The acidity and alkaline of fly ash could significantly affect the de novo synthesis of dioxins. It could not only influence the total concentration of dioxins

but also the distribution of congeners. In this study

different amounts of Ca(H2PO4)2 and CaO were added to control the pH values of model fly ashes. Experiments were conducted to study the effects of acidity and alkaline of fly ash on de novo synthesis of dioxins. The results suggested that the amount of dioxins increased and the I-TEQ values didn't change when pH values decreased from 5.5 to 4.3 with an addition of 5% (mass) Ca(H2PO4)2. When pH value decreased to 3.6 with an addition of 20% (mass) Ca(H2PO4)2

the total concentration of dioxins and I-TEQ value went down. While with additions of 0.4% (mass)

5% (mass) and 20% (mass) CaO

the pH values increased to 9.8

12.4 and 12.6 and the total concentration of dioxins went down as well as the I-TEQ values. Therefore

the alkaline environment of fly ash could be better to suppress the formation of dioxins than the acidic environment. Meanwhile

the correlations between the total concentrations of dioxins in fly ash and base/acid ratios were studied. The results revealed that the correlation value for fly ashes produced from fluidized bed was 0.95

however

there is no relationship for fly ashes which were collected from grate furnace.