Investigation On Electrostatic Distribution Characteristics And Charge Control In Silo

In the conveying,stirring,and fluidizing processes,the powders will inevitably obtain lots of charges due to the continuous interparticle and particle-wall contact and separation.A strong electric field will be established when the charged powders are loaded into the silo,which is easy to induce the electrostatic discharge and cause fire or dust explosion,threatening the production and personnel safety.A full understanding of electrostatic distribution characteristics and the charge control method in the silo is of great significance for preventing the electrostatic hazard of the charged powder and ensuring the safe and stable operation of the silo.However,the charging characteristics of the powder in the silo are not investigated systematically,the electric field distribution characteristics and the charge control mechanism are not clear,and the anti-static strategy of the charged powder still needs to be improved.In this paper,the electrostatic distribution characteristics and the charge control of the charged powder in the silo were studied.The bipolar charging characteristics and charge migration law in the particles system were obtained,and the electric field distribution characteristics calculated by the uniform and non-uniform space charge densities of the powder in the silo and the underlying mechanism were revealed.The critical parameters of electrostatic discharge were proposed.The electrostatic elimination characteristics of the bipolar direct-current(DC)electrostatic eliminator and the optimal configuration for complete elimination of charges on powder were obtained.Moreover,the detailed mechanism of electrostatic elimination in the silo was clarified.The key research contents and results are as follows,(1)A fluidized charging experiment system was constructed,and the large and small particles were separated by the wind separation technology.The charging characteristics of large and small particles under different operating parameters and physical parameters were obtained.The impact charging characteristics of particle-particle and particle-wall contacts were explored,and the law of charge migration during bipolar charging was revealed.The results show that the polyethylene particles of narrow size are negatively charged when they are fluidized alone.When the large and small particles are mixed and charged by fluidization,the net charge on the small particles is always negative,while the net charge on the large particles can change from negative to positive with increasing the mass fraction of the small particles in the mixture,and its charge-to-mass ratio can be greater than that obtained in fluidizing particles of narrow size alone.The wall material,size ratio,and the particle initial charge-to-mass ratio all affect the net positive charge of large particles.The amount and direction of charge transfer between particles are related to the difference in particle size.The charge transfer between particles of the same size is random.When the charge is transferred between particles of different sizes,negative charges are transferred from large particles to small ones,and the large particles are positively charged.The amount of charge transfer in one collision decreases with the increase of the size ratio.The direction of charge transfer between the particle and the wall is fixed,and the particle with a smaller work function tends to be negatively charged.The amount of charge transfer in a particle-wall collision increases with increasing the difference in their work functions.(2)Based on the electrostatic field model,the electric field formed by the charged powder in the cylindrical silo was simulated,and the reliability of the simulation results was verified by the experimental data.The characteristics of electric field distribution calculated by the uniform space charge density in the silo were revealed.The mechanism of effects of the powder heap shape,filling fraction,and the repose angle on the electric field distribution and the maximum field strength was clarified.The results show that the larger electric field in the silo is located on the heap surface and the silo wall and bottom,and the three locations have a larger probability of electrostatic discharge.The electric field distribution of flat heap,conical heap,inclined heap,and reverse conical heap is different from each other,and the position of maximum field strength is closely related to the heap shape.The change of the filling fraction will alter the position of the maximum field strength in the three locations.With the increase of the filling fraction,the maximum field strength on the heap surface goes through three stages:rising,steady situation,and sudden increase.At the moderate filling fraction,the order of the electrostatic discharge probability from highest to lowest is the conical heap,inclined heap,flat heap,and reverse conical heap.The larger repose angle leads to larger maximum field strength and a higher risk of electrostatic discharge.The influence of metal protrusion on the electric field distribution in the silo under different operating parameters was explored.It is found that the position of the maximum field strength in the silo is transferred from the powder space to the protrusion,and the electrostatic discharge probability on the protrusion is the largest.The results are in good agreement with the experimental phenomenon.When the diameter of the metal protrusion and the height between the protrusion and the heap surface decrease,the maximum field strength as well as the discharge probability increase.Based on the classical theory of electrostatic induction,the mechanism of the influence of the metal protrusion on the electric field distribution in the silo was clarified.Based on the simulation results,the corresponding anti-static discharge strategy was proposed in the case of the uniform space charge density in the silo.(3)The non-uniform space charge density distribution of the charged powder in the silo during particle segregation was obtained,and the distribution characteristics of the electric field in the silo during particle segregation were revealed.The variation rule of the maximum field strength calculated based on the non-uniform space charge was clarified.The mechanism of effects of particle segregation on electrostatic discharge probability under different operating parameters and physical properties was analyzed.The results show that the amount of large particles with the smaller charge-to-mass ratio increases as the thickness fraction of the large particle layer increases,and the electric field in the silo decreases,resulting in the reduction of the electrostatic discharge probability.When the space charge density of the large particle layer is smaller than that of the small particle layer,the maximum field strength and the electrostatic discharge probability of the segregation case are higher than that of the uniform case.The enhancement or weakening effect of particle segregation on the electrostatic discharge probability increases