Catalytic Oxidation Removal of Sulfur Dioxide by Ozone in the

Aug 29, 2012 - Mei-Yuan Nie, Chao Gu, Kai-Li Zhong, and Yun-Jin Fang*. State Key Laboratory of Chemical Engineering, East China University of Science ...
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Catalytic Oxidation Removal of Sulfur Dioxide by Ozone in the Presence of Metallic Ions Mei-Yuan Nie, Chao Gu, Kai-Li Zhong, and Yun-Jin Fang* State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, People’s Republic of China ABSTRACT: Desulfurization of flue gas by ozonation aided by the catalysis of a metallic ion was investigated experimentally. The results showed that the removal efficiency of SO2 was only 30% at a 1:1 molar ratio of O3/SO2 without the addition of a catalyst. However, the efficiency was substantially increased when a metallic ion was added to the absorption solution at the same ratio of O3/SO2. In particular, the performance was optimal using a Mn2+ catalyst. Under the optimal condition given as the absorption liquid temperature of 20 °C, gas flow rate of 10 L/min, molar ratio of 1.0 for O3/SO2, and Mn2+ concentration of 6.4 × 10−4 mol/L, the removal efficiency of SO2 was 99%. A reaction mechanism of SO2 with ozone in the liquid-phase oxidation catalyzed by Mn2+ was proposed. The advantage of ozonation desulfurization technology is that SO2 is directly transformed into a valuable product, sulfuric acid, which provides favorable environmental and economic benefit.

1. INTRODUCTION With the shortage of petroleum resources, coal is becoming a major source of energy supply, especially in China. However, the emissions of nitrogen oxides (NOx) and sulfur dioxide (SO2) caused by coal combustion result in serious air pollution and threaten industrial production and human life. Hence, great efforts have been made to develop the technologies of flue gas desulfurization (FGD) by coal-fired power plants. At present, limestone− gypsum is the most widely used FGD technology in the industry. However, this process has some shortcomings, such as the high investment and the secondary pollution of the desulfurization products.1 There are also other kinds of wet desulfurization and denitrification processes, for instance, strong oxidants, plasma, and wet complex metallic ions of Fe and Mn2,3 were used to desulfurize and denitrate simultaneously. The strong oxidants include KMnO4,4 ClO2,5 NaClO2,6 etc. Although NaClO2 and ClO2 have high efficiencies for the removal of SO2, NOx, and Hg0, there are many problems, such as the acidic waste liquid, equipment corrosion, high operating costs, etc. In recent years, O3, as one kind of clean and strong oxidant, has been widely used in water treatment and is found to be able to remove SO2, NOx, and Hg0 in the disposal of flue gas.7−12 When SO2 dissolves in water, SO2·H2O and HSO3− are formed and SO2 can also be oxidized into higher oxidative species,13 which can break the equilibrium between the dissolution of SO2 and achieve the removal of SO2. O3 may be used as an oxidant in the technology of simultaneous desulfurization and denitrification. In most practical flue gas, NO is the predominant nitrogen species of NOx. Unlike NO, NO2, NO3, N2O5, etc. are highly soluble in water, and these soluble species can be captured in downstream SO2 removal equipment, such as a wet flue gas desulfurization (WFGD) system. Therefore, the oxidation of NO is the first step of the simultaneous removal process. When O3 is injected into the removal equipment, NO in the flue gas can be oxidized into NO2, NO3, and N2O5, as shown in the following reactions:7,8 NO + O3 = NO2 + O2

(2)

NO2 + NO3 = N2O5

(3)

N2O5 + H 2O = 2HNO3

(4)

HNO3 + NaOH = NaNO3 + H 2O

(5)

