Investigation on the Removal of NO from SO2-Containing Simulated

Sep 4, 2012 - Yi Xing , Liuliu Li , Pei Lu , Jiansheng Cui , Qianli Li , Bojun Yan , Bo Jiang ... Anchao Zhang , Chengwei Li , Weibo Xing , Jun Song ,...
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Investigation on the Removal of NO from SO2‑Containing Simulated Flue Gas by an Ultraviolet/Fenton-Like Reaction Yangxian Liu,*,†,‡ Jun Zhang,§ Jianfeng Pan,*,† and Aikun Tang† †

School of Energy and Power Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, People’s Republic of China Key Laboratory of Water and Air Pollution Control of Guangdong Province, Guangzhou, Guangdong 510655, People’s Republic of China § School of Energy and Environment, Southeast University, Nanjing, Jiangsu 210096, People’s Republic of China ‡

ABSTRACT: The removal process of NO from SO2-containing simulated flue gas using an ultraviolet (UV)/Fenton-like reaction in a photochemical reactor was investigated. The effects of several operating parameters, such as the H2O2 concentration, Cu2+ concentration, solution pH, NO concentration, solution temperature, gas flow, SO2 concentration, and O2 concentration on NO removal were studied using a single-factor method. The liquid anions were measured by ion chromatography, and the material balances for NO and SO2 were also calculated. The results show that, with the increase in the Cu2+ concentration, NO removal efficiency significantly increases. With the increase in the H2O2 concentration, NO removal efficiency increases, but the changes gradually become smaller. NO removal efficiency greatly reduces with an increasing gas flow and NO concentration. An increasing solution temperature or SO2 concentration slightly decreases NO removal efficiency. An increase in the O2 concentration can promote the removal of NO. The anions in the liquid phase are mainly SO42− and NO3−. The calculated values of SO42− and NO3− are overall in agreement with the measured values.

1. INTRODUCTION The emissions of SO 2 and NOx have been a major environmental concern because of their hazardous effects on human health and ecosystems. Thus far, SO2 and NOx are mainly controlled by separately installing flue gas desulfurization and denitrification equipment at the rear of a coal-fired boiler, but this approach has not only large and complex systems but also high investment and operating costs. Recently, considerable attention has been given to the simultaneous removal technologies of SO2 and NOx from flue gas, considering the low investment and operating costs and the relatively smaller space for equipment.1,2 According to the status of the reaction process and reaction products, simultaneous removal technologies of SO2 and NOx can be divided into two classes: dry and wet.3,4 The dry technologies mainly include plasma removal, adsorption removal, photocatalytic oxidation, and ozonation oxidation.5−8 The wet technologies mainly include chemical oxidation, complex absorption, and reduction absorption.9−11 These technologies have shown good development and application prospects, but most of them are still in the laboratory stage because of existing technical or economic problems.12,13 Therefore, continuing to develop new and efficient simultaneous removal technologies of NOx and SO2 undoubtedly has important theoretical and practical significance. Wet flue gas desulfurization processes have been widely applied because of simple process and high desulfurization efficiency. However, because of the low solubility of NO in water, traditional wet flue gas desulfurization equipment is unable to achieve the simultaneous removal of NOx and SO2.3,4 Thus, studying some effective measures to strengthen the absorption of NO has become one of the most attractive research directions in flue gas pollution control filed. In the past © 2012 American Chemical Society

