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RESIDENCE TIME OF CONTAMINANTS RELEASED IN SURFACE COAL MINES: A WIND TUNNEL STUDY
Citation:
Thompson, R.S. RESIDENCE TIME OF CONTAMINANTS RELEASED IN SURFACE COAL MINES: A WIND TUNNEL STUDY. U.S. Environmental Protection Agency, Washington, D.C., EPA/600/A-93/229 (NTIS PB93236586).
Description:
The 1990 Clean Air Act Amendments direct the U.S. Environmental Protection Agency to evaluate and modify, as required, existing dispersion models for the prediction of dispersion of dust from surface coal mines. he application of mathematical air pollution dispersion models to the dispersion of dust from surface coal mines requires knowledge of not only the amount of dust generated in the mine, but the fraction of that generated that actually escapes from the mine. he escape fraction can be related to the residence time that released material will remain, on average, within the mine. A wind-tunnel study was performed to measure the residence time for a variety of rectangular mine shapes at a scale of 1:300. lthough it is not possible to model the dispersion of dust particles at this model scale, the residence time of a neutrally buoyant gas can be measured. his determined residence time can then be used in algorithms that relate residence time and particle size to escape fraction. wenty combinations of rectangular surface mine shape, source location and wind direction were studied in a simulated atmospheric boundary layer. or each case, a point source of a hydrocarbon tracer was placed at the floor level of the mine and a sample port was installed at the downwind lip of the mine. he sample port was connected to a flame ionization detector that was monitored with a personal computer. he source was turned on until the concentration at the sampler reached steady-state and then instantaneously turned off; the concentration was recorded as a function of time. orty such realizations were performed and averaged for each case to obtain the mean residence time. In addition, a pulsed-wire anemometer was used to measure velocities on the centerplane for two of the cases. ecirculating flow with low mean speeds and large turbulent fluctuations was observed. The concentration in the mine was found to follow an exponential decay function from which an exponential decay time constant (or residence time) was computed for each case. emi-empirical formula was found that related the residence time to the mine geometry and wind direction quite well. his formula can be used to estimate escape fraction in determining the source strength for the application of mathematical dispersion models.