Integrated Co-Electrolysis and Syngas Methanation for the Direct Production of Synthetic Natural Gas from CO₂ and H₂ O

The concept of an integrated power-to-gas (P2G) process was demonstrated for renewable energy storage by converting renewable electrical energy to synthetic fuels. Such a dynamically integrated process enables direct production of synthetic natural gas (SNG) from CO₂ and H₂ O. The produced SNG can be stored or directly injected into the existing natural gas network. To study process integration, operating parameters of the high-temperature solid oxide electrolysis cell (SOEC) producing syngas (H₂ +CO) mixtures through co-electrolysis and a fixed bed reactor for syngas methanation of such gas mixtures were first optimized individually. Reactor design, operating conditions, and enhanced SNG selectivity were the main targets of the study. SOEC experiments were performed on state-of-the-art button cells. Varying operating conditions (temperature, flow rate, gas mixture and current density) emphasized the capability of the system to produce tailor-made syngas mixtures for downstream methanation. Catalytic syngas methanation was performed using hydrotalcite-derived 20 %Ni-2 %Fe/(Mg,Al)O_x catalyst and commercial methanation catalyst (Ni/Al₂ O₃ ) as reference. Despite water in the feed mixture, SNG with high selectivity (≥90 %) was produced at 300 °C and atmospheric pressure. An adequate rate of syngas conversion was obtained with H₂ O contents up to 30 %, decreasing significantly for 50 % H₂ O in the feed. Compared to the commercial catalyst, 20 %Ni-2 %Fe/(Mg,Al)O_x enabled a higher rate of CO_x conversion.