The Behavior And Partitioning Of Heavy Metals During Thermal Treatment Of Municipal Solid Waste

With the rapid development of economy and accelerated process of urbanization, the amount of municipal solid waste(MSW) has increased substantially. Due to the advantages of high ability of minimization and eliminating the bacteria and virus, thermal treatment of MSW is gradually becoming major means of handling MSW. However, thermal treatment of MSW can give rise to secondary pollutions:fly ashes, acid gases, particulates, heavy metals and Dioxin, and so on. Therefore, it is increasingly turning into one of the major environmental problems that how to effectively dispose the MSW and control pollution emissions at the same time. Based on the research progress at home and abroad, the behavior and partitioning of Cd, Pb, Zn and Cu were studied systematically during the thermal treatment of model solid waste and representative MSW, which is of great significance in the understanding of the heavy metals behavior and control.To begin with, the influence of reaction times, redox conditions, water and mineral matrice on metal chlorides volatilization during thermal treatment of model solid waste in a fluidized bed reactor was investigated. The results showed that increasing oxygen concentration and water can inhibit the vaporization of heavy metals by forming involatile compound. Nevertheless, HC1can promote the heavy metal release by preventing the formation of metal aluminates and metal oxides. SO2also had positive effects on the vaporization of heavy metals.To thoroughly and deeply understand the kinetic vaporization behavior of heavy metals and study the diffusion mechanisms of heavy metals through the particles, a mathematical model was established to predict the behavior of CdCl2and studied the effect of different parameters. Through the simulation, it can be observed that diffusion coefficient played an important effect on the heavy metals vaporization. The vaporization amount increased rapidly with the diffusion coefficient increasd. The porosity of particles also had significant effect. The vaporization amount of increased from20%to40%when that porosity of particles increased from0.2to0.68. The smaller the diameter of particles, the higher the vaporization rate of heavy metals. The heavy metals in the different MSW components may behave differently during the thermal treatment, since the amount and form of heavy metals in the different waste fractions varied. On the basis of previous studies, different categories of MSW were sampled. The behavior of Cd, Pb, Cu and Zn in the Paper, Disposable bamboo chopsticks (DC), Garbage bags (GB), PVC pipe (PVC) and Tire were studied at the different reaction temperatures under the conditions of rapid and slow combustion. Moreover, pyrolysis experiments at900℃were conducted to outline the effect of redox conditions on heavy metals behaviors. It was found that incineration temperature made little difference in the vaporization of heavy metals in paper, DC and PVC. Nevertheless, the volatilization rate of heavy metals in GB and tire increased with the increase in incineration temperature. Among these five waste components, the volatilization rate of heavy metals in PVC and DC was highest; the paper lied next while GB and tire were lowest. The volatilization rate of heavy metals at condition of slow combustion was much lower than that at condition of rapid combustion. The volatilization rate of Cd and Zn in paper, GB and tire under the condition of pyrolysis was higher than that at combustion condition. Because PVC contained a significant amount of chlorine, more volatile metal chlorides could be formed under combustion conditions which contributed to the higher volatilization rate of heavy metals. Under N2atmosphere, the volatilization rate of Cu and Pb was much lower than that under combustion conditions for all wastes.Due to the complexity of MSW, there was the necessity to study the effects of co-combustion between different compositions of MSW on the heavy metals behaviors. The results revealed the nonlinear relationships between the partitioning of heavy metals and the fuel mixtures. The retention rate of Cd in residues showed a upward trend with the co-combustion of tire and GB. As to lead, the retention rate changed little with the tire and textile co-fired, while GB had negative effects on the vaporization of Pb. Compared with lead, the retention rate of zinc was a little higher. With the co-combustion of tire, PVC and GB, more zinc could be retained in the residues. The percentage of copper retained in the residues was increased significantly when the tire was co-fired, while that of copper decreased slightly with the PVC and GB co-fired, indicating that the copper in plastic waste would be vaporized more easily. Due to the disparity of the heavy metals partitioning between in industrial incinerator and in experimental reactor, a study of the physical and chemical characteristics of fly ash and bottom ash from a fluidized bed incinerator in Wuhan was carried out. The results showed that the heavy metals were preferentially concentrated in the finer bottom ashes and fly ash particles. BCR results demonstrated that Pb were mainly bound into the matrix and would not be easily released under natural conditions, while Cd compounds may be released easily into the natural environment. According to the chemical characterisation of residuals and the leachates, only the bottom ash can be landfilled directly. Nevertheless, fly ash must be treated before landfilled.