Effecting Mechanism Of Scraper Behavior On Stabilization And Separation Of An Air Dense Medium Fluidized Bed

In 2017,the proportion of coal in China's energy consumption was 60%.Associated minerals from raw coal have caused high energy consumption and environmental pollution in their production and utilization.Therefore,it is urgent to raise the washed ratio of raw coal.However,2/3 of China's coal recoverable reserves are in drought and scarce water areas,which limits the application of wet separation technology.Air dense fluidized bed is an efficient dry coal beneficiation technology,which meets requirements of coal ash reduction and quality improvement.The air dense fluidized bed is a very complicated gas-solid two-phase multi-scale material separation system.It is not only necessary to form a fluidized bed with a stable density,separation of incoming materials,but also internal scraper conveyors are required to eliminate products in time.So far,the effect of internal component disturbances on the stability and separation of fluidized beds has been almost blank.Therefore,it is very important for the system to study the action of the scraper on the stability and sorting mechanism of the air dense fluidized bed.In this paper,a continuous air heavy medium fluidized bed is designed and manufactured based on industrial fluidized bed structure parameters,strength check of scraper is also conducted.Through the combination of experiment and numerical calculation,particle stress is analyzed to study effects of scraper behavior on bed pressure drop,initial fluidization gas velocity,bed density distribution and expansion rate,and to reveal the sidewall effect of dynamic scraper and its effect on gas phase motion.Meanwhile,separation experiments of tracer particles are carried out in a continuous air dense fluidized bed to clarify the influence of scraper speed on separation results.With reference to structural parameters of an industrial air dense medium fluidized bed,a laboratory-scale continuous fluidized bed was designed.Mechanical calculation results showed that the chain tension in fluidized bed was1700N,and strength check results showed that the maximum bending moment was far less than allowable bending strength,which provides a theoretical basis for the stability of continuous fluidized and sorting experiments.Comparison between forces of particles in different scraper states,acceleration equations of different density coal particles moving in fluidized bed are proposed,and the influence of particle residence time in fluidized bed on the movement path and sorting results is revealed.Large Geldart B magnetite powder was used as the fluidizing medium and the Syamlal-O'Brien model was used as the drag equation.Particle restitution coefficient was set as 0.9.0.2 mm was used as the dominant particle size,and Euler-Euler multi-phase flow model was used in simulation.The result indicated that fluid bed stability was affected by promoting large bubble generation by increasing scraper spacing in a fluidized bed.For the distribution of pressure and density,simulation results demonstrate that bed pressure drop is about 4600 Pa when scraper speed is 0.It is significantly higher than 4400 Pa after scraper begins to move.The density distribution of fluidized bed is about 2000 kg/m³.By comparing experimental results with simulation results,numerical errors are within 3%¹1%,and numerical simulation results are in well agreement with experimental measurement values.The lateral movement of scraper inside the fluidized bed can improve fluidized bed fluidization characteristics and stability of the fluidized bed.The particle dynamics study represents that when scraper is static,medium solids increase in the axial center of the fluidized bed,and the movement on both sides decrease by degrees.On a local area,velocity fluctuates between positive and negative values.With the increase of scraper spacing,particle velocity fluctuates greatly.The lateral motion of the scraper can improve axial velocity of particle,but with the increase of scraper spacing,change of axial velocity of particle is not manifest.On both sides of the fluidized bed,the lateral velocity is mostly less than 0.1 m/s,and the lateral velocity of particles is small.The particles are dominated by axial motion.In the central region of fluidized bed,the lateral velocity of particles increases significantly in the axially upward direction.On the surface of the fluidized bed,the lateral velocity of the bed is as high as 0.35 m/s,and the direction of particle velocity below 0.15 m is opposite to the direction of particle velocity above0.15 m.With the increase of the scraper spacing,the velocity of particle motion is weakened.Experimental studies of fluidized characteristics of continuous air dense fluidized bed show that sliding friction generated by moving scrapers pushes medium moving,and results in a velocity gradient between different bed high media,which causes bed pressure drop lower than when the scraper is at rest,and the"steep peak"of pressure drop curve slows down.The impact of the single scraper on the gas fluidization effect is relatively small,and incipient fluidizing velocity increases with the increase of the scraper spacing.However,the change in the rotational speed of the scraper does not have a significant effect on the incipient fluidizing velocities.The relationship between bed expansion rate and scraper spacing and rotational speed is roughly opposite to the incipient fluidizing velocities.The compression of the media between the moving squeegee and the sidewall causes the pressure drop and the initial fluidizing gas velocity to increase,which deteriorates the fluidization effect to some extent.The high-speed dynamic analysis shows that under combined conditions of higher gas velocity and scraper speed,the gas appears obvious turbulent behavior inside the bed,which promotes a reasonable density gradient in vertical direction of fluidized bed.Bed density stability shows that bed density decreases with the increase of the squeegee speed,but its size changes little.The bed density is slightly worse than that of scraper speed exceeding 0.354 m/s.Separation experiments of tracer coal particles in continuous air dense fluidized bed at different scraper speeds and airflow rates were carried out,and sorting effect was evaluated by selecting possible deviation and ash removal rate of tail coal.Experimental results show that separation density and ash removal rate of the tail coal both show the trend of increasing first and then decreasing with the increase of scraper speed,and the sorting deviation decreases first and then increases.When the fluidizing gas velocity is14 m/s,the scraper spacing is 50 mm and the scraper speed is 0.3 m/s,fluidized bed has the best separation effect on tracer coals,with the E value of 0.055g/cm^-3.The optimal scraper movement speed determined in studies of the generation mechanism of air bubble and fluidization characteristics of the air dense medium fluidized bed was consistent with the scraper speed.