Hydrotreatment of algae-based HTL biocrude: Insights from bibliometric and process-oriented analysis

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Hydrothermal liquefaction of microalgae is widely regarded as a promising pathway toward the production of sustainable aviation fuels and other drop-in biofuel candidates. However, realizing this potential requires overcoming significant chemical and engineering barriers, particularly concerning the high concentration of refractory nitrogenous compounds and oxygenates in algal biocrudes. This study presents a comprehensive dual-approach investigation, combining a bibliometric analysis to map the evolutionary landscape of hydrothermal liquefaction research with a critical, process-oriented review of catalytic hydrotreatment. The bibliometric assessment reveals a strategic paradigm shift from fundamental biocrude production toward deep heteroatom removal and process integration. Critically evaluating the literature, this review highlights the severe limitations of conventional batch-reactor studies, emphasizing the urgent need for continuous-flow benchmarking to address hydrodynamic challenges and complex catalyst deactivation mechanisms. Furthermore, this study concludes that a two-stage hydrotreatment scheme—comprising mild stabilization followed by severe hydrocracking—currently represents one of the most promising strategies to prevent thermal repolymerization. Crucially, we demonstrate that achieving true “drop-in” status extends far beyond matching bulk elemental properties; upgraded fractions must rigorously satisfy precise international certification standards, necessitating negligible nitrogen content and strict aromatic speciation to balance seal swelling with particulate emissions. Finally, this review tempers optimistic life-cycle assessments, cautioning that reported net-negative carbon footprints are highly dependent on theoretical co-product credits, thereby redefining the realistic commercialization roadmap for algal biorefineries.