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Plasma Reforming and Coupling of Methane with Carbon Dioxide S. L. Yao,* M. Okumoto, A. Nakayama, and E. Suzuki Catalysis Science Laboratory, Research Institute of Innovative Technology for the Earth, 9-2 Kizugawadai, Kizu-cho, Soraku-gun, Kyoto 619-0292, Japan Received April 11, 2001. Revised Manuscript Received July 12, 2001
Plasma reforming and coupling of methane with carbon dioxide using a point-to-point type of reactor have been invested. A feed mixture of CH4 and CO2 could be converted mainly to CO, H2, and C2H2 at atmospheric pressure and without external heating except plasma heating. Under a condition of 200 mL/min of CH4 and CO2 (CH4:CO2 volume ratio, 50:50), a 2.5 mm discharge gap, and a pulse frequency of 10.3 kPPS, CH4 and CO2 conversion, CO and C2H2 selectivities, H2/CO ratio, and (CH4 + CO2) conversion efficiency were 65.9% and 57.8%, 85.9% and 11.3%, 0.99, and 2.4 mmol/kJ, respectively. C2H2 selectivity and H2/CO ratio could be moderated by changing CH4 concentration in the feed mixture. The influence of methane concentration and pulse frequency on product selectivity and plasma energy efficiency was evaluated. A brief economic evaluation of this process was given. The coproduction of acetylene of high value remarkably improved the production of synthesis gas (CO + H2) from carbon dioxide reforming of methane, which could also contribute the emission reduction of global warming gas CO2.
Introduction Dry reforming of methane to synthesis gas (CO + H2) has recently attracted considerable interest.1-10 Such a reaction was first described by Fischer and Tropsch using various catalysts such as Ni supported on silica at 860 °C.11 The high reaction temperature implied that this process is very costly.12 On the other hand, methane conversion leads to productions of ethylene, acetylene, and higher hydrocarbons.13-15 Recent plasma conversion of methane has gained especially remarkable progress by research groups such as Dr. Mallinson’s16-20 of The University of Oklahoma, Dr. Eliasson’s of ABB Corpo* To whom correspondence should be addressed. Tel.: (+81) 77475-2305. Fax: (+81) 774-75-2318. E-mail:
[email protected]. (1) Poirier, M. G.; Trudel, J.; Guay, D. Catal. Lett. 1993, 21, 99. (2) Chang, Y. F.; Heineman, H. Catal. Lett. 1993, 21, 215. (3) Tsang, S. C.; Claridge, J. B.; Green, M. L. H. Catal. Today 1995, 23, 3. (4) Wang, S.; Lu, G. Q.; Millar, G. J. Energy Fuels 1996, 10, 896. (5) Bradford, M. C. J.; Vannice, M. A. J. Catal. 1996, 142, 73. (6) Ross, J. R. H.; van Keulen, A. N. J.; Hegarty, M. E. S.; Seshan, K. Catal. Today 1996, 30, 193. (7) Bitter, J. H.; Seshan, K.; Lercher, J. A. J. Catal. 1998, 176, 93. (8) O’connor, A. M.; Ross, J. R. H. Catal. Today 1998, 46, 203. (9) Bradford, M. C. J.; Vannice, M. A. Catal. Today 1999, 50, 87. (10) Tsui, M.; Miyao, T.; Naito, S. Catal. Lett. 2000, 69, 195. (11) Fischer, F.; Tropsch, H. Brennstoff Chem. 1928, 9, 39. (12) Solbakken, A. In Natural Gas Conversion; Holmen, A., et al., Eds.; Elsevier Science Publisher: Amsterdam, 1991; p 447. (13) Keller, G. E.; Bhasin, M. M. J. Catal. 1982, 73, 9. (14) Matherne, J. L.; Culp, G. AIChE, Ann. Meeting, Chicago, 1990, paper 59f. (15) Lee, L. L.; Aitani, A. M. Fuel Sci. Int. 1991, 9 (2), 137. (16) Mallinson, R. G.; Sliepcevich, C. M.; Rusek, S. Am. Chem. Soc., Div. Fuel Chem. 1987, 32, 266. (17) Bhatnagar, R.; Mallison, R. G. In Methane and Alkane Conversion Chemistry; Bhasin, M. M., Slocum, D. W., Eds.; Plenum Press: New York, 1995; p 249. (18) Liu, C. G.; Marafee, A.; Hill, B. J.; Xu, G. H.; Mallinson, R.; Lobban, L. Ind. Eng. Chem. Res. 1996, 35, 5 (10), 3295. (19) Larkin, D. W.; Caldwell, T. A.; Lobban, L. L.; Mallinson, R. G. Energy Fuels 1998, 12, 740.
