Abstract
Gas pressure changes during the process of coal mine gas drainage and CBM recovery. It is of great importance to understand the influence of sorption pressure on gas diffusion; however, the topic remains controversial in past studies. In this study, four samples with different coal ranks were collected and diffusion experiments were conducted under different pressures through the adsorption and desorption processes. Three widely used models, i.e., the unipore diffusion (UD) model, the bidisperse diffusion (BD) model and the dispersive diffusion (DD) model, were adopted to compare the applicability and to calculate the diffusion coefficients. Results show that for all coal ranks, the BD model and DD model can match the experimental results better than the UD model. Concerning the fast diffusion coefficient Dae of the BD model, three samples display a decreasing trend with increasing gas pressure while the other sample shows a V-type trend. The slow diffusion coefficient Die of BD model increases with gas pressure for all samples, while the ratio β is an intrinsic character of coal and remains constant. For the DD model, the characteristic rate parameter kΦ does not change sharply and the stretching parameter α increases with gas pressure. Both Dae and Die are in proportion to kΦ, which reflect the diffusion rate of gas in the coal. The impacts of pore characteristic on gas diffusion were also analyzed. Although pore size distributions and specific surface areas are different in the four coal samples, correlations are not apparent between pore characteristic and diffusion coefficients.
Highlights
During the process of coal mine gas drainage and CBM recovery, the gas flow process can be divided into two stages
The diffusion coefficient represents the essential parameter of diffusibility and related studies show that it could be affected by temperature [1,2], moisture [3], pressure [4,5], gas type [6,7,8], sample size [6,9,10], and coal sample features [11,12]
The bidisperse diffusion (BD) model [5,8,9,14,20,21] assumes that the coal particle includes independent macropore micropore systems, which are represented by Da and Di respectively
Summary
During the process of coal mine gas drainage and CBM recovery, the gas flow process can be divided into two stages. Charrière et al [2] used CH4 and CO2 to conduct the adsorption kinetics experiments when the pressure is equal to 0.1 MPa and 5 MPa respectively They found that the diffusion coefficient increases with gas pressure. Cui et al [8] found that the diffusion coefficient of CO2 reduces when gas pressure is smaller than 3.6 MPa. Staib et al [4] conducted the adsorption kinetics experiments and analyzed the results using the BD model. To guarantee the comparability of the results, all the experiments are carried out under similar gas pressure range Both adsorption and desorption kinetics test are conducted. Four coal samples with different ranks are collected from typical mining areas in China, the test results are cross-compared to understand does the coal rank have impacts on the relationship between gas pressure and diffusion coefficients
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