Review on Nitrogen Transformation during Microalgae Thermochemical Liquefaction: Recent Advances and Future Perspectives

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.