Class 2 (2nd Engineer) MEKM 📅 Aug 2023

Exam Question

(a) Describe, with the aid of a sketch, an external system for reducing engine NOx emissions, explaining the chemistry of the process (8)

(b) Explain why Urea is used in the Selective Catalytic Reduction process instead of ammonia. (4)

(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 (4)

Reference Answer

### (a) Selective Catalytic Reduction (SCR) System
#### Concept and Operation
A Selective Catalytic Reduction (SCR) system is an advanced active emissions control technology used to reduce Nitrogen Oxides (NOx) emissions from marine diesel engines to comply with MARPOL Annex VI Tier III regulations. The system works by injecting a reducing agent, typically an aqueous urea solution (often referred to as AUS 32), into the hot exhaust gas stream upstream of a catalytic converter. The ammonia derived from the urea selectively reacts with NOx in the presence of the catalyst to convert it into harmless diatomic Nitrogen (N₂) and water (H₂O).
**The process flow is as follows:**

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