Release characteristics of mercury in fly ashes collected from coal-fired CFB power units during thermal treatment

Abstract

In order to investigate release characteristics of mercury in fly ashes collected from a coal-fired CFB power unit, the effect of temperature and time on mercury release characteristics of fly ash samples as well as release rules of mercury in fly ash samples were analyzed through the heat treatment experiments and temperature-programmed pyrolysis experiments. Moreover, the mechanism of mercury adsorption was examined based on the fitting results of first order kinetic model, Elovich kinetic model, Freundlich kinetic model and parabolic diffusion kinetic model. The results of heat treatment experiments showed that mercury release rate of the CFB fly ash was closely related to the heating temperature and time. The release rate of mercury in the CFB fly ash was lower than 0.2 at temperature below 200 ℃, while the release rate of mercury in the CFB fly ash was no less than 0.935 at the temperature over 300 ℃. The results of temperature-programmed pyrolysis experiments suggested that Mercury species present in the fly ash include HgCl2, Hg2Cl2, HgO, HgSO4, HgS (black) and HgS (red). HgS (black) and HgS (red) were the main mercury species present in the fly ash. The fitting results of kinetic equation showed that Elovich kinetic model was more applicable to reflect the Hg release from the fly ash, while the correlation coefficient R2 between the experimental and calculated data was greater than 0.95. It is indicated that this process controlled by multiple reaction mechanisms, and the activation energy changes greatly.