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.     

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.     

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.     

烟气NOx低温选择性催化还原催化剂研究进展

催化剂是选择性催化还原(SCR)脱硝技术的核心,其催化性能直接关系到脱硝效果的好坏。近年来,由于低温选择性催化还原法具有良好的经济性,引起各国环保研究工作者的关注,成为近年SCR研究的热点。本文概述了贵金属、金属氧化物、分子筛和碳基材料催化剂的低温(低于250℃)选择性催化还原法脱除NOx研究进展。其中,以碳基材料为载体的催化剂显示出良好的低温选择性和稳定性,具有很好的应用前景。     

Surface chemical characterization of deactivated low-level mercury catalysts for acetylene hydrochlorination

Mercury-containing catalysts are widely used for acetylene hydrochlorination in China. Surface chemical characteristics of the fresh low-level mercury catalysts and spent low-level mercury catalysts were compared using multiple characterization methods. Pore blockage and active site coverage caused by chlorine-containing organics are responsible for catalyst deactivation. The reactions of chloroethylene and acetylene with chlorine free radical can generate chlorine-containing organic species. SiO₂ and functional groups on activated carbon contribute to the generation of carbon deposition. No significant reduction in the total content of mercury was observed after catalyst deactivation, while there was mercury loss locally. The irreversible loss of HgCl₂ caused by volatilization, reduction and poisoning of elements S and P also can lead to catalyst deactivation. Si, Al, Ca and Fe oxides are scattered on the activated carbon. Active components are still uniformly absorbed on activated carbon after catalyst deactivation.     

Influence of iron species present in fly ashes on mercury retention and oxidation

The aim of this study was to assess the effect of iron species present in fly ashes produced from coal combustion on mercury retention and oxidation. To achieve this objective the work was divided into two parts. In the first part the relationship between the mercury and iron content in fly ashes of different origin and characteristics was evaluated. In the second, a series of fractions enriched in iron oxides were separated from the fly ashes to determine the effect of increasing iron content on mercury retention and oxidation. Special attention was paid to the influence of iron on mercury behavior in enriched carbon particles in fly ashes. From the results obtained it can be inferred that, in the range of fly ashes studied, iron species do not affect the retention of mercury and do not play any role in heterogeneous mercury oxidation.     

水泥窑低温SCR脱硝技术研究

介绍了研究水泥窑低温脱硝的必要性,阐述了水泥窑低温SCR脱硝工艺技术路线和工艺布置。通过对低温SCR脱硝技术与中温SCR脱硝技术对比,表明低温SCR脱硝技术具有安装条件可选择性强、成本费用小、催化剂用量少、维护方便等优点。并且通过对几种低温SCR脱硝催化剂的性能研究,结果表明需要在提高其活性、选择性、抗SO_2、K、Na等中毒能力、机械强度及稳定性、再生性能等方面对其进行改进。     

Detoxication of water containing 1,1-dimethylhydrazine by catalytic oxidation with dioxygen and hydrogen peroxide over Cu- and Fe-containing catalysts

A number of Cu- and Fe-hydroxide containing catalysts, supported on oxide carriers, were prepared to provide the removal of 1,1-dimethylhydrazine from aqueous solutions via its oxidation by hydrogen peroxide and air oxygen. The Cu-containing samples as well as Fe/ZSM-5 are the most active catalysts in this reaction. The reaction products were analyzed by gas chromatography and UV–Vis spectroscopy. The effect of nature of the oxidizer and catalyst, pH and temperature on both the reaction rate and product composition was studied.     

