Thermodynamic modeling of PCDD/Fs formation in thermal processes

Three thermodynamic databases of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), derived using the Group Additivity approach and two computational molecular modeling methods, Modified Neglect of Diatomic Overlap (MNDO) and Parametrized Model 3 (PM3), respectively, combined with the Scientific Group Thermodata Europe (SGTE) database have been used to model the formation of PCDD/Fs in thermal processes, such as iron ore sintering process. The predictions using the three different databases are compared, and similar thermodynamic conditions of PCDD/Fs formation are found. The comparison of the calculated values with measured results obtained from industrial iron ore sinter plant indicates that the PCDDs and PCDFs found in practice are not in equilibrium with each other. While within each dioxin and furan homologue equilibrium between the isomers appears to be established in industrial processes, reactions between dioxins and furans seem to be kinetically inhibited. This view has been supported by assuming no reaction at all between PCDFs and PCDDs in the simulation. With this assumption, both PCDFs and PCDDs reached partial pressures between 600 and 800 K in the order of magnitude actually found in practice. Taking this restriction into account, the conditions for PCDD/Fs formation were calculated as a function of oxygen partial pressure; temperature; concentrations of carbon, hydrogen, and chlorine; and C/H ratio.