Selective catalytic reduction (marine NOx Tier III)

Selective catalytic reduction (SCR) is the dominant abatement technology used to satisfy MARPOL Annex VI Tier III nitrogen oxide (NOx) limits aboard marine diesel and gas engines. The process injects a reductant - typically an aqueous urea solution - into the exhaust gas stream upstream of a catalyst bed, where NOx reacts with ammonia derived from the urea to form harmless dinitrogen (N2) and water vapour. No major structural changes to the engine itself are required, and conversion efficiencies above 90% are routinely achieved in service, making SCR the most widely installed route to Tier III compliance on new and retrofit vessels. Tier III applies whenever a ship constructed on or after 1 January 2000 operates inside a designated NOx Emission Control Area (NOx ECA): the North American ECA from 1 January 2016, the US Caribbean ECA from the same date, and the North Sea and Baltic Sea ECA from 1 January 2021. ShipCalculators.com provides purpose-built tools for SCR system analysis; the SCR urea consumption calculator and the NOx Tier checker are the primary entry points. The ShipCalculators.com calculator catalogue contains additional NOx, fuel, and emissions tools relevant to Tier III compliance planning.

Contents

Background and regulatory context

Origins of NOx control in shipping

Nitrogen oxides from combustion form via three established mechanisms: thermal (Zeldovich), prompt (Fenimore), and fuel-bound nitrogen. In marine diesel engines, thermal NOx dominates: at the extreme temperatures present during combustion, atmospheric nitrogen and oxygen react to produce nitric oxide (NO), which subsequently oxidises partially to nitrogen dioxide (NO2) in the exhaust. Collectively, NO and NO2 are referred to as NOx.

The health and environmental consequences of NOx emissions - ground-level ozone formation, photochemical smog, and contribution to acid deposition - attracted increasing regulatory attention during the 1990s. Shipping represented a concentrated NOx source along coastlines and in heavily trafficked straits. The IMO addressed marine NOx within MARPOL Annex VI, which was adopted at the 1997 MARPOL Protocol and entered into force on 19 May 2005. The NOx Technical Code, adopted alongside Annex VI, specified the test cycles and measurement methods by which engine manufacturers must demonstrate compliance and certify each engine with an Engine International Air Pollution Prevention (EIAPP) certificate.

MARPOL Annex VI established three progressive NOx tiers, each representing a substantial reduction over the previous:

Tier I applied to engines installed on or after 1 January 2000, setting limits that required relatively modest combustion optimisation.
Tier II applied to engines installed on or after 1 January 2011, requiring a roughly 20% reduction from Tier I and achievable by optimising fuel injection timing, injection pressure, and turbocharger matching.
Tier III applied to engines installed on or after 1 January 2016, operating inside NOx ECAs, requiring an approximately 75 to 80% reduction from Tier II limits and generally necessitating dedicated after-treatment technology or a fundamentally different combustion regime.

The switch from Tier II to Tier III within a NOx ECA creates a dual-mode operating requirement: the vessel runs in Tier II mode on ocean passages and must switch to Tier III mode upon entering the ECA. This mode-switching capability, and its documentation, forms a key part of the EIAPP certification regime.

Tier III NOx limit values

MARPOL Annex VI Regulation 13 specifies NOx limits as a function of rated engine speed n in revolutions per minute:

For Tier III, the limits are: at n below 130 rpm, the limit is 7.7 g/kWh; at n between 130 and 1,999 rpm, the limit is 9 × n to the power of −0.2 g/kWh (evaluated at the rated speed); at n at or above 2,000 rpm, the limit is 3.4 g/kWh.

For comparison, the Tier II limits are: below 130 rpm, 17.0 g/kWh; 130 to 1,999 rpm, 44 × n−0.23 g/kWh; at or above 2,000 rpm, 14.4 g/kWh.

A slow-speed two-stroke main engine running at 100 rpm must therefore achieve 7.7 g/kWh under Tier III, compared with 17.0 g/kWh under Tier II - a reduction of approximately 55% required from that specific engine speed class. At medium speeds around 500 rpm, the Tier III limit is approximately 9 × 500−0.2, which equals approximately 9 × 0.287 = 2.6 g/kWh, against a Tier II value of 44 × 500−0.23 of approximately 10.5 g/kWh - a reduction exceeding 75%. The MARPOL NOx Tier III calculator and MARPOL NOx Tier II calculator compute applicable limits for any rated speed, and the NOx Tier checker confirms which tier applies to a given vessel and area. The NOx ppm-to-mg/Nm³ converter converts between the concentration units used in on-board analysers and the g/kWh values in the regulations.

