Page 317 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

294 10 Post-combustion Air Emission Control

reactivity of the sorbent increased. This indicates that the performance of a steam-
reactivation process cannot be evaluated solely based on the pore structure. The
XRD and SEM analyses showed that the migration of CaO from inside outward
played a critical role in reactivation of the spent sorbent.

10.3.3 Dry FGD

Dry sorbent, typically lime powder, can be injected into the flue gas duct where the
gas temperatures are about 150–400 °C. In a power plant it is located before the
emission stack, and likely to be located between the air pre-heater and the partic-
ulate control devices.
Dry FGD can also be an add-one device that is referred to as a dry scrubber; it
does not use water or any liquid solvent. The scrubber is usually filled with caustic
granulates or pellets to create chemical reactions that remove the SO x emission
compounds. A dry scrubber can be placed before an exhaust gas economizer (EGE)
or used in conjunction with SCR units, which typically require exhaust gas tem-
peratures above 350 °C to reduce both SO x and NO x emissions.
Using caustic lime as an example, in a dry scrubber, the caustic lime (Ca(OH) 2 )
reacts with sulfur dioxide (SO 2 ) to form calcium sulfite, which is then oxidized to
form calcium sulfate dehydrate or gypsum:

SO 2 þ Ca OHð Þ ! CaSO 3 þ H 2 O ð10:17Þ
2
1
CaSO 3 þ = 2O 2 ! CaSO 4 ð10:18Þ
The oxygen is usually supplied from the air to reduce the operating cost. The
final product is likely to be gypsum (CaSO 4 • 2H 2 O) and there is no CO 2 .
The desulfurization product includes not only CaSO 4 particles but also unused
lime powder. They are then removed from the flue gas by the particulate air
cleaning devices downstream. This approach is relatively cheap and can easily be
added on to the older facilities without proper sulfur control.

10.3.4 Semi-Dry FGD

Since solid-gas reaction is slow and lime is not reactive enough, additional
humidification is needed in order to improve the desulfurization efficiency and the
effectiveness of sorbent usage. This approach is referred to as semi-dry FGD, where
a calcium-based sorbent is sprayed into the flue gas duct as a water slurry. The lime
in the spray will be converted to calcium hydroxide, and it is more reactive to SO 2
than CaO