CAS OpenIR
NH3-SCR催化剂与纤维陶瓷膜复合方法与性能研究
李伟曼
Thesis Advisor陈运法
2020-07-01
Degree Grantor中国科学院大学
Degree Name博士
Degree Discipline材料学
KeywordNox,Nh3-scr,过渡金属氧化物,分子筛催化剂,钒钛复合氧化物
Abstract

氮氧化物(NOx)是一类重要的大气污染物,可以导致雾霾、光化学烟雾和酸雨等污染。目前我国氮氧化物的固定源排放仍然占据较大比例,在燃煤电厂大多实现超低排放情况下,非电行业的NOx排放控制成为现阶段主要攻坚环节之一。氨选择性催化还原技术(NH3-selective catalytic reduction,NH3-SCR)是控制NOx排放的主要技术,已在电厂锅炉中广泛应用。然而非电行业工业锅炉烟气成分复杂(经常同时含有二氧化硫、氮氧化物、颗粒物等组分)和温度窗口偏低等特点限制了NH3-SCR技术在该行业中的快速应用。目前非电行业工业锅炉烟气的治理技术是将单一控制模块串联使用,存在着流程长、占地面积大、投资运行费用高等问题。多组分协同治理与功能化耦合技术是解决上述问题的有效途径,其中气体处理技术与除尘模块的耦合是其研究重点之一。现有的除尘模块多集中在不耐高温的布袋和自重大的碳化硅陶瓷上,亟需轻质高强且耐高温的载体。本论文以高孔隙率(68%)和轻质高强(表观密度为0.625 g/cm3)的纤维陶瓷膜为载体,设计制备了NH3-SCR催化剂与除尘陶瓷膜功能耦合模块,重点研究了SCR催化剂的结构设计、负载方法以及催化性能的优化,目的在于研发低阻力、高催化效率的复合净化模块。研究内容和主要结果如下:(1)以轻质高强低阻的纤维陶瓷膜为基体,通过浸渍法,调节锰铈硝酸盐前驱体的负载量和摩尔比,经过煅烧得到锰铈系列的脱硝陶瓷膜块,并考察了负载量和锰铈摩尔比对催化性能的影响。锰铈摩尔比为6:4时得到的催化剂陶瓷膜脱硝性能最佳,可以在120-250 oC保持90%以上的脱硝效率,主要原因是丰富的Mn4+、表面活性氧和酸性位点有利于低温SCR反应。此外,NO转化率会随着氧浓度的增加而增加,随着空速的增加而降低。但是由于纤维陶瓷膜上催化组分含量较少,且锰系催化剂性能限制,所得到的膜催化剂耐硫性较差。(2)以三种商用分子筛ZSM-5、25beta(数字为硅铝比)和40beta为载体,以铁和锰硝酸盐为前驱体,通过浸渍法制备了Fe-Mn/分子筛催化剂,考察了分子筛载体对于脱硝性能的影响,将脱硝性能最好的Fe-Mn/25beta催化剂负载到陶瓷膜上,得到催化陶瓷膜。以分子筛为载体的催化剂适用脱硝温度窗口明显拓宽,其中Fe-Mn/25beta催化剂在220-450 oC可以将NO完全转化,主要原因是Fe和Mn更容易进入到beta分子筛的内部形成孤立的Fe(III)和Mn物种。更多的铁锰元素会在ZSM-5分子筛的表面形成氧化物颗粒,导致Fe-Mn/ZSM-5耐硫性变差。将Fe-Mn/25beta催化剂负载到陶瓷膜载体中,发现相比于粉末催化剂,低温活性稍有下降,高温活性稍有上升,耐硫性下降。(3)以自制的多孔氧化硅(MCM,比表面积为1188.7 m2/g)为载体,乙酰丙酮钒和钛酸四丁酯为前驱体,通过溶胶-凝胶方法制备比表面积改善的催化剂,将催化剂粉末以浆液涂覆的方式引入到陶瓷膜中,得到了钒钛系催化陶瓷膜。结果表明,5V2O5-20TiO2/MCM粉体(数字为相比于MCM的质量百分比)可以在250-450 oC内保持90%以上的NO脱除效率;且在SO2存在时,脱硝效率从90%下降到80%(T = 330 oC);然而该系列的催化陶瓷膜的最高脱硝效率不到50%。(4)以正硅酸乙酯、钛酸四丁酯和乙酰丙酮氧钒为前驱体,采用一步溶胶-凝胶方法合成了钒钛硅系列复合氧化物;通过一步溶胶-凝胶(FC-sg)、两步溶胶凝胶(FC-2)和颗粒负载法(FC-p)三种负载方式将催化剂负载到陶瓷膜载体,对比研究了复合催化陶瓷膜的催化性能。结果表明,高硅含量可以提高催化剂粉末的比表面积,但是会降低钒物种的分散性,引起脱硝性能下降;高钒含量样品的脱硝温度窗口向低温区间移动。脱硝性能最好的配方为2V-90Ti10Si,可以在230-470 oC内达到90% 以上的NO转化率。从负载方式对脱硝性能的影响发现,在相同条件下的脱硝活性顺序为:FC-sg≈FC-p>FC-2;FC-2的效果最差,其原因是钒组分在制备过程中发生了迁移,在膜表面形成了聚集态的V2O5物种。;Nitrogen oxides (NOx) are kinds of the most important air pollutants, which can participate in the formation of haze, photochemical smog, acid rain, et al. The flue gas from stationary source still accounts for the majority of NOx in China. With the ultra-low emission in power industry, control of NOx from non-power industry is becoming crucial. Selective catalytic reduction of NOx by NH3 (NH3-SCR) is widely used in powder industry. However, the special temperature window and complexity of flue gas in non-power industry confines the use of NH3-SCR. The flue gas from industry boiler comprises particulates, sulfur oxide, and nitrogen oxides. In practical applications, the pollution control in flue gas is mostly completed by the combination of various separate units corresponding to one single pollutant. How to design the multifunctional units in which yielding the coupled effects of de-NOx and dust precipitation becomes more interesting. The dust precipitation modules comprise bag filters that intolerance to high temperature and SiC filter with large self-weight. Therefore, a support with light weight and high dust precipitation efficiency is needed. In this thesis, the fibrous ceramic membrane (porosity-68%, apparent density-0.625 g/cm3) with high strength were used as support, and loaded with NH3-SCR catalysts in order to obtain catalytic membrane with low pressure drop and high de-NOx efficiency. The catalysts composition, microstructure, depositing method and the catalytic performance were investigated. The main research content and conclusions were as follows.