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 particl es emitted from power stations. Arsenic, selenium, and antimony are present in the submicron particles presumably via a vaporization-condensation path way although the volatilities of these elements are very different. Previou s explanations include the volatility and the forms of occurrence of elemen ts in coals. Based on well-controlled experimental studies for selected coa ls, 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 vapori zation processes for these three elements consist of three consecutive proc esses: transport of molecules or atoms through the bulk pyrite liquid (melt ) to the melt/gas interface, vaporization of elements at the surface of mel ts, and transport of molecules/atoms through the pores of the char to the a tmosphere. The controlling step for vaporization of arsenic is diffusion th rough the melt. Diffusion processes in the melt and within the char pores t ogether determine the vaporization rates for selenium and antimony. (C) 200 1 by The Combustion Institute.