【作者】 王鹏；

【导师】 骆仲泱； 岑可法；

【作者基本信息】 浙江大学 ， 工程热物理， 2008， 博士

【摘要】 在我国,由于粗放型发展模式及以煤为主的能源结构和低效能源利用方式,使目前我国煤炭生产与消费量均居世界首位,煤烟型大气污染非常严重;另外,产生的量SO₂、NO_x,使81.6%的城市出现酸雨。由于严重的大气污染,个别城市已成为“卫星上看不见的城市”。研究大气污染现状,讨论污染控制和治理方案已经成为迫在眉睫的任务,其中可吸入颗粒物的排放和控制研究是相当重要的一部分。本文建立了燃煤锅炉烟气小颗粒等速采样系统和实验流程,使用US EPA method 5对燃煤锅炉实验台混合燃料稳定燃烧排放的烟尘气溶胶进行采样实验;并对循环流化床电厂电除尘器前后烟尘进行采样实验。测试得到烟尘气溶胶的浓度分布和粒径分布以及PM₁₀/PM_2.5在除尘前后烟尘所占的比例,并求得电除尘器对PM₁₀、PM_2.5、PM_1.0的分级除尘效率。燃煤排放的可吸入颗粒物通常富集各种重金属元素和PAHs、VOCs等有机污染物。通过实验研究,分析了燃煤锅炉烟道中可吸入颗粒物的重金属含量分布、多环芳烃浓度以及颗粒物的显微结构和微观孔隙特性等方面的物理化学特性,还分析了实验工况的变化对各种痕量污染物的影响,指出污染物在不同粒径颗粒上的富集规律。同时结合实验室研究,分析了常规静电除尘器和高压脉冲放电的复合静电除尘器对这些痕量污染物的脱除能力。声波团聚是对载尘气流进行中间预处理过程以提高传统除尘装置除尘效率的一种方法。本文搭建了声波团聚飞灰颗粒实验装置,主要研究声压、声波频率、停留时间、烟气温度、颗粒物浓度等试验参数对煤飞灰微粒声波团聚过程的影响。在激光粒度仪上进行了声波团聚飞灰颗粒的实验,在线测量飞灰颗粒在声波团聚过程中的粒径分布变化规律。在高压脉冲放电的条件下,由于比常规直流放电有着更高的放电电压,产生了大量的高能电子,纳米级颗粒的荷电量有了数量级的提升。本文通过实验分析了脉冲放电电压、电极反应器形式对各粒径段颗粒特别是纳米级颗粒的荷电规律,证实了纳米级颗粒的荷电量可以有很大提升。在此基础上,为提高传统除尘器对亚微米级颗粒的脱除效率,本文建立了一种脉冲预荷电直流收尘的复合式除尘系统,采用较低能耗的高压窄脉冲放电对颗粒物预荷电,大大提高了亚微米级颗粒的荷电量,结合传统的直流静电除尘器可以大幅提高对亚微米级颗粒的脱除效率,对小于1μm的颗粒脱除效率可以达到90%以上。主要分析了脉冲荷电的电源形式和电压等因素对脱除效率的影响,为改造传统静电除尘器提供了理论支持。NO是燃烧产生的重要气体污染物之一,脉冲放电与直流放电相结合的新型除尘方式在有效提高对PM_2.5脱除效率的同时,还具有氧化NO的能力。高压脉冲放电产生的高能电子可以产生强氧化性的自由基,对NO进行氧化,同时还可以直接打断N-O键,游离态的N原子大部分生成了N2分子。线筒式放电结构氧化NO的能力明显优与线板式放电,与除尘能力具有一致性。氧化效率随着电压的升高而升高,由于一定电压下对NO的处理量趋于定值,氧化效率与NO的初始浓度具有重要的关系,同时也受NO₂浓度的影响。在本文讨论的反应器中,当脉冲峰值电压达到50kV,NO初始体积百分比低于10-4时,NO的氧化率可以达到90%以上。氧化产物NO₂可以在湿法脱硫设备中脱除。因荷电粉尘凝并过程极为复杂,目前尚无电凝并除尘效率理论计算模型。但在电凝并复合除尘的工作过程中是先荷电然后进行凝并收尘,凝并和收尘在同一电场区域同时进行。于是,电凝并除尘效率实际上是凝并和收尘的综合除尘效率,只不过静电除尘器所收集的粉尘的粒度分布已不同于原来气体中粉尘的粒度分布。根据入口处颗粒的粒径分布和荷电量,可以通过理论计算得出无凝并状态下的收尘效率,将理论值和实际收尘效率对比即可估算带电颗粒在收尘电场中的凝并效率。本文建立了复合除尘系统的理论模型,集合常规除尘器除尘理论和颗粒在能量场中的凝并理论,分析了颗粒在电场中的运动轨迹,得到了半经验的颗粒凝并脱除效率公式,在凝并和脱除同时进行的情况下对两种行为的比重进行了分析。结果显示在高压脉冲荷电直流凝并收尘的复合除尘器中,提高收尘效率的主导因素在于大大提高了纳米级颗粒的带电量,从而提高了常规直流除尘器对于颗粒的收集效率,而颗粒凝并只有不到10%的影响效果。更多还原

