Hydrothermal processing can be used to treat wet biomass by converting the organic contaminants to gases. When the system is operated as a metal catalyzed process at nominally 350°C and 21 MPa (so-called low-temperature gasification), it can produce a methane/carbon dioxide product gas from water slurries of biomass.

Hydrothermal gasification is accomplished at the upper end of the process temperature range. It can be considered an extension of the hydrothermal liquefaction mechanisms that begin at the lowest hydrothermal conditions with subsequent decomposition of biopolymer fragments formed in liquefaction to smaller molecules and eventually to gas.