Energy & Fuels 2008, 22, 2341–2345
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Investigation of H2O and CO2 Reforming and Partial Oxidation of Methane: Catalytic Effects of Coal Char and Coal Ash Hongcang Zhou,†,‡ Yan Cao,† Houyin Zhao,† Hongying Liu,† and Wei-Ping Pan*,† Institute for Combustion Science and EnVironmental Technology, Western Kentucky UniVersity, Bowling Green, Kentucky 42101, and School of EnVironmental Science and Engineering, Nanjing UniVersity of Information Science and Technology, Nanjing 210044, People’s Republic of China ReceiVed October 27, 2007. ReVised Manuscript ReceiVed April 1, 2008
Methane reforming and partial oxidation was studied to evaluate the catalytic effects of coal chars and coal ashes on methane (CH4) conversion, sum selectivity (the sum of H2 and CO), and ratio selectivity (the ratio of H2/CO) in an atmospheric fluidized bed. The kinetics study presented the possibility of CH4 reforming and partial oxidation with a favorable H2/CO ratio, greater than 5. The higher H2/CO ratio in CH4 reforming and the partial-oxidation process can reduce the consumption of CH4 needed to adjust the H2/CO ratio during combined coal gasification and methane reforming. Coal ashes failed to be good candidates of catalysts on CH4 reforming and partial oxidation because of their very low specific surface area available for catalytic reactions. However, coal chars presented very promising catalytic performance on CH4 reforming and partial oxidation because of their larger specific surface area. In this study, no other constituents in coal fly ash or special surface properties of coal chars were correlated with the enhanced methane-conversion efficiency. It seems that the specific surface area is only variable in controlling methane-conversion efficiency.
1. Introduction It is well-known that many chemical products are synthesized through syngas (H2 and CO). The production of syngas is of great importance in the chemical industry because it is the raw material for methanol synthesis, Fischer-Tropsch (F-T) synthesis, and dimethyl ether (DME).1–3 Natural gas is an important resource for syngas production. With an insufficient supply and the rising price of petroleum, great importance has been attached to the research and development of natural gas reforming. Coal gasification is also a promising resource for syngas production because the carbon in coal can react with H2O to produce CO and H2. Therefore, natural gas reforming and coal gasification are two primary resources for the production of syngas and may become the new source of the modern chemical industries in the future instead of petroleum. The downstream synthesis of different chemical products requires syngas with different H2/CO ratios. The H2/CO ratio of syngas usually depends upon the H/C ratio of raw materials and the reaction routes of the syngas production. The desired H2/CO ratios for methanol synthesis and F-T synthesis of different chemical products are usually 1.5-2.2 Presently, syngas is mainly produced by H2O reforming of methane. However, syngas produced from H2O methane reforming has a H2/CO ratio between 3 and 4 higher than what is needed for the downstream synthesis processes and thus requires further adjustment to be used in methanol synthesis and F-T synthesis. Syngas produced from CO2 methane reforming and steam * To whom correspondence should be addressed. E-mail: wei-ping.pan@ wku.edu. † Western Kentucky University. ‡ Nanjing University of Information Science and Technology. (1) Bai, Z. Q.; Chen, H. K.; Li, W.; Li, B. Q. Int. J. Hydrogen Energy 2006, 31, 899–905. (2) Song, C. S.; Pan, W. Catal. Today 2004, 98, 463–484. (3) Li, Y. B.; Xiao, R.; Jin, B. S. Chem. Eng. Technol. 2007, 30, 91– 98.
gasification of coal cannot also be directly used in methanol synthesis or F-T synthesis because the H2/CO ratio is close to 1. Methane partial oxidation needs to be carefully controlled to obtain an available yield of H2 and CO, although this reaction can produce syngas with a H2/CO ratio close to 2. At the same time, CH4 partial oxidation requires pure O2 and thus increases the investment and operation costs of CH4 partial oxidation. How can we get a desired syngas to meet the demand of the modern chemical industry? Combined methane reforming and coal gasification is expected to easily produce syngas with the desired H2/CO ratio of 1.5-2 by changing feed composition.4–6 With the use of H2O and CO2, methane can be reformed to produce H2 and CO according to the following reactions shown in eqs 1 and 2. These two reactions are so-called methane reforming. With a supply of lower stoichiometric coefficients of oxygen, methane can be partially oxidized to produce H2 and CO according to the following reaction, which is shown in eq 3. Carbon monoxide can further react with an excessive supply of H2O to produce more H2. This reaction is called the water-gas shift reaction, as shown in eq 4. Carbon deposit is one of major problems during methane reforming and partial gasification. The possible carbon deposit reaction is shown in eq 5. CH4 + H2O S CO + 3H2
+205.9 kJ/mol
(1)
CH4 + CO2 S 2CO + 2H2
+247.1 kJ/mol
(2)
1 CH4 + O2 S CO + 2H2 2
-35.9 kJ/mol
(3)
(4) Wu, J. H.; Fang, Y. T.; Wang, Y.; Zhang, D. K. Energy Fuels 2005, 19, 512–516. (5) Haghighi, M.; Sun, Z. Q.; Wu, J. H.; Bromly, J.; Ng, E.; Wee, H. L.; Wang, Y.; Zhang, D. K. Proc. Combust. Inst. 2007, 31, 1983–1990. (6) Li, Y. B.; Jin, B. S.; Xiao, R. Korean J. Chem. Eng. 2007, 24, 688– 692.
