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There are several techniques for removal of oxides of nitrogen (NOx) depending on the composition/temperature/removal efficiencies.

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

0.175

10,000

0.35

1,000

3.5

100

35

10

350

1

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.