Page 321 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

298 10 Post-combustion Air Emission Control

absorption/adsorption methods have been introduced in Part 1. SO x /NO x removal
by electron beam radiation is briefly introduced by [13]. SO 2 /NO X can also be
captured by copper oxide spray mixed with ammonia. SO 2 is first captured CuO at
400 °C.

1
CuO þ SO 2 þ = 2O 2 ! CuSO 4 ð10:26Þ
CuO þ SO 2 ! CuSO 3 ð10:27Þ

CuO þ SO 3 ! CuSO 4 ð10:28Þ
Meanwhile, NO is reduced by ammonia to nitrogen, for which copper oxide is a
very good catalyst.

4NO þ 4NH 3 þ O 2 ! 4N 2 þ 6H 2 O ð10:29Þ
2NO þ 2NH 3 þ O 2 þ H 2 ! 2N 2 þ 4H 2 O ð10:30Þ

The copper sulfate (CuSO 4 ) product is transported to the two-stage heater/
regenerator section where it is reduced to Cu at 500° C. SO 2 is released and it also
can be used for sulfuric acid production.
More recently, researchers are trying to use wet scrubbers to absorb both SO 2 and
NOx simultaneously, in this approach, the caustic solution contains strong oxidizers
[39] and the oxidants include Fe(II)-EDTA and H 2 O 2 , penta- and hex-amminecobalt
(II) chloride [37, 38]. Penta- and hexa-amminecobalt(II) chloride seemed to be
superior over other two oxidants, but they are not stable and cannot be stored for
long time. More importantly, waste disposal is a major challenge when cobalt is
mixed with fly ash in the sludge and dissolved in the liquid solvent.

10.6 Control of Volatile Organic Compounds

The properties of the VOCs determine the suitable methods to capture or to oxidize
the VOCs in a gas stream. The most important parameter is the flammability limits,
also referred to as flammable limits of the VOCs/Air mixture. The lower flammable
limit (LFL) describes the mixture with the smallest fraction of combustible gas. The
upper flammable limit (UFL) defines the richest flammable mixture. These limits
define whether the air-VOCs mixture may ignite or not [5]. When the mixture
exceeds 25 % of the LFL, there is a high chance of ignition and it has to be handled
with caution. The values of LFL and UFL for several VOCs and other gaseous
organic compounds are available in literature [18]. For gas mixtures with VOC
contents below LFL, the most important options are VOC combustion, condensa-
tion, and carbon absorption. When the concentration of VOC is above the UFL the
gas can be oxidized in flares or boilers with air or steam.