Class 2 (2nd Engineer) MEKG 📅 Sep 2024

Exam Question

(a) Describe, with the aid of a sketch, an external system for reducing engine NOx emission, 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 Principle
Selective Catalytic Reduction (SCR) is an exhaust gas after-treatment technology used to reduce Nitrogen Oxides (NOx) emissions from marine diesel engines to comply with MARPOL Annex VI Tier III regulations. The system injects a reducing agent, typically an aqueous urea solution (also known as AdBlue or AUS 32), into the hot exhaust gas stream upstream of a catalyst. The urea decomposes into ammonia, which then reacts with NOx on the catalyst surface to convert it into harmless diatomic nitrogen (N₂) and water (H₂O).
#### System Components

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