Bio-Oil Production via Two-Stage and Direct Hydrothermal Liquefaction Process from High-Protein <i>Monoraphidium</i> sp. KMC4: A Comparative Study of Both Processes and an Insight into the Reaction Pathway

Hydrothermal liquefaction (HTL) technology is a process to produce substantial energy resources from potential algae feedstock. However, the high N and O content in wastewater-grown microalgae yields bio-oil with a significant number of heteroatoms, hindering its practical utilization. In this work, the bio-oil generated from direct high-temperature and two-stage hydrothermal liquefaction was compared for its elemental quality and yield. A bio-oil yield of 33.50% was achieved via direct HTL, with high N/C (0.05 mol/mol), H/C (1.36 mol/mol), and O/C (0.11 mol/mol) ratios. The implementation of two-stage hydrothermal liquefaction was applied to increase the H/C and to decrease the N/C of the feedstock, to increase the H/C and to decrease the N/C and the O/C of bio-oil. The aforementioned technology resulted in better bio-oil quality by the prior extraction of carbohydrates and protein components from algae biomass at lower pretreatment temperatures. The bio-oil from direct HTL had a significant number of nitrogen-based compounds (13.36%), i.e., amides, amines, and heterocyclic forms, while it was reduced to 1.98% in the two-stage hydrothermal liquefaction process, confirmed by GC-MS. Also, a reduction of 38% nitrogen in bio-oil was observed from direct HTL compared to two-stage HTL through elemental analysis. The GC-MS results aligned with the elemental analysis of bio-oil from the two-stage hydrothermal liquefaction. The bio-oil produced from two-stage HTL of pretreated microalgae exhibited improved properties, characterized by reduced N-heterocyclic compounds and better elemental composition. The study also formulated the hypothetical reaction pathway during the two-stage HTL. The findings obtained using the two-stage HTL greatly improved the bio-oil characteristics without the expense of reduced yield.