Chlorophenol Reactions on Fly Ash. 1. Adsorption/Desorption Equilibria and Conversion to Polychlorinated Dibenzo-p-dioxins

Abstract

Tetrachlorophenol (T4CP) reactions to PCDD products in the presence of MSWI fly ash were measured as a function of gas-phase precursor concentration, reaction time, and reaction temperature. Tetrachlorophenol concentrations ranged from 150 to 700 ng/mL in 10% O2, and temperatures ranged from 250 to 400 °C, which resulted in PCDD formation rates from about 0 to 35 μg of PCDD (g-fly ash)-1 min-1. Reaction conditions were found such that the reactor approached differential behavior, which led to the breakthrough of the T4CP and the desorption of adsorbed PCDD products into the gas-phase. At constant temperature, the surface coverage of chlorophenols on fly ash was found to be constant for reaction times between 2 and 120 min, suggesting that the system had quickly reached adsorption/desorption equilibrium. Yields of PCDD were observed to increase with increasing precursor concentration but either peaked, leveled off, or increased as the temperature was raised. Studies of T4CP reactions on different fly ashes showed a correlation between total conversion of T4CPto unknown productsand total yield of the trace reaction forming PCDD.

The formation of dioxins in combustion systems

“…The rate was found to be proportional to the concentration of precursor P at high concentrations, but to be second order at low concentrations. The model successfully predicted the low-temperature experimental data of Milligan and Altwicker [176]. The output was found to be sensitive to the number of active sites, which was assumed to be constant.…”

“…With precursors a first-order dependence on concentration has been reported by Cains et al [69] and Milligan and Altwicker [176]. However at low concentrations of precursor, this has been questioned by Huang and Buekens [89] and Penner et al [177], who have all postulated a second-order dependence.…”

Influence of metal oxides on PCDD/Fs formation from pentachlorophenol

“…Model studies also showed that PCDD/ Fs formation from precursors is likely the predominance of the two proposed pathways (ii) and (iii) (Dickson et al, 1992). Chlorophenols (ClPhs) are structurally closely related to PCDD/Fs and thought as important precursors for PCDD/Fs formation (Dickson and Karasek, 1987;Milligan and Altwicker, 1996). Some mechanisms of PCDD/Fs formation from ClPhs as precursors have been reported (Weber and Hagenmaier, 1999;Tuppurainen et al, 2000).…”

Inhibition of PCDD/Fs formation from dioxin precursors by calcium oxide

“…The decisive role of chlorophenols (CPs) and chlorobenzenes (CBs) could hardly be challenged in PCDD/Fs formation (Altwicker, 1996;Milligan and Altwicker, 1996;Weber and Hagenmaier, 1999;Iino et al, 2000;Catherine and Dellinger, 2003). These classes of compounds were always found together with PCDD/Fs in the effluents of incinerators and have been suggested as indicator compounds.…”