The International Convention for the Prevention of Pollution from Ships (MARPOL), 1973 as modified by the Protocol of 1978, also known as MARPOL 73/78, Annex VI Prevention of Air Pollution from Ships, requires the reduction of SOx emissions either by burning low sulfur fuel oil or cleaning the exhaust gas.
The use of scrubbers to clean the exhaust from marine engines using high sulphur residual oil and diesel fuels is a more economical option for reducing SOx air emissions required by Annex VI of the MARPOL International Convention for the Prevention of Pollution from Ships.
The EGCSs used today for sulphur control are commonly known as SOx scrubbers. In a wet scrubber, the exhaust gas is mixed with washwater, and the water-soluble components of the exhaust gas are removed by dissolution into the washwater. SOx scrubbers are capable of removing up to 95 percent of SOx from ship exhaust (EGCSA, 2010).
By removing sulphur from the exhaust, the scrubber also removes most of the direct sulphate particulate matter (PM). Sulphates are a large portion of the PM from ships operating on high sulphur fuels. By reducing the SOx emissions, the scrubber also controls much of the secondary PM that is formed in the atmosphere from these emissions.
Wash water generated by an EGCS can contain contaminants from three sources:
- Pollutants scavenged from the exhaust gas exiting the engine (combustion products, fuel and lubricants);
- The source of washwater used to clean the exhaust (seawater or freshwater); and
- The scrubber itself (dissolution of materials, possible reaction products and/or chemical additives).
The deployment of SOx scrubbers to treat emissions from diesel engines on large ocean going vessels has been so far limited to a handful of ships.
However, the use of SOx scrubbers in combination with high sulfur residual oil fuels may be an economically attractive option in Sulfur Oxide Emission Control Areas (SECAs) and worldwide.
As the deadlines for SOx reduction draw nearer, some vessels may begin using scrubbers to reduce SOx emissions rather than burning low sulfur fuel oil