碳基吸附剂和SCR催化剂在燃煤烟气脱汞中的应用进展

介绍了吸附和催化氧化脱汞技术。其中,分别重点介绍了CeCl_2浸渍活性焦炭对单质汞具有很好的吸附效果,CeO_2修饰的Mn Ox/TiO_2催化剂能够同时脱除烟气中的Hg和NO。     

燃煤烟气中单质汞吸附与氧化机理研究进展

燃煤电厂汞排放控制是当前的研究热点,实现燃煤烟气中汞的脱除涉及两个非常重要的过程：吸附与氧化。本文概述了汞在吸附剂表面吸附的相关理论和单质汞的催化氧化机理,结合碳基和非碳基吸附剂相关吸附特性的研究现状,对汞的吸附机理进行了讨论,回顾了SCR、碳基、以及金属和金属氧化物三种常见催化剂对单质汞的催化氧化性能并总结了其可能存在的机理。指出吸附剂表面活性位是决定其对汞吸附效果的关键因素,异相反应是单质汞氧化的重要途径,吸附和氧化是相辅相成的,不同催化剂不同气氛下氧化机理不同。并提出通过氯化物等物质改性能够提高吸附剂的吸附效果,深入研究汞的异相反应机理并开发经济有效的汞脱除方法是今后的研究方向。     

汞在MnO_x-CeO_2/γ-Al_2O_3催化剂表面的赋存形态分析

为了高效经济地控制燃煤电站汞的排放,探索新型SCR催化剂上汞的催化氧化具有重要的学术价值和广阔的应用前景。Hg0在Mn Ox-Ce O2/γ-Al2O3(Mn Ce15)催化剂上被氧化后会以多种形态存在于催化剂表面,为确定Mn Ce15催化剂上汞的化合物的赋存形态,使用程序升温热分解方法研究了纯汞化合物的分解曲线,并与在不同烟气条件下处理过的Mn Ce15催化剂的分解脱附曲线进行了对比。研究确认了反应中Hg O、Hg Cl2、Hg(NO3)2和Hg SO4的生成,并根据实验结果分析了相应的汞化合物生成的反应路径。结果表明,在模拟烟气条件下Hg Cl2是主要的异相催化反应产物,同时可以在催化剂表面检测到少量Hg O和Hg SO4的生成。这一结论可为研究SCR催化剂上Hg0的催化氧化机理提供基础。     

WFGD系统添加氧化剂除汞的研究进展

石灰石-石膏湿法脱硫系统(WFGD)是现有燃煤电厂主要采用的脱硫系统,利用已建成的WFGD实现烟气同时脱硫除汞是最为经济合理除汞途径。WFGD可吸收烟气中Hg2+,然而不能有效地将Hg0氧化为Hg2+,如何将烟气中Hg0转化为Hg2+是烟气同时脱硫除汞亟需解决的关键问题。本文主要对WFGD系统中添加氧化剂进行综述并提出些许展望。     

氯元素对燃煤烟气脱汞的影响研究进展

燃煤电厂是最主要的人为汞排放污染源,氯元素对汞的形态转化及脱除率有非常重要的影响。本文概述了燃煤电厂汞的释放特性和现有控制技术,从氯元素作为烟气组分、活性炭改性物以及燃料添加剂这3个方面详细阐述了氯对汞排放控制的影响。首先氯化氢作为烟气组分,对单质汞向氧化态汞的形态转化有促进作用,这有利于现有除尘、脱硫装置对烟气汞的脱除。含氯化合物改性活性炭吸附剂时,物理吸附和化学吸附同时存在,这能有效提高吸附剂对汞的吸附性能。氯化物作为燃煤添加剂也能有效促进烟气汞的氧化和脱除,其中氯元素在湿法脱硫废水中富集,如何把其利用到烟气汞的脱除对开发高效脱汞技术有重要的意义。同时,比较了以上3种氯添加方案的优缺点。最后指出,深入研究氯元素对汞作用机理是今后的研究方向。     

光催化脱硫脱硝脱汞技术研究进展

燃煤烟气中二氧化硫(SO2)、一氧化氮(NO)和汞(Hg)的大量排放已经造成严重的环境污染,影响人体健康.光催化技术是一种高级氧化技术,广泛应用于处理大气污染中的多种有害气体.基于光催化氧化技术对二氧化硫、一氧化氮和汞具有良好的脱除效果,本文综述了光催化氧化机理及光催化脱硫脱硝、光催化脱汞和同时脱硫脱硝脱汞技术的研究进...     

