Increasing longwall dimensions create new dilemmas ... Part 2

Staff Reporter

It may surprise some younger engineers that the basic equation for calculating frictional losses in ventilation circuits (Atkinson’s equation) was formulated in the 1850s, long before the introduction of PCs and network simulation programs. Compared to other spreadsheet models developed by these engineers on site, the calculation of quantity-velocity-pressure or gas emission versus production is trivial and can easily be applied to coal mine ventilation circuits. Most coal mine circuits do not involve a “Hardy Cross” split and therefore a network program is not necessarily required.

For acceptable differential pressures, the capacity of a mine ventilation circuit is determined principally by the number and length of mains headings and the total pressure-quantity duty of production panels. The most straightforward method of increasing ventilation capacity of the mine as a whole, or individual panels, is to develop additional headings. There are clearly operational and cost implications, however, this method is widely used in high production North American mines. It should be recognised that, if block lengths are increased significantly, the effect of additional mains headings on development ratio may be small but the effect on ventilation capacity is profound.

Three-heading gate roads provide a solution to high panel pressure differentials and facilitate several additional longwall circuit options. The additional development does raise operational, cost and mine scheduling problems, however, it is apparent that extending two-heading gateroads beyond circa 4km will be problematic in terms of applied pressure

differentials and intensity of gas drainage required. To avoid increasing the number of gateroad headings, one NSW mine has previously split longwall blocks with single headings developed between adjacent gate roads.

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.

Continued in Part 3.

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