Known methods for hydrothermal carbonization are very time intensive, as the carbonization reaction only proceeds gradually in the biomass used therefor. This is because of the different reaction conditions prevailing in different parts of the biomass. These also cause an inhomogeneous reaction product. To accelerate the method and to improve the result, the biomass is swirled inside the available reaction space with the aid of blower nozzles, which blow in the steam at a high speed so that the biomass is swirled. This ensures that the carbonization reaction can proceed uniformly and promptly after the biomass is introduced.