Plasma Catalytic Hybrid Reforming of Methane

Abstract

Introduction Hydrogen enriched fuel gases offer the potential for efficient low NOx combustion processes. The reduction of NOx-emission is caused by a reduction of the adiabatic flame temperature due to stable, lean combustion and in some cases due to steam addition or exhaust gas recirculation. In the past reduced NOx-emissions both from gas turbines and internal combustion engines were reported. At the same time the energy efficiency of internal combustion engines was improved for 15-50 % compared to pure gasoline operation. In stationary applications there is an additional potential for the reduction of specific CO2-emissions: If the hydrogen is generated by integrated reforming of fossil fuels, carbonaceous products of the reforming reaction like CO2, solid carbon or in the case of methane higher hydrocarbons may be separated prior to combustion and utilized as raw material e.g. in the chemical industry. For mobile application of PEM fuel cells requiring hydrogen as a fuel on-board generation of hydrogen is desirable because of the high energy storage density of gasoline or diesel fuel. Large scale production of hydrogen is performed e.g. by catalytic steam reforming of methane. However, this process requires a temperature of about 900 °C. At lower temperatures catalyst coking and poisoning can get a problem. For the generation of hydrogen enriched fuel gases a low temperature reforming process which is not sensitive to coking or poisoning would be desirable. Recently plasma reforming has been proposed for the efficient generation of hydrogen and higher hydrocarbons in a compact, light weight reactor. For large scale application arc based plasma torches heating the gas very rapidly to temperatures of several thousand degrees Celsius for complete fuel conversion are utilized. However, for small scale application and for incomplete fuel conversion nonthermal plasmas which avoid excessive gas heating are a much better choice. Non-thermal plasma (NTP) reforming induced by dielectric barrier discharges (DBD) has been shown to have the potential for the generation of hydrogen and higher hydrocarbons. In facilities where waste heat can be utilized to support NTPreforming an endothermic reforming process like methane steam reforming CH4 + H2O → CO + 3 H2, ∆H = 206 kJ/mole (1) rather than an exothermic one will be applied: However, neither the selectivity nor the energy efficiency of DBD induced methane steam reforming showed to be sufficient for practical application. For this reason plasma catalytic hybrid steam reforming of methane was investigated in the temperature range 200-600 °C.