Direct Methanation of CO2 in Biogas with Hydrogen from Water Electrolysis: The Catalyst and System Efficiency

Methanation of CO2 in biogas could realize CO2 utilization; meanwhile, the biogas is upgraded. In this study, a methanation catalyst was designed and the system efficiency considering a solid oxide electrolysis cell (SOEC) was evaluated. A layered nickel silicate catalyst was synthesized and modified by doping with trace amounts of precious metal Pt and alkaline-earth metal Mg. The conversion of CO2 reached 79% with 100% selectivity toward CH4 for model biogas with 50 vol % CO2 and 50 vol % CH4 over the Pt- and MgO-doped Ni/SiO2 catalyst at 350 °C. A simulation system consisting of SOEC and methanation of simulated biogas is built with commercial software Aspen Plus. The energy efficiency was analyzed and optimized through a heat exchange network, which achieved about 30% increase in efficiency. The energy utilization efficiency can be greater than 86% when the CO2 conversion in biogas is greater than 99% by adjusting the methanation temperature and H2/CO2 ratio. The energy efficiency in direct methanation of biogas exceeds 8% higher than that in methanation of CO2 separated from biogas.