| The fine particles formed by coal fired power plants in China are one of the main emission sources of atmospheric fine particulate matter, which is closely related to the large-scale and long-time's fog and haze weather in China. By using water film removing dust, the wet ESPs can avoid re-entrainment of dust effectively and have high corona current as well as the obvious effect of particle coagulation, which has distinguished technological advantage in controlling the emission of ultrafine particles. During the operation of the wet ESPs, on account of the deviation of the liquid surface tension and the processing plane degree of rigid collectors surface machines, the rigid collectors have general problems such as large amount of flushing water, uneven distribution of flushing water film, and the operation of electro-discharge stopping. Besides, the fiber fabric collectors have the problem that the tightness degree of woven fabric is instable causing the deformation and vibration of the fabric, which resulting in the change of the distance between the collectors and then causes the instability of the electric field.By analyzing the theory of liquid flow on solid surface, this paper concludes that there are two kinds of methods to improve the wetting properties of solid surface. The one is to overlay hydrophilic layer on the solid surface. The other one is to change the microstructure of the solid surface. The parameters that characterize the wetting characteristics of the solid surface should not be limited to the contact angle. As a result, in this paper, surface liquid holdup, surface saturated liquid holdup, surface flowrate, surface flowrate, the film rate, film rate in the hole, average film thickness and critical saturation time are proposed to measure the wetting characteristics of the collectors' surface of the wet ESPs based on the weight method and imaging methods. The liquid holding capacity plays an important role in cleaning the surface of the collectors of the wet ESPs. What's more, the critical saturation time determines the flushing time of the collectors.Under the condition of high humidity and high voltage of the wet ESPs, it's difficult to maintain effective for a long time using the conventional modification method. As a result, by coating the carbon steel surface with corrosion resistant epoxy resin layer and then coating other modified materials, we make hydrophilic modification on the surface. What's more, the wetting characteristics of the modified surface were studied in the aspects of surface coating materials, processing technology, weaving structure, temperature and so on. By single strand flow supply, we investigated the wetting characteristics of the circular holes and rectangular groove PMMA and FRP surfaces. We also analyzed the effects of the diameter of the circular hole, the wide and height of the rectangular groove, the thickness of the plate as well as feed water flow on wetting characteristics of the surfaces. In addition, we qualitatively discussed the liquid flow pattern of the circular holes and rectangular grooves PMMA and FRP surfaces.According to the characteristics of the plate, a test apparatus with modified rigid collectors and a test apparatus with the circular hole and rectangular groove PMMA and FRP collectors were set up for the removal of fine particles from the flue gas respectively. Analysis and researches were conducted on the performance of the collectors with film forming used for the removal of fine particles. We investigated the effect on the removal of fine particles from the water film of surface of the plate. We also studied the law of the influence of main operating parameters, such as the flue gas residence time, operating voltage, initial concentration and flushing water flow rate on the removal efficiency of fine particles. What's more, we also make a comparison between the ash deposition situation of the PMMA and the FRP with rectangular grooves grid collectors in the industrial applications. We have the main conclusion as following:(1) Established the wetting characteristic experiment table. Based on the theory of falling film on the vertical wall of the whole surface, using single strand flow supply, analyzed the wetting characteristics of the substrate, the corrosion resistance layer and the modified surface. Research findings:Epoxy resin and GFCI have the similar surface wetting characteristics to the substrate after being modified. Sand, glass beads and the GFF series have the similar wetting characteristics to each other after being modified, but their hydrophilic characteristics only get limited improvement. When the treatment process of the additional layer is changed, the modified surface of GFCII, GFCIII and GFCIV have good hydrophilicity. The weaving density and the weaving method of the glass fiber cloth have great influence on the wettability of the modified surface. If the fabric is thicker, it is more favorable for the modified surface to maintain the loose state and it is more favorable for the diffusion of the liquid film. It is not conducive to the diffusion of liquid film if the surface density of the fabric is too large or too small. If the surface density is too small, which means the fiber bundle is sparse so as not to form an effective capillary tube. If the surface density is too large, which means the fiber bundle is dense, so that it's not conducive to the diffusion of the liquid film. The glass fiber cloth with a thickness of 0.25mm and the surface density of 220g/m2 showed better wetting characteristics that the liquid holdup is high, the surface flowrate is small and the film rate is 100%. The effect of treatment temperature on the liquid diffusion power of the modified surface is different. When the treatment temperature is 40-70?, the liquid diffusion process on the surface is in line with the Hill distribution. The liquid diffusion process at 80? is in line with the DoseResp distribution. The liquid diffusion process at 90? is in line with the Exponential distribution. On the surface of 50?, the main driving force of liquid diffusion may be capillary force. As for the surface of 90?, the initial liquid diffusion may be the main effect of gravity. As for the other surfaces, these two forces promote and compete with each other. Finally, the film formation rate of the modified surface is 100%, and the wetting property is all better than that of the substrate carbon steel.