| Incineration plays an important role in the disposal and utilization of municipal solid wastes (MSW). However, there is a growing concern for the environmental impact of MSW incineration fly ashes in proper disposal and utilization, especially for the release of toxic substances such as heavy metals and dioxins. More recently, among all the final disposal of hazardous MSW Incinerator (MSWI) fly ashes, the thermal melting process becomes one of most significant research community.The emphasis of this study is on the fundamentals of the melting of MSWI fly ashes, as well as the verification of practical feasibility of gasifying & melting process of Chinese MSW. The melting mechanisms of MSWI fly ashes were investigated either in a micro-scale DSC-DTA system or in a laboratory-scale batch system.Firstly, various MSWI fly ashes, either domestics or abroad, were collected or sampled. The physical-chemical properties of these ash samples, such as major compositions (SiO2, Al2O3, CaO and Fe2O3), trace elements of hazards (dioxins and heavy-metals), melting point, porosity and morphology were successively analyzed by Electron Spectroscopy, Atom Absorbing Spectrum, GC-MS and Poremeter, respectively.Subsequently, the thermodynamics and kinetics of melting process of MSWI fly ashes were characterized on basis of experimental evidence. Experiments were conducted in a micro-scale high temperature DSC-DTA system. The heat of melting transition as well as the sensible heat during the process was measured. The theoretical model on basis of the first thermodynamic law was proposed to predict the heat change during the ash melting process. Reasonable agreement between the predicting and the experiments was obtained. In addition, the kinetics model of ash melting transition was regressed from the experimental DTA-DSC data. These aforementioned models form the basic fundamentals for the practical engineering and optimal design of melting process. For instance, as the GMP (gasfying & melting process) recently prevails as one of most potential disposals of MSW in developed nations, the incipient MSW Low-Heating-Value required for this incoming process was predicting staring from thermodynamic model. Moreover, the feasibilities of both IAMP (incineration + ash melting process) and GMP for Chinese MSW were discussed, based on the actual circumstance that the domestic MSW has high moisture and low heating-value.Finally, the laboratory-scale studies on the mechanisms of fly ash melting were conducted in a self-developed batch system. The influences of operational conditions on the melting characteristics such as volume-reduction ratio, molten-slag porosity and morphology, vitrification transiting temperature and thermal stability were investigated, while the considerable variables include process temperature, atmosphere, slag cooling mode and additional reagent.The control of secondary pollutants of melting process was considered. The degradation of dioxins during melting process was studied in lab-scale batch system. Experimental results indicate that the great part (>99.9%) of dioxins with MSWI fly ash degrades, and quite a few part remains in the molten slag while no dioxins was detected hi the exhausted gas-phase. With increasing temperature, the degradation efficiency of dioxins increases and nearly approaches to 100% at 1460℃. The degradation of dioxins under oxidized atmospheres is easier than that under inert ones. With addition of 10 liquid-ceramics, the complete degrading temperature reduces to 1100℃ from 1460℃.The partition of heavy metals (Ni, Cd, Cr, Cu, Pb, Zn) during melting process was also studied in lab-scale set-up. Influences of temperature, atmosphere, time, slag cooling mode and additional reagent on the solidification of heavy metals were discussed. It is concluded that the partition characteristics of during ash melting greatly differs for each kind of heavy metals. Generally, Ni, Cr and Zn can be classified as the in-vaporized metals, while Hg, Cd and Pb as the easily-vaporized metals.
|
PDF Full Text Request