A review on subcritical and supercritical water gasification of biogenic, polymeric and petroleum wastes to hydrogen-rich synthesis gas

Lignocellulosic feedstocks such as forestry biomass and agricultural crop residues can be utilized to generate biofuels and biochemicals. In addition, a large amount of non-plant residues or biogenic wastes is also generated worldwide that has huge potentials but remains underutilized. Converting these organic waste materials through thermochemical and biochemical processes into biofuels is widely regarded as a remedial approach to address waste management and clean energy problems. One of such thermochemical biomass-to-gas technologies is hydrothermal gasification in the presence of subcritical or supercritical water, which utilizes the unique fluid properties of water to disintegrate effectively the organic wastes into hydrogen-rich syngas. This paper reviews the thermophysical chemistry of subcritical and supercritical water as well as their role in generating syngas from the hydrothermal decomposition of biogenic, polymeric and organic wastes such as municipal solid waste, animal manure, food waste, industrial effluents, sewage sludge, mixed plastics, waste tires and petrochemical wastes. The paper also describes different technologies used for syngas cleaning and conditioning together with gas-to-liquid technology such as Fischer-Tropsch synthesis to produce hydrocarbon fuels. The current progress, challenges and knowledge gaps in the research and development of hydrothermal gasification of biogenic wastes are also discussed.