CO2 valorization into SNG through a methanation reactor combined with a multi-stages bio-polymeric membrane separation system: An experimental study, with exergy and cost analyses

Today, the multifaceted challenges posed by climate change and the persistent reliance on fossil fuels need to pursuit innovative and environmentally sustainable solutions. Carbon capture and utilization emerge as pivotal pillars for achieving the decarbonisation targets established within the European Green Deal framework. An emerging and attracting solution for the reduction of CO 2 emissions is represented by its thermochemical conversion into high added-value products, such as synthetic natural gas (SNG). This work proposes a novel approach consisting of the development of an integrated systems combining a fixed bed methanation reactor, operated at 300 °C and GHSV between 1,100 h −1 and 2,200 h −1 and packed with a Ru-Al 2 O 3 catalyst, with the aim of converting CO 2 into synthetic methane. The system further incorporates a subsequent membrane gas separation process, based on eco-friendly bio-polymeric membranes (PLA), prepared by using green solvents, useful to purify the CH 4 -rich stream into a biomethane-grade stream. At 300 °C and 4 bar, CO 2 conversion was 80 % and the gaseous mixture coming out from the fixed bed reactor was purified, recovering CH 4 with a concentration ≥ 97 % and CO 2 below 2 %, while recycling CO 2 and H 2 to the reactor inlet by a four-stages membrane gas separation system. The experimental campaign was supplemented by a thorough exergy analysis to evaluate the overall process efficiency, showing a maximum of exergy efficiency of 94 % for the methanation reactor at 300 °C and 4 bar, 95 % for the condenser at 300 °C and 2 bar, and 94.9 % for the membrane gas separation system at ambient temperature. The economic analysis, adopted to determine the final cost of the synthetic CH 4 production to make the proposed integrated system competitive in the CH 4 market, highlighted a value below $10/kWh operating between 9 and 10 h per day.