MICRO- AND NANO-CHEMICAL CHARACTERIZATION OF FLY ASH PARTICLES

Fly ash from a coal-fired power plant was investigated to gain an understanding of potential environmental and health impacts associated with its emission as aerosols into the atmosphere. The studied ash, produced by combustion of bituminous coal, contains as main chemical components: SiO2 (51.4 wt%), Al2O3 (22.1 wt%) and Fe2O3 (17.2 wt%), and has a mean particle size of 12 micrometers. Only 10% of ash is larger than 45 micrometers, therefore, much of the ash particulate is in the respirable range (<10 µm). The small size of the ash particles makes it necessary to use scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and analytical transmission electron microscopy (AEM) to study the physical and chemical properties of the individual ash particles.

The measured weight content of magnetic particles is about 10%. X-ray diffractometry revealed that the magnetic fraction consists mainly of hematite and magnetite, while the nonmagnetic portion is composed of glass, quartz, and mullite. Individual fly ash particles were characterized quantitatively by EPMA and AEM in order to determine their trace and major element contents. In addition to hematite and magnetite, which usually occur as cracked crystalline spherical particles, the magnetic fraction also contains a small amount of Fe-rich glass (Fe2O3 16 wt%). This glass appears mainly as homogenous solid spheres which sometimes have an Al-rich rim. The nonmagnetic glass has a variable composition of mainly Al2O3 and SiO2, is Fe-poor (Fe < 9 wt%) with generally low concentrations of Ca. The glass spheres also have variable physical attributes; some are solid others hollow. It is common for spheres to contain many tiny spheres or have an extremely Al-rich core. Ca-rich crystals, including lime, were also identified in the nonmagnetic fraction of the ash.

Up to five percent of ash may be emitted as aerosol into the atmosphere. As commonly observed, many toxic elements and heavy metals are highly enriched in the fly ash relative to the original coal, with the smallest particles being the most enriched. The microscopic size and large physical and chemical variations of fly ash particles make the combination of EPMA and TEM a very powerful approach to help determine the environmental hazard posed by this ash.