【Abstract】 Coal is the primary energy source in Chinese electrical power industry.Coal combustion is one of the important sources of Inhalable Particulate(PM₁₀) emission to the atmosphere.PM₁₀, especially PM_2.5 has become the most important air pollutant in our country according to the energy construction of China.It has attracted the world attention at present.Size distribution of particles emitted from coal fired boiler was performed according to US EPA Method 5.Particulate samples were extracted from flue gas of a pilot scale 0.5MW coal combustor test facility and circulating fluidized boiler（CFB） power plants using cascade impactor. Particle size distribution and segregated particulate concentration was measured on both experiments,the fraction of PM₁₀/PM_2.5 in flue gas particulates is calculated at upstream and downstream of the electrostatic precipitator or bag house and size-dependent capture efficiencies for PM_2.5 on ESP were derived in the thesis.Particles emitted from coal combustion are usually enriched toxic heavy metals and organic pollutants such as PAHs,VOCs.Several toxic heavy metals including Cd,Pb,Ni,Cu concentration in particulates was analyzed with atomic absorption spectrometer（AAS） and Hg was analyzed with atomic fluorescence spectrometer.The result shows the concentrations of medium volatility Cd and Pb increase with decreasing particle size and tends to enrich in submicron particles.Limestone powder injection in the pilot scale CFB boiler helps to absorb heavy metals gas and reduces heavy concentration in particles and change Hg distribution in combustion products.Both traditional ESP and combined ESP have ability to catch these pollutants.Acoustic agglomeration is an aerosol preconditioning procedure to improve the performance of conventional particle removal devices.Acoustic agglomeration experiments were done in traveling wave field with redispersed fly ash,and effect of parameters on agglomeration processes is investigated such as SPL,frequency,temperature,reaction time and dust loading concentration. Particle size distributions were measured on line with laser particle sizer MS-2000.PM_2.5 especially nano-particles can get higher charge amount under high voltage pulse discharge compared with DC discharge.The main component in particle causing this result is SiO₂. To improve traditional electrostatic precipitator（ESP） removal efficiency on particles smaller than lμm,a new type of fine particle removal system is set up by combine traditional ESP and pulse charge pretreatment technology.High voltage narrow pulse discharge increases the charge amount of fine particles and makes particles easier to be caught by traditional ESP.The efficiency to removal particles smaller than lμm can be much higher than traditional ESP.The influence of charge voltage and form are analyzed to give theoretically support to improve traditional ESP.NO is one of the most important pollutants during combustion.The combined electrostatic precipitator（ESP） has a character of de-NOx with the increasing efficiency of catching PM_2.5. During pulse corona discharge,the formation of ozone and free radical has important effect on NO oxidation.Electron with high energy can also break N-O directly and the dissociative N can form N2.Accordant with dust catch efficiency,line-cylinder chamber is better than line-plate chamber. With the increase of voltage,the de-NOx efficiency becomes higher.Under a certen voltage,the efficiency of de-NOx is strongly influenced by the initial concentration of NO and NO₂.In the reactor developed by this paper,the oxygenation efficiency can be higher than 90%when the voltage is 50kV and the initial NO volume percentage is less than 10^-4.The oxidation outcome NO₂ can be absorbed by wet gas desulphuration（WGD） technology.In the combined ESP,particles are charged first in the pulse electric field and then go to the DC field.Agglomeration and collection work at the same time in DC field.The total efficiency of combined ESP is influenced by these two processes.According to traditional agglomeration theory and ESP theory,a new combined ESP efficiency model is set up.The proportion of this two processes is analyzed by this model.According to the model,the main influence on collection efficiency is the charge ability of high voltage pulse power.The influcen of agglomeration is below 10%.更多还原