| Coal is the foundation of China’s energy and economic structure,occupies a huge proportion of the application of fossil energy,and dominates the lifeline of China’s national economy.The underground coal fire disaster will bring huge risks to the sustainable development of resources.At present,coal fire disasters frequently occur in the mined-out areas of shallow coal seams in western my country.The structure of the fractured air supply network and the mechanism of spontaneous combustion of leftover coal are the key factors in determining underground coal fires.Oxygen supply environment and spontaneous combustion,characteristics of leftover coal are the dominant factors that promote the development of fire zone.The mechanical properties and pore and fracture structure of the overlying coal seam,as well as the stress concentration in the mined-out area of the underlying coal seam due to roof caving and stratum collapse are all important factors for coal.The characteristics of spontaneous combustion have far-reaching effects,and the combined action of various factors dominates the development trend of high temperature areas of coal fires.Therefore,this paper studies the evolution of pore structure of high temperature coal rock mass and the dynamic characterization of fractures under load,discusses the influence of stress and temperature coupling field on spontaneous combustion of residual coal,and establishes a catastrophe model for predicting the development process of spontaneous coal combustion.The following research results were obtained:The evolution law of pore and fracture structure of coal and rock mass caused by high temperature environment is revealed.Through the test and analysis of SEM and mercury intrusion tester,the evolution law of pore and fracture structure of coal after different high temperature treatments was obtained.The results show that high temperature increases the pore size,pore depth and fracture roughness,and enhances the connectivity between pores and fractures.The coal samples treated at 400 appear obvious pore clusters and high temperature pores appear on the fracture walls;the temperature increases,most of the micropores and small pores evolve into mesopores,macropores,visible pores and fractures,and mesopores are the main pore type of high temperature coal samples,accounting for more than 35%.The mercury intrusion test identified the fracture wall with pores as the result of clogging the fractures by coal surface or high temperature collapsed particles.The development characteristics and mechanical properties of the fracture structure of the loaded high-temperature coal-rock mass was explored.The full stressstrain dynamic fracture scanning experiment of coal samples treated at different temperatures in different compression deformation stages was carried out by using the loaded coal and rock industry CT scanning system.The law of dynamic changes in the form of destruction.The results show that shear slip failure occurs in all coal samples,and the difference in specific heat capacity between minerals and coal is the main factor causing thermal stress expansion fractures.High temperature increases the strain interval and peak stress,reduces the elastic modulus,and changes the fracture initiation position,fracture morphology,penetration characteristics and fracture arrest end point.Temperature mainly weakens the mechanical properties of the top and bottom edges of coal and rock mass,and increases the number of fractures in parallel bedding.The coal sample heat treated at 200 has no through fractures,and the fracture volume is the smallest in the complete failure stage.According to the change of three-dimensional fracture of coal samples treated at different temperatures,the change of threedimensional fractal dimension is basically consistent,and the evolution of fractures is divided into a stable development stage,a slow growth stage and a fast growth stage.The regularity of spontaneous combustion of coal leftover in the goaf and the reason for the lag of spontaneous combustion characteristics are revealed.The thermalstress test was carried out on coal with five particle sizes by using a load-pressurized coal spontaneous combustion characteristic device.Based on the Hertz contact deformation principle,the thermodynamic hysteresis characteristics of crushed coal under uniaxial stress were theoretically analyzed.The results show that the stress has a significant hysteresis effect on the critical temperature of coal spontaneous combustion,heating rate,Graham’s Ratio-R2 temperature coefficient and pyrolysis temperature.With the increase of particle radius,the growth rate of exothermic intensity is inhibited,but there is no inhibiting effect on the critical temperature of minimum exothermic intensity.The extreme point and critical temperature of the coal self-ignition limit parameter show the same corresponding hysteresis under uniaxial stress.The particle size ratio obviously changes the limit parameters of coal spontaneous combustion,and the coal with large particle size will reduce the risk of spontaneous combustion.The stress-strain curve and the stress-porosity curve division stage found that the critical uniaxial stress of the porosity and strain of each coal sample was up to 4 MPa.In addition,temperature changing permeability is the main cause of hysteresis.The permeability stress sensitivity decreased with the increase of effective stress,increased with the increase of temperature,and decreased with the decrease of coal particle size.The hysteresis effect of uniaxial stress on coal spontaneous combustion increases first,then decreases,and finally increases.A model for predicting the development law of coal spontaneous combustion in goaf is constructed.According to the relationship between gas production and temperature,the apparent activation energy and oxygen consumption rate of coal spontaneous combustion were calculated,and the nonlinear development process of coal mass from slow oxidation to rapid oxidation under thermal-stress was expounded.And based on catastrophe theory,the catastrophe temperature and critical temperature were calculated,and four characteristic parameters were determined.They are:catastrophe temperature(characterized by CO)and(characterized by oxygen consumption rate),critical temperature(characterized by CO)and(characterized by oxygen consumption rate).The results show that the index gas concentration,apparent activation energy and oxygen consumption rate show a cubic function law of first increasing,then decreasing and then increasing with the increase of uniaxial stress.At 1.8 MPa,the apparent activation energy and various characteristic parameters are the lowest,the coal-oxygen reaction rate is the fastest,and the oxygen consumption rate is the highest.When the uniaxial stress is 5.5 MPa,the oxygen consumption rate is the largest,and the new fractures in the coal body are the most.Uniaxial stress has the greatest influence on the characteristic parameters of,the temperature index of coal spontaneous combustion slowly transitioning to rapid oxidation,and the abrupt change temperature characterized by CO concentration is the most accurate.There are 46 figures,13 tables and 121 references in this thesis. |
