Experimental Study On The Adsorption Characteristics Of CH₄ And CO₂ By Coal With Different Metamorphic Degree

By studying and calculating the thermodynamic characteristic parameters and the interaction between the coal molecular fragment model and different gas molecules in the process of coal adsorption of methane and carbon dioxide can systematically understand the difference in coal adsorption capacity of different gases,so as to better improve and develop the coal adsorption gas theory,and guide the on-site application of carbon dioxide to drive coalbed methane.In this paper,coal samples with different metamorphic degrees are taken as the research object,including Sijiazhuang coal mine,Yangliu coal mine and Xiaoqing coal mine.The basic parameters measurement,isothermal adsorption experiment,thermodynamic parameter calculation,software simulation and other methods are used to study the composition,pore development characteristics and functional group distribution of coal samples.The equivalent adsorption heat of methane and carbon dioxide and surface free energy changes of coal samples are calculated,the stable adsorption structure of coal molecules for methane and carbon dioxide molecules are simulated,and the differences in the adsorption capacity of coal for methane and carbon dioxide gas with different metamorphic degrees were analyzed.The research work mainly achieved the following knowledge:(1)In the experiment of basic physical property parameters,through the pore test of anthracite SJZ,bituminous YL and lignite XQ,it can be seen that each coal sample has an open pore shape,the micropores of each coal sample are well developed,and the specific surface area of YL coal sample is the smallest.The results of infrared spectrum show that with the increase of metamorphism degree,the content of hydroxyl in coal decreases gradually,the content of aromatic hydrocarbon increases gradually,there is no obvious absorption peak of aliphatic hydrocarbon in SJZ coal sample,and the content of oxygen-containing functional group in XQ coal sample is much higher than that in SJZ and YL.The results of isotherm adsorption experiments show that each isotherm adsorption curve shows an upward trend,and the curve becomes more and more gentle with the increase of pressure.The adsorption capacity of carbon dioxide on the same coal sample is higher than that on methane gas.However,with the increase of temperature,the adsorption capacity at the same pressure decreased gradually.With the increase of metamorphism degree,the adsorption capacity of coal sample for methane and carbon dioxide at the same temperature shows a trend of first decreasing and then rising,which is consistent with the change trend of specific surface area,so it can be seen that the specific surface area plays a decisive role in the gas adsorption capacity of coal sample.With the decrease of coal metamorphism degree,the ratio of carbon dioxide to methane limit adsorption capacity increases gradually,which indicates that the metamorphism degree decreases and coal is more sensitive to the change of gas types.(2)Using the formula to calculate the equal amount of adsorption heat and surface free energy,the results show that with the increase of the amount of adsorption,the heat released in the adsorption process gradually increases,and the value of the amount of equal adsorption heat continues to increase.With the increase of the degree of metamorphism,under the same adsorption amount,the adsorption heat becomes larger,indicating that the heat released during the adsorption process is more and the adsorption capacity is stronger.When the adsorption amount of the same coal sample is the same,the heat released by the adsorption of carbon dioxide gas is much larger than that of the methane gas,indicating that the coal sample has a stronger adsorption capacity for carbon dioxide gas.For the same coal sample,at a fixed temperature,the surface free energy increases with increasing pressure.For the same coal sample,under a fixed pressure condition,as the temperature increases,the coal surface free energy decreases.(3)The software simulation results show that as the degree of coal metamorphism increases,the dipole moment of the coal molecules becomes smaller,the benzene rings are arranged more closely,and the structural symmetry is enhanced,the adsorption energy is greater.This shows that as the degree of metamorphism increases,the adsorption capacity of coal molecules becomes stronger and the adsorption state is more stable.When coal molecules adsorb methane and carbon dioxide at the same time,carbon dioxide will occupy more adsorption sites and be closer to the coal molecules.The calculated value of the adsorption energy is greater.This shows that the adsorption is more stable than methane,and the adsorption ability of coal molecules to carbon dioxide molecules is stronger than that of methane molecules.The effect of displacing methane and extracting coalbed methane can be achieved by injecting carbon dioxide.(4)According to the various experiments taken,the differences in the adsorption capacity of coal with different metamorphisms for methane and carbon dioxide are comprehensively analyzed,and the competitive adsorption mechanism is further analyzed.According to the existing on-site study of carbon dioxide injection to promote methane extraction,the analysis found that the anthracite coal seam with high metamorphic degree has high gas content in the coal seam,and the effect of carbon dioxide injection to promote gas extraction is obvious.However,for low-metamorphic coal seams,there are few engineering practices,and the difficulties of carbon dioxide injection to displace coal seam gas need to be overcome urgently.This thesis consists of 58 figures,26 tables,and 78 references.