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NOX: NO and NO2

For years, combustion evaluation was concerned only with the oxygen in the air.  Air is made up of 20.9% oxygen, 78% nitrogen, and the balance is made up of trace gasses.  Nitrogen was considered an inert gas that simply stole heat from the combustion process, but did not enter into it.  Efficient combustion was expressed using terms such as stoichiometric, excess air, lean or rich, etc., all involving the fuel’s relationship with oxygen.

Combustion engineers knew that the nitrogen is not totally inert, but under the right conditions, would combine with oxygen forming nitric acid (NO) and nitrogen dioxide (NO2).  This was not considered important, since the amounts were small and had little affect on combustion efficiency calculations.

As various pollution laws were enacted, such as the Federal Clean Air and Water Act, NO and NO2 emitted in stack gasses became important as they were declared pollutants.  The EPA set limits on the amount of these gasses that would be allowed in stack emissions.  States can set their own limits as long as they are more stringent than the EPA’s limits.

The concentration of these trace compounds are measured in parts per million (PPM) instead of percentages.

To ease the measurements of NO and NO2, rather than attempt to measure them individually, they are combined and the term NOX is used to describe these pollutants.  In fact, the EPA always considers NOX as NO2, even though it is probably largely NO.

In summary, NOX is the sum of NO and NO2 in stack emissions, expressed in PPM.

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