- Email: [email protected]
Using dielectric barrier discharge and rotating packed bed reactor for NOx removal
Journal Pre-proofs Using Dielectric Barrier Discharge and Rotating Packed Bed Reactor for NOx Removal Chuang Liang, Yong Cai, Kuan Li, Yong Luo, Zhi Qian, Guang-Wen Chu, JianFeng Chen PII: DOI: Reference:
S1383-5866(19)32373-1 https://doi.org/10.1016/j.seppur.2019.116141 SEPPUR 116141
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
5 June 2019 21 August 2019 26 September 2019
Please cite this article as: C. Liang, Y. Cai, K. Li, Y. Luo, Z. Qian, G-W. Chu, J-F. Chen, Using Dielectric Barrier Discharge and Rotating Packed Bed Reactor for NOx Removal, Separation and Purification Technology (2019), doi: https://doi.org/10.1016/j.seppur.2019.116141
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Using Dielectric Barrier Discharge and Rotating Packed Bed Reactor for NOx Removal
Chuang Liang a, b, Yong Cai a, b, Kuan Li a, b, Yong Luo a, b*, Zhi Qian c*, Guang-Wen Chu a, b, Jian-Feng Chen a, b
a
Research Center of the Ministry of Education for High Gravity Engineering and
Technology and
b
State Key Laboratory of Organic-Inorganic Composites, Beijing
University of Chemical Technology, Beijing 100029, PR China c University
of Chinese Academy of Sciences, Beijing 100049, PR China
* Corresponding author. Tel: +86 10 64446466; Fax: +86 10 64434784. E-mail address: [email protected]; [email protected]
1
Abstract: Nitrogen oxides (NOx) are the main cause of photochemical smog and acid rain, which seriously endangers the ecological environment and human health. This phenomenon is particularly serious in developing countries like China. In this work, it is the first time to combine a dielectric barrier discharge (DBD) and a rotating packed bed (RPB) for NOx removal from the simulated flue gas. A DBD is employed to oxide part of NO to soluble NO2, which is conducive to the followed absorption process using the RPB with (NH2)2CO used as the absorbent and NaClO2 as the effective additive. NOx oxidation degree can reach more than 50% under the NOx concentration of 1000 ppm. Absorption results show that the NOx removal efficiency (η) dramatically increases with the increase of the NOx oxidation degree. Additionally, it increases with the enhancement of NaClO2 concentration and rotational speed of the RPB. As a result, NOx removal efficiency can be more than 85% at the optimal conditions. The obtained results forecast a bright prospect in the NOx removal from flue gas by combining DBD with RPB. Keywords: Oxidation, Rotating Packed Bed, NOx removal
2
1. Introduction Air pollution has drawn great public attentions in recent years all over the world, especially in developing countries [1-4]. Air pollutants are mainly generated during the combustion of fossil fuels by incinerators, automobiles, thermal power plants, etc. [5-7], which is especially common in developing countries like China. Among these pollutants, nitrogen oxides (NOx) are responsible for the formation of photochemical smog, which results in problems of many human organs, such as heart, lungs, and eyes. In addition, NOx participates in some key chain reactions and exhausts the ozone, leading to depletion of the ozone layer in the upper atmosphere [8]. Therefore, it is of great significance to remove NOx from gases. Over the past decades, corresponding effective treasures have been taken to reduce NOx emissions in the combustion and post-combustion [9, 10]. So far, many technologies, mainly including dry and wet methods, have been developed to reduce NOx emissions in China and significant improvements have been achieved [11, 12]. Dry methods mainly include selective catalytic reduction (SCR) [13] and selective non-catalytic reduction (SNCR) [14]. SCR can decrease NOx emissions effectively in the presence of catalyst and the heavy metals and sulfur compounds in the flue gas make the catalyst suffer from the risk of poisoning [15, 16, 17]. A high NOx removal efficiency was also achieved by SNCR without catalyst and the temperature were highly demanded during the process [18, 19]. As a result, wet methods have drawn much attention for its advantages of low cost and co-capture of other pollutants [20]. As we all known, NO2 is quite easily removed by wet scrubbing, while NO is much
3
more difficult to remove because it is sparingly soluble in aqueous solutions [21, 22]. Oxidation of NO and then absorption of NO2 from flue gas is considered as a valuable option in terms of high efficiency and low investment for the NOx removal [23]. A potential way to improve the NOx removal efficiency is to oxidize NO to soluble NO2 [24]. A large number of highly active oxidizing species (O·, ·OH, O3 etc.) can be generated in non-thermal plasma (NTP) [25]. Therefore, NTP could be used to deal with NOx, such as dielectric barrier discharge (DBD) [26-27], pulsed corona discharge [28, 29], direct current (DC) corona discharge [30, 31], etc. The feature of DBD is that a homogenous discharge with a low energy consumption is generated [32]. Furthermore, the existence of medium in the DBD can avoid spark formation inside the streamer channels [33]. Based on the above advantages, DBD was exploited to control the NOx oxidation process in this study. On the other hand, in the whole process of NOx removal, the NOx must be absorbed after oxidation. Rotating packed bed (RPB) is a novel gas-liquid reactor designed to generate high acceleration via centrifugal force, leading to the formation of thin liquid films and tiny liquid droplets. Consequently, RPB can enhance the mass transfer of the absorption process as well as reduce the initial investment cost [34, 35]. This study is the first time to combine DBD with RPB for the NOx removal. The objective of this work is to investigate the oxidation process in DBD and absorption process in RPB, respectively. In the oxidation process, we explored the effects of some operating parameters on the NOx oxidation degree, such as output frequency, peak voltage, residence time, oxygen content and the concentration of NOx. In the
4
absorption process, some parameters such as the NOx oxidation degree, urea concentration, NaClO2 concentration, rotational speed, gas-liquid ratio, and the concentration of NOx were also researched to optimize this process.
2. Experimental Section 2.1 Experimental materials Analytical reagent grade urea ((NH2)2CO) and sodium chlorite (NaClO2) were purchased from Beijing Reagent Factory of China. The gas of NO with a purity of 99.9% and N2 with a purity of 99.9% were purchased from Beijing Huatongjingke Gas Chemical Co., LTD., China. 2.2 Experimental setup DBD and RPB are two main reactors in this study. As shown in Figure 1, the coaxial DBD reactor consisted of stainless-steel tube as the inner electrode, quartz tube as the dielectric, and a metal mesh wrapped around the quartz tube as the outer electrode. The discharge gap was 3.5 mm and the length of the reactor was 150 mm. NTP was obtained in this DBD reactor with sinusoidal high voltage power input generated by a plasma generator in the range of 0-30 kV voltage and 5-25 kHz frequency (CTP-2000K, Nanjing Suman Electronics Co., Ltd, China). The RPB reactor used in this work mainly consisted of a packed rotator, a fixed casing, a liquid inlet, a liquid outlet, a gas inlet, and a gas outlet. The simulated flue gas continuously flowed through the system and was measured by flue gas analyzer (ECOM-EN2, RBR Analytical Instrument Institution, Germany) before and after the
5
absorption system to calculate the NOx removal efficiency. Figure 2 shows the experimental setup is divided into three parts, including gas-feeding system, oxidation system, and absorption system. Gas-feeding system mainly includes NO cylinder, N2 cylinder, air blower, and mixer. DBD reactor, function generator (peak voltage: 0-30 kV, frequency: 5-25 kHz), HV transformer, HV probe, and oscillography are belonging to the oxidation system. Absorption system is composed of RPB reactor, pump, and absorbent tank. 2.3 Experimental procedure The initial concentrations of NO, NO2, NOx and O2 could be changed by adjusting the mass flow meters. The mixed gas flow rate ranged from 0.5 to 3 m3·h-1. The concentrations of the NOx in gas mixture were set in the range of 400-1600 ppm (ppm was a volume-based unit, equivalent to ppmv). The high concentration was used to simulate the glass melting furnaces and the nitric acid plants, while the low concentration was used to simulate the cement kilns [36, 37, 38]. After mixed in the gas-feeding system, the simulated flue gas was introduced into the oxidation system. The main chemical processes in DBD plasmas are the reactions with atoms (O, N), the activated atoms and molecules. When the discharge energy is high enough, the molecules of N2 and O2 are activated to a higher energy state, even ionization after colliding with the high energy state electron, thus promoting a series of chemical reaction processes. Since the dissociation energy of O2 (5.2 eV per molecule) is lower than that of N2 (9.8 eV per molecule), it is easier for electrons to interact with O2 and produce strong oxygen species (·O and O3) at the
6
same discharge condition. These oxidizing species promote the oxidation reaction of NO [39]. According to previous studies [40-42], the ozone was believed to dominantly affect the oxidation of NO. Since N2O5 is easily decomposed to NO2, NO or N2 in discharge plasma [43]. The absorption experiment was performed as follows. The gas mixture stream from the outlet of DBD was continuously introduced into the RPB outer cavity and flowed inward by a pressure-driving force. Meanwhile, the absorbent entered the rotor by a liquid distributor, sprayed from the inner bed and then moved outward under the centrifugal force. The liquid distributor used in this study had two rows of holes with 1 mm diameter and four each row. Consequently, the gas and liquid were contacted counter-currently inside the RPB. Urea ((NH2)2CO) is a slightly alkaline reagent as well as a strong reducing reagent. Compared with other absorption solutions, urea is a cheap and non-toxic reactant which can be easily obtained. Reaction products of denitration using urea solution are carbon dioxide and nitrogen which can be directly released into the atmosphere [44]. As a consequence, urea was used as the basic absorbent. Meanwhile, NaClO2 was considered as an effective absorbent [45-48], but its industrial application is difficult because of the high price. In order to reduce the cost and improve removal efficiency of NOx, a complex absorbent that was made up of urea and NaClO2 was used to removal NOx from the simulated flue gas in this work. 2.4 Data analysis The NOx oxidation degree (α) and NOx removal efficiency (η) are defined as
7
follows: C2 (NO 2 ) 100% C2 (NO x )
(1)
C1 (NO x ) C3 (NO x ) 100% C1 (NO x )
(2)
Where C(NO), C(NO2) and C(NOx) are the concentrations of NO, NO2, and NOx, respectively, and the total concentration of NOx is the sum of NO and NO2. The subscript 1, 2, and 3 denote DBD inlet, the middle position of DBD outlet and RPB inlet, and RPB outlet, respectively.
