Doctor of Philosophy
Chemical and Biochemical Engineering
China University of Mining and Technology
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.
Summary for Lay Audience
Suspension of solid particles by an upward gas flow generally leads to a gas-solid fluidized bed, characterized by particles suspension and bed expansion, while the upward gas flow travels through the void space (voidage) between solid particles. At a relatively lower gas flowrate, the fluidized bed exhibits lower bed expansion with the appearance of gas bubbles, like boiling water. This bubbling fluidized bed which is also called Air Dense Medium Fluidized Bed (ADMFB) has similar properties like a liquid, and thus the buoyancy effect can be utilized for the dry gravity separation of particulate materials of different densities, e.g. raw coal dry beneficiation. According to Archimedes principle, the clean coal of less density than the fluidized bed will float on top of the bed, whereas the gangue of heavier density will sink to the bottom and thus can be removed from raw coal. Therefore, control of the density of fluidized bed is crucial for dry coal beneficiation by the ADMFB technology.
The density of a fluidized bed is corresponding to the mass of solid particles per unit bed volume. In order to adjust the bed density for more efficient coal beneficiation, binary mixtures of solid particles of different densities are proposed to replace the single particles in ADMFB system. Consequently, the fluidized bed density can be easily manipulated by changing the composition of the solids mixture. The objectives of this work is to study the fundamental theory and underlying mechanism of the ADMFB with binary mixtures, including the followings: (1) The minimum requirement of gas velocity for fluidization of the binary mixtures; (2) The gas distribution between the bubbles and emulsion phase; (3) Axial distribution of two types of solid particles in the fluidized bed; (4) Prediction of the fluidized bed density at different operating conditions; (5) Performance of coal beneficiation in a semi-industrial ADMFB with binary mixtures.
Fu, Zhijie, "Hydrodynamics Studies of Air Dense Medium Fluidized Bed with Binary Mixtures for Dry Coal Beneficiation" (2019). Electronic Thesis and Dissertation Repository. 6252.