Observations and Assessment of Fly Ashes from High-Sulfur Bituminous Coals and Blends of High-Sulfur Bituminous and Subbituminous Coals: Environmental Processes Recorded at the Macro- and Nanometer Scale

Fly ash was investigated with a variety of chemical, mineralogical, petrographic, and microbeam techniques from three coal-fired units at two Kentucky power plants. Two units burn high-sulfur Illinois Basin high volatile bituminous (hvb) coal, and the third unit burns a ∼70:30 blend of high-sulfur Illinois Basin hvb coal and low-sulfur, relatively high-CaO Powder River Basin subbituminous coal. With high-S, high-Fe coals in all of the blends, spinel (magnetite) is an important constituent in the fly ashes. Overall, the fly ashes are dominated by glass. Portlandite was noted in the high-Ca-coal-derived ash. Concentrations of Ba and Sr are highest in the latter fly ash, a function of the Powder River Basin coal source for a portion of the blend. Rare earth elements do not have a high concentration in any of the fly ashes and do not show any significant partitioning between the electrostatic precipitator (ESP) or baghouse rows in the individual generating units. In contrast to previously studied fly ashes from plants burning hvb coals and to other fly ash specimens in this study, the fly ash from the unit burning the Illinois Basin/Powder River Basin coal blend did not have nanoscale carbon on the surface of the spherical inorganic fly ash particles. The absence of carbon may be a function of the nature of the feed coal, with 30% derived from the non-caking sub-bituminous component in the coal blend, although some contribution of carbon derived from caking hvb coal would be expected. The fly ash carbon content is very low, suggesting that the amount of carbon rather than or along with the rank of the coal may be a determining factor in the absence of nanoscale carbon deposition on the surface of the fly ash particles.