few decades, many chemical oxidants, such as KMnO4, NaClO2, Na2S2O8, and H2O2, have been used to strengthen the absorption of NO using wet scrubbing,3,4,14−17 but thus far, these technologies are still in the laboratory exploration stage because of high use cost, serious secondary pollution, or low oxidation efficiency.12,13 An ultraviolet (UV)/Fenton reaction and an UV/Fenton-like reaction can produce strong oxidative ·OH free radicals to oxidize multi-pollutants, and it has many advantages, such as strong oxidation, simple and reliable equipment, and environmental protection, which have been widely used in the wastewater treatment field.18−23 However, until now, using an UV/Fenton reaction and an UV/Fenton-like reaction for the oxidation−absorption of NO from flue gas using wet scrubbing has rarely been reported in the literature. Because the chain reactions caused by adding Cu2+ are more moderate and controlled and have much higher effective use of H2O2 than adding Fe2+ or Fe3+,22 in this paper, the oxidation−absorption of NO from SO2-containing simulated flue gas using an UV/ Fenton-like (Cu2+/H2O2) reaction was studied in a lab-scale photochemical reactor. The effects of several operating parameters, such as the H2O2 concentration, Cu2+ concentration, solution pH, NO concentration, solution temperature, gas flow, SO2 concentration, and O2 concentration, on NO removal were investigated using a single-factor method. The anions in the liquid phase were measured by ion chromatography (IC), and the material balances for NO and SO2 were calculated. The reaction pathways of NO removal using an UV/ Fenton-like reaction were also discussed. The results will Received: January 7, 2012 Revised: August 31, 2012 Published: September 4, 2012 5430

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reaction. The simulated flue gas was dried using the dryer (22) and then enters into the flue gas analyzer (23) to determine the outlet concentrations of gas components. Each experimental run is kept for 20 min, and the concentration of pollutants in the gas outlet (b) was recorded every minute. The average concentrations within 20 min are used as the outlet concentrations of pollutants. The simulated flue gas was further treated by the exhaust pipe (24) and then discharged. The anions in solution were determined by IC (792 Basic IC, Metrohm, Switzerland) under the chromatographic conditions: anion dual 2 anion column, aluent, 1.0 mmol/L Na2CO3 + 1.5 mmol/L NaHCO3; flow rate, 0.80 mL/min; injection volume, 25 μL; column temperature, 303 K; and automatic regeneration suppression system, H2O and 60 mmol of H2SO4. 2.3. Removal Efficiency. The concentrations measured by the bypass (B) are the inlet concentrations of pollutants. The average concentrations within 20 min measured by the gas outlet (b) are the outlet concentrations of pollutants. The removal efficiency of pollutants is calculated by the following eq 1:

provide some guidance for the industrial applications of this technology.

2. EXPERIMENTAL SECTION 2.1. Experimental Apparatus. As shown in Figure 1, the experimental apparatus consists of three parts, mainly including a

Figure 1. Schematic diagram of the experimental system: (1−4) N2, NO, SO2, and O2 cylinders, (5−9) rotameters, (10) gas mixing tank, (11−12) gas valves, (13) constant temperature water bath, (14) bubble column reactor, (15) jacket heat exchanger, (16) mercury thermometer, (17) rubber plug, (18) quartz tube, (19) UV lamp, (20) gas distributor, (21) cooling circulating pump, (22) gas dryer, (23) flue gas analyzer, (24) exhaust pipe, (a) primary-gas path, (b) gas bypass, (a) gas inlet, (b) gas outlet, (c) cooling water inlet, (d) cooling water outlet, and (e) sampling outlet.

η=

C in − Cout × 100% C in

(1)

where η is the removal efficiency of pollutants (%), Cin is the inlet concentration of pollutants (ppm), and Cout is the outlet concentration of pollutants (ppm).

3. RESULTS AND DISCUSSION 3.1. Comparison of Different Reaction Systems. The comparative experiments in different reaction systems were carried out, and the results are shown in Figure 2. It can be seen

