Simultaneous Removal of NOx and SO2 through a

sustainability Article

Simultaneous Removal of NOx and SO2 through a Simple Process Using a Composite Absorbent Ping Fang 1,2 , Zijun Tang 1 , Xiongbo Chen 1,2 , Peiyi Zhong 1,2 , Jianhang Huang 1 , Zhixiong Tang 1 and Chaoping Cen 1,2, * 1

2

*

South China Institute of Environmental Sciences, Ministry of Environmental Protection, Guangzhou 510655, China; [email protected](P.F.); [email protected](Z.T.); [email protected](X.C.); [email protected](P.Z.); [email protected](J.H.); [email protected](Z.T.) The Key Laboratory of Water and Air Pollution Control of Guangdong Province, Guangzhou 510655, China Correspondence: [email protected]; Tel.: +86-020-8555-7692

Received: 22 October 2018; Accepted: 19 November 2018; Published: 22 November 2018

Abstract: In this work, the feasibility of the simultaneous removal of NOx and SO2 through a simple process using a composite absorbent (NaClO2 /Na2 S2 O8 ) was evaluated. Factors affecting the removal of NOx and SO2 , such as NaClO2 and Na2 S2 O8 concentrations, solution temperature, the initial pH of solution, gas flow rate, and SO2 , NO, and O2 concentrations were studied, with a special attention to NOx removal. Results indicate that a synergistic effect on NOx removal has been obtained through combination of NaClO2 and Na2 S2 O8 . NaClO2 in the solution played a more important role than did Na2 S2 O8 for the removal of NOx . The above factors had an important impact on the removal of NOx , especially the solution temperature, the initial pH of the solution, and the oxidant concentrations. The optimum experimental conditions were established, and a highest efficiency of NOx removal of more than 80% was obtained. Meanwhile, tandem double column absorption experiments were conducted, and a NOx removal efficiency of more than 90% was reached, using NaOH solution as an absorbant in the second reactor. A preliminary reaction mechanism for NOx and SO2 removal was deduced, based on experimental results. The composite absorbent has the potential to be used in the wet desulfurization and denitration process, to realize the synergistic removal of multi-pollutants. Keywords: NOx ; SO2 ; NaClO2 ; Na2 S2 O8 ; wet scrubbing

1. Introduction NOx (mainly NO) and SO2 released from the fossil fuels burning process, are becoming increasingly well-known as the precursors for photochemical smog and regional haze [1,2]. Coal-fired power plants, industrial boilers and furnaces are considered to be the main anthropogenic sources of these pollutants. Currently, wet flue gas desulfurization (WFGD) technology and selective catalytic reduction (SCR) technology are the mature commercial technologies for controlling SO2 and NOx , and they are widely used in the flue gas treatment of power plant boilers in worldwide [3]. At present, flue gas pollutants emitted from power plant are being effectively controlled. WFGD technology is often used to control SO2 emitted from other industrial sources, such as industrial boilers and industrial furnaces; however, there is a lack of cost-effective NOx treatment technology. Due to the high investment and operating costs, a complicated system, and the large area required, the application of the SCR system in the field of industrial boilers and furnaces flue gas treatment is limited. Therefore, it is urgent that a cost-effective NOx control technology is developed, and it is better to develop an economical, and simplified method to achieve synergistic removal NOx and SO2 . The subject of multi-pollutant (NOx , SO2 , etc.) cooperative control has been widely researched in recent years. According to a review of the literature, there are two major methods for realizing Sustainability 2018, 10, 4350; doi:10.3390/su10124350

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multi-pollutant removal: the dry method and absorption (solution scrubbing) [4]. The dry method includes catalysis oxidation [5], adsorbent adsorption [6], plasma degradation [7], photocatalytic methods [8], etc. The absorption method includes complex [9], oxidation [10], and reducing [11] absorption methods. Among these methods, the oxidation absorption method based on the WFGD process is generally considered to be one of cost-effective methods for controlling multi-pollutant in flue gas. Due to the low solubility of NO, in the wet oxidizing process, rapid and efficient oxidation of NO to NO2 is a key step. In order to increase the NO removal, a variety of reagents have been utilized to synergistically remove SO2 and NO, such as potassium permanganate (KMnO4 ) [12], potassium ferrate (K2 FeO4 ) [13], sodium chlorite (NaClO2 ) [14], sodium hypochlorite (NaClO) [15], UV/H2 O2 [16], ozone [17], and persulfate systems [18,19], and satisfactory SO2 and NO removal efficiencies have been achieved under laboratory conditions. In the oxidation absorption method, NaClO2 and Na2 S2 O8 are considered to be effective reagents for controlling multi-pollutant in flue gas. Fang et al. [20] developed a novel oxidation–absorption system for SO2 , NO, and Hg0 removal, and NaClO2 and NaOH were used as the oxidant and the absorbent, respectively. SO2 , NOx , and Hg0 removal efficiencies of more than 99, 82, and 95% could be obtained, respectively, under optimal experimental conditions. Hao et al. [21] developed a new process for gas-phase oxidation, combined with liquid-phase absorption. The gas phase oxidants (mainly ClO2 ) was produced by vaporizing the mixed solution of sodium chlorite and sodium persulfate at a temperature of 413 K. NO in the simulated flue gas was first oxidized to NO2 by the gas phase oxidants, then NO, NO2 , and SO2 were absorbed by the sodium humate solution. Finally, 82.7% of NO removal and 100% of SO2 removal could be reached under optimal conditions. Wang et al. [22] used a dual oxidant (H2 O2 /Na2 S2 O8 ) system to remove NO, and the highest NO removal efficiency (82%) was obtained under the optimal conditions. Khan et al. [23] used an aqueous solution of persulfate (0.01–0.2 M) to remove NO, and 92% of NO removal was obtained at 90 ◦ C and 0.1 M persulfate concentration. However, these technologies included some shortages, such as a complex system, high reagent consumption, and more demanding process conditions. Thus, how to simply and cost-effectively perform the cooperative absorption of SO2 and NO in a single reactor is an urgent problem that needs to be solved. According to our previous works [20], NaClO2 was a better oxidant in view of SO2 and NO removal, but there are issues with the actual usage of the reagent, such as its high price (10,000 ¥/t), its large rate of consumption, and its easy decomposition under acidic conditions. Na2 S2 O8 is easily soluble in water and environmentally friendly. It is a strong and nonselective oxidant, and its price is relatively low (5000 ¥/t); however, it needs to be activated by heating or other methods to reveal its strong oxidizing properties. According to the existing literature [23,24], the presence of Cl− could improve NO oxidation and absorption when using a persulfate solution to remove NOx . Hence, in this study, a composite absorbent consisting of NaClO2 and Na2 S2 O8 was utilized to simultaneously remove SO2 and NOx . The main purpose of this study is to evaluate the feasibility of simultaneous removal of SO2 and NOx by using a dual oxidant solution. Meanwhile, factors affecting the removal of NOx and SO2 , such as NaClO2 and Na2 S2 O8 concentrations, solution temperature, the initial pH of solution, gas flow rate, and SO2 , NO, and O2 concentrations were studied, with a special attention to NOx removal. The preliminary reaction mechanisms of SO2 and NOx removal were also hypothesized. 2. Materials and Methods 2.1. Reagents Reagents used in this study were bought from Guangzhou Chemical Reagent Factory (Guangzhou, China), and directly used without purification. Deionized water was used to prepare the solution. The composite absorbent used in the first absorber (Figure 1) was prepared by NaClO2 (80%, AR) and Na2 S2 O8 (98%, AR). CO(NH2 )2 (99%, AR), Ca(OH)2 (95%, AR), Na2 S2 O8 (98%, AR), and NaOH (96%, AR), were used in the second absorber when needed (Figure 1). The solution pH was adjusted using

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adjusted using 1 mol/L of H2SO4 and 1 mol/L of NaOH solutions. Anhydrous CaCl2 (96%, AR) was used as a dryer to protect the flue gas analyzer. 1 mol/L of H2 SO4 and 1 mol/L of NaOH solutions. Anhydrous CaCl2 (96%, AR) was used as a dryer to protect the flue gas analyzer.

