The Investigation On The Characteristics Of Volatilization Of Typical Trace Element During Coal Combustion

The trace elements emitted during the coal combustion will greatly damage the mankind and the environment, which are receiving increased attention throughout the world. Little information about the mechanism of the transformation of trace elements and the regulation of the behavior of trace elements was reported because the amount of the trace elements is small and they are difficult to be measured. This paper regarded the typical trace elements emitted during the coal combustion as the targets. Some researches on the characteristic of volatilization of trace elements through the model of vaporization of trace elements had been done in this thesis. Firstly, the model of the vaporization of trace elements was introduced. Vaporization of trace element associated with pyrite is mainly composed by three steps: (1) transport of molecules or atoms through the bulk liquid melt to the melt surface; (2) vaporization at the melt surface/interface; (3) transport of the vapor through the char pores to the char surface. The mass-transfer coefficients of Se at the temperature range of 500-1227 ℃were calculated through the model. The result shows that the mass transfer is determined by both the liquid-phase diffusion and gaseous diffusion through the pores, mainly the liquid-phase diffusion. Secondly, a basic experiment was performed based on the calculation mentioned above. The effects of coal type, temperature and atmosphere were taken accounted. The result shows that high temperature and high concentration of oxygen effectively promote the volatilization of As and Se. In addition, SO₂ also has some positive effect to the volatilization of the two type of trace element. Finally, experiment was performed on the DTF to investigate the partition of the trace elements in the Pingdingshan bituminous coal. The coal size, temperature and atmosphere varied in the experiment. A LPI was used to obtain size-segregated ash samples at furnace outlet. The metal content of the impactor stages were measured. The result shows that As, Se, Cr enriched in fine particles ( Dp<0.5μm ). Moreover, these trace elements more enriched in the fine particles with increased temperature and decreased oxygen concentration in the bulk gas.