Numerical investigation of mixing and segregation of biomass particles in bubbling fluidized beds using CFD-derived uniformity index

Journal article, 2027

Biomass is a promising carbon-neutral renewable energy resource, and bubbling fluidized beds are the dominant equipment for biomass thermochemical conversion. However, the significant density and size differences between biomass and bed materials cause severe particle segregation, leading to poor mixing and low energy efficiency. To address this issue, an Eulerian multi-fluid model was employed to investigate the mixing and segregation behaviors of biomass particles in bubbling fluidized beds. Three widely used drag models were validated against experimental data, and the Huilin-Gidaspow drag model was selected for its superior predictive accuracy. Two quantitative mixing indices, the overall uniformity index <β> and the local uniformity index β with its standard deviation σ, were proposed to characterize both overall and local mixing and segregation performance of biomass particles. The effects of superficial gas velocity, biomass mass fraction, biomass particle size, and bed diameter on mixing and segregation behaviors of biomass particles were systematically analyzed. The results show that increasing superficial gas velocity and reducing biomass particle size significantly improve mixing homogeneity. Most importantly, a remarkable scale-up effect was observed: Severe vertical segregation occurs in narrow laboratory-scale beds, whereas substantially improved mixing is observed when bed diameter is increased at the same bed height and under identical operating conditions. This work provides fundamental insights into biomass fluidization dynamics and valuable guidance for the design and scale-up of bubbling fluidized beds for biomass thermochemical conversions.