Exergetic assessment of CO2 methanation processes for the chemical storage of renewable energies

Abstract

One option in the power-to-gas scenario is the methanation (Sabatier) reaction using carbon dioxide from anaerobic digestion as a carbon source and hydrogen obtained by electrolysis. The exergetic efficiencies of four process configurations for the methanation are assessed in this contribution. The specifications of the German natural gas grid are used as product quality requirement for the produced methane. The configurations are analyzed on the system level, including the acquisition of the reactants, the chemical conversion process and finally the energy conversion of methane to electrical energy. The results of the analysis demonstrate that the mixture of methane and carbon dioxide from anaerobic digestion can be directly fed into the methanation. No prior removal of biogenic methane is necessary. This configuration is the most efficient process in terms of exergetic efficiency in this study. The process including the electrolysis, methanation, separation via pressure and temperature swing adsorption, and gas conversion to electricity has an overall energetic efficiency of 23.4%, without the excess heat contribution, covering the complete cycle from electricity over chemical storage back to electricity. The exergetic efficiency is higher, when taking the contribution of excess heat into account. The obtained efficiency for methanation is clearly higher than that reported in literature using methanol for energy storage.