|
FORMATION
OF NOx
For
high temperature operations, nitrogen in the air will react with oxygen to
form oxides of nitrogen, NO, and NO2.
The resulting NOx concentration depends on the highest temperature
and how quickly the gas cools. The
higher the temperature, the greater the formation of NOx.
The more rapidly the gas cools, the more of the NOx is permanently
formed. Sources such as
boilers, incinerators, and gas turbines all create NOx.
Other
operations which form NOx include chemical reactions such as those
involving nitric acid.
For
example; pickling lines for passivating stainless steel involve dipping
the metal parts into a bath containing dilute nitric acid.
The impurities are dissolved and the stainless steel receives an
oxide coat which is passive and resists further corrosion.
During this operation, the impurities react with the nitric acid to
form oxides of nitrogen. Because
of their low solubility, they come off the solution as vapor. |
NO
– NO2
Oxides
of nitrogen include nitric oxide and nitrogen dioxide.
There are other compounds which can be formed such as nitrous oxide
(N2O), but these are not normally encountered in either thermal
or industrial process operations.
NO is
a colorless gas with virtually no water solubility.
In the presence of excess oxygen, NO will slowly convert to
nitrogen dioxide. *Reference:
Air Pollution Control and Design Handbook – Part 2 Pgs: 672/673.
Table
24-11
|
NO
concentration in air
(ppm) |
Time
required for half NO to be oxidized
to NO2 (min) |
|
20,000
10,000
1,000
100
10
1 |
0.175
0.35
3.5
35
350
3,500 |
Nitrogen
dioxide is also a colorless gas, but dimerizes to N2O4.
As a result, the gas appears as a yellowish orange to dark red
brown color gas depending on the concentration. The higher the
concentration, the darker the color.
|
WHAT
IS UNIQUE ABOUT NOx?
Many
flue gas contaminants are cleaned using a variety of techniques.
The most common is wet scrubbing.
For example, removal of oxides of sulfur using wet
scrubbers.
Since scrubbers are low in initial cost, why aren't most
applications handled using wet scrubbers?
NO2
has a very low solubility in water.
However, NO2 will slowly dissolve.
Once
NO2 dissolves, it goes through an auto-oxidation step as
follows:
3NO2 + H2O →
2HNO3
+ NO↑
This
is the overall reaction. As
you can see, the absorption of nitrogen dioxide forms byproduct nitric
oxide (NO). Nitric Oxide is
very low in solubility so it escapes the scrubber as part of the exhaust
air.
There
are techniques for interfering with this reaction including oxidation and
reduction chemicals or the special wet phase catalyst packing media.
While
scrubbing is obviously possible, using special chemicals or other
techniques, it is a much more costly approach than a conventional
scrubber. Both initial cost
and operating costs are higher. Additionally,
wet scrubbing is not nearly as cost effective when the inlet gas
temperature is hot. Conditions
involving high temperatures or low concentrations – which limit
scrubbing effectiveness – are better suited to SCR
technology. |
