Exergo-economic analysis of a direct biogas upgrading process to synthetic natural gas via integrated high-temperature electrolysis and methanation

Biogas upgrading to synthetic natural gas (SNG) is a viable and appealing route for power-to-gas because it combines waste management with the use of the surplus electricity that might arise in energy systems having a considerable share of renewable energy sources in their production mix. In this work, the exergo-economic performance of a biogas upgrading process through integrated electrolysis and methanation is assessed in connection with the current market status to test which conditions could make the proposed option economically viable. Two different configurations, which differ mainly for the operating pressure of the electrolyser, are compared. The exergy efficiencies are high (>80%) and exergo-economic costs of the produced bio-SNG in the two analyzed configurations are 5.62 and 4.87 c€/kWhexergy, for low- and high-pressure respectively. Lower values would be required for the bio-SNG to compete with fossil natural gas. We show how both the input electricity price and the capacity factor have a substantial impact on the economic sustainability of the process. Eventually, the monetary exploitation of the oxygen produced by electrolysis and the participation to the emission trading scheme could contribute further to improve the economic attractiveness of the process.