N2O Emissions from Marine Engines

Nitrous oxide (N2O) occupies an awkward position in marine emissions regulation. MARPOL Annex VI Regulation 13 ignores it completely because the NOx Tier regime targets NO and NO2, not N2O. Yet the IMO LCA Guidelines adopted under MEPC.376(80) and refined by MEPC.391(81), and the EU FuelEU Maritime Regulation 2023/1805, both apply an IPCC AR5 GWP-100 of 265 to N2O in the well-to-wake intensity calculation. That GWP-100 of 265 means each kilogram of N2O has the climate impact of 265 kg of CO2 over a hundred years, and because ammonia-fuelled engines introduce a new, high-nitrogen fuel into marine combustion, N2O slip has become a first-order design constraint for the ammonia engine generation due from 2025 to 2027.

The N2O CO2-equivalent calculator converts measured N2O mass to CO2-equivalent using the GWP-100 from the applicable regulatory instrument. The N2O emissions calculator estimates N2O mass flow from engine power, fuel type, and operating point. For ammonia-specific work, the ammonia NOx and N2O slip calculator handles combined NOx + N2O slip estimation, and the ammonia well-to-wake intensity calculator integrates N2O slip into the full well-to-wake GHG intensity.

What N2O is and why the GWP matters for shipping

N2O is a stable, colourless gas with an atmospheric lifetime of roughly 109 years (IPCC AR5, Chapter 8). That lifetime exceeds the 100-year integration period used for GWP-100 calculations, which is why the GWP-20 and GWP-100 of N2O are nearly identical: 264 and 265 respectively in AR5. This insensitivity to integration period matters operationally because the regulatory choice between 20-year and 100-year GWP that creates large differences for methane simply does not apply to N2O.

The IPCC AR6 (2021) revised the GWP-100 of N2O upward slightly to 273. The IMO LCA Guidelines under MEPC.376(80) and MEPC.391(81) retained the AR5 value of 265, as did FuelEU Maritime Annex I. Owners should use 265 for regulatory compliance calculations under both regimes; the AR6 value of 273 may appear in voluntary or scientific reporting. The GFI compliance calculator and the well-to-wake blend calculator both use 265.

CO2-equivalent conversion

The CO2-equivalent of an N2O emission is:

where $\text{GWP}_{100} = 265$ under IPCC AR5 (IMO LCA Guidelines, FuelEU Maritime Annex I). For a vessel emitting 10 kg of N2O per hour from an ammonia dual-fuel engine at full load:

At a specific fuel consumption of roughly 150 g/kWh for a large two-stroke engine burning ammonia at 20,000 kW, that 10 kg/h of N2O slip corresponds to a slip rate of approximately 3.3 g-N2O per kg of NH3 fuel, or a TtW GHG intensity contribution from N2O alone of approximately $3.3 \times 265 / 18.6 \approx 47 \, \text{g CO}_2\text{e/MJ}$.

N2O also depletes stratospheric ozone. It is currently the most significant ozone-depleting substance emitted anthropogenically, because the Montreal Protocol and its amendments eliminated most chlorofluorocarbon (CFC) and hydrochlorofluorocarbon (HCFC) emissions. N2O is not a halogenated compound and does not fall under the Montreal Protocol. Its ozone-depletion dimension has no direct regulatory consequence for ship operators yet, but it reinforces the case that N2O emission reductions are desirable independently of the CO2-equivalent accounting.

N2O formation in marine combustion

Conventional distillate and residual fuel engines

Conventional marine diesel engines burning heavy fuel oil, VLSFO, or marine gas oil produce N2O at low concentrations, typically 1 to 5 ppm in the exhaust by volume. Three pathways contribute.

Prompt and thermal N2O. In the high-temperature flame front, nitrogen from the combustion air and fuel reacts with oxygen radical species to produce trace N2O. This pathway dominates in clean, low-nitrogen fuels. The rate increases at intermediate temperatures around 600 to 900 degrees C; above roughly 900 degrees C, N2O is thermally unstable and rapidly decomposes to N2 and O2.

