NOx is a generic term for mono-nitrogen oxides (NO and NO2). These oxides are produced during combustion, especially combustion at high temperatures.
When NOx and volatile organic compounds (VOCs) react in the presence of sunlight, they form photochemical smog, a significant form of air pollution, especially in the summer.
NOx formation is promoted by rapid fuel-air mixing. This produces high peak flame temperatures and excess available oxygen, which, in turn, promotes NOx emissions.
It is economically logical to consider NOx controls that achieve the lowest emission levels possible. These post-combustion control systems are referred to as selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR). In either technology, NOx is reduced to nitrogen (N2) and water (H2O) through a series of reactions with a reagent (or reagents) injected into the flue gas. The most common reagents used in commercial applications are ammonia and and urea for both SCR and SNCR systems.
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