Effect of down-tube structure on particle flow by CFD-DEM method

Biomass energy is one of the most important eco-friendly renewable resources to reduce environmental pollution. The effective pyrolysis in a down-tube structure reactor can directly catalyze the production of bio-oil from biomass, in order to recycle the heat carriers. Among them, the flow patterns of particles are crucial to optimize the efficiency of the down-tube operation and the quality of the bio-oil products. This study aims to clarify the flow patterns of ceramic beads and biomass particles within different down-tube structures. A comparative study was conducted on the impact of square and round down-tubes on particle axial velocity, particle collision rates, and biomass residence time in a cold state. Additionally, a systematic investigation was implemented to examine the effect of tube diameter on particle axial velocity and biomass residence time. A CFD-DEM coupled computational framework was established, where the fluid part used the FLUENT software for CPU multi-core parallel computation, and the solid particles were simulated using EDEM software. The simulations were velocity-validated using particle image velocimetry (PIV). The experimental results showed that better performance was achieved in the consistent test and simulation velocity for ceramic beads. In terms of electrostatic effects, the experimental velocity of biomass particles was consistently lower than the simulated one. However, the difference in velocity between biomass in simulations and experiments was kept within a narrow range after electrostatic removal operations on the down-tube walls, fully meeting experimental requirements. The structural differences in the down-tubes depended on the flow of particles. The mixed particles exhibited higher time-averaged axial velocities in round tubes than those in square ones. Particularly, the maximum axial velocity of ceramic beads in round tubes was about 10% higher than in square ones. The difference was observed in the particle axial velocity at the bends of the down-tubes under different tube diameters. The velocity of ceramic beads decreased by approximately 37%, 40%, and 45% at 60, 70, and 80 mm diameters, respectively; The biomass velocity decreased by about 49%, 52%, and 54%, respectively. The ceramic beads were introduced to change the comparative flow velocities of biomass particles in square and round tubes. The biomass velocity in round tubes was greater than that in square tubes. The ceramic beads reduced the flow velocity of biomass. The maximum axial velocity of biomass decreased by 41% and 33%, respectively, in both types of tubes. In square tubes, the biomass had a higher frequency of contact collisions with the walls, where about 23% of biomass particles were contacted with the ceramic beads, primarily flowing along the walls. In round tubes, there was a higher frequency of contact collisions between biomass and ceramic beads, where about 39% of biomass particles were contacted with the ceramic beads, resulting in a more uniform distribution of biomass and movement in the central part of the tube. Biomass particles shared a larger average residence time and more concentrated distribution in square tubes, compared with the round tubes. The ceramic beads were introduced to increase the dispersion of biomass residence time distribution, which increased by 10% and 17% in the square and round tubes, respectively. The findings can provide a strong reference for the particle flow states in the down-tube pyrolysis devices, in order to better design and optimize the down-tube structures.