Experimental Study On Pore Structure Evolution And Gas Desorption And Diffusion Characteristics In Coal Particles Under Low Frequency Mechanical Vibration

The occurrence conditions of coal seams in my country are extremely complex,and gas accidents have always been one of the important disasters that threaten the safety of coal mine production.Numerous examples of coal and gas outbursts show that mechanical vibration is an apparent factor that induces outbursts.In order to explore the desorption and diffusion characteristics of coal particle gas under the action of low-frequency mechanical vibration and its role in outburst development.In this paper,using the self-developed experimental platform of constant temperature gas adsorption and desorption under the condition of mechanical vibration,the pore structure of coal particles before and after mechanical vibration is refined and quantitatively characterized,and the influence characteristics of mechanical vibration on the internal pore structure of coal particles are obtained.The related mechanism of mechanics was analyzed.Then,desorption experiments were carried out on granular coals with different particle sizes and different equilibrium pressures under the condition of mechanical vibration at different frequencies.Influence mechanism;Finally,from the perspective of energy conservation,the role of gas expansion energy generated by gas desorption under mechanical vibration in the development of swell is analyzed,and the reasons for vibration induction and excitation swell are clarified from the perspective of gas energy.At the same time,the influence of mechanical vibration on the accuracy of gas content directly measured by drilling cuttings method in regional outburst prediction is analyzed.The main conclusions and understandings are as follows:（1）Mechanical vibration promotes the development of pores in coal as a whole,resulting in an increase in pore volume and specific surface area of pores in coal.Mechanical vibration has no obvious effect on the shape and connectivity of pores in coal,but it leads to an increase in the porosity of coal particles,a decrease in the tortuosity of pore channels,and a more straight pore channel,which is beneficial to gas diffusion and migration.The macropores and micropores in coal are more affected by mechanical vibration than mesopores and minipores.（2）The pore structure in coal has obvious monofractal and multifractal features.Mechanical vibration has obvious effects on the local density,heterogeneity,spatial complexity and surface roughness of pore distribution in coal.On the whole,the mechanical vibration leads to an increase in the complexity and surface roughness of macropores and mesopores in coal;an increase in the spatial complexity of small pores and micropores and a decrease in surface roughness.Mechanical vibration as a whole leads to a decrease in the local concentration of macropores of coal particles and a decrease in pore heterogeneity;an increase in the fluctuation of distribution of mesopores and an increase in heterogeneity;The density and local differences are greatly affected by mechanical vibration,but they do not show a uniform law.The multifractal Hurst index shows that mechanical vibration has an insignificant effect on pore connectivity in coal,and the connectivity changes of pores of different scales of different types of granular coals are quite different.（3）The multifractal Hurst exponent shows that mechanical vibration has a certain influence on the pore connectivity in coal,but the changes of pores in different coal samples at different stages are quite different,and the effect of vibration on pore connectivity is not obvious.However,mechanical vibration leads to an increase in porosity in coal,a decrease in tortuosity of pore channels,and a more straight pore channel,which is beneficial to gas diffusion and migration.The deformation,collapse or fracture of the coal matrix skeleton caused by the compressive stress generated by mechanical vibration on the coal matrix,and the loosening and exfoliation of the clastic particles on the surface of the interclastic pores and the clastic pores in the coal caused by the vibration are the main reasons for the evolution of the pore structure in the coal.（4）Mechanical vibration enhances the desorption and diffusion capacity of gas in pelletized coal.The gas desorption amount,desorption rate,desorption velocity,initial gas diffusion coefficient D₀,and initial effective diffusion coefficient D_eof vibrated pellet coal are all larger than those of non-vibrated coal samples,and they all increase with the increase of vibration frequency,but there are a certain characteristic frequency leads to a sudden increase in the desorption and diffusion capacity of coal particles when they are vibrated.During the entire diffusion process,the gas diffusion coefficient of the vibrated coal sample at the same time is always greater than that of the non-vibrated coal sample.（5）The mechanical vibration leads to the increase of pore volume in coal,the increase of the connectivity of macropores and mesopores,and the decrease of the concentration of pore size distribution,all of which are beneficial to the desorption and diffusion of gas.In addition,the"friction"and"throwing off"effects of mechanical vibration on gas molecules,the energy conversion during mechanical vibration,the"secondary pulverization effect"of mechanical vibration on coal particles,and the"frictional electrification"during mechanical vibration Both effects can affect the desorption and diffusion of gas.The improvement of gas desorption and diffusion capacity in coal caused by mechanical vibration is the coupling and superposition effect of the above factors.（6）The actual gas loss when the coal sample is exposed to vibration is significantly greater than that of the coal sample not affected by vibration;the error rate between the gas loss calculated by the t method and the power function method and the actual loss is significantly greater than that in the exposure stage not affected by vibration time error rate.The influence of vibration in the coal sample exposure stage will lead to large errors in the calculation of gas content loss,and the accuracy and reliability of the measurement results will be reduced,which will bring large errors to the determination of regional outburst prediction indicators.（7）Mechanical vibration increases the proportion of the initial gas desorption amount of granular coal,increases the gas expansion energy,and improves the gas transport capacity to the coal body.As a whole,the gas expansion energy increases with the increase of the vibration frequency.Compared with the non-vibrated particle coal,the outburst critical average particle size is larger,and the critical particle size increases with the increase of the vibration frequency.Under the action of mechanical vibration,granular coal can excite outburst at larger particle size and meet the requirements of desorption speed required for coal transportation.Figure[78]Table[37]Reference[207]...