Investigation on the mechanism of carbon structural evolution of Huolinhe lignite during different pretreatment processes

The depth understanding of the organic structure characteristics of coal and its evolution is beneficial to the clean and efficient coal directional conversion. This research probes the evolution path and mechanism of carbon structure of lignite during the hydrothermal transitions dewatering (HTD) and tetralin solvent thermal transitions dewatering (TTD). The structural evolution with these two transitions was explored using 13C NMR and XRD in an autoclave at different temperatures. The moisture content in lignite is effectively decreased during HTD and TTD processes, and this is mainly due to the removal of O-containing functional groups and the decrease of water holding capacity. The TTD process is achieved a higher dehydration and deoxidation for lignite compared with HTD process. This is possibly due to that the dehydrating energy of TTD process not only comes from the thermal energy, but also the tetralin is more likely to form hydrogen bonds with water. The aromaticity is increased during HTD and TTD processes, and this increase is possibly contributed by aromatization while the contribution of aromatic rings condensation was limited because the molar ratio of aromatic bridgehead carbon has changed little. Also, from the XRD data, the stacking of aromatic structural units are significantly promoted and are not obvious for the lateral extended size for crystal structural evolution during HTD and TTD processes. when the pretreatment temperature was at 310 °C, the aromaticity increase from 60.94% to 69.50% and 69.68% during HTD and TTD processes, respectively. The stacking height increase from 8.94 Å to 12.06 Å and 13.27 Å, and the stacking number increase from 2.49 to 3.51 and 3.86, the lateral sizes have changed little. The increase of aromaticity is finally manifested in aromatic units vertical stacking height and stacking layer number. These results indicate that the graphitization and ordering are promoted to some degree during HTD and TTD processes. Compared with the HTD, the TTD had the greater impact on the conversion of lignite carbon structure to high-rank coal. Those results are potentially useful to the clean and efficient utilization of lignite.