Investigation of H2O and CO2 Reforming and Partial Oxidation ofMethane: 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, BowlingGreen, 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 coalashes on methane (CH4) conversion, sum selectivity (the sum of H2 and CO), and ratio selectivity (the ratioof H2/CO) in an atmospheric fluidized bed. The kinetics study presented the possibility of CH4 reforming andpartial oxidation with a favorable H2/CO ratio, greater than 5. The higher H2/CO ratio in CH4 reforming andthe partial-oxidation process can reduce the consumption of CH4 needed to adjust the H2/CO ratio duringcombined coal gasification and methane reforming. Coal ashes failed to be good candidates of catalysts onCH4 reforming and partial oxidation because of their very low specific surface area available for catalyticreactions. However, coal chars presented very promising catalytic performance on CH4 reforming and partialoxidation because of their larger specific surface area. In this study, no other constituents in coal fly ash orspecial surface properties of coal chars were correlated with the enhanced methane-conversion efficiency. Itseems that the specific surface area is only variable in controlling methane-conversion efficiency.

1. Introduction

It is well-known that many chemical products are synthesizedthrough syngas (H2 and CO). The production of syngas is ofgreat importance in the chemical industry because it is the rawmaterial for methanol synthesis, Fischer-Tropsch (F-T) syn-thesis, and dimethyl ether (DME).1–3 Natural gas is an importantresource for syngas production. With an insufficient supply andthe rising price of petroleum, great importance has been attachedto the research and development of natural gas reforming. Coalgasification is also a promising resource for syngas productionbecause the carbon in coal can react with H2O to produce COand H2. Therefore, natural gas reforming and coal gasificationare two primary resources for the production of syngas and maybecome the new source of the modern chemical industries inthe future instead of petroleum.

The downstream synthesis of different chemical productsrequires syngas with different H2/CO ratios. The H2/CO ratioof syngas usually depends upon the H/C ratio of raw materialsand the reaction routes of the syngas production. The desiredH2/CO ratios for methanol synthesis and F-T synthesis ofdifferent chemical products are usually 1.5-2.2 Presently, syngasis mainly produced by H2O reforming of methane. However,syngas produced from H2O methane reforming has a H2/COratio between 3 and 4 higher than what is needed for thedownstream synthesis processes and thus requires furtheradjustment to be used in methanol synthesis and F-T synthesis.Syngas produced from CO2 methane reforming and steam

gasification of coal cannot also be directly used in methanolsynthesis or F-T synthesis because the H2/CO ratio is close to1. Methane partial oxidation needs to be carefully controlled toobtain an available yield of H2 and CO, although this reactioncan produce syngas with a H2/CO ratio close to 2. At the sametime, CH4 partial oxidation requires pure O2 and thus increasesthe investment and operation costs of CH4 partial oxidation.How can we get a desired syngas to meet the demand of themodern chemical industry? Combined methane reforming andcoal gasification is expected to easily produce syngas with thedesired H2/CO ratio of 1.5-2 by changing feed composition.4–6

With the use of H2O and CO2, methane can be reformed toproduce H2 and CO according to the following reactions shownin eqs 1 and 2. These two reactions are so-called methanereforming. With a supply of lower stoichiometric coefficientsof oxygen, methane can be partially oxidized to produce H2

and CO according to the following reaction, which is shown ineq 3. Carbon monoxide can further react with an excessivesupply of H2O to produce more H2. This reaction is called thewater-gas shift reaction, as shown in eq 4. Carbon deposit isone of major problems during methane reforming and partialgasification. The possible carbon deposit reaction is shown ineq 5.

CH4 +H2OSCO+ 3H2 +205.9 kJ/mol (1)

CH4 +CO2S 2CO+ 2H2 +247.1 kJ/mol (2)

CH4 +12

O2SCO+ 2H2 -35.9 kJ/mol (3)* 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.

(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.

Energy & Fuels 2008, 22, 2341–2345 2341

10.1021/ef700638p CCC: $40.75 2008 American Chemical SocietyPublished on Web 06/04/2008

H2O+COSCO2 +H2 -44.0 kJ/mol (4)

CH4SC+ 2H2 -74.9 kJ/mol (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 duringcoal gasification, CH4 in natural gas will react with H2O, CO2,and O2. That is to say, combined methane reforming and coalgasification involve CH4 reforming of H2O and CO2 and partialoxidation by O2. Presently, many references mainly focus onthe research of CH4 reforming of H2O and CO2 and partialoxidation by O2, especially in the presence of catalysts, suchas noble metals and their oxides supported on the carriers(normally metal or nonmetal oxides).8–14 Do coal char and coalash have obvious effects on H2O and CO2 reforming and partialoxidation of CH4 during combined CH4 reforming and coalgasification? Very few papers in the literature have dealt withthis topic until now.1,4,5,15,16

In this study, steam and CO2 reforming and partial oxidationof CH4 in the presence of coal chars and coal ashes wereperformed in a fluidized bed reactor. The catalytic effects ofcoal chars and coal ashes on methane reforming and partialoxidation were evaluated.

