| Combustible gases generated by coal spontaneous combustion in the goaf change the explosive limit of the gas mixture,and high-temperature heat source formed due to generated heat accumulation.In addition,gas emission and heat generation are in a dynamic evolution process.Goaf risk area is the result of the superposition and coupling of O2 field,combustible gas(CH4,CO,C2H4)field and temperature field.The dynamic evolution of multiple fields induces the real-time variability of the goaf risk area.It is meaningful for thermodynamic disasters monitoring and early warning to study this process.In this dissertation,based on the characteristics of porous media in the goaf,the law of coal spontaneous combustion gas emission and temperature rise,the combination of theoretical analysis,experimental study and numerical simulation was used to establish the model of porosity,particle size,permeability,inertial resistance coefficient,O2 consumption,gas emission and exothermic intensity of coal spontaneous combustion.The evolution characteristics of gas emission and the migration law of high temperature heat source in the process of coal spontaneous combustion were obtained through goaf thermodynamic disasters experiment.Thermodynamic disasters risk evaluation index system was established,and the risk area division method of goaf was determined.In addition,dynamic evolution law of goaf thermodynamic disasters risk area with different high temperature heat source locations was obtained.The main research results of the dissertation are as follows:(1)Porosity,particle size distribution of fractured rock,permeability and inertial resistance coefficient models of goaf were established to accurately characterize the porous media properties of the goaf.Based on the theory of overlying rock movement,the sigmoid function was introduced to reflect the distribution characteristics of the transverse three zones of the goaf,and the main roof as well as the fractured zone rock subsidence model were established.According to the relationship between grain size and stress of the broken rockmass,the constitutive relation of the broken rockmass in the goaf was constructed.Goaf porosity was found to show a dustpan shape distribution,and the largest porosity appeared at the natural accumulation area and roadway side influenced area.It was found that the particle size of the broken rockmass decreased exponentially with the increase of stress.The coupling equation of coefficient A and coefficient B of Ergun equation with particle size and porosity was established,and it was found that with the increase of porosity,coefficient A gradually increases and coefficient B gradually decreases.With the increase of particle size,the coefficient A gradually increases and the coefficient B gradually decreases.Permeability and inertial resistance coefficient shape factor of the broken rockmass were introduced to establish the correction models of permeability and inertial resistance coefficient in the goaf.The permeability and inertial resistance coefficient shape factor of the broken rockmass were found to have a good power function relationship with the grain size,and the correction model was proved to be applicable for broken rock mass with grain size of0.75?120 cm and porosity of 0.1?0.6.(2)The O2 consumption model,gas(CH4,CO,CO2,C2H4)release model and exothermic intensity model in the process of coal spontaneous combustion were established,and the influence laws of temperature and O2 concentration on each stage of coal spontaneous combustion were obtained.Through coal temperature programmed experiment,it was found that when coal temperature is low,the temperature is the key factor affecting the O2 consumption rate,and the difference of O2 consumption rate of coal under different O2 concentration conditions is small.As the coal temperature increases,the O2 consumption rate were affectd by temperature and O2 concentration jointly,and the O2 consumption rate of coal appears different growth rate under different O2 concentration conditions.As coal temperature continues to rise,O2 consumption rate of coal tends to level off,and the O2 concentration is the key factor affecting O2 consumption rate.The generation rate of CO and CO2 is a binary function of temperature and O2 concentration.When the coal temperature is low,temperature is the key factor affecting the generation rate of CO and CO2.As the temperature increases,the generation rate of CO and CO2 were affected by temperature and O2 concentration together.The concentration of CH4 and C2H4 change as a power function with coal temperature,and the generation rate of CH4 and C2H4 is a univariate function of temperature.Temperature is the key factor affecting the generation rate of alkene,and O2 has less effect on the generation of alkene.The numerical simulation results shows that a low temperature point appears inside the coal sample at the initial stage of heating.With the increase of heating temperature,two temperature limit zones appeared inside the coal sample.and a high temperature heat source and a low temperature point appeared at inlet and outlet respectively.With further increase of heating temperature,the low temperature point disappeared and high temperature point appeared only at the inlet.(3)The multi-field coupling evolution law of the goaf during coal spontaneous combustion was studied,and the evaluation method of thermodynamic disasters risk in the goaf was established,and the dynamic evolution law of the goaf thermodynamic disasters risk area under different high temperature heat source locations was obtained.Based on the characteristics of gas flow and heat and mass transfer characteristics in porous media,an experimental platform of thermodynamic disasters in the goaf was established,and the multi-field coupling evolution law in the goaf was obtained through experiments.It was found that the high temperature range formed by coal spontaneous combustion was limited to a small area around the spontaneous combustion point,and the effect of coal spontaneous combustion on O2 concentration field and CH4 concentration field was small,and an enriched area of CO and C2H4was formed around the heat source.The numerical simulation study found that the temperature disturbance generated by the heat source on the return side was stronger than that of the heat source on the inlet side and when the high-temperature heat source was on the air inlet side,the amount of CO and C2H4 generated by the heat source was higher than that on the air inlet side and middle of the goaf.Based on the gas explosion concentration index and the risk assessment index of coal spontaneous combustion process,a goaf risk evaluation index system was established,in which the risk of the goaf was divided into six levels according to risk value:very high risk,significant risk,large risk,general risk,low risk and safety.Risk area division method of the goaf was constructed based on the multi-physical field indexes of gas concentration and temperature.With the increase of coal oxidation exothermic intensity,the hazardous area of the goaf was expanded,when the high temperature heat source was on the inlet side,the increased hazard area of goaf was wider than that on the middle of the goaf and return side.The research results reveal the dynamic evolution mechanism of goaf dangerous area in the process of coal spontaneous combustion,and provide theoretical guidance for thermodynamic disasters early warning.During the research period,4 SCI papers were completed and 2 were officially published.2 national invention patents were applied and 1 was authorized.There are 117 figures,27 tables and 212 references in this dissertation. |
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