NOx is a generic term for the various nitrogen oxides produced during combustion. They are believed to aggravate asthmatic conditions, react with the oxygen in the air to produce ozone, which is also an irritant and eventually form nitric acid when dissolved in water. When dissolved in atmospheric moisture the result can be acid rain which can damage both trees and entire forest ecosystems.
Sources
Three primary sources of NOx formation in combustion processes are documented: prompt NO, fuel NO and thermal NO. Thermal NOx formation, which is highly temperature dependent, is recognized as the most relevant source when combusting natural gas.
In an internal combustion engine, a mixture of air and fuel is burned. When the mixture is tuned so as to consume every molecule of reactant (in this case fuel and oxygen) it is said to be "running at stoichiometry." With this burns, combustion temperatures reach a high enough level to actually burn some of the nitrogen in the air, yeilding various oxides of nitrogen, the results of which can be seen over major cities such as Los Angeles, CA in the summer in the form of brown clouds of smog.
Prevention
Technologies such as flameless oxidation (FLOX®) and staged combustion significantly reduce thermal NOx in industrial processes.
Regulation
The Environmental Protection Agency (EPA) regulates and enforces NOx emission limits in the U.S. in accordance to legislation passed by Congress.
The Kyoto Treaty, ratified by 54 nations in 1997, calls for a substantial world wide reduction of green house gas including NOx.
References