Figure Figure1.1. Schematic Schematicdiagram diagramof ofthe theexperimental experimentaldevice. device.

2.2. Equipment 2.2. Equipment The experimental apparatus consisted of a simulated flue gas generation system, a wet scrubbing The experimental apparatus consisted of a simulated flue gas generation system, a wet system, an online monitoring system and an exhaust gas adsorption system, as shown in Figure 1. scrubbing system, an online monitoring system and an exhaust gas adsorption system, as shown in SO2 , NO, O2 , and N2 gases were supplied by the compressed cylinders (Gas Co., Ltd. of Zhuo Figure 1. Zheng, Guangzhou, China), and metered by the mass flow controllers (Beijing Sevenstar Flow Co., SO2, NO, O2, and N2 gases were supplied by the compressed cylinders (Gas Co., Ltd. of Zhuo Ltd., Beijing, China). Various gases flowed into the Mixer 1 to produce the flue gas, where the total Zheng, Guangzhou, China), and metered by the mass flow controllers (Beijing Sevenstar Flow Co., flue gas flow (Q) was kept at 2 L/min. The two absorbers (4 cm i.d. × 65 cm length) were all made of Ltd., Beijing, China). Various gases flowed into the Mixer 1 to produce the flue gas, where the total borosilicate glass. The first absorber was filled with ceramic Raschig rings (1.25 cm i.d. × 1.25 cm length) flue gas flow (Q) was kept at 2 L/min. The two absorbers (4 cm i.d. × 65 cm length) were all made of of 20 cm height, heated by a temperature-controlled heating belt, and the second absorber was an borosilicate glass. The first absorber was filled with ceramic Raschig rings (1.25 cm i.d. × 1.25 cm empty column. The volume of the solution in the absorber was 1 L. The simulated flue gas was length) of 20 cm height, heated by a temperature-controlled heating belt, and the second absorber introduced into the first absorber through a gas distribution pipe fitted at the bottom of the absorber. was an empty column. The volume of the solution in the absorber was 1 L. The simulated flue gas When the tandem double column absorption experiments were conducted, the simulated flue gas was introduced into the first absorber through a gas distribution pipe fitted at the bottom of the flowed through two absorbers in sequence. The purified flue gas was then dried by anhydrous CaCl2 absorber. When the tandem double column absorption experiments were conducted, the simulated and then it was introduced into the detection system. In this work, SO2 concentration range was 0–2100 flue gas flowed through two absorbers in sequence. The purified flue gas was then dried by ppm, NO concentration range was 0–1000 ppm, O2 concentration range was 1–15% (v/v). The time anhydrous CaCl2 and then it was introduced into the detection system. In this work, SO2 for each set of experiments was 1 h. The experimental conditions were summarized and shown in concentration range was 0–2100 ppm, NO concentration range was 0–1000 ppm, O2 concentration Table 1. An ECOM-J2KN flue gas analyzer (RBR Company, Germany) was utilized to detect online the range was 1–15% (v/v). The time for each set of experiments was 1 h. The experimental conditions inlet and outlet SO2 , NO2 , NO, and O2 concentrations. An MP511 pH Detector (Shanghai Precision were summarized and shown in Table 1. An ECOM-J2KN flue gas analyzer (RBR Company, Instruments Co., Ltd., Shanghai, China) was used to detect the solution pH. Concentrations of NO3 − , Germany) was utilized to detect online the inlet and outlet SO2, NO2, NO, and O2 concentrations. An Cl− , ClO2 − , and ClO3 − were measured with an ion chromatography system (IC, Thermo ICS-1100, MP511 pH Detector (Shanghai Precision Instruments Co., Ltd., Shanghai, China) was used to detect America). The NOx and SO2 removal were calculated by Equation (1): the solution pH. Concentrations of NO3−, Cl−, ClO2−, and ClO3− were measured with an ion chromatography system (IC, Thermo ICS-1100, C −America). Coutlet The NOx and SO2 removal were calculated η = inlet × 100% (1) by Equation (1): Cinlet

− C C where η is the removal efficiency of NOη x or SO2 ; Cinlet and × Coutlet 100%are the inlet and outlet concentrations (1) C of NOx or SO2 , respectively. where η is the removal efficiency of NOx or SO2; Cinlet and Coutlet are the inlet and outlet concentrations of NOx or SO2, respectively.


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Table 1. Experimental conditions of the individual experiments. No.

Experiment

Experimental Conditions

1

Removal of SO2 and NO with different oxidant solutions

Single absorber, Q = 2 L/min, [NO] = 500 ppm, [SO2 ] = 1000 ppm, O2 = 10%, Tabsorption = 50 ◦ C

2

Effect of NaClO2 concentration

Single absorber, Q = 2 L/min, [NO] = 500 ppm, [SO2 ] = 1000 ppm, O2 = 10%, Tabsorption = 50 ◦ C, [Na2 S2 O8 ] = 0.5 wt.%, pH = 12

3

Effect of Na2 S2 O8 concentration

Single absorber, Q = 2 L/min, [NO] = 500 ppm, [SO2 ] = 1000 ppm, O2 = 10%, Tabsorption = 50 ◦ C, [NaClO2 ] = 0.2 wt.%, pH = 12

4

Effect of initial pH

Single absorber, Q = 2 L/min, [NO] = 500 ppm, [SO2 ] = 1000 ppm, O2 = 10%, Tabsorption = 50 ◦ C, [Na2 S2 O8 ] = 0.5 wt.%, [NaClO2 ] = 0.2 wt.%

5

Effect of solution temperature

Single absorber, Q = 2 L/min, [NO] = 500 ppm, [SO2 ] = 1000 ppm, O2 = 10%, [Na2 S2 O8 ] = 0.5 wt.%, [NaClO2 ] = 0.2 wt.%, pH = 12

6

Effect of gas flow rate

Single absorber, [NO] = 500 ppm, [SO2 ] = 1000 ppm, O2 = 10%, Tabsorption = 50 ◦ C, [Na2 S2 O8 ] = 0.5 wt.%, [NaClO2 ] = 0.2 wt.%, pH = 12

7

Effect of SO2 concentration

Single absorber, Q = 2 L/min, [NO] = 500 ppm, O2 = 10%, Tabsorption = 50 ◦ C, [Na2 S2 O8 ] = 0.5 wt.%, [NaClO2 ] = 0.2 wt.%, pH = 12

8

Effect of NO concentration

Single absorber, Q = 2 L/min, [SO2 ] = 1000 ppm, O2 = 10%, Tabsorption = 50 ◦ C, [Na2 S2 O8 ] = 0.5 wt.%, [NaClO2 ] = 0.2 wt.%, pH = 12

9

Effect of O2 concentration

Single absorber, Q = 2 L/min, [NO] = 500 ppm, [SO2 ] = 1000 ppm, Tabsorption = 50 ◦ C, [Na2 S2 O8 ] = 0.5 wt.%, [NaClO2 ] = 0.2 wt.%, pH = 12

