Physical and chemical processes relevant to the formation of chlorinated dioxins in municipal solid waste incinerators

Chlorinated dioxins consist of a group of compounds consisting of two benzene rings linked by two oxygen atoms. Counting positional isomers there are 75 chlorinated dioxins containing from one to eight chlorine atoms. The isomer 2,3,7,8 tetrachlorodibenzo-p-dioxin is one of the most toxic substances known and is of particular interest. It has been known for some time that municipal solid waste incinerators are capable of producing chlorinated dioxins in the stack gases and on the fly ash. Since dioxin is not initially present in municipal waste, it must be produced by the incinerator. It has been known empirically that if the incinerator is operated so that it provides a residence time of at least one second at 1800{dollar}spcirc{dollar}F that dioxin production is minimized. It has been speculated that this 1800{dollar}spcirc{dollar}F, one second criteria destroys dioxins.; It is demonstrated that the often quoted one second residence time at 1800{dollar}spcirc{dollar}F in the combustion chamber to minimize dioxin does not destroy dioxin as often thought; rather it destroys dioxin precursors which are then unavailable to form dioxin in the low temperature zone by surface catalyzed reactions on fly ash.