The current economic climate, together with advances in longwall equipment technology, is leading the Australian coal mining industry to significantly increase longwall block dimensions, compared to historical norms.
Based on 1999-2000 Joint Coal Board statistics for 34 mines, the average Australian longwall mine would currently be described as having a 200m wide face, a working section height of 3-3.5m, a block length of circa 2km and be mining at a depth of 260m. The range of annual production is relatively high and obviously dependent on numerous geological and operational factors, however, the average is currently around 2 million tonnes per annum.
Benefits of increasing the dimensions of longwall blocks are principally improved development ratios, higher recovery, improved utilisation of face equipment and fewer longwall moves. An indication of this effect is obtained by plotting annual 1999-2000 longwall production against volume of coal per block (working height x width x length) for each .
As working section height is typically constrained by geological factors, mining engineers are now seeking methods of increasing the length and width of longwall blocks. Although face widths are increasing, typically from 200m to 250m-plus, the favoured method is to extend block lengths. A Queensland mine is currently developing gate roads to 4.2km, and two projects in Queensland and New South Wales are considering block lengths of 5-6km. These changes are combined, in some cases, with a plan to increase annual longwall production to circa 7.5Mt, or a weekly average of about 165,000 tonnes over seven days.
Together with other service and coal transport systems, ventilation and seam gas management strategies have to be designed to manage the following hazards to within acceptable or, more often, prescribed standards:
* Seam gas emission CH4 CO2 H2S
* Seam gas outbursts
* Frictional ignition
* Spontaneous combustion
With respect to these hazards, some shallower mines in New South Wales operate in relatively benign conditions, while others, for example in central Queensland, must manage most of them to a high standard. Hazards associated with seam gas emission, windblast, heat and dust are, to various degrees, production dependent and therefore consequential risk will increase. Risks associated with other hazards, such as spontaneous combustion, will increase with block dimensions and the higher ventilation rates required to manage contaminant loads.
It is also recognised that some fundamental controls are common to all hazards, for example, gas monitoring and ventilation devices, while others conflict, for example, high ventilation rates for gas management resulting in higher differential pressures and therefore an increased risk of spontaneous combustion.
Nearly all Australian longwall mines use a two-heading gateroad ventilation circuit with various modifications to the conventional “U” configuration. Consequently, for a given development profile, the ventilation capacity of a panel is determined by limits of ventilation pressure and airway velocity together with the airway configuration used. In regimes where seam gas contents exceed residual (Q3) levels (1-1.5 cubic metres per tonne) gas emission is strongly dependent on development advance and longwall production rates. Consequently, the magnitude of these hazards to be managed will increase with longwall geometry and production rate. In addition, heat management is now becoming more problematic in Queensland mines, subject to high summer surface temperatures, as the power rating of face and conveyor equipment is increased.
The minimum ventilation rate to be delivered in development panels is typically determined by 130% of the open circuit capacity of auxiliary fan(s) employed. Ventilation rates may then need to be increased further in order to maintain acceptable concentrations of seam gas (CH4 and or CO2) in intake, face and return airways. Pre drainage or rib gas capture holes are then required when gas emission exceeds practicable ventilation capacity and or gas contents exceed outburst threshold limits.
The ventilation rate at the start of a panel, and pressure differential to be applied, depends on development profile, air quantity, leakage through cut through stoppings and length of panel.
For the “average” Australian mine, a 2km gateroad would require a ventilation duty at the start of the panel of around 40cu.mps at 275Pa to deliver 35cu.mps at the last cut-through. At 5km, this duty increases to around 54cu.mps at 830Pa. If the pillar length is reduced, to say 60m, for place changing, the 5km duty increases to 67cu.mps at 1.1kPa. Employing additional auxiliary fans will further increase ventilation requirements, resulting in differential pressures in excess of 1.5kPa.
It would be difficult to achieve these higher duties with most surface fan installations, number of mains headings and self imposed limits of differential pressure currently used in Australian coal mines. An additional issue in NSW, is the interpretation of regulations governing methane concentrations on entry to the hazardous zone. The limit is currently 0.25% CH4 in the intake airway 100m outbye of the last completed cut-through.
This is not the case in Queensland, where it is understood that, by defining “explosion risk zones” this limit can be raised to 0.5% CH4 providing appropriate controls are in place.
Significantly higher ventilation rates and or intensive gas drainage will be required to increase gateroad lengths at depth with this limiting methane concentration in the “hazardous zone”
For example, at 5km, 60cu.mps of air would be required to manage an average rib emission rate of 3 litres/s/100m to a limit of 0.25% CH4. In regimes of higher gas contents (>5cu.m/t) or enhanced permeability, rib emission rates may be significantly higher than this, even with effective pre-drainage. This issue is likely to be the major obstacle to significantly extending two-heading gate road lengths.
Originally published in the March 2001 edition of Australia's Longwalls.