NOx emission control areas

The IMO NOx ECA designation process requires a proposal from a member state or group of states, with supporting scientific evidence that local NOx reductions would produce measurable air quality benefits. The following NOx ECAs are in force as of the date of this article:

The North American ECA, which covers waters within approximately 200 nautical miles of the United States and Canadian coastlines (excluding the Gulf of Mexico), has applied Tier III limits since 1 January 2016 for vessels constructed on or after that date. The US Caribbean Sea ECA, covering waters around Puerto Rico and the US Virgin Islands, entered force under the same timeline. The North Sea and Baltic Sea ECA was adopted under MARPOL Annex VI and has applied Tier III from 1 January 2021 for ships built on or after that date. The Mediterranean Sea was adopted as a SOx ECA in 2021, but a separate NOx ECA proposal for the Mediterranean was under discussion as of early 2026 and had not yet entered into force.

Vessels trading on routes that never enter a NOx ECA are not required to meet Tier III, though Tier II remains the baseline for all engines installed on or after 1 January 2011 worldwide.

Principles of selective catalytic reduction

Chemical reactions

SCR exploits the thermodynamic favourability of ammonia (NH3) as a reductant for NOx over a suitable catalyst. The primary stoichiometric reactions are:

4NO + 4NH3 + O2 → 4N2 + 6H2O (standard SCR reaction, dominant when NO » NO2)

2NO2 + 4NH3 + O2 → 3N2 + 6H2O (for NO2-rich streams)

6NO2 + 8NH3 → 7N2 + 12H2O (fast SCR, proceeds at lower temperature when NO:NO2 ≈ 1:1)

In a marine diesel exhaust, NO typically constitutes 85 to 95% of the total NOx, making the first reaction dominant. However, where an oxidation catalyst is installed upstream to convert a proportion of NO to NO2, the fast SCR reaction can proceed at lower temperatures, which is relevant when exhaust temperatures are marginal for the catalyst.

Urea (CO(NH2)2) is not itself the reductant; it serves as a safe, stable ammonia precursor. Two decomposition steps occur within the exhaust duct before the catalyst bed: thermolysis, in which urea vaporises and decomposes to isocyanic acid (HNCO) and ammonia (NH3), and hydrolysis, in which HNCO reacts with water vapour to yield a second mole of NH3 and carbon dioxide (CO2). The net conversion is: CO(NH2)2 → 2NH3 + CO2. An aqueous urea solution at 32.5% concentration (mass fraction) - commercially known as AUS 32 in the automotive sector, or supplied as a marine-grade product certified to ISO 18611 - is the standard reductant fluid.

The stoichiometric ratio of urea solution to NOx governs reagent consumption. In practice, the normalised stoichiometric ratio α (the ratio of NH3 supplied to the NH3 theoretically required for complete reaction) is set between 1.2 and 1.5 to ensure adequate NOx conversion across varying load points, while not generating so much excess ammonia that slip becomes significant. The SCR urea consumption calculator takes engine load, exhaust NOx concentration, and target α to compute urea solution flow rate in litres per hour.

Catalyst types and operating temperature windows

Two principal catalyst chemistries are used in marine SCR systems:

Vanadium-titanium oxide catalysts consist of vanadium pentoxide (V2O5) and tungsten trioxide (WO3) coated on a titanium dioxide (TiO2) carrier, typically in a honeycomb or corrugated plate monolith form. These catalysts operate in an optimal temperature window of approximately 300 to 450°C. Below 300°C, conversion efficiency drops sharply because the surface reactions slow; above 450°C, the catalyst begins to sinter and the vanadium coating can volatilise. Vanadium catalysts are relatively inexpensive and have excellent durability in clean, low-sulphur exhaust, but sulphur trioxide (SO3) present in HFO exhaust reacts with ammonia to form ammonium bisulphate, which deposits on catalyst surfaces at temperatures below approximately 320°C, causing fouling.

Zeolite catalysts (typically copper-zeolite or iron-zeolite formulations) offer a broader operating window of approximately 180 to 550°C, making them attractive for low-temperature applications and for gensets whose load fluctuates widely. Zeolite SCR systems have been developed by several European and Korean suppliers and have entered marine service particularly on medium-speed engines. Their higher material cost compared with vanadium catalysts is partially offset by suitability over a wider load range.

In the marine context, the choice of catalyst chemistry is inseparable from the choice of SCR installation position relative to the turbocharger, as this determines the exhaust temperature reaching the catalyst.

