【作者】 吴辉；

【导师】 邱建荣；

【作者基本信息】 华中科技大学 ， 热能工程， 2011， 博士

【摘要】 汞(Hg)是一种剧毒的重金属元素,能在生物体内累积并通过食物链进入人体,造成中枢神经损伤及婴幼儿畸形,是一种全球污染物,其排放控制成为越来越被关注的热点。燃煤电厂是最大的人为汞排放源,然而煤燃烧过程中汞的排放规律和控制机理方面研究尚很浅薄,在我国更是如此。本文借助管式炉和沉降炉试验系统和燃煤烟气Hg在线分析仪,对煤粉燃烧过程中汞的释放迁徙及排放控制规律展开了实验和理论方面的研究,考察了汞在煤粉燃烧过程中的析出特性及其影响因素,探索了氧燃烧方式下的汞排放特征及脱除性能,分析了燃煤烟气组分对汞的氧化规律,并对实际燃煤电厂的汞排放进行了现场采样和模拟预测,以期为燃煤汞排放控制的理论研究和控制技术的开发奠定基础。在管式电加热炉试验系统上,考察了不同煤种的Hg析出温度、析出强度分布及其烟气中的Hg形态分布,并与煤中Hg的赋存形态进行了关联分析,提出了基于Hg的析出特性初步判断煤中主要汞赋存形态的实验思路与方法,分析了煤中Hg与S的依存释放特性。煤样的Hg析出呈多峰分布,第一个析出峰(P1)强度较高,在360℃～460℃左右；第二个析出峰(P2)和第三个析出峰(P3)的强度大小不一,P2峰在445℃～850℃,P3峰在930℃以上。析出峰的单质汞(Hg0(g))比例在38%～100%,呈现P1>P2>P3的规律。利用Hg的析出特性,结合浮沉实验,可对煤中主要的Hg赋存形态及其分布进行初步判断。煤中Hg与S存在一定依存关系。利用管式炉和沉降炉实验系统,在氧燃烧气氛(O2/CO2)下,考察了不同煤种的Hg排放浓度及形态分布,分析了O2浓度变化的影响,探索了煤粉添加CaO和Fe203的Hg控制机理。O2/CO2气氛下,煤中Hg的初始析出受到一定抑制,P1峰的强度和温度均有所降低。烟气中的汞浓度及形态分布因煤样而异。O2浓度升高至30%时,烟煤的总汞(HgT(g))排放浓度显著增加,Hg0(g)浓度基本维持不变。O2/CO2气氛可促进CaO对Hg的脱除,强化Fe2O3对Hg0(g)的催化氧化；O2浓度升高不利于CaO脱HgT(g),但30%的O2可明显促进Fe2O3对Hg0(g)的氧化吸附。利用模拟燃煤烟气,研究了燃煤烟气组分HCl/C12/SO2/NO/NO2对Hg0(g)形态转化的影响规律,探讨了烟气组分间的协同作用。HCl和Cl2是最主要的Hg0(g)氧化剂,HCl与Hg0(g)的反应在600℃以上才发生,Cl2在室温下即能显著氧化Hg0(g)。反应温度高于400℃后,烟气中的Cl2开始分解。NO对Hg0(g)的氧化与其浓度有关,Hg0(g)氧化率随反应温度升高而降低。O2、NO2和SO2对Hg0(g)的氧化率均较低,不超过10%。两种烟气组分共存显示了Hg0(g)氧化的协同作用。HCl/NOx和Cl2/NOx对Hg0(g)的氧化情况相似：200℃～600℃范围的Hg0(g)氧化显著增强,600℃～1000℃范围的Hg0(g)氧化有所减弱；加入O2有利于Hg0(g)氧化。SO2明显抑制HCl对Hg0(g)的氧化。400℃时SO2和NO2共存可获得48%～63%的Hg0(g)氧化率,温度升高到1000℃后,Hg0(g)氧化率大幅降低。采用美国EPA推荐的OHM方法和EPA方法26A,对燃煤电厂的汞及卤素化合物的排放进行了现场同时采样,调查了燃煤电厂烟气中的汞形态分布及排放浓度,分析了Hg0(g)随烟气流程的氧化情况,探讨了HF、HCl、NO和SO2与汞形态转化的关联,建立了基于BP神经网络的燃煤电厂汞排放预测模型并与现场采样结果进行了比较。烟气中气态总汞的浓度为6～28μg/Nm3,随着烟气流程,烟气中气相总汞的浓度降低,氧化态汞的比例从41%升高到74%。汞的排放因子为5.63 g/1012J(13.11b/1012Btu)。燃煤烟气中的卤素化合物主要为HF和HCl, Cl2与HBr浓度极低,未检测到Br2。烟气中HF、HC1、SO2和NO的浓度与氧化态汞的比例均显示了一定的正相关性。对本次采样结果的预测显示模型具有一定的精度。更多还原

