Investigation of hydrochar properties and bio-oil composition from two-stage hydrothermal treatment of dairy manure

[Display omitted] •Maximal nitrogen removal was obtained at 200 °C for 1 h during the first-stage HTT.•Higher energy recovery of hydrochars could be obtained from the two-stage HTT.•Microspheres with better thermostability and larger size were found at two-stage HTT.•Lower N-containing compounds content was detected in bio-oils from the two-stage HTT. In this study, a two-stage hydrothermal treatment (HTT) method was used to increase nitrogen removal to aqueous phase and reduce N-containing compounds in dairy manure bio-oil. The physico-chemical properties of hydrochar were also investigated. During the first-stage HTT process, the maximal nitrogen removal of 48.11 % was obtained at 200 °C for 1 h. The retained solids were subjected to the second-stage HTT process at 240–320 °C for 0.5–4 h. The results showed that the energy recovery (43.91–67.86 %) and the nitrogen recovery (54.35–69.43 %) of hydrochars obtained from the two-stage HTT were significantly higher than that from the direct HTT of dairy manure. The changes of H/C and O/C atom ratio indicated that decarboxylation reaction predominated in the second-stage HTT with the temperature increased from 240 to 280 °C. Decarboxylation and dehydration played a dominate role in the second-stage hydrothermal process with the residence time increased from 0.5 to 4 h. The scanning electron microscope analysis revealed that microspheres with better thermostability and larger size could be obtained from the two-stage HTT process. And the reaction temperature has more remarkable effect on the surface morphology of hydrochars than the residence time. Additionally, the N-containing compounds content of bio-oil from the two-stage HTT process was significantly lower than that from the direct HTT of dairy manure. Meanwhile, more acids and aldehydes compounds could be detected. These findings suggest that the two-stage HTT could be considered as a potential technology for treating dairy manure and producing value-added hydrochar, low nitrogen bio-oil and N-rich aqueous phase.