2. Experimental Section

2.1. Experimental Apparatus. Figure 1 shows a schematicdiagram of the fluidized bed reactor for methane reforming andpartial oxidation in this study. The methane reforming and partialoxidation experiment system consists of four parts: electric heatingfurnace, fluidized bed reactor, steam generator, and control unit.

The fluidized bed reactor is made of quartz and is 600 mm longwith a porous quartz plate of 20 mm in diameter placed 300 mmfrom the bottom. The temperature was measured 30 mm above theporous quartz plate. The steam generator is composed of a syringepump, a syringe, and a heating tube. The heating tube includes astainless-steel tube, heating tape, glass bead, and septum. Theseptum has a sealed function for water and gas. The glass beadscan promote the conversion of water into steam because they canprovide surface area for vaporization nucleation and preventsuperheating and bumping for water. The heating tape was used toheat the whole steam-generator system. This steam generator canprovide the desired flow rate of steam for the experiment. The flowrates of methane, air, carbon dioxide, and nitrogen are controlledby the mass flow controller (MFC).

2.2. Experimental Materials. CH4, CO2, and N2 used in thisstudy are high pure gases. The air used in this study is general,compressed air. Coal carbonization and char activation were carriedout in the fluidized-bed reactor, as shown schematically in Figure1. The carbonization temperature was 450 °C. After 10 g of coalwas 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 atroom temperature, the reactor was heated to the desired temperature.After 30 min of carbonization time, the electric furnace power wasshut off. During the purge, carbonization, and cooling stages, N2

flow was constant to prevent char oxidation. The prepared charsamples were activated by steam following the carbonizationprocedure in the same reactor at 800 °C for 30 min. Water injectionwas controlled by a syringe pump, steamed in a preheater, and thencarried in a flow of N2 at 320 mL/min. The coal ashes used in theexperiment were sampled from the coal-fired plant. Two commercialgasification chars were derived from commercial integrated gas-ification combined cycle (IGCC) processes.

Figure 1. Schematic diagram of the test facility.

2342 Energy & Fuels, Vol. 22, No. 4, 2008 Zhou et al.

2.3. Experimental Procedure. During the experiment, N2 wasintroduced into the fluidized-bed reactor for 1 h to avoid theinterference of oxygen during methane reforming and partialoxidation. After the activated char (or fly ash) was added into thereactor, it was preheated to 200 °C and the heating tube waspreheated to 150 °C. Simultaneously, methane and steam generatedfrom the steam-generating system (or carbon dioxide or air) wereintroduced into the fluidized-bed reactor. The flow rate of methane(or carbon dioxide or air) can be adjusted by the mass-flowcontroller, while the flow rate of steam can be adjusted by thesyringe 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 differenttemperatures. After the experiment, the fluidized-bed reactor mustbe cooled to room temperature. Therefore, the power of the systemwas shut off.

2.4. Method of Analysis. The compositions of fuel gas sampleswere analyzed by a gas chromatograph (Shimadzu Model GC-8A)with a thermal conductivity detector (TCD) and an injectorconnected to a Carboxen-1000 column 60/80 (mesh range) of 15ft × 1/8 in. stainless steel (2.1 mm inner diameter). Chromatographycalibration was performed with standard gas mixtures of H2, CO,O2, N2, and CO2, and the standard deviation curve of the typicalcomponent was drawn. Argon was used as the carrier gas at a flowrate of 40 mL/min. The temperature of the chromatography columnwas 70 °C, and the temperature of TCD was 110 °C. The porousproperties 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 steamreforming and partial oxidation of CH4 was calculated as describedbelow:

XCH4(%))

CCO +CCO2

CCH4+CCO +CCO2

× 100 (6)

where XCH4 is CH4 conversion and CCO, CCO2, and CCH4 are thecontents of CO, CO2, and CH4 in fuel gas, respectively.

The selectivity in methane reforming and partial oxidation wasdescribed by two modes. One called the sum selectivity is thecontent sum of H2 and CO in fuel gas, and the other called theratio selectivity is the ratio of H2/CO in fuel gas during methanereforming and partial oxidation.

