Underground » Ventilation, Gas Drainage and Monitoring
This report should be compulsory reading for those responsible for mining coal with significant gas content.
This report has been prepared as a document which describes outbursts and their mechanism in an environment where most Australian Coal miners have never seen an outburst. Its first function is hopefully therefore to educate. Its second function is to suggest ways by which outburst conditions may be predicted and to advise on methods to prevent them occurring.
The reason that outbursts are extremely rare in Australia is the success that the mining industry has had in preventing them. Gas drainage and a conservative protocol has virtually eliminated the occurrence of outbursts. However the cost of this is sometimes quite great in as far as areas that cannot be drained cannot be mined, or can only be mined by shotfiring. These limitations have severely slowed or halted development in some cases. Hence an attempt is made in this report to put forward methods to assist in determining whether outbursts can occur under varying conditions.
In an endeavour to provide a basis of prediction as to whether an outburst will occur it takes the approach of examining the total energy that may be released in an outburst. The sources of energy considered are the strain energy that may be released in failure of the coal and in the release of gas. Two modes of gas release are considered, one from pore space and one from diffusion. In the latter case a new model is developed to describe the potential energy release from diffusing particles. In the Australian context it is considered that the elements of energy release due to gas dominate.
The critical factors that contributing to energy release in an outburst are:
- Gas Content/Gas Pressure
- Diffusion Coefficient
- Sorption Isotherm
- Particle Size
The less critical factors are:
- Free pore space
- Stress
- Stiffness
A determination of the total potential energy that may be released from an outburst is considered to be a better method of determining the outburst risk than current single parameter methods in use. It is proposed that this should be developed to replace current systems. Such a development is likely to enable mining in gassy but hard coals but may impose different restrictions on mining fine gouge material.