【作者】 常景彩；

【导师】 马春元；

【作者基本信息】 山东大学 ， 热能工程， 2011， 博士

【摘要】 湿式静电除尘技术具有高效去除微细颗粒物、低压损、防止二次携带等优点,目前已经广泛应用于煤气生产、硫酸制造、磷矿加工等行业。然而目前的湿式静电技术仍存在一些问题,集中体现在集尘极材料选择和表面水膜分布不均引发的系列问题上。例如集尘极表面水膜不均引起的“干斑点”和“火花放电”故障、大部分设备采用的停机冲洗操作缺陷、集尘极采用耐腐蚀材料造成的设备成本较高等问题,限制了湿式静电技术的大规模工业应用。针对上述问题,本文提出以柔性绝缘疏水纤维织物用作集尘极材料,旨在解决传统湿式静电技术电气与设备故障根源。通过研究柔性集尘极表面水膜形成过程、内在动力与液滴浸入蔓延规律,论证水在柔性疏水纤维织物集尘极表面形成均匀水膜的可行性；通过分析S03\酸雾气溶胶核化、吸收竞争机制,揭示燃煤烟气S03进入湿法脱硫塔前后的形态转化与吸收规律；以S03转化物粒度分布特征为据,建立酸雾气溶胶深度净化小型试验台,通过测量不同给水率条件下柔性绝缘集尘极电阻率变化及V—I曲线,揭示柔性集尘极表面水膜特性与静电场性能之间的耦合关系,提出柔性集尘极应用于燃煤脱硫烟气深度净化的理论分析模型,指导中试试验系统设计；通过中试试验研究得出柔性集尘极应用于燃煤脱硫烟气深度净化的组合操作参数,并在2×220t/h锅炉上实现工业化应用。(1)采用浸润质量实时监测法,在柔性集尘极毛细浸润实验台上,测量了三种浸润液体对两种疏水性纤维织物(丙纶织物：840A；涤纶织物：728)浸润过程中集尘极质量的变化；利用浸润过程分步吸收、材料表面胶化处理方式,得到液体浸润织物过程存在的两种独立浸润途径,揭示了水在柔性绝缘疏水性纤维织物集尘极表面形成均匀水膜的可行性及内在动力。研究发现：液体对柔性集尘极毛细浸润过程,可分为浸入、毛细吸附、液固分离、织物表面液体蒸发四个阶段;织物编织结构、材料表面自由能、液体性质等参数对织物毛细浸润特性影响显著；当液体浸润同一织物,经向和纬向浸润规律表现相同,水浸润840A、728织物过程分别符合ExpAssoc指数分布和Hill分布；液体浸润量由织物纤维间隙毛细吸附量和织物表面空穴容液量两个相互独立部分组成,毛细吸附与空穴充满过程同时发生并存在质量交换；纤维间隙毛细吸附效应、表面空穴内液体产生的Laplace压力与液体本身静压差、表面空穴内液体之间的压力差为柔性疏水绝缘纤维织物集尘极表面水膜的形成提供动力。(2)以液滴扩散基础理论为指导,利用FASTCAM—10kC高速摄像系统,拍摄不同性质液滴在不同柔性集尘极材料表面的浸入扩散过程。运用数据无量纲化处理方法对拍摄内容统计分析,得到液体、织物参数对液滴在集尘极表面浸入扩散特性的影响规律。研究发现：液体性质、材料表面自由能决定二者之间的接触角数值,水液滴与840A、728的接触角分别为136.2°、52.5°,乙醇与840A、728的接触角分别为41.9°、46.5°；接触角越大,液滴浸入扩散速度越小,达到平衡时间越长；液滴浸入扩散过程可分为浸入扩散初期和浸入扩散后期两个阶段；在浸入扩散初期,液滴重力对扩散速度及体积浸入速度影响显著,在浸入扩散后期,液体粘度、惯性力、材料表面能起主导作用；水液滴在728表面及乙醇液滴在728、840A表面的扩散形状均为椭圆形,扩散过程分别符合ExpAssoc和Hill分布；水液滴在840A正、反面的浸入时间较长,扩散形状为圆形,直径略小于液滴最大直径；织物表面纹理结构、液滴体积和性质、织物厚度、材料表面自由能对液滴浸入扩散特性产生明显影响。(3)在S03形态转化实验中,采用03氧化S02法产生S03,利用激光相位多普勒测速系统对S03转化物进行粒度测量,得到典型湿法脱硫温度下S03转化物多分散性粒度分布特征,并为酸雾气溶胶深度净化试验颗粒物粒径选择提供依据。研究发现：在温度56°CS03与反应器内残存少量空气接触时,S03与水分子快速结合转化为H2SO4分子,而不是以S03分子形态存在；温度是影响硫酸蒸气核化凝结的关键因素；硫酸蒸气凝结成核形成的酸雾气溶胶颗粒物粒径的估计值在[0,40]、[41,70]这两个区间的概率为96%,两段区间内颗粒物平均粒径分别为18μm和48μm;SO3进入湿法脱硫塔后全部以酸雾气溶胶形态存在。