Formation and Decomposition Mechanisms of Dioxins in High Temperature Packed Bed Processes

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In 1999, an experimental apparatus of a packed bed reactor with gas flow was made which has a gas supplying system quantitatively controlling the concentration of chloro-benzenes or chloro-phenols of the main gas. A number of interesting results were obtained, e.g., the peak of dioxins formation is observed at about 300℃ and about 2% of O_2, and formation rate of highly chlorinated dioxins promoted by the HCl addition to the gas through the bed.Based on the above results, in 2000, the packed bed combustion process was first applied to the remediation of soils highly contaminated by dioxins. The validity of the process was verified using a laboratory-scale simulator on the bases of process efficiency. The sample was soil contaminated by dioxins (13,100 pg/g-dry soil, 330 pg-TEQ/g-dry soil) and different conditions, e.g., admixing ratios of coke, limestone and water for pre-granulation, were examined. The results showed that high efficiencies of dioxins removal (more than 98.9% of removal ratio) were obtained in all conditions. Further, pretreatment soils such as pre-drying and pre-granulation can give extremely high efficiencies, i.e., more than 99.9%.Further, quenching experiments were conducted to observe in detail the transport phenomena of dioxins and relating compounds in the bed during combustion. The macroscopic phenomena of dioxins removal with the proposed process was clarified.