Vaporization of arsenic, selenium and antimony during coal combustion

Accumulation of toxic trace elements generated by coal-fired power stations presents a serious threat to the environment. Field testing and laboratory studies have revealed the existence of trace elements in submicron particles emitted from power stations. Arsenic, selenium, and antimony are present in the submicron particles presumably via a vaporization-condensation pathway although the volatilities of these elements are very different. Previous explanations include the volatility and the forms of occurrence of elements in coals. Based on well-controlled experimental studies for selected coals, this paper establishes the first quantitative physicochemical model for vaporization of arsenic, selenium, and antimony during coal pyrolysis and combustion. Advanced characterization methods found that the three elements are associated with pyrites in the coals burned for this study. The vaporization processes for these three elements consist of three consecutive processes: transport of molecules or atoms through the bulk pyrite liquid (melt) to the melt/gas interface, vaporization of elements at the surface of melts, and transport of molecules/atoms through the pores of the char to the atmosphere. The controlling step for vaporization of arsenic is diffusion through the melt. Diffusion processes in the melt and within the char pores together determine the vaporization rates for selenium and antimony.