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

Yu, Song, Bo, Jiang, Meijun, Qu
Energy & fuels 2018 v.32 no.3 pp. 3085-3096
activation energy, adsorption, carbon dioxide, coal, diffusivity, energy, gases, methane, molecular dynamics, nitrogen, temperature
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 ΔE 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.