Underground » Detection and Prevention of Fires and Explosions
Spontaneous combustion is a significant hazard in some coal mines. In Australia there is currently no remote sensing technique practically available for Australian coal mine operators to locate underground spontaneous combustion of coal remotely from the surface.
The main objectives of the project were to investigate and demonstrate a surface-based radon technique developed by Taiyuan University of Technology and used in Chinese coal mines to detect the location of underground spontaneous combustion.
Principle, operation and application of the radon technique
Radon-222 gas occurs naturally as a decay product of the long-lived uradium-238 that is a common rare element in rock/coal/soil strata. In the event that spontaneous combustion of coal occurs underground, the radon emanation ratio from coal, and its migration through the overlying coal/rock strata increase significantly. This results in an elevated concentration of radon above where the spontaneous combustion of coal is occurring underground. The elevated radon concentrations are detected and used to locate the underground area of spontaneous combustion of coal. This is the basic principle of the radon technique for detecting the location of underground sponcom from surface.
Operational aspects of this radon technique are fairy simple. It employs an alpha detector and alpha cups. The detector is a portable, battery powered "alpha counting" detector with a pulse ionisation chamber for detecting alpha radiation of radon and its daughters. The alpha cup is an open-end plastic cup with a sorbent coating on its internal surface so that radon and its daughters are adsorbed and deposited on the internal surface of the cup. An operation including field test and results processing may only take between 2 to 4 days to complete depending upon the size of a detection area.
The radon technique has been used about twenty times for remotely locating spontaneous combustion in Chinese coal mines with a great success.
Field demonstration
Field demonstration of the radon technique was carried out in two separate areas at Dartbrook mine to locate the suspected sponcom of coal in these areas. The first area covers a surface area of 103,395m2 directly above LW7 goaf. The cover depth in the area is about 340m. The second area of 10,000m2 is centered at CDH003 hole directly above LW2-3 goaf. The cover depth in the area is about 200m.
Test results in the LW7 area indicated that there existed four zones of coal heating. These zones were A, B, C and D. Zone A was detected to be an oxidation zone and the extent of its influence was about 20m from the LW7 stop line. Zones B, C and D showed some degree of heating, however the results indicated that the heating in these zones was not as intensive as that in Zone A. Results also indicated that the area of heatings might have been quite extensive prior to the treatments of the sponcom. This implied that the treatments of sponcom in the LW7 goaf were effective. These results are consistent with those based on analysis of gas monitoring data.
Test results in the CDH003 area indicated that there existed one zone of coal heating. The zone was located in the coal pillar between LW2 and LW3. Results indicated that the heating in this zone was of fairly low intensity. It appears that the identified zone is about 50m off the likely heating location in the area. This discrepancy poses a challenge to the radon technique, however it also provides an opportunity to undertake further study on the radon technique.
Future research
To apply the radon technique in Australian coal mines with greater confidence, detailed studies of the temperature dependence of radon emanation ratio from coal and its upright movement in strata should be undertaken in future research projects. The studies are of fundamental importance to the underlying science of the radon technique. Outcomes of the studies would lead to developing and applying the radon technique in Australian coal mines.
Temperature dependence of radon emanation ratio from coal is the core science upon which the radon technique for locating sponcom of coal is based. However there is very little data available to illustrate the dependence. The dependence should be investigated by considering the effect of a number of factors such as coal type, moisture content, particle size, etc.
The effect of sponcom on radon movement through strata underpins the principle of the radon technique. Although modeling radon transport in soil has been undertaken by a number of investigators, there is very little published literature available for modeling the effect of coal sponcom on movement of radon through strata.
Bench-scale testing is required to investigate the temperature dependence of radon emanation ratio from coal, while numerical modeling is necessary to study the effect of sponcom on radon movement, in particular its upright movement through strata.