Evolution of N-Containing Compounds during Hydrothermal Liquefaction of Sewage Sludge

In this study, complementary techniques, including ultrahigh-resolution Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) and X-ray photoelectron spectroscopy (XPS), were applied to characterize the transformation of nitrogen components during hydrothermal liquefaction (HTL) of sewage sludge. Results showed that 3–35% of nitrogen in sludge was transferred into the biocrude product when using HTL reaction temperatures ranging from 200 to 350 °C, with the remaining nitrogen partitioning to aqueous, solid, and gaseous coproducts. N-containing organics in biocrude shifted to structures with higher H:C ratios when HTL reaction temperatures were increased, whereas an opposite trend was observed for O:C ratios of N-containing chemicals in biocrude and aqueous products. Accordingly, weighted averages of double-bond equivalents (DBEw) and aromatic index (AImod,w ) of N-containing products in biocrude decreased as reaction temperature increased, while AImod,w increased for products in the aqueous phase. These trends are consistent with cyclization/aromatization reactions, promoting the formation of N-containing products that partition predominantly to the aqueous phase. XPS analysis revealed that the proportion of amine-N in biocrude increased sharply with reaction temperature, whereas heterocyclic-N (pyridinic-N and pyrrolic-N) and nitrile-N structures were mainly concentrated in solid residues. Improved molecular insights can serve as the basis for optimizing nitrogen management and recovery operations during HTL of sewage sludge.