Abstract: The alkalinity of the solids, slag and ash, which occur in a combustion plant such as an incinerator is utilized for the flue gas scrubber, while at the same time the storability and the alkaline flue gas washing stage are improved. For this purpose, the scrubbing liquor is removed from the tank of a flue gas scrubber for SO.sub.2, so that it can be used for the separate scrubbing of slag and ash in, for example, stirrer vessels. Subsequently, the slag is separated in a sieve according to particle size, and the fine portion, together with the ash, is supplied to a dewatering stage, from which they pass to a stirrer vessel, in order to be re-washed there with scrubbing liquor from tank emanating from a flue gas scrubber for HCl. In a subsequent washing stage, the solids are freed of chlorides. The scrubbing liquor from the dewatering stage is returned to the tank, while the scrubbing liquor from the settling tank is returned to another tank.

Abstract: Flue gases (6) discharged from a flue gas desulfurizer (5) are fed to an absorption tower (7) and the heat recovered by the absorption is transferred to the flue gases and/or the absorbent (8). The water which has been absorbed by the absorbent is expelled therefrom as vapor by desorption, e.g., by a stripping with flue gas in a preliminary scrubber (44), which precedes the flue gas desulfurizer (5). By means of a pump (16) the absorbent (8) is circulated between the absorption tower (7) and the preliminary scrubber (44). The flue gases (4) discharged from the boiler furnace (1) flow through an air preheater (2), the preliminary scrubber (44), the flue gas desulfurizer (5) and the absorption tower (7), in that order, and are then discharged into the open through a chimney (19). The combustion air (3) is supplied to the boiler furnace (1) through a heat exchanger (45) incorporated in the circulating system for the absorbent (8) and through the air preheater (2).