Selective Catalytic Reduction - Chemistry

Chemistry

The NOx reduction reaction takes place as the gases pass through the catalyst chamber. Before entering the catalyst chamber the ammonia, or other reductant (such as urea), is injected and mixed with the gases. The chemical equation for a stoichiometric reaction using either anhydrous or aqueous ammonia for a selective catalytic reduction process is:

4NO + 4NH3 + O2 → 4N2 + 6H2O + CO2
2NO2 + 4NH3 + O2 → 3N2 + 6H2O
NO + NO2 + 2NH3 → 2N2 + 3H2O

With several secondary reactions:

2SO2 + O2 → 2SO3
2NH3 + SO3 + H2O → (NH4)2SO4
NH3 + SO3 + H2O → NH4HSO4

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

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

The ideal reaction has an optimal temperature range between 630 and 720 K, but can operate from 500 to 720 K with longer residence times. The minimum effective temperature depends on the various fuels, gas constituents, and catalyst geometry. Other possible reductants include cyanuric acid and ammonium sulfate.

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