Process Optimization and Performance Enhancement of CO2 Biomethanation in Continuous Stirred-Tank Reactor (CSTR)

Power-to-Methane (PtM) offers a promising route for storing surplus renewable electricity by converting H₂ and CO₂ into CH₄. However, the performance and optimization of CO₂ biomethanation under low organic concentrations are unexplored – a scenario common in areas with abundant curtailed wind/solar power but limited organic waste. To address this gap, this study systematically optimized three key process parameters in continuous stirred-tank reactor (CSTR). Increasing agitation intensity from 80 to 160 rpm raised the volumetric methane production rate (VMP) by 13.8% and the gas–liquid mass transfer coefficients (kLa) by 66.7%, achieving a peak CH₄ content of 94%. Raising the gas recirculation rate to 1200 mL·min⁻¹ enabled a VMP of 1.07 L CH₄·L⁻¹·d⁻¹. The optimal H₂/CO₂ ratio was 4:1, maintaining stable pH (7.31–7.51) and low VFAs. Microbial analysis revealed that enhanced mass transfer strongly enriched hydrogenotrophic methanogens (especially Methanobacterium, up to 74.8% relative abundance), suppressed acetoclastic pathways (Methanosaeta <2%), and concurrently inhibited hydrolytic/acidogenic bacteria (e.g., Firmicutes, Bacteroidota, Cloacimonadota), indicating a trade-off between methanogenic activity and microbial functional diversity. Overall, this study demonstrates that a CSTR can achieve efficient CO₂ biomethanation under low organic concentrations through integrated optimization of agitation, gas recirculation, and feed ratio. The results provide critical data support for PtM deployment in regions with high renewable energy surplus but limited organic waste, enabling grid-to-gas energy storage, industrial CO₂ utilization, and carbon emission reduction.