新型宽温度窗口催化剂选择性催化还原NO_x的机理研究

发布时间：2018-03-05 01:28

本文选题：选择性催化还原　切入点：氮氧化物　出处：《浙江大学》2017年博士论文　论文类型：学位论文

【摘要】：随着社会经济的迅速发展,能源消费水平逐年上升,特别是京津冀、长三角、珠三角等重点地区,能源消费强度远高于全国平均水平。高强度的能源特别是煤炭的消费带来了严重的环境污染问题。氮氧化物(NO_x)是一种主要的大气污染物,其排放会造成酸雨、光化学烟雾等问题,给环境和人类健康带来极大的影响。选择性催化还原(SCR)技术是目前治理烟气中氮氧化物最为高效的技术。催化剂是SCR技术的关键。SCR催化剂在我国推广应用面临的一大挑战在于其对锅炉负荷的适应性。我国燃煤电厂负荷波动频繁,在机组低负荷运行时,传统的V2O5-WO3/TiO2催化剂低温活性不足,导致氮氧化物排放超标、氨逃逸增加、甚至引起环保设备的故障。传统的应对策略是采用省煤器旁路改造,这一方法成本高,而且会导致锅炉热效率降低。本文针对燃煤电厂全负荷脱硝的难题开展研究,提出了宽温度窗口催化剂设计的理论依据并开发了系列宽温度窗口 SCR催化剂配方,系统研究了催化剂的构效关系,反应机理,抗硫性能和促进硫酸氢铵分解的机制,得到了适用于低温实际烟气条件下可长时间稳定运行的宽温度窗口催化剂。全文得到的主要结果如下:一、研究了宽温度窗口脱硝催化剂的构效关系。从SCR反应机理出发,采用具有良好氧化还原性能的氧化铈作为催化剂氧化位点以及具有强酸性的氧化铌作为酸性位点,采用共沉淀法合成了系列铈铌复合氧化物SCR催化剂。当Ce:Nb原子摩尔比为1:1时,其在200～450℃C温度区间内具有80%以上的催化活性。研究建立了催化剂的结构-理化性质-催化活性的关联,揭示了不同价态的稀土/过渡金属氧化物的掺杂可以产生短程活化作用,生成更多的配位不饱和氧物种,并首次发现了这种配位不饱和氧物种是决定低温活性的关键因素。二、研究了催化剂表面的关键活性物种和反应路径。研究发现在催化剂表面,SCR反应遵循Eley-Rideal机理和Langmuir-Hinshelwood机理。在L-H机理中,NO_2和单齿硝酸盐物种具有良好的催化活性,而双齿硝酸盐以及桥式硝酸盐则不具备反应活性;而在E-R机理中,Lewis酸性位点起到主要作用。通过定量计算不同温度区间内L-H机理和E-R机理对反应的贡献,发现L-H机理对反应的低温活性更为重要,而E-R机理则是主要负责高温活性。根据前两部分的研究内容提出了宽温度窗口 SCR催化剂设计理论依据,并制备了一系列的催化剂进行验证,发现在250℃下的脱硝活性都显著高于商业催化剂,而在400℃时活性都在97%以上,证实了该理论具有普遍适应性。三、研究了催化剂抗SO_2中毒的机理。研究发现铈铌复合氧化物催化剂易受SO_2影响,200ppm SO_2的加入使得铈铌复合氧化物催化剂活性在20 h内降低了接近40%,且该失活现象为不可逆现象;而氧化铁的加入可有效提高铈铌催化剂抗SO_2中毒能力:20h后,催化活性依旧保持在92%左右。通过表征测试,发现在氧化铁改性的催化剂表面所形成的硫酸盐具有较低的热稳定性,同时与硫酸化后的铈铌复合氧化物催化剂相比,硫酸化后的铈铌铁催化剂反应中L-H比例依旧高达32%,远高于硫酸化后铈铌催化剂反应中L-H比例(6%)。这说明低温下氧化铁的添加可有效保证L-H机理的进行,因此提高了催化剂在低温下抗SO_2中毒的能力。四、研究了催化剂表面硫酸氢铵分解的机理。针对SCR催化剂低温区间内硫酸氢铵沉积问题,制备了 TiSi复合载体,并研究了 TiSi复合载体对于硫酸氢铵在低温下分解的促进作用。研究发现,当硫酸氢铵负载在催化剂表面时,铵物种主要以NH_4~+形式存在,而硫物种主要以双齿硫酸盐形式存在。硫酸氢铵在升温过程中遵循如下的分解规律:低温下NH_3首先释放出来,硫物种则固定在催化剂表面;随着温度的进一步升高,硫酸盐开始分解为SO_2。与Ti载体相比,TiSi载体表面的硫酸氢铵中的NH_4~+更容易在较低温度下分解并释放出NH_3,而SO_4~(2-)具有较高的热稳定性。根据这一研究结果,设计了系列载体均可促使硫酸氢铵在较低温度下分解。基于本论文的研究所开发的催化剂应用于温州电厂300MW机组,在35%和100%负荷下氮氧化物排放浓度均低于50 mg/m~3。
[Abstract]:With the rapid development of economy, energy consumption increased year by year, especially in Beijing Tianjin Hebei, Yangtze River Delta, Pearl River Delta and other key areas, the energy consumption intensity is much higher than the national average. High strength energy especially coal consumption brought serious environmental pollution. The nitrogen oxide (NO_x) is one of the major pollutants in the atmosphere, its emissions will cause acid rain, smog and other issues, a great impact to the environment and human health. The selective catalytic reduction (SCR) technology is currently the treatment of flue gas of nitrogen oxides in the most efficient technology. The catalyst is the key technology of SCR.SCR catalyst is its adaptability to boiler load in a big challenge the popularization and application in China. The frequent fluctuation of coal-fired power load in the low load operation of the unit, the V2O5-WO3/TiO2 catalyst activity at low temperature than conventional nitrogen oxide emissions exceed the standard result. , ammonia escape increases, even cause the failure of environmental protection equipment. The traditional strategy is the use of bypass reconstruction of economizer, the cost of this method is high, but also leads to a decrease in the thermal efficiency of the boiler. Aiming at the problem of coal-fired power plant full load denitrification research, puts forward the theoretical basis for a wide temperature window catalyst design and development a series of wide temperature window of SCR catalyst, structure-activity relationship, catalyst system and reaction mechanism of the sulfur resistance, and promote the mechanism of ammonium hydrogen sulfate decomposition, which are suitable to the actual conditions of flue gas in low temperature under long time stable wide temperature window catalyst run. The main results are as follows: first, this thesis. Effect on wide temperature window of denitration catalyst. Starting from the SCR reaction mechanism, the cerium oxide has good redox properties as catalysts for the oxidation of strong acid sites and oxygen Niobium as acidic sites, using a series of cerium niobium composite oxide SCR catalyst was synthesized by co precipitation method. When the Ce:Nb atom ratio is 1:1, with