【Abstract】 Mercury is an extremely toxic trace element, can occumulated in organisms and enter hunman body through food chain. Mercruy poisoning can cause neurologic damage and infantile deformity. As a global pollutant, mercury has gained more and more attention. Coal-fired power plant is the main anthropogenic emission source of mercury. However, detail mechanisms about mercury emission and transportation during coal combustion remained unknown, especially in China.Mercury emission, transformation and control characteristics during coal combustion were investigated by tube furnace and drop tube furnace, utilizing mercury continuous emission monitor (Hg CEM). Studies of mercury about the emission characteristics and influences during heating, the speciation and control performance under oxy-fuel combustion, the oxidation by other flue gases, as well as the emission from a power plant were conducted in order to provide theoretical and practical information on mercury control during coal combustion.Mercury emission characteristics, including temperature and tensity, as well as speciation in flue gas during coal heating were investigated on a tube furnace. The mercury emission characteristics were analysed and used to identify the mercury form and distribution in coal. The relationship between mercury and coal sulfur was also studied. Results showed that Hg emission concentration displayed a multi-peak distribution. The first peak (P1) had the highest emission tensity at the temperature of 360℃-460℃, the second peak (P2), appeared at 445℃-850℃while the third peak (P3) corresponding to temperature higher than 930℃. The concentration of mercury released in P2 could be higher or lower than P3, depending on the coal type. The Hg0(g) percentage in separate Hg peaks was in the range of 38%-100%, in the order of P1>P2>P3. With the mercury emission characteristics and sink-float seperation process, the main forms of mercury and its content in coal were predicted. Mercury in coal is associated with sulfur, for some coals mercury and SO2 released at the same temperature.Mercury emission and speciation during coal combustion under O2/CO2 atmosphere were conducted on a tube furnace and a drop tube furnace. Influences of O2 concentration and CaO/Fe2O3 were also investigated. Results showed that the initial mercury emission was inhibited with both the tensity and temperature of P1 weakened. Mercury concentration and speciation in flue gas differed for different coals. When O2 increased to 30%, the HgT(g) concentration of bituminous coal notablely increased, while Hg (g) concretion was nearly not affected by O2 variation. Mercury control by CaO and the catalytic oxidation of Hg (g) by Fe2O3 can be promoted by O2/CO2 atmosphere. However, the increase of O2 concentration would prompt HgT(g) emission under CaO addition and prompted Hg0(g) oxidation and adsorption by Fe2O3.Homogeneous Hg0(g) oxidation by a single as well as two flue gas components of HCl/Cl2/SO2/NO/NO2 from 200℃-1000℃were investigated on a bench-scale quartz reactor. Results showed that HCl and Cl2 were the main Hg0(g) oxidants:Hg0(g) oxidation by HCl only occurred above 600℃, while Cl2 can significantly oxidize Hg0(g) even at room temperautre. Cl2 sarted to decompose at temperture higher than 400℃. Hg0(g) oxidation by NO was affected by NO concentration and decreased at higher temperature. O2, NO2 and SO2 alone exhibited limited Hg0(g) oxidation, less than 10%. The flue gas interactions effected Hg0(g) oxidation. Hg0(g) oxidation by HCl/NOx or Cl2/NOx was enhanced at 200℃-600℃and inhabited at 800℃～1000℃, wihle the addition of O2 further promoted Hg0(g) oxidation. SO2 strongly inhibited Hg0(g) oxidation by HCl. The coexist of SO2 and NO2 at 400℃can oxidize 48%～63%of the total Hg0(g) and the percent of Hg0(g) oxidized decrease sharply at 1000℃.A field measurement was conducted on a 200 MW pulverized coal fired boiler, using Ontario Hydro method (OHM) and EPA Method 26A to determine the speciation of Hg and halides in postcombustion flue gases, respectively. A model based on BP neural network was established and trained by ICR data in order to predict the mercury emission from real power plants and was compareded with this field data. Results indicated that, the total gas phase mercury (HgT(G)) in flue gas was 6-28μg/Nm3. As the flue gas cooling down, the percentage of oxidized mercury in total gas phase mercury (Hg2+(g)/HgT(G)) increased from 41% to about 74% across the electrostatic precipitator (ESP) outlet. The main halides measured in flue gas were HF and HCl, while the concentration of Cl2 and HBr were extremely low and no Br2 was detected in flue gas. Acid flue gas components, such as HCl, HF, SO2 and NO, showed a certain extent of promotion on Hg oxidation. The measured mercury emission factor (EMF) in this test was 5.63 g/1012J (13.1 1b/1012Btu). The BP neural network model was used to predict the mercury emission from this 200MW pulverized coal fired boiler, and results showed certain practical merits.更多还原