Fuel-nitrogen oxidation. Residual fuels (HFO, VLSFO) typically contain 0.2 to 0.6 percent nitrogen by mass as organically bound nitrogen compounds. Partial oxidation of this nitrogen during combustion produces N2O alongside NO and N2. The fraction converted to N2O is small relative to NO, but residual fuel nitrogen content is high enough that the fuel-nitrogen route is measurable in exhaust analyses of HFO-burning engines.

Cool-zone recombination. In the quench zone of the combustion chamber, where charge gases cool rapidly, nitrogen radical chemistry can produce N2O. This is the same mechanism that produces N2O in post-combustion zones in large gas turbines.

For a 20,000 kW slow-speed two-stroke engine burning VLSFO at approximately 160 g/kWh, exhaust N2O at 3 ppm (by volume, dry) translates to roughly 0.02 to 0.05 g-N2O per kg of fuel. The CO2 from fuel calculator handles CO2 accounting under MEPC.364(79); N2O is additive to that CO2 figure in the well-to-wake intensity calculation.

LNG and methanol dual-fuel engines

LNG dual-fuel engines burning natural gas produce slightly lower N2O than their HFO counterparts because natural gas contains near-zero fuel-bound nitrogen. The flame-front pathway still applies, so N2O is not zero, but it is broadly similar in magnitude and not a primary concern.

Methanol dual-fuel engines have zero fuel-bound nitrogen (methanol is CH3OH) and produce N2O only through the thermal pathway. Exhaust N2O from methanol engines is comparable to or slightly lower than LNG engines.

Neither engine type introduces N2O as a significant driver of WtW intensity under current regulatory default values.

SCR systems and N2O as an aftertreatment byproduct

Selective catalytic reduction (SCR) systems used for NOx Tier III compliance inject urea-water solution upstream of the catalyst. Urea ((NH2)2CO) thermally decomposes and hydrolyses to ammonia (NH3) and CO2; the ammonia then reacts with NOx over the vanadium-titanium or copper-zeolite catalyst to produce N2 and water. The principal reaction is:

Two side reactions produce N2O.

The NH3 + NO2 pathway. At operating temperatures below approximately 300 degrees C, the reaction $2\, \text{NH}_3 + 2\, \text{NO}_2 \rightarrow \text{N}_2\text{O} + \text{N}_2 + 3\, \text{H}_2\text{O}$ becomes a measurable contributor. Marine SCR systems are particularly susceptible during part-load and port-manoeuvring conditions when exhaust temperatures fall.

Ammonia oxidation at high temperature. When urea is over-injected or catalyst temperature rises above 450 to 500 degrees C, ammonia oxidation produces NO and, in a side branch, N2O.

Modern marine SCR catalysts from Johnson Matthey, BASF, and Haldor Topsoe (now Topsoe) are formulated to minimise N2O slip: typical values are 1 to 5 ppm in the exhaust at the design operating point. Worn or aged catalysts, or systems operating outside their temperature window, can produce 5 to 20 ppm. For regulatory purposes, FuelEU Maritime Annex I tables include N2O in the default TtW emission factors for engine-plus-SCR combinations; owners certifying vessel-specific (non-default) values must measure and certify SCR N2O.

The quantitative impact is modest on a conventional vessel: 5 ppm N2O in the exhaust of a 20,000 kW engine represents roughly 0.1 to 0.3 g-N2O/kg-fuel, or about 1 to 2 g-CO2e/MJ against a total WtW intensity of 85 to 95 g-CO2e/MJ for VLSFO. It does not materially affect compliance margins for the FuelEU 2025 to 2035 targets, but it counts and must be included in monitored reports.

N2O from ammonia dual-fuel engines

This is where N2O becomes a first-order issue. Ammonia (NH3) is 82.4 percent nitrogen by mass. Every kilogram of ammonia fuel contains 0.824 kg of nitrogen. Even if 99.9 percent of that nitrogen exits as N2, the residual 0.1 percent N as N2O corresponds to roughly $0.824 \times 0.001 \times (14/44) \times ... \approx 0.12 \, \text{g-N}_2\text{O per kg-NH}_3$, which at GWP-100 = 265 and net calorific value of approximately 18.6 MJ/kg gives approximately 1.7 g-CO2e/MJ from N2O alone. A 0.5 percent N slip rate (early prototype figures) gives about 8.5 g-CO2e/MJ. For reference, the FuelEU 2025 GHG intensity limit for alternative fuel vessels is set relative to a fossil baseline of approximately 91.16 g-CO2e/MJ.