catalytic activity of more than 80% at 200~450 DEG C temperature range. The research establishes the relationship of catalyst structure physicochemical properties and catalytic activity, reveals different valence the rare earth doped transition metal oxides can produce short-range activation, generate more coordinatively unsaturated oxygen species, and for the first time found the coordinatively unsaturated oxygen species is a key factor in determining the low temperature activity. Two, studies the key active species and reaction path of catalyst surface. The study found that on the surface of the catalyst. SCR reaction follows the Eley-Rideal mechanism and Langmuir-Hinshelwood mechanism. In the mechanism of L-H, NO_2 and monodentate nitrate species has good catalytic activity, and bidentate nitrate and nitrate salt bridge Do not have the reaction activity; and in the E-R mechanism, Lewis acid sites play a major role. Through the calculation of different temperature range L-H mechanism and E-R mechanism of contribution to the reaction mechanism of L-H, found that activity in low temperature reaction is more important, and the mechanism of E-R is mainly responsible for the high temperature activity. According to the contents of the first two the author put forward a wide temperature window of SCR design based on the theory of catalyst, catalyst and preparation of a series of verification, found that the denitrification activity at 250 DEG C were significantly higher than that of the commercial catalyst, and at 400 degrees C activity in more than 97%, confirmed that the theory has universal adaptability. Three, study on the mechanism of resistance SO_2 poisoning catalyst. Study found that cerium niobium oxide catalyst susceptible to SO_2 200ppm because of the addition of SO_2 within 20 h by nearly 40% active cerium niobium composite oxide catalyst, and the deactivation phenomenon can not be Inverse phenomenon; and iron oxide could effectively improve the anti poisoning ability of cerium niobium catalyst SO_2 20h, catalytic activity still remained at around 92%. The characterization tests that sulfate formed in the catalyst surface iron oxide modified with low thermal stability, compared with cerium niobium composite oxide catalyst after sulfation cerium niobium iron catalyst, reaction of sulfated L-H in the proportion is still as high as 32%, far higher than the proportion of cerium niobium catalyst reaction after L-H sulfuric acid (6%). This shows that low temperature iron oxide addition can effectively ensure the L-H mechanism, so as to improve the ability of the catalyst under low temperature resistance to SO_2 poisoning. Four. Study on the mechanism of ammonium hydrogen sulfate catalyst surface decomposition. For the deposition of ammonium hydrogen sulfate SCR catalyst at low temperature range, the TiSi composite support was prepared, and the effects of TiSi composite carrier for ammonium hydrogen sulfate at low temperature Under the decomposition of the role. The study found that when ammonium hydrogen sulfate load on the surface of the catalyst, ammonium species mainly exists in the form of NH_4~+ and sulfur species mainly exist in the form of bidentate sulfate. Ammonium hydrogen sulfate in the heating process follow the decomposition rules as follows: low temperature NH_3 first released sulfur species is fixed at the surface of the catalyst; as the temperature increases, the sulfate decomposition of SO_2. and Ti compared to the carrier, the ammonium hydrogen sulfate TiSi carrier surface in NH_4~+ more easily in the low temperature decomposition and release of NH_3, and SO_4~ (2-) has high thermal stability. According to the results of this study, designed a series of carrier can promote the decomposition of ammonium hydrogen sulfate at low temperature. The development of this research based on the catalyst was applied in Wenzhou power plant 300MW unit, in the 35% and 100% load NOx concentrations were lower than 50 mg/m~3.

【学位授予单位】：浙江大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：X773;O643.3

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