摘要
NO是煤燃烧过程中产生的主要污染气体之一,对人类健康和环境有严重的危害,因此研究NO的有效控制技术具有重要的学术价值和现实的工程实践参考价值。作者提出了一种基于铁直接催化脱除NO气体的有效方法,并进行了详细的实验研究和机理分析。
首先概述了当前脱硝技术的研究进展,并着重介绍了再燃和选择性催化还原两种代表性的脱硝技术,同时指出了它们存在的问题和当前研究热点,前人在铁及其氧化物在NO脱除方面的研究工作做了讨论。在此基础上,提出了本课题的研究目的和研究方法。
本文采用了铁丝卷作为催化脱除NO的铁样品。实验在一个陶瓷管反应器中以氮气为基础的NO体积浓度在0.05%-0.1%的模拟烟气进行。陶瓷管内径2.5cm,由程序控温的电加热炉加热。加热段长度为30cm,烟气流量为1.5L/min。在实验中,使用了几组不同长、宽尺寸的铁丝卷,铁丝卷网孔的大小为6mm×6mm,铁丝直径0.5mm。铁丝网首先卷成直径略小于陶瓷管内径的卷,然后水平放在加热段的中央。实验温度为300℃~1200℃。对CO,CO2和O2等不同气体对铁脱硝效果的影响特性进行了测试。出口烟气的变化通过一个在线气体分析仪(型号:ECOM-J2KN,德国产)监测。铁样品实验前后的表面微观结构和组分的变化通过XRD和扫面电镜进行了分析。
本文研究结果得出的主要结论如下:
(1)通过在惰性氛围中铁催化还原NO的实验研究,表明铁对NO有很好的催化还原效果,并确定在实验条件下由长×宽=160mm×80mm的铁丝网卷成的铁丝卷对NO的催化还原效果最佳,680℃以上时NO脱除效率可达95%。
(2)通过在还原性氛围中铁催化还原NO的实验研究,表明CO能够很好的促进铁对NO的催化还原。在1000ppm氛围中,600℃时铁对NO的催化还原效率可达到95%,相比惰性氛围,温度下降了180℃;同时表明CO浓度越高,对铁催还还原NO效果越好。
(3)通过在氧化性氛围中铁催化还原NO的实验研究,表明02能够强烈的抑制铁对NO的催化还原,氧气浓度越高这种抑制作用越强烈。1050℃时,惰性氛围中铁对NO的脱除效率高达99%,氧气浓度为0.5%时铁对NO的脱除效率为90%,氧气浓度为1%时铁对NO的脱除效率为80%,氧气浓度为2%铁对NO的脱除效率降至60%,氧气浓度为4%时NO脱除效率更是降至35%。
(4)通过在CO2氛围中铁催化还原NO的实验研究,发现CO2对铁催化还原NO有一定的抑制作用,但温度高于700℃时,CO2的抑制作用已经不明显。
(5)在模拟烟气(16.8%CO2,1%-2%O2,0.05%NO)条件下,铁丝卷在一定温度时,脱除NO的效率可超过90%。结果表明,在CO浓度为4%、O2浓度为1%时,铁对NO的催化还原效果最佳,650℃C时NO脱除效率可达90%以上。
最后,本文通过扫描电镜(SEM)技术和X衍射(XRD)技术研究反应后铁样品的表面微观形态和组分变化。测试结果表明,在惰性和还原性气氛中,铁样品表面多孔、蓬松的结构,有助于铁对NO的吸附,使NO能够渗入铁样品内层,促进铁对NO的脱除;相反,在氧化性和模拟烟气氛围中,铁样品表面致密、光滑的表面结构则不利于铁对NO的催化还原。XRD测试结果表明,在惰性、氧化性和模拟烟气氛围中铁均被氧化为Fe+2Fe2+3O4、Fe2O3,而在还原性气氛中铁被氧化成FeO,CO2氛围中铁被氧化为FeO.Fe+2Fe2+3O4、Fe2O3。
Nitrogen oxide (NO) is a major pollutant emission from coal-fired boilers, which is very harmful to human health and the environment. It is very important to study effective and low cost technologies of NO reduction from the point of view of both academic understanging and industrial application. In this thesis, the author proposed a novel method to reduce NO emission which is based on iron catalysis and conducted detailed experimental tests and mechanism analysis.
