【作者】 陈燕；

【导师】 李彩亭；

【作者基本信息】 湖南大学 ， 环境工程， 2009， 硕士

【摘要】 随着经济的发展,我国能源消耗量不断增加,NOx的排放量也持续增加,如不加强控制,NOx将对我国大气环境造成严重污染。国内外最常用也是最有效的烟气NOx脱除技术是NH₃-选择性催化还原（SCR）。本实验即选择SCR法以NH3为还原剂在催化剂的作用下将烟气中NOx还原成N2。目前工程中常用的SCR催化剂主要成分是V₂O₅-WO₃/TiO₂系列,商业SCR催化剂在较高温度（300～400℃）下才有好的催化活性,催化剂主要布置在空气预热器之前,能满足反应温度要求,然而该位置的高灰和二氧化硫容易使催化剂中毒。低温SCR装置可以置于除尘和脱硫之后,不仅可比较容易的用于现有锅炉,而且可以缓解粉尘和SO₂对催化剂的堵塞和毒化,避免烟气的预热耗能,降低脱硝成本。催化剂是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。因此,研究和开发能布置在除尘和脱硫之后（烟道气的温度大约为100℃～130℃）的低温SCR催化剂已迫在眉睫。活性碳纤维具有发达的孔结构,是一种良好的吸附剂,同时也是一种很好的催化剂,常常被用作催化剂的载体;Fe具有独特的化学性质,是较好的助催化剂组分;氧化铈（CeO₂）最重要的性质是作为氧的存储器,在氧化和还原条件下能通过Ce⁴⁺和Ce³⁺的转换来存储和释放氧;它也能够促进NO转化成NO2,来提高NH₃选择还原NO活性。三者均被广泛地应用于催化净化NOx的脱硝领域。但目前用ACF负载复合型金属氧化物CeO2-Fe2O3低温SCR催化剂的研究,尚不多见。本研究用活性炭纤维（ACF）作载体,负载CeO₂以及CeO₂-Fe₂O₃,采用等体积浸渍法,制备了不同负载量的CeO₂/ACF和不同负载量配比的CeO₂-Fe₂O₃/ACF两个系列的催化剂。用所制备的催化剂进行SCR实验,发现在单独负载CeO₂时,负载量为12%的催化剂在80℃～300℃温度范围内表现出了较好的催化活性;而混合负载时,由于加入了助催化剂,催化活性明显比相同负载量单独负载CeO₂时的要好,尤其是在较低温度范围内（80℃～200℃）活性更佳,10%CeO₂-2%Fe₂O₃/ACF催化剂表现出了最佳的催化活性和稳定性。进而对10% CeO₂-2%Fe₂O₃/ACF催化剂进行了煅烧温度影响实验,同时制备了200℃、300℃和400℃三种煅烧温度下的催化剂,得知300℃为最佳的煅烧温度;分别研究了O2、NH3对10%CeO₂-2%Fe₂O₃/ACF催化剂催化活性的影响,得出O₂和NH₃的存在对SCR反应的NOx净化效率有较大影响,那是因为NOx必须在有O₂的情况下才能被更好的还原成N₂;同时做了该催化剂的催化活性与时间关系的考察,结果显示10% CeO₂-2%Fe₂O₃/ACF催化剂具有较长的使用寿命,超过13小时,较以往研究的脱硝催化剂有很大程度的改进。另外,我们还通过比表面积（BET）测定、扫描电镜（SEM）和X射线衍射实验（XRD）等方法对催化剂的物理化学性能进行了分析:BET结果表明助催化剂Fe₂O₃的加入改变了催化剂的孔结构,也许孔径增大是CeO₂-Fe₂O₃/ACF催化剂活性变大的主要原因之一;SEM结果显示,负载在催化剂载体上的金属氧化物颗粒物分布均匀这说明氧化铈分散性较好,能够很好的负载在ACF上,使得催化剂性能更佳;XRD衍射实验发现CeO₂和Fe₂O₃在12%CeO₂/ACF和10%CeO₂-2%Fe₂O₃/ACF中多以非晶状态存在,没有检测出明显特征峰,从而推断出CeO₂和Fe₂O₃是无定形的。更多还原

