【作者】 朱继保；

【导师】 施耀； 闫克平；

【作者基本信息】 浙江大学 ， 环境工程， 2010， 博士

【摘要】 燃煤产生的可吸入颗粒物粒径小、数量多、比表面积大等特征,因而在大气中停留时间长,且容易吸附各种有害的有机或无机化合物,对人体健康危害极大。因此,细颗粒物的控制尤其是燃煤电厂颗粒物控制成为了大家关注的焦点之一。我国是以燃煤为主的能源大国,电除尘器是最重要的电厂除尘设备之一,然而目前约90%的电除尘器排放在50mg/Nm3以上,未能有效地控制细颗粒物的污染。根本原因是目前运行的电除尘系统不能对PM2.5细微颗粒进行有效的荷电和由于气流离子风和振打等因素导致的二次扬尘、电气控制等因素导致的收集电场场强降低等。针对于以上因素,本文将重点研究了几种静电收集细颗粒物的控制技术,总结静电收集技术的主要技术原理、应用领域和技术突破难点,并最终寻找出一种适合目前工业除尘器安装应用的技术路线途径。首先研究了极板收集灰的过程以及灰的分布和堆积情况,在自行设计的闭循环试验台上研究电除尘器电极与极板灰堆积特性的相关性。本研究选用扁钢芒刺型放电极,电极间距在22 mm和330 mm间可调以及单根放电极来研究粉尘的收集规律。颗粒物入口浓度为约180 g/m3,运行电压为46kV条件下收集颗粒物,系统风量为725 m3/h。实验结果表明,电极间距与半极板宽度之比接近1:1时电流密度最大,而当极板上收集粉尘厚度4mm后,电晕电流减小约0.4mA,同时击穿电压也上升4kV。粉尘在极板上的堆积形状为矩形,其尺寸与放电极间距相对应,针尖对应位置厚度最高,边缘较薄。极板上粉尘的堆积密度约0.9g/cm3几乎不受电极间距的影响,而同一放电极间距时,出口电场灰的堆积密度则降低至入口电场的一半。当电极间距继续增大,边缘处厚度大幅度降低,第一第二电场收集的灰相对减少,从而后三电场灰厚度增加至1至2mm,不利于二次扬尘的控制。其次在自行设计的闭循环实验装置上进行了细颗粒物的荷电、预荷电及细颗粒的双极性预荷电凝并及收集。深入研究预荷电的原理及其影响因素,期望能够用于高气体流速10 m/s下的细颗粒荷电,并利用电除尘器现有的导流区域实现带电细颗粒物的有效地正负凝并,而不必额外添加凝并室,减少工业应用成本和设计难度。在本电除尘实验系统中,细颗粒物的个数浓度收集效率最高只能达到94%-95%,而增加SCA(比集尘面积)值或增加电场数并不能提高细颗粒物的收集效率。采用双极性预荷电器后,颗粒物物的分级收集效率显著提高,在最佳运行电压情况下可以达到98%。最后研究了电袋复合除尘技术,分别在实验室小试以及中试实验平台上进行,电除尘器位于复合除尘器的第一电场,预先去除约80%至90%的颗粒物。电源的选型上采用了单相、三相电源对比研究了对细颗粒物的控制以及对复合除尘器的适应性。当采用三相电源替代传统单相电源时,400mm间距的除尘器实际运行电压从55kV提高到了71kV,平均电流从31mA提高到了62mA。细颗粒分级收集效率以及颗粒物的迁移速度得到显著提高,采用单相电源时细颗粒迁移速度约17cm/s,而采用三相电源时最大值能达到35cm/s。对于粒径为0.03-0.1μm和0.1-2.5μm的颗粒物,分级效率能够从85%分别提高到95%和92%；而对于粒径为2.5-8.0μm的颗粒物,则从87%提高到98%。根据实验结果分析显示,细颗粒物的分级收集效率不能直接采用Deutsch公式直接计算,而采用修正后的公式log(?)可以很好地与实际结果相吻合,α和β为修正因子。更多还原

【Abstract】 Fine particles can stay long in atmosphere with large number, and always adsorb lots of harmful compounds. Therefore fine particle emission from the coal fired power plants has become a big problem for the harm to the human health. Coal is the primary energy source in China and electrostatic precipitation (ESP) has become the primary apparatus for particle collection. However, about 90% of the ESPs can not effectively ccapture the fine particles with the mass emission above 50mg/Nm3. For this reason, we focus our studies on the behavior of fine particle collection with ESP, and new technique to improve fine particle collection for industrial power plant.This paper firstly presents the characteristics of the dust layer collected with a laboratory electrostatic precipitator. Experiments are performed with different electrode distances varies from 22 mm to 330 mm, and also with a single discharge electrode. Discharge last for 40 to 50 seconds under 46kV voltage,150 g/m3 inlet dust concentration and 700 m3/h flow rate. Ash layer thickness collected on the plate is about 4 mm in average. We observed that typical shape of the collected dust is rectangle and the dust in the center is much thicker. The density at the center, however, almost keeps constant.In addition, a laboratory ESP together with a bipolar pre-charger has been designed for studying charge-induced agglomeration and fine particle collection. In terms of particle numbers, the ESP collection efficiency drops to its minimum of near 90% for particles with diameters of near 0.2μm and 3μm. For other particles, its value is around 94%-95%. By using the bipolar pre-charger, the grade efficiency can be significantly increased for all particle sizes due to the charge-induced particle agglomeration. The grade collection efficiency rises to about 95%-98%for all size particles.Industrial investigations on fine particle grade collection efficiency of an ESP are performed with a hybrid ESP and fabric filter (FF) on a 30MW coal-fired boiler. Gas flow rates, mass inlet concentration and gaseous temperature are 20000-40000Nm3/h,15g/Nm3 and 110℃, respectively. The ESP specific collection area (SCA) ranges from 10 to 20m2/m3/s. Both single-phase and three-phase transformer-rectifiers (TRs) are used for energizing the ESP. When changing the single-phase TR to the three-phase TR, the maximum average secondary voltage is increased from 55kV to 71kV and average corona current rises from 31mA to 62mA without spark breakdown. As a result, both particle grade collection efficiencyη(r) and migration velocities are significantly improved. With the single-phase TR, the velocity is around 17cm/s for all particles. With three-phase TR, its maximum value is about 35cm/s. For particles within 0.03-0.1μm and 0.1-2.5μm, the efficiencies rise from about 85% to 95% and 92%, respectively. For particles of around 2.5-8.0μm, they rise from about 87% to 98%. Moreover, experiments show that a revised Deutsch equation log(?) gives a good approximation via the average electric field Ea, the specific collection area S and two correction coefficientsαandβ.更多还原