Carbon recovery from biogas through upgrading and methanation: A techno-economic and environmental assessment

Reducing the use of fossil fuels is an essential measure to counteract the rise in greenhouse gas emissions. In this context, biofuels and e-fuels make an important contribution to achieving climate neutrality targets, especially if their distribution can take place within existing infrastructure, as in the case of methane. The aim of this work is to carry out a techno-economic and environmental assessment of the combined production of biological and synthetic methane in a wastewater treatment plant (WWTP). Methane yield from biogas, usually associated only with biogas upgrading, is enhanced by recovering CO2 to produce additional synthetic natural gas (SNG) through a methanation process. The analysis is applied to a medium-sized WWTP in Italy, whose biogas production profile is known throughout the year. In the current scenario, SNG is not competitive on the gas market. The investment costs of the technologies and the electricity price are then varied in order to better investigate the profitability of SNG production. The results show that, considering long-term cost projections and an electricity price of about 50 €/MWh, SNG can become competitive, with a production cost of 1.4 €/Sm3. Finally, the environmental competitiveness of SNG (direct and indirect CO2 emissions) with respect to fossil natural gas is investigated: results are shown as a function of the carbon intensity of grid electricity and the share of local renewable energy. To make SNG environmentally sustainable, the renewable share must increase to 46% or, alternatively, the carbon intensity of grid electricity must decrease to 187 gCO2eq/kWh.