At present, the cost of ozone is expensive because of the production technology. Therefore, reducing O3 consumption in the oxidation process of SO2 or NO is essential to reduce the operating cost. Halstead et al.14 found that metallic ions of Fe3+ and Mn2+ can promote the aqueous oxidation of sulfites by O3. In the water treatment15 and atmospheric chemistry,16 a lot of transition-metal ions can catalyze the oxidation of sulfites, such as Co2+, Fe3+, Mn2+, etc. Among them, Fe3+ and Mn2+ have the best catalytic performance.17,18 In this paper, referring to the advanced oxidation technology of ozone in water treatment as well as to some atmospheric chemistry research, SO2 removal was investigated using the liquid-phase oxidation of ozone under catalysis of metallic ions in the desulfurization process. In comparison to the previous work in the literature, in this paper, the ozonation desulfurization technology directly converts SO2 into a high-value product, sulfuric acid, rather than a low-value sulfate product, such as gypsum. For example, there is an excess of gypsum byproduct from FGD systems at coal-fired power plants every year in China, which cannot be effectively used and which can cause the secondary pollution to groundwater,19 because natural gypsum resources are abundant in China and the quality of FGD gypsum is inferior to natural gypsum. Therefore, the desulfurization technology by ozone in this paper is favorable in view of the environmental and economic benefits.

2. EXPERIMENTAL SECTION 2.1. Simulated Gas. The 5% (v/v) SO2 in nitrogen was supplied from a cylinder and was further diluted with N2 to the desired Received: December 29, 2011 Revised: August 21, 2012 Published: August 29, 2012

(1) © 2012 American Chemical Society

NO + O3 = NO3 + O2

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Ozone Technology Service Company), where O2 (0.1−0.6 L/min) passed through the flow meter and then O3 was produced through discharging electricity. The ozone distributing heads with small pores were located in the flask.

concentrations before being fed to the absorber. The feed concentration of SO2 ranged from 600 to 1500 ppm (by volume), which were the common concentration ranges of SO2 in the flue gas. 2.2. Material and Reagent. Manganese sulfate [chemically pure (CP)], iron(II) sulfate [analytical reagent (AR)], nickel(II) chloride hexahydrate (CP), cupric sulfate anhydrous (CP), cobalt(II) chloride hexahydrate (AR), and zinc nitrate hexahydrate (AR) were purchased from commercial suppliers. 2.3. Analytical Method. The initial concentration of SO2 in the simulated flue gas was monitored by the sulfur dioxide analyzer manufactured by TESTO Company (Germany, model 325-I, electrochemical method). The concentration of ozone was measured by the standard iodine method (CJ/T 3028.2-1994). It was observed first in this paper’s experiments that the existence of the ozone in the flue gas could affect the measurement of the sulfur dioxide concentration by the electrochemical method. The measured SO2 concentration in the presence of ozone in the vent gas of the absorption tower would be lower, and the SO2 removal efficiency would be higher than the actual value. Therefore, a sulfur mass balance was used in the determination of the SO2 removal. The concentration of sulfuric acid was measured by chemical titration to determine the S moles in the absorption solution after the absorption of SO2 for a period of time. Then, the average SO2 removal efficiency in the desulfurization process was calculated. 2.4. Absorption Process. A self-made experimental system was adopted to carry out the experiments. Experiments were performed in a packed (spiral glass packing) column (35 mm inner diameter and 1000 mm height) tower. The schematic diagram of the experimental apparatus is shown in Figure 1. Experimental facility consisted of an

3. RESULTS AND DISCUSSION 3.1. Measurement of the Ozone Concentration. The amount of ozone produced by the ozone generator was related to the input amount of oxygen; therefore, the accurate amount of ozone in the flue gas could be determined on the basis of the standard curve of oxygen flow with ozone (as shown in Figure 2).

Figure 2. Curve of the ozone concentration.

The standard iodine method (CJ/T 3028.2-1994) was used to measure the amount of ozone in the solution, and then the ozone concentration was calculated according to the flow rate of flue gas. The measuring principles are as follows: O3 + 2KI + H 2O → O2 + I 2 + 2KOH

(6)

I 2 + 2Na 2S2 O3 → 2NaI + Na 2S4 O6

(7)

According to the experimental condition, after converting concentration of O3 was calculated as CO3 =

24CV V0

(mg/L)