SCR installation position

The two principal installation positions - pre-turbo and post-turbo - have distinct temperature profiles and engineering implications:

Pre-turbocharger SCR (also called “hot SCR”) is preferred for slow-speed two-stroke main propulsion engines. The exhaust temperature upstream of the turbocharger turbine is approximately 330 to 420°C at normal operating loads, comfortably within the vanadium catalyst window. Positioning the SCR before the turbocharger exploits this heat and avoids the substantial drop in exhaust temperature that occurs as the gas expands through the turbine. The drawback is that the SCR reactor must withstand higher pressures and temperatures, requiring heavier pressure-vessel construction, and the reactor housing is often integrated into the exhaust manifold path within an already congested engine room. Urea injection must occur well before the catalyst bed - typically at least 0.3 to 0.5 seconds of residence time upstream - to allow complete thermolysis and hydrolysis of urea before the gas reaches the catalyst.

Post-turbocharger SCR is standard for medium-speed four-stroke gensets and auxiliary engines. The exhaust temperature downstream of the turbocharger turbine typically ranges from 250 to 370°C at full load, falling to 200°C or less at low load. Post-turbo installation is mechanically simpler because the reactor sits in the atmospheric-pressure exhaust duct, allowing lighter construction, and the lower gas velocity gives longer catalyst residence time. The temperature limitation at low load can be addressed by thermal insulation, catalyst inlet temperature control, or by accepting a reduced conversion efficiency at low loads where NOx output is also reduced. For vessels with highly variable power demand, such as dynamic positioning vessels or cable layers, the potential for low-load NOx ECA operation without a comfortably warm catalyst requires careful system analysis.

On ships that combine a slow-speed main engine with medium-speed gensets, separate SCR systems are normally fitted to each engine category. Sharing a single SCR reactor across multiple exhaust streams is possible in principle but rarely practiced commercially due to the control complexity of mixing exhaust at different temperatures and NOx concentrations.

Urea supply and handling system

AUS 32 and ISO 18611

The reductant used in all commercial marine SCR systems is an aqueous urea solution, most commonly at 32.5% urea by mass. At this concentration the solution has a eutectic freezing point of approximately −11°C, minimising the risk of freezing during cold-weather operations and at the same time keeping urea concentration high enough to limit tank and pipe sizing. In automotive applications this solution is marketed under the tradename AdBlue (in Europe) and DEF (Diesel Exhaust Fluid, in North America), where it is covered by ISO 22241. The marine version is addressed by ISO 18611, which defines chemical purity requirements adapted for the marine environment, including limits on biuret (a urea decomposition product), aldehydes, and metals that could cause catalyst deactivation.

Suppliers such as Yara Marine (through its product Uviex Marine) and others produce ISO 18611-certified marine urea solution and operate supply chains through major bunkering ports. Vessels must confirm the availability of urea supply along their trading route before commissioning an SCR system without a backup Tier III pathway.

Urea storage and consumption rates

A typical Panamax bulker main engine of 9,000 kW running at 85% maximum continuous rating (MCR) produces exhaust NOx of approximately 15 g/kWh (typical Tier II output). Reducing this to below the applicable Tier III limit of approximately 7.7 g/kWh (at 100 rpm) requires removing roughly 7.3 g/kWh from 7,650 kW output, meaning approximately 55,845 g/h (56 kg/h) of NOx must be converted. At a stoichiometric ratio α of 1.3, the required urea supply is approximately 1.3 × 56 / 0.325 / 0.567 kg/h of urea solution, where 0.567 is the mass fraction of NH3 in the urea, working out to roughly 395 litres per hour of AUS 32 solution. Across a 10-day North Atlantic crossing where a ship spends three days inside the North Sea NOx ECA, total urea consumption at these parameters is of the order of 28,000 litres. Purpose-built urea storage tanks are therefore sized for port-to-port passages, commonly 20 to 100 m³, and are constructed from stainless steel or high-density polyethylene due to the mild corrosivity of urea solution towards ordinary mild steel. The SCR urea consumption calculator quantifies these figures for any specific engine and voyage scenario.

Urea solution can degrade if stored above 35°C for extended periods, converting to biuret and other species that may deposit on catalyst surfaces or reduce reductant effectiveness. Tank temperature sensors and insulation are standard features on SCR urea storage systems, and tanks in tropical trades are sometimes fitted with cooling arrangements.

