Geochemistry and Mineralogy of Coal-Fired Circulating Fluidized Bed Combustion Fly Ashes

The fuel, bed ash, and fly ash were sampled from a circulating fluidized bed combustion (CFBC) unit at two times. The first sampling was a high-sulfur (S) coal-only run, and the second sampling coincided with an experimental burn of up to 10% switchgrass (Panicum virgatum) pressed pellets mixed with a high-S coal. The latter blend had a higher moisture content and a lower heating value than the coal-only fuel. Given the time between the samplings and the special needs for the experimental run, unavoidable changes in the coal and limestone complicate comparisons of the bed ash and fly ash chemistry between the sampling times. The bed ash is dominated by CaO and SO3, and the fly ash has a higher CaO content than would be expected for a pulverized-coal burn of the same coal. The fly ash chemistry bears a superficial resemblance to class C fly ashes, but given the different combustion conditions and consequent differences in the ash mineralogy, the fly ash should not be considered to be a class C ash. The bed ash mineral assemblages consist of anhydrite, mullite, portlandite, and anorthite, while the fly ash has less portlandite and more anorthite than the bed ash.