AbstractEstablishing an appropriate NOX production and decomposition model is of practical significance to the grate firing of biomass fuel. This study used the CHEMKIN software package to simulate the combustion process of biomass pyrolysis and char combustion. Through rate of production (ROP) analysis and simplification based on GRI-Mech3.0, the 15-step reaction mechanism of volatile-N converting into NOX and the 12-step reaction mechanism of char-N converting into NOX were specified. It was found that in the NOX generated from the fuel, N was mainly in the form of NO and N2O. HCN and NH3 were the important intermediate products. NH3 was mainly converted into NO and some converted into N2O, while HCN mainly consumed NO and produced N2O. According to the transfer characteristics of the biomass fuel nitrogen, the NOX production and decomposition model of the biomass fuel nitrogen in grate furnace firing was established. A simulation computation on the NOX production was implemented for an actual furnace. The established model was confirmed reliable through the comparison of field test data and simulation results.