【Abstract】 With the economic development, China’s energy consumption has been increasing and nitrogen oxides（NOx） emissions have continued to grow, if not further control, NOx will cause serious pollution of atmospheric environment of our country. The most commonly and effective technology of NOx removing in flue gas used at home and abroad is the NH3-Selective Catalytic Reduction（SCR）. We just selected SCR to reduce NOx in the flue gas into N₂ with NH3 as a reductan and under the influence of the catalyst in this study. The main component of SCR catalyst in the current project is V₂O₅-WO₃/TiO₂ series, and commercial SCR catalyst have good catalytic activity only at high temperature （300℃～400℃）,with catalyst mainly deployed before the air preheater where it can meet the temperature requirements, but the location of the high ash and sulfur dioxide may easily cause catalyst poisoned. By placing low-temperature SCR device after the dust removal and desulfurization, it can be not only relatively easy for the existing boilers, but also ease the congestion and the poison of dust and SO₂, as well as avoid the flue gas preheat energy consumption and reduce the cost of denitrification. Catalyst is the core of SCR technology, and its catalytic performance is directly related to the effects of denitrification. Therefore, it’s extremely urgent to research and develop a low-temperature SCR which can be arranged after the dust removal and desulfurization （temperature of flue gas is about 100℃～130℃）. Active carbon fiber （ACF） is not only used as a good absorbent, but also a good catalyst because ACF has developed pore structure, so often it is also used as a catalyst carrier; Fe has a unique chemical properties, as well as a good catalyst components; the most important nature of oxidation cerium （CeO₂） is as a storage of oxygen, through the conversion of Ce⁴⁺ and Ce³⁺ to store and release oxygen in the conditions of oxidation and reduction; and with it NO converted to NO₂ can be promoted to improve the NH₃-selective reduction activity of NO. The above-mentioned three are widely used in catalytic areas of the NOx purification. But low-temperature SCR catalysts such as ACF load with CeO₂-Fe₂O₃ are still rare.In this study, the activated carbon fiber （ACF） was the support, and CeO₂ and CeO₂-Fe₂O₃ was loaded on it, with equal volume impregnation method,two series catalysts were prepared : different loading of CeO₂/ACF and different load ratio of CeO₂-Fe₂O₃/ACF. It was found that as the temperature increased from 80℃to 300℃, the 12%CeO₂/ACF catalyst showed good catalytic activity for SCR when single load; While mixed load, due to adding cocatalyst, the catalytic activity is significantly higher than single load with the same loading of CeO₂, especially in the lower temperature range（80℃～200℃）, the activity is much better, 10%CeO₂-2% Fe₂O₃/ACF catalyst showed the best catalytic activity and stability. Therefore experiments of calcination temperature effction was took for the catalyst of 10%CeO₂-2% Fe₂O₃/ACF, meantime another three kinds of catalyst were prepared, at different calcination temperature which were 200℃, 300℃and 400℃, and it was found that 300℃was the best calcination temperature. The effect of the O₂ and NH₃ for the catalytic activity of 10%CeO₂-2% Fe₂O₃/ACF catalyst were respectively studied,from which we can learn the presence of O₂ and NH₃ do much influence to the NOx purification efficiency in SCR reaction,that is because only in the presence of O2 can NOx be better reduced to N₂. In the meantime,the catalytic activity and time relationship were also studied, and the result presented 10% CeO₂-2%Fe₂O₃/ACF had longer lifetime,and when surpassed thirteen hours it also showed much more improvement contract to previous research. In addition, a combination of various physicochemical techniques containing surface area （BET） measurement, X-ray diffraction （XRD） and scanning electron microscopy （SEM） were used to characterize the physico-chemical properties of the catalysts. The reason of CeO₂-Fe₂O₃/ACF catalyst activity becomes larger might be that pore diameter got larger with the injection of cocatalyst Fe₂O₃, which changes the pore structure in the BET study.Cerium oxide’s good dispersivity make it better loaded on ACF, so SEM showed the metal oxide particles distributed evenly on the carrier had best catalytic performance.And from CeO₂-Fe₂O₃/ACF XRD-diffraction we conclude that both CeO₂ and Fe₂O₃ are amorphous for 12%CeO₂/ACF and 10%CeO₂-2%Fe₂O₃/ACF were amorphous state, no obviously characteristic peaks were seen.更多还原