How does a closed-loop scrubber work?

A closed-loop scrubber is an exhaust gas cleaning system (EGCS) that controls air pollution on a vessel. In order to comply with the strict IMO 2020 sulfur cap, this method is critical as it reduces sulfur oxides (SOx) emissions from marine exhaust gas streams by as much as 98 %. However, a closed-loop scrubber consists of various elements and complex processes, and let us dive into how a closed-loop scrubber works.

When a ship operates, its engine, auxiliary engine, or boiler generates exhaust caused by combustion processes. Before the shipping industry focused on desulfurization, the exhaust gas went directly into the atmosphere, which caused acid rain, harming people, wildlife, and the planet. Today, the exhaust gas can be cleaned with an exhaust gas cleaning system employing unique scrubber technology. A closed-loop scrubber can remove up to 98 % of sulfur from marine exhaust gases, ensuring a sustainable operation. And more essentially, it ensures compliance with the IMO 2020 sulfur cap by lowering SOx emissions from 3.5 % to less than 0.1 %, allowing ships to operate within the ECA.

There are three types of scrubbers; open-loop, closed-loop, and hybrid. Read more about them and how they differ here.

Closed-Loop Scrubber

The exhaust gas streams move through a scrubber tower in a closed-loop scrubber. The exhaust gas intake is always at the bottom of the scrubber tower as this design ensures that the SOx emissions in the exhaust gas stream pass through the scrubbing liquid to deliver the most thorough scrubbing. The exhaust gas is scrubbed with a watery substance utilizing many nozzles inside the scrubber tower. A closed-loop scrubber typically employs seawater with added alkalies such as sodium hydroxide (NaOH), also known as caustic soda, or magnesium hydroxide (MgOH) to scrub the exhaust gases. Yet, a closed-loop scrubber can also utilize seawater. The many nozzles ensure that the alkaline-dosed seawater is distributed effectively. By scrubbing the exhaust gas, 98 % of sulfur is removed from the exhaust gases. The SOx are turned into sulfates such as Na₂SO₄, which are harmless to the environment. In addition, nitrogen oxides (NOx) and particulate matter are also reduced. Learn more about what a scrubber removes here.

Closed-Loop Scrubber Process

At the top of the scrubber tower, a continuous emission monitoring system (CEMS) is placed to monitor the emission outlet concerning the IMO 2020 sulfur limit to ensure constant compliance. The cleaned exhaust gas stream can pass out of an inlet at the top of the closed-loop scrubber.

The wash water will move downstream to the process tank. Through automatic monitoring and control, alkali is automatically dosed directly into the process tank to maintain the process pH and, thereby, the SOx removal efficiency. Dosage can also be inline before the scrubber. The alkaline and the specific amount highly depend on where and how the vessel operates. This pH adjustment is the primary environmental advantage as it ensures to neutralize the sulfuric wash water. Additionally, the water quality is reached by maintaining a low temperature by cooling the scrubber water with fresh seawater in a heat exchanger.

A small amount of the wash water is bled off from the process tank to a water treatment unit (WTU) to remove the accumulated impurities from the scrubber wash water. The treatment of scrubber wash water consists of suspended solids removal. The WTU will treat the bleed flow from the scrubber’s process tank to remove polycyclic aromatic hydrocarbons (PAH), particulate matter (PM), and harmful elements stemming from the combustion process, such as oil, ash, and soot.

A water monitoring unit (WMU) is attached to continuously monitor and control pH, temperature, turbidity, and PAHs in relation to IMO regulations on scrubber water discharge.

The scrubber sludge is led to a holding tank, where it is stored until land-based disposal for a fee. The sludge can be dewatered with a filter press for further solid and liquid separation to reduce the weight of the sludge. Reduced sludge weight will lead to reduced sludge handling costs. This will deliver a dry filter cake consisting of up to 80 % dry matter, easy to dispose of onshore. The rejected water is then recirculated back into the scrubbing process.

Marine Knowledge E-book

If you want to learn more about the marine industry, you can download our Marine Industry Knowledge e-book.

Learn everything worth knowing about marine scrubbers, wash water treatment, and how to comply with the IMO 2020 sulfur cap.

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