Retention mechanisms of ash compositions on toxic elements (Sb, Se and Pb) during fluidized bed combustion

Both laboratory simulation experiments and thermodynamic equilibrium calculations were conducted to illustrate the retention mechanisms of ash components on three toxic elements (Sb, Se and Pb). The experiment was carried out in a fluidized bed reactor with the combustion temperature in the range of 500-1000 °C. The results shown that the redistribution behavior of these toxic elements was greatly proportional to the combustion temperature, ash components and amounts. The adding of Fe, Ca, Mg, Na and K favour the retention of Sb and Se, while the presence of Si was represented against Sb and Se capture. Si, Al, Fe, Na and K have an appreciable effect on the retention of Pb, while the Pb capture by Ca and Mg was limited. The partial retention of these elements in ashes may be attributed to both physisorption and chemisorption. Among them, the formation of non-volatilized compounds may be the primary contribution for the retention of Sb, Se and Pb (except for Al). The negative Sb and Se captured by Si may be ascribed to the interactions between Si and other components which promote the retention of Sb and Se. The highly Pb retention by Si and Al in ashes may be explained by the formation of PbO·SiO2, PbO·Al2O3 and PbO·Al2O3·2SiO2. Fe- and Al-based materials may be supposed to the appreciate additives for the capture potential of all the selected elements.