Study On Promoting The Removal Of Fine Particles And SO₃ In Coal-Fired Flue Gas By The Coupling Of Turbulent And Chemical Agglomeration

The fine particles and SO₃ in the flue gas from coal-fired power plants are harmful to the atmospheric environment and human health.As the environmental policies and standards become more and more stringent,it is still an important issue to study how to effectively and synergistically remove the fine particles and SO₃ in the flue gas from coal-fired power plants.For the fine particles in coal-fired flue gas,due to the limit of the dust removal mechanism of traditional electrostatic precipitator(ESP),the collection efficiency of fine particles with small sizes is relatively low.For the SO₃ in coal-fired flue gas,as there is no special equipment for its removal,it mainly relies on other pollutant treatment equipment for synergistic removal,resulting in low removal efficiency of SO₃,and a large amount of SO₃ escape into the atmosphere.Therefore,the coupling of turbulent and chemical agglomeration technology was proposed to promote the efficient and synergistic removal of fine particles and SO₃ in coal-fired flue gas,and it was systematically studied by means of experiment and numerical simulation.In this paper,the agglomeration and removal of fine particles in coal-fired flue gas by single turbulent agglomeration were investigated firstly.The effects of vortex size,dimension and coupling mode of vortexes with different scales on the agglomeration and removal of fine particles were systematically studied.The interaction mechanism between the structure of turbulent agglomerator,the turbulent flow field property,the flue gas parameter and the agglomeration effect of fine particles was also explored.Furthermore,the turbulent agglomeration model of fine particles was proposed.It was found that the agglomeration of fine particles in the turbulent flow field consisted of two agglomeration modes:the collision agglomeration of fine particles in the backflow area and the capture agglomeration of fine particles by coarse particles in the main flow area.The small-scale and three-dimensional vortexes in the turbulent flow field can enhance the collision and agglomeration between coarse and fine particles in the flue gas,so as to enhance the agglomeration and removal effect of them,which can make the agglomeration efficiency of fine particles increased to 26.3%,and the number and mass concentrations of fine particles after ESP were decreased by 34.8%and 38.2%respectively.The coupling of vortexes with different scales can enhance the collision agglomeration between fine particles and the capture agglomeration between coarse and fine particles,extend the residence time of particles in turbulent flow field,and enlarge the fluctuation range of coarse particles in the direction perpendicular to the flue gas flow,so as to improve the contact and collision probability between particles with different sizes.It can make the agglomeration efficiency of fine particles increase to 31.3%,and reduce the number and mass concentrations of fine particles after ESP by 40.9%and 44.2%respectively.In addition,the concentration of fine particles,the temperature of flue gas and the velocity of flue gas all had influence on the final agglomeration effect.Based on the research about single turbulent agglomeration of fine particles,the agglomeration and removal effect of fine particles under the coupling effect of turbulent and chemical agglomeration were further studied.The effect of coupling mode of the two agglomeration technologies,size and dimension of vortex in the turbulent flow field,coupling of vortexes with different scales and the flue gas parameters on the agglomeration and removal of fine particles were investigated.The coupling agglomeration mechanism of fine particles and the effect mechanism of turbulent flow field characteristics on the coupling agglomeration of fine particles were also explored.The results showed that the chemical-turbulent agglomeration was better than single turbulent agglomeration method for the agglomeration and removal of fine particles,and the promotion effect of chemical-turbulent agglomeration mode was better than turbulent-chemical agglomeration mode.At the same time,in the process of chemical-turbulent coupling agglomeration,the vortexes with large-scale and two-dimensional structure in the turbulent flow field can promote the relative motion between chemical droplets and fine particles,expand the capture area of chemical droplets to fine particles,and increase the collision probability between the chemical droplets and fine particles,so as to improve the agglomeration and removal effect of fine particles,which can make the agglomeration efficiency of fine particles increased to 45.8%,and the number and mass concentrations of fine particles after ESP were decreased by 53.3%and 60.1%respectively.On the other side,the coupling of vortexes with different scales can make the turbulent flow field more chaotic,enhance the relative motion of particles with