Separated two-stage hydrothermal liquefaction of livestock manure for high-quality bio-oil with low-nitrogen content: Insights on nitrogen migration and evolution

[Display omitted] •ST-HTL enhanced the yield and quality of bio-oil compared to conventional HTL.•ST-HTL decreased the N partitioning to bio-oil by 32.83%.•Maillard reaction and acylation reaction was weakened during the ST-HTL process.•ST-HTL improved the saturation degree of bio-oil.•The N removal mechanism of bio-oil by ST-HTL was firstly proposed. The practical utilization of hydrothermal liquefaction (HTL)-derived bio-oil encounters limitations due to its high nitrogen (N) content, specifically when derived from protein-rich feedstock such as algae and livestock manure. In this study, a separated two-stage HTL (ST-HTL) was developed for producing bio-oil with high yield and low N-content. The main objective of this study was to reveal the N migration and evolution during the HTL process for a better understanding of the detailed mechanism. The separated two-stage HTL yielded the highest bio-oil, 32.90 wt%, with a maximum energy recovery of 68.47 %. In conventional HTL-derived bio-oil (280 °C), 38.85 % of N and 56.22 % of C (carbon) was transferred to the bio-oil, while 26.04 % of N and 61.34 % of C (carbon) in swine manure was transferred into the ST-HTL-derived bio-oil, which indicated a 32.83 % reduction in N-transfers and 8.35 % increase in C-transfers. GC–MS analysis revealed that the bio-oil from ST-HTL contained a higher proportion of fatty acids and a lower proportion of nitrogenous compounds compared to conventional HTL (280 °C). In addition, FT-ICR MS revealed that the ST-HTL-derived-bio-oil was more saturated than conventional HTL-derived bio-oil (280 °C). The underlying mechanism behind the increased bio-oil yield and reduced N content attained through ST-HTL are discussed in detail, thereby revealing its intrinsic merits. Thus, it is the first study to propose the N migration and evolution pathways during ST-HTL through detailed characterizations, thus, demonstrating its potential for improving the feasibility of processing high-protein livestock manure and promising practical applications.