The chemistry of chlorine in combustion systems and the gas-phase formation of chlorinated and oxygenated pollutants

Abstract

A major concern during the combustion of chlorinated fuels is the formation of chlorinated organic pollutants, which are potentially dangerous to human health and to the environment. Observed combustion byproducts include suspected carcinogenic and toxic compounds, such as chlorobenzenes, chlorophenols, and polychlorinated dioxins, furans and biphenyls. AH of these pollutants are regulated by US-EPA emission standards. This research studies the conditions which favor the gas-phase formation of these chlorinated byproducts. Under fuel-lean conditions, oxygen and chlorine compete for the available hydrogen to form H2O and HCl respectively. Laboratory reactor experiments show that, at short residence times (<100ms), relatively large amounts of chlorine atoms and Ch can be present under oxygenrich conditions. A detailed kinetic model of the post-flame chemistry of chlorocarbon combustion is validated in the course of this study, and applied to the prediction of the levels of Cl atoms at the different stages of practical furnaces and incinerators. It is found that atomic chlorine can be present in the combustor gas at levels in the range of0.5 to 2.5%, at temperatures as low as 400-500°C. In the absence of other reactants, these radicals undergo recombination and form equivalent amounts of Ch in the final quenched products. Changes in the concentrations of chlorine products are shown to occur in response to the addition of trace organic species to the flue gas of chlorocarbon combustion. When small amounts of a combustible species (ethylene, methane or carbon monoxide) are injected into the hightemperature (T> 1100°C), oxygen-rich products of methyl chloride combustion, OH and 0 radicals, generated in the oxidation of the injected fuel, transform significant amounts of HCl to Cl radicals. If the chlorine-containing combustion gas is first cooled to an intermediate temperature (T<800° C), the effect of the hydrocarbon injection is to consume Cl radicals in reactions of H abstraction and to reduce the final concentration of Ch in the quenched products. Chlorine radicals and Ch can react with organic compounds and form stable chlorinated byproducts. Cl· can readily abstract H atoms from stable hydrocarbons, forming carbon-based radicals. At high temperatures, these radicals are promptly consumed in oxidation reactions, and low temperatures are needed for the formation of stable chlorinated species. However, the notion that kinetic constraints severely affect homogeneous pathways at the low temperatures has led several authors to doubt that significant contributions to the formation of chlorinated and oxy-chlorinated pollutants could come from gas-phase reactions. Homogeneous reactions involving free radicals are not slowed by decreasing temperature. The present research shows that chlorine atoms may survive temperature reductions and, if they mix with organic species at temperatures in the range of 500 to 900°C. they promptly react forming large amounts of chlorinated and oxygenated adducts. The gas-phase reactions of benzene with the chlorine species present in combustion products are shown to form chlorinated benzenes and phenols, with a yield of chlorinated products up to 12% of the total benzene consumed. It is shown that the presence of atomic chlorine is required to initiate these reactions and, therefore, conditions which increase the levels of Cl radicals in the combustion gas enhance the formation of chlorinated pollutants. Not only are chlorinated benzenes and phenols hazardous products per se, but they are also known precursors for the formation of poly-chlorinated dibenzodioxins, dibenzofurans and biphenyls, via low-temperature (300-500° ) catalytic pathways. Therefore, gas-phase reactions can explain the formation of a large fraction of the organic pollutants released from practical systems.