simulated flue gas system, a photochemical reactor, and an analytical system. The photochemical reactor consists of a bubble column reactor (14), a jacket heat exchanger (15), a mercury thermometer (16), a rubber plug (17), a quartz tube (18), an UV lamp (19), and a gas distributor (20). The bubble column reactor (14) (height, 40 cm; inside diameter, 8 cm) is made by Perspex. The gas distributor (20) (average pore size, 30 μm) is installed at the bottom of the bubble column reactor (14) to distribute simulated flue gas. The UV lamp (19) (PL-L36, wavelength, 254 nm), as an excitation light source, is placed in the bubble column reactor (14). A constant temperature water bath (13) (DCW-1015, Ningbo Jiangnan Instrument Factory) with a cooling circulating pump (21) is used to control the solution temperature. The solution temperature is measured using a mercury thermometer (16). The gas dryer (22) is used to dry the simulated flue gas. The flue gas analyzer (23) (MRU Vario Plus, Germany) is used to determinate the concentrations of pollutants. The exhaust pipe (24) is used to absorb the residual pollutants in exhaust to protect the environment. 2.2. Experimental Procedures. The H2O2 solutions were prepared using the commercial 30% H2O2 solution [Shanghai Chemical Reagent Co.; analytical reagent (AR)] and the deionized water. The volume of H2O2 solution used for each experiment was 600 mL. Cu2+ was supplied by adding CuSO4 (Nanjing Chemical Reagent Co.; AR), and SO42− was supplied by adding Na2SO4 (Nanjing Chemical Reagent Co.; AR). According to the required Cu2+ or SO42− concentration, CuSO4 or Na2SO4 was weighed and added to the H2O2 solution prepared. The pH value of solution was adjusted by adding 0.5 mol/L HCl and 0.5 mol/L NaOH solutions and was determinated using an acidimeter (Shanghai Leici Instrument Co.; PHB-3). Four kinds of gases, N2, O2, SO2, and NO (Nanjing Specialty Gas Production Plants; high-purity gases), were used to make the simulated flue gas. The flows of simulated flue gas and the concentrations of pollutants were regulated using the rotameters. The inlet concentrations of pollutants were measured using the flue gas analyzer through the gas bypass B. The solution temperatures are adjusted to the temperatures required by the constant temperature water bath (13) and the mercury thermometer (16). When the solution temperatures are kept stable, first opening the valve (11) and closing the valve (12) and then turning on the UV lamp (19), the simulated flue gas enters the bubble column reactor (14) through the primary road (A) to make a gas−liquid absorption

Figure 2. Comparison of different reaction systems. Conditions: SO2 concentration, 1500 ppm; NO concentration, 400 ppm; O 2 concentration, 6.0%; gas flow, 800 mL/min; solution temperature, 298 K; solution pH, 3.2; Cu2+ concentration, 0.007 mol/L; H2O2 concentration, 1.5 mol/L; and UV radiation intensity, 0.012 W/mL.

that NO can achieve 13.1, 28.5, 52.0, and 68.2% removal efficiencies in H2O2 (B), Cu2+/H2O2 (C), UV/H2O2 (D), and an UV/Fenton-like reaction (E), respectively, but even little NO is removed under the sole UV radiation (A), showing that an UV/Fenton-like reaction has the strongest oxidation for NO. In addition, similarly, little SO2 is removed under the sole UV radiation (F), but SO2 is almost completely removed in H2O2 (G), Cu2+/H2O2 (G), UV/H2O2 (G), and an UV/ Fenton-like reaction (G) because of its high solubility in water. Thus, the following contents in this paper mainly focus on the removal of NO using an UV/Fenton-like reaction. 3.2. Effects of Cu2+ and SO42− Concentrations on NO Removal Efficiency. The effects of Cu 2+ and SO 4 2− concentrations on NO removal efficiencies were studied, and the results are shown in Figure 3. It can be seen that when the Cu2+ concentration increases from 0 to 0.01 mol/L, the NO removal efficiency significantly increases from 52.8 to 71.9%. However, when the SO42− concentration increases from 0 to 0.01 mol/L, the NO removal efficiency only slightly increases 5431

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Figure 3. Effects of Cu2+ and SO42− concentrations on NO and SO2 removal efficiencies. Conditions: SO2 concentration, 1500 ppm; NO concentration, 400 ppm; O2 concentration, 6.0%; gas flow, 800 mL/ min; solution temperature, 298 K; solution pH, 3.2; UV radiation intensity, 0.012 W/mL; and H2O2 concentration, 1.5 mol/L.

