SCR Retrofit on Two-Stroke Marine Engines

Background

Selective catalytic reduction (SCR) is the established NOx control technology for stationary power plants and on-road heavy-duty vehicles. Its adaptation to marine slow-speed two-stroke engines began in the 2000s and is now a routine option for Tier III compliance.

SCR works by adding aqueous urea solution (32.5% urea in water, called AdBlue or DEF) to the exhaust stream. Urea decomposes to ammonia (NH3) at exhaust temperatures, and ammonia reacts with NOx over a catalyst to form nitrogen and water:

4 NO + 4 NH3 + O2 → 4 N2 + 6 H2O 2 NO2 + 4 NH3 + O2 → 3 N2 + 6 H2O

Modern marine SCR systems achieve 80-95% NOx conversion, sufficient to bring Tier II engines into Tier III compliance.

This article covers SCR retrofit components, installation, operational characteristics, and the strategic comparison with EGR.

Components

Urea storage

Aqueous urea solution is stored in dedicated tanks:

• Service tank: 5-20 m³, providing daily supply
• Storage tanks: 50-200 m³, providing reserve for typical voyage
• Heat tracing: urea solution freezes at -11°C; tanks and lines require heating in cold climates
• Tank cleanliness: urea is sensitive to contamination; dedicated piping and pumps prevent cross-contamination

Dosing system

A pump and metering valve deliver urea to the dosing point in the exhaust stream:

• Dosing pump: small displacement pump, typically 5-50 L/h capacity
• Metering valve: precision valve that pulses urea as a function of NOx setpoint
• Compressed air: some designs use air-assisted dosing for better atomisation
• Heating: dosing line heated to prevent urea precipitation

Mixer

The dosing point introduces urea into the exhaust stream. Mixing devices ensure good distribution:

• Static mixers: vanes or baffles that swirl the exhaust gas
• Turbulent mixing: relying on natural exhaust turbulence
• Combination: mixer plus mixing chamber length

Adequate mixing is essential; uneven distribution leads to uneven catalyst utilisation and ammonia slip.

Catalyst

The catalyst block contains the active SCR catalyst:

• Substrate: ceramic honeycomb (cordierite) or metallic foil with channels
• Active material: vanadium pentoxide (V2O5) or zeolite-based catalyst
• Coating: catalyst applied as a wash coat on the substrate

Modern marine SCR catalysts achieve 80-95% NOx conversion at design conditions.

Heating

SCR is effective only above approximately 280°C exhaust temperature. Below this, urea decomposition is incomplete and catalyst activity drops. For low-load operation:

• Engine exhaust may be too cold: at low load, exhaust temperatures may drop below 280°C
• Auxiliary heating: some installations include electric heaters or fuel burners to maintain catalyst temperature
• Low-temperature catalysts: emerging catalyst formulations work down to 200°C

Sensors

A typical installation includes:

• NOx sensor: upstream and downstream of catalyst
• NH3 sensor: downstream of catalyst (ammonia slip detection)
• Temperature sensors: catalyst inlet, outlet, multiple points
• Pressure sensors: pressure drop across catalyst
• Urea quality sensor: in storage and supply

Control system

The SCR control system manages:

• Urea dosing rate based on NOx and exhaust temperature
• Bypass valve operation for low-temperature conditions
• Heating system control
• Diagnostics and alarms
• Integration with engine control system

Catalyst sizing

Volume requirements

The catalyst volume is sized for the engine’s NOx output and target conversion:

• Space velocity: exhaust flow per unit catalyst volume (typically 5,000-15,000 1/h)
• Catalyst volume: typically 0.5-2.0 m³ for a large slow-speed two-stroke engine
• Catalyst length: typically 0.5-1.5 m of axial flow path
• Catalyst cross-section: matched to exhaust pipe diameter

Cell density

Catalyst substrates have a defined cell density (cells per square inch, CPSI):

• Coarse: 100-200 CPSI, lower pressure drop, lower surface area
• Standard: 300-400 CPSI, balance of pressure drop and reactivity
• Fine: 400-600 CPSI, highest reactivity, more pressure drop

Marine SCR typically uses 200-400 CPSI.

Pressure drop

The catalyst introduces pressure drop into the exhaust system:

• Typical: 30-100 mbar at full load
• Effect on engine: slightly increased back-pressure, ~0.5-1 g/kWh SFOC penalty
• Bypass: some installations include bypass valves to reduce drop at low load

Operational characteristics

Activation

SCR is typically active continuously while the engine is running and exhaust temperatures are adequate:

• Activation takes seconds (urea dosing starts, catalyst already warm if engine running)
• Bypass deactivates SCR if temperature drops below threshold
• Operator can manually disable SCR if needed (rare)

Urea consumption

Urea consumption depends on engine load and target conversion:

• Stoichiometric ratio: typically 1:1 NH3 to NO molar ratio at the catalyst inlet
• Mass ratio: 0.4-0.6 grams urea solution per gram of NOx
• Daily consumption: 50-200 kg urea per cylinder per day at full load with active SCR

For a typical 8-cylinder large slow-speed engine at 70% MCR with continuous SCR:

• NOx output: ~150 kg/day
• Urea consumption: ~75-90 kg/day
• Per voyage (5-15 days): 400-1,500 kg

