Energy Fuels 2009, 23, 4782–4786 Published on Web 07/08/2009
: DOI:10.1021/ef900272n
Effects of the Catalyst and Reaction Conditions on the Integrated Process of Coal Pyrolysis with CO2 Reforming of Methane† Jiahe Liu, Haoquan Hu,* Lijun Jin, and Pengfei Wang State Key Laboratory of Fine Chemicals, Institute of Coal Chemical Engineering, School of Chemical Engineering, Dalian University of Technology, 129 Street, Dalian 116012, People’s Republic of China Received March 28, 2009. Revised Manuscript Received June 17, 2009
Our previous works showed that the tar yield of coal pyrolysis can obviously be improved by integrated CO2 reforming of methane to coal pyrolysis in a fixed-bed reactor consisting of an upper catalyst layer and a lower coal layer. In this work, the effects of catalyst supports (MgO, Al2O3, SiO2, and NaY) and reaction conditions on tar and water yields, CH4 conversion in pyrolysis of Chinese Pingshuo coal, and the carbon deposition on different catalysts were investigated. The results indicated that the catalyst support has an important effect on the integrated process and MgO is the best among the studied supports. A higher tar yield, lower water yield, and lower carbon deposition can be obtained with Ni/MgO as the catalyst. The tar yield increases with the increase of the pyrolysis temperature, holding time, CO2/CH4 ratio, and CH4 flow rate, respectively, while the char yield decreases with an increasing pyrolysis temperature.
combination of free radicals from coal pyrolysis and CHx groups from CO2 reforming of methane. Therefore, the catalyst for CO2 reforming of methane is the key to improve the tar yield. Supported Ni catalysts were always used to activate CH4 for its high activity.5-8 However, a serious problem is their deactivation, owing to coke formation.9 Reduced NiO-MgO solid solution catalyst showed high resistance to carbon formation. Ruckenstein et al. investigated CO2 reforming of methane over Ni/alkaline earth metal oxide catalysts. The results showed that, in comparison to Ni/ CaO, Ni/SrO, and Ni/BaO catalysts, the Ni/MgO catalyst provided high CH4 and CO2 conversions, which remained unchanged for 120 h.10 Hu et al. studied NiO/MgO catalysts prepared by impregnation and mechanical mixing of powders of NiO and MgO. The reduced catalyst prepared by impregnation showed high activity and stability in CO2 reforming of methane because more NiO-MgO solid solution was formed in the catalyst.11 Tomishige et al. investigated carbon formation behavior in CO2 reforming of methane over reduced Ni0.03Mg0.97O solid solution and Ni/Al2O3 catalysts. They found that the reduced Ni0.03Mg0.97O solid solution catalyst
Introduction In comparison to coal pyrolysis under inert gas, hydropyrolysis, coal pyrolysis under H2, effectively improves tar yield and quality because of chemical reactions between H2 and the free radicals cracked from coal. However, it is unprofitable for large-scale operation because of the high cost of pure hydrogen. Methane, as the main component of natural gas, was considered to be the preferable H2 substitute because of its high H/C ratio. Smith et al. studied IBCSP No. 5 coal pyrolysis under various gas mixtures in a microbalance reactor. The results showed that weight loss was larger in pyrolysis of the coal sample under CH4/NO and CH4/O2 and the yields of C2 and C3 hydrocarbons were higher than that under other atmospheres.1 Steinberg and Fallon found that the total liquid yield under CH4 was 6 times that under He when New Mexico sub-bituminous coal pyrolysis was performed at 900 °C.2 Liu et al. studied pyrolysis of Datong coal under CH4/O2 with Ni/Al2O3 catalyst at 650 °C and found that the tar yield was 2.3 times that under H2.3 In our recent work, a novel and effective integrated process of coal pyrolysis and CO2 reforming of methane over Ni/MgO catalyst was developed to improve tar yield.4 At an appropriate condition, the tar yield could increase by 60% compared to that in hydropyrolysis, which was ascribed to the
