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燃煤电厂固定污染源的颗粒物排放是造成大气污染的重要原因。近年来,我国各大燃煤电厂进行了除尘设备的升级改造,大大降低了颗粒物的排放浓度。国内外相继出台相关测试标准方法以便能更准确监测大气中的固定源污染物,如固定污染源排气中颗粒物测定与气态污染物采样方法(GB/T 16157-1996)[1]、固定污染源废气低浓度颗粒物的测定重量法(HJ 836-2017)[2]和可凝结颗粒物测试标准(US EPA Method 202)[3]等。固定污染源排放的一次总颗粒物(total particulate matter,TPM)是由可过滤颗粒物(filterable total particulate matter,FPM)与可凝结颗粒物(condensable particulate matter,CPM)共同组成[4-5]。我国现行颗粒物采样方法大多针对FPM,而忽视了颗粒物中的CPM。这意味着TPM的排放水平被低估,故目前我国关于固定源颗粒物的测试结果(即FPM)并不能代表固定源颗粒物的真实排放情况。CORIO等[6]对燃煤烟气实测得出,燃煤锅炉PM10中CPM占76%。LI等[7]的研究结果表明,CPM占总TPM的83.20%。而YANG等[8]对多家燃煤电厂的测试结果表明,CPM在TPM中占50%以上。以上研究表明,CPM在总TPM中占比越大,其浓度会随着颗粒物粒径的减小和排烟温度的升高而逐渐提高[9]。
CPM在固定污染源烟道内为气体状态,但其排放入大气后,随着温度的降低,会立即凝结成颗粒物状态。CPM对环境的影响是由于其物理形态及化学组成[10-13]所导致。从物理形态上看,CPM的粒径一般小于1 μm,由气态物质凝聚而成,以气溶胶的形式存在于环境空气中,通过在大气中凝结和扩散对区域空气质量产生不利影响,影响到大气能见度,甚至致霾[14];从化学组成上看[15-16],CPM主要为无机组分,富集了各种重金属(例如砷、硒、铅、铬等)和多环芳烃(polycyclic aromatic hydrocarbon,PAHs)等致癌物质与基因毒性诱变物质,对环境和人类有极大危害;同时,由于CPM比表面积较大,吸附性较强,可携带各种污染物进入大气环境中,催化大气中的某些化学反应[17]。因此,有必要详细分析CPM中无机组分的排放特性。
测定CPM排放量对完整测算燃煤电厂向大气环境中排放的颗粒物含量意义重大,并已成为污染物排放监测的重要内容,而如何高效率捕集CPM为当前的研究重点。目前,捕集并测试CPM的方法有冲击冷凝法[3]和稀释冷凝法[18]2种。李兴华等[19]对稀释冷凝法用到的稀释采样系统进行了研制、优化与应用。周楠等[20]对固定源稀释通道的设计与外场测试进行研究,对稀释比、停留时间等因素进行了优化。然而,上述采样系统无法实现FPM与CPM的分开采样,且采样装置体积较大,不便携带。YANG等[8]利用冲击冷凝法对5个固定源(包括发电厂)的FPM和CPM排放情况进行了现场监测,但并未进行化学分析。LI等[7]研究了某1 030 MW燃煤电厂排放烟气中CPM的物理和化学特性,同时对比分析了CPM与FPM的化学成分。而在实际应用中,该方法易受SO2等水溶性气体的影响。TSUKADA等[21]、杨柳等[22]对“湿式冲击冷凝法”进行了改进,用一级螺旋冷凝管与冷凝液吸收瓶代替冲击瓶组,可有效减少SO2等可溶性气体的溶解,但未设置后置CPM滤膜,故不能保证充分捕集颗粒态的CPM。同时,国内燃煤电厂烟气温度跨度较大(50~110 ℃)、湿度较高,且易在采样枪中析出大量冷凝物质。为解决采样装置换热效果随水槽中冷凝水温度上升而快速减弱;烟气采样位点较高,现场采样空间受限,现有采样设备体积较大、不便携带等问题,亟需开发一种适合国内固定污染源实际情况的CPM采样装置和测试方法,以优化国内固定污染源CPM的监测及研究工作。
本研究旨在优化设计一种体积轻巧、便于携带的CPM采样装置,对3台国家大气污染防治重点区域内的600 MW等级以上超低排放燃煤机组进行CPM排放状况实测,并分析CPM中无机离子的分布与排放特征,以期为燃煤电厂的颗粒物污染监测提供参考。
可凝结颗粒物采样系统的优化设计及实测应用
Optimal design and determination study of condensable particulate matter sampling system
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摘要: 燃煤电厂烟气中可凝结颗粒物(CPM)主要由无机成分组成,其排放会影响大气环境。为有效捕集烟气中CPM并研究其无机组分排放特性,采用自主优化设计的CPM采样装置,就国家大气污染防治重点区域内3台600 MW等级以上超低排放燃煤机组开展了烟气中CPM实测研究,同时对采样装置的有效性、可靠性进行了探讨。该装置利用半导体材料进行制冷,与常规采样装置相比体积轻巧、便于携带、捕集效率高、冷凝效果好,且使用成本低。研究结果表明:燃煤电厂中A、B、C 3个机组烟囱排口的CPM质量浓度分别为2.22、3.02、3.83 mg·m−3;CPM无机水溶性离子中,阴离子主要有SO42−、Cl−等,阳离子主要有Ca2+、Na+、NH4+等;其中,SO42−质量浓度占比均大于50%,在水蒸气存在的情况下,会与NH4+产生硫酸氢铵和硫酸铵;而硫酸氢铵易与烟气中的水蒸气结合形成酸雾,会腐蚀烟气管道,还会因其黏附性较强附着飞灰等杂质在管道表面沉积附着,加剧管道的腐蚀和结垢。本研究结果可为探究超低排放燃煤电厂烟气中CPM的排放特性提供参考。