Dosing and injection systems

The urea dosing system consists of a supply pump, a flow meter, a control valve, and one or more injector nozzles mounted in the exhaust duct. The nozzle must atomise the solution into fine droplets - typically below 100 micrometres Sauter mean diameter - to achieve complete evaporation before the catalyst face. A mixer static element or a dedicated mixing duct is installed downstream of the injector to promote homogeneous distribution of ammonia across the full cross-section of the catalyst bed. Uneven distribution causes local ammonia deficiency (under-reduction) and local excess (ammonia slip). The control system adjusts urea injection rate in response to signals from the engine management system: engine load, speed, and where fitted, an upstream NOx analyser. Feed-forward control on engine load alone is sufficient for steady-state operation but may be supplemented by closed-loop NOx feedback during load transients.

Catalyst management and ageing

Ammonia slip

The term “ammonia slip” refers to unreacted NH3 in the exhaust downstream of the SCR catalyst. Because ammonia is itself an irritant and a regulated atmospheric pollutant, slip must be controlled. Port regulations and class notations commonly require slip below 10 ppm (by volume, dry). Fresh catalysts with good dispersion of active sites can achieve this at α values up to 1.5. As catalyst activity declines with age, achieving the same NOx conversion requires a higher α, which in turn increases the risk of slip. Well-maintained systems are designed to hold slip below 5 ppm during normal in-ECA operation. The ammonia slip calculator for fuel-ammonia engines addresses a related but distinct scenario (ammonia slip from ammonia-fuelled combustion), and the principles of slip measurement apply equally to SCR systems.

Catalyst ageing mechanisms

Catalyst activity declines due to several mechanisms:

Thermal sintering occurs when catalyst surfaces are exposed to temperatures above the design maximum for sustained periods, causing the active vanadium sites to agglomerate and lose surface area. Operating a vanadium SCR above approximately 500°C accelerates this process irreversibly.

Sulphur poisoning is specific to vanadium catalysts operating on high-sulphur exhaust. Sulphur trioxide present in HFO combustion products reacts with NH3 above the catalyst surface to form ammonium sulphate and ammonium bisulphate. Ammonium bisulphate is a viscous liquid at temperatures around 240 to 320°C that coats and blocks catalyst pores. At temperatures above approximately 350°C it decomposes, so the problem is most acute during low-load operation or cold start. Operating an HFO-fuelled engine through a NOx ECA at loads below 25 to 30% MCR without an adequate temperature management strategy carries a risk of progressive catalyst fouling. The standard operating protocol, where practicable, is to run the engine at a minimum load when entering the ECA to heat the catalyst before reducing to service load.

Phosphorus and potassium compounds from lubricating oil additives can deposit on catalyst surfaces and cause deactivation; proper cylinder oil selection and controlled cylinder oil feed rates reduce this risk.

Physical fouling by soot, ash, and unburned fuel particles can partially block catalyst channels. Soot blowing - using compressed air or steam to dislodge deposited material - is a standard maintenance procedure on large SCR reactors, typically performed at regular intervals. On some systems, water washing of catalyst modules is performed during dry-dock overhauls.

Catalyst service life on a properly operated and maintained system running on distillate fuel (marine gas oil or ultra-low-sulphur fuel oil) is typically 16,000 to 32,000 hours before the catalyst modules require replacement or regeneration. On HFO with sulphur content above 0.10% by mass, this life may be substantially shorter. Suppliers offer periodic catalyst activity measurement services using small sample cores removed during dry dock to track degradation and schedule replacement economically.

Sulphur content and fuel constraints

Vanadium SCR systems designed for use with HFO require that the fuel sulphur content be managed carefully. Class society guidance and manufacturer recommendations commonly specify a maximum fuel sulphur content of 0.50% for continuous SCR operation with vanadium catalysts, and some systems are qualified only for operation with fuel at or below 0.10% sulphur. This constraint links the SCR system directly to the vessel’s fuel strategy: a ship trading routes that include both North Sea NOx ECA passages and open-sea HFO bunkering must plan fuel switches from VLSFO or ULSFO to HFO at or before the NOx ECA boundary if the SCR is sulphur-sensitive. This interaction is addressed in the context of the IMO 2020 sulphur cap and the broader discussion of exhaust gas cleaning systems, which includes scrubbers that manage SOx compliance separately. The EGCS SOx scrubber calculator is relevant where an open-loop scrubber is considered alongside SCR.

Integration with marine diesel engines

Two-stroke main engines

Low-speed two-stroke main propulsion engines present the most demanding SCR integration challenge due to their physical scale, pre-turbo positioning requirement, and mode-switching operational profile. The most common engineering approach involves mounting the SCR reactor in the exhaust gas path between the exhaust valve and the turbocharger turbine inlet. This requires routing large-bore exhaust ducting through the reactor housing, maintaining the high-temperature seal integrity, and providing adequate support for the combined weight of the reactor and catalyst modules, which on large engines may be 20 to 40 tonnes.

