Nitrogen Migration and Transformation during Hydrothermal Liquefaction of Livestock Manures

Nitrogen flow and fate critically affects the hydrothermal liquefaction (HTL) of protein-rich feedstock such as livestock manure and algae. It also impacts the downstream process of HTL aqueous and oil products. Here, we reveal the migration and transformation pathways of nitrogen during HTL of typical livestock manures using combined gas chromatography-mass spectroscopy (GC-MS) and Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis. Over 37% of nitrogen in the manure migrated to the aqueous phase in all HTL experimental trials, except for beef manure. GC-MS results indicated that the nitrogen compounds in the biocrude oil were mainly long chain amides, whereas in the aqueous phase the compounds were mainly small molecules of pyrazines, pyrroles, and pyridines. FT-ICR MS identified that N₁O₁, N₂, and N₂O₁ species were dominant in the biocrude oil, while the nitrogen-containing compounds in the aqueous phase primarily took the form of N₂O₂ and N₂O₃. Five reaction pathways were proposed for the transformation of nitrogen during HTL. This study first characterized the transformation of nitrogenous compounds during HTL of livestock manures, which could be greatly beneficial to biocrude production, oil quality, and aqueous utilization in future studies.