Using the crossing point method to assess the self-heating behavior of Indonesian coals

Indonesian coal, although generally low in sulfur and ash content, is a lower rank coal with a relatively high moisture content (5-18%). Consequently, it has a greater propensity to self-heating behavior, and the aim of this paper is to assess the hazard for three typical coals that are used both domestically and for export. Experimental tests are reported using a new technique (the crossing point method) to determine the activation energy and reactivity of the low-temperature slow oxidation reaction. This uses the transient temperature profiles to deduce the activation energy and reactivity of the sample. The method relies on finding the center temperature at the point when a flat profile is observed (called the crossing temperature) and has been used successfully to ascertain the kinetic parameters of three Indonesian coal samples. It is found that the method is valid as long as the coal experiment is not greatly supercritical when it is then difficult to determine an appropriate crossing temperature. The method has the distinct advantage over the traditional Frank-Kamanetskii approach, in that only one oven experiment is needed for any one point on the kinetic plot, and that the results are not heavily dependent on the fan setting of the oven, or on sample size. A comparable experiment using the Frank-Kamanetskii approach of finding the critical temperature for different sample sizes showed good agreement for estimating activation energy.