Effect of weathering of coal and organic dusts on their spontaneous ignition

Weathcring of coal and other cellulosic dusts occur duc to the process of wetting and subsequent drying, or by subjecting them to a temperature higher than the ambient temperature for prolonged time periods. The first type of weathering occurs in a wetted storage. The second type of weathering occurs when a dust processing unit stores and maintains the dust deposit at an elevated temperature. As a result of weathering, the physical and thermal properties of the dust may change. Therefore, the weathered dust is expected to ignite at a different temperature, when compared to that of a fresh sample. In this study, a standard test method (ASTM E 2021) is used to determine the safe surface temperature, which would avoid self-heating of a weathered combustible dust deposit. Weathering of two types coal dust samples - (a) wheat flour, (b) Pittsburgh seam coal and (c) powder river basin coal are carried out by two techniques; namely, (a) controlled wetting and drying of coal dust and (b) subjecting the coal dust to a uniform temperature, 70 °C below the minimum surface ignition temperature obtained using a fresh sample, until no further change in temperature occurred at any height in the dust layer. The implications of the observed results on industrial safety measures related to combustible dust layers are discussed.