Study On Gas Adsorption Characteristics In Coal Nanopore

This thesis researched on the pore structures of coal samples from 12 different mines and different coal ranks by using a combination of mercury intrusion,CO2adsorption,low-temperature nitrogen adsorption,scanning electron microscopy,and small-angle X-ray scattering experiments.The structural parameters such as the number of micropores,mesopores and macropores,pore size distribution characteristics,and pore shape were analyzed in detail based on the advantages of each method.The types of chemical functional groups,atom substitution forms,aromatic layer structures and the existing form of C,N,O,and S of 12 different coals were quantitatively and qualitatively analyzed by using ¹³C-NMR,FTIR,HRTEM,XPS,and XRD.Combined with the results of elemental analysis,the 12 molecular structures of coals with the different ranks were drawn.By analyzing the difference between the model and the experimental ¹³C-NMR spectrum,the 12 molecular structures were corrected.Based on the theories of molecular mechanics and molecular dynamics,numerical simulations were used to simulate the adsorption of CO2,N2,and CH4 in single molecular layer structure and nano-pores with the different sizes of coal by means of GCMC（giant canonical ensemble Monte Carlo）.The characteristics of gas adsorbed on the single molecular layer were discussed.The differences between dry coal samples and water-containing coal samples for gas adsorption were analyzed,and the main factors affecting the gas capacity were obtained.The research of this thesis will help to better understand the relationship between the nano-pore structure of coal,the competitive adsorption of mixed gas,and the influence factors of gas adsorption in coal.It will improve the gas adsorption theory and gas drainage of low permeability coal seams in the future.Besides,it has an important guiding significance on the extraction of natural gas.The results obtained in this thesis are as follows:（1）The multi-scale pore structure characteristics of coal were given.The pore size distributions of 12 coal samples from micropores to macropores and microscopic images of pores were obtained by analyzing the results of mercury injection,CO2adsorption,low-temperature nitrogen adsorption,small-angle X-ray scattering,and picture information taken by a scanning electron microscope.The experimental results showed that the pore size distributions obtained by the mercury intrusion method are mainly above 1000 nm;the pore size distributions of the micropores obtained by the CO2 adsorption test are mainly between 0.3～1.5 nm,and the majority of the pores are 0.5nm,0.6nm and 0.8nm.The low-temperature nitrogen adsorption experiments used HK,DFT,and BJH methods to characterize the micropore,full pore,and mesopore size distributions of 12 coal samples.However,the BJH curves of the 12 coal samples are very different due to the different degrees of coal metamorphism.There are multiple peaks in the HK curves,and the DFT method also shows that there are abundant micropores below 1 nm in 12 coal samples.Scanning electron microscope pictures of coal were used to analyze the pore shape,solidity of coal,and the degree of damage during coalification.Small-angle X-ray scattering can get more accurate pore size distribution characteristics of 12 coal samples without destroying the pore structure of coal.The higher the coal rank is,the more the pores were developed.（2）Combined with various experimental measurement methods,the relevant structural parameters and chemical functional group types of coal were tested,the molecular structures of 12 coal samples with the different ranks were plotted,and structural optimization and energy minimization calculations were performed on them.A series of coal molecular structure experiments were carried out in this paper.Among them,12 group structural parameters characterizing coal were calculated from the¹³C-NMR spectrum of coal;the functional groups contained in coal were qualitatively analyzed from the infrared spectrum,verifying the results of nuclear magnetic resonance experiments;lattice fringes of 12 coal samples were obtained from the processed high-resolution transmission electron microscope photos,and the number of aromatic rings in the coal was calculated;The existing forms of C,N,O,and S in coal and the size of the aromatic layer were obtained from the XPS and XRD experiments Based on the results of the above experiments,the molecular structure of 12 coal samples were drawn using Materials Studio software.The calculated ¹³C-NMR of the model and experimental ¹³C-NMR were compared and analyzed.The models were continuously revised to determine the molecules of 12 coal samples structure.（3）The adsorption of single-component and mixed-component gas in coal was simulated.Firstly,the giant canonical ensemble Monte Carlo method was used to simulate the adsorption of H2O,CO2,CH4,and N2 by the monolayers of 12 coal samples.The research found that the adsorption isotherms of the 12 coal samples had basically similar trend.Although the molecular structures of the 12 coal samples are different,the differences in the gas adsorption capacity of the monomolecular layer structure of different coal samples were not obvious.The difference in the adsorption amount was mainly caused by the difference in the surface area of the molecular structure.The relationship between the adsorption capacity of coal to four gases is H2O>CO2>CH4>N2.Subsequently,the molecular structure model of Matigou coal sample was used to study the multi-gases adsorption under different molar ratios.By comparing and analyzing the adsorption isotherms,it was found that the adsorption amount of H2O was very high under the condition of a low molar ratio.When the molar ratio was large,the adsorption amount of other gases was almost zero.In the simulation of competitive adsorption of CO2 with CH4 and N2,it was found that the competitive adsorption advantage of CO2 was obvious.Under the condition that the molar ratio of CO2 was lower than CH4 or N2,CO2 also showed a large adsorption capacity,but the competitive adsorption advantage of CO2 is smaller than that of H2O.When the molar ratio of CO2:CH4:N2 was 2:3:5,the adsorption amount of CH4 was basically zero,which could provide some guiding significance for solving the problem of low gas drainage in low-permeability coal seams.（4）The effects of gas and moisture adsorption on coal gas adsorption in dry coal samples were simulated and analyzed,and the factors affecting gas adsorption capacity were analyzed in detail.The adsorption of CH4,CO2 and N2 on pores with pore sizes of 1,2,5 and 10 nm in dry coal samples was simulated.The study found that the adsorption of these three gases increased first,then decreased and lastly increased with the increment of the nanopore size.The adsorption amount of gas in pores of 1nm and2nm is more than ten times that of gas in pores of 5nm and 10nm,indicating that micropores can adsorb a large amount of gas.There is no linear relationship between the increasing gradient of the amount of gas adsorbed and the increase in pore size.As the pore size increases,the gradient of gas adsorption in micropores is much larger than that in mesopores.When the pore size changes from micropores to mesopores,the amount of gas adsorbed in the pores will suddenly drop,due to that the pore diameter is less than 2 nm,the gas is adsorbed in the form of micropore filling,and a part of gas in the 5nm and 10 nm pores appears as multimolecular layer adsorption.The other gas affected by the external pressure is limited to the middle area of the pore,and the gas in the restricted movement area is not really adsorbed.It was found that for the problems of low coal seam permeability and difficulty in gas extraction in ultra-deep mines,coal seam gas permeability can be increased utilizing coal seam water injection,and gas recovery rate can be improved.