| Wet electrostatic precipitators (wet ESPs) have exhibited satisfied performance for fine particulates control in some applications such as sulfuric acid and Phosphorus production. Moreover, wet ESPs may provide much higher average corona power levels compared to dry ESPs, because they are constantly being cleaned by flowing water, and can prevent "back corona". However, unexpected corrosion and fouling of collector surfaces may limit the wide applicability of current ESPs, and the research on ESP technology has tried to find more improved methods to solve these imperfections. In most wet ESPs, the collector surface is continuous and rigid. Flushing water passing over the surface tends to "bead" due to both surface tension effects as well as initial geometric surface imperfections which is not beneficial to particle precipitation. Based on this background, the flexible collection electrode was put forward. This paper summarized the possibility, internal power and laws of evolution for water transferring in flexible hydrophobic collection electrodes. In fact, large amounts of SO3 and acid mists would be produced in coal-fired process which would cause serious corrosion of pipeline and air pollution. The characteristics of SO3 conversion, absorption and particle size distribution of aerosol had been investigated in this paper, too.The effects of several parameters on the removal of pollutants were studied in lab and pilot scales experimental system. A modified efficiency formula was put forward to instruct actual industrialization for wet ESP with single flexible collection electrode. Flexible collection electrode applied in advanced purification for coal-fired flue gas following wet flue gas desulfurization (WFGD) has been put into service in 2×220t/h utility boilers since 2010.(1) The imbibition of flexible hydrophobic and insulated collection electrode was studied at the laboratory bench by monitoring quality change. The different ways of imbibition were separated successfully by superficial treatment.The transfer rules of water under capillary penetration in polypropylene (840A) and terylene (728) were analyzed. The results show that all capillary process could be divided into four phases. The immersion and solid-liquid separation were related to the balance between wetting tension and buoyancy of water. The capillary absorption process fully revealed the dynamic interaction of transfer process for water in fabrics. The evaporation of water film emphasized the influence of environmental conditions on experimental data. The effects of several parameters on capillary penetration were obvious, such as assemblages into strands of fabrics, properties of liquids and surface free energy of materials. The capillary processes for water and ethanol absolute penetrating 840A and 728 were in accord with the law of ExpAssoc and Hill. It can be concluded that two different ways exist which occurred independently and simultaneously in the process of imbibition. One was volume rising inside the yarns, and the other was surface filling of alveoli. There were three ways providing power for water transferring in fabrics. The yarns between filaments of fabrics drove the liquid up, and the liquid rose to the top of fabrics. If the yarns were saturated and the surface was dried, the liquid transferred to surface retention. The inherent kinetics was pressure difference between yarns and alveoli. Because of different size of alveoli, there existed mass exchanges among alveoli.(2) An experimental system was designed to investigate internal dynamics of drop spreading for water and ethanol absolute on 840A and 728 using FASTCAM—10kC system. The results show that the contact angles for water on 728 and 840A were 52.5°and 136.2°,and the contact angles for ethanol absolute on 728 and 840A were 41.9°and 46.5°. It was observed that overall spreading process could be divided into two stages. The processes of drop spreading for water and ethanol absolute on 728 were in accord with the law of ExpAssoc and Hill, and the diffusion shapes were elliptical. The processes of drop spreading for ethanol absolute on 840A were in accord with the law of Hill. The shapes for ethanol absolute and water on 840A were elliptical and circular. The effects of several parameters on drop spreading were obvious, such as assemblages into strands of fabrics, property and size of droplet, thickness of fabric and surface free energy of material.(3) A multi-purpose experimental system was established in Chapter 4. SO3 was produced by means of oxidation of SO2 by O3, and the conversions of SO3 and particle size distributions were investigated. The results show that SO3 translated into acid mists under certain conditions at 56℃. The characteristics of particle size distribution of acid mists were measured by Particle Dynamic Analyzer. The decisive parameter for SO3 conversion was temperature under simulation conditions. The actual particle size distribution of SO3 formed three intervals such as [0,40], [41,70] and [71,100], and the confidence levels were 79%,17%,4% in sequence. The mean diameters of former two groups were 18um and 48μm. All SO3 would translate into acid mists as soon as entering into WFGD.