10.1021/ef700638p CCC: $40.75 2008 American Chemical Society Published on Web 06/04/2008
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H2O + CO S CO2 + H2 CH4 S C + 2H2
-44.0 kJ/mol
-74.9 kJ/mol
Zhou et al.
(4) (5)
During coal gasification in the presence of H2O and/or air, the reactants in the gasifier consist of CO, H2, O2, H2O, CH4, and CO2.7 Once natural gas is introduced into the gasifier during coal gasification, CH4 in natural gas will react with H2O, CO2, and O2. That is to say, combined methane reforming and coal gasification involve CH4 reforming of H2O and CO2 and partial oxidation by O2. Presently, many references mainly focus on the research of CH4 reforming of H2O and CO2 and partial oxidation by O2, especially in the presence of catalysts, such as noble metals and their oxides supported on the carriers (normally metal or nonmetal oxides).8–14 Do coal char and coal ash have obvious effects on H2O and CO2 reforming and partial oxidation of CH4 during combined CH4 reforming and coal gasification? Very few papers in the literature have dealt with this topic until now.1,4,5,15,16 In this study, steam and CO2 reforming and partial oxidation of CH4 in the presence of coal chars and coal ashes were performed in a fluidized bed reactor. The catalytic effects of coal chars and coal ashes on methane reforming and partial oxidation were evaluated. 2. Experimental Section 2.1. Experimental Apparatus. Figure 1 shows a schematic diagram of the fluidized bed reactor for methane reforming and partial oxidation in this study. The methane reforming and partial oxidation experiment system consists of four parts: electric heating furnace, fluidized bed reactor, steam generator, and control unit.
Figure 1. Schematic diagram of the test facility.
The fluidized bed reactor is made of quartz and is 600 mm long with a porous quartz plate of 20 mm in diameter placed 300 mm from the bottom. The temperature was measured 30 mm above the porous quartz plate. The steam generator is composed of a syringe pump, a syringe, and a heating tube. The heating tube includes a stainless-steel tube, heating tape, glass bead, and septum. The septum has a sealed function for water and gas. The glass beads can promote the conversion of water into steam because they can provide surface area for vaporization nucleation and prevent superheating and bumping for water. The heating tape was used to heat the whole steam-generator system. This steam generator can provide the desired flow rate of steam for the experiment. The flow rates of methane, air, carbon dioxide, and nitrogen are controlled by the mass flow controller (MFC). 2.2. Experimental Materials. CH4, CO2, and N2 used in this study are high pure gases. The air used in this study is general, compressed air. Coal carbonization and char activation were carried out in the fluidized-bed reactor, as shown schematically in Figure 1. The carbonization temperature was 450 °C. After 10 g of coal was added to the fluidized bed, the reactor was connected to a N2 supply at a flow rate of 800 mL/min. After 30 min of purging at room temperature, the reactor was heated to the desired temperature. After 30 min of carbonization time, the electric furnace power was shut off. During the purge, carbonization, and cooling stages, N2 flow was constant to prevent char oxidation. The prepared char samples were activated by steam following the carbonization procedure in the same reactor at 800 °C for 30 min. Water injection was controlled by a syringe pump, steamed in a preheater, and then carried in a flow of N2 at 320 mL/min. The coal ashes used in the experiment were sampled from the coal-fired plant. Two commercial gasification chars were derived from commercial integrated gasification combined cycle (IGCC) processes.
