Subcritical water conversion of biomass to biofuels, chemicals and materials: a review

The use of fossil fuels has been essential to the development of society, but has also contributed partly to global warming. For example, carbon dioxide emissions from fossil fuels and industries have increased by 60% since 1990, calling for the recycling of modern biomass in the context of a carbon neutral economy. Here we review the hydrothermal conversion of biomass into biofuels, chemicals and biomaterials with emphasis on subcritical water properties, hydrolysis of biomass, steam explosion, fractionation, carbonization, liquefaction, gasification, and fractionation of bio-oil. We observe that hydrothermal conversion of biomass in the presence of water at subcritical conditions produces value-added compounds with high process efficiency. Subcritical water allows rapid reaction rates, low mass transfer resistance, and gas-like diffusivity.