Direct Methanation of CO<sub>2</sub> in Biogas with Hydrogen from Water Electrolysis: The Catalyst and System Efficiency
Liu Zhang, Fangping Ma, Qing Zeng, Fang Peng, Qiang Hu, Kejing Wu, Yixue Yang, Houfang Lu, Bin Liang
Abstract
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.