Effect of non-isothermal adsorption model on gas diffusion in coal particle

Abstract

Gas diffusion is of great importance for coalbed methane (CBM) recovery and CO2 storage in coal seam. Most of the existing models of gas diffusion in coal particles are derived under isothermal conditions, while the coupled effect between gas adsorption/desorption and temperature variation is often neglected. The effect of accurate non-isothermal adsorption model on gas diffusion simulation also needs further study. In this work, isothermal adsorption tests of CO2 in anthracite/bitumite particles at different temperatures are conducted. The fitting results of Langmuir/Freundlich adsorption models are analyzed and the optimal non-isothermal adsorption models for anthracite/bitumite are obtained respectively. Non-isothermal desorption-diffusion tests of CO2 in anthracite/bitumite particles are carried out and corresponding non-isothermal desorption-diffusion model is also proposed, based on which the effect of non-isothermal adsorption model on gas diffusion in coal is numerically analyzed comprehensively. The results show that the non-isothermal Langmuir/Freundlich adsorption models are optimal for anthracite/bitumite respectively. Based on the optimal adsorption models, simulated gas diffusion amounts agree well with the experimental results and consistent diffusion coefficients can be back calculated. For sub-optimal adsorption model with R2 > 0.9, its deviation from adsorption data would significantly affect the accuracy of gas desorption-diffusion simulation. Moreover, if ‘diffusion ratio’ is taken as evaluation index, the inaccurate diffusion amount caused by inaccurate adsorption model in numerical simulation may be ignored, and the diffusion coefficient determined by ‘diffusion ratio’ would deviate from the true value.