Mercury transformation across particulate control devices in six power plants of China: The co-effect of chlorine and ash composition

This paper presents the results of field measurements on mercury speciation in six power plants of China by applying the Ontario hydro method. During the tests, flue gas was sampled simultaneously before and after particulate control devices (electrostatic precipitator and fabric filter baghouse) along with the pulverized coal, bottom ash and fly ash sampling. The amount of oxidized mercury in gas phase before and after ESP/FF suggests that mercury oxidation after combustion is a kinetically controlled process. The comparison of mercury speciation in different power plant indicates a clear relationship with coal type, especially the chlorine concentration and the basic ash compositions in coal. Both of the factors are analyzed quantitatively in this study. A new parameter C (ratio of chlorine in coal to base/acid ratio) has been introduced to evaluate the co-effect of the two factors above on mercury speciation.     

Bimodal fly ash size distributions and their influence on gas-particle mass transfer during electrostatic precipitation

Electrostatic precipitators (ESPs) have previously been demonstrated to achieve substantial (up to 60–70%) removal efficiency of mercury from coal-fired power plants (CFPPs). However, a high degree of scatter exists in the pilot- and full-scale data, suggesting an incomplete understanding of the mechanism by which mercury is adsorbed within an ESP, particularly by native fly ash. The present analysis explores the influence of bimodal particle size distributions (PSDs) on the gas-particle mass transfer underlying mercury adsorption by fly ash within an ESP. The analysis is motivated by the recent discovery by other investigators of bimodal fly ash PSDs resulting from coal combustion. Results of the present analysis show that, relative to similar monomodal PSDs, bimodal PSDs exhibit greatly increased gas-particle mass transfer potential during electrostatic precipitation. For bimodal PSDs, gas-particle mass transfer potential increases with increasing particle mass in the second mode and decreasing geometric mean diameter and geometric standard deviation of the second mode. A supplemental analysis compares the mercury removal potential of native fly ash, injected fly ash, and injected powdered activated carbon (PAC) during their collection within an ESP, using representative mercury adsorption capacities and particle mass loadings for each. Results showed only marginal differences in mercury removal efficiency between the three sorbents.     

An update on DOE's Phase II and Phase III mercury control technology R&D program

The U.S. Department of Energy's National Energy Technology Laboratory, under the Office of Fossil Energy's Innovations for Existing Plants Program, carried out a comprehensive Hg research and development program for coal-fired power generation facilities since the mid-1990s. Working collaboratively with the U.S. Environmental Protection Agency, the Electric Power Research Institute, power plant operators, state and local agencies, and a host of research organizations and academic institutions, the Program identified the major factors that affect mercury speciation and capture in coal combustion flue gas and funneled this knowledge into the development of a suite of mercury control technologies for the diverse fleet of U.S. coal-fired power plants. The high performance observed during full-scale field testing has given coal-fired power plant operators the confidence to begin deploying technology. As of March 2009, more than 130 full-scale activated carbon injection systems have been ordered by the U.S. coal-fired power generators. These contracts include both new and retrofit installations and represent over 55 GW of coal-based electric generating capacity.     

100MW燃煤电厂非碳基吸附剂喷射脱汞实验研究

选取一个100 MW燃煤电厂对氯化铜改性氧化铝和氯化铜改性沸石进行喷射脱汞实验,应用EPA 30B标准方法对静电除尘器(ESP)前后烟气中的汞价态分布进行了采样和测试.研究了吸附剂喷射量对烟气中汞脱除率的影响.现场测试的汞平衡结果为77.1%～111.5%.经修正的汞平衡结果表明,吸附剂喷射量越大,脱汞率越高.改性氧化铝的脱汞率最高可达30.6%,改性沸石的脱汞率最高可达29.2%.喷射非碳基吸附剂后烟气中元素汞(Hg⁰)显著下降,在喷射量为0.22 g·m^-3时,两种吸附剂可将烟气中Hg⁰比例由40%降低至22%左右,减少的Hg⁰主要转化为Hg^P.非碳基吸附剂与湿法脱硫(WFGD)系统协同使用可以有效减少Hg⁰向大气中的排放.     

