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MARPOL Annex VI — Air Pollution: SOx, NOx & Exhaust Gas Cleaning

SOx limits (0.50% / 0.10%), EGCS scrubbers, Tier I/II/III NOx curves, SCR urea reactors, and Bunker Delivery Note sampling.

11 min read
Intermediate
Safety
Key Principles at a Glance 6 points
  • MARPOL Annex VI governs atmospheric emissions, controlling Sulphur Oxides (SOx), Nitrogen Oxides (NOx), Ozone-Depleting Substances (ODS), Particulate Matter, and Volatile Organic Compounds (VOCs).
  • The global fuel sulphur cap is 0.50% m/m (since 1 Jan 2020); inside Emission Control Areas (ECAs), the cap drops to 0.10% m/m, achievable via Ultra-Low Sulphur Fuel Oil (ULSFO), marine gas oil (MGO), or an approved Exhaust Gas Cleaning System (EGCS / Scrubber).
  • NOx standards apply to marine diesel engines >130 kW (excluding emergency/lifeboat engines); limits tighten across Tier I (2000), Tier II (2011), and Tier III (2016 for ECAs), with Tier III requiring Selective Catalytic Reduction (SCR) or LNG dual-fuel.
  • Fuel changeover entering an ECA must be strictly managed to prevent thermal shock to fuel injection pumps; temperature change rates must never exceed 2°C per minute.
  • Bunker Delivery Notes (BDNs) must be retained on board for 3 years, accompanied by the sealed MARPOL representative sample retained for at least 12 months.
  • Certificates include the International Air Pollution Prevention (IAPP) Certificate (5 years), the Engine International Air Pollution Prevention (EIAPP) Certificate with its NOx Technical File, and the IEE Certificate.

1. Sulphur Oxides (SOx) Limits & Hybrid EGCS Scrubber P&ID

MARPOL Annex VI Regulation 14 controls emissions of Sulphur Oxides (SOx) and Particulate Matter (PM). When marine fuels containing sulphur are atomized and combusted inside diesel cylinders or boiler furnaces, the fuel-bound sulphur oxidizes directly into sulphur dioxide (SO2) and sulphur trioxide (SO3). In the atmosphere, SOx reacts with moisture to form sulphuric acid (H2SO4), precipitating as acid rain and forming hazardous sub-micron particulate aerosols.

Annex VI regulates fuel sulphur content through a dual-zone regime:

  • Global Waters (Outside ECAs): Since 1 January 2020 (the "IMO 2020 Sulphur Cap"), fuel sulphur content must not exceed 0.50% m/m (down from the historic 3.50%).
  • Emission Control Areas (ECAs): Inside designated ECAs (Baltic Sea, North Sea, North American ECA, US Caribbean, and Mediterranean Sea from May 2025), fuel sulphur must not exceed 0.10% m/m.
0.50% S Global Sulphur Cap
0.10% S ECA Sulphur Limit
≤2°C / min Fuel Changeover Thermal Ramp
3 Years BDN Retention Onboard

Under Regulation 4 (Equivalents), a ship may continue burning high-sulphur heavy fuel oil (HFO up to 3.50% S) provided it operates an approved Exhaust Gas Cleaning System (EGCS / Scrubber) that reduces exhaust emissions to an equivalent SO2/CO2 ratio (≤4.3 for 0.10% S equivalence; ≤21.7 for 0.50% S equivalence):

RAW EXHAUST FROM ENGINE (300°C, SO2 & SOOT) → SCRUBBER ABSORPTION TOWER (EGCS) VENTURI QUENCH STAGE PACKED BED CONTACTOR (HIGH SURFACE AREA) DEMISTER (DROPLET SEPARATOR) → CLEAN PLUME (SO2 ≤ 0.10% S EQUIV, WHITE STEAM) CEMS RECIRCULATION TANK Closed Loop Fluid NaOH DOSING Caustic Soda (50%) COOLER HEAT EX Seawater Cooled WATER TREATMENT Separates Soot & Ash ↓ Sludge to Tank (Cat E) SW PUMP ← Sea Chest WATER MONITOR (WQM) pH ≥ 6.5 • PAH • Turbidity • DO SCRUBBER OPERATING MODES OPEN LOOP MODE: • Uses seawater alkalinity • High power consumption • Banned in many ports CLOSED LOOP MODE: • Doses NaOH to freshwater • Zero washwater discharge • Legal in all ports & ECAs
Figure 1: Marine Exhaust Gas Cleaning System (EGCS / Scrubber) Hybrid Closed/Open Loop P&ID. In open-loop mode, natural seawater alkalinity neutralizes SO2 before passing continuous water quality monitoring (pH ≥6.5, PAH, turbidity). In closed-loop mode, fresh water dosed with 50% sodium hydroxide (NaOH) neutralizes acids in a closed circuit, cooled by seawater heat exchangers and cleaned by a bleed-off centrifuge to eliminate effluent discharge in port.

2. Nitrogen Oxides (NOx): Tier Curves & SCR Urea Injection Cutaway

MARPOL Annex VI Regulation 13 governs Nitrogen Oxides (NOx) emissions. Unlike SOx, which depends entirely on fuel chemistry, NOx is an engine combustion artifact formed via the thermal mechanism (Zeldovich mechanism) when atmospheric nitrogen and oxygen react under intense peak temperatures (>1100°C) and cylinder pressures.