MAN Energy Solutions and WinGD both offer certified SCR packages qualified to operate with their respective engine families. MAN ES designates its two-stroke SCR installation as a pre-turbine SCR and has certified it in combination with the ME-C and ME-GI engine families. Urea injection begins when exhaust temperature upstream of the reactor reaches approximately 290°C, which typically corresponds to around 25 to 30% MCR load. Below this load, the system reverts to a standby or dosing-suspended mode, and the engine produces Tier II emissions; entering a NOx ECA at such low loads is operationally constrained unless the engine is accelerated briefly to warm the catalyst.

Dual-mode EIAPP certification allows the engine to hold a single certificate that records both the Tier II (SCR off) and Tier III (SCR active) emission values for the applicable NOx test cycles (E2 for constant-speed operation, E3 for propeller curve operation). The IMO NOx Technical Code 2008, adopted under MEPC.177(58), defines the E2 and E3 test cycles and requires that the certified Tier III values be demonstrated on a test bed or verified by on-board engine parameter log (EPL) checks. MARPOL Annex VI Regulation 13 and associated MEPC resolutions specify the documentation that must be carried on board to demonstrate the mode in use to port state control.

Four-stroke medium-speed engines

Medium-speed four-stroke engines used in diesel-electric propulsion, shaft-generator sets, and auxiliary generating sets are typically fitted with post-turbo SCR systems. The reactor is installed in the exhaust duct downstream of the turbocharger turbine and exhaust gas boiler (if fitted), or downstream of the silencer. The lower exhaust temperatures at this position - and the greater temperature variability associated with genset load swings from harbour manoeuvring to sea passage - make system design more sensitive to thermal performance.

Wärtsilä and Alfa Laval offer SCR systems sized for medium-speed engines in the range of 2 to 20 MW per engine. Hug Engineering, a specialist SCR supplier, provides systems across a wide range of engine makes. Mitsubishi Shipbuilding and CSIC (China Shipbuilding Industry Corporation) supply SCR systems primarily for vessels built in their respective yards. Hyundai Heavy Industries has developed proprietary SCR systems for its HiMSEN engine family.

For diesel-electric vessels, the NOx ECA compliance mode may require all running generators to have their SCR systems active simultaneously. Load management strategies that minimise the number of running generators during ECA transits can reduce the operational complexity of multi-engine SCR management, but must be balanced against vessel manoeuvring safety margins.

Waste heat interactions

The interaction between SCR systems and waste heat recovery systems deserves attention. In a conventional arrangement, exhaust gas boilers recover heat from the post-turbo exhaust for fuel oil heating, fresh water generation, or steam services. If the SCR is post-turbo and positioned upstream of the exhaust gas boiler, the catalyst must be sized for the higher-temperature stream; if positioned downstream of the boiler, the exhaust may be too cool for reliable SCR operation. Pre-turbo SCR bypasses this issue entirely since it operates in the high-temperature zone, but it prevents any waste heat recovery from the high-temperature gas before it is used for turbocharger driving.

Some vessels with combined SCR and waste heat recovery install a bypass duct allowing the high-temperature exhaust to route around the waste heat boiler when SCR is active, reverting to full waste heat recovery outside the ECA. This increases system complexity and cost but optimises both NOx compliance and fuel economy depending on the operating mode.

EIAPP certification and documentation

Engine International Air Pollution Prevention certificate

Every marine diesel or gas engine installed on a ship of 130 gross tonnage or above, or with output exceeding 130 kW, must carry an EIAPP certificate issued by or on behalf of the flag state administration. For engines intended to operate at Tier III within a NOx ECA, the EIAPP must record the Tier III emission values achieved on the applicable test cycle. Where an SCR system is required to achieve those values, the SCR system is recorded as part of the certified engine system, and specific SCR parameters - urea solution specifications, dosing rates, catalyst type and serial numbers, minimum operating temperature - are recorded in the Engine’s Technical File (ETF) and On-Board NOx Verification Procedure (EIAPP Supplement).

Modifications to any component recorded in the ETF that could affect NOx output require a survey and endorsement of the EIAPP. Replacing catalyst modules with units from a different supplier or of a different specification, for example, is an EIAPP-affecting modification. This creates an administrative link between catalyst maintenance scheduling and class survey planning.

