Dioxin formation in stretched flames

Numerical simulations of stretched laminar twin premixed flames are carried out in order to understand the gasphase formation of dioxins in situations analogous to the post combustor and following regions of a typical incinerator. A previously developed chemical kinetic mechanism, that describes dioxin formation in terms of some generic species, is used in the calculations. The results indicate a temperature region favorable to the gasphase production of dioxins that lies between 1100 and 1500 K, as well as a weak dependence of dioxin formation on the oxygen concentration in the flames. The effect of oxygen is better described by observing the consumption of some of the generic species. Though no measurements of dioxin concentration in idealized flows, such as those simulated, are available, the results are in qualitative accord with total measurements of dioxin concentration (due to both gas- and solid-phase processes) obtained by other investigators. The numerical predictions identify some flames that are ignited, in which dioxin consumption takes place, and others which are unignited, in which significant dioxin production occurs as the result of largely isothermal mixing. The calculations indicate that unignited flames containing no initial fuel favor dioxin formation significantly over those that contain some initial quantity of fuel. Finally, some implications regarding incinerator practice are discussed.