Class 2 (2nd Engineer) MEKM 📅 Sep 2022

Exam Question

(a) Describe, with the aid of a sketch, an external system for reducing engine NOx emissions, explaining the chemistry of the process

(b) Explain why Urea is used in the Selective Catalytic Reduction process instead of ammonia

(c) Explain why the exhaust gas quality must be monitored before and after the Selective Catalytic Reduction unit, stating how such monitoring influences operation of the SCR unit.

Reference Answer

Selective Catalytic Reduction is a means of converting nitrous oxides in the exhaust with the help of a catalyst into diatomic nitrogen and water.
A reductant Anhydrous Ammonia (NH3), Aqueous Ammonia (Ammonium Hydroxide) or Urea (Carbamide) solution is added to a stream of exhaust gas and is adsorbed onto a catalyst. Carbon Dioxide (CO2) is a reaction product when urea is used as the reductant.
The chemical equation for the reaction using either anhydrous aqueous ammonia for the process is
4NO + 4NH3 + O2 = 4N2 + 6H2O

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