SCR Retrofit on Two-Stroke Marine Engines

Selective catalytic reduction (SCR) is one of two main after-treatment technologies for meeting the MARPOL Annex VI Tier III NOx limit inside designated emission control areas. SCR injects an aqueous urea solution into the exhaust upstream of a catalyst bed, where urea-derived ammonia reduces NOx to nitrogen and water at conversion efficiencies of 90 to 95 percent. On slow-speed two-stroke main engines, the critical engineering choice is reactor placement: upstream of the turbocharger turbine (high-pressure SCR) to exploit the hotter pre-turbine gas, or downstream (low-pressure SCR) where the exhaust has already expanded and cooled.

The Tier III compliance driver

The MARPOL Annex VI Tier III NOx limit is the direct regulatory trigger for SCR retrofits on vessels trading in NOx emission control areas (NOx ECAs). MARPOL Annex VI Regulation 13, revised under IMO MEPC.177(58) and subsequent amendments, sets NOx limits that vary with rated engine speed n in rpm. At engine speeds below 130 rpm, the applicable Tier III limit is 7.7 g/kWh. That speed bracket covers all large slow-speed two-stroke main propulsion engines, which typically run at 80 to 120 rpm. Compare this with the Tier II baseline of 17.0 g/kWh for the same speed bracket: Tier III demands roughly a 55 percent reduction, which no amount of combustion tuning alone can deliver on a conventional diesel cycle. After-treatment, alternative fuels, or exhaust gas recirculation are the only practical routes.

The MARPOL NOx Tier III limit calculator computes the applicable limit for any rated speed, and the NOx Tier checker confirms which tier applies for a given vessel and operating area.

Tier III applies when a vessel constructed on or after 1 January 2016 operates inside a NOx ECA. The North American NOx ECA has applied Tier III from 1 January 2016. The North Sea and Baltic Sea NOx ECA followed from 1 January 2021. Ships constructed before those dates are nominally Tier I or Tier II engines by certification year, but when they enter a NOx ECA on routes that require it, flag state administrations and port state control authorities expect either a valid Tier III EIAPP or a documented operational exemption. The emission-control areas article covers the geographic boundaries and the administrative procedures in detail.

For a ship that was never designed for Tier III, the retrofit window is the drydock cycle. Most owners schedule SCR or EGR retrofits during the first or second special survey after the NOx ECA compliance date becomes commercially unavoidable for their trade route. Delaying the retrofit beyond that point risks port state control detention and loss of charter income in restricted areas.

Tier III and the dual-mode operating requirement

A vessel fitted with SCR holds two certified emission states recorded on the Engine International Air Pollution Prevention (EIAPP) certificate: a Tier II mode (SCR off or bypassed) for open-ocean passages and a Tier III mode (SCR active and dosed) for ECA operation. IMO NOx Technical Code 2008 (MEPC.177(58)) defines the E2 and E3 test cycles used to verify both modes. The Technical File and On-Board NOx Verification Procedure must record the SCR system parameters, urea specification, dosing rates, and minimum activation temperature against which port state control officers can check compliance. The test-imo-nox-technical-code calculator supports weighted emission factor calculation per those test cycle procedures.

SCR chemistry: urea, ammonia, and the catalyst

Why urea rather than direct ammonia injection

Pure ammonia is toxic, flammable above 15 percent volume in air, and tightly restricted aboard vessels under SOLAS. Aqueous urea solution at 32.5 percent urea by mass (commercially known as AUS 32 in automotive applications, marketed under trade names including AdBlue and DEF, and specified for marine use under ISO 18611) is non-flammable, mildly alkaline, and safe to handle with standard chemical-resistant materials. It serves as a stable shipboard ammonia precursor.

Two sequential decomposition reactions convert urea to ammonia within the hot exhaust duct before the catalyst bed. First, thermolysis breaks urea into isocyanic acid (HNCO) and ammonia (NH3) as the droplets vaporise. Second, hydrolysis of HNCO with water vapour in the exhaust produces a second mole of NH3 and a mole of CO2. Net: one mole of urea yields two moles of ammonia.

