Underground » Ventilation, Gas Drainage and Monitoring
This project is an extension of Project C6024 (Wold and Choi, 1999). The project aims to develop a high level numerical model for the simulation of coal and gas outbursts, for the purposes of:
- improved application of outburst risk criteria under variable mining conditions
- improved application of outburst control measures
For outburst risk analysis and for outburst control and management, it is important to be able to assess both the likelihood of an outburst event and the consequence in case such an event does happen. The previous project focused on the former, and this project focuses on the latter - the likely volume of coal that can be expelled, and the volume of gas that can be emitted, taking into account the influence of gas composition. This project also seeks to gain a better understanding of the cause of the observed differences between CH4 and CO2 outbursts.
Model development
The work involves mainly theoretical development and enhancement of an existing numerical model. Due to the lack of laboratory and field data on coal fragmentation and on the kinetics of gas desorption in coal, the new model developed is based mainly on existing theories and most recent understanding of the processes that have been published in the literature in related fields. In the modelling studies, pre-initiation quasi-static yielding type failure of coal is distinguished from the post-initiation failure of outburst coal, as the latter is much more dynamic and violent, involving fragmentation of the coal and rapid release of gas. The quasi-static yielding failure is modelled using macroscopic plasticity theory with softening, and the fragmentation process is modelled using continuum damage mechanics. The effects of particle size on the rate of mass transfer between adsorbed gas and free gas during fragmentation are studied using both the more conventional two-stage desorption diffusion processes and a new hypothesis based on the kinetics of desorption. The violence of the model outburst is estimated using the momentum of the outburst coal. The potential volume of gas that can be released immediately after an outburst is estimated based on the amount of desorbable gas in the ejected coal and the gas flux from the faces. The new hypothesis appears to be able to better explain the difference in violence between CO2 and CH4 outbursts.
Model results
The model results do not indicate that coal seams rich in CO2 are more outburst prone than seams rich in CH4. On the contrary, because of the higher adsorption capacity of coal to CO2 relative to CH4 at the same partial pressure, the results suggest that outbursts tend to be initiated at higher gas content for CO2 compared to CH4. On closer examination, the observational data collected by Lama (1996) do not appear to show unequivocally that outburst will occur at a lower gas content for CO2 compared to CH4.
The model results suggest that CO2 outbursts tend to be more violent mainly because of the greater adsorption capacity of CO2 under the same partial pressure compared to CH4. After an outburst has been initiated, there is a higher rate of mass transfer of CO2 from the adsorbed state to the free state, the greater amount of free CO2 provides more energy to fragment the coal and a greater drag force to act on the fragmented coal, leading to a more violent outburst compared to CH4
The likelihood of outbursts appears to be less in stronger coal. Also the degree of violence decreases with an increase in coal strength. It appears that, if the strength of the coal is high enough, the outburst process can be subdued.
Damage reduces the strength and the stiffness of the coal. The results show that, during an outburst, a substantial amount of energy can be released suddenly, together with transfer of load to the adjacent more intact material. This can cause the adjacent coal to fail and burst. This would suggest that the likelihood of outbursts may increase when the coal is more friable.
The model results have enabled a better understanding of the influence of some of the factors and processes on post-initiation outburst evolution, and the possible cause for the more violent CO2 outbursts compared to CH4. Methods of predicting the potential volume of outburst coal and gas have also been developed. In the absence of laboratory data, great effort has been spent to ensure that the new models are based on sound physical principles. However, in order to validate the model and to apply the model to field problems, further work is required.