10

Tandem double column absorption experiments

Q = 2 L/min, [NO] = 500 ppm, [SO2 ] = 1000 ppm, O2 = 10%; Tabsorption = 50 ◦ C, [Na2 S2 O8 ] = 0.5 wt.%, [NaClO2 ] = 0.2 wt.%, pH = 12 (the first absorber); Tabsorption = 25 ◦ C, [reagent] = 5 wt.% (the second absorber)

11

Product analysis

Single absorber, Q = 2 L/min, [NO] = 500 ppm, [SO2 ] = 1000 ppm, O2 = 10%, Tabsorption = 50 ◦ C, [Na2 S2 O8 ] = 0.5 wt.%, [NaClO2 ] = 0.2 wt.%, pH = 12

3. Results and Discussion 3.1. Simultaneous Removal of SO2 and NOx with Different Oxidant Solutions Based on the literature and on our previous works [12,20,22,23], NaClO2 , KMnO4 , and Na2 S2 O8 were considered as better NO oxidants. Thus, in this study, the contrast experiments for different oxidants on SO2 and NOx removal were investigated, and the results are displayed in Figures 2 and 3. In this set of experiments, the concentrations of the single oxidants in the solutions were all 0.2 wt.%, and the initial pH values of NaClO2 + NaOH, KMnO4 , and Na2 S2 O8 + NaOH solutions were 12.0, 8.0, and 12.0, respectively. The results indicate that all oxidant solutions had a good level of efficiency of removal on SO2 , where the removal efficiency was close to 100%. However, different oxidant solutions had different effects on NOx removal, and the capacity on the NOx removal of three oxidant solutions was NaClO2 > KMnO4 > Na2 S2 O8 , with the average NOx removal efficiencies being at 48.69%, 48.13%, and 23.98%, respectively. Although the average efficiency of NOx removal by KMnO4 solution was close to that of the NaClO2 solution, NOx removal efficiency by the KMnO4 solution decreased, with the reaction time increasing. Thus, it can be speculated that the NOx removal efficiency by the NaClO2 solution is significantly better than that of KMnO4 solution with the increase of reaction time. However, a NOx removal efficiency of approximately 50% using the NaClO2 solution is relatively low. Hence, in this study, the authors hope to improve the denitration performance of NaClO2 solution

of reaction 2018, time.10,However, a REVIEW NOx removal efficiency of approximately 50% using the NaClO2 solution Sustainability x FOR PEER 5 of 18 is relatively low. Hence, in this study, the authors hope to improve the denitration performance of NaClO 2 solution (0.2 wt.%)a NO by adding a certain amount of Na2S2O8 50% (0.1 wt.%), and the results are of reaction time. However, x removal efficiency of approximately using the NaClO 2 solution illustrated Figures 2 and 3. The results thathope SO2 and NOx could efficientlyperformance absorbed, and is relativelyinlow. Hence, in this study, theshow authors to improve the be denitration of Sustainability 2018, 10, 4350 5 of 18 nearly 100% of the SO 2 and 73.51% of the NO x were simultaneously removed. NO x removal using a NaClO2 solution (0.2 wt.%) by adding a certain amount of Na2S2O8 (0.1 wt.%), and the results are dual oxidant than removal by2 using a single oxidant solution. SO2 removal illustrated in solution Figures 2was andmuch 3. Thebetter results show that SO and NO x could be efficiently absorbed, and remained constant at about 100% inofallthe tests, thesimultaneously NOx removal was affectedNO byx many factors, nearly 100% of the SO 2 and 73.51% NObut x were removed. removal usingso a (0.2 wt.%) by adding a certain amount of Na2 S2 O8 (0.1 wt.%), and the results are illustrated in Figures 2 the emphasis was placed on the discussion of NO x removal by using a NaClO 2 + Na 2 S 2 O 8 solution, in dual oxidant solution was much better than removal by using a single oxidant solution. SO2 removal and 3. The results show that SO2 and NOx could be efficiently absorbed, and nearly 100% of the SO2 subsequent experiments. remained constant at about 100% in all tests, but the NOx removal was affected by many factors, so and 73.51% of the NOx were simultaneously removed. NOx removal using a dual oxidant solution the emphasis was placed on the discussion of NOx removal by using a NaClO2 + Na2S2O8 solution, in was much better than removal by using a single oxidant solution. SO2 removal remained constant at subsequent experiments. about 100% in all tests, but the NOx removal was affected by many factors, so the emphasis was placed on the discussion of NOx removal by using a NaClO2 + Na2 S2 O8 solution, in subsequent experiments.

Figure 2. Effect of different oxidants on NOx removal. Figure 2. Effect of different oxidants oxidants on on NO NOxx removal.

Figure Figure 3. 3. Effect of different different oxidants oxidants on on SO SO22 removal.

3.2. Concentration 3.2. Effect Effect of of NaClO NaClO22 Concentration Figure 3. Effect of different oxidants on SO2 removal. Several at at various NaClO (0, 0.05, 0.1, 0.15, 0.20, and 0.30 wt.%) 2 concentrations Several sets setsofofexperiments experiments various NaClO 2 concentrations (0, 0.05, 0.1, 0.15, 0.20, and 0.30 3.2. Effect of NaClO 2 Concentration were out to study the effect of NaClO on NOx removal efficiency at the 2 concentration wt.%)carried were carried out to study the effect of NaClO 2 concentration on NO x removal efficiency at the same persulfate level (0.5 wt.%). Figure 4 shows that the NO removal efficiency sharply increased sameSeveral persulfate (0.5 wt.%). at Figure 4 shows that the NOxx removal efficiency sharply sets level of experiments various NaClO 2 concentrations (0, 0.05, 0.1, 0.15, 0.20,increased and 0.30 with the increment of NaClO2 concentration at first, and then it gradually increased; when the wt.%) were carried out to study the effect of NaClO2 concentration on NOx removal efficiency at the NaClO2 concentration increased to 0.20 wt.%, a maximum NOx removal efficiency of 80.61% was same persulfate level (0.5 wt.%). Figure 4 shows that the NOx removal efficiency sharply increased obtained, and thereafter, NOx removal efficiency decreased from 80.61% to 72.83% with the increase of NaClO2 concentration from 0.2 wt.% to 0.3 wt.%. NaClO2 is a strong oxidant that can oxidize NO to NO2 , which then oxidizes NO2 to nitrate (Equations (2) and (3)). It can also directly oxidize


