This paper presents the results of an experimental study on a 300-MW boiler unit fired with Thai lignite. Effects of operating conditions (excess air ratio and unit load) and fuel quality on the boiler heat losses and thermal efficiency as well as on the gaseous (CO2, CO, NOx and SO2) and particulate matter (PM) emissions from the boiler unit are discussed. The boiler thermal efficiency was weakly affected by the excess air ratio, unit load and fuel lower heating value, varying from 90.3 to 92.3% for wide ranges of the above variables. In all the tests, the NOx, SO2 and PM emissions were below the national emission standards for these pollutants. Quite low level of the SO2 emission was secured by the high-efficiency flue gas desulphurization system. The CO emissions of rather small values were detected only at extremely low excess air ratios. The emission rate and specific emission (i.e. per MWh of electricity produced) for NOx, SO2 and CO were quantified using experimental emission concentrations of the pollutants. Meanwhile, the emission characteristics for CO2 were determined with the use of fuel-C and fuel consumption by the boiler. In addition, the emission rate and specific emission for PM were estimated by taking into account the actual fuel-ash content and fuel consumption by the boiler, as well as the effects of SO2 adsorption by fly ash in the boiler gas ducts and overall ash-collecting efficiency of the electrostatic precipitators and flue gas desulphurization system. Elevated CO2 and NOx emissions from the 300-MW boiler units firing Thai lignite are of great concern. (C) 2006 Elsevier B.V. All rights reserved.