Report: APP CMHS Project 1

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3.1.3. Gas Monitoring and Control

Gas Monitoring

Gas monitoring is a common thread across a number of the high risk occurrences in underground coal mining. It also plays a role in the prediction and prevention of such events. Specifically, gas monitoring is relevant to identifying flammable and noxious gases, fire, potential for explosion, spontaneous combustion, indicators of outburst and in some cases health impacts (e.g. hydrogen sulphide, carbon monoxide, carbon dioxide and sulphur dioxide). The four broad categories of gas monitor and gas monitoring systems are – hand held and machine mounted gas monitors, telemetric real time monitoring systems and tube bundle sampling and monitoring systems.

Portable Monitors

Electronic hand held gas monitors with multi-gas detection capabilities have all but replaced the locked oil flame safety lamp. All statutory officials are required to carry a suitable gas monitor at all times that they are underground, with relevance to the potential gases that may be present. The current suit of monitors are modular and allow for a customer to specify the gases of interest to them and to have a unit that can detect up to 6 gases simultaneously, in some cases. Many units have additional features such as: the ability to provide Time Weighted Average (TWA) and Short Term Exposure Level (STEL) readings; data logging capability; and remote sampling pumps. Such units are provided by international companies with the onus being on the mine to select the units that best suit their particular application. The use of colourimetric stain tubes is still in existence though with such a wide range of electronic instrumentation, not as prevalent or necessary, except in the case of high concentrations.

List of typical gas monitoring apparatus as exempted for use in New South Wales coal mines:

Typical portable (hand held) gas monitoring equipment suppliers include Drager MiniWarn, Drager X-am7000, Odalog and iTX.

Machine Mounted Monitors

Machine mounted gas monitors are proprietary apparatus fitted to all mining machinery to comply with legislation. In this regard there is nothing unique to any particular country or application. Typical providers and/or units include Trolex, Drager, Austdac, Ampcontrol and Bacharach.

Critical locations in coal mines are monitored using two systems and based on two differing philosophies – tube bundle and telemetric. Both systems rely on a computer control system, typically SCADA (Supervisory Control and Data Acquisition) to interrogate and present critical data. Such systems collect data from various sensors at a mine and then send this data to a central computer which then manages and controls the data. Most mines use a combination of tube bundle and telemetry systems in order to utilise the strengths and manage the weaknesses of each one.

Tube Bundle Monitoring

Tube bundle gas sampling and analysis is used to monitor goaf areas, operating longwall or pillar extraction panels and selective areas such as conveyor belts in return roadways. The system is used where there is likely to be a need for continual monitoring even in the event of mine evacuation and electric power isolation from the mine. The ability of the system to operate continuously in a wide range of conditions provides a reliable means of data collection from underground areas for analysis. Tube bundle systems range from 20 to 40 tubes depending on the size of the mine and can sample atmospheres from up to 8000 m away via the tubes and vacuum pumps. Sample tubes are typically installed in the most gas rich areas of the mine – low pressure areas of goafs and return airways. Gas analysers are installed on the surface in a central location with a computer control system and communication system such as Wi-Fi back to the office complex. The tube bundle system has the disadvantage of a lag between the time of sampling and the time of analysis, due to the transmission time of the gas sample through the tube.

Gas trends collected from the tube bundle sampling process, instantaneous analysis and interpretation systems form the main information source used in the decision making process for critical gas conditions. Typical gas trends and ratios used for the assessment of longwall goafs are a mixture of air free values, ratios of one gas to another and raw gas value amounts. The tube bundle system has the advantage of continuing to monitor the environment when personnel have withdrawn and electric power has been removed from the mine.

The other important attribute for the use of the tube bundle system is the ability to integrate gas chromatograph analysis if needed. Gas chromatography is needed to be able to determine if hydrogen or higher order hydrocarbons such as ethylene is present – indicating that coal distillation or combustion is occurring.

Typical tube bundle gas monitoring system providers include SICK Maihak, Aust Dynamic Technologies, Gas Chromatograph and Agilent. Quality gas monitoring operations include Crinum, Broadmeadow and North Goonyella Coal Mines.

Telemetric Monitoring

The final gas monitoring approach is that of telemetry, where specific gas analysers are installed at strategic locations underground to provide immediate detection of a specific gas and communicate the reading telemetrically to a surface computer/control room. Telemetric systems have the advantage of being instantaneous, so provide the ability for rapid response. They are typically installed in conveyor belt and return roadways where there is a need to detect and respond quickly. The disadvantage is that they no longer function if people are withdrawn and power has to be removed from the mine.

