TRANSMISSION ELECTRON MICROSCOPY STUDY OF THE SITES OF MERCURY AND OTHER TRACE ELEMENTS IN A KENTUCKY FLY ASH

A combination of high-resolution transmission electron microscopy, scanning transmission electron microscopy, and electron energy-loss spectroscopy (HRTEM-STEM-EELS) is a powerful tool in the study of elemental associations in nm-scale particles. HRTEM-STEM-EELS were used to study fly ashes produced from the combustion of an eastern Kentucky coal blend at a wall-fired pulverized coal power plant in southeastern Kentucky. The fly ash was collected from individual hoppers in each row of the pollution-control system, with multiple hoppers sampled within each of the three rows of the electrostatic precipitators (ESP). Temperatures within the ESP array range from about 200 C at the entry to the first row to <150 C at the exit of the third row. HRTEM-STEM-EELS study demonstrated the presence of nano-scale (10's nm) graphitic carbon agglomerates with typical soot-like onion-ring structures, juxtaposed and intergrown with slightly larger aluminosilicate spheres. These agglomerates form an ultra thin deposit on the coarser fly ash particles routinely observed via optical microscopy and scanning electron microscopy (SEM). The STEM-EELS analyses revealed that the soot-like carbon agglomerates host even finer (< 3 nm) particles, some of which appear to be Fe-spinels. Elemental analysis indicated an association of Hg, possibly in sulfide form, with carbon. Arsenic, Se, Pb, Co, and traces of Ti and Ba are associated with these Fe-rich particles.