Steels_ Metallurgy and Applications, Third Edition

366 Steels: Metallurgy and Applications

on all new coal-fired power stations and also for the retrofitting of such plant

on certain existing coal-fired stations. Following privatization, PowerGen have

completed the installation of FGD at Ratcliffe and National Power have FGD

operational at Drax power station. The main function of FGD plant is to remove

at least 90% of the SO2 from boiler exhaust gases and, ideally, the process should

also generate a saleable by-product.

Most FGD plants operate on the basis of a wet scrubbing process in which

SO2 is obsorbed in a suspension of lime (CaO) or limestone (CaCO3) in water:

CaCO3 -I- SO2 -.>. CaSO3 -t-CO2

In the above reaction, calcium sulphite (CaSO3) forms as a sludge and having
no commercial value would have to be disposed of to landfill. However, by
incorporating an oxidation stage in the process, the calcium sulphite is converted
to gypsum:

CaSO3 -I- O -t- 2H20 -+ CaSO4.2H20

Coal-fired

boiler

Flue

gas

Electrostatic

precipitator

r

t

Reheater

Demister

Absorber

Chloride

pre-scrubber

Heat

exchanger

l Stack

Bypass duct

Figure 4.40 Schematic layout for flue gas desulphurization plant