Partitioning of arsenic, selenium, and cadmium during the combustion of Pittsburgh and Illinois #6 coals in a self-sustained combustor

An important environmental issue is the emission of semi-volatile toxic metals, such as arsenic, selenium, and cadmium, from the combustion of coal. These materials may vaporize in the hot portions of the combustor then return to the solid phase in cooler zones of the process downstream. Understanding the mechanisms by which toxic metals partition between the vapor and solid phases is an important step for predicting and mitigating the effect of these metals upon the environment. Particulate ash samples were withdrawn from a 17-kW pilot-scale downflow combustor in which pulverized coal was burned under self-sustaining conditions. The samples were size segregated in a Berner low pressure impactor, and then analyzed using neutron activation. This research approach has suggested mechanisms, which govern the partitioning of arsenic, selenium, and cadmium in practical pulverized coal combustion processes. The results suggest that volatilization and subsequent transfer of selenium to submicron particle surfaces appears to be an important post-combustion phase mechanism for Illinois #6 coal but not for Pittsburgh seam coal. Most of the selenium in the Pittsburgh submicron fly ash, cadmium in Illinois #6 submicron fly ash, and arsenic in both Pittsburgh and Illinois #6 submicron fly ash enters the post-combustion zone in the solid phase. The dominant heterogeneous partitioning mechanism for transformation to large, supermicron particles is the reaction of metal vapor on the surface or within the pores of an ash particle. The results also suggest that the rate of transformation is dominated either by an exterior surface reaction-controlled regime or a pore diffusion-controlled regime. A relationship between the concentration of solid phase arsenic, selenium, and cadmium to calcium in supermicron particles was also observed, suggesting the formation of As–Ca, Se–Ca, and Cd–Ca reaction products.