Flue gas desulphurisation system
In a flue gas desulphurisation system (FGD), sulphur compounds are removed from the exhaust emissions of fossil-fuelled power stations. This is done by means of an industrial process through the addition of absorbents. This can remove up to 95 % of the sulphur dioxide from the flue gas, since the current threshold value for SO2 in the EU is 200 mg/Nm3 (Nm3 = normal cubic metre).
The wet process has become the main method of flue gas desulphurisation in large, fossil-fuelled power plants. In this method, the flue gases are steam-saturated with the absorbent in aqueous solution. Substances such as ammonia or sodium sulphite are used as absorbents; however the use of lime or limestone slurry (wet limestone scrubbing) is also widespread. The uncleaned flue gas is sprayed in a scrubber tower (absorber tower) with a mixture of water and limestone (scrubbing slurry), whereby most of the sulphur dioxide is bonded by chemical reaction.
See Fig. 1 Flue gas desulphurisation system
Fig. 1 Flue gas desulphurisation system: Wet process (schematic); lime stone scrubbing
After a number of chemical reactions, gypsum is finally produced in a suspension. After dewatering, this leaves gypsum with up to 10 % residual moisture, which provides a valuable product for the construction material industry.
The pumps used in the individual process stages are absorber circulating pumps (scrubber pumps, which - because of the high solids content and the aggressiveness of the fluid handled - are designed as non-clogging impeller pumps with special lining) and FGD auxiliary pumps (for lime
and gypsum slurries in duplex materials).
See Fig. 2 Flue gas desulphurisation system
Fig. 2 Flue gas desulphurisation system: Absorber circulating pump with channel impeller and CPS (CeramicPolySiC) lining