Study On Fracture Development And Percolation Characteristics Of Thermal Damage Of Coal Body

The coalfield fire has become a global catastrophe,the scope of its distribution is wide,the consequences of the fire are serious in our country.Not only a large number of coal resources have been burned,but the local fragile ecological environment is seriously damaged.In the evolution process of it,the high temperature results in the thermal damage fracture of the coal and rock,and it provides the passage for oxygen and gas to make the coal fire burning,resulting in the burning center along the seam to go further deep development.The development degree of the internal fissures in the coal mine directly affects the air leakage,which determines the development degree of the fire.The fracture of burning area is mainly divided into the initial fracture,the thermal damage fracture and the fracture caused by stress transfer in the burned area.To the deep coal seam with the internal fissures not developed and the early of the fire with little fracture caused by stress transfer in the burned area,the thermal damage fracture has become a key to the development of fire area.Therefore,this paper studies the hole and fracture characteristics of the thermal damage coal and corresponding seepage characteristics to promote the understanding of the thermal damage fracture evolution law in the fire,and provide some guidance for fire control.This paper focuses on the coal fire coal to carry out uniaxial mechanical,splitting,SEM,mercury,seepage and temperature programmed experiments,simulating the distribution characteristics of coal temperature field by the COMSOL5.0 simulation software,analyzing the coal thermal damage mechanism combined with experimental results and analyzing its influence mechanism on the development of coal spontaneous combustion combined with the development process of mining goaf fire.The main research contents and results are as follows:The new SANS microcomputer controlled electro-hydraulic servo pressure tester and the corresponding split grinding tool were applied to carry out the uniaxial compression test and splitting test of the thermally damaged coal.The thermal damage temperature of the coal body is 25 ?~300 ?.The study found that after heat treatment the compressive strength,elastic modulus,tensile strength of coal with the change of temperature is divided into 25 ?~100 ?,100 ?~200 ?,200 ?~300 ? three characteristic temperature region.with the increase of temperature,the failure of coal by splitting failure mode into split tension set,the tensile shear failurecombination.Scanning electron microscope(SEM)and mercury pressure test(Hg)were used to study the characteristics of holes and fracture development of coal body after thermal damage.It was found that with the increase of temperature,the evolution of the holes and fracture in the inner of the coal body is obvious.With the increase of temperature,the holes and fracture system of coal surface is changed.The number and length of the fracture increase with the increase of temperature,but the fracture rate and width increase first and then decrease with the increase of temperature.Before 300 degree,thermal cracking is the main reason for the expansion of fractures.With the increase of temperature,the surface pores of coal samples are independent to each other,and finally form pore clusters,and the size and quantity increase constantly.The internal pore connectivity of coal samples increases first and then decreases,finally increases.With the increase of temperature,the porosity of coal body is increasing in general.The thermal damage causes the surface of the coal body to become more rough,and the degree of unsmoothness is obviously improved.The percolation test of coal body after heat damage was carried out by a team percolation device.It was found that under low effective stress state,coal permeability increases first and then decreases with increasing temperature,and then increases.Under higher effective stress state,coal permeability decreases first and then increases with temperature increasing.Under different temperature conditions,the permeability of coal samples decreases with the increase of nitrogen pressure,which is caused by the slippage effect.By calculation,25 ? ~200 ? with temperature increasing the coal absolute permeability decreased first and then increased,the slippage factor increased first and then decreased,the average crack radius decreased first and then increased,It indicated that the slipping effect was gradually obvious first and then gradually weakened.The oxidation characteristics of coal after heat damage have been studied by temperature programmed experiments of coal sample,the distribution characteristics of coal temperature field have been simulated by the COMSOL5.0 simulation software,the coal thermal damage mechanism has been analyzed combined with the previous research results,and its influence mechanism on the development of coal spontaneous combustion has been analyzed based on the development process of goaf fire.By temperature programmed experiments of coal sample found that the oxidation enhancement of coal samples after thermal damage.The numerical simulation resultsshowed that when the temperature rises to the same temperature,the time required for the first thermal damage is longer;There is little difference in the spatial distribution of temperature between the first thermal damage and the second thermal damage when the coal sample reaches 300 degrees.The thermal damage changed the pore and fracture structure,mechanical properties and permeability characteristics of the coal,which changed the fracture field and airflow field at the edge of the fire,thus the coal spontaneous combustion fire was developed to the deep.The research results can be used as a reference for the field fire protection work.