Knowledge Management System Of Institute of process engineering,CAS
|关键词||择性催化还原 低温脱硝 V2o5-wo3/tio2 钒前驱体 涂覆型蜂窝体催化剂|
氮氧化物是全球大气污染主要污染物之一，引起光化学烟雾、酸雨、臭氧层破坏等环境问题，严重影响人们的生存环境和生活质量，引起了世界各国的广泛关注。针对固定源和移动源燃烧排放，各国制定了日益严格的排放标准。氨气选择性催化还原技术（NH3-SCR）以NH3作为还原剂，已经广泛应用于燃煤电厂、工业锅炉、柴油车尾气等NOx的脱除，其中，V2O5-WO3/TiO2是应用广泛的商业SCR催化剂，活性温度窗口为300-400 ℃。与发达国家不同，中国有30%的工业锅炉的尾气排放温度为150-300 ℃，例如，供暖锅炉、水泥窑和烧结窑等；且电厂锅炉低负荷工作时，烟气出口温度通常低于300 ℃，利用现有的商业催化剂进行脱硝无法满足排放标准。此外，虽然我国较快地实施了国家汽车第四、五阶段排放标准，SCR系统的应用在一定程度上降低了NOx的排放量，但存在NOx脱除率偏低的问题，导致在实际道路排放中NOx存在不同程度超标的问题，例如，城市公交车平均车速较低、怠速工况比例高，发动机长时间处于低负荷运行工况，排气温度偏低，从而导致SCR系统的NOx脱除率偏低，未能满足排放标准限值。因此，针对当前商业SCR催化剂的不足，本课题开发了宽活性温度窗口（220-400 ℃）、抗硫抗水中毒的V2O5-WO3/TiO2催化剂，重点优化了其低温催化活性。从载体优化和催化剂改性两方面入手，研究制备方法、钒前驱体种类对催化剂结构形貌、活性组分表面形态以及脱硝活性的影响，最后，通过浆料涂覆法分别制备适用于柴油车尾气脱硝和燃煤等工业锅炉烟气脱硝的蜂窝体催化剂。本课题针对低温V2O5-WO3/TiO2催化剂的开发与应用研究，获得如下主要结果：1. TiO2和WO3/TiO2载体优化、改性。采用常压水解法制备介孔TiO2载体，添加活性炭硬模板剂可以提高载体的比表面积、调整介孔结构及孔径分布，优化制备工艺参数：活性炭添加量10%，并于500 ℃空气气氛中焙烧。然后，在介孔TiO2载体优化制备工艺基础上进行氟掺杂，结果表明：F掺杂可提高催化剂的低温催化活性。掺杂F后，催化剂的比表面积、酸性位量均下降，晶粒尺寸变大；但催化剂的V和W表面原子浓度升高，与载体之间的相互作用增强，促进其在载体表面的富集。F前驱体NH4F未能完全进入催化剂掺杂，而是在样品洗涤过程中进入废液，带来含氟废液处理问题。2. 不同钒前驱体V2O5-WO3/TiO2催化剂的低温脱硝性能优化。以VO(acac)2为钒前驱体采用溶剂热法制备SCR催化剂，在220-450 ℃含有SO2和H2O的烟气中，脱硝活性稳定。对使用前后催化剂进行FTIR和TG表征，结果表明，亚硫酸铵盐或硫酸铵盐在催化剂表面的生成和分解形成了动态平衡，确保即使温度低至220 ℃，还可获得稳定脱硝活性。为了避免有机溶剂的影响和使用，采用不同钒前驱体与WO3-TiO2载体直接研磨制备催化剂，其脱硝活性差别较大。以VO(acac)2为钒前驱体的催化剂，不仅具有较好的低温脱硝活性，还具有优异的抗硫抗水中毒性能。XPS结果表明，以VO(acac)2为钒前驱体的催化剂，钒表面原子浓度较高，且有70%的还原态V2O5 (V3+和V4+)；Raman和H2-TPR结果表明，聚合态钒物种和钒的还原能力影响催化剂的低温催化活性。具体地，钒表面原子浓度越高、聚合态钒物种越多、钒的还原温度越低，催化剂的低温脱硝活性越高。机械研磨法制备V2O5-WO3/TiO2催化剂不仅阐释了不同钒前驱体促进SCR脱硝反应的机理，也提供了一种高效率、低成本、操作简单，且具有较好低温活性的粉体脱硝催化剂的制备技术。3. 柴油车氧化催化器DOC与NH3-SCR两段工艺集成与优化。SCR催化剂对CO和HC的脱除率较低，且仅对具有较好脱硝活性的V2O5-WO3/TiO2催化剂进行Pd掺杂，并不能将两者的优势结合起来，不能同时有效脱除NO、CO和HC。而DOC催化剂具有较高的CO和HC脱除率，但NO脱除效果较差，不适用于富氧条件下NO脱除反应。针对三种污染物同时脱除率较低的问题，集成和优化DOC+NH3-SCR两段工艺。结果表明：两段工艺优化后，NO、CO、HC同时脱除率较高，DOC氧化层能将部分NO氧化为NO2，提高了SCR低温脱硝活性；此外，CO或HC及其中间产物，进入下游SCR催化还原层，可作为还原剂参与NO催化反应，获得了高于理论值的NO转化率。此外，涂覆量为23 wt.%的蜂窝体催化剂具有与商业国Ⅴ催化剂相当的脱硝活性，适当提高涂覆量，具有满足国Ⅴ排放标准的潜力。4. 涂覆型整体式催化剂抗磨损特性考察与寿命预测。通过粘结剂筛选等工艺探索制备了涂覆型整体式SCR催化剂。结果表明：当V2O5-WO3/TiO2催化剂涂层厚度为260 μm，250 ℃时，可以获得与100% V2O5-WO3/TiO2粉末挤出成型蜂窝体相当的脱硝活性。在沙含量为300 g/m3、表观线速度为10 m/s空气中进行4-h磨损实验，涂覆蜂窝体催化剂的脱硝活性仍约为其初始值的92%，表现出优异的抗磨损性能。根据相当时间计算公式，预测涂覆型催化剂可在飞灰含量为1 g/m3的实际烟气中服役一年或更长时间，且其脱硝活性下降量仅为8%。证明了该涂覆型V2O5-WO3/TiO2整体式催化剂具有广泛的工业应用前景，例如玻璃厂等烟气中飞灰含量低于1 g/m3的工业锅炉。5. 涂覆型整体式SCR催化剂宏观动力学。求得不同条件下的总反应速率常数Ko、气体扩散速率常数Kg以及催化反应速率常数Ks，结果表明，该催化剂的脱硝反应活化能约为30 kJ/mol，指前因子A约为20307 s-1，低于商业催化剂的活化能，从而获得较好的低温脱硝活性。根据宏观动力学参数，可以预测任意NO浓度下达到目标NO转化率所需要的催化剂装载量，指导中试设计以及实际工业应用。
Nitrogen oxides (NOx) remain as one of the major sources of air pollution, which greatly contributes to the greenhouse effect, ozone depletion and formations of photochemical smog and acid rain. Many countries have made stringent emission limits for flue gases from combustion facilities and vehicles. The selective catalytic reduction (SCR) of NOx with NH3 is worldwide used as the most effective technology for removal of NOx from stationary sources such as power boilers and combustion furnaces. The V2O5-WO3/TiO2 catalysts have been widely used for decades, with a relatively narrow temperature window of 300-400 ℃. In China, unlike