3. Results and discussion 3.1 Oxidation process by DBD 3.1.1 Influence of peak voltage on the NOx oxidation degree Output frequency (f) and peak voltage (Vpp) applied on DBD reactor have a prominent influence on electric discharge [49]. Therefore, the influence of output frequency on peak voltage was firstly investigated. Figure 3 shows that Vpp of the DBD reactor was maximized at 8.5 kHz when input voltage was 10, 20, 30 and 40 V, respectively. The possible reason is that the total impedance of the plasma generator was minimum when the operating frequency approached the resonance point (8.5 kHz), and the output power of the plasma generator reached the maximum. Consequently, the output frequency was fixed at 8.5 kHz in the following investigation. As mentioned above, besides output frequency, peak voltage also has prominent
8
influence on electric discharges. Figure 4 shows that, at different gas flow rates, the NOx oxidation degree increased slightly and then increased sharply. NOx oxidation degree reached more than 50% when the peak voltage was 28 kV. With the peak voltage further increased, the oxidation degree began to decrease. The reasons may be as follows: i) when peak voltage was relatively low, the discharge energy could hardly activate N2 and O2 to higher energy state; ii) when peak voltage exceeded 25 kV, the discharge energy might be higher than the dissociation energy of O2 (5.2 eV per molecule), but still lower than that of N2 (9.8 eV per molecule). As a result, reactions took place in this process mainly promoted the NO oxidation (R2, R3 and R9 in Table 1); iii) besides O2, N2 could be also dissociated when peak voltage was higher than 28 kV. Consequently, the NO2 reductions (R8 and R10 in Table 1) were simultaneous with the oxidation process of NO. Therefore, the peak voltage was fixed at 28 kV in the following study. 3.1.2 Influence of average residence time on the NOx oxidation degree The average residence time could be represented as follows:
tr
VDBD G
(3)
where VDBD is the volume of DBD and G represents gas flow rate. As shown in Figure 5, it can be concluded that the average residence time had a minimal influence on the NOx oxidation degree within the range of the investigation. Compared with the time for high-energy electrons to collide with gas molecules to produce free radicals and the time required by a series of reactions, the residence time was long enough [50]. Hence, the oxidation degree was almost constant at about 50% in the range of average 9
residence time from 0.04 s to 0.10 s. Shorter average residence time represented larger gas flow rate, which benefited the further industrial application. 3.1.3 Influence of oxygen content on the NOx oxidation degree Since oxygen content of the actual flue gas is different due to various sources, it is essential to explore the influence of oxygen content on the NOx oxidation degree. As displayed in Figure 6, the oxidation degree increased from 23% to 52% when the oxygen content increased from 4% to 21%. Under the given discharge condition, the energy input to the DBD reactor was constant. When O2 content increased, more O2 could be activated to generate more O3 and O radicals, providing a greater oxidizing source for the reaction system, allowing the oxidation atmosphere (formed by O3 and O radicals) to be maintained for a longer time [43]. The higher the oxygen content existed in the flue gas, the higher the NOx oxidation degree could be achieved. 3.1.4 Influence of NOx concentration on the NOx oxidation degree As shown in Figure 7, the oxidation degree decreased from 70% to 42% when the NOx concentration increased from 400 to 1600 ppm. The reason is similar as the above explanation. Under the given condition, the energy input to the DBD reactor was constant and the number of active species (mainly O radicals and O3) were invariable. As a consequence, the higher the NOx concentration of flue gas was, the lower the NOx oxidation degree was. According to previous researches [51-52], NOx oxidation process in DBD could be described as follows: Oxygen was converted into active oxygen species after colliding with high-energy electrons generated by discharge. Subsequently, active
10
oxygen species with strong oxidizing property reacted with part of NO to generate NO2. 3.2 Absorption process by RPB 3.2.1 Influence of the NOx oxidation degree on the NOx removal efficiency It is successful to regulate and control the NOx oxidation degree in the oxidation process. Therefore, in the absorption process, we firstly explored the relation between the NOx oxidation degree and the NOx removal efficiency in order to determine the operating parameter of oxidation process. It is fairly obvious that the NOx removal efficiency increases with the increase of the NOx oxidation degree in Figure 8. When the oxidation degree was about 50%, the NOx removal efficiency could be reached more than 62% at different gas flow rates (1.5 m3·h-1, 2 m3·h-1, 2.5 m3·h-1). 3.2.2 Influence of absorbent concentration on the NOx removal efficiency When the concentration of NOx retained constant (1000 ppm), the influence of urea concentration on the NOx removal efficiency was experimentally investigated, and the results are shown in Figures 9 and 10. It could be concluded from Figure 9 that different concentrations of urea solution has little influence on the NOx removal efficiency at the beginning of the experiments (~10 min). As time went on, the NOx removal efficiency would decline faster at lower concentration of urea solution (Figure 10). This means that the high concentration of urea solution could keep the NOx removal efficiency stable (~28%) for at least one hour, which was beneficial to the cycled utilization of urea solution. When urea solution was used as single absorbent, the NOx removal efficiency
11
could reach about 28%. As mentioned above, NaClO2 is considered as an extremely effective additive to improve the removal efficiency. Experimental results shown in Figure 11 indicate that adding NaClO2 into urea would greatly improve the NOx removal efficiency from ~28% to ~56%. Meanwhile, when the concentration of NaClO2 increased from 0.02 to 0.08 mol·L-1, the NOx removal efficiency increased rapidly and then became relatively stable. This phenomenon may be interpreted as follows: although the NOx oxidation degree was enhanced to ~50% by DBD, NO still took quite a large percentage in NOx. NaClO2 could not only oxide NO to NO2, but also promoted the absorption process, especially when the NaClO2 concentration was below 0.08 mol·L-1. Gas-liquid contact time might become the main limitation when the NaClO2 concentration exceeded 0.08 mol·L-1. Based on previous researches [53, 54], the absorption process in RPB could be described as follows: urea reacted with NO2 to generate carbon dioxide and nitrogen. Meanwhile, NaClO2 reacted with NO and NO2 to generate sodium nitrate and sodium chloride. The chemical reaction between NOx and urea/NaClO2 could promote NOx absorption in the liquid. Hence, NOx removal was achieved under the combination of DBD and RPB. 3.2.3 Influence of rotational speed on the NOx removal efficiency Figure 12 shows that, at different NaClO2 concentrations, the NOx removal efficiency increases sharply with the increase of the rotational speed of RPB. When the NaClO2 concentration was 0.08 mol·L-1, the NOx removal efficiency could reach more than 65% at 1600 r/min, and the NOx removal efficiency tended to level off
12
even increasing rotational speed continuously. The possible reason is that increasing rotational speed contributes to more drastic dispersion effects of the packing in RPB on the liquid, which results in the increase of gas-liquid contact area and liquid surface renewal rate. Both of them were conducive to the NOx absorption. When rotational speed exceeded 1600 r/min, the liquid residence time would be shorter than the necessary reaction time required to absorb NOx. The direct consequence was that the liquid phase had not enough time to fully contact with the gas phase. Hence, the upward trend of NOx removal efficiency began to level off. 3.2.4 Influence of gas-liquid ratio on the NOx removal efficiency In our study, we fixed the gas flow rate and changed the liquid flow rate to investigate the influence of gas-liquid ratio on the NOx removal efficiency. It can be seen from Figure 13 that the NOx removal efficiency gradually decreases with the increase of the gas-liquid ratio. When the gas-liquid ratio was 50, the NOx efficiency could reach more than 68%. The reason is that higher gas-liquid ratio results in the decreasing liquid holdup and effective gas−liquid contact area [50]. When the mass transfer driving force is constant, the mass transfer rate is limited. The total amount of NOx dissolved into the absorbent reduces, which results in the decrease of the NOx removal efficiency. Therefore, lower gas-liquid ratio is beneficial to increase the NOx removal efficiency. When the gas flow rate is constant, lower gas-liquid ratio means larger amount of liquid that will increase operating costs. A suitable gas-liquid ratio should be chosen based on the actual operating conditions. 3.2.5 Influence of NOx concentration on the NOx removal efficiency
13
As mentioned earlier, NOx oxidation degree exerted a significant effect on the NOx removal efficiency. The NOx removal efficiency decreased from 84% to 62% when the NOx concentration increased from 400 to 1600 ppm. At the given oxidation condition, oxidation degrees of different NOx concentrations are distinct. It is evident that NOx removal efficiency decreases with the increase of NOx concentration from Figure 14. The reason could be that when the denitration capacity of the solution was constant, increasing NOx concentration caused a reduction in the amount of absorbed NOx per volumetric gas [55].
4. Conclusions This work studied the NOx removal for the first combination between DBD and RPB. The NOx oxidation degree was regulated by DBD, and the NOx removal was achieved by RPB. In the oxidation process, we mainly investigated the influence of several parameters, such as peak voltage, oxygen content, residence time and NOx concentration on the oxidation degree. When the NOx concentration was 1000 ppm, NOx oxidation degree could reach more than 50%. NOx removal efficiency was evaluated in terms of oxidation degree, urea concentration, NaClO2 concentration, rotational speed, gas-liquid ratio, and NOx concentration in the absorption process. Results showed that the NOx removal efficiency could reach more than 62% at different gas flow rates when the oxidation degree was about 50%. High concentration of urea solution could keep the NOx removal efficiency stable for a long time, which is beneficial to the circulation of absorbent. For the purpose of further
14
improving the removal efficiency of NOx, NaClO2 was added as an effective absorbent. The results showed that the NOx removal efficiency improved about 28%. Under the optimal conditions, NOx removal efficiency could be more than 85%. The obtained results forecast a bright prospect in the removal of NOx from flue gases by the combination between DBD and RPB.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 21725601, 21676009, and 21436001) and University of Chinese Academy of Sciences (No. Y8540XX222).
