N2O5 Formation Mechanism during the Ozone-Based Low

May 18, 2016 - With advantage of the solubility improving from NO into a higher valence state of nitrogen oxides, especially N2O5, the ozone-based ...
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N2O5 Formation Mechanism during the Ozone-Based LowTemperature Oxidation deNOx Process Fawei Lin, Zhihua Wang,* Qiang Ma, Yong He, Ronald Whiddon, Yanqun Zhu, and Jianzhong Liu State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, Zhejiang 310027, People’s Republic of China ABSTRACT: With advantage of the solubility improving from NO into a higher valence state of nitrogen oxides, especially N2O5, the ozone-based low-temperature oxidation deNOx process was investigated, especially the N2O5 formation mechanism. The temperature ranging from 60 to 150 °C and O3/NO ratios and residence time changing were investigated by a well-designed experiment. N2O5 was detected by Fourier transform infrared spectroscopy (FTIR). A 24-step mechanism was also proposed, specially for describing the formation of N2O5. Results demonstrated that the formation of N2O5 was greatly influenced by the temperature and residence time. N2O5 could be formed at relatively low temperatures, such as 60−80 °C, within 3−5 s when O3/ NO > 1.0. There was no N2O5 detected when the temperature was higher than 130 °C as a result of the decomposition of NO3. The proposed mechanism could give a good prediction of the experimental results with kinetic simulation. To speed up the N2O5 formation process and reduce the O3 dosage and O3 slip, a type of MnOx-based catalyst was developed. Results showed that the MnOx-loaded spherical alumina catalyst had obviously a positive effect on the formation of N2O5, with more than 90% NO converted into N2O5 at 80 °C within 0.24 s and O3/NO = 1.5, with less than 15 ppm of O3 leftover.

1. INTRODUCTION Sulfur dioxide (SO2) and nitrogen oxides (NOx), as the major gas-phase pollutants emitted from fossil fuel combustion, have raised great environmental problems. As well-known, SO2 can be effectively removed from exhaust by the wet flue gas desulfurization (WFGD) process as a result of its high solubility.1 NOx is usually removed by selective catalytic reduction (SCR)2 or selective non-catalytic reduction (SNCR)3 methods with ammonia to reduce NOx into N2. However, drawbacks, such as ammonia slip,4 air preheater fouling, catalyst poisoning,5 strict temperature window, and expensive cost, are big problems for those NOx control technologies. Alternatively, a new kind of low-temperature oxidation method has been proposed by converting insoluble NO (>95% NOx) into more soluble forms, such as NO2, NO3, and N2O5 with ozone.6−9 As a result of the acid/base neutralization reaction process, SO2 and NOx can be simultaneously removed within the WFGD tower. From conversion of element mercury Hg0 into oxidized mercury Hg2+, Hg2+ can be efficiently removed by WFGD.9,10 The volatile organic compounds (VOCs) can also be removed at the same time.11 Therefore, the ozone-based low-temperature oxidation technology becomes an attractive method for flue gas cleanup with the ability of simultaneously removing SO2, NOx, Hg, and VOCs within one process. The simultaneous removal of SO2 and NOx by ozone oxidation has been investigated previously.9,12−19 During the process, NO was first converted to NO2 when the molar ratio of O3/NO was less than 1.0.9 However, the NO2 absorption efficiency was usually lower than expected in limestone slurry at typical WFGD conditions.18 To improve the NO2 absorption efficiency, a lot of work have been performed, such as additive development like sodium sulfite (Na2SO3),12,17 sodium humate,13 sodium sulfide (Na2S),14 pyrolusite,15 calcium sulfite (CaSO3),16 etc. These additives can increase the sulfite ion S(IV) concentration, which can improve the NO2 absorption © XXXX American Chemical Society

