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N2O Emissions from Marine Engines

Nitrous oxide (N2O) is a potent greenhouse gas with an IPCC AR5 100-year global warming potential (GWP-100) of 265 (or 298 with climate-carbon feedback) and is the third-largest contributor to anthropogenic radiative forcing after CO2 and CH4. N2O is also a stratospheric ozone-depleting substance, currently the most significant ozone-depleting emission outside the Montreal Protocol scope. In the marine sector, N2O is emitted from marine engines through three principal pathways: (i) incomplete combustion of nitrogen-containing fuels, principally ammonia (NH3) (the upcoming 2025 to 2027 ammonia dual-fuel engine generation has projected N2O slip rates of 0.05 to 0.5% of nitrogen input), and to a lesser extent some biofuels with high nitrogen content; (ii) urea-SCR aftertreatment slip on conventional Tier III engines, where the urea reductant breaks down through side reactions to produce trace N2O (typically 1 to 5 ppm in the exhaust); (iii) hot-fuel-injection NOx-formation chemistry in conventional diesel engines, principally at high load and high cylinder-peak temperatures (typically 2 to 5 ppm in the exhaust). With the planned introduction of ammonia-fuelled engines from 2025 to 2027 and the wider deployment of SCR aftertreatment under NOx Tier III, N2O is increasingly material to the well-to-wake intensity of marine fuels and to FuelEU Maritime, IMO Net-Zero Framework GFI and EU ETS compliance (the EU ETS extension to N2O takes effect from 2026). ShipCalculators.com hosts the principal computational tools: the N2O slip calculator, the ammonia engine N2O penalty calculator, the N2O FuelEU intensity calculator, the SCR N2O slip calculator, the WtW intensity calculator and the GFI compliance calculator. A full listing is available in the calculator catalogue.

Contents

Background

Why N2O matters

N2O (nitrous oxide) is a long-lived atmospheric trace gas with a present-day atmospheric concentration of approximately 335 parts per billion by volume (ppbv) and an atmospheric lifetime of approximately 109 years. Three properties make it particularly concerning for the marine sector:

  • High GWP-100: at 265 (IPCC AR5), each tonne of N2O has the climate-warming impact of 265 tonnes of CO2 over a 100-year horizon. The GWP-20 is 264 (essentially the same as GWP-100 because N2O’s atmospheric lifetime is much longer than the integration period); this insensitivity to integration period means that policy choices about CH4 vs CO2 equivalency that depend on integration period do not similarly affect N2O.
  • Stratospheric ozone depletion: N2O is the most significant anthropogenic ozone-depleting substance currently emitted, after CFCs were largely eliminated under the Montreal Protocol. N2O is not yet under Montreal Protocol control (the Montreal Protocol covers halogenated compounds; N2O is unhalogenated). Its long atmospheric lifetime means that emissions today contribute to stratospheric ozone depletion for many decades.
  • Long atmospheric residence: a tonne of N2O emitted today will continue to contribute to atmospheric warming and to ozone depletion for over a century. This contrasts with CH4 (lifetime 12 years), where emission reductions translate to atmospheric benefit on decadal timescales.

For marine fuel regulation, the key policy reality is that N2O is already in scope of the FuelEU Maritime intensity calculation (FuelEU Annex II tabulates N2O emission factors for each fuel category) and enters the EU ETS scope from 2026 (under the 2023 EU ETS Directive amendment). The IMO Net-Zero Framework GHG Fuel Intensity standard from 2027 includes N2O within the GFI calculation. Marine N2O is therefore a regulated pollutant, not a niche concern.