(8)

where V represents the standard aqueous consumption of Na2S2O3 (mL), C is the standard aqueous concentration of Na2S2O3 (mol/L), and V0 is the total volume of the oxygen in 10 min through the measuring instrument (L). In the experiments, C = 0.1008 mol/L was used. The relationship of the ozone concentration with oxygen flow is shown in Figure 2. 3.2. Effect of Different Catalysts on the Removal Efficiency of Sulfur Dioxide. At room temperature and the catalyst concentration of 5.16 × 10−4 mol/L, the performance of different catalysts on the removal of SO2 in the simulated flue gas was investigated. The removal efficiencies of sulfur dioxide under different catalysts are shown in Figure 3. As seen from Figure 3, the SO2 removal efficiency is only about 30% without ionic metal catalysts, while the SO2 removal efficiency is increased obviously under catalysis of metallic ions. Among these catalysts, the catalytic effect of manganese is relatively better than the other metals. The SO2 removal efficiency can reach 94.47% with the addition of the Mn2+ catalyst. In comparison to manganese, other metal catalysts have less desulfurization effect and the removal efficiencies using them

Figure 1. Experimental apparatus for removing SO2: 1, cylinder of SO2; 2, cylinder of N2; 3, cylinder of O2; 4, flow meter; 5, mixing bottle; 6, ozone generator; 7, gas valve; 8, flue gas analyzer; 9 and 12, thermometer; 10, absorption tower; 11, tower bottoms; and 13, peristaltic pump. absorption tower, a flask (500 mL), and a constant flow pump. The temperature of absorption liquid was controlled by an automatic feedback controller in the heater. The absorber was operated with a continuous feed of N2 (8−20 L/min), SO2 (concentration of 5%, 0.1−0.3 L/min) at the bottom, and a continuous feed (41 mL/min) of scrubbing solution over the top. Experiments were carried out under atmospheric pressure. O3 was produced by an O3 generator (manufactured by Zhengzhou Zhongren 5591

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Many mechanisms were reported on the oxidation of SIV species catalyzed by transition-metal ions. Three main reaction mechanisms are proposed: (1) non-radical mechanism, (2) free-radical mechanism, and (3) a combination of non-radical and free-radical mechanisms. One such free-radical mechanism is originally proposed by Bassett and Parker20 in 1951 for the manganese catalysis. The mechanism favors the formation of metal−sulfur(IV) complexes followed by binding of dioxygen and subsequent intermolecular electron transfer from coordinated sulfur(IV) to oxygen, mediated by the metal ion. On basis of the mechanism proposed by Bassett and Parker, we supposed the process of SO2 liquid-phase ozonation catalyzed by Mn2+ as follows. The following four equilibria are present in SO2 solution: O3(g) ⇌ O3(l)

Figure 3. SO2 removal efficiency with different catalysts (temperature, 20 °C; gas flow, 10 L/min; SO2 concentration, 600 ppm; liquid flow rate, 41 mL/min; and molar ratio of O3/SO2, 1:1).

are all less than 62%. The color of the absorption liquid is changed from initial colorless to brown and to purple during the removal of SO2 with manganese; therefore, the manganese ion can be changed into the different chemical valence by ozone oxidation and SO2 reduction in the process of the absorption. When the catalytic effect of different metal catalysts is compared, manganese has the better catalytic performance than the other metal catalysts. Therefore, Mn2+ is chosen as the catalyst for SO2 removal from the flue gas in the following experiments. 3.3. Effect of Mn2+ Concentrations on Sulfur Dioxide Removal. In the reaction, a 1:1 molar ratio of O3/SO2 with a gas flow rate of 10 L/min was introduced into the reaction system. The effect of different Mn2+ concentrations on sulfur dioxide removal efficiency was investigated at 20 °C. The sulfur dioxide removal efficiencies with different Mn2+ concentrations are shown in Figure 4.

(9)

SO2 (g) + H 2O ⇌ SO2 ·H 2O

(10)

SO2 ·H 2O ⇌ H+ + HSO3−

(11)

HSO3− ⇌ H+ + SO32 −

(12)

SO32−

2+

In Mn aqueous solution, a intermediate complex will form in solution and promotes the oxidative reaction of sulfur(IV). 2Mn 2 + ⇌ Mn2 4 +

(13)

Mn2 4 + + SO32 − ⇌ [Mn2(SO3)]2 +

(14)

[Mn2(SO3)]2 + + O3 ⇌ [Mn2(SO3)O3]2 +

(15)

[Mn2(SO3)O3]2 + → 2Mn 2 + + SO4 2 − + O2

(16)

