Flue Gas Desulphurisation

Exhaust gasses can be treated to remove sulphur before the gasses are emitted. The basic chemical process is generally to mix the gases with a compound containing calcium, so the SOx is converted to Calcium Sulphate. Calcium sulphate – gypsum – is recovered and disposed of (in landfill) or used in building materials. It is a significant component of plasterboard and plays a role in cement manufacture.

On-land, the calcium often comes from limestone or chalk. After mining and transport it is made into a slurry, and mixed with the exhaust gasses. The mixing technology varies, and may comprise a “spray curtain” or “bubble bath” through which the exhaust gasses pass. This is generally known as a scrubbing process. The reactions to form sulphates from SOx are very fast and efficient, and can often be achieved in compact single pass equipment. 95% reduction of SOx is readily achieved, so scrubbing the exhaust from 4% S fuel is equivalent to using 0.2%S fuel. Further processing is often required to remove the water that may have joined the gas-stream.

In plants near coasts, the spray material can be sea water – hence the technology become know as SeaWater Scrubbing or SWS. Seawater already includes high levels of sulphates so the additional sulphur, once diluted, has negligible impact, generally too small to detect.

Ships clearly have the advantage of plenty of seawater, but face special challenges:

• High gas speeds and limited space for the scrubber. However, current technology can replace the conventional silencers and so can fit within current space constraints. Clearly, the equipment needs to cope with the movement of the ship. On land, space is rarely such a significant constraint and the platform does not rock about.
• Acidity and high temperatures. The acidity of the scrubbing water is increased, so the materials used must be particularly resistant to acid.
• Cleaning the scrubbing water. A significant benefit of the scrubbing process is that it also removes many other materials from the flue gas, in particular particulates. However, these materials cannot safely be discharged so the scrubbing water has to be treated.

These challenges do not present insuperable obstacles, but, in the absence of commercial incentives and a secured market, only a very few people have so far tried.