Page 318 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

10.3 Flue Gas Desulfurization 295

CaO þ H 2 O ! Ca OHÞ ð10:19Þ
ð
2
Consequently, the desulfurization reaction becomes

Ca OHÞ þSO 2 ! CaSO 3 þ H 2 O ð10:20Þ
ð
2
The reaction would be even faster before complete water evaporation due to the
ions in liquid phase.
A potential problem associated with the semi-dry FGD is the corrosion of duct
under humidified environment. Typically these systems are operated with a Ca/S
ratio near 2, with typical SO 2 concentrations of a few 1000 ppmv from coal
combustion.
Another important parameter is the approach temperature, which is defined as
how far the gas temperature is above the saturation temperature for the water in the
gas. The lower the approach temperature, the longer it takes for water to be
vaporized completely, and consequently, a higher sulfur uptake by the sorbent.

10.4 NO x Reduction Using SCR and SNCR

NO x control from the flue gas can be completed by dry and wet approaches too. Dry
approaches include (SCR), selective noncatalytic reduction (SNCR), and adsorp-
tion, whereas wet approaches are, similar to wet FGD, by wet scrubbers. This
section focuses on dry approach of SCR/SNCR.

10.4.1 Selective Catalytic Reduction

SCR is one of the most successful technologies for NO x removal from flue gases
[7, 24, 34]. Typical catalysts include V 2 O 5 or WO 3 on a TiO 2 support. SCR
operates at temperatures of 350–400 °C. For a power plant the location is right
before the air pre-heater, where ammonia is injected into the furnace and reacts with
NO to form water and nitrogen as products:

NH 3 þ NO ! N 2 þ H 2 O ð10:21Þ

Typical NO x reduction efficiency is in the range of 90–95 % when SCR is
operated at an ammonia injection molar ratio of NH 3 /NO of about 0.8. The NH 3
concentration after removal is below 5 ppm. It was reported that commercial SCR
units reduce NO x emissions by 40–70 %, depending on upstream NO x concentra-
tions and the local allowable NO x emissions [32].