Transmission electron microscopy study of the sites of mercury, selenium, and arsenic in a kentucky fly ash

A combination of high-resolution transmission electron microscopy, scanning transmission electron microscopy, and electron energy-loss spectroscopy (HRTEM-STEMEELS) is a powerful tool in the study of elemental associations in nm-scale particles. HRTEMSTEM- EELS were used to study fly ashes produced from the combustion of an eastern Kentucky coal blend at a wall-fired pulverized coal utility boiler 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 o 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 nanoscale (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.