TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: In the method, the mass ratio of the biomass and water/biocrude in the hydrothermal carbonization apparatus is 1:3 to 1:9, where a raw biomass with 5-80 wt.% moisture is used in the hydrothermal carbonization process. The size of a biomass for hydrothermal carbonization is 1-20 mm long and 1-10 mm thin. The hydrothermal carbonization apparatus is a subcritical water reactor; and is used in batch or continuous mode. The hydrothermal carbonization apparatus is operated at temperatures of 230-350 degrees C, and at pressure of 500-3000 psi. The total residence time of the biomass and water/biocrude in the hydrothermal carbonization apparatus is 0.1-5 hours. The CO2-rich gas is released from an exhaust valve of the hydrothermal carbonization apparatus. The biocrude and biochar are separated by a liquid/solid separator. The pH of the biocrude for recycling is 2-7. The organic carbon content of the biocrude for recycling is 0.5-10 wt.%. EXAMPLE - A method (test) for conversion of biomass to biochar was carried out as follows: a subcritical water carbonization with biocrude recycling was carried out, by feeding biomass (45.5 g-dry basis) and aqueous biocrude as the feed to the reactor, which resulted in the production of 33.3 g (dry basis) biochar with a heating value of 24.9 MJ/kg (HHV-dry basis) and a carbon content of 70.0 wt.% (dry basis). In this case the biomass energy input was 0.892 MJ and biochar energy output was 0.829 MJ, giving an energy yield of 92.9% and a mass yield of 73.2%. For the control experiment (without biocrude recycling), biomass (45.5 g-dry basis) and water were used as the feed to the reactor, which resulted in the production of 27.5 g (dry basis) biochar with a heating value of 26.1 MJ/kg (HHV-dry basis) and a carbon content of 70.0 wt.% (dry basis). In this case the biomass energy input was 0.892 MJ and biochar energy output was 0.718 MJ, giving an energy yield of 80.5% and a mass yield of 60.3%.