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Show Introduction In spray-fired incinerators, liquid wastes are chemically removed by exposing them to high temperature oxidizing environments for a specified time. Typical spray combustion models treat the spray as a fraction of a continuous flow , thereby describing the global, or "averaged" combustion process. However, since hazardous waste disposal requires destruction efficiencies as high as 99.9999%, it is precisely the non-average, or individual droplet behavior of the process which must be understood to assure destruction. Individually burning droplets at the edge of sprays, and any oversize droplets which penetrate the flame zone, may degrade the destruction efficiency. These droplets either escape into the environment, or must be treated by an after-burner, neither of which is desirable. In this paper, we focus our attention on a particular class of hazardous wastes: chlorinated hydrocarbons. These wastes are very common in industry, where they are found in dry-cleaning fluids, paint solvents, and other industrial processesl . Although chlorine does not inhibit burning as dramatically as other halogens such as bromine and iodine, the chemical reaction mechanisms are similar to other halogens. The chlorine inhibits burning by consuming hydrogen atoms necessary for chain branching reactions. The hydrogen atoms preferentially form HCl, competing with the chain branching reaction, (H + O2 ---+OH + 0)2. Senkan S also suggests that soot formation from the chlorinated hydrocarbon could suppress the oxidation process, making it difficult to maintain combustion. Recently initiated research indicates that blending these wastes with hydrocarbon fuels may enhance combustion, and therefore thermal destruction, by providing an ignition source, increasing the burning rate, and possibly delaying extinction of the waste droplets. The objective of this study is to apply our understanding of droplet combustion processes to problems in hazardous waste incineration. We use numerical simulations of droplets to predict ignition, burning rates, droplet trajectories, and extinction. Our numerical predictions are compared with available experimental results. Numerical Models The numerical studies presented in this paper have two objectives: first to identify and correlate parameters affecting the required residence time of droplets in a waste incinerator, and second to study fundamental combustion processes in multicomponent waste droplets. Two different numerical simulations are performed. Initial and boundary conditions are kept as similar as possible to allow comparison between models. Experimental conditions from Ref. 4 are simulated: a cold (SOOK) droplet of either pure hydrocarbon (nonane) or a blend of nonane and tetrachloroethane (C2H2 Ct.), reacting in a hot oxidizing environment (1200K air) . The initial droplet diameter in the experiments ranges from 250JLm to 300JLm. The numerical simulations are for initial droplet diameters of 250JLm. The Reynolds number based on free-stream conditions for the experimental data is on the order of two. We choose to study this particular blend of waste and fuel because the components have approximately the same boiling temperature. Therefore, no preferential volatility effects are present, making other combustion mechanisms easier to distinguish . One-Dimensional Multicomponent Model: The first model is a one-dimensional, spherically symmetric, full Navier-Stokes solution of multicomponent, burning droplets6 . This model approximates the flame chemistry by using two-step, finite-rate kinetics, and includes variable thermodynamic and transport properties in the gas phase. A variable grid is used to model the very high gradients near the droplet surface and the equations are non-dimensionalized 2 |
