Development of a spontaneous heating liability index of coal from kinetic parameters

Spontaneous heating of coal leading to fires is a serious problem in the coal mining industry. These fires create several problems, such as accidents, loss of coal reserves, sterilization of resources, diminution of the heating value and coking properties of coal, environmental pollution, etc.
In India, the crossing point temperature (CPT) method is used for estimating the susceptibility of coal to spontaneous heating; a low susceptibility is always predicted for coals with a high moisture content. However, in reality, these coals are highly susceptible. Some researchers have used other experimental techniques for assessing the proneness of coal to spontaneous heating. Most of the research findings on this aspect are empirical in nature. With this in mind, an effort was made for this paper to develop a spontaneous heating liability index (SHLI) of coal, based on a mathematical approach using kinetics of low-temperature oxidation. The rate of this oxidation is a function of the temperature-dependent rate constant and the reaction model. The effect of temperature on the rate constant is introduced through the use of an Arrhenius equation. Finally, the energy transferred through coal during reaction is predicted by applying the Frank-Kamenetskii model for conductive heat flow. After incorporating all the concepts of coal oxidation and using a mathematical approach, SHLI was developed and is presented in this paper. The parameters required for computation of the SHLI are the enthalpy change or heat of reaction, specific heat of coal, rate constant and time required to reach a certain temperature. All of these parameters were obtained by differential scanning calorimetric (DSC) studies.
In order to test the validity of this index, 31 coal samples representing both fiery and non-fiery coal seams of different ranks were collected from different Indian coalfields. DSC studies were conducted on all the samples using a Perkin-Elmers DSC-7 calorimeter. The kinetic parameters and specific heat values obtained from DSC experiments are used to compute the SHLI values. In addition, the susceptibility of these samples to spontaneous combustion was ascertained by standard crossing-point temperature measurement, and the moisture percentage of all the samples was obtained by proximate analysis. A comparative study was carried out between the SHLI and crossing-point temperature, along with moisture percentage. There are cases in which the SHLI and crossing-point temperature do not predict similar results. In these cases, the spontaneous heating tendency of these coal seams have been noted in the field conditions and compared with the spontaneous heating tendency predicted by both the SHLI and crossing-point temperature. Finally, it was concluded that the SHLI developed and reported in this paper predicts the susceptibility of coal to spontaneous heating more accurately than crossing-point temperature. This index has a sound theoretical base and the results are reproducible. The SHLI values obtained in the present study vary in wider range, i.e. between 2.40 to 150.93, thus providing a broad range for classifying coals with respect to their proneness to spontaneous heating. The findings of this study will be very useful to mine planners and engineers in taking preventive measures in avoiding a high occurrence of fires due to spontaneous heating.
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