The Role of Chlorine in Dioxin Formation

Abstract

There is poor correlation between total chlorine in waste streams and formation of polychlorinated dibenzodioxin and polychlorinated dibenzofuran (PCDD/F) during waste combustion. This is because the active chlorine (Cl) species are strongly dependent upon combustion conditions. For homogeneous conditions, trace amounts of a hydrocarbon species (benzene) injected into the effluent from complete combustion of a mixed chlorocarbon fuel (ethylene and chloromethane) results in formation of stable, oxygenated and chlorinated compounds. This occurs over a broad range of temperatures (400–900°C), provided that a fraction of the system chlorine is in the Cl radical form. Cl is the only form in which chlorine can react, in gas-phase, with stable hydrocarbon species, and these reactions are very fast, even in the low temperature regime. Molecular chlorine (Cl2) may subsequently participate in reactions with carbon-based radicals, and these reactions are the primary source of chlorinated products. Hydrogen chloride (HC1)—the major chlorine species in the products—can react with oxidizing radicals (eg, OH) and promptly form significant amounts of Cl. Gas temperature has a great influence on the final distribution of products. At 800–900°C, practically all of the benzene which was attacked by Cl is converted to carbon monoxide and small unsaturated hydrocarbons by subsequent reactions with oxygen. At about 750°C, measurable concentrations of chlorobenzenes are formed. At lower temperatures (400–600°C), chlorophenols become a large fraction (up to 15%) of the total reacted benzene. Heterogeneous reactions result in the formation of Cl2 through catalyzed reactions (most actively by copper salts) and promotion of carbon-Cl bond formation. The latter is dependent on the ash surface species and type, ash adsorptive characteristics, temperature, and presence of regenerative Cl in the waste combustion products.