MARPOL Annex VI NOx & SOx Emission Limits – Regulations, Tiers & Compliance Measures
Maritime Emissions Regulation · Technical Reference
NOx & SOx Emission Limits
MARPOL Annex VI — Regulations 13 & 14 | Applicable Standards for Marine Vessels
- MARPOL 73/78
- ANNEX VI
- REG. 13 — NOx
- REG. 14 — SOx
- IMO 2020 COMPLIANT
Regulation 13
NOx — Oxides of Nitrogen Limits
Applicable to all marine diesel engines with a power output greater than 130 kW installed on ships constructed on or after 1 January 2000. Limits are expressed in total weighted NOx (as NO₂) in grams per kWh and are a function of rated engine speed (n, rpm).
| Tier | Ship build date | Applicable area | Limit at n < 130 rpm | Limit at 2000+ rpm |
|---|---|---|---|---|
| Tier I | On/after 1 Jan 2000 | Global (all waters) | 17.0 g/kWh | 9.8 g/kWh |
| Tier II | On/after 1 Jan 2011 | Global (all waters) | 14.4 g/kWh | 7.7 g/kWh |
| Tier III | On/after 1 Jan 2016 | NECAs only | 3.4 g/kWh | 1.96 g/kWh |
Tier I formula 45 · n−0.2 (130 ≤ n < 2000)
Tier II formula 44 · n−0.23 (130 ≤ n < 2000)
Tier III formula 9 · n−0.2 (130 ≤ n < 2000)
Designated NECAs (NOx Emission Control Areas): North American ECA, US Caribbean ECA, Baltic Sea ECA and North Sea ECA (Tier III effective 1 May 2021). Outside NECAs, Tier III ships revert to Tier II compliance. Compliance verified via the EIAPP Certificate and NOx Technical File.
Regulation 14
SOx — Oxides of Sulphur Limits
Controlled through maximum permitted sulphur content in fuel oil. Applies to all ships without exception. Stricter limits apply within designated Sulphur Emission Control Areas (SECAs).
| Period | Global sulphur cap (% m/m) | SECA sulphur cap (% m/m) |
|---|---|---|
| Before 1 January 2012 | 4.50 % | 1.50 % |
| 1 January 2012 – 31 December 2019 | 3.50 % | 0.10 % (from 1 Jan 2015) |
| From 1 January 2020 (IMO 2020) | 0.50 % | 0.10 % |
Designated SECAs: Baltic Sea, North Sea, North American ECA, US Caribbean ECA.
Compliance options: (1) Low-sulphur fuel oil — VLSFO ≤ 0.50% S globally, MGO ≤ 0.10% S in SECAs; (2) Approved Exhaust Gas Cleaning System (EGCS/scrubber); (3) Alternative fuel — LNG, methanol, ammonia (inherently sulphur-free).
Regulation 18: Bunker Delivery Notes (BDN) must be retained for 3 years; MARPOL fuel samples for 12 months. Fuel oil changeover log entries are mandatory on SECA entry/exit.
Compliance options: (1) Low-sulphur fuel oil — VLSFO ≤ 0.50% S globally, MGO ≤ 0.10% S in SECAs; (2) Approved Exhaust Gas Cleaning System (EGCS/scrubber); (3) Alternative fuel — LNG, methanol, ammonia (inherently sulphur-free).
Regulation 18: Bunker Delivery Notes (BDN) must be retained for 3 years; MARPOL fuel samples for 12 months. Fuel oil changeover log entries are mandatory on SECA entry/exit.
Compliance Strategies · Technical & Operational
Measures to Reduce NOx & SOx Emissions
NOx — Technical Measures
TECH
Selective Catalytic Reduction (SCR): Urea (AdBlue) injected into exhaust reacts with NOx over a catalyst to produce harmless N₂ and H₂O. Achieves 80–95% NOx reduction; primary Tier III solution.
TECH
Exhaust Gas Recirculation (EGR): 20–40% of exhaust recirculated into charge air, diluting O₂ and lowering peak combustion temperature. Reduces NOx by 30–50%; widely used on two-stroke crosshead engines.
