The characteristics of the flow, combustion, temperature and NOx emissions in a 500 MWe tangentially fired pulverized-coal boiler are numerically studied using comprehensive models, with emphasis on fuel and thermal NOx formations. The comparison between the measured values and predicted results shows good agreement, which implies that the adopted combustion and NOx formation models are suitable for correctly predicting characteristics of the boiler. The relations among the predicted temperature, O-2 and CO2 mass fractions are discussed based on the calculated distributions. The predicted results clearly show that NOx formation within the boiler highly depends on the combustion processes as well as the temperature and species concentrations. The results obtained from this study have shown that overfire air (OFA) operation is an efficient way to reduce the NOx emissions of the pulverized-coal fired boiler. Air staging combustion technology (OFA operation) adopted in this boiler has helped reduce fuel NOx formation as well as thermal NOx formation under the present simulated conditions. The decrease in the formation of fuel NOx is due to the decreased contact of the nitrogen from the fuel with the oxygen within the combustion air, while the decrease in thermal NOx formation is caused by a decrease in temperature. The detailed results presented in this paper may enhance the understanding of complex flow patterns, combustion processes and NOx emissions in tangentially fired pulverized-coal boilers, and may also provide a useful basis for NOx reduction and control. (C) 2009 Elsevier Ltd. All rights reserved.