PETROLOGY AND CHEMISTRY OF FLY ASHES FROM A WESTERN KENTUCKY POWER PLANT: LEGACY OF SHIFTING FUEL SUPPLIES

A utility power plant in western Kentucky has changed coal supplies over the past 30 years. Originally built to burn high-sulfur Illinois Basin coals, the fuel supply was switched to low-sulfur Central Appalachian coal in the 1980's, low-sulfur western United States bituminous coal in the later 1990's, to the current blend of western United States bituminous coal and subbituminous Powder River Basin coal. The overall objective of the fuel switching was to lower sulfur dioxide emissions and avoid installing flue gas desulfurization in the nine pulverized-fuel150-MW units.

The changing coal supply has influenced the nature of the fly ash. In all cases, the ash petrology is dominated by Al-Si glass. Combustion of the high-S coal produced a fly ash with a moderate content of Fe-oxides. The glass content of the fly ash increased with the shift away from high-S coal. Carbon content of the fly ash generally increased following the 1990's installation of low-NOx burners. The nature of the fly ash carbon changed with the introduction of subbituminous coal to the fuel blend. Carbons from bituminous coals take the form of inertinite, the macerals passing through the boiler with little or no physical change, and isotropic and anisotropic cokes derived from the thermoplastic melting and repolymerization of vitrinite and low-reflectance semifusinite. Carbons from subbituminous coals are generally chars which have not passed through a plastic phase. Subbituminous chars have a greater capacity to adsorb Hg. The distribution of volatile trace elements other than Hg generally follows a path of enrichment towards the cooler parts of the ash collection system.