According to the above reaction equations, the removal efficiency of sulfur dioxide is related to the concentration of Mn2+. A high concentration of Mn2+ favors the formation of a complex, [Mn2(SO3)]2+, which is easily oxidized by ozone; hence, a high Mn2+ concentration in aqueous solution can enhance the removal efficiency of sulfur dioxide. It can also be seen from the reaction equations 15 and 16 that only one oxygen atom in the ozone molecule participates in the oxidative reaction and the other two oxygen atoms are released in the form of an O2 molecule in the reaction. Therefore, theoretically, the oxidation of 1 mol of SIV requires 1 mol of ozone. 3.4. Effect of Different Molar Ratios of O3/SO2 on Sulfur Dioxide Removal. At the gas flow of 10 L/min and the Mn2+ concentration of 5.16 × 10−4 mol/L, the effects of the O3/SO2 molar ratio on the SO2 removal efficiency were investigated at 20 °C. The experimental results are reported in Figure 5. As shown in Figure 5, the SO2 removal efficiency increases with the increase of the molar ratio of O3/SO2. When the molar ratio of O3/SO2 is 0.2, the SO2 removal efficiency is only about 60%. As it increases to 0.5, the SO2 removal efficiency can arrive at 80%. Continuing to raise the molar ratio of O3/SO2 to 1.0, an efficiency of 99% is achieved. This can also be explained from eqs 15 and 16, and it is consistent with the theoretical analysis. According to desulfurization emission standards, the SO2 removal efficiency of 90−95% is required. To reach this efficiency, the molar ratio of O3/SO2 of about 0.6−0.8 is sufficient. It does not require 1.0, and thus, it can reduce the consumption of O3. The theoretical power consumption of the ozone generator is 0.82 kW h kg−1 of O3,21 but the typical commercial corona discharge ozone generator requires about 6−8 kW h kg−1 of O3. Thus, there is much space for the development of the ozone

Figure 4. Sulfur dioxide removal efficiency at different concentrations of Mn2+.

As shown in Figure 4, when the Mn2+ concentration is less than 5.16 × 10−4 mol/L, the SO2 removal efficiency gradually increases with the increase of the Mn2+ concentration, and once the concentration reaches 5.16 × 10−4 mol/L, the SO2 removal efficiency approaches an asymptotic value. The removal efficiency of SO2 is quite low at the low concentration of Mn2+ of 3.87 × 10−4 mol/L. However, when the Mn2+ concentration is 6.4 × 10−4 mol/L, the SO2 removal efficiency reaches 99%. 5592

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At low Mn2+ concentrations, the SO2 removal efficiencies decrease rapidly with the rise of sulfuric acid concentrations (Figure 6). At a Mn2+ concentration of 5.16 × 10−4 mol/L, the SO2 removal efficiencies are in the range of 94.47−60% when the sulfuric acid concentrations are in the range of 0−2.0 mol/L. However, SO2 removal efficiencies are much less dependent upon the Mn2+ concentration at high concentrations of Mn2+. For example, at a high concentration of Mn2+ of 1.29 × 10−3 mol/L, the SO2 removal efficiencies decrease only from 99 to 91.7% when the sulfuric acid concentration is in the range of 0−2.0 mol/L. On the basis of the above eqs 9−16, the proposed mechanism of SO2 liquid-phase ozonation catalyzed by Mn2+, it can be known that there are two effects of the initial sulfuric acid concentration on the SO2 removal efficiency: the pH value of the aqueous solution and mass transport. On the basis of the literature,23,24 the decomposition rate of ozone in aqueous solution is affected by the pH value. When the pH value is greater than 6, the decomposition rate of ozone is very fast. As the pH value varies from 3 to 6, the decomposition rate of ozone decreases with the decrease of the pH value; in other words, the stability of ozone in the aqueous solution increases. However, when the pH value is less than 2, the decomposition rate of ozone speeds up. In this experiment, the pH value of the aqueous solution is less than 2 when the initial concentration of sulfuric acid is higher than 0.005 mol/L. Thus, while the concentration of sulfuric acid increases, the decomposition rate of ozone speeds up and the amount of ozone, which participates in the oxidative reaction, decreases slightly. Therefore, the removal efficiency of SO2 will decrease slightly because of the decomposition rate of ozone increasing slightly with the increase of the sulfuric acid concentration. Schwartz and Freiberg25 proposed seven steps that occurred when SO2 reacted with oxygen to form sulfate in an aqueous solution. Similarity, the sequence of steps of SO2 reacting with O3 to form sulfuric acid can be supported to consist of the following processes: (a) transport of SO2 (and O3) within the gas phase to the gas−liquid interface, (b) establishment of local solubility equilibrium (Henry’s law) at the interface, (c) hydrolysis of SO2 and partial ionization to HSO3− and SO32−, (d) transport of these SIV species, H+, and O3 within the aqueous phase, (e) oxidation of the SIV precursors to SVI, (f) sulfuric acid generated on the interface diffusing to the bulk of liquid, with subsequent adjustment of the ionization equilibrium, and (g) mass transport resulting from the concentration gradients introduced by steps e and f. In the seven steps of liquid-phase oxidation of SO2, the first step of the process is likely to be the key step. In the presence of a catalyst, SO2 hydration and ionization take place more easily in aqueous solution. According to the two-film theory, SO2 absorption is a gas−liquid-film-controlling process and the absorption of O3 is a liquid-film-controlling process. To enhance the removal efficiency of SO2, the sulfuric acid generated on the interface must diffuse to the bulk liquid quickly. However, the existence of initial sulfuric acid in the bulk liquid makes a concentration gradient form between the bulk liquid and the liquid film. The concentration gradient hinders the diffusion of sulfuric acid generated at the interface to the bulk liquid. According to eqs 10−12, the high concentration of sulfuric acid in the liquid film, namely, the high H+ concentration, will reduce the concentration of SO2·H2O and the dissolution of SO2 will decrease. Therefore, the removal efficiency of SO2 decreases.