Figure 4. Effects of the H2O2 concentration and NO and SO2 removal efficiencies. Conditions: SO 2 concentration, 1500 ppm; NO concentration, 400 ppm; O2 concentration, 6.0%; gas flow, 800 mL/ min; solution temperature, 298 K; solution pH, 3.2; Cu 2+ concentration, 0.007 mol/L; and UV radiation intensity, 0.012 W/mL.

from 52.8 to 54.6%. The results show that Cu2+ is the main catalyst, instead of SO42−. On the basis of the basic principles of an UV/Fenton-like reaction,19,22 an increasing Cu2+ concentration can increase the yield of ·OH free radicals, thereby strengthening the removal of NO. The related process can be expressed by the following chain reactions (eq 2−8):19,22,24,25

with the further increase in the H2O2 concentration from 1.5 to 2.0 mol/L, NO removal efficiency only slightly increases from 67.1 to 69.9%. The results30,31 show that H2O2 is the releasing agent of ·OH free radical; thus, an increasing H 2 O 2 concentration can effectively increase the yield of ·OH free radical by the following photolysis reaction 11, enhancing the removal of NO.

Cu 2 + + H 2O2 → Cu+ + HO2 ·+H+ +

Cu + H 2O2 → Cu

Cu

2+

2+

(2)

+ ·OH + OH +

+ H 2O2 → [Cu(HO2 )] + H

−

(3)

+

(5)

[Cu(HO2 )]+ + hν → Cu+ + HO2 ·

(6)

2.0 × 1010 M−1 s−1

NO + HO2 ·→ HNO3

(7)

1.0 × 1010 M−1 s−1

H 2O2 + ·OH → O2− ·+H 2O

(12)

Cu

−

SO4 ·+H 2O2 → HSO4 + HO2 ·

1.2 × 10 M

2+

8

+ ·OH → CuOH

3.5 × 10 M

−1 −1

s

4.2 × 109 M−1 s−1

·OH + HO2 ·→ H 2O2 + O2

(13) (14)

1.0 × 1010 M−1 s−1 (15)

HO2 ·+HO2 ·→ H 2O2 + O2

1.0 × 106 M−1 s−1 (16)

3.4. Effects of the NO Concentration on NO Removal Efficiency. The effects of the NO concentration on NO removal efficiency are shown in Figure 5. It can be seen that, when the NO concentration increases from 200 to 1200 ppm, NO removal efficiency greatly decreases from 73.4 to 46.9%. The effects of the NO concentration on NO removal efficiency are usually attributed to two reasons. On the one hand, increasing the NO concentration will increase the amount of NO through the reactor per unit time, which can decrease the relative molar ratio of ·OH to NO, thereby being able to reduce NO removal efficiency.29 On the other hand, according to the two-film theory, the absorption rate of NO can be expressed by the following eq 17:37

1.4 × 107 M−1 s−1 7

2+

·OH + ·OH → H 2O2

(9) −

2.7 × 107 M−1 s−1

(8)

Furthermore, for the effects of SO42−, the results26 show that SO42− can react with ·OH free radical to produce SO4−· free radical (redox potential is up to 2.60 eV) by the following reaction 9. The SO4−· free radical can react with H2O2 to produce HO2· free radical by the following reaction 10.27 The HO2· free radical can effectively oxidize and remove NO according to the above reaction 8 because the reaction rate constant between them is up to 1.0 × 1010 M−1 s−1.25 In fact, here, SO42− may play a role in the capture of intermediates of free radicals, which can effectively prevent the collapse and termination of free radicals from each other, thereby increasing the effective use of free radicals. Thus, NO removal efficiency increases with the addition of SO42−. Similar results28,29 also are obtained in the process of studying the effects of SO42− on pollutant degradation using UV/H2O2, UV/Fenton, and UV/ Fenton-like reactions. SO4 2 − + ·OH → ·SO4 − + OH−

(11)

However, with the addition of excessive H2O2, a lot of ·OH free radicals will be produced in a short time; thus, at this time, the following several side reactions 12−16 may significantly occur in solution, which will result in a great self-loss in ·OH free radicals.30−36 Therefore, the further increase in the H2O2 concentration only has a small impact on NO removal efficiency.

