Report: APP CMHS Project 1




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3.1.7. Spontaneous Combustion Control

Current Technologies

Spontaneous combustion is similarly addressed through prevention, detection and extinguishment, if possible.

Prevention of spontaneous combustion is dependent on an appreciation of qualities, such as the propensity of the particular coal, ventilation design, and speed of mining. The simplest and safest way to preclude the occurrence of spontaneous combustion in a longwall operation is to maintain a consistent speed of retreat that will ultimately bury and smother any potential heating. This must be coupled with the timely installation of high quality seals.

Detection relies upon the knowledge and experience of people conducting inspections and the sophistication, extent and reliance on electronic sampling and analysis systems. Sampling and detection must be applied at areas of lowest ventilation pressure, where the richest, undiluted gas levels will occur. Similarly, analysis of gaseous indicators and ratios must be constant and automated to allow rapid response to an escalating situation, if needed.

The extinguishing of heating due to spontaneous combustion relies on the exclusion of oxygen, or in locating the source, and if possible, directly extinguishing as for a fire. For a longwall operation, exclusion of oxygen entails the introduction of inert gas, such as nitrogen, or in some cases the introduction of flyash. In a bord and pillar operation, if the heating cannot be located, sealing the area may be possible either with ventilation control devices, for roadways or injecting grout into suspected air leakage paths in pillars.

The effective management of spontaneous combustion (spon com) involves the development of defined and documented systems for monitoring and managing ventilation and goaf environments. These systems are maintained by the performance of regular reviews of goaf conditions with respect to pre set trigger levels. Integral to the proactive management of spontaneous combustion is the capability to inject inert gas into the goaf at a specific location to prevent the formation of conditions likely to allow spontaneous combustion or in the case of an event, the control of the situation until more permanent measures can be put in place.

Those that entail a disciplined management process – such as removal of stowage, maximising coal recovery, reducing frictional resistance of roadways and self inertising using seam gas; will not be considered here. What will be considered, are the technological controls that are implemented as proactive or reactive measures. Key control measures such as pressure rated seals, pressurised seals, goaf pressure balancing, Ventilation Control Device (VCD) leakage reduction and artificial inertisation. The important step of monitoring has been covered in Section 3.4.

Seals

It is common place for panel goafs to be sealed as extraction progresses and certainly when the panel is completed. These seals are pressure rated depending on the application through testing undertaken at the NIOSH Lake Lynn Experimental Mine testing facility, Pittsburgh. The over-pressure required to be resisted is:

  • 345 kPa if persons remain underground when an explosive mixture exists and there is a possibility of spontaneous combustion or incendive spark or other ignition source

  • 140 kPa for where an explosive mixture exists and there is a potential ignition source but there are no persons underground

  • 35 kPa if the level of flammable gas is insufficient to reach the lower explosive limit.

Such seals include:

Goaf Pressure Balancing

Text book goaf pressure balancing covers the technique of constructing pressure chambers at each seal site then connecting all pressure chambers around a goaf to the same (or similar) pressure. This reduces the differential pressure around the goaf and reduces the tendency and magnitude of air ingress.

An enhancement of this technique is the injection of inert gas such as nitrogen into the constructed pressure chamber. This ensures that if there is any differential pressure that creates an air leakage path, nitrogen, rather than oxygen leaks into the goaf with reduced spontaneous combustion potential.

Ventilation control device leakage reduction

Ventilation control devices (VCD's) such as seals and stoppings are installed in roadways that have been excavated and often later exposed to the stresses of secondary mining processes. Air leakage, that can exacerbate spontaneous combustion in the pillar interface, often occurs around these structures and through coal skin surface cracking. Applications of epoxy based paints and other flexible adhesives are being used to proactively reduce this potential, as well as improving the air leakage characteristics of VCD’s.

Artificial inertisation

    A number of spontaneous combustion prone longwall mines have implemented the practice of continuous ‘trickle feed’ of nitrogen gas into the goaf, some distance behind the active face. This is typically conducted at the critical zone in the goaf where oxygen levels are still high enough to support combustion, but heat removal due to ventilation flow is at a minimum. Inert gas generators are typically used for this technique. The inertisation systems include:

  • The GAG jet engine – capable of introducing 20 m3/s of exhaust gas rich in Nitrogen into the mine workings, typically from the surface or a large borehole;

  • Mine Shield Nitrogen evaporation plant – used to inject ~ 2 m3/s evaporated pure nitrogen via borehole and pipeline into remote areas of the mine. Ideally for panel inertisation;

  • Tomlinson Boiler – capable of producing ~ 1/2 m3/s of flue gas rich in nitrogen into mine workings via a borehole;

  • Floxal or other pressure swing – molecular sieves, typically used for injection of nitrogen via borehole into operating extraction panels as a preventative measure.

The inert gases are simply introduced through piping into seals behind the face or via surface to seam boreholes at specific locations into the goaf. Detailed descriptions of active inertisation techniques can be found in ACARP Report C12020 (www.acarp.com.au).

Application Sites

Clearly, the application of such technology is only relevant to spontaneous combustion prone seams. This currently means seams in Bowen Basin and West Moreton particularly Moura, Newlands, Amberley, Goonyella and Collinsville seams and seams in the Greta and Great Northern coal measures in the Hunter coalfields.

Technology Gaps/Needs

Significant research into the particular qualities for combustion is underway (at the University of Queensland and Simtars) and needs ongoing support to achieve a greater level of knowledge for individual coals. Given an accurate knowledge of the requisite conditions and susceptibility, a mine can be designed in such a way as to minimise this potential. By far the greatest challenge regarding spontaneous combustion is in detection, locating and control of deep goaf heatings. Radon technology for locating heating in a goaf (from the surface) and fire suppressant gels, (though relatively untried in this country) offer significant potential for detection and control success.


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