Abstract: The environmental impact of the Condensable Particulate Matter (CPM) emission, in flue gas, from coal-fired power plants is innegligible. However, the CPM collection methods existed are not well suited to the actual situation of the fixed stationary source within the country. Inorganic component is the main component of CPM. In order to capture CPM in flue gas effectively and study its inorganic components emission characteristics, we adopted the self-designed CPM sampling device to carry out the CPM measurement research in flue gas, and at the same time, the effectiveness and reliability of this CPM sampling device are discussed. The test objects are three ultra-low emission coal-fired power plants in the national key field of air pollution prevention and control. The CPM sampling device uses semiconductor materials for refrigeration, which has light in size, easy to carry, high collecting efficiency, good condensation effect, and low cost compared with the conventional sampling device. The study shows that the CPM concentration at the chimneys of A, B and C coal-fired power plants is 2.22, 3.02 and 3.83 mg·m−3. The anions in CPM inorganic water-soluble ions, are mainly SO42− and Cl−, while the cations are mainly Ca2+, Na+ and NH4+. The mass concentration of SO42− accounts for more than 50%. In the presence of SO42−, ammonium hydrogen sulfate and ammonium sulfate will be produced when the water vapor is present with NH4+. Ammonium hydroxide is easy to combine with the water vapor in the flue gas to form acid mist, which will not only cause corrosion to the flue gas pipeline, but also fly ash and other impurities because of its strong adhesion. The deposition and attachment on the surface of the pipeline will aggravate the corrosion and scaling of the pipeline. Therefore, it is of great significance to study the effective capture and emission characteristics of CPM in coal-fired power plants.
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表 1 燃煤机组基本信息
Table 1. Basic information of the tested coal-fired power plants
机组编号 装机容量/MW 各机组的污染物控制技术 A 600 SCR+WFGD+ESP+MGGH B 900 SCR+WFGD+ESP+MGGH C 1 000 SCR+WFGD+LT-ESP+GGH 注:SCR(selective catalytic reduction)为选择性催化还原技术;WFGD(wet flue gas desulphurization)为湿法烟气脱硫技术;ESP(electrostatic precipitator)即电除尘器;LT-ESP(Low temperature electrostatic precipitator)即低低温电除尘器;MGGH(Mitsubishi gas-gas heater)指低低温烟气换热系统;GGH(gas-gas heater)指回转式烟气换热系统。 表 2 3台机组烟气中一次颗粒物排放情况
Table 2. Primary particulate matter emission in flue gas of three units
机组编号 CPM/ (mg·m−3) FPM/ (mg·m−3) TPM/ (mg·m−3) (CPM/TPM)
/%烟气流速/ (m·s−1) 烟气温度
/℃A 2.22 0.1 2.32 95.7 16.5 77.8 B 3.02 2.5 5.52 54.7 20.3 77.3 C 3.83 5.4 9.23 41.5 24.8 69.2 -
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