Experimental Study on NO Removal Using Non-thermal Plasma Oxidation-alkali Absorption

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1 Experimental Study on NO Removal Using Non-thermal Plasma Oxidation-alkali Absorption Shuilan Ding 1, Qi Yu, Yingzhou Zhang 1, Yuan Liu 1, Chunxue Xie 1, Gang Yu*, 1, 3 1 School of Environment Engineering, Wuhan Textile University, Wuhan , China School of Chemistry and Chemical Engineering, Wuhan Textile University, Wuhan , China 3 Department of Man-Machine and Environment Engineering, College of Aerospace Engineering; Nanjing University of Aeronautics and Astronautics, Nanjing 10016, China Abstract: A non-thermal plasma oxidation-alkali absorption NO removal system is designed by connecting sequentially an alkali aqueous solution absorption device to a gas discharge reactor. An experimental system of simulated flue gas NO removal using non-thermal plasma oxidation and Ca(OH aqueous solution absorption was established and influences of the concentrations of O and H O vapor on this kind of NO removal method were studied experimentally. The experimental results show that oxidation of NO in non-thermal plasma can be promoted by H O vapor and the overall NO removal efficiency under this condition is higher than that of the system without H O vapor added, which can be up to 7%. Due to the existence of O in the present system, Ca(OH aqueous solution can play a certain role in improving the NO removal efficiency and the NO removal efficiency can increase by 10%-14% under this condition using the alkali absorption. When O is added into the system and Ca(OH aqueous solution absorption device is connected sequentially to the non-thermal plasma reactor, the NO removal efficiency is enhanced after H O vapor is added in the present system, which can obviously increase by 7%-17%. Keywords: non-thermal plasma oxidation, alkali absorption, nitric oxide (NO removal Introduction In the power plant boiler, NO x produced by coal combustion (NO more than 95% is one of the main flue gas pollutants and its emission concentration needs to be controlled. For flue gas denitrification, in addition to the mainstream methods of selective catalytic reduction (SCR and the selective noncatalytic reduction (SNCR, there are liquid absorption method, microorganism absorption method, hot carbon reduction method catalytic decomposition method, liquid film method, SNRB process denitration technology, feedback type oxidation denitration technology, activated carbon adsorption method, plasma method and so on. However the above methods may have been eliminated, or at the laboratory research stage. They are difficult to be put into large-scale industrial applications (1. From the intrinsic reaction process in the nonthermal plasma, it can be known that NO can be removed through the pathways of oxidation and reduction. When NO is removed through the reduction pathway, it can be reduced into N by N atom. When NO is removed by oxidation pathway, it will be *Corresponding author; address: @qq.com oxidized into higher-order oxide NO by oxidant O 3, ClO, KMnO 4 (, or the products of pulse discharge, such as O, HO and free radicals (3-5. Then through the subsequent reactor, NO can be absorbed by water or alkali solution (, or can be transformed into HNO 3 while meeting H O in the flue gas. When the participation of NH 3 or other neutral particle is considered, NO can further be transformed into NH 4 NO 3 aerosols which can be collected by dust collector (3-5. Compared with the NO oxidation pathway, the NO reduction pathway has the advantage that its products do not need to be dealt with further, however, the SCR flue gas denitration process, one of the NO reduction methods, still remains some defects of higher investment and operation costs. Therefore, the NO oxidation method still may be an efficient option. Among numerous flue gas denitration techniques, non-thermal plasma method has attracted many researchers due to its outstanding advantages of simple equipment, less investment, easy operation and feasibility of integrating multi-pollutant removal processes into one (6-8. Actually, in the process of NO plasma removal, both NO reduction removal and NO oxidation removal are coexisting. When the 114 J. Adv. Oxid. Technol. Vol. 18, No. 1, 015 ISSN Science & Technology Unauthenticated Network, Inc. Download Date 10/0/17 :04 PM

