Study On Stress Distribution And Flowability During Hopper Discharge

Pulverized coal gasification technology has received extensive attention due to its cleaning and high conversion efficiency.Dense-phase pneumatic conveying of pulverized coal at high pressure is the key technology in coal gasification.However,the powder in the hopper is easy to agglomeration,arching and stagnation during discharge,which result in the flow unstability of the powder.Discharge unstability of hopper seriously affects the safety and operation of gasifier.Due to the complexity of stress distribution and flowability during hopper discharge,the theory in this field has not been fully understood.In this paper,a three dimensional hopper was designed and built to study the wall normal stress distribution and flowability of the powder during hopper discharge at atmospheric pressure.The stress distribution and flowability of the powder in the hopper at the initial stage of discharge,stable discharge and unsteady discharge were obtained.The results can provide theoretical basis and data support for the development of pulverized coal gasification technology.Glass beads,corn powder and anthracite were applied to investigate the discharge characteristics in the three dimensional hopper.The results showed that the flowabilities of glass beads and anthracite were well.The mass flow rate of corn powder was the smallest and the flowability was the worst.The wall normal stress of the glass beads at the junction was greatest during discharge.The wall normal stress showed regular fluctuations.The maximum overpressure position occured at the junction.The closer to the outlet,the smaller the overpressure coefficient.The dynamic response of glass beads at the discharge start was fastest.With the decrease in particle size,the overpressure coefficient for the cylindrical section increased while decreased for the conical section.The dynamic response of smaller particles at the discharge start was faster than that of larger particles.The wall normal stress of smaller particles was in good agreement with the theoretical calculation during discharge.Increasement of the outlet diameter can significantly improve the flowability of corn powder but has weak effect on the flowabilities of glass beads and anthracite.The stress fluctuation frequency at the junction was linear with the mass flow rate.Increasement of the outlet diameter can diminish the fluctuation period,relieve the overpressure coefficient at the junction and accelerate the dynamic response speed.The cone angle had a great influence on the stress peak and overpressure coefficient at the junction.The severe fluctuation only occured in the hopper with a small cone angle.With the increase of cone angle,the mass flow rate decreased and the flowability of the powder deteriorated.With the increase of height-to-diameter ratio,the mass flow rate remained stable while the wall normal stress at the junction increased gradually and the stress fluctuation appeared.Anthracite coals with different moisture content and particle size were used to carry out the experimental research.The results showed that with the increase of moisture content,the flowability of the powder deteriorated.The fluctuation range of wall normal stress increased and the wall normal stress showed a severe fluctuation.With the increase of moisture content,the overpressure coefficient for the cylinder section decreased.While the overpressure coefficient for the conical section increased,especially in the area above the outlet.With the decrease in particle size,the flowability of the powder deteriorated.Uncontrollable dumping hehaviors appeared at the end of the discharge.The wall normal stress at the junction and the overpressure coefficient of the bottom in the cylinder section reduced.The overpressure coefficient for conical section and the upper in the cylinder section increased.Increasement of the outlet diameter,decreasement of the cone angle and height-to-diameter ratio can inhibit the generation of a stable arch,which improved flowability of viscous powders.With the increase of outlet diameter,the overpressure coefficient for the cylinder section and the outlet of the hopper increased.The size of cone angle had little effect on the overpressure coefficient for the cylinder section.With the increase of the cone angle,the flowability of the powder deteriorated and the wall normal stress increased at the junction.When the height-to-diameter ratio was low,the mass flow rate was stable.With the increase of the height-to-diameter ratio,the stable arch appeared during discharge.The wall normal stress at the junction gradually falled during disacharge.