Marine engine N2O sources

In conventional marine diesel engines burning HFO, VLSFO or MGO, N2O emissions are typically very low: approximately 1 to 5 ppm in the exhaust, equivalent to approximately 0.001 to 0.005 g-N2O per g-fuel. The N2O comes from:

  • Hot-zone NOx formation: trace N2O is formed in the high-temperature flame front through nitrogen-oxygen radical chemistry. Quantitatively small.
  • Fuel-nitrogen oxidation: marine fuels typically contain 0.1 to 0.6% nitrogen (by mass) which can be partially oxidised to N2O during combustion.
  • Cool-zone reactions: trace N2O is formed in the cooler parts of the combustion chamber through nitrogen-radical chemistry.

In LNG dual-fuel engines burning natural gas, N2O is broadly similar to diesel (slightly lower because LNG has near-zero fuel-nitrogen content; trace formation in flame front is similar).

In methanol dual-fuel engines, N2O is broadly similar to diesel (methanol has zero fuel-nitrogen).

In ammonia dual-fuel engines (in commercial development for 2025 to 2027 service entry), N2O is fundamentally different. Ammonia (NH3) is a nitrogen-containing fuel: each kilogram of NH3 contains 0.82 kg of nitrogen. Combustion of ammonia therefore creates much greater opportunity for N2O formation through fuel-nitrogen pathways. The expected N2O slip rates from ammonia engines (0.05 to 0.5% of N input) translate to significant CO2eq emissions due to the high GWP of N2O.

In biofuel-blended engines, some biofuels (notably FAME from rapeseed or soybean oil) have higher fuel-nitrogen content than petroleum-derived fuels and may produce slightly elevated N2O emissions; quantitatively small relative to ammonia.

In SCR aftertreatment systems (used for NOx Tier III compliance), urea is used as the reductant; urea decomposition produces ammonia, which reduces NOx to N2 and water but can also produce trace N2O through side reactions. SCR catalyst formulations can be optimised to minimise N2O slip; older or worn catalysts can produce significantly higher N2O slip than new catalysts.

N2O versus N2 chemistry

Combustion of any nitrogen-containing fuel produces a mixture of:

  • N2 (molecular nitrogen, climatically inert; the desired end product).
  • NO (nitric oxide, the principal NOx species, regulated under NOx Tier I, II, III).
  • NO2 (nitrogen dioxide, a secondary NOx species).
  • N2O (nitrous oxide, the climate concern).
  • HCN and other minor species (very low concentration).

The branching ratio between these products depends on:

  • Combustion temperature: higher temperatures favour N2 and NO; lower temperatures favour N2O.
  • Air-fuel ratio: lean (excess air) combustion favours N2 and NO; very lean operation can favour N2O.
  • Residence time: long residence at high temperature favours conversion of N2O to N2 (N2O is metastable above approximately 600 °C).
  • Catalyst composition: in SCR, certain catalyst formulations favour N2 over N2O; others favour N2O (undesirable).

The engineering challenge for ammonia engines is to maintain combustion conditions that favour N2 production over N2O production across the full operating envelope, while also maintaining acceptable NOx levels and acceptable engine efficiency.

N2O from ammonia dual-fuel engines

Engine architectures under development

The principal ammonia dual-fuel engine architectures under commercial development are:

  • MAN ME-LGI-A (low-pressure ammonia variant of MAN ME-LGI): commercial pilot delivery 2025; series production from 2026.
  • MAN ME-AM (high-pressure direct-injection ammonia variant of MAN ME-GI): commercial pilot delivery 2026; series production from 2027.
  • Wartsila 25/27 DF Ammonia: commercial pilot delivery 2025; series production from 2026.
  • Wartsila 31DF Ammonia: under development for commercial delivery from 2026 to 2027.
  • Win GD X92DF-A (low-pressure ammonia variant of X-DF): commercial pilot delivery 2025; series production from 2026.
  • Win GD X-DF-A (high-pressure ammonia variant): under development for commercial delivery from 2027 to 2028.

Each architecture has different N2O slip characteristics, predominantly driven by the injection pressure (high-pressure direct injection allows tighter combustion control and lower N2O), the air-fuel ratio strategy and the use of pilot diesel.