2 Stop Stop S. Ding et al. Alkali absorption equipment 50HZ 0V Pressu re regula tor highvoltage power supply Plasma reactor Flue gas analyzer water Stop reducing flowmeter nitrogen nitric oxide reducing flowmeter mixing tank oxygen reducing flowmeter Figure 1. Schematic diagram of experiment system. participation of O and H O vapor is considered, NO plasma oxidation can be more dominant than NO plasma reduction (9. Thus the non-thermal plasma can also be used to conduct NO oxidation removal. When the plasma NO oxidation removal is implemented, products of NO oxidation can be absorbed sequentially by connecting an alkali aqueous solution absorption device to a gas discharge reactor. There have been some studies on the method of combining plasma NO oxidation combined and alkali absorption. By combining plasma oxidation and chemical absorption, Kuroki T and others studied NO x removal and pointed out that this method is a potential and effective one to reduce the costs in flue gas treatment; Yamamoto T and others (11 pointed out that the operation cost of NO removal is only about 1/3-1/5 of that of the traditional selective catalytic removal technology. These studies indicated that nonthermal plasma oxidation-alkali absorption NO removal system, a kind of NO sequential removal process, is feasible to remove NO. Therefore, in order to promote the practical application of this NO sequential removal technology, it is necessary to carry out some detailed researches on the respect role of plasma oxidation process and alkali absorption process in the whole NO sequential removal process. In this paper, through the experimental studies on non-thermal plasma oxidation-alkali absorption NO sequential removal process, the influences of oxygen concentration, water vapor concentration and NO initial concentration on NO plasma oxidation and absorption of NO oxidation products by the alkali were obtained. This study can help us explore the respective role of the plasma oxidation and the alkali absorption in the whole NO sequential removal technology, and the further optimization of this technology in future. Experimental System and Solutions In order to study how O, water vapor, Ca(OH solution, and other factors affect plasma oxidationalkali absorption NO sequential removal, the experimental device was established in Figure 1. The concentration of Ca(OH solution in the alkali absorption device is mol/l; nitrogen, nitric oxide and oxygen were produced by Nanjing Shangyuan Industrial Gas Factory in Nanjing, China. The purities of NO, N and O are 4000ppm (the rest is N, % and 99.% respectively. A coaxial structure was adopted in the plasma reactor as Figure.1. The total length of the quartz glass cylinder is 180 mm. The inner diameter of the J. Adv. Oxid. Technol. Vol. 18, Unauthenticated No. 1, Download Date 10/0/17 :04 PM

3 cylinder is 15 mm and the outer diameter is 17 mm. The inner electrode is a copper bar and its diameter is 8 mm. In order to make sure that the copper bar is installed in the center of the quartz glass tube, both ends of the copper bar are fixed in the middle of two plugs respectively and the two plugs are stuffed in the two ends of the quartz glass tube. The copper wire net is covered on the outer surface of the quartz glass tube as the outer electrode and its length is 10 mm. The voltage regulator model is TDGC-1 provided by Nanjing Suman Electronics Co. Ltd., Nanjing, China. Its rated power, rated frequency, rated input voltage and rated output voltage are 1 KVA, 50 Hz, 0 V and 0-50 KV respectively. The real input voltage and frequency of the voltage model are 0 v and 50 Hz, which are the parameters of electric supply commonly used in China Laboratory. Its output voltage is just the input voltage of the high voltage pulse power supply. That is to say, the output voltage of the voltage regulator can be used as its input voltage in the high voltage pulse power supply, which can transform the low input voltage varying between 0 V and 0 V into very high voltage. The high voltage pulse power supply was also provided by Nanjing Suman Electronics Co. Ltd., Nanjing, China, and its model is CTP-000K. Its output voltage is applied on both discharge electrodes and the inter-electrode voltage will be varying between 0 KV and 30 KV according to the discharge load which may vary immediately even if minor change of any system operating condition, and this minor change may cause the change of inter-electrode voltage. Voltage and current waveforms were measured by using oscilloscope (Tektruding and high voltage probe (P6015A. Voltage and current waveforms have showed as Figure.. The Picture of discharge plasma has showed as Figure.3. The concentrations of main gas components were measured by the UK KANE KM9106 portable flue gas analyzer. Analysis of Experimental Data NO removal efficiency is calculated by the following equation: C C NO before after, C NO before NO where C NO before and C NO after are the concentrations of NO before switching on the plasma reactor and after switching on the plasma reactor respectively. 