15
Nomenclatures
CNaClO2 = NaClO2 concentration (mol·L-1) CNOx = NOx concentration (ppm) CO2 = O2 content (%) f = output frequency of DBD (kHz) G = gas flow rate (m3·h-1, L·min-1) G/L = gas-liquid ratio k298K = reaction rate constant at 298 K (L·mol-1·s-1) L = liquid flow rate (L·h-1) N = rotational speed of RPB (r/min) t = operating time (min) tr = residence time in DBD (s) Vpp = peak voltage of DBD (kV) VDBD= volume of DBD, 27.214×10-3 (L) wurea = mass fraction of urea in the absorbent (wt%)
Greek Symbols α = NOx oxidation degree (%) η = NOx removal efficiency (%)
16
References [1] C. Wang, A. Engels, Z. Wang, Overview of research on China's transition to low-carbon development: the role of cities, technologies, industries and the energy system, Renew. Sust. Energ. Rev. 81 (2018) 1350-1364. [2] Y. Osaka, K. Iwai, T. Tsujiguchi, A. Kodama, X. Li, H. Y. Huang, Basic study on exhaust gas purification by utilizing plasma assisted MnO2 filter for zero-emission diesel, Sep. Purif. Technol. 215 (2019) 108-114. [3] D. Matthews, S. Solomon, Reversing excess atmospheric CO2-response, Science 340 (2013) 1523-1523. [4] Z. F. Lu, D. G. Streets, Increase in NOx emissions from Indian thermal power plants during 1996-2010: unit-based inventories and multisatellite observations, Environ. Sci. Technol. 46 (2012) 7463-7470. [5] J. H. Ye, J. Shang, Q. Li, W. W. Xu, J. Liu, X. Feng, T. Zhu, The use of vacuum ultraviolet irradiation to oxidize SO2 and NOx for simultaneous desulfurization and denitrification, J. Hazard. Mater. 271 (2014) 89-97. [6] W. Y. Sun, S. L. Ding, S. S. Zeng, S. J. Su, W. J. Jiang, Simultaneous absorption of NOx and SO2 from flue gas with pyrolusite slurry combined with gas-phase oxidation of NO using ozone, J. Hazard. Mater. 192 (2011) 124-130. [7] Y. X. Liu, J. Zhang, C. D. Sheng, Y. C. Zhang, L. Zhao, Simultaneous removal of NO and SO2 from coal-fired flue gas by UV/H2O2 advanced oxidation process, Chem. Eng. J. 162 (2010) 1006-1011. [8] Z. Wei, X. Li, L. Xu, C. Tan, Optimization of operating parameters for low NOx
17
emission in high-temperature air combustion, Energ. Fuel. 26 (2012) 2821-2829. [9] T. Saikaew, P. Supudommak, L. Mekasut, P. Piumsomboon, P. Kuchonthara, Emission of NOx and N2O from co-combustion of coal and biomasses in CFB combustor, Int. J. Greenh. Gas Con. 10 (2012) 26-32. [10] S. Mahmoudi, J.Baeyens, J. Seville, NOx formation and selective non-catalytic reduction (SNCR) in a fluidized bed combustor of biomass, Biomass Bioenerg. 34 (2010) 1393-1409. [11] Z. Ma, J. Deng, Z. Li, Q. Li, P. Zhao, L. G. Wang, Y. Z. Sun, H. X. Zheng, L. Pan, S. Zhao, J. K. Jiang, S. X. Wang, L. Duan, Characteristics of NOx emission from Chinese coal-fired power plants equipped with new technologies, Atmos. Environ. 131 (2016) 164-170. [12] H. Z. Tian, P. P. Qiu, K. Cheng, J. J. Gao, L. Lu, K. Y. Liu, X. G. Liu, Current status and future trends of SO2 and NOx pollution during the 12th FYP period in Guiyang city of China, Atmos. Environ. 69 (2013) 273-280. [13] M. Shelef, Selective catalytic reduction of NOx with N-free reductants, Chem. Rev. 95 (1995) 209-225. [14] D. I. Sandberg, M. Lamberti-Pasculli, J. M. Drake, T. H. Christensen, Life cycle assessment of selective non-catalytic reduction (SNCR) of nitrous oxides in a full-scale municipal solid waste incinerator, Waste Manage. 31 (2011) 1184-1193. [15] Z. Z. Ma, J. G. Deng, Z. Li, Q. Li, P. Zhao, L. G. Wang, Y. Z. Sun, H. X. Zheng, L. Pan, S. Zhao, J. K. Jiang, S. X. Wang, L. Duan, Characteristics of NOx emission from Chinese coal-fired power plants equipped with new technologies, Atmos.
18
Environ. 131 (2016) 164-170. [16] H. Rojel, A. Jensen, P. Glarborg, K. D. Johansen, Mixing effects in the selective noncatalytic reduction of NO, Ind. Eng. Chem. Res. 39 (2000) 3221-3232. [17] M. H. Zhang, B. H. Huang, H. X. Jiang, Y. F. Chen, Research progress in the SO2 resistance of the catalysts for selective catalytic reduction of NOx, Chinese J. Chem. Eng. 25 (2017) 1695-1705. [18] S. S. Daood, M. T. Javed, B. M. Gibbs, W. Nimmo, NOx control in coal combustion by combining biomass co-firing, oxygen enrichment and SNCR, Fuel 105 (2013) 283-292. [19] J. Baleta, H. Mikulcic, M. Vujanovic, Z. Petranovic, N. Duic, Numerical simulation of urea based selective non-catalytic reduction deNOx process for industrial applications, Energ. Convers. Manage. 125 (2016) 59-69. [20] R. T. Guo, J. K. Hao, W. G. Pan, Y. L. Yu, Liquid phase oxidation and absorption of NO from flue gas: a review, Sep. Sci. Technol. 50 (2015) 310-321. [21] Y. G. Adewuyi, S. O. Owusu, Aqueous absorption and oxidation of nitric oxide with oxone for the treatment of tail gases: process feasibility, stoichiometry, reaction pathways, and absorption rate, Ind. Eng. Chem. Res. 42 (2003) 4084-4100. [22] M. Petrissans, A. Petrissans, A. Zoulalian, Experimental study and modelling of mass transfer during simultaneous absorption of SO2 and NO2 with chemical reaction, Chem. Eng. Process. 44 (2005) 1075-1081. [23] R. Ochoagonzalez, P. Cordoba, M. Diazsomoano, O. Font, M. A. Lopezanton, C. Leiva, M. R. Martineztarazona, X. Querol, C. F. Pereira, A. Tomas, P. Gomez, P.
19
Mesado, Differential partitioning and speciation of Hg in wet FGD facilities of two Spanish PCC power plants, Chemosphere 85 (2011) 565-570. [24] Y. Cai, Y. Luo, G. W. Chu, W. Wu, X. Yu, B. C. Sun, J. F. Chen, NOx removal in a rotating packed bed: oxidation and enhanced absorption process optimization, Sep. Purif. Technol. (2019) 115682. [25] K. Shang, X. Wang, J. Li, H. Wang, N. Lu, N. Jiang, Y. Wu, Synergetic degradation of acid orange 7 (AO7) dye by DBD plasma and persulfate, Chem. Eng. J. 311 (2017) 378-384. [26] M. B. Chang, S. C. Yang. NO/NOx removal with C2H2 as additive via dielectric barrier discharges, AIChE J. 47 (2001) 1226-1233. [27] B. M. Obradovic, G. B. Sretenovic, M. M. Kuraica, A dual-use of DBD plasma for simultaneous NOx and SO2 removal from coal-combustion flue gas, J. Hazard. Mater. 185 (2011) 1280-1286. [28] H. H. Kim, K. Takashima, S. Katsura, A. Mizuno, Low-temperature NOx reduction processes using combined systems of pulsed corona discharge and catalysts, J. Phys. D Appl. Phys. 34 (2001) 604. [29] L. Huang, D. Yongxia, Removal of SO2 and NOx by pulsed corona combined with in situ Ca(OH)2 absorption, Chinese J. Chem. Eng. 19 (2011) 518-522. [30] T. Fujii, Y. Aoki, N. Yoshioka, M. Rea, Removal of NOx by DC corona reactor with water, J. Electrostat. 51 (2001) 8-14. [31] J. S. Chang, K. Urashima, Y. X. Tong, W. P. Liu, H. Y. Wei, F. M. Yang, X. J. Liu, Simultaneous removal of NOx and SO2 from coal boiler flue gases by DC corona
20
discharge ammonia radical shower systems: pilot plant tests, J. Electrostat. 57 (2003) 313-323. [32] C. Q. Wang, G. X. Zhang, X. X. Wang, Comparisons of discharge characteristics of a dielectric barrier discharge with different electrode structures, Vacuum 86 (2012) 960-964. [33] A. Fridman, A. Chirokov, A. Gutsol, Non-thermal atmospheric pressure discharges, J. Phys. D Appl. Phys. 38 (2005) 1-24. [34] K. Guo, F. Guo, Y. Feng, J. F. Chen, C. Zheng, N. C. Gardner, Synchronous visual and RTD study on liquid flow in rotating packed-bed contactor, Chem. Eng. Sci. 55 (2000) 1699-1706. [35] K. Yang, G. W. Chu, H. K. Zou, B. C. Sun, L. Shao, J. F. Chen, Determination of the effective interfacial area in rotating packed bed, Chem. Eng. J. 168 (2011) 1377-1382. [36] M. Flamme, Low NOx combustion technologies for high temperature applications, Energ. Convers. Manage. 42 (2001) 1919-1935. [37] K. Skalska, J. S. Miller, M. Wilk, L. Stanisław, Nitrogen oxides ozonation as a method for NOx emission abatement, Ozone-Sci. Eng. 34 (2012) 252-258. [38] A. T. McQueen, S. J. Bortz, M. S. Hatch, R. L. Leonard, Cement kiln NOx control, IEEE T. IND. APPL. 31 (1995) 36-44. [39] E. Stamate, C. Irimiea, M. Salewski, Investigation of NOx reduction by low temperature oxidation using ozone produced by dielectric barrier discharge, Jpn. J. Appl. Phys. 52 (2013) 129-142.
21
[40] Y. S. Mok, V. Ravi, H. Kang, B. S. Rajanikanth, Abatement of nitrogen oxides in a catalytic reactor enhanced by nonthermal plasma discharge, IEEE T. Plasma Sci. 31 (2003) 157-165. [41] J. Li, W. H. Goh, X. Yang, R. T. Yang, Non-thermal plasma-assisted catalytic NOx storage over Pt/Ba/Al2O3 at low temperatures, Appl. Catal. B-Environ. 90 (2009) 360-367. [42] B. Eliasson, U. Kogelschatz, Modeling and applications of silent discharge plasmas, IEEE T. Plasma Sci. 19 (1991) 309-323. [43] Y. J. Zhang, X. L. Tang, H. H. Yi, Q. J. Yu, J. G. Wang, F. Y. Gao, Y. M. Gao, D. Z. Li, Y. M. Cao, The byproduct generation analysis of the NOx conversion process in dielectric barrier discharge plasma, RSC Adv. 6 (2016) 63946-63953. [44] P. Fang, C. P. Cen, Z. X. Tang, P. Y. Zhong, D. S. Chen, Z. H. Chen, Simultaneous removal of SO2 and NOx by wet scrubbing using urea solution, Chem. Eng. J. 168 (2011) 52-59. [45] T. W. Chien, H. Chu, Removal of SO2 and NO from flue gas by wet scrubbing using an aqueous NaClO2 solution, J. Hazard. Mater. 80 (2000) 43-57. [46] E. Sada, H. Kumazawa, I. Kudo, T. Kondo, Absorption of NO in aqueous mixed solutions of NaClO2 and NaOH, Chem. Eng. Sci. 33 (1978) 315-318. [47] J. C. Wei, Y. B. Luo, P. Yu, B. Cai, H. Z. Tan, Removal of NO from flue gas by wet scrubbing with NaClO2/(NH2)2CO solutions, J. Ind. Eng. Chem. 15 (2009) 16-22. [48] B. R. Deshwal, S. H. Lee, J. H. Jung, B. H. Shon, H. K. Lee, Study on the removal of NOx from simulated flue gas using acidic NaClO2 solution, J. Environ.