rate Research Ltd,21 and some other groups.22-25 We have very recently reported that a high-frequency pulsed plasma can obviously improve the energy efficiency of methane conversion.26 Such an approach implied that this kind of plasma can be used for production of acetylene and hydrogen from methane. Our other report has shown that the energy efficiency of such a plasma can be improved for methane oxidative coupling and reforming with carbon dioxide by using a coaxial cylindrical (CAC) type of reactor.27 Furthermore, we also found that a point-to-point (PTP) type of reactor used for methane conversion has a higher energy efficiency than that of the CAC reactor.28 In this study, we investigated methane reforming and coupling with carbon dioxide using the PTP reactor. An economic evaluation of this process was also given. Experimental Section The PTP reactor is shown in Figure 1, which was simply composed of a Pyrex tube and two stainless steel electrodes of sharp terminals. The pulse power supply and related mea(20) Larkin, D. W.; Lobban, L. L.; Mallinson, R. G. 1st International Conference on Gas Processing; AIChE Spring National Meeting, GA, March 5-9, 2000; p 10. (21) Zhou, L. M.; Xue, B.; Kogelschatz, U.; Eliasson, B. Energy Fuels 1998, 12 (6), 1191. (22) Gesser, H. D.; Hunter, N. R.; Probawono, D. Plasma Chem. Plasma Process. 1998, 18 (2), 241. (23) Bromberg, L.; Cohn, D. R.; Rabinovich, A. Energy Fuels 1998, 12, 11. (24) Huang, A.; Xia, G.; Wang, J.; Suib, S. L.; Hayashi, Y.; Matsumoto, H. J. Catal. 2000, 189, 349. (25) Okumoto, M.; Takahshima, K.; Katsura, S.; Mizuno, A. Thermal Sci. Eng. 1999, 7 (3), 23. (26) Yao, S. L.; Nakayama, A.; Suzuki, E. AIChE J. 2001, 47 (2), 413. (27) Yao, S. L.; Ouyang, F.; Nakayama, A.; Suzuki, E.; Okumoto, M.; Mizuno, A. Energy Fuels 2000, 14, 910. (28) Yao, S. L.; Suzuki, E.; Meng, N.; Nakayama, A. Plasma Chem. Plasma Process., in press.
10.1021/ef010089+ CCC: $20.00 © 2001 American Chemical Society Published on Web 08/23/2001
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Figure 1. Point-to-point reactor. Pyrex tube: 15 (o.d.) × 12 (i.d.) × 800 (length) mm3. Anode and cathode, stainless steel wire of sharp terminals. Gap distance, 2.5 mm. surement system were described in detail elsewhere.27,29 The peak pulse voltage was fixed at 17 kV under a condition without loads. A feed mixture of methane and carbon dioxide was introduced into the upper part of the vertically placed reactor at a flow rate of 200 mL/min. Carbon compounds in the effluent from the lower part of the reactor were analyzed with an online gas chromatograph equipped with a 2 m Porapak N and FID. The products were pretreated with a methanizer (MT-221, GL Science) to convert carbon compounds to related alkanes prior to detection by FID. H2 concentration was measured with another online gas chromatograph equipped with a 2 m activated carbon and TCD. All experiments were carried out at atmospheric pressure without external heating except plasma heating. The overall conversions were defined as
CH4 conversion ) moles of CH4 before reaction - moles of CH4 after reaction moles of CH4 before reaction × 100% CO2 conversion ) moles of CO2 before reaction - moles of CO2 after reaction moles of CO2 before reaction × 100% CO selectivity was defined as
CO selectivity ) moles of CO produced × moles of CH4 converted + moles of CO2 converted 100% The selectivity of C2H2 was defined as
C2H2 selectivity ) 2 × moles of C2H2 produced × moles of CH4 converted + moles of CO2 converted 100% The energy injection for discharge was calculated from waveforms of voltage and cathode current using the following integration formula:
Vi+1 + Vi Ii+1 + Ii (ti+1 - ti) 2 2
∑
P)F
where P, F, Vi, Ii, and ti are, respectively, energy injection in watts, pulse frequency in pulses per second (PPS), discharge (29) Yao, S. L.; Nakayama, A.; Suzuki, E. AIChE J. 2001, 47 (2), 419.
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Figure 2. Influence of pulse frequency on methane (O) and carbon dioxide (]) conversions. CH4 concentration in the feed gas was 50%.
