In this paper, an experimental study has been performed for mixed (solid + gas) fuelled boiler operating at 7000 kPa pressure and 778.15 K temperature with a nominal capacity of 25 kg s(-1) in order to find improvement in the boiler efficiency and to reduce its environmental emissions. In this scope, while the boiler was working under operating conditions, temperature, pressure, velocity, and combustion gas measurements were performed and the measured data were used to establish mass and energy balance. Main efficiency loses were identified as operation of the boiler with high excess air factor (39.78%), leakage air in the rotary air hater and surface thermal losses. Calculations, were performed with the use of measured data, estimated that the boiler and second law efficiency were calculated as 85.3 and 39.6% respectively. When the injected heat value on water is 67 433.65 kW in the boiler, by the reduction of excess air factor is 550 kW and by the prevention of leakage air is 1251.6 kW, including totally saving amount is 1801.7 kW. Financial value of this saving was calculated about 309 465$.