CFD simulation of mesoscale structures in mono and bidispersed fluidized bed pyrolysis reactors

Pyrolysis of biomass and organic matter in fluidized bed reactors have become popular in recent years due to the efficient conversion of biomass to bio-fuels. Among others, the presence of mesoscale structures produced by the transient nature of gas solid flow governs the performance of fluidized bed reactors. In the present study, monodispersed system with the particle size of 155 mu m and bidispersed system with particle sizes of 128 and 1500 mu m are simulated by the multi-fluid model with the kinetic theory of granular flow closure laws. The objective is to investigate the effect of size distribution and solid volume fraction on the hydrodynamics of gassolid flows. The ANSYS Fluent (R) software is used for the simulations and the slip velocity, heterogeneity index as well as mixing index are investigated. A two dimensional fully periodic domain is used to investigate the slip velocity at five different solid concentrations in dilute range. The slip velocity, which represents the interaction between solid particles and gas, calculated by simulation is higher than that calculated through RichardsonZaki's correlation. The mixing index is found to be minimum for a solid volume concentration of 5%. The heterogeneity index is significantly less than 1 for both cases with bidisperesed system having lower value as compared to monodispersed system. The image analysis of solid fraction contours exhibits that mesoscale structures are dense and non-uniform for solid fractions of 0.05 and are small and uniform of solid fractions of 0.025 and 0.075.

Automated summary

This study investigates the hydrodynamics of gas-solid flows in fluidized bed reactors by simulating monodisperse and bidisperse systems with different particle sizes and solid volume fractions. The results show that slip velocity, heterogeneity index, and mixing index are influenced by the size distribution and solid volume fraction.