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汞是一种全球性污染物，严重危害人体健康和生态环境。燃煤电厂是我国汞污染物排放最重要的来源之一，燃煤烟气中汞的脱除具有重要意义。气态零价汞（Hg0）、气态二价汞（Hg2+）以及颗粒态汞（Hgp）是燃煤烟气中汞的主要存在形态。电厂的污染物控制设备能高效脱除Hg2+和Hgp，但对Hg0去除效率低，因此强化Hg0向Hg2+转化是燃煤电厂汞排放控制的关键。研究表明：烟气中Hg2+占比与燃煤中的卤素含量呈正相关，而我国燃煤中卤素含量普遍低，造成烟气中Hg0占比高，因此向燃煤中额外添加卤素化合物是一种强化Hg0氧化的方法；此外，在燃煤电厂烟气SCR脱硝过程中，Hg0能够在SCR催化剂上发生催化氧化生成Hg2+，但是对于广泛使用的V基SCR催化剂，其对Hg0的氧化作用有限，因此，可以通过对催化剂改性强化Hg0氧化。在本文中，一方面，研究了燃煤中添加CaBr2对烟气中Hg0的氧化作用，并对氧化机制进行了推测，由于发现该方法会增加烟气中HBr的浓度，进一步研究了HBr可能带来的不利影响从而为实际工业应用提供指导；另一方面，通过理论分析，选取CeO2对传统V基SCR催化剂改性，强化Hg0在SCR催化剂上氧化，通过活性评价以及反应前后催化剂的表征，揭示了Hg0在催化剂表面的反应机理，并且研究了工艺参数对催化剂性能的影响。本论文取得的主要成果如下：（1）对全国19个煤样中的汞含量、氯含量进行了分析，研究了煤中汞的析出规律，并筛选出褐煤样品进行了CaBr2添加脱汞实验，结果表明：CaBr2添加能有效加强Hg0氧化，当添加量为200 μg/g时，Hg0析出量减少了40.4%；CaBr2的添加会增加烟气中HBr的浓度，HBr浓度与Hg0析出量之间存在强负相关性，推测HBr生成活性Br原子将Hg0氧化。（2）研究了CaBr2添加脱汞产生的HBr对V2O5-WO3/TiO2催化剂脱硝性能的影响，结果表明：在反应温度低于300 °C时，HBr了抑制催化剂的脱硝活性，当烟气中HBr浓度为50 ppm，反应温度为200 °C和250 °C时，NO转化率分别降低了10%和20%。XPS和In-situ FTIR的研究结果表明：HBr会与SCR脱硝反应中的氧化还原活性位点V=O发生反应：HBr + V5+=O → Br-V4+-OH，从而抑制了催化剂脱硝活性。（3）创新性的提出了加强NO在催化剂上的吸附将同时有利用提高催化剂脱硝活性和Hg0的催化氧化，并选取CeO2对传统的V2O5/TiO2催化剂进行改性，通过浸渍法合成了V1Ti和V1Ce5Ti催化剂。活性评价结果表明：一方面，CeO2改性提高了V2O5/TiO2催化剂低温区间脱硝活性，在最佳反应温度区间250-400 °C内，NO转化率均超过95%；另一方面，Hg0在催化剂上的氧化得到显著提升。Hg-TPD实验结果揭示了Hg(NO3)2是Hg0的氧化产物，HCl在反应中起到了促进Hg(NO3)2脱附的作用；综合实验结果，提出了Hg0在催化剂上的硝酸盐氧化路径，NO和O2首先在催化剂表面吸附并反应生成硝酸盐物种，进一步与Hg0反应生成Hg(NO3)2，在HCl作用下Hg(NO3)2从催化剂上脱附完成催化循环，催化剂表面生成的双齿硝酸盐物种在Hg0氧化过程中起到了重要作用。（4）探究了工艺参数对催化剂性能的影响，结果表明：SO2和H2O(g)对V1Ce5Ti催化剂SCR脱硝活性无明显影响；在400 °C时SO2会抑制Hg0在V1Ce5Ti催化剂上的氧化，XPS分析结果表明SO2会与催化剂表面活性组分CeO2发生反应；Hg0的氧化率随空速的增加而降低，在空速远高于常规SCR操作空速的条件下(260000 h-1)，在200–400 °C温度范围内，Hg0氧化率均超过80%；催化剂在48 h的稳定性评价过程中活性无明显下降。;Mercury is a global pollutant which has serious effects on human health and ecological environment. Coal-fired power plants are one of the dominant sources of mercury emission in China, and mercury emission control for coal-fired power plants is of great significance. Elemental mercury (Hg0), oxidized mercury (Hg2+) and particulate-bound mercury are main existing form of mercury in coal-fired flue gas. Hg0 and Hg2+ can be effectively removed by the air pollution control devices in power-plant while Hg0 is difficult to be removed. Therefore, enhancing Hg0 oxidation into Hg2+ play a key role in mercury control in coal-fired power plants.Previous studies suggest that there is a positive correlation between the proportion of Hg2+ in flue gas and the halogen content in coal. The low halogen content in Chinese coal results in high Hg0 concentration in flue gas. Consequently, adding extra halogen component in coal is a useful way to promote Hg0 oxidation. Additionally, Hg0 can be oxidized on SCR catalyst during flue gas denitration process, however, the oxidation rate is quiet low over the widely used V-based SCR catalyst. Hence, high efficiency Hg0 oxidation can be achieve by modifying present catalyst.In this research, on the one hand, extra CaBr2 was added to coal to enhance the Hg0 oxidation in flue gas. The promotion mechanism was proposed. On the other hand, based on the theoretical analysis, CeO2 was selected to modify the traditional V-based SCR catalyst. The catalytic mechanism of Hg0 oxidation