Marine SOx scrubbers, also known as Exhaust Gas Cleaning Systems (EGCS), are used on ships to reduce sulfur oxide emissions from marine engine and boiler exhaust gases. Instead of relying only on low-sulfur fuel, an approved EGCS can clean exhaust gas before it is released into the atmosphere.
Marine scrubbers are commonly classified by their washwater operating method into three configurations: open-loop, closed-loop and hybrid systems. Each type uses a different approach to treating and managing washwater, which affects installation cost, operating cost, discharge requirements and operational flexibility.
Understanding the differences between these marine SOx scrubber types is important when selecting an exhaust gas cleaning system for a vessel. The choice depends on the vessel’s operating routes, water conditions, discharge restrictions, available installation space, operating costs, and required flexibility.
Table of Contents
1. What Is a Marine SOx Scrubber?
A marine SOx scrubber is an exhaust gas treatment system designed to remove sulfur oxides from the exhaust generated by marine engines and boilers.
When ships burn fuel containing sulfur, combustion produces sulfur oxides, mainly sulfur dioxide (SOâ‚‚). If released directly into the atmosphere, these emissions contribute to air pollution and acid deposition. A wet scrubber reduces SOx by bringing the exhaust gas into contact with a liquid scrubbing medium.
In a typical wet marine scrubber, exhaust gas enters a scrubber tower and is contacted with water through a spray system. The SOx is absorbed into the washwater and converted into acidic compounds. Depending on the scrubber configuration, the resulting acidity is neutralized by the natural alkalinity of seawater or by an alkaline reagent such as sodium hydroxide (NaOH).
Marine SOx scrubbers are therefore also commonly referred to as:
- Marine scrubbers
- SOx scrubbers
- Ship scrubbers
- Exhaust Gas Cleaning Systems
- Marine EGCS
- Ship exhaust gas cleaning systems
- Marine desulfurization systems
These terms describe closely related technologies, although marine SOx scrubber and EGCS are generally more widely used in international marine engineering applications.
2. How Does a Marine SOx Scrubber Work?
Although open-loop, closed-loop, and hybrid systems have different washwater arrangements, their basic gas-cleaning principle is similar.
Step 1. Exhaust Gas Enters the Scrubber
Exhaust gas from the ship’s main engine, auxiliary engine, or boiler is directed into the scrubber tower.
Depending on the system design, the exhaust gas may first be cooled before entering the main scrubbing section. Cooling helps create suitable conditions for gas-liquid contact.
Step 2. Washwater Is Sprayed Into the Gas Stream
Washwater is distributed through spray nozzles inside the scrubber tower.
The nozzles create a controlled spray pattern and distribute water across the exhaust gas flow. Good liquid distribution is important because the effectiveness of wet scrubbing depends heavily on gas-liquid contact.
Step 3. SOx Is Absorbed by the Washwater
As the exhaust gas passes through the spray zone, sulfur oxides come into contact with the water droplets.
The SOx dissolves into the washwater and forms acidic compounds. The resulting acidity then needs to be neutralized.
Step 4. The Acidic Washwater Is Neutralized
The neutralization method depends on the scrubber configuration.
In an open-loop system, the natural alkalinity of seawater provides the neutralizing capacity. In a closed-loop system, an alkaline reagent such as NaOH is commonly added to the circulating washwater.
Step 5. Cleaned Exhaust Gas Leaves the System
After the scrubbing process, the treated exhaust gas passes through the remaining sections of the system and exits through the exhaust outlet.
A properly designed system must maintain stable gas flow, water flow, spray coverage, and monitoring conditions to achieve consistent emission performance.
Step 6. Washwater Is Discharged or Recirculated
This is the major difference between the three main scrubber configurations.
- Open-loop:washwater is generally discharged after appropriate treatment and monitoring.
- Closed-loop:washwater is recirculated, with a controlled bleed-off stream treated and managed.
- Hybrid:Â the system can switch between open-loop and closed-loop operation.
3. Main Types of Marine SOx Scrubbers
There are three main types of marine SOx scrubbers currently installed on ships: Open-Loop Marine Scrubber, Closed-Loop Marine Scrubber, and Hybrid Marine Scrubber.
(1) Open-Loop Marine Scrubber
An open-loop marine scrubber uses seawater as the primary scrubbing medium. Seawater is pumped into the scrubber and sprayed into the exhaust gas stream.
Because seawater has natural alkalinity, it can neutralize the acidic compounds generated when SOx dissolves into the washwater. After the scrubbing process, the washwater is treated and monitored before discharge in accordance with applicable requirements.
How Does an Open-Loop Scrubber Work?
The basic process is:
Seawater intake → Pump → Spray nozzles → Scrubber tower → Washwater treatment/monitoring → Discharge
The system normally requires:
- Seawater supply system
- Seawater pumps
- Scrubber tower
- Spray nozzles
- Washwater monitoring system
- Exhaust gas monitoring system
- Control system
- Discharge system
Advantages of Open-Loop Scrubbers
Open-loop systems have a relatively simple operating arrangement because they use seawater directly rather than continuously dosing an alkaline reagent.
