Investigation of non-linear gas adsorption in coal based on density-gradient driven flow:Experiments and numerical solutions

Coal gas adsorption and its mathematical characterization plays a theoretical basis to estimate coal seam gas content,forecast coalbed gas production,classify the coal / gas outburst. When theoretically describing coal gas absorp- tion,only considering adsorbed gas to compute gas content in coal and only involving Fick’s mass influx may lead to an erroneous prediction due to the pore size in coal ranging widely and it should be a multi-mechanism process in multi-scale pores. In order to theoretically characterize the gas adsorption process in high accuracy,a series of experi- ments were conducted under different initial pressures with different grain size samples,and the changing curves of the gas adsorption amount with time were obtained. Following this,recalling to the derivation of Fick model,the concept of gas density was employed to compute gas content consisting of adsorbed gas and free gas,the non-linear gas adsorption model (Density model) was proposed with an assumption that the gas transport in coal was density-gradient driven flow,and the numerical model of Density model was presented by using the finite difference method. Numerical code to solve the two models was developed independently. Afterward the two sets of simulated results obtained by numerical solving the Fick model and Density model were compared to the measured date,and it shows Density model owns a higher accuracy over Fick model in terms of modeling gas adsorption,implying the proposed Density model may be more effective in describing the non-linear gas transport behaviors in coal.