Hydrothermal liquefaction is pyrolysis in hot compressed water of around 300°C and 10 MPa. Biomass is converted into gas, liquid and solid, like common pyrolysis in gas phase. The light tar fraction, such as pyroligneous, can be dissolved in water, and the heavy tar fraction can be obtained in the mixing with char. That is, products are gas, aqueous, and oily material.

aqueous soluble fraction is dissolved, and extraction is applicable. At above 150°C, hydrolysis occurs, and biomass polymers, such as cellulose, hemicellulose, protein, and so on, are degraded into monomer. At around 200°C and 1 MPa, solid like biomass is changed to slurry (liquidization), and oily product, however, is not obtained. At severer condition of 300°C and 10 MPa, liquefaction occur, and oily product is obtained. When reaction condition is changed such as reaction time or catalyst, main product can be changed to char (hydrothermal carbonization). At around critical point and with catalyst, biomass can be gasified (see Chapter 4.5).