process. With the help of additive, nearly 95% NOx can be removed. However, the running cost of the additives becomes a big obstacle for industrial application. To improve the NOx removal efficiency, excess ozone was usually injected to the system, in which N2O5 could be formed to help the absorption process. In our previous tests, it was found that the NO2 concentration decreased gradually when O3/NO exceeded 1.0. Meanwhile, the NOx removal efficiency was increased obviously after wet scrubbing. It is known that the solubility of nitrogen oxide increases with its valence state. Especially, N2O5, the anhydride of nitric acid, which is the most soluble species of NOx, can be absorbed by wet scrubbing without any additives.20,21 However, there is still a lack of a sophisticated detection method for N2O5. The formation mechanism of N2O5 was seldom reported. Excess ozone in the system will increase the running cost and also cause ozone slip. Therefore, understanding the formation mechanism of N2O5 well is essential to improve the deNOx efficiency and reduce the cost. An experimental setup equipped with a gas supplier, an ozone generator and analyzer, and a Fourier transform infrared spectroscopy (FTIR) gas analyzer was assembled in this work to investigate the N2O5 formation mechanism. The gas composition, such as NO, NO2, and N2O5, can be detected by the infrared absorption spectrum. A modified chemical kinetics mechanism was proposed and verified by the experimental data. Jogi et al.22 reported that TiO2 powder could improve NOx oxidation in medium-scale experiments. In this paper, the spherical alumina loaded with MnOx was prepared to shorten the required residence time and reduce O3 dosage and O3 leftover. Received: April 8, 2016 Revised: May 16, 2016

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DOI: 10.1021/acs.energyfuels.6b00824 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Experimental setup for the N2O5 formation research. R7, NO3 + O = O2 + NO2, R8, NO3 = O2 + NO, R14, NO2 + NO3 = O2 + NO + NO2, R17, NO3 + NO3 = 2NO2 + O2, R23, NO3 + O3 = 2O2 + NO2, and R24, NO2 + NO3 = N2O5, were included. N2O5 formation reaction R24, NO2 + NO3 = N2O5, was included and modified. The reaction kinetics parameters were obtained from the National Institute of Standards and Technology (NIST) kinetic database. Modeling was performed using both the previous mechanism 24 and the new mechanism applied to a closed homogeneous batch reactor in Chemkin Pro Release 15083. A comparison between the kinetic model and experiments was carried out to explore the formation mechanism of N2O5.

2. EXPERIMENTAL SECTION 2.1. Experimental System. Figure 1 illustrates the experimental setup for the ozone-based low-temperature oxidation deNOx process. The simulated flue gas was mixed from bottled nitrogen, oxygen, and NO supplied by Jingong Gas Co., Ltd. (N2, 99.999%; O2, 99.999%; NO, 5%; balance N2). Ozone was generated with a dielectric barrier discharge (DBD) device manufactured by Qingdao Guolin Co. (model CF-G-3-10g, >10 g/h). The inlet ozone concentration was continuously monitored with an ozone analyzer (BMT-964 BT, OSTI, Inc., 0−200 g/Nm3, ±0.1 g/Nm3). The NO/N2 and O3/O2 gas streams were injected at different points to avoid the reaction before entering the reactor. Flow rates of the gas streams were controlled by an individual mass flow controller (MFC, Alicat Scientific, Inc.). The initial NO concentration was set around 200 ppm. The N2O5 formation mechanism was investigated by the deep oxidation reactor, which is made by quartz with 36.4 mm internal diameter and 40 cm length. The residence time was varied by altering the flow rate of the total gas stream. The residence time was set around 5 s for all sections, except residence time changing. The oxidation process with a catalyst was carried out at 80 °C in the catalytic oxidation reactor. The total flow rate was 2 L/min, and the catalyst dosage was 2.2 g. The total residence time was 0.24 s, with 0.12 s on catalyst. The catalyst is spherical alumina (with 2−3 mm diameter, Sinopharm Chemical Reagent Co., Ltd.) loaded with MnOx, which was prepared by the impregnation method. A total of 11.14 g of Mn(CH3COO)2 was dissolved in deionized water. A total of 50 g of spherical alumina was immersed into the solution for 24 h and then dried at 110 °C for 12 h. Finally, it was calcined at 400 °C for 3 h under an air atmosphere in a muffle furnace. The mass content of Mn in the resulted spherical material was about 5.39%, measured by the energy-dispersive spectroscopy (EDS) method. The reactor was loaded in an electric-heated tube furnace (Yifeng Furnace Co., Ltd.) with a digital temperature controller and K-type thermocouple. The concentrations of NO, NO2, and O2 in the outlet gas stream were measured quantitatively by a FTIR gas analyzer (DX4000, Gasmet). Although N2O5 cannot be measured quantitatively using the existing flue gas analyzer, its infrared absorption spectra can be obtained.6−8,19 A second ozone analyzer (model 205, 2B Technologies; 0−200 ppm, ±1 ppb) was used to detect the leftover ozone concentration after the reaction chamber. 2.2. Kinetic Modeling. There are some kinetic mechanisms that have been reported in the literature for the reactions between NOx and O3.7,8,14,23 In our previous works, a 20 species, 65-step kinetic mechanism has been proposed (henceforth “previous mechanism”).24 In general, these mechanisms were employed to study the oxidation process for O3/NO < 1.0. The consumption reactions of NO3 and N2O5, which are critical for deep oxidation when O3/NO > 1.0, were neglected in these mechanisms. Thus, a modified chemical kinetics model (shown in Table 1) was developed here, including necessary reactions involving NO3 and N2O5 (henceforth “new mechanism”). NO3 formation reactions R3, O3 + NO2 = NO3 + O2, and R15, NO2 + O = NO3, and NO3 consumption reactions R5, NO + NO3 = 2NO2,

