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Flammability Limit Measurements for Dust and Gases: Ignition Energy Requirements and Pressure Dependencies.

Hertzberg-M; Cashdollar-KL; Zlochower-IA
21st Symp International on Combustion the Combustion Institute 1986 :303-313
This paper presents data on the flammability limits of Pittsburgh seam bituminous coal dust, polyethylene powder, and methane in air at pressures in the range of 0.5 to 3 bar. Explosibility test chambers of 20 and 120 l were used, and ignitability limitations were overcome with efficient pyrotechnic ignitors with nominal energies of 500 to 5,000 j. The propagation criterion used was based on the maximum explosion pressure and the size-normalized maximum rate of pressure rise. The latter is a dynamic criterion that tends to minimize "overdriving" effects as the 20-l data are taken to their asymptotic limits at high ignition energies. The measured lean limits in air at atmospheric pressure are 90 g/m3 for the coal dust, 35 g/m3 for polyethylene, and 4.9 Vol pct for methane. The rich limit for methane is 18-19 vol pct, whereas the dusts have no rich limits out to concentrations as high as 4,000 g/m3. A linear, lean-limit pressure dependence was measured for the dusts which was essentially the same as the pressure dependence measured for methane when all are expressed in comparable units: namely, mass concentration of fuel per unit volume of air. This observation further confirms a lean limit dust flame propagation mechanism that is controlled by the gas-phase reaction rate. The limit concentrations are then determined by the dust loading required to generate a lean limit concentration of pyrolysis products in the volatiles-air mixture, and the dust behaves as an equivalent "homogeneous", premixed gas, regardless of the initial pressure.
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OP 60-88
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21st Symp. (International) on Combustion. The Combustion Institute, 1986, PP. 303-313