Expected N2O slip rates

N2O slip rates from ammonia engines are characterised in g-N2O per kg of NH3 fuel input, or equivalently as a percentage of nitrogen input. Engine manufacturer bench-test projections for the upcoming generation:

  • HPDF ammonia (MAN ME-AM, Win GD X-DF-A): 0.5 to 2.0 g-N2O/kg-NH3 (approximately 0.05 to 0.2% N input).
  • LPDF ammonia (MAN ME-LGI-A, Win GD X92DF-A): 1.5 to 4.0 g-N2O/kg-NH3 (approximately 0.15 to 0.4% N input).
  • Medium-speed ammonia four-stroke (Wartsila 25/27 DF Ammonia, Wartsila 31DF Ammonia): 2.0 to 5.0 g-N2O/kg-NH3 (approximately 0.2 to 0.5% N input).

For comparison, ammonia (energy density approximately 18.6 MJ/kg) has TtW combustion CO2 emissions of approximately 0 g-CO2/kg-NH3. Therefore the entire TtW GHG intensity of an ammonia engine comes from N2O slip and from any pilot diesel. A 2 g-N2O/kg-NH3 slip rate translates to approximately 530 g-CO2eq/kg-NH3 from N2O alone, or approximately 28.5 g-CO2eq/MJ.

Combined with the WtT intensity:

Ammonia pathwayWtT (g-CO2eq/MJ)TtW from N2O at 0.2% slip (g-CO2eq/MJ)WtW (g-CO2eq/MJ)
Grey ammonia12117138
Blue ammonia251742
Green ammonia (RFNBO)51722

A 0.2% N slip rate is the design target for the upcoming HPDF generation; a 0.05% slip rate (the “stretch” target requiring N2O catalyst aftertreatment) would reduce the N2O contribution to approximately 4 g-CO2eq/MJ. The N2O slip rate is therefore the dominant uncertainty in the WtW intensity of green ammonia, and a critical engineering metric for the ammonia engine programme.

Mitigation: combustion-side approaches

Engine manufacturers have several combustion-side approaches to reduce N2O:

  • High-pressure direct injection: tighter combustion control reduces N2O formation. The HPDF architecture (MAN ME-AM, Win GD X-DF-A) is favoured over LPDF on N2O grounds.
  • Optimised pilot fuel injection timing: the pilot diesel ignition source can be timed to favour N2 over N2O production.
  • Air-fuel ratio control: avoiding very lean operation reduces N2O.
  • Combustion chamber design: chambers that maintain longer residence at high temperature favour conversion of N2O to N2.

These combustion-side approaches collectively reduce N2O slip from approximately 0.5% N (early prototypes) to approximately 0.2% N (current commercial bench-test target) without aftertreatment. Further reductions require N2O-specific aftertreatment.

Mitigation: N2O catalyst aftertreatment

A dedicated N2O abatement catalyst can be installed in the exhaust system to decompose N2O to N2 and O2:

$$ 2 N_2 O \rightarrow 2 N_2 + O_2 $$

Suitable catalyst formulations are typically rhodium-based or copper-zeolite based. Effective catalysts achieve 80 to 95% N2O conversion at exhaust temperatures of 350 to 500 °C; conversion is much lower at lower temperatures.

The N2O catalyst is conventionally installed downstream of the SCR (NOx control) catalyst, both because the SCR can produce trace N2O that the downstream catalyst should remove, and because the N2O catalyst should not be exposed to the higher temperatures and ammonia concentrations upstream of the SCR.

Capital cost of N2O catalyst aftertreatment: USD 200,000 to USD 500,000 per engine. Sulphur poisoning is a concern (similar to MOC for methane slip); marine N2O catalysts require very low sulphur in the upstream exhaust stream.

