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Australian Mine Methane Drainage Case Study. Three (3) Heading Gate Roads Used. Anglo’s Grasstree Mine.

Australian Mine Methane Drainage Case Study. Three (3) Heading Gate Roads Used. Anglo’s Grasstree Mine.

Gas Drainage Guidelines UN 2016

UN Best Practice Guidance for Effective Methane Drainage and Use in Coal Mines has a number of case studies Example 3 is an Australian Mine and by the reference to Moura No 2 Disaster a Queensland.

This is on page 86 of 108

It talks about how the introduction of 3 heading longwall gate roads because of increasing methane load.

The original plan to employ three-heading gate roads was correct in providing a longwall ventilation circuit capacity of 100 to 120 m3/s (2,000 to 2,400 l/s CH4 at the return limit of 2.0%)

Grasstree Mine is the unnamed mine used in the Example.

Grasstree did not stick with the 3 heading gate roads and reverted back to the standard 2 headings employed by all other Australian Mines.

Only Anglo can explain the reasons for abandoning what was being touted as best practice by the United Nations.

Grasstree is one of the Anglo Sister Mines to Grosvenor.

Grasstree Mine is specifically name under the Grosvenor Inquiry Terms of Reference

 inquire into the incidents described in subparagraphs a. to e.

  1. the 11 high potential incidents that occurred at Grasstree mine (operated by Anglo Coal (Capcoal Management) Pty Ltd) involving exceedances of methane (>2.5%) in and around the longwall on various dates between 1 July 2019 and 5 May 2020;

Is it just a c0-incidence that Grasstree after abandoning 3 heading gate roads had 11 methane HPI’s runs second only to Grosvenor with 27 in 10 months?

Anglo must had some reason why they did not keep 3 headings at Grasstree and after the International recognition as best practice introduce 3 headings at its other gassy underground coal mines in Queensland?

One thing is for certain we will never find out at the Grosvenor Board of Inquiry.

The question will never be asked.

The statement below and the comments about drainage boreholes and their introduction should have been asked of Grasstree Management when they were on the stand last year.

But like anything else, the deliberately restrictive time frames imposed by Minster Lynham will make sure that this goes straight through to the keeper

Recognising that, in future blocks, gas emission to ventilation net of 85% goaf capture will still prove problematic for the ventilation system, the mine is now attempting to also pre-drain thicker roof target seams using approximately 2.0 km long holes drilled along longwall axes. These holes will serve initially as predrainage and after under-working as goaf drainage holes targeting close face gas emission. Conventional multiple seam completion frack wells may also be considered should additional predrainage be required above future deeper workings.

Initial conditions:

A new series of longwall blocks is located in a 2.8 m-high seam with methane contents ranging from 8 to 14 m3/t. Depth of cover is 250 m to 500 m with surface access generally unconstrained by surface features.

In situ gas content must be reduced to or below 7.5 m3/t to satisfy the outburst prevention code.

There is a single floor seam and eight roof seams containing 10 m to 15 m of coal within the nominal caving zone.

Longwall blocks are 350 m wide and up to 3.6 km in length (Figure 9.5), with a planned production rate of 200,000 tonnes per week.

High potential gas emission values led the mine to develop three-heading gate roads on longwalls from the outset in order to provide a high volumetric capacity ventilation system for gas dilution. A three-heading gate road allows substantially more air to be provided for gas dilution to the return end of a longwall face, without increasing face air velocities, compared with a conventional U-ventilated system.

This is currently the only mine in Australia to employ three-heading gate roads.

Gas control problems: Gas emission predictions indicate likely specific emissions of 15 to 30 m3/t from coal seam sources. At planned production rates, this would equate to 3,500 to 7,000 l/s CH4, generally increasing with depth.

However, previous studies at an adjacent mine demonstrated substantial extraneous gas which could significantly increase the total emission rates. Emissions from the first three longwalls were controllable within the existing design but were higher than expected for the relatively shallow depths. Extrapolation to the deeper longwalls indicated feasibility stage predictions would be exceeded with emissions possibly reaching 9,500 l/s.

Solutions: Development phase outburst and frictional ignition limits are reached using a combination of surface to inseam medium radius drilling (MRD) techniques supplemented with underground directional holes and compliance holes that are cored for gas content testing. The initial pit bottom area was pre- drained with tight radius drilling (TRD) techniques.

The original plan to employ three-heading gate roads was correct in providing a longwall ventilation circuit capacity of 100 to 120 m3/s (2,000 to 2,400 l/s CH4 at the return limit of 2.0%). It is important to note that, following the Moura disaster of 1994 where 11 miners died, coal mine regulations, guidelines, and custom and practice in Queensland prevent mines from employing a full U.S.-style flood ventilation bleeder system. However, controlled bleed with due consideration to the location of potentially explosive mixtures and control of spontaneous combustion is possible.

The realistic dilution capacity of a bleeder system in these blocks is well below total longwall gas emission rates and alternative strategies are required. To date, the mine has successfully employed conventional surface to goaf drainage holes (300 millimetre diameter at 50 m spacing located on the tailgate return side) to reduce the gas emission load on the ventilation system. This strategy has achieved an average 80% capture (goaf drainage plus ventilation) with peaks of about 85% at high gas stream purity (>90% CH4).

The gas collection infrastructure is on the surface, using 450 millimetre diameter pipes, including that from vertical connections to underground directional holes. All surface gas streams from underground predrainage, surface MRD predrainage, and goaf holes are exhausted to a mobile goaf drainage plant and central pump station from where about 2,200 l/s of gas is discharged to 16 x 2.0 MW gas engines with the balance flared. The site policy is to avoid direct discharge of captured gas if at all possible.

Recognising that, in future blocks, gas emission to ventilation net of 85% goaf capture will still prove problematic for the ventilation system, the mine is now attempting to also pre-drain thicker roof target seams using approximately 2.0 km long holes drilled along longwall axes. These holes will serve initially as predrainage and after under-working as goaf drainage holes targeting close face gas emission. Conventional multiple seam completion frack wells may also be considered should additional predrainage be required above future deeper workings.

The original plan to employ three-heading gate roads was correct in providing a longwall ventilation circuit capacity of 100 to 120 m3/s (2,000 to 2,400 l/s CH4 at the return limit of 2.0%). It is important to note that, following the Moura disaster of 1994 where 11 miners died, coal mine regulations, guidelines, and custom and practice in Queensland prevent mines from employing a full U.S.-style flood ventilation bleeder system. However, controlled bleed with due consideration to the location of potentially explosive mixtures and control of spontaneous combustion is possible.

The realistic dilution capacity of a bleeder system in these blocks is well below total longwall gas emission rates and alternative strategies are required. To date, the mine has successfully employed conventional surface to goaf drainage holes (300 millimetre diameter at 50 m spacing located on the tailgate return side) to reduce the gas emission load on the ventilation system. This strategy has achieved an average 80% capture (goaf drainage plus ventilation) with peaks of about 85% at high gas stream purity (>90% CH4).

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