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NO removal from flue gas by using chlorine dioxide solution Shujun Sun, Suxia Ma, Bingchuan Yang, Jie Wang, and Rongji Cui Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b01694 • Publication Date (Web): 02 Sep 2019 Downloaded from pubs.acs.org on September 2, 2019
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NO removal from flue gas by using chlorine dioxide solution Shujun Sun, Suxia Ma*, Bingchuan Yang, Rongji Cui, Jie Wang
Department of Thermal Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China
ABSTRACT: NO removal from flue gas was conducted on a self-made bubbling reactor with ClO2. Results indicated that ClO2 has excellent oxidizing power to NO, and its actual reaction molar ratio is close to the theoretical molar ratio. At temperature of 80 °C and ClO2/NO = 0.6, the NO removal efficiency can reach 96.7%. The electron spin resonance and inhibitor (sodium formate and isopropanol) addition experiments indicated that the ClO2 and ·OH radical in the solution played an important role in NO oxidation. The mechanism of NO removal by ClO2 was proposed through the analysis of oxidation products and relative references.
Keywords: NO; chlorine dioxide; OH radical; oxidize 1 ACS Paragon Plus Environment
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1. Introduction NO is one of the main pollutants in the flue gas of coal-fired power plants and has caused great damage, such as photochemical smoke formation and ozone layer destruction, to the natural environment and our daily life.1, 2 NO removal from coal-fired flue gas is a major concern in power plant development. At present, selective catalytic reduction (SCR) has been used in power plants for NO removal from flue gas; however, this method has several disadvantages, such as high investment and operating cost, large occupying area, catalyst poisoning, and huge economic burden on the power plant.3,
4
Considering that NO and NO2 are insoluble and soluble in water, respectively, oxidation– absorption has attracted extensive attention; in this method, NO is first oxidized to NO2 and then can be removed by the alkali-absorption tower.5 The oxidant choice determines the NO removal efficiency. Compared with traditional oxidation methods, such as KMnO4 oxidation,
6
NaClO2 oxidation,7,
8
NaClO oxidation,9 and O3 oxidation,10
advanced oxidation processes (AOPs) has gradually attracted great attention; these techniques use ultraviolet (UV) light, catalyst, microwave, and other auxiliary means of inducing the oxidant to produce active free radicals.11-15 The generation of active free radicals can reduce the amount of oxidant and greatly improve the removal efficiency. However, the auxiliary methods may also increase the removal cost or narrow the application scope. 2 ACS Paragon Plus Environment
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The ·OH (2.7 eV) radical with a strong oxidizing ability is one of the common free radicals in AOPs. Fenton and Fenton-like systems mainly use ·OH radical to remove pollutants from flue gas. The catalytic generation path of ·OH radical in the liquid phase is illustrated by Eqs. (1)–(2): 16, 17 Fe3+ + H2O2 → Fe2+ + HO2· (O2−·) + H+ (2H+),
(1)
Fe2+ + H2O2 → Fe3+ + ·OH + OH−.
(2)
The catalytic generation path of ·OH radical in the gas–solid phase is expressed by Eqs. (3)–(4)): 12, 16
≡Fe (Ш) − OH + H2O2 → ≡Fe (П) − OH + HO2,
(3)
≡Fe (П) − OH + H2O2 → ≡Fe (Ш) − OH + ·OH + H2O.
(4)
In the liquid phase catalytic system, 90% NO removal efficiency can be obtained by generating ·OH radical under optimal conditions, but the secondary pollution of iron ions also occurs. Although more than 80% removal efficiency can be obtained in the gas–solid heterogeneous system, the further application of this process is limited by the complicated catalyst preparation. The UV/H2O2 system is also adopted to generate ·OH for NO removal from the flue gas and obtain desirable removal efficiency.14, 18 When the H2O2 solution is mixed with Na2S2O8, SO4·− and ·OH are produced for the NO removal, and the removal efficiency is 79.6%.19,
20
The generation paths of SO4·− and ·OH are
expressed in Eqs. (5)–(8): 3 ACS Paragon Plus Environment
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S2O82− + heat/H2O2 → 2SO4·−,
(5)
SO4·− + H2O2 → ·OH + HSO5−,
(6)
SO4·− + H2O ↔ ·OH + H+ + SO42−,
(7)
SO4·− + ·OH → HSO5−.