Their main advantages include:
- Relatively simple system configuration
- Lower chemical consumption
- Lower operating complexity
- Generally lower CAPEX and OPEX than closed-loop systems
- Suitable for vessels spending significant time in open seawater with suitable alkalinity
Alfa Laval notes that open-loop systems can offer lower CAPEX and OPEX, although low water alkalinity or local restrictions on washwater discharge can limit where they can be operated.
Limitations of Open-Loop Scrubbers
The main limitation is operational flexibility.
Open-loop systems depend on suitable seawater conditions and permitted washwater discharge. Some ports, coastal areas, or jurisdictions may impose additional restrictions on scrubber washwater discharge.
Therefore, a vessel that operates across multiple regions needs to consider its actual sailing profile before selecting an open-loop system.
(2) Closed-Loop Marine Scrubber
A closed-loop marine scrubber recirculates washwater instead of continuously using and discharging seawater.
Freshwater or treated water is circulated through the scrubber, while an alkaline reagent is added to maintain the required neutralization capacity. Sodium hydroxide (NaOH) is one commonly used reagent.
How Does a Closed-Loop Scrubber Work?
A typical process is:
Circulation tank → Pump → Heat exchanger → Spray nozzles → Scrubber tower → Water treatment → Circulation tank
A controlled amount of water may be removed as bleed-off. The bleed-off is treated to ensure its quality complies with applicable discharge requirements, or it can be retained on board where required by operating conditions and system design.
Main Components
A closed-loop system may include:
- Scrubber tower
- Circulation tank
- Circulation pumps
- Spray nozzles
- Heat exchanger
- Water treatment system
- Chemical dosing system
- Washwater monitoring system
- Exhaust gas monitoring system
- Control system
Advantages of Closed-Loop Scrubbers
The main advantage is greater operational flexibility in areas where open-loop discharge is restricted.
Other benefits include:
- Recirculation of washwater
- Reduced dependence on seawater alkalinity
- Better suitability for restricted discharge areas
- Greater control over the scrubbing medium
- Suitable for vessels operating in waters with low alkalinity
Limitations of Closed-Loop Scrubbers
Closed-loop systems are more complex than open-loop systems.
They generally require additional equipment for:
- Chemical dosing
- Water circulation
- Water treatment
- Heat management
- Monitoring
This can increase both initial investment and operational requirements.
Therefore, closed-loop operation may be more suitable when a vessel frequently operates in areas where open-loop discharge is restricted or when greater control over washwater conditions is required.
(3) Hybrid Marine Scrubber
A hybrid marine scrubber combines open-loop and closed-loop operating modes in one system.
This configuration allows the vessel to operate in seawater-based open-loop mode when conditions permit and to switch to closed-loop operation when required.
How Does a Hybrid Scrubber Work?
In open-loop mode, the system uses seawater and relies on its natural alkalinity.
In closed-loop mode, the system recirculates washwater and uses an alkaline reagent to maintain the required scrubbing conditions.
This makes hybrid systems particularly useful for vessels operating on routes that include different types of waters and discharge restrictions. Wärtsilä and Alfa Laval both describe hybrid systems as configurations capable of operating in both open-loop and closed-loop modes.
Advantages of Hybrid Scrubbers
The main advantage is flexibility.
A hybrid scrubber can provide:
- Open-loop operation where discharge is permitted
- Closed-loop operation where required
- Greater route flexibility
- Reduced dependence on a single operating mode
- Better adaptability to changing operating conditions
Limitations of Hybrid Scrubbers
The additional flexibility comes with greater system complexity.
A hybrid system requires components and controls for both operating modes, which can increase:
- Initial equipment cost
- Installation requirements
- System complexity
- Maintenance requirements
For vessels operating across multiple compliance zones, however, this additional complexity may be justified by the greater operational flexibility.
4. Open-Loop vs Closed-Loop vs Hybrid Marine Scrubbers
The following table provides a simplified comparison of the three major marine SOx scrubber configurations.
Feature | Open-Loop | Closed-Loop | Hybrid |
Scrubbing Medium | Seawater | Freshwater/treated water + alkaline reagent | Seawater or recirculated water |
Water Operation | Once-through | Recirculated | Both |
Chemical Requirement | Generally low/none | Typically required | Depends on operating mode |
Washwater Management | Discharge after treatment/monitoring | Recirculation + controlled bleed-off | Depends on operating mode |
System Complexity | Lower | Higher | Highest |
Operating Flexibility | Limited | High | Very high |
Dependence on Seawater Alkalinity | Higher | Lower | Depends on mode |
Suitability for Restricted Areas | May be limited | Better suited | Highly flexible |
Typical Cost Level | Lower | Higher | Higher |
Best Application | Suitable open-sea routes | Restricted or low-alkalinity waters | Mixed routes and varied operating conditions |
The most appropriate configuration depends less on the scrubber type alone and more on the vessel’s route, operating profile, discharge requirements, available space, fuel strategy, and lifecycle cost.
