The Speciation And Partitioning Of Heavy Metal During Municipal Solid Waste Thermal Treatment

As the speedy development of national economy and the escalation of people’s living standard, the amount of solid waste has increased substantially. With the accelerated process of urbanization, land resource has become exiguous. How to dispose the solid waste (mainly municipal solid waste) effectively has become a bottleneck of ecological urban construction and sustainable economic and social development. This dissertation aim to control heavy metal pollution, and launched a series of experiment and theoretic study on the speciation and partitioning of heavy metal during solid waste thermal treatment in oxygen controlled atmosphere.The zinc speciation of two kinds of fly ash particles was characterized and comparatively studied using high resolution synchrotron X-ray absorption fine structure (XAFS). It is found that zinc chloride is the major zinc species in a moving grate incinerator but willemite dominates in the fluidized bed incinerator. Zinc chloride and zincite were enriched in the fine particle samples, whereas willemite and zinc sulfate were present at higher concentrations in the larger particle samples. Test results show the whole process of zinc speciation change from evaporation to condensation. With a higher ratio of sulfur/chloride in the furnace, less chloride will be formed. Silica, alumina, aluminosilicates, and calcium-based compounds are potential sorbents for transforming zinc to less harmful species. To prevent toxic zinc species contained in fine particles from escaping into the atmosphere, wet scrubbers are more suitable for cleaning flue-gases in moving grate incineration systems, while improving the efficiency of dust removal is more important for fluidized bed incineration systems. The valence and chemical speciation of Cr in the bottom ash was also studied using XAFS. We analysed the effect of mineral additives on the valence and speciation change of Chromium. It is found that Chromium is oxidized to Cr2O3 at low temperature and to CrO3 at high temperature. The presence of HCl will promote the formation of Cr(VI). The evaporation of Cr is inhibited with the addition of FeS2, CaO,(NH4)2SO4 and kaolinite. The main part of Cr in the bottom ash is Cr(III). Using FeS2,(NH4)2SO4 and kaolinite as additives can decrease the proportion of Cr(VI). Although CaO can inhibited Cr evaporation, but a lot of Cr(VI) will be formed and leach out at the same time. (NH4)2SO4 and kaolinite will increase the proportion of leachable Cr(VI) slightly, FeS2 can decrease the proportion of leachable Cr(VI) and Cr leaching concentration.We have investigated, theoretically and experimentally, the effects of several important flue gas species and mineral sorbents on the speciation change and partitioning behaviour of five major heavy metals (Pb, Cd, Cr, Zn and Cu). Test results show that except Cu, the evaporation rates of metals are higher under a reductive rather than an oxidative atmosphere. The release trends observed suggest that HCl can promote heavy metal release by preventing the formation of metal aluminates, especially for Zn and Pb. Oxygen has a negative influence on the effect of HCl. When SO2 is present, it can reduce the formation of chlorides, but it is still a promotor of metal evaporation. NH3 can inhibit metal vaporization indirectly, and using SNCR in the furnace can help reduce metal contamination simultaneously. The presence or addition of SiO2- or Al2O3- containing minerals should be effective for metal volatilization control during MSW incineration. The BCR sequential extraction results show that fly ash generated at low equivalence ratio has higher environmental risks. With a higher equivalence ratio, the acid and water soluble Pb, Zn, Cr decreased; reducible Cu decreased 9.95% and Cd, Pb, Cr increased slightly; oxidizable metals decreased and residual metals increased except Pb.We have investigated the municipal solid waste gasification in a downdraft gasifier and a two-stage moving grate incinerator, detected the metal pollution emission and the leaching toxicity of fly ash and bottom ash. The test results show that with a higher equivalence ratio, Ni, Ba, Hg has a lower leaching toxicity, while Cu, Pb, Cd and Zn is more leachable. Gasification bottom slag is not hazardous waste, but fly ash requires special disposal.Experimental study was established to investigate the effect of combination process of flotation with acid washing and use biomass ash as fusion assistant on reducing the solidification cost of fly ash. Through flotation, small ash particles which are more leachable and have higher metal concentration will be enriched in the float. The acid washing time of non-float products can be reduced a lot. Flotation process can improve the disposal efficiency and enhance the economic efficiency. By adding biomass ash as fusion assistant, the melting heat for fly ash can be reduced a lot. When the mass ratio of rice husk ash is one-third, the melting heat is reduced about 300 kJ/kg. If wood ash was mixed half to half with solid waste fly ash, the melting heat of mixed sample can be 700 kJ/kg lower than the melting heat of pure fly ash.