Biomass energy promises to replace traditional fuels as a mean of sustainably energy generation. However the combustion of biofuels still struggle with low efficiencies and the release of toxic organic compounds, this scenario may change with either the development of new technology or the improvement of long-established techniques. From all the known techniques to convert biomass into energy, the circulating fluidized bed combustion (CFBC) has proven to be a successful choice since its mass and heat transfer capabilities allows the combustion of low grade fuels with lower emissions. Also, the introduction of a new variable in the system, through the recirculation of solids, grants a better control over the heat transfer occurring inside the vessel. For that reason, the CFBC is a favourable technique for large scale energy production from biomass fuels. On the other hand, the circulating fluidized bed combustion is a very omplex process, making its design and operation challenging. To approach the inherent intricacy of this problem, a modelling of this system is proposed in this work and the software program IPSEpro was adopted, which due to its flexible programming and equation-oriented solution strategy indicates a sensible choice for the tasks this work is set to accomplish. Additionally, IPSEpro brings the possibility to reliably represent the solids flow which is a great advantage, comparing to the standard modelling of a CBFC, since it enables a more accurate representation of the recirculation flow. The first goal of this work is to develop a model and to carry out a simulation analysis under pre determined varied conditions, based upon principles of parametric experimental design, to seek the parameters that most affect the process by using a design of experiments analysis. Furthermore, the developed model is to be validated taking as basis a valid example: the results presented by Wöß in his work, describing the pilot plant constructed in Gumpoldskirchen in a cooperation between Messer Austria GmbH and the Austrian Research Promotion Agency (FFG), and realized by Höltl during his PHD thesis. After having the model validated by a concrete example, the next step is to apply the model to a simulation of a co-firing device using a combinationof biomass and waste. A range of technologies could be applied for the co-firing process, however, the use low grade fuels is the most attractive feature of this process, and in this case the CFBC may be the most suitable technology for this application.