3. RESULTS AND DISCUSSION 3.1. Deep Oxidation of NO by O3. The deep oxidation process of NO by O3 was first conducted by the experiment at a temperature of 80 °C. The NO, NO2, and O3 concentrations for various O3/NO ratios are shown in Figure 2a. There is a linear increase of NO2 and decrease of NO with the O3/NO ratio at O3/NO < 1.0. When O3/NO > 1.0, the decrease of NO2 can be found, whereas residual O3 appears. The results mean that further conversion of NO2 is taken place but not complete with excess O3 leftover, which is both not economic and not environmentally friendly. The corresponding infrared absorption spectra of the gas stream are shown in Figure 3 varied with O3/NO ratios from 0.73 to 2.25. As the O3/NO ratio increases larger than 1.0, the absorption peaks at 887, 1326, and 1720 cm−1 appear and keep increasing, while the peak at 1600 cm−1 keeps decreasing. According to the spectra library,25,26 the peaks at 887 and 1326 cm−1 are corresponding to HNO3 and the peak at 1720 cm−1 is corresponding to N2O5, while the peak at 1600 cm−1 is corresponding to NO2. The spectra confirm that NO2 is oxidized into N2O5 when O3/NO > 1.0, and some N2O5 subsequently converts to HNO3 by combination with water vapor in the gas stream. In addition, the absorption peak of O3 can be observed at 1501 cm−1 19 when O3/NO > 2.0, which is so weak compared to other components. Actually, the O3 peak should appear when O3/ NO > 1.0, according to the results in Figure 2. The discordance may be attributed to the disturbance of the nitrogen oxides in the analysis unit of FTIR. The experimental results running at 130 °C are shown in Figure 2b. There is no big difference with Figure 2a when O3/ NO < 1.0. When O3/NO > 1.0, it is completely different with Figure 2a; the NO2 concentration does not decrease along with O3/NO, and residual O3 is close to zero. The results mean that NO2 does not further convert to N2O5 at the temperature of 130 °C with no O3 leftover. The deep oxidation process between NO and O 3 is greatly dependent upon the B

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28 28 28 29 30 32 33 33 34 36 37 39 2603 0 0 12100 −1788 0 2504 −238 63190 6260 0 0 0 1.4 0 0 0 0 0 0 0 1 −0.75 0.2 10 109 1011 106 1013 107 1010 1012 1013 109 1015 1011

× × × × × × × × × × × × O3 + NO = O2 + NO2 NO + NO2 = N2O3 NO2 + NO2 = N2O4 NO3 = O2 + NO O + O + M = O2 + M O3 + N = O2 + NO NO2 + NO3 = O2 + NO + NO2 NO2 + O = O2 + NO NO + NO = O2 + N2 N + O2 = NO + O NO + O = NO2 NO2 + NO3 = N2O5

Reaction rate constants expressed as k = ATn exp(−E/RT) (cal, cm, mol, and s).