(2) Improved the wetting characteristic experiment table. Using single strand flow supply, analyzed the wetting characteristics of the PMMA and FRP surface with holes and grooves. Research findings:the PMMA surface is light, but the wetting property is slightly worse. However, the water film on the surface of FRP is easy to expand and the wetting property is better. When the diameter of the hole in PMMA surface is 2,3,4mm, the film rate of the hole can up to 100%. Increasing the plate thickness and the water flow rate, the saturated liquid holdup of the surface, film rate and liquid stay on the surface after cutting off the water supply increased. When the size of the hole is 5 or 6 mm, the water flow rate will increase and the water film in the surface and hole will fluctuate violently, which leads to the low film rate. Film blanking of the water appears on the surface beneath the hole, which becomes water drops. With the thickness of the plate increasing, the water film in the hole is diffused to the thicker part of the plate. The water film can guide the water film on the unbroken surface to converge to the hole. Besides, the water film on the unbroken surface can guide the water film on the hole to leave the hole. This phenomenon depends on the competition between the surface tension of the water film and the gravity of the water film. With wide side increasing, the surface saturated liquid holdup increases and the rate of film increases to the PMMA surfaces with rectangular groove. The water film rate in the hole also increases. It is much easier to realize the integrity of the water film when the width of the rectangular groove is 10mm. When the width of the rectangular groove is 20mm and the height is 2mm, the integrity of the water film can also be achieved. The integrity of the water film generally appears when the flow length is smaller and the water flow is extreme small or extreme large. This phenomenon shows that on the one hand, the fluctuation of a water film can enhance the water film's expansion along the width of the rectangular groove, on the other hand, the fluctuation of a water film can also enhance the water film's expansion to the thicker part of the plate. These two aspects complement each other and promote each other. With the wide side increasing, the saturated liquid holdup of the surface decreases, the water film rate decreases and the film water in the holes decreases to the FRP with the rectangular groove. When the thickness of the plate is 2mm and the width of the rectangular groove is 3mm, the surface has better wetting characteristics. If the thickness of the plate increases to 5mm, the surface has better wetting characteristics and the expansion of water film beneath the groove is obvious when the length of height of the rectangular groove is 4mm. The film rate in the hole of the FRP is lower than that of PMMA, which may be related to the microstructure around the groove. In the case of single stream supply, the pattern of the flow on the surface of the PMMA is mainly dominated by the pass flow and back flow exists. As for FRP, the pattern is pass flow when the upstream side is long meanwhile pass flow, around flow, bias flow, flow near the wall and confluence after around flow will appear when the height of the rectangular groove is short. The patterns of flow on the surface are more complex especially when the height of the rectangular groove is 4mm.(3) A small test table for removing fine particles from flue gas with a hydrophilic modified rigid electrode plate was builded. Research findings:When the residence time is 4s, at the same voltage level, the fine particles collection efficiency of the modified rigid plate is higher than that of the conventional rigid plate. With the applied voltage of 44kV, the modified rigid plate can achieve the collection efficiency of the conventional rigid plate with the applied voltage of 60kV. It is due to the fact that the use of a modified rigid plate can produce a larger corona current and the charge rate of dust particles is relatively fast, which is beneficial for the fine particles'collection. The fiber layer on the surface of modified rigid plate can reduce recoil current and the electrical resistance of particles. With continuous uniform of the surface water film, no anti-corona, no re-entrainment of dust, the evaporation of the water film which can improve the humidity of flue gas as well as the increasing of particle charging and migration velocity, the efficiency of particle removal can be improved.0.04t/(h·m2), which is the minimum water consumption to form a stable uniform water film on a modified rigid electrode can efficiently remove the particles whose size is less than 0.48m?. With the increase of gas temperature, the corona current increases gradually and the thermophoretic force is enhanced gradually, which is beneficial to improve the efficiency of particles removal. However, the increasing viscosity of the gas has a restraining effect on the removal of the particles. Prolonging the residence time and increasing the working voltage can increase the efficiency of particle removal. But the inlet concentration of particulate matter and the flushing water flow rate have little effect on the efficiency of the particles collection.(4) A test table for removing fine particles from the flue gas by using the FRP collectors was set up. Experiments of the current-voltage characteristics of the collectors using the simulated flue gas were carried on. Compared and studied the mechanism of the efficiency of particles removal of the water film on the surface of 3 kinds of FRP with grooves. Research findings:The characteristics in electric field of the 3 types FRP collectors with grooves are basically similar. The total collection efficiency of the fine particles of all collectors increased with the increase of the voltage. The total collection efficiency of the fine particles with FRP collectors with the circular grooves is greatly improved. When the residence time is 4s, the total collection efficiency of the fine particles of the 3 types FRP collectors with grooves is higher than that of the conventional rigid plate for any given voltage. When the working voltage is more than 40kV, the performance of FRP collectors with a transverse groove is similar to the collectors with circular holes and both of them perform better than the FRP collectors with vertical grooves. The FRP collectors with grooves are suitable for the conditions of low wind speed and long residence time. When the water film is distributed in the hole, it can prevent particles from backmixing after passing through the groove collectros. The performance of the FRP collectors with circular holes is better than that of the FRP with transverse grooves. The FRP with vertical grooves doesn't have the obvious advantage to particles collection.(5) PMMA cannot be competent for the working environment with wet ESPs. The FRP collectors with grooves have excellent performance in the working environment with wet ESPs without ash piece on the surface and ash clogging in the hole. |
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