in developed countries, a considerably huge amount of NOx is from a large number of coal-fueled industrial furnaces, such as heating boilers, cement klins, sintering machines and so on. The flue gas temperatures are mostly in a range of 150℃ to 300℃ and out of the working temperature window of commercial SCR catalysts for denitration (de-NOx). Moreover, Euro Ⅵ diesel buses equipped with SCR systems failed to mitigate on-road NOx emissions to meet the expected regulation. The NOx emission factor of Euro Ⅴ diesel buses was 37% lower than that of Euro Ⅵ diesel buses, but it still exceeded the Euro Ⅴ standard by 180%. The real-word NOx emission factors were sensitive to changes of average speed. In other words, NOx emissions are higher than the Euro Ⅵ/Ⅴ limits because of the low exhausting temperatures.Considering the drawbacks of current SCR technology for denitration at low tempeartures, it is urgent to explore how to reduce the NOx emission at low flue gas temperatures at 200 to 400 ℃, which will have considerable applications not only to industrial combustion facilities but also to diesel engines. Systematic investigation has been performed to clarify the influences of supports, preparation method and vanadium precursors on catalytic activity and its resistance to SO2 and H2O poisoning. In this work, fundamental studies on catalytic mechanism and industrial application are carried out for V2O5-WO3/TiO2 catalyst, and the main research achievements are listed as follows:1. Optimization and modification of the TiO2 and WO3/TiO2 supports. The TiO2 support was prepared through atmospheric hydrolysis method. The active carbon (AC) was added as template which would facilitate the formation of mesoporous. Based on the preparation of TiO2 support, the catalyst using F-doped supports exhibited higher catalytic activity, especially for low-temperature denitration reaction. And for the F-doped catalyst there were higher surface atomic concentrations of V and W. But the excess F would was washed into the solution, causing issues of treatment of waste liquid。2. Improved low-temperature activity of V2O5-WO3/TiO2 for denitration using different vanadium precursors. A catalyst made according to the solvothermal method using vanadyl acetylacetonate (VO(acac)2) as the vanadium precursor was obatained to have high catalytic activity and good resistance to poisoning of SO2 and H2O at 220-450 ℃. Via FTIR and TGA analyses it clarified that over the catalyst a dynamic balance between the formation and decomposition of ammonium sulfite or sulfate is possibly built at low temperatures, for example, 220 ℃. To eliminate the influence of organic solvent, the study tests grinding as a safety method for V2O5/WO3-TiO2 catalyst preparation. Catalysts were prepared by grinding method using different vanadium precursors, and were further evaluated and characterized to clarify the relationship between surface morphology of vanadium oxides and performance. Catalyst characterization revealed that the VO(acac)2 precursor promoted the formation of polymeric vanadia species and low valence of vanadium oxides to decrease the difficulty of NO