The fluidized bed boiler of the company Sappi located in Gratkorn is mainly fired with hard coal and sewage sludge, providing energy for the pulp and paper production on site. In order to increase the efficiency of the boiler an extensive material stream analysis was initially conducted and results were visualized within a balance. As a result high content of unburnt carbon in the fly ash was found to be the most crucial parameter affecting efficiency. Due to its importance, specific operational tests were implemented to assess the influence of various changes on this parameter. Subsequently different measures were derived from acquired findings in order to ensure better combustion efficiency. Thereby also other technical issues such as NOx-emissions or the characteristics of fluidization were taken into account. Among others, cyclone efficiency, quantity of air distribution and flue gas recirculation as well as the operation of the ash cooler were eventually identified as possible areas of improvement.