IMO NOx Tiers Explained

Tier III applies only when operating in designated NOx Emission Control Areas (NECAs). Current NECAs include the North American ECA (NAZ) and the US Caribbean ECA. The Baltic Sea and North Sea NECAs entered force in 2021.

SCR: How It Achieves 80% NOx Reduction

Selective Catalytic Reduction injects a urea-water solution (AUS40) into the exhaust gas stream upstream of a catalyst. The urea thermally decomposes to ammonia, which reacts with NOx over the vanadium or zeolite catalyst surface:

4 NO + 4 NH₃ + O₂ → 4 N₂ + 6 H₂O   (standard SCR reaction)

SCR efficiency depends on exhaust temperature (optimal range 300–600°C), AUS40 dose rate, catalyst condition, and backpressure. A properly maintained SCR system achieves 85–95% NOx removal — well within Tier III requirements.

AUS40 Consumption Rate

AUS40 consumption is typically 5–10% of fuel oil consumption by volume, depending on engine load, NOx baseline, and Tier III reduction target. For a vessel consuming 30 MT/day HFO at sea, expect 1.5–3.0 MT/day AUS40 consumption when operating in a NECA.

Port planning: ensure adequate AUS40 stock for the full NECA transit leg, plus 20% reserve. Running out of AUS40 in a NECA forces engine derate or route deviation — both expensive outcomes.

Port State Control (PSC) Requirements

PSC officers in NECA ports inspect SCR system compliance. Required documentation:

• Engine EIAPP Certificate (Engine International Air Pollution Prevention)
• Technical File specifying SCR operating parameters and AUS40 specification
• AUS40 COA (Certificate of Analysis) for current stock onboard
• Bunker Delivery Note or supply record showing AUS40 source and ISO 18611-1 conformity
• SCR bypass log (if bypass mode is fitted — requires Port Authority notification in many jurisdictions)

MarineUrea.com provides COA and ISO 18611-1 conformity statements with every supply to support PSC documentation requirements.

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