CHEMICAL AND MINERAL VARIATION OF COAL AND COAL COMBUSTION PRODUCTS FROM A WESTERN POWER PLANT UTILIZING POWDER RIVER BASIN COAL

Coal quality, composition of stack emissions, and coal combustion products (CCPs) have become major environmental concerns as the rate of national coal utilization increases. Because of increasing emphasis on environmental issues, information on the quality of coal has become essential for understanding the potential for future domestic utilization. This study evaluates the chemical and mineralogical variations of feed coal and related CCPs from a western power plant that burns Powder River Basin coal.

A sampling plan was developed and a schedule was coordinated so that the pulverized feed coal samples could be matched as closely as possible with the CCPs that were generated from that coal. During a one month interval, 78 samples were collected from this plant. These samples included pulverized feed coal, economizer fly ash, fly ash, and bottom ash, as well as a sample of the lime used in the dry scrubber unit. An extensive suite of coal quality analyses, as well as mineralogical analyses, were performed on all samples taken from different phases of the coal utilization process. Ash content of the feed coal ranges from 6.55-12.6 percent, sulfur content ranges from 0.51-0.80 percent, and the calorific values range from 7,430-8,230 Btu/lb. Elemental variation indicates that when compared to the feed coal, the fly ash is slightly enriched in Cl, Hg, Se, and Ca, whereas the economizer and bottom ash are higher in Si, Al, Fe, K, and Ti.

Quantitative X-ray diffractometry, using a Rietveld-based computational method, was run for several series of samples to determine the mineralogy. Although the economizer and bottom ash have a similar mineralogy, the analyses show that the economizer fly ash contains trace amounts of anhydrite and is higher in gehlenite and quartz, whereas the bottom ash is higher in feldspar and pyroxene.The economizer and bottom ashes are different from the fly ash in that the fly ash does not contain feldspar, pyroxene, or hematite. The fly ash contains periclase and wustite, rather than hematite, and the Ca-based minerals hannebachite, ettringite, and calcite, which formed with the addition of lime downstream of the economizer. The lime had trace amounts of calcite and quartz. Most of the samples analyzed for mineral content in this study contain from 40 to 90 percent amorphous material.