Nitrogen Oxides (mainly NO and NO2), or NOx, is the generic term for a group of highly reactive gases, which contain nitrogen and oxygen in various amounts and chemical configurations. Most of the nitrogen oxides are colorless and odorless.
However, one very common pollutant, nitrogen dioxide (NO2) along with other particles in the air can often be seen as a reddish-brown layer of smog over heavily populated areas.
Nitrogen oxides form when fuels are burned at high temperatures, as in a combustion process. The primary sources of NOx are motor vehicles, electric utilities, and other industrial, commercial, and residential sources.
Simple NOx
The most basic way to measure NOx is “Simple NOx” with a single sensor. NO (nitric oxide), the primary component of NOx, is measured directly with a gas sensor.
The NO to NO2ratio is then assumed based on the type of combustion fuel and process, from which the NO:NO2 ratio historical data has been gathered.
The NOx is then calculated using the measured NO and assumed NO:NO2 ratio.
Total NOx
A more accurate and precise way to determine NOx is to provide actual measurements for both NO and NO2.
When both NO and NO2 are both directly measured with gas sensors, there is no longer an assumed NO:NO2 ratio. The “Total NOx” is the sum of the measured NO plus the measured NO2.
True NOx
NOx, especially NO2, are water soluble gases. When measuring NOx, it is important to make sure that condensation does not build up and allow the NO2 to bubble form the gas phase into the condensate.
Minimizing the transit time from the flue to the analyzer and drying the flue gas immediately as it enters the analyzer will prevent the NO2 bubbling.
An internal thermoelectric chiller can then quickly dry the flue gas to avoid dilution of NO2 into the condensate to allow for a flue gas sample that is most representative of the emissions being released.
This “True NOx” method using a chiller with NO & NO2 gas sensors is the most efficient and accurate way to measure NOx with a portable gas analyzer.
Common methods for measuring NO and NO2 include sensors using electrochemical, non-dispersive infrared (NDIR), and chemiluminescence technologies.