3. Results and Discussion

3.1. Steam Methane Reforming. Commercially, the steammethane reforming needs a catalyst to promote the reactionkinetics. The most popular commercial catalyst for steammethane reforming is NiO with a large specific surface area. Inthis study, the kinetics of methane reforming and partialoxidation was evaluated in a fluidized bed reactor. Catalytic

effects of coal chars and coal ashes from gasification andcombustion processing on steam methane reforming wereevaluated. Methane conversion efficiency, sum selectivity, andratio selectivity (H2/CO) of steam methane reforming bydifferent coal chars and coal ashes are shown in Figures 2–4,respectively. A blank test without any catalysts was conductedto compare the catalytic effects of different coal chars and coalashes on steam methane reforming. Powder River Basin (PRB)ACC and Lignite ACC are the gasification chars derived fromlow-rank coals, for which the specific surface areas are higherat 649.3 and 359.9 m2/g, respectively. Kentucky (KY) bit-3 SCCis a carbonization char from the pyrolysis process with a lowerspecific surface area at 3.03 m2/g. Two commercial gasificationchars come from commercial IGCC processes. Their specificsurface areas are lower because they become slag after highertemperature treatment in the gasifier.

3.1.1. Effect of Coal Chars and Fly Ashes on MethaneConVersion Efficiency. As indicated in Figure 2, the temperatureis a major factor in CH4 conversion efficiency. The increase ofCH4 conversion efficiency is nearly 25% when there is atemperature increase from 700 to 900 °C for PRB ACC.Methane conversion efficiencies by coal chars are all greaterthan that in the blank test, which confirms the occurrence ofcatalytic effects by coal chars. It seems that coal chars with a

(7) Zhou, H. C.; Jin, B. S.; Zhong, Z. P.; Huang, Y. J.; Xiao, R. EnergyFuels 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 2. CH4 conversion of methane steam reforming by differentcoal chars and coal ashes.

Figure 3. Sum selectivity (H2 plus CO) of methane steam reformingby different coal chars and coal ashes.

Catalytic Effects of Coal Char and Coal Ash Energy & Fuels, Vol. 22, No. 4, 2008 2343

higher specific surface area, such as PRB ACC and LigniteACC, result in higher CH4 conversion efficiencies and carbon-ization char and commercial chars result in lower CH4 conver-sion efficiencies, which are comparable to that in the blank test.It was also found that there is a greater catalytic effect on steammethane reforming by coal chars than by fly ashes, which werederived from the same coals. Similarly, the specific surface areasof coal chars are generally higher than those of coal ashes, whichpossibly explains the difference between catalytic effects by coalchars and coal ashes.

3.1.2. Effect of Coal Chars and Fly Ashes on the SumSelectiVity (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 sumselectivity of steam methane reforming. The increase of sumselectivity of steam methane reforming is nearly 35% by thetemperature increase from 700 to 900 °C for PRB ACC. Thesum selectivity also increases with the increase of the specificsurface area of coal chars. However, the variation of sumselectivity is not greater by variation of the specific surface areathan that by the temperature variation. The sum selectivity ofcoal chars is also greater than that of fly ashes, possibly becauseof the same reasons as that of CH4 conversion efficiency. Thistrend is apparent in higher temperatures (900 °C) than in lowertemperatures (700 °C).

3.1.3. Effect of Coal Chars and Fly Ashes on the RatioSelectiVity (H2/CO) of Syngas. The ratios selectivity (H2/CO)produced from steam methane reforming by different coal charsand fly ashes are shown in Figure 4. The temperature seems tobe negatively correlated to the ratios selectivity (H2/CO) forboth coal chars and coal ashes. Two reactions may impact theratio selectivity (H2/CO) during steam methane reforming. Underthe lower temperature range, the ratio selectivity (H2/CO)increases with generating H2 and consumption of CO by thewater-gas shift reaction. Under the higher temperature range,the methane decomposition reaction, as indicated in eq 5, couldincrease the concentration of H2 in the produced syngas despitethe restriction of the water-gas shift reaction. Because of theinterference of the methane decomposition reaction, the ratioselectivity (H2/CO) is generally larger than that of the stoichio-metric factor of eq 1. Although the higher ratio selectivity (H2/CO) is expected for the cogasification process, the formationof soot is not expected because it is difficult to burn out.

3.1.4. Effect of the Ratio of Steam/Methane on SteamMethane Reforming. As indicated in Figure 5, steam suppliedwith the ratio of steam/methane (RSM) at 2 and 3, which arehigher than the stoichiometric factor, does not help in theabatement of soot formation at 900 °C because the ratioselectivity (H2/CO) is still higher than the stoichiometric factorof the syngas product in eq 1. As expected, the increase of RSMdoes increase the CH4 conversion efficiency and sum selectivityin this study because steam methane reforming is a process withthe kinetics control. Higher partial pressure of steam willincrease process kinetics. However, this impact is limited.Because of energy penalties, we do not suggest a higher steamratio, which is applied in the steam methane reforming process.