with the increase of the difference in the space charge density of the large and small particle layers.The enhancement effect of the particle segregation on the electrostatic discharge probability in the bipolar charging condition is much higher than that in the unipolar charging condition.Based on the discrete element method(DEM),the particle segregation flow pattern during filling the silo with the binary mixture of particles was simulated.The space charge density distribution of the powder during the particle segregation was obtained in combination with the charging characteristics of the particles.The results show that there are both large and small particles in the particle layer when the particle segregation happens,which agrees well with the experimental results.In the unipolar charging condition,the electric field distribution of the segregation case is similar to that of the uniform case: the larger electric field is located at the heap surface and the silo wall and bottom,and the difference in electrostatic discharge probability of the two cases is small.In the bipolar charging condition,the space charge density of the small particle layer is always negative,and the positive space charge density of the large particle layer increases with the increase of the positive charge on the large particles.Meanwhile,the position of the larger electric field on the silo wall transfers to the junction of large and small particle layers.The electric field in the bipolar charging condition is smaller than that in the unipolar charging condition,implying a smaller electrostatic discharge probability in the bipolar charging condition.The smaller the net charge of the particles in the bipolar charging condition,the higher the enhancement effect of particle segregation on the electrostatic discharge probability.When the volume fraction of small particles is small,the enhancement effect of particle segregation on the electrostatic discharge probability is greater.As the repose angle increases,the degree of electric field distortion and the difference in space charge density in the large and small particle layers increase,resulting in an increased electrostatic discharge risk.Based on the simulation results,the corresponding strategies for reducing the electrostatic discharge probability of charged powders in the particle segregation case were proposed.(4)Based on the charge dissipation function,the non-uniform space charge density distribution in the powder space was obtained,and the electric field in the silo was simulated.The mechanism of effects of operating parameters and physical parameters on the electric field distribution in the silo was revealed.The critical time number of electrostatic discharge and the critical air gap distance of sudden increased electrostatic discharge probability in the later stage of the silo filling process were obtained,and the amplification rule of critical parameters was explored.The results show that the larger electric field in the silo occurs at the heap surface and the silo wall and bottom when the time number is small.The closer to the bottom of the silo,the more charge is dissipated.With the increase of the time number,the position of the maximum electric field migrates to the heap surface,and the electric field strength and the electrostatic discharge probability decrease.At any time number,the maximum field strength on the heap surface and the critical time number all experience three stages of sudden increasestabilization-sudden increase with the decrease of the air gap distance.The maximum field strength on the heap surface increases with the increase of the repose angle when the time number is small,while it increases first and then decreases with the increase of the repose angle when the time number is large.The critical time number at the critical air gap distance increases first and then decreases slightly with the increase of the repose angle.With the increase of the height-diameter ratio of the silo,the maximum field strength on the heap surface increases first and then decreases.The silo with a height-diameter ratio of at least 2 has a lower risk of electrostatic discharge.The differences in the critical air gap distance and critical time number of the silo with different height-diameter ratios are small.When the silo is scaled up,the maximum field strength on the heap surface,the critical air gap distance,and the critical time number are all increased in proportion to the scale.Lowering the initial space charge density of powder results in a proportional reduction in the maximum field strength and critical time number,which reduces the risk of electrostatic discharge.(5)An experimental system for electrostatic elimination of charged powders was constructed,and the laboratory test of bipolar DC electrostatic eliminator and research on electrostatic elimination characteristics in the industrial silo were carried out.The electrostatic elimination characteristics and the influence mechanism under different applied voltages,airflow velocities of the ionizer,and the powder initial charge-to-mass ratios were obtained.The results show that the corona discharge at a higher applied voltage leads to reverse charging of particles,while the elimination efficiency at the smaller applied voltage is lower.For the negatively charged powder,when the upstream ionizers of the electrostatic eliminator are applied with a strong positive voltage,the elimination efficiency decreases with the increase of the negative voltage of the downstream ionizers.The negative voltage to achieve complete elimination is much smaller than the positive voltage.The optimal voltage configuration of the bipolar DC electrostatic eliminator is insensitive to the airflow velocity of the ionizer.The difference in the reverse charging limit of the positive and negative ionizers was compared through the repeated elimination experiment of powder.The results show that increasing the airflow velocity of the ionizer has the same enhancement effect on the elimination ability of the positive and negative corona discharges.For bipolar charged powders,the optimal voltage configuration of the electrostatic eliminator has good adaptability to the powders with a net negative charge but has a poor performance on eliminating the powders with a net positive charge.The photon number characteristics of positive and negative corona discharges were tested by an ultraviolet image detector,which proved the existence of weak discharges at lower voltages,and revealed the static elimination mechanism of the bipolar DC electrostatic eliminator.In the experiment on the industrial silo,the Faraday cup and image intensifier were used to obtain the powder charge-to-mass ratio and electrostatic discharge images before and after switching on the eliminator.The results show that the bipolar DC electrostatic eliminator can reduce the charge of the charged powder effectively and the electrostatic discharge probability,completely suppressing the electrostatic discharge caused by the charged powder in the industrial silo under the optimal voltage configuration.