Abstract: A fire-tube boiler which is suitable for operation with powdered ash-containing fuels and automatically maintains itself clean in operation, has inlet openings, widening in trumpet shape, of flue tubes in the wall which separates the boiler drum from a reversing chamber. The fire tube is so dimensioned that, with due consideration of the boiler capacity, the flue gases at the outlet from the fire tube are cooled by a safety margin to below the ash-softening point of the corresponding fuel. The number and inside diameter of the flue tubes are so dimensioned that a dynamic pressure which is not less than 40 Pa is established in them. A blast tube which blows approximately tangentially over the partition wall is arranged on the reversing chamber in order to keep the inlet openings free of deposits of ash.

Abstract: Combustion waste gases of coal dust comprising SO.sub.2 and fly ash are completely and dry desulphurized by very quickly heating up the fly ash to a high temperature which, however, is below the sintering temperature of the fly ash, and cooling the combustion waste gases to a temperature, the distance of which to the water dew point is relatively small and is below 25.degree. C. Thereby, SO.sub.2 is bound into the ash, so that the combustion waste gases become free of SO.sub.2. (FIG.