Effect of Noncovalent Bonds on the Thermal Extraction of Subbituminous Coals

Abstract

This work examined the effect of noncovalent bonds on the thermal extraction of subbituminous coal. The bonds were released by pretreatment involving acid treatment, O-acetylation, and O-methylation. The pretreated coal was subjected to mild thermal extraction at 360 °C for 1 h with flowing solvents. Acid treatment increased the extraction yield of Wyodak Anderson subbituminous coal with a nonpolar solvent, light cycle oil (LCO), from 35 to 43 wt % (dry-ash-free coal basis). Furthermore, O-acetylation of the acid-treated coal increased the yield to 48 wt %. The former increment can be explained by the release of cation-bridging cross-links, whereas the latter can be explained by the release of hydrogen bonds among phenolic hydroxyls. In addition, O-methylation released the hydrogen bonds among carboxyls and increased the extraction yield to 53 wt %, which was a further 5 wt % increase in the yield over that of the O-acetylated coal. In contrast, the extraction yield with a polar solvent, crude methylnaphthalene oil (CMNO), was unchanged on O-acetylation, indicating that the effect of releasing hydrogen bonds among phenolic hydroxyls on the extraction yield was small. These results suggest that the polar solvent itself can release weaker noncovalent bonds, such as hydrogen bonds among phenolic hydroxyls, whereas nonpolar solvents hardly release even weak noncovalent bonds. Therefore, the extraction of subbituminous coals is strongly dependent on noncovalent interactions, such as ionic cross-links and hydrogen bonds. The extraction yield can be enhanced greatly by releasing such noncovalent interactions.