Application Of SLD Model And Molecular Dynamics In Gas Adsorption Of Different Coal Types

In this paper,five coal samples of anthracite,high-volatile bituminous coal,medium volatile bituminous coal,sub-bituminous coal and lignite are selected from the hot areas of coalbed methane to conduct pure gas and mixed gas adsorption experiments in different water conditions.The mathematical model predicts isothermal adsorption.The curves and molecular dynamics simulations confirm each other to study the adsorption properties of different coal rank coal samples for various pure gases and mixed gases,and provide valuable reference for coalbed methane mining under different coal types in different regions.From the experimental measurement and mathematical model fitting to the molecular dynamics research stage,the water content in the water-containing coal sample was studied as a separate factor.When volumetric method is used to measure adsorption data:the four states of water（equilibrium water,capillary water,inherent water,and adsorbed water）are theoretically analyzed before the experiment,and through reasonable simplification(equilibrium moisture of bituminous coal and anthracite is considered to be equal to coal seam moisture,Correction coefficients are applied to the equilibrium moisture results of low-order lignite to reduce the effect on the adsorption data;moisture treatment,simplification and calculation are performed in the experiment;error analysis reduces the impact after the experiment（do not consider adsorption expansion,and do Good uncertainty analysis）.When introducing the SLD theory to build a mathematical model:pre-process the water-containing coal sample data for the most soluble CO₂,modify coefficients for other gas distributions,and perform phase inspection analysis for 5 coal samples.When using the molecular dynamics method to construct a typical coal sample model:water molecules suitable for the water content in this paper are added to the upper and lower coal seams of the coal molecular model.The modified SLD model was used to predict the isothermal adsorption curves of pure gas adsorption for different coal rank coal samples.A PR-EOS co-volume correction coefficient was introduced for each coal sample and pure gas adsorption.The degree is higher than the existing mathematical model;the modified SLD model is also used to predict the isothermal adsorption curves of different coal rank coal sample mixed gas adsorption（including binary mixed gas and ternary mixed gas）,which introduces mutual The action parameter Cij correlates the mixed gas adsorption behavior of the system to a standard error of experimental error,and allows the less-adsorbed components in the diluted mixture to correlate with each other within the experimental uncertainty range,making the range of use wider and fitting Degree is higher than other existing mathematical models.By constructing a modified atomic model（C,H,N,O）,molecular model（CH₄,CO₂,N₂,C₂H₆）and experimentally determined C,H,O,N,and S,a modified version of the wiser model,The three-dimensional and three-dimensional maps of the gas distribution under different pressures and different water contents were visually displayed using the constructed coal molecular model,and then the concentration distribution,mean square displacement,energy distribution,and simple isothermal adsorption curve were simulated to calculate experimental data and mathematics.The models confirm each other.Through experimental methods,mathematical models and molecular simulations,they have confirmed each other,combined with the accuracy improvement brought about by the separate treatment of water factors,and concluded:（1）The improved SLD model can fit pure gas adsorption isotherms of different coal rank coal samples with different water states in a wide range of pressure and temperature.In order to improve the fit in a wider pressure range,the SLD-PR covolume parameter needs to be introduced,which effectively changes the density of the adsorbed phase.The introduction of the PR-EOS covolume improves the prediction of the excess adsorption and the density of the adsorbed mass phase,which makes the SLD model fit better;the optimized representation of the improved SLD model for the mixed gas isotherm adsorption curve requires a suitable interaction parameter C_ij.The use of the C_ij parameter allows the less adsorbed components in the diluted mixture to be correlated within the experimental uncertainty range.（2）A typical coal molecular model constructed by molecular dynamics can visually display three-dimensional and two-dimensional plan views of gas distribution under different pressures and different water contents.The concentration distribution,mean square displacement,energy distribution,and simple isothermal adsorption curve of the method were calculated by molecular dynamics simulations to confirm with experimental data and mathematical models,and the results are in agreement with the experimental conclusions.（3）Under the same conditions of adsorption pressure,the adsorption amount of gas increases with the increase of coal rank,and the isothermal adsorption curves of different coal rank coal samples are similar.When the gas is in a gas state,the equilibrium pressure increases.Large,the excess adsorption capacity of coal samples gradually increased;（4）The adsorption amount of CO₂ in the water-containing coal sample is significantly smaller than that in the dry coal sample,and there is not much difference between the adsorption amount of the other gas-containing coal samples and the dry coal sample;（5）When the temperature is the same:the amount of CO₂ adsorption is much larger than CH₄,N₂,C₂H₆,and CO₂ has obvious adsorption advantages compared to the other three gases.During the adsorption process,the intermolecular interaction between CO₂ and coal molecules is stronger and dominates.The adsorption capacities of C₂H₆,N₂,and CH₄ in dried coal samples were not significantly different,but the difference in adsorption capacity increased significantly in the presence of water.