Behavior of Fine Particles in a Fluidized Bed of Coarse Solids. Final Report

摘要

The residence time of carbon particles in a fluidized bed is a major factor determining combustion efficiency. Particles carried out of the system incompletely burnt in the flue gases are in the size range 100 to 400 microns; the dispersion and entrainment of these fines forms the subject of this report. Most of the work was done in equipment 0.29m diameter by 5m tall using solids between 45 and 5000 micron fluidized at velocities up to 5 m/s in beds with and without tubes. Carryover rates, the concentration of solids in the freeboard, bed density profiles and solids mixing characteristics were measured. It was found that a vertical concentration gradient of fines which exists throughout the bed depends on both the relative and absolute size of fines and coarse. It was also discovered that fines and coarse interact in the freeboard; the larger the carryover flux of fines the greater the height to which coarse particles are carried. The bed density decreased with increasing distance above the distributor, and the presence of tubes in the bed modified the density profile significantly; under certain circumstances two separate fluidized beds were formed. Correlations are presented which allow carryover rates and particle residence times to be calculated as a function of gas and particle properties. (ERA citation 07:003950)