Impact of polydispersity on the flow dynamics in the riser of a circulating fluidized bed

Abstract

Circulating fluidized bed (CFB) has been commonly operated with polydisperse particles with a wide particle size distribution (PSD), which significantly affects solid transportation and the resulting thermochemical performance. Dense gas-solid flows in a pilot-scale CFB riser are simulated via the multiphase particle-in-cell (MP-PIC) coupled with a heterogeneous drag model. Gas-solid hydrodynamics together with the particle-scale information with a wide PSD is discussed. The results show that in the riser, the non-uniformly spatial distribution of solid phase appears with the presence of a core-annulus structure and a dilute-upper dense-bottom profile. The wide PSD gives rise to the axial segregation phenomenon with the accumulation of large particles in the bottom region. Enlarging the superficial gas velocity and PSD width respectively decreases and increases the segregation intensity of solid phase. As compared with gas velocity, the PSD width performs limited impacts on solid concentration, particle-scale information along the axial direction, and solid residence time. The horizontal solid dispersion coefficient is at the scale of 10−3 m2/s, while the vertical dispersion is at the scale of 1 m2/s. For a wide PSD, particles with larger sizes have larger slip velocities but smaller dispersion coefficients in the riser.