| At present,China is the largest producer and consumer of coal in the world.With the adjustment of energy structure,the proportion of coal in total energy consumption gradually decreases.However,due to the increase in total energy consumption in China,the absolute coal consumption is still huge.The clean,efficient,and sustainable use of coal not only concerns China's energy security and stability,but also serves as the foundation and guarantee for the construction of China's socialist ecological civilization.The development and application of clean coal technology is an inevitable choice for China's energy strategy.In order to adapt to the significant regional differences in the distribution of China's coal resources and water resources,a series of lump coal dry cleaning technologies have been effectively promoted and applied,but there has been no major breakthrough in the dry separation of the 6 mm fine coal.Studies have shown that the application of mechanical harmonic vibration to a conventional gas-solid dense medium fluidized bed can enhance gas-solid contact,inhibit bubble merger,and improve fluidization quality.Therefore,in this dissertation,the quasi-dispersion mechanism of vibrated dense medium fluidized bed(VDMFB)tending to form a uniform fluidized bed with microbubbles and the strengthening process of fine coal separation effect were studied.Firstly,the physical parameters and the grading principle of the pulverized coal and magnetite powder that make up a homogeneous binary dense medium were studied,and the migration rule and the mixing process were explored.The results showed that,under suitable conditions,the mechanical vibration was used as the dominant factor to drive the dense medium in the side wall area to rise rapidly and gradually spread to the center area after reaching the bed surface.At the same time,the dense medium particles in the middle part of the bed bottom moved upward at relatively low velocity under the driving of bubbles,and synchronously diffused in axial and radial directions.The bed impact force of the two opposite directions in the middle and upper beds was relatively small,the macroscopic directional movement of the bed was gentle,and the one-way additional force and backmixing interference to the settlement process of the separated fine coal were weak.In addition,the critical state transition characteristics of the pressure drop curve of the bed under different vibration excitations were investigated,and a critical fluidization gas velocity prediction model was established,which can provide gas velocity operation criteria for the in-depth study of fluidized bed for fine coal separation.By analyzing the power spectral density and coherence of the bed impact force and pressure signal,the density disturbance behavior of the fluidized bed was explained.The composite excitation method of simple harmonic vibration and updraft and bed response law were clarified,and the synergetic effect of vibration and airflow on improving mechanical properties of particle system was explained.The attenuation law of the wave energy in the bed was studied,and the dynamic evolution process of the emulsification phase,bubble phase and particle agglomeration was analyzed.The bubble evolution model in different regions was established,a method for calculating the gas-phase distribution ratio was proposed,and the spatial distribution and interphase interaction of the emulsification phase and the bubble phase were clarified,and the quasi-dispersion mechanism of the VDMFB revealed.It is that the competition and coordination between mechanical vibration energy and ascending gas flow formed the transmission and dissipation of vibration characteristics from bottom to top,resulting in regular oscillation of particles and quasi-periodic bulging of voidage along the axial direction.The non-uniform diffusion of vibration in the radial direction created a radial vibration shear force.Axial and radial impact forces collided against the wall of the bubble,distorting the bubble,inhibiting the bubble from merging,and causing the bubble to burst rapidly.The high-frequency fracturing of bubbles and the low-frequency merger promoted the diffusion of gas into the emulsification phase from the bubble phase,thereby weakening the bed fluctuation,improving the fluidization quality,and forming a uniform fluidization environment with microbubbles.Based on the Hilbert-Huang transform,the intrinsic mode function IMF component in the time-domain pressure signal was extracted to characterize the oscillation mode and nonlinear characteristics of the pressure signal.The energy distribution law of the frequency domain IMF under different fluidization state was studied.The identification method of homogeneous microbubble fluidized bed was established.An axial identification zone was established for the harmonic vibration excitation to effectively restrict the bed flow pattern transition.According to the fluctuation law of bed fluidization quality under different operating conditions,the regional difference of fluidization characteristics were determined,and the evaluation of vibration to improve the fluidization quality was proposed.Based on the periodic unsteady-state fluctuation characteristics of fluidized bed,the separation mechanism fine coal with similar sedimentation behavior in VDMFB were explained.The response surface methodology was used to study the influence of operational factors.The differences in the causes of the test results were clarified.A prediction model for predicting ash segregation of products based on operating parameters was established.Based on the E?0.15 g/cm~3 as the evaluation index,the suitable operating range for separating fine coal in VDMFB was determined,which provided a uniform and stable fluidization environment for efficient fine coal separation. |
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