Sulphur Removal During Combustion
A number of technologies to prevent the production and release of SO2 during combustion have been developed over the past decade, but very few have achieved wide commercial application to date. The most developed are the Fluidised Bed Combustion (FBC) process and the integrated Gasification Combined Cycle (IGCC) system.
Fluidised Bed Combustion
This process involves the combustion of coal in a bed of inert material such as sand, with air being blown up from beneath the bed at high velocities. As velocity increases individual particles begin to be forced upwards until they reach a point at which they remain suspended in the air stream. The bed in this state behaves like a liquid and can be described as fluidised. Tubes containing water are immersed in the bed to absorb the generated heat (this water is converted to steam which is used to drive the steam turbine and thus produces electricity). The fluidised movement within the combustion chamber results in a greater heat transfer efficiency to the water filled tubes and therefore operating temperatures are lower than in a conventional system. SO2 emissions can be controlled in this system by adding a sorbent (a substance used to absorb any SO2 present, for example lime or limestone) to the bed of inert material. The limestone effectively absorbs the SO2 as it is released from the coal and retains it within the ash, which is removed regularly. The low combustion temperatures allow efficient combustion to take place without causing the ash to soften, thereby allowing easy removal of the ash containing the absorbed SO2.
The FBC can achieve in the region of 80 - 90% SO2 removal. Two main disadvantages of this system are firstly the large quantities of sorbent required (approximately twice that of an FGD system (see later) to achieve the same SO2 removal), and secondly the large quantities of strongly alkaline waste produced, which is generally disposed of in landfill.