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NO to nitrate (Equation (4)) [20]. So, even the introduction of 0.05 wt.% of NaClO2 had a very pronounced effect, where the NOx removal efficiency went increased from 29.16% without any NaClO2 to 54.68% with NaClO2 . Moreover, Na2 S2 O8 in the solution could be activated to generate sulfate-free radicals (SO4 •− ) and hydroxyl radicals (OH• ), and a large number of chlorine-free radicals (Cl• , Cl2•− , ClOH•− ) could be produced through complex reactions via the reactions of Equations (5)–(8) [23–25]. The chlorine-free radicals could react with dissolved NO to form nitrates and nitrites (Equations (9)–(11)). Therefore, the addition of Na2 S2 O8 could increase the total number of reactive radical species, and enhance the NOx removal efficiency. In addition, as the reaction proceeded, the pH of the solution decreased, H+ could catalyze the decomposition of ClO2 − to generate the strong oxidizing ClO2 (Equations (12)–(14)). ClO2 could oxidize NO to NO2 and nitrate (Equations (15)–(17)) [20], leading to the increase of NOx removal. However, with an increasing concentration of NaClO2 , the NO concentration in the exhaust decreased, but the NO2 concentration increased. A large amount of NO2 was produced by the reactions (Equations (2), (10) and (15)) due to the high NaClO2 concentration. On the one hand, the high NaClO2 concentration was not conducive to the absorption of NO2 [20]; on the other hand, a large amount of NO2 escaped from the system, due to the limited gas–liquid contact time. The study found the average emission concentrations of NO2 were 10.56, 13.44, 39.35, 64.17, 87.18, and 158.24 ppm when the NaClO2 concentrations were 0, 0.05, 0.1, 0.15, 0.20, and 0.30 wt.%, respectively. Obviously, the NO2 concentration in the exhaust significantly increased when the NaClO2 concentration was more than 0.2 wt.%. Thus, there was an optimal NaClO2 concentration range of 0.15–0.2 wt.%, and the best NaClO2 concentration was determined as 0.2 wt.%. 2NO(l) + ClO2− (aq) → 2NO2 (l) + Cl− (aq)

(2)

4NO2 (l) + ClO2− (aq) + 4OH− (aq) → 4NO3− (aq) + Cl− (aq) + 2H2 O(l)

(3)

4NO(l) + 3ClO2− (aq) + 4OH− (aq) → 4NO3− (aq) + 3Cl− (aq) + 2H2 O(l)

(4)

SO4 •− (aq) + Cl− (aq) ↔ SO4 2− (aq) + Cl• (aq)

(5)

Cl• (aq) + Cl− (aq) ↔ Cl2•− (aq)

(6)

OH• (aq) + Cl− (aq) ↔ ClOH•− (aq)

(7)

ClOH•− (aq) + Cl− (aq) → Cl2•− (aq) + OH− (aq)

(8)

ClOH•− (aq) + NO(l) → HNO2 (aq) + Cl− (aq)

(9)

Cl2•− (aq) + NO2− (aq) → NO2 (l) + 2Cl− (aq)

(10)

ClOH•− (aq) + NO2 (l) → HNO3 (aq) + Cl− (aq)

(11)

8H + (aq) + 8ClO2− (aq) → 6ClO2 (l) + Cl2 (l) + 4H2 O(l)

(12)

4H + (aq) + 5ClO2− (aq) → 4ClO2 (l) + 2H2 O(l) + Cl− (aq)

(13)

4ClO2 − (aq) + 2H+ (aq) → Cl− (aq) + 2ClO2 (l) + ClO3 − (aq) + H2 O(l)

(14)

5NO(l) + 2ClO2 (l) + H2 O(l) → 5NO2 (l) + 2HCl(aq)

(15)

5NO2 (l) + ClO2 (l) + 3H2 O(l) → 5NO3− (aq) + Cl− (aq) + 6H+ (aq)

(16)

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5NO2 (l) + ClO2 (l) + 3H2 O(l) → 5NO3 (aq) + Cl (aq) + 6H aq

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5NO2 (l) + ClO2 (l) + 3H2 O(l) → 5NO3 (aq) + Cl (aq) + 6H aq

(16)


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Figure 4. Effect of NaClO2 concentration on NOx removal. Figure Figure4.4. Effect Effect of ofNaClO NaClO22 concentration concentration on on NO NOxx removal. 3.3. Effect of Na2S2O8 Concentration

The results 5 illustrate the effect of Na2S2O8 concentration on NOx removal efficiency 3.3. of Sin OFigure Concentration 3.3. Effect Effect of Na Na22S 22O88Concentration at a constant NaClO2 concentration of 0.2 wt.% and at 50 °C. The addition of Na2S2O8 could effectively The results in Figure illustrate of of NaNa O2O onon NO efficiency at x removal 2 S22S 8 concentration The the results Figure55efficiency, illustratethe theeffect effect 8 concentration NO x removal efficiency enhance NOxinremoval and Na2S2O 8 concentration had an important influence on the ◦ aatconstant NaClO of 0.20.2 wt.% and atat 5050C. The addition S2S22O 2 concentration 88 could a constant 2 concentration wt.% additionofofNa Na2the Osolution could effectively effectively absorption ofNaClO NOx. Figure 5 showsofthat whenand the Na 2S°C. 2O8 The concentration in increased enhance the NO removal efficiency, and Na S O concentration had an important on the x 22S22O88 concentration had an important influence enhance the NO x removal efficiency, and Na influence onand the from 0.00 wt.% to 0.1 wt.%, the NOx removal efficiency rapidly increased from 48.69% to 72.34%, absorption of NO . Figure 5 shows that when the Na S O concentration in the solution increased x 2 2 8 absorption of NO x. Figure 5 shows when the NaNa 2S22O inrange the solution increased then it slightly increased from 72.34%that to 80.61% in the S28Oconcentration 8 concentration of 0.1 wt.% to 0.5 from 0.00 wt.% to to0.1 0.1wt.%, wt.%,the theNO NO efficiency rapidly increased from 48.69% to 72.34%, x removal from 0.00 wt.% x removal efficiency rapidly increased from 48.69% to 72.34%, and wt.%. However, further increases in Na2S2O8 concentration resulted in a slight drop in NOx removal and then it slightly increased from 72.34% to 80.61% inNa the2SNa S2 O8 concentration range of 0.1 to wt.% 2 concentration then it slightly increased from 72.34% to 80.61% in the 2 O 8 range of 0.1 wt.% 0.5 efficiency, which then maintained stability. For example, the NOx removal efficiencies were 78.96% to 0.5 wt.%. However, further increases in Na S O concentration resulted in a slight drop in NO x 2 2 8 wt.%. However, in Na2S2O8 concentration in a slight drop in NOx removal and 79.42% whenfurther the Naincreases 2S2O8 concentrations were 0.7 wt.% resulted and 0.9 wt.%, respectively. removal efficiency, which then maintained stability. For example, the NO removal efficiencies were x efficiency, which then maintained stability. For example, the NOx removal efficiencies were 78.96% 78.96% and 79.42% when wt.% and 0.9respectively. wt.%, respectively. 2 S2 O8 concentrations and 79.42% when the Na2the S2ONa 8 concentrations were 0.7were wt.%0.7 and 0.9 wt.%,

Figure5.5. Effect Effectof ofNa Na22SS22O Figure O88 concentration concentration on on NO NOxxremoval. removal. 5. Effect of Naoxidant, 2S2O8 concentration x removal. Na2 S2 O8 is a strongFigure and nonselective and it canon beNO activated by heat and alkaline [21], •− to produce an intermediate sulfate free radical (SO4 ) (Equation (17)) [23,26], then SO4 •− reacts with hydroxide ions or water molecules to produce a sulfate ion and hydroxyl radicals (OH• ) (Equations (18) and (19)). The reaction rate constant of Equation (19) (k = 7 × 107 M−1 s−1 ) is significantly greater than that of Equation (18) (k = 6.6 × 102 M−1 s−1 ). So, the conversion of SO4 •− to OH• via Equation (19)


