Kinetic modeling of PCDD/F formation based on carbon degradation reactions

Combustion experiments in a laboratory-scale fixed bed reactor were performed to determine the role of temperature and time in polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated dibenzofuran (PCDF) formation, allowing a global kinetic expression to be written for PCDD/F formation due to soot oxidation in fly ash deposits. Rate constants were calculated for the reactions of carbon degradation, PCDD/F formation, desorption, and degradation. For the first time, values for activation and thermodynamic parameters for the overall reactions have been calculated for PCDD/F formation, desorption, and destruction reactions. Good agreement was found between the calculated rate constants for carbon degradation and for PCDD/F formation, indicating that the two processes have a common rate-determining step. Moreover, PCDD/F formation was found to be still active after long reaction times (24 h). These results points out the importance of carbon deposits in the postcombustion stages that can account for emissions long after their formation (memory effects). The calculated formation rates were 7-15 times higher than those reported in the literature from fly ash-only experiments, indicating the importance of both soot and a continuous source of chlorine. A comparison between full-scale incinerator rates and model calculated rates indicates that our model based on carbon degradation kinetic can be a tool to estimate emissions. © 2008 American Chemical Society.