Co3O4 based catalysts for NO oxidation and NOx reduction in fast SCR process

Reaction activities of several developed catalysts for NO oxidation and NO_x (NO + NO₂) reduction have been determined in a fixed bed differential reactor. Among all the catalysts tested, Co₃O₄ based catalysts are the most active ones for both NO oxidation and NO_x reduction reactions even at high space velocity (SV) and low temperature in the fast selective catalytic reduction (SCR) process. Over Co₃O₄ catalyst, the effects of calcination temperatures, SO₂ concentration, optimum SV for 50% conversion of NO to NO₂ were determined. Also, Co₃O₄ based catalysts (Co₃O₄-WO₃) exhibit significantly higher conversion than all the developed DeNO_x catalysts (supported/unsupported) having maximum conversion of NO_x even at lower temperature and higher SV since the mixed oxide Co-W nanocomposite is formed. In case of the fast SCR, N₂O formation over Co₃O₄-WO₃ catalyst is far less than that over the other catalysts but the standard SCR produces high concentration of N₂O over all the catalysts. The effect of SO₂ concentration on NO_x reduction is found to be almost negligible may be due to the presence of WO₃ that resists SO₂ oxidation.     

商用蜂窝式SCR催化剂脱除NO_x试验研究

为了评价燃煤电厂广泛采用的选择性催化还原工艺对不同形态NO_x的脱除效果,以某商用蜂窝式SCR催化剂为例,在SCR脱硝试验装置上研究了氧量、温度、空速、氨氮摩尔比等反应条件对NO、N_2O和NO_2脱除过程的影响。结果表明,氧量可以促进NO氧化以及NO与氨的催化还原反应;高温可以促进NO的脱除和氨气氧化为N_2O;空速升高会导致NO脱除率先升高后降低;氨氮摩尔比提高在促进NO脱除的同时会增加氨逃逸;与NO_2可以完全脱除相反,N_2O与氨气不发生反应。因此,为真正实现NO_x的超净排放,应适当控制锅炉运行参数避免N_2O的生成。     

Investigation on elemental mercury oxidation mechanism by non-thermal plasma treatment

Converting elemental mercury into divalent compound is one of the most important steps for mercury abatement from coal fired flue gas. The oxidation of elemental mercury was investigated in this paper using dielectric barrier discharge (DBD) non-thermal plasma (NTP) technology at room temperature. Effects of different flue gas components like oxygen, moisture, HCl, NO and SO₂ were investigated. Results indicate that active radicals including O, O₃ and OH all contribute to the oxidation of elemental mercury. Under the conditions of 5% O₂ in the simulated flue gas, about 90.2% of Hg⁰ was observed to be oxidized at 3.68 kV discharge voltage. The increase of discharge voltage, O₂ level and H₂O content can all improve the oxidation rate, individually. With O₂ and H₂O both existed, there is an optimal moisture level for the mercury oxidation during the NTP treatment. In this test, the observed optimal moisture level was around 0.74% by volume. Hydrogen chloride can promote the oxidation of mercury due to chlorine atoms produced in the plasma process. Both NO and SO₂ have inhibitory effects on mercury oxidation, which can be attributed to their competitive consumption of O₃ and O.     

Low-temperature SCR of NO with NH3 over noble metal promoted Fe-ZSM-5 catalysts

We have reported previously the excellent performance of Fe-exchanged ZSM-5 for selective catalytic reduction (SCR) of NO with ammonia at high temperatures (300–400 °C). In this work, we found that the reaction temperature could be decreased to 200–300 °C when a small amount of noble metal (Pt, Rh, or Pd) was added to the Fe-ZSM-5. The SCR activity follows the order Pt/Fe-ZSM-5 > Rh/Fe-ZSM-5 > Pd/Fe-ZSM-5 at 250 °C. On the Pt promoted Fe-ZSM-5, 90% NO conversion was obtained at 250 °C at GHSV = 1.1 × 10⁵ h^–1. Moreover, the noble metal improved the resistance to H₂O and SO₂. The presence of H₂O and SO₂ decreased the SCR performance only very slightly.     

乙腈加氢制乙胺的催化剂性能研究

以乙腈和氢气为原料,在过量氨和催化剂的作用下,进行催化反应合成乙胺,考察各种组分加氢催化剂对反应结果的影响,根据最终测试结果选择镍为主催化剂,加入镧为助催化剂,以γ-Al2O3为载体,采用浸渍法制备催化剂,转化率可达到93.16％,总收率达到92.99％,选择性为98.95％.     

