Explosion inhibitors reduce the risk of coal dust explosions in underground mine roadways. Self-heating and spontaneous combustion of coal occurs under favorable conditions. It is a complex phenomenon caused by certain and uncertain factors. Explosion of coal causes loss of coal resources and caking. Open fires and noxious gases are from explosion of coal dust. Coal consists of carbonaceous material. At low temperatures its oxidized in the presence of oxygen rich air. Coal and oxygen reaction at low temperature is exothermic. Some reaction sequences are endothermic. Low temperature oxidation is the main source of heat. Spontaneous ignition from microbial metabolism, coal-water chemistry and oxidation of pyrite is possible. Heat evolves when moisture bonds with dry coal particles. Explosion inhibitors dissipate heat generated by coal oxidation via conduction, convection and radiation. Coal has poor thermal conductivity. As a result it stores excessive heat. Explosive inhibitors reduce the temperature and the rate of reaction of coal oxidation. In Queensland, Australia Recognised Standard 5 – Quality of incombustible dust, sampling and analysis of roadway dust in underground coal mines regulates explosion inhibitors used in underground coal mines. The article will discuss Recognised Standard 5. Focus will be on dust explosion suppression, sampling and road dust analysis. We will sum up with Global Road Technology’s coal dust suppression solutions with reference to our recent e-book, which is available on all digital platforms. 

Recognised Standard 5

Recognised Standard 5 – Quality of incombustible dust, sampling and analysis of roadway dust in underground coal mines was issued under the authority of the Minister for Natural Resources and Mines. This was under Section 37(3) of the Coal Mining Safety and Health Act 1999. It is a technical standard for the application and monitoring. Refers to use of Stonedust or other explosion inhibitors in underground mine roadways. This is to reduce the risk of dust explosion to an acceptable level.

The three core areas of Recognised Standard 5 are:

  • incombustible stonedust properties 
  • sampling methods for roadway dust 
  • methods of analysis of roadway dust 

The application framework tackles accumulation of fine coal dust within underground mine roadways. Coal dust has the potential to start explosions in underground workings. Dust in the roadways is an ignition source which can cause dust buoyancy in the atmosphere. A chain reaction commences as the roadway dust continues to ignite. Rate of reaction reduces when incombustible content is present in the roadway dust. Monitoring explosive inhibitor levels reduces the chances of explosions taking place. The standard outlines a method of representative sampling and testing roadway dust samples. Ensuring use of suitable incombustible material is as specified in the regulations. 

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What is stonedust?

Incombustible stonedust used in dust explosion suppression is actually limestone. It must conform with physical properties, size range limitations and specifications in the standard. Stonedust must be light in colour and contain less 3% by mass free silica according to MDG3006 MRT5. Fineness of the limestone dust is determined by MDG3006 MRT5. It must not be less than 95% by mass through a 250 micrometer sieve. The dry dust which passes the sieve, must not be less than 60%. It must not be more than 80% by mass passing through a 75 micrometer sieve. The supplier of Stonedust must supply evidence of compliance on request. Sampling is through strip sampling or spot sampling. Strip samples are from a transverse strip around the edges of the roadway. Sampling width should be consistent. Spot samples are from a series of spots. The area of spot taking must at least be 0.1m2 of the roadway floor. Collection of roadway dust must be from layers to a depth not greater than 5mm. Sample material moisture is critical. If water can be squeezed from it, the sample should be discarded. Sampling zones are according to the percentage of incombustible dust required in each zone. The different zones and frequency of sampling are as follows:

  • 85% – sampled weekly 
  • 80% – sampled monthly 
  • 70% – sampled three months

What is road dust analysis referring to?

Road dust analysis uses laboratory, portable instrument and colorimetric methods. The urgency of analysis is as soon as practicable possible after collection. The laboratory method is to determine the fraction of incombustible material. Sample drying is at 140 degrees celcius. Mass loss to moisture is then recorded. Residue heating is between 480 and 520 degrees celcius. Coal must be completely charred. The incinerated residue is weighed. The incombustible content is the sum of the moisture and the incinerated residue. Expressed as a percentage of the total sample mass. Portable instrument analysis gives similar results to the laboratory method. Method of sample preparation is instrument dependent and calibration is very important. Colorimetric method compares each dust sample with a prepared reference colour sample. The reference colour sample is of a known incombustible matter content. The incombustible matter content is from the 70%, 80% and 85% samples. If the colour of the sample is the same or lighter than the reference sample. The incombustible content in the sample is compliant with the required total percentage of incombustible matter. The opposite is true. If the colour of the sample is darker than the reference sample. There is non-compliance of the sample. The sample is referred to laboratory for analysis. 

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Recognised standard 5 is the benchmark for decision-making, with the safety and health of coal mine workers as a priority. Use of coal dust explosion inhibitors preserves lives. Complimentary to materials such as stonedust, Global Road Technology offers coal dust suppression solutions. These coal dust suppression solutions target coal chemistry. Coal chemistry relates to oxidation reactions from self-heating reactions. Coal is susceptible to igniting if explosion inhibitors are not used to reduce the rate of reaction. The economic value of coal is lost if coal self-ignites. It also poses danger to the health and safety of coal mine workers in underground coal mines. GRT Activate binds coal dust particles at the source. The loss of coal dust to activities in the coal mine can result in it gathering in roadways. Its presence in roadways can result in spontaneous combustion. Explosion inhibitors such as limestone are used to reduce the rate of reaction. Sampling uses different methods. Roadway dust testing also uses different methods. Coal dust suppression needs to happen before it reaches the roadway. In underground coal mines, GRT Activate UG can be used to ensure that the water sprayed for dust control and spark arrest actually captures coal dust, and drops it out of suppression. This prevents inhalation risk as well as eliminating it as a source of explosion or road dust contaminant.. It does not affect the calorific value of coal. It targets coal dust particles without affecting it as a source of energy. It reduces the risk associated with coal dust explosions. 

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