5. Marine SOx Scrubber System Components
A marine scrubber is more than just a tower. It is an integrated exhaust gas treatment system containing several interconnected components.
Component | Main Function |
Scrubber Tower | Provides the main gas-liquid contact area |
Spray Nozzles | Distribute washwater through the exhaust gas |
Seawater Pump | Supplies seawater in open-loop operation |
Circulation Pump | Circulates washwater in closed-loop operation |
Heat Exchanger | Controls washwater temperature |
Water Treatment System | Removes contaminants from circulating water |
Chemical Dosing System | Adds alkaline reagent when required |
Monitoring System | Monitors exhaust gas and washwater parameters |
Control System | Controls pumps, valves, dosing, and operating modes |
Mist Eliminator | Helps reduce liquid carryover from the gas stream |
The design and sizing of each component affect the overall performance of the scrubber.
6. Role of Spray Nozzles in Marine SOx Scrubbers
Spray nozzles are critical components in wet marine scrubbers because they determine how washwater is distributed inside the scrubber tower.
The objective is not simply to spray water into the tower. The spray system must provide sufficient and uniform liquid coverage across the exhaust gas flow.
Poor spray distribution can create areas with insufficient water-gas contact, potentially reducing scrubbing performance. Proper nozzle selection therefore needs to consider several parameters.
â‘ Flow Rate
The nozzle must provide the required washwater flow at the available operating pressure.
â‘¡ Spray Pattern
The spray pattern determines how effectively the nozzle covers the scrubber cross-section.
Full cone, hollow cone, and other spray configurations may be considered depending on the scrubber design.
â‘¢ Spray Angle
The spray angle needs to match the tower geometry and nozzle installation position.
â‘£ Droplet Distribution
The size and distribution of droplets influence gas-liquid contact and residence behavior inside the scrubber.
⑤ Material Compatibility
Marine scrubbers expose spray components to water, acidic compounds, chemicals, and potentially abrasive contaminants. Corrosion-resistant materials may therefore be important depending on the application.
â‘¥ Clogging Resistance
Washwater can contain suspended solids and contaminants. A nozzle design with an appropriate flow passage can help reduce clogging and simplify maintenance.
For this reason, selecting a suitable marine scrubber spray nozzle should be treated as part of the overall scrubber design rather than as a minor component decision.
7. How to Choose the Right Marine Scrubber Type?
There is no single scrubber configuration that is ideal for every vessel. Consider the following factors before making a selection.
- Vessel Operating Routes
Where does the vessel spend most of its operating time?
A ship operating primarily in open seawater may have different requirements from one that frequently enters ports or restricted waters.
- Washwater Discharge Requirements
Local requirements can affect whether open-loop operation is practical.
If discharge restrictions are significant along the vessel’s route, closed-loop or hybrid operation may provide greater flexibility.
- Seawater Alkalinity
Open-loop systems rely on the natural alkalinity of seawater. Therefore, water conditions should be considered when evaluating open-loop operation.
- Available Installation Space
A retrofit project must consider the available space for the scrubber tower, pumps, tanks, treatment equipment, piping, control systems, and associated components.
- CAPEX and OPEX
Initial equipment cost should not be the only consideration.
A proper evaluation should also consider:
- Chemical consumption
- Pumping power
- Maintenance
- Water treatment
- Spare parts
- Expected operating hours
- Fuel cost strategy
- Required Operational Flexibility
If a vessel operates under highly variable conditions, a hybrid system may offer advantages because it can switch between open-loop and closed-loop modes when appropriate.
8. FAQs
Q1. What is the difference between open-loop and closed-loop scrubbers?
Open-loop scrubbers use untreated seawater and discharge the wash water directly to the sea, while closed-loop scrubbers use freshwater dosed with alkali, recirculate it, and only discharge small amounts of treated bleed-off water.
Q2. Which scrubber type is most common on ships?
Open-loop scrubbers are the most widely installed type due to their lower cost and simpler design, though many ports and emission control areas now restrict or ban their discharge.
Q3. Can a hybrid scrubber switch between modes at sea?
Yes, hybrid scrubbers can switch between open-loop and closed-loop operation depending on local water alkalinity and port discharge regulations.
Q4. What role do spray nozzles play in a desulfurization tower?
Spray nozzles atomize the scrubbing liquid into fine droplets that maximize surface contact with the flue gas, directly affecting SOx removal efficiency and pressure drop across the tower.
9. Conclusion
Marine SOx scrubbers are mainly classified into open-loop, closed-loop, and hybrid systems, each offering different advantages depending on vessel routes, washwater requirements, and operating conditions.
Choosing the right system requires consideration of compliance, operating costs, and flexibility. Proper spray nozzle selection is also essential for achieving uniform washwater distribution and effective gas-liquid contact inside the scrubber.
