Hydrodynamics Studies of Air Dense Medium Fluidized Bed with Binary Mixtures for Dry Coal Beneficiation
Abstract
A systematic and comprehensive study of fluidization hydrodynamics and separation properties was conducted in a bench-scale and a semi-industrial Air Dense Medium Fluidized Bed (ADMFB) systems for dry coal beneficiation. In order to achieve the adjustment of fluidized bed density for efficient dry gravity separation, various types of binary mixtures of solid particles were tested and used as the medium materials in ADMFB. Fluidization hydrodynamics including minimum fluidization velocity, fluidized bed expansion, solids mixing/segregation, and bed density distribution were carefully investigated. A series of continuous experiments of raw coal dry beneficiation were successfully implemented in a semi-industrial ADMFB system with binary mixtures magnetite and fine coal particles.
Minimum fluidization velocity of binary mixtures of solid particles was experimentally studied considering the effects of particle size ratio, particle density ratio, and mixture composition of solid materials. A new correlation has been developed for the accurate prediction of minimum fluidization velocity of binary mixtures used in ADMFB or other similar fluidized bed systems. Besides, an attempt was made to study the effects of bed inventory on the incipient fluidization, and the correlation proposed by Wen and Yu was modified to precisely predict the minimum fluidization velocity as a function of bed inventory. Combining of these two correlations would improve the accuracy of the estimation for various binary systems.
Fluidized bed expansion behavior was carefully investigated in terms of the two-phase theory which predicts the distribution of gas flow in bubbling fluidized beds. Since the original two-phase theory was verified to overestimate the bubble flowrate in most cases, a correction factor (Y) was introduced for the modification. The expansions of fluidized beds containing single and binary mixtures of solid particles were inspected to reveal the influences of particle properties and operating conditions on the correction factor (Y). The contribution to estimate the parameter Y for Geldart Group B and D particles was formulated based on the available experimental data in literature and the present work.
Particle mixing and segregation behavior of ADMFB with binary mixtures were investigated to achieve a relatively uniform gas-solid suspension for efficient coal beneficiation. The effects of operating parameters on the mixing and segregation pattern were examined, including particle properties, mixture composition, superficial gas velocity, and fluidized bed height. Moreover, a mixing index was employed to evaluate the mixing and segregation performance for identifying the appropriate conditions for ADMFB operations.
The distribution of bed density in an ADMFB with Geldart Group B and D particles was studied both theoretically and experimentally. A new correlation based on the modified two-phase theory was derived to predict the distribution of fluidized bed density, considering particle properties and fluidization characteristics. An examination of the bed density distribution for fluidizing single and binary mixtures of Geldart Group B and/or D particles at various operating conditions has been made to validate the proposed correlation with a good agreement.
The performance of dry coal beneficiation in a semi-industrial ADMFB with binary mixtures was evaluated by the variations of ash content and calorific value, considering the effects of feed coal size, operating gas velocity, and mixture composition of solid particles. These continuous operations of coal beneficiation are used to validate the ADMFB operation using binary mixtures of solid particles as medium materials.