Underground » Health and Safety
The concentration of methane is relatively easily measured on a coal face, and therefore can be controlled to prevent the development of dangerous conditions, but it is much more difficult to measure airborne dust concentrations.
As a result, very little is known about the concentration of total airborne dust in the vicinity of a coal face, nor with regard to the total amount of dust produced on a face and its fallout distribution in the return ventilation system.
Coal dust can contribute to an underground explosion in two different ways. The more usual way is that a smaller explosion of methane takes place initially which causes a blast of air to travel along the mine roadways lifting coal dust from the floor, ribs and roof. The flame of the initial methane explosion ignites the suspended coal dust particles which burn very rapidly producing large volumes of hot and toxic gases.
Further coal dust is lifted into the air ahead of the burning coal dust cloud, and the coal dust explosion will continue until there is no longer a supply of coal dust. Clearly the total amount of coal dust in the mine roadways is an important contributing factor in the risk associated with the coal dust explosions.
Coal dust may also increase the risks of an underground explosion by forming, with methane, a hybrid atmosphere which is explosive, but in which neither the coal dust or methane concentrations separately constitute an explosion hazard. This phenomenon is not well understood and is difficult to investigate.
Research continues to determine the nature of hybrid explosive mixtures of coal and methane, but very little is known about the concentrations of dust that occur in coal mine and which might contribute to the formation of an explosive hybrid mixture.
Objective & Method
The intention of this research project was to investigate the concentrations and rates of production of total airborne dust in and around a producing coal face.
To do this a number of different methods of sampling coal mine air to determine the total airborne dust concentration were utilised or developed for use underground.
Most significant of these was a modified cyclone dust sampling probe based on the BCURA stack sampling cyclone, which was tested and successfully used for this purpose. Reproducible and accurate results were obtained with this instrument within the range of dust concentrations expected to occur on a longwall face.
The collection efficiency was found to be about 85-90% so long as care was taken to maintain isokinetic sampling and the inlet air velocity was above 2.3 metres/second. The drawbacks encountered were the reliance on a compressed air supply to operate the cyclone which severely hampered the use of the instrument, and the long sampling period of up to 3 minutes which limited the sensitivity to short term peaks.
A filter paper dust probe was also designed in an attempt to overcome the problem associated with the cyclone dust probe's reliance on compressed air. Measurements were also taken with a Hund respirable dust meter as a cross reference against respirable dust concentrations. Due to safety considerations in moving around an operating longwall face, almost all measurements were taken in the tailgate return roadway.
The average total dust concentrations were all extremely low, with all the mine sites having averages below 1 gram/m³. The peak dust concentration observed was 5.8 grams/m³, which was recorded on only one occasion. Significant levels of over 1 gram/m³ were observed when the longwall shearer was at the tailgate end of the face near to the sampling position.
Because of the difficulties of operating the sampling equipment on an operating longwall face, it was not possible to conduct intensive sampling close to the shearer. It is probable that higher dust concentrations exist nearer the shearer than measured here. It is also probable that higher dust concentrations could occur over short periods of time which could not be detected with the available equipment due to the extended sampling period of 1 to 3 minutes.
None of the samples obtained in this project were of a concentration to suggest that they represent a directly explosive mixture of coal dust in air throughout the actions of coal cutting and breakage alone. However, more intensive sampling closer to the main areas of coal breakage would probably show much higher dust concentrations and these warrant further investigation.
Particle size analysis of the samples obtained showed that the average was about 25-50µm varying from mine to mine. This is considerably lower than 200µm which is accepted as the upper limit for stone dust.
There is concern that differential settling rates between coal and stone dust in return airways could result in high concentrations of coal dust despite application of large quantities of stone dust. Consideration should be given to investigations of the effect of relative particle size differences between stone dust and coal dust on the prevention of explosion propagation.
It was also noted that respirable dust was approximately 10% of the total airborne dust in the samples analysed.
Of most significance was the calculation of estimated minimum stone dust application rates. Based on the average dust concentrations observed, the average coal dust burden entering the panel returns was estimated and used to estimate minimum stone dust application rates. For three mines in which sampling was undertaken this varied from about 17 to 150kg/hour, depending upon the volatile content.
An estimate of the current rule of thumb practice suggests that current application rates are about 20kg/hour, at the lower end of the calculated requirements. This is of some considerable concern and may indicate that current stone dusting practices are inadequate for the level of airborne coal dust carried into the returns from a longwall face. The level of hazard that this represents is significant and is worthy of further investigation.
The use of water sprays as a means of dust suppression was investigated by examining the operation of two types of spray under varying conditions of airflow, dust concentration and water flow. The dust suppression efficiencies were found to be similar under similar conditions with efficiencies of 75-85%. The major differences were in the water flow rates and pressures required to operate the sprays.
The TF spiral spray, manufactured by Bete, .requires a large flow rate of 40 to 60 litres/minute at low pressures of 0.6 to 1.1 bar.
The Conflow drum spray and the Conflow drum spray, manufactured by Senior operates at 5 litres/minute but requires a water pressure of 12 bar. Operationally the drum spray would be prone to blockage due to the very small orifice size of 1/16", compared with a minimum orifice size of 5mm on the spiral spray.
Areas suggested for further investigation include:
- continued development of instrumentation for the instantaneous and continuous determination of total airborne concentrations;
- investigation of the occurrence of peak dust concentrations and their origins;
- investigation of the settlement characteristics of coal dust and stone dust in face returns roadways; and
- the factors affecting the behaviour of stone dust as an effective means of explosion suppression such as relative particle size, and settlement characteristics.
There is evidence from this study that the average coal dust particle size is considerably less than the maximum stone dust size. The effect of this on the efficiency of stone dust inertisation is not clear and would be an area of considerable interest. This would also require a better knowledge of the stone dust particle size distribution and the relative depositional rates of stone dust and coal dust occurring in the panel return roadways