Mitigation: ammonia slip catalyst (ASC)

A related concern in ammonia engines is ammonia slip (unburned NH3 escaping through the exhaust). An ammonia slip catalyst (ASC) oxidises unburned NH3 to N2 and water; the ASC can also produce trace N2O if not properly designed. Modern ASC formulations (typically platinum-vanadium or rhodium-based) achieve high NH3 conversion with low N2O selectivity.

The integrated exhaust aftertreatment train for an ammonia dual-fuel engine therefore typically comprises:

  1. Exhaust gas cleaning system (sulphur scrubber): only required if pilot diesel is non-LSMGO. See exhaust gas cleaning system.
  2. SCR (selective catalytic reduction): NOx control to Tier III levels.
  3. ASC (ammonia slip catalyst): NH3 slip control.
  4. N2O abatement catalyst: N2O conversion to N2.

The integrated train adds significant exhaust system complexity and cost (combined approximately USD 500,000 to USD 1,500,000 per engine for the full aftertreatment train).

N2O from SCR aftertreatment systems

SCR side reactions

SCR systems use urea-water solution as the reductant for NOx control. Urea ((NH2)2CO) decomposes to ammonia and isocyanic acid (HNCO):

$$ (NH_2)_2 CO \rightarrow NH_3 + HNCO $$

The HNCO is hydrolysed to additional ammonia. The ammonia then reduces NOx over the SCR catalyst (typically vanadium-titanium, copper-zeolite, or iron-zeolite):

$$ 4 NH_3 + 4 NO + O_2 \rightarrow 4 N_2 + 6 H_2 O $$

The SCR side reactions that produce N2O include:

  • Ammonia oxidation by oxygen at high temperature: 4 NH3 + 5 O2 → 4 NO + 6 H2O (NOx reformation), with subsequent reaction with N2O byproduct.
  • NH3 + NO2 → N2O + H2O: a side reaction of the standard SCR chemistry, particularly at lower temperatures.
  • Trace NHCO oxidation to N2O: at higher temperatures.

Modern marine SCR catalysts produce typical N2O slip of 1 to 5 ppm in the exhaust at design operating point. Older or worn catalysts can produce 5 to 20 ppm.

Compliance impact

The N2O slip from SCR aftertreatment of a typical NOx Tier III marine engine is approximately 0.1 to 0.5 g-N2O per kg of fuel input, or approximately 0.5 to 2.5 g-CO2eq/MJ at GWP-100 = 265.

For a vessel with WtW intensity of approximately 90 g-CO2eq/MJ on conventional fossil fuel (using FuelEU Annex II default values), the SCR N2O slip adds approximately 0.5 to 3% to the WtW intensity. The contribution is small but measurable, and is captured in the FuelEU Annex II default values.

For owners certifying a vessel-specific (lower than default) WtW intensity, the SCR N2O slip must be measured and certified.

Catalyst optimisation

SCR catalyst manufacturers (Johnson Matthey, BASF, Albemarle, Haldor Topsoe, Catalytic Combustion Corporation, Hitachi Zosen) have developed catalyst formulations optimised for low N2O slip. The principal levers are:

  • Catalyst chemistry: copper-zeolite favours low N2O over vanadium-titanium.
  • Operating temperature: maintaining the catalyst above 350 °C reduces N2O formation. Active heating may be required at low load.
  • Urea injection rate: avoiding over-injection (which leads to ammonia slip and downstream N2O) is important.

Marine SCR catalyst lifetimes are typically 5 to 10 years; replacement at survey intervals provides the opportunity to upgrade to lower-N2O formulations.