Figure 5. Effect of different molar ratios of O3/SO2 on sulfur dioxide removal (gas flow, 10 L/min; Mn2+ concentration, 5.16 × 10−4 mol/L; SO2 concentration, 600 ppm; and liquid flow rate, 41 mL/min).

production technology. At present, the electricity price for desulfurization is about $4.76 for each 100 kW h in the power plant of China. On the basis of 95% removal efficiency of SO2, it needs about 570 kg of ozone or 4560 kW h of electricity to remove 1000 kg of SO2 in flue gas; therefore, the expense of power is about $217.0/ton of SO2. However, the removed SO2 can produce 2.9 tons of 50 wt % sulfuric acid, and the value of 2.9 tons of sulfuric acid is about $230. Thus, the total desulfurization cost is only about $50/ton of SO2, which involves other operating costs, and is much cheaper than $230/ton of SO2 of the lime gypsum method.22 3.5. Effect of the Sulfuric Acid Concentration on Sulfur Dioxide Removal. Because SO2 in the flue gas is removed by absorption in the solution to produce sulfuric acid, is there any effect of the initial sulfuric acid concentration in the solution on the removal of SO2? The effects of the sulfuric acid concentration on the SO2 removal efficiency were studied under the different concentrations of Mn2+. Figure 6 describes

Figure 6. Effect of sulfuric acid concentrations on SO2 removal efficiencies [temperature, 20 °C; SO2 concentration, 600 ppm; liquid flow rate, 41 mL/min; molar ratio of O3/SO2, 1; Mn2+, (■) 5.16 × 10−4 mol/L, (●) 7.74 × 10−4 mol/L, (▲) 1.03 × 10−3 mol/L, and (□) 1.29 × 10−3 mol/L].

the SO2 removal efficiencies as a function of different sulfuric acid concentrations. 5593

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Meanwhile, the SO2 removal efficiency increases with the increase of the Mn2+ concentration. As shown in the above eqs 13−15, the increase of the Mn2+ concentration will lead to the increase of the [Mn2(SO3)]2+ concentration. The equilibrium will move in the direction of the forward reaction with the increase of the [Mn2(SO3)]2+ concentration, which is beneficial for the absorption of SO2. Hence, the SO2 removal efficiency can increase with the increase of the Mn2+ concentration, and this is very important to obtain the high concentration sulfuric acid product, meanwhile ensuring the removal efficiency of SO2. 3.6. Effect of the Absorption Liquid Temperature on Sulfur Dioxide Removal. In the presence of the manganese catalyst, the effects of different temperatures of absorption liquid on the SO2 removal efficiencies were considered. The relationship between the SO2 removal efficiency and absorption liquid temperature is depicted in Figure 7.

