Study On Mechanism Of Heat And Moisture Transfer On Electrical Field And Fine Particles

To meet the strict emission standards,wet electrostatic precipitators(ESPs)are widely used.There is a uniform flowing water film over the collecting electrode in wet ESPs,deposited particles were washed by flowing water constantly.Wet ESPs are used to remove particles in flue gas with a wide range of gas temperature and humidity.Some of those are used to control particles from fugitive emission sources in industry(20?200?,Unsaturated flue gas).Some of those matching with the wet flue gas desulfurization(WFGD)system are used to control fine particles,sulfuric acid aerosols,heavy metals and so on(50?80?,Saturated flue gas).The others are used as the final-stage coupled with the dry ESPs to enhance the collection efficiency of sub-micron particles(80?150 ?,Unsaturated flue gas).Water molecules escape from water film into the main flue gas with the coupled effect of temperature,humidity,velocity and electrical field.There is an un-uniform gas humidity distribution in the space between discharge and collecting electrode.The charactertics of corona discharge,particle charging,migration and deposition changed with the effect of heat and moisture transfer between wet collecting electrode and flue gas.In view of this,a lab-scale wet membrane-based ESP experimental bench was setup.Firstly,the migration of water molecules in the water film with the coupled effect of temperature,humidity,velocity and electrical field was studied experimentally.Secondly,the flue gas humidity distribution and development in the discharge space along the flue gas flow were also studied.Eventually,the charactertics of corona discharge,particles charging,migration,deposition,agglomeration with the effect of heat and moisture exchange between wet collecting electrode and flue gas were all discussed in this paper.Some main conclusions we obtained are as following:1)Based on the differences of the discharge current distribution between the wet and dry collecting electrode,an experimental study about the effect of gas humidity and water film on the discharge characterizes were conducted.The results showed than the ESP can work at a large voltage range.The differences of discharge current density between the wet and dry collecting electrode were analyzed quantitatively.Meanwhile,a numerical simulation of the process of corona discharge in the wet ESPs under different conditions was carried out using the Comsol Multiphysics software.Especially,the effect of the water film on the electrical field,ion temperature,charged particles(electrons,ions)were mainly discussed.The discharge current of the wet ESPs is 50?80%higher than the dry ESPs.The ratio of the discharge current between the wet ESPs and dry ESPs ? is in the range of(1?3)corresponding to the discharge current density distribution.The dry electrode has a more uniform discharge current distribution than the wet collecting electrode.The electrical field intensity and electron temperature in the wet ESPs is higher than the dry one and the number concentration of charged particles(electrons,ions)in the wet ESPs is about 2 times of the dry one.2)An experimental study about the weight loss of water film over collecting electrode in the wet ESPs was conducted.A water film weight loss model used in the electrical field was proposed.Besides,a particularly research on the coupled effect of electrical field,temperature field,velocity field on the weight loss charactertics of water film were all investigated.The results showed that water molecules could be polarized and the water film surface was swept directly by ion wind.The process of heat and moisture transfer between wet collecting electrode and flue gas was promoted.The process of water film evaporation at a constant speed in electrical field and the process can be inhibited by the solute dissolve in the water film.In electrical field,the crystal precipitated firstly in the surface of water film and then the evaporation process could be divided into two processes constant and decreasing.The turbulence intensity of the flue gas was promoted with the couple effect of electrical field and velocity.And water molecules have a higher kinetic energy at a higher temperature.So the evaporation rate of the water film over the collecting electrode surface is higher than that of with the couple effect of electrical field and velocity.The effect of electrical filed on the process of heat and moisture transfer between wet collecting electrode and flue gas will be more significant at lower temperature and gas velocity.3)Particle charging,deposited particle observation and size distribution were all studied in the dry and wet ESP in this part.The mechanism of particle charging,packing,and abscission in the wet membranes ESP were investigated based on the electron transfer in the deposited particles.Particularly,the effects of the water film on these processes were clarified.The results showed that the water film had a better effect on the particle charging process,especially in the diffusion dominated case for the particle size less than 0.1 ?m.The particles can be classificated separate in the electrical field.Under some positions(L=200 cm),the PM2.5 content in the deposited particles of the wet collecting electrode(80.09%)is higher than the dry one(77.32%).The water film over the collecting electrode had a better effect on the fine particles removal efficiency in the electrical field.Dust layer patterns on the dry ESP collecting electrode surface strongly correlated to the discharge current distribution.The thicker and closely packed dust layer directly occurring with regions where the discharge current is biggest.While,the dust layer patterns packed loosely on the wet ESP collecting electrode surface and it has no regular arrangement.4)In the process of heat and moisture transfer between wet collecting electrode and flue gas,the particle size distribution and removal charactertics as well as the flue gas humidity evolution were all studied in this part.Besides,the effects of water molecules on particles agglomeration were specified and a particle agglomeration model was proposed based on the particles surface morphology.Water molecule diffused into the unsaturated flue gas from the water film and be charged,then it was driven back to the water film over the collecting electrode.These two aspects contradict to each other.Eventually,a uniform distribution humidity field was formed.In this process,the number concentration of submicron particle(Di<0.1 ?m)decreased while the number concentration of micron particles(Di>0.1 ?m)increased.Results showed than the submicron particles can adhere to each other with the effect of water molecules.At low heat and moisture exchange rate,the integration between fine particles and fine particles to coarse particles are the dominant model.The aggregates size was small(Di? ?m).At high heat and moisture transfer rate,the aggregated particles size continue to grow up and form a beaded-like aggregate.Meanwhile,the integration between coarse particles cannot be ignored and the aggregates size are bigger(Di?3 ?m).