多壁碳纳米管负载FeS2的制备及脱汞性能分析

采用硝酸铁与硫脲在溶剂热反应条件下制备FeS₂,利用物理浸渍法将FeS₂负载至多壁碳纳米管（MWCNTs）上,借助扫描电镜（SEM）、X射线衍射（XRD）、X光电子能谱（XPS）等分析测试手段对制备样品的结构特性进行探究,通过固定床反应器研究模拟烟气氛围下吸附剂对烟气中单质汞的吸附特性。实验考察了不同FeS₂负载量、烟气初始汞浓度、床层温度以及O₂、NO和SO₂对吸附剂脱汞效率的影响。结果表明,制备出的FeS₂分散性较好,呈球状晶体,表面均匀覆盖着MWCNTs,成团簇状。当负载量为10%、反应温度为70℃时,FeS₂/MWCNTs的吸附效果最好,最高脱除效率能达到100%,60min后脱除效率仍有80.3%。TPD脱附曲线和XPS分析结果进一步表明烟气中的Hg⁰被氧化成Hg²⁺以HgS的形式附着在吸附剂表面,证实吸附剂以化学吸附为主。此外,汞脱除效率随初始汞浓度的增加而降低,汞吸附容量却随之增加,60min最高能达到5.1μg/g。酸性气体NO和SO₂的存在,占据了吸附剂表面的活性点位,不利于Hg⁰的吸附,但低浓度NO对吸附剂的整体效果影响不大,抗NO性能较好。     

湿式氧化法脱除燃煤烟气中多污染物的研究进展

燃煤烟气排放的大量气态污染物危害严重,分级治理系统庞大、投资昂贵。湿式脱硫法目前应用广泛,在此基础上加以改进开发同时脱硫脱硝脱汞技术具有必要性和可行性。扼要总结了国内外湿式氧化法在烟气中SO2、NO和Hg0脱除方面的研究概况,包括次氯酸盐氧化法、过硫酸氧化法、H2O2氧化法。介绍了各方法的研究现状、特点、存在问题等,为该方法实现更高效的烟气净化效率提供参考。     

改性燃煤飞灰吸附氧化脱汞机理研究进展

元素汞是燃煤电站汞污染防治工作的难点,研究开发高效价廉的汞吸附剂已被作为科技攻关重点方向。燃煤飞灰经卤化物改性后脱汞潜力强、成本低,但必须研究掌握其吸附氧化Hg0的作用机制。本文系统综述了飞灰作为脱汞材料在国内外燃煤烟气汞污染控制方面的研究进展。论述了物化特性、烟气组分、卤素等因素对飞灰脱汞性能的影响,指出未燃尽炭和无机矿物组分表面活性位上的非均相反应是Hg0氧化脱除的关键。分析了残炭和磁珠作为载体材料的优缺点,并讨论了不同改性剂和改性方法对飞灰直接改性的影响。总结了HBr改性飞灰对Hg0的吸附氧化机理,并提出利用量子化学理论与宏观实验研究相结合的方式探索Hg0在飞灰表面上的成键特征及反应动力学特性应作为今后的研究方向。     

燃烧温度对煤中汞析出特性及形态分布的影响

在管式炉实验装置上研究了燃烧温度对燃煤汞析出特性的影响,利用在线烟气分析仪和EPA Method 26A湿化学法对燃煤烟气中O_2/SO_2/NO/CO/HCl/Cl_2等进行同步定量分析,探讨不同燃烧温度下烟气组分的变化及其在烟气降温过程中对汞形态迁移转化的影响规律。结果表明:在高温燃烧条件(≥750℃)下,煤中汞基本全部逸出,析出率≥97.9%,且随燃烧温度升高而递增;基于氧量分析,煤中汞的析出在挥发分燃烧阶段即开始发生;随着燃烧温度的升高,烟气中元素态汞(Hg0)份额逐渐降低,而氧化态汞(Hg~(2+))份额相应增大,900℃时接近50%;燃烧温度对烟气中各组分浓度影响较大,随温度升高,烟气中NO、HCl及Cl_2呈现不同程度的上升趋势,SO_2含量基本持平,而CO含量逐渐降低;对痕量元素汞而言,NO、HCl及Cl_2通过直接或间接氧化作用在烟气降温过程中促进Hg0向Hg~(2+)转化,烟气中还原性气体组分CO对Hg0的氧化有一定抑制作用,SO_2对汞形态分布的影响则不甚明显。     

