Study On Gas Adsorption And Diffusion Of Soft And Hard Coal Based On Molecular Simulation

The difference of molecular structure of soft and hard coal and its influence on the difference of adsorption,desorption and diffusion.It is an important aspect of prevention and cure of coal seam gas disaster and feasibility evaluation of coalbed methane development.It is a necessary means to discover and explain the macroscopic laws from the microscopic point of view.At present,most of the researches on the adsorption and diffusion characteristics of coal are focused on the physical experiments,but few on the combination of microscopic angle and macroscopic property.In this thesis,the typical soft and hard coal samples of anthracite,coking coal and fat coal are selected,and X-ray diffraction experiments are carried out to find out the difference of molecular structure parameters of soft and hard coal,referring to the unit cell parameters of graphite in crystal structure database ICSD.The supercrystalline structure models with different pore sizes and water content were constructed and optimized to reflect the molecular structure differences of soft and hard coals.The isothermal adsorption of methane on soft and hard coals at 1nm?3nm and 5nm pore size was simulated by using the giant regular Monte Carlo method.The characteristics of adsorption difference between soft and hard coals with the same pore size were analyzed,and the mechanism of the difference was explained from the aspects of molecular structure and intermolecular force.The adsorption characteristics of methane at different temperature and moisture content were analyzed,and the influence of these two factors on methane adsorption in soft and hard coal was revealed.The gas diffusion law of soft and hard coal was simulated by molecular dynamics method?MD?.The diffusive characteristics of typical soft and hard coals at the same temperature and same pore size are analyzed.The diffusivity of methane in 1nm and 5nm pore sizes in soft and hard coals is simulated.The diffusion coefficients of three groups of typical soft and hard coals at different temperatures are calculated.The activation energy of three groups of coal samples was calculated by Arrhenius equation fitting.The results show that the supercell structure model reflects the differences of soft and hard coal extensibility?La?,stacking capacity?Lc?and interlayer spacing(d₀₀₂)in terms of unit cell multiple and layer number.the excess adsorption capacity is the largest at 1nm pore size,3nm followed and the lowest at 5 nm.in the same pore diameter and temperature,and the excess adsorption capacity of methane in soft coal decreases with the increase of water content,compared with that of hard coal at the same pore size and temperature,the excess adsorption capacity of methane in soft and hard coal decreases with the increase of water content.When the water content reaches a certain degree,the adsorption capacity of coal to methane basically does not change,because the adsorption sites of coal structure model are basically occupied by water molecules.With the increase of temperature,the adsorption capacity of soft and hard coal decreases.And soft coal is always higher than hard coal,and it is also found that the difference between the adsorption capacity of soft and hard coal decreases gradually.In the process of calculating diffusion coefficient of soft and hard coal,it is found that the diffusion coefficient of methane in coal body decreases with the increase of moisture content.With the increase of simulated temperature,the calculated diffusion coefficient is fitted The diffusivity of soft coal is higher than that of hard coal.by Arrhenius equation to get the activation energy of three groups of soft and hard coal at different temperatures.The results show that the activation energy of soft coal is smaller than that of hard coal.It shows that methane diffuses more easily in soft coal.