Mechanism And Application For Simultaneous Removal Of PM And SOx From Wet Flue Gas Enhanced By External Fields

Improving the air quality is not only an urgent desire for the whole nation,but also is an inherent requirement for the implementation of the new development concept.Air pollutants generated during the energy utilization process are considered to be important for air pollution.Among them,coal is the most important component in China's primary energy consumption structure.Removal of pollutants from coal-fired power plant is the key to eliminate pollutant emission since it burns most of the coal.This paper focuses on the key process of efficient and simultaneous removal of PM,SOx,and other pollutants from coal-fired flue gas.The simultaneous removal mechanism enhanced by external fields was investigated with the experimental and theoretical methodsFirst of all,the growth of sulfuric acid mist under the effects of collision,agglomeration,and condensation was investigated since the diameter of sulfuric acid mist is significantly small and the number concentration is high.After revealing the disctribution of sulfuric acid mist in the growth tube,the effects of gas temperature,humidity,residence time and discharge parameters on the evolution of sulfuric acid mist were studied.The acid mist growth can be promoted by condensation effects when reducing gas temperature or adding water vapor.Under the condition of an external electric field,the charge promoted the collision and agglomeration between acid mists and caused an increase of the acid mist concentration near 0.230?m.However,when the applied electric field strength was too high,the trapping effect became more prominent,resulting in the decreasing acid mist concentration of all particle sizes with the increase of the voltage.The reduction of acid mist reached a maximum value(47.5%)when the applied voltage was 8 kV.Secondly,based on the different characteristics of particles and sulfuric acid mists,a multi-polluted simultaneous removal experimental system was designed and constructed.The enhanced mechanism of migration and trapping mechanism of the two were studied under multi-fields.It is found that the electric field strength and ion density are two key factors affecting particle removal,and reducing gas temperature is effective to improve the fine particle removal efficiency.When the flue gas temperature decreased from 45.4 ? to 35.1 ?,the median size(Dso)of sulfuric acid mist increased by 27.5%.On this basis,a method of enhancing the charging of fine particles by using pre-charger was proposed.With the assistance of the pre-charger,the particle charge amount was more than doubled and the effective migration velocity of PM0.1,PM1.0,and PM2.5 increased by 69.9%,65.7%and 34.2%,respectively.When the pre-charger was used for removal of sulfuric acid mist,the corona suppression was minimized.With assistance of the pre-charger,the removal efficiency of sulfuric acid mist can be as high as 95.8%,and the corresponding emission concentration was lower than 2 mg/m3.Moreover,the emission concentration of sulfuric acid mist could be lower than 1 mg/m3 with application of a heat exchanger.Thirdly,to realize simultaneous removal of SO2,the mechanism of SO2 migration and conversion under corona discharge conditions were studied,and the influence of corona discharge on SO2 mass transfer absorption was revealed.When the SO2 concentration was 77 mg/m3,the enhanced absorption effect by corona discharge can be as high as 13.8%.The conversion amount of SO2 by oxidation effects is negligible,while the number concentration of sulfuric acid mist formed during corona discharge can amount to 8×106/cm3.Fractional aerosol size distribution indicated that mist penetration ratio presented a U-shaped curve.Moreover,the correlation between mist formation and corona discharge was analyzed.The formation of sulfuric acid mist could only be detected when the specific energy density exceeded a threshold value.This threshold specific energy density increased with the increasing gas temperature,and it increased from 16.7 J/m3 to 154.4 J/m3 when the gas temperature increased from 28.7 ? to 54.6 ?.Fourthly,the experimental results showed the complexity of the removal of particle and sulfuric acid mist in electrostatic fields.The coupling mechanism among discharge,charging,flow,and migration during the removal of fine particles and sulfuric acid mist was investigated.With the particle concentration increasing from 0 to 200 mg/m3,the electric field strength on the electrode surface was inhibited from 1.32×106 V/m to 1.24×106 V/m,while the electric field strength on the plate surface was raised from 7.1×105 V/m to 8.3×105 V/m.By contrast,the ion density within the whole domain decreased when particle space charge considered,and it decreased by more than 40%when the particle concentration was 50 mg/m3.The correlation between collection efficiency and corona current was discussed.A criterion for efficiency deterioration,under corona suppression conditions,was first proposed for instructing electrostatic precipitator(ESP)design and operation.Finally,based on the experimental and theoretical results,two pilot-scale experimental systems,including a particle removal system and a SO3 removal system were designed and constructed.The removal of PM and SO3 were studied under pilot parameters similar to actual conditions.The data was provided for the selection of key parameters in the design of the wet electrostatic precipitator(WESP).The combined wet electrostatic precipitator was developed and has been successfully applied in a 220 t/h thermal power plant and a 1000 MW coal-fired power plant.Continuous monitoring results showed that the PM emission level was significantly reduced to below 1 mg/m3,which provided foundation for the further promotion and application of the combined WESP.