ASPECTS OF THE CHEMISTRY OF CALCIUM-SULFATE IN COAL ASH

Equimolar mixtures of iron sulfide, calcium carbonate and other selected additives were ignited in air at 1200 K, and it was shown that about 70% of the sulfur was retained in the residue as calcium sulfate. These reaction conditions were selected to represent those applicable when calcium carbonate is used in fluidized bed combustion. Calcium oxide is efficient in trapping sulfur oxides. It was further shown, however, that under the same well defined reaction conditions alumino-silicates react with CaSO4, to some extent, to release a proportion of the sulfur (approximately 30%). This result is consistent with a previous conclusion that acidic oxides in coal reduce the efficiency of calcite for sulfur oxide removal from flue gases.