Power-to-methane via co-electrolysis of H2O and CO2: Reactor operation and system simulation

Abstract

The traditional pathway of electrolysis based power-to-methane systems consists of water (liquid or vapor) electrolysis followed by CO2 methanation. The use of solid-oxide electrolysers allows an alternative pathway, i.e., steam and CO2 electrolysis followed by syngas methanation which can be more efficient than the traditional pathway but requires additional considerations related to cost and operation. The operation of an evaporating water-cooled CO2 methanation reactor in syngas methanation is first validated: (1) H2 concentrations between 10.3 and 21.0 % are measured with the lowest concentration occurring at higher reactant pressure and cooling water pressure and lowest reactant flow rate. (2) Increasing the CO concentration in the reactant, while supplying the stochiometric ratio of H2 and a ratio of H2O to CO of 1, increases the measured hot spot temperature but it remains under 700 °C. However, the changes in H2 concentration at the outlet are within the error of the injected reactant.Furthermore, an idealized power-to-methane system is analyzed, using various system layouts, to identify the impact of the steam production’s energy source, as well as the additional steam requirement for syngas methanation, on the system’s heating value efficiency and on the key operating variables. Restricting the internal steam production to the cooling system of the reactor causes a decrease in maximum efficiency from 94.1% to 91.1%. Directly compressing the humid mixture to be injected into the reactor as it exits the solid-oxide electrolyzer partially alleviates the stress of restricting the evaporation process to reach a maximum efficiency of 92.4%. When including heat losses in the reactor, avoiding the condensation offered a gain of up to 1.2% compared to condensing the water before the compression step.