Particle mass flux in the freeboard of a fluidized bed boiler

Artikel i vetenskaplig tidskrift, 1989

The flux of descending particles has been measured in the freeboard of a fluidized bed boiler. The lateral and vertical distributions of the flux were studied and the influence of the superficial gas velocity and the static bed height analyzed. A tube bundle located in the bed surface region made it possible to investigate the effects of tubes on the entrainment of particles from the surface of the bed. The particle flux was relatively constant across the freeboard except near the wall, where it was about twice as large as the average. In the vertical direction, the flux decreased exponentially with height except close to the bed, where the rate of decay was less than at higher elevations. The tube bundle was found to inhibit the splashing of bed particles when the bed is below the tubes. A model is used for the estimation of the vertical particle flux and concentration profiles in the freeboard. It is based on separate treatment of small and coarse particles and calculation of trajectories of ejected bed particles. The calculated freeboard values describe the experimental data satisfactorily except close to the bed, where a considerable discrepancy is observed.