(8)
However, the high price (9800 ¥/t) of persulfate has hindered the further development of this process. Chlorine-containing radicals, including ClO·, Cl·, and Cl2−·, combined with ·OH have been widely studied in the field of water treatment due to their good cost-efficient performance.21-24 Although few studies on NO removal from flue gas by chlorine-containing radicals are available, the method’s feasibility has been confirmed in relevant experimental results. More than 90% removal efficiency can be obtained in the UV/Ca(ClO)2 removal system.25 The main UV catalytic reaction in the solution is shown in Eqs. (9)–(11):
HClO + hv → ·OH + Cl·,
(9)
ClO− + hv → ·O− + Cl·,
(10)
·O− + H2O →·OH + OH−.
(11)
·OH production greatly contributes to the removal efficiency. When UV was combined with NaClO2, the ClO· and ·OH radicals worked together for NO removal.
26
The free
radical generation path is calculated as follows: 4 ACS Paragon Plus Environment
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ClO2− + hv → ClO· + O·−
(12)
O·− + H2O ↔ HO· + OH−
(13)
The above-mentioned studies reported that the existence of one kind of free radical in the oxidation system often stimulates the production of various free radicals, which will interact and further enhance the oxidizing ability. One of these many oxidants is ClO2, a free radical. Several scholars experimented on NO removal by liquid ClO2. In such studies, the good oxidation performance of ClO2 to NO was mainly attributed to ClO2 itself.27,
28
The present study is the first to show that not only ClO2, which has an
oxidation effect on NO, but HO· radicals, which can be generated without auxiliary means in solution, greatly contribute to NO oxidation together. The mechanism of HO· radical generation in solution and NO oxidation by HO· radical are proposed. The effects of different reaction conditions on NO removal by ClO2 oxidation are comprehensively explored. The traditional bubbling bed reaction device, which cannot precisely control the reactant molar ratio, has been improved. Our results indicated that higher than 96% oxidation efficiency can be obtained with the use of an extremely low molar ratio (ClO2/NO=0.6) under appropriate conditions.
2. Experimental apparatus and methods 5 ACS Paragon Plus Environment
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2.1. Experimental apparatus and procedure
Figure 1. Experimental apparatus. 1, N2 cylinder; 2, SO2 cylinder; 3, NO cylinder; 4, ClO2 solution; 5, pressure valve; 6, globe valve; 7, pressure gage; 8, mass flow meter; 9, gas mixer; 10, peristaltic pump; 11, tubular furnace; 12, quartz tube; 13, Ca(OH)2 solution; 14, CaCl2; 15, flue gas analyzer; 16, flow controller; 17, temperature controller.
The experimental apparatus for removing NO from flue gas in ClO2 solution is shown in Fig. 1. The required concentrations of NO, N2, and SO2 (Jining Xieli Special Gas Company) were produced with a compressed gas cylinder. The gas flow rates can be effectively controlled with mass flow meters (Beijing Sevenstar Electronics Company). The buffer bottle allowed the gas to be sufficiently mixed before flowing into the bottom of the quartz tube (bubble reactor: length = 500 mm; inside diameter = 20 mm). A porous partition was placed in the lower quarter of the quartz tube to disperse the gas. The quartz tube was then placed in a tubular furnace (height = 350 mm; inside diameter = 24 mm; 6 ACS Paragon Plus Environment
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outside diameter = 160 mm) for heating. The solution temperature in the tube was precisely controlled with a thermocouple (HYD-7411, Shanghai Huoyu Instrument Company, China.). The initial pH of the ClO2 solution was adjusted by H2SO4 (1 mol/L) and NaOH (1 mol/L) and measured by pH-electrode (PHS-3E Shang Hai Electrical Instrument Company, China). The prepared ClO2 solution continuously flowed into the reactor containing 30 mL of deionized water via the peristaltic pump (DG-1, Baoding Longer Precision Pump Company, China) to oxidize NO. The reacted gas was then absorbed by the saturated Ca(OH)2 solution. Finally, the tail gas was detected with a flue gas analyzer (Testo350, 0–5000 ppm, accuracy: 5% rel., Testo Instruments International Trade Company, Germany).
