Environmental Engineering Reference
In-Depth Information
Incorporated with Ozone Oxidization and Dual-Tower Scrubbing
absorption, oxidization, and byproduct recycle are then conducted in the two
scrubbing towers.
Fig. 4.16 shows the schematic of the setup used for a real flue gas treatment
based on the method in [27]. Flue gas emitted from a furnace is cooled in a heat
exchanger 2 and then passes through scrubbing towers 3 and 11, respectively,
where SO
2
and NO
x
are respectively absorbed and recycled by alkali liquids. The
remaining flue gas is sent back to the heat exchanger 2 for heating before being
discharged into the chimney. High-concentration ozone is generated in an ozone
generator 4 by blowing in air or oxygen. The partial ozone entering the liquid
phase in a scrubbing tower 3 oxidizes tetravalent-S components. Sulfur is recycled
as sulfate with high valences. To save the ozone consumption, a replaceable way
for oxidizing tetravalent-S components is to fan air into the scrubbing tower 3.
The left ozone is sent into the flue gas flow before entering a scrubbing tower 11.
The molar ratio of this ozone input to NO
x
in the flue gas flow is set at 0.5 - 1.5,
depending on the denitrification products and absorbent liquid in the scrubbing
tower 11. Byproducts (i.e., nitric acid through water recycling absorption and
nitrate/nitrite through alkali liquor absorption, respectively) can be obtained by
transforming the low-valence and weak-water-solubility NO
x
(with NO accounting
for the major share and a little NO
2
) into the high-valence and
high-water-solubility oxidized forms (such as NO
2
and N
2
O
5
).
The byproduct of nitric acid can be obtained through water absorption in the
scrubbing tower 11 with the O
3
/NO molar ratio of around 1.5. The experimental
investigation
[28]
into producing nitric acid through ozone solely oxidizing NO in
flue gas has indicated that increasing the molar ratio of O
3
/NO produces more
nitric acid and simultaneously restrains the nitrous acid generation. Within the
common exhaust gas temperature range, O
3
and NO
3
concentrations decrease as
the reactive time in the gas phase prolonged because of two negative reactions (i.e.,
O
3
reacting with NO
3
and the NO
3
molecule reacting with itself). As a result, the
nitric acid production decreases. In the practical application, O
3
and NO
3
should
enter the wet removal device as soon as they thoroughly mix, thereby shortening
greatly their residence times before entering the scrubbing tower 11. Similarly, the
injecting temperature should be as low as possible to enhance the nitric acid yield
because a low flue gas temperature facilitates to reduce the consumption of O
3
and
NO
3
in gas phase reactions. Because the N
2
O
5
absorption is independent of the pH
value, the nitric acid yield is affected slightly by the pH value of the solution,
favoring an acid solution recycling scrubbing process.
Search WWH ::
Custom Search