Abstract: 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.