2.2. Data process The removal efficiency under different conditions was read at the 20th minute. The results of each experiment were measured threefold and then averaged. The calculation method of NO removal efficiency is shown in Eq. (15): 𝜂(%) =
𝐶𝑖𝑛 ― 𝐶𝑜𝑢𝑡 𝐶𝑖𝑛
(14)
× 100%,
where η is the NO removal efficiency, and Cin and Cout are the NO inlet and outlet concentrations, respectively.
2.3. Analytical methods
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The reagents used in experiments were of analytical grade (Aladdin Chemical Reagent Co., Ltd.). The saturated Ca (OH) 2 and anhydrous CaCl2 were employed as the absorbent and dryer, respectively. The H2SO4 (6 mol/L) and NaClO2 (8 mol/L) were reacted to prepare ClO2 gas (Eq. (14)), which was absorbed by deionized water to prepare ClO2 solution. Sodium formate (SF) and isopropanol (IP) were used as the radical inhibitors. Starch solution (5 g/L), sodium thiosulfate (0.01 mol/L), and potassium iodide solution (100 g/L) were used for the titration of ClO2 concentration in solution.
10NaClO2 + 5H2SO4 → 8ClO2 + 5Na2SO4 + 2HCl + 4H2O
(15)
Ion products in the spent solutions were analyzed with ion chromatography (Dionex ICS5000, America). Electron spin resonance (ESR) detection was conducted at 25 °C and 40 °C with an ESR spectrometer (JEOL FA300) combined with the spin trap agent 5,5-dimethyl-l-1-pyrroline N-oxide (DMPO). The ClO2 solution concentration was titrated threefold by using iodometry. The average value was then determined.
3. Results and discussion 3.1. Effect of molar ratio (ClO2/NO) and reaction temperature on NO removal efficiency
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Figure 2. Effect of molar ratio (ClO2/NO) on NO removal efficiency. Flue gas flow: 350 mL/min; NO concentration: 370 mg/m3; pH of ClO2 solution: 7.
Figure 3. ESR test of ClO2; pH of ClO2: 7.
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Figure 2 shows the effect of molar ratio (ClO2/NO) and temperature on NO removal efficiency. The removal efficiency sharply increased with the molar ratio ClO2/NO from 0.15 to 0.6. When the molar ratio was more than 0.6, the removal efficiency slowly increased. In the temperature range of 20 °C–80 °C, the NO removal efficiency increased with the reaction temperature. The removal efficiency was stabilized at 96.7% under 80 °C and ClO2/NO = 0.6. The reaction equation (Eq. (16)) of ClO2 and NO showed that 0.6 is the theoretical molar ratio of the reaction.28 5NO + 3ClO2 + 4H2O → 5HNO3 + 3HCl
(16)
These results indicated that ClO2 has excellent oxidizing power to NO, and its actual reaction molar ratio is close to the theoretical value. ESR (electron spin resonance spectroscopy) test results are usually used as direct evidence for determining the presence of free radicals and distinguishing the types of free radicals in solution.16,22 ESR test with DMPO as the spin trapping agent was employed in detecting the free radicals in chlorine dioxide solution. When ·OH radical is present in the solution, the DMPO added during the detection will react with ·OH as follows reaction: DMPO + ·OH → DMPO-OH
(17)
In previous works, a four-line peak with an intensity ratio of 1:2:2:1 is the typical spectrum shape of DMPO-OH adducts, suggesting •OH was produced.16, 22, 25 As shown in Fig. 3, a four-line peak with an intensity ratio of 1:2:2:1 manifested, and aN = aH = 1.49 10 ACS Paragon Plus Environment
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G was identified, thereby indicating the presence of ·OH radical. Furthermore, Figs. 3 and 4 reveal that the peak intensity was significantly strengthened with the increase in temperature and ClO2 concentration. This finding indicated that the content of ·OH radical in the solution increases with the temperature and ClO2 concentration. The standard electrode potential has been considered as one of the important indicators representing the oxidation performance. However, the standard electrode potential of ClO2 (1.56 eV) 28 is lower than that of other oxidants, such as H2O2 (1.776 eV).17 The oxidation capacity of H2O2 is worse than that of ClO2 at the same concentration. It is the ·OH radical in ClO2 solution that plays a key role in the NO oxidation.