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becomes more important under alkaline conditions [23]. Both SO4 •− and OH• are very strong oxidants in aqueous solution. They can oxidize NO dissolved in water to nitrite, which further oxidizes to NO2 and, finally, the NO dissolved in water is oxidized to nitrate, as represented by the generalized Equations (20)–(26), respectively. The amount of SO4 •− generation in the solution increased with the increase of Na2 S2 O8 concentration at the constant reaction temperature. Therefore, the NOx removal efficiency increased as the Na2 S2 O8 concentration increased. However, a further increase of Na2 S2 O8 concentration could lead to the production of a large amount of SO4 •− . Then, the self-recombination and the intercombination of oxidative free radicals, and the scavenging reactions by free radicals with the remaining persulfate ions might become significant (Equations (27)–(30)). Those scavenging reactions could compete with the NOx removal reactions to consume a large amount of free radicals, finally leading to the decrease of NOx removal efficiency [23,26,27]. Thus, considering the NOx removal and economic costs, the optimum Na2 S2 O8 concentration range of 0.3–0.5 wt.% and the optimal concentration of 0.5 wt.% were selected. S2 O8 2− (aq)

∆/alkaline

→

2SO4 •− (aq)

(17)

SO4 •− (aq) + H2 O(l) → H+ (aq) + SO4 2− (aq) + OH• (aq)

(18)

SO4 •− (aq) + OH− (aq) → SO4 2− (aq) + OH• (aq)

(19)

OH• (aq) + NO(l) → H+ (aq) + NO2− (aq)

(20)

SO4 •− (aq) + H2 O(l) + NO(l) → HSO4 − (aq) + H+ (aq) + NO2− (aq)

(21)

S2 O8 2− (aq) + NO2− (aq) → SO4 2− (aq) + SO4 •− (aq) + NO2 (l)

(22)

SO4 •− (aq) + NO2− (aq) → SO4 2− (aq) + NO2 (l)

(23)

NO2 (l) + OH• (aq) → H+ (aq) + NO3− (aq)

(24)

S2 O8 2− (aq) + NO(l) + H2 O(l) → 2HSO4 − (aq) + NO2 (l)

(25)

SO4 •− (aq) + OH• (aq) + NO(l) → HSO4 − (aq) + NO2 (l)

(26)

OH• (aq) + OH• (aq) → H2 O2 (aq)

(27)

SO4 •− (aq) + SO4 •− (aq) → S2 O8 2− (aq)

(28)

SO4 •− (aq) + S2 O8 2− (aq) → S2 O8 •− (aq) + SO4 2− (aq)

(29)

OH• (aq) + S2 O8 2− (aq) → S2 O8 •− (aq) + OH− (aq)

(30)

3.4. Effect of Initial pH The effect of the solution initial pH ranging from 6.0−12.0 on NOx removal was studied. Figure 6 illustrates that NOx removal efficiency was much higher under alkaline rather than acidic conditions, and the NOx removal efficiency sharply increased with the increase of the initial pH. When the initial pHs of the solutions were 6.0, 8.0, 10.0, and 12.0, the NOx removal efficiencies were 3.54, 18.46, 38.36, and 80.61%, respectively. Meanwhile, it was discovered that the NO concentration gradually increased, and the NO2 concentration gradually decreased in the exhaust with the increasing of initial pH. The average emission concentrations of NO were 0, 4.89, 11.22, and 20.39 ppm, and the average emission concentrations of NO2 were 482.30, 402.81, 269.84, and 87.18 ppm, when the initial pHs of the solution were 6.0, 8.0, 10.0, and 12.0, respectively. The research found that the fractional conversion of NO to NO2 could reach 100% at low pH. The literature reported that H+ could catalyze the decomposition of S2 O8 2 − to form SO3 and HSO4 − [23,27]. Hence, increasing the H+ concentration would promote the decomposition of S2 O8 2− , and inhibit the generation of free radicals, finally resulting in a decrease in the amount of reactive radicals produced, and the decrease of NOx removal. Meanwhile, H+ can also catalyze the decomposition of ClO2 − to generate strongly oxidizing

ppm, and the average emission concentrations of NO2 were 482.30, 402.81, 269.84, and 87.18 ppm, when the initial pHs of the solution were 6.0, 8.0, 10.0, and 12.0, respectively. The research found that the fractional conversion of NO to NO2 could reach 100% at low pH. The literature reported that H+ could catalyze the decomposition of S2O82− to form SO3 and HSO4− [23,27]. Hence, increasing the H+ concentration Sustainability 2018,would 10, 4350promote the decomposition of S2O82−, and inhibit the generation of free radicals, 9 of 18 finally resulting in a decrease in the amount of reactive radicals produced, and the decrease of NOx removal. Meanwhile, H+ can also catalyze the decomposition of ClO2− to generate strongly oxidizing ClO (Equations (12)–(14)) [20,28]. When the initial pH decreased from 12.0 to 6.0, the solution ClO22 (Equations (12)–(14)) [20,28]. When the initial pH decreased from 12.0 to 6.0, the solution color color varied from colorless to yellowish green, and then its color gradually deepened. result varied from colorless to yellowish green, and then its color gradually deepened. The resultThe indicated indicated large of amounts of ClO being produced and dissolving into the solution under 2 were that large that amounts ClO2 were being produced and dissolving into the solution under low pH low pH conditions. Dissolved ClO could react with dissolved NO to preferentially form NO2 2 with dissolved NO to preferentially form NO2 (Equation (15)) conditions. Dissolved ClO2 could react (Equation (15)) [29]. Although the NO2 produced could be further oxidized to nitrate, it was more likely [29]. Although the NO 2 produced could be further oxidized to nitrate, it was more likely to escape to escape from theunder solution low pH conditions [20],to leading to the increase of NO2 concentration from the solution lowunder pH conditions [20], leading the increase of NO2 concentration and the and the decrease of NO concentration in the exhaust with the decrease of solution pH [20]. the When the decrease of NO concentration in the exhaust with the decrease of solution pH [20]. When initial initial pH solution of the solution increased, NOx removal increased. This is because more free reactive pH of the increased, the NOthe x removal increased. This is because more free reactive radicals • radicals were generated (especially OH ) the inhibition the decomposition O8 2−ClO and • were generated (especially OH ) throughthrough the inhibition of the of decomposition of S2Oof82−S2and 2− − ClO under alkaline conditions. In particular, alkaline conditions could promote the conversion •of 2 under alkaline conditions. In particular, alkaline conditions could promote the conversion of SO • • •− •− SOOH by Equation (19). Meanwhile, escape amount • OH is more reactive than•SO4 4 • to to byOH Equation (19). OH is more reactive than SO [23]. [23]. Meanwhile, the the escape amount of of NO decreased significantly under alkaline conditions. So, NO removal increased with the increase x 2 NO2 decreased significantly under alkaline conditions. So, NOx removal increased with the increase of initial initial pH. pH. The The study study indicates indicates that that this this new new process process is is more more suitable suitable for for operation operation under under alkaline alkaline of conditions, so so in in this this study, study, the conditions, the best best pH pH was was determined determined to to be be 12.0. 12.0.

removal. Figure 6. Effect of initial solution pH on NOxx removal.