Regulation 13 applies to each marine diesel engine >130 kW installed on a ship constructed on or after 1 January 2000 (or undergoing a major conversion). It explicitly exempts emergency diesel generators, lifeboat engines, and engines used solely in emergencies. NOx limits are indexed to the engine's rated crankshaft speed ($n$, in rpm):

IMO NOx EMISSION LIMIT CURVES: TIER I, TIER II & TIER III vs ENGINE SPEED (rpm) 0 4 8 12 16 g/kWh 130 rpm 500 rpm 1000 rpm 2000 rpm TIER I (17.0 g/kWh → 9.8 g/kWh) TIER II (14.4 g/kWh → 7.7 g/kWh) TIER III (3.4 g/kWh → 2.0 g/kWh) — 80% REDUCTION (REQUIRES SCR / LNG) 2-STROKE M/E MEDIUM SPEED A/E TIER III COMPLIANCE: In-cylinder optimization alone cannot reach Tier III; engines require Selective Catalytic Reduction (SCR) or gas.
Figure 2: IMO NOx Emission Limit Curves (Tiers I, II, and III) vs Rated Engine Speed. Limits drop inversely with crankshaft rpm: Tier I applied globally from 2000; Tier II enforced an approximate 20% cut from 2011; Tier III mandates an aggressive 80% reduction inside ECAs, requiring downstream catalytic conversion or alternative fuels.

Selective Catalytic Reduction (SCR) De-NOx Engineering:

To reach Tier III levels inside ECAs, diesel engines employ Selective Catalytic Reduction (SCR). Aqueous urea solution (40% AUS40) is injected directly into hot exhaust gases, generating ammonia (NH3) that converts toxic NOx into pure nitrogen and water vapor across vanadium-titanium catalyst blocks:

SELECTIVE CATALYTIC REDUCTION (SCR) DE-NOx REACTOR CUTAWAY RAW EXHAUST → UREA DOSER AUS40 (40% Urea) MIXING DUCT Hydrolysis Reaction: Urea → 2 NH3 + CO2 CATALYST 1 CATALYST 2 SOOT BLOWER LANCE CATALYTIC REDUCTION REACTION: 4 NO + 4 NH3 + O2 → 4 N2 + 6 H2O • Operating Window: 300°C – 450°C • Below 300°C: Ammonium bisulfate blocks pores • Ammonia Slip Sensor: < 10 ppm NH3 CLEAN → TEMPERATURE INTERLOCK: SCR units automatically bypass below 300°C to prevent catalyst poisoning by sticky ammonium salts.
Figure 3: Marine Selective Catalytic Reduction (SCR) De-NOx Reactor Mechanical Cutaway. Aqueous urea (AUS40) is injected into exhaust gases upstream of the reactor, vaporizing and hydrolyzing into ammonia (NH3). Inside the ceramic honeycomb catalyst blocks (operating at 300–450°C), ammonia selectively reacts with NO and NO2, yielding inert nitrogen (N2) and water vapor while keeping ammonia slip below 10 ppm.

3. Low-Sulphur Fuel Changeover & Bunker Delivery Note (BDN) Auditing

Ships trading into Emission Control Areas that lack exhaust scrubbers must physically switch from Very Low Sulphur Fuel Oil (VLSFO 0.50% S) to Ultra-Low Sulphur Fuel Oil (ULSFO 0.10% S) or Marine Gas Oil (MGO) prior to crossing the ECA geographic boundary.

Thermal Shock & Fuel Pump Plunger Seizure

Heated HFO runs at 130–140°C to reach injection viscosity, while cold MGO enters at 30–40°C. Switching too quickly causes rapid differential thermal contraction of fuel injection pump barrels around plungers, seizing the fuel racks and blacking out the ship. Chief Engineers must strictly enforce a temperature change rate of no more than 2°C per minute using a fuel blending heat exchanger.

Bunker Delivery Note (BDN) & Representative Sampling (Regulation 18):

Every commercial bunkering operation requires an official Bunker Delivery Note (BDN) and verified physical samples:

1
BDN Statutory Retention (3 Years): The BDN must be kept on board readily available for inspection and preserved for a minimum of 3 years after the fuel oil has been delivered.
2
MARPOL Representative Sample (12 Months): The delivery must be accompanied by a sealed representative sample taken at the ship's bunkering manifold via a continuous-drip sampler. Signed by both barge master and chief engineer, it must be retained for at least 12 months until the fuel is substantially consumed.
3
Four Mandatory Bunkering Samples: Standard bunkering practice takes four sealed bottles:
1. MARPOL Sample (retained on board for PSC inspection)
2. Ship's Onboard Retained Sample
3. Fuel Testing Laboratory Sample (sent for independent lab testing)
4. Bunker Barge / Supplier Sample
ECA FUEL CHANGEOVER SYSTEM P&ID (≤2°C/min THERMAL RAMP) VLSFO TANK (0.50% S) Pre-heated to 90°C ULSFO / MGO (0.10% S) Ambient ~35°C FUEL COOLER Ramp ≤ 2°C / min PUMP → ENGINE MANDATORY CHANGEOVER LOGGING (REGULATION 14.6): Log date, time, position, low-sulphur fuel volume, and crew sign-off BEFORE entering ECA. MARPOL CONTINUOUS-DRIP SAMPLER BUNKER MANIFOLD SPOOL PIECE Needle Valve SAMPLE SEAL #8942 • Continuous drip throughout full bunkering • Divided into 4 bottles (750 ml each) • Retained onboard for ≥ 12 months SURVEYOR CHECK: Port State Control will verify the BDN sulphur statement against the physical MARPOL seal number and log coordinates.
Figure 4: ECA Fuel Changeover System P&ID and MARPOL Manifold Continuous-Drip Sampler. To avoid fuel injection pump seizure, fuel coolers limit the changeover temperature drop to ≤2°C/min. At bunkering, a needle-valve sampler takes a representative drip throughout delivery, filling four tamper-sealed bottles retained with the Bunker Delivery Note (BDN) for 3 years.