Molecular Dynamic Simulation of Self- and Transport Diffusion for CO₂/CH₄/N₂ in Low-Rank Coal Vitrinite

Abstract

The self-, Maxwell-Stefan-, and transport diffusions of CO2, CH4, and N2 in coal vitrinite macromolecules were simulated through molecular dynamics. Results indicated that these diffusion coefficients increase slowly when T < 340 K while rapidly at T > 340 K independent of the adsorbate numbers and types. The self- ([CO2] > [N2] > [CH4] in order) and transport diffusion coefficients ([N2] > [CO2] > [CH4] in order) decrease with increasing adsorbate number. The diffusion activation energy (ΔE) of vitrinite-n CO2 (5.07, 5.73, and 15.96 kcal/mol for vitrinite-5 CO2, vitrinite-10 CO2, and vitrinite-22 CO2 respectively) is lower than vitrinite-n CH4 (8.15, 8.97, and 17.09 kcal/mol for vitrinite-5 CH4, vitrinite-10 CH4, and vitrinite-17 CH4 respectively). At the saturation adsorption state, the ΔE of vitrinite-7 N2 (12.03 kcal/mol) is the lowest compared with vitrinite-22 CO2 and vitrinite-17 CH4, indicating that the diffusion process for N2 is the easiest to inspire among these three gases. The swelling ratio ([CO2] > [CH4] > [N2] in order) increases with the increasing temperature, indicating that high temperature is conducive for the swelling equilibrium. While the Δ E of pressure dependence first decreases with increasing pressure until the peak pressure (0.5–1.0, 1.5–2.0, and 2.5–3.5 MPa for CO2, CH4, and N2 respectively) and then increases significantly, indicating that the diffusion energy barrier decreases with increasing pressure.

Molecular Dynamics Simulation of Diffusion Behavior of CH4, CO2, and N2 in Mid-Rank Coal Vitrinite

“…The diffusion coefficients of CO 2 , CH 4 , and N 2 simulated by Song et al [11] in low-rank coal vitrinite molecules were 27.83-79.56 × 10 −11 , 16.23-38.45 × 10 −11 , and 42.37-86.45 × 10 −11 m 2 /s, respectively. Zhao et al [12] obtained the diffusion coefficients of CO 2 and CH 4 based on the Wiser coal molecular simulations, which were 77.3 × 10 −11 m 2 /s and 36.3 × 10 −11 m 2 /s, respectively.…”

“…Therefore, the difference in the number of chemical groups in each model is the main reason for the difference in the simulated diffusion results. On the other hand, the different force field selections and parameter settings in the simulations will also lead to large differences in the simulation results, as reported by Song et al [11], who based their work on the Dreiding force field, and by Hu et al [36], who used the Compass II force field. In molecular dynamics, the self-diffusion requires a long simulation time to ensure a stable linear relationship between MSD and time.…”

“…However, it is very challenging to quantitatively investigate the microscopic diffusion behavior of gas in coal, especially in the micropores. Systematic research on the diffusion process and mechanisms of CH 4 , CO 2 , and N 2 at different geometric scales in coal seams can both efficiently enhance both CBM production and the implementation of CO 2 capture, utilization, and sequestration (CCUS) [11,12].…”

“…The macroscopic diffusion process for the CH 4 , CO 2 , and N 2 into coal is closely related to the ranks [24]. However, the previous efforts have mainly concentrated on the low-rank coals (Ro, max = 0.58-0.62% [11,12]). Although, the Wiser [27] bituminous coal model was also adopted for the studies on the self-and transport diffusions, this model was constructed via the gasification and liquefaction processes of coal gas and has a lower fidelity for the macromolecules of bituminous coal.…”

Coalbed methane diffusion and water blocking effects investigated by mesoscale all-atom molecular dynamic simulations

“…[6][7][8][9] More recent investigations utilize larger coal models containing thousands of atoms and MD simulations to estimate the adsorption capability of coal and the diffusivity of methane under various temperatures, pressures, and concentrations. [10][11][12][13][14][15][16][17] One of the limitations of the aforementioned MD studies 10-17 is that they usually simulate the micropore (<2 nm) structures. According to the classication proposed by the International Union of Pure and Applied Chemistry (IUPAC), pores within the range of 2-50 nm are considered to be mesopores.…”