Both steps require adequate gas temperature and residence time: below roughly 160 to 180°C thermolysis stalls, leaving liquid urea deposits in the duct. Adequate mixing length between the injection nozzle and the catalyst face is therefore a mandatory design constraint, typically providing at least 0.3 to 0.5 seconds of gas residence time for complete decomposition.

The catalytic reduction reactions

Over the catalyst bed, ammonia reduces nitrogen oxides through several reactions. In marine diesel exhaust, nitric oxide (NO) constitutes 85 to 95 percent of the total NOx, making the standard SCR reaction dominant:

Where NO2 is present (for example, downstream of an oxidation catalyst), the fast SCR reaction proceeds at lower temperature:

The stoichiometric ratio $\alpha$ (the ratio of NH3 supplied to the NH3 theoretically required for complete NOx conversion) is set between 1.2 and 1.5 in practice. Running $\alpha$ below 1.0 leaves NOx unconverted; running it too high causes ammonia to pass through the catalyst unreacted, producing ammonia slip in the exhaust. The SCR urea consumption calculator takes engine load, exhaust NOx concentration, and target $\alpha$ to compute urea solution flow rate in litres per hour.

Vanadium-titanium versus zeolite catalysts

Two principal catalyst chemistries serve marine SCR systems, and the choice between them directly shapes the thermal constraints on the installation.

Vanadium-titanium oxide catalysts consist of vanadium pentoxide (V2O5) and tungsten trioxide (WO3) coated on a titanium dioxide (TiO2) carrier, formed into a ceramic honeycomb or corrugated-plate monolith. Their active temperature window runs from approximately 300 to 450°C. Below 300°C the surface reactions slow sharply; above 450°C the vanadium coating starts to sinter and can volatilise, permanently reducing active surface area. These catalysts are the industry standard on large two-stroke main engines because their cost, durability, and performance within that window are well proven. The critical liability is sulphur: vanadium catalysts are susceptible to ammonium bisulphate fouling at temperatures below roughly 320°C, an issue discussed in detail below.

Zeolite catalysts, typically copper-zeolite (Cu-ZSM-5, Cu-SSZ-13) or iron-zeolite (Fe-ZSM-5) formulations, offer a wider operating window of roughly 180 to 550°C. This lower-temperature capability makes them attractive for applications where exhaust temperatures are marginal, including some post-turbo installations and medium-speed gensets with wide load swings. Their higher material cost relative to vanadium catalysts has limited penetration on the large two-stroke main engine market, but zeolite formulations have entered service on auxiliary engines and on two-stroke engines running below 25 percent MCR where vanadium catalysts would bypass.

High-pressure SCR versus low-pressure SCR on two-stroke engines

The defining engineering choice for a two-stroke SCR retrofit is reactor position relative to the turbocharger turbine. This single decision determines the exhaust temperature reaching the catalyst bed, the pressure at which the reactor must operate, its physical construction, and its integration with the engine’s waste heat recovery and turbocharger system.

The temperature problem specific to slow-speed two-stroke engines

A slow-speed two-stroke diesel engine running at 85 percent MCR delivers exhaust gas to the turbocharger turbine inlet at approximately 330 to 420°C. After the gas expands through the turbine, temperature drops to roughly 250 to 330°C at full load, and to 200°C or below at loads under 25 percent MCR. That post-turbine temperature range overlaps uncomfortably with the lower bound of the vanadium catalyst window, and at anything below about 30 percent MCR it falls outside it entirely.

Medium-speed four-stroke engines, in contrast, run hotter exhaust with better temperature stability across a wider load range. Post-turbo SCR on four-stroke engines is generally straightforward. On slow-speed two-stroke engines, post-turbo placement requires careful thermal management to keep the catalyst alive, and high-pressure pre-turbo placement is the standard engineering response.

High-pressure (pre-turbo) SCR: the MAN ES and WinGD approach

In a high-pressure SCR installation, the reactor is inserted into the exhaust gas path between the exhaust valves (or exhaust receiver) and the turbocharger turbine inlet. The gas at this position is at manifold pressure, typically 3 to 5 bar absolute, and at the temperature noted above: 330 to 420°C at normal operating loads. Both MAN Energy Solutions and WinGD adopted this arrangement for their two-stroke SCR packages, and it is now the standard configuration for slow-speed main engine SCR retrofits.

