Doctor of Philosophy (PhD)
Chlorinated aromatics undergo surface-mediated reactions with metal oxides to form environmentally persistent free radicals (EPFRs) which can further react to produce polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). Previous work using laboratory-made fly ash surrogates composed of transition metal oxides deposited on silica powder has confirmed their ability to mimic fly ash in the production of PCDD/Fs. However, little is known about the propensity of alumina and aluminosilicates to form PCDD/Fs. Cooperative catalysis between transition metals in the formation of PCDD/Fs has also seen little research. A fly ash sample containing both alumina and mullite, an aluminosilicate, was tested for PCDD/F formation ability and compared to PCDD/F yields from the thermal degradation of 2-monochlorophenol (2-MCP) precursor over γ-alumina, α-alumina, and mullite. A series of fly ash surrogates with varying amounts of co-deposited iron (III) oxide and copper (II) oxide, both known to increase PCDD/F formation individually, were also investigated. A packed-bed flow reactor was used to investigate the thermal degradation of 2-MCP over the various catalysts at 200-600 ⁰C. Fly ash gave similar PCDD/F yields to surrogates made with similar transition metal content. γ-alumina, which is thermodynamically unfavorable, was very catalytically active and gave low PCDD/F yields despite a high destruction of 2-MCP. Mullite and α-alumina, the thermodynamically favorable form of alumina, yielded higher concentrations of dioxins and products with a higher degree of chlorine substitution than γ-alumina. The data suggests that certain aluminas and aluminosilicates, commonly found in fly ash, are active in the formation of PCDD/Fs in the post-flame cool zones of combustion systems and should be considered as additional catalytic surfaces active in the process. The bimetallic surrogates were found to be extremely catalytically active, suggesting synergistic effects between Fe and Cu in real incineration systems. Under oxidative conditions, the bimetallic surrogates completely catalytically oxidize the 2-MCP precursor and exhibit low yields of PCDD/Fs. Under pyrolytic conditions, the bimetallic surrogates give extremely high yields of PCDD/Fs. The comparisons between transition metal and non-transition metal effects on PCDD/F formation represents a new step forward in our understanding of PCDD/F emissions from incineration systems.
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Potter, Phillip M., "The Contribution of Fly Ash Components to PCDD/F Formation" (2014). LSU Doctoral Dissertations. 3917.
McCarley, Robin L