(5) Wang, S. B.; Lu, G. Q. A comprehensive study on carbon dioxide reforming of methane over Ni/γ-Al2O3 catalysts. Ind. Eng. Chem. Res. 1999, 38 (7), 2615–2625. (6) Rezaei, M.; Alavi, S. M.; Sahebdelfar, S.; Yan, Z. F. Nanocrystalline zirconia as support for nickel catalyst in methane reforming with CO2. Energy Fuels 2006, 20 (3), 923–929. (7) Luengnaruemitchai, A.; Kaengsilalai, A. Activity of different zeolite-supported Ni catalysts for methane reforming with carbon dioxide. Chem. Eng. J. 2008, 144 (1), 96–102. (8) Kumar, P.; Sun, Y. P.; Idem, R. O. Comparative study of Ni-based mixed oxide catalyst for carbon dioxide reforming of methane. Energy Fuels 2008, 22 (6), 3575–3582. (9) Castro Luna, A. E.; Iriarte, M. E. Carbon dioxide reforming of methane over a metal modified Ni-Al2O3 catalyst. Appl. Catal., A 2008, 343 (1-2), 10–15. (10) Ruckenstein, E.; Hu, Y. H. Carbon dioxide reforming of methane over nickel/alkaline earth metal oxide catalysts. Appl. Catal., A 1995, 133 (1), 149–161. (11) Hu, Y. H.; Ruckenstein, E. The characterization of a highly effective NiO/MgO solid solution catalyst in the CO2 reforming of CH4. Catal. Lett. 1997, 43 (1-2), 71–77.
†
Progress in Coal-Based Energy and Fuel Production. *To whom correspondence should be addressed. Telephone/Fax: þ86-411-39893966. E-mail:
[email protected]. (1) Smith, G. V.; Wiltowski, T.; Phillips, J. B. Conversion of coals and chars to gases and liquids by treatment with mixtures of methane and oxygen or nitric oxide. Energy Fuels 1989, 3 (4), 536–537. (2) Steinberg, M.; Fallon, P. T. Make ethylene and benzene by flash methanolysis of coal. Hydrocarbon Process. 1982, 61 (11), 92–96. (3) Liu, Q. R.; Hu, H. Q.; Zhu, S. W. Integrated process of coal pyrolysis with catalytic partial oxidation of methane. 2005 International Conference on Coal Science and Technology, Okinawa, Japan, Oct 9-14, 2005. (4) Liu, J. H.; Hu, H. Q.; Jin, L. J.; Wang, P. F.; Zhu, S. W. Integrated coal pyrolysis with CO2 reforming of methane over Ni/MgO catalyst for improving tar yield. Fuel Process. Technol. 2009, in press, doi: 10.1016/j. fuproc.2009.05.003. r 2009 American Chemical Society
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Energy Fuels 2009, 23, 4782–4786
: DOI:10.1021/ef900272n
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showed high resistance to carbon formation and the selectivity to carbon formation was much lower than that of the Ni/Al2O3 catalyst.12 The aim of this work was to examine the effects of catalyst supports and calcination temperature of Ni/MgO catalysts on tar and water yields, CH4 conversion, and carbon deposition on catalysts under different reaction conditions in the integrated process of coal pyrolysis and CO2 reforming of methane.
Table 1. Proximate and Ultimate Analyses of the PS Coal Sample proximate analysis (wt %) Mad
Ad
Vdaf
C
H
N
S
Oa
2.23
17.93
37.19
80.41
5.20
1.38
1.06
11.95
a
By difference.
Catalyst Characterization. X-ray diffraction (XRD) patterns of different catalyst samples were obtained on a DMAX2400 diffractor with Cu KR radiation. The amount of carbon deposition on Ni/MgO catalysts calcined at 500-800 °C during the integrated process was measured in a thermogravimetric analyzer (Mettler Toledo model TGA/SDTA851e) by heating the used catalysts to 500 °C in a flow of air (60 mL/min). The amount of carbon deposition on Ni/Al2O3, Ni/SiO2, and Ni/ NaY catalysts was roughly measured without using a thermogravimetric analyzer because of the serious carbon deposition on the catalysts, which made the catalyst samples non-uniform.