× × × × × × × × × × × × 4.82 8.43 1.08 1.02 1.51 4.31 1.26 1.32 5.12 1.30 1.45 6.03 O3 + O = 2O2 O3 + NO2 = NO3 + O2 NO + NO3 = 2NO2 NO3 + O = O2 + NO2 N2O5 + H2O = 2HNO3 O3 = O2 + O NO + N = N2 + O NO2 + O = NO3 NO3 + NO3 = 2NO2 + O2 NO2 + N = N2 + O + O NO + M = N + O + M NO3 + O3 = 2O2 + NO2 R1 R3 R5 R7 R9 R11 R13 R15 R17 R19 R21 R23

Figure 2. NO oxidation process varied with the O3/NO ratio at (a) 80 °C and (b) 130 °C. Dash dot lines, previous mechanism; solid lines, new mechanism; and scattered points, experimental data.

temperature. To see the complete influence of the temperature, the experiments were repeated at various temperatures ranging from 60 to 150 °C with O3/NO = 2.0. The infrared absorption spectra of the outlet gas stream are shown in Figure 4. It can be seen that the absorption peak of NO2 (1600 cm−1) is very weak when the temperature is lower than 80 °C. With the increase of the temperature, the peak of NO2 keeps increasing, especially for the temperatures above 90 °C, while the peaks of N2O5 (1720 cm−1) and HNO3 (887 and 1326 cm−1) decline. The peaks of N2O 5 and HNO3 even disappear when the temperature exceeds 130 °C. The results confirm that almost no further N2O5 can be formed in the deep oxidation process when the temperature exceeds 130 °C, which is in accordance with the experimental results in Figure 2b. From these observations, the optimal temperature for N2O5 formation should be between 60 and 80 °C. 3.2. Effect of the Temperature on N2O5 Formation. As mentioned above, the temperature is a crucial factor for N2O5 formation. To see more detail, kinetic modeling was carried out with both the previous mechanism and new mechanism and compared to experimental results, as shown in Figure 2. It can be found that the modeling results by the two mechanisms all give satisfactory agreement with experimental results at 80 °C in Figure 2a. However, there is much difference for the

a

R2 R4 R6 R8 R10 R12 R14 R16 R18 R20 R22 R24 28 28 28 28 28 31 33 33 33 35 37 38 4094 4908 −219 0 0 22300 −199 0 4869 0 148000 0 10 1010 1013 1013 103 1014 1013 1013 1011 10−1 1015 106

0 0 0 0 0 0 0 0 0 0 0 0

number reference E n

12

A reaction number

Table 1. N2O5 Formation Mechanism in the NO/O3 Systema

reaction

8.43 1.62 6.02 2.50 1.89 6.03 2.71 3.92 3.10 6.41 1.31 3.66

A

11

n

E

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that most influenced the NO3 production rate at 80 and 130 °C are shown in Figure 5. At both temperatures, reaction R3, O3 +

Figure 3. Infrared absorption spectra varied with the O3/NO ratio.

Figure 5. NO3 production rates with varying residence times at (a) 80 °C and (b) 130 °C.

NO2 = NO3 + O2, has the greatest positive influence in NO3 production. Reaction R15, NO2 + O = NO3, begins to have a small contribution to NO3 formation when the temperature increases to 130 °C. NO2 then combines with NO3 to generate N2O5 through reaction R24, NO2 + NO3 = N2O5. NO3 is the key intermediate species for N2O5 formation. Meanwhile, NO3 has two main consumption pathways: reactions R5, NO + NO3 = 2NO2, and R14, NO2 + NO3 = O2 + NO + NO2. When the temperature increases to 130 °C, these NO3 decomposition reactions accelerate to a considerable level compared to that at 80 °C. Hence, NO3 comes back to NO2 and NO with consumption of O3. Sun et al.8 also pointed out that the reverse reaction of reaction R24, NO2 + NO3 = N2O5, accelerated continuously along with the temperature. Therefore, the N2O5 concentration declines sharply as a result of both NO3 and its decomposition at such a high temperature. The new mechanism can give a good prediction for the N2O5 formation process. Hereafter, the detailed analysis will be carried out by kinetic modeling with the new mechanism. In real case application, ozone is usually injected in front of the WFGD tower. For power plants and industrial boilers, the flue gas temperature at this position is usually below 200 °C. Therefore, the N2O5 formation characteristic was investigated

Figure 4. Infrared absorption spectra with different temperatures at O3/NO = 2.0.