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.     

Experimental study on mercury transformation and removal in coal-fired boiler flue gases

This paper reported mercury speciation and emissions from five coal-fired power stations in China. The standard Ontario Hydro Method (OHM) was used into the flue gas mercury sampling before and after fabric filter (FF)/electrostatic precipitator (ESP) locations in these coal-fired power stations, and then various mercury speciation such as Hg⁰, Hg²⁺ and Hg^P in flue gas, was analyzed by using EPA method. The solid samples such as coal, bottom ash and ESP ash, were analyzed by DMA 80 based on EPA Method 7473. Through analysis the mercury speciation varied greatly when flue gas went through FF/ESP. Of the total mercury in flue gas, the concentration of Hg²⁺ is in the range of 0.11–14.76 μg/N m³ before FF/ESP and 0.02–21.20 μg/N m³ after FF/ESP; the concentration of Hg⁰ ranges in 1.18–33.63 μg/N m³ before FF/ESP and 0.77–13.57 μg/N m³ after FF/ESP, and that of Hg^P is in the scope of 0–12.11 μg/N m³ before FF/ESP and 0–0.54 μg/N m³ after FF/ESP. The proportion of Hg²⁺ ranges from 4.87%–50.93% before FF/ESP and 2.02%–75.55% after FF/ESP, while that of Hg⁰ is between 13.81% – 94.79% before FF/ESP and 15.69%–98% after FF/ESP, with that of Hg^P is in the range of 0%–45.13% before FF/ESP and 0%–11.03% after FF/ESP. The mercury in flue gas mainly existed in the forms of Hg⁰ and Hg²⁺. The concentrations of chlorine and sulfur in coal and flue gas influence the species of Hg that are formed in the flue gas entering air pollution control devices. The concentrations of chlorine, sulfur and mercury in coal and the compositions of fly ash had significant effects on mercury emissions.     

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.     

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.     

Influence of SO3 on mercury removal with activated carbon: Full-scale results

Activated carbon injection is considered one of the most cost-effective options for mercury control at PRB-fired power plants. However, roughly 30% of sites firing PRB coal use SO₃ for flue gas conditioning. The presence of SO₃ in flue gas can decrease mercury capture by activated carbon, sometimes dramatically. Overcoming activated carbon performance limitations caused by SO₃ conditioning for units with this configuration is essential to enable these plants to cost-effectively meet pending mercury emission regulations. Ameren's Labadie Unit 2 fires PRB coal and uses SO₃ to enhance particulate capture in the electrostatic precipitator (ESP). Full-scale sorbent injection tests at Labadie were conducted from 2005–2007. Six sorbents were tested at SO₃ injection concentrations ranging from 0 to 10.7 ppm. Sorbent performance was evaluated at two injection locations (the air preheater (APH) inlet and outlet). Native mercury capture on fly ash was typically less than 15%. When the mercury sorbents were injected downstream of the air preheater, the SO₃ concentration resulted in a decrease in mercury capture from 85% (no SO₃ injection) to 17% (SO₃ injection set at 10.7 ppm). Mercury sorbents were more effective when injected upstream of the air preheater. With the SO₃ system off, mercury removal increased from 75% when injecting 5.1 lb/MMacf of brominated carbon at the APH outlet, compared to 95% when injecting at the inlet. With the SO₃ system on, test results indicated an increase from about 30% injecting at the outlet to 58% injecting at the inlet. Tests evaluating the ADA-ES patented onsite milling process showed that 85% mercury capture was achieved injecting 4 lb/MMacf of milled activated carbon compared to a requirement of 10 lb/MMacf to achieve the same removal using as-received carbon, representing a 60% reduction in activated carbon consumption. No changes in opacity, APH and ESP performance, or other balance-of-plant effects were observed in these tests.     

