Background
Many ships currently operating outside ECAs were built before IMO Tier III requirements applied (i.e. before 2016) and use Tier II engines. When such ships need to enter ECAs (North America, North Sea, Baltic, US Caribbean, Mediterranean), they must comply with Tier III NOx limits. Three options exist:
- Avoid ECAs: route the ship around ECA boundaries, accepting longer voyages
- Retrofit EGR or SCR: bring the engine into Tier III compliance
- Replace the engine with a Tier III-compatible model: extreme cost, rarely chosen
EGR retrofit is the most common pathway when the ship’s trade pattern mandates ECA entry. EGR systems can be installed during a drydock visit without major engine modifications, and they provide Tier III compliance with manageable operational impact.
This article covers EGR retrofit architecture, installation procedures, operating characteristics, and the strategic decisions that determine retrofit timing and scope.
EGR fundamentals
Why EGR reduces NOx
NOx forms in high-temperature combustion regions where atmospheric nitrogen reacts with oxygen. The Zeldovich mechanism describes the dominant pathway:
N2 + O2 → 2 NO at high temperature
NOx formation rate increases exponentially with temperature above approximately 1500 K. By recirculating exhaust gas (which is mostly inert N2 and CO2 after combustion) into the cylinder inlet, the cylinder charge is diluted, reducing oxygen partial pressure and lowering peak combustion temperature.
Typical EGR effects:
- Recirculation rate: 25 to 35 percent of exhaust mass flow
- Oxygen content reduction in scavenge air: from 21% to 16-18%
- Peak combustion temperature reduction: 100-200 K
- NOx reduction: 70-80 percent
These effects bring NOx from Tier II level (~14.4 g/kWh) down to Tier III level (~3.4 g/kWh).
High-pressure vs low-pressure EGR
High-pressure EGR
In high-pressure EGR, exhaust gas is extracted before the turbocharger turbine and recirculated directly to the scavenge receiver. The hot gas, at exhaust manifold pressure, is cooled and mixed with charge air without additional pumping.
Advantages:
- No additional fan or blower required
- Direct flow without pressure barriers
Disadvantages:
- Hot gas (~400°C) at high pressure (~4 bar)
- Difficult to clean (SOx removal at high pressure is challenging)
- Pressure drop across cooler must be carefully managed
High-pressure EGR is uncommon in marine practice except in some new-build installations where the architecture is integrated from design.
Low-pressure EGR
In low-pressure EGR, exhaust gas is extracted after the turbocharger turbine, where it is at near-atmospheric pressure and cooler (~250°C). The gas is scrubbed, cooled, and pumped back to the scavenge receiver via a small EGR blower.
Advantages:
- Lower temperature simplifies cooling and scrubbing
- Atmospheric pressure simplifies scrubber design (similar to caustic scrubbers used for IMO sulphur compliance)
- Easier integration with existing engines
Disadvantages:
- Requires a blower or fan for recirculation
- More complex flow paths
- More components to maintain
Low-pressure EGR is the dominant retrofit architecture.
Retrofit components
Exhaust extraction
A bypass duct is added to the exhaust system, downstream of the turbocharger turbine. A control valve regulates the EGR rate. Typical extraction is 25-35 percent of exhaust flow.
EGR scrubber
A scrubber removes SOx and particulates from the exhaust gas before recirculation. Without scrubbing, recirculated SOx would attack engine components.
Scrubber types:
- Open-loop water scrubber: seawater sprayed through the gas, removing SOx and particulates. Simple but produces wash water that must be discharged or treated.
- Closed-loop caustic scrubber: alkaline solution recirculated, neutralising SOx. Higher capex but no overboard discharge.
- Water-mist scrubber: fine water droplets injected into the gas stream
For EGR retrofits on ships also seeking SOx compliance, an integrated scrubber may serve both purposes.
EGR cooler
The scrubbed gas is cooled before mixing with charge air. Typical inlet temperature 250-350°C; outlet temperature 50-80°C. Cooling water (often the engine’s main cooling water circuit) absorbs the heat.
EGR blower
A small centrifugal fan moves the recirculated gas from the scrubber/cooler to the scavenge receiver. Flow rate typically 5,000-15,000 m³/h depending on engine size; pressure rise typically 0.1-0.3 bar.
Mixing at scavenge receiver
The recirculated gas is introduced into the scavenge receiver, where it mixes with the turbocharger discharge to form the cylinder inlet charge. A venturi mixer or simple injection point ensures good mixing.