Figure 8. Effect of the gas flow rate on the SO2 removal efficiency (temperature, 20 °C; catalyst concentration, 5.16 × 10−4 mol/L; SO2 concentration, 600 ppm; liquid flow rate, 41 mL/min; and molar ratio of O3/SO2, 1:1).

therefore, the amount of SO2 oxidized in the liquid film also increases little. In this case, the SO2 removal efficiency will decrease with the increase of the gas flow. Therefore, the ozone mass-transfer process is the key step for the liquid-phase oxidation of SO2. To increase the SO2 removal efficiency in the case of the gas flow rate increasing, it is necessary to increase the solubility of ozone in the absorption solution, and the increase of ozone solubility can be achieved by increasing the gas−liquid contact area.

4. CONCLUSION In this study, an effective method, removal of SO2 from flue gas by ozonation in the presence of a metallic ion catalyst, was developed. It is a simple process in which SO2 can directly transform into a valuable product, sulfuric acid, and it does not generate secondary pollution. The desulfurization of flue gas by ozonation must be catalyzed by a metallic ion, especially Mn2+. Otherwise, in the absence of catalyst, the removal efficiency of SO2 is only about 30%, even if the molar ratio of 1.0 of O3/SO2 is used. The SO2 removal efficiency increases with the increase of the Mn2+ concentration and the molar ratio of O3/SO2. When the concentration of Mn2+ increases to 6.4 × 10−4 mol/L and the molar ratio of O3/SO2 is 1:1, the SO2 removal efficiency reaches 99%. The catalytic reaction mechanism of SO2 liquid-phase ozonation catalyzed by Mn2+ is also proposed, and the ozone mass-transfer process was found to be the key step for the liquid-phase oxidation of SO2. The process can be operated in a coal-fired power plant at typical flue gas temperatures of 120−150 °C. In the WFGD process, the flue gas temperature can be cooled by the evaporation of water in the absorption liquid and the exit gas temperature will be decreased to 55−70 °C. The temperature of the absorption liquid is about 50 °C; in this case, the SO2 removal efficiency by ozonation is still able to reach 90% or more. In this method, the sulfuric acid product at a high concentration can be produced, meanwhile ensuring the removal efficiency of SO2 using a suitable Mn2+ concentration, and the favorable environmental and economic benefit is obtained.

Figure 7. Effect of the absorption liquid temperature on SO2 removal (gas flow, 10 L/min; Mn2+ concentration, 5.16 × 10−4 mol/L; SO2 concentration, 600 ppm; liquid flow rate, 41 mL/min; and molar ratio of O3/SO2, 1:1).

As indicated in Figure 7, the SO2 removal efficiency gradually decreases with the rise of the absorption liquid temperature. As we know, the desulfurization process of ozone catalytic oxidation is a chemical absorption process. The higher absorption liquid temperature improves the oxidative reaction rate of SO2 but also promotes the decomposition of ozone. Meanwhile, a high temperature can also reduce the solubility of SO2 in the liquid phase; therefore, the higher absorption liquid temperature is disadvantageous to the removal of SO2. 3.7. Effect of the Gas Flow Rate on Sulfur Dioxide Removal. The gas flow rate will affect the SO2 removal efficiency, and results are presented in Figure 8. As shown in Figure 8, with the increase of the gas flow rate, the SO2 removal efficiency decreases rapidly. The SO2 removal efficiency decreases from 94.47 to 50% when the gas flow rate increases from 10 to 30 L/min. The effect of the gas flow rate on SO2 removal can be explained as follows. The solubility of SO2 in water is moderate, but the solubility of ozone in water is low. The mass-transfer rate of ozone from gas to liquid is controlled by liquid-film resistance according to the two-film theory. In the absorption process, the amount of SO2 will increase when the gas flow rate increases. Although the molar ratio of SO2 to ozone does not change, the amount of ozone transferred to the absorption liquid from gas increases little with an increasing gas flow rate;

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 5594

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work is supported by Shanghai Sanqing Environmental Protection Technology Co., Ltd. REFERENCES

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