A CFD investigation has been carried out about the performance of a 600 MWe tangentially coal-fired boiler, focusing on the reduction of NOx attainable by using overfire air. To this purpose, a comprehensive combination of NOx chemistry models has been used, coupled with the numerical simulation of fluid and particle flow, solid fuel combustion and heat and mass transfer. Predicted values of gas temperature and species concentration have been adopted to validate the model against actual measurements from the full-scale boiler, under conventional and overfire air arrangements. A reasonable agreement has been attained in most cases. Additionally, modelling sensitivity has been evaluated against variations in some fuel-dependent parameters hard to measure or estimate (devolatilisation rates, nitrogen content in volatiles and char, reburning rates). As a result, an analysis tool is available to study the response of this kind of boilers to a variety of coal feedstock and combustion conditions, in a feasible and economic manner. (c) 2007 Elsevier Ltd. All rights reserved.