The dynamics of large particles in a four-compartment interconnected fluidized bed

In order to investigate the potential of a four-compartment interconnected fluidized bed for the combustion of biomass, the behaviour of cylindrical pellets in a bed material of glass ballotini was characterized as a function of the operational parameters. This involved (a) studying the distribution of pellets over the compartments after a given fluidization time, (b) measuring the pellet circulation time and (c) following the dynamics of a radio-active pellet using Positron Emission Particle Tracking (PEPT). The circulation rate of the bed material itself was also measured to relate it to that of the pellets. The results show that control of the material circulation rate can be achieved either by varying the excess gas velocity in the two vigorously fluidized ('fast') compartments or the aeration rate in the two slow ones, the latter option being the better. The fast compartments were found to act as ideal mixers, while little mixing took place in the slow ones at the aeration rates used. Segregation of pellets relative to the bed material was observed, mainly in the slow compartments, in some cases leading to pellets getting stuck low in the slow beds. It was found that the best option for avoiding problems with segregation and controlling the circulation rate of both pellets and bed material was to aerate the slow beds at superficial velocities under the minimum fluidization velocity. The draw-back in doing so is that dead-spaces will develop in these beds. (C) 1999 Elsevier Science S.A. All rights reserved.