Insights into the evolution of fuel-N to NOx precursors during pyrolysis of N-rich nonlignocellulosic biomass

Pyrolysis of N-rich nonlignocellulosic biomass is a promising way to their energy utilization, in which the evolution of fuel-N may result in serious emission of N-containing pollution due to abundant nitrogen content in them. Understanding of specific evolution pathways of fuel-N to NOx precursors is significant. Based on two typical species with highest nitrogen content from different nitrogen origins - cultivated chlorella microalgae (CMA) and penicillin mycelia waste (PMW), characteristics of NOx precursors and relevant nitrogen functionalities in chars/tars with the temperature were fully analyzed and compared. Subsequently, evolution mechanisms of fuel-N to NOx precursors and their possible reaction pathways were in detail elucidated. Results revealed that overwhelming similarities on the evolution mechanisms were observed for two species based on their main similar nitrogen functionalities - N-A (amino-N/amide-N/amine-N) types, which was characterized by different pyrolysis stages. Around 80% of NH3-N yield was produced at devolatilisation stage, which was attributed to cyclization of more stable N-A into pyridinic-N/pyrrolic-N in chars, bond clearage of more liable N-A into amine-N in tars and further cyclization of amine-N into heterocyclic-N in tars. On the contrary, More than 90% of HCN-N yield was generated at secondary reaction stage due to a basically equal contribution from char-N and tar-N. The relevant pathways were ring scission of pyrrolic-N (chars) while ring-opening of heterocyclic-N (chars) and dehydrogenation of imines (tars). Besides, the additional mild HCN-N yield (devolatilisation stage) and limited NH3-N yield (secondary reaction stage) originated from dehydrogenation of amine-N in tars and hydrogenation of pyridinic-N in chars/bimolecular reaction of amine-N in tars, respectively. Particularly, two biomass species had an opposite sequence on the increasing rate of NH3-N yield (CMA > PMW) at devolatilisation stage and HCN-N yield (PMW > CMA) at secondary reaction stage. As a consequence, these findings could be well applied for N-rich nonlignocellulosic biomass containing similar fuel-N types, and to some extent provide guidance on the control of nitrogenous pollution emission for their energy utilization.