Effect Of Atmosphere And Metal Compounds On Dioxins In Fly Ash At Low Temperature

In recent decades, due to the rapid development of social economy, more and more wastes are produced, even leading to a terrible consequence-garbage siege. Waste incineration is widely applied for its advantages of reduction efficiency, resource-recycling and harmless. Nevertheless, waste incineration can produce secondary pollutants, including dust, heavy metal, and dioxins as well as other kinds of organic pollutants, even resulting in numerous "mass incidents" of the emission of dioxins. Fly ash from waste incineration accounts for about 3%-5% of the total amount of waste incinerated, considered as one of the Hazard Wastes. Furthermore, the amount of dioxins on fly ash accounts for 58%-88% of the total dioxins generated during waste incineration. Thus, studying the factors influencing dioxins formation and desorption contributes to a further understanding of the formation mechanism and disposal method of dioxins, helping to achieve near-zero emissions of dioxins.Many researches focused on the influence on dioxins concentration distribution in surroundings, but less involved the influence of heavy metals, especially the influence of the long-term operation on surrounding soil and human health. Therefore, the author concentrated on the influence of atmosphere and metal catalysts on dioxins formation as well as the concentration of heavy metals in soil near the incineration plant. Several conclusions are as following:(1) Atmosphere has important influence on dioxin formation, from reducing to inert and then oxidizing atmosphere, the amount of dioxin formation increases rapidly, the increase can be more than one hundred times; the oxygen concentration increase can promote the chlorination reaction, meanwhile the value of PCDD/PCDF also increased; it has some "preference" and "selectivity" during the formation process, especially the T4CDD isomers:T1368 and T1369; the study also found that reducing atmosphere has good inhibition on dioxin formation compared with inert atmosphere, the inhibition rate is about 90%, The probable inhibition mechanism is that some NH3 can be produced when N2 and H2 are heated together, then the NH3 counteract the catalytic action of CuCl2; precursor synthesis is dominanted in the gas phase while de novo synthesis is dominanted in the solid phase.(2) Comparing the concentration of dioxin under different conditions,the order of catalytic ability of metals are as following:CdO, ZnO,MFA< NiO,CdCl2,Cr2O3<CuO,ZnCl2<NiCl2,CrCl3<CuCl2 In this study, metal chlorides have the ability to promote dioxin formation, the promote ability ranged from 10-3000 times under different atmosphere,CuCl2 is the strongest catalyst, as to NiCl2 and CrCl3,the amount of dioxin is about 5%-10% comparing with CuCl2,their effect on dioxin formation can’t be ignored; meanwhile CdO and ZnO show some inhibitory effect on dioxin formation. We find that Cr2O3 and CrCl3 conduce to the formation of PCDD while CdO and CuCl2 conduce to the formation of PCDF by analyzing the isomers and value changing of PCDD/PCDF.The others’effect change with the O2 concentration. Except CuCl2 sample, the low chlorinated dioxins(tetra- and penta-) account for 75%-90% of all dioxins in other metal compounds samples, T4CDD accounts for as high as 60%,it indicates that the effect on dioxins has selectivity while the reaction condition changing by analyzing the distribution characteristics of isomers.(3)The functional mechanism of Nickel and Chromium during dioxin formation process are studied by the XANES technique and PCA analysis.It indicates that the redox happens on Nickel and Chromium compounds and metal compounds with different valence state are generated; as to Nickel, Principal Component Analysis (PCA) indicates that both Ni2O3 and NiCl2 probably play an important role in the formation of dioxins, an oxidation-reduction and oxychlorination cycle involving model fly ash is proposed to define the formation of possible Ni-species。As to Chromium, Cr2O3 and CrCl3 play the role of the Cl-transfer and oxidation-reduction, CrCl3 plays an important role in the formation of dioxins.(4) The effect of temperature and atmosphere on dioxin thermal desorption are studied,it found that the desorption efficiency and degradation efficiency are increased with the increase of temperature under inert atmosphere,about 30% to 95%; as to reducing atmosphere,the efficiency was about 40% and 90% under 200℃ and 300℃ respectively,it is a little better than inert atmospherre under the same temperature;When the temperature is 400℃, due to the effect of formation, degradation efficiency decreases obviously, only about 80%, But the fly ash can meet the standard of landfill after treatment; we notice that the desorption, degradation and formation played different predominant role under different temperature by analyzing the correlation analysis and distribution characteristics of isomers under various conditions.(5) A long-term monitoring of metal concentration in the soil around a medical waste incinerator in the north of Zhejiang province was conducted,the concentration and distribution characteristics of 10 metals are obtained; the average concentration of metals in soil of 2011,2012 and 2014 is 615.70mg/kg,596.96mg/kg and 622.64mg/kg respectively.Comparing with the secondary standard of agricultural land, Cd and Pb are out of the limits, the worst is about 581.91% of the standard; meanwhile the situation of Hg and Zn is not optimistic,more attention should be paid; While comparing with secondary standard of residential land, no one exceeds the limit. Assessment of environment and human health risk are done based on the potential ecological risk index method of Hakanson and Preliminary Remediation Goals of USEPA, The results show that the environment risk of metals in soil from strong to weak as following:Hg>Cd>As>Pb>Cu>Ni>Zn>Co>V, The annual comprehensive ecological risk evaluation index is in a state of medium and strong. Arsenic is out of the limits both on human non-cancer and cancer risk.The distribution characteristics of metals in fly ash and open burning ash are also studied. The principal component analysis indicates that the influence of incinerator on metals in surrounding soil is not obvious.