Nitrogen Transformation during Pyrolysis of Various N‑Containing Biowastes with Participation of Mineral Calcium

Nitrogen (N) is one of the nutrients embedded in biowastes and its speciation in solid–liquid–gas pyrolysis products is closely related to environmental issues such as the greenhouse effect, acid rain, and eutrophication. This study investigated the distribution and evolution of N species during pyrolysis of various biowaste matrixes, and the roles of exogenous mineral calcium (CaCl₂) in regulating N transformation were explored. The results showed that these biowastes show a similar tendency for the conversion of protein-N (70–90% in biowaste) into char-N (25–35%), oil-N (15–30%), and gas-N (30–60%). Exogenous Ca did not alter total char-N yield, while it promoted the conversion of protein/pyridine-N into pyrrole/quaternary-N, which could mitigate N loss as a cause of eutrophication; Ca catalyzed the cracking of N-containing macromolecules in bio-oil, especially amine, and therefore, drove N migrating from the liquid phase to gas phase. This would benefit the recycle of bio-oil as a fuel. In gas, the significant decrease of harmful HCN accompanied by a remarkable increase of NH₃ and N₂ were observed. Detection by thermogravimetric analysis–Fourier transform infrared spectrometry–gas chromatography mass spectrometry confirmed that mineral Ca intensified N-related reactions, including dehydration, decarboxylation, dehydrogenation, and deamination of protein-N. This study could guide pyrolytic production and subsequent application of biochar/biofuel, as well as exhaust gas collection regarding N recovery and pollution control.