This article reviews the hydrothermal liquefaction of biomass with the aim of describing the current
status of the technology. Hydrothermal liquefaction is a medium-temperature, high-pressure thermochemical
process, which produces a liquid product, often called bio-oil or bi-crude. During the hydrothermal
liquefaction process, the macromolecules of the biomass are first hydrolyzed and/or degraded
into smaller molecules. Many of the produced molecules are unstable and reactive and can recombine
into larger ones. During this process, a substantial part of the oxygen in the biomass is removed by
dehydration or decarboxylation. The chemical properties of bio-oil are highly dependent of the biomass
substrate composition. Biomass constitutes of various components such as protein; carbohydrates, lignin
and fat, and each of them produce distinct spectra of compounds during hydrothermal liquefaction. In
spite of the potential for hydrothermal production of renewable fuels, only a few hydrothermal technologies
have so far gone beyond lab- or bench-scale.