22
Sci-China. 20 (2008) 33-38. [49] X. L. Tang, F. Y. Gao, J. G. Wang, H. H. Yi, S. Z. Zhao, B. W. Zhang, Y. R. Zuo, Z. X. Wang, Comparative study between single- and double-dielectric barrier discharge reactor for nitric oxide removal, Ind. Eng. Chem. Res. 53 (2014) 6197-6203. [50] R. Dorai, J. K. Kushner, Consequences of propene and propane on plasma remediation of NOx, J. Appl. Phys. 88 (2000) 3739-3747. [51] M. M. Santillan, A. Vincent, E. Santirso, J. Amouroux, Design of a DBD wire-cylinder reactor for NOx emission control: experimental and modelling approach, J. Clean. Prod. 16 (2008) 198-207. [52] B. M. Obradovic, G. B. Sretenovic, M. M. Kuraica, A dual-use of DBD plasma for simultaneous NOx and SO2 removal from coal-combustion flue gas, J Hazard. Mater. 185 (2011) 1280-1286. [53] A. Pant, S. J. Schmieg. Kinetic model of NOx SCR using urea on commercial Cu−zeolite catalyst, Ind. Eng. Chem. Res. 50 (2011) 5490-5498. [54] A. Pourmohammadbagher, E. Jamshidi, H. Ale-Ebrahim, S. Dabir, Study on simultaneous removal of NOx and SO2 with NaClO2 in a novel swirl wet system, Ind. Eng. Chem. Res. 50 (2011) 8278-8284. [55] B. C. Sun, M. P. Sheng, W. L. Gao, L. L. Zhang, M. Arowo, Y. Liang, L. Shao, G. W. Chu, H. K. Zou, J. F. Chen, Absorption of nitrogen oxides into sodium hydroxide solution in a rotating packed bed with preoxidation by ozone, Energ. Fuel. 31 (2017) 11019-11025.
23
[56] P. H.Wine, J. M. Nicovich, R. J. Thompson, A. R. Ravlshankara, Kinetics of atomic oxygen (3PJ) reactions with hydrogen peroxide and ozone, J. Phys. Chem. 87 (1983) 3948-3954. [57] I. Wangberg, E. Ljungstrom, B. Olsson, J. Davidssont, Temperature dependence of the reaction NO3+NO2 → NO+NO2+O2 in the range from 296 to 332 K, J. Phys. Chem. 96 (1992) 7640-7645. [58] K. Skalska, J. S. Miller, S. Ledakowicz, Trends in NOx abatement: a review, Sci. Total Environ. 408 (2010) 3976-3989. [59] M. Schieferstein, K. Kohse‐Höinghaus, F. Stuhl, Temperature dependence of the rate constants of the reaction O+ NO+ M→ NO2+ M (M= He, NO, N2, CH4), Ber. Bunsenges. phys. Chem. 87 (1983) 361-366. [60] P. O. Wennberg, J. G. Anderson, D. K. Weisenstein, Kinetics of reactions of ground state nitrogen atoms (4S3/2) with NO and NO2, J. Geophys. Res-Atmos. 99 (1994) 18839-18846. [61] Y. S. Mok, I. S. Nam, Reduction of nitrogen oxides by ozonization-catalysis hybrid process, Korean J. Chem. Eng. 21 (2004) 976-982.
24
Figure Captions Figure 1. Schematic diagram of DBD reactor. Figure 2. Experimental setup. Figure 3. Influence of output frequency on the peak voltage. Figure 4. Influence of peak voltage on the NOx oxidation degree. Figure 5. Influence of residence time on the NOx oxidation degree. Figure 6. Influence of oxygen content on the NOx oxidation degree. Figure 7. Influence of NOx concentration on the NOx oxidation degree. Figure 8. Influence of NOx oxidation degree on the NOx removal efficiency. Figure 9. Influence of urea concentration on the NOx removal efficiency. Figure 10. Influence of reaction time on the NOx removal efficiency. Figure 11. Influence of NaClO2 concentration on the NOx removal efficiency. Figure 12. Influence of rotational speed on the NOx removal efficiency. Figure 13. Influence of gas-liquid ratio on the NOx removal efficiency. Figure 14. Influence of NOx concentration on the NOx removal efficiency.
Table Legends Table
1.
Main
reactions
which
happen
in
DBD.
25
Figure 1. Schematic diagram of DBD reactor.
26
Sampling point-C1
Tail gas
HV probe
HV transformer
DBD reactor
Gas Mixer
R0 Mass flow meter Valve
AC
Function generator
RPB reactor
Sampling point-C3
Pump
Cm Oscillograph
Air NO
N2
Gas distribution system
Sampling point-C2
Waste Tank
Oxidation system by DBD
Absorbent Tank
Absorption system by RPB
Figure 2. Experimental setup.
27
30 10 V 20 V 30 V 40 V
25
Vpp (kV)
20 15 10 5 0
7
8
9
10
11
f (kHz)
Figure 3. Influence of output frequency on the peak voltage.
28
55 50
f=8.5 kHz, cNOx=1000 ppm 3
45
α (%)
-1
G=2.5 m h 3 -1 G=2 m h 3 -1 G=1.5 m h
40 35 30 25 20
22
24
26
28
30
Vpp (kV)
Figure 4. Influence of peak voltage on the NOx oxidation degree.
29
65 f=8.5 kHz,Vpp=28 kV cNOx=800 ppm
60
cNOx=1000 ppm
α (%)
55 50 45 40
0.04
0.05
0.06
0.07
0.08
0.09
0.10
tr (s)
Figure 5. Influence of residence time on the NOx oxidation degree.
30
55 f=8.5 kHz , Vpp=28 kV 3 -1 G=2m h , cNOx=1000 ppm
50 45
α (%)
40 35 30 25 20
4
8
12
16
20
cO2(%)
Figure 6. Influence of oxygen content on the NOx oxidation degree.
31
75 f=8.5 kHz , Vpp=28 kV 3 -1 G=2m h
70 65
α (%)
60 55 50 45 40 400
600
800
1000
1200
1400
1600
cNOx (ppm)
Figure 7. Influence of NOx concentration on the NOx oxidation degree.
32
70
-1
L=40 Lh , cNOx=1000 ppm, N=1600 rpm -1
curea=10 wt%, cNaClO =0.08 molL 2
3
(%)
-1
G=2.5 m h 3 -1 G=2 m h 3 -1 G=1.5 m h
60
50
40 25
30
35
40
45
50
55
a (%)
Figure 8. Influence of NOx oxidation degree on the NOx removal efficiency.
33
40 N=1600 rpm, cNOx=1000 ppm 3
-1
-1
G=2 m h , L=40 Lh , t=10 mins
(%)
35
30
25
20
2
4
6
8
10
w urea (wt%)
Figure 9. Influence of urea concentration on the NOx removal efficiency.
34
30 28 26
(%)
24 wurea
22
10 wt% 8 wt% 6 wt% 4 wt% 2 wt%
20 18 16 10
20
30
40
50
60
t (min)
Figure 10. Influence of reaction time on the NOx removal efficiency.
35
70
(%)
60
50
40
3
-1
-1
wurea=10 wt%, G=2 m h , L=40 Lh N=1600 rpm, cNOx=1000 ppm
30 0.00
0.02
0.04
0.06
0.08
0.10
0.12
-1
cNaClO (mol L ) 2
Figure 11. Influence of NaClO2 concentration on the NOx removal efficiency.
36
70 65
(%)
60 3
55
-1
-1
wurea=10 wt%, G=2 m h , L=40 L h -1
cNaClO =0.08 mol L 2
50
-1
cNaClO =0.06 mol L 2
-1
45
cNaClO =0.02 mol L 2
800
1200
1600
2000
2400
N (rpm)
Figure 12. Influence of rotational speed on the NOx removal efficiency.
37
70 68
(%)
66 64 -1
62 60
wurea=10 wt%, cNaClO =0.08 molL 2
3
-1
N=1600 rpm, G=2 m h , cNOx=1000 ppm 40
50
60
70
80
90
100
G/L
Figure 13. Influence of gas-liquid ratio on the NOx removal efficiency.
38
90 85
3
-1
2
80
(%)
-1
G=2 m h , L=40 L h , N=1600 rpm -1 wurea=10 wt%, cNaClO =0.08 mol L
75 70 65 60
400
600
800
1000
1200
1400
1600
cNOx (ppm)
Figure 14. Influence of NOx concentration on the NOx removal efficiency.
39
Table 1. Main reactions which happen in DBD. Number R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15
Reactions O3+O→2O2 NO+NO3→2NO2 O2+NO3→N2O5 O3→O2+O O3+N→O2+NO O+O+M→O2+M N+O2→NO+O NO+N2O→NO2+N2 NO+NO→O2+N2 NO+O+M→NO2+M N+NO2→N2O+O NO+N→N2+O NO+O3→NO2+O2 NO2+O3→NO3+O2N N2O5→NO2+NO3
k298K(L·mol-1·s-1) 8.0×10-15 1.566×1010 8.431×108 2.023×10-2 6.00×104 3.872×108 4.742×104 5.265×10-21 1.388×10-36 9.091×10-31 1.214×10-14 3.764×10-14 2.576×109 8.36×107 1.159×1017
Refs. Wine et al. [56]. Wangberg et al. [57]. Wangberg et al. [57]. Skalska et al. [58]. Skalska et al. [58]. Skalska et al. [58]. Skalska et al. [58]. Skalska et al. [58]. Skalska et al. [58]. Schieferstein et al. [59]. Wennberg et al. [60]. Wennberg et al. [60]. Mok et al. [61]. Mok et al. [61]. Mok et al. [61].
