SCRUBBER SYSTEMS: OPERATING PRINCIPLES
Scrubbers have become an important, established technology in the maritime business since the introduction in 2015 of SOx-emission ECA zones in Europe and in North America.
HOW DO THEY WORK?
Exhaust gas streams are passed inside the scrubber, where an alkaline scrubbing material is present to neutralise the acidic nature of the gasses and remove any particulate matter from the exhaust.
The used scrubbing material is then collected with wash water, which may be stored or disposed of immediately as the effluent. The cleaned exhaust is passed out of the system and into the atmosphere.
The scrubbing material is chosen for its ability to remove specific impurities – such as SOx or NOx – by chemical reactions. For de-sulphurization purposes, marine scrubbers use lime or caustic soda in order to produce sulphur-based salts after treatment, which can be easily discharged as they are safe for the environment.
Scrubbers may use sea water, or fresh water with added calcium/sodium sorbents, or pellets of hydrated lime, as the scrubbing medium due to their alkaline nature.
Types of scrubbers
On the basis of their operation, marine scrubbers can be classified into Wet and Dry versions.
Dry scrubbers employ solid lime as the alkaline scrubbing material, which removes sulphur dioxide from exhaust gasses. Wet scrubbers use water, which is sprayed into the exhaust gas for the same purpose.
Wet scrubbers are further classified into closed-loop or open-loop. Closed-loop scrubbers can use fresh water or sea water as the scrubbing liquid. When fresh water is used, the quality of water surrounding the ship has no effect on the performance and effluent emissions of the scrubber. Open-loop scrubbers consume sea water in the scrubbing process.
Hybrid scrubbers can utilise both closed and open running modes, at the same time or by switching between the two. Seawater hybrid scrubbers can operate in closed or open mode, with seawater used as the scrubbing medium.