A computational model for analysis and minimizing the fuel and environmental costs was applied to a 310 MW fuel oil fired boiler. The effects of the main operating variables (load and excess air ratio) on these costs were studied in this work. The emission rates of the gaseous pollutants (NOx, SO3, SO2 and CO) were estimated with the use of emission models considering the boiler furnace as a control volume. The predicted emissions for NOx, SO2 and CO were compared with the values obtained in experimental tests on this boiler operated at different loads and various excess air ratios. The major (i.e. fuel, CO2 and SO2) cost items are weakly dependent on the excess air ratio and mainly affected by the boiler load. Meanwhile, the effects of the excess air ratio, as well as boiler load, on the minor costs (associated with NOx, SO3 and CO emissions) are found to be noticeable. Optimum and "compromise" excess air ratios were quantified for the boiler loads in the range of 80-100%. Switching the boiler to firing the fuel at the "compromise" excess air ratio leads to a reduction in the total (fuel and environmental) costs for this boiler. (C) 2003 Elsevier Ltd. All rights reserved.