Underground » Detection and Prevention of Fires and Explosions
This project was intended to quantify the nature of coal and stone dust fallout in Australian coal mines and to use the results to determine the factors affecting optimal stone dusting strategies. A fairly simple method of collecting roadway dust fallout samples was developed and refined while undertaking fallout measurements along the longwall tailgate return in nine Australian coal mines. Analysis of the samples and results obtained showed a wide range of fallout rates from mine to mine. The results were then analysed to determine overall fallout characteristics of coal and stone dust and then to help develop a spreadsheet model of dust fallout in a longwall tailgate using various stone dusting options and coal dust fallout rates. An Explosion Risk Factor was developed to allow different outcomes to be compared. This is based on the length of roadway in which a combustible layer of dust accumulates, the proportion of time that this exists and the average incombustible content of the layer.
The main observations and conclusions made in this study are:
- The total estimated rates of coal dust fallout were seen to vary from about 10 kg/ktonne of coal produced to 699 kg/ktonne, or 0.001 to 0.07% of total longwall face production.
- Both stone dust and coal dust fallout rates along the tailgate were found to fall exponentially with distance away from the source or face.
- Stone dust tends to fall out earlier than coal dust due to its coarser size and higher density. This causes the incombustible content to rise away from the duster. Almost all of the stone dust distributed by the trickle duster was seen to fallout within 100 to 150m of the source.
- Most importantly, as a result of the modelling studies, it is concluded that there is no single "best" solution for all circumstances. When modelling was applied across the range of coal dust fallout rates observed during the sampling phase, it was clear that in some circumstances it may be possible to dispense with all stone dust control methods apart from pre-production campaign dusting of the tailgate. At the other extreme of high coal dust fallout rates extensive campaign dusting or very heavy trickle dusting will be required to avoid extensive accumulations of explosible dust.
- The threshold coal dust fallout rate at which it may be possible to dispense with all forms of stone dusting apart from pre-production campaign dusting of the tailgate, is about 29 kg/ktonne. A number of survey locations had fallout rates less than this.
- For higher rates of coal dust fallout the "optimum" stone dust application rates required to achieve satisfactory inerting were found greatly depending upon the total rate of coal dust fallout and the strategy of stone dusting employed. Typical values in the order of 1000kg/ktonne may be required for mines with very high coal dust makes. Most mines however, would require application rates in the order of 250 to 500 kg/ktonne.
- One option identified for further investigation is the development of matched stone dust which displays the same fallout characteristics as coal dust. Modelling results indicated that a matched stone dust for use in a trickle duster would minimise the stone dust application required to achieve satisfactory inerting.
- There is a significant problem associated with the depth of sampling of roadway dust samples. It has been shown that the depth of dust lifted in an explosion is about 2 to 4mm. Roadway dust sampling in NSW permits sampling to a depth of 5mm. The Explosion Risk Factor at 5mm layer thickness can be substantially less than at 2mm. It has also been shown that it is possible to have a satisfactory result from roadway dust sampling at 5mm and a significant explosion risk at 2mm. Consideration should be given to better methods of roadway dust sampling that do not tend to underestimate the hazard due to accumulation of combustible roadway dust.
Finally it can be said that techniques have been developed that allow the quantification of coal dust fallout rates and the prediction of the effectiveness of various stone dusting strategies. These can be applied to any mine to determine the "optimum" strategy to minimise the risks associated with the propagation of coal dust explosion. This can result in significant cost savings and reduced risks compared to the current methods of determining stone dusting methods.