Experimental Section Coal Sample. A Chinese sub-bituminous coal, Pingshuo (PS) coal, ground to -100 mesh, was used in the pyrolysis experiments. The analyses of the coal sample are shown in Table 1. Catalyst Preparation. MgO (powder), Al2O3 (20-40 mesh), SiO2 (20-40 mesh), and NaY (20-40 mesh) were selected as supports to prepare nickel-loading catalyst. The catalysts were prepared by incipient wetness impregnation of the supports using Ni(NO3)2 3 6H2O as a precursor in the content of 10 wt % Ni. After drying at 110 °C for 12 h, the catalysts were calcined at 800 °C for 4 h, except for the Ni/NaY catalyst at 450 °C, to avoid the destruction of the crystal structure. The Ni/MgO catalyst powder was pressed, crushed, and sieved to 20-40 mesh. Ni/MgO, Ni/Al2O3, and Ni/SiO2 catalysts were reduced at 850 °C, and the Ni/NaY catalyst was reduced at 500 °C, for 4 h in a stream of 15% H2/N2 before use. Apparatus and Procedures. The coal pyrolysis under the gas mixture of CH4 and CO2 was carried out in a fixed-bed reactor, as shown in Figure 1. The fixed-bed reactor, constructed of stainless steel with a length of 150 mm and an internal diameter of 18 mm, contains an upper catalyst layer and a lower coal layer. About 1 g of catalyst and 5 g of coal were loaded in the two layers, respectively, which was separated by quartz wool. The height of the catalytic bed is about 4 mm. A thermowell, made of stainless steel of 2.5 mm outer diameter and equipped with a movable thermocouple, was placed in the center of the coal bed to monitor the pyrolysis temperature. The gas mixture of CH4 and CO2 in a CO2/CH4 ratio of 0.25-1.25 and CH4 flow rate of 100-400 mL/min was introduced into the reactor from the upside. The gas mixture passed first through the catalyst layer, then to the coal layer, and out of the reactor with gas and liquid products of coal pyrolysis. The reactor was heated to a desired temperature (500-800 °C) within 10 min, and the holding time, which is the time for the reactor kept at the desired temperature, is 0-40 min. For comparison, the coal pyrolysis under H2 and N2 was also performed at the same experimental conditions as those under CH4/CO2, except without catalyst and gas (H2 or N2) flow rate of 400 mL/min. The liquid products involving tar and water out of the reactor were condensed in a glass bottle immersed in a cool trap at -15 °C, and then the water in the liquid products was separated according to American Society for Testing and Materials (ASTM) D95-05e1, using toluene as a solvent. In this case, the yields of tar and water can be separately obtained. The gas product was collected and analyzed to obtain gas composition by gas chromatography (GC7890T) equipped with two channels (5A molecular sieve and GDX502) and a thermal conductivity detector. The conversion of CH4, CCH4, in coal pyrolysis under CH4/CO2 was calculated using the following formula: CCH4 ð%Þ ¼
ultimate analysis (wt %, daf)
Results and Discussion Effect of Catalyst Supports. The tar and water yields, CH4 conversion in pyrolysis of PS coal under CH4/CO2, and carbon deposition on catalysts with different supports are summarized in Table 2. Among these four supports, MgO provides a higher tar yield, lower water yield, and CH4 conversion, which reaches 33.5 wt %, 25.8 wt %, and 16.8%, respectively. The carbon deposition on the Ni/ MgO catalyst is 4.3 wt %, while the carbon deposition on Ni/Al2O3, Ni/SiO2, and Ni/NaY catalysts is higher than 100 wt %, respectively, which caused catalyst deactivation, reactor plugging, and catalyst breakdown within 30 min. The carbon formation reactions in the integrated process may be as follows:13 CH4 f C þ 2H2
ΔH 0 ¼ þ 75:2 kJ=mol
ð1Þ
2CO f C þ CO2
ΔH 0 ¼ -173:0 kJ=mol
ð2Þ
