Abstract The current and future energy policy aims at increasing the share of renewable energy in world?s energy supply. One possibility to enhance energy production by renewable sources within a short term is co-combustion. This means co-firing biomass and waste with fossil fuels at existing power plants originally designed to fire fossil fuels. In Central Europe, the main interest lies in co-firing biomass and waste at pulverised coal boilers, because these plants form the basis of the currently utilised thermal fuel conversion techniques. Co-combusting supplementary fuels with coal is an economical option to increase the share of renewable energy because usually no large investments are required before implementation at large-scale utilities. Moreover, in many countries fiscal advantages are granted for renewable fuels. Co-firing can contribute to the increasing waste disposal problem by offering alternatives to landfills and waste incineration plants. When striving after higher renewable fuel shares, supplementary fuel replacement ratios at power plants increase and various problems can arise due to the differences between coal and the secondary fuels. Technical barriers that must be studied and overcome include matters related to fuel supply, fuel handling, changed combustion conditions in the boiler, ash quality and emissions. For this, the fundamental combustion properties of supplementary fuels compared with those of coal must be carefully studied. In order to better understand and predict the (co-)combustion behaviour of different biomass and waste fuels, a combination of experimental work and modelling should be used. Therefore, in the first part of this dissertation, pyrolysis of supplementary fuels is experimentally investigated by laboratory scale set-ups i.e. thermogravimetric analyser and heated wire mesh. The second part of the thesis concentrates on Computational Fluid Dynamics (CFD) modelling of co-combustion in a bench-scale, electrically heated pulverised fuel combustion reactor. Combustion sub-models are validated against experimentally determined concentration profiles and particle burnout data. The CFD model is then applied to optimise co-firing regarding supplementary fuel type, share and particle size.@en