Journal of Loss Prevention in the Process Industries
Assessing the self-heating behaviour of Callide coal using a 2-metre column
Introduction
Self-heating poses a significant hazard for both opencut and underground coal mine operations in Australia. Since the Moura disaster in 1994, which resulted in 11 fatalities (Windridge, 1996), the hazard has been managed with varying degrees of success as sporadic heatings continue to occur. One of the major problems facing the coal industry is producing reliable hazard management plans that require identification of the self-heating propensity in advance of mining, during transportation and stockpiling. Coal users such as power utilities face a similar problem when storing the coal in stockpiles and bunkers.
One of the major criticisms levelled at small-scale (100–200 g samples) laboratory methods used to rate the propensity of coal to self-heat is that they often bear little resemblance to the actual conditions under which the process occurs in nature (Cliff, Rowlands, & Sleeman, 1996). For example the crossing point temperature or relative ignition temperature tests are always obtained at elevated temperatures, where the main heat source initially is provided externally to the coal. However, these methods do provide a rapid measure for rating coals as to their propensity to self-heat. The closest simulation to actual conditions, at small-scale, is provided by the adiabatic self-heating test developed by Humphreys (1979). This test is routinely used in the assessment of coal self-heating propensity of Australian and New Zealand coals (Blazak, Beamish, Hodge and Nichols, 2001, Beamish, Barakat and St George, 2001). The index parameter obtained is known as R70, and is the average adiabatic self-heating rate of the coal from 40–70 °C, expressed as °C h−1. Generally, coals with R70 greater than 0.8 °C h−1 are considered to be highly prone to self-heating (Moreby, 1997).
Large-scale (16 t) self-heating tests have been successfully applied at the Queensland Safety in Mines Testing and Research Station (SIMTARS), with the limitation that it can take 6–12 months to generate one result (Cliff, Davis, Bennet, Galvin, & Clarkson, 1998). The time taken for these tests is dependent on the physical parameters used in the test (e.g. coal size, moisture content etc.) and the rank of coal, which affects the reactivity. Smith, Miron and Lazzara (1991) performed large-scale spontaneous combustion studies using a 13 short ton chamber, with high-volatile C bituminous coals. One of these coals reached thermal runaway near the centre of the coalbed after 23 days from an initial starting temperature of 30 °C.
Bulk sample testing has also been successfully applied using medium-scale (40–1000 kg) test equipment. Stott (1980) reported the use of a 5 m long, 0.6 m diameter vertical container in the US. This experiment ran for 5 months and the coal temperature only rose to 45 °C due to insufficient insulation of the outside of the column by normal means. It was recommended that a similar smaller apparatus be constructed with approximate dimensions of 2 m long and 0.5 m diameter. Chen (1991) followed these recommendations under the supervision of Stott, and built a so-called ‘Full-Scale Experiments Apparatus’, which was 2 m long and 0.3 m in diameter. The equipment was used to study New Zealand coals ranging in rank from lignite to high volatile bituminous (Stott & Chen, 1992).
This paper presents the results from recommissioning a 2-m self-heating column at The University of Queensland (UQ), using subbituminous coal from the Callide Coalfields, Queensland. The coal used in the column was also tested with the UQ small-scale adiabatic oven to obtain self-heating rate (R70) index values.
Section snippets
The University of Queensland 2-m self-heating column
Arief (1997) developed a column self-heating test apparatus at UQ measuring 2 m long and 0.2 m in diameter (Fig. 1), which was a modified version of the column used by Chen (1991). As suggested by Chen (1991), and from a practical viewpoint, the diameter of the column was reduced so that the amount of sample needed to undertake the experiments was relatively small and the column could be loaded and unloaded by one person. The column was used with as-received coal samples that would incorporate
Sample details and preparation
A 60x40x40 cm block of coal was supplied in May 2000 from the Dunn Creek Mine operating in the Callide Coalfields, Queensland. The block was stored in the laboratory freezer 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 −75 mm and placed in the UQ 2-m self-heating column for testing as a two-layer sequence. A sieve analysis was
Results of column self-heating
Analytical results for the two layers used in the column are contained in Table 2. The coal is subbituminous A in rank, with ash content less than 11% on a dry basis and high as-received moisture content (>11%). The initial self-heating rate (R70) index values for each layer are 5.90 °C h−1 for the frozen-stored block and 2.73 °C h−1 for the unfrozen-stored block. These values indicate the coal is still highly reactive even after being stored for over 12 months. An R70 index value for fresh
Discussion
The initial hotspot generation in the unfrozen-stored coal appears contrary to the R70 self-heating rate indices for each layer. This is most likely due to the moisture states of the two layers. As the frozen-stored coal thawed in the column, it would have retained a higher surface moisture and inherent moisture compared to the unfrozen-stored layer which would have reached an air-dried moisture state consistent with the laboratory conditions. Vance (1993) and Hamilton (2001) show that as
Conclusions
Subbituminous coal from Callide Coalfields has been used to successfully recommission a 2-m, adiabatic self-heating column. The results obtained show that:
- 1.
Initial heating develops at the top of the column due to moisture adsorption.
- 2.
A final hotspot developed at 60 cm from the air inlet consistent with the higher self-heating rate index of the coal at this location. Hence, it is possible to use the column to reconstruct seam profiles and determine layering effects within a seam.
- 3.
The heating that
Acknowledgements
Callide Coalfields Pty Ltd have been a major sponsor of this research thanks to the generous foresight of Wes Nichols. Iain Hodge obtained the coal block used in the column and personally transported it from Callide to the University of Queensland. Owynne Radford provided the sound technical expertise needed to ensure the column became operational again.
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