Liquefaction is referred to as liquefaction of gases, the process of condensing a gas into a liquid. It consists of direct or indirect processes in thermochemical conversion of biomass. Indirect processes are not typically defined as a thermochemical process but as chemical upgrading, such as Fisher-Tropsch processes. Direct liquefaction involves rapid pyrolysis to produce bio-oils and/or condensable organic vapors. The hydrothermal liquefaction (HTL) involves direct liquefaction of biomass, with the presence of water and perhaps some catalysts, to directly convert biomass into liquid oil, with high pressure and medium temperature. Research is continuing in this area.
The exact pathways of HTL to produce crude oil from biomass remain unclear. The following reactions are believed to be typical: