Impacts of acid gases on mercury oxidation across SCR catalyst

A series of bench-scale experiments were completed to evaluate acid gases of HCl, SO₂, and SO₃ on mercury oxidation across a commercial selective catalytic reduction (SCR) catalyst. The SCR catalyst was placed in a simulated flue gas stream containing O₂, CO₂, H₂O, NO, NO₂, and NH₃, and N₂. HCl, SO₂, and SO₃ were added to the gas stream either separately or in combination to investigate their interactions with mercury over the SCR catalyst. The compositions of the simulated flue gas represent a medium-sulfur and low- to medium-chlorine coal that could represent either bituminous or subbituminous. The experimental data indicated that 5–50 ppm HCl in flue gas enhanced mercury oxidation within the SCR catalyst, possibly because of the reactive chlorine species formed through catalytic reactions. An addition of 5 ppm HCl in the simulated flue gas resulted in mercury oxidation of 45% across the SCR compared to only 4% mercury oxidation when 1 ppm HCl is in the flue gas. As HCl concentration increased to 50 ppm, 63% of Hg oxidation was reached. SO₂ and SO₃ showed a mitigating effect on mercury chlorination to some degree, depending on the concentrations of SO₂ and SO₃, by competing against HCl for SCR adsorption sites. High levels of acid gases of HCl (50 ppm), SO₂ (2000 ppm), and SO₃ (50 ppm) in the flue gas deteriorate mercury adsorption on the SCR catalyst.     

KBr和KI改性黏土脱除模拟烟气中的单质汞

黏土是一种非常廉价的天然矿物,为了考察溴和碘改性黏土（膨润土）脱除烟气中单质汞的特性,采用浸渍法分别制备了KBr和KI改性黏土（KBr-clay和KI-clay）。在固定床吸附实验台上进行了脱除Hg⁰的实验研究,考察不同的负载量、吸附温度以及烟气中SO₂和水蒸气等对KI-clay和KBr-clay脱除单质汞的影响。研究结果表明：改性后黏土的比表面积、孔隙结构未发生显著的改变,表面官能团也没有发生本质的变化。KI-clay和KBr-clay脱汞能力大大提高,在同等实验条件下,KI-clay脱汞性能优于KBr-clay。研究表明单质汞在改性黏土表面产生了化学吸附,烟气中低浓度SO₂的存在对汞脱除有一定的促进作用,在相同的实验条件下,SO₂ 对KBr-clay促进作用更加明显,但水蒸气在低温下对脱汞具有很强的抑制作用,提高吸附温度可以消弱水蒸气的负面影响。     

介质阻挡放电中气体成分对NOx脱除的影响

利用介质阻挡放电(DBD)产生低温等离子体进行烟气的脱硝实验,研究了在乙烯存在的条件下,温度和其他烟气成分对NO_x脱除率的影响。结果表明:随着温度的升高,NO脱除速率增快;模拟烟气中加入CO₂,在能量密度较低时,CO₂作为电负性分子会降低自由基的生成,导致NO的脱除率降低,随着能量密度的升高,CO₂对NO脱除的影响减小;模拟烟气中加入水后可以产生更多的OH、HO₂等自由基,促进NO的氧化;SO₂的加入会与自由基O反应,使初始反应中O与C₂H₄的反应速率减弱,从而影响了NO的氧化速率,但O₃、HO₂等强氧化自由基会优先与NO反应,因此SO₂的加入不会影响NO最终的脱除率。     

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.     

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.     

烟气气相组分及Ca（OH）2对KMnO4氧化NO的影响机理

在固定床反应器中考察了KMnO₄氧化烟气中NO的过程,分析了烟气组分H₂O、O₂及SO₂对NO氧化过程的影响规律,得到了Ca（OH）₂对KMnO₄氧化NO的影响机理。实验结果表明,H₂O是KMnO₄氧化NO的必要条件;在含H₂O条件下,O₂可以提高NO氧化率。SO₂与氧化剂反应生成无水钾镁钒类复盐K₂Mn₂（SO₄）₃对NO氧化具有负面作用;Ca（OH）₂的加入提高了氧化剂表面的固体碱度从而促进氧化过程进行;通过添加Ca（OH）₂可以降低SO₂对NO氧化过程的负面影响。根据气体成分和产物分析可知,KMnO₄在钙基吸收剂表面氧化烟气中NO的机理可能是KMnO₄以离子态将吸附在氧化剂表面的NO和SO₂氧化为NO₂和SO₃,生成的NO₂、SO₃再传递到氧化位临近的碱性位被吸收。     

Investigations on bromine corrosion associated with mercury control technologies in coal flue gas

