Report: APP CMHS Project 1

CSIRO advises that the information contained in this comprises general statements based on scientific research. The reader is advised and needs to be aware that such information may be incomplete or unable to be used in any specific situation. No reliance or actions must therefore be made on that information without seeking prior expert professional, scientific and technical advice. To the extent permitted by law, CSIRO (including its employees and consultants) excludes all liability to any person for any consequences, including but not limited to all losses, damages, costs, expenses and any other compensation, arising directly or indirectly from using this publication (in part or in whole) and any information or material contained in it.

3.1.2. Explosion Control

Current Technologies

Explosions are the result of a number of sources; explosives, gas ignition from sources such as hot work, electrical sparking or frictional ignition, and dust-cloud ignition. In this respect an explosion is a secondary occurrence following the loss of control of primary hazards such as fuel (gas or coal dust) and ignition sources such as electricity, welding or heat from cutting machines.

The main preventative measures relate to the removal or control of either the fuel and/or the ignition source. The control of gas is covered within the next section of the report. Typical fuels that can propagate an explosion are coal dust, introduced gases (acetylene) or diesel fuels and flammable lubricants. Introduced gases, diesel fuels and lubricants are managed by systems and procedures and will not be covered in this report.

Current practice and technologies in managing the potential explosion of coal dust in the Australian coal mines include dust control measures such as water infusion in coal seams, water sprays, use of dust scrubbers and limestone dusting, and control of ignition sources.

Water infusion into coal seams can reduce air borne dust generation by up to 50 % given sufficient lead time and ideal seam conditions. Typical installations rely on mine water pressure being applied to the gas drainage holes. The use of packers at ~20 m intervals along a hole has been found to assist in diverting the water deeper into the drained coal and further reducing airborne dust.

Water sprays are widely used to control coal dust at coal faces, chain haulage, lump breakers, conveyor belts and conveyor transfer points. Coal dust control with water sprays have focused on spray optimisation and design, and use of additives in spray water to improve the effectiveness of the dust suppression.

Dust scrubbers have been shown to provide a significant reduction in airborne dust and are currently installed on longwall crushers, continuous miners and auxiliary fans.

The application limestone dust to roadways is recognised as the most effective way to suppress an explosion. Water or limestone dust barriers are also used to limit the extent of a gas explosion by providing an extinguishant that is raised into suspension by a shock wave. There is also some use of dust consolidation (products such as Dustex) to reduce the propensity for dust being raised into suspension. Wet stone dust is applied to conveyor belt roadways to suppress roadway dust levels. Whilst it is recognised that this cannot be raised into suspension to limit an explosion, additives are being developed that will assist in the drying and anti-caking, so that it will become an effective stone dust application process that will suppress an explosion.

There are three primary ignition sources in underground coal mines, namely electricity, friction, and incendive sparking (spontaneous combustion is covered separately). The control of each source is described as follows:

  • Electrical ignition sources are controlled by a combination of engineering and management systems. All control circuits are limited to a level of intrinsic safety; whereby a spark is not capable of igniting methane. All other electrical sources are protected by explosion proof enclosures, such that any ignition of gas is not capable of igniting an external gaseous atmosphere.

  • Frictional ignition typically occurs due to conveyor belting or rollers running in coal dust or, gas ignition at the cutting face caused by sparking from a cutter pick. Fire Resistant Antistatic (FRAS) material is used for conveyor belt and roller lagging and management and housekeeping standards if implemented and maintained will manage this issue. There is scope however, for alternative methods of coal haulage.

  • Incendive sparking occurs due to high quartz rock striking steel or steel striking light metal alloys such as aluminium. Ignition of rock on steel typically occurs in a goaf area. Such areas usually have insufficient oxygen to sustain an ignition, however the possibility remains and has been attributed to more than one explosion. Aluminium is either banned or limited in its magnesium content or in some special cases coated.

Application Sites

Mines that have considerable drainage regimes also implement water infusion, particularly mines in the Bulli seam (e.g. West Cliff and Appin). Continuous miners have an optional scrubber where they are used in a series of ventilation situations such as ‘cut and flit’; in the Centennial mines in the Newcastle coalfields. Longwall equipment suppliers provide a scrubber option for the crusher. Research work is currently underway in developing a scrubber for the shearer. All mines utilise sprays wherever coal is cut or loaded; and limestone dust is applied and maintained on all roadways, as required by legislation.

Technology Gaps/Needs

Two major areas requiring further improvements include gas capture and dust suppression. Gas capture is also related to gas hazards and will be discussed in Section 3.1.3. The improvements in dust suppression include:

  • improved suppression of airborne dust due to gas drainage – a more effective means of water (or wetting agent) injection into gas drainage holes is required. This potentially means improved or earlier gas drainage that allows for longer infusion time and more thorough re-hydration of the coal;

  • cost effective wetting or containment (foams etc.) of airborne coal dust to improve the effectiveness of water sprays;

  • achievement of effective wet stone dusting system and product; and

  • safe, economical, and automatic fire and dust suppression systems for continuous miners and shearers.

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