Co-pyrolysis of soybean straw and spirulina platensis for N-containing chemicals and N-doped carbon materials production: Unraveling the nitrogen migration mechanism and involved chemical reactions

In this study, the influences of soybean straw (SS) to spirulina platensis (SP) mass ratios and pyrolysis temperature on the evolution properties of N-containing species in pyrolytic tri-products and the nitrogen migration mechanism and involved chemical reactions during the co-pyrolysis processes were comprehensively investigated. Results indicated that co-pyrolysis of SS ang SP significantly facilitated the transformation of nitrogen into N bio-oil and N char from N gas . Increasing the amount of SP led to a decrease in the concentration of NH 3 and HCN in N gas , while their concentrations increased gradually with temperature increasing. NH 3 and HCN formation was due to H radicals attacking the N-sites of N-heterocyclic, with NH 3 formation being more strongly influenced by this process. Regarding N char , increasing the ratio of SP enhanced nitrogen incorporation reactions and promoted formation of pyridinic-N and pyrrolic-N. Conversely, decreasing the ratio of SP enhanced hydrogenation and condensation reactions, which favored the formation of quaternary-N. With increasing temperature, protein-N decomposed significantly, while pyridinic-N, pyrrolic-N, and quaternary-N remained stable. Additionally, pyridinic-N can convert to quaternary-N at temperature > 500 °C. Increasing the SP ratio greatly decreased oxygenates content while increased that of N-containing compounds in N bio-oil . The content of heterocycle-N (34.31 %) and nitriles (9.22 %) in bio-oil peaked at a ratio of 1:3, attributed to SP promoted the condensation and diazotization cyclization reactions. Furthermore, increasing the reaction temperature (from 400 to 800 °C) significantly increased the yields of heterocycles-N, while the yields of amines and amides decreased sharply due to dehydration reaction. As analyzed above, the possible nitrogen migration mechanism and involved chemical reactions were elucidated. These findings provides insights for generating high value-added N-containing chemicals and N-doped carbon materials during N-rich pyrolysis.