Efficient adsorptive separation of CH4-CO2 mixture in coal nanopores is important for CO2-enhanced coalbed (CO2-ECBM) recovery and CO2 sequestration. However, The understanding of the adsorption selective behavior and its controlling mechanisms is still limited. The low-pressure N2/CO2 adsorption, Fourier transform infrared spectroscopy, and gravimetric dynamic adsorption measurements were conducted on coals with six metamorphisms to characterize the micropore structural, chemical properties and CH4-CO2 mixture adsorption isotherms. The results show that the selectivity for coal samples at maximum vitrinite reflectance Ro,max ≥ 1.78 % shows an asymmetric inverse parabola trend with the increase of pressure, while it follows a general convex decreasing tendency for coal samples at Ro,max < 1.78 %. Compared with chemical structures, the micropore volume primarily determines the adsorption selectivity, while the super-micropore volume displays the highest negative correlation with selective coefficients. Two adsorption competitive mechanisms are summarized from thermodynamics: For low-rank coal, the decreased qstCO2/qstCH4 but with values much greater than 1 leads to a monotonous decrease in selectivity; For high-rank coal, qstCO2/qstCH4 is slightly larger than 1, CO2 filling in ultra-micropore compensates for the competitive adsorption on medium and super-micropore surfaces at the low-pressure range, resulting in the initial enhancement of the inverse parabola-curved adsorption selectivity.

中文翻译：

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受限 CH4-CO2 混合物在煤中的吸附选择性：理化结构及热力学竞争机制的影响


煤纳米孔中 CH4-CO2 混合物的高效吸附分离对于 CO2 增强煤层 （CO2-ECBM） 回收和 CO2 封存非常重要。然而，对吸附选择性行为及其控制机制的理解仍然有限。对 6 种变质作用的煤进行低压 N2/CO2 吸附、傅里叶变换红外光谱和重力动态吸附测量，表征了微孔结构、化学性质和 CH4-CO2 混合物吸附等温线。结果表明，在最大镜质体反射率 Ro，max ≥ 1.78 % 时，煤样的选择性随压力的增加呈不对称的逆抛物线趋势，而在 Ro，max < 1.78 % 时，煤样的选择性呈一般凸下滑趋势。与化学结构相比，微孔体积主要决定吸附选择性，而超微孔体积与选择性系数呈最高负相关。从热力学中总结出两种吸附竞争机制：对于低阶煤，qstCO2/qstCH4 降低但值远大于 1 导致选择性单调降低;对于高阶煤，qstCO2/qstCH4 略大于 1，超微孔中的 CO2 填充补偿了低压范围内对中超微孔表面的竞争性吸附，从而初步增强了逆抛物线-弯曲吸附选择性。