Occurrence and volatilization behavior of Pb, Cd, Cr in Yima coal during fluidized-bed pyrolysis☆

The volatilization behavior of Pb, Cd, Cr and the influence of coexisting mineral matters were investigated during pyrolysis of Yima coal in a fluidized-bed reactor at temperatures ranging from 500 to 900 °C. The modes of occurrence of Pb, Cd, Cr in Yima raw coal and two char samples were determined using density fractionation, demineralization and sequential chemical extraction methods. Lead in Yima coal is mostly associated with mineral matter in various forms, mainly pyrite, sulfates and monosulfides. Large part of cadmium is associated with pyrite. Chromium in Yima coal is mainly associated with organic matter. Different trends are observed for various forms of trace metals during pyrolysis. Lead associated with pyrite, sulfates and carbonates, lead in water soluble and ion exchangeable forms, and cadmium associated with pyrite are all unstable under pyrolysis conditions. During Yima coal pyrolysis, the volatilities of lead and cadmium vary greatly with pyrolysis temperature, while chromium volatility in Yima raw coal only has slight changes over the temperature range (500–900 °C) studied. The volatility of Pb, Cd, Cr in demineralized Yima coal (YimaD) is much higher than that of Yima coal during pyrolysis. New thermally stable forms of Pb, Cd and Cr are formed during pyrolysis of Yima, whereas the sources of them are different. The interactions between chromium and its coexisting mineral matters in Yima coal retard its vaporization during pyrolysis.