Mild hydrotreatment as a stabilization step for nitrogen-rich sewage sludge biocrude in a pilot-scale trickle-bed reactor

Biocrude from sewage sludge contains high levels of nitrogen- and oxygen, making it unstable and unsuitable for direct use as fuel. This study investigates mild hydrotreatment as a stabilization step using a sulfided commercial NiMo/Al 2 O 3 catalyst in a pilot-scale trickle-bed reactor. Treatments were performed at 200–280 °C and 120 barg with liquid hourly space velocities (LHSV), ranging from 0.5 to 2 h −1 . Elemental analysis showed efficient chlorine and sulfur removal (up to 85 % and 60 %, respectively) and 51 % oxygen removal, while nitrogen remained persistent with less than 25 % conversion at 280 °C. Micro carbon residue (MCR) decreased with temperatures up to 240 °C, indicating stabilization, but increased at higher temperatures due to oligomerization. Simulated distillation confirmed formation of heavier fractions above 240 °C. LHSV had a smaller effect, suggesting potential for increased throughput. Supercritical fluid chromatography-high-resolution mass spectrometry (SFC-HRMS) revealed removal of quinolines and methoxy-oxygenates, with concurrent formation of amines and fatty acid amides. Mild hydrotreatment at 240 °C effectively reduced chemical reactivity while preserving catalytic integrity and process stability. The results establish process boundaries for industrial stabilization of nitrogen-rich biocrudes and support staged upgrading strategies for full fuel conversion. • Sewage sludge derived from biocrude was hydrotreated at temperatures up to 280 °C. • The pilot plant was operated for 1010 h with limited catalyst deactivation. • The temperature in the stabilization reactor should not exceed 240 °C. • Molecular characterization revealed that intermediate compounds were formed at moderately low temperatures (e.g., amines and fatty acid amides).