Evolution of nitrogen functionalities in relation to NOx precursors during low-temperature pyrolysis of biowastes

Abstract

Nitrogen functionalities in biowastes appear to differ in species and thermal stabilities due to various nitrogen origins. Their evolution at primary pyrolysis is essential to the formation of NOx precursors while the relevant mechanisms and characteristics on this haven’t been clearly elucidated. Low-temperature pyrolysis of two biowaste categories involving agricultural ones (bean straw, BS; rice straw, RS; and wheat straw, WS) and industrial ones (medium-density fiberboard waste, MFW; penicillin mycelia waste, PMW; and sewage sludge, SS) were conducted to investigate evolution characteristics and pathways of nitrogen functionalities in relation to NOx precursors. Results demonstrated that N-A with various types (amino-N/amide-N/amine-N) together with N-IN (inorganic-N) were main nitrogen functionalities in two raw categories with a sequential thermal stability of N-IN (PMW, SS) < N-A (MFW) < N-A (PMW, SS) < N-A (agricultural ones), determining their evolution pathways. Subsequently, some of NH3-N was initially formed through decomposition of N-IN which was vanished at 250–300 °C. Furthermore, the majority of NH3-N was continuously produced by deamination of labile N-A and subsequent amine-N in tars. Simultaneously, their weak dehydrogenation would form a little HCN-N (<2.5 wt%). Two heterocyclic-N in chars were found to be firstly formed by stable N-A and then converted into quaternary-N together with some NH3-N. Hence, NH3-N was the more dominant species with a sequential yield of industrial ones ≫ agricultural ones. Accordingly, due to the most thermally unstable nitrogen functionalities in industrial biowastes, suitable low-temperature pyrolysis (250–300 °C) was found to be an excellent pretreatment technique for them to realize an effective nitrogen removal at a small cost of energy loss (e.g. 63% for MFW and 31% for PMW within 25% energy loss).