The temperature advantage is clear. At these conditions, a vanadium catalyst operates comfortably within its optimal window for all load points above roughly 25 percent MCR. Urea injection occurs upstream of the reactor, and the elevated gas temperature and turbulence aid thermolysis and hydrolysis of the urea droplets. MAN Energy Solutions’ certified pre-turbine SCR packages for the ME-C and ME-GI engine families specify a minimum exhaust temperature of approximately 290°C for urea dosing to begin, corresponding to about 25 percent MCR.

The structural demands are the engineering trade-off. The reactor vessel must be rated for elevated pressure and temperature, requiring heavier pressure-vessel construction than a post-turbo unit. On a large bore two-stroke engine (say, a MAN 12G95ME-C engine producing around 73,000 kW), the SCR reactor housing can weigh 25 to 45 tonnes installed, must be integrated into an exhaust path that was not designed for it on retrofit vessels, and must provide for catalyst module removal during scheduled maintenance without dismantling the turbocharger system. Access planning is a significant part of the retrofit engineering scope.

A secondary consequence is that the high-pressure gas passing through the reactor sees a pressure drop of 30 to 80 mbar across the catalyst bed. This back-pressure acts on the turbocharger, slightly reducing scavenging efficiency and imposing a fuel consumption penalty of roughly 0.5 to 1.5 g/kWh, depending on catalyst condition and gas velocity. That penalty is measured against the baseline during the EIAPP certification process and is accepted as part of the Tier III compliance package.

Low-pressure (post-turbo) SCR on two-stroke engines

Post-turbo SCR on a slow-speed two-stroke is mechanically simpler: the reactor sits in the atmospheric-pressure exhaust duct downstream of the turbine, allowing lighter construction and easier access. The gas velocity is lower, giving longer catalyst residence time per unit reactor length. But the temperature limitation is real. To operate a vanadium catalyst post-turbo on a slow-speed two-stroke at full load, the exhaust must remain above roughly 300°C throughout the load range of ECA operation.

Where the trade profile guarantees high-load ECA transits, post-turbo vanadium SCR can work. Vessels on short ECA crossings at near-full manoeuvring load may find post-turbo placement acceptable. The risk is during slow steaming, harbour approach at reduced power, or coastal passages at 40 to 60 percent MCR, where post-turbo temperatures may drop below the catalyst activation threshold and the SCR must be bypassed. Bypassing inside a NOx ECA is non-compliant; the operator must either accelerate briefly to heat the catalyst or accept the compliance exposure.

Low-temperature zeolite catalysts at post-turbo positions extend the operational window to about 180°C catalyst inlet temperature, which covers more of the slow-steaming load range. Several European SCR suppliers, including Hug Engineering, offer zeolite post-turbo packages qualified for slow-speed two-stroke auxiliary service and increasingly for main engine auxiliary generator sets.

Comparison table: high-pressure versus low-pressure SCR on two-stroke engines

Urea supply, storage, and dosing system

AUS 32 and ISO 18611 quality requirements

The standard reductant is aqueous urea solution at 32.5 percent urea by mass (AUS 32). At this concentration the eutectic freezing point is approximately -11°C, a useful property for cold-weather operation. ISO 18611 (specifically ISO 18611-1:2014 for urea solution quality) defines marine-grade purity requirements covering biuret content (a urea decomposition product that reduces catalyst activity), aldehydes, and trace metals including phosphorus and calcium that can deposit on catalyst surfaces.

Marine-grade AUS 32 supply chains have grown substantially since North American NOx ECA Tier III enforcement began in 2016. Yara Marine Technologies supplies its Uviex Marine certified product at major bunkering ports on North American, Northern European, and East Asian routes. Operators on routes with thin AUS 32 infrastructure must arrange forward supply arrangements or tank capacity sufficient for multi-port autonomy. A vessel without a backup Tier III compliance pathway (such as EGR or an alternative-fuel engine) that runs out of urea inside a NOx ECA is exposed to port state control action.