Regulatory treatment

FuelEU Maritime

FuelEU Maritime applies the AR5 GWP-100 = 265 for N2O in the WtW intensity calculation. The Annex II default values include explicit accounting for N2O for each fuel category:

Fuel categoryN2O TtW (g-CO2eq/MJ)
HFO/VLSFO with no SCR0.5
HFO/VLSFO with Tier III SCR1.0
LSMGO/MGO with no SCR0.5
LSMGO/MGO with Tier III SCR1.0
LNG (all engine types)0.3
Methanol (all engine types)0.4
Ammonia HPDF (low N2O slip)5 to 15 (engine-dependent)
Ammonia LPDF (medium N2O slip)15 to 30
Ammonia medium-speed four-stroke20 to 50

A vessel certifying engine-specific N2O slip can use the lower values; a vessel relying on default uses the higher values.

IMO Net-Zero Framework GFI

The IMO Net-Zero Framework similarly applies AR5 GWP-100 = 265 in the GFI calculation. The IMO LCA Guidelines (MEPC.391) provide default N2O values broadly aligned with FuelEU Annex II. The default values are reviewed periodically.

EU ETS extension to N2O (from 2026)

The EU ETS Directive amendment of 2023 (Directive (EU) 2023/959) brings N2O into ETS scope from 2026 for vessels above 5,000 GT. The vessel must monitor and report N2O emissions, and surrender EUAs for the corresponding CO2eq. At 2024 EUA prices of approximately EUR 70 to EUR 100 per t CO2, and a typical N2O emission of 3 to 50 t-CO2eq per year per vessel (depending on engine type), the additional ETS cost is approximately EUR 250 to EUR 5,000 per year per vessel. For ammonia-fuelled vessels with high N2O slip, the cost can be significantly higher (potentially EUR 50,000 to EUR 200,000 per year).

Voluntary frameworks

The Poseidon Principles, Sea Cargo Charter and RightShip GHG Rating frameworks all apply N2O in their respective WtW intensity scoring. The treatment is broadly aligned with FuelEU and IMO Net-Zero Framework.

In-service measurement methodology

Continuous emission monitoring

The most accurate N2O measurement is continuous emission monitoring by Fourier-transform infrared (FTIR) spectroscopy, which provides simultaneous measurement of CO2, CO, NO, NO2, N2O, NH3, CH4 and water vapour. Marine FTIR systems (Sintrol, FCT, Servomex, MKS Instruments, Bosean, IRcon) are commercially available.

Sampling location is typically at the funnel, downstream of all aftertreatment systems. The sample line must be heated to prevent water condensation that would absorb water-soluble species.

Capital cost of marine FTIR: USD 80,000 to USD 200,000 per system.

Periodic spot measurement

Annual or semi-annual spot measurement by a third-party measurement contractor is an alternative. Typical campaign cost: USD 25,000 to USD 60,000 per vessel.

Engine-modelled estimation

The lowest-cost approach is engine-modelled estimation, in which N2O is estimated from engine-bench-test data combined with operating profile data. Suitable for non-ammonia fuels where N2O is small; less reliable for ammonia engines where the variability is large.

Implications for owners, charterers and insurers

Owners

Vessel owners specifying ammonia dual-fuel engines for newbuild orders must understand the N2O slip implications. The choice between HPDF and LPDF ammonia architectures is partly driven by N2O slip considerations. The decision to install N2O catalyst aftertreatment (additional USD 200,000 to USD 500,000 per engine) is increasingly informed by FuelEU and IMO Net-Zero Framework cost projections.

Owners of conventional fossil-fuel vessels with SCR aftertreatment have less direct exposure but should monitor SCR N2O slip and consider upgrade to lower-N2O catalyst formulations at survey intervals.

Charterers

Long-term charterers will increasingly require certified N2O slip rates from owners as the FuelEU and IMO Net-Zero Framework calculations mature. The contract framework is evolving in parallel with the BIMCO FuelEU Maritime Clause development.

Insurers

Marine insurers are integrating N2O into hull and P&I underwriting for Poseidon Principles signatories. The insurance implications of ammonia bunkering and onboard storage (ammonia toxicity and corrosivity) are a separate concern from the N2O emission concern.

