| Coal fires represent a worldwide hazard and exist in all major coal-producing countries.China suffers the most serious coal fires in the world.Coal fires generate huge amounts of heat energy while burning numerous coal resources.However,for a long time,the heat energy in the coal fire areas has always been treated as part of the disaster itself during the fire-control process,without being considered from the aspect of heat exaction and utilization.So the heat as a resource is totally abandoned,which results in a huge loss of the heat energy in coal fire areas.Besides,the heat pipes or borehole heat exchangers are mainly adopted to extract heat in coal fire areas which occupy a relatively small cooling areas for a single borehole,thus making it difficult to achieve the effective fire control in the large-scale high-temperature zone of coal fire areas.Therefore,in order to realize an effective combination of heat exaction and cooling in coal fire areas,the forced convective heat-extraction and cooling method is proposed in the present dissertation.In this method,a heat transfer medium(HTM)is adopted that would be injected into the underground high-temperature zone and then extracted to attain heat extraction and cooling effect on high-temperature coal and rock.Based on this thought and method,the investigations in this dissertation start from the characteristics of coal reaction kinetics under lean-oxygen condition,then a multi-field mathematical model for coal fire was established,and the numerical and experimental investigations on heat-exaction and cooling characteristics using forced convection were conducted.Based on these characteristics obtained,the forced convective heat-extraction and cooling technology was developed and its field tests were carried out.The main results and conclusions are as follows:The variation of coal reaction kinetics characteristics with oxygen concentration under lean-oxygen conditions was revealed.The thermogravimetry and heat flow of the coal samples during oxidation and combustion process under different lean oxygen concentrations were tested.The reaction kinetic parameters(apparent activation energy and pre-exponential factor)under different reaction stages and oxygen concentrations were analyzed.The results show that in the water evaporation stage,the apparent activation energy and pre-exponential factor of coal decreased first and then gradually stabilized with the increase of conversion rate.The relationship between apparent activation energy and oxygen concentration was a linear function under a same conversion rate,while the pre-exponential factor was a cubic or an exponential relationship of oxygen concentration.In the oxygen-adsorption and weight-increment stage and the high-temperature combustion stage,the apparent activation energy and the pre-exponential factor remained relatively stable as conversion rate increased.Under a same conversion rate,the apparent activation energy,pre-exponential factor were linear and exponential relationships of the oxygen concentration,respectively.The Thermal-Hydraulic-Chemical multi-field coupled mathematical model for coal fire areas was established.Base on the multi-field coupling relationship between the temperature field,vadose field,chemical reaction of coal oxidation and combustion and the porosity and permeability in coal fire areas,combined with coal reaction kinetics characteristics under lean-oxygen conditions,the Thermal–Hydraulic–Chemical multi-field coupled mathematical model for coal fire areas was established.A coal-fire evolution system was also built to conduct the corresponding experiments with the aim to verify the mathematical model.The results show that the equivalent average velocity of the propagation of the coal fire front obtained based on the model proved to be in good agreement with the experimental results.The model can overcome the shortcomings of the existing models in the simulation of reaction kinetic characteristics of coal under lean-oxygen conditions and the variation in permeability of coal and rock,which can reflect the evolution of coalfield fires more realistically.The model provides a practical and feasible approach for analyzing the forced convective heat-extraction and cooling process in a large-scale coal fire area.The characteristics of forced convective heat-extraction and cooling in high-temperature coal were expounded.Base on the Thermal–Hydraulic–Chemical multi-field coupled mathematical model,the borehole layouts,the cooling effect on the temperature of coal fire zones as well as the variation of heat extraction rates were studied when using nitrogen as HTM for forced convection.On this basis,a method to improve the heat transfer performance by adding water mist into the nitrogen was proposed,and the cooling effect of the mixed HTM on high-temperature coal was investigated under different nitrogen flow rates and coal permeability values.The research shows that a borehole layout of multihole injection and oriented type proves to be suitable for the forced convective heat-extraction and cooling in the coal fire areas.The temperature of the extracted HTM decreased as nitrogen rates increased,and the heat-extraction rate was more stable for relatively low HTM injection rates.Forced convection using nitrogen can effectively cool high-temperature coal and rock and control the propagation of coal fires at the same time.The coal fires presented a trend of“reversal evolution”after the initiation of forced convection,with the maximum temperature of the coal fire zones and the average temperature in the residual coal zone being cubic and quadratic function relationships of the heat-extraction time,respectively.Adding water mist can effectively improve the cooling performance of nitrogen under different flow rates and coal permeability values.The improved amount of cooling effects of water mist first increased and then decreased with time increase,and the effects were more obvious when the nitrogen flow rates were relatively small.At the initial stage,the mixed HTM showed a better cooling effect in the coal with a high permeability,but the cooling rate increased and the cooling effect became more significant in the coal with a low permeability as heat exchange time increased.The forced convective heat-extraction and cooling technology for coal fire areas was developed.A self-suctioning water mist generator was designed for the forced convection using water mist and nitrogen mixed HTM.The structural and operation parameters of the generator were then optimized,providing the device with the ability to produce efficient atomization at a relatively low pressure without the need for compressed air.On this basis,the forced convective heat-extraction and cooling system using water mist and nitrogen mixed HTM was developed.The system was constituted of three main parts:the water mist production system,low-temperature nitrogen conversion system and heat extraction system.Combined with the HTM injection and extraction method of multihole injection and oriented type proposed during the numerical simulation process,the field tests of the technology was successfully carried out in Sandaoba coal fire areas in Xinjiang.The results indicate that after 10h of forced convection using water mist and nitrogen mixed HTM,the maximum temperature of the surface region around the center of heat-extraction and cooling area decreased by56.6%,the average temperature of a deep borehole(with depth of 55m)decreased by61.0%and the temperatures of a HTM injection borehole at all depth was decreased to the values below 100?(with maximum temperature decrease of 246?).This technology effectively suppressed coal fires and realized the heat extraction at the same time.The average temperature of the extracted HTM increased by 197.6?(with maximum temperature increase of 203?),and the corresponding heat extraction rate was about 1.24×10~4 W. |
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