Urea consumption: worked example

A 9,000 kW slow-speed main engine at 85 percent MCR produces roughly 7,650 kW of brake output. At a Tier II NOx specific emission of 15 g/kWh, the engine emits approximately 115,000 g/h (115 kg/h) of NOx. To reduce this to below the applicable Tier III limit of 7.7 g/kWh requires converting roughly 56 kg/h of NOx through the SCR catalyst.

At a normalised stoichiometric ratio $\alpha$ of 1.3, the NH3 required is 1.3 times the stoichiometric quantity. Since AUS 32 contains 32.5 percent urea by mass, and one mole of urea yields two moles of NH3 (molecular weight 17 g/mol) from a urea molar mass of 60 g/mol, the mass conversion factor is 0.567 kg NH3 per kg urea. Working through: approximately 400 litres per hour of AUS 32 at a density of roughly 1.09 kg/litre. Across a 72-hour North Sea ECA crossing at this load, the system consumes around 28,800 litres. That volume drives the tank sizing: most retrofits specify 20 to 100 m3 of AUS 32 storage, depending on port-to-port voyage length. The SCR urea consumption calculator quantifies these figures for any engine and voyage configuration.

Tank construction and temperature management

Urea solution is mildly corrosive to mild steel. Storage tanks are constructed from 316L stainless steel or, for lower-cost installations, high-density polyethylene (HDPE). Tanks in tropical trades require temperature management: AUS 32 begins to decompose above 35°C, converting urea to biuret, which reduces reductant effectiveness and can deposit on catalyst surfaces. Insulation and, on some installations, chilled water cooling are fitted to storage tanks in hot-climate trades. Conversely, the -11°C eutectic freeze point requires heat tracing on tanks and all urea supply lines on vessels trading in Arctic or Northern European winter conditions.

Dosing and injection system

The dosing system consists of a supply pump, a flow meter, a modulating control valve, and one or more injector nozzles mounted in the exhaust duct upstream of the catalyst. The nozzle atomises urea solution into droplets with a Sauter mean diameter below 100 micrometres to promote complete evaporation and thermolysis before the gas reaches the catalyst face. A static mixer or dedicated mixing chamber downstream of the injection point distributes ammonia evenly across the full catalyst cross-section.

Uneven ammonia distribution across the catalyst face produces local areas of under-dosing (where NOx conversion is insufficient and the Tier III limit is locally exceeded) and over-dosing (where excess NH3 passes through the catalyst as ammonia slip). Modern SCR control systems use computational fluid dynamics (CFD) results from the mixing duct design to verify distribution uniformity, and some installations fit a grid of upstream ammonia concentration probes during commissioning to validate the CFD predictions.

The dosing control logic receives load and speed signals from the engine management system, computes the expected NOx mass flow from a calibrated engine map, applies the target $\alpha$, and sets the urea flow accordingly. Feed-forward control on engine load suffices for steady-state operation. Closed-loop control using an upstream NOx analyser is added on some systems to handle transient load changes and catalogue aging-related changes in engine NOx output.

SCR reactor sizing and catalyst design

Space velocity and catalyst volume

The fundamental sizing parameter is space velocity (SV), defined as the volumetric exhaust flow divided by the catalyst volume, expressed in units of h$^{-1}$. A higher space velocity means less catalyst contact time per unit volume. Vanadium catalysts for marine SCR typically operate at space velocities of 5,000 to 15,000 h$^{-1}$ at design conditions. The required catalyst volume for a 9,000 kW two-stroke engine at full load is typically 1.5 to 3.0 m3, depending on target conversion efficiency, exhaust gas composition, and allowable pressure drop.

Cell density of the monolith substrate, measured in cells per square inch (CPSI), determines the balance between pressure drop and reactive surface area. Marine SCR typically uses 200 to 400 CPSI. Coarser cells (100 to 200 CPSI) give lower pressure drop and are more resistant to particulate bridging, which is important for HFO operation where soot loads are higher. Finer cells (300 to 400 CPSI) give greater surface area and higher activity at the same volume but are more vulnerable to fouling by soot and ash.

The catalyst block is housed in a pressure vessel (for high-pressure SCR) or an insulated duct casing (for low-pressure SCR). Catalyst modules are designed for removal via a maintenance access hatch, typically requiring a free-standing platform or crane access during dry dock. For large pre-turbo reactors, module removal may require partial dismantling of the exhaust gas ducting, and this access sequence is defined in the EIAPP Technical File.