Ammonia slip

Excess urea injection or unburned NH3 escapes the catalyst as ammonia slip. Modern SCR aims for slip below 10 ppm (approximately 5-15 mg/m³). High slip indicates:

• Over-dosing
• Catalyst aging or poisoning
• Temperature out of optimal range
• Maldistribution (uneven flow through catalyst)

Catalyst aging

SCR catalysts gradually lose activity over time:

• Hydrothermal aging: high-temperature exhaust causes sintering of active sites
• Poisoning: heavy metals (vanadium from fuel, sulphur compounds) bind to active sites
• Mechanical damage: thermal shock, vibration
• Plugging: particulate accumulation in cells

Typical catalyst life: 30,000-60,000 hours of operation. Replacement cost: USD 100,000-300,000 depending on size.

Bypass operation

When exhaust temperature is below 280°C (low load, slow steaming), the SCR is bypassed:

• Bypass valve diverts exhaust around the catalyst
• Urea dosing stops
• Engine NOx output rises to Tier II level (acceptable outside ECAs but not in)
• Operator must avoid sustained low-load operation in ECAs

Capex and opex

Capex breakdown

Typical retrofit costs (USD millions):

Operating costs

Annual operating costs (3,000 hours of SCR-active operation):

• Urea consumption: ~250 tonnes × USD 250-400/tonne = USD 60-100k/year
• Catalyst replacement amortisation: ~USD 30-100k/year
• Maintenance: ~USD 50-100k/year
• SFOC penalty (slight): ~USD 30-100k/year
• Total annual: USD 170-400k

SCR vs EGR comparison

The two technologies are roughly comparable in capex and complexity. Choice typically depends on:

• Operating profile: SCR works poorly at low loads, EGR works at all loads
• Cylinder oil costs: EGR raises cylinder oil costs; SCR does not
• Fuel quality: SCR is more tolerant of fuel quality variation
• Operator preference: based on previous experience and crew familiarity

Some installations combine EGR (for low-load operation) with SCR (for high-load operation), achieving the best of both technologies at higher capex.

Installation procedure

A typical SCR retrofit takes 4 to 6 weeks of drydock time:

Week 1-2: Preparation and equipment staging

• Survey of engine room and exhaust system layout
• Catalyst housing fabrication or installation prep
• Urea storage tank installation (typically in dedicated location)
• Initial removals to clear paths

Week 2-4: Equipment installation

• Catalyst block installed in exhaust system
• Mixer fitted upstream of catalyst
• Urea dosing system installed
• Control system installed
• Heat tracing installed on urea lines

Week 4-5: Connections and integration

• Exhaust ducting connections
• Urea piping
• Cooling water (if used)
• Electrical connections
• Engine control system integration

Week 5-6: Commissioning and certification

• System testing at standstill
• Engine running tests at multiple loads
• NOx emission verification
• Class society survey
• EIAPP certificate update

Strategic considerations

SCR retrofit suits ships with:

• High-load operating profile (consistent above SCR temperature threshold)
• HFO operation with scrubber (SCR may not require additional scrubbing)
• Older engines where EGR’s cylinder oil and corrosion effects would be problematic
• Sufficient onboard space for urea tanks

EGR retrofit may be preferred for:

• Low-load or variable-load operating profile
• Ships without scrubber (EGR doesn’t add scrubber requirement)
• Smaller engines where catalyst cost is disproportionately high
• Operators with EGR experience

Industry experience

SCR retrofit volume has grown alongside EGR:

• 2016-2018: limited adoption
• 2018-2020: increasing volume, particularly for ships with scrubbers
• 2020-2024: routine for high-load profile ships
• 2024+: continuing as ECA boundaries expand

Multiple suppliers offer complete retrofit packages. Class societies have established certification procedures.

Related Calculators

• SCR Catalyst Sizing Calculator
• Urea Consumption Calculator
• SCR Retrofit Cost Calculator
• Ammonia Slip Calculator
• NOx Reduction Calculator
• Tier III Compliance Cost Calculator

See also

• EGR Retrofit on Two-Stroke Marine Engines
• Tier III Compliant Two-Stroke Engines
• Two-Stroke Marine Diesel Engine Fundamentals
• MAN B&W ME-C Electronic Control Overview

Additional calculators:

• SCR Urea Consumption
• EGR Rate for Tier III Compliance
• MARPOL Annex VI - NOx Tier III Limit
• Engine - Pcomp vs Pmax Ratio

Additional formula references:

• Emis Scr Urea Consumption
• Fuel Ammonia Nox Slip
• Nox Tier Compliance Check
• System Main Engine Slow Speed 2 Stroke

Additional related wiki articles:

• Selective catalytic reduction (marine NOx Tier III)

References

• IMO. (2008). MARPOL Annex VI: Regulations for the Prevention of Air Pollution from Ships.
• MAN Energy Solutions. (2023). SCR System Operation and Maintenance Manual. MAN Energy Solutions.
• DNV. (2022). Marine Emission Compliance: SCR Implementation. DNV.
• Yara Marine. (2022). SCR Retrofit Guide for Marine Engines.
• Lloyd’s Register. (2022). SCR System Approval Guidelines.