CFD modeling of tapered circulating fluidized bed reactor risers: Hydrodynamic descriptions and chemical reaction responses

A comprehensive two-dimensional transient Eulerian model combined with the kinetic theory of granular flow was developed to obtain the hydrodynamic and chemical reaction behaviors in tapered circulating fluidized bed reactor risers. In this study, the focus was on the chemical reactions and its behaviors inside three different riser geometries. The model was verified by using an experimental dataset from the literature, and was then used for both predicting the hydrodynamic behaviors and computing the system turbulent properties. The tapered-out riser improves the system turbulence or mixing which can be explained by the dispersion coefficients. On the other hand, the tapered-in riser increases the solid particle residence time and gives a more uniform temperature distribution, because it does not have sufficient force to support the weight of the particles. The same riser geometries but with the addition of the chemical reaction were then used for evaluating the previously proposed criteria that the riser geometry should be chosen with respect to the characteristics of the reactions. Reactions with a medium reaction rate were best suited to the typical riser, whilst reactions with a fast and slow reaction rate best fitted the tapered-out and tapered-in risers, respectively.