Comparison of Open-Loop, Closed-Loop, and Hybrid EGCS

An Exhaust Gas Cleaning System (EGCS), commonly known as a scrubber, is installed on ships to remove sulphur oxides (SOx) from exhaust gases. It enables vessels to comply with international emission regulations while continuing to use high-sulphur fuel oil (HSFO).

1. Open-Loop EGCS

Principle of Operation

In an open-loop system, seawater is used directly as the scrubbing medium. The natural alkalinity of seawater neutralizes the sulphur oxides present in the exhaust gas. After scrubbing, the wash water is monitored and then discharged back into the sea.

Advantages

• Simple system design with fewer components.
• Lower capital investment compared with other scrubber systems.
• No requirement for chemicals such as caustic soda (NaOH).
• Lower operating and maintenance costs.
• Smaller equipment footprint and reduced space requirements.

Disadvantages

• Performance depends on the alkalinity of seawater.
• Less effective in fresh water, brackish water, or areas with low seawater alkalinity.
• Wash water is discharged overboard, creating environmental concerns.
• Many ports and coastal states restrict or prohibit open-loop discharge.
• Limited operational flexibility due to regulatory restrictions.

Preferred Applications

• Ocean-going vessels operating mainly in open seas.
• Regions with high seawater alkalinity.
• Ships seeking lower installation and operating costs.

2. Closed-Loop EGCS

Principle of Operation

A closed-loop system continuously circulates fresh water in a closed circuit. An alkaline chemical, usually sodium hydroxide (NaOH), is added to neutralize sulphur oxides. The wash water is cleaned, treated, and reused repeatedly. Only a small amount of treated bleed-off water is discharged or stored onboard for disposal ashore.

Advantages

• Independent of seawater alkalinity.
• Can operate effectively in seawater, fresh water, and brackish water.
• Suitable for ports where open-loop discharge is prohibited.
• Significantly reduces wash water discharge to the environment.
• Provides more consistent and reliable scrubbing efficiency.

Disadvantages

• Higher installation cost.
• More complex system with additional pumps, tanks, and treatment equipment.
• Requires storage, handling, and dosing of NaOH.
• Increased maintenance requirements.
• Higher power consumption.

Preferred Applications

• Ships frequently visiting ports with open-loop restrictions.
• Vessels operating in rivers, estuaries, and low-alkalinity waters.
• Ships requiring higher environmental compliance.

3. Hybrid EGCS

Principle of Operation

A hybrid EGCS combines the features of both open-loop and closed-loop systems. The operator can switch between the two modes depending on the operating area, water conditions, and local environmental regulations.

Advantages

• Provides maximum operational flexibility.
• Capable of worldwide operation under different environmental regulations.
• Open-loop mode allows economical operation in open seas.
• Closed-loop mode enables compliance in restricted ports and inland waters.
• Greater adaptability and reliability in varying operating conditions.

Disadvantages

• Highest capital and installation cost.
• More complex design and control systems.
• Requires additional space and equipment.
• Higher maintenance costs.
• Increased crew training requirements.

Preferred Applications

• Ships engaged in global trade with varying environmental regulations.
• Large container ships, tankers, and bulk carriers.
• Vessels requiring uninterrupted compliance in all operating areas.

Comparison of Open-Loop, Closed-Loop, and Hybrid EGCS