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UNDERSTANDING PCDD/F FORMATION AND SOURCES
Citation:
Gullett*, B K. UNDERSTANDING PCDD/F FORMATION AND SOURCES. Presented at 2nd Internatioal Conference on Combustion, Incineration/Pyrolysis, and Emission Control, Cheju, Korea, 9/5-7/02.
Description:
Recent work at the U. S. EPA has examined fundamental concepts regarding formation and minimization of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs/Fs) in combustion scenarios. This paper summarizes some of these main findings, drawing on work from multiple studies. Results have shown both fuel- and deposit-carbon sources resulting in PCDD/F formation. Fuel carbon sources can immediately impact emissions, and wall deposits can be a persistent source of emissions. Their relative reactivity and role in determining emissions are a function of the system combustion history. Significant levels of emissions can persist, resulting in lagged effects from operational changes. Formation of PCDDs/Fs can occur at high temperatures (> 600 degrees C), apparently followed by subsequent chlorination, although the inability of reactive chlorine (CI-) to persist to these temperatures and the inability to draw mass balances with precursors leave this question for further study. In relatively clean, non-deposit- laden systems, PCDD/F formation appears most likely in marginal stoichiometry regimes. The requirement of sufficient oxygen (O2) for oxychlorination reactions competes with the need for a supply of unoxidized fuel carbon to form the carbon structure of the PCDD/F or its precursors. This scenario is akin to open burning situations, where fluctuating stoichiometries are effected by variation in wind (O2 supply) and the proximity of volatilizing fuels. The presence of high concentrations of sulfur dioxide has a declining influence on PCDD/F levels. While this may be due partially to gas-phase reaction with Cl species, catalytic suppression has not yet been fully explored. Efforts to understand phase partitioning suggest that the highly sorptive nature of some fly ash/filter residues biases PCDD/F toward the solid phase. Early indications suggest that gas- phase PCDD/F is dominant at T > 400 degrees C, resulting from surface formation followed by vaporization.