Hydrothermal conversion of biomass to fuels and energetic materials

•Hydrothermal processes avoid the energy intensive step of biomass drying.•Hydrogen or methane can be formed throughout hydrothermal biomass gasification.•Hydrothermal liquefaction produces oils with a high heating value.•Lignin can be hydrothermally liquefied to valuable chemicals.•Hydrothermal carbonization produces biochars and other interesting materials.

Available biomass, preferentially residues, can be divided in two groups: biomass with a high or natural water content (‘wet’ or ‘green’ biomass) and biomass with low water content such as wood and straw. In ‘dry’ biomass gasification processes, originating in most coal processing technologies, biomass of low water content is necessary to avoid the energy loss by water evaporation. In contrast, hydrothermal processes need water as reaction medium; therefore, these processes are preferentially used for wet or ‘green’ biomass.In this review paper we will describe the main research directions in the hydrothermal conversion of biomass into fuels and carbon throughout gasification to produce H2 or CH4, liquefaction to produce crude oils and phenols from lignin as well as carbonization to produce carbonaceous materials which can be either used as fuels (carbon negative chars) or interesting energetic materials (hydrothermal carbons).

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (245 K)Download as PowerPoint slide