Figure 3. Selectivities of C2H2 (0), CO (O), and other products (]) at various pulse frequencies. Conditions are the same as those in Figure 2. voltage in volts, cathode discharge current in amperes, and discharge time in seconds. The conversion efficiency of CH4 and CO2 was used to show the energy efficiency of this plasma and given in millimoles of CH4 and CO2 converted per kilojoules of energy injected as
conversion efficiency ) millimoles of CH4 and CO2 converted per second P/1000
Results and Discussion The dry reforming of methane with CO2 is usually carried out at a ratio 1:1 of CH4 to CO2, yielding a 1:1 mixture of H2 and CO (eq 1)
CH4 + CO2 ) 2 CO + 2H2
(1)
Here, we first investigated the influence of pulse frequency at a CH4 concentration of 50%. Conversions of CH4 and CO2 increased with increasing pulse frequency (Figure 2). CH4 conversion was higher than CO2 conversion, as there were some CO2 formation reactions:
OH + CO ) H + CO2
(2)
The selectivity of CO increased with the increase in pulse frequency, but selectivities of C2H2 and other products such as C2H4, C2H6, and higher hydrocarbons decreased (Figure 3). In this study, we found that the selectivity of C2H2 was higher than that of C2H4. This finding deferred from that with a coaxial cylindrical type of reactor on which we reported previouslysnamely, that C2H4 is the main product after CO.27 This difference came from the geometry of the reactor. When using the CAC reactor, the effluent from the discharge space
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Figure 4. Ratios of H2 to CO at various pulse frequencies. Conditions are the same as those in Figure 2.
Figure 5. Conversion efficiency of CH4 and CO2 at various pulse frequencies. Conditions are the same as those in Figure 2.
Figure 6. Ratios of H2 to CO at various CH4 concentrations. Pulse frequency was 4 kPPS.
Figure 7. Influence of methane concentration on methane (O) and carbon dioxide (]) conversions. Conditions are the same as those in Figure 6.
is kept at a relatively high temperature, which enhances the reaction of C2H2 and H2 to form C2H4; some C2H2 reactions with H such as eqs 3 and 4 also contribute C2H4 formation. This implied the rapid quenching of the effluent was important
C2H2 + H ) C2H3
(3)
C2H3 + H ) C2H4
(4)
The ratio of H2/CO in the product is shown in Figure 4. The H2/CO ratio peaked at a pulse frequency of 2 kPPS. The H2/CO ratio was 0.99, lower than 1.0 at 10.3 kPPS due to water production. The conversion efficiency of CH4 and CO2 decreased at high pulse frequencies (Figure 5). The conversion efficiency of CH4 and CO2 was higher using the PTP reactor than using the CAC reactor (about 0.5-0.7 mmol/kJ27), especially at a low pulse frequency range. This finding was similar to that of CH4 conversion.26 One main product of CH4 conversion is CH3OH from the reforming of CH4 with H2O (eq 5) and the reaction of CO and H2 (eq 6) in industry:
CH4 + H2O ) CO + 3H2
(5)
CO + 2H2 ) CH3OH
(6)
Since the synthesis gas from dry reforming of CH4 (eq 1) cannot be directly used for CH3OH production, the reaction yielding a high H2/CO ratio is required. We then carried out CH4 reforming with CO2 at various CH4
Figure 8. Selectivities of C2H2 (O), CO (]), and other products (4) at various CH4 concentrations. Conditions are the same as those in Figure 6.
concentrations and at a fixed pulse frequency of 4 kPPS. The H2/CO ratio increased remarkably with increasing CH4 concentration (Figure 6). A ratio of H2/CO of 2 could be obtained at a CH4 concentration of 67%. The conversions of CH4 and CO2 at various CH4 concentrations are shown in Figure 7. Conversions of both CH4 and CO2 peaked around a CH4 concentration of 60-70%. This satisfied the CH3OH production since a CH4 concentration of 67% could give a H2/CO ratio of 2. The selectivity of each product is illustrated in Figure 8. CO selectivity decreased, but C2H2 increased with increasing CH4 concentration. Since C2H2 is an important raw material and H2 is an clean energy source, Figure 8 implied that we could get these important chemicals by changing CH4 concentration.
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Figure 9. Conversion efficiencies of CH4 and CO2 at various CH4 concentrations. Conditions are the same as those in Figure 6.
