The role of chlorine in dioxin formation

There is poor correlation between total chlorine in waste streams and forma tion of polychlorinated dibenzodioxin and polychlorinated dibenzofuran (PCD D/F) during waste combustion. This is because the active chlorine (Cl) spec ies are strongly dependent upon combustion conditions. For homogeneous cond itions, trace amounts of a hydrocarbon species (benzene) injected into the effluent from complete combustion of a mixed chlorocarbon fuel (ethylene an d chloromethane) results in formation of stable, oxygenated and chlorinated compounds. This occurs over a broad range of temperatures (400-900 degrees C), provided that a fraction of the system chlorine is in the Cl radical f orm. CZ is the only form in which chlorine can react, in gas-phase, with st able hydrocarbon species, and these reactions are very fast, even in the lo w temperature regime. Molecular chlorine (Cl-2) may subsequently participat e in reactions with carbon-based radicals, and these reactions are the prim ary source of chlorinated products. Hydrogen chloride (HCl)-the major chlor ine species in the products-can react with oxidizing radicals (eg, OH) and promptly form significant amounts of Cl. Gas temperature has a great influe nce on the final distribution of products. At 800-900 degrees C, practicall y all of the benzene which was attacked by Cl is converted to carbon monoxi de and small unsaturated hydrocarbons by subsequent reactions with oxygen. At about 750 degrees C, measurable concentrations of chlorobenzenes are for med. At lower temperatures (400-600 degrees C), chlorophenols become a larg e fraction (up to 15%) of the total reacted benzene. Heterogeneous reaction s result in the formation of Cl-2 through catalyzed reactions (most activel y by copper salts) and promotion of carbon-Cl bond formation. The latter is dependent on the ash surface species and type, ash adsorptive characterist ics, temperature, and presence of regenerative Cl in the waste combustion p roducts.