Catalyst management: ageing, fouling, and sulphur sensitivity

Ammonia slip and its limits

Ammonia slip is the concentration of unreacted NH3 in the exhaust downstream of the catalyst. Because ammonia is itself a regulated atmospheric pollutant and an irritant at port, class society notations and port regulations commonly require slip below 10 ppm by volume (dry). Fresh, well-distributed catalysts operating within their temperature window achieve this at $\alpha$ values up to 1.5. As catalyst activity declines with age, achieving the same NOx conversion demands a higher $\alpha$, which increases slip risk. Operators track ammonia slip using a downstream NH3 analyser or periodic grab samples; rising slip at constant dosing is a reliable indicator of catalyst degradation.

Ammonia slip above 10 ppm can also cause secondary reactions in the exhaust downstream of the SCR. In the presence of SO3 and water vapour, excess NH3 forms ammonium sulphate particles, which can deposit on exhaust gas economisers and silencers downstream, causing corrosion and blockage.

Thermal sintering and high-temperature excursions

Vanadium catalysts sinter when exposed to exhaust temperatures above approximately 500°C for sustained periods. Sintering agglomerates the active vanadium particles, reducing active surface area irreversibly. On a slow-speed two-stroke, high-temperature excursions can occur during engine load surges, scavenging pressure anomalies, or late injection events that produce extended combustion. Exhaust temperature monitoring at the reactor inlet with automatic bypass actuation when the temperature exceeds the catalyst rated maximum is a standard protective feature on MAN ES and WinGD pre-turbo SCR packages.

Sulphur poisoning and ammonium bisulphate fouling

Sulphur poisoning is the dominant catalyst degradation mechanism on vessels burning heavy fuel oil (HFO) or very-low-sulphur fuel oil (VLSFO) with sulphur content above 0.10 percent. Sulphur dioxide (SO2) in the exhaust is partially oxidised to SO3 over the catalyst surface. SO3 reacts with NH3 to form ammonium sulphate ((NH4)2SO4) at temperatures below approximately 240°C and ammonium bisulphate (NH4HSO4) at temperatures in the range 240 to 320°C. Ammonium bisulphate is a viscous, sticky liquid at these temperatures that coats catalyst channels and blocks pores, progressively reducing activity.

At catalyst temperatures above about 350°C, ammonium bisulphate decomposes and desorbs from the catalyst surface, so the fouling is thermally reversible. The standard operating remedy is to run the engine at a load sufficient to keep the catalyst inlet temperature above 350°C whenever possible during ECA transits. For vessels that must operate at low loads inside an ECA (for example, during slow maneuvering or waiting at anchor), some operators schedule a brief full-load “bake-out” run to desorb accumulated ammonium bisulphate before reverting to reduced speed.

Most vanadium SCR manufacturers specify a maximum fuel sulphur content of 0.50 percent for continuous SCR operation, and several qualifications are limited to 0.10 percent sulphur. This constraint links the SCR system directly to the vessel’s fuel strategy. A vessel trading routes that mix North Sea NOx ECA passages (where 0.10% SOx limit applies under MARPOL Annex VI Reg. 14) and open-ocean HFO bunkering must switch to VLSFO or ULSFO before or at the ECA boundary, aligning SOx compliance with SCR catalyst protection. The MARPOL Annex VI Reg. 14 sulphur cap article covers the fuel-sulphur regulatory framework.

Phosphorus and lubricating oil deposits

Phosphorus and calcium from cylinder lubricating oil additives deposit on catalyst surfaces and cause permanent deactivation. Controlled cylinder oil feed rates and oil formulations with low phosphorus additive packages reduce this risk. MAN Energy Solutions’ Alpha Lubricator system enables variable cylinder oil dosing down to approximately 0.6 g/kWh on modern ME-C engines, reducing lubricant consumption and the associated catalyst contamination. The cylinder lubrication systems for two-stroke engines article covers these feed-rate strategies.

