| With the development of the national economy,the people's consumption level and the demand for a better life are constantly increasing,which rises the energy consumption of production and development.The energy production and consumption structure of China are mainly coal-fired,which leads to serious air pollution problems.Among them,the coal-fired power plants in China is the main sources of air pollutants.Controlling the emission process of pollutants become an important measure for smog and haze controlling.In light of this,China promulgated the most stringent flue gas emission standard,"Emission Standard of Air Pollutants for Thermal Power Plants," in 2011,and the mid-eastern China,where the thermal power plants are dense and the concentration of pollutants is high,was requested to meet the ultra-low emissions.Wet electrostatic precipitation is an effective way to realize the deep removal of multi-pollutants,which is an important technology to realize the ultra-low emission for the coal-fired power plants.However,the high sulfur-content coal was used in some power plants.The part of SO2 would be oxidized to SO3 and flow into the wet electrostatic precipitator,which has a serious impact on the particle removal process.In this paper,the effect of particle space charge on corona discharge was studied.Then,the characteristics of current density distribution on the collecting plate for the different discharge electrode configuration were studied based on the current distribution test platform,which could provide a theoretical basis for the optimization of electrode configuration.Finally,based on the pilot-scale wet ESP,the SO3 removal under different working conditions and with different electrode configurations were investigated.First of all,the effect of particle space charge on corona discharge was studied based on the numerical simulation.The effect of different particle size and different particle concentration on the electric field parameters and the particle transport behavior were analyzed and compared.The results show that when the particle concentration is 100mg/m3,the particle size decreases from 5?m to 0.1 ?m,the field strength decreases within the ionization zone near the electrode,while the electric field intensity near the collecting plate increases.The area of high ionic charge density decreases more than 70%and the local peak density decreases nearly 80%.With the increase of the voltage,the proportion of the particle space charge decreases,but the slope become small with the decrease of particle size.With the increase of particle concentration,the proportion of the particle space charge rises,but as the particle become large,the increase rate is significantly reduced.At the same time,the ionic wind in the electric field become weak with the decrease of the particle size due to the influence of the particle space charge.In addition,the migration velocity of submicron particle in electric field decreases by 47.4%with considering the particle space charge effect.Finally,based on the simulation results and previous studies,the modified current equation of Cooperman's model was provided with considering the particle space charge,and the fitting correlation coefficient reached 0.996,which met the expected requirements.Additionally,the discharge characteristics of 23 kinds of electrodes and their corresponding current density distribution characteristics were studied based on the current distribution test platform.The results show that the zigzag-disk electrode have the highest discharge intensity because the number of tips is the largest and the uniform distribution.In the corresponding axial current density distribution,the current density peaked in the corresponding region of the tip.However,the local current density is 0 in the rod segment between two tips.In terms of the electrode with needles,as the increases of needle's length and decreases of the needle's spacing,the discharge intensity increases.However,the profile of the circumferential current density becomes narrow as the increase of the needle length,which tends to be nonuniform.The reduction of needle's spacing increased the total number of needles,and the local current density provided by each needle was reduced,which reduced the utilization of the needle.At the same time,the change of tip angle has little effect on the discharge characteristics,but the corresponding circumferential current density changes significantly.The distribution profile gradually become wide and the peak value decreases while the uniformity increases.In addition,the characteristics of discharge and the current density distribution of the four kinds of the electrodes in SO3 were compared.With the increase of SO3 concentration and the flow rate,the discharge intensity is weakened.The corresponding axial current density is consistent with the volt-ampere characteristic,but the distribution shows an increasing tendency of sharply first and then gently.Finally,the SO3 removal characteristics and particle size distribution under different conditions were investigated based on the wet electrostatic precipitator pilot-scale test platform.Compared with the fly ash,the SO3 concentration increased from 0 to 179.6mg/m3,the current decreased from 7.08mA to 0.96mA,while the corresponding current of fly ash is only reduced to 6.91mA when the concentration reached 414.29mg/m3.It is easy for SO3 to form sulfuric acid aerosols that with small particle size and high concentration in high humidity environment.The concentration of SO3 reaches 4.49×107cm-3 at 179.6mg/m3 and the median diameter is 0.149?m.At the same time,when the humidity increases from 75%to 100%,the corresponding corona current further decreases with the particle number concentration rising.In addition,the SO3 removal efficiency is only 34.33%when the gas flow rate reaches 4.8m/s,which shows the effect of flue gas flow rate on the removal efficiency of SO3 is particularly remarkable.When the gas flow rate is 2.4 m/s and choosing the best electrode configuration,the maximum SO3 removal efficiency could reach 90.13%. |
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