The deployment of thermosyphons to extract heat from burning coal mine waste

After drilling to establish the hottest part of a coal mine spoil pile, two stainless steel thermosyphons were installed to assess their ability to cool the hot ground. One device was a single-phase coaxial thermosyphon (water down through the outer annulus) with forced circulation, and the other was a two-phase (liquid plus vapour) device that relied on thermal buoyancy. The heated mine spoil into which the devices were installed was found to have experienced combustion that had since ceased, leaving a localised hotspot. Natural ground cooling was found to be unexpectedly rapid and exacerbated by the intrusion of a water table. After 36 successful pumped trials, performed over 14 months, the study found that neither device could significantly affect ground temperature, and that any cooling they could achieve was insignificant compared to the background cooling at the site. Also, the single-phase thermosyphon was ineffective at reheating cooled water when operated continuously; consequently the rate of energy extraction was not useful. Further, the heat extraction potential was so limited that it did not exceed the unintended losses from the aboveground system elements, so most acquired heat was unintentionally lost. The combination of vertical, lateral and temporal ground temperature variations made interpretation of the data difficult and rendered the devices ineffective. Vertical stratification of ground heat meant a significant amount of the heat acquired by the device was redistributed before it could be brought to the surface. The study concludes that given the limited fuel in the waste, the rapid natural ground cooling, and the ineffectiveness of either type of device to remove heat, there is little to be gained in trying to extinguish fire in waste piles when it occurs.