116 J. Adv. Oxid. Technol. Vol. 18, No. 1, 015 Figure.1. plasma reactor. Figure.. Voltage and current waveforms applied to the electrodes. Figure.3. Picture of discharge plasma. Principles of NO Oxidation and Alkali Absorption When the plasma reactor is switched on, a pulsed discharge will occur. The high-energy electrons in the pulsed discharge will collide with some gas molecules to generate ions and free radicals. These ions and radicals can induce a series of chemical reactions, so that NO is removed in certain conditions within a short time. Since the reactions between nitric oxides and alkaline solution are mainly through NO NO Ca( OH Ca( NO HO and 4NO Ca( OH Ca( NO Ca( NO3 HO, NO solely cannot react with alkali solution. Only the products of NO oxidation can be absorbed by alkaline solution. Therefore, the oxidation mechanism of NO is crucial for alkali absorption. Only when the oxygen is present, can oxygen generate O and O 3 in the plasma reactor. NO reacts with these oxidizing substances mainly through Unauthenticated Download Date 10/0/17 :04 PM

4 NO O M NO M. Only when the water vapor is present, will water vapor be activated and generate OH in the plasma reactor. NO reacts with OH mainly through and NO OH M HNO M, HNO OH NO HO NO O3 NO O. When the oxygen and water vapor exist in the plasma reactor at the same time, in addition to the above reactions, the following reactions can also occur in the plasma reactor: OH O 3 HO O NO HO NO OH Effect of Water Vapor on Plasma Denitration In this study, the method of adding water vapor allows the gas mixture at the outlet of the mixing tank into a bubbling device with some water. After flowing through the water, the gas mixture will carry a small amount of water vapor into the plasma reactor. With a small amount of water vapor and without alkali absorption device, the effects of water vapor on plasma denitration were studied. Experimental conditions: The input voltage value of the high frequency high voltage power is 00 V, and the output voltage frequency is 8 khz; The total flow of gas mixture is 300 L/h; The initial concentrations of NO are 1000 ppm, 000 ppm and 3000 ppm respectively. The concentrations of NO and NO are measured by flue gas analyzer before and after switching on the plasma reactor. Figure 3.1, Figure 3.3 and Figure 3.5 show how the NO removal rate changes as the oxygen concentration does, under two conditions of considering and non-considering the participation of water vapor. Figure 3., Figure 3.4 and Figure 3.6 show how the generation of NO changes as the oxygen concentration does, under two conditions of considering and non-considering the participation of water vapor. Discussions of the experimental results are as follows: (1 When the oxygen concentration is 0, the main electron-collision reaction is that N is transformed into N atom produced by collision between highenergy electron and N. Due to the presence of water vapor, the main electron-collision reaction is that H O is activated and generate OH by pulsed discharge. Then following reactions mainly occur in the plasma reactor: N NO N NO OH M HNO O M HNO OH NO HO N OH NO H Obviously, when water is considered, NO is oxidized while it is also generated in large quantities at the same time. When there is no oxygen, adding water, the following reactions will occur to achieve the oxidation of NO removal: NO OH M HNO M HNO OH NO HO If there are no O and H O (vapor in the gas stream, the reaction is reducing NO into N using N atom. The following reactions mainly occur in the plasma reactor: e N N e N NO N O Through the above reactions, most of NO will be reduced and NO reduction efficiency will be more than 90% (1-13. when the oxygen concentration is 0%, the NO removal efficiency with water is lower than that without water. However, the problem is that, combustion exhaust gas include some water and some oxygen under normal conditions. Such substances and some oxidation groups which are produced by electroncollision reaction, are easier to oxidize NO, and then the ability of NO reduction will be weaken, which will make it hard to Perform reduction pathway. In order to realize the oxidation of NO removal, the paper is to study how to oxidize more NO. ( If the oxygen is added into the system, whether adding water vapor or not, the NO removal rate presents a downward trend, the reason is that the O can generate O and O 3 under the condition of the pulse discharge. The following reactions mainly occur in the plasma reactor: N O M NO M N O NO O NO O M NO NO O 3 NO O Obviously, the increase of the amount of O can promote the NO removal, nevertheless a great number of N is translated into the NO at the same time. (3 When water vapor is added, the NO removal rate is higher than that of a system without water and the maximum increase is 7% due to the following reactions in the plasma reactor: M J. Adv. Oxid. Technol. Vol. 18, Unauthenticated No. 1, Download Date 10/0/17 :04 PM