3.1.5. Effect of Gas Hourly Surface Velocity (GHSV) onSteam Methane Reforming. Figure 6 shows the impact ofvariations of the GHSV on CH4 conversion efficiency, sumselectivity (H2 plus CO), and ratio selectivity (H2/CO) duringthe methane reforming process. The increase of the GHSVresults in the decrease of the CH4 conversion efficiency. Thetrend of sum selectivity is similar. The main reason may bethat the residence time of reactants in the fluidized bed reactoris shortened by the increase of GHSV. At the same time, thereaction load on coal char increases with GHSV, which willdecrease 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).

Figure 4. Ratio selectivity (H2/CO) of steam methane reforming bydifferent coal chars and coal ashes.

Figure 5. Effects of RSM on CH4 conversion efficiency, sum selectivity(H2 plus CO), and ratio selectivity (H2/CO) under steam methanereforming by KY bit-3 chars.

Figure 6. Effects of GHSV on CH4 conversion efficiency, sumselectivity (H2 plus CO), and ratio selectivity (H2/CO) under steammethane reforming by KY bit-3 chars.

2344 Energy & Fuels, Vol. 22, No. 4, 2008 Zhou et al.

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 sumselectivity (H2 plus CO) increases by increasing RCM. It seemsthat higher RCM increases the kinetics of CO2 methanereforming. However, this impact is limited and can not becompared to the impact of the temperature. The temperatureshould be the most significant positive impact factor on CO2

methane reforming. From Figure 7, it can also be seen that theconcentration of CO is higher than that of H2 at high RCM andbed temperatures. The reverse of the water-gas shift reactionis also an endothermic reaction, which is favored at hightemperatures. The outcome of this reaction causes the concen-tration of CO to increase in the syngas and the concentrationof H2 to decrease. The increase of RCM means more carbondioxide entering into the reactor to participate in the reaction,which can increase the partial pressure of CO2 and thus makereactions faster to generate the concentration of CO and decreasethe concentration of H2.

3.3. Methane Partial Oxidation. Figures 8 and 9 show theeffects of temperature, GHSV, and the ratio of oxygen/methane(ROM) on three parameters [CH4 conversion efficiency, sumselectivity, and ratio selectivity (H2/CO)] during methane partialoxidation. At the invariable GHSV and ROM, all parametersincrease 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 andROM, all parameters just slightly increase with the GHSVincrease. This may indicate fast kinetics of methane or CH4

partial oxidation. At the invariable temperature and GHSV, thedecrease of ROM from 1/2 to 1/3 leads to a small increase of thesum selectivity (H2 plus CO) in produced gas during methanepartial oxidation. The methane conversion increases with theincrease of ROM, while the ratio selectivity (H2/CO) and thesum selectivity (H2 plus CO) have a reverse change rule above900 °C. The high ROM means more oxygen will participate inthe reaction of the direct or partial oxidation of methane andmore methane will be consumed during the direct or partialoxidation of methane. The increase of oxygen may burn COand H2 into CO2 and H2O, which will reduce the concentrationof CO and H2 in produced gas. Simultaneously, the high ROMmeans more nitrogen is available in syngas, which dilutes theconcentrations of H2 and CO. Because the adjustability of thepartial oxidation of methane by oxygen is not ideal and there isa possibility that it could consume H2 and CO, injecting CH4

in the cogasification process should select a reaction zone whereoxygen is not available for CH4 burnout or partial oxidation.

4. Conclusions

The methane reforming and partial oxidation experimentshave been performed in a fluidized bed reactor. The experi-mental results show the possibility of CH4 reforming and thepartial oxidation with a favorable H2/CO ratio, which is greaterthan 5. The higher H2/CO ratio in CH4 reforming and partialoxidation process means less CH4 needed to adjust the H2/COratio 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 specificsurface area, while coal chars present very promising catalyticperformance on CH4 reforming and partial oxidation becauseof their larger specific surface area. In this study, no otherconstituents in coal fly ash or special surface properties of coalchars were correlated with the enhanced CH4 conversionefficiency. It seems that the specific surface area is the onlyvariable in controlling methane conversion efficiency.

Acknowledgment. This work was supported by the KentuckyGovernor’s Office Energy Policy (KGOEP) (S-06014932).

EF700638P

Figure 7. Effects of RCM on selectivity (H2 plus CO) and ratioselectivity (H2/CO) under CO2 methane reforming by KY bit-3 char.

Figure 8. Effects of GHSV on methane partial oxidation.

Figure 9. Effects of ROM on methane partial oxidation.

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