Marine emissions regulations: Promoting green practices
MARINE SCRUBBER SYSTEMS: OPERATING PRINCIPLES
Marine 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 marine scrubber, where an alkaline scrubbing material is present to neutralize the acidic nature of the gasses and remove any particulate matter from the exhaust. 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.
In most marine scrubbers, the design is such that the scrubbing liquid moves downstream; however, scrubbers with an upstream movement of scrubbing liquids are also available. The used scrubbing material is collected with wash water, which is discharged into the open sea after being treated in a separator to remove any sludge, while the cleaned exhaust passes out of the system.
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 to produce sulphur-based salts after treatment, which can be easily discharged as they are safe for the environment.
Marine scrubbers may use seawater or freshwater with added calcium/sodium sorbents, or pellets of hydrated lime, as the scrubbing medium due to their alkaline nature.
Types of marine scrubbers
Based on 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 does not affect the performance and effluent emissions of the scrubber. Open-loop scrubbers consume sea water in the scrubbing process.
Hybrid scrubbers can utilize 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.
TYPES OF MARINE SCRUBBER Systems
Open-loop scrubber systems uses seawater as the scrubbing and neutralizing medium; additional chemicals aren’t required. The exhaust stream from the engine or boiler passes into the marine 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, isn’t effective and cannot be used.
For these reasons, an open-loop marine scrubber is not considered a suitable technology for areas such as the Baltic, where salinity levels are not high.
Closed-loop scrubber systems work on similar principles to the open-loop system: it uses fresh water treated with a chemical (usually uhsodium 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 freshwater 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 processor or buffer tank, a holding tank through which discharge to the 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.
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