The CH4 decomposition reaction 1 and CO disproportionation reaction 2 result in coke accumulation on the catalyst surface. A large amount of carbon deposition on Ni/Al2O3, Ni/SiO2, and Ni/NaY catalysts makes the catalyst deactivate, which decreases the tar yield and increases the CH4 conversion by the CH4 decomposition reaction 1. When H2 produced from reaction 1 reacts with CO2 by reverse water-gas shift and methanation reactions,4 the water will be formed. This may explain the high water yield over Ni/ Al2O3, Ni/SiO2, and Ni/NaY catalysts. Figure 2 displays the XRD patterns of Ni/MgO, Ni/ Al2O3, and Ni/SiO2 catalysts calcined at 800 °C and the Ni/NaY catalyst calcined at 450 °C. The Ni/MgO catalyst shows no NiO peaks in the XRD patterns. Five major peaks at 36.96°, 42.96°, 62.36°, 74.72°, and 78.68° for 2θ are identified as MgO and/or MgNiO2 in the XRD of the Ni/ MgO catalyst.14 Because NiO has the lattice parameters and bond distance close to those of MgO, NiO and MgO can form a solid solution.10 The small Ni particles reduced from
Ftotal, in cin -Ftotal, out cout 100% Ftotal, in cin
where Ftotal,in and Ftotal,out represent the inlet and outlet flow of the total gas and cin and cout represent the inlet and outlet concentration of CH4 in the total gas, respectively.
(13) Luo, J. Z.; Yu, Z. L.; Ng, C. F.; Au, C. T. CO2/CH4 reforming over Ni-La2O3/5A: An investigation on carbon deposition and reaction steps. J. Catal. 2000, 194 (2), 198–210. (14) Furusawa, T.; Tsutsumi, A. Comparison of Co/MgO and Ni/ MgO catalysts for the steam reforming of naphthalene as a model compound of tar derived from biomass gasification. Appl. Catal., A 2005, 278 (2), 207–212.
(12) Tomishige, K.; Chen, Y. G.; Fujimoto, K. Studies on carbon deposition in CO2 reforming of CH4 over nickel-magnesia solid solution catalysts. J. Catal. 1999, 181 (1), 91–103.
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Figure 1. Schematic for the experimental apparatus: (1) gas cylinder, (2) valve, (3) mass flow controller, (4) mass flow indicator, (5) pressure gauge, (6) temperature controller, (7) temperature indicator, (8) furnace, (9) reactor, (10) cool trap, (11) decompress valve, and (12) gas flowmeter. Table 2. Tar and Water Yields, CH4 Conversion of PS Coal under CH4/CO2, and Carbon Deposition on Ni Catalysts with Different Supportsa catalyst support MgO Al2O3 SiO2 NaY
tar yield (wt %, daf)
H2O yield (wt %, daf)
conversion of CH4 (%)
carbon deposition (wt %)
33.5 30.2 28.0 29.7
25.8 34.4 40.9 72.6
16.8 24.6 27.5 39.3
4.3 >100 >100 >100
a Conditions: pyrolysis temperature, 750 °C; holding time, 30 min; CO2/CH4 ratio, 1:1; CH4 flow rate, 400 mL/min; 10 wt % Ni/MgO, Ni/Al2O3, Ni/ SiO2, and Ni/NaY catalysts.
NiO-MgO solid solution remain highly dispersed on the catalyst surface, which inhibits carbon deposition effectively.15 In addition, the strong interaction of Ni and MgO prohibits the formation of the large clusters of Ni needed for coke generation.11 Besides, the basicity of MgO preventing CH4 decomposition also makes the catalyst show higher resistance to coke formation.16 Therefore, the Ni/MgO catalyst shows better performance, and higher tar yield and lower water yield and CH4 conversion over the Ni/MgO catalyst were obtained, as shown in Table 2. In XRD patterns of the Ni/Al2O3 catalyst, no NiO but NiAl2O4 and Al2O3 phases are observed for the strong NiO-A12O3 interaction at high calcination temperatures.17 A higher content of carbon deposition on the Ni/Al2O3 catalyst than that on the Ni/MgO catalyst can be ascribed to the Lewis acid sites of Al2O3 that facilitates the cleavage of C-H bonds of CH4 and enhances carbon accumulation at high temperature.18 NiO phases are observed in XRD patterns of Ni/SiO2 and Ni/NaY catalysts. It is known that the NiO species is
Figure 2. XRD patterns of catalysts with different supports.
