银改性复合氧化物催化氧化烟气中零价汞的研究
发布时间：2018-01-12 09:32
本文关键词：银改性复合氧化物催化氧化烟气中零价汞的研究 出处：《上海交通大学》2015年博士论文 论文类型：学位论文
【摘要】：燃煤过程所造成的汞排放及污染问题,近年来已引起国际社会的广泛关注。为此,我国修订了《火电厂大气污染物排放标准》(GB13223-2011),并对燃煤电厂的汞排放提出了具体限制要求。将烟气中的零价汞(Hg~0)氧化为二价汞(Hg2+),继而通过现有的空气污染控制装置进行联合去除,是目前去除Hg~0的较好的方法。针对我国燃煤氯含量普遍较低及传统催化剂较窄的活性温度窗口等问题,开发高效及宽活性温度窗口的催化剂具有重要意义。本文通过掺杂银(Ag)作为改性组分,旨在研制能对Hg~0高效转化的催化剂,通过氧化实验对其去除Hg~0的性能进行评价,并考察了温度及烟气组分对催化剂性能的影响,同时通过物理化学表征以及汞和氯的活性分析,进而探讨催化剂在不同温度区间氧化Hg~0的机制,为催化剂的实际应用提供理论依据。论文主要得到了以下结论:1.采用Ag改性V2O5-TiO2(V-Ti)催化剂来提高催化剂氧化Hg~0的性能,并通过添加分散剂来获得均匀分散的银纳米颗粒以进一步提高催化剂的氧化能力。研究发现在催化剂制备过程中,添加聚乙烯吡咯烷酮(PVP)可以进一步提高银纳米颗粒的分散度,且制备的银纳米颗粒尺寸更小,更有利于Ag,V和Ti元素的协同作用而具有最高的催化效率,而当添加离子液体([bmim][BF4])后,则产生较大的银纳米颗粒,导致催化剂的比表面积变小,催化氧化Hg~0性能比Ag-V-Ti稍差些;银物种主要以金属银的形态存在,其本身的氧化性能不高,主要作为助催化剂来提高活性组分钒的氧化能力。2.采用银改性一系列选择性催化还原(SCR)催化剂,并考察了复合催化剂的氧化Hg~0性能。结合表征结果及氧化实验,发现V2O5-MoO3-TiO2(V-Mo-Ti)氧化的效率比V2O5-WO3-TiO2(V-W-Ti)高,元素钼具有抗硫的特性,加银能进一步增强V-Mo-Ti的性能,5ppmv(1)HCl时,掺杂0.5%Ag可以达到90%的氧化Hg~0效率,催化效率提高20-40%左右,活性温度窗口也得到拓宽(150-450℃);Ag和Mo增强了催化剂吸附Hg~0的能力;Ag-Mo/V-Ti在只有氧气(O2)和氮气(N2)存在时也具有一定的氧化能力,且低温性能更好,HCl是反应的主要催化组分;Ag-Mo/V-Ti氧化Hg~0在低温时发生Semi-Deacon反应,而高温时发生Full-Deacon反应,此外,吸附的氧气和活性氧也可以氧化Hg~0生成氧化汞,继续和氯化氢反应生成氯化汞。3.采用银改性MoO3-TiO2(Mo-Ti)催化剂,并考察了复合催化剂的氧化Hg~0性能。当结合银和钼后,Ag(2%)-Mo-Ti在各个温度区间氧化Hg~0的性能都有显著的提高,然而催化剂在低温时性能更高,更适合在低温下操作。组分分析表明,HCl是主要的催化组分,催化剂在O2不存在时也有很高的催化效果,此外,催化剂有很好的抗硫能力;低温时,吸附的活性氯和吸附的Hg~0通过LangmuirHinshelwood机制发生反应;而且,银和钼结合生成钼酸银,有利于晶格氧转为化学吸附氧进而参与反应,也提高了催化剂的氧化性能。4.采用银改性CeO2-TiO2(Ce-Ti)催化剂,并考察了复合催化剂的氧化Hg~0性能。结合表征结果及氧化实验,发现银纳米颗粒负载在载体二氧化钛表面,且银主要是以金属银的形式存在,它能使元素铈保持更多的高价态Ce(IV),提高了催化剂的氧化能力并降低催化剂的氧化还原温度;Ag-Ce-Ti在高温时有更强的氧化能力,且HCl是主要的催化组分,HCl在低温时以活性氯的形式参与反应,400℃左右才有氯气产生并参与反应;催化剂在低温时抗硫性能较好,高温时SO2极大的抑制Hg~0的氧化;结合Ag、Mo和Ce,催化剂在各个温度区间氧化Hg~0的效率都很高,且抗硫性能良好。5.引入AgI-TiO2来去除烟气中的Hg~0,并通过掺杂Ag进一步提高催化剂的性能及稳定性。AgI-TiO2具有很高的氧化Hg~0效率,在350℃时通入5ppmvHCl效率达到100%,强于KI-Ti;掺杂Ag能显著延长AgI-TiO2催化剂高性能的时间,2%的Ag含量较合适,氧化Hg~0的效率在10h后仍能达到90%以上;HCl是反应的主要组分,且O2是必要的;通入氯化氢时,银对氯化氢有催化作用,导致活性氯的产生,然后氧化碘离子为活性碘,活性碘和Hg~0形成中间体Hg-I*进而加速Hg~0的氧化,然后,氯转化中间体为最终产物HgCl2,活性碘和银又生成碘化银,保持Ag(2%)-AgI-Ti中的碘含量;此外,Ag(2%)-AgI-Ti具有很好的热稳定性以及化学稳定性。6.对催化剂的NH3氧化及脱硝性能进行了分析,发现Ag(5%)-Ti在各个温度下都具有很高的NH3氧化能力,且不同的温度时氧化NH3的机理不同,低温时N2O产量较多,而高温时NO产量较多;催化剂的脱硝性能随温度呈先升高后降低的趋势,在250℃时的脱硝效率最高,高温时对脱硝会产生不利的影响,总体来说Ag适用于逃逸NH3的去除。同样,掺杂Ag能提高V-Mo-Ti、Mo(5%)-Ti和Ce(5%)-Ti的NH3氧化能力,且在高温时提高Mo(5%)-Ti和Ce(5%)-Ti催化剂氧化NH3的选择性能;掺杂Ag后,低温时V-Mo-Ti催化剂的脱硝性能提高,而高于300℃时,催化剂的脱硝性能又降低;在Mo(5%)-Ti和Ce(5%)-Ti上掺杂Ag后,不利于催化剂脱硝反应的进行;Ag(2%)-AgI-Ti氧化NH3的能力随温度的升高而增强,催化剂氧化NH3主要转化为NO,而两者之间几乎不发生反应,不适合用于脱硝反应中。本文制备的催化剂在低含量HCl时都具有较高的氧化Hg~0能力,且活性温度窗口都较宽,具有较强的实际应用前景。
