Shigeatsu Nakazawa

Thermodynamic evaluations on the formation of dioxins and furans in combustion gas

Abstract The formation of toxic PCDD/Fs are discussed thermodynamically for COHCl quaternary combustion gas to elucidate the effects of temperature and gas composition. In most cases, these toxic gases are formed at a temperature below 400°C, representing the isomers of furans and tetra dioxin (4CDD) in similar amounts, but those of other dioxin isomers are much lower.The amounts of PCDD/Fs species formed depend greatly on the gas composition, but are always associated with the formation of chlorobenzenes. When the system contains excess oxygen such as O/C > 1, PCDD/Fs decrease sharply.Hydrogen and water vapor are quite effective in decomposing dioxins and furans to form benzene and hydrocarbons. The derived amounts of PCDD/Fs are much higher than those observed in practice because of kinetic reasons. Taking account of kinetic factors, reasonable amounts of these toxic gases are derived.Especially, the reaction of carbon deposition has always been neglected in thermodynamic calculation of PCDD/Fs because of kinetic reason, but both of solid carbon and PCDD/Fs must be coexisting in small amounts.

Explanation of the thermal formation of dioxins from thermodynamic viewpoint

The thermal formation of dioxins is typical of recent environmental issues relating to metallurgy. To analyze the behavior of these toxic species, computer assisted thermodynamic assessments were extensively carried out. In the homogeneous gas phase equilibria of the C−O−H−Cl system, the formation of dioxins is usually observed below 300–350°C, but is observed at higher temperature under the condition of oxygen deficiency. It has become clear that the presence of enough oxygen and hydrogen, as well as a decrease in chlorine content, would be desirable to prevent dioxin formation. From a strict thermodynamic standpoint, dioxins are meta-stable products observed under the retardation of the carbon deposition reaction which must be very slow at low temperature. The effects of some inhibitors or accelerators on the formation of dioxins are also discussed.