Mercury control technologies are often associated with adding halogens to the flue gas to enhance oxidation of elemental mercury. The present research was to evaluate the corrosion characteristics of iron in a flue gas containing bromine to simulate mercury control applications in coal-fired utility plants. An AISI 1008 cold rolled steel was exposed to a synthetic flue gas (7.1 vol% O₂, 14.3 vol% CO₂, 2.0 vol% H₂O, 51 ppmv HBr, 510 ppmv SO₂, 51 ppmv NO_x, and the balance N₂). Exposure times ranged from 30 days to 6 months. Metal coupons were exposed with simulated flue gases at 300°, 150°, and 80 F (149°, 66°, and 27 °C), respectively. The corroded coupons were analyzed using scanning electron microscope and micrometer measurements to determine the deposit chemistry and corrosion loss. The corrosion products consisted mainly of iron oxides and iron bromide. A mechanism for HBr corrosion is suggested. Bromine dew point corrosion took place on metal surfaces at temperatures below or close to the dew point of HBr, while active oxidation occurred at higher temperatures.     

UV/H2O2氧化联合Ca(OH)2吸收同时脱硫脱硝

在小型紫外光-鼓泡床反应器中,对UV/H₂O₂氧化联合Ca(OH)₂吸收同时脱除燃煤烟气中NO与SO₂的主要影响因素[H₂O₂浓度、紫外光辐射强度、Ca(OH)₂浓度、NO浓度、溶液温度、烟气流量以及SO₂浓度]进行了考察。采用烟气分析仪和离子色谱仪分别对尾气中的NO₂和液相阴离子作了检测分析。结果显示:在本文所有实验条件下,SO₂均能实现完全脱除。随着H₂O₂浓度、紫外光辐射强度和Ca(OH)₂浓度的增加,NO的脱除效率均呈现先大幅度增加后轻微变化的趋势。NO脱除效率随烟气流量和NO浓度的增加均有大幅度下降。随着溶液温度和SO₂浓度的增加,NO脱除效率仅有微小的下降。离子色谱分析表明,反应产物主要是SO₄^2-和NO₃^-,同时有少量的NO₂^-产生。尾气中未能检测到有害气体NO₂。     

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.     

SO_2对钙基吸收剂吸收NO的作用机理

针对低温条件下SO2对Ca(OH)2吸收NO的影响进行了实验研究,分析了烟气中O2和H2O对SO2促进Ca(OH)2吸收NO的影响。实验结果表明,当烟气不含SO2时,Ca(OH)2对NO基本无吸收作用;烟气中SO2的存在对NO吸收具有促进作用。H2O和O2对SO2促进NO吸收有显著影响;当烟气不含O2时,即使大量的SO2被吸收,NO吸收效率仍较低;只有SO2与O2和H2O共存才能促进NO吸收。脱硫产物CaSO3对NO无氧化作用;NO、H2O和SO2未在吸收剂表面产生可分解释放NO2的大分子中间配合物。分析认为在脱硫过程中产生了可以促进NO与O2反应的非稳定中间活性组分。     

Removal of elemental mercury from simulated coal-combustion flue gas using a SiO2–TiO2 nanocomposite

A novel silica–titania (SiO₂–TiO₂) nanocomposite has been developed to effectively capture elemental mercury (Hg⁰) under UV irradiation. Previous studies under room conditions showed over 99% Hg⁰ removal efficiency using this nanocomposite. In this work, the performance of the nanocomposite on Hg⁰ removal was tested in simulated coal-fired power plant flue gas, where water vapor concentration is much higher and various acid gases, such as HCl, SO₂, and NO_x, are present. Experiments were carried out in a fix-bed reactor operated at 135 °C with a baseline gas mixture containing 4% O₂, 12% CO₂, and 8% H₂O balanced with N₂. Results of Hg speciation data at the reactor outlet demonstrated that Hg⁰ was photocatalytically oxidized and captured on the nanocomposite. The removal efficiency of Hg⁰ was found to be significantly affected by the flue gas components. Increased water vapor concentration inhibited Hg⁰ capture, due to the competitive adsorption of water vapor. Both HCl and SO₂ promoted the oxidation of Hg⁰ to Hg(II), resulting in higher removal efficiencies. NO was found to have a dramatic inhibitory effect on Hg⁰ removal, very likely due to the scavenging of hydroxyl radicals by NO. The effect of NO₂ was found to be insignificant. Hg removal in flue gases simulating low rank coal combustion products was found to be less than that from high rank coals, possibly due to the higher H₂O concentration and lower HCl and SO₂ concentrations of the low rank coals. It is essential, however, to minimize the adverse effect of NO to improve the overall performance of the SiO₂–TiO₂ nanocomposite.     

改性桑树枝焦对模拟烟气中汞的吸附性能

采用固定床热解、蒸汽活化和改性剂(H₂O₂、ZnCl₂和NaCl)浸渍等方法制得不同的桑树枝焦。在固定床吸附实验台上,研究了蒸汽活化、改性剂、吸附温度和烟气组分等对改性桑树枝焦汞吸附性能的影响。结果表明:蒸汽活化显著提高了桑树枝热解焦的比表面积,H₂O₂改性可进一步提高桑树枝蒸汽活化焦比表面积并改善其孔隙结构参数,ZnCl₂和NaCl改性则降低了桑树枝蒸汽活化焦的比表面积、D-R微孔容积和总孔容。10%H₂O₂和30%H₂O₂浸渍改性桑树枝焦的单位汞吸附量分别是蒸汽活化焦的2.02倍和1.77倍;相同改性剂浓度下,ZnCl₂改性焦的单位汞吸附量比NaCl改性焦稍好;随着ZnCl₂浓度增大,改性桑树枝焦的汞吸附性能增强,MT600-A-ZnCl₂(5%)桑树枝焦的单位汞吸附量达到29.55 μg·g^-1,是蒸汽活化焦的3.37倍。在吸附温度为60～120℃范围内,H₂O₂改性焦的汞吸附效率及单位汞吸附量随着吸附温度的升高而下降,而ZnCl₂改性焦的单位汞吸附量则随着吸附温度提高呈现先增大后减小的趋势,最佳吸附温度为90℃。烟气中SO₂和NO组分对汞吸附性能有一定的抑制作用,随着SO₂和NO浓度的增加,汞吸附效率和单位汞吸附量均稍有下降。     

Gas-phase elemental mercury capture by a V2O5/AC catalyst

Gas-phase elemental mercury (Hg⁰) capture by an activated coke (AC) supported V₂O₅ (V₂O₅/AC) catalyst was studied in simulated flue gas and compared with that by the AC. The study on the influences of V₂O₅ loading, temperature, capture time and flue gas components (O₂, SO₂, H₂O and N₂) shows that the Hg⁰ capture capability of V₂O₅/AC is much higher than that of AC. It increases with an increase in V₂O₅ loading and is promoted by O₂, which indicates the important role of V₂O₅ in Hg⁰ oxidation and capture; it is promoted slightly by SO₂ but inhibited by H₂O; it increases with an increase in temperature up to 150 °C when Hg desorption starts. X-ray photoelectron spectroscopy analysis and sequential chemical extraction experiments indicate that the main states of Hg captured on V₂O₅/AC are HgO and HgSO₄. Temperature programmed desorption experiments were also made to understand the stability of the Hg captured.     

Cobenefit of SO3 reduction on mercury capture with activated carbon in coal flue gas

Parametric experiments were carried out to study the interactions of mercury, SO₃, and injected activated carbon (AC) in a coal flue gas stream. The levels of SO₃ vapor in flue gas were altered by individually varying flue gas temperature, moisture, or sodium fume injection in the flue gas. Meanwhile, mercury emissions with AC injection (ACI) upstream of an electrostatic precipitator (ESP) were evaluated under varied SO₃ concentrations. SO₃ measurements using a condensation method indicated that low temperature, high moisture content, and sodium fume injection in flue gas shifted SO₃ partitioning from the vapor to particulate phase, subsequently improving mercury capture with ACI. 0.08 g/m³ of DARCO^® Hg-LH injection only provided approximately 20% mercury reduction across the ESP in a bituminous coal flue gas containing 28 ppm SO₃, but mercury capture was increased to 80% when the SO₃ vapor concentration was lowered less than 2 ppm. Experimental data clearly demonstrate that elevated SO₃ vapor is the key factor that impedes mercury adsorption on AC, mainly because SO₃ directly competes against mercury for the same binding sites and overwhelmingly consumes all binding sites.     

Catalytic removal of SO2, NO and HCl from incineration flue gas over activated carbon-supported metal oxides

Activated carbon-supported copper, iron, or vanadium oxide catalysts were exposed to incineration flue gas to investigate the simultaneous catalytic oxidation of sulfur dioxide/hydrogen chloride and selective catalytic reduction of nitrogen oxide by carbon monoxide. The results show that AC-supported catalysts exhibit higher activities for SO₂ and HCl oxidation than traditional γ-Al₂O₃-supported catalysts and the iron and vanadium catalysts act as catalysts instead of sorbents, and can decompose sulfate with evolution of SO₃ and then regenerate for more SO₂ adsorption to take place. The AC-supported catalysts also display a high activity for NO reduction with CO generated from a flue gas incineration process and the presence of SO₂ in the incineration flue gas can significantly promote catalytic activity. Using CO as the reducing agent for NO reduction is more effective than using NH₃, because NH₃ may be partially oxidized in the presence of excess O₂ (12 vol%. in the incineration flue gas used) to form N₂, which can decrease the overall extent of NO reduction.     

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.     

Characteristics of the mercury vapor removal from coal combustion flue gas by activated carbon using H2S

Removal of Hg⁰ vapor from the simulated coal combustion flue gases with a commercial activated carbon was investigated using H₂S. This method is based on the reaction of H₂S and Hg over the adsorbents. The Hg⁰ removal experiments were carried out in a conventional flow type packed bed reactor system in the temperature range of 80–150 °C using simulated flue gases having the composition of Hg⁰ (4.9 ppb), H₂S (0–20 ppm), SO₂ (0–487 ppm), CO₂ (10%), H₂O (0–15%), O₂ (0–5%), N₂ (balance gas). The following results were obtained: in the presence of both H₂S and SO₂, Hg removal was favored at lower temperatures (80–100 °C). At 150 °C, presence of O₂ was indispensable for Hg⁰ removal from H₂S–SO₂ flue gas system. It is suggested that the partial oxidation of H₂S with O₂ to elemental sulfur (H₂S+1/2O₂=S_ad+H₂O) and the Clause reaction (SO₂+2H₂S=3S_ad+2H₂O) may contribute to the Hg⁰ removal over activated carbon by the following reaction: S_ad+Hg=HgS. The formation of elemental sulfur on the activated carbon was confirmed by a visual observation.     

The PCO process for photochemical removal of mercury from flue gas

A promising technology has been developed to capture and remove elemental mercury species from coal-fired power plants. Powerspan Corp. has licensed the technology and initiated a bench and pilot test program to develop the Photochemical Oxidation, or PCO™, process for commercial application with subbituminous and lignite fuels.The process has the potential to serve as a low cost mercury oxidation technology that will facilitate elemental mercury removal in a downstream SO₂ scrubber, wet electrostatic precipitator (WESP), or baghouse. It uses 254-nm (nanometer) ultraviolet light from a mercury lamp to produce an excited state mercury species in the flue gas, leading to oxidation of elemental mercury. This paper presents results of Powerspan's initial bench-scale testing on a simulated flue gas stream. Preliminary testing conducted in Powerspan's bench-scale facility showed greater than 90% oxidation and removal of elemental mercury. The process also has potential to serve as a low cost method for the removal of mercury from waste incinerator flue gases.     

The PCO process for photochemical removal of mercury from flue gas

A promising technology has been developed to capture and remove elemental mercury species from coal-fired power plants. Powerspan Corp. has licensed the technology and initiated a bench and pilot test program to develop the Photochemical Oxidation, or PCO™, process for commercial application with subbituminous and lignite fuels.The process has the potential to serve as a low cost mercury oxidation technology that will facilitate elemental mercury removal in a downstream SO₂ scrubber, wet electrostatic precipitator (WESP), or baghouse. It uses 254-nm (nanometer) ultraviolet light from a mercury lamp to produce an excited state mercury species in the flue gas, leading to oxidation of elemental mercury. This paper presents results of Powerspan's initial bench-scale testing on a simulated flue gas stream. Preliminary testing conducted in Powerspan's bench-scale facility showed greater than 90% oxidation and removal of elemental mercury. The process also has potential to serve as a low cost method for the removal of mercury from waste incinerator flue gases.     

Investigation of flue-gas treatment with O3 injection using NO and NO2 planar laser-induced fluorescence

Direct ozone (O₃) injection is a promising flue-gas treatment technology based on oxidation of NO and Hg into soluble species like NO₂, NO₃, N₂O₅, oxidized mercury, etc. These product gases are then effectively removed from the flue gases with the wet flue gas desulfurization system for SO₂. The kinetics and mixing behaviors of the oxidation process are important phenomena in development of practical applications. In this work, planar laser-induced fluorescence (PLIF) of NO and NO₂ was utilized to investigate the reaction structures between a turbulent O₃ jet (dry air with 2000 ppm O₃) and a laminar co-flow of simulated flue gas (containing 200 ppm NO), prepared in co-axial tubes. The shape of the reaction zone and the NO conversion rate along with the downstream length were determined from the NO-PLIF measurements. About 62% of NO was oxidized at 15d (d, jet orifice diameter) by a 30 m/s O₃ jet with an influence width of about 6d in radius. The NO₂ PLIF results support the conclusions deduced from the NO-PLIF measurements.