[Abstract]:Mercury emission and pollution problems caused by coal combustion, in recent years has caused widespread concern in the international community. Therefore, China revised the "thermal power plant air pollutant emission standards" (GB13223-2011), and put forward specific requirements to limit mercury emissions from coal-fired power plants. The flue gas in Hg0 oxidation (Hg~0) two mercury (Hg2+), and then through the existing air pollution control device for combined removal, is a good method of the removal of Hg~0. According to China's coal chlorine content is generally low and narrow the traditional catalyst active temperature window and other issues, has important significance to the catalyst efficiency and hair wide active temperature window. This paper the silver doped (Ag) as a modified component, aimed at the development of catalyst for Hg~0 energy efficient conversion, the performance was evaluated by the removal of Hg~0 on the oxidation experiment, and the effects of temperature and smoke components on the catalyst performance At the same time, through physical and chemical characterization and activity of mercury and chlorine analysis, and to explore the mechanism of oxidation of Hg~0 catalysts in different temperature range, to provide a theoretical basis for the practical application of catalyst. The main results are as follows: 1. using Ag modified V2O5-TiO2 (V-Ti) catalyst to improve the performance of catalytic oxidation of Hg~0, and by adding dispersant to obtain silver nanoparticles dispersed in order to further improve the oxidation ability of the catalyst. The study found that in the preparation process of the catalyst, adding polyvinylpyrrolidone (PVP) can improve the dispersion of silver nanoparticles and silver nanoparticles, smaller size preparation, more conducive to the synergistic effect of V and Ti Ag. The element has the highest catalytic efficiency, and when adding ionic liquid ([bmim][BF4]), produces larger silver nanoparticles, resulting in the surface area of the catalyst decreases, catalytic oxidation Hg~0 performance is slightly worse than Ag-V-Ti; silver species existed mainly in the form of metallic silver oxide, its performance is not high, mainly as a catalyst to improve active component vanadium oxidation ability of.2. by modification of a series of selective catalytic reduction (SCR) catalyst was studied and the oxidation performance of Hg~0 composite catalyst. According to the characterization results and oxidation experiments, found that V2O5-MoO3-TiO2 (V-Mo-Ti) oxidation efficiency of V2O5-WO3-TiO2 (V-W-Ti), has the characteristics of sulfur element molybdenum, silver can further enhance the performance of V-Mo-Ti, 5ppmv (1) HCl, doped 0.5%Ag can reach 90% Hg~0 oxidation efficiency, improve the efficiency of catalytic activity about 20-40%. The temperature window has been widening (150-450 C); Ag and Mo enhanced the ability of adsorption of Hg~0 catalyst; Ag-Mo/V-Ti (O2) only in oxygen and nitrogen (N2) is also an oxidant, low temperature and better performance, HCl Is the main component of the catalytic oxidation of Hg~0; Ag-Mo/V-Ti Semi-Deacon reaction at low temperature, and high temperature Full-Deacon reaction, in addition, the adsorption of oxygen and active oxygen can also Hg~0 oxidation reaction to mercuric oxide, and hydrogen chloride