Hydrothermal carbonization of organic wastes to carbonaceous solid fuel – A review of mechanisms and process parameters

HydroThermal Carbonization (HTC) is a thermochemical process by which biomass is treated in aqueous environment under temperatures of 180–300 °C and self-generated pressure to produce solid fuel as hydrochar from wide range of organic wastes such as garbage, bio solids, agro residues and organic wastes from industrial operations. It is an economically efficient process as it eliminates the need for pre-drying and generates energy rich carbonaceous products. This paper reviews the hydrochar formation mechanisms of hydrolysis, dehydration, decarboxylation, condensation, polymerization and aromatization in terms of the critical hydrothermal parameters such as temperature, residence time, solid load, pressure, pH and aqueous conditions to derive fundamental understanding in the improvements required in HTC process design for waste to energy conversion. The identification of design protocols of HTC for solid fuel production is expected to expedite further research and field level scale up of this technology.