Earlier ACARP research found that many of the index parameters used for identifying the propensity of coal to self-heat gave conflicting results. To finetune these index parameters needs further testing of a range of coals, using rigid testing procedures.

Being conducted at the University of Queensland’s (UQ) Department of Mining, Minerals & Materials Engineering, the 24 month long project has just begun the scale-up to a medium-scale (50 kg) adiabatic 2-metre column, and early results are encouraging.

Senior lecturer, Basil Beamish, who is overseeing the project, said earlier work on 150 gram coal samples shows that the results from this small-scale testing of coal can be applied to predict self-heating rate behaviour at a larger scale. Importantly, the research proves that the relationship between coal composition and self-heating rates can be determined under adiabatic test conditions.

It is expected that as a result of this work, there will be a reduction in both self-heating incidents and loss of product. A further outcome will benefit the management of coal stockpiles at mine sites, in transit and at ports. Beamish said moist coal sitting on dry coal had been found to have a significant effect on self-heating initiation. In blended stockpiles high self-heating rate coal is frequently next to low self-heating rate coal. By ‘layering’ the coal in the medium-scale column, Beamish hopes to determine the effects of blending and layering on hot spot initiation.

In the most recent test a block of coal was tested in a two-layer sequence in the medium-scale column. The coal from Dunn Creek Mine at Callide was supplied to UQ in May 2000. The block was stored in the laboratory freezer (to retard oxidation) until July 2000, when it was cut in half. One half of the block was left intact at room temperature and the other half was cut into slabs and put back into the freezer. In September 2001, both halves were crushed to -75mm and placed in the self-heating column.

R70 values were obtained for each layer using the UQ small-scale adiabatic oven. As the frozen-stored block was prepared frozen, the column was allowed to stand for one day to thaw the coal. Once air was introduced to the column, oxidation began almost immediately and a hot spot appeared in the unfrozen-block layer between 120-140cm from the column inlet. This was quite noticeable by Day 5.

“Initially this seemed to contradict the R70 values for each layer, which were 5.90 °C/h for the frozen-stored coal and 2.73 °C/h for the unfrozen-stored coal,” Beamish said. “However, the thawed coal layer had a higher starting moisture content than the unfrozen-block layer due to surface moisture from the thawing process. The self-heating curves show that once the surface moisture was removed, the coal with the higher R70 value took over as the hot spot generating layer. By Day 10, the temperature rise curves had crossed over and the new hot spot was easily identifiable at 60-80cm from the inlet.

“After Day 11, several intermittent technical problems occurred with the heating elements on the outside wall of the column, resulting in loss of adiabatic conditions for short periods. These problems were not sufficient to hinder the self-heating process. By Day 16, the hot spot in the frozen-stored layer advanced to the stage where it began to starve the upper parts of the column of air. The original hot spot had all but disappeared at this stage,” Beamish said.

A moisture build up in the outlet tube for two days resulted in retardation of the self-heating rate. Once this blockage was removed the hot spot developed rapidly approaching thermal runaway in about 10-12 hours. The test was terminated at this point.

A second recommissioning test is currently in progress with the coal layers reversed in the column. The results from the first test appear to fit a finite difference numerical model proposed by US researchers, Beamish said. This aspect will be investigated in 2002 as part of a Mining Undergraduate thesis, jointly supervised with CSIRO.

The outcomes of this project can be transferred across the industry to assist with hazard assessment of greenfields projects including underground operations. In longwall operations the model could be used to assist with hazard management plans for spon com in goaf areas by testing self-heating of goaf coal in the column. The results from this test could be used to calibrate the model for an individual mine, thus enabling various scenarios to be run simulating mine conditions.

Coal from other mines will be needed for this study. Anyone interested in this work should contact: Dr Basil Beamish, Senior Lecturer in Mining Engineering, Department of Mining, Minerals and Materials Engineering, The University of Queensland, Brisbane, Qld 4072 or e-mail b.beamish@minmet.uq.edu.au.