Insight into the molecular structural evolution of a series of medium-rank coals from China by XRD, Raman and FTIR

Abstract

Fifteen coals of various rank with Ro, max=0.53–2.23 % were selected from seven coalfields in China. The samples were demineralized using HCl-HF solution, and the demineralized coals were thoroughly characterized using XRD, Raman and FTIR techniques. Structural parameters were determined from spectrum deconvolution, and correlated with Ro, max to investigate the structural changes around the second coalification jump. XRD indicated that the evolutionary process has three stage characteristics with Ro, max=0.53–0.99 %, 0.99–1.47 %, and 1.47–2.23 %, respectively. For Ro, max=0.53–0.99 %, the aromatic ring lamellae of crystallites became smaller and develop vertically; For Ro, max=0.99–1.47 %, the microcrystalline structure occurred reconstruction; For Ro, max=1.47–2.23 %, aromatization strengthened, and the lateral ductility and vertical stacking of aromatic layers enhanced with increasing coalification. The correlations of G band position, AD/A(GR+VL+VR), and AS/A(GR+VL+VR) with Ro, max revealed that abrupt changes occurred at Ro, max=0.84 %, and 1.32 %. FTIR revealed that aromaticity and condensation of aromatic rings enhanced, whereas the C-factor decreased as coalification deepened, which reflected their coal rank dependency. The CH2/CH3 ratio showed the minimum value at Ro, max = 1.47%, while the A-factor and C=O/C-O ratio showed the maximum values at Ro, max = 0.84% and 1.32%, respectively. As Ro, max=1.32 %, the content of cyclic OH, and the content of free hydroxyl show the minimum values, while self-associated OH reaches the maximum value. The structural parameters of medium-rank coals derived from different spectral methods have abrupt changes during coalification process, which preferably response to the second coalification jump. These findings can provide the basis for exploring the coal coking mechanism of medium-rank coals, and are potentially useful to efficient, clean and directional coal conversion.