A simplified model for dioxin and furan formation in municipal-waste incinerators

Abstract

We have used the temperature-time profile derived from a one-dimensional energy-balance model, in conjunction with the kinetic estimates of Shaub and Tsang, to estimate the production of dioxins from chlorinated precursors in the gas phase and on fly ash. In agreement with earlier studies, we find that dioxin production on particulates is much larger than gas-phase formation. The scaling relation for dioxin production on fly ash shows that the adsorbed concentration in ng/g is proportional to the square of the gas-phase precursor concentration and inversely proportional to the particulate radius. Calculated dioxin outputs for flue-gas-fly-ash mixtures and for bottom ash fall in the general range observed for some of the available municipal-waste incinerator data. We suggest modifications needed in the model parameters to obtain agreement with the best of the newer incinerators. Work by Vogg et al on dioxin production, after maintenance of fly ash with adsorbed precursors for 2 h at relatively low temperatures, is accounted for quantitatively with the Shaub-Tsang model by introducing minor changes in the kinetic parameters. This slow, low-temperature conversion to dioxin may occur in low-temperature furnace sections, the baghouse or the electrostatic precipitator and may contribute to the dioxin concentration in bottom ash.