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In order to further clarify the adsorption thermodynamic characteristics of CH4 and CO2 in coal, the giant canonical ensemble Monte Carlo method was used to simulate the adsorption behavior of CH4 and CO2 in coal at different temperatures and different moisture content. The results show that the adsorption capacity of CH4 and CO2 in coal molecules decreases with the increasing temperature and moisture content. By comparing the adsorption amounts of the two components, it is found that both high temperature and the presence of water molecules are not conducive to displace CH4 by CO2; the average adsorption heat of CH4 is negatively correlated with temperature and water content, and average adsorption heat of CO2 has a negative correlation with temperature and a positive correlation with water content. Water molecules are more inclined to react with CO2; the overall adsorption potential of CH4 and CO2 is negatively correlated with temperature and moisture content. When the pressure is low, the adsorption potential of CO2 increases slightly with the increasing of temperature; the adsorption entropy of CH4 and CO2 increases with the increasing of temperature and decreases with the increasing of moisture content. The influence of water on CO2 is stronger than CH4. When two components are injected, the existence of water molecules makes the whole system more stable, which is not conducive to CH4 output.

As an efficient and clean energy, coalbed methane development and utilization have deep significance in promoting energy conservation and emission reduction, reducing greenhouse gas emissions. Therefore, molecular simulation was utilized to study the influence of N2/CO2 on the adsorption and diffusion of methane in coal under different gas injection methods and to elucidate the influence of varying gas injection methods on the efficiency of coalbed methane extraction, which provides a basis for the efficient development of coalbed methane. The results show that the adsorption effect of gases in coal decreases with the increase of temperature and increases with the rise of pressure, and the adsorption performance of the three gases in coal shows the law of CO2 > CH4 > N2. In addition, the injection of CO2/N2 had an obvious inhibition effect on CH4 adsorption, and the inhibition effect of CO2 was more significant, and the inhibition effect on CH4 adsorption reached the maximum when the two gases were mixture injected. In terms of diffusion, compared with separate injection, mixed injection of N2 + CO2 promotes CH4 diffusion more effectively, which can be reflected in the relative concentration distribution and velocity distribution. The injection of N2 helps to increase the porosity of coal, and the injection of CO2 and N2 + CO2 will lead to the decrease of porosity, but the mixed gas injection has less effect than the injection of CO2 alone. [ABSTRACT FROM AUTHOR]

As a clean energy source, coalbed methane (CBM) produces almost no exhaust gas after combustion, and its extraction and efficient utilization play a key role in supporting sustainable development. Therefore, molecular dynamics simulations were used to research the diffusion of CH4 in coal after injecting CO2/N2 in different sequences and to clarify the efficiency of CBM extraction under different injection sequences, so as to contribute to sustainable development. The results show that the adsorption amounts of CO2 and N2 in different injection sequences are obviously different. To narrow the gap between the two injection amounts, the injection pressure of N2 can be appropriately increased and that of CO2 can be reduced, or N2 can be injected preferentially instead of CO2. When CO2 is injected first, the interaction energy between CH4 and coal is stronger and increases slightly with displacement time as a whole. The interaction energy curve of the N2 injection decreases, and the displacement effect becomes worse and worse. From the diffusion and relative concentration distribution of CH4, it can be seen that the diffusion of CH4 molecules outside the grain cell is more obvious when N2 is injected first. In terms of the number of CH4 molecules diffusing outside the crystal cell, it is less when CO2 is injected first than when N2 is injected first. The average value of the velocity distribution of CH4 increases slightly when CO2 is injected first and decreases significantly when N2 is injected first, but the average value is overall higher for N2 injection first. From the difference in diffusion coefficients before and after the gas injection, it can be seen that the decrease in permeability due to the expansion of the coal matrix by CO2 is more obvious than the increase in permeability due to the contraction of the coal matrix by N2. [ABSTRACT FROM AUTHOR]

In this paper, heat injection and CO2 injection are combined, and the influence of coal seam parameters on CO2-ECBM is analyzed to improve the production of CH4 and CO2 reserves and the effective control of both greenhouse gases. A multi-physical field coupling model of CO2-ECBM was established based on Darcy's law, Fick's law of diffusion, the extended Langmuir model for adsorption, and the equation of state. Numerical simulation of CO2-ECBM under different coal seam parameters was carried out by COMSOL Multiphysics. The results show that increasing the injection pressure of the CO2 injection well and the initial pressure of the coal seam can effectively increase the gas pressure and concentration gradient, which has a positive effect on improving the extraction concentration of CH4 and the sequestration concentration of CO2 in the coal seam. The increase of the initial temperature of the coal seam will promote the desorption and diffusion of the binary elemental gas, resulting in a decrease in the concentration of coalbed methane and a decrease in the displacement effect. In the process of displacement, the greater the initial permeability, the greater the fracture opening of the coal seam, which is more conducive to the seepage transport of the gas. The closer to the position of the injection well, the better the displacement effect and the lower the permeability rate ratio. [ABSTRACT FROM AUTHOR]

In this paper, THM model and coal molecular model are established to study the gas adsorption and desorption behavior in coal seam at different injection pressures. The results show that, the total energy decreases during the adsorption process and increases in the desorption process. CO 2 has a stronger adsorption capacity which is in a dominant position in the competition adsorption process, while N 2 is in a weak position. The order of the diffusion coefficient is N 2 >CH 4 (CH 4 –N 2) > CH 4 (CH 4 –CO 2) > CO 2. The diffusion coefficient does not necessarily increase with the increase of injection pressure. At the same injection pressure, the relative concentration of CH 4 in the CH 4 –CO 2 system is greater than that in the CH 4 –N 2 system, and injection of CO 2 to promote CH 4 desorption is significantly better than the injection of N 2. With the increase of injection pressure, the average relative concentration of CH 4 /CO 2 /N 2 in the vacuum layer increased. The optimal injection pressure for N 2 injection to promote CH 4 desorption is 2 MPa, and the reasonable injection pressure for CO 2 injection is 1–3 MPa. • THM model and coal molecular model are established to study the gas adsorption and desorption at different injection pressures. • The total energy of the system decreases during the adsorption process and increases in the desorption process. • The diffusion coefficient does not necessarily increase with the increase of gas injection pressure. • Relative concentration of CO 2 in the vacuum layer is lower than N 2 , the adsorption performance of CO 2 is greater than N 2. [ABSTRACT FROM AUTHOR]