Biomethane production through the power to gas concept: A strategy for increasing the renewable sources exploitation and promoting the green energy transition

Direct catalytic methanation of biogas using hydrogen from electrolysis is a promising pathway to store the electricity from renewable power plants according to the Power to Gas concept. This type of methanation offers technical and economic advantages over the methanation of carbon dioxide separated from biogas, since it eliminates the upgrading step. This work is focused on the sizing and performance analysis of a Power to Biomethane plant based on biogas direct catalytic methanation process. The plant consists of two main sections: i) the Renewable Island, in which an anaerobic digestion plant and a renewable power plant (Photovoltaic plant or Wind farm) supply the biogas and the electricity; ii) the Biomethane Production Island, in which the hydrogenation of the carbon dioxide in the biogas, in presence of methane, occurs. This study, by means of an optimization procedure based on a multi-objective approach, aims to evaluate the optimal size of the Power to Biomethane plant that assures the maximum biomethane production and the maximum exploitation of the renewable electricity. Two case studies, referring to the installation of a Photovoltaic plant and a Wind farm as renewable power plants, have been analyzed. Results have highlighted that, for obtaining an annual biomethane production in the range of 2550–3150 kNm3 and by starting from a biogas availability of 500 Nm3/h, the sizes of the renewable power plants must be in the range 15–20 MW and in the range 19–23 MW for the Wind farm and the Photovoltaic plant, respectively. In terms of performance indicators, even if the PV plant has the highest electricity storage factor (up to 69 %), the Wind-based plant has both the highest plant load factor (up to 81%) and the equivalent full load operating hours (up to 6081), so that it is more favorable for the development of the Power to Biomethane concept.