Computational fluid study of radial and axial segregation characteristics in a dual fluidized bed reactor system

In this work, computational fluid dynamics is adopted to simulate steam-biomass gasification in a dual fluidized bed reactor system. Under the Eulerian-Lagrangian multiphase framework, the large eddy simulation and multiphase particle-in-cell models are selected to track the gas and solid phases, respectively. The compositions of the gaseous products numerically obtained at the gasifier outlet are compared with the experimental data for model validation. Then, the radial and axial segregation of heat carrier with a wide particle size distribution together with the spatial variation of the microscale properties of solid phase are explored. The results show that the apparent size-induced segregation of heat carrier results in the distribution of large particles in the bottom region of both the combustor and gasifier, while the accumulation of fine particles in the freeboard of the gasifier. The mass of heat carrier distributed in the combustor is nearly 10% of that in the gasifier. The particles in the combustor have larger Reynolds number and dispersion coefficient as compared with that in the gasifier. For a specific particle size distribution, reducing the particle size enlarges the temperature, heat transfer coefficient, particle Reynolds number. Different solid residence behaviors appear in different components of the bed. A wider size distribution of heat carrier slightly enlarges the mean residence time in the combustor but obviously diminishes it in the gasifier, and also evidently reduces the solid circulation.