Engine architectures and expected slip rates

Two broad injection architectures cover the commercial ammonia engine market.

High-pressure direct-injection (HPDF) engines. MAN Energy Solutions’ ME-LGIA (direct adaptation of the ME-LGI methanol engine, low-pressure injection) and the forthcoming ME-AM (high-pressure direct injection), plus WinGD’s X-DF-A, all inject ammonia at pressures high enough to achieve tight mixing and near-stoichiometric combustion in the ignition zone. MAN’s published bench-test data for the ME-LGIA pilot programme (2024 delivery to Amon Maritime/Avance Gas) indicate N2O slip in the range of 0.5 to 2.0 g-N2O per kg-NH3 at full load, with higher values at part load.

WinGD’s X-DF-A uses a two-stroke low-pressure port injection architecture derived from the X-DF series. WinGD has published a target of below 2 g-N2O per kg-NH3 for the commercial series entering service from 2026, pending full-scale certification testing.

Medium-speed four-stroke engines. Wartsila’s 25/27 DF Ammonia and 31DF Ammonia use port-injection at lower pressures than the two-stroke HPDF engines. Pre-commercial test data indicate N2O slip in the range of 2 to 5 g-N2O per kg-NH3 at rated load, with variation across the load envelope.

Converting slip rates to WtW intensity terms at GWP-100 = 265 and NCV = 18.6 MJ/kg:

These N2O contributions are additive to the well-to-tank (WtT) intensity of the ammonia pathway. For green ammonia produced via electrolysis with renewable electricity, the IMO LCA Guidelines default WtT intensity is approximately 5 to 8 g-CO2e/MJ. A WinGD X-DF-A engine at 2 g/kg-NH3 N2O slip adds 28.5 g-CO2e/MJ in TtW N2O alone, bringing the WtW total to approximately 33 to 37 g-CO2e/MJ. That is still below the FuelEU 2025 fossil baseline but approaches the 2035 target of a 14.5 percent reduction from the reference (roughly 77.9 g-CO2e/MJ), depending on the trajectory.

For blue ammonia (SMR with carbon capture, WtT approximately 20 to 30 g-CO2e/MJ), high N2O slip can place the total WtW above the fossil baseline, eliminating the carbon benefit of the fuel switch. The N2O slip rate is therefore the swing factor in the commercial viability of blue ammonia as a transition fuel.

Why N2O forms preferentially at low combustion temperatures

N2O is metastable in hot gas. Above approximately 900 degrees C, the thermal decomposition $2\,\text{N}_2\text{O} \rightarrow 2\,\text{N}_2 + \text{O}_2$ is fast enough that N2O formed in the flame front does not survive to the exhaust. Below 600 to 900 degrees C, the decomposition slows sharply and N2O can persist.

In ammonia combustion, lean burn conditions (excess air) are necessary to manage the low flame temperature of ammonia (adiabatic flame temperature approximately 1,850 degrees C, compared to approximately 2,230 degrees C for methane). Lean burn reduces combustion temperature, which shifts the selectivity of nitrogen oxidation from $\text{N}_2$ and NO toward N2O. This is the fundamental tension: the combustion conditions that keep flame temperatures safe for engine hardware (lean, cool) are also the conditions that maximise N2O formation.

Engine makers address this by using a small amount of pilot fuel (marine gas oil or LSMGO) to achieve a hot ignition source that locally raises temperature before the ammonia charge ignites. The pilot fraction for HPDF engines is 3 to 7 percent of energy input; for LPDF engines it is 5 to 15 percent. The pilot fuel combustion produces some CO2 on a TtW basis, but the key role is combustion stability, not energy supply.

Combustion-side N2O reduction strategies

Engine makers have developed four approaches to reduce N2O at the source rather than in aftertreatment.

Pilot timing optimisation. Advancing or retarding pilot injection timing changes the local temperature history in the cylinder, shifting the N2–N2O branching ratio. MAN and WinGD each have proprietary tuning maps that balance N2O, NOx, and efficiency.