(1) Using manganese and cerium nitrates as raw materials, Mn-Ce oxides catalysts were loaded on fibrous ceramic membrane with light weight, high strength and low pressure drop, in order to obtain catalytic ceramic membrane. The effects of loading weights and molar ratios of Mn/Ce on catalytic performance were investigated. The catalytic membrane with Mn/Ce molar ratio of 6/4 performed a best de-NOx performance, with more than 90% de-NOx efficiency in the temperature range of 120-250 oC; mainly because the abundance of Mn4+, surface oxygen species, and acid sites facilitate the low temperature SCR. The de-NOx efficiency of catalytic membrane increased with oxygen concentration, and decreased with space velocity. Due to a small amount of catalysts on membrane, and the poor anti-sulfur characteristics of manganese catalysts, the catalytic performance of Mn-Ce based membrane decreased when introducing SO2 in flue gas. (2) The Fe-Mn/zeolite catalysts were prepared by employing ZSM-5, 25beta and 40beta as supports and iron nitrates, manganese nitrates as precursors through a co-precipitation method. The influence of support on de-NOx and its mechanism were analyzed. Fe-Mn/25beta catalyst showed the best catalytic performance, and it was loaded onto the membrane to obtain catalytic membrane with good low temperature SCR and anti-sulfur performance. Fe-Mn/25beta can obtain 100% NO conversion in a wide temperature window of 220-450 oC, mainly beacause Fe and Mn elements can easily enter the matrix of beta zeolites. However, more metal oxides aggregated on the surface of ZSM-5, leading to poor anti-sulfur performance of Fe-Mn/ZSM-5. Fe-Mn/25beta powder were loaded onto ceramic membrane, and compared with powder catalysts, its high temperature performance increased, low temperature and anti-sulfur performance decreased.(3) The vanadium-titanium composites with large specific surface area were obtained by employing porous silica (MCM, BET = 1188.7 m2/g) as supports and vanadyl acetylacetonate, tetrabutyl titanate as precursors through a sol-gel route. The powder catalysts were then loaded onto ceramic membrane to obtain vanadium titanium based catalytic membrane. It was shown that 5V2O5-20TiO2/MCM (the number denoted as loading weight of V2O5 and TiO2 on MCM) performed more than 90% NO conversion in 250-450 oC. The NO conversion decreased from 90% to 80% when introducing SO2 into flue gas (T = 330 oC). However, when loading the powder onto ceramic membrane, the NO conversion of catalytic membrane decreased to less than 50%. (4) The vanadium-titanium-silicon porous composites were synthesized by using tetraethyl orthosilicate, tetrabutyl titanate and vanadyl acetyacetonate as raw materials through a sol-gel route. The catalysts with best SCR performance were loaded onto ceramic membrane by one-step sol-gel method (FC-sg), two-step sol-gel method (FC-2) and particle loading method (FC-p) to obtain catalytic membrane. The specific area of composites was increased with the contents of silica; however, more silica would lead to worse dispersion of vanadium oxide, thus leading to NO conversion decrease. The de-NOx temperature window moved to low temperature range with high vanadium loading. The catalysts with wide temperature window (230-470 oC with more than 90% NO conversion) was 2V-90Ti10Si. The influence of loading methods were investigated, and it was shown the catalytic performance followed FC-sg≈FC-p>FC-2. The FC-2 sample showed the worst catalytic performance, mainly because the vanadium species removed to membrane surface, leading to aggregated V2O5 species. 

Language中文
Document Type学位论文
Identifierhttp://ir.ipe.ac.cn/handle/122111/49684
Collection中国科学院过程工程研究所
Recommended Citation
GB/T 7714
李伟曼. NH3-SCR催化剂与纤维陶瓷膜复合方法与性能研究[D]. 中国科学院大学,2020.
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