FORMATION AND DESTRUCTION MECHANISM OF POLYCHLORODIBENZO-P-DIOXINS AND POLYCHLORODIBENZOFURANS IN THERMAL PROCESSES

Abstract

We studied the possible correlation between the rate of oxidation of native carbon in fly ash from thermal processes, measured as a decrease in the content of total organic carbon, TOC, and the formation of PCDD/F by de novo synthesis. The results of experimental tests at a laboratory scale are reported and discussed as oxidation of the native carbon in fly ash and as formation of primary products, CO and CO2, and by-products in trace amounts, PCDD and PCDF, of the oxidation reaction. Considering the trend of experimental data for the disappearance of the native carbon, it was hypothesized that the decrease of TOC is the result of two processes taking place simultaneously on the surface of fly ash: the dissociative chemisorption of oxygen followed by the formation of CO2 form the oxygenated complexes C(O) and the direct oxidation of native carbon. The oxidation of the oxygenated complexes is the determining step of the oxidation process. As a result, it was hypothesized that a small fraction of C(O) is involved in the formation of PCDD/F. The processing of data by means of two kinetic models, one for the decrease of TOC and one for the formation of PCDD/F, demonstrates that the formation of the oxygenated complex, C(O), represents the kinetic determining step for the de novo synthesis of PCDD/F. On the basis of the PCDD/F formation mechanism, interventions for the prevention of the formation of these micropollutants were carried out at an industrial scale.