MICROCHEMICAL CHARACTERIZATION OF FLY ASH PARTICLES

Fly ash from a coal-fired power plant has been investigated in order to gain an understanding of potential environmental and health impacts associated with its emission into the atmosphere. The power plant combusts bituminous coal from the Illinois basin, and the resulting ash is a low-calcium (2.8% wt % CaO), tan-gray Class-F fly ash. It contains as main chemical components SiO2 (51.4 wt%), Al2O3 (22.1 wt%) and Fe2O3 (17.2 wt%; ignited weight basis). As commonly observed, many toxic elements and heavy metals are highly enriched in the fly ash relative to the original coal. The studied sample contains, for example, considerable amounts of Be (16.4 ppm), Cu (106 ppm), Zn (578 ppm), As (40.4 ppm), Cd (2.6 ppm), Hg (18 ppm), Pb (71 ppm), and U (21.8 ppm). These concentrations are higher by factors of up to 50 relative to typical Illinois basin coal.

Since one to five percent of ash is emitted as aerosol into the atmosphere and deposited downwind from the power plant, it is important to study the chemical composition and physical form of individual fly ash particles. The mean particle size is 12 micrometers, and only 10% of the ash is larger than 45 micrometers. Scanning electron microscopy revealed mostly spherical particles, both solid and hollow. The measured weight content of magnetic particles (hematite, magnetite) is 21.8% (Diamond, 1985). X-ray diffractometry showed that the nonmagnetic fraction consists of glass, mullite, and quartz. Individual fly ash particles were characterized by quantitative electron probe microanalysis in order to determine their trace element contents. Two particle types were studied so far: one is Al- and Si-rich, the other is iron oxide. One of the most important findings is that the iron oxides are significantly richer in uranium than the Si-Al-rich particles (610 vs. 360 ppm U). This suggests that emissions of U-enriched aerosols could be considerably reduced by inserting an appropriate magnetic trap into the pathway of the flue gas.

Much of the aerosols emitted from coal-burning power plants is in the respirable range (< 10 micrometers). Because of their enrichment in toxic elements, the fly ash particles pose a hazard to the human lungs. A better characterization of these aerosols should help in reducing environmental and health impacts of coal-fired power plants.