Study on vaporization behavior of heavy metals in fly ash derived fromcoal combustion during thermal treatment

In order to investigate the vaporization behavior of heavy metals in fly ash derived from coal combustion in circulating fluidized bed boiler during thermal treatment process to reduce volume of fly ash, taking Cu, Pb, Zn and Cr from fly ash as study objects, this paper aimed to clarify the occurrence of heavy metals in fly ash using sequential extraction and their vaporization propensities using a lab-scale horizontal electrical heated furnace, and the effects of thermal treatment temperatures from 900 to 1 400 ℃ and reaction atmosphere(air or N2) on vaporization behavior of heavy metals in fly ash was analyzed. The vaporization mechanisms of these four metals were elucidated based on the results from thermodynamic equilibrium calculation. The results indicated that Cu, Pb, Zn and Cr mainly existed in carbonate fraction, Fe-Mn oxides fraction and residual fraction. With the increasement of reaction temperature would facilitate the vaporization of heavy metals, their vaporization ratio followed a sequence of Pb>Cu≈Zn>Cr. Heavy metals vaporized as their chloride at relative low temperature while the formation of gaseous metallic oxides or metal element was responsible for the vaporization of heavy metals at higher temperature during thermal treatment. Reaction atmosphere was insignificant in the vaporization of Cu and Pb whereas it was influential on the vaporization of Zn and Cr. The shift of air to N2 atmosphere promoted the vaporization of Zn but prevented the vaporization of Cr.