Figure 4. Inhibitor added test.
The ·OH radical scavenger (sodium formate and isopropanol) addition experiment was adopted in this work to obtain the effect of ·OH radical on oxidizing NO. The 11 ACS Paragon Plus Environment
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experimental results are shown in Fig. 4. Approximately 20% IP (isopropanol) solution, which can inhibit ·OH radical16, was added to the ClO2 solution and compared with the ClO2 solution without inhibitor. The removal efficiency was decreased by approximately 29.2% at 40 °C. When 20% SF (sodium formate) solution that can also inhibit ·OH radical was added,
29
the 42.3% removal efficiency declined at 40 °C. This phenomenon
is due to the scavenger reacting with the •OH radicals in the solution, causing the •OH radicals to fail in oxidizing NO. Such occurrence indirectly demonstrates the presence of •OH during the removal process. Different inhibitors have distinct inhibitory capacities for the same free radical. Accordingly, the test can prove the presence of ·OH radical, and at least 42.9% of the removal effect on NO was caused by it at 40 °C. When the same ·OH inhibitor (SF) was used, the inhibitory effect (22.9%) at 40 °C was better than that (42.3%) at 20 °C. This finding is consistent with the ESR test results. The path of free radical production might be calculated as follows (Eqs. (18)–(23)): 26, 29 ClO2· + H2O ↔ HClO2 + ·OH,
(18)
ClO2· + ·OH ↔ HOOClO,
(19)
ClO2·+ ClO2· ↔ Cl2O4,
(20)
HOOClO ↔ H+ + OOClO−,
(21)
OOClO− ↔ ClO·+ O2·−,
(22)
ClO·+ ·OH ↔ HClO2.
(23)
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The chlorine dioxide gas can be stably present in water and this stabilization is a dynamic equilibrium state. From a macroscopic point of view, the degree of progress of the abovementioned reactions (Eqs. (18)–(19)) is insufficient. ·OH radical, an intermediate product of the abovementioned disproportionation reaction, has been in continuous production and disappearance. However, when the reducing agent NO is added to the above mentioned reaction system, it is easily oxidized by the intermediate ·OH radical. The reason is that the redox potential of the ·OH radical (2.8 eV) is considerably higher than that of chlorine dioxide (1.56 eV), which greatly promotes the forward progress of reaction 18.
28,17
This phenomenon causes ·OH radical to become a
key factor in NO oxidation. Equations (18)–(23) reveal that hydroxyl radicals were generated in the solution with the presence of substantial ClO2 molecules. Such phenomenon results in an increased removal efficiency. It can be seen from Fig.2 that temperature in the range from 20 °C to 40 °C contributed the most to efficiency improvement. After 40 °C, the temperature effect gradually reduced. The effect of temperature on NO removal efficiency is due to two main reasons. According to Arrhenius’s law: 16
(
E
)
(24)
K(T) = 𝐴0exp ― RT ,
where T is the absolute temperature, K; K(T) is the reaction rate constant, min−1; A0 is the frequency factor constant, min−1; E is the reaction activation energy, J/mol; and R is the molar gas constant, J/(mol/K). 13 ACS Paragon Plus Environment
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The increase in temperature enhanced the collisions of activated molecules per unit time, and the reaction rate of oxidizing NO and generating ·OH radical was accelerated. By contrast, the NO solubility in aqueous solution decreased with increasing temperature, thereby reducing the contact time between NO and ClO2 solution. Such occurrence was non-conducive to the NO removal. In addition to the ·OH radical, HClO2 and free radicals, such as ClO· (1.5–1.8 eV) and O2·− (1.6 eV)
23
could also oxidize NO. The content of these radicals may be
particularly low as detected in the ESR test. In summary, the NO oxidation by ClO2 solution is also a type of AOPs.
3.2. Effect of initial pH on removal efficiency
Figure 5. Effect of initial pH on removal efficiency. Flue gas flow: 350 mL/min; NO concentration: 370 mg/m3; reaction temperature: 40 °C; mole ratio (ClO2/NO): 0.4.
