Computational modelling of NOx formation in a 275 MW(e) utility boiler was investigated. The aim was to explore the use of simplified NOx chemistry when applied to a 3-D utility furnace, and ascertain if the correct exit NO trends and magnitudes could be obtained under a range of furnace operating conditions. The furnace was front-wall-fired with 24 burners in six groups of four. Furnace air-staging in the form of overfired air was available in the furnace. Combustion and modelling studies were carried out with two coals of different fuel-nitrogen content. A number of NOx control measures were investigated, including: burners out of service, overfired air and excess air. The computational framework included: Lagrangian particle tracking; turbulent particle dispersion; the discrete transfer thermal radiation model; the standard k-epsilon model for flow turbulence; and a simple NOx chemistry model. The NOx model contained a hydrocarbon re-burn mechanism, and the model also included the NOx precursors NH3 and HCN. A sensitivity analysis was included, to test the relative importance of selected NOx modelling parameters. The computational grid size necessary to carry out a 3-D NOx analysis of the combustor was also investigated. Furnace temperature, heat-transfer and exit NOx predictions were compared with site measurements. Detailed axial in-flame measurements of oxygen, gas temperature and NO were compared with predictions in the near-burner field of the furnace. Thermal NOx was shown to contribute greater than 20% of the total exit NO. Reasonable predictions could be obtained by means of simplified NOx chemistry combined with a relatively coarse computational grid. The predicted furnace exit NO concentrations differed by 0-30% from those measured.