(4) On the multi-purpose equipment, the different absorption rate of SO3 was investigated by different absorbents. The absorptivities of different absorbents from low to high were CaCO3(30%)< desulfurizer<NaOH (0.25mol/L)<deionized water. All dissolved particles in absorbents decreased the solubility of sulfuric acid aerosol. The absorption rate of SO3 by desulfurizer was 64.21% and not influenced when gas temperature increased from 50℃to 85℃. A portion of acid mists was not absorbed by desulfurizer. As a result, a large amount of micro-fine sulfuric acid aerosol formed, and the forming speed of micro-fine sulfuric acid aerosol was much higher than that of dissolution in desulfurizer, thus caused corrosion and fouling of subsequent equipments and chimney.(5) A patented device was designed, and a bench scale experimental system was set up to investigate the removal of sulfuric acid aerosol by flexible collection electrode designed for gas flow rates of 450—1500m3h-1. Single polypropylene and terylene fabrics served as collection electrodes were introduced into a new wet ESP. The specific resistance and V-I curves of different collection electrodes were illustrated. The effects of several important parameters on collection efficicency of sulfuric acid aerosol were analyzed. The results demonstrated that gas flow distribution became uniform, and the no-uniformity decreased obviously with three deflectors. The RMS without deflector was 0.209, but in contrast RMS with three deflectors was 0.135. The behaviors of flexible collection electrodes were consistent with typical FRP using a thimbleful of water penetrating them via capillary flow at 0.147L/m2. The collection efficiencies by 840A and 728 were higher than that by FRP under certain conditions. The collection efficiency had linear relationship with specific surface area (SCA) and mass concentration. The collection efficiency increased with increasing electric field strength, particles average diameter and with decreasing gas temperature. As long as there was any water on the collector surface, any particle would exhibit similar collection efficiencies, whether of high resistivity or not. Based on analysis of main factors, a modified efficiency formula was put forward for advanced purification with single flexible hydrophobic and insulated collection electrode.(6) A pilot-scale unit was built up to demonstrate potential advantages of flexible collection electrode for flue gas following Ca-based WFGD. It was a counter current structure, designed for gas flow rates of 9000—25000m3h-1. The removal of sulfuric acid aerosol, slurry droplet and fine particle aerosol were tested under actual flue gas following fabric filter system and WFGD. The collection efficiencies by 840A and 728 amounted to 99% under certain conditions. The collection efficiencies by barbed wires were higher than those by diamond-shaped wires in the device, the deviation between them was 7—10% at higher gas flow rate, in contrast was 1—4% at lower gas flow rate. The concentrations of sulfuric acid aerosol, slurry droplet and fine particle aerosol were lower than 10mg/m3,15mg/m3 and 5mg/m3 at outlet, respectively. The pH value of removal liquid was usually in the range of 1.55—1.97, had infiltrated flexible collection electrodes for seven months. When the device was taken apart, the surfaces of collection electrode and emitter electrode were verified to be very clean and smooth due to continuous flushing by removal liquid. Single terylene collection electrodes presented significant resistance to acid corrosion. As long as the collecor electrode could be kept wet, there would be little maintenance cost. The optimum combination of advanced purification with flexible collection electrode is 728 collection electrode combined with barbed wires for fuel gas following Ca-based WFGD, the distance between two parallel collection electrodes is 400mm, the optimum gas treatment time is 2—2.5s, the recommended value of SCA is 16—20m2/(m3/s) at 65—70kV. The normal operation of industrialization and the higher gas transparency verified the validity of these results.(7) Another pilot-scale unit was built up to demonstrate potential advantages of flexible collection electrode for flue gas following NH3-based WFGD. It was a downstream structure, and designed with pre-washing system. The results demonstrated that the collection efficiencies for ammonia aerosol and ammonium aerosol by 728 amounted to 89.24% and 90.86% under certain conditions. The concentrations of NH3 and SO2 were influenced obviously by pre-washing rate at inlet. The relationship between collection efficiency of ammonia and pre-washing rate was not obvious. In contrast, the relationship between ammonium collection efficiency and pre-washing rate appeared linear growth. The effect of gas flow rate on collection efficiency decreased when pre-washing rate was higher during the course of performance tests. The optimum combination of advanced purification for gases following NH3-based WFGD is 728 collection electrode combined with barbed wires, The recommended pre-washing rate is 0.09—0.18L/(m3·h), the optimum gas treatment time should be lower than 2s, the optimal SCA is 18—22m2/(m3/s) at 60kV.Flexible collection electrode applied in advanced purification is satisfied for controlling aerosol emissions at higher face velocity, thereby be of great benefit to large-scale industrial use, and to meet more rigorous air quality standards. Flexible collection electrode applied in advanced purification for coal-fired fuel gas following WFGD gains some advantages of higher efficiency, higher gas transparency, thorough dust cleaning, continuous operation, lower cost, easy to be accepted which may become dominating control technology of coal-fired pollutants. |
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