Hot gases are drawn off from the combustion chamber of the furnace by a hot gas grinding unit (8) and supplied to a drier. The sludge is introduced in controlled quantities into the drier (5) and dried and/or degasified directly by the hot gases. The dried sludge is then supplied to the hot gas grinding unit with controlled hot gas intake. The prepared sludge is now supplied as a mixture with the hot gases and vapours after the coal dust grinder to the burner pipe and burned with the prepared coal dust. The after grinding temperature of the coal dust/conveyor gas mixture is controlled through the amount of vapours supplied.