Control system
The EGR control system manages:
- Recirculation rate based on engine load and ambient conditions
- Scrubber operation (water flow, dosing)
- Blower speed
- Cooler bypass for temperature control
- Integration with engine control system for activation/deactivation
Sensors
A typical EGR installation adds:
- Mass flow meter on EGR line
- Oxygen sensor in scavenge receiver
- Pressure sensors at multiple points
- Temperature sensors throughout the gas path
- pH sensor in scrubber (closed-loop systems)
Installation procedure
A typical EGR retrofit takes 4 to 8 weeks of drydock time:
Week 1-2: Preparation
- Yard receives and stages EGR equipment
- Survey of engine room layout for component placement
- Pipe and cable routing planning
- Initial removals of engine room components to clear paths
Week 2-4: Equipment installation
- EGR scrubber installed, typically in the funnel or upper engine room
- EGR cooler installed, typically near the engine
- EGR blower installed
- Exhaust extraction duct fitted
- Recirculation duct routed to scavenge receiver
- Control system cabinet installed
Week 4-6: Connections
- Cooling water connections to EGR cooler
- Wash water/caustic connections to scrubber
- Electrical connections throughout
- Engine control system integration
- Sensor installations
Week 6-7: Commissioning
- Hydraulic and electrical testing
- Standalone system testing
- Engine integration testing
- Performance verification at multiple loads
- NOx emission verification
Week 7-8: Class survey and certification
- Class society survey of installation
- EIAPP certificate update
- Sea trial verification
- Documentation for certification
Capex and opex
Capex breakdown
Typical retrofit costs (USD millions):
| Component | Cost |
|---|---|
| Scrubber | 0.5-1.5 |
| Cooler | 0.2-0.5 |
| Blower | 0.2-0.4 |
| Ducting and pipework | 0.4-0.8 |
| Control system | 0.3-0.6 |
| Sensors and instrumentation | 0.1-0.3 |
| Engine modifications | 0.1-0.4 |
| Drydock labor | 0.5-1.5 |
| Class certification | 0.1-0.2 |
| Project management | 0.2-0.4 |
| Total | 2.6-5.6 |
Operating costs
Annual EGR operating costs (assuming 3,000 hours of EGR-active operation per year):
- SFOC penalty: ~3-5 g/kWh × engine load × hours = ~USD 100-300k/year
- Scrubber consumables: caustic, fresh water, sludge disposal = ~USD 20-100k/year
- Maintenance: spare parts, service intervals = ~USD 50-150k/year
- Cylinder oil increase: 5-10% increase in feed rate = ~USD 30-100k/year
- Total annual: USD 200-650k
Operational considerations
Activation and deactivation
EGR is active only in ECAs. The system is activated:
- A few miles before entering an ECA
- Activation takes 5-10 minutes (ramp up recirculation)
- Engine remains operational throughout
Deactivation when leaving the ECA takes similar time.
Cylinder oil and lubrication
EGR introduces several lubrication challenges:
- Cold corrosion rises with EGR because the recirculated gas brings residual sulphuric acid into the cylinder
- Cylinder oil BN may need to be increased
- Feed rate typically rises 5-10% during EGR operation
- Liner and ring wear monitoring requires extra attention
SFOC penalty
The SFOC penalty during EGR operation is approximately 3-5 g/kWh, varying with load. Higher EGR rates produce larger penalties. The penalty results from:
- Slower combustion in lower-oxygen environment
- Charge air heating from EGR cooler bypass
- Pressure drop across EGR system
Scrubber maintenance
The scrubber requires regular maintenance:
- Wash water flow checks and cleaning
- pH adjustment (closed-loop systems)
- Sludge removal
- Pump and valve inspection
- Eventually, scrubber media replacement
Blower maintenance
The EGR blower runs whenever EGR is active. Routine maintenance includes:
- Bearing inspection and lubrication
- Impeller cleaning
- Vibration monitoring
- Eventually, blower replacement (typically 60,000-100,000 hours)
Strategic considerations
When to retrofit
Operators consider EGR retrofit when:
- ECA exposure expected for the ship’s remaining life
- Cost of avoiding ECAs (longer routes) exceeds retrofit cost
- Drydock visit can accommodate the retrofit duration
When not to retrofit
EGR retrofit may not make sense if:
- ECA exposure is rare and route adjustments are feasible
- Ship is approaching scrap age (insufficient remaining life to amortise capex)
- Engine condition is poor (retrofit may not be effective)
Alternative paths
- SCR retrofit as an alternative to EGR
- Engine replacement during major overhaul to a Tier III-compliant model
- Trade route adjustment to avoid ECAs
- Operating restriction: ship sold to operators on non-ECA routes
Industry experience
EGR retrofit has been increasingly common since 2016:
- 2016-2018: limited adoption, awaiting commercial maturity
- 2018-2020: rising adoption as commercial systems became proven
- 2020-2024: routine for ECA-trading vessels
- 2024+: continuing as fleet renewal continues
The technology is now mature, with multiple suppliers (Wartsila, MAN, Yanmar, others) offering complete retrofit packages. Class societies have established procedures for certification.
Related Calculators
- EGR Effectiveness Calculator
- EGR Retrofit Cost Calculator
- SFOC Penalty Calculator
- NOx Reduction Calculator
- Tier III Compliance Cost Calculator
- Scrubber Sizing Calculator
See also
- Tier III Compliant Two-Stroke Engines
- MAN B&W ME-C Electronic Control Overview
- WinGD X-DF Dual-Fuel Engine Architecture
- Two-Stroke Marine Diesel Engine Fundamentals
References
- IMO. (2008). MARPOL Annex VI: Regulations for the Prevention of Air Pollution from Ships.
- MAN Energy Solutions. (2023). EGR System Operation and Maintenance Manual. MAN Energy Solutions.
- DNV. (2022). Marine Emission Compliance: EGR Implementation. DNV.
- Wartsila. (2022). EGR Retrofit Guide for Marine Engines. Wartsila Corporation.
- Lloyd’s Register. (2022). EGR System Approval Guidelines.