Mechanisms of Low-Temperature Processes of Biomass Conversion (A Review)

Torrefaction and hydrothermal carbonization are low-temperature thermochemical procedures for the biomass conversion to biocoal, a carbon-neutral analog of fossil coal. Biocoals, compared to untreated biomass, exhibit hydrophobic properties, increased energy density, and calorific value similar to that of brown coals. The two processing methods differ essentially in that hydrothermal carbonization is performed in the presence of a large amount of water as reaction medium; hence, the biocoal formation mechanisms will be different for each process. Papers dealing with specific features of low-temperature heat treatment of biomass and with regular trends in conversion of biomass structural components (cellulose, hemicellulose, lignin) in the course of torrefaction and hydrothermal carbonization are considered in the review.