This paper reports the application of a full three-dimensional mathematical model to a fuel-oil fired power station boiler of the Portuguese Electricity Utility. Special emphasis is placed on the formation of nitric oxide. Models for thermal and fuel-NO formation are included and several variants of the models are employed. The predicted results are compared with measurements. Concentrations of O2, CO2and CO are in good agreement with the data whereas the temperature, although in qualitative agreement, is underestimated along the profile crossing the flame region. It is shown that fuel NO is an important source of the total NO formed. Nevertheless, the NO mole fraction is underestimated. Several hypotheses were checked to identify the source of discrepancy and the most probable ones are the temperature underestimation in the flame region and the experimental uncertainties.