First, recent development of NO control technologies was comprehensively reviewed based on literature research. Two typical methods, e.g., reburning and selective catalysis reduction (SCR) were particularly introduced. The shortcomings and the hot topics related to reburning and SCR were discussed in detail. The previous research work on NO reduction by iron and its oxides were discussed and the present research methods were then proposed.
In this thesis, Iron mesh rolls were selected as iron samples to catalytically reduce NO. Experiments were carried out in a ceramic tube reactor with a simulated flue gas consisting 0.05%-0.1%NO in a nitrogen base. A one-dimensional ceramic tube of inner diameter of 2.5 cm was used as the reactor and was electrically heated in the temperature programmed furnace. The total heated length of the reactor was 30 cm. The flow rate of the flue gas was 1.5 L/min. Several groups of iron meshes with different length and width were used in the tests. The size of the basic mesh unit was 6mm X 6mm and the diameter of the mesh wire was 0.5 mm. The mesh was first rolled into a mesh roll a little thinner than the tube inner diameter and then the mesh roll was horizontally set in the center of the heated tube. The tests were conducted in the temperature range of 300℃to 1200℃. The effect of different gas agents including CO, CO2 and O2 were examined. An online analyzer (ECOM-J2KN, Germany) was used to monitor the gas species. The surface micro-structure and component changes of the rion samples were analyzed by X-ray diffraction (XRD) and scanning electrical microscope (SEM) methods.
The following main conclusions can be drawn from the results.
(1) In inert atmosphere, the results showed that iron mesh roll was very effective to catalytically reduce NO to N2. The sample with a size of length X width =160mm×80mm had the best effect under the experimental conditions. NO removal efficiency was higher than 95% above 680℃.
(2) CO can promote the NO catalytic reduction by iron. When 0.1% CO was fed into the reactor, the NO reduction efficiency by iron approached 95% at 600℃, 180℃temperature drop for the similar NO reduction efficiency compared to that in inert atmosphere. Results showed that the higher the concentration of CO, the higher NO reduction efficiency by iron became.
(3) Experimental results demonstrated that O2 had a negative effect on NO reduction by Iron, At 1050℃, NO removal efficiency was about 99% in inert atmosphere, while it dropped to be 90% when 0.5% O2 was fed into the reactor. As oxygen concentration was 1%, NO removal efficiency dropped to 80%, when oxygen concentration was 2%, NO removal efficiency dropped to 60%, when oxygen concentration is 4%, NO removal efficiency dropped to 35%.
(4) Results showed that CO2 had a negative effect on NO catalytic reduction, but negative effect will be negalected when the temperature is higher than 700℃.
(5) In simualtied flue gas consisting 16.8% CO2, 1%-2% O2 and 0.05% NO, iron mesh roll was also effective to reduce more than 90% NO at certain temperature conditions. The results showed that when the CO concentration was 4%,02 concentration was 1%, the NO removal efficiencywas up to 90% above 650℃.
Finally, author tested the oxide Iron by scanning electron microscopy (SEM) techniques and X-ray diffraction (XRD) technique. SEM Test results showed that the porous, fluffy surface structure of the iron mesh roll formed in inert and reducing atmosphere helped iron absorption of NO, so NO can penetrate into the inner layer of iron samples easily, and promoted NO removal by Iron; on the contrary, the dense, smooth surface structure of the iron mesh roll formed in oxidizing and simulated gas atmosphere was harmful to the catalytic reduction of NO by iron. XRD results showed that iron was oxidized to Fe+2Fe2+3O4 and Fe2O3, in an inert, oxidizing and simulated gas atmosphere, FeO in the reducing atmosphere, and FeO, Fe+2Fe2+3O4 and Fe2O3 in CO2 atmosphere, respectively.
引文
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