Environmental Engineering FOURTH EDITION

402 ENVlRONMENTALENGINEEIUNG

Reheat

SQ from fossil-fuel burning off-gas was first controlled by trapping as calcium sulfate
in a lime-limestone mixture. This mixture was produced by calcining limestone and
injecting lime into the scrubber and adding limestone to the boiler. Although the SO2
was absorbed as CaSO3 and CaSO4, the lime-limestone absorbing material created a
solid waste disposal problem of staggering proportions.
Flue-gas desulfurization methods that trap SO2 as sulfite instead of sulfate allow
regeneration of the absorbing material, and thus mitigate the solid waste disposal
problem. A typical method is single-alkali scrubbing, for which the reactions are

so2 recovery

The concentrated SO2 that is recovered from this process can be used industrially in
pulp and paper manufacture and sulfuric acid manufacture. Figure 20-16 is a diagram
for single-alkali scrubbing.
A modification of this system that is both more efficient and less wasteful is
double-alkali scrubbing, in which a primary scrubbing cycle dissolves the SO2 in a
sodium hydroxide solution, forming sodium bisulfite by the reaction

. - ------ - - - - -
Absorber

SO2 + NaOH + NaHSO3.

• Evaporator Na2S03 (s) Makeup Na2S03
  Surge

Addition of lime in the second step regenerates the NaOH and produces calcium

sulfite.

Flue __. gas

tank -

NaHSd3

soh. vvv

Dissolving

7 tank

Figure 20-16. Simplified diagram for single-alkali scrubbing of flue gas with

regeneration.