NOVELTY - The method comprises supplying a biomass by a conveying device (3) to a preheating device (4) in which the biomass is heated, where the heated biomass is supplied to a reactor (5) in which the biomass is dehydrated and carbonized, and removing a carbon sludge, originating from a carbonization process, from the reactor. The reactor is heated and/or temperature-stabilized by a passage of gases or vapors. The gases are flue gases obtained from a combustion process or a water vapor. The gases or vapors are conducted through a heat exchanger (6) provided in an inner space of the reactor.    USE - The method is useful for the hydrothermal carbonization of biomass in the passage (claimed), where: the biomass is animal biomass and plant biomass such as green waste, wood chips, plants and plant residues, straw, silage, residues from agriculture, paper and sewage sludge; and the hydrothermal carbonization is an exothermic reaction.    ADVANTAGE - The method allows rapid hydrothermal carbonization of the biomass with high efficiency and less consumption of heat.    DETAILED DESCRIPTION - The method comprises supplying a biomass by a conveying device (3) to a preheating device (4) in which the biomass is heated, where the heated biomass is supplied to a reactor (5) in which the biomass is dehydrated and carbonized, and removing a carbon sludge, originating from a carbonization process, from the reactor. The reactor is heated and/or temperature-stabilized by a passage of gases or vapors. The gases are flue gases obtained from a combustion process or a water vapor. The gases or vapors are conducted through a heat exchanger (6) provided in an inner space of the reactor and through other heat exchanger after passing through the reactor. Dehydrated water in the fluid state is removed from the reactor. The dehydrated water is immediately supplied to the preheating device and mixed with the biomass in the preheating device. The preheating device is supplied to the dehydrated water having a temperature equal to or substantially equal to the temperature of a biomass-water-mixture in the reactor. The removing step is carried out in dependence of the volumetric flow of the biomass feed, a reactor temperature and/or a reactor pressure. The dehydration and/or carbonation process takes place in process steps that are partly carried out in different reactors connected together in series. The step of transferring a reaction product from the reactor to the other reactor takes place by the conveying device or by pressure difference between the reactors. The carbon sludge obtained from the reactor is conducted by a filter device (8) for the separation of contaminants. An INDEPENDENT CLAIM is included for a device for the hydrothermal carbonization of biomass in the passage.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a device for the hydrothermal carbonization of biomass in the passage.    Conveying device (3)    Preheating device (4)    Reactor (5)    Heat exchanger (6)    Filter device. (8)