Solid oxide electrolysis cells (SOECs) enable the integration of H2O/CO2 co-electrolysis and methanation reactions in a single reactor. The product gas may therefore potentially be sent directly to the existing natural gas pipelines. However, current power-to-methane (P2M) routes are not able to achieve acceptable CO2-to-CH4 conversion in a single SOEC reactor for direct integration into natural gas networks. In order to obtain higher CO2-to-CH4 conversions, a two-dimensional multiscale electro-thermo tubular SOEC model was developed to describe in detail the P2M processes along the whole cell and optimize the materials, flow modes, and operating conditions for CH4 production. The numerical simulations revealed that the stabilized Zr-based SOEC is unable to offer a sufficiently high electrochemical performance at methanation-favoring temperatures (250-650 degrees C). Such a temperature mismatch limits the peak CH4 production ratio in the tubular-stabilized Zr-based SOEC (with an inlet CO2 content of 20) to only 13 at 1.5 V. In contrast, a tubular strontium and magnesium doped lanthanum gallate (LSGM)-based SOEC showed remarkable performance at or below 650 degrees C, improving the CH4 production ratio to over 50 at 550 degrees C and 1.3 V for the same inlet gas composition. However, the operating temperature was still too high to obtain an acceptable CO2-to-CH4 conversion for direct supply to natural gas pipelines. The model revealed that the H2O/CO2 co-electrolysis and methanation reactions can be synchronously enhanced by feeding a cold gas in counter-flow mode under pressurized conditions. At 29 bar, the tubular LSGM-based SOEC could achieve a CO2-to-CH4 conversion ratio of 98.7, with an electricity-to-gas efficiency of 94.5 in the thermal neutral mode. The produced gas can be directly sent to the existing natural gas pipelines so that the curtailed renewable power can be stored more efficiently and utilized over a wider area.
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