Circulating Fluidized Beds in Three Different Scales

Due to its excellent heat and mass transfer characteristics, circulating fluidized bed has gained increasing interest in industrial applications. Although it has been studied widely, most available data are obtained from the lab scale and pilot scale models due to the restriction and complexity of experimental data acquisition in large scale models. Furthermore, scale-up of fluidized beds is difficult because it is widely known that the hydrodynamics of small fluidized beds can be significantly different from the hydrodynamics of larger beds. So, the data from small scale cannot be used in the large scale directly.

With the development of computational algorithm and computational capacity, computational fluid dynamics (CFD) is becoming as a powerful and efficient tool for studying the flow behaviour of gas and solids in CFBs design, optimization and scale-up. In this study, computational particle fluid dynamics (CPFD) based on the multi-phase particle-in-cell (MP-PIC) methodology is used to model the solid circulation behaviour in the three full-loop CFB systems. In the three systems, they all have the rectangular cross section risers, but the first one is in the lab-scale with the dimension of 6.7 cm x 20.1 cm x 200 cm and one cyclone; the second one is in the pilot scale with the dimension of 42 cm x 92 cm x 580 cm and six cyclones; the third one is in the industrial scale with the dimension of 2823 cm x 922.5 cm x 5000 cm and three cyclones. Homogeneous drag model Wen-Yu or heterogeneous drag model energy minimization multi-scale (EMMS)/matrix drag models are employed in the simulation and the results will be compared with the available experiment data.

In the lab-scale CFB, the simulated pressure drop of the riser is 3500 Pa when the primary inlet air flow is 2.7 m/s and static bed height is 35 cm, and both the simulated and experimental results show that double bubbles appear in the cross section of the bottom region and the shape is irregular and stretched along the long side. In the pilot-scale CFB, the simulated pressure drop of the riser is 5976 Pa when the primary inlet air flow is 4 m/s and bed material is 200 kg, and Wen-Yu drag model based simulation shows the drag force between the gas and the solid particles is over-predicted while EMMS drag model under-predicts the force. In the industrial scale CFB, the simulated pressure drop of the riser is 7400 Pa when the primary inlet flow is 180 kg/s and static bed height is 100 cm, and half of the pressure drop happens in the bottom of the riser.