A review on the NOx precursors release during biomass pyrolysis

Pyrolysis, as a means of utilizing biomass resources, converts fuel-N in biomass into N-containing compounds, such as pyrrole-N, pyridine-N and nitrile-N. These compounds can be further decomposed into NOx precursors (mostly NH3, HCN and HNCO), which will cause a series of air pollution after oxidation and emission. Many reviews focused on unitary aspects of biomass such as wood fiberboard or N-containing models, and there is no comprehensive and critical review on the formation pathways of NOx precursors during biomass pyrolysis. Therefore, this review focuses on the effects of different reaction parameters such as thermal factors (temperature and heating rate), atmosphere and biomass co-pyrolysis on NOx precursors release, discusses the strategies taken to control emission of NOx precursors in the current process, summarizes the nitrogen migration pathways during biomass pyrolysis. Finally, we outlook the development of catalysis, theoretical calculation and co-pyrolysis reaction in the field of NOx precursors. It is hoped that this review can provide suggestions for the preparation of high-quality pyrolysis products (such as high nitrogen biochar and low nitrogen bio-oil) and the reduction of NOx precursor emissions.