performance of the two mechanisms at 130 °C in Figure 2b, especially when O3/NO > 1.0. The new mechanism gives an accurate prediction for the behavior of NO2 and O3, while the old mechanism gives a completely wrong tendency when O3/ NO > 1.0. Reactions R2, O3 + NO = O2 + NO2, and R22, NO + O = NO2, are the dominant pathways through which NO is converted into NO2 when O3/NO < 1.0. When O3/NO > 1.0, NO3 begins to be produced. Reactions from the kinetic model D

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This excess ozone will be consumed and self-decomposed into O2 when the reaction temperature is higher enough, as shown in Figure 6c. There is almost no O3 leftover at all of the O3/NO ratios when the temperature is higher than 130 °C. 3.3. Effect of the Residence Time on N2O5 Formation. The NO2 concentrations changing with various ratios of O3/ NO at different residence times (0.4, 0.9, 1.5, and 3.9 s) were measured by experiments and modeling at 80 °C, as shown in Figure 7. It can be seen that the NO2 profiles show almost the

Figure 7. NO2 concentration with variation of O3/NO at different residence times. Scattered points, experimental data; lines, modeling results.

same linearly line when O3/NO < 1.0 at the four residence times. The results mean that the conversion of NO into NO2 is dominate and fast enough. When O3/NO > 1.0, the NO2 concentration diminishes along with O3/NO. For these four residence times, the longer the reaction time, the lower the NO2 concentration at the same O3/NO. This indicates that the conversion of NO2 to N2O5 is a slow reaction, which cannot reach chemical equilibrium within a short reaction time, such as 0.4 s, at 80 °C. The modeling results give relatively good agreement with experimental data. The detailed formation characteristic of N2O5 varied with the residence time at different temperatures was calculated and compared in Figure 8. It can be seen that the N 2 O 5 concentration increases along with the residence time between 60 and 80 °C. When the temperature is higher than 80 °C, the N2O5 concentration shows an initial increase and then decrease along with the residence time. Meanwhile, the peak of N2O5

Figure 6. Concentration profiles of NO2, N2O5, and O3 as a function of the temperature.

From panels a and b of Figure 6, it can be seen that almost 90% NO can be oxidized to N2O5 over the temperature range of 60−80 °C when O3/NO = 2.0. The NO2 concentration increases with the rising temperature when O3/NO > 1.0, while N2O5 decreases. Except for the NO3 decomposition mentioned before, the ozone destruction accelerates sharply at higher temperatures by reactions R1, O3 + O = 2O2, and R11, O3 = O2 + O. The proportion of ozone that is available for oxidation decreases. In industrial application, a heat recovery exchanger can be added to control the temperature, which has been successfully applied in a carbon black drying kiln furnace flue gas treatment.27 The global reaction for N2O5 formation is 2NO + 3O3 = N2O5 + 3O2, which gives a stoichiometric ratio of 1.5 to oxidize 1 mol of NO into N2O5. Practically, as a result of kinetics limitation, excess O3 is usually added to shift the chemical reaction toward N2O5. Modeling and experimental results illustrate that a molar ratio of O3/NO > 2.0 is needed for >90% conversion to N2O5, resulting in a large amount of O3 leftover, which is both not economic and not environmentally friendly.

Figure 8. N2O5 formation as a function of the residence time. E

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changes occur in the sensitivity trends. First, the fast reactions (reactions R2, R5, and R24) come to steady state more quickly; therefore, their influence on N2O5 production stabilizes more rapidly. Second, the positive influence from reaction R3 reduces and switches to a negative influence on N2O5 production after ∼1.6 s of reaction time. Finally, reaction R14, NO2 + NO3 = O2 + NO + NO2, shows an increasingly negative impact on N2O5 production. Thus, it is a combination of reactions R3 and R14 that accounts for the suppression of N2O5 formation at a higher temperature. 3.5. N2O5 Formation with a Catalyst. As mentioned above, the long residence time, excess O3 dosage, and O3 slip are crucial problems for industrial application of deep oxidation. Therefore, a Mn-based catalyst, i.e., spherical alumina loaded with MnOx, was developed and tested for the N2O5 formation. The oxidation process was carried out at 80 °C with NO, NO2, and O3 monitored at the exhaust. As shown in Figure 10, when

