Destruction and formation of organic micropollutants in incineration process

In this paper are presented the results obtained from a lab-scale investigation carried out with a system for Thermal Diagnostic Studies (STDS) aimed to study the effect of some process variables during incineration. The study has been focused on (I) gas phase dioxins formation during precursors thermal degradation, (II) thermal degradation of toxic organic compounds, (III) products of incomplete combustion (PICs) formation during thermal degradation of urban sludge spiked with toxic organics, (IV) PICs formation during process failure modes, (V) polynuclear aromatic hydrocarbons (PAHs) formation during urban sludge thermal degradation and (VI) influence of conditioning polymer on PICs emission during sludge incineration. The study about gas phase dioxins formation during precursors thermal degradation has been carried out with 2, 4, 6-trichloro- and 2, 4, 6 -tribromo-phenol that were thermal degraded at temperatures between 300 and 8000 C in an air atmosphere. Both phenols showed the formation of the same tetra-halo-dioxin isomers that were further degraded at higher temperature. Furthermore, chlorine-containing dioxins showed higher thermal stability than bromine-containing dioxins. The study about thermal degradation of toxic organic compounds has been carried out with chlorobenzene, tetrachloroethylene and toluene that were thermal degraded at temperatures between 300 and 10000C in an inert as well as air atmosphere. Results show that in all experimental conditions tetrachloroethylene and toluene are the most and less thermal stable compounds respectively. Also, all compounds are more thermal resistant during pyrolytic experiments and less thermal resistant when they are treated as a whole mixture. The study about PICs formation during thermal degradation of urban sludge spiked with toxic organics has been carried out by thermally degrading urban sludge alone or spiked with the above reported three organics at different conditions of temperature and oxygen concentration. Results show that the presence of the three organics doesn't alter the number and quantity of PICs emitted. The sludge presence caused a decrease and an increase of tetrachloroethylene and toluene thermal stability respectively while that of chlorobenzene remains constant. The study about PICs formation during process failure modes has been carried out by simulating three process failure modes (thermal, mixing and residence time) during sludge incineration. Results show that temperature is the principal variable controlling emissions. In fact, below 7000 C a great number of PICs are emitted at all oxygen concentrations. At higher temperatures oxygen concentration (mixing failure mode) controls the emissions causing a great number of PICs to be formed even at 11000 C with a low air concentration. Finally, when proper temperature and oxygen concentration values are reached, the influence of residence time becomes evident causing a PICs decrease from 2 to 3 s. The study about PAHs formation during urban sludge thermal degradation has been carried out by thermally degrading urban sludge at various conditions of temperature and oxygen concentration. Results show that PAHs are mainly formed at 10000 C and low oxygen concentration. In particular, bene zene is formed in concentration 100 times higher than anthracene, fluorene and cyano-naphthalene, and 10 times higher than naphthalene, benzonitrile, acenaphthylene, fluoranthene, phenanthrene and pyrene. The study about influence of conditioning polymer on PICs emission during sludge incineration has been carried out by thermally degrading a conditioning polymer (Percol 787), alone or spiked onto urban sludge at various dosages (75-300 ppm), at different conditions of temperature and oxygen concentration. Results show that the conditioning polymer alone gives rise to a great number of PICs that are completely masked by its dilution in the sludge