Study On Density Regulation And Particle Segregation Enhancement Mechanism Of A Dense Phase Pulsed Gas-Solid Fluidized Bed

For a long time,coal has been the cornerstone of China’s society and rapid economic development,serving as a stabilizer and ballast for national energy security.The clean and efficient utilization of coal is a major national strategic need and an important way to achieve the goal of"carbon peaking and carbon neutrality".However,the wet coal separation technology consumes a lot of water,and it is difficult to apply and popularize in the arid and water-deficient areas in western China with abundant coal reserves.The air dense medium fluidized bed dry separation technology,which requires no water,can compensate for the shortcomings of wet separation technology and promote high-quality development of the coal industry.While coarse coal particles larger than 6 mm can be efficiently separated by dry separation,ordinary fluidized beds cannot effectively separate fine coal particles smaller than 6 mm.By introducing forced vibration of pulsating airflow into gas-solid fluidized beds,bubble size can be reduced,gas-solid contact efficiency can be increased,material channeling and short-circuiting can be minimized,bed density stability can be improved,and efficient separation of fine coal particles can be achieved.This dissertation conducts research on particle fluidization characteristics and gas-solid distribution change law,vibration-airflow synergistic strengthening particle separation theory,bed stability and fine-grained coal separation strengthening mechanism in turn,reveals the critical fluidization mechanism of heavy mass in a pulsed gas-solid fluidized bed,and explores the energy dissipation mechanism of fluidized bed.In addition,the reason of vibration energy inhibiting bubble growth is clarified,the mechanism of particle separation and mixing is clarified under the synergistic effect of vibration energy and airflow,the mechanism of strengthening the separation effect and enhancing the bed density stability in a pulsed fluidized bed is investigated,and the operating parameters of fine grain coal separation are optimized.It provides a theoretical basis for the effective separation of fine-grained coal for a pulsed fluidized bed.Based on the spring-damping model,the characterization of the packed particle state is studied.By dimensional analysis and combining with particle force analysis,the impact mechanism of vibration energy on the minimum fluidization velocity is investigated,and a theoretical model for minimum fluidization velocity applicable to pulsed gas-solid fluidized beds is derived.The model is validated by testing the minimum fluidization velocity of particles and collecting literature data from previous studies,and its predictive accuracy is compared with that of traditional minimum fluidization velocity models.The energy dissipation characteristics of particles with different properties are studied,and as particle size and density increase,energy dissipation intensifies and becomes less sensitive to external excitation energy.The probability density distribution of bubble diameter under different particles and operating conditions in a pulsed fluidized bed is studied,the effect of gas velocity and pulsation frequency on the average bubble rising velocity is explored,and a bubble rising velocity model suitable for pulsed fluidized bed is established to improve the dynamics of bubbles.The calculation provides a basis for the theory of gas-solid distribution in a pulsed fluidized bed.At the same time,the wake in the pulsed fluidized bed is listed as the single phase,and the existing state,composition ratio,and changing law of the emulsion phase,bubble phase,and wake under different operating conditions are studied,and the gas-solid distribution model of the pulsed fluidized bed is established.The bed expansion characteristics under different pulsation frequencies and gas velocities are studied,and a prediction model of bed expansion height is established,which provides a theoretical basis for the accuracy of the bed density.Based on the dry coal separation technology in a pulsed fluidized bed,the separation and mixing mechanism of binary particles are discussed.The voidage of the bed at different pulsation frequencies is analyzed,and the correlation formula of voidage of the emulsion phase under particulate fluidization is established.The mechanism of the Brazil nut and reverse Brazil nut effect is explored,proving that the change of pulsation frequency and particle properties both affected the force state of particles and then caused the variation of particle separation direction.In addition,the competing dominant mechanisms of fluid forces in the separation process are elucidated,and the particle separation phase diagram and model are proposed,which can predict the separation and mixing behavior of particles accurately after experimental verification.The separation and mixing mechanism of particles in a pulsed fluidized bed is discussed.It is found that changing the pulsation frequency can alter the voidage in a fluidized bed,which can be used to control particle mixing and segregation.Furthermore,particle characteristics and particle proportions can affect the relationship between buoyancy,drag,and gravity and change particle behavior.In addition,an improved phase diagram and a model of particle segregation in a pulsed fluidized bed are developed,which can accurately predict segregation and mixing.This work can provide a theoretical basis for controlling the mixing and segregation of particles.According to the gas-solid bubbling fluidization characteristics of the pulsed fluidized bed,combined with the modified two-phase theory of fluidization,the density distribution of the pulsed gas-solid fluidized bed is theoretically deduced,the effect of pulsating energy on the density distribution of the bed is discussed in depth,and the response mechanism of fluidized bed to pulsating energy excitation is described.The fluidized bed density calculation model reveals the changing pattern of bed density at different operating conditions.At the same time,starting from the analysis of the separation force balance of the selected materials in the gas-solid separation fluidized bed during the fluidized dry separation process,the factors affecting the separation of materials according to the bed density are deeply explored.The theoretical model of fluidized bed separation density is established based on force balance.The separation density model is calibrated through the fine coal dry separation experiment to realize an efficient and reliable fluidized dry separation control mechanism,and the value of E can reach 0.095 g/cm~3.The dissertation consists of 121 figures,14 tables,and 185 references in this thesis.