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Extended Abstract In recent years, the effective use of low grade fossil fuel and the diversity of fuels are required due to the exhaustion of fossil fuels, global warming and the nuclear accident caused by the Tohoku earthquake in Japan. The studies of brown coal, biomass, biofuel, etc. are being conducted to expand the use of these fuels throughout the world. We have paid attention to biogas among such low grade fuels. Biogas is a carbon neutral fuel to be generated from garbage, livestock manure, sewage sludge, etc. CO2 emitted by its combustion is not converted to greenhouse gases. On the other hand, biogas is primarily composed of 60% CH4 and 40% CO2, and has a low calorific value. Low calorific value does not make biogas easy to apply to a conventional energy utility system. To utilize biogas, it is necessary to develop the combustion promotion technology of biogas or upgrade biogas.

Biogas, composed of ∼60% CH 4 and ∼40% CO 2 , is a carbon-neutral fuel but suffers from low flame stability. Thus, for effective utilization, the combustion reaction must be enhanced or biogas must be upgraded. Herein, microwave plasma-assisted combustion was utilized for partial oxidation reforming of biogas to enhance the reforming reaction and to upgrade biogas. The characteristics of the biogas reforming process and effects of the nozzle shape were investigated using four different nozzles. The small thin nozzle suppressed microwave attenuation to the greatest extent and thus most effectively promoted the reforming reaction. Microwave plasma not only reformed CH 4 into H 2 and CO but also decomposed CO 2 (which does not have any calorific value) into CO (which has calorific value); thus, the maximum cold gas efficiency achieved with the small thin nozzle was 112.7%, and the biogas was upgraded. Biogas reforming using O 2 as an oxidizer was also evaluated to improve the cold gas efficiency, but the cold gas efficiency undesirably decreased in this reaction relative to that using air with N 2 as the oxidizer because the reforming reactions are enhanced by increasing the active radicals like H and OH in the latter.