As the industry standard for NOx control technology, selective catalytic reduction systems (SCR) converts toxic nitrogen oxides with the aid of a catalyst into diatomic nitrogen and water. Mixing the exhaust air with a gaseous reagent, anhydrous ammonia, aqueous ammonia or urea, while passing the homogenous mixture over a catalyst bed the nitrogen oxides are absorbed.

Proper size, composition and space velocity are three integral elements in the loading of a catalyst, where as the temperature range, viscosity and anticipated NOx levels are the basis for the design. Throughout this process, Ammonia slip is carefully controlled, meeting environmental requirements. Afterwards the prepared reagent and control system are integrated into plant operations.

Stoichiometric reaction using either anhydrous ammonia or aqueous ammonia process is:

4NO + 4NH3 + O2 → 4N2 + 6H2O
2NO2 + 4NH3 + O2 → 3N2 + 6H2O
NO + NO2 + 2NH3 → 2N2 + 3H2O

The reaction for urea instead of anhydrous ammonia or aqueous ammonia process is:

4NO + 2(NH2)2CO + O2 → 4N2 + 4H2O + 2CO2

The ideal reaction has an optimal temperature range between 674.33 °F and 836.33 °F, but can operate from 440.33 °F to 836.33 °F with longer residence times. The minimum effective temperature depends on the various fuels, gas constituents and catalyst geometry.