reduction reactions over the catalyst. The study would provide a new and low-cost technology to prepare the V2O5/WO3-TiO2 catalyst with good de-NOx activity.3. Integration and optimization of Diesel Oxidation Catalyst (DOC) and SCR two-stage process. The performances to removal of NO, CO and HC by NH3-SCR and DOC catalysts were studied. The conversions of CO and HC were too low over SCR catalyst, which cannot be improved by doping Pd. Thus, a two-stage process combining DOC and SCR was tested. The results showed that all species were simultaneously removed, and NO was oxidized to NO2 via DOC to benefite denitration at low temperatures. Moreover, CO or HC and its intermediate products can participate in SCR reaction as reductant to obtaine higher NO conversion than theoretical value based on NH3/NO ratio. Comparing with a Euro Ⅴ monolith catalyst, a monolith with a catalyst coating layer of 23 wt.% enabled the similar NOx reduction. Thus, with suffuciatly increased coating catalyst, the coated monolith can meet the Euro Ⅴ emission standard.4. Abrasion resistance and prediction of durable hours of washcoated monolith catalyst. Monolith SCR catalysts coated with V2O5-WO3/TiO2 powder were prepared by varying binder and coating thickness. Comparing with a monolith extruded with 100% V2O5-WO3/TiO2 powder, a coated monolith with a catalyst-coating layer of 260 mm in thickness exhibited the similar initial NOx reduction activity at 250℃. After 4-h abrasion (attrition) in an air stream containing 300 g/m3 fine sands (50-100 mm) at a superficial gas velocity of 10 m/s, the catalyst still has the activity as a 100% molded monolith does in a 24-h activity test and it retains about 92% of its initial activity at 250℃. Estimation of the equivalent durable hours at a fly ash concentration of 1.0 g/m3 in flue gas and a gas velocity of 5 m/s demonstrated that this coated monolith catalyst is capable of resisting abrasion for 13 months without losing more than 8% of its initial activity. The result suggests the great potential of the coated monolith for application to de-NOx of flue gases with low fly ash concentrations from, such as glass and ceramics manufacturing processes.5. Apparent chemical kinetics of washcoated honeycomb catalyst. The effects of gas linear velocity (LV), gas hourly space velocity (GHSV) and channel size on NO conversion were investigated to seek an apparent chemical kenetic model for prediction of denitration rate and also for reactor design of industrial application. The overall rate constant Ko, mass transfer coefficient Kg, reaction rate constant Ks and kinetic parameters (Ea and A) were calculated. The results showed that the active energy for washcoated honeycomb catalyst was about 30 kJ/mol, lower than the value of commercial SCR catalysts, indicating that the denitration reaction was easier to happen over the self-made monolith catalyst. Finally, the simulation calculation was performed under arbitrary conditions to provide valuable information for design of industrial SCR system.
|甘丽娜. 低温V2O5-WO3/TiO2脱硝催化剂开发与应用研究[D]. 北京. 中国科学院研究生院,2016.|
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