New emissions rules being introduced in 2015 means ship owners worldwide are faced with a choice on how to comply. From January 2015 sulphur emissions limits will reduce 0.1%, leaving ship owners with two options to comply with the new regulations – switch to a low sulphur content fuel or use scrubbers to remove sulphur particulates.
Scrubbing systems remove the sulphur content of fuel from the exhaust gases after they’ve been burnt, meaning vessels can go on using their existing fuel.
The process of scrubbing exhausts has been used since the 1930s in industrial plants and marine vessels. The scrubbing process uses a fluid containing alkaline material which can absorb SOx and neutralise it. After this process the clean exhaust gases are released and the resulting waste product, or sludge, is stored on board and transferred on shore.
There are two types of scrubbing– wet scrubbing systems and dry scrubbing systems.
Wet Scrubbing Systems
Wet scrubbing systems use a combination of three types of water in the process of removing SOx from exhaust gases:
- Seawater is used in some processes, because of its natural alkaline content. Water is drawn from the sea, used in exhausts to absorb sulphur from exhaust emissions before being returned to sea. Before it’s returned any oil and solid matter is removed.
- Freshwater is often used on vessels where the natural alkalinity of seawater is not sufficient to react to the sulphur found in exhaust emissions. This process requires the addition of caustic soda (NaOH) which reacts with and absorbs sulphurous gases. Freshwater scrubbing reduces SOx emissions by 97.15%
- A hybrid scrubbing system can also be used, which is a combination of both the above methods. Using this system can improve SOx emissions cleaning performance by 98-100%
Dry Scrubbing Systems
On-board dry scrubbing systems use granular hydrated (slaked) lime, which is converted to calcium sulphate, a by-product with commercial value in shore-side industry.
Scrubber units generally consist of a vessel or vessels that enable the exhaust streams from one or more engines or boilers to intimately mix with the ‘washwater’ or dry chemical.
Scrubber vendors typically quote the maximum sulphur content of the fuel that can be consumed by an engine so that emissions equivalent to using 0.1% sulphur fuel can be achieved. This varies between 3% and no upper limit, which in reality means that very high sulphur oxide removal rates of over 98% are possible.
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