Simulation of mercury emission control by activated carbon under confined-bed operations

Mercury emissions from coal-fired power plants have been a great environmental and regulatory concern due to the toxic nature of mercury and the significant amount of emissions from these plants. An effective method for controlling mercury emission is to employ activated carbon to adsorb mercury from the combustion flue gas. In this study, an activated carbon mercury sorption model was applied to simulate a confined-bed mercury emission control process. Model simulations were performed to generate dynamic mercury concentration profiles and the corresponding profiles of mercury uptake by activated carbon at various bed locations under various process conditions. The simulation parameters included flue gas flow rate, inlet mercury concentration, and adsorption bed temperature. The study has demonstrated the applicability of the model for simulating the process and provided insights into the mercury control process especially the effects of flue gas flow rate, inlet mercury concentration, and activated carbon bed temperature on the process. Such information is critically needed in the design and operation of a mercury emission control process involving activated carbon adsorption.     

UV/H_2O_2高级氧化工艺脱汞试验研究

烟气中的汞污染是燃煤电厂污染源之一,将烟气中的Hg~0氧化脱除是脱汞的重要思路。采用光化学鼓泡反应器研究UV/H_2O_2高级氧化工艺脱汞过程,结果表明,增加UV的功率可以提高Hg~0去除率;随着H_2O_2浓度提高,Hg~0去除率先上升后趋于下降;随着Hg~0初始浓度提高,Hg~0去除率呈下降趋势;而Hg~0去除率与溶液体积成正比。该UV/H_2O_2高级氧化工艺可为燃煤电厂烟气脱汞提供新思路。     

燃煤电厂协同脱汞研究进展及强化措施

燃煤电厂是大气汞排放的重要源头,但是我国目前尚无完善的烟气汞控制方案。本文简要综述了国内外烟气脱汞技术研究现状,统计了国内污控设备（包括脱硝设备、除尘设备和脱硫设备）的装机容量。指出污控设备对烟气汞具有一定的协同脱除作用,但是受到我国煤质及运行条件等因素的制约,效果并不理想。本文结合国内某燃煤电厂的实测情况,提出了以下强化措施：①通过添加溴盐溶液,提高选择性催化还原（SCR）对烟气汞的氧化效率;②通过粉末活性炭与溴盐联合使用,强化静电除尘器（ESP）对烟气汞的协同脱除效率,脱汞效率可达90%以上;③通过精确控制脱硫浆液的pH值以及定期外排脱硫浆液,以降低其中汞的再释放率,维持湿法脱硫工艺（WFGD）稳定的烟气汞协同脱除效率;④通过优化和调整锅炉运行条件,提高现有污控设备体系的协同脱汞能力。     

超低排放燃煤电厂中SO3的迁移及脱除特征

燃煤烟气中的SO₃会对机组运行及大气环境造成不利影响。为研究燃煤电厂SO₃排放特征,本文采取异丙醇吸收法对某300MW超低排放机组污染物控制装置进出口SO₃采样,以分析SO₃在燃煤机组中的迁移及脱除特性。结果表明：炉膛燃烧过程以及选择性催化还原装置（selective catalytic reduction,SCR）均将部分SO₂转化为SO₃,炉膛燃烧生成SO₃的质量浓度为SO₂的0.86%,SCR内SO₂/SO₃转化率为0.45%。烟气经过空气预热器,SO₃浓度降低了5.7%;静电除尘器（electrostatic precipitator,ESP）脱除SO₃效果较差,主要由于ESP内烟温在110℃以上,H₂SO₄酸雾凝结量较少;双级湿法脱硫装置（wet flue gas desulfurization,WFGD）对SO₃脱除效率达到81.3%,比国内单级脱硫装置SO₃脱除效果高30%～50%;湿式静电除尘器（wet electrostatic precipitator,WESP）脱除SO₃效率为23.0%。机组烟囱排放SO₃质量浓度为2.025mg/m³（标准）,SO₃排放因子EF为0.034kg/t。     