On-board NOx verification

The NOx Technical Code 2008 provides two approaches for verifying Tier III performance in service without a full test-bed measurement: the parameter check method and the simplified measurement method. Under the parameter check method, the operator records key engine parameters (fuel injection timing, charge air pressure, temperature, NOx-relevant parameters of the SCR system) during a defined measurement run, compares them against the certified ETF values, and demonstrates that parameters are within the specified tolerance bands. The simplified measurement method allows portable NOx analysers to be used during a voyage to cross-check exhaust concentrations against the certified values.

Port state control officers may request demonstration of the EIAPP and ETF, inspection of the SCR system, review of the urea consumption log, and in some cases a simplified on-board NOx measurement. The test IMO NOx Technical Code calculator supports pre-inspection preparation and weighted emission factor calculation per the NOx Technical Code test cycle procedures.

DNV and Lloyd’s Register class notations

Classification societies have developed specific notations for SCR-equipped vessels. DNV issues the SCR notation (appended to the main class notation) after verifying that the installed system meets the declared Tier III performance and that the documentation, survey access provisions, and maintenance procedures are in order. Lloyd’s Register issues the NOx-SCR notation under a similar regime. Both notations include requirements for annual and five-yearly surveys of the SCR system, including catalyst inspection, and for the maintenance of records showing urea solution quality and quantity on board.

Alternative Tier III compliance pathways

SCR is not the only route to Tier III. Three principal alternatives exist:

Exhaust gas recirculation

High-pressure exhaust gas recirculation (EGR) dilutes the charge air with cooled exhaust gas, reducing the peak combustion temperature and thereby suppressing thermal NOx formation. MAN Energy Solutions developed the HP-EGR (high-pressure EGR) system for its ME-C two-stroke engines, in which a proportion of the exhaust gas leaving the receiver is returned to the scavenge air receiver via a blower, cooler, and water-treatment scrubbing unit. The scrubbing unit removes SOx from the recirculated gas and produces a wash water stream requiring management similar to that of an EGCS scrubber. NOx reductions of 50% or more are achievable, sufficient to meet Tier III in many cases without a catalyst.

EGR avoids the operational issues of urea supply and catalyst management but adds complexity to the engine itself and generates wash water. Its capital cost is broadly comparable to SCR on large two-stroke engines. The EGR Tier III rate calculator quantifies the recirculation ratio required to achieve a target NOx reduction for a given engine baseline. On vessels trading routes that include both NOx ECAs and areas where sulphur-compliant fuel must be carried, EGR requires separate scrubbing of the recirculated exhaust to handle the SOx; an alternative is to switch to low-sulphur fuel within the ECA, removing the need for gas scrubbing in the EGR loop.

LNG and dual-fuel engines

Natural gas combustion by the Otto cycle or lean-burn premixed combustion does not generate the peak temperatures characteristic of diffusion-flame diesel combustion, and methane-fuelled engines consequently produce substantially lower NOx than diesel engines at comparable outputs. Dual-fuel engines in gas mode can achieve Tier III compliance without any after-treatment. This is one of the primary regulatory drivers behind the growth of LNG-fuelled shipping, alongside the SOx compliance benefit of natural gas’s negligible sulphur content. The LNG as marine fuel article discusses the broader fuel economics and supply chain considerations. Dual-fuel engines operating in diesel (oil) mode fall back to Tier II emissions, so outside a NOx ECA they comply without gas operation, but inside a NOx ECA in diesel mode they require a supplemental Tier III measure unless a NOx ECA operational credit has been obtained.

Methanol dual-fuel engines, covered in the methanol as marine fuel article, can also achieve low NOx in certain combustion modes, but methanol marine engines are not universally certified for Tier III in methanol mode without supplemental measures. Ammonia combustion engines, discussed in ammonia as marine fuel, produce NOx from fuel-bound nitrogen as well as thermal NOx, requiring SCR or other treatment even in gas mode; the ammonia NOx slip calculator addresses this specific scenario.

Internal engine measures

Several internal measures can achieve partial NOx reductions from Tier II baselines: retarded fuel injection timing, water-in-fuel emulsification, direct water injection into the cylinder, charge air humidification, and two-stage turbocharging with Miller cycle. These measures alone typically cannot bridge the full gap from Tier II to Tier III for conventional diesel combustion, and they carry fuel consumption penalties that must be weighed against the cost of after-treatment alternatives. However, they may be combined with SCR to reduce the required NOx conversion duty of the catalyst, allowing a smaller catalyst volume or extending catalyst service life.

