Study On Competitive Adsorption Mechanism Of Pingdingshan Bituminous Coal For Gas Under Different Moisture Conditions

Coal seams are not only rich in gas resources,but also full of a large amount of water.The existence of water will have an impact on the adsorption of coalbed methane,which will affect gas extraction,carbon dioxide sequestrations,coal spontaneous combustion,etc.If the conditions can be reasonably changed to allow a large amount of inert gas to be adsorbed and displace oxygen,methane and other gases that are easy to cause disasters in coal seams,it can achieve the win-win effect of greenhouse gas storage and prevention of coal spontaneous combustion at the same time.Therefore,it is significant to explore the influence of water content on gas competitive adsorption.Taking Pingdingshan bituminous coal as adsorbent,the dry coal model and four coal molecular models with different water content were constructed.With oxygen,methane,carbon dioxide and nitrogen as adsorbents,the adsorption behavior of coal samples with different water content on single component and mixed component gases under the pressure of 298.15 K and 0～10 MPa was simulated by Grand Canonical Monte Carlo method.The results show that at a certain temperature,the adsorption capacity of gas increases with the increase of pressure.In the low-pressure stage,the gas quickly fills the pores,the adsorption capacity increases,the gas molecules increase,and the intermolecular force increases,so that the adsorption isotherm tends to be stable and reaches the adsorption equilibrium state.In the single component system,the relationship between the adsorption capacity of coal and gas is:CO₂>O₂>CH₄>N₂.Under the condition of low water content,H₂O molecules have little effect on the gas adsorption capacity.With the increase of water content,the adsorption capacity of single component gas decreases.And there is obvious competitive adsorption between H₂O molecules and gas molecules.Among them,water has the most obvious effect on the adsorption of CH₄ and the least effect on the adsorption of CO₂,and the addition of moisture does not change the type of gas adsorption curve.In the mixed system of O₂/N₂,O₂/CH₄ and O₂/CO₂,the addition of water reduces the adsorption capacity of gas.Among them,CO₂ has the greatest impact on the adsorption capacity of O₂,showing obvious competitive adsorption advantages,followed by CH₄ and N₂.Therefore,the effect of carbon dioxide injection on preventing coal spontaneous combustion is better than that of nitrogen injection.Coal pore structure is the main place for gas adsorption behavior.The pore structure measurement test shows that Pingdingshan bituminous coal has developed pores,mainly macropores and mesopores,with less micropores.However some ultramicropore structures in coal are difficult to measure by the test method.Therefore,different probe molecules are introduced to simulate the ultramicropore structure in coal.It is found that the ultramicropore are mainly ink bottle shaped pores.The smaller the diameter of probe molecules are,the more complex the pore structure detects.The larger the pore surface area and free volume are,the more effective adsorption space and adsorption points of adsorbed gas have.And there is a negative correlation between gas adsorption capacity and molecular dynamics diameter.Fourier transform infrared spectroscopy shows that there are obvious differences in the content of functional groups in coal samples with different moisture content.The content of functional groups changes with the change of moisture content,but the change law of each functional group is different.By simulating the saturated adsorption state of coal molecules under different conditions,it finds that the bond length of the four gases does not change before and after adsorption,and the adsorption heat is less than 40 k J/mol.Therefore,the adsorption behavior of coal to gas belongs to physical adsorption.After entering the coal structure,the gas molecules are mainly adsorbed near the oxygen-containing functional groups,and the water is added to compete with the gas for the adsorption sites near the oxygen-containing functional groups.The higher the water content is,the less heat is released by gas adsorption,which means the increase of water content inhibits the adsorption of coal to oxygen.