ROLE OF OXYGEN IN FORMATION OF POLYCHLORINATED DIBENZO-P-DIOXINS/DIBENZOFURANS FROM CARBON ON FLY-ASH

Under an oxygen-deficient flow, no PCD D/F is formed from a carbon/fly ash mixture, but with a mixture of 1% O-2 in N-2 de nova synthesis of PCDD/F sets in. The O-2 concentration was varied 0-10%, formation was highest at 5-10% O-2. The [PCDD]:[PCDF] ratio rises with increasing [O-2], but the average degree of chlorination of PCDD and PCDF does not depend on the O-2 concentration. Especially 3,4,6,7-substituted PCDD/F congeners are formed. Without a gas flow, carbon/fly ash mixtures can give PCDD/F formation too: the O-2 now enters the mixture by diffusion. This formation process can continue for several hours. Isomer distributions do not change with increased reaction times. Formation rates under conditions without a gas flow are higher than with a gas flow present, suggesting that O-2 diffusion into the fly ash bed is more efficient than the supply of O-2 through a gas flow.