Soot blowing and physical cleaning

Particulate matter and soot from HFO combustion can partially block catalyst channels, increasing pressure drop and reducing activity. Soot blowing using compressed air or steam at regular intervals is standard practice on large pre-turbo SCR reactors; many systems perform automated soot blow sequences every 24 to 72 hours of operation. During dry dock, water washing of catalyst modules removes heavier deposits that compressed air cannot dislodge.

Catalyst service life on a properly operated system burning distillate fuel (MGO or ULSFO) is typically 16,000 to 32,000 hours before modules require replacement or regeneration. On HFO above 0.50% sulphur, this life can be materially shorter. Catalyst module suppliers including Hug Engineering and specialist catalyst manufacturers offer periodic activity testing using small core samples removed during dry dock, enabling operators to track degradation curves and schedule replacement economically rather than conservatively.

Replacement catalyst modules for a large main engine SCR system cost approximately USD 200,000 to 500,000 per replacement cycle, including labour and dry-dock coordination. Some suppliers offer catalyst regeneration services, thermally treating aged modules to desorb sulphur compounds and restore a proportion of the original activity, at roughly half the cost of full replacement.

Retrofit engineering and installation

Pre-retrofit survey and feasibility

An SCR retrofit starts with an engine room survey to assess exhaust duct routing, available mounting points, structural load capacity, space for catalyst housing, urea storage tank location, and crane access for catalyst module removal. On many older vessels, engine rooms were designed with no allowance for post-combustion treatment equipment, so the retrofit engineer must resolve conflicts with existing equipment: exhaust gas boilers, waste heat recovery systems, silencers, and structural framing.

Pre-turbo SCR installations require access to the exhaust gas path between the exhaust receiver and the turbocharger turbine inlet, a zone that is physically congested on most slow-speed engine designs. The turbocharger may need temporary removal or repositioning of access equipment. Structural calculations must verify the deck load capacity for a reactor assembly that can weigh 25 to 45 tonnes. High-pressure exhaust flanges must meet the class society’s welded joint and pressure-vessel inspection requirements.

Urea storage tanks of 20 to 100 m3 are typically installed in void spaces, ballast tank top flats, or dedicated tank rooms. The preferred location is close to the engine room to minimise urea supply line length, reducing the heat-tracing scope and pump capacity required.

Drydock timeline and major activities

A complete high-pressure SCR retrofit on a large slow-speed two-stroke typically requires four to six weeks of drydock time. The first week covers detailed survey, fabrication preparation, and equipment staging. Weeks two and three involve structural works: cutting access openings in engine room casings, reinforcing deck structures, and pre-assembling exhaust duct sections. Weeks three to five cover equipment installation: catalyst housing, urea storage tanks, dosing system, heat-tracing on urea lines, exhaust ducting connections, and electrical and control system cabling. The final week is commissioning: no-load system tests, engine run-up at multiple load points to verify NOx conversion across the E3 propeller curve test cycle, ammonia slip measurement, and class society survey attendance for EIAPP endorsement.

EIAPP certification and the Technical File

The SCR system becomes part of the certified engine system once the class society (acting on behalf of the flag administration) endorses the EIAPP. The engine’s Technical File records: the SCR system as fitted, the AUS 32 specification and minimum onboard quantity, the urea dosing map at each test cycle load point, the minimum exhaust temperature for SCR activation, the catalyst type and module serial numbers, and the maintenance schedule. Any modification to a component listed in the Technical File, including substitution of catalyst modules from a different supplier, requires a formal Technical File amendment and class survey.

Port state control officers are entitled to inspect the EIAPP, the Technical File, the urea consumption log, and the SCR system condition during a port state control examination. A vessel found operating inside a NOx ECA in Tier II mode without valid documentation of an approved Tier III system faces detention and potential referral to the flag administration for investigation. The nox-tier-i-ii-iii article covers the enforcement framework and documentation requirements in detail.

ECA approach and mode switching

The SCR catalyst requires time to reach operating temperature from standby. A vanadium catalyst that has been idle for several hours (or after a cold-start following dry dock) takes 20 to 40 minutes of engine operation to reach 290°C at the catalyst inlet. Vessel operations must initiate catalyst warm-up at least 30 to 45 minutes before the ECA boundary. MAN ES and WinGD SCR control systems include an ECA approach mode: the system monitors GPS position against a stored ECA boundary database and triggers the warm-up sequence automatically at a configurable distance from the boundary (commonly 50 to 100 nautical miles for vessels at 12 to 15 knots).

