SO_x emission levels can be calculated from coal sulphur content. Ash chemistry and ash content have some influence, as some absorption of SO₂ takes place in the ash. The level of absorption increases with increasing alkalinity levels in ash (ie. high levels of CaO, MgO, etc.) and increasing ash content, however it is unlikely that absorption levels would ever exceed 10-15% of the total SO₂ produced. SO₂ emissions are additive and therefore can be calculated from the sulphur content of the component coals with any SO₂ absorption onto the ash estimated from the ash chemistry of the component coals.

SOx emissions may be reduced by direct injection of a sorbent directly into the furnace chamber and/or by flue gas desulphurisation (FGD) systems attached to the back end of the boiler. All FGD systems involve intimately mixing the SOx emissions with an absorbing chemically reactive material, which converts them into a solid or liquid product. This product may have a marketable value.