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This study investigated the transformation of N‑heterocycle compounds during sewage sludge pyrolysis process. The reaction pathways for evolution of the products especially the N-containing organics such as amino acids‑N were analyzed in detail. NH3 is the main N-containing gaseous product during the pyrolysis and its concentration was increased with the increasing temperature. Phenolic compounds, hydrocarbons and heterocyclic compounds were the main component in liquid products. The in situ FT-IR study showed that the labile inorganic ammonium salt decomposition proceeded to completion below 500 °C. Above 500 °C N‑heterocycle compound was formed and then it was decomposed, while above 600 °C nitrile started to decompose. NH3 formation main originates from the decomposition of N‑heterocyclic compounds and nitrile. Proteins during the pyrolysis, as verified by XPS, were mainly converted into inorganic nitrogen. In addition, the Py-GC–MS results showed that large amount of N‑heterocyclic compounds were found in amino acid‑N pyrolysis products. The transformation paths for the amino acids with different chemical structures were distinct. Except N‑heterocyclic amino acids, long chain aliphatic amino acid also can form N‑heterocyclic compounds through cyclization. In addition, the decomposition of the intermediates of N‑heterocyclic compounds form small molecular compounds or while the polymerization of the intermediates forms macromolecule compounds. The amino acids with heterocycle structure were disrupted to form heterocycle and aliphatic intermediate. The plausible mechanism of sewage sludge pyrolysis was proposed.
•NH3 was mainly originated from labile protein and inorganic ammonium salt in the range of 240–480 °C.•The heterocyclic and aliphatic amino acid can form N‑heterocyclic compounds during pyrolysis.•The ESI FT-ICR MS provided an insight to observe heavy compounds-N transfer.