Molecular Simulation Of Adsorption And Diffusion Of Methane In Deformed Coal

Deformed coal is a kind of coal that the primary structure has been damaged; even the internal chemistry constituents and structure have been changed. The deformed coal has close relationship with gas outburst. The objective of this work is to explore microscopic reasons to gas outburst by comparing the differences of adsorption and diffusion for methane in deformed coal and corresponding undeformed coal using molecular simulation of grand canonical Monte Carlo (GCMC) and molecular dynamic (MD) simulation. The main contents are listed as follows:The models of deformed coal and corresponding undeformed coal were constructed according to the XRD data of coals excavating from the same mines and seams combined with the cell parameter of graphite in Inorganic Crystal Structure Database (ICSD).The Henry constants for methane adsorbed in the two models of coal from 283 to 323K were calculated by GCMC, and the heats of adsorption were obtained. It is found that, in all the coal samples with different ranks from fat-coal to anthracite, there is little differences in the heats of adsorption between deformed coal and corresponding undeformed coal, which shows that the thermal effect of adsorption for methane in coal will not dominant cause gas outburst.Two pairs of deformed coal and corresponding undeformed coal with great tectonic differences were selected, and the adsorption isotherms of methane at 298K and 303K were calculated. It is found that the deformed coal adsorbs more methane than corresponding undeformed coal at the same temperature.The effects of temperature, coal rank, and water molecule on the adsorption of methane were also studied. The adsorption capacity of methane in the both coal samples decreased regularly with the increase of temperature in the range of 283K to 323K. The empirical formula of temperature-adsorption was obtained by linear regression, which shows that the deformed coal was more sensitive to temperature. The adsorption capacity of methane increased with ranks; moreover, there is a little influence of temperature at low rank, and more significant influence with ranks. The adsorption capacity of methane is decreased due to the water molecular easily adsorbed in coal causing water molecular occupy adsorption sites prior to methane. The adsorption capacity of methane decreased more obviously with the increase of water molecule increase, but would not affect essentially in quantity.The diffusion of methane in deformed coal and corresponding undeformed coal was studied by MD. The diffusion coefficients were obtained by analyzing mean square displacement (MSD). It exhibits that deformed coal is more beneficial to the diffusion of methane.The pair of deformed coal and corresponding undeformed coal were selected to study the diffusion activation energy in diffusion process. The diffusion coefficients of methane in the two coals were calculated at 288, 298, 308, 318 and 328K, respectively. The diffusion activation energy of methane in deformed coal is smaller than that in undeformed coal by fitting Arrhenius equation, that is, the deformed coal is more beneficial to the diffusion of methane which is considered as the microscopic reasons of gas outburst.