The results of measurements and experiments on two power plant units are presented. The treated coal is lignite with lower heating values ranging between 9 and 10 MJ/kg and with contents of ash about 20% and moisture about 38%. The influences of significant lignite composition components on heating value, boiler losses, boiler specific production of steam, own consumption of electric power, power consumption for coal grinding and flue gas desulphurisation etc, are discussed. The results of analyses and measurements are graphically presented.