Mercury speciation and emission from the coal‐fired power plant filled with flue gas desulfurization equipment

Mercury speciation and emission from two Chinese coal‐fired power stations equipped with flue gas desulfurization device were investigated. Research results reveal that Hg⁰ is the main form in the flue gas in Plant 1; Hg²⁺ is the main form in the flue gas in Plant 2. Most of mercury was emitted to the atmosphere, which was about 77–98%, and the elemental mercury released to atmosphere ranged 73–94% approximately. A pot of mercury is adsorbed by bottom ash, electrostatic precipitator (ESP) ash, and gypsum in Plant 1. However, most mercury, the scale of which is 75–83.2%, is collected by ESP ash, and only 7.0–12.2% mercury is emitted to the atmosphere in Plant 2. The mercury removal by NID semi‐desulfurization system is higher than wet flue gas desulfurization (WFGD) desulfurization system.     

Kinetic modeling of H2O2-enhanced oxidation of flue gas elemental mercury

Mercury emissions from coal-fired power plants account for 40% of the anthropogenic mercury emissions in the U.S. The speciation of mercury largely determines the amount of mercury capture in control equipments. Conversion of insoluble Hg⁰ into more soluble Hg²⁺ facilitates its removal in scrubbers. Past studies suggest that an added supply of OH radicals possibly enhance the mercury oxidation process. This study demonstrates that the application of H₂O₂, as source of OH radicals, accelerates the oxidation of Hg⁰ into Hg²⁺. A detailed kinetic reaction mechanism was compiled and the reaction pathways were established to analyze the effect of H₂O₂ addition. The optimum temperature range for the oxidation was 480–490 °C. The sensitivity analysis of the reaction mechanism indicates that the supply OH radicals increase the formation of atomic Cl, which accelerates the formation of HgCl₂ enhancing the oxidation process. Also, the pathway through HOCl radical, generated by the interactions between chlorine and H₂O₂ was prominent in the oxidation of Hg⁰. The flue gas NO was found to be inhibiting the Hg⁰ oxidation, since it competed for the supplied H₂O₂. Studying the interactions with the other flue gas components and the surface chemistry with particles in the flue gas could be important and may improve the insight into the post combustion transformation of mercury in a comprehensive way.     

Heterogeneous oxidation of mercury in simulated post combustion conditions

Heterogeneous mercury oxidation was studied by exposing whole fly ash samples and magnetic, nonmagnetic, and size-classified fly ash fractions to elemental mercury vapor in simulated flue gas streams. Fly ash from sub-bituminous Wyodak-Anderson PRB coal and bituminous Blacksville coal were used. Scanning electron microscopy, X-ray diffraction, thermogravimetric analyses, and BET N₂ isothermal sorption analyses were performed to characterize the fly ash samples. Mercury speciation downstream from the ash was determined using the Ontario Hydro method. Results showed that the presence of fly ash was critical for mercury oxidation, and the surface area of the ash appears to be an important parameter. However, for a given fly ash, there were generally no major differences in catalytic oxidation potential between different fly ash fractions. This includes fractions enriched in unburned carbon and iron oxides. The presence of NO₂, HCl, and SO₂ resulted in greater levels of mercury oxidation, while NO inhibited mercury oxidation. The gas matrix affected mercury oxidation more than the fly ash composition.     

