| Abstract
| - Tests in a 10 kg h-1 pilot scale incinerator burningmedical waste showed that the amounts of PCCD/F formedwhen cooling from 900 °C to ambient were almostproportional to the residence time in the 400−200 °Crange. This suggests that gas-phase adsorption controlrather than kinetic control was operating. A generalizedmodel of PCCD/F formation was developed for a chlorine-rich gas phase, based on diffusion of HCl to the fly ashsurface. It was necessary to incorporate a “sticking factor”α, which specifies the fraction of molecules that areretained on the ash surface and subsequently enter intoreaction. The calculation combines rates for the formationand destruction mechanisms that are effective in therange of 400−225 °C. Application of the model requiresknowledge of the cooling regime, the specific surface areaof fly ash, its concentration at the sampling point, andthe initial surface concentration of PCCD/F. It assumes thatall the PCCD/F emitted from waste incinerators isassociated with the fly ash. The model satisfactorilysimulates the upper temperature window for PCCD/Fformation and the effect of cooling regime on rate. Whentested on a number of commercial incinerators withvarying levels of gas cleaning efficiency, the value of αwas found to depend on the downstream cleaning facilities,being 4−5 × 10-8 for gas cleaning systems involvingonly solids removal (wet scrubbing) and 1−3 × 10-9 forthose utilizing adsorption processes for gas-phasecomponents as well (lime, carbon). The latter appears tobe the true value for surface formation processes. Thepresence of sulfur in the fuel forces the value of α downward.
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