NOVELTY - Thermal processing of solid fuels, preferably coal dust, which is produced via hydrothermal carbonization of a biomass, comprises post-treating an exhaust gas stream (17) in a combustion chamber in such a manner that a safe treatment, preferably minimum temperature or minimum duration of contaminants present in the exhaust stream is ensured, where a minimum duration of 2 seconds of the flowing particles of a fuel is ensured by the mechanical design of a combustion chamber or a fluidized bed furnace or operating conditions.    USE - The additive is useful for hydrothermal carbonization of the biomass, preferably sewage sludge. The method is useful for thermal processing of solid fuels, preferably coal dust.    ADVANTAGE - The method: allows use of less important biomass or biomass-loaded with contaminants; uses the additive, which increases thermal conductivity of the biomass, and thus it performs improved and cost-efficient heat-exchange, and reduces viscosity of the biomass, and thus it improves pumping ability; has safe treatment and short treatment time, and requires small reaction volume, and thus it is simple and cost-efficient; ensures sufficient supply of heat energy in a required temperature range of up to 300 degrees C, and thus it allows maximum energy balance and carbon dioxide balance; provides nutrients including phosphorus to the plants via recovery of the precipitate; and exhibits efficient combustion by removal of unburnt coal particles.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) hydrothermal carbonization of biomass, comprising providing a combustion- or a cogeneration plant for the required heat energy of the exhaust stream and/or a cooling circuit, and drying the final product by spray drying;    (2) an additive for the hydrothermal carbonization of the biomass comprising glycerol, propanetriol, triglycerides, crude glycerol and/or their derivatives in an amount of 3-100 wt.%, based on the total additive;    (3) purifying waste water of hydrothermal carbonization of the biomass, comprising subjecting the waste water to electrolysis; and    (4) using liquid coal sludge or wastewater of hydrothermal carbonization of the biomass, comprising contacting the exhaust gas of a device for performing thermal processing of the biomass, preferably fuel from the hydrothermal carbonization of the biomass with the liquid coal sludge or wastewater of hydrothermal carbonization of the biomass via spraying, blowing, bubbling or gas scrubbing.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic representation of the device for performing thermal processing of solid fuels.    Extender screw pump (1)    Tubular heat exchanger (2)    Hydrothermal carbonization reactor (3)    Nozzle (6)    Exhaust gas stream (17)