Sulphur occurs in coal in three forms:

• pyrite;
• organically bound sulphur; and
• sulphates.

During combustion, both the organic sulphur and pyrite are oxidised to sulphur dioxide (SO2). Depending on the combustion conditions, a small amount of sulphur trioxide (SO3) may also be formed. The sulphates represent a small fraction of the total sulphur in coal and have no significant role in the combustion process itself or in contributing to emissions. The amount of sulphur emitted from coal combustion is a complicated function of the relative amounts of pyrite and organic sulphur in the coal and the combustion conditions. Roughly speaking, 5–10% of the sulphur may be retained in the fly ash. The remainder, in the absence of emissions control, escapes to the atmosphere as SOx, mainly SO2.

Environmental legislation to control SO2 emissions has been adopted in more than 30 countries, while fuel switching to lower-sulphur coal had been chosen by many power generators in order to achieve compliance. However, where fuel switching is not an option or is inadequate, FGD is used to control SO2 emissions in pulverised coal-fired power plants. In commercial use since the early 1970s, FGD has become the most widely used technique to control SO2 emissions, next to the firing of low-sulphur coal. Numerous FGD systems are currently in use, with additional systems under development.

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