| Coal mining plays an important role for China's economic development. Coal waste,which is produced during coal mining and processing, is souring with increasing of the demand for coal production. The open-air coal waste dumps easily occur spontaneous combustion, leading to air pollution, underground water pollution, soil pollution and so on. Meanwhile, the accidents, such as collapse, landslide and explosion, pose a threat to people's life and safety. Some practices on controlling spontaneous combustion in coal waste dumps in China usually fail after a long treatment time. The real reason is that spontaneous combustion distribution is not known, especially for underground fire source locations. Controlling spontaneous combustion usually fails even though some effective fire-fighting measurements are taken. Therefore, it is necessary to study deep temperature distribution and deep heat source distribution in the process of self-heating in coal waste dumps, which gives an early warning for spontaneous combustion and provides a guidance for locating fire sources.A field simulation experiment in the process of self-heating was designed and conducted based on the published studies and the theories of thermal physics, heat and mass transfer, soil science, surveying, solid heat conduction and so on. The experimental results revealed deep temperature distribution in coal waste dumps and the depth was divided into different ranks based on degree of influence of heat sources temperature and ambient temperature. The depth was divided into surface layer,boundary layer and inner layer. A mathematic function of the relationship between heater temperatures and the maximum areas affected by the heater, and between heater temperatures and the furthest propagation distance was proposed with the knowledge of deep temperature distribution. On the basis of qualitative and quantitative analysis, a mathematic model of the relationship between heat intensity and depth was built for unsteady heat conduction, and a model for predicting heat depth and temperature was respectively proposed for steady heat conduction. If a high temperature heat source is inside coal waste dumps, it can cause thermal anomalies on the surface. For this case, a methodology of integrating infrared thermal imager and close-range photogrammetry was proposed to monitor and locate surface temperature anomalies areas for coal waste dumps. Some discussions and results are as follows.(1) In order to simulate inside coal waste dumps in a similar way, a simulation experiment was designed and conducted to reveal deep temperature distribution in the horizontal direction.?For different heater temperatures in each horizontal layer, the areas affected by the heat source temperature gradually expanded, and the temperature in the affected zones slowly increased over time. The affected zones became stable after the heat source temperature to be constant for a certain time.?For each horizontal layer (0-0.6m), the radius affected by the heater was approximately 0.3m for each horizontal layer. The affected zones showed an obvious temperature increase,and the temperature without the affected radius fluctuated between 15 ? and 25 ?. The study above showed that coal waste has a poor thermal conductivity and thermal physical properties. Therefore, it is necessary to insert temperature probes reasonably into coal waste dumps to observe temperature changes for a long time for locating the heat source. Meanwhile, the temperature of each horizontal layer was decreasing with increasing distance from the heat source,indicating that heat conduction was gradually decreasing.?The results showed that the temperature located 0.1m above the heat source was higher than that located at the same plane as the heat source. The reason was that uneven air density was formed due to uneven deep temperature distribution in coal waste dumps, leading to convection inside the interstice. The convection turned to heat dispersion, because of the small interstice. The mechanism enhanced heat transfer above the heat source, leading to higher temperature. Therefore, it can be concluded that the convection and heat dispersion in the vertical direction were stronger than that in the horizontal direction.? A polynomial function was proposed to describe the relationship among heater temperatures, depth and temperature rise rate, and the maximum error is 0.86 and the minimum error is 0.00 for the simulative temperature rise rate.?For the analysis of temperature loss rate, it rapidly went down with depth 0-0.2m,and then the temperature loss rate increased with increasing depth 0.2-0.6m, which indicated that thermal dispersion was the strongest with depth 0.2m.(2) The experimental results revealed deep temperature distribution in the vertical direction.?The results of the temperature distribution located above the heat source showed that the heater had a significant effect on the temperature from the 0m layer to 0.6m layer. The heater had a small effect on the temperature from the 0.6m layer to lm layer.Effect of the heat source on the temperature from the lm layer to 1.5m layer became weaker than that below the 1m layer, and the ambient temperature began to affect this area, especially for the low heat source temperature. This phenomenon revealed that it was not easy to locate heat source when coal waste dumps were in the stage of initial oxidation exothermic reaction. Therefore, it is necessary to monitor temperature changes by inserting additional measuring points into coal waste dumps for a long time to determine whether there was a high temperature heat source. The temperature followed the same trend as the ambient temperature for the layers 1.5-1.8m, indicating that taking internal factors and external