responsible for coke formation;19 therefore, the extremely high content of carbon deposition on Ni/SiO2 and Ni/NaY catalysts may be attributed to the large NiO particle ensembles, which decreases the catalytic activity. Effect of the Calcination Temperature. The Ni/MgO catalyst shows higher tar yield and lower carbon deposition because of high dispersion of reduced Ni species, interaction of Ni and MgO, and the basicity of the support surface.20 Therefore, the Ni/MgO catalyst was further studied upon catalyst preparation. Table 3 shows the tar and water yields, CH4 conversion in pyrolysis of PS coal under CH4/CO2, and carbon deposition on Ni/MgO catalysts with different calcination temperatures. It can be seen that higher tar and water yields and CH4 conversion over the Ni/MgO catalyst
(15) Requies, J.; Cabrero, M. A.; Barrio, V. L.; Guemez, M. B.; Cambra, J. F.; Arias, P. L.; Perez-Alonso, F. J.; Ojeda, M.; Pe~ na, M. A.; Fierro, J. L. G. Partial oxidation of methane to syngas over Ni/MgO and Ni/La2O3 catalysts. Appl. Catal., A 2005, 289 (2), 214–223. (16) Horiuchi, T.; Sakuma, K.; Fukui, T.; Kubo, Y.; Osaki, T.; Mori, T. Suppression of carbon deposition in the CO2-reforming of CH4 by adding basic metal oxides to a Ni/Al2O3 catalyst. Appl. Catal., A 1996, 144 (1-2), 111–120. (17) Jiang, H. T.; Li, H. Q.; Zhang, Y. Tri-reforming of methane to syngas over Ni/Al2O3;Thermal distribution in the catalyst bed. J. Fuel Chem. Technol. 2007, 35 (1), 72–78. (18) Nagaoka, K.; Seshan, K.; Aika, K.; Lercher, J. A. Carbon deposition during carbon dioxide reforming of methane;Comparison between Pt/Al2O3 and Pt/ZrO2. J. Catal. 2001, 197 (1), 34–42. (19) Roh, H. S.; Jun, K. W.; Park, S. E. Methane-reforming reactions over Ni/Ce-ZrO2/θ-Al2O3 catalysts. Appl. Catal., A 2003, 251 (2), 275– 283.
(20) Chen, Y. G.; Tomishige, K.; Yokoyama, K.; Fujimoto, K. Catalytic performance and catalyst structure of nickel-magnesia catalysts for CO2 reforming of methane. J. Catal. 1999, 184 (2), 479–490.
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Table 3. Tar and Water Yields, CH4 Conversion of PS Coal under CH4/CO2, and Carbon Deposition on Ni/MgO Catalysts Calcined at Different Temperaturesa calcination temperature (°C)
tar yield (wt %, daf)
H2O yield (wt %, daf)
conversion of CH4 (%)
carbon deposition (wt %)
800 700 600 500
33.5 28.2 29.5 27.0
25.8 22.0 23.8 20.6
16.8 10.5 12.7 10.8
4.3 5.8 6.5 6.1
a
Conditions: pyrolysis temperature, 750 °C; holding time, 30 min; CO2/CH4 ratio, 1:1; CH4 flow rate, 400 mL/min; 10 wt % Ni/MgO catalysts.