Banks and finance

Ship-finance banks signed up to the Poseidon Principles include N2O in their portfolio WtW intensity reporting. The portfolio impact of ammonia dual-fuel newbuilds depends critically on the certified N2O slip rate.

Future outlook

N2O emission standards

The IMO is developing N2O emission standards for ammonia and other nitrogen-containing fuel engines, intended to come into effect from approximately 2028 to 2030. The standards are expected to set maximum N2O slip rates by engine type, with progressive tightening over time.

Catalyst development

Marine N2O catalyst development is accelerating in response to the ammonia engine programme. Several catalyst manufacturers (Johnson Matthey, BASF, Haldor Topsoe, Mitsui Mining and Smelting) are running marine N2O catalyst trials with engine manufacturers. Commercial marine N2O catalysts are expected to be available from approximately 2025 to 2026.

Continued engine refinement

Engine manufacturers continue to refine the combustion-side N2O reduction. Targets for the second-generation ammonia engines (delivery 2027 to 2030) are approximately 0.05 to 0.10% N slip without aftertreatment; with N2O catalyst aftertreatment, effective slip rates of approximately 0.005 to 0.010% N are expected.

Convergence with methane slip regulation

The regulatory framework for N2O is developing in parallel with the methane slip framework. The two issues share many similarities (both are non-CO2 GHG slip from marine engines, both require aftertreatment for full mitigation, both are captured in FuelEU and IMO Net-Zero Framework via AR5 GWP-100). The integrated approach is expected to develop through MEPC.391 LCA Guidelines updates and through subsequent FuelEU Annex II revisions.

Stratospheric ozone considerations

The Montreal Protocol parties have considered (but not yet adopted) extending the protocol to N2O. If extended, marine N2O would face a parallel regulatory regime focused on stratospheric ozone impact rather than just climate impact. The likelihood of Montreal Protocol extension is currently low but increases as N2O emissions grow.

See also

Marine fuels

Engines, exhaust and machinery

Regulatory and reporting frameworks

Voluntary frameworks

Operational and technical efficiency

Hydrostatics, stability and ship types

Conventions, codes and class

Calculators

References

  • IMO Resolution MEPC.391 (in adoption 2025): Life Cycle Assessment Guidelines for Marine Fuels. International Maritime Organization, 2025.
  • IMO Resolution MEPC.328(76): 2021 Revised MARPOL Annex VI. International Maritime Organization, 2021.
  • Regulation (EU) 2023/1805 of the European Parliament and of the Council of 13 September 2023 on the use of renewable and low-carbon fuels in maritime transport (FuelEU Maritime). Official Journal of the EU, 2023.
  • Regulation (EU) 2023/959 of the European Parliament and of the Council of 10 May 2023 amending Directive 2003/87/EC (EU ETS Maritime). Official Journal of the EU, 2023.
  • IPCC. Fifth Assessment Report (AR5) Working Group I: Climate Change 2013. Intergovernmental Panel on Climate Change, 2013.
  • DNV. Ammonia as a marine fuel: Position Paper. DNV Maritime, 2022.
  • DNV. Maritime Forecast to 2050. DNV Energy Transition Outlook, 2023.
  • ICCT. The role of ammonia in shipping decarbonisation. International Council on Clean Transportation, 2023.
  • MAN Energy Solutions. MAN B&W ME-LGI-A and ME-AM Engine Specifications. MAN Energy Solutions, 2024.
  • Wartsila Marine Solutions. Wartsila 25/27 DF Ammonia Engine Specifications. Wartsila, 2024.
  • Win GD. Win GD X92DF-A Engine Specifications. Win GD, 2024.

Further reading

  • IRENA. A pathway to decarbonise the shipping sector by 2050. International Renewable Energy Agency, 2021.
  • ICS. Catalysing the Fourth Propulsion Revolution. International Chamber of Shipping, 2022.
  • IEA. The Future of Hydrogen. International Energy Agency, 2019.