Stratified charge control. Localised fuel-rich zones near the pilot create high-temperature pockets where N2O decomposes before the charge cools. HPDF architectures achieve better stratification than LPDF, which is one reason HPDF shows lower N2O slip in bench tests.

Exhaust gas recirculation (EGR). Recirculating a fraction of exhaust gas into the intake reduces peak combustion temperature and, counterintuitively, can reduce N2O by suppressing the intermediate radical reactions that produce it. EGR is more established for NOx control in methane engines; its application to ammonia is under active development.

Air-fuel ratio management. Avoiding the lambda values (excess-air ratio) where N2O selectivity peaks. The optimal lambda window varies by engine type and load, and requires load-dependent control maps.

These combustion-side measures collectively push the target from early-prototype slip rates of 3 to 5 g/kg-NH3 toward commercial targets of 1 to 2 g/kg-NH3 without aftertreatment.

N2O abatement catalysts

When combustion-side reduction is insufficient, a dedicated N2O decomposition catalyst can be installed in the exhaust line. The net reaction is:

This is thermodynamically favourable but kinetically slow without a catalyst. Suitable catalyst families include rhodium-based monoliths and copper-zeolite (Cu-ZSM-5 or Cu-SSZ-13) formulations. At exhaust temperatures of 350 to 500 degrees C, well-maintained catalysts achieve 80 to 95 percent N2O conversion. Below 300 degrees C (part-load or port manoeuvring), conversion efficiency drops sharply, which is an operational limitation.

The N2O catalyst is placed downstream of the SCR (NOx catalyst) and the ammonia slip catalyst (ASC). The SCR operates at higher temperatures and requires ammonia as a reductant; placing the N2O catalyst downstream means it sees a cleaner gas stream and avoids interference with SCR chemistry. The ASC converts residual NH3 slip to N2; it also produces trace N2O via the reaction $4\,\text{NH}_3 + 3\,\text{O}_2 \rightarrow 2\,\text{N}_2 + 6\,\text{H}_2\text{O}$ (main) and the side reaction $4\,\text{NH}_3 + 4\,\text{O}_2 \rightarrow 2\,\text{N}_2\text{O} + 6\,\text{H}_2\text{O}$ (minor). So the ASC itself contributes to N2O, and the N2O catalyst should sit downstream of both.

The integrated aftertreatment train for an ammonia dual-fuel main engine therefore runs: SCR (NOx control) + ASC (ammonia slip control) + N2O catalyst. Capital cost estimates from catalyst manufacturers in 2024 are USD 200,000 to USD 500,000 per engine for the N2O catalyst stage alone. The full SCR + ASC + N2O train is estimated at USD 500,000 to USD 1,500,000 per engine, depending on engine size and whether open-loop or closed-loop dosing control is specified. Sulphur in the exhaust poisons rhodium-based catalysts; engines burning pilot fuel with sulphur content above 0.1 percent (m/m) require a sulphur scrubber upstream.

Regulatory treatment: how N2O is counted

FuelEU Maritime (Regulation 2023/1805, from 2025)

FuelEU Maritime applies to vessels above 5,000 GT on voyages to, from, or between EU ports. The GHG intensity of energy used on board (the “WtW intensity”) is calculated using the methodology in FuelEU Annex I, which applies GWP-100 = 265 for N2O (from IPCC AR5) and GWP-100 = 28 for CH4 (biogenic) and 82.5 for CH4 (fossil), also from AR5. The methane slip article covers the CH4 side.

FuelEU Annex I default TtW emission factors include N2O for each fuel category. For fuels with negligible N2O (diesel, LNG, methanol), the default N2O contribution is small. For ammonia, no single default applies yet; vessels using ammonia must monitor and certify N2O slip directly under the FuelEU monitoring plan. The regulation entered application on 1 January 2025; the first compliance year for surrendering FuelEU certificates is 2025 (reported by 31 March 2026).

IMO LCA Guidelines (MEPC.376(80) and MEPC.391(81))

The IMO adopted the 2023 IMO GHG Strategy under MEPC.377(80), which sets a trajectory toward net-zero GHG emissions from international shipping by or around 2050. The strategy introduces the GHG Fuel Intensity (GFI) standard from 2027, building on the IMO LCA Guidelines.

