Study On Instability Disaster Mechanism Of Combination Of Intact Coal And Tectonic Coal Based On Unsteady Diffusion

The coal resources are still the main pillar energy industry in China.The development of industries is based on coal resources as the main energy supply.This makes the coal resources of shallow coal seams have been exhausted,thus promoting the extension of coal mining to the deep.However,the characteristics of high ground stress,high gas content,high gas pressure and low coal seam permeability are often manifested in deep coal seams,thus increasing the risk of coal and gas outburst disasters.In addition,due to the influence of geological structure,there are many phenomena of intact coal and tectonic coal stratification in coal seams.According to the current research,tectonic coal is a necessary condition for coal and gas outburst.Therefore,in order to further clarify the role of tectonic coal in coal and gas outburst disasters,based on the theoretical basis of adsorption science,rock mechanics,seepage mechanics,diffusion dynamics and fractal theory,theoretical analysis,experimental verification and numerical simulation are used to study the instability and disaster mechanism of the composite coal of intact coal and tectonic coal with unsteady diffusion.The main conclusions are as follows.（1）According to the results of scanning electron microscope,mercury intrusion porosimetry and low-temperature liquid N₂ adsorption test,the effects of fragmentation on pore shape,pore size distribution and fractal dimension of intact coal and tectonic coal were analyzed.The influence paths of fragmentation on different types of pores in coal samples was proposed.Based on the relationship between fractal dimensions and pore parameters,the critical particle size range of tectonic coal was determined.It was determined to be 0.25-1 mm.Finally,the influence of the coupling of fragmentation and tectonism on tectonic coal was discussed.It was found that tectonic coal is more prone to matrix damage than intact coal under the dual coupling effect,which leads to rapid gas desorption in micropores as the main place for gas storage.At the same time,the gas migration resistance in the large pores as the migration channel is reduced,and the phenomenon that there are many tectonic coals with high crushing degree in the outburst site was generated.（2）Based on the experimental data of gas desorption and the discussion of the current steady-state and unsteady-state gas diffusion models,the general form of simplified diffusion model of coal particle gas considering steady-state and unsteady-state gas diffusion was established.The accuracy of the model was verified by using the gas diffusion data of intact coal and tectonic coal under different conditions,and the reliability of the model in the calculation of gas loss was discussed.Based on the results of verification,it is found that the calculation error range of the model for intact coal is 0.02%～2.53%,and the calculation error range for tectonic coal is 0.15%～10.80%.The n value in the general form of simplified diffusion model was determined as the index of gas state discrimination.Based on pure steady diffusion（n=0.25）and pure unsteady diffusion（n=0.5）,the gas diffusion state is divided into three zones,namely ultra-negative unsteady diffusion zone,negative unsteady diffusion zone and positive unsteady diffusion zone.By discussing the influence mechanism of pressure,particle size and tectonism on gas diffusion state,it was found the main reasons for controlling the change of effective diffusion coefficient with time include the influence of diffusion length（IDL）,the influence of free gas pressure（IGP）and the influence of absorbed gas thickness（IGT）.The effective diffusion coefficient of intact coal with particle size of 1-3 mm is dominated by IGT,and the gas diffusion in intact coal and tectonic coal with other particle sizes is mainly controlled by IDL and IGP.（3）According to the discussion results of gas seepage control model,combined with the relationship between fractal theory and matrix damage,the coal permeability model considering matrix damage was established.Taking the composite coal seam of intact coal and tectonic coal in Daning Coal Mine as the research object,the COMSOL-Multiphysics numerical simulation model was established.Based on the results of simulation,the unsteady-state gas diffusion mode and the damage change of coal particle size were verified.The change of matrix gas pressure under steady-state and unsteady-state gas diffusion was compared.The changes of horizontal stress,plastic failure zone and gas pressure in front of roadway after mining disturbance were discussed.It is found that the coal samples in the composite coal seam produces mutual restriction,thus increasing the horizontal tensile stress at the interface of the two coal seams.In addition,the presence of tectonic coal leads to a decrease in the ability of coal in front of the roadway to resist gas outburst,and tectonic coal affects the migration of gas in its adjacent intact coal,thereby increasing the risk of coal and gas outburst.（4）Based on the general form of simplified diffusion model and damage coefficient of coal,the calculation expression equations of elastic energy and gas expansion energy considering unsteady gas diffusion and coal damage were proposed.According to the calculation results and numerical simulation results of COMSOL-Multiphysics,it is found that the order of magnitude of the total outburst energy of tectonic coal is 10¹～10² MJ.The ratio of elastic energy to total outburst energy under different initial conditions is 0.21%-35.57%.By analyzing the total outburst energy change results of intact coal,tectonic coal and composite coal seam,it is found that the total outburst energy of single coal seam of tectonic coal is 89.77～1021.71 times that of single coal seam of intact coal.The total outburst energy of tectonic coal in composite coal seam is 8.24～93.32 times that of single coal seam of intact coal,and the former ratio is much larger than the latter.When tectonic coal exists in coal seam,the total outburst energy of intact coal in composite coal seam also increases.In addition,the existence of tectonic coal makes the adsorbed gas in the intact coal seam in the composite coal seam accelerate desorption of gas when the composite coal was disturbed by mining,resulting in a huge adsorbed gas expansion energy,which in turn increases the outburst tendency of the composite coal seam.