3.5. Effect Effect of of Solution Solution Temperature Temperature 3.5. ◦ C) to Several experiments experimentswere werecarried carriedout out various temperatures 40,60, 50,and 60, 70 and Several at at various temperatures (25,(25, 40, 50, °C)70 to study study the effects of solution temperature on NOx removal, andresults the results are shown in Figure 7. the effects of solution temperature on NO x removal, and the are shown in Figure 7. As As depicted in Figure 7, NO removal efficiency decreased slowly at first, then it decreased rapidly as x depicted in Figure 7, NOx removal efficiency decreased slowly at first, then it decreased rapidly as ◦ C. The NO the temperature increased, with the turning point point of the temperature increased, with the turning of the the solution solution temperature temperature being being 50 50 °C. The NOxx removal efficiencies were 91.77%, 84.48%, 80.61%, 66.29%, and 49.94% when the solution temperatures removal efficiencies were 91.77%, 84.48%, 80.61%, 66.29%, and 49.94% when the solution ◦ were 25, 40, 50,were 60, and respectively. The°C, research also found NO concentration temperatures 25,7040,C,50, 60, and 70 respectively. Thethat research also found decreased that NO and NO2 concentration increased in the exhaust with theinincrease of the solution temperature. concentration decreased and NO2 concentration increased the exhaust with the increase of the The average emissionThe concentrations of NOconcentrations were 33.72, 31.24, 20.39, and 0.0020.39, ppm,0.00, andand the solution temperature. average emission of NO were0.00, 33.72, 31.24, average of concentrations NO2 were 7.43,of46.36, 87.18,7.43, 168.55, and 250.30 ppm,and when the 0.00 ppm,emission and the concentrations average emission NO2 were 46.36, 87.18, 168.55, 250.30 ◦ C, respectively. The fractional conversion of NO solution temperature changed from 25 to 70 ppm, when the solution temperature changed from 25 to 70 °C, respectively. The fractional increased with increasing temperature, which is consistent with the results reported by Khan et al. [23]. Higher temperatures can significantly promote the activation of Na2 S2 O8 to produce a large amount of sulfate free radicals [30], and then to generate a large number of other free radicals, such as OH• . These free radicals can oxidize NO to nitrite and nitrate, to promote NO removal. However, the high temperatures could also promote ClO2 generation [20], leading to the fractional conversion of NO and an increase in the generation of NO2 . NO2 can easily escape from solution into gas, due to the high temperature and limited gas–liquid contact time, so that the NOx removal efficiency is decreased.

conversion of NO increased with increasing temperature, which is consistent with the results reported by Khan et al. [23]. Higher temperatures can significantly promote the activation of Na2S2O8 to produce a large amount of sulfate free radicals [30], and then to generate a large number of other free radicals, such as OH • . These free radicals can oxidize NO to nitrite and nitrate, to promote NO removal. the high temperatures could also promote ClO2 generation [20], leading 10 to ofthe SustainabilityHowever, 2018, 10, 4350 18 fractional conversion of NO and an increase in the generation of NO2. NO2 can easily escape from solution into gas, due to the high temperature and limited gas–liquid contact time, so that the NOx The influence of solution temperature on NOxofremoval ultimately determined by the two removal efficiency is decreased. The influence solutionistemperature on NOx removal is above ultimately aspects. The results indicate that the increase of absorption temperature is not conducive to the determined by the above two aspects. The results indicate that the increase of absorption temperature NO Thus, it shows that NaClO dual oxidant solution a more important role x removal. 2 in is not conducive to the NOx removal. Thus, it the shows that NaClO 2 in the plays dual oxidant solution plays than Na S O for the NO removal. The temperature of the absorption liquid of WFGD is usually x 2 2 8 a more important role than Na2S2O8 for the NOx removal. The temperature of the absorption liquid ◦ C. Considering the actual situation, in this study, the solution temperatures of approximately 40–50 approximately of WFGD is usually 40–50 °C. Considering the actual situation, in this study, the ◦ the new process were all selected as 50 solution temperatures of the new processC.were all selected as 50 °C.

Figure Figure 7. 7. Effect Effect of of solution solution temperature temperature on on NO NOxx removal. removal.

Effect of of Gas Gas Flow Flow Rate Rate 3.6. Effect is is shown in Figure 8. Results indicate that that the gas The effect effect of ofgas gasflow flowrate rateononNO NO x removal shown in Figure 8. Results indicate theflow gas x removal rate had an important effect on NO removal, and NO removal decreased with the increasing of the flow rate had an important effect on x NOx removal, and x NOx removal decreased with the increasing − 1 −1 gasthe flow rate. When gas flow rateflow increased from 1.0 tofrom 3.0 L1.0 ·minto 3.0 , theL·min NOx removal of gas flow rate.the When the gas rate increased , the NOefficiencies x removal were 88.28%, 83.30%, 80.61%, 72.07%, and 56.24%, Zhao et al. [2] also the efficiencies were 88.28%, 83.30%, 80.61%, 72.07%, andrespectively. 56.24%, respectively. Zhao et al.found [2] alsothat found removal efficiencies of SO , NO , and NO decreased as the gas flow rate increased. Meanwhile, it was that the removal efficiencies of 2 2 SO2, NO2, and NO decreased as the gas flow rate increased. also found that thealso average NO concentration sharply in the exhaust fromin10.67 ppm to Meanwhile, it was found that the average increased NO concentration increased sharply the exhaust −1 . The gas–liquid contact 195.5610.67 ppmppm withto the increase gas flow rate from 3.0 Lrate ·min time from 195.56 ppmofwith the increase of 1.0 gastoflow from 1.0 to 3.0 L·min−1. The gas– decreased withtime the decreased increase inwith the gas rate.in On the one hand, NO in theNO gasmolecules could not liquid contact theflow increase the gas flow rate. Onmolecules the one hand, have into theto solution. the other hand,On thethe NO byNO the2 in thethe gasnecessary could not time have to thedissolve necessary time dissolveOn into the solution. other hand, the 2 generated reactions was taken out of the further reaction to form nitrates. Therefore, NOx generated by the reactions wassystem taken without out of the system without further reaction to form the nitrates. removal efficiency reduced with an increase in the gas rate. in the gas flow rate. Therefore, the NOxwas removal efficiency was reduced with anflow increase 3.7. Effect of SO2 Concentration The effect of SO2 concentration on NOx removal in the range of 0–2100 ppm was investigated. Figure 9 indicates that the SO2 concentration in the flue gas had a very significant effect on NOx removal; the NOx removal efficiency was first rapidly increased, and then it decreased with the increase of SO2 concentration. For example, when the SO2 concentration increased from 0 to 500 ppm, the removal of NOx rapidly increased from 62.05% to 83.37%, then with the SO2 concentration further increasing from 500 ppm to 2100 ppm, the NOx removal efficiency decreased from 83.37% to 70.66%. Results indicate that the removal of NOx could be promoted at low SO2 concentration conditions, but higher concentrations of SO2 inhibited NOx removal. SO2 is very soluble in aqueous solution, and it dissolves into solution to generate HSO3 − , which can react with NO2 − to generate HON(SO3 )2 2− and ONSO3 − (Equations (31)–(33)) [30–32]. Meanwhile, dissolved NO can also directly react with SO3 2− to form ON(SO3 )2− and − ON(NO)(SO3 )2− (Equations (34) and (35)) [32]. These reactions promote the dissolution and oxidation of NO, while reducing the consumption of free radicals and oxidants,


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finally promoting NO removal. On the other hand, HSO3 − and SO3 2− also can react with NO2 to form NO2 − (Equations (36) and (37)) [32], then NO2 − can react with oxidants or free radicals to form NO3 − , finally resulting in the inhibition of NO2 volatilization from the solution, increasing NOx removal. The research found that the average NO2 concentration in the exhaust decreased from 167.89 to 73.72 ppm when the SO2 concentration changed from 0 to 500 ppm. This finding also confirmed the existence of the above reaction process. When the SO2 concentration was further increased, the NOx removal efficiency decreased. The main reason for this outcome was that severe competitive reactions of NO and SO2 for the limited oxidants and free radicals existed in the solution. As the solubility of SO2 is significantly greater than that of NO, so dissolved SO2 preferentially reacts with oxidants and free radicals in the solution (Equations (38)–(40)), and inhibits the oxidation of NO, finally resulting in a decrease of NOx removal. Meanwhile, the pH of the solution drops sharply with the large amount of SO2 dissolved, which will promote the decomposition of S2 O8 2− [23,27], and this is not conducive to NOx removal. In addition, the dissolved SO2 also can react with NO3− to form NO (Equation (41)); the NO can escape from the solution, leading to the decrease of NOx removal [20]. NO2− (aq) + H + (aq) + 2HSO3− (aq) ↔ HON(SO3 )22− (aq) + 2H2 O(l)

