The significance of gas release on the mixing of larger particles in bubbling fluidized beds

The mixing of larger particles (e.g., fuel particles) in a bubbling fluidized bed is governed by buoyancy and drag, which correspondingly promote segregation and internal circulation. Prior studies have shown that gas released from such particles during drying or devolatilization can generate so-called endogenous bubbles, which exert an effective lift force on the particle. However, the impact of gas release under bubbling conditions and with over-bed particle feeding, remains unexplored. This study investigates how gas release influences the mixing of such particles in a bubbling fluidized bed. Experiments were conducted in a fluid-dynamically downscaled model simulating biomass pyrolysis at 700 °C in a bed of silica sand fluidized with flue gas using magnetic particle tracking and gas-releasing tracers. Different behaviors were observed depending on the fluidization velocity. At low fluidization velocities (u 0 /u mf ≤ 2), buoyant particles sink slightly deeper by locally reducing the concentration of surrounding suspension, whereas heavy particles experience inhibited sinking resulting in preferred positions closer to the bed surface. As fluidization velocity increases beyond u 0 /u mf ≥ 3.5, the effect of gas release on axial mixing diminishes, while the lateral mixing becomes more pronounced, with the dispersion coefficient enhanced by up to 40%. These findings inform industrial fluidized-bed design.