Fluidized bed combustion - modeling and mixing

Doctoral thesis, 2008

First demonstrated on industrial scale in 1976, fluidized bed combustion (FBC) rapidly became an established technology for solid fuel combustion with a current installed capacity of about 30 GWe and power plants ranging from 10 to 460 MWe. The widespread use of the FBC technology both under so-called circulating and bubbling conditions (i.e. with and without external recirculation of solids) is generally considered to be due to two main advantages with respect to other competing combustion technologies: the high fuel flexibility and possibilities of cost efficient reduction of SOx and NOx emissions. Yet, the relative lack of knowledge within some of the phenomena governing the process represents a major obstacle for the further development of the FBC technology. Efficient scale-up and development of the FBC technology depends on the possibility to increase the knowledge on the main processes involved in FBC boilers. A combination of experimental work and formulation of mathematical models is important to increase the knowledge of the process. In this work, macroscopic submodels for key processes in FBC (fluid dynamics, combustion and heat transfer) are formulated based on experimental data. These submodels are then linked to form a comprehensive model for large-scale fluidized bed combustion which is compared to experimental data not used in the validation of the submodels. The submodels include treatment of important phenomena such as fluctuations in the gas phase, separate convective and radiative heat transfer accounting for absorption in the particle suspension, influence of the pressure drop across the gas distributor and corner effects in the solid phase. These phenomena were not included in previous comprehensive FBC models and are the focus of this thesis. The comparison between the model and experimental data from large-scale fluidized bed boilers have been carried out for a wide range of data categories covering the solid phases (inert bed material and fuel), the gas phase and the temperature field. The modeled data generally shows good agreement with experimental data. Although the model developed is primarily aimed at circulating fluidized bed boilers, its principles and formulation can also be applied to bubbling beds and gasifiers as well as new FBC technologies such as oxy-fuel or chemical looping combustion.