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Show - 8 - Fig. 4 differ in oxygen levels by 5%). Increasing the chlorine level further for a stoichiometric flame, the second Hame can be extinguished (top picture of Fig. 5). The Hame now emits toxic products from the first reaction zone. Increasing the oxygen level from 34% to 36% ignites the second reaction wne (center picture, Fig. 5). Higher levels of oxygen bring the second reaction zone closer to the inner combustion cone. The influence of the H/Cl ratio is quite dramatic; while for the stoichiometric flame with an WCl ratio of 0.6, the second flame cannot be ignited at oxygen levels of 34%, the lean flame (equivalence ratio of 0.75, Fig. 3) with an H/Cl ratio of unity is about to flashback (Fig. 3). Figure 6 shows non-dimensional flame speeds of the first-stage flame as a function of oxygen fraction in the oxidant for a H/Cl ratio of unity, a C/O ratio of 0.5, and equivalence ratios of 1 and 0.75, respectively. These relative flame speeds were obtained from images of the flame cone-angle [18]. For the stoichiometric flame, the relative flame speed increased strongly with oxygen fraction. It is interesting to note that this trend was also observed for the lean flame (Fig. 6). Discussion Two-stage premixed flames have been observed for trichloroethylene [13] and dichloromethane flames [19] . As shown above, hydrocarbons premixed with molecular chlorine may exhibit two-stage flame structures. For example, the cases presented in Fig. 2, which exhibited two-stage flame structures, had WCl and ClCl ratios which correspond to dichloromethane. It is consistent with our results that premixed dichloromethane indeed show two-stage behavior [19]. Furthennore, in the course of the experiments no two-stage flames were encountered for H/Cl ratios much larger than unity. This is in agreement with the observation reported on monochloromethane flames [19]. These results imply that mixtures of hydrocarbons and chlorine may model the macroscopic behavior of the combustion of complex mixtures of hazardous wastes which may have various levels of bound chlorine. Although it is likely that some intennediate species fonned in the flame may differ, the total amounts as well as the nature of emissions of toxic byproducts from chlorinated hydrocarbon flames may well be dominated by the presence or absence of a second-stage flame. Therefore, the |