Tube bundle and telemetric systems can be observed in most mines in Australia. All Queensland mines must have a chromatograph installed on site as well. These chromatographs are supported by 24 hr service agreements with technical expertise and the ability to directly connect to the mine computer system. The organisations such as Simtars and Coal Mines Technical Services also have mobile gas analysis laboratories that can be rapidly deployed to a mine that needs additional gas analysis support.

Gas Control

Current Technologies

Coal seams in gassy mines can contain up to 20 m3/t of gas insitu and may yield up to 80 m3/t of specific gas – i.e. 80 m3 of gas per tonne of coal mined. Gas control consists of pre or post drainage strategies, aside from the ventilation component. Ventilation can remove up to 1.2 m3/s of gas at a peak. Pre-drainage typically consists of in-seam drainage, to remove gas levels above the threshold that established conventional ventilation systems can accommodate. There is a trend towards surface to in-seam (SIS) drainage, where there is sufficient lead time, access and confidence in the investment. The gas removal by the SIS methods generally requires some synergy with a gas producer, with the knowledge and capability to remove the gas as a cost benefit to both the mining and gas companies. Such drainage is very much limited by the permeability of the seam and the subsequent area of influence of individual drainage holes. There is some use of hydro-fracturing for enhancing the release of gas from coal seams.

Directional drilling is used for in-seam drainage of seam gas in many Australian coal mines. Advances in technology have occurred over the past 10 years as a result of collaboration between industry, equipment suppliers and contracting companies. This has provided great benefits that are nearing the limit for current gas levels and seam conditions. Further advances in drill hole control and direction would provide greater benefits and optimisation, but there is probably more benefit from other gas stimulation and capture techniques. Holes with a depth of up to 1800 m have been achieved but the majority are in the range 250 – 400 m.

A range of developing technologies have been identified for increasing gas capture such as gas injection and mineral content removal. As gas management is and will continue to be one of the major safety considerations for underground coal mining, the future development of these technologies is imperative for the Australian industry. Significant opportunity for improvement is available from existing technology within the gas and petroleum industries. The injection of nitrogen in particular seems quite encouraging and the early trials in Canada, USA and Japan have yielded increased gas flow rates from areas of low permeability. Field trials are planned to inject nitrogen from membrane technology generators into alternate in-seam holes to active suction holes and assist in drainage.

Gas post drainage at best practice can remove around 4 to 5 m3/s of gas. With increasing depth and multiple seam conditions there will be a continuing need for post drainage. SIS drilling and drainage applied to surrounding seams provides a potential reduction in high total gas content. Current technology for post drainage includes bleeder roadways, gas drainage ranges connected to the goaf seals, and surface drainage wells. A significant amount of research has identified the best location for surface drainage wells, however, these are at times limited by the added gas load of other surrounding gas bearing strata such as overlying sandstones. This can neutralise the benefit of post drainage of the goaf and not aid in the reduction of face gas levels.

Gas monitoring, other than hand held personal or machine mounted gas monitors, comprises either Telemetric real time systems or Tube Bundle sampling systems. The latter usually incorporates a gas chromatograph as well. All technologies are covered in some detail in the Stage 2 report. Gas monitoring will continue to be a critical factor in detecting and understanding the critical hazards of various mine gases, spontaneous combustion, fire, and outbursts.

Application Sites

Of the 30 longwall mines in Australia, approximately 20 require some form of gas drainage. Successful application of the above gas control technologies can be found at BHPB’s West Cliff and Appin mines in New South Wales, and Anglo Coal’s Grasstree mine in Queensland.

Technology Gaps/Needs

  • Technology for soft or structured difficult drilling conditions and for maintaining hole integrity.

  • Continuing development of surface to in-seam drilling techniques, such as coiled-tube drilling, that can be applied to more urban mining areas.

  • The application of coiled tube drilling for in-seam drainage.

  • Water jet drilling combined with tight radius drilling applied to multiple seams.

  • Improved pre-drainage capability, such as hydrofracture, nitrogen injection, mineral leaching, particularly from low permeability reserves.

  • The practice of bleeder roadways above or below extraction panels as applied in China should be considered and modelled as a potential aid to post drainage capability, particularly if pre-drainage becomes more difficult, as has occurred in Chinese mines as mining seams become deeper.

  • Broader scale sampling that provides a clearer more accurate picture of the underground environment – rapid sampling time, ability to continue operation of real time monitoring in an evacuation situation, more sampling points throughout the mine.

  • Continued development of analytical software to provide improved interrogation of conditions.

  • Development of analytical equipment for detection of aromatic compounds for early detection of heatings.

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