MEPC.376(80) (July 2023) adopted the LCA Guidelines that define how well-to-wake GHG intensity is calculated, including N2O at GWP-100 = 265 (AR5). MEPC.391(81) (April 2024) updated those guidelines with revised default values for certain fuel pathways. The GFI calculation therefore includes N2O from the outset; ships burning ammonia with measurable N2O slip will carry a GFI penalty from the N2O term that cannot be offset by zero-CO2 combustion alone.

The N2O GWP-100 in the IMO LCA context applies to both tank-to-wake (TtW) N2O from direct engine emissions and to any upstream N2O in the well-to-tank (WtT) pathway. For ammonia produced by the Haber-Bosch process, the upstream production and purification stages can also emit trace N2O; this is captured in the WtT default values in the guidelines.

EU ETS extension (Regulation 2023/957, from 2024 reporting)

Regulation 2023/957 amended Directive 2003/87/EC (the EU ETS Directive) to bring maritime transport into the ETS and to extend the scope of the maritime MRV to include N2O and CH4 from 2024 reporting (covering the calendar year 2024, with EUAs surrendered in 2025 for the ship’s pro-rata share depending on EU voyage fraction). The EU MRV Regulation was updated in parallel via Regulation 2023/957 to require N2O and CH4 monitoring from 2024.

For a conventional vessel with SCR-derived N2O of approximately 0.5 g/kg-fuel and annual fuel consumption of 5,000 tonnes, the total N2O ETS exposure is roughly 2.5 tonnes of N2O, equal to 662 tonnes of CO2-equivalent. At 2025 EUA prices of approximately EUR 60 to EUR 70 per t-CO2e, this represents approximately EUR 40,000 to EUR 46,000 per year for a vessel whose voyages were fully within EU scope. The voyage fraction (50 percent of intra-EU voyages, 100 percent of EU-to-EU) substantially reduces the effective exposure for most deepsea vessels; a vessel making three EU roundtrips per year may have an effective EU-voyage fraction of 15 to 25 percent.

For an ammonia-fuelled vessel at 3 g-N2O/kg-NH3 with 5,000 tonnes annual ammonia consumption, the CO2-equivalent exposure is approximately 3,975 tonnes CO2e from N2O alone: roughly EUR 240,000 to EUR 280,000 per year in ETS costs at those EUA prices.

What MARPOL Annex VI does not cover

MARPOL Annex VI Regulation 13, read with the NOx Technical Code 2008 (NTC 2008), sets Tier I, II, and III limits for total NOx (the sum of NO and NO2) from marine diesel engines above 130 kW installed after specified dates. N2O is explicitly excluded from the NOx definition in the NTC 2008; the limit applies to $\text{NO} + \text{NO}_2$ expressed as NO2-equivalent. See the NOx Tier I, II, III article for the applicable limits by engine speed and entry-into-service date.

This means that an SCR system optimised for NOx Tier III compliance could in principle produce elevated N2O as a byproduct without violating MARPOL Annex VI. The regulatory pressure to control SCR-derived N2O comes entirely from FuelEU Maritime and the EU ETS, not from MARPOL.

Measurement and monitoring

Continuous emission monitoring with FTIR

The most accurate method for in-service N2O measurement is Fourier-transform infrared (FTIR) spectroscopy. A marine FTIR system samples a heated, filtered stream of exhaust gas downstream of all aftertreatment, and simultaneously measures CO2, CO, NO, NO2, N2O, NH3, SO2, CH4, and water vapour. Measurement uncertainty for N2O is typically 5 to 10 percent of the reading for concentrations above 5 ppm; below 1 ppm the measurement uncertainty increases substantially, which is relevant for high-efficiency N2O catalysts.

Commercial marine FTIR systems are available from Sintrol, Servomex, MKS Instruments, and Bosean, among others. Capital cost is USD 80,000 to USD 200,000 per system. Heated sample lines (to 180 degrees C) are required to prevent water condensation, which would dissolve NH3 and N2O and underread both species.

Periodic third-party spot measurement

Annual or voyage-specific spot measurement by an accredited measurement contractor is an alternative to continuous monitoring for vessels where N2O is expected to be low (conventional non-SCR engines). A typical measurement campaign costs USD 20,000 to USD 50,000 per vessel per visit and covers one or more engine load points. The resulting data support FuelEU monitoring plan certification for the fuel-specific N2O emission factor.