(4)

[Cu(HO2 )]+ → Cu+ + HO2 ·

NO + ·OH → HNO2

H 2O2 + hν → 2·OH

−1 −1

s

(10)

3.3. Effects of the H2O2 Concentration on NO Removal Efficiency. The effects of the H2O2 concentration on NO removal efficiency are shown in Figure 4. It can be seen that little NO is removed when the H2O2 concentration is 0. However, when the H2O2 concentration increases from 0 to 1.5 mol/L, NO removal efficiency increases from 0 to 67.1%. Then,

NNO = kNO,G(pNO − pNO, i )

(17) −2 −1

where NNO is the absorption rate of NO (mol m s ), kNO,G is the gas-phase mass-transfer coefficient of NO (mol s−1 m−2 Pa−1), pNO is the NO partial pressure in the gas body (Pa), and pNO,i is the NO partial pressure in the phase interface (Pa). 5432

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pH increases from 1.31 to 9.22, NO removal efficiency increases significantly from 67.1 to 86.6%. The effects of solution pH on the removal of NO can be explained by the following reasons. On the one hand, in aqueous solution, HO2− can be produced by the following hydrolysis reaction 18 of H2O2:2,13,29−31 H 2O2 ↔ HO2− + H+ ·OH + HO2− → H 2O + O2−·

(18)

7.5 × 109 M−1 s−1 (19)

Figure 5. Effects of the NO concentration on NO and SO2 removal efficiencies. Conditions: SO2 concentration, 1500 ppm; O2 concentration, 6.0%; gas flow, 800 mL/min; solution temperature, 298 K; solution pH, 3.2; Cu2+ concentration, 0.007 mol/L; UV radiation intensity, 0.012 W/mL; and H2O2 concentration, 1.5 mol/L.

−

HO2 is an effective scavenger of ·OH free radical, which can consume ·OH free radicals by the above reaction 19.2,13,29−31 From the reaction 19, it can be seen that increasing solution pH (OH− concentration) can increase the yield of HO2− by promoting the shift to the right of the chemical reaction equilibrium, thus inhibiting the removal of NO. However, on the other hand, some researchers2,13,24,25,29,38,39 suggested that the removal paths of NO by the oxidation of ·OH and H2O2 mainly included the reactions 7, 8, and 20−22, which can generate a large number of H+ in solution.

From the eq 17, it can be seen that an increasing NO concentration (NO partial pressure) can raise the mass-transfer driving force of NO, thereby promoting the removal of NO. As shown in the above results, here, the front negative factors may play a major role. However, it is noteworthy that NO removal efficiency greatly decreases, but the total removal amount of NO greatly increases with the increase of the NO concentration, as shown in Figure 6, such that the total mass of NO removal greatly increases from 0.32 to 1.21 mg when the NO concentration increases from 200 to 1200 ppm, increasing by 278.2%.

HNO2 + ·OH → HNO3 + ·H

6.0 × 109 M−1 s−1 (20)

HNO2 + H 2O2 → HNO3 + H 2O

(21)

2NO + 3H 2O2 → 2HNO3 + 2H 2O

(22)

−

Thus, when OH is added to the solution, it can absorb the product H+ generated by the reactions 7, 8, and 20−22 through the following acid−base neutralization reaction 23, thereby increasing the absorption rate of NO.2,13,29 H+ + OH− → H 2O

(23)

3.5. Effects of Solution pH on NO Removal Efficiency. The effects of solution pH on NO removal efficiency were studied, and the results are shown in Figure 7. When solution

In summary, the effects of solution pH on the removal of NO are controlled by these positive and negative factors together. However, here, the latter positive factors may play a major role. Therefore, with the increase of solution pH, NO removal efficiency increases. 3.6. Effects of the Solution Temperature on NO Removal Efficiency. The effects of the solution temperature on NO removal efficiency are shown in Figure 8. It can be seen that, when the solution temperature increases from 288 to 338 K, NO removal efficiency decreases from 68.5 to 61.1%. The increase in the solution temperature can raise the chemical reaction rate according to the famous Arrhenius equation,40 thereby being able to promote the removal of NO. However,

Figure 7. Effects of solution pH and NO and SO2 removal efficiencies. Conditions: SO2 concentration, 1500 ppm; NO concentration, 400 ppm; O2 concentration, 6.0%; gas flow, 800 mL/min; solution temperature, 298 K; Cu2+ concentration, 0.007 mol/L; UV radiation intensity, 0.012 W/mL; and H2O2 concentration, 1.5 mol/L.

