Page 312 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

10.3 Flue Gas Desulfurization 289

CaCO 3 ! CO 2 þ CaO ð10:5Þ

The simplified reactions for desulfurization are

Ca OHÞ þSO 2 ! CaSO 3 þ H 2 O ð10:6Þ
ð
2
The liquid after reaction is collected at the bottom of the scrubber and is partially
recycled. The rest of the used solution stream is fed to the thickener to convert
calcium sulfite to calcium sulfate by oxidation.
1
CaSO 3 þ 2H 2 O þ = 2O 2 ! CaSO 4 þ 2H 2 O ! CaSO 4 2H 2 O ð10:7Þ
Limestone/lime FGD is sensitive to acidic or alkaline components in the flue gas.
Acidic components reduce the pH value and change the equilibrium for SO 2
absorption (Eq. (10.6)). Therefore, a pre-scrubber is useful to remove the acidic
components, fly ash, and mercury from the flue gas. It also improves the quality of
the gypsum. Alternatively, organic buffers can be applied to maintain a constant pH
of the solution. This also lowers the capital cost by lowering the absorber size and
pumping accessories.

10.3.1.1 Other Chemical Sorbents

There are other sorbents that can also react with SO 2 in the flue gas. For example,
Mg(OH) 2 (magnesium hydroxide) can react with SO 2 and produce MgSO 3 (mag-
nesium sulfite):

Mg OHÞ þ SO 2 ! MgSO þ H 2 O ð10:8Þ
ð
2 3
Similarly, caustic (NaOH) can react with SO 2 producing sodium sulfite
2NaOH þ SO 2 ! Na 2 SO 3 þ H 2 O ð10:9Þ

Seawater is a natural alkaline to absorb SO 2 . When SO 2 is absorbed into the
+
seawater, it reacts with oxygen and water to form sulfate ions SO 4 − and free H .
+
And surplus of H is offset by the carbonates in seawater pushing the carbonate
equilibrium to release CO 2 gas:
1
SO 2 þ H 2 O þ = 2O 2 ! SO 2 þ 2H þ ð10:10Þ
4
þ
HCO þ H ! H 2 O þ CO 2 ð10:11Þ
3