Carbonate-catalyzed reverse water-gas shift to produce gas fermentation feedstocks for renewable liquid fuel synthesis

Abstract

Carbon-neutral liquid fuel generation is essential for decarbonizing sectors that cannot readily electrify. Recently commercialized acetogenic gas fermentation offers an alternative to conventional biofuels that circumvents efficiency limitations and land requirements, provided the requisite H2/CO feedstocks can be generated efficiently using renewable inputs. CO2 electrolysis is under development for this application but suffers from scalability challenges and impurity sensitivity. We describe an alternative that utilizes dispersed alkali carbonates as reverse water-gas shift (RWGS) catalysts to convert H2 and CO2 to an appropriate ratio of CO/CO2/H2 for acetogenic fermentation. Using a fixed-bed reactor operating at industrially relevant space velocity, we demonstrate equilibrium RWGS conversion starting at 430°C that remains stable over days, even with 50 ppm H2S. The combination of carbonate-catalyzed RWGS, water electrolysis, and gas fermentation could convert electricity to ethanol with nearly 50% energy efficiency (higher heating value), providing a compelling option for renewable liquid fuel production.