Changes in the Nature of Midwestern Fly Ash Over the Past Two Decades- I. Chemistry, Mineralogy and Beneficiation Potential

Changes in the Nature of Midwestern Fly Ash Over the Past Two Decades- I. Chemistry, Mineralogy and Beneficiation Potential

Grand Rapids, Michigan

Changes in the Nature of Midwestern Fly Ash Over the Past Two Decades: I. Chemistry, Mineralogy, and Beneficial Potential Authors Dr. Bob Jewell - United States - University of Kentucky Center for Applied Energy Research Ms. Anne Oberlink - United States - University of Kentucky Center for Applied Energy Research Dr. Thomas Robl - United States - University of Kentucky Center for Applied Energy Research Abstract Environmental regulations regarding coal combustion have become more stringent over time. The percentage of plants equipped with scrubbers in Kentucky increased from ~4% in 1978 to ~48% in 1997 to the point where flu gas scrubbers are essential to all plants regardless of coal type. Although still of importance in the production of coke, the ability to produce a “compliant” low-sulfur coal is no longer an advantage for central Appalachian coal mines in the electrical generation market. The past two decades have seen a substantial shift to higher sulfur and lower-cost Illinois basin coal in much of the Midwest. This has resulted in shifts in the overall composition of fly ash. The collection and examination of fly ash from six Kentucky power plants found a substantial increase in Fe2O3 (~14% to 30%) compared to previous collections where fly ash as low as 5% Fe2O3 was collected from plants that used low sulfur coal. This increase in Fe2O3 was concomitant with a decrease in Al2O3 (~16-19% compared to ~27-30%) and SiO2 (~36-45% compared to ~54-58% for low sulfur coals). Much of the higher sulfur content of the Illinois basin coal is in the form of pyrite, which, upon combustion, produces a fly ash predominantly composed of magnetite, Fe3O4. This has resulted in denser fly ash that is darker in color, ranging from olive brown to dark brown to almost black, compared to the grays and buff colors of the low-sulfur coal ash. More significantly, the increase in magnetite is at the cost of reduced glass content, which is essential to the pozzolanic reactivity of the fly ash.