drops a lot with the rising temperature. This is attributed to the elevated decomposition of NO3 and N2O5, as mentioned above. As shown in Figure 8 and mentioned above, the N2O5 formation is relatively slow at a low temperature, such as 60 °C, that needs ∼5 s to achieve saturation. With an increase to 80 °C, the required residence time becomes a little shorter (∼3 s), while the saturated concentration of N2O5 drops. With the increase of the temperature, the required residence time becomes shorter but the formation of N2O5 is inhibited. Thus, methods to accelerate the N2O5 formation at a low temperature are of the greatest importance in developing the ozone-based low-temperature oxidation deNOx process. 3.4. Sensitivity Analysis of N2O5 Formation. The effect of individual reactions on N2O5 formation was evaluated by performing a sensitivity analysis on the new mechanism. The sensitivity coefficients of the five most influential reactions from the new mechanism at 80 and 130 °C are shown in Figure 9.

Figure 10. NO oxidation process with a catalyst at various O3/NO ratios. Solid lines, spherical alumina loaded with Mn; dash dot lines, only spherical alumina.

O3/NO < 1.0, there is no obvious difference between the oxidation process with and without a catalyst. However, the NO2 concentration decreases faster along with the O3/NO ratio at 0.24 s compared to results in Figure 7 without a catalyst when O3/NO > 1.0. Almost 90% NO is converted into N2O5 at the stoichiometric ratio O3/NO = 1.5, with less than 15 ppm of O3 leftover. For comparison, the oxidation process with only spherical alumina (not loaded with Mn) is also shown in Figure 10. It can be observed that less than 50% NO is converted into N2O5 at O3/NO = 1.5. This illustrates that the arrangement of spherical alumina also has a positive effect on N2O5 formation, which can be attributed to the high surface area of alumina as well as the better steams mixing. However, the loading of Mn improves the formation efficiency more obviously. The results mean that N2O5 can be formed efficiently with the help of a Mn-based catalyst to speed up the chemical reaction process. In the catalytic oxidation process, NO2 will first be adsorbed on the catalyst surface, followed by conversion to NO3 with the help of absorbed O3/O radicals. MnOx is believed to accelerate the decomposition of O3 into an O radical at the catalyst surface, which may help the formation of NO3 and then N2O5. The catalytic reaction mechanism undoubtedly needs further investigation.

Figure 9. Sensitivity coefficient of N2O5 with variation of the residence time at (a) 80 °C and (b) 130 °C.

Examining the 80 °C reaction analysis, reactions R2, O3 + NO = O2 + NO2, R5, NO + NO3 = 2NO2, and R24, NO2 + NO3 = N2O5, have fast chemistry and show the greatest influence during the first 0.2 s of reaction. Reaction R3, O3 + NO2 = NO3 + O2, has a positive sensitivity coefficient that slowly tails off as reaction time accrues. This means that NO3 formation is the most important factor in N2O5 production at long reaction times. When the reaction temperature is set to 130 °C, several F

