A review on the NOx precursors release during biomass pyrolysis

•N-A and inorganic-N are major sources of NOx precursors.•NH3 and HCN dominate NOx precursors during biomass pyrolysis.•DKP produces NOx precursors by ring-condensation and direct CC/CN breakage.•NH3 is generated at low temperatures, and HCN is produced at high temperatures.•Staged pyrolysis and co-pyrolysis can reduce the N-related emissions.