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 degrees 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 (AI(mod,w)) of N-containing products in biocrude decreased as reaction temperature increased, while AI(mod,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.