Energy & Fuels 1998, 12, 1191-1199
1191
Nonequilibrium Plasma Reforming of Greenhouse Gases to Synthesis Gas L. M. Zhou,† B. Xue, U. Kogelschatz,* and B. Eliasson ABB Corporate Research Ltd., 5405 Baden, Switzerland Received March 4, 1998
The use of dielectric-barrier discharges (DBDs) is a mature technology originally developed for industrial ozone production. In this article, it is demonstrated that DBDs are also an effective tool to convert the greenhouse gases CH4 and CO2 to synthesis gas (syngas, H2/CO) at low temperature and ambient pressure. The synthesis gas produced in this system can have an arbitrary H2/CO ratio, mainly depending on the mixing ratio of CH4/CO2 in the feed gas. Specific electric energy, gas pressure, and temperature hardly influence syngas composition. The amount of syngas produced strongly depends on the electric energy input. CO2-rich mixtures prevent carbon and wax formation. At fixed specific input energies, the maximum amount of syngas with low H2/CO molar ratio is produced from a mixture of CH4:CO2 ) 20:80. In a mixture of CH4:CO2 ) 80:20, as high as 52 mol of H2 and 14 mol of CO have been obtained from 100 mol of feed gas at a specific input energy of 87 kW h/(N m3). CH4 conversion reaches 64%, and CO2 conversion is 54%. High temperatures lead to wax formation and carbon deposition in CH4-rich feeding mixtures. Low gas pressures favor syngas production.
Introduction Carbon dioxide is a major man-made greenhouse gas. Anthropogenic CO2 emissions to the atmosphere originate mainly from combustion of fossil fuels (i.e., coal, natural gas, and oil). About one-third of the global emissions of carbon dioxide originates from power stations generating electricity. In addition to causing an increase in the global mean temperature, this emission also constitutes an extensive waste of natural C-source. One of the ideas presented so far is to remove CO2 from the flue gas of power plants and use it in a catalytic process to synthesize useful chemicals such as, e.g., methanol.1,2 The recovery of CO2 from the flue gas of a power plant has already been demonstrated on a medium scale in a coal-fired power plant built in Oklahoma. The hydrogenation of CO2 to methanol has been investigated on a laboratory scale with copperbased catalysts or/and dielectric-barrier discharges (DBDs). At about 220 °C and moderate pressures of less than 30 bar, a methanol yield of 5-10% with a selectivity of about 40% per single pass was obtained in a packed-bed reactor.3,4 These yields are already close to equilibrium values. To overcome this thermodynamic limitation, we inserted a catalyst into a DBD gap.5 The simultaneous presence of the discharge shifts the tem* To whom correspondence should be addressed. † On leave from Xi’an Jiaotong University, Xi’an, P. R. China. (1) Eliasson, B. CO2 Chemistry: An Option for CO2 Emission Control. In Carbon Dioxide Chemistry: Environmental Issues; Paul, J., Pradier, C. M., Eds.; The Royal Society of Chemistry: Cambridge, 1994; pp 5-15. (2) Eliasson, B.; Egli, W.; Kogelschatz, U. Pure Appl. Chem. 1994, 66, 1275-1286. (3) Bill, A.; Eliasson, B.; Killer, E. 11th World Hydrogen Energy Conference (HYDROGEN96), Stuttgart, Germany, 1996; Vol. II, 19891995. (4) Bill, A.; Wokaun, A.; Eliasson, B.; Killer, E.; Kogelschatz, U. Energy Convers. Manage. 1997, 38, S415-S422.
perature region of maximum catalyst activity from 220 to 100 °C. In this temperature range, the theoretical yield is 34% at 8 bar. So, in principle, there is much more room for improvement. Hydrogenation of CO2 requires hydrogen. This technology will only be attractive if hydrogen is produced in a cheap way and without additional CO2 emission.1 Methane (the chief constituent of natural gas) is the second major gas responsible for the greenhouse effect. It contributes about 12% of the total greenhouse forcing. Due to its longer lifetime, the time-integrated warming potential of CH4 per unit mass is much higher than that of CO2. Methane can be converted directly to methanol by partial oxidation6-8 or to ethane and ethylene by oxidative coupling.9 The methanol yield, so far, is not of economical interest since good partial oxidation conditions are difficult to maintain. Methane can also be looked upon as an abundant and hydrogen-rich source gas. It can be converted to synthesis gas (syngas: a gas mixture containing mainly of H2 and CO), an important chemical feedstock. Syngas can then be turned into useful chemicals, e.g., methanol and other valuable oxygenated compounds. There are three main routes for the conversion of methane to syngas: steam reforming, partial oxidation, and carbon dioxide reforming, producing syngas with H2/CO molar ratios of >3,
