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Combustion experiments in a laboratory-scale fixed bed reactor were performed to determine the role of temperature and time in PCDD/F 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/IF 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 hours). These results points out the importance of carbon deposits in the post-combustion stages that can account for emissions long after their formation (memory effects). The calculated formation rates were 7 to 15 times higher than those reported in the literature from fly-ash-only experiments, indicating both the importance of 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.

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journal article

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