Numerical Study on Biogas Refining System Combined with Proton-Conducting Solid Oxide Electrolyzer

Biogas is one of the most globally distributed carbon-neutral fuels to be used as a local biomass energy source for local society energy demand. However, since the biogas from organic waste is typically a mixture of 60% methane and 40% carbon dioxide, the biogas has a lower energy density compared with coal, oil, LNG, and etc. In this study, biogas refining system including methanation reactor and proton-conducting electrolysis cell has been proposed and investigated based on the numerical calculation including mass and heat balance analysis. Using this system, hydrogen and CO2 derived from electrolysis cell and biogas, respectively, will be converted into methane via methanation reaction. Carbon deposition is a well-known problem to be resolved when carbon-rich gas is supplied to the cell cathode. Based on the calculation results, the gas humidification was found to be necessary to prevent carbon deposition at the cathode inlet. However, carbon deposition was predicted to still occur at the cathode outlet even with the gas humidification when a conventional oxide-ion conducting electrolyte is applied to the electrolysis cell due to the enhanced carbon activity caused by oxide-ion transport from the cathode to the anode. In the case of the proton-conducting electrolyte, proton transport from the anode to the cathode was found to mainly promote reverse water-gas shift reaction and methanation reaction without any carbon deposition. Even though the calculation results should be verified by comparing with experimental results, the numerical study suggested that the biogas refining system combined with p-SOEC is a promising technology for generating carbon-neutral methane with a strongly suppressed risk of carbon deposition.