CeO2改性再生SCR催化剂的单质汞氧化特性

对国内某1000MW燃煤发电机组失活选择性催化还原（SCR）催化剂进行CeO₂改性再生。对再生前后样品进行N₂吸附-脱附、扫描电子显微镜（SEM）、X射线荧光光谱（XRF）、傅里叶变换红外光谱（FTIR）对比表征分析。在自制固定床反应系统上对CeO₂改性再生催化剂（CeReCat）进行Hg⁰氧化性能测试,同时研究了SO₂、H₂O、NO和NH₃对Hg⁰氧化性能的影响。结果表明,CeO₂改性再生方法可有效清洗失活SCR催化剂表面杂质,恢复催化剂表面活性位点和孔隙结构,可使Ce、V两种活性元素得到有效负载。CeO₂改性后的样品Hg⁰氧化性能显著提升,3.0 CeReCat具有最佳Hg⁰的氧化效率。此外,烟气中加入600μL/L SO₂后,3.0 CeReCat仍具有高达74.4%的Hg⁰氧化效率,抗SO₂性能较好。烟气中的NO可轻微促进Hg⁰的氧化。由于竞争吸附作用,烟气中的H₂O和NH₃会抑制Hg⁰的氧化。CeO₂改性再生催化剂置于SCR系统下层时,由于烟气NH₃浓度较低而具有较高Hg⁰氧化效率,具有良好的应用前景。     

Water tolerance of DeNOx SCR catalysts using hydrocarbons: Findings, improvements and challenges

The recent developments on the effect of H₂O on deNO_x performance of a variety of SCR catalysts selectively removing NO_x by hydrocarbons in excess oxygen have been reviewed. In particular, the water tolerance of the catalyst is summarized to illustrate a common deactivation behavior of SCR catalyst for the reduction of NO by hydrocarbons under wet feed gas mixture. Earlier proposals elucidating the possible cause of the catalyst deactivation under wet conditions are discussed, focusing mainly on the catalyst characteristics. A promising way, which can improve the water tolerance and the hydrothermal stability of zeolite-based SCR catalyst, is also described.     

Influence of reaction products of K-getter fuel additives on commercial vanadia-based SCR catalysts: Part I. Potassium phosphate

Commercial vanadia-based full-length monoliths have been exposed to aerosols formed by injection of K₃PO₄ (dissolved in water) in a hot flue gas (T > 850 °C) from a natural gas burner. Such aerosols may form when burning fuels with high K- and P-content, or when P-compounds are mixed with biomass as a K-getter additive. The formed aerosols have been characterized by using both a SMPS system and a low pressure cascade impactor, showing a dual-mode volume-based size distribution with a first peak at around 30 nm and a second one at diameters >1 μm. The different peaks have been associated with different species. In particular, the particles related to the 30 nm peak are associated to condensed phosphates, whereas the larger particles are associated to potassium phosphates. Two monoliths have been exposed during addition of 100 and 200 mg/Nm³ K₃PO₄ for 720 and 189 h, respectively. Overall, deactivation rates up to 3%/day have been measured. The spent catalysts have been characterized by bulk chemical analysis, Hg-porosimetry and SEM-EDX. NH₃-chemisorption tests on the spent elements and activity tests on catalyst powders obtained by crushing the monoliths have also been carried out. The catalyst characterization has shown that poisoning by K is the main deactivation mechanism. The results show that binding K in K–P salts will not reduce the rate of catalyst deactivation.     

