The Chemical and Alignment Structural Properties of Coal: Insights from Raman, Solid-State 13C NMR, XRD, and HRTEM Techniques.

Abstract

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. 13C nuclear magnetic resonance (13C 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 %Ro = 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 %Ro > 2.07%, the distribution shifted to longer fringes. Moreover, all the samples showed a regional orientation, and when %Ro > 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 %Ro < 2.07%, the stacking appeared in the form of two or three layers. However, five-layer stacking merely appeared in the sample with %Ro = 2.47%. In addition, based on the Raman data, the evolution of carbon disorder was divided into three stages: %Ro = 0.39–1.23%, 1.23–2.07%, and 2.07–2.47%, and aromatization caused the overall disorder to decrease. The 13C 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 d002 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.