| Polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) are unwanted byproducts formed in certain industrial processes and as a product of incomplete combustion when chlorine and carbon are present. Studies show that in a combustion process, PCDD/F are generated in low temperature post combustion regimes, with fly ashes acting as catalysts. These low temperature catalytic reactions are also believed to be major pathways for the formation of PCDD/F in other thermal treatments such as the Catalytic Extraction Processing (CEP), new waste management process proposed as an alternative to incineration.; A reaction system that can be operated as an aerosol flow reactor as well as a fixed bed reactor has been designed and implemented to investigate the potential of PCDD/F formation in the CEP and incineration environments. Several important parameters controlling the PCDD/F formation/destruction in both CEP and incineration processes have been identified, namely temperature, reaction time, and oxygen content in the gas stream. Under de novo conditions, a half order dependence of PCDD/F formation rate on oxygen partial pressure has been observed on incinerator fly ash and 1/3 for CEP dust. It has also been proven that at a temperature window of around 300 -325°C PCDD/F generation yield is the highest. A multi-step reaction kinetic modeling is proposed to explain these experimental observations.; Model mixtures have been used to further study the roles of individual components of the catalysts. Experimental results indicate that copper and iron are active in catalyzing PCDD/F formation and destruction, and nickel and calcium are not. These observations can be explained by a Mars-van Krevelen mechanism in that multi-valence states are required to provide redox couples in order for the redox cycle to proceed. The Mars-van Krevelen mechanism also suggests that lattice oxygen is the active species in the oxidation process. The role of the gaseous oxygen is to regenerate or maintain the oxidized state of the catalyst. Important issues remaining to be solved are also discussed. |