SCR脱硝催化剂抗砷中毒改性优化与再生研究进展

选择性催化还原（SCR）是目前应用最为广泛的烟气脱硝技术,催化剂是整个SCR脱硝系统的核心。在实际应用过程中,催化剂存在各种失活问题,其中砷中毒是催化剂失活的重要原因之一。本文详细阐述了SCR脱硝催化剂砷中毒的物理和化学失活机理,其中物理失活是由于As₂O₃在催化剂表面沉积、氧化造成催化剂孔道堵塞所致,而化学失活是由于砷氧化物破坏催化剂酸位点、改变活性基团形态、降低催化剂氨吸附及氧化还原能力所致。然后,系统介绍了抗砷中毒SCR脱硝催化剂的研发路线以及现有抗砷中毒催化剂优化改进的主要技术手段,主要包括调整催化剂孔隙结构、优化催化剂化学配方和烟气侧砷氧化物吸附固化等,其中MoO₃是优选的催化剂活性助剂,金属元素（如Bi、In、Sn、Mg）是主要的抗砷助剂,钙基物质是典型的烟气侧砷氧化物吸附添加剂。最后,对砷中毒废弃催化剂的再生技术进行了简要介绍,包括湿法清洗、热还原法、复合再生等,在实际工业应用中,主要以物理清扫、湿法清洗配合活性组分添加的复合再生方式实现中毒催化剂再生。本文可对未来抗砷中毒SCR脱硝催化剂的研发与优化提供重要支撑。     

微波热解制备g-Fe2O3催化剂及其SCR脱硝性能

以FeSO₄·7H₂O[Fe(NO₃)₃·9H₂O]为铁源,采用新型微波热解法制备γ-Fe₂O₃[a-Fe₂O₃]催化剂样品,通过XRD、N₂等温吸附-脱附、压汞法等实验手段对催化剂样品晶相、微观孔结构等进行表征;考察两种催化剂样品的NH₃-SCR脱硝性能,通过归一化处理得到两种催化剂在不同温度下的本征脱硝反应速率,同时对比研究了γ-Fe₂O₃与钒系催化剂的脱硝活性;研究氨氮比、氧浓度等运行参数对γ-Fe₂O₃催化剂NH₃-SCR脱硝性能的影响规律,并对其抗硫抗水性能进行考察.结果表明:采用新型微波热解法可得到纯度较高的γ-Fe₂O₃催化剂,其介孔分布合理且大孔数量丰富;同时γ-Fe₂O₃催化剂表现出优于a-Fe₂O₃催化剂的脱硝性能,400℃时最大NO_x转化率达到96%,300、325、350℃下单位面积脱硝速率达到a-Fe₂O₃催化剂的3倍左右;γ-Fe₂O₃催化剂具备优良的抗硫抗水性能,其最佳氨氮比为1、最佳氧体积分数为3.5%.     

Effect of Cu doping on the SCR activity of CeO2 catalyst prepared by citric acid method

CeO₂–CuO catalyst prepared by citric acid method was investigated for selective catalytic reduction of NO with NH₃. The activity of the CeO₂ catalyst was enhanced about 8–27% in the temperature range of 125–225 °C at a space velocity of 28,000 h⁻¹ by the addition of Cu. It was found that the state of Cu species had great impact on the SCR performance of CeO₂–CuO catalyst. Cu²⁺ can enhance the low temperature activity of SCR reaction, while CuO would promote NH₃ oxidation before SCR reaction at high temperature, which would cause the decrease of its high temperature SCR activity.     

合成氯乙烯低汞催化剂制备

采用不同工艺条件制备氯乙烯低汞催化剂,利用固定床反应器对催化剂进行活性评价,筛选转化率和选择性较高的催化剂制备条件。结果表明,与等体积浸渍法相比,由于过饱和浸渍法制得的催化剂负载更多的Hg Cl2,在反应前期,催化剂活性较低,随着反应时间的延长,催化剂被完全激活,活性超过等体积浸渍法制备的催化剂。浸渍时间、水浴温度和溶液p H均影响活性炭对Hg Cl2的吸附量,在浸渍时间6 h、水浴温度60℃和p H=1条件下,控制Hg Cl2质量分数为5.35%,120℃干燥20 h制备的催化剂具有较理想的活性。复合助剂的加入有助于提高催化剂活性与热稳定性,当n(Hg Cl2)∶n(Ce Cl3)∶n(KCl)=1∶1∶1时拥有更高活性,添加更多助剂堵塞活性炭孔道,降低催化剂活性。