Controlling Mechanisms of Dust Layer Ignition and Flame Propagation in Dust Clouds

Global Congress on Process Safety
April 3, 2012 - 12:00am

Initiation and propagation of dust deflagrations are extremely complex phenomena due to the interaction between solid particles and the gaseous flame front. In comparison with premixed gas deflagration, a dust-oxidizer deflagration depends on the rate of evolution of volatiles, the mixing of these volatiles with the oxidizer surrounding the particles, coupling of the particles and gas phase oxidation as well as radiative energy exchange between the flame and its surroundings. Due to these complications, a comprehensive mathematical theory to predict deflagration mechanisms of dust clouds is at present beyond reach. Although vast amount of testing, both small scale (20 liter explosion vessel) and large scale tests have been done over the last 50 years, most theories that connect the data to models are heavily empirical and the problem has never been analyzed from a fundamental viewpoint.

This study will identify the controlling parameters of dust layer ignition, and deflagration mechanisms. A modified ASTM E2021 hot surface apparatus is used to analyze the parameters controlling the ignition of dust layers. The parameters are then be used to predict ignition of a dust deposit in a realistic geometry such as a 2 dimensional wedge. To study flame propagation in dust clouds a novel premixed-dust-air burner is designed to measure the burning velocity of a hybrid mixture of Pittsburgh seam coal dust, with typical particle sizes in the range of 75 to 90 µm and methane-air. The results show that the addition of coal dust in methane-air premixed flame reduces the burning velocity. Two competing effects are considered to explain these trends. The first effect is due to volatile release, which increases the overall equivalence ratio and thus, the burning velocity. The second is the heat sink effect that the coal particles take up to release the volatiles. This process reduces the flame temperature and accordingly the burning velocity also. A mathematical model is developed considering these effects and it is seen to successfully predict the change of laminar burning velocity for various cases with different dust concentrations and equivalence ratios of the gas mixture. Furthermore, the implication of this study to one specific fugitive dust problem in coal mines is discussed.

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