Figure 3. XRD patterns of Ni/MgO catalysts calcined at different temperatures.
calcined at 800 °C were obtained than those over Ni/MgO catalysts calcined at 500-700 °C. Figure 3 presents the XRD patterns of Ni/MgO catalysts calcined at 500-800 °C. No distinct differences in XRD intensities are found when Ni/MgO catalysts were calcined below 700 °C. However, when increasing the calcination temperature to 800 °C, the intensities of XRD peaks increase, suggesting that more NiO-MgO solid solution was formed. The NiO-MgO solid solution has highly dispersed Ni particles after reduction. The more the NiO-MgO solid solution, the better the dispersion of the Ni particle and the catalytic activity will be improved correspondingly.11 This may be the explanation of the Ni/MgO catalyst calcined at 800 °C providing higher tar and water yields and CH4 conversion compared to those calcined at 500-700 °C. Effect of the Pyrolysis Temperature. Figure 4 shows the effect of the temperature on tar, water, and char yields in pyrolysis of PS coal under CH4/CO2, H2, and N2 and CH4 conversion and gas composition in pyrolysis of PS coal under CH4/CO2. The tar and water yields and CH4 conversion increase greatly with the increase of the pyrolysis temperature. At the pyrolysis temperature of 750 °C, the tar yield under CH4/CO2 is 1.6 and 1.8 times that under H2 and N2, respectively, while the water yield represents 23.3 and 24.4 wt % increment over that under H2 and N2, respectively. The fact that the free radicals cracked in coal pyrolysis are stabilized evidently when combining with the CHx groups dissociated from CH4 in CO2 reforming of methane leads to the significant increase of tar yield under CH4/CO2. In addition, the reactions of the reverse water-gas shift and methanation are responsible for the extremely high water yield under CH4/ CO2. A strongly endothermic reaction of CO2 reforming of methane is the explanation that the CH4 conversion enhances with the increase of the pyrolysis temperature.4 The char yield under CH4/CO2, H2, and N2 at 750 °C is 69.5, 62.3, and 65.8 wt %, respectively, and decreases with an increasing pyrolysis temperature. The highest char yield
Figure 4. Effect of the temperature on tar, water, and char yields of PS coal pyrolysis under CH4/CO2, H2, and N2, and CH4 conversion and gas composition under CH4/CO2 (conditions: holding time, 30 min; CO2/CH4 ratio, 1:1; CH4 flow rate, 400 mL/min; 10 wt % Ni/ MgO catalyst calcined at 800 °C).
under CH4/CO2 could be ascribed to the carbon deposition on char.21 The gas composition under CH4/CO2 includes H2, CO, C2H4, and C2H6 besides CH4 and CO2. The content of H2 and CO, much higher than that of C2H4 and C2H6, increases with the increase of the pyrolysis temperature. Effect of the Holding Time. According to the mechanism for improving tar yield under CH4/CO2, the formation of free radicals cracked from coal is one of the main factors. It is expected to produce more free radicals from coal by increasing the holding time at the pyrolysis temperature. Figure 5 presents the tar and water yields and CH4 conversion in pyrolysis of PS coal under CH4/CO2 at 750 °C with different holding times. The results show that an obvious increase in tar and water yields occurs as the holding time increases to 30 min, while the CH4 conversion remains almost unchanged. It may be concluded that the tar yield increases through the combination of CHx with more free radicals (21) Haghighi, M.; Sun, Z. Q.; Wu, J. H.; Bromly, J.; Wee, H. L.; Ng, E.; Wang, Y.; Zhang, D. K. On the reaction mechanism of CO2 reforming of methane over a bed of coal char. Proc. Combust. Inst. 2007, 31 (2), 1983–1990.
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Figure 5. Effect of the holding time on tar and water yields and CH4 conversion of PS coal pyrolysis under CH4/CO2 (conditions: pyrolysis temperature, 750 °C; CO2/CH4 ratio, 1:1; CH4 flow rate, 400 mL/min; 10 wt % Ni/MgO catalyst calcined at 800 °C).