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2.3. Experimental Procedure. During the experiment, N2 was introduced into the fluidized-bed reactor for 1 h to avoid the interference of oxygen during methane reforming and partial oxidation. After the activated char (or fly ash) was added into the reactor, it was preheated to 200 °C and the heating tube was preheated to 150 °C. Simultaneously, methane and steam generated from the steam-generating system (or carbon dioxide or air) were introduced into the fluidized-bed reactor. The flow rate of methane (or carbon dioxide or air) can be adjusted by the mass-flow controller, while the flow rate of steam can be adjusted by the syringe pump. When the fluidized-bed reactor was heated to 700 °C and stabilized for half an hour, the fuel gas sample was collected. Then, the fluidized-bed reactor was heated to 800, 900, and 950 °C in turn. After the above run, the fuel gas was sampled at different temperatures. After the experiment, the fluidized-bed reactor must be cooled to room temperature. Therefore, the power of the system was shut off. 2.4. Method of Analysis. The compositions of fuel gas samples were analyzed by a gas chromatograph (Shimadzu Model GC-8A) with a thermal conductivity detector (TCD) and an injector connected to a Carboxen-1000 column 60/80 (mesh range) of 15 ft × 1/8 in. stainless steel (2.1 mm inner diameter). Chromatography calibration was performed with standard gas mixtures of H2, CO, O2, N2, and CO2, and the standard deviation curve of the typical component was drawn. Argon was used as the carrier gas at a flow rate of 40 mL/min. The temperature of the chromatography column was 70 °C, and the temperature of TCD was 110 °C. The porous properties including Brunauer-Emmett-Teller (BET) surface area, pore volume and average pore diameter of fly ashes, pyrolysis chars, and activated char samples were measured by nitrogen adsorption/ desorption isotherms with a Micrometritics instrument ASAP 2020. 2.5. Methods of Data Processing. CH4 conversion in steam reforming and partial oxidation of CH4 was calculated as described below: XCH4 (%) )
CCO + CCO2 CCH4 + CCO + CCO2
× 100
Figure 2. CH4 conversion of methane steam reforming by different coal chars and coal ashes.
(6)
where XCH4 is CH4 conversion and CCO, CCO2, and CCH4 are the contents of CO, CO2, and CH4 in fuel gas, respectively. The selectivity in methane reforming and partial oxidation was described by two modes. One called the sum selectivity is the content sum of H2 and CO in fuel gas, and the other called the ratio selectivity is the ratio of H2/CO in fuel gas during methane reforming and partial oxidation.
3. Results and Discussion 3.1. Steam Methane Reforming. Commercially, the steam methane reforming needs a catalyst to promote the reaction kinetics. The most popular commercial catalyst for steam methane reforming is NiO with a large specific surface area. In this study, the kinetics of methane reforming and partial oxidation was evaluated in a fluidized bed reactor. Catalytic (7) Zhou, H. C.; Jin, B. S.; Zhong, Z. P.; Huang, Y. J.; Xiao, R. Energy Fuels 2005, 19, 1619–1623. (8) Matsumura, Y.; Nakamori, T. Appl. Catal., A 2004, 258, 107–114. (9) Hou, K. H.; Hughes, R. Chem. Eng. J. 2001, 82, 311–328. (10) Mo, L. Y.; Zheng, X. M.; Jing, Q. S.; Lou, H.; Fei, J. H. Energy Fuels 2005, 19, 49–53. (11) Rice, S. F.; McDaniel, A. H.; Hecht, E. S.; Hardy, A. J. J. Ind. Eng. Chem. Res. 2007, 46, 1114–1119. (12) Ruckenstein, E.; Hu, Y. H. Ind. Eng. Chem. Res. 1998, 37, 1744– 1747. (13) Pistonesi, C.; Juan, A.; Irigoyen, B.; Amadeo, N. Appl. Surf. Sci. 2007, 253, 4427–4437. (14) El-Bousiffi, M. A.; Gunn, D. J. Int. J. Heat Mass Transfer 2007, 50, 723–733. (15) Chen, W. J.; Sheu, F. R.; Savage, R. L. Fuel Process. Technol. 1987, 16, 279–288. (16) Sun, Z. Q.; Wu, J. H.; Haghighi, M.; Bromly, J.; Ng, E.; Wee, H. L.; Wang, Y.; Zhang, D. K. Energy Fuels 2007, 21, 1601–1605.
