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.
Read more about this topic: Selective Catalytic Reduction
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