Figure 7. Effect of the CH4 flow rate on tar and water yields and CH4 conversion of PS coal pyrolysis under CH4/CO2 (conditions: pyrolysis temperature, 750 °C; holding time, 30 min; CO2/CH4 ratio, 1:1; 10 wt % Ni/MgO catalyst calcined at 800 °C).
and 5 and results in the increase of CH4 conversion. When CHx generated from reaction 3 combines with the free radicals cracked from coal, the free radicals are stabilized and the tar yield is enhanced accordingly. Effect of the CH4 Flow Rate. The contact time of the gas mixture and Ni/MgO catalyst has a significant influence on the integrated process. The tar and water yields and CH4 conversion in pyrolysis of PS coal under CH4/CO2 at different CH4 flow rates are shown in Figure 7. The tar and water yields increase with an increasing CH4 flow rate from 100 to 300 mL/min and change a little at above 300 mL/min. CH4 conversion decreases sharply from 64.0 to 16.8% as the CH4 flow rate varies from 100 to 400 mL/min. A high CH4 flow rate shortens the contact time of the gas mixture with the Ni/ MgO catalyst, which makes CH4 conversion decrease.22 However, with the increase of the CH4 flow rate, the resistance that prevents the gas mixture from contacting the coal particles will be reduced and CHx penetrates the coal particle and combines with free radicals cracked from coal to form tar, which results in the increase of the tar yield.
Figure 6. Effect of the CO2/CH4 ratio on tar and water yields and CH4 conversion of PS coal pyrolysis under CH4/CO2 (conditions: pyrolysis temperature, 750 °C; holding time, 30 min; CH4 flow rate, 400 mL/min; 10 wt % Ni/MgO catalyst calcined at 800 °C).
cracked from coal within 30 min of the holding time and the effect is slight after 30 min for the reduction of free radicals cracked from coal. Effect of the CO2/CH4 Ratio. The effects of the CO2/CH4 ratio on tar and water yields and CH4 conversion in pyrolysis of PS coal under CH4/CO2 are shown in Figure 6. It can be seen that the tar and water yields and CH4 conversion increase significantly with the increase of the CO2/CH4 ratio in a range below 1.00. The mechanism for CO2 reforming of methane is complex. The likely reaction steps proposed by investigators can be summarized as follows:13 CH4, s f CHx, s þ ð4 -xÞHs
ð3Þ
CO2, s f COs þ Os
ð4Þ
CH4, s þ Os f CHx Os þ ð4 -xÞHs
ð5Þ
CO2, s þ xHs f CHx Os þ Os
ð6Þ
CHx Os f CO þ xHs
ðs ¼ surfaceÞ
Conclusions The Ni/MgO catalyst calcined at 800 °C presents a higher tar yield and lower water yield, CH4 conversion, and carbon deposition than Ni/Al2O3, Ni/SiO2, and Ni/NaY catalysts in Pingshuo coal pyrolysis under CH4/CO2, which can be ascribed to the high dispersion of reduced Ni species, interaction of Ni and MgO, and the basicity of the support surface. The tar and water yields increase with the increase of the pyrolysis temperature, holding time, CO2/CH4 ratio, and CH4 flow rate, respectively, which reach 33.5 and 25.8 wt % at the conditions of 750 °C, 30 min holding time, 1:1 CO2/CH4 ratio, and 400 mL/min CH4 flow rate. Carbon deposition results in the increase of the char yield. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (20576019 and 20776028), the National High Technology Research and Development Program of China (863 Program), the Ministry of Science and Technology (2008AA05Z307), and the Key Program in Major Research Plan for the West of China, the National Natural Science Foundation of China (90410018).
ð7Þ
CH4 is decomposed to reactive carbon species (CHx, x = 0-3) on the metallic sites, which are oxidized by the oxygencontaining species originated from CO2. Increasing the CO2/ CH4 ratio is favorable for augmenting the amount of Os, which accelerates CH4 decomposition through reactions 3
(22) Tomishige, K. Syngas production from methane reforming with CO2/H2O and O2 over NiO-MgO solid solution catalyst in fluidized bed reactors. Catal. Today 2004, 89 (4), 405–418.
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