Analysis of gas adsorption and diffusion characteristics of coal particles in confined space and model adaptation differences

There is a difference between the gas adsorption diffusion coefficient and the desorption diffusion coefficient,and the adsorption diffusion coefficient under constant volume conditions obtained by the single-hole diffusion model is not appropriate.In order to compare the fitting difference of the two models and correctly evaluate the gas diffusion process under different boundary conditions,the experiments of industrial analysis,isothermal adsorption and dynamic adsorption diffusion kinetics experiments were used to study the gas adsorption equilibrium process in coal particles and the induced variation of diffusion coefficients under different initial pressures.The changes were studied experimentally.The results show that the adsorption pressure of the test coal sample under constant volume conditions gradually decreases with the adsorption time in a logarithmic curve,and the length of the adsorption equilibrium time is determined by the pore structure in coal.In addition,based on the real gas state equation,the pressure drop law can be converted into the adsorption law,and the constant volume and constant pressure diffusion models are used to fit the gas adsorption and diffusion law.It can be found that the diffusion coefficients obtained from the constant-pressure diffusion model is two times larger than that obtained from the constant-volume diffusion model,and this magnification increases with the gas pressure increases.The constant-pressure boundary gives the gas molecules a free space to diffuse,while in the constant-volume boundary,the gas concentration gradually increases.The boundary of constant pressure is equivalent to a space where gas molecules can easily escape.Compared with the boundary of constant volume with gradually increasing concentration,the diffusion is less difficult and the diffusion coefficient is greater.Finally,according to the difference between the constant pressure diffusion coefficient and the constant volume diffusion coefficient,the application range of the two diffusion coefficients in the experiment and in the project is determined:the common constant-pressure diffusion model is appropriate for describing adsorption or desorption laboratory experiments in a fixed pressure environment,and gas emission law in high-permeability zone of coal seams.In contrast,the constant-volume diffusion model is appropriate for describing adsorption or desorption laboratory experiments in a fixed volume environment,as well as the gas emission law isolated by low-permeability coal,such as the region isolated by high stress concentration area,or faults and folds where tectonically deformed coal commonly exists.