TECH
Miller Cycle & Injection Timing: Early intake valve closing and retarded fuel injection timing reduce charge temperature and peak cylinder pressure, limiting thermal NOx by 10–25%.
TECH
Humid Air Motor (HAM) / Water Injection: Humidified scavenging air or direct water injection into the combustion chamber reduces flame temperature. Achieves 30–70% NOx reduction.
TECH
Alternative Fuels (LNG, Methanol): LNG combustion inherently produces lower NOx due to lower stoichiometric flame temperature. Dual-fuel engines permit fallback to liquid fuel.
SOx — Technical Measures
TECH
Open-Loop Scrubber (EGCS): Seawater scrubs SO₂ from exhaust and is discharged back to sea after alkalinity treatment. Permits HSFO use; prohibited in some port states and enclosed waters.
TECH
Closed-Loop Scrubber: NaOH-dosed freshwater recirculated through scrubber; wash water retained onboard for shore disposal. Compliant in all ports and SECAs.
TECH
Hybrid Scrubber: Capable of switching between open-loop and closed-loop modes, providing flexibility across varying port-state wash water discharge regulations.
TECH
VLSFO / MGO / LSFO: Very Low Sulphur Fuel Oil (≤ 0.50% S) globally and Marine Gas Oil (≤ 0.10% S) within SECAs provide direct compliance without additional hardware.
TECH
Alternative Fuels (LNG, Methanol, NH₃): Inherently sulphur-free; provide simultaneous SOx and (partially) NOx compliance. LNG offers near-zero SOx and significant GHG reduction potential.
NOx — Operational Measures
OPS
Slow Steaming: Reduced engine load lowers combustion temperatures and NOx output. Effective combined with load-optimised injection timing to prevent incomplete combustion.
OPS
NECA Mode Switching: SCR or EGR activation procedure verified and completed before NECA entry. A written switchover log is mandatory; failure to activate constitutes non-compliance.
OPS
Engine Maintenance & Tuning: FIE maintenance (nozzle condition, timing) within NOx Technical File parameters. Regular indicator cards and performance analysis detect compliance drift.
OPS
Voyage Planning: Routing to minimise transits in NECAs where Tier III applies. Shore-side optimisation reduces unnecessary NECA exposure for non-Tier III equipped vessels.
SOx — Operational Measures
OPS
Fuel Changeover Procedure: Changeover from HSFO to LSFO/MGO completed before SECA entry. Position, time, and tank figures recorded in the Fuel Oil Changeover Log (Reg. 14.6).
OPS
Bunker Management: Procuring compliant fuel, verifying BDN sulphur content, retaining MARPOL representative samples onboard for 12 months for PSC inspection.
OPS
EGCS Monitoring & Records: Continuous pH/PAH/turbidity monitoring of scrubber wash water; data logger records retained. Scrubber approval document and discharge data must be available for PSC.
OPS
SEEMP & ISM Compliance: Fuel management integrated into Ship Energy Efficiency Management Plan. Operator instructions, drills, and checklists for SECA entry/exit under the ISM Code SMS.
IMO 2020 Global Sulphur Cap — Key Impact
From 1 January 2020, the global sulphur cap was reduced from 3.50% to 0.50% m/m, representing an 85% reduction in allowable sulphur content. The shipping industry responded through a combination of VLSFO uptake, scrubber retrofits, and accelerated LNG adoption, resulting in an estimated reduction of 8.5 million tonnes of SO₂ emissions annually.
Compliance Summary
| Measure | NOx reduction | SOx reduction | Tier III capable | Alt. fuel req. |
|---|---|---|---|---|
| SCR (urea catalyst) | 80 – 95% | — | ✔ | — |
| EGR | 30 – 50% | — | ✔ (combined) | — |
| Miller cycle / retarded timing | 10 – 25% | — | — | — |
| EGCS Scrubber (open/closed) | — | > 97% | — | — |
| VLSFO / MGO fuel switch | — | Full compliance | — | — |
| LNG / methanol / ammonia | Significant | Full compliance | ✔ (LNG + SCR) | ✔ |
| Slow steaming (operational) | Moderate | Proportional | — | — |