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

Abstract

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.

A Review of Hydrothermal Biomass Liquefaction: Operating Parameters, Reaction Mechanism, and Bio-Oil Yields and Compositions

“…Usman et al summarized the oil production of different biomass hydrothermal liquefaction and the integration of machine learning in hydrothermal optimization. 19 Yin et al summarized the detection and analysis methods of hydrothermal liquefaction, the effects of raw material properties and operating conditions on the nitrogen content and types of nitrogen organic compounds in bio-oil. 20 Ameri et al summarized the potential effects of bio-oil produced from different biomass sources on the physical aging, chemical aging and moisture damage of asphalt.…”

Thermal and Oxidative Stability of Biocrude Oil Derived from the Continuous Hydrothermal Liquefaction of Spirulina

“…Microalgae, as a renewable biomass, have been identified as a promising feedstock for biofuel production due to their high photosynthetic efficiency, ability to grow without occupying arable land, rapid growth rate, abundant organic matter content, and strong capability to absorb CO 2 [1][2][3]. Hydrothermal liquefaction (HTL) is a thermochemical conversion technology that converts biomass with low value and high moisture into biocrude oil with high energy density under the conditions of subcritical water temperatures of 250-375 • C and pressures of 4-22 MPa [4,5].…”

“…The physiochemical properties were the difference between the biocrude oil from the batch reactor and the continuous hydrothermal reactor. Therefore, this study aims to (1) expose the biocrude oil produced from Spirulina through a continuous hydrothermal reactor to different thermal (4 • C, 15 • C, and 35 • C) and oxidation (air and N 2 ) storage environments over a period of three months and (2) assess the changes in the viscosity, HHV, boiling point distribution, and chemical distribution to determine how the thermal and oxidation environments influence the properties of biocrude produced from a continuous hydrothermal reactor. The findings of this study will contribute to a better understanding of the potential of biocrude as a sustainable alternative to fossil crude oil, thereby promoting its industrial applications as a biofuel.…”

Effects of Lipids and Type of Amino Acid in Protein in Microalgae on Nitrogen Reaction Pathways during Hydrothermal Liquefaction

“…Generally, the nitrogen components in microalgae are originally presented as proteins [6]. It was, thus, not surprising when the content of proteins in microalgae was high and lipids was low, as this suggested that NOCs' concentration in biocrude was evidently high [7]. Our previous study focused on high-protein (>50 wt%) microalgae (Chlorella sp.…”