| Emission problems have hindered the acceptance of incineration as a viable solid waste treatment method, particularly due to releases of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F). Experiments in our laboratory suggest that PCDD/F are not formed during combustion, but instead in the lower temperature (250-400{dollar}spcirc{dollar}C) post-combustion zone of the incinerator. Metal species found in incinerator fly ash--the residual material formed from non-combustibles in the waste stream and entrained in the flue gas--appear to be catalyzing PCDD/F formation at these low temperatures.; Two distinct and separable heterogeneous pathways for PCDD/F formation on fly ash were studied in our laboratory: reactions involving native carbon in the fly ash matrix as a precursor, and those involving chemically-similar gas-phase precursors. The first pathway has been referred to as de Novo synthesis, where unlike complexes of elemental carbon, oxygen, chlorine, and hydrogen in fly ash are somehow combined in the presence of gas-phase oxygen to form chlorinated aromatic compounds such as PCDD/F. De Novo synthesis of {dollar}sp{lcub}13{rcub}{dollar}C-labelled PCDD/F was observed when {dollar}sp{lcub}13{rcub}{dollar}C-activated carbon was added to fly ash and reacted with air at 250-350{dollar}spcirc{dollar}C. Optimum temperatures for de Novo synthesis were found to range from 300-325{dollar}spcirc{dollar}C.; The second pathway involves chemically-similar gas-phase precursors, such as chlorophenols, formed initially as products of incomplete combustion (PICs). These precursors appear to react catalytically with elements in the fly ash to produce PCDD molecules. Reactions of chlorophenol precursors--in particular 2,3,4,6-tetrachlorophenol--with fly ash were studied in the laboratory as a function of precursor concentration and temperature. Optimum PCDD formation temperatures ranged from 300-400{dollar}spcirc{dollar}C. Chlorophenols were found to be highly adsorptive on fly ash, while satisfying a Freundlich adsorption isotherm. The catalytic mechanism for chlorophenol conversion to PCDD on fly ash was determined to be consistent with a Langmuir-Hinshelwood mechanism. |
