Three-dimensional modeling of a large-scale CFB-furnace

Abstract

Ensuring sufficient oxygen distribution in a large-scale circulating fluidized bed boiler (CFB) furnace is a challenging task. Secondary-airs (SA) must penetrate suitably to assure efficient combustion, protect walls from corrosion and keep emissions under control. Therefore, it is vital to know how SAs penetrate and affect into the process. Measurements are often impossible to complete. In this thesis, transient SA’s penetration into a large-scale CFB-furnace was studied. Computational fluid dynamics (CFD) program ANSYS Fluent was used. Reactions were neglected. Results corresponded literature findings. Coarse mesh together with homogeneous gas-solid drag model resulted into overpredicted drag and solids circulation. CFD SA penetration results from horizontal plane were connected to the three-dimensional comprehensive semi-empirical model called CFB3D. Predictions effects to CFB3D-model’s accuracy were investigated with comparison to a reference case where SA penetration bases on a correlation. Horizontal oxygen and temperature profiles were smoothened making emission predictions easier. It is shown that correlation based penetration should be corrected transversely. In higher levels dispersion coefficients must be decreased to prevent excessive gas spreading. Effects of fuel spreading were shortly tested with no influence on results. Literature survey showed that dispersion coefficients could be determined using CFD. In future, to obtain more accurate results whether fine mesh, drag correction or heterogeneous gas-solid drag model must be used. By changing realizable k-ε to swirl-modified Re-Normalization Group (RNG) k-ε turbulence model overprediction of external solids flow can be further decreased. Depending on particle size distributions (PSD) shape at least two characteristic particle sizes must be used to obtain decent pressure profile. Mesh accuracy and number of cells can be optimized by using hexahedral cells.