Hydrothermal liquefaction is the conversion of solid biomass into gaseous and/or liquid products in the presence of water. Liquefaction consists of the catalytic thermal decomposi­tion of large molecules to unstable shorter species that polymerize again into a bio-oil.

Biomass is mixed with water and basic catalysts like sodium carbonate, and the process is carried out at lower temperatures than pyrolysis (252-472 °C) but higher pressures (50-150 atm) and longer residence times (5-30 min.). These factors combine to make lique­faction a more expensive process; however, the liquid product obtained contains less oxygen (12-14%) than the bio-oil produced by pyrolysis and typically requires less extensive processing (Elliott, 2007; Inoue et al., 1999; Karagoz et al., 2006; Kruse et al., 2003; Minowa et al., 1997).