Figure 8. Effects of the solution temperature and NO and SO2 removal efficiencies. Conditions: SO2 concentration, 1500 ppm; NO concentration, 400 ppm; O2 concentration, 6.0%; gas flow, 800 mL/ min; solution pH, 3.2; Cu2+ concentration, 0.007 mol/L; UV radiation intensity, 0.012 W/mL; and H2O2 concentration, 1.5 mol/L.

Figure 6. Effects of the NO concentration on NO removal mass. Conditions: SO2 concentration, 1500 ppm; O2 concentration, 6.0%; gas flow, 800 mL/min; solution temperature, 298 K; solution pH, 3.2; Cu2+ concentration, 0.007 mol/L; UV radiation intensity, 0.012 W/ mL; and H2O2 concentration, 1.5 mol/L.

5433

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3.8. Effects of the SO2 Concentration on NO Removal Efficiency. The effects of the SO2 concentration on NO removal efficiency are shown in Figure 11. It can be seen that

with the increase of the solution temperature, the solubility of NO in solution will decrease,37,41 thereby reducing NO removal efficiency. The two reasons jointly dominate the removal of NO, but the effects of the latter may be bigger than that of the former. Therefore, NO removal efficiency decreases with the increase of the solution temperature. 3.7. Effects of the Gas Flow on NO Removal Efficiency. The effects of the gas flow on NO removal efficiency are shown in Figure 9. It can be seen that gas flow has a great impact on

Figure 11. Effects of the SO2 concentration on NO and SO2 removal efficiencies. Conditions: NO concentration, 400 ppm; O2 concentration, 6.0%; gas flow, 800 mL/min; solution temperature, 298 K; solution pH, 3.2; Cu2+ concentration, 0.007 mol/L; UV radiation intensity, 0.012 W/mL; and H2O2 concentration, 1.5 mol/L.

when the SO2 concentration increases from 0 to 2500 ppm, NO removal efficiency decreases from 70.5 to 61.9%. On the basis of the following reactions 24−29,17,42−44 SO2 can consume the ·OH free radicals and H2O2 reacting with NO; thus, with the increase in the SO2 concentration, NO removal efficiency decreases. Owusu et al.44 also found that adding SO2 with high concentrations had an obvious inhibitory effect on the removal of NO when they used the ·OH free radicals produced by the ultrasonic cavitation effect in the liquid phase to simultaneously oxidize and remove NO and SO2 from flue gas by wet scrubbing.

Figure 9. Effects of the gas flow and NO and SO2 removal efficiencies. Conditions: SO2 concentration, 1500 ppm; NO concentration, 400 ppm; O2 concentration, 6.0%; solution temperature, 298 K; solution pH, 3.2; Cu2+ concentration, 0.007 mol/L; UV radiation intensity, 0.012 W/mL; and H2O2 concentration, 1.5 mol/L.

NO removal efficiency. With the increase in the gas flow from 400 to 2000 mL/min, NO removal efficiency greatly decreases from 75.8 to 43.6%. The effects of the gas flow on NO removal efficiency can be explained as follows. On the one hand, the increase in the gas flow can increase the gas−liquid mass-transfer rate by strengthening the disturbance in the gas−liquid two phase, thereby increasing the absorption rate of NO.13,29 However, on the other hand, the increase in the gas flow makes the residence time (reaction time) of NO in the reactor reduce. In addition, the amount of NO through the reactor per unit time greatly increases, which can decrease the relative molar ratio of ·OH to NO,13,29 thereby being able to reduce NO removal efficiency. Here, the latter may play a leading role; thus, NO removal efficiency decreases with the increase of the gas flow. However, similarly, it can be seen in Figure 10 that the total removal amount of NO greatly increases with the increase of the gas flow, such that the total mass of NO removal greatly increases from 0.33 to 0.94 mg when the gas flow increases from 400 to 2000 mL/min, increasing by 184.9%.