(31)

NO2− (aq) + H+ (aq) + HSO3− (aq) ↔ ONSO3− (aq) + 2H2 O(l)

(32)

ONSO3− (aq) + HSO3− (aq) → HON(SO3 )22− (aq)

(33)

NO(aq) + SO23− (aq) → ON(SO3 )2− (aq)

(34)

NO(aq) + ON(SO3 )2− (aq) → − ON(NO)SO3− (aq)

(35)

2NO2 (aq) + HSO3− (aq) + H2 O(l) → 2NO2− aq + SO24− (aq) + 3H+ (aq)

(36)

2NO2 (aq) + SO23− (aq) + H2 O(l) → 2NO2− aq + SO24− (aq) + 2H+ (aq)

(37)

S2 O8 2− (aq) + SO2 (l) + 2H2 O(l) → 2HSO4− (aq) + H2 SO4− (aq)

(38)

SO23− (aq) + 2OH• → H2 O(l) + SO24− (aq)

(39)

2SO2 (l) + ClO2− (aq) + 2H2 O(l) → 4H+ (aq) + 2SO24− (aq) + Cl− (aq)

(40)

3SO2 (l) + 2NO3− (aq) + 2H2 O(l) → 2NO(l) + 3SO24− (aq) + 4H+ (aq)


(41) 11 of 18

Figure Figure 8. 8. Effect Effect of of gas gas flow flow rate rate on NOxx removal. removal.

3.7. Effect of SO2 Concentration The effect of SO2 concentration on NOx removal in the range of 0–2100 ppm was investigated. Figure 9 indicates that the SO2 concentration in the flue gas had a very significant effect on NOx removal; the NOx removal efficiency was first rapidly increased, and then it decreased with the increase of SO2 concentration. For example, when the SO2 concentration increased from 0 to 500 ppm,

aq

(40)

3SO2 (l) + 2NO3 (aq) + 2H2 O(l) → 2NO(l) + 3SO24 aq + 4H aq

(41)

2SO2 (l) + ClO2 aq + 2H2 O(l) → 4H

aq + 2SO24 aq + Cl


12 of 18

Figure Figure 9. 9. Effect Effect of of SO SO22 concentration concentration on on NO NOxx removal. removal.

3.8. Effect Effect of of NO NO Concentration Concentration 3.8. Figure 10 10 indicates indicatesthat thatthe theeffect effectofofNO NO concentration results x removal. Figure concentration onon NONO x removal. The The results showshow that that NO removal was slightly affected by NO concentration. NO removal increased slowly from x x increased slowly from 78.33% NOx removal was slightly affected by NO concentration. NOx removal 78.33% to with 80.61%, with an of increase of NO concentration from to and 500 ppm, thendecreased it slowly to 80.61%, an increase NO concentration from 100 to 500100 ppm, then itand slowly decreased from 80.61% to 74.04%, with the NO concentration changing from 500 to 1000 ppm. from 80.61% to 74.04%, with the NO concentration changing from 500 to 1000 ppm. The keyThe stepkey in step in the removal of NO by using oxidation absorption method is to promote the NO dissolution. the removal of NO by using oxidation absorption method is to promote the NO dissolution. The The driving ofmass the mass transfer ofNO the absorption NO absorption increases concentration in flue driving forceforce of the transfer of the increases withwith NO NO concentration in flue gas gas increasing. Hence, the amount of dissolved NO increases with an increase of NO concentration increasing. Hence, the amount of dissolved NO increases with an increase of NO concentration in in flue Because amount of oxidants radicals relatively sufficient the flue gas.gas. Because the the amount of oxidants and and free free radicals werewere relatively sufficient whenwhen the NO NO concentration wasthan less 500 than 500 therefore ppm, therefore NOx removal increased with an concentration was less ppm, the NOthe x removal slightly slightly increased with an increase increase of NO concentration. However, theconcentration NO concentration in the exceeded500 500ppm, ppm, of NO concentration. However, when when the NO in the flueflue gasgas exceeded although the theamount amountofofdissolved dissolvedNO NOincreased, increased, but more amount of NO absorbed by although but more amount of NO waswas not not absorbed by the the solution and run out of the reactor with the flue gas due to the limited gas-liquid contact time. solution and run out of the reactor with the flue gas due to the limited gas-liquid contact time. Meanwhile, the the consumption consumption rates rates of of the the oxidants oxidants and and free free radicals radicals increased increased with with NO NO concentration concentration Meanwhile, increasing, and and the the pH pH value value of of the the absorbent absorbent solution solution also also dropped dropped very very quickly. quickly. These These reasons reasons all all increasing, are not conducive for the NO removal. Therefore, the combined result is that the absolute amount of are not conducive for the NO removal. Therefore, the combined result is that the absolute amount of NO removal removal increased, increased, but but the the NO NO removal removal efficiency efficiency slightly slightly reduced. reduced. For For example, example, when when the the NO NO NO concentrations in 1000 ppm, thethe NONO efficiencies were 80.61% and x removal concentrations in flue fluegas gaswere were500 500ppm ppmand and 1000 ppm, x removal efficiencies were 80.61% 74.04%, and and the absolute amount of NO werewere 86.8586.85 mg and mg, respectively. and 74.04%, the absolute amount of removal NO removal mg 159.54 and 159.54 mg, respectively. 3.9. Effect of O2 Concentration The influence of O2 concentration on NOx removal, ranging from 1 to 15% (v/v), was investigated, and the results are shown in Figure 11. The results demonstrate that O2 concentration in the flue gas had a certain effect on NOx removal. When the O2 concentration increased from 1 to 15% (v/v), the NOx removal increased slowly from 79.11 to 81.74%. This is because increasing the O2 concentration could lead to an increase in the O2 concentration in the liquid phase, which increased the concentration of the oxidants in the liquid phase. However, compared with NaClO2 and Na2 S2 O8 , the oxidation of O2 was so weak in the experimental conditions that the NOx removal increased slowly with an increase of O2 concentration.


13 of 18


13 of 18

Figure 10. Effect of NO concentration on NOx removal.

3.9. Effect of O2 Concentration The influence of O2 concentration on NOx removal, ranging from 1 to 15% (v/v), was investigated, and the results are shown in Figure 11. The results demonstrate that O2 concentration in the flue gas had a certain effect on NOx removal. When the O2 concentration increased from 1 to 15% (v/v), the NOx removal increased slowly from 79.11 to 81.74%. This is because increasing the O2 concentration could lead to an increase in the O2 concentration in the liquid phase, which increased the concentration of the oxidants in the liquid phase. However, compared with NaClO2 and Na2S2O8, the oxidation of O2 was so weak in the experimental conditions that the NOx removal increased slowly with an increase of O2 concentration. Figure 10. Effect of NO concentration on NOx removal. removal. x

3.9. Effect of O2 Concentration The influence of O2 concentration on NOx removal, ranging from 1 to 15% (v/v), was investigated, and the results are shown in Figure 11. The results demonstrate that O2 concentration in the flue gas had a certain effect on NOx removal. When the O2 concentration increased from 1 to 15% (v/v), the NOx removal increased slowly from 79.11 to 81.74%. This is because increasing the O2 concentration could lead to an increase in the O2 concentration in the liquid phase, which increased the concentration of the oxidants in the liquid phase. However, compared with NaClO2 and Na2S2O8, the oxidation of O2 was so weak in the experimental conditions that the NOx removal increased slowly with an increase of O2 concentration.