新形势下燃煤电厂汞排放问题探讨

燃煤电厂汞污染已经成为继SO2污染之后的又一重大污染问题。燃煤烟气中汞污染的控制研究是目前重要的环保课题之一,开发高效、低成本、无二次污染的烟气脱汞技术已成为研究重点。本文综述了国内外在汞的监测方法、汞在燃煤电厂中的迁移规律以及汞污染物控制措施等方面的研究进展,并做了简要的分析与总结,对未来燃煤烟气汞污染控制技术的发展进行了展望。     

Effect of selective catalytic reactor on oxidation and enhanced removal of mercury in coal-fired power plants

The present study investigated the variation of mercury (Hg) speciation within the air pollution control devices (APCDs) in bituminous coal-fired power plants. The effect of selective catalytic reduction (SCR) system, which is mainly installed for NOx removal, on elemental Hg (Hg⁰) oxidation and enhancement of Hg removal within APCDs, was studied. Hg speciations in flue gas at the inlet and outlet of each APCDs, such as SCR, cold-side electrostatic precipitator (CS-ESP) and flue gas desulphurization (FGD), were analyzed. Sampling and analysis were carried out according to Ontario Hydro Method (OHM). Overall Hg removal efficiency of APCDs, on average, was about 61% and 47% with and without SCR system, respectively. In the flue gas, Hg was mainly distributed in gaseous (elemental and oxidized) form. The oxidized to elemental Hg partitioning coefficient increased due to oxidation of Hg⁰ across the SCR system and decreased due to the removal of oxidized Hg (Hg²⁺) across a wet FGD system. Hg⁰ oxidation across the SCR system varied from 74% to 7% in tested coal-fired power plants. The comparative study shows that the installation of an SCR system increased Hg removal efficiency and suppressed the reemission of captured Hg⁰ within a wet FGD system.     

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.     

A technique to control mercury from flue gas: The Thief Process

The Thief Process is a mercury removal process that may be applicable to a broad range of pulverized coal-fired combustion systems. This is one of several sorbent injection technologies under development by the U.S. Department of Energy for capturing mercury from coal-fired electric utility boilers. A unique feature of the Thief Process involves the production of a thermally activated sorbent in situ at the power plant. The sorbent is obtained by inserting a lance, or thief, into the combustor, in or near the flame, and extracting a mixture of partially combusted coal and gas. The partially combusted coal or sorbent has adsorptive properties suitable for the removal of vapor-phase mercury at flue gas temperatures that are typical downstream of a power plant preheater. One proposed scenario, similar to activated carbon injection (ACI), involves injecting the extracted sorbent into the downstream ductwork between the air preheater and the particulate collection device of the power plant. Initial laboratory-scale and pilot-scale testing, using an eastern bituminous coal, focused on the concept validation. Subsequent pilot-scale testing, using a Powder River Basin (PRB) coal, focused on the process development and optimization. The results of the experimental studies, as well as an independent experimental assessment, are detailed. In addition, the results of a preliminary economic analysis that documents the costs and the potential economic advantages of the Thief Process for mercury control are discussed.     

320MW燃煤电厂痕量元素的分布、脱除及排放特性

燃煤电厂痕量元素的排放已经引起了世界的广泛关注。在一台配置选择性催化还原（SCR）+静电除尘器（ESP）+湿法脱硫装置（WFGD）的320MW燃煤电厂上进行了12种痕量元素（Cr、Mn、Co、Ni、Cu、Zn、As、Mo、Cd、Sb、Ba、Pb）排放特性的实验研究,使用了US EPA Method 29对4个测点烟气痕量元素进行同时取样,考察了痕量元素在电厂中的分布、协同脱除以及在烟囱中的排放。结果表明：锅炉、SCR、ESP、WFGD和整个系统的痕量元素质量平衡率均在可接受的范围内。这12种痕量元素主要分布在底渣和飞灰中,分别占据底渣、ESP灰、WFGD脱除及烟囱排放痕量元素总量的1.90%～27.6%和72.3%～98.0%,然而,它们在烟囱和被WFGD脱除的部分所占比例较少,两者之和仅占0.11%～0.66%。ESP和WFGD对烟气痕量元素的脱除率分别为99.39%～99.95%和40.39%～78.98%,SCR+ESP+WFGD对烟气痕量元素的总体脱除率为99.79%～99.99%。ESP对痕量元素较高的脱除效率是APCDs系统具有较高的协同脱除效率的主要原因。烟囱排放的痕量元素浓度及排放因子分别为0.01～12.88 μg·m^-3和（0.002～4.57）×10^-12 g·J^-1。应进行更多的燃煤电厂痕量元素排放的研究,以便为中国燃煤电厂痕量元素的排放预测模型的建立以及相关标准的制定提供参考。     

