Li Shike, Zhu Yanming, Wang Yang, Liu Jing
Key Laboratory of Coalbed Methane Resource and Reservoir Formation on Process, Ministry of Education, China University of Mining and Technology, Xuzhou 221008, China.
School of Resources and Geosciences, China University of Mining and Technology, Xuzhou 221116, China.
ACS Omega. 2021 Apr 19;6(17):11266-11279. doi: 10.1021/acsomega.1c00111. eCollection 2021 May 4.
The chemical and alignment structures of coal impacts coalbed methane behavior: adsorption, desorption, and diffusion. Recently, the research on accurate characterization techniques for coal structure has received widespread attention. In particular, spatial alignment is critical for the molecular modeling of coal. However, due to the great challenges of quantification, spatial alignment has often been ignored in previous studies. In this study, high-resolution transmission electron microscopy (HRTEM) was employed to quantitatively characterize the fringe length, orientation, and stacking distributions of these five coal samples with different ranks. Raman spectroscopy was utilized to investigate the overall structural disorder of the coal molecules. C nuclear magnetic resonance (C NMR) was conducted to characterize the chemical structures of coals, and XRD experiments recorded the transition of the microcrystallite structure. The results show that in the range of % = 0.39-2.07%, the distributions of the aromatic structural units were similar: mainly composed of fringes of size equivalent to naphthalene and 2 × 2 and 3 × 3 rings. When % > 2.07%, the distribution shifted to longer fringes. Moreover, all the samples showed a regional orientation, and when % > 2.07%, there was significantly higher alignment. The degree of stacking of fringes were limited, most of which appeared in the form of a single layer. When % < 2.07%, the stacking appeared in the form of two or three layers. However, five-layer stacking merely appeared in the sample with % = 2.47%. In addition, based on the Raman data, the evolution of carbon disorder was divided into three stages: % = 0.39-1.23%, 1.23-2.07%, and 2.07-2.47%, and aromatization caused the overall disorder to decrease. The C NMR data indicated that the chemical structure also transitioned in stages, with aliphatic carbon and oxygen-containing groups gradually decreasing and aromatic carbon increasing. Meanwhile, the XRD data supported increased organization (lower values) with maturities. Thus, this study provides quantitative information about the spatial alignment and the size of aromatic rings, which helps to improve a comprehensive understanding of the chemical structure of coal and coalbed methane behaviors.
煤的化学结构和取向结构会影响煤层气的行为，包括吸附、解吸和扩散。近年来，关于煤结构精确表征技术的研究受到广泛关注。特别是，空间取向对于煤的分子建模至关重要。然而，由于量化面临巨大挑战，空间取向在以往研究中常常被忽视。在本研究中，采用高分辨率透射电子显微镜（HRTEM）对这五个不同煤级的煤样的条纹长度、取向和堆积分布进行定量表征。利用拉曼光谱研究煤分子的整体结构无序性。进行碳核磁共振（C NMR）以表征煤的化学结构，X射线衍射（XRD）实验记录微晶结构的转变。结果表明，在% = 0.39 - 2.07%范围内，芳香结构单元的分布相似：主要由尺寸相当于萘以及2×2和3×3环的条纹组成。当% > 2.07%时，分布转向更长的条纹。此外，所有样品均呈现区域取向，当% > 2.07%时，取向明显更高。条纹的堆积程度有限，大部分以单层形式出现。当% < 2.07%时，堆积以两层或三层形式出现。然而，五层堆积仅出现在% = 2.47%的样品中。此外，基于拉曼数据，碳无序的演化分为三个阶段：% = 0.39 - 1.23%、1.23 - 2.07%和2.07 - 2.47%，芳构化导致整体无序度降低。C NMR数据表明化学结构也分阶段转变，脂肪族碳和含氧基团逐渐减少，芳香碳增加。同时，XRD数据支持随着成熟度增加组织性增强（更低的 值）。因此，本研究提供了关于空间取向和芳香环尺寸的定量信息，有助于增进对煤的化学结构和煤层气行为的全面理解。
