Thermodynamic modeling of PCDD/Fs formation in thermal processes

Three thermodynamic databases of polychlorinated dibenzo-p-dioxins and dibe nzofurans (PCDD/Fs), derived using the Group Additivity approach and two co mputational molecular modeling methods, Modified Neglect of Diatomic Overla p (MNDO) and Parametrized Model 3 (PRM), respectively, combined with the Sc ientific Group Thermodata Europe (SGTE) database have been used to model th e formation of PCDD/Fs in thermal processes, such as iron ore sintering pro cess. Th e predictions using the three different databases are compared, an d similar thermodynamic conditions of PCDD/Fs formation are found. The comp arison of the calculated values with measured results obtained from industr ial iron ore sinter plant indicates that the PCDDs and PCDFs found in pract ice are not in equilibrium with each other. While within each dioxin and fu ran homologue equilibrium between the isomers appears to be established in industrial processes, reactions between dioxins and furans seem to be kinet ically inhibited. This view has been supported by assuming no reaction at a ll between PCDFs and PCDDs in the simulation. With this assumption, both PC DFs and PCDDs reached partial pressures between 600 and 800 K in the order of magnitude actually found in practice. Taking this restriction into accou nt, the conditions for PCDD/Fs formation were calculated as a function of o xygen partial pressure; temperature; concentrations of carbon, hydrogen, an d chlorien; and C/H ratio.