Detailed study on self- and multicomponent diffusion of CO₂-CH₄ gas mixture in coal by molecular simulation

详细信息    查看全文

• 作者：Haixiang Hu^a ; ^{hxhu@whrsm.ac.cn" class="auth_mail" title="E-mail the corresponding author} ; Lei Du^b ; ^c ; Yanfei Xing^a ; Xiaochun Li^a ; ^{xcli@whrsm.ac.cn" class="auth_mail" title="E-mail the corresponding author}
• 关键词：Multicomponent diffusion ; Mutual diffusion coefficients ; Self-diffusion ; Coal bed methane ; Molecular simulation
• 刊名：Fuel
• 出版年：2017
• 出版时间：1 January 2017
• 年：2017
• 卷：187
• 期：Complete
• 页码：220-228
• 全文大小：1981 K

文摘

Gas diffusion plays a key role in CO₂-enhanced recovery of coal bed methane (ECBM), where more than one types of gases coexist and multicomponent gas diffusion occurs. Such process is now usually described by non-coupled two-component gas diffusion equations which exclude the interactions between gases. Self-diffusion and mutual diffusion of CO₂–CH₄ mixture are investigated through molecular simulation for the first time. The self-diffusion coefficients of CO₂ and CH₄ decrease with gas concentration but increase with temperature. The mutual diffusion coefficients of binary gas mixture of CO₂–CH₄ in coal are computed through Maxwell–Stefan diffusion theory. A 2D diffusivity matrix |D| (with diagonal element D_i and non-diagonal element D_ij) is obtained to depict the mutual diffusion of the gas mixture. It is found that CO₂ (CH₄) diffusion is coupled with CH₄ (CO₂). The diffusion coupling strength of CO₂ and CH₄ decreases with increasing gas concentration. Temperature positively affects D_ij but minimally influences D_i, resulting in large ratios of D_ij/D_i at high temperatures. It means that CO₂–CH₄ diffusion correlation interactions, which are not present in non-coupled pure gas diffusion equations, are necessary to analyze gas mixture diffusion in coal. In this case, non-coupled pure gas diffusion equations are inadequate for description of CO₂–CH₄ mixture diffusion. The coupling between the gases can be ignored only at very high temperatures (T > 400 K), which means a large depth of coal bed in ECBM engineering.