燃煤电厂吸附剂喷射脱汞技术的研究进展

燃煤汞污染已引起广泛关注。燃煤电厂控制汞排放最成熟可行的技术是烟道活性炭喷射技术,但该技术在我国燃煤电厂的广泛应用还存在较多的科学问题,因为活性炭对烟气汞的脱除是包含吸附、扩散、传质及化学反应在内的多元化过程,因此,针对燃煤电厂吸附剂喷射脱汞技术的研究已成为当前的热点课题。本文从吸附剂喷射脱汞技术原理、脱汞吸附剂的评价方法、汞吸附机理研究以及吸附剂喷射脱汞数学模型方面评述了燃煤电厂吸附剂喷射脱汞技术近些年取得的研究进展,并在此基础上提出了开发廉价高效、可再生的脱汞吸附剂,全面深入研究吸附剂的脱汞机理以及开发简单、精确的吸附剂喷射脱汞数学模型等后续研究方向,可为我国燃煤电厂吸附剂喷射脱汞技术的开发提供一定指导。     

Characteristics and composition of fly ash from Canadian coal-fired power plants

Fly ashes were collected from the electrostatic precipitator (ESPs) and/or the baghouse of seven coal-fired power plants. The fly ashes were sampled from power plants that use pulverized subbituminous and bituminous feed coals. Fly ash from bituminous coals and limestone feed coals from fluidized-bed power plant were also sampled. The fly ashes were examined for their mineralogies and elemental compositions. The fly ashes from pulverized low sulfur coals are ferrocalsialic, those from high sulfur coals are ferrosialic and the fly ashes from the fluidized bed coals are ferrocalcic. The concentrations of As, Cd, Hg, Mo, Ni, and Pb in fly ash are related to the S content of the coal. Generally, those feed coals with a high S content contain higher concentrations of these elements. The concentrations of these elements are also greater for baghouse fly ash compared to ESP fly ash for the same station. The S content of fly ash from high S coal is 0.1% for pulverized ESP fly ash and 7% for baghouse fly ash from the fluidized bed, indicating that most of the S is captured by fly ash in the fluidized bed. The baghouse fly ash from the fluidized bed has the highest content of Cd, Hg, Mo, Pb, and Se, indicating that CaO, for the most part, captures them. Arsenic is captured by calcium-bearing minerals and hematite, and forms a stable complex of calcium or a transition metal of iron hydroxy arsenate hydrate [(M²⁺)₂Fe₃(AsO₄)₃(OH)₄·10H₂O] in the fly ash. Most elements in fly ash have enrichment indices of greater than 0.7 indicating that they are more enriched in the fly ash than in the feed coal, except for Hg in all ESP ashes. Mercury is an exception; it is more enriched in baghouse fly ash compared to ESP. Fly ash collected from a station equipped with hot side ESP has a lower concentration of Hg compared to stations equipped with cold side ESP using feed coals of similar rank and mercury content. Fly ash particles from fluidized bed coal are angular and subangular with cores of quartz and calcite. The quartz core is encased in layer(s) of calcium-rich aluminosilicates, and/or calcium/iron oxides. The calcite core is usually encased in an anhydrite shell.