40
Highlights
1. A DBD reactor and an RPB reactor were firstly combined for the NOx removal. 2. The DBD reactor was employed to improve the NOx oxidation degree. 3. The NOx removal efficiency was enhanced by the RPB. 4. Optimizations for the NOx removal efficiency were presented systematically.
41
S1383-5866(19)32373-1 https://doi.org/10.1016/j.seppur.2019.116141 SEPPUR 116141
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
5 June 2019 21 August 2019 26 September 2019
Please cite this article as: C. Liang, Y. Cai, K. Li, Y. Luo, Z. Qian, G-W. Chu, J-F. Chen, Using Dielectric Barrier Discharge and Rotating Packed Bed Reactor for NOx Removal, Separation and Purification Technology (2019), doi: https://doi.org/10.1016/j.seppur.2019.116141
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Using Dielectric Barrier Discharge and Rotating Packed Bed Reactor for NOx Removal
Chuang Liang a, b, Yong Cai a, b, Kuan Li a, b, Yong Luo a, b*, Zhi Qian c*, Guang-Wen Chu a, b, Jian-Feng Chen a, b
a
Research Center of the Ministry of Education for High Gravity Engineering and
Technology and
b
State Key Laboratory of Organic-Inorganic Composites, Beijing
University of Chemical Technology, Beijing 100029, PR China c University
of Chinese Academy of Sciences, Beijing 100049, PR China
* Corresponding author. Tel: +86 10 64446466; Fax: +86 10 64434784. E-mail address: [email protected]; [email protected]
1
Abstract: Nitrogen oxides (NOx) are the main cause of photochemical smog and acid rain, which seriously endangers the ecological environment and human health. This phenomenon is particularly serious in developing countries like China. In this work, it is the first time to combine a dielectric barrier discharge (DBD) and a rotating packed bed (RPB) for NOx removal from the simulated flue gas. A DBD is employed to oxide part of NO to soluble NO2, which is conducive to the followed absorption process using the RPB with (NH2)2CO used as the absorbent and NaClO2 as the effective additive. NOx oxidation degree can reach more than 50% under the NOx concentration of 1000 ppm. Absorption results show that the NOx removal efficiency (η) dramatically increases with the increase of the NOx oxidation degree. Additionally, it increases with the enhancement of NaClO2 concentration and rotational speed of the RPB. As a result, NOx removal efficiency can be more than 85% at the optimal conditions. The obtained results forecast a bright prospect in the NOx removal from flue gas by combining DBD with RPB. Keywords: Oxidation, Rotating Packed Bed, NOx removal
2
1. Introduction Air pollution has drawn great public attentions in recent years all over the world, especially in developing countries [1-4]. Air pollutants are mainly generated during the combustion of fossil fuels by incinerators, automobiles, thermal power plants, etc. [5-7], which is especially common in developing countries like China. Among these pollutants, nitrogen oxides (NOx) are responsible for the formation of photochemical smog, which results in problems of many human organs, such as heart, lungs, and eyes. In addition, NOx participates in some key chain reactions and exhausts the ozone, leading to depletion of the ozone layer in the upper atmosphere [8]. Therefore, it is of great significance to remove NOx from gases. Over the past decades, corresponding effective treasures have been taken to reduce NOx emissions in the combustion and post-combustion [9, 10]. So far, many technologies, mainly including dry and wet methods, have been developed to reduce NOx emissions in China and significant improvements have been achieved [11, 12]. Dry methods mainly include selective catalytic reduction (SCR) [13] and selective non-catalytic reduction (SNCR) [14]. SCR can decrease NOx emissions effectively in the presence of catalyst and the heavy metals and sulfur compounds in the flue gas make the catalyst suffer from the risk of poisoning [15, 16, 17]. A high NOx removal efficiency was also achieved by SNCR without catalyst and the temperature were highly demanded during the process [18, 19]. As a result, wet methods have drawn much attention for its advantages of low cost and co-capture of other pollutants [20]. As we all known, NO2 is quite easily removed by wet scrubbing, while NO is much
3
more difficult to remove because it is sparingly soluble in aqueous solutions [21, 22]. Oxidation of NO and then absorption of NO2 from flue gas is considered as a valuable option in terms of high efficiency and low investment for the NOx removal [23]. A potential way to improve the NOx removal efficiency is to oxidize NO to soluble NO2 [24]. A large number of highly active oxidizing species (O·, ·OH, O3 etc.) can be generated in non-thermal plasma (NTP) [25]. Therefore, NTP could be used to deal with NOx, such as dielectric barrier discharge (DBD) [26-27], pulsed corona discharge [28, 29], direct current (DC) corona discharge [30, 31], etc. The feature of DBD is that a homogenous discharge with a low energy consumption is generated [32]. Furthermore, the existence of medium in the DBD can avoid spark formation inside the streamer channels [33]. Based on the above advantages, DBD was exploited to control the NOx oxidation process in this study. On the other hand, in the whole process of NOx removal, the NOx must be absorbed after oxidation. Rotating packed bed (RPB) is a novel gas-liquid reactor designed to generate high acceleration via centrifugal force, leading to the formation of thin liquid films and tiny liquid droplets. Consequently, RPB can enhance the mass transfer of the absorption process as well as reduce the initial investment cost [34, 35]. This study is the first time to combine DBD with RPB for the NOx removal. The objective of this work is to investigate the oxidation process in DBD and absorption process in RPB, respectively. In the oxidation process, we explored the effects of some operating parameters on the NOx oxidation degree, such as output frequency, peak voltage, residence time, oxygen content and the concentration of NOx. In the
4
absorption process, some parameters such as the NOx oxidation degree, urea concentration, NaClO2 concentration, rotational speed, gas-liquid ratio, and the concentration of NOx were also researched to optimize this process.
2. Experimental Section 2.1 Experimental materials Analytical reagent grade urea ((NH2)2CO) and sodium chlorite (NaClO2) were purchased from Beijing Reagent Factory of China. The gas of NO with a purity of 99.9% and N2 with a purity of 99.9% were purchased from Beijing Huatongjingke Gas Chemical Co., LTD., China. 2.2 Experimental setup DBD and RPB are two main reactors in this study. As shown in Figure 1, the coaxial DBD reactor consisted of stainless-steel tube as the inner electrode, quartz tube as the dielectric, and a metal mesh wrapped around the quartz tube as the outer electrode. The discharge gap was 3.5 mm and the length of the reactor was 150 mm. NTP was obtained in this DBD reactor with sinusoidal high voltage power input generated by a plasma generator in the range of 0-30 kV voltage and 5-25 kHz frequency (CTP-2000K, Nanjing Suman Electronics Co., Ltd, China). The RPB reactor used in this work mainly consisted of a packed rotator, a fixed casing, a liquid inlet, a liquid outlet, a gas inlet, and a gas outlet. The simulated flue gas continuously flowed through the system and was measured by flue gas analyzer (ECOM-EN2, RBR Analytical Instrument Institution, Germany) before and after the
5
absorption system to calculate the NOx removal efficiency. Figure 2 shows the experimental setup is divided into three parts, including gas-feeding system, oxidation system, and absorption system. Gas-feeding system mainly includes NO cylinder, N2 cylinder, air blower, and mixer. DBD reactor, function generator (peak voltage: 0-30 kV, frequency: 5-25 kHz), HV transformer, HV probe, and oscillography are belonging to the oxidation system. Absorption system is composed of RPB reactor, pump, and absorbent tank. 2.3 Experimental procedure The initial concentrations of NO, NO2, NOx and O2 could be changed by adjusting the mass flow meters. The mixed gas flow rate ranged from 0.5 to 3 m3·h-1. The concentrations of the NOx in gas mixture were set in the range of 400-1600 ppm (ppm was a volume-based unit, equivalent to ppmv). The high concentration was used to simulate the glass melting furnaces and the nitric acid plants, while the low concentration was used to simulate the cement kilns [36, 37, 38]. After mixed in the gas-feeding system, the simulated flue gas was introduced into the oxidation system. The main chemical processes in DBD plasmas are the reactions with atoms (O, N), the activated atoms and molecules. When the discharge energy is high enough, the molecules of N2 and O2 are activated to a higher energy state, even ionization after colliding with the high energy state electron, thus promoting a series of chemical reaction processes. Since the dissociation energy of O2 (5.2 eV per molecule) is lower than that of N2 (9.8 eV per molecule), it is easier for electrons to interact with O2 and produce strong oxygen species (·O and O3) at the
6
same discharge condition. These oxidizing species promote the oxidation reaction of NO [39]. According to previous studies [40-42], the ozone was believed to dominantly affect the oxidation of NO. Since N2O5 is easily decomposed to NO2, NO or N2 in discharge plasma [43]. The absorption experiment was performed as follows. The gas mixture stream from the outlet of DBD was continuously introduced into the RPB outer cavity and flowed inward by a pressure-driving force. Meanwhile, the absorbent entered the rotor by a liquid distributor, sprayed from the inner bed and then moved outward under the centrifugal force. The liquid distributor used in this study had two rows of holes with 1 mm diameter and four each row. Consequently, the gas and liquid were contacted counter-currently inside the RPB. Urea ((NH2)2CO) is a slightly alkaline reagent as well as a strong reducing reagent. Compared with other absorption solutions, urea is a cheap and non-toxic reactant which can be easily obtained. Reaction products of denitration using urea solution are carbon dioxide and nitrogen which can be directly released into the atmosphere [44]. As a consequence, urea was used as the basic absorbent. Meanwhile, NaClO2 was considered as an effective absorbent [45-48], but its industrial application is difficult because of the high price. In order to reduce the cost and improve removal efficiency of NOx, a complex absorbent that was made up of urea and NaClO2 was used to removal NOx from the simulated flue gas in this work. 2.4 Data analysis The NOx oxidation degree (α) and NOx removal efficiency (η) are defined as
7
follows: C2 (NO 2 ) 100% C2 (NO x )
(1)
C1 (NO x ) C3 (NO x ) 100% C1 (NO x )
(2)
Where C(NO), C(NO2) and C(NOx) are the concentrations of NO, NO2, and NOx, respectively, and the total concentration of NOx is the sum of NO and NO2. The subscript 1, 2, and 3 denote DBD inlet, the middle position of DBD outlet and RPB inlet, and RPB outlet, respectively.