Calculation-based estimation

For non-ammonia fuels, FuelEU Maritime and IMO MRV rules permit the use of default emission factors (the calculation method) rather than direct measurement. Default N2O factors are specified in FuelEU Annex I for each fuel category; the values reflect fleet-average data and carry a conservatism margin. Vessels relying on defaults cannot claim credit for lower-than-default N2O, but also do not carry measurement and calibration costs.

For ammonia-fuelled vessels, calculation-based estimation from engine bench-test data is acceptable during the initial commissioning period, but on-vessel measurement is required for certification under FuelEU Maritime and for ETS monitoring under Regulation 2023/957. The EU MRV regulation requires that the monitoring plan specify the method used for each emission type, including N2O.

Sampling location and representativeness

The regulatory requirement is to measure N2O in the exhaust after all onboard abatement, at the stack. Two practical difficulties affect representativeness. First, the exhaust flow in a large two-stroke engine is pulsed, not continuous, and the pulse frequency can interact with the sampling line delay to skew readings. Marine FTIR installations need an adequately sized mixing volume before the sample extraction point. Second, on ammonia engines, the NH3 concentration in the exhaust can be high enough (50 to 200 ppm before the ASC) that NH3 breakthrough into the FTIR cell causes a measurement interference with N2O at the infrared absorption bands where both species overlap. Purpose-designed marine FTIR systems for ammonia engines include a NH3 pre-scrubber to remove NH3 before the FTIR cell while preserving N2O; this component is not standard in FTIR systems designed for conventional diesel exhaust.

A calibration protocol is needed for each FTIR installation. N2O calibration gases at concentrations representative of expected exhaust levels (1 to 50 ppm for SCR-equipped diesel engines; 5 to 200 ppm for ammonia engines without N2O catalyst) should be supplied by an accredited provider with traceability to national standards. Annual calibration verification is required under the FuelEU monitoring plan for continuous monitoring systems.

Quality assurance for N2O monitoring plans

FuelEU Maritime Article 8 and Annex I require that vessels above 5,000 GT submit a monitoring plan to their administering authority (the relevant EU member state, typically the flag state or the port state for non-EU flag vessels) before the start of each reporting period. The monitoring plan must specify, for each GHG species including N2O: the measurement method (measurement, calculation, or default), the monitoring equipment and its calibration frequency, the uncertainty threshold, and the procedure for handling data gaps.

For ammonia-fuelled vessels, the EU monitoring plan must specify N2O as a directly monitored species using the measurement method (not default), because no default emission factor applicable to ammonia is currently specified in FuelEU Annex I for N2O. The administering authority reviews and approves the plan before the ship operates under FuelEU. Classification societies including DNV, Lloyd’s Register, and Bureau Veritas offer monitoring plan verification services for FuelEU compliance; this verification step is not legally required but is commercially expected for vessels in long-term charter.

N2O in the context of the NOx-N2O tradeoff

One underappreciated complication in ammonia engine design is that the combustion conditions favouring low NOx are not the same as those favouring low N2O. In conventional diesel combustion, low NOx is achieved by reducing peak flame temperature, typically through cooler charge temperature, higher exhaust gas recirculation rates, or retarded injection timing. Each of those measures also lowers the temperature in the post-flame zone, which is exactly where N2O is most stable and where it accumulates. The NOx Tier III requirement therefore creates a mild opposing pressure: achieving low NOx may, if done crudely, increase N2O.

This tradeoff is manageable in conventional diesel engines because both NOx and N2O are present at low concentrations; a small increase in N2O to achieve NOx Tier III via SCR (rather than in-engine measures) carries negligible CO2-equivalent consequences. For ammonia engines, the tradeoff is more material. The high fuel-nitrogen content means that N2O is generated in quantities large enough that it drives GHG intensity on its own; simultaneously, the ammonia combustion chemistry already runs lean, which suppresses NOx formation below what a comparable diesel engine would produce. The practical result is that ammonia engines tend to be closer to Tier III compliance in-cylinder than diesel engines but carry a larger N2O penalty.