Figure 3. Sum selectivity (H2 plus CO) of methane steam reforming by different coal chars and coal ashes.
effects of coal chars and coal ashes from gasification and combustion processing on steam methane reforming were evaluated. Methane conversion efficiency, sum selectivity, and ratio selectivity (H2/CO) of steam methane reforming by different coal chars and coal ashes are shown in Figures 2–4, respectively. A blank test without any catalysts was conducted to compare the catalytic effects of different coal chars and coal ashes on steam methane reforming. Powder River Basin (PRB) ACC and Lignite ACC are the gasification chars derived from low-rank coals, for which the specific surface areas are higher at 649.3 and 359.9 m2/g, respectively. Kentucky (KY) bit-3 SCC is a carbonization char from the pyrolysis process with a lower specific surface area at 3.03 m2/g. Two commercial gasification chars come from commercial IGCC processes. Their specific surface areas are lower because they become slag after higher temperature treatment in the gasifier. 3.1.1. Effect of Coal Chars and Fly Ashes on Methane ConVersion Efficiency. As indicated in Figure 2, the temperature is a major factor in CH4 conversion efficiency. The increase of CH4 conversion efficiency is nearly 25% when there is a temperature increase from 700 to 900 °C for PRB ACC. Methane conversion efficiencies by coal chars are all greater than that in the blank test, which confirms the occurrence of catalytic effects by coal chars. It seems that coal chars with a
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Figure 5. Effects of RSM on CH4 conversion efficiency, sum selectivity (H2 plus CO), and ratio selectivity (H2/CO) under steam methane reforming by KY bit-3 chars. Figure 4. Ratio selectivity (H2/CO) of steam methane reforming by different coal chars and coal ashes.
higher specific surface area, such as PRB ACC and Lignite ACC, result in higher CH4 conversion efficiencies and carbonization char and commercial chars result in lower CH4 conversion efficiencies, which are comparable to that in the blank test. It was also found that there is a greater catalytic effect on steam methane reforming by coal chars than by fly ashes, which were derived from the same coals. Similarly, the specific surface areas of coal chars are generally higher than those of coal ashes, which possibly explains the difference between catalytic effects by coal chars and coal ashes. 3.1.2. Effect of Coal Chars and Fly Ashes on the Sum SelectiVity (H2 Plus CO) of Syngas. The sum selectivity (H2 plus CO) of methane steam reforming is shown in Figure 3. Similarly, the temperature is a major factor on the sum selectivity of steam methane reforming. The increase of sum selectivity of steam methane reforming is nearly 35% by the temperature increase from 700 to 900 °C for PRB ACC. The sum selectivity also increases with the increase of the specific surface area of coal chars. However, the variation of sum selectivity is not greater by variation of the specific surface area than that by the temperature variation. The sum selectivity of coal chars is also greater than that of fly ashes, possibly because of the same reasons as that of CH4 conversion efficiency. This trend is apparent in higher temperatures (900 °C) than in lower temperatures (700 °C). 3.1.3. Effect of Coal Chars and Fly Ashes on the Ratio SelectiVity (H2/CO) of Syngas. The ratios selectivity (H2/CO) produced from steam methane reforming by different coal chars and fly ashes are shown in Figure 4. The temperature seems to be negatively correlated to the ratios selectivity (H2/CO) for both coal chars and coal ashes. Two reactions may impact the ratio selectivity (H2/CO) during steam methane reforming. Under the lower temperature range, the ratio selectivity (H2/CO) increases with generating H2 and consumption of CO by the water-gas shift reaction. Under the higher temperature range, the methane decomposition reaction, as indicated in eq 5, could increase the concentration of H2 in the produced syngas despite the restriction of the water-gas shift reaction. Because of the interference of the methane decomposition reaction, the ratio selectivity (H2/CO) is generally larger than that of the stoichiometric factor of eq 1. Although the higher ratio selectivity (H2/ CO) is expected for the cogasification process, the formation of soot is not expected because it is difficult to burn out.
Figure 6. Effects of GHSV on CH4 conversion efficiency, sum selectivity (H2 plus CO), and ratio selectivity (H2/CO) under steam methane reforming by KY bit-3 chars.