factors into consideration when applying shallow temperature to predict deep temperature, or avoiding this range. The depth was divided into surface layer, boundary layer and inner layer, and the surface layer referred to 0-0.3m from the surface, which was significantly affected by the ambient temperature.?Comparing the temperature changes in the center line and 0.2m parallel to the center line, it can be seen that the zone in the 0.2m layer in the vertical direction had the strongest thermal dispersion than the other zones. The temperature away from 0.2m center line was twice lower than that in the center line. If applying injection method,such as grout injection, gel injection and insert gas injection, to extinguish fires after borehole drilling, the effective way to determine the fire source temperature and locate the fire source was to drill vertically. If drilling at an angle, it would reduce the accuracy of predicting fire source temperature and location, even leading to the wrong prediction.?For the analysis of temperature loss rate, it is difficult for a point heat source to diffuse heat to outside in the process of self-heating. When a lot of point heat sources came to an area, the accumulative heat reached to self-ignition, and then spontaneous combustion occurred. Therefore, it is concluded that the initial spontaneous combustion should occur in a small area, not in a point.(3) Analysis of the relationship between heater temperatures and the maximum areas affected by the heater,between heater temperature and furthest propagation distance quantitatively.Based on the analysis of deep temperature distribution and temperature changes qualitatively, a power function was proposed between heater temperatures and the maximum areas affected by the heater: f(x) = -296.7x-1.072 +2.089. A power function was proposed between heater temperature and the furthest propagation distance:f(x) = 2.05*10-6x2.274 +0.6506. It is effective to predict the maximum affected areas and the furthest propagation distance with the knowledge of these function, providing a reference for detecting and locating heat sources.(4) The models to predict heat source temperature and location were proposed.?When heat conduction inside coal waste dumps was unsteady heat conduction,the model of the relationship between heat temperature and depth was proposed:T = a·eb(x-d) + C. This model can predict the minimum depth for deep potential heat sources, and predict heat source temperature for different depths.?When heat conduction inside coal waste dumps was steady heat conduction, the model to predict heat source depth was proposed based on heat conduction theory. The results showed that the maximum error was 0.087m, and the minimum error was 0.006m.?When heat conduction inside coal waste dumps was steady heat conduction, the model to predict heat source temperature was proposed. The procedure was divided into three steps. Firstly, the depth of heat source was predicted based on the model above.Secondly, the surface energy balance equation was used to predict the temperature for the upper boundary layer, and then the depth and the temperature for the upper boundary layer were as an initial value. Finally, based on the exponential function:T = Td ·b·ea-(R-d) + c between heat source temperature and depth, the heat source temperature was predicted. The results showed that the maximum error was 6.82 ? and the maximum relative error was 2.62%.(5) A methodology of integrating infrared thermal imager and close-range photogrammetry was proposed to monitor and locate surface temperature anomalies areas for coal waste dumps.?The surface temperature followed a similar trend as ambient temperature and solar radiation temperature. With increasing distance from the surface of coal waste dumps, the effect of ambient temperature on shallow temperature showed a downward trend. For the case that an inner heat source leads to surface thermal anomalies, it is necessary to propose a method to detect and locate thermal anomalies areas based on infrared thermal imager and close-range photogrammetry. It is easy to find and locate thermal anomalies through this method, and then applying the models above to predict heat source temperature and location. The results showed that the maximum point location error was 0.014m, and the minimum point location error was 0.005 for this method.? A conceptual model for predicting deep temperature was proposed. The procedure was described as follows. The temperature anomalous areas were determined by the surface temperature model, and then the upper boundary layer temperature was predicted by the surface energy balance equation. Finally, the heat sources temperature and location were predicted by deep temperature model. This method can locate the fire sources and control spontaneous combustion.Controlling spontaneous combustion for coal waste dumps is a long and tough task.The most important thing is to know more about deep temperature distribution and temperature changes, and then applying models to locate heat sources. This research gave an understanding of controlling deep temperature distribution and temperature changes, and provided a reference for predicting and locating heat sources. Because internal heat sources inside coal waste dumps are very complicated, and the experimental conditions are difficult to simulate inside heat conduction in the same way,the results and conclusions studied in this paper are limited. The study on different heater temperature and different heater location needs to be studied in the future. It could provide more scientific references for locating heat sources in coal waste dumps,achieving harmonious development of coal mine environment. |
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