NOx precursors evolving during rapid pyrolysis of lignocellulosic industrial biomass wastes

Nitrogen functionalities play an important role in the formation of NOx precursors during pyrolysis of lignocellulosic industrial biomass wastes (IBWs) while the influences haven't been clarified yet. Based on the rapid pyrolysis of three typical ones - medium-density fiberboard waste (MFW), Chinese herb residue (CHR) and tea stalk waste (TSW) in a horizontal tubular quartz reactor, characteristics of NOx precursors and their relationships with nitrogen functionalities at different temperatures were elucidated with the help of spectrophotometry, XPS and GC-MS technologies. The results demonstrated that the overwhelming nitrogen functionalities in lignocellulosic IBWs were amide-N types with sequential thermal stabilities of caffeine > protein > polyamide. At 300-550 degrees C, more stable amide-N tended to form heterocyclic-N in chars while more labile one was inclined to produce new kinds of amine-N in tars and NH3-N (12-17 wt% yield) in gases. Meanwhile, the weak dehydrogenation (into HCN and nitrile-N) and the sustained polymerization of amine-N (into heterocyclic-N) in tars also occurred at this stage. At high temperatures (> 550 degrees C), HCN-N (10-16 wt% yield) was produced by ring scission of heterocyclic-N in tars while 70% of NH3-N (similar to 16 wt% total yield) was mainly generated by hydrogenation of heterocyclic-N in chars. In addition, the remaining 30% of NH3-N yield was attributed to the simultaneous thermal cracking of amine-N. Despite the similar evolution pathways of amide-N for three lignocellulosic IBWs, NH3-N yield was produced with sequence of MFW > CHR > TSW at low temperatures while HCN-N yield was generated with the opposite order at high temperatures due to different thermal stabilities of amide-N types. It might be concluded that thermal stabilities of amide-N together with subsequent nitrogen functionalities evolved were key factors controlling the formation of NOx precursors during lignocellulosic IBWs pyrolysis. (C) 2017 Elsevier Ltd. All rights reserved