(4)在S03酸雾气溶胶吸收特性实验台上,测量了室温下不同吸收剂对S03酸雾气溶胶的吸收率,对比分析了不同温度下,钙基湿法脱硫循环浆液对S03酸雾气溶胶吸收率的变化,并对S03酸雾气溶胶在吸收剂内的核化、吸收过程存在的竞争机制进行了机理分析和实验验证。实验测得不同吸收剂对S03酸雾气溶胶的吸收率为：CaCO3(30%):60.79%,脱硫循环浆液：64.21%, NaOH(0.25mol/L):67.02%,去离子水：67.21%；吸收剂中各类溶解性粒子的存在降低了SO3酸雾气溶胶的溶解度；温度在50～85℃变化时,脱硫循环浆液对SO3酸雾气溶胶的吸收率轻微波动；实际运行中燃煤产生的S03进入脱硫塔时全部转化为H2SO4并形成大量亚微米级凝结核,酸雾气溶胶的核化速度远远高于在吸收剂内的溶解速度,最终形成大量微细硫酸雾悬浮于气相中,而不能被脱硫浆液全部吸收；净烟气中的酸雾含有未脱尽的硫酸雾部分。(5)以机理研究成果为基础,建设了柔性集尘极应用于酸雾气溶胶深度净化小型试验台。重点在该试验台上考察了集尘极材料、比收尘面积、场强、颗粒粒径、气体温度、进口浓度、水膜蒸发率等参数对酸雾气溶胶脱除率的影响规律。利用FLUENT对静电场内导流板的设计进行优化,优化后电场内速度均方根降为0.139,有利于柔性集尘极的张紧和垂直；当绝缘集尘极给水率为0.03L/m2时,材料电阻率降为半导体材料范畴；给水率为0.147Lm~-1h~-1时,测得728、840A、玻璃钢(FRP)三种集尘极的电场特性相似,起晕电压均为22kV,30kV以后,电晕电流728>840A>FRP;试验工况下,840A对酸雾气溶胶的脱除效率为99.4%；比收尘面积、进口浓度与酸雾气溶胶脱除率成线性增加关系,场强与效率之间的关系符合Logistic模型,脱除率随温度的升高成指数衰减,表面水膜蒸发率对酸雾气溶胶脱除率无明显影响；通过各影响参数分析,对传统效率公式进行了修正,提出了柔性集尘极应用于燃煤脱硫烟气深度净化的理论分析模型。(6)在柔性集尘极应用于钙基湿法脱硫烟气深度净化中试试验台上,发现试验工况下728和840A集尘极对浆液滴、微细粉尘气溶胶的脱除率均在99%以上；锯齿芒刺线的工作性能优于星形放电极,二者在低流量条件下偏差为1%-4%,在高流量下二者偏差为7-10%；试验装置出口烟气酸雾含量<10mg/m~3、浆液滴气溶胶<15mg/m~3,微细粉尘气溶胶<5mg/m~3；低pH值静电脱除液体作为唯一水膜供给水源,柔性集尘极低温高湿环境运行7个月后,性能持续表现优异；728集尘极+芒刺线极配方式合理,两电极表面保持高度清洁；柔性集尘极应用于钙基湿法脱硫烟气深度净化的工业化应用中,推荐参数为：极配方式为728集尘极+芒刺线；气体与水膜接触方式为逆流接触；极间距为400mm；气体停留时间范围为2s-2.5s；运行电压在65-70kV时,比收尘面积推荐值为16-20m~2/(m~3/s)；工业化装置的正常运行验证了研究成果的正确性。(7)建立柔性集尘极应用于湿式氨法脱硫烟气深度净化的中试试验台,明确了适用于湿式氨法脱硫烟气深度净化技术的特性参数。试验工况下,728集尘极对氨、铵盐类气溶胶的脱除率分别为89.24%和90.86%；预洗涤水量对装置进口烟气NH3和SO2含量均产生明显影响；预洗涤水量的大小对氨气溶胶的脱除率无明显影响,铵盐类气溶胶脱除率随预洗涤水量增加成线性增加；烟气流量越大,氨气溶胶和铵盐类气溶胶的脱除率均相应降低,预洗涤水量越大,烟气流量对污染物脱除率的影响越小；静电脱除液体及过饱和烟气冷凝液完全满足集尘极给水率要求；柔性集尘极应用于湿式氨法脱硫烟气深度净化的工业化应用中,当气体与水膜接触方式为顺流接触时,推荐参数为：预洗涤水量范围为0.09-0.18L/m3·h,气体停留时间<2s,运行电压在60kV时,比收尘面积推荐值为18-22m~2/(m~3/s)。柔性集尘极应用于燃煤脱硫烟气深度净化技术,高烟气流速下具有气溶胶脱除率高、烟羽能见度高、集尘极表面水膜均匀、清灰彻底、无间断在线操作、运行成本低等优点及现场应用优势,使其有望成为协同脱除WFGD、SCR系统运行副产物的主流控制技术之一。更多还原

【Abstract】 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.更多还原