| Gas is not only a kind of high-quality clean energy with abundant reserves,but also one of the main disasters that harm the safety production of coal mines.Gas extraction is the main method to ensure the safety mining of coal mines.Low-rank bituminous coal is widely distributed.Because the coal seam is subjected to stress for a long time,and the low-rank bituminous coal has high water content,the gas migration in the coal seam will inevitably be affected.The study of gas desorption characteristics of low-rank bituminous coal under the constraints of water and stress and the exploration of the mechanism of various influencing factors on the gas diffusion capacity in coal can provide theoretical basis for the prediction of gas outburst risk,gas extraction and gas disaster prevention in low-rank bituminous coal mining area.In this thesis,the coal sample of the 3 coal seam of Guojiahe Mine is taken as the research object,and the multi-disciplinary research methods such as gas diffusion dynamics,molecular dynamics,surface physical chemistry and so on are comprehensively applied.Theoretical analysis,experimental measurement,model calculation,molecular simulation and other means are adopted to conduct multi-scale and systematic analysis on the desorption and diffusion characteristics of coal mass and its influence mechanism under the condition of water and stress constraints.The main conclusions of this thesis are as follows:（1）The pore structure of coal samples was measured from the microscopic point of view.The full-aperture distribution curve of coal samples is obtained.The micropore specific surface area of low-rank bituminous coal is more than 80%.A large number of micropore volume and specific surface area are the main reasons for its strong gas adsorption and desorption ability.After the water treatment method in this thesis and long time drying treatment,the pore volume changes little,and the specific surface area has a certain decrease.It is proved that the method of water preparation is reasonable and practical.At the same time,the drying time of coal sample should be controlled reasonably during the experiment.（2）The law of adsorption and desorption diffusion of granular coal gas is revealed from the perspective of macroscopic experiment.The water content of low-rank bituminous coal increases from 6.77%to 11.50%,and a decreases from 26.5156 to20.2561 m3/t,but the b has a relatively small change,ranging from 0.7351 to 0.9123MPa-1.The increase of adsorption equilibrium pressure will increase the limit gas desorption amount of coal,and the increase of water will decrease the limit gas desorption amount of coal.The adsorption equilibrium pressure of granular coal increases from 0.74 MPa to 2 MPa,and the gas desorption amount increases by 90.89%in 120 min.The adsorption equilibrium pressure can promote the gas desorption of granular coal,but it is not the main factor affecting the diffusion characteristics.The moisture content of granular coal increased from 6.77%to 11.50%,and the desorption capacity decreased by 43%in 120 min.Under the condition of low adsorption equilibrium pressure,the constant diffusion coefficient decreases by 59.9%and the initial value of time-varying diffusion coefficient decreases by 61.7%.Under the condition of high adsorption equilibrium pressure,the constant diffusion coefficient decreases by 48.6%and the initial value of time-varying diffusion coefficient decreases by 48.4%with the increase of water content.The water content of coal sample will lead to the weakening of coal gas desorption and diffusion capacity.The inhibition effect of water on gas desorption increased with the increase of adsorption equilibrium pressure.After fitting analysis,the time-varying diffusion model is more suitable for full stage fitting.Water and adsorption equilibrium pressure have little influence on the attenuation coefficient of the time-varying diffusion coefficient of gas,and the constant diffusion coefficient is close to the average value of the time-varying diffusion coefficient in the whole-time process under the corresponding conditions.（3）The influences of confining pressure,water pressure and gas pressure on the desorption and diffusion capacity of coal gas are expounded from the macroscopic perspective.The simplified flat plate model based on the flat plate matrix was used to describe the gas diffusion characteristics and the time-varying diffusion equation was selected to modify it.When the stress reaches the range of middle and high stress,the seepage process has little influence on the apparent diffusion coefficient,and the apparent gas diffusion coefficient is close to the effective diffusion coefficient.Confining pressure has obvious inhibition effect on gas desorption and diffusion of coal body.The larger confining pressure is,the smaller the gas desorption amount,desorption rate and diffusion coefficient of coal body are.When confining pressure increased from 5 MPa to 30 MPa,gas desorption capacity decreased by 59.57%,constant diffusion coefficient decreased by 84.46%,and initial value of time-varying diffusion coefficient decreased by 87.46%.Under the condition of stress constraint,the inhibition effect of water on the desorption and diffusion of coal body gas is stronger.The desorption amount and desorption rate of 120 min of 11.26%coal column is decreased by 56.62%and 13.63%,respectively,compared with 6.53%coal column.The plate type diffusion coefficient of 6.53%moisture content sample is 2.78 times that of 11.26%moisture content sample,and the initial value of the time-varying diffusion coefficient of the plate type is 2.74 times that of the coal sample.The higher the adsorption equilibrium pressure is,the larger the gas desorption capacity and diffusion coefficient will be.When the adsorption equilibrium pressure increases from 0.74MPa to 3 MPa,the desorption capacity of coal pillar in 120 min increases by 248.36%.The time-varying diffusion coefficients of plate type and plate type are 2.97 times and 3.60times of those before pressure boost,respectively.From the perspective of influencing mechanism,the influencing factors of coal gas desorption and diffusion under the conditions of water and stress are summarized,and the influence of water,adsorption equilibrium pressure,confining pressure and form on the actual production of gas desorption and diffusion is summarized.The prevention and control of coal and gas outburst should reasonably consider the actual situation to predict.（4）The small molecular configuration of low-rank water-containing bituminous coal is constructed from the molecular scale and its diffusion characteristics are expounded.Compared with the self-diffusion coefficients of different water matrix configurations,the simulated methane diffusion process is similar to the real coal diffusion process.With the increase of water content,the simulated diffusion coefficient shows a sharp decrease at first,and then a slow change.The water content increases from 1.54%to 11.42%,and the diffusion coefficient decreases by 59.93%.By comparing the measured and simulated diffusion coefficients,the moisture content of coal body increases from 7.73%to 11.26%,the measured diffusion coefficient decreases by 49.34%,and the simulated diffusion coefficient decreases by 28.28%.When the moisture content of coal changes,the change rate of simulated diffusion coefficient is obviously smaller,while the change rate of measured diffusion coefficient is more obviously affected by the moisture content.The difference between the two is mainly caused by the difference of scale and process resistance.There are 77 figures,23 tables and 107 references in this thesis. |
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