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Show 1. INTRODUCTION In a fluidized bed system the gas is blown upward through a distributor to fluidize the bed and expand its volume. A fluidization vessel (Fig. 1) usually consists of three zones. At the bottom of the bed is the distributor or grid region where gas is forced to flow upward in the form of gas jets. Any amount of air flow rate in excess of the minimum fluidization velocity flow in the form of gas bubbles through the bed. Above this zone is the dense or bubbling phase. In this bubbling zone, bubbles grow by coalescence and rise to the surface of the bed. As bubbles break at the surface of the bed, particles are thrown up above the bed surface and are entrained by the flowing gas stream. The third zone of the vessel, between the surface of the dense bed and the top of the bed where the gas stream exits, is called the freeboard. In this zone, some particles are carried far above the bed surface and are elutriated while others disengage from the flow and fall back to the bed. In a wood-burning fluidized bed combustion (FBC) system, calculation of the freeboard height is one of the most important design problems. The freeboard region provides the space for additional reaction between the projected particles and gas and also for the disengagement of solids. During the operation of a fluidized bed, if the freeboard height has not been calculated correctly, a large amount of fine particles could be elutriated continuously. Therefore, it may become necessary to recycle the (1) Minimum fluidization velocity is the minimum requirement of gas to counterbalance the frictional force with the weight of particles. 2.3.3 |