Mercury speciation and its emissions from a 220 MW pulverized coal-fired boiler power plant in flue gas

Distributions of mercury speciation of Hg⁰, Hg²⁺ and Hg ^P in flue gas and fly ash were sampled by using the Ontario Hydro Method in a 220 MW pulverized coal-fired boiler power plant in China. The mercury speciation was varied greatly when flue gas going through the electrostatic precipitator (ESP). The mercury adsorbed on fly ashes was found strongly dependent on unburnt carbon content in fly ash and slightly on the particle sizes, which implies that the physical and chemical features of some elemental substances enriched to fly ash surface also have a non-ignored effect on the mercury adsorption. The concentration of chlorine in coal, oxyge nand NO _x in flue gas has a positive correlation with the formation of the oxidized mercury, but the sulfur in coal has a positive influence on the formation of elemental mercury. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

Catalytic performance of CuCl<Subscript>2</Subscript>-modified V<Subscript>2</Subscript>O<Subscript>5</Subscript>-WO<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> catalyst for Hg<Superscript>0</Superscript> oxidation in simulated flue gas

CuCl₂-SCR catalysts prepared by an improved impregnation method were studied to evaluate the catalytic performance for gaseous elemental mercury (Hg⁰) oxidation in simulated flue gas. Hg⁰ oxidation activity of commercial SCR catalyst was significantly improved by the introduction of CuCl₂. Nitrogen adsorption, XRD, XRF and XPS were used to characterize the catalysts. The results indicated that CuCl₂ was well loaded and highly dispersed on the catalyst surface, and that CuCl₂ played an important role for Hg⁰ catalytic oxidation. The effects of individual flue gas components on Hg⁰ oxidation were also investigated over CuCl₂-SCR catalyst at 350 oC. The co-presence of NO and NH₃ remarkably inhibited Hg⁰ oxidation, while this inhibiting effect was gradually scavenged with the decrease of GHSV. Further study revealed the possibility of simultaneous removal of Hg⁰ and NO over CuCl₂-SCR catalyst in simulated flue gas. The mechanism of Hg⁰ oxidation was also investigated.     

Analysis of mercury species present during coal combustion by thermal desorption

Mercury in coal and its emissions from coal-fired boilers is a topic of primary environmental concern in the United States and Europe. The predominant forms of mercury in coal-fired flue gas are elemental (Hg⁰) and oxidized (Hg²⁺, primarily as HgCl₂). Because Hg²⁺ is more condensable and far more water soluble than Hg⁰, the wide variability in mercury speciation in coal-fired flue gases undermines the total mercury removal efficiency of most mercury emission control technologies. It is important therefore to have an understanding of the behaviour of mercury during coal combustion and the mechanisms of mercury oxidation along the flue gas path. In this study, a temperature programmed decomposition technique was applied in order to acquire an understanding of the mode of decomposition of mercury species during coal combustion. A series of mercury model compounds were used for qualitative calibration. The temperature appearance range of the main mercury species can be arranged in increasing order as HgCl₂ < HgS < HgO < HgSO₄. Different fly ashes with certified and reference values for mercury concentration were used to evaluate the method. This study has shown that the thermal decomposition test is a newly developed efficient method for identifying and quantifying mercury species from coal combustion products.     

Mercury emissions from six coal-fired power plants in China

Mercury emission field measurements based on the Ontario Hydro Method (OHM) were conducted for six coal-fired power plants in China. The mercury mass balances for the six power plants varied from 100.3% to 139.5% of the input coal mercury for the whole system. About 0.02%–1.2% of the mercury remained in the bottom ash. In the first five power plants equipped with pulverized coal boiler, most of the mercury was emitted from the stack to the atmosphere. The plants with Electrostatic Precipitator (ESP) system emitted more Hg⁰ than Hg²⁺, while the plants with the Fabric Filter (FF) emitted less Hg⁰ than Hg²⁺. Virtually all of the Hg^P enter the ESP or the FF was removed. The FF systems had better Hg⁰ and Hg²⁺ removal efficiencies than the ESP systems. The flue gas desulfurization (FGD) system removed up to 78.0% of Hg²⁺ and only 3.14% of Hg⁰ in the flue gas, while 8.94% of the original mercury in the coal was removed by the FGD system. The average mercury removal efficiencies of the ESP systems was 11.5%, that of the FF systems was 52.3% and that of the combined ESP + FGD system was 13.7%, much lower than the average removal efficiencies of pollution control device systems in US plants which have been used in previous studies of Chinese mercury emission inventory. Hg⁰, rather than Hg²⁺ as assumed in previous estimates, has been found to be the dominant species emitted in the atmosphere. The average emission factor was found to be 4.70 g/TJ (10.92 bl/Tbtu), which is much higher than for US plants burning bituminous coals due to the high mercury content in the Chinese coal and the low mercury removal efficiency of air pollution control devices of power plants.     

