The phenomenon of spontaneous combustion can be problem where coal is found, stored or transported. Most studies regarding the spontaneous combustion of coal have been undertaken specifically through study of the situation at the minesite, at points of stockpile or utilisation facilities. It is not clear, however, whether there is a relationship between the propensity of coal to spontaneous combustion as it is moved within a materials handling system from the mine through washing and transportation to the utilisation facilities.
The main aim of this thesis is to investigate the trend in the liability of a unit coal to spontaneous combustion within the coal stream from the mine to the utilisation plant. Several tests and experiments have been undertaken to determine the liability of coal to spontaneous combustion at various points within the coal stream. The major tests were a newly developed "practical test" for spontaneous combustion, an autoignition test and spontaneous combustion oven tests. In addition, coal analyses, including petrographic, proximate and elemental analyses were undertaken to examine the relationship between spontaneous combustion and these parameters.
The "practical test apparatus" was developed as a modification of a device developed by a previous researcher. This apparatus was designed to model the behaviour of a stockpile of large mass and differs from the original apparatus in that the tested sample is in an "as received" condition to simulate field conditions closely and to avoid changing some parameters, including moisture content, due to sample preparation. This is one major factor that makes the present study different from previous studies which normally use crushed samples.
The new design for the practical test apparatus is one of the contributions of the “present study to spontaneous combustion research. The principle of this test is to measure 1· the temperature rise of coal under adiabatic conditions. This is achieved ·by passing regulated rates of airflow through an insulated, vertically mounted, cylindrical tube of 2 m length and 200 mm diameter. A comparison of recorded temperature rise of each sample was made to determine different liability of tested coals to spontaneous combustion. Details of the apparatus design are provided m the thesis and the results of tests undertaken are discussed.
The autoignition tests and spontaneous combustion oven tests were carried out using standard apparatus available in the laboratory. An apparatus developed by Grewer was used in the autoignition tests. Tested coals and a reference substance were heated in reaction tubes in a furnace at a constant rate of temperature rise with air moving until the coal temperature reached the reference temperature, established as the autoignition temperature. Different autoignition temperatures of the tested coals indicate different propensities to spontaneous combustion. A spontaneous combustion index can be developed using the results of this test.
In the spontaneous combustion oven tests, the assessment was based on the analyses of combustion gases produced from the coal samples when heated gradually in an oven from room temperature up to certain preset temperatures. The relative tendency of coal to spontaneous combustion was assessed by comparing the composition of the outlet gas stream of each coal sample.
The results obtained from the various tests were discussed to determine the trend in the propensity of coal to spontaneous combustion within the coal flow from the mine to the utilisation plant. They were also correlated with the results of the coal analyses to draw the relationship between spontaneous combustion of coal and parameters including ash content, maceral composition and particle size. The future application of the apparatus developed in this study and the implications of the present study for the coal industry are discussed.
The practical test for spontaneous combustion indicated that coal taken at the power station was the most liable to spontaneous combustion, while no substantial difference was recorded between coal sampled at the washing plant and run of mine. The auto ignition test, on the other hand, showed a clear difference in the propensity of tested coal to spontaneous combustion. Liability of coal to spontaneous combustion from high to low is coal taken at the power station followed by that taken at the washing plant and then run of mine. In general, the results of the spontaneous combustion oven tests showed that coal taken at the power station and run of mine was the most and the least liable to spontaneous combustion respectively.
The relationship between spontaneous combustion and maceral analysis showed that coal with a high vitrinite composition correlates with a high susceptibility to spontaneous combustion. In contrast, the higher the ash content of coal, the less liable is spontaneous combustion. The results of the gas analyses to determine the relationship between spontaneous combustion and coal particle size showed that no consistent conclusions could be made. However, from sieve analysis it was shown that coal which has the smallest average size was the most susceptible to spontaneous combustion. Samples of coal were from the Ebenezer and Jeebropilly mines and the Swanbank power station, Queensland, Australia.