Experimental investigation of the lateral mixing of large and light particles immersed in a fluidized bed

Artikel i vetenskaplig tidskrift, 2023

Fluidized bed reactors for solid fuel conversion are characterized by the presence of a small fraction of fuel particles that are significantly larger (usually 1–2 orders of magnitude larger) and lighter (2–20-fold less dense) compared to the bulk solids. This difference in physical properties strongly influences the mixing of the fuel particles and therefore affects the mass, momentum and heat transfers between the fuel particles and the surrounding bed. This work uses Magnetic Particle Tracking (MPT) to acquire highly resolved trajectories for single tracer particles immersed in a bubbling fluidized bed operated under ambient conditions and with a cross-sectional area of 0.45 m2. This bed size is sufficiently large to abrogate the influence of the reactor walls, allowing data post-processing to study the free movement of the tracer particle, which has not been available to date. This required the enhancement of the MPT system from that in previous works: 12 sensors and a communication protocol in series are here applied, which showed good performance in both spatial accuracy (1 mm) and time resolution (100 Hz). The bed material used in the experiments was glass beads (mean particle size of 106 µm, particle density of 2,486 kg/m3). Two different tracer particles, with diameters of 18 mm but with different densities (572 kg/m3 and 1,015 kg/m3) were used to mimic the sizes and densities of the solid fuel particles. Fluidization velocity was varied within 0.2–0.7 m/s and two fixed bed heights (50 mm and 130 mm) were tested. Based on the trajectories, dispersion coefficients were calculated for quantitative evaluation of the solids mixing. The results reveal that increased bed height yields higher dispersion coefficients with a higher sensitivity for fluidization velocity. The properties of the tracer particles appear, within the tested range, to exert little impact on its lateral dispersion. From the velocity maps generated, a swirling pattern was observed in the vicinity of the walls, while zones of preferential ascendent or descendant movement were observed in the cross-section centre, although clearly defined mixing cells were not exhibited.