Table 5 Different scrubber systems and relevant features

Open loop seawater scrubbers

Water and Sulfur react to form Sulfur acid, which is neutralized with alkaline components in the sea water. Filters separate particles and oil from the mixture before the cleaned water is sent back into the sea. It typically uses seawater as the scrubbing medium and requires relatively large space on board. The negative characteristic of an open loop system is its greater energy consumption compared to a closed loop system, but there is no need for chemical additives like caustic soda in a closed loop system. Increase of sulfur in seawater would impact the water quality.

Closed loop scrubbers

This type uses freshwater with the addition of an alkaline chemical (such as caustic soda). Therefore, no wash-water is produced that would have to be pumped into the sea. This type requires more space than open loop systems.

Dry scrubbers

It uses a dry chemical, such as calcium hydroxide and Sulfur is locked in, meaning it cannot burden the biosphere at sea anymore. It does not use any liquids in the process but exhaust gases are cleaned with hydrated lime-treated granulates. Exhaust gas flows through granulated limestone. It combines with the Sulfur to form gypsum, which can then be disposed of on land. The storage room has to be created on board for granulate, which reduces cargo capacity. An advantage of a dry scrubber is its lower energy consumption compared to a wet scrubber.

Hybrid scrubbers

It gives the possibility to either use a closed loop or open loop technology. Hybrid scrubbers are generally used as an open loop system when the vessel is operating in the open sea and as a closed loop system when operating in harbor or estuaries, where water discharge is prohibited.