Pore Structure Effect On Adsorption/desorption Dynamics Of Methane In Coal

Fragmentation always occurs in the disaster of coal and gas outburst. Thus, Pore damage evolution during coal being broken should be studied for explaining the mechanism of coal and gas outburst.The research method of multidisciplinary approach of surface chemistry, fluid mechanics, and capillary mechanics are used to establish the micro model of gas migration in coal pore of MC、YQW and BLS coal mines, fractal geometry and mathematical statistics are used to establish the micro model of gas migration in coal pore and the macro model of gas migration in coal particles. Numerical analysis combing with the laboratory experiment was used to observe the evolution of pore damage evolution mechanism. The adsorption isotherms and desorption kinetic curves of coal with different particle size are measured in the laboratory to verify the micro and macro model and the conditions for the particle size that shold be meet at the moment of coal and gas outburst. The main research conclusions are as follows:1) Particle size distribution at the maximum distance of the outburst points: weight percent of micron grade particle size for coal samples is between 0.66%~1.46%; weight percent of ten micron grade particle size for coal samples is between 2.59%~7.44%; weight percent of hundred micron grade particle size for coal samples is between 40.71%~62.76%; weight percent of micro meter grade particle size for coal samples is between 33.19%~52.15%.2) Numerical modeling combing with the laboratory experiments was used to study the pore damage evolution mechanism. Numerical modeling results shows that the total specific surface area experiences an outstanding increase with broken, as the external specific surface area experiences a rapid increase, and the internal specific surface area experiences a slow decrease. The laboratory experiment result shows that the pore volume increases with the decrease of coal particle size, and the ratios of the pore volume to the minimum pore volume is between 1.1715~28.5. The variation of specific surface area is relatively complex; the changing rule of the specific surface area of MC and YQW coal sample with particle size is like a ‘U’shape with the higher left side, and the specific surface area for BLS coal samples will increase with the decrease of particle size, and the ratios of the specific surface area to the minimum is between 1.0346~3.7807. The semi quantitative research of the pore shape: pore shape in the section of >2nm is almost cylindrical with two opened ends. pore shape in the section of <2nm is irregular. In addition, calculated total pore length of single coal decrease with the increase of pore radius, and decrease with the decrease of particle size, thus, “ink bottle” pore existed in large particle size coal disappears with coal broken, the existence of “ink bottle” represents larger Adsorption capacity for methane and gas emission ability.3) Establishment of micro model of gas migration in coal pore. Parameters in this model contains: Environmental parameters of adsorption equilibrium pressure and temperature, pore structure parameters of pore length and pore radius,adsorption parameters of surface tension and adsorption constant b. from the perspective of pore size, the effect of upper paramaters on gas adsorption/desorption performances., and the results show that the gas equilibrium amount is larger with higher above-mentioned parameters. The incidence of each parameter on the gas migration equilibrium amount is: pore radius > pore length > adsorption equilibrium pressure ≈ adsorption constant b. In addition, the increase of the pore radius r, the adsorption constant b and the adsorption equilibrium pressure can decrease the migration equilibrium time of gas in coal pore, and the incidence of above-mentioned three parameters is: pore radius > adsorption equilibrium pressure > adsorption constant b. The increase of the length L would obviously increase the migration equilibrium time of gas in coal pore. The results of gas adsorption/desorption performances after pore shape damage(A:one open end to two open ends; B:damage at the position of 1/2; C: damage at the position of 1/4;D:damage of “ink bottle ”at the positon with different-diameter;E: one open end to two open ends for the “ink bottle”) from the perspective of micro perspective show that the pore shape damage has no influence on the gas migration equilibrium amount, and it mainly influence the gas migration equilibrium time. The incidence is: A=B>E>C>D。4) Establishment of macro model suitable for gas migration in coal particles- Fractal Theory Based Fractional Diffusion Model, the euclidean space dimensionality in Fick diffusion law is replaced by fractal dimension, and the diffusion coefficient in Fick diffusion law is replaced by the diffusion coefficient suitable fractal porous media, and the fractional order is used to replace the integer order in Fick diffusion law. The initial and boundary conditions of the Fick model are used to solve the FFDModel. And this model was verified from the theoretical aspect. The desorption rate will increase rapidly,and the increasing ration is between 4~16, when the particle size becomes 1/2 of before5) Experimental verification for the micro and macro model by conducting the adsorption isotherms and the desorption kinetic curves experiment. Partition phenomenon exists in the adsorption isotherms. And the adsorption isotherms get closer to the top as the particle size becoming smaller. Equilibrium time will be rapidly increased as the particle size becoming smaller. The minimum value is 5min, and the maximum value is a couple days. Combining with the micro model, the pore number can be obtained. Pore structure distribution combing with the micro model will realize the transition from micro to macro phenomenon.6) Particle size for the outburst occurrence: Pore structure distribution combing with the micro model and the expansion energy of gas will obtain the distribution of expansion energy for coal with different particle size.combing with the condition of outburst occurrence, the conclusion of desorption rate in irregular particle size can not promote the occurrence of outburst. Thus micro size coal samples should exist in the preparation process of coal and gas outburst.