FLUIDIZED BED COMBUSTION

When air is passed through a .fixed or packed bed of particles, air simply percolates through the interstitial gaps between the particles. As the air flow rate through the bed is steadily increased, a point is eventually reached at which the pressure drop across the bed becomes equal to the weight of the particles per unit cross-sectional area of the bed. This critical velocity is called 'the minimum fluidization velocity, UrnC' at which the bed is said to be incipiently fluidized. As the air velocity is increased further, the particles are buoyed up and imparted a violently turbulent fluidlike motion, with the drag forces exerted by the fluid on the particles exceeding their weight. There is a high degree of particle mixing and equilibrium between gas and particles is rapidly established. This is called a fluidized bed.  '  air supplied by a centrifugal blower is passed through a perforated or porous plate, called the distributor, and then a bed of particles of wide size distribution. A few distributors are shown in Fig. 5.36(b). The air flow rate is regulated by a bypass valve along with a control valve, and it is measured by a rotameter. Dividing the mass flow rate, so measured, by the product of the bed cross-sectional area and density of air, the superficial velocity of air, U, is estimated. For each mas's flow rate or superficial velocity, which is gradually increased, the pressure drop across the bed is measured. Figure 5.37(a) demonstrates the variation of bed pressure drop with superficial velocity. The pressure drop /!.p varies with the superficial velocity linearly along AB till it approaches WIA"where Wis the weight ofparticles in the bed and AI is the bed cross-sectional area. This is the fixed bed regime. With further increase in air.