Getting the most from spontaneous combustion testing

BULK coal self-heating tests conducted at the University of Queensland's Spontaneous Combustion Testing Laboratory are rapidly becoming a preferred option for input to mine plans, writes Dr Basil Beamish.
Getting the most from spontaneous combustion testing Getting the most from spontaneous combustion testing Getting the most from spontaneous combustion testing Getting the most from spontaneous combustion testing Getting the most from spontaneous combustion testing

Non-gassy coal reaches the intense oxidation stage very rapidly

Staff Reporter

The technique has been developed through ACARP project C12018. To date, the test has been successfully used to assess 45kg of coal core obtained from a greenfields project area. Recently the range of coals tested has been extended to New Zealand mining operations. Results obtained in 2004 are now being put to practical use as a new longwall comes into production in New South Wales.

The results obtained closely simulate the three stages of hot spot development.

Initial warming (by far the longest stage of the heating)

Evaporation and condensation of water (creating a maximum temperature plateau effect around 80-90C, during which time the hot spot migrates slightly downwind before starting to migrate upwind)

Intense oxidation (once the coal becomes dry locally the maximum temperature rises rapidly, with the hot spot continuing to migrate upwind towards the air source and increasing in size)

If left unchecked, the hot spot would soon lead to local ignition. Safety devices on the equipment, though, limit the coal temperature to less than 250°C, with most tests terminated at 220°C.

Generally, with the airflow rates used, the test is completed in less than a month, and the time to final hot spot development is measured on the as-received coal. The progress of the hot spot development is monitored by thermocouples measuring the coal temperature. The corresponding gas evolution history is recorded from gasbag samples analysed by Simtars using Gas Chromatography.

The gas evolution results can be interpreted in two ways. Firstly, the gas indicators can be plotted against the maximum temperature of the hot spot to show the general trends that occur. Secondly, the gas indicators can be plotted with the hot spot temperature history to help identify the stages of hot spot development.

As part of the assessment, additional R70 data is obtained on the coal using the UQ adiabatic oven. Nominally, three results are obtained on samples taken from the as-received coal. This provides a measure of the reactivity of the coal, which is then checked against the bulk coal results.

A major difference has been identified between gassy and non-gassy (gas-drained, partially dry) coal from the bulk testing. The non-gassy coal reaches the intense oxidation stage very rapidly, as shown in the figure, and exhibits a different gas evolution pattern to the gassy coal. This has major implications for mines practising seamgas drainage or underground mines that will be developed into pre-drained coal.

ACARP project C12018 is scheduled for completion at the end of August this year. Details on the bulk testing can be obtained from: Dr Basil Beamish, b.beamish@minmet.uq.edu.au

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