Study On Adsorption And Diffusion Law Of CO₂ Gas In Low Permeability Coal

Gas accident is one of the main disasters that affect the safety of coal mining.Gas extraction in low permeability coal seams is difficult and inefficient,so CO₂ displacing coalbed methane technology is generally used to control gas in coal seams.However,in the process of CO₂ displacing coalbed methane,the diffusion of CO₂ will significantly affect the gas displacement volume and extraction efficiency.In view of this,this paper usesd theoretical analysis,physical experiment,molecular simulation and other means,and carried out the mercury injection experiment and CO₂ adsorption experiment of coal.The influence of pressure and temperature on CO₂ diffusion in coal was mainly studied.Combined with molecular simulation,the diffusion law obtained from physical simulation experiment was verified.The main research results are as follows:The mercury injection test was conducted on the experimental coal sample.According to the pore classification method proposed by Hodote,the pore volume of the experimental coal sample was mainly composed of transition pores and micropores,accounting for 39.43%and 57.58%,followed by macropores and mesopores.The adsorption curves of CO₂ gas at different temperatures are consistent with the Langmuir monolayer adsorption model.At the same temperature,with the increase of pressure,the adsorption capacity of CO₂ gas increases.Both a and b values of adsorption constants decreased with the increase of temperature.In the physical simulation experiment process of CO₂ gas diffusion in columnar raw coal,under constant pressure,with the increase of ambient temperature,the cumulative diffusion of CO₂ gradually increases with the extension of diffusion time.The diffusion velocity of CO₂ decreases with time.The diffusion coefficient of CO₂ increases with the increase of temperature.Under constant temperature,with the increase of pressure,the cumulative diffusion of CO₂ gradually increases with the extension of diffusion time.The diffusion velocity of CO₂ decreases with time.The diffusion coefficient of CO₂ increases gradually with the increase of pressure.The adsorption and diffusion process of CO₂ in coal molecules based on molecular simulation.When the simulated temperature is 15?,25?,30? and 35?,the coal molecules can adsorb 14,13,12 and 11 CO₂ molecules respectively by using Grand Canonical Monte Carlo(GCMC).With the increase of temperature,the adsorption amount of CO₂ in coal molecules decreases gradually.The self-diffusion coefficient,corrected diffusion coefficient and transfer diffusion coefficient of CO₂ in coal molecules all increase with the increase of temperature and pressure.