3. Results and discussion 3.1 Oxidation process by DBD 3.1.1 Influence of peak voltage on the NOx oxidation degree Output frequency (f) and peak voltage (Vpp) applied on DBD reactor have a prominent influence on electric discharge [49]. Therefore, the influence of output frequency on peak voltage was firstly investigated. Figure 3 shows that Vpp of the DBD reactor was maximized at 8.5 kHz when input voltage was 10, 20, 30 and 40 V, respectively. The possible reason is that the total impedance of the plasma generator was minimum when the operating frequency approached the resonance point (8.5 kHz), and the output power of the plasma generator reached the maximum. Consequently, the output frequency was fixed at 8.5 kHz in the following investigation. As mentioned above, besides output frequency, peak voltage also has prominent
8
influence on electric discharges. Figure 4 shows that, at different gas flow rates, the NOx oxidation degree increased slightly and then increased sharply. NOx oxidation degree reached more than 50% when the peak voltage was 28 kV. With the peak voltage further increased, the oxidation degree began to decrease. The reasons may be as follows: i) when peak voltage was relatively low, the discharge energy could hardly activate N2 and O2 to higher energy state; ii) when peak voltage exceeded 25 kV, the discharge energy might be higher than the dissociation energy of O2 (5.2 eV per molecule), but still lower than that of N2 (9.8 eV per molecule). As a result, reactions took place in this process mainly promoted the NO oxidation (R2, R3 and R9 in Table 1); iii) besides O2, N2 could be also dissociated when peak voltage was higher than 28 kV. Consequently, the NO2 reductions (R8 and R10 in Table 1) were simultaneous with the oxidation process of NO. Therefore, the peak voltage was fixed at 28 kV in the following study. 3.1.2 Influence of average residence time on the NOx oxidation degree The average residence time could be represented as follows:
tr
VDBD G
(3)
where VDBD is the volume of DBD and G represents gas flow rate. As shown in Figure 5, it can be concluded that the average residence time had a minimal influence on the NOx oxidation degree within the range of the investigation. Compared with the time for high-energy electrons to collide with gas molecules to produce free radicals and the time required by a series of reactions, the residence time was long enough [50]. Hence, the oxidation degree was almost constant at about 50% in the range of average 9
residence time from 0.04 s to 0.10 s. Shorter average residence time represented larger gas flow rate, which benefited the further industrial application. 3.1.3 Influence of oxygen content on the NOx oxidation degree Since oxygen content of the actual flue gas is different due to various sources, it is essential to explore the influence of oxygen content on the NOx oxidation degree. As displayed in Figure 6, the oxidation degree increased from 23% to 52% when the oxygen content increased from 4% to 21%. Under the given discharge condition, the energy input to the DBD reactor was constant. When O2 content increased, more O2 could be activated to generate more O3 and O radicals, providing a greater oxidizing source for the reaction system, allowing the oxidation atmosphere (formed by O3 and O radicals) to be maintained for a longer time [43]. The higher the oxygen content existed in the flue gas, the higher the NOx oxidation degree could be achieved. 3.1.4 Influence of NOx concentration on the NOx oxidation degree As shown in Figure 7, the oxidation degree decreased from 70% to 42% when the NOx concentration increased from 400 to 1600 ppm. The reason is similar as the above explanation. Under the given condition, the energy input to the DBD reactor was constant and the number of active species (mainly O radicals and O3) were invariable. As a consequence, the higher the NOx concentration of flue gas was, the lower the NOx oxidation degree was. According to previous researches [51-52], NOx oxidation process in DBD could be described as follows: Oxygen was converted into active oxygen species after colliding with high-energy electrons generated by discharge. Subsequently, active
10
oxygen species with strong oxidizing property reacted with part of NO to generate NO2. 3.2 Absorption process by RPB 3.2.1 Influence of the NOx oxidation degree on the NOx removal efficiency It is successful to regulate and control the NOx oxidation degree in the oxidation process. Therefore, in the absorption process, we firstly explored the relation between the NOx oxidation degree and the NOx removal efficiency in order to determine the operating parameter of oxidation process. It is fairly obvious that the NOx removal efficiency increases with the increase of the NOx oxidation degree in Figure 8. When the oxidation degree was about 50%, the NOx removal efficiency could be reached more than 62% at different gas flow rates (1.5 m3·h-1, 2 m3·h-1, 2.5 m3·h-1). 3.2.2 Influence of absorbent concentration on the NOx removal efficiency When the concentration of NOx retained constant (1000 ppm), the influence of urea concentration on the NOx removal efficiency was experimentally investigated, and the results are shown in Figures 9 and 10. It could be concluded from Figure 9 that different concentrations of urea solution has little influence on the NOx removal efficiency at the beginning of the experiments (~10 min). As time went on, the NOx removal efficiency would decline faster at lower concentration of urea solution (Figure 10). This means that the high concentration of urea solution could keep the NOx removal efficiency stable (~28%) for at least one hour, which was beneficial to the cycled utilization of urea solution. When urea solution was used as single absorbent, the NOx removal efficiency
11
could reach about 28%. As mentioned above, NaClO2 is considered as an extremely effective additive to improve the removal efficiency. Experimental results shown in Figure 11 indicate that adding NaClO2 into urea would greatly improve the NOx removal efficiency from ~28% to ~56%. Meanwhile, when the concentration of NaClO2 increased from 0.02 to 0.08 mol·L-1, the NOx removal efficiency increased rapidly and then became relatively stable. This phenomenon may be interpreted as follows: although the NOx oxidation degree was enhanced to ~50% by DBD, NO still took quite a large percentage in NOx. NaClO2 could not only oxide NO to NO2, but also promoted the absorption process, especially when the NaClO2 concentration was below 0.08 mol·L-1. Gas-liquid contact time might become the main limitation when the NaClO2 concentration exceeded 0.08 mol·L-1. Based on previous researches [53, 54], the absorption process in RPB could be described as follows: urea reacted with NO2 to generate carbon dioxide and nitrogen. Meanwhile, NaClO2 reacted with NO and NO2 to generate sodium nitrate and sodium chloride. The chemical reaction between NOx and urea/NaClO2 could promote NOx absorption in the liquid. Hence, NOx removal was achieved under the combination of DBD and RPB. 3.2.3 Influence of rotational speed on the NOx removal efficiency Figure 12 shows that, at different NaClO2 concentrations, the NOx removal efficiency increases sharply with the increase of the rotational speed of RPB. When the NaClO2 concentration was 0.08 mol·L-1, the NOx removal efficiency could reach more than 65% at 1600 r/min, and the NOx removal efficiency tended to level off
12
even increasing rotational speed continuously. The possible reason is that increasing rotational speed contributes to more drastic dispersion effects of the packing in RPB on the liquid, which results in the increase of gas-liquid contact area and liquid surface renewal rate. Both of them were conducive to the NOx absorption. When rotational speed exceeded 1600 r/min, the liquid residence time would be shorter than the necessary reaction time required to absorb NOx. The direct consequence was that the liquid phase had not enough time to fully contact with the gas phase. Hence, the upward trend of NOx removal efficiency began to level off. 3.2.4 Influence of gas-liquid ratio on the NOx removal efficiency In our study, we fixed the gas flow rate and changed the liquid flow rate to investigate the influence of gas-liquid ratio on the NOx removal efficiency. It can be seen from Figure 13 that the NOx removal efficiency gradually decreases with the increase of the gas-liquid ratio. When the gas-liquid ratio was 50, the NOx efficiency could reach more than 68%. The reason is that higher gas-liquid ratio results in the decreasing liquid holdup and effective gas−liquid contact area [50]. When the mass transfer driving force is constant, the mass transfer rate is limited. The total amount of NOx dissolved into the absorbent reduces, which results in the decrease of the NOx removal efficiency. Therefore, lower gas-liquid ratio is beneficial to increase the NOx removal efficiency. When the gas flow rate is constant, lower gas-liquid ratio means larger amount of liquid that will increase operating costs. A suitable gas-liquid ratio should be chosen based on the actual operating conditions. 3.2.5 Influence of NOx concentration on the NOx removal efficiency
13
As mentioned earlier, NOx oxidation degree exerted a significant effect on the NOx removal efficiency. The NOx removal efficiency decreased from 84% to 62% when the NOx concentration increased from 400 to 1600 ppm. At the given oxidation condition, oxidation degrees of different NOx concentrations are distinct. It is evident that NOx removal efficiency decreases with the increase of NOx concentration from Figure 14. The reason could be that when the denitration capacity of the solution was constant, increasing NOx concentration caused a reduction in the amount of absorbed NOx per volumetric gas [55].
4. Conclusions This work studied the NOx removal for the first combination between DBD and RPB. The NOx oxidation degree was regulated by DBD, and the NOx removal was achieved by RPB. In the oxidation process, we mainly investigated the influence of several parameters, such as peak voltage, oxygen content, residence time and NOx concentration on the oxidation degree. When the NOx concentration was 1000 ppm, NOx oxidation degree could reach more than 50%. NOx removal efficiency was evaluated in terms of oxidation degree, urea concentration, NaClO2 concentration, rotational speed, gas-liquid ratio, and NOx concentration in the absorption process. Results showed that the NOx removal efficiency could reach more than 62% at different gas flow rates when the oxidation degree was about 50%. High concentration of urea solution could keep the NOx removal efficiency stable for a long time, which is beneficial to the circulation of absorbent. For the purpose of further
14
improving the removal efficiency of NOx, NaClO2 was added as an effective absorbent. The results showed that the NOx removal efficiency improved about 28%. Under the optimal conditions, NOx removal efficiency could be more than 85%. The obtained results forecast a bright prospect in the removal of NOx from flue gases by the combination between DBD and RPB.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 21725601, 21676009, and 21436001) and University of Chinese Academy of Sciences (No. Y8540XX222).
