Methane Emission Prediction
Earlier investigations in this ECSC-supported project, suggested a link between the height of the methane gas emission zone around longwall workings and the fracture zone. However, fracture data was limited both in quantity and in scope of the depths and face lengths worked. IMC has now used FLAC (fast lagrangian analysis of continua) computer modelling to calculate the heights of fracture for a wider range of depths and face lengths. The model producing the main source of data is that for a vertical transverse cut through a district, the cut being parallel with the face. The model is two dimensional and does not include any effects due to the advancing face. Strata are included up to the surface and for a significant distance into the floor. The model has been configured not to calculate the post failure behaviour of the waste, the main interest being in the shape and size of the fracture zones delineating the goaf.
The results show a similar pattern of fracture to that demonstrated by other computer and physical models; that is, a trapezoidal zone delineated by two bounding fractures at the district edges, leaning in towards a horizontal fracture which closes the zone. In the floor (and in some cases the roof) the horizontal bounding fracture is absent. The height of fracture has been taken to be the height of the horizontal bounding fracture although the sub-vertical fractures generally exceed this level by about 13%. Although there are no horizontal fractures in the floor the ratio between the sub-vertical fractures in the roof and floor is approximately 3.4:1; slightly higher than the presumed value of 3:1.
The new data was analysed by plotting the height against the product of the depth and the face length. There is good agreement with previously acquired fracture data, but indications of higher zones of fracture than heights of emission. It is now clear that the height of fracture behaves in much the same way as the height of emission and therefore that both are controlled by the same mechanisms.
IMC's work in this area, supported by the ECSC, has progressed on three fronts.
Auxiliary Ventilation Systems for Continuous Miner Headings
The current trend towards the rapid development of mine drivages using roadheaders and continuous miners to serve conventional and multi-entry longwall retreat faces has exacerbated the environmental conditions experienced within these headings. RJB Mining identified an operational need, and has gained ECSC funding to carry out a study, to investigate the range of alternative auxiliary ventilation layouts and configurations which effect good and safe environmental and climatic control within these development headings.
The project is re-examining recent and current UK auxiliary ventilation practice and investigating the suitability, and applicability to British conditions, of alternative auxiliary ventilation configurations currently employed elsewhere in world mining (including the EU, the USA, Canada, South Africa and Australia). A series of scale model and gallery tests of various alternatives, including on-board primary exhaust overlap fan systems, will be used to correlate the development of a CFD computer model of the ventilation flow and pollutant dispersion. This correlated model will be used to study alternatives and their potential to promote efficient dilution and dispersion of dust, gas, heat and humidity at the face and along the length of the heading.
During 1998, a series of full scale experiments have been carried out in the Welbeck Colliery training gallery. These were first carried out without a continuous miner in the heading and then repeated with the miner at the head end. Anemometry and tracer gas was used to ascertain the airflow patterns at the head end and over the machine. A 1/10th scale model has been used to compare the data obtained from flow visualisation, pressure measurement and laser Doppler anemometry with data from CFD models. The work continues.
Further work has been carried out by IMC, with funding from the Coal Authority, on developing methods for assessing the gas emission potential for closed and closing collieries. Fundamental to the calculations is the premise that gas emission only occurs from those seams disturbed by mining and those which were able to release gas during the mining process.
Assessing the gas emission potential involves zoning the area around a colliery, and other linked sealed collieries, into areas with the same extraction history and calculating their area. Combining their area and the gas content data allows an assessment of the remaining gas with the potential for release to the atmosphere. If such gas is not vented then pressure will build up in the workings to the point where gas could escape from any existing, unplanned routes. If the gas is vented or pumped the calculations provide a comparative assessment of the volumes of gas that may be released.
The currently available data indicates that the gas pressure within old sealed workings increases in exponential form, suggesting that the flow of recharge decreases in line with the closeness to some limiting pressure within the mine void. Studies indicate that a sealed mine, or a mine under pump test conditions, can be modelled as a void of a fixed volume connected to the surface vent, with a gas feed into the void which is proportional to the difference between the pressure within the void and some limiting gas pressure. This finding is an extension of previous findings regarding descriptions of breathing voids under atmospheric pressure. The coefficient of proportionality between the pressure difference and the flow into the void can differ greatly for different sites. It is this coefficient which determines the suction pressure required to be applied to a mine to obtain any required flow. It is suspected that the coefficient is likely to increase with time as the driving pressure decreases within the strata.
The combination of assessment of the potential reservoir and the behaviour of gas flows after shut in, and/or pump tests gives a rounded view on the potential gassing future of abandoned mined sites and the potential pressures developed.
The studies have also highlighted the great importance of water levels on the potential gas emission from mines. Rising water levels may pressurise the gas and force gas within void space into the atmosphere. Conversely, the rate of flow of water may be much smaller than the rate of flow of gas from the mine, and by flooding gas sources the potential gas emission can easily be decreased due to sealing of coal seams by water pressure.