Factors governing spontaneous ignition of combustible dusts

Joshi, Kulbhushan Arvind

Etd

Factors governing spontaneous ignition of combustible dusts

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The problem of self-heating of combustible dusts accumulated on hot surfaces has caused several fires and dust explosions. The current test standards (ASTM E 2021, EN50281-2-1) used to ensure safe environment for a given dust, define a safe temperature of the flat hot surface for certain dust layer thickness. Since in these standards, measurement of temperature is taken along the centerline, they mainly represent a simplified scenario of one-dimensional heat transfer. A need to investigate behavior of spontaneous ignition in dust deposits in complex geometries forms the motivation of this work. The effect of hot surface geometry is experimentally studied by devising wedge-shaped configurations having angles of 60o and 90o. Results show that ignition always occurred around the top region in the case of 60o wedge, and in the top and middle regions in the case of 90o wedge. These trends are explained by investigating three parameters affecting the ignition behavior, namely, the heat transfer from the hot plate to the dust, the rate of heat transfer between different regions within the dust and the minimum volume of dust required to produce sufficient heat release. A mathematical method has been proposed to predict the ignition behavior of dust deposit subjected to any boundary conditions arising due to geometrical confinement. Further, numerical simulations have been carried out to simulate the conjugate heat transfer in the interface of dust surface and air. Both analyses, mathematical and numerical, compare well with the experimental data. Furthermore, in the standard test method, ASTM E- 2021, a metal ring is used to contain the sample dusts. It is observed from experimental and numerical simulations that the resultant temperature field is not one-dimensional as desired since the corner part ignites first due to heat transfer from both the bottom plate and the metal ring, which is at almost same temperature as that of bottom plate. Theories those describe the thermal ignition in these standard tests, use the assumption that the heat flow is unidirectional. Therefore, a better substitute to the metal ring has been proposed as a ring made out of an insulating material (having low thermal conductivity). This makes the heat transfer to the dust layer phenomenally one-dimensional. Another leg of the experiments have been carried out to investigate the effect of weathering of combustible dusts on their spontaneous ignition process. Two types of weathering methods, heat- and moisture-weathering are used. Sample preparation and weathering quantification methods follow the standard test procedure. Thermogravimetric analysis has been employed to understand the variation in weight loss of fresh, heat-weathered and moisture-weathered samples of coal and organic dusts. Preliminary results show that heat weathering increases the hazard level for organic (wheat) dust. In summary, the current research work mainly involves modification of the standard test method such as ASTM E-2021 to include an insulated ring instead of a metal ring to ensure one-dimensional heat transfer and extending the test method to include wedge-shaped geometries. The spontaneous ignition of combustible dust in the new setups is investigated thoroughly. Furthermore, mathematical and numerical models have been proposed to simulate the experimental tests. Finally, the effect of two types of weathering processes on the characteristics of spontaneous ignition has been studied. In all the cases, results are thoroughly discussed with the explanation of the physics involved.

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