SO2 + H 2O ↔ HSO3− + H+

(24)

HSO3− ↔ SO32 − + H+

(25)

HSO3− + ·OH → ·SO3− + H 2O

4.5 × 109 M−1 s−1 (26)

SO32 − + ·OH → ·SO3− + H 2O

5.1 × 109 M−1 s−1 (27)

HSO3−

+ H 2O2 → SO4

2−

+ H 2O

SO32 − + H 2O2 → SO4 2 − + H 2O

(28) (29)

3.9. Effects of the O2 Concentration on NO Removal Efficiency. The effects of the O2 concentration on NO removal efficiency were studied, and the results are shown in Figure 12. NO removal efficiency increases from 66.2 to 69.6% when the O2 concentration increases from 0 to 10.0%. The results39,45,46 show that, as a capture intermediate, O2 usually plays an important role in restraining the combination and destruction of free radicals,2,29 thereby being able to increase the effective use of free radicals. Thus, adding O2 is beneficial for promoting the removal of NO. 3.10. Removal of SO2. As shown in Figures 3−12, under different experimental conditions, NO removal efficiency changes but SO2 achieves 100% removal. Some results1−3,12−18 18 show that SO2 is much easier to be removed by wet scrubbing because of its high solubility in solution compared to NO.21 In addition, in this paper, to achieve a high NO removal efficiency, a relative large liquid−gas ratio was set during the experiments. Thus, under all experimental conditions, SO2 has a complete removal. Many other scholars1−3,12−18 also obtained

Figure 10. Effects of the gas flow on NO removal mass. Conditions: SO2 concentration, 1500 ppm; NO concentration, 400 ppm; O2 concentration, 6.0%; solution temperature, 298 K; solution pH, 3.2; Cu2+ concentration, 0.007 mol/L; UV radiation intensity, 0.012 W/ mL; and H2O2 concentration, 1.5 mol/L. 5434

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solution. Besides, to understand the transfer paths of nitrogen in NO and sulfur in SO2, on the basis of the results determined by IC, the material balances for NO and SO2 were also calculated. On the basis of the mass conservation of nitrogen element in NO and sulfur element in SO2, the calculated values of NO3− and SO42− in solution can be calculated by the following eqs 30 and 31: Ccal,total = Ccal,oxidation + Ccal,addition Ccal,oxidation =

Figure 12. Effects of the O2 concentration on NO and SO2 removal efficiencies. Conditions: SO2 concentration, 1500 ppm; NO concentration, 400 ppm; gas flow, 800 mL/min; solution temperature, 298 K; solution pH, 3.2; Cu2+ concentration, 0.007 mol/L; UV radiation intensity, 0.012 W/mL; and H2O2 concentration, 1.5 mol/L.

3.12. Liquid Reaction Products. To understand the composition of reaction products, the anions in solution were determined using IC, and the results are shown in Table 1. The results show that NO3− and SO42− are the major anion products in solution, suggesting that NO and SO2 are mainly removed by oxidation. The toxic byproducts NO2− and SO32− are not found in solution because of their instabilities in H2O2 Table 1. Reaction Products and the Material Balance for the Removal of NO and SO2 SO32−

NO3−

NO2−

measured ion concentration (mg/L) calculated ion concentration (mg/L) relative error (%)

768.8 843.5 8.8

0

18.9 20.1 5.9

0

(31)

