A review on nitrogen transformation mechanism during biomass pyrolysis

The thermochemical conversion of biomass produces NO_x precursors (e.g. NH₃, HCN, and HCNO) and high-value-added nitrogenous products (e.g. pyrroles, pyridines, and indoles). The control of NO_x precursors and regulation of nitrogenous chemicals are of great significance for environmental protection and biomass resource utilisation. However, the nitrogen transformation mechanism of biomass pyrolysis is not clear at present, which is not conducive to the regulation of pyrolysis products. In this review, the elemental and biochemical compositions and the N transformation directions of common N-rich biomass have been introduced. The advances in nitrogen transformation characteristics during typical model compounds (glutamic acid, aspartic acid, phenylalanine, proline and 2,5-diketopiperazines) and major biomass components pyrolysis have been summarized. The effects of experimental conditions such as pyrolysis parameters (temperature, heating rate), catalysts, and atmospheres on nitrogen migration have been analysed. Finally, nitrogen regulation strategies and future research directions on nitrogen reduction in bio-fuels and nitrogen enrichment in nitrogenous products have been proposed. This review can provide suggestions for NO_x precursor control, high-quality nitrogenous chemical and multifunctional N-doped carbon product preparation during thermochemical conversion of biomass.