Characteristics of municipal solid waste incinerator fly ash promoting the formation of polychlorinated dibenzo-p-dioxins.

摘要

Studies conducted over the past two decades have shown that polychlorinated dibenzo-p-dioxins (PCDDs) are present in significant amounts on municipal solid waste (MSW) incinerator fly ash. More recent studies have shown that PCDDs are produced via catalytic activity of the fly ash itself. It is unknown which particular characteristics of the fly ash are actually responsible for the catalytic activity.; This research project is aimed at determining which physical or chemical characteristics of MSW incinerator fly ash promote the formation of PCDDs. Thirteen fly ash samples were collected from U.S. MSW incinerators, with no distinction being made as to incinerator feed type or operating conditions. Complete physical and chemical characterizations of the fly ash samples were performed. The concentration of PCDDs on each sample was determined. An attempt was made to correlate physical and chemical characteristics to the PCDD concentrations. Physical characteristics did not correlate with PCDD concentrations. However, some elemental concentrations did correlate. Positive correlations were obtained for chlorine, potassium, sodium, sulfur, and zinc.; The correlations were developed assuming that no other factors such as incinerator feed type or operating conditions were affecting PCDD formation. To insure that PCDD concentrations were due primarily to the catalytic activity of the fly ash and not other factors, laboratory reaction experiments were performed. Cleaned fly ash samples were used as catalysts and pentachlorophenol was used as a model precursor feed. The experiments proved that the differences in catalytic activity of the fly ash was the primary factor in PCDD formation. This validated the correlations developed using the raw fly ash data.; The final portion of this research used synthetic catalysts in statistically designed experiments to further examine the catalytic activity of several selected species. These species were copper hydroxide, sodium hydroxide, and zinc nitrate. Again, reaction experiments were performed. These experiments showed positive catalytic activity by zinc nitrate. Both copper hydroxide and sodium hydroxide showed negative effects. A general reaction pathway was proposed for the formation of PCDD in MSW incinerators. A mechanism was also proposed for the formation of PCDDs from pentachlorophenol as catalyzed by zinc nitrate.