Process Simulation and Energy Integration of a Power-to-Gas Plant for CO2 Methanation Using Renewable Hydrogen

Power-to-Gas (PtG) technology, which converts surplus renewable electricity into storable chemical energy via water electrolysis and subsequent CO₂ methanation, represents a strategic pathway toward grid-scale energy storage and defossilization of the gas sector. This study presents a comprehensive process simulation and energy integration analysis of a PtG plant combining an alkaline water electrolyzer unit with a fixed-bed catalytic methanation reactor for synthetic natural gas (SNG) production. The process flowsheet is developed in Aspen Plus, incorporating electrolyzer performance correlations at varying load factors (40–100%), a multi-stage adiabatic methanation reactor train operating over Ni/Al₂O₃ catalysts, an intermediate water condensation and gas recycle loop, and a downstream gas upgrading unit to meet SNG grid-injection specifications. Thermodynamic equilibrium calculations and published Langmuir–Hinshelwood kinetics are employed to model reactor behavior across temperatures of 250–450°C and pressures of 1–10 bar. Energy integration is performed using pinch analysis to identify optimal heat exchanger network configurations that maximize internal heat recovery from the highly exothermic methanation reaction (ΔH = −165 kJ/mol) and minimize auxiliary heating demands. The simulation yields an SNG purity of 97.2 vol% CH₄ and a CO₂ conversion rate exceeding 98% under nominal full-load operation. System-level round-trip energy efficiency reaches 68% with full thermal integration, declining to approximately 57% at the 40% minimum load factor, highlighting the trade-off between operational flexibility and energy performance. These results significantly enhance the assessed techno-economic viability of PtG deployment. The proposed simulation framework constitutes a foundational tool for the design, optimization, and scale-up of renewable energy storage systems based on catalytic CO₂ valorization.