Transformation of Nitrogen and Evolution of N-Containing Species during Algae Pyrolysis

Transformation and evolution mechanisms of nitrogen during algae pyrolysis were investigated in depth with exploration of N-containing products under variant temperature. Results indicated nitrogen in algae is mainly in the form of protein-N (∼90%) with some inorganic-N. At 400-600 °C, protein-N in algae cracked first with algae pyrolysis and formed pyridinic-N, pyrrolic-N, and quaternary-N in char. The content of protein-N decreased significantly, while that of pyrrolic-N and quaternary-N increased gradually with temperature increasing. Pyridinic-N and pyrrolic-N formation was due to deamination or dehydrogenation of amino acids; subsequently, some pyridinic-N converted to quaternary-N. Increasing temperature decreased amides content greatly while increased that of nitriles and N-heterocyclic compounds (pyridines, pyrroles, and indoles) in bio-oil. Amides were formed through NH₃ reacting with fatty acids, that underwent dehydration to form nitriles. Besides, NH₃ and HCN yields increased gradually. NH₃ resulted from ammonia-N, labile amino acids and amides decomposition, while HCN came from nitrile decomposition. At 700-800 °C, evolution trend of N-containing products was similar to that at 400-600 °C. While N-heterocyclic compounds in bio-oil mainly came from pyrifinic-N, pyrrolic-N, and quaternary-N decomposition. Moreover, cracking of pyridinic-N and pyrrolic-N produced HCN and NH₃. A mechanism of nitrogen transformation during algae pyrolysis is proposed based on amino acids decomposition.