Modern marine diesel engines operate under strict environmental regulations established by the International Maritime Organization (IMO), particularly Tier II and Tier III NOx limits. To comply with these standards, ship engines are equipped with exhaust gas emission control systems.
.jpg "What is EGR and SCR on a ship? Troubleshooting and Fault Finding")
The two main technologies used onboard are:
- EGR (Exhaust Gas Recirculation)
- SCR (Selective Catalytic Reduction)
Both systems are designed to reduce nitrogen oxides (NOx), but they work in fundamentally different ways.
As an example, the article considers the MITSUI-MAN B&W 6S50ME-C9.5-EGRBP main engine with an EGR system and auxiliary Daihatsu 6DE-18 Tier III 630kW engines with an SCR system.
Principle of Operation
EGR reduces NOx formation inside the engine cylinder. A portion of exhaust gases is redirected back into the engine intake, which results in:
- Lower oxygen concentration
- Reduced combustion temperature
Since NOx is primarily formed at high temperatures, this significantly decreases its formation.
.JPG "EGR MOP (Main Operating Panel)")
EGR with Scrubber and NaOH
On marine engines, EGR is often combined with a gas cleaning system (scrubber). Exhaust gases contain harmful components such as:
- Sulfur oxides (SOx)
- Particulate matter (soot)
- Acidic compounds
Before recirculation, these gases must be cleaned to avoid engine damage.
.jpg "NaOH tank on the vessel (temperature and level control)")
.jpg "NaOH tank on the vessel")
Role of NaOH (Sodium Hydroxide)
NaOH is used in the scrubber to neutralize acidic components:
Chemical reactions:
- SO₂ + 2NaOH → Na₂SO₃ + H₂O
- SO₃ + 2NaOH → Na₂SO₄ + H₂O
This process:
- Removes acidic compounds
- Prevents corrosion
- Protects engine components
.jpg "EGR (sensors & control valves)")
Main Components of EGR System
- EGR valve
- Scrubber (gas cleaning unit)
- EGR cooler
- Circulation water system
- NaOH dosing unit
- Pumps and pipelines
- Sensors (temperature, pressure, pH)
.jpg "pH control system")
.jpg "pH control system")
.jpg "pH control system")
pH sensors calibration (Main Engine EGR system) (video)
Common EGR Failures
1. Fouling and Deposits
- Soot and unburned particles accumulate
- Salt deposits (from NaOH reactions)
Symptoms:
- Reduced engine efficiency
- Increased exhaust temperature
- Unstable operation
2. EGR Valve Malfunction
- Stuck open → excessive recirculation
- Stuck closed → increased NOx emissions
3. Scrubber System Issues
- Poor gas cleaning
- Blocked spray nozzles
- Dirty circulation water
4. Incorrect pH Level
- Low pH → insufficient SOx neutralization
- High pH → excessive NaOH consumption
5. Corrosion and Scaling
- Caused by poor water treatment
- Leads to damage of coolers and pipelines
.jpg "EGR blower (ABB motor)")
.jpg "EGR blower (ABB VFD)")
- EGR (Exhaust Gas Recirculation) (video part 1)
- EGR (Exhaust Gas Recirculation) (video part 2)
- EGR (Exhaust Gas Recirculation) (video part 3)
- EGR (Exhaust Gas Recirculation) (video part 4)
- EGR (Exhaust Gas Recirculation) (video auto mode)
EGR Troubleshooting
Key diagnostic steps:
- Check pH level of scrubber water
- Inspect EGR valve operation
- Monitor:
- Temperature
- Pressure
- Flow rates
- Inspect scrubber:
- Nozzle condition
- Deposits and sludge
.jpg "EGR drain tank (SEMCO liquid level indicator)")
- Analyze engine performance:
- Smoke emissions
- Fuel consumption
EGR Maintenance and Repair
- Cleaning of scrubber and pipelines
- Removal of deposits and sludge
- Adjustment of NaOH dosing
- Calibration or replacement of sensors
- Repair or replacement of EGR valve
.jpg "WTS (Water Treatment System) Separator")
.jpg "WTS (Water Treatment System) Control Panel")
.jpg "WTS (Water Treatment System)")
.jpg "WTS (Water Treatment System)")
Principle of Operation
SCR reduces NOx after combustion, in the exhaust system. A solution of Urea (32.5%) is injected into the exhaust gas stream. At high temperatures, Urea decomposes: CO(NH₂)₂ → NH₃ + CO₂
.jpg "SCR System (Selective Catalytic Reduction)")
The ammonia (NH₃) reacts with NOx in the catalyst:
- NOx → N₂ (nitrogen)
- H₂O (water)
Main Components of SCR System
- Urea tank
- Dosing unit (pump)
- Injection nozzle
- SCR reactor (catalyst)
- NOx sensors
- Control system
.jpg "Urea pump")
Common SCR Failures
1. Urea-Related Problems
- Low level or empty tank
- Poor quality Urea
- Crystallization
.jpg "Urea tank")
Symptoms:
- System alarms
- Reduced engine power
- Non-compliance with emissions
2. Injector and Dosing Issues
- Blocked injector
- Pump malfunction
- Incorrect dosing
3. NOx Sensor Failure
- Incorrect readings
- Fault codes
- Poor system performance
4. Catalyst Degradation
- Contamination
- Thermal damage
- Aging
.jpg "SCR Catalyst")
.jpg "SCR Catalyst")
.jpg "SCR Catalyst (damper)")
5. Temperature Problems
- Low exhaust temperature → ineffective reaction
- High temperature → catalyst damage
SCR Troubleshooting
- Check Urea level and quality
- Inspect injection system
- Monitor NOx sensor readings
- Analyze exhaust temperature
- Check system alarms and error codes
.jpg "SCR Unit")
.jpg "SCR Unit")
.jpg "Temperature and Humidity sensor")
SCR Maintenance and Repair
- Cleaning or replacing injector
- Flushing system to remove crystallization
- Replacing faulty sensors
- Repairing dosing unit
- Replacing catalyst if required
.jpg "SCR injector")
.jpg "Cleaning injector")
.jpg "Cleaning injector")
Comparison of EGR and SCR
Combined Use on Ships
Many modern vessels use both EGR and SCR systems to meet IMO Tier III requirements.
- EGR reduces NOx formation
- SCR removes remaining NOx
This combination provides maximum emission reduction.
.jpg "SCR operation")
EGR and SCR are essential technologies in modern marine engineering. EGR reduces NOx by lowering combustion temperature but requires complex gas cleaning using NaOH and careful maintenance. SCR provides highly efficient NOx reduction using Urea but depends on proper dosing and system condition.
Effective operation of both systems requires:
- Regular monitoring
- Proper maintenance
- Accurate troubleshooting
Understanding these systems is critical for marine engineers to ensure compliance with environmental regulations, reliable engine operation, reduced maintenance costs.
Manuals and videos! Any troubleshooting on a vessel is impossible without equipment operation and repair manuals. If you need troubleshooting manuals and videos for specific EGR, SCR, and scrubber systems, I recommend our closed Telegram channel "Marine Engineering Manuals".
I hope this article was helpful. If you have any problems with EGR, SCR, or scrubbers on your vessel, please let me know in the comments, and I'll try to help.
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