Hydrothermal Liquefaction of Nitrogenous Feedstocks: Biocrude Product Optimization and Reaction Mechanistic Insights

Hydrothermal liquefaction (HTL) valorizes wet waste feedstocks by converting them into marketable products as energy-rich, biocrude. However, nitrogenous food and agricultural wastes impart N-heteroatoms into the biocrude, impeding its energy content and means of use as a drop-in fuel. This study investigated N-migration pathways in HTL of binary mixture (guaiacol–glutamic acid) and ternary mixture (glucose–guaiacol–glutamic acid) representative of lignin–protein and cellulose–lignin–protein-rich feedstocks under the influence of reaction conditions, particularly pH. A well-defined set of HTL experiments following a full-factorial central composite design was conducted over the reaction parameters: temperature (280–370 °C), time (10 °C to 60 min), and pH (2–12). Response surface analysis with multicriteria optimization was used to determine the maximum energy recovery in biocrude. The optimal conditions were identified for HTL of the binary mixture (283 °C, 9.8 min, pH 2) and ternary mixture (367 °C, 9.8 min, pH 2), under which biocrude oil exhibited the highest energy recovery, mass yield, and carbon yield, along with minimal nitrogen and oxygen contents. The divergence in optimal temperature arose because increasing temperature in the binary system mainly accelerated guaiacol (representing lignin) degradation and oil loss, while the ternary system glucose conversion at elevated temperatures generated oil-soluble furans, cyclic ketones, and heterocycles. Experiments revealed that guaiacol (representing lignin) remained relatively stable under acidic conditions, while basic pH transformed the reactant into different organic components (phenolics, cyclic ketones, and organic acids). N-elemental yield and composition were higher in biocrudes under acidic pH and elevated temperature due to N-heterocyclic formation (pyrrolidinones). Novel synergistic pathways revealed polycyclics (indanols, indenes, etc.) formation from binary mixture interaction in basic media. Similarly, the ternary mixture evolved N-heterocyclics (pyridines, pyrazines, pyrroles, indoles, etc.) and O-heterocyclics (benzofurans, coumarins, etc.). Detailed reaction pathways were proposed with aided product characterization, elucidating the roles of temperature, pH, and feedstock interactions in directing nitrogen speciation and organic partitioning across product phases.