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For hydrothermal liquefaction of dry biomass to produce liquid fuels, water needs to be added or the aqueous phase products can be recycled. This paper focuses on understanding the relationship between hydrothermal degradation of the dry biomass and oil formation under the condition of accumulative recycling of the aqueous phase. Completely dried corn stalk and deionized water were used for the hydrothermal liquefaction (HTL) experiment. The aqueous products for subsequent recycling were not diluted. It was demonstrated that the recycling of the aqueous can promote the enrichment of organic acids and the conversion of ketones and phenols in the aqueous, improving the yield and quality of Bio-crude oil. After recycling, the yield of Bio-crude oil increased from 20.42% to 24.31% continuously, and the oxygen content decreased from 13.34% to 9.90%. Although the process was accompanied by solid deposition and had a negative impact on the hydrothermal degradation efficiency, the formation of carbon microspheres during the deposition enhanced the utilization of nondegradable solids, while the formation of metal salt particles had a positive impact on oil production. After three rounds of recycling, the solid deposition effect was weakened. At this time, oil production and solids degradation can be promoted simultaneously.

Producing biocrude from wet biomass such as microalgae via hydrothermal liquefaction has been attracting extensive attention. However, those biocrude usually contained high nitrogen and oxygen and produced massive nitrogen oxides during downstream utilization like combustion. Because hydrothermal liquefaction is a complex and dynamic transformation process, the pathway of nitrogen during microalgae hydrothermal liquefaction has not yet been clearly described. This work critically and comprehensively reviewed the effects of detecting and analyzing method, microalgae property and operating condition on the nitrogen contents, and the types of nitrogen organic compounds of biocrude. The mechanism of nitrogen transformation in the hydrothermal liquefaction process was systematically summarized; the dominant reactions in each stage were described in detail. Moreover, the nitrogen removal methods from biocrude were proposed from two perspectives: feedstock and product, respectively. Finally, challenges and future perspectives are proposed in view of the existing research gaps.