Engine makers are targeting this jointly: MAN’s combustion development for the ME-LGIA explicitly targets minimising both NOx and N2O by choosing injection strategies that achieve a short, hot ignition kernel (favourable for NO conversion to N2, and high temperature for N2O decomposition) followed by a lean, cool burn-out zone (favourable for low NOx). WinGD’s approach for the X-DF-A emphasises staged combustion and a split injection pattern that controls the temperature history over the full stroke. Whether these approaches will hold across the full operating envelope, including part-load and transient conditions on a long voyage, is a question that only in-service monitoring data can answer.

The SCR interaction in ammonia engines

SCR in ammonia dual-fuel engines plays a different role than in diesel engines. In diesel engines, the SCR handles residual NOx from combustion. In ammonia engines, NOx and N2O both need to be managed; the conventional SCR handles NOx, the ASC handles NH3 slip, and the N2O catalyst handles N2O. But the boundaries are not clean.

An SCR system operating on exhaust from an ammonia engine sees a different gas composition than one designed for diesel: the ammonia concentrations upstream of the SCR can be much higher, especially during combustion upsets or load changes, because NH3 is both the fuel and the potential reductant slip species. Over-reduction of NOx by excess NH3 in the SCR can produce N2O via the $2\,\text{NH}_3 + 2\,\text{NO}_2 \rightarrow \text{N}_2\text{O} + \text{N}_2 + 3\,\text{H}_2\text{O}$ pathway. Under-dosing leads to NOx breakthrough. The SCR control system for an ammonia engine must therefore handle a wider range of NH3/NOx ratios than a diesel SCR, and the risk of N2O production from the SCR itself is higher.

This is an active area of catalyst development. Copper-zeolite SCR formulations are generally preferred over vanadium-titanium for ammonia-engine applications because they show lower N2O selectivity across a wider temperature range. The operating temperature window for the combined SCR + ASC + N2O catalyst train on an ammonia engine (approximately 300 to 450 degrees C) is narrower than for a diesel SCR, which creates a stronger argument for exhaust heat management systems that keep the catalyst train in its optimal window during port approach and slow-steaming.

How N2O compares to methane slip as a regulatory concern

N2O and methane slip share structural similarities: both are non-CO2 greenhouse gas species that can escape from marine engines via incomplete combustion or aftertreatment chemistry, both are counted in the FuelEU and IMO LCA WtW intensity, and both require measurement or certification beyond the conventional CO2 MRV framework. The methane slip article covers the methane side in detail.

The key difference is fuel specificity. Methane slip is primarily an issue for LNG dual-fuel engines because LNG is almost pure methane; non-LNG fuels produce negligible methane slip. N2O is primarily an issue for ammonia dual-fuel engines because ammonia is a nitrogen-rich fuel; conventional diesel, LNG, and methanol engines produce N2O only at trace levels from air-nitrogen and fuel-nitrogen. The regulatory frameworks treat them symmetrically (both counted via AR5 GWP-100 in FuelEU and IMO LCA) but the engineering challenges are distinct.

One practical difference: methane has a GWP-100 of 28 (biogenic) to 29.8 (fossil) under AR5, which is roughly one-ninth the GWP-100 of N2O (265). A methane slip rate of 1 g/g-LNG is thus much less damaging in CO2-equivalent terms than a N2O slip rate of 0.1 g/g-NH3. Ammonia engine designers are working to tight tolerances because even sub-percent N2O slip rates produce large CO2-equivalent penalties.

GHG impact table: N2O in the well-to-wake context

The table below shows how N2O slip integrates into the full WtW calculation for selected fuel and engine combinations, using IMO LCA default WtT values and the GWP-100 values in MEPC.376(80) / MEPC.391(81).

The N2O catalyst row illustrates the scale of abatement available: a 90 percent efficient catalyst reduces the N2O TtW contribution by a factor of ten, from 28.5 to 2.8 g-CO2e/MJ. For green ammonia, that brings the WtW total below 10 g-CO2e/MJ, which is competitive with the projected FuelEU 2045 target (around 14 g-CO2e/MJ for a 70 percent reduction from the fossil reference). Without the catalyst, even green ammonia at 2 g/kg-NH3 slip sits above the 2040 target band.