15
Nomenclatures
CNaClO2 = NaClO2 concentration (mol·L-1) CNOx = NOx concentration (ppm) CO2 = O2 content (%) f = output frequency of DBD (kHz) G = gas flow rate (m3·h-1, L·min-1) G/L = gas-liquid ratio k298K = reaction rate constant at 298 K (L·mol-1·s-1) L = liquid flow rate (L·h-1) N = rotational speed of RPB (r/min) t = operating time (min) tr = residence time in DBD (s) Vpp = peak voltage of DBD (kV) VDBD= volume of DBD, 27.214×10-3 (L) wurea = mass fraction of urea in the absorbent (wt%)
Greek Symbols α = NOx oxidation degree (%) η = NOx removal efficiency (%)
16
References [1] C. Wang, A. Engels, Z. Wang, Overview of research on China's transition to low-carbon development: the role of cities, technologies, industries and the energy system, Renew. Sust. Energ. Rev. 81 (2018) 1350-1364. [2] Y. Osaka, K. Iwai, T. Tsujiguchi, A. Kodama, X. Li, H. Y. Huang, Basic study on exhaust gas purification by utilizing plasma assisted MnO2 filter for zero-emission diesel, Sep. Purif. Technol. 215 (2019) 108-114. [3] D. Matthews, S. Solomon, Reversing excess atmospheric CO2-response, Science 340 (2013) 1523-1523. [4] Z. F. Lu, D. G. Streets, Increase in NOx emissions from Indian thermal power plants during 1996-2010: unit-based inventories and multisatellite observations, Environ. Sci. Technol. 46 (2012) 7463-7470. [5] J. H. Ye, J. Shang, Q. Li, W. W. Xu, J. Liu, X. Feng, T. Zhu, The use of vacuum ultraviolet irradiation to oxidize SO2 and NOx for simultaneous desulfurization and denitrification, J. Hazard. Mater. 271 (2014) 89-97. [6] W. Y. Sun, S. L. Ding, S. S. Zeng, S. J. Su, W. J. Jiang, Simultaneous absorption of NOx and SO2 from flue gas with pyrolusite slurry combined with gas-phase oxidation of NO using ozone, J. Hazard. Mater. 192 (2011) 124-130. [7] Y. X. Liu, J. Zhang, C. D. Sheng, Y. C. Zhang, L. Zhao, Simultaneous removal of NO and SO2 from coal-fired flue gas by UV/H2O2 advanced oxidation process, Chem. Eng. J. 162 (2010) 1006-1011. [8] Z. Wei, X. Li, L. Xu, C. Tan, Optimization of operating parameters for low NOx
17
emission in high-temperature air combustion, Energ. Fuel. 26 (2012) 2821-2829. [9] T. Saikaew, P. Supudommak, L. Mekasut, P. Piumsomboon, P. Kuchonthara, Emission of NOx and N2O from co-combustion of coal and biomasses in CFB combustor, Int. J. Greenh. Gas Con. 10 (2012) 26-32. [10] S. Mahmoudi, J.Baeyens, J. Seville, NOx formation and selective non-catalytic reduction (SNCR) in a fluidized bed combustor of biomass, Biomass Bioenerg. 34 (2010) 1393-1409. [11] Z. Ma, J. Deng, Z. Li, Q. Li, P. Zhao, L. G. Wang, Y. Z. Sun, H. X. Zheng, L. Pan, S. Zhao, J. K. Jiang, S. X. Wang, L. Duan, Characteristics of NOx emission from Chinese coal-fired power plants equipped with new technologies, Atmos. Environ. 131 (2016) 164-170. [12] H. Z. Tian, P. P. Qiu, K. Cheng, J. J. Gao, L. Lu, K. Y. Liu, X. G. Liu, Current status and future trends of SO2 and NOx pollution during the 12th FYP period in Guiyang city of China, Atmos. Environ. 69 (2013) 273-280. [13] M. Shelef, Selective catalytic reduction of NOx with N-free reductants, Chem. Rev. 95 (1995) 209-225. [14] D. I. Sandberg, M. Lamberti-Pasculli, J. M. Drake, T. H. Christensen, Life cycle assessment of selective non-catalytic reduction (SNCR) of nitrous oxides in a full-scale municipal solid waste incinerator, Waste Manage. 31 (2011) 1184-1193. [15] Z. Z. Ma, J. G. Deng, Z. Li, Q. Li, P. Zhao, L. G. Wang, Y. Z. Sun, H. X. Zheng, L. Pan, S. Zhao, J. K. Jiang, S. X. Wang, L. Duan, Characteristics of NOx emission from Chinese coal-fired power plants equipped with new technologies, Atmos.
18
Environ. 131 (2016) 164-170. [16] H. Rojel, A. Jensen, P. Glarborg, K. D. Johansen, Mixing effects in the selective noncatalytic reduction of NO, Ind. Eng. Chem. Res. 39 (2000) 3221-3232. [17] M. H. Zhang, B. H. Huang, H. X. Jiang, Y. F. Chen, Research progress in the SO2 resistance of the catalysts for selective catalytic reduction of NOx, Chinese J. Chem. Eng. 25 (2017) 1695-1705. [18] S. S. Daood, M. T. Javed, B. M. Gibbs, W. Nimmo, NOx control in coal combustion by combining biomass co-firing, oxygen enrichment and SNCR, Fuel 105 (2013) 283-292. [19] J. Baleta, H. Mikulcic, M. Vujanovic, Z. Petranovic, N. Duic, Numerical simulation of urea based selective non-catalytic reduction deNOx process for industrial applications, Energ. Convers. Manage. 125 (2016) 59-69. [20] R. T. Guo, J. K. Hao, W. G. Pan, Y. L. Yu, Liquid phase oxidation and absorption of NO from flue gas: a review, Sep. Sci. Technol. 50 (2015) 310-321. [21] Y. G. Adewuyi, S. O. Owusu, Aqueous absorption and oxidation of nitric oxide with oxone for the treatment of tail gases: process feasibility, stoichiometry, reaction pathways, and absorption rate, Ind. Eng. Chem. Res. 42 (2003) 4084-4100. [22] M. Petrissans, A. Petrissans, A. Zoulalian, Experimental study and modelling of mass transfer during simultaneous absorption of SO2 and NO2 with chemical reaction, Chem. Eng. Process. 44 (2005) 1075-1081. [23] R. Ochoagonzalez, P. Cordoba, M. Diazsomoano, O. Font, M. A. Lopezanton, C. Leiva, M. R. Martineztarazona, X. Querol, C. F. Pereira, A. Tomas, P. Gomez, P.
19
Mesado, Differential partitioning and speciation of Hg in wet FGD facilities of two Spanish PCC power plants, Chemosphere 85 (2011) 565-570. [24] Y. Cai, Y. Luo, G. W. Chu, W. Wu, X. Yu, B. C. Sun, J. F. Chen, NOx removal in a rotating packed bed: oxidation and enhanced absorption process optimization, Sep. Purif. Technol. (2019) 115682. [25] K. Shang, X. Wang, J. Li, H. Wang, N. Lu, N. Jiang, Y. Wu, Synergetic degradation of acid orange 7 (AO7) dye by DBD plasma and persulfate, Chem. Eng. J. 311 (2017) 378-384. [26] M. B. Chang, S. C. Yang. NO/NOx removal with C2H2 as additive via dielectric barrier discharges, AIChE J. 47 (2001) 1226-1233. [27] B. M. Obradovic, G. B. Sretenovic, M. M. Kuraica, A dual-use of DBD plasma for simultaneous NOx and SO2 removal from coal-combustion flue gas, J. Hazard. Mater. 185 (2011) 1280-1286. [28] H. H. Kim, K. Takashima, S. Katsura, A. Mizuno, Low-temperature NOx reduction processes using combined systems of pulsed corona discharge and catalysts, J. Phys. D Appl. Phys. 34 (2001) 604. [29] L. Huang, D. Yongxia, Removal of SO2 and NOx by pulsed corona combined with in situ Ca(OH)2 absorption, Chinese J. Chem. Eng. 19 (2011) 518-522. [30] T. Fujii, Y. Aoki, N. Yoshioka, M. Rea, Removal of NOx by DC corona reactor with water, J. Electrostat. 51 (2001) 8-14. [31] J. S. Chang, K. Urashima, Y. X. Tong, W. P. Liu, H. Y. Wei, F. M. Yang, X. J. Liu, Simultaneous removal of NOx and SO2 from coal boiler flue gases by DC corona
20
discharge ammonia radical shower systems: pilot plant tests, J. Electrostat. 57 (2003) 313-323. [32] C. Q. Wang, G. X. Zhang, X. X. Wang, Comparisons of discharge characteristics of a dielectric barrier discharge with different electrode structures, Vacuum 86 (2012) 960-964. [33] A. Fridman, A. Chirokov, A. Gutsol, Non-thermal atmospheric pressure discharges, J. Phys. D Appl. Phys. 38 (2005) 1-24. [34] K. Guo, F. Guo, Y. Feng, J. F. Chen, C. Zheng, N. C. Gardner, Synchronous visual and RTD study on liquid flow in rotating packed-bed contactor, Chem. Eng. Sci. 55 (2000) 1699-1706. [35] K. Yang, G. W. Chu, H. K. Zou, B. C. Sun, L. Shao, J. F. Chen, Determination of the effective interfacial area in rotating packed bed, Chem. Eng. J. 168 (2011) 1377-1382. [36] M. Flamme, Low NOx combustion technologies for high temperature applications, Energ. Convers. Manage. 42 (2001) 1919-1935. [37] K. Skalska, J. S. Miller, M. Wilk, L. Stanisław, Nitrogen oxides ozonation as a method for NOx emission abatement, Ozone-Sci. Eng. 34 (2012) 252-258. [38] A. T. McQueen, S. J. Bortz, M. S. Hatch, R. L. Leonard, Cement kiln NOx control, IEEE T. IND. APPL. 31 (1995) 36-44. [39] E. Stamate, C. Irimiea, M. Salewski, Investigation of NOx reduction by low temperature oxidation using ozone produced by dielectric barrier discharge, Jpn. J. Appl. Phys. 52 (2013) 129-142.
21
[40] Y. S. Mok, V. Ravi, H. Kang, B. S. Rajanikanth, Abatement of nitrogen oxides in a catalytic reactor enhanced by nonthermal plasma discharge, IEEE T. Plasma Sci. 31 (2003) 157-165. [41] J. Li, W. H. Goh, X. Yang, R. T. Yang, Non-thermal plasma-assisted catalytic NOx storage over Pt/Ba/Al2O3 at low temperatures, Appl. Catal. B-Environ. 90 (2009) 360-367. [42] B. Eliasson, U. Kogelschatz, Modeling and applications of silent discharge plasmas, IEEE T. Plasma Sci. 19 (1991) 309-323. [43] Y. J. Zhang, X. L. Tang, H. H. Yi, Q. J. Yu, J. G. Wang, F. Y. Gao, Y. M. Gao, D. Z. Li, Y. M. Cao, The byproduct generation analysis of the NOx conversion process in dielectric barrier discharge plasma, RSC Adv. 6 (2016) 63946-63953. [44] P. Fang, C. P. Cen, Z. X. Tang, P. Y. Zhong, D. S. Chen, Z. H. Chen, Simultaneous removal of SO2 and NOx by wet scrubbing using urea solution, Chem. Eng. J. 168 (2011) 52-59. [45] T. W. Chien, H. Chu, Removal of SO2 and NO from flue gas by wet scrubbing using an aqueous NaClO2 solution, J. Hazard. Mater. 80 (2000) 43-57. [46] E. Sada, H. Kumazawa, I. Kudo, T. Kondo, Absorption of NO in aqueous mixed solutions of NaClO2 and NaOH, Chem. Eng. Sci. 33 (1978) 315-318. [47] J. C. Wei, Y. B. Luo, P. Yu, B. Cai, H. Z. Tan, Removal of NO from flue gas by wet scrubbing with NaClO2/(NH2)2CO solutions, J. Ind. Eng. Chem. 15 (2009) 16-22. [48] B. R. Deshwal, S. H. Lee, J. H. Jung, B. H. Shon, H. K. Lee, Study on the removal of NOx from simulated flue gas using acidic NaClO2 solution, J. Environ.
