Geochemical partitioning from pulverized coal to fly ash and bottom ash

Abstract

Fly ash and bottom ash chemistry is a function of the chemistry and mineral assemblages of the feed coal in addition to coal pulverization, boiler type and size, combustion parameters and combustion efficiency, and configuration of the ash collection system. Pulverization eliminates <1% of the raw feed coal, but, in reducing the amount of pyrite and hard rock in the feed coal, eliminates more that 10% of the As, Hg, and Se, among other hazardous elements, from the feed to the boiler. Pulverization also levels the variation in the boiler feed, smoothing the natural variation in ash chemistry. Arsenic is more concentrated in the cooler back rows of electrostatic precipitator (ESP) arrays where the fly ash particles are smaller. Mercury is captured by fly ash carbons; the fundamental factors being cool flue gas temperatures; the amount of carbon in the fly ash; and the distribution of fly ash carbon forms and their surface area. Selenium, a volatile element, exhibits some semblance of flue gas temperature control, but some ESP arrays have their highest Se concentrations in the relatively warmer 1st ESP row. The lanthanides, Y, and Sc (REYSc) are not volatile elements; their distribution is controlled by the fragmentation of the REYSc-bearing minerals at combustion temperatures, the incorporation of the fine minerals in Al-Si glass and other neoformed fly ash constituents, and the partitioning of those constituents among the ash-collection rows and by particle size within the individual samples.