was investigated by activity test and characterization. The effects of operation parameters were also investigated.The main results of this paper are as follows:(1) Mercury release rules in coal were studied by analyzing the mercury and chlorine contents in 19 coal samples nationwide. Lignite samples were screened out for mercury removal experiments by adding CaBr2. The results showed that the addition of CaBr2 can significantly inhibit Hg0 release. The release of Hg0 decreased by 40.4% with 200 μg/g CaBr2 addition in coal. The addition of CaBr2 increased the concentration of HBr in flue gas. The strong negative correlation between HBr concentration and Hg0 release amount suggest that HBr can generate active Br atoms to oxidize Hg0.(2) The potential negative influence of HBr generated by CaBr2 addition was study. The effect of HBr on the performance of V2O5-WO3/TiO2 SCR catalyst was studied, the results indicated that the HBr inhibited the activity of catalyst when reaction temperature was below 300 °C. When 50 ppm was added in flue gas, the NO conversion rate was reduced by 10% and 20% at 200 °C and 250 °C, respectively. Further studies by XPS and in-situ FTIR revealed that HBr can react with V=O sites through the following equation: HBr + V5+=O → Br-V4+-OH, which affected the redox cycle in SCR reaction, therefore inhibiting the activity of the catalyst.(3) It was innovatively proposed that enhancing the NO adsorption on the catalyst will simultaneously improve SCR activity and Hg0 catalytic oxidation on catalyst. CeO2 was selected to modify traditional V2O5/TiO2 catalyst. V1Ti and V1Ce5Ti catalyst was synthesized by impregnation method and the activity of the catalysts was measured. Results show that, on the one hand, CeO2 improved the activity at low temperature. NO conversion exceed 95% within the optimum operating temperature range from 250 °C to 400 °C. On the other hand, the Hg0 oxidation was significantly improved. Hg-TPD results indicated that Hg(NO3)2 was the oxidation product. HCl facilitated the desorption of Hg(NO3)2. A nitrate pathway for Hg0 oxidation was proposed. NO and O2 first adsorbed on catalyst and generated nitrate which furtherly react with Hg0 to form the Hg(NO3)2. Subsequently, Hg(NO3)2 desorbed from catalyst with the aid of HCl and accomplished the catalytic cycle. The bidentate nitrate on the surface of the catalyst plays an important role in the oxidation of Hg0.(4) The effects of operation parameters on performance of catalyst were investigated and results showed that: SO2 and H2O(g) had no significant effect on the SCR performance over V1Ce5Ti while inhibited Hg0 oxidation at 400 °C. XPS analysis showed that SO2 can react with CeO2. Hg0 oxidation rate decreases with the increase of gaseous hourly space velocity (GHSV). The Hg0 oxidation rate were more than 80% even under the GHSV as high as 260000 h-1. What’s more, no obvious activity reduction occurred during stability test as long as 48 h. These results suggested CeO2 modified SCR catalyst had industrial application prospect.
|杨阳. 基于形态调控的燃煤烟气中零价汞的脱除研究[D]. 中国科学院大学,2019.|
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