5 Figure 3.1. The changing curves of the NO removal rate when the NO initial concentration is 1000 ppm. Figure 3.. The changing curve of the generation of NO when the NO initial concentration is 1000 ppm. Figure 3.3. The changing curves of the NO removal rate when the NO initial concentration is 000 ppm. Figure 3.4. The changing curve of the generation of NO when the NO initial concentration is 000 ppm. Figure 3.5. The changing curves of the NO removal rate when the NO initial concentration is 3000 ppm. Figure 3.6. The changing curve of the generation of NO when the NO initial concentration is 3000 ppm. Figure 3. The influences of water vapor on NO plasma removal rate and NO generation. NO OH M HNO OH NO HO NO HNO HO NO OH NO OH M HNO3 M These reactions spur NO to be oxidized into NO, and through reacting with OH, NO is transformed into HNO 3 to achieve the aim of removing NO. M If there is water in the system, OH will be easily generated through the collisions between high-energy electrons and H O. The oxidizing ability of OH is much stronger than that of O and O 3. The reaction between NO and OH is NO OH M HNO M and this NO oxidation reaction is prominent and strong. In addition, when the water exists, the product of this NO oxidation reaction can be easily transformed into 118 J. Adv. Oxid. Technol. Vol. 18, No. 1, 015 Unauthenticated Download Date 10/0/17 :04 PM

6 HNO 3 stably. Thus NO is easier to be oxidized than that without water,which shows the effect of water is great. This is the reason why the NO removal rate is higher than that of a system without water. (4 Due to spontaneousness of the reaction between NO and O, the amount of NO will increase with the addition of oxygen in the absence of plasma reactor. NO concentration curve shows a trend of increase along with the amount of oxygen, Here the following reaction occurs: NO O NO This reaction can easily occur in atmosphere and cause the acid rain through the further reaction between NO and H O. Compared with the condition that there is no water vapor added and the plasma reactor is switched on, the concentration of NO will be obviously higher, up to 50 ppm, when there is water vapor added and the plasma reactor is switched on. It can be explained due to the fact that more NO is oxidized into NO by OH. When the initial concentration of NO is 000 ppm or 3000 ppm, it is found that the NO generates in the condition of no water and no oxygen after opening the plasma reactor. This is because some water vapor and some oxygen are attached to the electrode surface. In a word, with the increase of O, the water vapor would lead more NO into NO and the oxidation of NO is promoted. Effect of Alkali Absorption on Denitrification Rate (1 Under the Condition with Oxygen Only The above figures show how the denitrification rate changes along with the increase of the amount of oxygen under the condition without alkali absorption. After reactions in the plasma reactor, the gas mixture is introduced into the Ca(OH solution whose concentration is mol/l. Experimental conditions: the input voltage value of the high frequency high voltage power is 00 V, and the output voltage frequency is 8 khz; the total flow of gas mixture is 300 L/h; the initial concentration of NO are 1000 ppm, 000 ppm and 3000 ppm respectively. The concentrations of NO and NO are measured by the flue gas analyzer before and after switching on the plasma reactor. Figure 4.1, Figure 4.3 and Figure 4.5 show how the NO removal rate changes, when accompanied by the changing of oxygen concentration. Figure 4., Figure 4.4 and Figure 4.6 show how the generation of NO changes, when accompanied by the changing of oxygen concentration. Discussions of the Experimental Results are as Follows: Figure 4.1, Figure 4.3 and Figure 4.5 show that, after adding the calcium hydroxide solution, the NO removal efficiency rate is higher than that of system without calcium hydroxide solution. When oxygen is added gradually, the following reactions will occur in the plasma reactor: N O NO O, N O M