| dc.description.abstract | Incineration is considered as the major method for waste treatment in Taiwan and the residues of this process are bottom ash and fly ash (FA). Due to the high concentration of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzo-furans (PCDFs), fly ash generated from 24 large-scale municipal waste incinerators (MWIs) in Taiwan can not be reused or recycled and is finally subjected to sanitary landfill. Therefore, the increasing amount of fly ash generated from incineration process combines with the lack of landfill space has caused intense public concerns. This study focuses on the removal of PCDD/Fs in fly ash generated from MWIs via catalytic pyrolysis process. Since the chloride content may affect PCDD/Fs reduction efficiency via pyrolysis, two pretreatment methods, i.e., water washing and carbonated water washing, are applied to reduce the chloride content in FA and the effect of chloride content on PCDD/Fs reduction achieved with pyrolysis is evaluated. Being pyrolyzed at temperature of 350oC for 1 hour, the TEQ removal efficiencies of 96.7%, 98.5% and 96.2% for PCDD/Fs are achieved for unwashed fly ash (23.4% Cl), water washed fly ash (7.10% Cl) and carbonated water washed fly ash (2.70% Cl), respectively. In addition, catalytic pyrolysis reveals significantly high PCDD/Fs TEQ removal efficiencies. Specifically, the removal efficiencies of PCDD/Fs achieved are 64.1% and 91.3% under pyrolysis at 350oC in 15 minutes with Pd/γ-Al2O3 and Pd/C as catalyst, respectively, which are significantly higher than that of thermal pyrolysis (35.4%). Activated carbon is proven to obtain higher activity compared with γ-Al2O3 as catalyst support in pyrolysis system. Various transition metals catalyst are prepared and applied in the catalytic pyrolysis system developed. The results indicate that PCDD/Fs removal efficiencies via pyrolysis at 350oC in 15 minutes with Fe/C, Co/C, Ni/C and Cu/C as catalysts are 91.1%, 91.1%, 93.7% and 94.5%, respectively. It indicates that PCDD/Fs concentrations decrease gradually with the reduction of catalyst size. Moreover, PCDD/Fs concentrations in FA are measured as 235, 231, 101 and 77.5 pg I-TEQ/g after being pyrolyzed with Ni/C catalyst sizes of 10-18 mesh, 20-40 mesh, 70-120 mesh and 120-200 mesh, respectively. In order to understand the mechanism of PCDD/Fs removal with catalytic pyrolysis, concentration and transformation of all 210 PCDD/Fs congeners need to be defined. A cost-effective clean-up method is successfully developed to reduce the solvent volume and working time required to analyze mono- to octa-CDD/Fs. In untreated FA, mono- to tri-CDD/Fs homologues account for 63.8% while tetra- to octa-CDD/Fs homologues account for 36.2% of total PCDD/Fs. After thermal pyrolysis, the contribution of mono- to tri-CDD/Fs homologues is increased to 88.3% while that of tetra- to octa-CDD/Fs homologues is reduced to 11.7%. During thermal pyrolysis, the concentrations of mono- to tri-CDDs increase by 130 pmol/g while those of tetra- to octa-CDDs decrease by 74.7 pmol/g. The results also indicate that 42.5% mono- to tri-CDDs measured in treated FA can be formed during thermal pyrolysis from other pathway rather than merely via dechlorination from tetra- to octa-CDDs. On the other hand, the concentration of mono-CDFs increases by 88.6 pmol/g while those of di- to octa-CDFs decrease by 478 pmol/g. This result indicates that most of PCDFs reduction during thermal pyrolysis is due to dechlorination of highly chlorinated congeners to low chlorinated congeners and the final product of PCDFs would be non-chlorinated dibenzofuran if the reaction time is sufficient. It is suggested that PCDD/Fs can be formed via the condensation of various precursors such as chlorophenol and chlorobenzene on FA surface. Thus, with the presence of appropriate catalyst in pyrolysis system, these precursors can be decomposed into other chemical compounds instead of forming PCDD/Fs. Inhibition of precursor condensation could be the main reason for the higher PCDD/F removal efficiency achieved with catalytic pyrolysis compared with thermal pyrolysis. | en_US |