Page 191 - New Trends In Coal Conversion

154 New Trends in Coal Conversion

applications. Fig. 6.6 illustrates the PFD of a conventional wet FGD process based on
limestone scrubbing.
The sorbent typically consists of a slurry containing 20% wt calcium carbonate
(CaCO 3 ) in water with pH equal to 7. Once the slurry is prepared, it is pumped into
the top of the absorber. In this unit, calcium carbonate reacts with SO 2 from the flue
gas in countercurrent with the injected slurry, as Reaction 3 indicates:

SO 2 þ CaCO 3 /CaSO 3 þ CO 2 (R3)

Spray towers are used in most absorber designs, but other equipment such as
packed and plate towers have also been used (Poullikkas, 2015). Forced oxidation
of the calcium sulfite occurs in the bottom of the absorber where air is injected and
dissolved into the solution. The pH in the bottom of the absorber is maintained

Process
water

Humidity eliminators

Desulfurization Sprayers
Stack
Flue gas Feeding of limestone slurry

Air
CaSO 2H O Limestone
4 2
Gypsum slurry filtration
Nebulizer collectors

Hydrocyclones
Filter water
re-circulation

Dilution process
Limestone slurry
Aqueous phase
FGD-gypsum of gypsum slurry
Figure 6.6 Process flow diagram of a limestone wet scrubbing process. FGD, flue gas
desulfuration
Adapted from C ordoba, P., 2015. Status of Flue Gas Desulphurisation (FGD) systems from coal-
fired power plants: Overview of the physic-chemical control processes of wet limestone FGDs.
Fuel 144, 274e286.