Since May 2005, specific environmentally friendly MARPOL regulations have been in force that stipulates that emissions from main and auxiliary machinery are kept within specific limits. They require, for instance, reduction of sulphur oxide combinations (SOx), carbon dioxide (CO2) and nitrogen oxide combinations (NOx). At the MPEC 58th it was decided that exhaust gas treatment system is approved for reducing sulfur (SOx) emissions from ships.
Some option available for reducing SOx emission today are: 1. switch fuel to a fuel with low sulfur content, 2. switching to natural gas as a fuel and 3. introduce an exhaust gas treatment plant, scrubber, which I intend to describe below.
Scrubber – Exhaust Gas Treatement
Scrubber is a system that is designed to by the use of water wash the exhaust gases from main, auxiliary and boilers to remove sulfur dioxide (SO2) which is a toxic gas, that is directly harmful to human health. There different designs in the market today however they could be divided into two categories, open and closed types. Power consumption of operating a scrubber system is typically between 1-2% of main engine power.
Open type Scrubber
The open type uses sea water to wash the exhaust gases. The wash water is then treated and discharged back to sea, with the natural chemical composition of the seawater being used to neutralize the results of SO2removal. Open seawater typically systems use 45m3/MW h for scrubbing.
Closed type Scrubber
The closed type uses fresh water in “closed” fresh water circuit that is treated with an alkaline chemical such as caustic soda neutralization and scrubbing. The wash water is re-circulated and the losses is made up with additional freshwater. A small quantity of the wash water is bled off to a treatment plant before discharge to sea. Typically closed freshwater systems have a discharge rate of 0.1- 0.3m3/MW h. The system could also be designed with a holding tank for zero discharge for a certain period.