Last Updated: Apr 2012
What is it?
An exhaust gas cleaning system - often referred to as scrubbing technology - is a air pollution control device that is used to remove some particulates and/or gases from exhaust streams.
How it works
There are a number of systems gaining industry attention (visit the Exhaust Gas Cleaning Systems Association (EGCSA) for some of the companies developing the systems).
MARPOL Annex VI sets limits on sulphur oxides (SOx) and nitrogen oxides (NOx) from ship exhausts and prohibits deliberate emissions of ozone depleting substances. Exhaust gas cleaning systems act as an air pollution control device that can be used to remove some particulates and/or gases from industrial exhaust streams.
Scrubbers are an accepted technology that have been used on land-based power plants since the 1930s. Sea water scrubbers have been used at sea since the 1960s, but originally went under the name 'inert gas generators'.
Scrubbers traditionally use liquid to wash unwanted pollutants from the ship's funnel. The main pollutants removed are SOx and particulate matter (PM), and in some cases small amounts of NOx, and carbon dioxide (CO2).
The removal of CO2 using scrubbers has been contested. However, one Singapore-based company previously claimed that its CSNOx system (link: http://www.sustainableshipping.com/news/i90271/Ecospec_on_mission_to_Copenhagen) can remove carbon dioxide (CO2) by about 75% using electrified seawater.
The basic principle of scrubbers is that the flue gases are exposed to sprayed water, seawater or a solution which dissolves the target emissions or particulate matter with the scrubbing solution.
Pollutant material removed from the exhaust is carried in the washwater. SOx then react with the seawater to form stable compounds. When discharged, the moderately acidified water will typically mix with the ambient water.
Scrubbing provides a short circuit to the the whole SOx cycle, and returns the sulphur to the sea. To prevent environmental harm due to the efflux water, the US Environmental Protection Agency and the International Maritime Organization (IMO) have adopted regulations for the mixing of scrubbing effluents with ambient seawater.
Dry scrubbing, unlike wet scrubbing, does not saturate the flue gas stream that is being treated with moisture.
In the so-called dry sorption process the exhaust gas is fed through a fixed-bed filter filled with calcium hydroxide granulate. This neutralises sulphur oxide such as sooty particles. NOx is then split catalytically in a catalytic converter, just as it is in cars.
Advanced maritime emissions systems differ as they do not need the ship to be modified and can treat ocean-going vessels while at anchorage or berthed for unloading and loading cargo. The ship's exhaust gas is captured by a bonnet that attaches to and surrounds the ship's stack. The captured emissions are sucked through a duct to an emission treatment subsystem for removal of toxic pollutants.