Power-to-Methane via H2O/CO2 Co-electrolysis Integration: A Conceptual Performance Assessment on Methanation Off-gas Recirculation

Abstract

Power-to-Methane (PtM) using electrolytic technologies has been a motive of interest in the recent years, due to the benefit of producing sustainable methane from renewable energy sources such as solar and wind. Solid oxide electrolysis cell (SOEC) reactors have emerged as a promising electrolytic option because of their high electrical efficiency, scalability for large hydrogen production systems, and the potential for direct H2O/CO2 co-electrolysis for syngas production – thereby avoiding additional process units for CO2 reduction. Co-electrolysis operation is still understudied in wider applications like PtM. Thus, modelling methods such as off-gas recirculation can lead to improvements in process efficiency and overall carbon utilization. While, with the experimentally validated models, operational regimes avoiding damaging electrolysis reactor operation can be found and investigated. This contribution presents a system concept modelling study of SOEC for H2O/CO2 co-electrolysis coupled with a methanation reactor. Specifically, three steady state system configurations are assessed regarding the recirculation of the methanation off-gas stream. The first configuration or base case, consists of an open system with no off-gas recirculation. The second configuration involves the off-gas recirculation back to the SOEC, while in the third configuration, the recirculation is fed back to the methanation reactor. The study was performed using an inhouse component-oriented modelling framework written in python which is experimentally supported by the research group’s extensive experimental facilities. This approach ensures smooth implementation and modification of simplified process system configurations, while producing reliable and reproducible fluid and thermodynamic process calculations. The output generated can be used to quickly determine safe operation regimes for electrolysis operation. The results presented quantify the advantages and disadvantages of the three system configurations in terms of yield, and energy and exergy efficiencies – upon optimization of key operational variables. Additionally, the influence of the variation the SOEC feed composition and pressure, and the corresponding impact on the stack carbon deposition and performance is shown. Finally, the modelling library and results obtained are expected to facilitate the formulation and characterisation of conceptual system designs for PtM relevant technology, as demonstrated in this work.