Implications for shipowners, operators, and charterers

Newbuild specification decisions

Owners ordering ammonia dual-fuel newbuilds must decide on engine architecture (HPDF vs LPDF), pilot fuel type and fraction, and whether to pre-install an N2O abatement catalyst or leave space and connections for future retrofit. The FuelEU compliance trajectory to 2045 argues for N2O catalyst pre-installation on vessels expected to burn ammonia beyond 2030. The incremental capital cost of USD 200,000 to USD 500,000 per engine is modest against the ETS and FuelEU penalty exposure from unabated N2O slip over a 25-year vessel life.

Charter party implications

Long-term charter parties increasingly specify a ceiling WtW GHG intensity for the chartered vessel. FuelEU-linked clauses (BIMCO FuelEU Maritime clauses are under development as of mid-2025) require the owner to certify N2O slip rates and provide monitoring data to the charterer. A vessel with certified low N2O slip from an N2O catalyst commands a stronger position in charter negotiations for ammonia-capable tonnage.

ETS and FuelEU cost allocation

The EU ETS for shipping and FuelEU Maritime both create financial exposures proportional to WtW GHG intensity. Under FuelEU, non-compliance penalties are EUR 2,400 per tonne of CO2-equivalent shortfall (the “excess bunker” charge). For an ammonia vessel with 2 g/kg-NH3 N2O slip consuming 5,000 tonnes NH3 per year on EU-scope voyages, the N2O-derived FuelEU shortfall against the 2030 target (estimated at approximately 80 g-CO2e/MJ) could represent several hundred tonnes of CO2-equivalent per year in excess shortfall, carrying penalty exposure in the tens of thousands of euros annually.

Limitations

The N2O slip data cited in this article for commercial ammonia engines are from engine maker bench-test programmes conducted in 2023 to 2024; full-scale in-service data do not yet exist at scale (the first commercial ammonia dual-fuel vessels were entering sea trials in late 2024 and 2025). Bench-test conditions may not fully replicate the load profiles, fuel quality variations, and auxiliary system interactions of in-service operation.

The regulatory default N2O emission factors in FuelEU Maritime Annex I are subject to revision as in-service data accumulate; MEPC.391(81) explicitly provides for periodic update of LCA default values. Owners making capital decisions based on current default values carry revision risk.

N2O catalyst efficiency data for marine applications are primarily from land-based industrial catalysts scaled to marine conditions; marine-specific certified test data are limited to a small number of pilot installations as of mid-2025.

The GWP-100 value of 265 (AR5) is the currently mandated figure for EU and IMO regulatory calculations. The AR6 value of 273 is higher; any future regulatory update to AR6 would increase the CO2-equivalent cost of N2O emissions by approximately 3 percent.

The stratospheric ozone dimension of N2O is noted in this article but has no current direct regulatory consequence for ship operators. That could change if the Montreal Protocol parties extend the protocol to N2O, but this is not under active negotiation as of mid-2025.

N2O from biofuel blends is not addressed in depth here. Some biofuels (notably FAME from nitrogen-rich feedstocks) have higher fuel-bound nitrogen than petroleum distillates, but the N2O contribution remains small relative to ammonia and is covered by the FuelEU Annex I default values for those fuel categories.

See also

Marine fuels and emissions

• Ammonia as marine fuel
• Ammonia marine engines overview
• Methane slip from LNG engines
• LNG as marine fuel
• Methanol as marine fuel
• Biofuels in shipping
• Well-to-wake intensity
• RFNBO under EU rules
• Heavy fuel oil
• Marine gas oil
• Black carbon and Arctic shipping

Regulations and frameworks

• MARPOL Annex VI
• NOx Tier I, II, III
• FuelEU Maritime
• IMO Net-Zero Framework
• IMO GHG Strategy
• EU ETS for shipping
• EU MRV Regulation
• EEXI, EPL and ShaPoLi
• CII corrective action plan
• Emission control areas
• UK ETS for shipping
• Poseidon Principles

Engine systems

• Marine diesel engine
• Exhaust gas cleaning system

Calculators

• N2O emissions calculator
• N2O CO2-equivalent calculator
• Ammonia NOx and N2O slip calculator
• Ammonia well-to-wake intensity calculator
• Well-to-wake blend intensity calculator
• GFI compliance calculator
• GFI attained calculator
• CO2 from fuel (MEPC.364(79))