Study On The Mechanism Of Thermal Damage And Numerical Simulation For Evolution Of Coalfield Fire Area

Coalfield fires can destroy a large number of coal resources,produce toxic gases,destroy soil vegetation,groundwater,cause surface subsidence,and cause great damage to human health and environment.China is one of the most serious coal fires in the country,involving Xinjiang province,Neimeng province and other 5 provinces.One of the most significant reasons for the spontaneous combustion and continuous spread of coal fires is that the expanded fracture of overlying strata supported the gas transport channel under the high temperature conditions.This paper primarily focused on the thermal damage mechanism of coal and rock.The theoretical analysis,laboratory experiments and numerical simulation were carried out to investigate the influence of thermal damage on the pore structure,mechanical properties and seepage characteristics.Furthermore,the failure mechanism of coal and rock thermal damage is discussed,and the thermal-solid coupling in coal fire zone is established.Then,the distribution characteristics of temperature field,stress field and displacement field are analyzed by numerical simulation.Finally,the plastic deformation region of coal fire goaf is divided according to the criterion of plastic deformation region and numerical simulation results.The main research results are as follows:（1）The SEM and NMR were carried out to analyze the influence of thermal damage on the evolution of pore structure.Based on fractal theory,the fractal dimension was calculated to quantificationally describe the evolution of coal pore structure.Results showed that,the high temperature can open the coal pore.With the rising temperature,the connectivity of pore is growing and the fracture is expanded and improved.After the coal treated by 300℃,the number,depth and length of coal cracks increased significantly.As the temperature rises,the pores in coal sample are developed and expanded,the porosity increases.The number of small holes and middle holes in coal decreases gradually,and the number of large holes increases gradually.The pore structure of coal samples changed greatly at the temperature range from 200 to 300 degrees.D_mNMR,D_MNMR and D_NMR decreased with the increase of treatment temperature.The fractal dimension of micro-pores changed more greatly than that of large and medium-sized pores,indicating that the effect of high temperature on micro-pores was greater than that of large and medium-sized pores.（2）The macrostructure characteristics and mechanical property of coal after thermal damage were tested.Then,the influence mechanism of thermal damage on the coal mechanical property were analyzed.Results showed that,the high temperature can cause thermal expansion of coal particles,resulting in cracking effect of coal.Most of the cracks are distributed vertically and connect with each other.The fracture development is faster along the bedding direction than the vertical bedding direction.The fracture extending along the bedding direction are connected with the surrounding fractures to form the main fracture of the coal body.After the coal treated by high temperature,the uniaxial compressive strength first decreases slowly and then declines steeply.The tensile strength reduces with the rising temperature.The stress-strain curve of coal after treated by high temperature can be divided into 4stages:Compaction stage,line elasticity stage,crack evolution stage and failure stage.The strength of the coal body decreases sharply and the peak stress drops significantly when the treatment temperature is 300℃.There is a temperature threshold between400°C and 500°C,and its mechanical properties are drastically reduced.The elastic modulus of coal decreases with the growing temperature.The Poisson’s ratio of coal first increases smoothly and then decreases slowly,while the Poisson’s ratio of sandstone decreases with the increase of temperature.（3）The three-axis experimental device were used to test the gas permeability for damage-thermal coal samples under different conditions.Then,the influence mechanism of thermal damage on the gas permeability were revealed.Results showed that,under the constant stress conditions,the permeability reduces with the rising gas pressure.Before 0.6MPa,the decreasing trend of permeability is more obvious.After0.6MPa,the permeability gradually tends to be smooth.Under the constant axial pressure and constant gas pressure conditions,the gas permeability declines with the growing confining pressure.Under the constant confining pressure and constant gas pressure conditions,the permeability reduces with the rising axial pressure.In addition,it is a positive relationship between the permeability and temperature.Temperature effect plays a significant role in permeability,and the permeability of XG and DL coal increase with the treatment temperature.Finally,the influence mechanism of dehydration effect,gas production and thermal stress effect on the gas permeability were analyzed.（4）The thermo-solid coupling mathematical model of coal and rock was constructed.The distribution characteristics and dynamic changes of temperature field,stress field and displacement field were analyzed by COMSOL numerical simulation software.The plastic deformation area of coal and rock is obtained by combining the failure criterion of coal and rock.Results showed that,in the heating stage（0⁶0d）,the high temperature region above 800K mainly concentrated in the arc zone near the burning surface and the area near the burning surface of the direct roof and the direct bottom about 0.25m.At this time,the vertical stress near the combustion surface increases.The maximum horizontal stress increased by about 1.5e⁶Pa.The vertical displacement of the direct roof and the main roof increases first and then decreases.The horizontal displacement of the direct roof mainly occurs near the boundary of the fire zone near the coal wall,and the moving direction is positive.In the combustion stage（61d-240d）,the range of high temperature region above 800 K is basically unchanged,and the vertical stress near the burning surface has little change.The horizontal stress at the contact point between the burning surface and the direct roof reaches 2.1e⁶Pa and the maximum horizontal displacement on the burning surface reaches 3.8mm.In the cooling stage（after 241 days）,the temperature of the original combustion surface decreased from 1273.15K to below 900K within 10 days.After 6months,the temperature center shifted to the direct bottom,and the temperature dropped below 480K.At this time,the vertical stress near the combustion surface decreased slightly,and the horizontal stress between the combustion surface and the direct bottom contact point reached above 2.6e⁶Pa,the maximum horizontal displacement point shifts to the direct top fracture,and the horizontal displacement decreases gradually from the top to the bottom of the combustion surface and appears negative value at the bottom.The plastic zone of roof rock in coal-fired goaf mainly distributes in the range of coal-seam roof.The plastic zone gradually enlarges over time.About 360 days,the plastic zone is almost semi-circular,and the range is almost the largest from the longitudinal section,at the same time,the plastic zone appears in the upper part of the whole combustion zone,the middle part of the plastic zone almost reaches the top and bottom of the main.the maximum plastic zone of the whole process from the cross-section.