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.
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. On the basis of their operation, marine scrubbers can be classified into Wet and Dry versions.
Types of scrubbers
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.
Inside a wet scrubber, the scrubbing liquid used may be sea water or fresh water with chemical additives. The most commonly used additives are caustic soda (NaOH) and Limestone (CaCO3). Scrubbing liquid is sprayed into the exhaust gas stream through nozzles to distribute it effectively.
In most scrubbers, the design is such that the scrubbing liquid moves downstream; however, scrubbers with an upstream movement of scrubbing liquids are also available.
The exhaust intake is either on the side or the bottom of the tower. The designs ensure that the sulphur oxides present in the exhaust are passed through the scrubbing liquid, reacting with it to form sulphuric acid.
When diluted with alkaline seawater, sulphuric acid (which is highly corrosive in nature) can be neutralized. The wash water is discharged into the open sea after being treated in a separator to remove any sludge and the cleaned exhaust passes out of the system.
Mist eliminators are used in scrubbing towers to remove any acid mist that forms in the chamber by separating droplets that are present in the inlet gas from the outlet gas stream. MARPOL regulations require that the wash water be monitored before being discharged to ensure that its PH value is not too low.
This system uses seawater as the scrubbing and neutralizing medium; additional chemicals aren’t required. The exhaust stream from the engine or boiler passes into the scrubber and is treated only with alkaline seawater – the volume of this seawater depends upon the size of the engine and its power output.
This system is extremely effective but requires a large pumping capacity as it uses a large amount of seawater. An open-loop system works well when the seawater used for scrubbing has sufficient alkalinity, but seawater with a high ambient temperature, as well as fresh or brackish water, aren’t effective and cannot be used.
For these reasons, an open loop scrubber is not considered a suitable technology for areas such as the Baltic, where salinity levels are not high.
The closed-loop system works on similar principals to the open-loop system: it uses fresh water treated with a chemical (usually sodium hydroxide) instead of seawater as the scrubbing media. The SOx from the exhaust gas stream is converted into harmless sodium sulphate.
Before being re-circulated for use, the wash water from a closed-loop scrubber system is passed through a process tank where it is cleaned. Ships can either carry fresh water in tanks or generate the required water from freshwater generators on board.
Small amounts of wash water are removed at regular intervals to holding tanks, where fresh water can be added to avoid the build-up of sodium sulphate in the system. A closed-loop system requires almost half the volume of wash water than that of the open-loop version, but the system does require more tanks.
These include a process or buffer tank, a holding tank through which discharge to sea is prohibited, as well as a storage tank capable of regulating its temperature between 20º and 50ºC for the sodium hydroxide, which is usually used as a 50% aqueous solution.