Environmental Engineering Reference
In-Depth Information
thereby reducing the NO
2
content that participates in the redox reactions. As a
result, the NO
3
concentration greatly increases, whereas that of NO
2
decreases
rapidly. Fig. 4.14 also shows that the SO
2
removal efficiency generally decreases
slightly with the molar ratio of O
3
/NO all the while. The explanation of the mild
SO
2
removal change within the molar ratio 0.7 - 1.0 is the same as that in the
corresponding discussion about Fig. 4.13 and is not repeated here. While for the
circumstances with the molar ratio above 1.0, on one side, the nitric acid generated
in the liquid phase blocks the SO
2
absorption because of an increase in the
hydrogen ion concentration, on the other hand, a mild reaction between ozone and
SO
2
in water
[25,26]
can increase a little the SO
2
removal. The combined result is
that the SO
2
removal efficiency still varies slightly. Fig. 4.15 also shows that the
SO
4
2
concentration increases through the NO
2
oxidization behavior as the molar
ratio increases within 0.7 - 1.0. Increasing further the molar ratio, on the contrary,
decrease the SO
4
2
concentration because of the lessened NO
2
oxidization with
tetravalent-S components (see the rapidly-decreasing NO
2
concentration pattern).
However, the SO
4
2
concentration reduces less obviously compared with the NO
2
concentration because of the partial tetravalent-S components oxidized by ozone
and the NO
3
dissolved in water.
4.7 Simultaneous Desulfurization and Denitrification Scheme
Incorporated with Ozone Oxidization and Dual-Tower
Scrubbing
As discussed previously, the typical flue gas components of SO
2
and NO
x
change
to solubilized salt ions of NO
3
, NO
2
, and SO
4
2
after a combined process of
ozone oxidization and liquid phase absorption in a scrubbing tower. It was found
that the NO
3
concentration greatly increased owing to the N
2
O
5
absorption in the
solution when the molar ratio of O
3
/NO exceeds 1.0. With the NO
x
(
>2)
absorption solely appearing in the solution, nitric acid, nitrate, and nitrite are
consequently generated. To achieve simultaneously desulfurization and
denitrification in flue gas and recycle the absorption products in scrubbing towers,
a flue gas treatment method combined ozone oxidization and two-tower scrubbing
and its setup thereof were put forward
[27]
. The key operational principle of the
proposed setup is that the separation of sulfur and nitrogen is accomplished based
on the different solubility levels between NO and SO
2
. Accordingly, the respective
x
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