3.1.4. Effect of the Ratio of Steam/Methane on Steam Methane Reforming. As indicated in Figure 5, steam supplied with the ratio of steam/methane (RSM) at 2 and 3, which are higher than the stoichiometric factor, does not help in the abatement of soot formation at 900 °C because the ratio selectivity (H2/CO) is still higher than the stoichiometric factor of the syngas product in eq 1. As expected, the increase of RSM does increase the CH4 conversion efficiency and sum selectivity in this study because steam methane reforming is a process with the kinetics control. Higher partial pressure of steam will increase process kinetics. However, this impact is limited. Because of energy penalties, we do not suggest a higher steam ratio, which is applied in the steam methane reforming process. 3.1.5. Effect of Gas Hourly Surface Velocity (GHSV) on Steam Methane Reforming. Figure 6 shows the impact of variations of the GHSV on CH4 conversion efficiency, sum selectivity (H2 plus CO), and ratio selectivity (H2/CO) during the methane reforming process. The increase of the GHSV results in the decrease of the CH4 conversion efficiency. The trend of sum selectivity is similar. The main reason may be that the residence time of reactants in the fluidized bed reactor is shortened by the increase of GHSV. At the same time, the reaction load on coal char increases with GHSV, which will decrease the catalytic effect of coal char. The ratio selectivity (H2/CO) also decreases with the increase of the temperature. The reason has been described in the above text. At above 900 °C, the variation of GHSV does not impact the ratio selectivity (H2/CO).
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Figure 9. Effects of ROM on methane partial oxidation. Figure 7. Effects of RCM on selectivity (H2 plus CO) and ratio selectivity (H2/CO) under CO2 methane reforming by KY bit-3 char.
Figure 8. Effects of GHSV on methane partial oxidation.
3.2. CO2 Methane Reforming. The investigation of CO2 methane reforming by varying of RCM (ratio of CO2/methane) is shown in Figure 7. As indicated in Figure 7, the sum selectivity (H2 plus CO) increases by increasing RCM. It seems that higher RCM increases the kinetics of CO2 methane reforming. However, this impact is limited and can not be compared to the impact of the temperature. The temperature should be the most significant positive impact factor on CO2 methane reforming. From Figure 7, it can also be seen that the concentration of CO is higher than that of H2 at high RCM and bed temperatures. The reverse of the water-gas shift reaction is also an endothermic reaction, which is favored at high temperatures. The outcome of this reaction causes the concentration of CO to increase in the syngas and the concentration of H2 to decrease. The increase of RCM means more carbon dioxide entering into the reactor to participate in the reaction, which can increase the partial pressure of CO2 and thus make reactions faster to generate the concentration of CO and decrease the concentration of H2. 3.3. Methane Partial Oxidation. Figures 8 and 9 show the effects of temperature, GHSV, and the ratio of oxygen/methane (ROM) on three parameters [CH4 conversion efficiency, sum selectivity, and ratio selectivity (H2/CO)] during methane partial oxidation. At the invariable GHSV and ROM, all parameters increase when the bed temperature increases from 700 to 900 °C. However, the ratio selectivity (H2/CO) is always less than
1, as shown in Figure 8. At the invariable bed temperature and ROM, all parameters just slightly increase with the GHSV increase. This may indicate fast kinetics of methane or CH4 partial oxidation. At the invariable temperature and GHSV, the decrease of ROM from 1/2 to 1/3 leads to a small increase of the sum selectivity (H2 plus CO) in produced gas during methane partial oxidation. The methane conversion increases with the increase of ROM, while the ratio selectivity (H2/CO) and the sum selectivity (H2 plus CO) have a reverse change rule above 900 °C. The high ROM means more oxygen will participate in the reaction of the direct or partial oxidation of methane and more methane will be consumed during the direct or partial oxidation of methane. The increase of oxygen may burn CO and H2 into CO2 and H2O, which will reduce the concentration of CO and H2 in produced gas. Simultaneously, the high ROM means more nitrogen is available in syngas, which dilutes the concentrations of H2 and CO. Because the adjustability of the partial oxidation of methane by oxygen is not ideal and there is a possibility that it could consume H2 and CO, injecting CH4 in the cogasification process should select a reaction zone where oxygen is not available for CH4 burnout or partial oxidation. 4. Conclusions The methane reforming and partial oxidation experiments have been performed in a fluidized bed reactor. The experimental results show the possibility of CH4 reforming and the partial oxidation with a favorable H2/CO ratio, which is greater than 5. The higher H2/CO ratio in CH4 reforming and partial oxidation process means less CH4 needed to adjust the H2/CO ratio during combined coal gasification and methane reforming. Coal ashes failed to be a good candidate for a catalyst on CH4 reforming and partial oxidation because of their very low specific surface area, while coal chars present very promising catalytic performance on CH4 reforming and partial oxidation because of their larger specific surface area. In this study, no other constituents in coal fly ash or special surface properties of coal chars were correlated with the enhanced CH4 conversion efficiency. It seems that the specific surface area is the only variable in controlling methane conversion efficiency. Acknowledgment. This work was supported by the Kentucky Governor’s Office Energy Policy (KGOEP) (S-06014932). EF700638P
