Exhaust Gas Cleaning Systems

The three main types—open-loop, closed-loop, and hybrid scrubbers—each offer distinct advantages and applications.

Open-loop scrubbers are cost-effective and suitable for use in open ocean waters, closed-loop systems provide compliance in sensitive or regulated waters, and hybrid scrubbers offer flexibility for ships operating in diverse environments. The choice of scrubber depends on the ship’s operating regions, environmental regulations, and the owner’s commitment to environmental sustainability.

A. Open-Loop Scrubbers

Open-loop scrubbers use seawater directly to remove SOx from the exhaust gases. The exhaust gases are funneled through the scrubber tower, where they come into contact with seawater. The sulfur oxides react with the seawater, forming sulfuric acid, which is then neutralized by the natural alkalinity of the seawater.

The treated water, now containing neutralized sulfur compounds, is discharged back into the ocean, provided it meets environmental discharge standards.

Open-loop scrubbers are commonly used on ships operating in areas with high alkalinity seawater, such as open oceans, where the environmental impact of discharging treated water is minimal. They are relatively simple in design and operation, with lower installation and operating costs compared to other types of scrubbers. Their use is restricted in certain areas, such as coastal waters, ports, and sensitive environmental regions, where discharge regulations are stricter.

B. Closed-Loop Scrubbers

Closed-loop scrubbers use a recirculating water system, typically with an alkaline additive (e.g., sodium hydroxide), to neutralize sulfur oxides in the exhaust gases. The treated water is then reused in the system after passing through a water treatment unit.

Unlike open-loop systems, closed-loop scrubbers do not discharge process water into the sea. Instead, they retain the wash water on board, treating and reusing it, with only a minimal amount of sludge or waste that needs to be disposed of at port facilities.

Closed-loop scrubbers are ideal for vessels operating in areas with strict environmental regulations, such as coastal waters, ports, and emission control areas (ECAs). They provide greater flexibility in complying with discharge regulations and are effective in all types of water conditions, including freshwater and low alkalinity seawater.

Closed-loop systems are more complex and costly to install and operate due to the need for additional components, such as a water treatment system and storage tanks for sludge and treated water.

C. Hybrid Scrubbers

Hybrid scrubbers combine the features of both open-loop and closed-loop systems, allowing the operator to switch between modes depending on the ship’s location and environmental regulations. In open-loop mode, the system operates like a traditional open-loop scrubber, using seawater for scrubbing and discharging the treated water overboard. In closed-loop mode, the system recirculates and treats the wash water internally, with no discharge to the sea.

The ability to switch between modes provides operational flexibility, enabling compliance with varying regional and global environmental regulations.

Hybrid scrubbers are suitable for vessels that operate in diverse regions, including areas with stringent discharge regulations and open waters where discharge is permitted. They offer the benefits of both open-loop and closed-loop systems, providing operational flexibility and ensuring compliance with a wide range of environmental regulations. Hybrid systems are typically more expensive to install and maintain due to their complexity and the need for additional components.

Exhaust gas cleaning systems, or scrubbers, play a vital role in reducing sulfur oxide emissions from ships, helping the maritime industry meet stringent environmental regulations.