90 33.53 2.81

20.50 37.63 6.35 32.48 3.04


9.7

10.4

9.8

9.9

10.0

10.1

CH4 conversion (%)

78.2

70.9

75.7

78

69

68.7

average carbon conversion (%)

70

71

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coal gasification and coal-bed methane reforming to be feasible. At the moderate temperature of ∼1000 °C, significantly high methane conversions were achieved, with a favorable quality of syngas being produced. The methane conversions were up to 90% and ∼78% for Coal A and Coal B, respectively. Carbon conversions of 68%70% were achieved. The syngas that is produced has a composition of 35%-39% H2, 31%-34% CO, and 20%22% CO2. The concentration of H2 is higher than that of CO. Further studies to understand the mechanisms and kinetics of the complex reactions involved are necessary to identify the optimum processing conditions

Wu et al.

for the combined coal gasification and gas-reforming process. Acknowledgment. This work has been supported under the Innovation Funds Scheme of the Institute of Coal Chemistry, Chinese Academy of Sciences (ICC, CAS), and the Outstanding Overseas Chinese Talent Funds Scheme of the Chinese Academy of Sciences (CAS). The authors also wish to thank the reviewers for their professional comments and suggestions, which have helped strengthen the manuscript. EF049853T

Combined Coal Gasification and Methane Reforming for Production of

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