NO M NO O M NO M, NO O3 NO O There are both NO generation and NO transformation. of course, the quantity of NO transformation is much more than that of NO generation. When there is Ca(OH solution absorption, the gas mixture in the Ca(OH solution will undergo the following reactions which can promote the NO removal: NO NO Ca( OH Ca( NO HO 4NO Ca( OH Ca( NO Ca( NO3 HO As shown in the above figures, when the concentration of NO is 1000 ppm, with the addition of O and the absorption of calcium hydroxide solution, the NO removal efficiency can be increased by 10%- 14% under this condition than that of the system without alkali absorption; When the concentration of NO is 000 ppm or 3000 ppm, the NO removal rates are only slightly higher than that without alkaline solution. Thus, the higher the concentration of NO is, the stronger inhibitory effect it shows. From the above figures of NO curves, in the presence of oxygen and plasma, when there is alkali absorption, the amount of NO is significantly reduced than that of system without alkaline solution. When the concentration of NO is 3000 ppm, the amount of NO can be reduced up to 400 ppm. Therefore, a lot of NO is absorbed by alkaline solution. It is demonstrated that adding alkali absorption has a promoting effect on NO removal efficiency. ( Under the Condition with Water Vapor Added In the above experimental system, there is a bubbling device with some water. Let the gas mixture flows through the bubbling device, and then enter the plasma reactor. Through the bubbling device, a small amount of water vapor will get into the plasma reaction system. By this system, the characteristics of nonthermal plasma denitration under the condition of containing a small amount of water vapor and alkali absorption can be studied. Figure 5.1, Figure 5.3 and Figure 5.5 show how the NO removal efficiency changes, when accompanied by the changing of oxygen concentration, under two J. Adv. Oxid. Technol. Vol. 18, Unauthenticated No. 1, Download Date 10/0/17 :04 PM

7 Figure 4.1. The changing curves of the NO removal rate when the NO initial concentration is 1000 ppm. Figure 4.. The changing curve of the generation of NO when the NO initial concentration is 1000 ppm. Figure 4.3. The changing curves of the NO removal rate when the NO initial concentration is 000 ppm. Figure 4.4. The changing curve of the generation of NO when the NO initial concentration is 000 ppm. Figure 4.5. The changing curves of the NO removal rate when the NO initial concentration is 3000 ppm. Figure 4.6. The changing curve of the generation of NO when the NO initial concentration is 3000 ppm. Figure 4. The influences of adding alkali absorption on NO plasma removal rate and NO generation. conditions of considering and non-considering the participation of water vapor. Figure 5., Figure 5.4 and Figure 5.6 show how the generation of NO changes, when accompanied by the changing of oxygen concentration, under two conditions of considering and non-considering the participation of water vapor. Discussions of the Experimental Results are as Follows: On the whole, Figure 5.1, Figure 5.3 and Figure 5.5 show that the NO removal efficiency is much better than that without adding water vapor when there are the absorption of calcium hydroxide solution, oxygen and water vapor added. 10 J. Adv. Oxid. Technol. Vol. 18, No. 1, 015 Unauthenticated Download Date 10/0/17 :04 PM

8 Figure 5.1. The changing curves of the NO removal rate when the NO initial concentration is 1000 ppm. Figure 5.. The changing curve of the generation of NO when the NO initial concentration is 1000 ppm. Figure 5.3. The changing curves of the NO removal rate when the NO initial concentration is 000 ppm. Figure 5.4. The changing curve of the generation of NO when the NO initial concentration is 000 ppm. Figure 5.5. The changing curves of the NO removal rate when the NO initial concentration is 3000 ppm. Figure 5.6. The changing curve of the generation of NO when the NO initial concentration is 3000 ppm. Figure 5. The influences of adding water vapor alkali absorption on NO plasma removal rate and NO generation. When the concentration of oxygen is 0%, with the help of water vapor, the following main reaction will also happen: N OH NO H When the amount of oxygen is 0%, the NO removal efficiency with water vapor added is lower than that without water vapor added. When some oxygen is added into the system, with some water vapor added and connecting alkali absorption device, the NO removal efficiency is higher than that of system without water vapor and the NO removal efficiency can be increased by 7%-17%. Before connecting sequentially an alkali aqueous solution absorption device, a When there is no water, the following reactions mainly occur: NO O M NO M, NO O3 NO O b When the water vapor is added, in addition to the above reactions, the following reactions will also occur: OH O HO 3 O NO OH M HNO M, NO HO NO OH J. Adv. Oxid. Technol. Vol. 18, Unauthenticated No. 1, Download Date 10/0/17 :04 PM