Economic and practical considerations

Capital and installation costs

A complete SCR system for a medium-sized vessel - say, a Handymax bulk carrier with a 7,500 to 9,000 kW main engine - is estimated to carry a capital expenditure of approximately US$1 to 3 million (2020s prices) inclusive of engineering, equipment, and installation. This range reflects the substantial variation in reactor size, catalyst volume, urea system complexity, and integration scope. Two-stroke pre-turbo installations at the upper end of this range; post-turbo auxiliary engine SCR systems for a single genset can be at the lower end.

Physical footprint is a consistent constraint. A catalyst reactor for a 9,000 kW diesel engine typically occupies a volume of approximately 3 × 3 × 5 m, weighing 20 to 40 tonnes including the catalyst modules. This mass and volume must be accommodated in an engine room designed without SCR in mind on retrofit projects, requiring structural reinforcement, revised gas ducting, and in some cases relocation of other equipment. New-build vessels with SCR specified from the design stage can integrate the reactor into the funnel casing or exhaust gas path with much less difficulty. Access for catalyst module removal and replacement during dry dock must be verified at the design stage.

Installation also requires supply pipework and storage for urea solution, an electrical supply for dosing pumps and control system, and integration with the engine management system’s load signal. Control panels and safety interlocks (low urea level alarm, high ammonia slip alarm if an analyser is fitted, high catalyst differential pressure alarm for fouling detection) add to the electrical installation scope.

Operating costs

Urea solution is the primary consumable cost. At the consumption rates described in the urea storage section above, a vessel spending 2,000 to 3,000 hours per year inside NOx ECAs with a main engine of 7,500 kW might consume 50,000 to 100,000 litres of urea solution annually. Supply costs vary by port and region but have historically ranged from approximately US$0.30 to 0.60 per litre for marine-grade AUS 32, giving an annual reagent cost of approximately US$15,000 to 60,000 depending on trading pattern. This figure is typically modest relative to fuel costs on the same voyage.

Catalyst replacement is a larger periodic cost. Catalyst modules for a large main engine SCR may cost of the order of US$200,000 to 500,000 per replacement cycle (every 16,000 to 32,000 operating hours under favourable conditions), including labour, dry-dock coordination, and catalyst disposal. Some suppliers offer catalyst regeneration services in which aged modules are thermally treated and reactivated, at lower cost than full replacement.

Norway operates a NOx fund (Næringslivets NOx-fond) under which Norwegian-flagged and Norwegian-trading vessels can receive subsidies for NOx reduction investments including SCR installation. The Norway NOx fund calculator models the expected subsidy and payback period. Vessels trading in Norwegian waters should evaluate this scheme as part of the SCR investment case.

Mode-switching operations

The dual-mode operational profile - Tier II at sea, Tier III in the ECA - requires a defined mode-switching procedure documented in the vessel’s shipboard operation manual and referenced in the EIAPP. The mode switch is not instantaneous: the SCR catalyst requires time to reach operating temperature from cold, typically 20 to 40 minutes for a system that has been idle. Vessels approaching a NOx ECA must therefore initiate catalyst warm-up well before the ECA boundary, either by commencing urea injection at reduced rates to warm the catalyst, or by running an electrical catalyst heater (fitted on some post-turbo systems) during approach.

Modern SCR control systems include an ECA approach mode that monitors GPS position, compares it against an ECA boundary database, and triggers the warm-up sequence automatically at a user-defined distance from the boundary. This automation reduces the risk of entering a NOx ECA with an unprepared SCR and issuing a non-compliant exhaust, which would expose the operator to port state control sanctions.

SCR addresses only NOx. Marine vessels are subject to multiple concurrent exhaust gas regulations under MARPOL Annex VI, and the interaction between SCR and other treatment technologies is a normal feature of modern newbuild design:

Sulphur oxides (SOx) are controlled by fuel sulphur limits or by exhaust gas cleaning systems (EGCS, also called scrubbers). The IMO 2020 sulphur cap capped global fuel sulphur at 0.50% from 1 January 2020. Within SOx ECAs (Baltic, North Sea, North American, US Caribbean), the limit is 0.10%. An EGCS that satisfies the SOx rules does not affect NOx, and an SCR that satisfies NOx rules does not affect SOx; the two systems operate in parallel where both are fitted. The physical integration of both systems in a common exhaust path requires careful thermal analysis to ensure the SCR catalyst operates within its temperature window after any heat exchange in the EGCS. The scrubber SO2/CO2 calculator and EGCS SOx scrubber calculator support EGCS performance calculations.

