Research On Ash Layer Characteristics And Numerical Simulation Of Particle Collection In A High-temperature Wire-cylinder Electrostatic Precipitator

Advanced clean coal technologies,such as the coal-staged conversion poly-generation system,integrated gasification combined cycle(IGCC)are important methods to solve the problem of energy and environment.High-temperature dust removal is a key issue for these technologies.Electrostatic precipitators(ESP)have a collection efficiency of greater than 0.99,wide operating ranges of flue gas volume and particle size,negligible pressure loss,reliability and ease of operation,high temperature ESP is a potential technology for high-temperature dust removal.However,reports on high temperature ESP are limited to date,and more investigations are needed.There are many problems in the test and application,such as back corona discharge and ash cake cleaning.Without solving these problems,high temperature ESP cannot meet the requirements of the advanced clean coal technologies.This present work studies the characteristics of ash layers and numerical simulation of particle collection and deposition in a high-temperature wire-cylinder electrostatic precipitator at temperatures from 350? to 700?,to provide theoretical guidance for a long-term and stable application of high temperature ESP.Firstly,the characteristics of the ash layer on the internal surface of the anode pipe of a wire-cylinder ESP were studied at temperatures ranging from 350? to 700?,including ash deposition forms,growth of ash layer and the effect of ash layer on back corona discharge.There are four typical ash deposition forms:the belt form,the slope with ribs form,the slope form and the slope with a thick bottom edge form.Ash layer thickness generally decreases with increasing height.When T ?500?,ash belts form under low port voltages,and with increasing port voltage,they will overlap each other to form ash ribs.When T? 500?,particles are deposited in the smooth slope form if the port voltage is great enough.When T?700?,a thick bottom ash edge occurs.Ash deposition forms can vary under different operating conditions.As the operating time increases,the thickness growth rate at a given point decreases,and the ash layer height increases because of the repulsive electrical force between the ash layer and the particles.Back corona discharge always occurs on the thickest portion of the ash layer first.The back corona discharge onset voltage decreases nearly linearly with increasing ash layer thickness,from 19787 V to 17197 V as the maximum thickness of the ash layer increases from 0.34 to 2.02 mm when T =500?,Up =17200 V and min = 650 mg/Nm3.To investigate the particle tracing and trapping process in a high-temperature wire-cylinder electrostatic precipitator,a 3D numerical model is developed to simulate the particle dynamic field and particle collection process.The commercial computational fluid dynamics software FLUENT is used to solve the numerical model,and the simulation results agree well with the experimental results.Increasing the port voltage(Up)and residence time is an effective means to improve collection efficiency.For example,a residence time of over 7 s can always result in a collection efficiency of over 0.9.The collection efficiency for particles larger than 6 ?m is nearly 100%when the temperature is 620? and the port voltage is 17310 V.The collection efficiency for submicron particles is only about 40%.The height of the cylindrical precipitator should be large when many small particles are present.Multistage ESP technology should be adopted to achieve high collection efficiency for micron-and submicron-sized particles.A scale-up method is also proposed.The collection efficiency for 1 ?m particles increases from 0.44 to 0.69 and then to 0.82 when the number of stages increases from one to two and then to three,respectively.The initial position of particles can much affect particle trajectory,e.g.particles of 10 ?m diameter will deposit at a low height in a comparatively short time if they enter the ESP nearby the anode tube.Ash layer thickness decreases uniformly with increasing height,and most ash particles(>80%)are collected on the lower half of the ESP.Using a multistage ESP with a short pre-stage ESP is a probable means of prolonging the cleaning period and reducing the ash-cleaning workload.