An experimental study into the hot surface ignition of coal dust layers from the Callide Coalfields has been conducted. The Australian Standard for classification of hazardous areas subject to such combustible dusts (AS 61241.3) identifies the potential for ignition of combustible dust layers that build up on surfaces with the potential to heat up. Hence electrical equipment installed in hazardous areas has to be designed to operate below the minimum ignition temperature of the dust relevant to that area even under abnormal conditions (such as overload conditions). For this reason it is important to establish certain critical parameters for the dust that will be likely to affect its minimum layer ignition temperature (LIT) and thus the resulting temperature rating for electrical equipment in the area of deposition.
The minimum LIT of a coal dust, or any self-heating dust layer, theoretically has no absolute ignition temperature. However, a minimum LIT can be defined for a given set of conditions that include layer thickness and particle size. The effect of layer thickness has already been well established in previous studies, all concluding that minimum LIT varies in an inverse manner with layer thickness. Consequently, this paper will present new results on investigations using a standard hot plate ignition apparatus to test the effect of particle size and coal composition for the Callide coals. Samples from the Trap Gully (TG), Dunn Creek (DC) and The Hut (TH) pits have been tested since they are known to exhibit varied self-heating potentials across a broad range of mineral matter constituents in the coal.
From this test work it has been established that the minimum LIT of a coal dust is quite reproducible for fresh samples, with ignition temperatures determined to 5oC rather than the 10oC as outlined by the International Standard test procedure (IEC 1241.2.1). The results also display that as particle size decreases so does the minimum LIT, with a 20°C fall in minimum LIT observed as particle top size went from -250μm to -75μm. This variation is considered insignificant in relation to the effect of layer thickness changes, where only a small change in the layer thickness will produced the same rise or fall in minimum LIT as a large change in particle size.
It has also been concluded from the experiments that a relationship exists between minimum LIT and mineral matter, and a rank association is evident across the three pits tested. This occurrence is clearly observable on an ash (dry basis) verus minimum LIT plot, where a rise in the ash level in the sample will produce a higher minimum LIT. Consideration of these trendlines along mineral matter compositional relationships displayed that the type of mineral matter contained within the sample indeed had an effect on the minimum LIT. Cores that were kaolinite dominant had a higher temperature to ignition than cores of increasing amounts of siderite and quartz (approximately 15°C higher for a 25% fall in kaolinite across the Trap Gully coals) with all else being equal. This is due to the higher heat capacity of kaolinite as compared to the other minerals found in the Callide core samples. The use of the 5°C interval for determination of minimum LIT facilitated these observations to be made, that would have been missed under the standard test procedure.
Other results for volatile matter (dry basis) supported these findings, producing the same mineral matter compositional trendlines that were observed in the ash (dry basis) analysis. A linear relationship also existed between minimum LIT and selfheating rates (R70), thus indicating minimum LIT could be used as another indicator of the coals propensity to spontaneously combust.
These findings will give coal mine, power station and coal port terminal operators a better understanding and graphical method of predicting a coals minimum LIT. This will allow for fast and accurate response to an identified hazard, using common geological data such as ash content and mineral matter composition, which will reduce the potential for losses in equipment, production and even life dramatically.