Boosting methane yields in biogas plants

Biogas plants play an important role in defossilization — anaerobic bacteria in these plants break down biomass to form biogas which, on average, comprises up to 60% methane and more than 40% CO2. While the biogas is used to generate electricity and heat in combined power-and-heating (CPH) units or can be upgraded to natural gas quality and fed into the natural gas network, the CO2 has not been utilized to date. Instead, it is typically released to the atmosphere. Now, as part of the ICOCAD projects, researchers from the Fraunhofer Institute for Microengineering and Microsystems (IMM, Mainz, Germany; www.imm.fraunhofer.de) are developing a way to convert CO2 into additional methane. The underlying chemical reaction — the Sabatier process; CO2 + 4H2 → CH4 + 2H2O — was discovered more than a hundred years ago, but to date it has not been used for direct upgrading of biogas. This endothermic reaction requires a high temperature (around 400°C), which not only reduces the maximum CO2 conversion through thermodynamic equilibrium, but also favors the undesired competing reverse water-gas shift reaction. Also, sulfur compounds generated in the biogas plant can poison the catalyst. The goal of the ICOCAD I project was to improve…