Quantitative analysis of high temperature toxic metal sorption rates using aerosol fractionation

Abstract

The airborne emission of toxic metals, such as lead, arsenic, and cadmium, from coal-fired boilers and incinerators is a major problem. These metals, which are considered to be semivolatile, are vaporized in the flame and subsequently form submicron particles. Since submicron particles are difficult to remove, one strategy to prevent emission is to inhibit ultra-fine particle formation within the combustor. Recent work has demonstrated that the sorbents such as kaolinite and lime are able to capture semivolatile metal vapors at high temperatures, thus preventing nucleation and condensation at lower temperatures. However, to develop practical scale technology, the rate of metal sorption by sorbents must be quantified. In this paper, an aerosol size fractionation approach is proposed to determine the extent of metal vapor-sorbent interactions above the metal dew point. The applicability of this methodology is illustrated by determining the rate of capture of cadmium by powdered, kaolinite sorbent at high combustion temperatures. An estimate of the rate of reaction was found to be 0.8 x 109 cm3 gas/mol sorbent· s. A simple competitive reaction model was constructed to investigate the competition between sodium and cadmium vapor sorption on a dispersed kaolinite substrate. Previous work had demonstrated that the presence of sodium hindered the sorption of cadmium by kaolinite. Results of the competition model suggest that sodium not only competes for available sorbent but also inhibits the sorption of cadmium when both metals are present.