22
Sci-China. 20 (2008) 33-38. [49] X. L. Tang, F. Y. Gao, J. G. Wang, H. H. Yi, S. Z. Zhao, B. W. Zhang, Y. R. Zuo, Z. X. Wang, Comparative study between single- and double-dielectric barrier discharge reactor for nitric oxide removal, Ind. Eng. Chem. Res. 53 (2014) 6197-6203. [50] R. Dorai, J. K. Kushner, Consequences of propene and propane on plasma remediation of NOx, J. Appl. Phys. 88 (2000) 3739-3747. [51] M. M. Santillan, A. Vincent, E. Santirso, J. Amouroux, Design of a DBD wire-cylinder reactor for NOx emission control: experimental and modelling approach, J. Clean. Prod. 16 (2008) 198-207. [52] B. M. Obradovic, G. B. Sretenovic, M. M. Kuraica, A dual-use of DBD plasma for simultaneous NOx and SO2 removal from coal-combustion flue gas, J Hazard. Mater. 185 (2011) 1280-1286. [53] A. Pant, S. J. Schmieg. Kinetic model of NOx SCR using urea on commercial Cu−zeolite catalyst, Ind. Eng. Chem. Res. 50 (2011) 5490-5498. [54] A. Pourmohammadbagher, E. Jamshidi, H. Ale-Ebrahim, S. Dabir, Study on simultaneous removal of NOx and SO2 with NaClO2 in a novel swirl wet system, Ind. Eng. Chem. Res. 50 (2011) 8278-8284. [55] B. C. Sun, M. P. Sheng, W. L. Gao, L. L. Zhang, M. Arowo, Y. Liang, L. Shao, G. W. Chu, H. K. Zou, J. F. Chen, Absorption of nitrogen oxides into sodium hydroxide solution in a rotating packed bed with preoxidation by ozone, Energ. Fuel. 31 (2017) 11019-11025.
23
[56] P. H.Wine, J. M. Nicovich, R. J. Thompson, A. R. Ravlshankara, Kinetics of atomic oxygen (3PJ) reactions with hydrogen peroxide and ozone, J. Phys. Chem. 87 (1983) 3948-3954. [57] I. Wangberg, E. Ljungstrom, B. Olsson, J. Davidssont, Temperature dependence of the reaction NO3+NO2 → NO+NO2+O2 in the range from 296 to 332 K, J. Phys. Chem. 96 (1992) 7640-7645. [58] K. Skalska, J. S. Miller, S. Ledakowicz, Trends in NOx abatement: a review, Sci. Total Environ. 408 (2010) 3976-3989. [59] M. Schieferstein, K. Kohse‐Höinghaus, F. Stuhl, Temperature dependence of the rate constants of the reaction O+ NO+ M→ NO2+ M (M= He, NO, N2, CH4), Ber. Bunsenges. phys. Chem. 87 (1983) 361-366. [60] P. O. Wennberg, J. G. Anderson, D. K. Weisenstein, Kinetics of reactions of ground state nitrogen atoms (4S3/2) with NO and NO2, J. Geophys. Res-Atmos. 99 (1994) 18839-18846. [61] Y. S. Mok, I. S. Nam, Reduction of nitrogen oxides by ozonization-catalysis hybrid process, Korean J. Chem. Eng. 21 (2004) 976-982.
24
Figure Captions Figure 1. Schematic diagram of DBD reactor. Figure 2. Experimental setup. Figure 3. Influence of output frequency on the peak voltage. Figure 4. Influence of peak voltage on the NOx oxidation degree. Figure 5. Influence of residence time on the NOx oxidation degree. Figure 6. Influence of oxygen content on the NOx oxidation degree. Figure 7. Influence of NOx concentration on the NOx oxidation degree. Figure 8. Influence of NOx oxidation degree on the NOx removal efficiency. Figure 9. Influence of urea concentration on the NOx removal efficiency. Figure 10. Influence of reaction time on the NOx removal efficiency. Figure 11. Influence of NaClO2 concentration on the NOx removal efficiency. Figure 12. Influence of rotational speed on the NOx removal efficiency. Figure 13. Influence of gas-liquid ratio on the NOx removal efficiency. Figure 14. Influence of NOx concentration on the NOx removal efficiency.
Table Legends Table
1.
Main
reactions
which
happen
in
DBD.
25
Figure 1. Schematic diagram of DBD reactor.
26
Sampling point-C1
Tail gas
HV probe
HV transformer
DBD reactor
Gas Mixer
R0 Mass flow meter Valve
AC
Function generator
RPB reactor
Sampling point-C3
Pump
Cm Oscillograph
Air NO
N2
Gas distribution system
Sampling point-C2
Waste Tank
Oxidation system by DBD
Absorbent Tank
Absorption system by RPB
Figure 2. Experimental setup.
27
30 10 V 20 V 30 V 40 V
25
Vpp (kV)
20 15 10 5 0
7
8
9
10
11
f (kHz)
Figure 3. Influence of output frequency on the peak voltage.
28
55 50
f=8.5 kHz, cNOx=1000 ppm 3
45
α (%)
-1
G=2.5 m h 3 -1 G=2 m h 3 -1 G=1.5 m h
40 35 30 25 20
22
24
26
28
30
Vpp (kV)
Figure 4. Influence of peak voltage on the NOx oxidation degree.
29
65 f=8.5 kHz,Vpp=28 kV cNOx=800 ppm
60
cNOx=1000 ppm
α (%)
55 50 45 40
0.04
0.05
0.06
0.07
0.08
0.09
0.10
tr (s)
Figure 5. Influence of residence time on the NOx oxidation degree.
30
55 f=8.5 kHz , Vpp=28 kV 3 -1 G=2m h , cNOx=1000 ppm
50 45
α (%)
40 35 30 25 20
4
8
12
16
20
cO2(%)
Figure 6. Influence of oxygen content on the NOx oxidation degree.
31
75 f=8.5 kHz , Vpp=28 kV 3 -1 G=2m h
70 65
α (%)
60 55 50 45 40 400
600
800
1000
1200
1400
1600
cNOx (ppm)
Figure 7. Influence of NOx concentration on the NOx oxidation degree.
32
70
-1
L=40 Lh , cNOx=1000 ppm, N=1600 rpm -1
curea=10 wt%, cNaClO =0.08 molL 2
3
(%)
-1
G=2.5 m h 3 -1 G=2 m h 3 -1 G=1.5 m h
60
50
40 25
30
35
40
45
50
55
a (%)
Figure 8. Influence of NOx oxidation degree on the NOx removal efficiency.
33
40 N=1600 rpm, cNOx=1000 ppm 3
-1
-1
G=2 m h , L=40 Lh , t=10 mins
(%)
35
30
25
20
2
4
6
8
10
w urea (wt%)
Figure 9. Influence of urea concentration on the NOx removal efficiency.
34
30 28 26
(%)
24 wurea
22
10 wt% 8 wt% 6 wt% 4 wt% 2 wt%
20 18 16 10
20
30
40
50
60
t (min)
Figure 10. Influence of reaction time on the NOx removal efficiency.
35
70
(%)
60
50
40
3
-1
-1
wurea=10 wt%, G=2 m h , L=40 Lh N=1600 rpm, cNOx=1000 ppm
30 0.00
0.02
0.04
0.06
0.08
0.10
0.12
-1
cNaClO (mol L ) 2
Figure 11. Influence of NaClO2 concentration on the NOx removal efficiency.
36
70 65
(%)
60 3
55
-1
-1
wurea=10 wt%, G=2 m h , L=40 L h -1
cNaClO =0.08 mol L 2
50
-1
cNaClO =0.06 mol L 2
-1
45
cNaClO =0.02 mol L 2
800
1200
1600
2000
2400
N (rpm)
Figure 12. Influence of rotational speed on the NOx removal efficiency.
37
70 68
(%)
66 64 -1
62 60
wurea=10 wt%, cNaClO =0.08 molL 2
3
-1
N=1600 rpm, G=2 m h , cNOx=1000 ppm 40
50
60
70
80
90
100
G/L
Figure 13. Influence of gas-liquid ratio on the NOx removal efficiency.
38
90 85
3
-1
2
80
(%)
-1
G=2 m h , L=40 L h , N=1600 rpm -1 wurea=10 wt%, cNaClO =0.08 mol L
75 70 65 60
400
600
800
1000
1200
1400
1600
cNOx (ppm)
Figure 14. Influence of NOx concentration on the NOx removal efficiency.
39
Table 1. Main reactions which happen in DBD. Number R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15
Reactions O3+O→2O2 NO+NO3→2NO2 O2+NO3→N2O5 O3→O2+O O3+N→O2+NO O+O+M→O2+M N+O2→NO+O NO+N2O→NO2+N2 NO+NO→O2+N2 NO+O+M→NO2+M N+NO2→N2O+O NO+N→N2+O NO+O3→NO2+O2 NO2+O3→NO3+O2N N2O5→NO2+NO3
k298K(L·mol-1·s-1) 8.0×10-15 1.566×1010 8.431×108 2.023×10-2 6.00×104 3.872×108 4.742×104 5.265×10-21 1.388×10-36 9.091×10-31 1.214×10-14 3.764×10-14 2.576×109 8.36×107 1.159×1017
Refs. Wine et al. [56]. Wangberg et al. [57]. Wangberg et al. [57]. Skalska et al. [58]. Skalska et al. [58]. Skalska et al. [58]. Skalska et al. [58]. Skalska et al. [58]. Skalska et al. [58]. Schieferstein et al. [59]. Wennberg et al. [60]. Wennberg et al. [60]. Mok et al. [61]. Mok et al. [61]. Mok et al. [61].
40
Highlights
1. A DBD reactor and an RPB reactor were firstly combined for the NOx removal. 2. The DBD reactor was employed to improve the NOx oxidation degree. 3. The NOx removal efficiency was enhanced by the RPB. 4. Optimizations for the NOx removal efficiency were presented systematically.
41