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(12) Chen, L.; Lin, J. W.; Yang, C. L. Environ. Prog. 2002, 21 (4), 225−230. (13) Hu, G. X.; Sun, Z. G.; Gao, H. Y. Environ. Sci. Technol. 2010, 44 (17), 6712−6717. (14) Mok, Y. S.; Lee, H. J. Fuel Process. Technol. 2006, 87 (7), 591− 597. (15) Sun, W. Y.; Wang, Q. Y.; Ding, S. L.; Su, S. J. Chem. Eng. J. 2013, 228, 700−707. (16) Wang, Z. H.; Zhang, X.; Zhou, Z. J.; Chen, W. Y.; Zhou, J. H.; Cen, K. F. Energy Fuels 2012, 26 (9), 5583−5589. (17) Yamamoto, T.; Okubo, M.; Nagaoka, T.; Hayakawa, K. IEEE Trans. Ind. Appl. 2002, 38 (5), 1168−1173. (18) Zheng, C. H.; Xu, C. R.; Zhang, Y. X.; Zhang, J.; Gao, X.; Luo, Z. Y.; Cen, K. F. Appl. Energy 2014, 129, 187−194. (19) Stamate, E.; Irimiea, C.; Salewski, M. Jpn. J. Appl. Phys. 2013, 52 (5S2), 05EE03. (20) Chang, W. L.; Bhave, P. V.; Brown, S. S.; Riemer, N.; Stutz, J.; Dabdub, D. Aerosol Sci. Technol. 2011, 45 (6), 665−695. (21) Skalska, K.; Miller, J. S.; Ledakowicz, S. Sci. Total Environ. 2010, 408 (19), 3976−3989. (22) Jogi, I.; Stamate, E.; Irimiea, C.; Schmidt, M.; Brandenburg, R.; Hołub, M.; Bonisławski, M.; Jakubowski, T.; Käar̈ iäinen, M.-L.; Cameron, D. C. Fuel 2015, 144, 137−144. (23) Asif, M.; Kim, W. S. Ozone: Sci. Eng. 2014, 36 (5), 472−484. (24) Wang, Z. H.; Zhou, J. H.; Fan, J. R.; Cen, K. F. Energy Fuels 2006, 20 (6), 2432−2438. (25) Mogili, P. K.; Kleiber, P. D.; Young, M. A.; Grassian, V. H. Atmos. Environ. 2006, 40 (38), 7401−7408. (26) Wangberg, I.; Etzkorn, T.; Barnes, I.; Platt, U.; Becker, K. H. J. Phys. Chem. A 1997, 101 (50), 9694−9698. (27) Ma, Q.; Wang, Z.; Lin, F.; Kuang, M.; Whiddon, R.; He, Y.; Liu, J. Energy Fuels 2016, 30, 2302−2308. (28) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Crowley, J. N.; Hampson, R. F.; Hynes, R. G.; Jenkin, M. E.; Rossi, M. J.; Troe, J. Atmos. Chem. Phys. 2004, 4, 1461−1738. (29) Johnston, H. S.; Cantrell, C. A.; Calvert, J. G. J. Geophys. Res. 1986, 91, 5159−5172. (30) Tsang, W.; Hampson, R. F. J. Phys. Chem. Ref. Data 1986, 15, 1087. (31) Heimerl, J. M.; Coffee, T. P. Combust. Flame 1979, 35, 117− 123. (32) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, R. F.; Kerr, J. A.; Troe, J. J. Phys. Chem. Ref. Data 1989, 18 (2), 881−1097. (33) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R.; Kolb, C. E.; Molina, M. J. JPL Publ. 1997, 1−266. (34) Yuan, E. L.; Slaughter, J. I.; Koerner, W. E.; Daniels, F. J. Phys. Chem. 1959, 63 (6), 952−956. (35) Phillips, L. F.; Schiff, H. I. J. Chem. Phys. 1965, 42 (9), 3171− 3174. (36) Baulch, D. L.; Drysdale, D. D.; Horne, D. G. Symp. Combust., [Proc.] 1973, 14, 107−118. (37) Tsang, W.; Herron, J. T. J. Phys. Chem. Ref. Data 1991, 20 (4), 609−663. (38) Hjorth, J.; Notholt, J.; Restelli, G. Int. J. Chem. Kinet. 1992, 24 (1), 51−65. (39) Hahn, J.; Luther, K.; Troe, J. Phys. Chem. Chem. Phys. 2000, 2 (22), 5098−5104.

4. CONCLUSION The N2O5 formation mechanism during the ozone-based lowtemperature oxidation deNOx process was proposed and validated against experimental results. N2O5 was produced from a series of reactions: NO oxidized by O3 into NO2, NO2 oxidized by O3 into NO3, and finally NO2 combining with NO3 to generate N2O5. The results showed that N2O5 formation was deeply influenced by the temperature and residence time. N2O5 decreased as the temperature increased with the same O3/NO ratio and could hardly be detected at the temperature exceeded 130 °C. Almost 90% NO could be converted into N2O5 at O3/ NO = 2.0 under the temperature range of 60−80 °C, which needs 3−5 s to achieve saturation. The spherical alumina loaded with MnOx was prepared as the catalyst for N2O5 formation, which could greatly speed up the reaction process with less O3 dosage and O3 slip. Almost 90% NO could be converted into N2O5 at 80 °C in 0.24 s at O3/NO = 1.5, with