Mineralogy and Chemical Composition of High-Calcium Fly Ashes and Density Fractions from a Coal-Fired Power Plant in China

To understand the formation mechanism of high-calcium fly ashes, the mineralogical, physical, and chemical properties of several high calcium fly ashes and their different density fractions (<1.0, 1.0−2.5, 2.5−2.89, and >2.89 g/cm3) from a coal-fired power plant were characterized by X-ray diffractometry (XRD), field scanning electron microscopy equipped with energy dispersive X-ray analysis (FSEM-EDX), and X-ray fluorescence spectroscopy (XRF). The occurrence of calcium in coal was determined using sequential extraction tests. The results show that the carbonate-bonded calcium is the dominant species in Xiaolongtan coal, and the ion-exchangeable calcium only occupies 19.2% of total calcium. The major calcium-bearing minerals in low temperature ash (LTA) of the feed coal, lignite from the Yunnan province, include calcite, bassanite, and dolomite. The fly ashes examined contained aluminosilicates with a high concentration of calcium oxide. The major minerals include mullite, quartz, lime, anhydrite, and gehlenite, and the minor minerals are comprised of hematite, magnetite, akermanite, portlandite, and larnite. Minerals in the density faction less than 1.0 g/cm3 consist of lime, calcite, anhydrite, and clay; between 1.0−2.5 g/cm3, quartz, mullite, anhydrite, and gehlenite; between 2.5−2.89 g/cm3, anhydrite, lime, gehlenite, hematite, and quartz; and greater than 2.89 g/cm3, larnite, gehlenite, anhydrite, brownmillerite, and some heavy minerals. In accordance with the microstructural characteristics of the fly ash particles, high-calcium fly ash can be classified into several groups, namely hollowed smooth particles, dense particles, agglomerate particles, porous particles, plerosphere, and other particles with complex surface characteristics. On the basis of chemical composition, high-calcium fly ashes can be classified into four groups namely: calcium oxide, calcium sulfates, Ca−Al−Si compounds, and Ca−S−X (X: Fe, Al, Si, Mg, etc.) compounds. Calcium oxide and calcium sulfates are mainly derived from the original calcium-bearing minerals in coal, whereas Ca−Al−Si and Ca−S−X compounds are formed by the secondary reaction of CaO and CaSO4.