1. Introduction2. Boiler efficiency and thermal losses3. Modernisation to reduce flue gas loss 4. Reduction of nitrogen oxide emissions 5. Modernisation of fuel grinding systems 6. Replacing coal with other fuels7. Adaptation of boilers for biomass burning 8. Harmful phenomena in modernised boilers9. Conversion of an existing boiler to a condensing boiler 10. Increasing flexibility of boiler operation 11. Interactions between emission reduction systems