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The suitable solution pH is beneficial to the free radical activity and can also reduce the equipment corrosion to a certain extent. The effect of the initial pH of the ClO2 solution on the NO removal was investigated and is shown in Fig. 5. The NO removal efficiency was slightly affected by the different pH levels of solution. With the initial pH changed from 3 to 11, the removal efficiency of NO decreased by approximately 4%. Such finding fully confirmed that the ClO2 solution has good pH adaptability for NO removal. The following reasons can explain this phenomenon: (1) The pH of the ClO2 solution itself was close to neutral, and the initial pH adjustment required only a small amount of H2SO4 and NaOH to be added. (2) Although it was unfavorable for the ·OH radical activity under alkaline conditions,
17
NaClO2 generated through disproportionation reaction (Eq.
(24)) from a small part of ClO2 and NaOH under alkaline conditions also has good oxidation performance for NO. 27 (3) When the reaction progressed, the amount of acidic ions, such as NO3− and NO2− forming in the solution had gradually increased. Accordingly, the overall pH of the solution shifted to acidity, and the ClO2 oxidation performance was improved under the low pH condition. When using the ClO2 solution for NO removal, the pH can be neutral to reduce the oxidant corrosion of the relevant equipment. 2ClO2 + 2NaOH → NaClO2 + NaClO3 + H2O
(25)
3.3. Effect of SO2 concentration on removal efficiency. 15 ACS Paragon Plus Environment
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Figure 6. Effect of SO2 concentration on removal efficiency. Flue gas flow: 350 mL/min; NO concentration: 370 mg/m3; reaction temperature: 40 °C; mole ratio (ClO2/NO): 0.5; pH of ClO2 solution: 7.
In addition to NO, SO2 is also one of the common pollutants in coal-fired flue gas. Hence, the effect of SO2 content in flue gas on the NO removal must be explored. As shown in Fig. 6, the increase in the SO2 content led to a gradual decrease in the NO removal efficiency. When the SO2 content in the flue gas varied from 0 mg/L to 800 mg/L, the NO removal efficiency decreased from 63.4% to approximately 5%. This outcome was mainly due to the presence of SO2 that consumed a portion of the oxidant (Eqs. (26)) and thereby reduced the ClO2/NO molar ratio; ClO2 preferentially oxidizes SO2 in the presence of SO2 and NO.
27
But SO2 is easily soluble in water, and the
products promote the NO2 absorption. The reactions are as follows (Eqs. ((27)–(30)): 16 ACS Paragon Plus Environment
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5SO2 + 2ClO2 + 6H2O → 5H2SO4 + 2HCl,
(26)
SO2 + H2O → HSO3− + H+,
(27)
2NO2 + H2O → HNO3 + HNO2,
(28)
2HNO2 + 2HSO3−→ 2HSO4− + N2O + H2O,
(29)
HNO2 + 2HSO3− → HADS + H2O.
(30)
3.4. Effect of NO concentration on removal efficiency
Figure 7. Effect of NO concentration on removal efficiency. Flue gas flow: 350 mL/min; reaction temperature: 40 °C; pH of ClO2 solution: 7.
The NO concentration in outlet flue gas often varies with the change of boiler load. Such NO concentration is an important index in the application. As shown in Fig. 7, the NO removal efficiency continuously decreased with the increase in the NO concentration 17 ACS Paragon Plus Environment
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in flue gas. This increase of NO concentration led to the reduction of ClO2/NO molar ratio, which is the main factor causing the decline of removal efficiency. The contact time between the gas and ClO2 solution was limited and the increase of NO molecules in the flue gas also decreased the contact time of each NO molecule with the ClO2.
3.5. Effect of flue gas flow on removal efficiency
Figure 8. Effect of flue gas flow on NO removal efficiency. NO concentration: 370 mg/m3; reaction temperature: 40 °C; mole ratio (ClO2/NO): 0.5; reaction temperature: 40 °C; pH of ClO2 solution: 7.
The variation of flue gas flow during the reaction also has a certain effect on the removal efficiency. As shown in Fig. 8, the removal efficiency was 87.1% when the flue gas flow rate was 150 mg/L. When the flue gas flow varied from 250 mg/L to 450 mg/L, the removal efficiency slightly decreased. When the flue gas flow rate was increased to 18 ACS Paragon Plus Environment
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550 mg/L, the removal efficiency dropped to 38%. This phenomenon was due to the increase of flue gas flow that reduced the NO concentration in a unit volume. Consequently, the average contact time of the NO gas molecules and ClO2 decreased, which is non-conducive to the NO removal. The flue gas increase also led to an increase in mass transfer resistance between the flue gas and the solution, which is also disadvantageous for the NO removal. Therefore, an appropriate flue gas flow must be selected for NO removal.
