Biogas, composed of ∼60% CH4 and ∼40% CO2, 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 CH4 into H2 and CO but also decomposed CO2 (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 O2 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 N2 as the oxidizer because the reforming reactions are enhanced by increasing the active radicals like H and OH in the latter.
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