By combining biomethanation with the utilization of renewable H2 produced via water electrolysis using renewable wind and solar power, the flue gas CO2 can through this process be converted to biomethane of natural gas grid quality. The employment of renewable based H2 from water electrolysis and further on to grid quality biomethane enables coupling of the power grid and the gas grid, which supports the conversion and subsequent storage of renewable electricity in the form of renewable natural gas. Natural gas is today used globally for a range of domestic and industrial purposes but is today mainly of petrochemical origin.