Effect of low temperature hydrothermal liquefaction on catalytic hydrodenitrogenation of algae biocrude and model macromolecules

Staged, low and high-temperature subcritical liquefaction were used to pretreat algae and generate a biocrude primarily characterized by free fatty acids and unsaturated hydrocarbons with reduced nitrogen heteroatom levels. Subsequently, catalytic hydrodenitrogenation and deoxygenation (HDN/HDO) was conducted using ruthenium (5% Ru/carbon) and cobalt molybdenum catalysts. Ru on carbon was the most effective HDO catalyst and generated the lowest level of nitrogen and heteroatoms in the resulting oil. A CoMo-S catalyst generated higher nitrogen and heteroatom levels, and resulted in a higher TAN value and lower heating value, probably due to the low sulfur levels in the biocrude. The highest quality oil was generated from a raceway strain using Ru/C and HTL pretreatment (225 °C, 15 min) with a repeated batch HDN/HDO step (3.24% N, 8.2% O, TAN of 12, 1.25% water, HHV 40 MJ/kg) and had a boiling point range of ~ 20% kerosene, ~ 30% distillate fuel oil, and ~ 50% gas oil. A 57% reduction in nitrogen content of the oil was realized when repeated HDN/HDO was coupled with HTL pretreatment using Ru/C (4.3 wt.%N), relative to 7.5%N for single stage HTL/HDO. Final yields for the highest quality oil generated via the coupled HTL/HDO process ranged from 15 to 22%. Recovery and reuse of the catalyst (Ru/C) resulted in a significant decline in activity potentially caused by coking and metals deposition.