Despite the rapid development of renewable energy systems, fossil fuels dominate the world's primary energy supply. Further improvement of the coal-fired power plants is needed due to many reasons, including the environmental impact and economic operation, particularly of the ones operating on low-grade solid fuels, such as lignite. The thermal power plant operation is commonly evaluated on the basis of the energy analysis, combustion efficiency and the level of generated harmful emissions. The objective of this work is to present a methodology for analyzing opportunities and measures for the efficiency improvement of coal-fired steam generators, based on the integration of zone thermal calculation method, exergy method, and computational fluid dynamics (CFD) technique, while studying the impact of some measures on the emission of NOx. The work deals with an analysis of the energy and environmental performances of a lignite-fired steam generator, which is a part of a 315 MW power generation unit. The zone thermal calculation method is used as a basis for exergy analysis and application of an engineering method for the calculation of NOx formation. The impacts of the air and fuel redistribution, excess air level optimisation and flue gas recirculation rate on NOx concentration in flue gases are presented.