where Ccal,total is the total calculated concentration of NO3 and SO42− (mg/L), Ccal,oxidation is the calculated concentration of NO3− and SO42− produced by oxidizing NO and SO2 (mg/L), Ccal,addition is the calculated concentration of SO42− produced by adding CuSO4 (mg/L), η is the removal efficiency of NO and SO2 (%), Cin is the inlet concentration of NO and SO2 (mg/ m3), Q is the gas flow (mL/min), t is the reaction time (min), M1 is the molar mass of NO3− and SO42− (g/mol), M2 is the molar mass of NO and SO2 (g/mol), and VL is the solution volume (L). As shown in Table 1, most of the low-valence nitrogen elements (+2) in NO and the low-valence sulfur elements (+4) in SO2 transform into the high-valence nitrogen elements (+5) in NO3− and the high-valence sulfur elements (+6) in SO42−, respectively. The relative errors between measured and calculated values for SO42− and NO3− are 8.8 and 5.9%, respectively. 3.13. Removal Pathways of NO Using an UV/Fentonlike Reaction. On the basis of the results,18−23 there are often three reaction pathways for the removal of pollutants using an UV/Fenton-like reaction, including UV decomposition removal, H2O2 oxidation removal, and ·OH oxidation removal, but the oxidation removal of ·OH free radical usually plays a main role. As shown in Figure 2, NO only achieves a removal efficiency of 13.1% in H2O2 and even little NO is removed in UV, but using an UV/Fenton-like reaction achieves a NO removal efficiency of 68.2%. The results show that the UV decomposition removal of NO fails to occur, the oxidation removal of ·OH free radical plays a main role, and the oxidation removal of H2O2 only plays a secondary role in the removal of NO using an UV/Fenton-like reaction. The analysis results of liquid reaction products indicate that NO3− is the final anion product in solution, further showing that NO is mainly removed by oxidation. In summary, on the basis of the results, although several other side reactions may also possibly occur in solution, the major reaction pathways for NO removal using an UV/Fenton-like reaction can be presumably concluded as follows: (a) the ·OH free radicals produced by a Fenton-like chain reaction and photolysis of H2O2 according to eqs 2−6 and 11; (b) the removal of NO by the oxidation of free radicals according to the reactions 7, 8, and 20; (c) the removal of NO by the oxidation of H2O2 according to the reactions 21 and 22; and (d) the termination of chain reactions by the reactions 12−16.

Figure 13. Outlet concentrations of NO and NO2 with time. Conditions: SO2 concentration, 1500 ppm; NO concentration, 400 ppm; O2 concentration, 6.0%; gas flow, 800 mL/min; solution temperature, 298 K; solution pH, 3.2; Cu2+ concentration, 0.007 mol/ L; H2O2 concentration, 1.5 mol/L; and UV radiation intensity, 0.012 W/mL.

SO42−

ηC inQtM1 × 10−6 M 2VL −

similar results in the process of studying the simultaneous removal of NO and SO2 by using wet scrubbing. In the next works, we will individually re-study the absorption of SO2 using an UV/Fenton-like reaction by redesigning the experimental system and adjusting the gas−liquid mass-transfer characteristics and volume of the reactor. 3.11. Determination of NO2 Outlet Concentrations. It is well-known that NO2 is a more harmful gas compared to NO; thus, it is very necessary to detect its yield in the oxidation process of NO. The outlet concentration of NO2 was measured using a flue gas analyzer, and the results are shown in Figure 13. It can be seen that NO is detected, but NO2 is not detected, showing that the oxidation process of NO using an UV/ Fenton-Like reaction will not produce secondary-gas pollution.

ion category

(30)

4. CONCLUSION With the increase of the Cu2+ concentration, NO removal efficiency significantly increases. With the increase in the H2O2 concentration, NO removal efficiency increases, but the changes become smaller gradually. NO removal efficiency greatly reduces with an increasing gas flow and NO 5435

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Energy & Fuels

Article

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concentration. An increasing solution temperature or SO2 concentration slightly decreases NO removal efficiency. The increase in the O2 concentration can promote the removal of NO. NO2 is not be detected in exhaust. The liquid anions are mainly SO42− and NO3−, suggesting that NO and SO2 are mainly removed by oxidation. Most of the low-valence nitrogen elements in NO and the low-valence sulfur elements in SO2 transform into the high-valence nitrogen elements in NO3− and the high-valence sulfur elements in SO42−, respectively. The oxidation removal of ·OH free radical plays a main role, and the oxidation removal of H2O2 only plays a secondary role in the removal of NO using an UV/Fenton-like reaction.

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +86-0511-89720178. E-mail: [email protected] (Y.L.); [email protected] (J.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (51206067), the New Teacher Fund for the Doctoral Program of Higher Education of China, Open Foundation for Key Laboratory of Water and Air Pollution Control of Guangdong Province of China (2011A060901002), the Fund for Senior Personnel of Jiangsu University (12JDG042), and the Jiangsu “Six Personnel Peak” TalentFunded Projects (2011-ZBZZ-27).

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