The mode-switch procedure is documented in the vessel’s shipboard operations manual and cross-referenced in the EIAPP. The officer of the watch records the mode switch time, GPS position, urea system status, and catalyst inlet temperature in the engine log. These entries are the primary operational evidence of Tier III compliance that a port state control officer will review.

Capital cost and operating economics

Capital expenditure breakdown

A complete high-pressure SCR retrofit on a Handymax or Panamax vessel with a 7,500 to 9,000 kW main engine typically costs USD 1 to 3 million in 2020s prices. The range reflects reactor size, catalyst volume, urea system scope, and integration complexity. Larger vessels with engines above 20,000 kW may see retrofit costs approaching USD 4 to 5 million. The main cost components and representative ranges for a mid-size retrofit:

Operating costs

The primary annual operating cost is urea solution. A vessel spending 2,000 to 3,000 hours per year inside NOx ECAs with a 7,500 kW main engine consumes roughly 50,000 to 100,000 litres of AUS 32 annually. At 2024 supply prices of approximately USD 0.35 to 0.65 per litre for marine-grade AUS 32 in Northern European ports, this gives an annual reagent cost of USD 17,500 to 65,000, which is modest relative to the fuel costs on the same voyage.

Catalyst amortisation is a larger periodic cost. Assuming replacement every 20,000 hours at USD 300,000 per replacement cycle, the annual amortised cost is roughly USD 30,000 to 45,000 for a main engine running 3,000 hours per year. Maintenance labour (dosing system servicing, sensor calibration, soot-blowing verification) adds perhaps USD 15,000 to 30,000 per year. The fuel consumption penalty from the catalyst pressure drop (0.5 to 1.5 g/kWh on SFOC) translates to roughly USD 10,000 to 40,000 per year depending on fuel price and hours at load.

Norway’s NOx fund (Næringslivets NOx-fond) provides subsidies for NOx reduction investments, including SCR retrofits, for vessels trading in Norwegian waters under an agreement with the Norwegian Environment Agency. The Norway NOx fund calculator models subsidy eligibility and payback period for this scheme.

SCR versus EGR on slow-speed two-stroke engines

SCR and EGR are the two main Tier III retrofit paths for slow-speed two-stroke engines, with broadly comparable capital cost but different operational profiles. SCR has a lower SFOC penalty and no scrubber water requirement, but it loses Tier III coverage below roughly 25 percent MCR due to catalyst temperature constraints. EGR works at all loads but adds wash water management, higher cylinder oil feed rates, and increased cold-corrosion risk.

The table below compares the two technologies across the factors that most commonly determine the choice.

The practical decision depends on the vessel’s load profile in the ECA. A container ship or bulk carrier transiting the North Sea NOx ECA at near-full load benefits from SCR’s lower fuel penalty and simpler water management. A harbour tug, short-sea ferry, or dynamic-positioning vessel that spends time at low loads within an ECA needs EGR’s all-load coverage, or a combined SCR-plus-EGR installation that uses EGR at low loads and switches to SCR above the catalyst activation threshold.

Some shipowners with high-ECA-exposure vessels have installed both systems. In this arrangement, EGR handles NOx control at loads below 25 percent MCR, while pre-turbo SCR handles the higher load range where the SFOC advantage matters. Capital cost is roughly additive, but the combined system eliminates all the load-range compliance gaps that either technology alone leaves open.

The EGR retrofit on two-stroke engines article covers the EGR architecture, scrubbing water treatment, and operational procedures in detail.

Limitations

Several practical constraints bound SCR performance on two-stroke engines, and operators who ignore them face compliance risk, accelerated catalyst degradation, or unexpected operating costs.

Low-load coverage is the most commonly encountered limitation. Below roughly 25 percent MCR, pre-turbo exhaust temperatures on slow-speed two-stroke engines drop to or below the vanadium catalyst activation threshold. The SCR system bypasses, engine emissions rise to Tier II levels, and the vessel is non-compliant inside a NOx ECA. Vessels with operating profiles that include low-load ECA operation (slow approach channels, pilot boarding areas, anchorage waiting periods) must plan for this gap through EGR, engine acceleration sequences to heat the catalyst before reducing power, or an alternative Tier III measure.

