Steels_ Metallurgy and Applications, Third Edition

Stainless steels 367

Gypsum is more environment-friendly than the calcium sulphite sludge and also

represents a commercially useful product in the building industry.

The various process stages in a typical FGD are shown schematically in

Figure 4.40. The flue gas leaves the boilers at a temperature of about 130"C

and passes through an electrostatic precipitator for the removal of solid particles,

e.g. fly ash. The gas then passes to a heat exchanger where it is cooled to about

90"C before entering a chloride prescrubber. UK coals contain high levels of

chlorine (typically 0.25 wt%) and chloride ions are removed from the flue gas

by a spray containing calcium chloride, limestone or dilute hydrochloric acid.

The gas stream leaves the chloride prescrubber at a temperature of about 50"C

and then enters the SO2 absorber section.

As indicated earlier, the SO2 is reacted with alkalis in this section to form

gypsum. On leaving the absorber, the gas passes on to a mist eliminator for the

removal of entrained droplets and then via a reheater to the stack.

Lane and Needham 12 have reported on the temperature and chemical conditions

in the various stages of the FGD process and these are shown in Table 4.16.

Prior to the heat exchanger, the gas is hot and dry and therefore the operating

conditions are not aggressive. Therefore materials such as mild steel and Cor-Ten

(weathering steel) perform adequately up to this stage.

Table 4.16 Operating conditions in flue gas desulphurization plant

Zone Temperature Chloride ion Acidity

(*C) content (ppm) (pH)

Comments

Electrostatic precipitator 120-150
Inlet duct 120-150
Reheater 88
(heat extraction)

Low

Inlet/quench zone 88-49 High Low

Prescrubber 49 Up to 350000

Absorber 49 5-30000

Outlet duct 49-80 Variable

1.5-20

4.5 -6.5

Low

Dry gas

Dry gas

Below dewpoint of

flue gas plus entrained

particulate matter not

removed in ESP

Highly abrasive, scale

deposits and

accumulation of

chloride ions

Conditions will be

process dependent

Conditions will be

process dependent

Temperature below

dewpoint, slurry

carryover and mixing

with raw gas can

lead to scale

deposits and high

chloride levels

After Lane and Needham. 12