Energy release estimation key to managing outburst risk

OUTBURSTS at underground coal mines are failures which are accompanied by significant energy release, according to a report by ACARP.

Lou Caruana

The report provides a basis for estimating this energy release, though a less than certain one for the case of gas contained in pore space.

It should be considered on the basis of energy per unit volume and energy over the size of (un)expected structure, according to ACARP. This value will depend on the diffusion coefficient, gas content/pressure and particle size that is likely to exist or be generated.

The report highlights serious deficiencies in some of the common procedures to measure gas content and casts doubts on the validity of isotherms. It is also at issue with some of the adopted outburst thresholds.

“Energy release does not occur without failure and the failure mode and fragmentation that accompanies it are key to the energy release that follows,” ACARP said.

“If coal has already failed because it has been structurally altered, as in a fault gouge, then the sudden release of energy from expanding gas is greatly facilitated and a major outburst can be expected.”

Outbursts from such situations are by far the most common, and can lead to thousands of tonnes of material and more than a hundred thousand cubic metres of gas being released if major structures are intersected.

The less certain case is when the coal is apparently solid but actually contains a multitude of small scale structures which will lead to its failure and fragmentation.

These have been the subject of major study in this report. The approach has been to examine coals in polished section for such structures which have indeed been found to exist.

Also coals were tested by sudden desorption by a new method and by using a Protodyakanov drop hammer as used in Russia and China. This work has established that some coals will indeed fragment.

Failure in an outburst situation is greatly facilitated by the removal of material from the face of the outburst by gas. This unloads the face and prevents the failure choking off on material that is broken.

Once the fragmentation process has occurred the prime energy release to drive the outburst is provided by desorption of gas.