Reduction of Sulfur from Fuel Oil and Exhaust Gasses
The two main pollutants are sulfur oxides (SOx) and nitrogen oxides (NOx), and several methods are being used reduce these emissions.
Reduction of Sulfur Content in Heavy Fuel Oil (HFO)
Heavy fuel oil is a by-product of the process of the refining of crude oil and is supplied to the industry from the residue of the distilling columns, known as “bottoms." This is in fact the bottom of the barrel and contains a few nasty contaminates, among which is sulfur.
So, this HFO can be reformulated to reduce the sulfur content before we load it as ships bunkers- but at a cost.
This increase in price, along with the general rise in fuel prices, is too much for some of the shipping companies. However there is another method of removing the sulfur, by post combustion treatment through scrubbing the exhaust gas fumes explained below.
Use of Scrubbers to Reduce Sulfur Oxides from Exhaust Gasses
Scrubbing the exhaust gasses does not come cheap either, but once the plant is installed the ship's engines can be run on the normal, cheaper HFO.
The scrubber is a vertical pressure vessel that is normally located in the funnel as there is usually space to fit it there, as well as being convenient to the exhaust piping to the silencer before exit to atmosphere.
There are several types of gas water scrubbers suitable for use in ships.
- Seawater Washed
- Fresh Water Washed
Both systems use the same principle of spraying water onto the outgoing exhaust gas streams as they pass up through the scrubber perforated plates, with the resulting gasses being up to 99% sulfur free, regardless of the sulfur content in the fuel being combusted.
The seawater scrubber works on the principle that seawater is alkaline from containing insoluble calcium salts. It is these salts that react with the sulfur dioxide in the exhaust gasses, neutralizing them before falling down into a storage tank. Modern plants supply more seawater to this solution, with the added alkali further reducing the SOx levels in the water contained in the tank.
There is an added bonus when scrubbing to remove SOx as the particulates in the gas are also removed. These are collected by seawater spray, before these too fall into the storage tank.
An efficient wash-water plant removes most of the solids before returning the solution to the sea; the remaining solids are pumped ashore for disposal while in port.
The freshwater scrubber works in a similar manner, only this time sodium hydroxide (NaOH) is added to the freshwater in a mixing/holding tank. The resultant solution is then sprayed into the exhaust gasses as they pass up through the scrubber, once again neutralizing the SOx in the gasses.
The resultant solution is then subjected to water wash treatment, with the clean water being pumped overboard with the effluent/solids mix going to a sludge tank for disposal ashore.
A typical fresh water gas scrubber is shown below. (Please click on the image to enlarge.)