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.

The production of fuel hydrocarbons from CO₂-neutral raw materials is a promising task at present. The thermal dissolution of biochar obtained by the method of hydrothermal carbonization of sawdust was studied. The dissolution of biochar in tetralin (hydrogen donor solvent) was studied at different temperatures (350–450 °C) and with two types of dilution of the mixture with tetralin: 1/3 and 1/4. The process proceeded without a catalyst. It was found that the samples subjected to thermal dissolution at temperatures of 425–450 °C had the highest conversion and yield of liquid products. The reaction temperature also had a significant effect on the composition of liquid products. It was found that an increase in the reaction temperature led to a significant increase in benzenes, both in the direct and in the hexane fraction. A benzene yield of more than 50% was observed for both fractions at a temperature of 450 °C. It was also suggested that the possible positive effect of abietates on the homogenization of the reaction mixture contributed to high conversion in the process. The biochar/tetralin ratio effects the yield and composition of the liquid products as well. An increase in the tetralin concentration in the mixture during thermal dissolution led to an increase in the conversion and yield of hydrocarbon fractions for fuel purposes. This is undoubtedly due to the large amount of elemental hydrogen involved in the hydrogenation of the reaction mixture.

The effect of temperature on the torrefaction (mild pyrolysis) of peat was studied. The process was carried out under anaerobic conditions at atmospheric pressure and a temperature of 230–300°C. It...

The effect of mild pyrolysis methods (hydrothermal carbonization and torrefaction) on the physicochemical properties of biocoal was studied. It was established that biocoal obtained by...

The effect of the temperature of hydrothermal carbonization of peat on the yield and composition of the resulting biocoal was studied. It was found that an increase in the temperature from...