Study On Power Function Model Of Methane Diffusion Coefficient In Coal Particles

The movement of methane in coal is divided into three stages,namely desorption,diffusion and seepage.Because the coal matrix have the special pore structure,diffusion of methane plays an important role in the whole process.The comprehensive analysis of mathematical physics models and experiments is an important way to understand the law of diffusion in depth.It can provide theoretical basis and application basis for improving the accuracy of gas disaster prediction and improving the extraction rate of coalbed methane(gas),which is of great significance for guiding the prediction and prevention of gas disasters and the exploration and development of coalbed methane.Firstly,Using coal particle methane desorption device,different time of methane desorption in coal particles was measured,the methane diffusion coefficient of coal particles at different time periods was calculated,the variation of diffusion coefficient with time was analyzed,the best quantitative relation between the diffusion coefficient and the time was found.Secondly,according to the Fick law,combined with variation of diffusion coefficient,the power function model of methane diffusion coefficient in coal particles was established,the errors between the classical diffusion model,the exponential diffusion model and the power function diffusion model are compared and analyzed.Finally,the methane desorption and diffusion characteristics of coal particles under different conditions were studied,and the variation of diffusion coefficient in the process of methane desorption and diffusion under different conditions was analyzed.The following conclusions are drawn:(1)Based on variation characteristic of diffusion coefficient with the time,power function model of methane diffusion coefficient in coal particles is constructed and it can be used to describe the whole desorption process of coal particles,it make up for the shortcomings of the classical model in the earlier experimental.(2)The process of methane adsorption in coal is divided into two stages,namely,initial stage fast adsorption and subsequent slow adsorption.The initial stage of rapid adsorption is mainly concentrated in the first 500 s,followed by slow adsorption until the adsorption equilibrium is reached.In the whole process,the diffusion coefficient is attenuated with time.With the increase of adsorption equilibrium pressure,the attenuation degree of diffusion coefficient is more obvious,the adsorption amount and the average diffusion coefficient increase gradually,and the average diffusion coefficient is negatively correlated with the time.(3)Through the methane desorption experiment,it is found that the adsorption equilibrium pressure has a significant influence on the desorption diffusion.With the increase of adsorption equilibrium pressure,the cumulative diffusion increase gradually,and the diffusion coefficient show a gradual change.The 0.5mm-1.0mm particle size of coal sample coefficients are greater than 0.25mm-0.5mm particle size of coal sample diffusion coefficient under the same pressure;The amount of coal methane diffusion in structure coal is two times as much as the amount of coal methane diffusion in the primary structure coal under the same conditions in 60 minutes.The methane diffusion coefficient of structure coal is always greater than the methane diffusion coefficient of the primary structure coal.Combined with the results of experiment on adsorption of liquid nitrogen at low temperature,The results of the analysis show that: The main reason for the large diffusion coefficient of methane in structure coal is that it has larger pore volume and specific surface area.