Particulate matter (PM) from diesel combustion is not directly regulated under MARPOL but is subject to local port state air quality ordinances in some jurisdictions. SCR does not remove PM; a diesel particulate filter (DPF) is required for PM reduction. DPFs are uncommon on large marine engines but are occasionally fitted on small high-speed harbour craft engines subject to California ATCM regulations.

Carbon dioxide (CO2) is not addressed by SCR. The suite of IMO energy efficiency measures - EEDI/EEXI for design efficiency and CII for operational efficiency - address CO2 through fuel economy rather than after-treatment. The CII rating calculator and related tools in the what is CII article cover that regulatory stream. The SFOC (specific fuel oil consumption) article addresses the fuel economy metric at the centre of CO2 management.

The combustion-based Zeldovich NOx calculator computes thermal NOx formation from first principles, useful for understanding the upstream NOx generation that SCR must handle.

Current developments

Low-temperature SCR for low-load operation

A persistent limitation of vanadium SCR is the difficulty of maintaining Tier III performance during low-load operation, which is increasingly common as ship operators slow-steam to reduce fuel costs and CO2 emissions. Below approximately 25% MCR, exhaust temperatures on two-stroke engines can drop below the minimum SCR activation temperature. Research and commercial development programmes have addressed this through several approaches: electrically heated catalyst elements that maintain catalyst temperature during low-load transit, hydrogen peroxide or plasma-assisted pre-oxidation of NO to NO2 (enabling fast SCR at lower temperatures), and the development of low-temperature zeolite formulations with activity starting at approximately 150°C. Some suppliers have reported successful in-service Tier III compliance at loads down to 15% MCR using advanced thermal management combined with zeolite catalysts.

Combined SCR and particulate filter systems

Several research groups and a small number of commercial suppliers have developed combined SCR-plus-DPF (diesel particulate filter) units, sometimes referred to as SCRF (SCR on Filter), in which the SCR catalyst is washcoated onto a porous ceramic filter substrate. Regeneration of the filter by passive oxidation (using NO2 from the exhaust) is combined with simultaneous NOx reduction. These systems are well established in the heavy-duty road vehicle market and are being scaled for marine applications, particularly for small ferries, harbour tugs, and inland waterway vessels subject to both NOx and PM regulations.

Integration with CII optimisation

The interaction between NOx compliance mode-switching and CII management under MARPOL is a developing operational consideration. Slow steaming reduces both fuel consumption and CO2 emissions, improving CII performance, but as noted above may compromise SCR thermal performance. Vessels must balance these factors. The slow steaming and CII article addresses the CII consequences of speed reduction, and the relationship to SCR thermal management is an emerging area of integrated operational planning.

Ammonia-fuel implications

As the marine gas turbine and emerging ammonia combustion engine research programmes indicate, vessels transitioning to ammonia fuel face a distinct SCR requirement: controlling NOx from fuel-nitrogen conversion rather than purely thermal NOx. Ammonia-fuelled engines may require higher reductant dosing rates and different catalyst formulations. Several engine manufacturers have committed to qualifying their ammonia engine platforms with integrated SCR systems.

References

IMO MEPC.177(58), “Revised MARPOL Annex VI and NOx Technical Code 2008”, adopted October 2008.
MARPOL Annex VI, Regulation 13, “Nitrogen oxides (NOx)”, as amended.
IMO MEPC.251(66), “Amendments to MARPOL Annex VI (Designation of the Baltic Sea and the North Sea Emission Control Areas for NOx Tier III control)”, adopted 2014, entry into force 1 January 2021.
IMO Resolution MEPC.73(38) and subsequent amendments establishing the North American Emission Control Area.
ISO 18611-1:2014, “Road vehicles - Marine diesel exhaust fluid (AUS 32) - Part 1: Requirements”.
NOx Technical Code 2008, Appendix II, “Test cycles and weighted emission factors for marine diesel engines”.
MAN Energy Solutions, “MAN B&W ME-C engine with SCR System - Project Guide”, various editions.
Wärtsilä, “Exhaust Gas After-treatment: SCR Technology”, technical documentation.
DNV GL (now DNV), “Class Notation SCR: Selective Catalytic Reduction Systems”, Rules for Ships.
Hug Engineering AG, “Marine SCR Systems - Technical Overview”, product documentation.

External links

MARPOL Annex VI and NOx Technical Code 2008 (IMO) - official IMO page on air pollution from ships
Yara Marine Technologies - SCR systems - supplier of marine urea and SCR systems
Hug Engineering AG - specialist marine SCR supplier