| In recent years,with the rapid development of our economy,fine particle pollution is increasingly serious.Coal-fired power plants are the main source of fine particles.However,conventional dust removal technologies have low removal efficiency of fine particle.Therefore,it is considered to use multi-field synergy to achieve efficient removal of fine particles.The most important part of multi-field synergy is to find proper single field.In single field useage,electric field has a significant influence on particulate matter.Direct currect(DC)electric field can reduce large particles by electrostatic precipitation.Pulsed electric field can reduce small particles by electric agglomeration.Acoustic agglomeration can make fine particles oscillating and colliding to large partciels.Since the sole effect of these two fields has been widely studied,and the combination of acoustic field and electric field is currently not reported.Therefore,this dissertation mainly foucs on experimental and theoretical research on agglomeration and revomal of fine partciels under coupling effect of acoustic field and electric field.Firstly,the full particle size distribution of coal-fired fly ash is detected by DP-02 Lazer Particle Size Analyzer.The agglomeration effect of fine particles is characterized by the volumn fraction of particulate matters before and after.The results indicate that under single field effect,the increase of median diameter is most obvious with acoustic field,followed by pulsed electric field,and DC electric field has little agglomeration effect on fine particles.The effect of multi-field synergy is better than the sole effect of single field,and the enhancement of combination between acoustic field and pulsed electric field is better than that of acoustic field combined with DC electric field.Secondly,the efficient agglomeration and removal of fine particles is studied in a wire-duct reactor under the effect of acoustic filed coupled with DC electric field and acoustic field coupled with pulsed electric field,repectively.Penetration efficiency is utilized to characterize the agglomeration and removal effect.The results show that acoustic wave cannot change the discharge characteristics of electric field.Under single field effect,DC electric field corresponds to the lowest penetration efficiency of fine particles,followed by acoustic field,and pulsed electric field is the worst.Under multi-field synergy effect,the fine particle penetration efficiency is further reduced when acoustic field is combined with either DC electric field or pulsed electric field.When acoustic wave is applied in DC electric field,the effectiveness of sound on the improvenment of particle removal diminished as the voltage increases.The influence of sound pressure level(SPL)and acoustic frequency becomes more complicated in the combined fields.There exists an optimal frequency and SPL for a given discharge voltage,and the optimal frequency and SPL will change with the voltage changes.When acoustic field combining with pulsed electric field,the greater the SPL,the larger the pulsed voltage,the higher the pulsed frequency,the lower the penetration efficiency.In this situation,the optimal acoustic frequency is consistent with that of acoustic agglomeration soly.Lower acoustic frequency makes better improvement on multi-field synergy.Furthermore,the optimal frequency do not change with the change of pulsed voltage and pulsed frequency.Spray droplets can enhance the agglomeration and removal effect of both acoustic field and electric field,while appropriate droplet size and droplet concentration should be selected in order to achieve the best enhancement.The addition of surfactants in droplets can improve the enhancement effect.The charge captured by fine particles increases when acoustic field is applied in electric field,and the greater the SPL,the smaller the acoustic frequency,the larger the charge.Then,analyzing the four main forces acted on the fine particles in the coupling effect of acoustic field and electric field enhanced by spray droplets,i.e.acoustic driven force,electric force,Coulomb force,liquid bridge force.The theoretical results show that under multi-field synergy,the fine particle trajectory is more complicated,and the collision efficiency and agglomeration volume becomes larger.Based on the theoretical studies,an approximate model for the agglomeration kernel of bipolarly charged particles in external acoustic and electric fields is established.The total agglomeration kernel is devided into three parts,namely Coulomb agglomeration kernel,acoustic agglomeration kernel,electric agglomeration kernel.Parameters such as electric field intensity,SPL and acoustic frequency are studied to investigate the influence on agglomeration kernel.Finally,the efficient agglomeration and removal of fine particles is studied in a wire-plate reactor under the effect of acoustic filed simultaneously coupled with both DC electric field and pulsed electric field.With negative DC corona discharge as pre-collection in the front and pulsed corona discharge as electric agglomeration in the middle and negative DC corona discharge as collection in the behind,the lowest penetration efficiency is 1.7%combining both acoustic field and spray droplets.Based on the experimental study,a highly efficient agglomeration and removal system of fine particles depending on electrostatic effect is designed.The system consists of four aeras namely negative DC corona discharge pre-collection aera,bipolar charged agglomeration and collection aera,pulsed corona discharge electric agglomeration aera,negative DC corona discharge collection aera.It mainly includes three systems like electric field system,acoustic field system,and spray droplet system.It can achieve a high removal efficiency of both total mass concentration of particulate matter and number concentration of fine particle.The system is easy for upgrading in existing electrostatic precipitator equipment with a wide application prospect. |
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