An Empirical Model for the De Novo Formation of PCDD/F in Medical Waste Incinerators

Tests in a 10 kg h - 1 pilot scale incinerator burning medical waste showed that the amounts of PCCD/F formed when cooling from 900 °C to ambient were almost proportional to the residence time in the 400−200 °C range. This suggests that gas-phase adsorption control rather than kinetic control was operating. A generalized model of PCCD/F formation was developed for a chlorine-rich gas phase, based on diffusion of HCl to the fly ash surface. It was necessary to incorporate a “sticking factor” α, which specifies the fraction of molecules that are retained on the ash surface and subsequently enter into reaction. The calculation combines rates for the formation and destruction mechanisms that are effective in the range of 400−225 °C. Application of the model requires knowledge of the cooling regime, the specific surface area of fly ash, its concentration at the sampling point, and the initial surface concentration of PCCD/F. It assumes that all the PCCD/F emitted from waste incinerators is associated with the fly ash. The model satisfactorily simulates the upper temperature window for PCCD/F formation and the effect of cooling regime on rate. When tested on a number of commercial incinerators with varying levels of gas cleaning efficiency, the value of α was found to depend on the downstream cleaning facilities, being 4−5 × 10 - 8 for gas cleaning systems involving only solids removal (wet scrubbing) and 1−3 × 10 - 9 for those utilizing adsorption processes for gas-phase components as well (lime, carbon). The latter appears to be the true value for surface formation processes. The presence of sulfur in the fuel forces the value of α downward.