Increasing longwall dimensions create new dilemmas ... Part 3

Staff Reporter

Two-heading circuits traditionally start by circulating most, if not all, of the mine ventilation capacity from intake airways through main headings, production panels and then back to an exhaust shaft located in close proximity to intake portals. As the mains extend, additional inbye exhaust shafts may be required to maintain acceptable pressure differentials and surface fan duty.

An alternative is to off-load return airways with higher capacity back return shafts that exhaust a significant fraction (20-30%) of the circuit volume.

When it is impracticable to mine additional gateroad headings, it may be appropriate to consider underground booster fans and or increase surface fan pressures above what is currently considered the norm. Booster and high pressure (>3kPa) surface fans have been successfully used elsewhere but do introduce additional hazards that must be controlled to acceptable standards. These methods will become more appropriate in deeper mines or those with restricted surface access. The application of passive or active pressure balancing of seals will also be required to manage goaf atmospheres as pressure differentials increase.

Pre-drainage by underground directional drilling is widely used to reduce seam gas contents prior to development and or longwall production. It is effective to a point, but is costly, requires underground access, involves significant lead times and has limited application for seams remote from the working section. Surface techniques for draining seam gas are being developed and applied by those seeking to generate power from coalbed methane rather than mine coal. Some of these techniques (tight radius drilling) allow for multiple seam pre-drainage from single holes and would be suitable for gas management at many coal mine locations. It is important that the power generation and coal mining industries continue to integrate their requirements for mutual benefit.

Where surface access permits, the most cost effective gas drainage method can be via surface holes into active or sealed goafs (goaf drainage). These techniques have been widely developed and will continue to be essential for gas management in the future.In Australia, refrigeration of ventilation is used exclusively by the metalliferous industry operating some deeper (>1100m) mines. In these mines, the combined effect of high surface temperatures (>24OC wet bulb), heating of intake air by auto compression and high virgin strata temperatures (>50OC) would lead to unacceptable intake temperatures for a significant period, if not all, of the year. Consequently, refrigeration is unavoidable and considered an essential part of the ventilation system.

Coal mines, subject to the same high surface temperatures in central Queensland, are currently much shallower (

However, the capacity of coal mine ventilation circuits to deliver higher ventilation volumes is limited and the principal source of heat is face equipment. Virgin strata temperatures are around 32OC at current depths of mining.

The fundamental problem is that air entering development and longwall panels are 25-26OC wet bulb during mid summer. Limiting face temperatures of 29O effective (30-31OC wet bulb) are then only 4-5OC wet bulb above those in the intake. Face and return temperatures are determined by the addition of heat from face equipment and volume of air used. For example, as an approximation, 50kW of heat will raise the temperature of 10cu.mps air by 1OC wet bulb. If the maingate intake temperature is 26OC wet bulb, the face volume is 40cu.mps and consumed equipment power is 1400kW then the tailgate return temperature would be 33OC wet bulb.

As with intensive pre-drainage of seam gas, refrigeration is normally considered a last resort for management of heat due to capital and operating costs involved. However, it is apparent that significantly increasing the power rating of face equipment and the limited ability to increase circulating volumes of air will make refrigeration more attractive as a solution for heat management in the future.

Heat loads are increasing significantly due to the power rating of face equipment, required to meet production demands, and outbye conveyor equipment required to transport coal over increasing distances. It must be recognised that, in a thermodynamic sense, mine equipment does very little real work (change potential or kinetic energy) and therefore the majority of consumed power manifests as heat. For example, if a longwall panel is using 1200kW of electrical power, approximately 1050kW will directly heat the air. This is the main reason for high tailgate return temperatures encountered in some Queensland mines.

Another impact of heat on two-heading longwall ventilation capacity is the traditional use of a “homotropal” belt road where one of the maingate intakes is effectively converted to a return. This may be effective in reducing heat load to face ventilation, but significantly reduces the ventilation capacity of a two heading maingate for gas management.

Solutions to ventilation and gas management issues resulting from increased longwall block size and production rates, fall in to two categories. Firstly, those methods currently employed in the Australian industry (directional drilling, underground post drainage, surface goaf drainage, homotropal belt road etc) which are accepted and can be applied with increased intensity and or capacity. And second, methods that are available but not currently employed, either due to risk management concerns (booster fans, intake ventilation past balanced goaf seals and higher differential pressures) or because the cost benefit and ability to apply techniques effectively have not been widely proven (three heading gate roads or refrigeration plant).

Perhaps the first step in solving these issues is for mine designers to recognise the difficulties currently faced in the management of pertinent hazards and to ensure that a realistic assessment of the consequences of their longwall design are obtained.

Originally published in the March 2001 edition of Australia's Longwalls.