This study aims to predict the pyrolysis behavior of the biomass particles interacting with the high-temperature ceramic balls in a down-tube reactor. A numerical simulation framework was also proposed for the accurate fitting of the transient heat transfer. The modeling approach was established on the distributed activation energy model (DAEM), particularly for the heterogeneous distribution of the activation energies within biomass. Multiphysics processes were also integrated, including Hertzian contact conduction, gas film conduction, convection, radiation, and mass transfer. An energy balance of the coupled ordinary differential equation was constructed after prediction. Thermogravimetric analysis (TGA) data were employed to calibrate the kinetic parameters. The Gaussian, Lorentzian, and logistic distribution functions were used to examine the distributions of the activation energy. Among them, the Lorentzian function also exhibited the superior fitting accuracy. The better performance was achieved in the mean absolute error (MAE) of 0.011 and root mean square error (RMSE) of 0.013. Thereby, the tail behavior of the pyrolysis kinetics was extended for better performance. Simulation results reveal that the biomass particles shared the extremely rapid heating at the initial stage. The instantaneous heating rates reached up to 2 136 °C/s. However, a time lag was observed between the thermal equilibrium and the pyrolysis reactions, indicating the kinetic limitations beyond thermal driving forces. Heat transfer analysis indicated that the heat conduction (via both direct Hertzian contact and gas film pathways) and convection were dominant in the energy exchange between biomass particles and ceramic spheres. Whereas the radiation was also neglected at high temperature, although the contribution rate was only about 10%-15%. Energy balance decomposition further confirmed that approximately 85%-90% of the total heat transfer was from conduction and convection. While the pyrolysis reaction was markedly endothermic, the cumulative energy consumption was delayed for the full conversion. The parameter sensitivity analysis systematically quantified the influence of the critical factors on the pyrolysis dynamics. Ceramic ball temperature and biomass particle radius emerged as the most influential parameters on both heating rate and conversion efficiency. Reaction enthalpy and collision probability also contributed to the pyrolysis. Whereas, the radiative view factor exerted only marginal effects, due to its relatively small share of the overall heat transfer. Specifically, the ceramic ball temperature substantially accelerated the particle heating for the nearly theoretical conversion (≈83.3%). While the particle size was reduced to avoid the thermal inertia for less reaction lag. The overall process efficiency was also highlighted to determine the reactor operation and particle-scale parameters. Fast pyrolysis was realized in the down-tube reactors using DAEM multiphysics. The finding can provide a robust computational tool for engineering optimization. The framework was utilized to accurately reproduce the transient temperature evolution, mass conversion, and reaction rates. Furthermore, the solid-solid contact efficiency was enhanced to adjust the particle dimensions for the external heating intensity during reactor operation. The appropriate modification of the boundary conditions was obtained to extend into the biomass conversion reactors, such as the fluidized beds or rotary kilns. Particle group hydrodynamics can be incorporated to expand the prediction at the reactor scale. Overall, the transient model can bridge the kinetic heterogeneity and multiphysics heat transfer using computational fluid dynamics (CFD) simulations. The experimental data were validated to identify the key parameters on the pyrolysis efficiency. The findings can also provide practical data support and engineering insights to optimize the down-tube reactors for sustainable biomass utilization.

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.

Biomass particles can often be confined to the insufficient disturbance and mixing flow within the down-tube pyrolysis reactor. In this study, a systematic investigation was implemented to explore the influence of an inclined platform inside the down-tube on the flow behavior of ceramic balls and biomass particles. The particle mixing was also analyzed using experimental and simulation. The parameters of the inclined platform were treated as the experimental variables, including the position, tilt angle, and height. While the degree of particle dispersion was taken as the evaluation criterion. The particle flow was then simulated using computational fluid dynamics (CFD) coupled with the discrete element method (DEM). particle image velocimetry (PIV) was employed to verify the simulation. The results indicate that the height of the inclined platform shared the most significant effect on the dispersion degree of the particles, followed by the platform’s position and tilt angle. The optimal working parameters were determined after optimization, where the bottom of the inclined platform was positioned 245 mm from the corner of the down-tube, with a height of 27 mm and a tilt angle of 149°. The degree of particle dispersion increased by 50.24% under these optimal conditions, indicating the better homogeneity of the particle mixture. The inclined platform was introduced to significantly enhance the flow characteristics of the particles. Furthermore, the axial average velocities of the biomass particles and ceramic balls decreased by 14.38% and 11.43%, respectively, compared with the conditions without the inclined platform. Concurrently, the average residence time of the particles increased by 20.00% and 5.75% for the biomass particles and ceramic balls, respectively. As such, the inclined platform effectively extended the residence time of the particles within the down tube. Thus, the pyrolysis reactions were enhanced for the high efficiency of the reactor. Moreover, the inclined platform also altered the flow characteristics of the particles. In the absence of the inclined platform, the particles exhibited a distinct centripetal flow, leading to the uneven particle distribution with the segregation, where some particles were denser at the bottom and sparser at the top. With the aid of the inclined platform in place, the parabolic flow of particles was obtained to effectively disrupt the segregation for the more uniform distribution of particles. The homogeneity of the particle mixture further facilitated the efficiency of the pyrolysis reaction. It was of significant importance to optimize the pyrolysis performance of reactors. The inclined platform also increased the mixing and flow behavior for the better homogeneity of the particle mixture. The findings can offer new insights into the design and optimization of down-tube reactors in the further development and application of biomass pyrolysis. Especially, the high efficiency of biomass energy utilization also contributed to the pyrolysis reactions. The findings can hold great potential for the practical implications of biomass energy conversion.