different sizes and chemical droplets,and prolong the residence time of particles in turbulent flow field,so as to further enhance the agglomeration and removal effect of fine particles.When the turbulent agglomerator containing different-scale vortexes was installed in the coupling agglomeration system,the agglomeration efficiency of fine particles was 49.5%,and the number and mass concentrations after ESP can be reduced by 59.5%and 64.2%respectively compared with that without agglomeration pretreatment,which were better than turbulent agglomerators containing single large-scale or small-scale vortexes.In addition,with the increase of fine particle concentration,the agglomeration and removal efficiencies of them decreased gradually;with the increase of flue gas temperature and flow rate of chemical solution,the agglomeration and removal efficiencies of fine particles firstly increased and then decreased;with the increase of flue gas velocity,the agglomeration and removal efficiencies of fine particles gradually increased.Based on the research about chemical-turbulent coupling agglomeration of fine particles,it was innovatively proposed to combine the particle turbulent agglomeration technology with the spray evaporation technology of desulphurization wastewater to removal fine particles efficiently and achieve zero discharge of desulphurization wastewater simultaneously.The characteristics of evaporation products from desulphurization wastewater,the effect of spray evaporation of desulphurization wastewater on the agglomeration and removal of fine particles,and the effect of turbulent flow field on the evaporation of wastewater droplets were studied experimentally and theoretically.The results showed that a small amount of prismatic crystal particles less than 1.0?m were precipitated from desulphurization wastewater by evaporation,and the agglomeration and removal of fine particles can be effectively promoted.After the spray coupling turbulent agglomeration system,the agglomeration efficiency of fine particles increased to 39.3%,which was10.9%higher than that of single turbulent agglomeration,and the number and mass concentrations of fine particles after ESP decreased by 46.5%and 38.9%respectively.Besides,the turbulent flow field generated by the turbulent agglomerator can increase the relative velocity between the wastewater droplets and flue gas,improve the heat and mass transfer rate of the wastewater droplets during the evaporation process,thus promoting the evaporation of desulphurization wastewater.For the droplets with diameter of 110?m and 120?m,the distances required for full evaporation can be reduced by 383 mm and 543 mm respectively.Finally,a low-low temperature electrostatic precipitator(LLT-ESP)experimental system was applied to systematically study the condensation and removal characteristics of SO₃ in the process of LLT-ESP.Based on this,the turbulent agglomeration technology and the cooling water spray coupled with turbulent agglomeration technology were proposed to further promote the synergistic removal of SO₃ and fine particles.In addition,an engineering experiment was carried out in a 660 MW coal-fired unit to study the coupling of turbulent agglomeration with LLT-ESP.It was found that in the process of LLT-ESP,the heterogeneous and homogeneous condensation of gaseous H₂SO₄ occurred simultaneously,and they tended to condense on fine particles with diameter less than 0.1?m.Besides,the concentration of fly ash in flue gas and flue gas temperature had great influence on the removal efficiency of SO₃.The arrangement of turbulent agglomerator behind the heat exchanger can promote the deposition of sulfuric acid droplets on fine particles,lead to the agglomeration and growth of fine particles,so as to promote the synergistic removal of SO₃ and fine particles.When the turbulent agglomerator was arranged in the experimental system,the removal efficiency of SO₃ increased from69.7%to 82.9%and the mass removal efficiency of fine particles increased from 90.1%to 97.6%.In the process of cooling water spray coupled with turbulent agglomeration,the cooling water spray can also make gaseous H₂SO₄ condense,and the subsequent turbulent agglomeration system can promote the collision and agglomeration of sulfuric acid droplets and fine particles,thus effectively promoting the synergistic removal of SO₃ and fine particles.When the cooling water spray system and turbulent agglomeration system were arranged in front of ESP,the removal efficiency of SO₃ increased from3.5%to 84.5%,and the mass removal efficiency of fine particles increased from 85.4%to 98.2%.The LLT-ESP system coupled with turbulent agglomeration device installed in a 660 MW coal-fired unit had good dust removal effect and can effectively remove fine particles in flue gas.It can not only meet the technical requirement of"PM_2.5 emission at the outlet of electrostatic precipitator should be less than 10 mg/Nm³",but also make the total dust concentration at the outlet of electrostatic precipitator was lower than 10 mg/Nm³.