Numerical investigation of the back-mixing and non-uniform characteristics in the three-dimensional full-loop circulating fluidized bed combustor with six parallel cyclones

Highlights: • CFD-DEM for high-fidelity simulation of the 3D full-loop CFB with six cyclones. • Particle-scale data on solid dispersion, force, mass distribution and back-mixing. • Solid dispersion behavior significantly different among the three directions. • Solid loading in the middle cyclone twice that of the two adjacent ones. • 50% and 48% of the total mass distributed respectively in the riser and standpipes. -- Abstract: The design and operation of circulating fluidized bed (CFB), which offers excellent heat and mass transfer, remain challenging because of the lack of a comprehensive understanding. CFD-DEM is used here for the high-fidelity simulation of the three-dimensional full-loop CFB with six parallel cyclones. Detailed information on the particle-scale information (i.e., solid dispersion, particle rotational speed, fluid and collision forces, mass distribution, and backing-mixing intensity) throughout the CFB are presented for the first time. Interesting results include: (i) the solid dispersion behavior is significantly different among the three directions; (ii) the solid loading in the middle cyclone on one side of the riser is twice that of the two adjacent ones, and this non-uniform distribution underscores the need for full-loop simulation for the CFB with multiple cyclones; (iii) the solid dispersion coefficients in all three directions, fluid force, collision force and particle rotational speed are similar among the six cyclones and correspondingly the standpipes; and (iv) approximately 50% and 48% of the total material are distributed respectively in the riser and standpipes. The results obtained provide valuable insights regarding the mass loading, particle-scale characteristics and non-uniform distribution of the solid phase throughout the entire CFB.