9 When the gas mixture flows into the Ca(OH solution absorption device, the following reactions will occur: 4NO O H O HNO 4 4NO 3O HO 4HNO3, 4HNO3 4NO O HO N NO NO NO HNO3 Ca( OH Ca( NO3 HO Because of the addition of water vapor, both of NO and NO are oxidized into HNO 3. Through the neutralization of alkali, the decomposition of HNO 3 is reduced, and the new NO which generated by the decomposition of HNO 3 will be reduced. More NO is removed through oxidation. From the above figures, under the conditions of adding oxygen, switching on the plasma reactor and connecting an alkali absorption device, when water vapor is added, the amount of NO will be increased. If the initial concentration of NO is 1000 ppm, 65 ppm of NO can be increased. Through comparing the Figure 5., Figure 5.4 and Figure 5.6 with the Figure 3., Figure 3.4 and Figure 3.6, the following can be obtained: a When the initial concentration of NO is 1000 ppm, in case of switching on the plasma reactor, with oxygen and water added, the NO concentration maintains at ppm; and after connecting an alkali absorption device, the NO concentration slowly maintains at ppm. b When the initial concentration of NO is 000 ppm, before connecting alkaline solution absorption device, the NO concentration maintains at ppm; and after connecting an alkaline solution absorption device, the NO concentration maintains at ppm. c When the initial concentration of NO is 3000 ppm, before connecting alkaline solution absorption device, the NO concentration maintains at ppm; and after connecting alkaline solution absorption device, the NO concentration maintains at ppm. Finally, it can be concluded that connecting alkaline solution absorption device can greatly reduce the amount of NO. Adding oxygen, water vapor and alkali absorption have a positive effect on the NO removal efficiency. Conclusion Through the above experiments, the characteristics of this kind of joint denitration process were studied and the following conclusions can be obtained: (1 In case of no alkali absorption, while adding the water vapor into the system, the increase of O 3 concentration would transform NO into NO and promote the oxidation removal of NO. ( In case of connecting an alkali absorption device, in order to achieve the goal of removing NO, NO needs to be oxidized at first and then react with alkaline solution. When only oxygen is added, the experimental results show that adding the alkali absorption has a promoting effect on the plasma denitration; When the water vapor is added and Ca(OH solution absorption device is connected sequentially to the plasma reactor, the NO removal can be significantly improved with O added and the NO removal efficiency can be increased by 7%-17%. Therefore, non-thermal plasma oxidation-alkali absorption is a potential method for denitration. Acknowledgement The authors acknowledge the financial support of the Key Project of Natural Science Foundation (No. 014CFA11 and Excellent Middle & Young Creative Team of Hubei Province (No. T0107. References (1 Sun, K.; Zhou, C.; Xu, H. Electric power environmental protection 005, 1(1, 7-. ( Gao, F.; Yang, J. Environmental protection science 007, 33(3, (3 Yu, G.; Yu, Q.; Zeng, K-S. Journal of Environmental Sciences 005, 17(5, (4 Wang, Z.; Zhou, J.; Wen, Z. Journal of Zhejiang University: Engineering Science 008, 41(5, (5 Han, J.; Xu, M. Power engineering 003, 3(6, (6 Zhang, L.; Liu, T.; Dang, W. et al. Spectroscopy and spectral analysis 007, 7(4, (7 Lin, H.; Gao, X.; Luo, Z. et al. Fuel 004, 83(10, (8 Vinogradov, J.; Rivin, B.; Sher. E. Energy 007, 3(3, (9 Yu, Q.; Zeng, K.; Zhang, Z. et al. Journal of Chemical Engineering 008, 59(1, (10 Kuroki, T.; Takahashi, M.; Okubo, M. et al. Industry Applications, IEEE Transactions on 00, 38(5, (11 Yamamoto, T.; Rajanikanth, B.S.; Okubo, M. Industry Applications, IEEE Transactions on 003, 39(6, (1 Yu, G.; Yu, Q.; Jiang, Y-L. et al. Journal of environmental sciences 005, 17(3, (13 Yu, G.; Yu, Q.; Jiang, Y-L. et al. Journal of environmental sciences 005, 17(4, Received for review May13, 014. Revised manuscript received July 8, 014. Accepted July 9, J. Adv. Oxid. Technol. Vol. 18, No. 1, 015 Unauthenticated Download Date 10/0/17 :04 PM