Investigation of physico-chemical properties of hydrochar and composition of bio-oil from the hydrothermal treatment of dairy manure: Effect of type and usage volume of extractant.

Hydrothermal treatment for dairy manure into value-added hydrochar and bio-oil is a potential technology for its resource utilization. During the process of treatment, extractant is applied to the separation of hydrochar and bio-oil. In this study, three polar extractants (ethyl acetate, dichloromethane, diethyl ether) and two nonpolar extractants (n-hexane and petroleum ether) were used, and the physico-chemical properties of hydrochar and the composition of bio-oil were investigated. Compared with nonpolar extractants, polar extractants could extract the bio-oil absorbed on the hydrochar exterior and interior surface, resulting in more mass loss of hydrochar and better extraction performance on the production of bio-oil. The decrease of H/C atomic ratio and the increase of O/C atomic ratio indicated the demethanation tendency to occur during the extraction process, and enhanced the hydrochar stability. The scanning electron microscope and specific surface area analysis revealed that polar extractant had a more positive effect than nonpolar extractant on the occurrence of disperse spherical microparticles and the augment of hydrochar specific surface area. The bio-oil from polar extractant mainly consisted of N-containing compounds, ketones, phenols and acids, while the bio-oil from nonpolar extractant mainly consisted of esters, alkanes and aromatics. These results reveal that the hydrochar extracted by polar solvent exerts a greater potential for the production of carbon-based material.