Hydrothermal gasification of waste biomass and plastics into hydrogen-rich syngas: a review

The current global greenhouse gas emissions have increased by over 90% since 1860 primarily due to our overreliance on fossil fuels, petrochemicals and their derivatives. Production of petrochemical plastics is also reaching 400 million metric tons in 2023. The lack of effective thermochemical processes for converting wet feedstocks and complex residues such as plastics is calling for hydrothermal gasification as an efficient approach to producing syngas. The demand for hydrogen production through greener approaches is also rising to compete with the commercial steam reforming of natural gas. Here, we review the conversion of biomass and plastics by hydrothermal gasification into hydrogen-rich syngas with a focus on the process parameters influencing the conversion of a variety of feedstock types. Parameters influencing hydrothermal gasification of biomass and plastics include temperature, pressure, reaction time, feedstock concentration, catalysts and reactor types. Several synergetic effects also influence product distribution during the co-processing of biomass and plastics during hydrothermal gasification. Processes that impact biomass conversion to syngas are hydrolysis, water-gas shift, methanation, hydrogenation, steam reforming and polymerization.