generated by mercuric chloride.3. Silver Modified MoO3-TiO2 (Mo-Ti) catalyst, and the effects of the oxidation performance of Hg~0 composite catalyst. When combined with silver and molybdenum, Ag (2%) can significantly improve -Mo-Ti performance in each temperature range of Hg~0 oxidation catalysts, but at low temperature is higher, more suitable for low temperature operation. Group analysis showed that HCl is the main constituent of catalyst, catalyst has catalytic effect very high, when there is no O2 in the catalyst has good anti sulfur ability; when the temperature is low, the adsorption of chlorine and the adsorption of Hg~0 by LangmuirHinshelwood reaction mechanism; moreover, silver and molybdenum combine to form molybdenum Acid silver, in favor of lattice oxygen into the chemical adsorption oxygen and then participate in the reaction, but also improve the oxidation performance of.4. catalysts modified by silver catalyst CeO2-TiO2 (Ce-Ti), and the effects of the oxidation performance of Hg~0 composite catalyst. According to the characterization results and oxidation experiments revealed that silver nanoparticles loaded on the carrier surface of titanium dioxide and silver mainly is there the silver form, it can keep high valence cerium more Ce (IV), improve the oxidation ability of catalyst and reduce the oxidation of the catalyst reduction temperature; Ag-Ce-Ti has stronger oxidation ability in high temperature, and HCl is the main catalytic group, HCl at low temperature with active chlorine in the form of reaction at the temperature of 400 DEG C to produce chlorine and sulfur resistance reaction; good catalyst at low temperature, the oxidation inhibition of Hg~0 SO2 greatly at high temperature; combined with Ag, Mo and Ce, the catalyst at different temperature range The efficiency of Hg~0 is very high, and the sulfur resistance of good.5. AgI-TiO2 is used to remove the Hg~0 in the flue gas, and by doping Ag to further improve the performance and stability of.AgI-TiO2 catalyst with Hg~0 oxidation efficiency is very high, at 350 degrees C in 5ppmvHCl efficiency reached 100%, stronger than the KI-Ti doped Ag can significantly prolong the time; AgI-TiO2 catalysts with high performance, the content of Ag 2% is appropriate, the efficiency of oxidation of Hg~0 can reach more than 90% after 10h; HCl is the main component of the reaction points, and O2 is necessary; the hydrogen chloride, silver has catalytic effect on hydrogen chloride, resulting in active chlorine, iodine and oxidation ion is the active iodine, then oxidized active iodine and the formation of Hg~0 intermediates Hg-I* and accelerated Hg~0, the conversion of chlorine intermediates to the final product HgCl2, the activity of iodine and silver and silver iodide generated, Ag (2%) iodine content in -AgI-Ti; in addition, Ag (2%) -AgI-Ti has very good 鐑ǔ瀹氭，
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