3.6. Reaction pathways
Figure 9. Ion chromatograph of spent ClO2 solution.
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As shown in Fig. 9, NO3− and NO2− appeared in the spent ClO2 solution after absorbing NO. This finding revealed that NO was mainly oxidized to NO3− during absorption because the NO2− peaks are not as apparent as the NO3− peak. Analysis of the reaction products in the solution indicated that the oxidative removal mechanism of NO was as follows: (1)
Generation of the related oxidants, as shown in Eqs. (18)–(23)
(2)
NO removal (Eqs. (31)–(38)): 17, 25, 26, 28, 29
5NO + 2ClO2 + H2O ↔ 5NO2 + 2HCl
(31)
5NO2 + ClO2 + 3H2O ↔ 5HNO3 + HCl
(32)
·OH + NO ↔ HNO2
(33)
·OH + NO ↔ NO2 + H·
(34)
·OH + NO2 ↔ HNO3
(35)
NO + ClO· → NO2 + Cl
(36)
ClO2− + 2NO ↔ 2NO2 + Cl−
(37)
O2−· + 2NO → 2NO2
(38)
(3) NO2 absorption in Ca(OH)2 solution: 2NO2 + H2O ↔ HNO3 + HNO2
(39)
Ca (OH) 2 + 2HNO3 → Ca (NO3)2 + 2H2O
(40)
Ca (OH) 2 + 2HNO2 → Ca (NO2)2 + 2H2O.
(41)
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4. Conclusions In this work, NO removal from flue gas using ClO2 solution was conducted. The ClO2 solution can effectively oxidize NO in the flue gas at a low molar ratio (ClO2/NO). During NO oxidation, ·OH radicals generated by the ClO2 solution, which greatly promoted the oxidation reaction. When the reaction temperature increased from 20 °C to 80 °C, the efficiency was also improved. Approximately 80 °C is a suitable reaction temperature, which is also the reaction temperature in the alkali-absorption tower. The ClO2 solution also has good pH adaptability. The NO in the flue gas was mainly converted to NO3−, followed by NO2−. The reaction mechanism between NO and the oxidizing groups in the solution was proposed. The generated ·OH radicals in the ClO2 solution require neither catalyst for catalyzing nor UV light for energy boosting compared with other oxidizing agents in AOPS. Accordingly, the complicatedness of technology and the running cost are reduced to a great extent. Hence, the prospect for its application is promising.
AUTHOR INFORMATION Corresponding Author *Tel.
/fax: + 86 351 6010281 21 ACS Paragon Plus Environment
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[email protected] ORCID Suxia Ma: 0000-0001-7238-4197 NOTES The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the NSFC-Shanxi Coal-based Low Carbon Joint Fund Key Project in Shanxi Province, China (U1710251). We especially thank Shanxi Department of Science and Technology. REFERENCES [1] M. Hallquist, J. Munthe, M.; Hu, T.Wang, C.K. Chan, J. Gao, J. Boman, S. Guo, Å.M. Hallquist, J. Mellqvist, J. Moldanova, R.K. Pathak, J.B.C. Pettersson, H. Pleijel, D. Simpson, M. Thynell. Photochemical smog in China: scientific challenges and implications for air-quality policies, National Science Review, 3(4) (2016) 401. [2] T.J. Wang, K.S. Lam, M. Xie, X.M. Wang, G. Carmichael, Y.S. Li, Integrated studies of a photochemical smog episode in Hong Kong and regional transport in the Pearl River Delta of China, Tellus B: Chemical and Physical Meteorology, 58 (2017) 31-40. [3] L.F. Hao, D.S. Xing, R.L. Cao, The environment influences evaluation on SCR flue-gas, Applied Mechanics and Materials, 71-78 (2011) 3156-3159. [4] Z.Y. Liang, X.Q. Ma, H. Lin, Y.T. Tang, The energy consumption and environmental impacts of SCR technology in China, Applied Energy, 88 (2011) 1120-1129. 22 ACS Paragon Plus Environment
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