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

Master MARPOL Annex VI: SOx 0.50%/0.10% caps, exhaust scrubbers (EGCS), NOx Tier I/II/III curves, SCR urea systems, fuel changeover, and BDN samples.

By the Chartandkeel editorial team · Updated October 2026 · About our sources

Published by ChartandKeel Updated

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.
  • Global Sulphur Cap: 0.50% S
  • ECA Sulphur Limit: 0.10% S
  • Fuel Changeover Thermal Ramp: ≤2°C / min
  • BDN Retention Onboard: 3 Years

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):

Hybrid exhaust gas scrubberEngine exhaust rises through a spray scrubber and demister. An open-loop seawater feed or closed-loop circulation pump and cooler supplies sprays. Closed-loop water returns to the circulation tank with alkali dosing. Washwater is checked before permitted discharge. DemisterClean exhaustEngineexhaustGas risesWater fallsSeawater supplyOpen-loop feedCOOLERWashwaterCIRCULATIONClosed-loop tankRecirc. pumpALKALIMetered doseWATER CHECKTreatment / monitorPermitted dischargeBlue: seawater / discharge · Teal: closed-loop circulation · Orange: exhaust
Figure 1: Hybrid exhaust gas scrubber — Exhaust rises through the washwater sprays and demister. Separate paths show open-loop seawater supply and closed-loop recirculation, cooling, dosing, and water checking.

EGCS open-loop, closed-loop and hybrid operation

An open-loop EGCS pumps seawater through the scrubber. Its natural alkalinity neutralises absorbed sulphur oxides and the washwater is treated and discharged subject to the system approval, monitoring limits and local discharge restrictions. A closed-loop EGCS recirculates treated water, normally dosing an alkali such as sodium hydroxide; a controlled bleed-off is treated and retained or discharged only where permitted. A hybrid EGCS can change between these modes, but mode changes require the maker's approved procedure and operating limits.

Both modes must achieve the applicable Annex VI equivalent SO2/CO2 performance and maintain their required monitoring and recordkeeping. Depending on the approved system, operators monitor washwater pH, PAH, turbidity and temperature, as well as discharge/bleed-off conditions. Restrictions on EGCS washwater discharge differ by port, coastal State and operating area: never assume that open-loop discharge is allowed everywhere or that closed-loop means no effluent is ever generated.

Operational check: before entering a restricted area, verify current local washwater rules, select the approved mode, confirm sensors and treatment plant status, check the log/recording function and retain the required EGCS records. Use the ship's approved EGCS Technical Manual and Onboard Monitoring Manual for actual limits and mode changes.

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):

NOx certification limits by rated speedComputed logarithmic-speed chart and exact piecewise table for all three NOx tiers. Tier III is qualified by designated NOx emission control areas, construction dates and exemptions.137 • NOₓ limits versus rated engine speedMARPOL Annex VI, regulation 13 • weighted NOₓ as NO₂, g/kWh0510152050130500100020003000NOₓ (g/kWh)Rated crankshaft speed n (rpm • logarithmic scale)Tier I9.8Tier II7.7Tier III2.0Limit, g/kWhn < 130130 ≤ n < 2000n ≥ 2000Tier I17.045 × n⁻⁰·²9.8Tier II14.444 × n⁻⁰·²³7.7Tier III3.49 × n⁻⁰·²2.0Tier I: ships built 2000–2010; Tier II: ships built from 2011, subject to reg. 13. Tier III: only applicable engines in designated NOₓ Tier III ECAs; construction-date thresholds and exemptions apply. Not every SOₓ ECA is a NOₓ ECA. Certification limits—not a live engine-load trace or a prescribed treatment method.
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)Urea is injected into the exhaust mixing duct upstream of two catalyst elements. Ammonia reacts with nitrogen oxides on the catalyst to produce nitrogen and water. The reactor has a cleaning lance and an outlet monitor. UREA SUPPLYMetered injectionExhaust inTreated gasMixing ductCatalyst converts NOx with ammoniainto nitrogen and waterCleaning lanceOutlet monitorCATALYST ELEMENTSDose and temperature control keep the reactor within its approved operating range.
Figure 3: Selective catalytic reduction (SCR) — Urea is dosed upstream of the mixing duct. The catalyst uses the resulting ammonia to convert nitrogen oxides into nitrogen and water; the drawing also shows cleaning and monitoring connections.

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.

Danger: 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
Fuel changeover & bunker samplingTwo fuel service tanks connect through isolation valves to a common conditioning unit, pump and engine supply. A separate bunker manifold sampler shows a needle valve taking a continuous drip into a sealed sample bottle. CONTROLLED FUEL CHANGEOVERCONTINUOUS-DRIP SAMPLERHEAVY FUELService tankDISTILLATEService tankCONDITIONERHeat / coolEngineBlend graduallyControl temperature / viscosityBunker manifoldNeedle valveSealed sampleLeft: engine fuel supply · Right: a representative drip sample throughout bunkering.
Figure 4: Fuel changeover & bunker sampling — The left circuit shows fuel selection and conditioning before engine supply. The separate right-hand detail shows continuous sampling from the bunker manifold through a needle valve into a sealed bottle.

Standards & sources referenced

MEO question bank
MARPOL
Topic 7 of 8
  1. MARPOL 73/78: The Convention & Its Six Annexes
  2. MARPOL Annex I — Prevention of Pollution by Oil
  3. MARPOL Annex II — Noxious Liquid Substances in Bulk
  4. MARPOL Annex III — Harmful Substances in Packaged Form
  5. MARPOL Annex IV — Prevention of Pollution by Sewage from Ships
  6. MARPOL Annex V — Prevention of Pollution by Garbage from Ships
  7. MARPOL Annex VI — Air Pollution: SOx, NOx & Exhaust Gas Cleaning
  8. MARPOL Annex VI — Emission Reduction, EEDI, EEXI & CII
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