The energy efficiency shown in Figure 9 shows that the conversion efficiency of CH4 and CO2 did not change obviously at a CH4 concentration higher than 70% but increased below 70%. The high energy consumption of a plasma usually limits its application in industry. Here, we used the results shown in Figures 6-9 to calculate the energy consumption in the discharge process and value of products C2H2 and CH3OH to show electricity consumption by assuming: (i) Energy efficiency of the pulse power supply: 80%30 (ii) Electricity price: $0.05 $/kWh (iii) Prices of products and raw materials: C2H2, $5 $/kg31; CH3OH, $0.2/kg32; CH4, $0.035/kg32; CO2, 1 kW h/kg.33 (iv) C2H2 could be 100% recovered. (v) H2 and CO could be 100% recovered and used for CH3OH production. Mixture of H2, CO, CO2, and CH4 can be separated with a pressure swing adsorption (PSA) gas separation system.34 Cost for separation of H2, CO, CO2, and CH4 was ignored. For convenient of comparison, a ratio of the electricity cost used for discharge to the value of products of C2H2 and CH3OH (eq 7) was used.
η)
electrcity cost value of C2H2 and CH3OH
(7)
As a result, shown in Figure 10, the electricity cost used for discharge was lower than the product value at CH4 concentrations higher than 43%. At 67% CH4 concentration, which gave a H2/CO ratio of 2 and a C2H2/CO ratio of 0.375, the η value became 0.36. The electricity consumption for the separation and purification of acetylene, ethylene, and hydrogen in the arc plasma of methane is less than 22% of total electricity consumption,35 and the energy consumption (30) Civitano, L. Achievable Level; NATO ASI Series G34: Ecological Sciences; Plenum: New York, 1993; p 103. (31) Up to $9-10/kg (cylinder) in Japanese market. 12394’s Chemicals, The Chemical Daily Co. Ltd.: 1994, p 217. (32) Parkyns, N. D.; Warburton, C. I.; Wilson, J. D. Catal. Today 1993, 18, 385. (33) Energy required to remove and recover CO2 from a coal-fired power plant. Halmann, M. M.; Steinberg, M. Greenhouse Gas Carbon Dioxide Mitigation, Science and Technology; Lewis Publishers: USA, 1999; p 137. (34) Mikami, K.; Ikumi, S.; Shishikura, S.; Matsuzaki, T.; Hokkedo, M.; Ibaragi, S. Tech. Rep. Mitsui Eng. Shipbuilding 1997, 161, 22.
Figure 10. Ratios of the electricity cost used for discharge to the product value of C2H2 and CH3OH (η) at various CH4 concentrations.
Figure 11. Ratios of the electricity cost used for discharge to the product value of C2H2 and CH3OH (η) as a function of acetylene price.
for CH3OH production from CO and H2 is less than 35% of total energy consumption in CH3OH production from CH4. Thus, the η value (including separation cost) of our process would be less than 0.46: ) electricity cost × (100% (discharge) + 22% (separation)) + CH3OH value × 35% value of C2H2 + CH3OH
) 0.36 × (100% + 22% ) + (1 - 0.375) × 0.2 × 35% 0.375 × 5 + (1 - 0.375) × 0.2 ) 0.44 + 0.02 This result suggested that the electricity cost used for discharge accounted for as high as 78% () 0.36/0.46), and it would be appropriate to ignore the separation cost for CO, H2, CO2, and CH4. If the price of CH3OH is $0.1/kg, the η value increases to 0.37. Since the high value of acetylene is the origin of this process, the sensitivity of the acetylene price is shown in Figure 11. The η value could be improved by increasing acetylene price. At acetylene prices lower than $3.4/kg, the η value could be kept at levels lower than 0.5. Conclusions CH4 reforming and CH4 coupling with CO2 have been carried out using a high-frequency pulsed plasma and (35) Gladisch, H. Hydrocarbon Process. Petrol. Refiner 1962, 41 (6), 159.
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a PTP reactor. Under a condition of 50:50 CH4:CO2 volume ratio and 10.3 kPPS pulse frequency, CH4 and CO2 conversion, CO and C2H2 selectivities, H2/CO ratio, and (CH4 + CO2) conversion efficiency were 65.9% and 57.8%, 85.9% and 11.3%, 0.99, and 2.4 mmol/kJ, respectively. The amounts of H2 and C2H2 were adjustable by changing the CH4 concentration in the feed gas. A raw feed gas of 67% CH4 and 33% CO2 could give a mixture containing mainly C2H2 and synthesis gas having a H2/ CO ratio of 2 suitable for CH3OH production. Because of the high value of C2H2, this process has economical
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potential for natural gas conversion to more valuable chemicals and is potentially contributable to the emission reduction of greenhouse gas CO2. Acknowledgment. We thank the New Energy and Industrial Technology Development Organization (NEDO) for financial support. Yao is grateful to NEDO for the fellowship support. EF010089+
