Study On Hydrothermal Treatment Of Heavy Metals And PCDD/Fs In MSWI Fly Ash

Municipal solid waste incineration (MSWI) has the advantages of both mass and volume reduction, and energy recovery; Waste-to-energy incineration has been considered as one of mainstream strategies for municipal solid waste (MSW) management in China. However, emissions of toxic hazardous materials from the MSWI, such as persistent organic pollutants (POPs), and heavy metals etc., have attracted wide attention. The majority of those hazardous compounds are usually enriched in fly ash, so it has been classified as a hazardous waste due to its high heavy metal contents and significant traces of toxic polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs). Therefore, MSWI fly ash must be treated before disposal in landfills, or secondary utilization. This dissertation was funded by national basic research program of China (973program, No2011CB201500), and launched a series of fundamental experiment and theoretic studies in the issue of hydrothermal treatment, such as high energy consumption, high demand of equipment, secondary environmental pollution, and single pollutant treatment, as well as lacking of theoretical direction. The objectives of the current research involved as follows:1). In order to provide the basis for typical MSWI fly ash choices, characteristics of fly ash which collected from different locations of the incineration system were studied, including studying on toxicity characteristic leaching procedure (TCLP) of heavy metals and distributions of dioxins in the fly ash samples. The results indicate that the maximum leaching concentrations of heavy metals occur at bag filters (BFs) fly ash samples, which also content high levels of dioxins. In addition, BF fly ash was identified as hazardous waste based on the standards of landfill and identification of hazardous waste. Moreover, there are similar congener and I-TEQ fingerprint profiles of PCDD/Fs in fly ash samples collected from different locations. However, PCDD/F homologue fingerprint profiles are significant different. In addition, Individual congener concentration has good correlation both with I-TEQ and total dioxin concentration, based on linear curve fit.2). Two types of fly ash samples are choosed based on the mainsteam MSW incineration and air pollution control devices (APCDs). And then hydtrothermal stabilization of heavy metals in the two typical fly ash collected from circulating fluidized bed (CFB) incineration and stoker incineration plants which have different incineration technologies is investigated. In addition, the speciation distributions of heavy metals in MSWI fly ash before and after the hydrothermal process evaluated using a modified Bureau Communautaire de Reference (BCR) sequential extraction procedure. The resuts indicate that the dominant fraction of heavy metals in CFB original fly ash is residual, and hydrothermal process further improve heavy metals in the ash to transform from the first three steps into the residue fraction. Conversely, the dominant fraction of heavy metals in stoker original fly ash is the first three fractions. The sepeciation distributions of heavy metals except for Pb element have no obvious change after treatment. In addition, the analysis results of contamination factors (ICFs) and risk assessments (RACs) reveal that hydrothermal process is benefit to stabilize heavy metals both in two types of MSWI fly ash, especially for CFB fly ash.3). Fly ash collected from CFB plant was furtherly treated by the hydrothermal processes under different conditions, in order to better investigate the major effect factors and stabilization mechanism of hydrothermal process. The results show that the concentrations of treated fly ash in all TCLP leachates are far below the regulatory limits under all experimental conditions. Especially, the stabilization efficiency of heavy metals exceeds95%, and the qualities of remaining water meet the national wastewater discharge standards under optimal conditions. Zeolite-like minerals, including sodalite and geopolymer, are synthesized during the hydrothermal treatment. Stabilization of heavy metals could be attributed to absorption on and ion exchange with zeolite-like minerals synthesized from MSWI fly ash, while precipitation and physical encapsulation simultaneously promote heavy metal stabilization.4). The distribution and degradation mechanism of PCDD/F decomposition by hydrothermal process were furtherly investigated according to the results of hydrothermal stabilization of heavy metals in MSWI fly ash. The results indicate that the degradation efficiency of PCDD/Fs is accelerated by oxidative degradation and with temperature increasing; for example,88.31%of the total PCDD/F was removed at mild temperature150℃in2h under oxidative condition, while only38.45%was destroyed at the same temperature in12h under non-oxidative condition. The results shows that aerating oxygen is benefit to the formation of OH radical and OH radical can accelerate the degradation of PCDD/Fs during hydrothermal process. Moreover, the formation of low chlorinated PCDDs and PCDF isomers and PCB isomers during the OCDD and OCDF hydrothermal degradation, suggesting that there were coexisting hydrodechlorination reactions, reciprocal transformation between PCDDs and PCDFs, and PCBs isomers are intermediates during hydrothermal degradation of PCDD/Fs.5). Reaction mechanisms of OCDD with OH radical under hydrothermal environment have been studied using density functional theory (DFT) and polarized continuum mode (PCM). Additionally, the information of the stationary points including the reactants, intermediates, transition states and products are calculated. The reaction pathway of OH radical and OCDD is following, firstly, the complex compounds of OH radical and OCDD are formed, and then OH attack the C atoms at the position of2,3,7,8and1,4,6,9separating Cl atoms from OCDD. In addition, OH radical can easily attack the C atom near to the O atom in dioxin ring to from OH radical adduct, and this reaction is no barrier reaction, releasing112.90kJ/mol free energy of activation, and then decompose to the substituted phenoxy radical P3by the fused-ring C-O bond cleavage.Overall, results of this study indicate that the hydrothermal technique is a promising and seemingly effective way for stabilizating heavy metals and degradation dioxins in MSWI fly ash. Aerating oxygen during the hydrothermal process could decrease reaction temperature and time, and improve reaction efficiency significantly, as well as reduce the demands of hydrothermal equipements. Therefore, hydrothermal treatment of MSWI fly ash shows a good application in industry. In addition, the treated MSWI fly ash could be used as environmental materials, such as adsorbent and acid neutralizer. The harmless disposal and higher value application of MSWI fly ash are realized in the study.