Evaluation of alternative processes of CO₂ methanation: Design, optimization, control, techno-economic and environmental analysis

Converting carbon dioxide (CO₂) into substitute natural gas (SNG) has received renewed interests recently. It not only realizes large scale CO₂ conversion, but also provides a synthetic source for those countries lacking natural gas. This study firstly attempts to explore and evaluate alternative processes for CO₂ methanation, in which four schemes (six processes overall) based on adiabatic and/or non-adiabatic fixed-bed reactors are focused. Rigorous process design, optimization, waste heat recovery, economic and environmental evaluation, and control are covered. From this work, the most promising configuration (i.e. Scheme 4B) uses only two reactors. A counter-current cooling, non-adiabatic reactor equipped with internal recycle goes first, and an adiabatic reactor follows. This process reveals great CO₂ reduction potential (i.e. CO₂-e: −3.338 kg/kg), as compared with other previously developed processes converting CO₂ to value-added chemicals (i.e. CO₂-e: −0.154 to 2.242 kg/kg). More importantly, better operability of the first reactor is indicated by the generated temperature profile. Finally, a suitable control strategy is developed for this scheme, which results in satisfactory closed-loop responses under throughput and composition disturbances. For future work, clarifying how the feed impurities influence the reaction kinetics, and developing a less expensive hydrogen source, will be recommended.