Study On Influencing Factors And Models Of Diffusion Coefficient Based On Measurement Method Of Concentration

Coal seam gas diffusion follows Fick’s law of diffusion,in which the gas diffusion coefficient characterizes the ability of gas to diffuse in the coal seam.The accurate measurement of the diffusion coefficient and the study of its influence law can further clarify the physical mechanism of the gas desorption process,and accurately determine the gas content.Issues such as the optimization of drainage technology are of great significance.In this paper,the traditional backcalculation diffusion coefficient method has the model dependence problem and the problem that the influence of temperature,pressure,stress and gas type on the diffusion coefficient is not thoroughly studied.The steady-state method under different influencing factors is carried out to measure the diffusion coefficient.The change law and transient method of granular coal diffusion experiment,the research results are as follows:(1)The experimental results show that the diffusion coefficient of helium and methane has a negative correlation with the effective volume stress,and the effective stress has a small effect on the diffusion coefficient;the diffusion coefficient of helium gas is quadratic with the increase of inlet pressure(concentration)According to the parabolic law,the methane diffusion coefficient presents a power function decay law as the intake pressure(concentration)increases;the helium diffusion coefficient and methane diffusion coefficient have a positive correlation with temperature,and satisfy the Arrhenius formula.(2)The measured diffusion coefficient of helium gas by steady-state method,the initial diffusion coefficient of exponential model,and the average diffusion coefficient of Barrel model are all parabolic laws with the gas equilibrium pressure;the measured diffusion coefficient of methane steady-state method and the gas equilibrium pressure show a power function decay,While the exponential model calculates the initial diffusion coefficient and the Barrell model calculates the relationship between the average diffusion coefficient and the gas equilibrium pressure,which fluctuates greatly.(3)Based on the measured diffusion coefficient and the change law of pressure(concentration)and temperature based on the steady-state method,combined with Fick’s diffusion law,a variable diffusion coefficient diffusion model considering the change of coal gas concentration and temperature is established,and the COMSOL numerical calculation software is used to calculate.(4)The constant temperature desorption diffusion and variable temperature desorption diffusion experiments of granular coal were carried out,and the results were compared and verified with the predicted value of the variable diffusion coefficient model.The results show that the variable diffusion coefficient model can describe the process of gas diffusion in coal matrix well whether it is constant temperature diffusion or variable temperature diffusion.The paper has 39 pictures,13 tables,and 83 references.