Hydrothermal carbonization of sawdust with the bio-oil of same origin impacts evolution of structures of hydrochar

The high tendency towards polymerization is a bottle-neck issue in upgrading of bio-oil, which, however, is highly desirable in co-hydrothermal carbonization (HTC) of biomass with bio-oil for enhancing production of hydrochar. In this study, co-HTC of bio-oil of the same origin with poplar sawdust was conducted at 180 to 260 degrees C. The results showed that the HTC in a water medium containing 10% bio-oil could enhanced the yield of hydrochar by ca. 36% at 220 or 260 degrees C through the cross-polymerization of the sugar-derived furans and ketones/aldehydes as well as the phenolics in bio-oil with the sawdust-derived organics. Furans/sugars in bio-oil showed much more contribution to the increase of hydrochar yield than the phenolics like guaiacol. The highly reactive organics in bio-oil even could react with the crystal region of cellulose at 180 degrees C, and affected property of hydrochar at especially the higher temperatures by promoting deoxygenation, enhancing carbonization, eliminating polar functionalities like -OH, retaining C=O for continued polymerization, etc. The co-existence of aliphatic organics and phenolics in bio-oil renders the simultaneous formation of highly volatile fraction and aromatic structures in the hydrochar. These created the carbon-rich hydrochar with higher crystallinity, lower hydrophilicity and improved comprehensive combustion index.