Environmental Engineering Reference
In-Depth Information
The removal of high-concentration NO
x
through solution absorption usually
appears in the industrial generation of nitric acid
[3-7]
, nitrate, or nitrite
[8-11]
. Under
these circumstances, the high NO
x
concentration results in a high gas-liquid
concentration gradient and the formation of some high-solubility diazo substances,
such as N
2
O
4
and N
2
O
3
(with a high solubility coefficient of 1.6 and 26
mol/(L⋅atm) (25 °C)
[12]
, respectively). Therefore, the mass transfer process keeps
in good conditions and the absorption ratio is usually high. However, the removal
of low-concentration NO
x
with such a solution absorption method is different. In
this aspect, the published work
[13-16]
focused on using reductive substances (such
as solutions with Na
2
SO
3
or Na
2
S) to enhance the mass transfer rate and little
attention was paid on the pH-value effect, product analysis, and interactions of
SO
2
and NO
2
. A highlight of employing O
3
for the simultaneous removal of
multi-pollutants in flue gas is that the existence of NO
3
or N
2
O
5
, generated at a
case with excessive O
3
, may greatly increase the mass transfer in te solution.
However, no report on the liquid-phase absorption of these high-valence NO
x
substances is available because the multi-pollutants removal technology with O
3
has not yet been developed. Some related information in this aspect is discussed in
this chapter.
In reactions between NO
x
and the absorption liquid, the mass transfer rate
calculation follows the two-membrane theory for liquid phase chemical reactions
and can be expressed as
[17]
N
A
=
×
k
L
×
H
×
P
i
(4.1)
where
is the increasing factor of absorption reactions,
k
L
is the liquid-membrane
mass transfer coefficient,
H
is the gas solubility coefficient, and
P
i
is the gas
partial pressure at the gas-liquid interface. Eq. (4.1) suggests that besides the
gas-liquid mixing conditions, two main factors from the properties of gas itself
affects the adsorption rate, i.e., the gas solubility coefficient
H
and reaction rate
between gas and the liquid phase. Essentially, a WFGD solution mainly contains
H
+
, Ca
2+
, OH
, SO
2
·H
2
O, SO
3
2
, HSO
3
, CO
3
2
, and HCO
3
, of which two factors
may affect on the high-valence NO
x
absorption rate, i.e., the pH value (determined
by concentrations of H
+
and OH
) and the solubilized tetravalent S components in
the solution (such as SO
2
·H
2
O, SO
3
2
, and HSO
3
).
Generally, NO
2
is the dominant oxidation product of NO
x
when ejecting O
3
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