Life-cycle sustainability of thermochemical biogas upgrading via CO2 methanation with green hydrogen

Biomethane is expected to play an increasingly important role in the decarbonization of energy systems and the integration of renewable gases into existing infrastructures. This study investigates an innovative thermochemical pathway for biogas upgrading based on CO2 methanation with renewable hydrogen, developed within the BIOMETHAVERSE project. Unlike conventional upgrading technologies based on CO2 separation, the proposed approach directly converts the CO2 in the biogas into biomethane, increasing biomethane yield while avoiding the energy intensive upgrading process and potential methane slip. A comprehensive assessment combining process modelling, techno-economic analysis, and environmental and social life cycle assessment is performed for three representative feedstocks: manure, biowaste, and maize. Process simulations show a CO2 conversion of 98.6 % and a 67 % increase in biomethane output compared to conventional upgrading. Under current conditions, the levelized cost of biomethane (180–240 €/MWh) remains dominated by hydrogen production, which accounts for more than 50 % of total costs. Sensitivity analysis identifies a break-even electricity price of 14, 28 and 83 €/MWhel for manure, biowaste and maize respectively, while electrolyzer cost reductions can further decrease production costs by 10–25 %. Environmental results highlight the critical role of feedstock selection: manure and biowaste exhibit significantly lower impacts than maize, with manure achieving net negative GHG emissions. Methanation outperforms conventional upgrading when renewable electricity is used, while grid-based scenarios remain penalized by hydrogen-related emissions. Social LCA indicates that impacts are largely driven by feedstock supply chains, with additional risks associated with electricity-intensive hydrogen production. Overall, thermochemical methanation represents a promising pathway to enhance biomethane production and integrate renewable hydrogen, although its sustainability strongly depends on electricity decarbonization and feedstock choice.