The results of an experimental study of the excess-air-dependent heat losses, as well as gaseous emissions (NOx, SO2 and CO), on a 150 MW boiler firing Thai lignite are discussed. The NOx emissions were found to increase with the higher excess air ratios; the NOx values in the flue gas (at 6% O-2) ranged from 257 to 325 ppm, whilst the excess air ratio varied from 1.06 to 1.32 at the economizer outlet. Owing to the highly-efficient operation of the flue gas desulfurization units, the SO2 emissions from the unit were maintained at a relatively low level, 50-76 ppm for the above excess-air ratios, whereas they accounted for about 3100-3300 ppm at the inlet of the FGD units. The CO emissions were determined for the extremely low excess air ratios. Two approaches for the optimization of the excess air ratio were analyzed in this study. For the first, i.e. the conventional approach, the optimization was carried out based on minimizing the total excess-air-dependent heat losses. The second, the environmentally friendly approach, proposed in this work, was aimed at minimizing the "external" costs (or the costs of damage done by the boiler emissions to the environment and humans). As shown in this paper, the lignite firing. at the optimal excess air results in a lower environmental impact by the boiler unit. (C) 2003 Elsevier Science Ltd. All rights reserved.