CFD modeling of gas–solids flow in a large scale circulating fluidized bed furnace

•CFD simulations performed for a large scale CFB and compared with measurements.•Ergun/Wen–Yu drag model simulates better with a finer mesh.•Coarse mesh with EMMS requires adjustment of the particle size.•The pressure profile does not change if the number of solid phases is increased.•Subgrid-scale models for coarse mesh are needed for solid–solid drag.

A Eulerian multiphase model was used to study the hydrodynamics of the gas–solids flow in a circulating fluidized bed (CFB) furnace. Alternative modeling approaches were tried to evaluate their effects on the predicted solids distribution in the furnace. Due to high computational costs, large scale calculations are commonly performed by using a coarse mesh, which leads to too uniform solids flow field and an overestimated gas–solid drag force. Thus, a suitable form of correction of the gas–solid drag is needed for coarse mesh simulations. This study presents fluid dynamics simulations of a 235 MWe CFB furnace by two sets of interphase momentum exchange coefficients: the model based on the homogeneous flow condition and the model based on the energy-minimization multi-scale (EMMS) approach. The calculations with one solid phase included studies with two mesh resolutions and different particle sizes. Additional simulations were performed using two solid phases and four solid phases to analyze the effect of polydispersity of particles. The model results were compared with the measured vertical pressure profile of the furnace and the locally measured solid concentration and velocity profiles. The results show that with a homogeneous model, the model matches better with measurements if the mesh resolution is finer. The EMMS approach can be used to improve the calculations in a coarse mesh if the average particle size can be determined correctly for the model. With multiple solid phases, the vertical and horizontal segregation could be simulated. However, increasing the number of solid phases did not change the total solid concentration and pressure profiles if the Sauter mean diameter of the total system was constant. The results indicate that with a coarse mesh, similar drag corrections would be necessary between the solid phases that have been developed for gas–solid drag.

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