Yang Fluidization, Solids Handling, and Processing

Fluidized Bed Scale-up

85

Figure 41. Tube configuration used for bed expansion studies. Note: all dimensions in cm scaled to equivalent sizes in the combustor. (From Glicksman et al., 1991b.)

86 Fluidization, Solids Handling, and Processing

Figure 42. Bed expansion for TVA 20 MWe FBC and for scale model, shown in Fig. 39. (From Glicksman et al., 1989.)

Figure 43. Particle accumulation in slumped zone adjacent to active bed: air velocity through active region = 151 cm/s; air velocity through slumped region = 7.3 cm/s; numbers above heater are heat transfer coefficient in W/m2 °C after 15–30 min. (From Glicksman et al., 1992.)

Fluidized Bed Scale-up

91

10.0HYDRODYNAMIC SCALING OF CIRCULATING BEDS

Given the success in scaling bubbling beds, research has progressed to the scaling of circulating fast beds. The initial research has focused on the verification of both the full and the simplified scaling relationships for circulating beds. The verification using data from combustors was complicated by the difficulty in accurately measuring the recycle rate of solids, an important parameter. Table 7 presents the range of scaling parameters for experimental studies undertaken for circulating beds. Figure 48 shows the range in terms of the Froude number based on bed diameter and the Reynolds number based on particle diameter and the superficial gas velocity.

Horio et al. (1989) experimentally verified their proposed circulating fluidized bed scaling laws. The solid-to-gas density ratio was maintained constant in the tests, thus they effectively verified the simplified set of scaling laws. Two cold scaled CFBs, fluidized using ambient air, were used in the verification. Good agreement in the axial solid fraction profiles was obtained for most of the conditions tested. A “choking-like transition” was found to occur for cases with higher solids fluxes and lower gas superficial velocities. A discrepancy in the “choking” transition point for the two beds was attributed to differences in the geometry of the bed exit and the solids recycle lines. The transition point was found to be very sensitive to the particle size ratio. An optical probe was used to verify similarity in the annular flow structures and the cluster velocities.

Ishii and Murakami (1991) evaluated the CFB scaling relationships of Horio et al. (1989) using two cold CFB models. Solids flux, pressure drop, and optical probe measurements were used to measure a large number of hydrodynamic parameters to serve as the basis for the comparison. Fair to good similarity was obtained between the beds. Dependent hydrodynamic parameters such as the pressure drop and pressure fluctuation characteristics, cluster length and voidage, and the core diameter were compared between the two beds. The gas-to-solid density ratio was not varied between the beds. As seen in Table 7, the dimensionless solids flux decreased as the superficial velocity was increased because the solids flux was held constant.

92 Fluidization, Solids Handling, and Processing

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