Influence of Drag Laws on Pressure and Bed Material Recirculation Rate in a Cold Flow Model of an 8 MW Dual Fluidized Bed System by Means of CPFD

Authors: Stephan Kraft ^a,b, Friedrich Kirnbauer ^a, and Hermann Hofbauer ^b
a Bioenergy 2020+ GmbH, Wiener Strasse 49, A-7540 Güssing, Austria
^b TU Wien, Institute of Chemical Engineering, Getreidemarkt 9/166, Vienna, Austria

Source: Kraft, S.; Kirnbauer, F.; Hofbauer, H. Influence of Drag Laws on Pressure and Bed Material Recirculation Rate in a Cold Flow Model of an 8 MW Dual Fluidized Bed System by Means of CPFD. Particuology 2018, 36, 70–81.

Abstract: A cold flow model of an 8 MW dual fluidized bed (DFB) system is simulated using the commercial computational particle fluid dynamics (CPFD) software package Barracuda. The DFB system comprises a bubbling bed connected to a fast fluidized bed with the bed material circulating between them. As the hydrodynamics in hot DFB plants are complex because of high temperatures and many chemical reaction processes, cold flow models are used. Performing numerical simulations of cold flows enables a focus on the hydrodynamics as the chemistry and heat and mass transfer processes can be put aside. The drag law has a major influence on the hydrodynamics, and therefore its influence on pressure, particle distribution, and bed material recirculation rate is calculated using Barracuda and its results are compared with experimental results. The drag laws used were energy-minimization multiscale (EMMS), Ganser, Turton–Levenspiel, and a combination of Wen–Yu/Ergun. Eleven operating points were chosen for that study and each was calculated with the aforementioned drag laws. The EMMS drag law best predicted the pressure and distribution of the bed material in the different parts of the DFB system. For predicting the bed material recirculation rate, the Ganser drag law showed the best results. However, the drag laws often were not able to predict the experimentally found trends of the bed material recirculation rate. Indeed, the drag law significantly influences the hydrodynamic outcomes in a DFB system and must be chosen carefully to obtain meaningful simulation results. More research may enable recommendations as to which drag law is useful in simulations of a DFB system with CPFD.