Graduate Thesis Or Dissertation
 

Experimental studies of a partially defluidized bed with immersed heat exchange tubes

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  • The object of this research study is to investigate and provide some fundamental information about bed slumping in a partially defluidized bed combustor. Due to the difficulty in making measurements and observations at high temperature, a dynamically similar cold phase fluidized bed was used to model the high temperature case. Experiments were conducted in a 0.305 m by 2.44 m cold fluidized bed which has plexiglass windows along the sides to provide adequate visualization of the fluidizing phenomena. The heat exchange tube array comprising six staggered rows of horizontal tubes 0.0125 m O.D. was placed in the bed. Zirconia sand particles of diameter d[supscript p] = 305 microns were used to simulate the mixture of 1200 micron limestone and coal particles. Gas velocities used in the experiment were varied from minimum fluidizing velocity to six times minimum fluidizing velocity. On defluidizing half of the fluidized bed, bed material from the active portion of the bed began transferring to the slumped side of the bed, this caused a ridge of solids to form and grow on the slumped surface. Correlations were then developed to describe the changing contours of the slumped bed surface. A major factor in the entrainment of bed material from the fluidized beds is the ejection of particles when bubbles break the bed surface. The initial velocities of ejected particles can be associated with the velocity of rising bubbles from potential flow theory. A simple model was developed to predict the motions and trajectories of ejected particles in the freeboard region. This model predicted the rate of bed material transfer from the active side to the defluidized section of the bed. These predictions were tested by experiment and showed good agreement. A correlation of the experimentally found particle density function for transferred material is presented. This can be used to determine the total amount of bed material transfer at any defluidization time. The study of the gas flow in the slumped portion of the bed was also conducted by measuring the pressure profiles in both sections of the bed. The amount of gas bypass through the slumped bed was determined by the Ergun equation and these pressure profiles. The results indicate that the percentage of the gas bypass through the slumped section does not depend on the superficial velocity. Necessary conditions required to completely refluidize the slumped section of the bed after a long period of defluidization were obtained from the pressure-time trace study. It was found that complete refluidization of the slumped section could not be achieved by using superficial refluidizing velocities of less than two times minimum fluidizing velocity. Study of the lateral mixing of solids in a wide shallow fluidized bed was conducted by using the ferrite tracer technique. A simple unsteady diffusion equation was used successfully to model the dispersion of solids from the refluidized bed to the active bed. The lateral dispersion coefficient of solids was found to depend on the superficial velocity and so was the rate of the solids mixing. Finally, the results obtained from the room temperature bed were converted to those in the high temperature case by using similarity analysis. Comparison of the converted results with the available data from the high temperature bed indicated good agreement, and thus, a successful simulation.
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