Understanding the effects of sulfur on mercury capture from coal-fired utility flue gases

Coal combustion continues to be a major source of energy throughout the world and is the leading contributor to anthropogenic mercury emissions. Effective control of these emissions requires a good understanding of how other flue gas constituents such as sulfur dioxide (SO₂) and sulfur trioxide (SO₃) may interfere in the removal process. Most of the current literature suggests that SO₂ hinders elemental mercury (Hg⁰) oxidation by scavenging oxidizing species such as chlorine (Cl₂) and reduces the overall efficiency of mercury capture, while there is evidence to suggest that SO₂ with oxygen (O₂) enhances Hg⁰ oxidation by promoting Cl₂ formation below 100 °C. However, studies in which SO₂ was shown to have a positive correlation with Hg⁰ oxidation in full-scale utilities indicate that these interactions may be heavily dependent on operating conditions, particularly chlorine content of the coal and temperature. While bench-scale studies explicitly targeting SO₃ are scarce, the general consensus among full-scale coal-fired utilities is that its presence in flue gas has a strong negative correlation with mercury capture efficiency. The exact reason behind this observed correlation is not completely clear, however. While SO₃ is an inevitable product of SO₂ oxidation by O₂, a reaction that hinders Hg⁰ oxidation, it readily reacts with water vapor, forms sulfuric acid (H₂SO₄) at the surface of carbon, and physically blocks active sites of carbon. On the other hand, H₂SO₄ on carbon surfaces may increase mercury capacity either through the creation of oxidation sites on the carbon surface or through a direct reaction of mercury with the acid. However, neither of these beneficial impacts is expected to be of practical significance for an activated carbon injection system in a real coal-fired utility flue gas.     

Novel sorbents for mercury emissions control from coal-fired power plants

The authors have successfully developed novel efficient and cost-effective sorbents for mercury removal from coal combustion flue gases. These sorbents were evaluated in a fixed-bed system with a typical PRB subbituminous/lignite simulated flue gas, and in an entrained-flow system with air simulating in-flight mercury capture by sorbent injection in the ductwork of coal-fired utility plants. In both systems, one of the novel sorbents showed promising results for Hg⁰ removal. In particular, this sorbent demonstrated slightly higher efficiencies in Hg⁰ removal than Darco Hg-LH (commercially available brominated activated carbon) at the similar injection rates in the entrained-flow system. The other novel sorbent showed excellent Hg⁰ oxidation capability, and may enable coal-fired power plants equipped with wet scrubbers to simultaneously control their mercury and sulfur oxides emissions. In addition, fixed-bed results for this sorbent showed that co-injection of a very small amount (∼10%) of raw activated carbon could eliminate almost all of the mercury generated by reactions of Hg⁰ with the sorbent.     

Catalytic oxidation and capture of elemental mercury from simulated flue gas using Mn-doped titanium dioxide

Titanium dioxide (TiO₂) and Mn-doped TiO₂ (Mn(x)-TiO₂) were synthesized in a sol-gel method and characterized by BET surface area analysis, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Gasphase elemental mercury (Hg⁰) oxidation and capture by the Mn-doped TiO₂ catalyst was studied in the simulated flue gas in a fixed-bed reactor. The investigation of the influence of Mn loading, flue gas components (SO₂, NO, O₂, and H₂O) showed that the Hg⁰ capture capability of Mn(x)-TiO₂ was much higher than that of pure TiO₂. The addition of Mn inhibits the grain growth of TiO₂ and improves the porous structure parameters of Mn(x)-TiO₂. Excellent Hg⁰ oxidation performance was observed with the catalyst with 10% of Mn loading ratio and 97% of Hg⁰ oxidation was achieved under the test condition (120 °C, N₂/6%O₂). The presence of O₂ and NO had positive effect on the Hg⁰ removal efficiency, while mercury capture capacity was reduced in the presence of SO₂ and H₂O. XPS spectra results reveal that the mercury is mainly present in its oxidized form (HgO) in the spent catalyst and Mn⁴⁺ doped on the surface of TiO₂ is partially converted into Mn³⁺ which indicates Mn and the lattice oxygen are involved in Hg⁰ oxidation reactions.