Urea supply chain security is an operational constraint that is easy to overlook at the retrofit planning stage. Vessel routes that include ports with thin marine AUS 32 availability require either enlarged onboard storage or confirmed advance delivery arrangements. An SCR-equipped vessel without urea is a Tier II vessel, and entering a NOx ECA without an active Tier III system risks port state control detention.

Sulphur sensitivity constrains the fuel oil choices compatible with the vanadium catalyst. On routes where the cheapest compliant option for open-sea passages is a high-sulphur HFO burned behind an open-loop EGCS, the SCR system must be compatible with that sulphur level or the fuel must be switched before SCR activation. Many SCR catalyst qualifications are limited to 0.50% sulphur, and some to 0.10%; using the system outside its certified sulphur range can void the catalyst warranty and the EIAPP.

Catalyst replacement cost and scheduling interact with the vessel’s dry-dock cycle. A catalyst reaching end of life between special survey dry docks requires either an off-cycle dry-dock entry for replacement (expensive) or an approved temporary bypass arrangement (complex to certify). Operators planning an SCR retrofit should model the expected catalyst life against their dry-dock schedule and choose catalyst specifications that align the replacement interval with scheduled dockings.

The interaction with waste heat recovery also deserves attention. A pre-turbo SCR reactor placed in the high-temperature exhaust stream absorbs heat and delivers cooler gas to the turbocharger turbine. This reduces the energy available for turbocharging and may reduce exhaust gas boiler output if the post-turbo exhaust is cooler than the original design assumed. In some installations, the exhaust gas boiler must be resized or supplemented by an auxiliary boiler when SCR is active, adding capital cost and weight not always anticipated in the initial feasibility study. The waste heat recovery system article covers the thermodynamic interactions.

Temperature management during startup and low-load operation can also conflict with SFOC-optimised slow steaming strategies. Running the engine at 25 to 30 percent MCR to warm the SCR catalyst before an ECA boundary, or accelerating briefly to desorb ammonium bisulphate fouling, uses more fuel than a pure slow-steaming approach and temporarily increases CO2 output. Vessel operators balancing CII ratings against NOx ECA compliance may find these temperature management requirements create competing objectives in the operational plan.

See also

• Selective catalytic reduction (marine NOx Tier III) - detailed treatment of the SCR technology, chemistry, and catalyst management
• EGR retrofit on two-stroke engines - the principal alternative Tier III retrofit pathway
• Tier III compliant two-stroke engines - new-build design approaches and engine family survey
• NOx Tier I, II and III - regulatory framework and limit values
• MARPOL Annex VI Reg. 13 NOx Tier - the specific regulatory text and its amendments
• Emission control areas - geographic scope of NOx ECAs and enforcement
• Two-stroke marine diesel engine fundamentals - engine architecture context for SCR integration
• Cylinder lubrication systems for two-stroke engines - lube oil additive impact on catalyst life
• Waste heat recovery system - interaction with SCR placement in exhaust path
• IMO 2020 sulphur cap - fuel-sulphur framework constraining SCR catalyst compatibility
• Exhaust gas cleaning system - scrubber technology often installed alongside SCR
• Slow steaming and CII - CII implications of speed reduction interacting with SCR thermal management
• SCR urea consumption calculator - models urea demand from engine load and NOx parameters
• NOx Tier checker - confirms applicable NOx tier for a given vessel and area
• MARPOL NOx Tier III limit calculator - computes Tier III limit from rated engine speed
• MARPOL NOx Tier II limit calculator - computes Tier II baseline for comparison
• EGR Tier III rate calculator - models exhaust gas recirculation as alternative Tier III pathway
• Test IMO NOx Technical Code calculator - weighted emission factor calculation for NOx Technical Code test cycles
• Norway NOx fund calculator - models NOx fund subsidy and payback for SCR investments
• Combustion Zeldovich NOx calculator - thermal NOx formation from first principles
• NOx Tier compliance check calculator - quick compliance status check