The partitioning of arsenic during pulverized coal combustion

An important issue facing the coal-fired electric utility industry is the environmental impact resulting from the release of arsenic contained in the coal. Depending on the forms of occurrence in the coal and transformation mechanism involved, arsenic-containing compounds may exit the furnace in the vapor, as part of submicron size aerosol particles, or as part of larger, supermicron particles. The specific path by which arsenic transforms from the vapor phase to the condensed phase is important in assessing the toxicity of the waste product. This paper documents research exploring the partitioning of arsenic during the combustion of five well-characterized pulverized coals under commercially relevant conditions in a well described laboratory combustion environment. Insight is gained into the timing of partitioning because this combustion system allows comparison of furnace vapor and particulate at different locations in the furnace. The partitioning of arsenic is governed primarily by the extent of arsenic volatilization during combustion. Volatilized arsenic will heterogeneously transform to both submicron and supermicron particles primarily by reaction with active cation sites. For many coals, arsenic appears to be highly reactive with active calcium sites to form calcium arsenates. The transformation process is typically dominated by surface reactivity rather than by gas film diffusion. Due to the similarities in the solubility characteristics of As2O3 and calcium arsenate, leaching studies provide little insight into the form of arsenic present on fly ash surfaces.