The relationship between fly ash chemistry and the thermal formation of polychlorinated pollutants during waste incineration
Abstract: The thermal formation of polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), biphenyls (PCBs), and naphthalenes (PCNs) is a major problem in waste incineration. Ideally, rather than relying on air cleaning systems and treatment techniques, their formation should be minimized or, if possible eliminated. The work presented in this thesis was conducted to obtain a deeper understanding of the thermal formation of PCDDs, PCDFs, PCBs, and PCNs during incineration using a 5 kW laboratory scale incinerator and two artificial wastes that were designed to reflect regional differences in waste composition.The first part of the thesis focuses on the validation of a recently-developed flue-gas sampling probe with enhanced cooling capabilities. Artifact formation of PCDDs and PCDFs can occur during the sampling of hot flue gases if the cooling is insufficient. The new probe was successfully used to collect samples at 700 °C without biasing the measured POP levels. The thermal formation of PCDDs, PCDFs, PCBs, and PCNs in the post-combustion zone of the incinerator was then studied by collecting flue gas samples at 400 °C, 300 °C, and 200 °C during the incineration of the two artificial wastes. Highly chlorinated POPs were formed in larger quantities when burning the waste with the higher content of metals and chlorine, which suggests that high metal levels in the waste favor the chlorination of less chlorinated POPs or otherwise facilitate the formation of highly chlorinated polyaromatics, possibly via the condensation of highly chlorinated phenols. The concentrations of these pollutants and the abundance of highly chlorinated homologues increased as the flue gas cooled. Fly ash particles play an important role in thermal POP formation by providing essential elements (carbon, chlorine, etc.) and catalytic sites. The chemical and mineralogical properties of fly ash samples were studied by X-ray diffraction (XRD), Fourier Transform Infrared (FTIR), Scanning Electron Microscopy/Energy Dispersive X-ray (SEM/EDX), and X-ray photoelectron spectroscopy (XPS) to determine their impact on thermal POP formation. Orthogonal Partial Least Squares (OPLS) modeling was used to identify correlations between the observed POP distributions and the physicochemical data. This investigation provided new insights into the impact of fly ash chemistry on thermal POP formation.In addition, the POP isomer distribution patterns generated during waste combustion were examined. These patterns are used to “fingerprint” mechanisms of POP formation. It was found that wastes containing large quantities of metals and chlorine favored the formation of highly chlorinated homologues including the very toxic 2,3,7,8-congeners. The data suggest that reducing fly ash emissions might increase the SO2 content of the flue gas and thereby suppress the Deacon process and the formation of harmful highly chlorinated aromatic species.
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