Figure 11. Effect of O22 concentration on NOxx removal. removal.

3.10. Tandem Double Column Absorption Experiments 3.10. Tandem Double Column Absorption Experiments The study found that when using a single absorber for absorption experiments, a maximum NO The study found that when using a single absorber for absorption experiments, a maximum NOxx removal of 80.61% could be obtained, and so that the average concentration of NO in the exhaust was removal of 80.61% could be obtained, and so that the average concentration of 2NO2 in the exhaust about 80 ppm at the optimal experimental conditions. Compared to NO, NO2 is more soluble, and it is was about 80 ppm at the optimal experimental conditions. Compared to NO, NO2 is more soluble, more easily absorbed by the solution. Therefore, in order to increase the NOx removal, several tandem and it is more easily absorbed by the solution. Therefore, in order to increase the NOx removal, several double column experiments were conducted. Na2 S2 O8 Na solution (5 wt.%), NaOH solution tandem double absorption column absorption experiments were conducted. 2S2O8 solution (5 wt.%), NaOH (5 wt.%), CO(NH ) solution (5 wt.%), and Ca(OH) solution (5 wt.%) were used to absorb the NOx 2 2 solution (5 wt.%),2 CO(NH 2)2 solution (5 wt.%), and Ca(OH)2 solution (5 wt.%) were used to absorb from the first absorber, and the results are illustrated in Figure 12. The results show that the NOx the NO x from the first absorber, and the results are illustrated in Figure 12. The results show that the removal efficiency could be significantly increased by using a tandem double column absorption Figure 11. Effect of O2 concentration on NOx removal. process, compared to using the single column absorption process. The absorption capacities of the four kinds of absorbents were in the order of Na2 S2 O8 > NaOH > Ca(OH)2 > CO(NH2 )2 , with NOx 3.10. Tandem Double Column Absorption Experiments removal efficiencies of 92.41, 91.12, 84.06, and 82.66%, respectively. The NOx removal efficiency by when using a single absorber absorption experiments, maximum NOx usingThe Nastudy as an that absorbent was higher than that offor the other three absorbents.a This is because 2 S2 O8found removal could obtained, and so that the for average concentration of NO 2 in the exhaust Na2 S2 O8 of as 80.61% an oxidant hasbe a good efficiency of removal both NO and NO2 . For alkaline absorbents, was about 80pH ppm optimal effect experimental Compared tovalues NO, NO is moreCa(OH) soluble, the solution hadatathe significant on NOx conditions. removal. The initial pH of 2NaOH, 2, and itCO(NH is more2 )easily absorbed by the solution. Therefore, in order to increase the NO x removal, several 2 solution were 13.6, 9.6, and 8.4, respectively. This was because the amount of NO2 tandem double column absorption experiments were conducted. Na2S2O8 solution (5 wt.%), NaOH solution (5 wt.%), CO(NH2)2 solution (5 wt.%), and Ca(OH)2 solution (5 wt.%) were used to absorb the NOx from the first absorber, and the results are illustrated in Figure 12. The results show that the

remove NO, and NO removal of 72% could be obtained [16]. Using NaClO2 as absorbent, and using a wet scrubber combined with a plasma electrostatic precipitator as reactor, the NO removal of 94.4% could be reached, but the absorption device of this method was complicated [33]. Therefore, this method has advantages in terms of NOx removal efficiency compared to other methods. The main difference 2018, between the method and the WFGD technology is the composition of the absorbent. Sustainability 10, 4350 14 of 18 Therefore, the investment cost of the method in practical engineering applications is comparable with that of WFGD technology. The operating cost of the method may be higher than that of WFGD in the composition of NOx was significantly higher than that of NO. Thus, NOx removal increased technology due to the high cost of the absorbent of this method. However, this method can with the solution pH increasing. Considering the NOx removal and economic costs, NaOH is a better simultaneously remove SO2 and NOx efficiently. Therefore, considering the economic and absorbent, due to its high initial pH. environmental aspects, this method has a good application prospect in the field of flue gas treatment.

Figure 12. Tandem double column absorption experiments.

In the Analysis study, when using a single column and double column to absorb NOx , the NOx removal 4. Product efficiencies of 80.61% and more than 90% could been obtained, respectively. The NOx removal In this study, the ionic products in solution were detected by using an ion chromatography efficiencies obtained in this work are significantly higher than that of our previous work (53.05%) [12]. system. As the peak times of persulfate ion and sulfate ion are almost the same, it is impossible to In other methods, a dual oxidant (H2 O2 /Na2 S2 O8 ) has been used to remove NO, and the highest measure persulfate ions and sulfate ions by the ion chromatography method when both are present. NO removal efficiency (82%) was obtained [22], but the concentrations of H2 O2 and Na2 S2 O8 were Fortunately, it is clear that the final decomposition product of persulfate is sulfate [21–23], and the 0.3 mol/L and 0.1 mol/L respectively which were higher than this work. UV/H2 O2 was also utilized SO2 that dissolves in the solution is eventually oxidized to sulfate in the oxidation system. Therefore, to remove NO, and NO removal of 72% could be obtained [16]. Using NaClO2 as absorbent, and using a wet scrubber combined with a plasma electrostatic precipitator as reactor, the NO removal of 94.4% could be reached, but the absorption device of this method was complicated [33]. Therefore, this method has advantages in terms of NOx removal efficiency compared to other methods. The main difference between the method and the WFGD technology is the composition of the absorbent. Therefore, the investment cost of the method in practical engineering applications is comparable with that of WFGD technology. The operating cost of the method may be higher than that of WFGD technology due to the high cost of the absorbent of this method. However, this method can simultaneously remove SO2 and NOx efficiently. Therefore, considering the economic and environmental aspects, this method has a good application prospect in the field of flue gas treatment. 4. Product Analysis In this study, the ionic products in solution were detected by using an ion chromatography system. As the peak times of persulfate ion and sulfate ion are almost the same, it is impossible to measure persulfate ions and sulfate ions by the ion chromatography method when both are present. Fortunately, it is clear that the final decomposition product of persulfate is sulfate [21–23], and the SO2 that dissolves in the solution is eventually oxidized to sulfate in the oxidation system. Therefore, the study focused on the other anions. The IC analysis results of the ionic components in the solution before and after the reaction are shown in Table 2.


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Table 2. Analysis of ionic components in the absorption solutions (mg/L).

Initial End

NO2 −

NO3 −

Cl−

ClO2 −

ClO3 −

— —

— 142.58

95.888 145.515

901.050 845.950

119.975 85.425

Results show that only Cl− , ClO2 − , and ClO3 − were detected in the solution before the reaction (irrespective of persulfate and sulfate). However, NO3 − , ClO2 − , Cl− , and ClO3 − were detected after the reaction. From a comparison of ionic composition before and after the reaction, Cl− concentration significantly increased, while ClO2 − and ClO3 − concentration decreased. Equation (42) could be used to explain why the amount of ClO3 − was decreased. According to the calculation results of Cl material balance, the amounts of elemental Cl in the solution before and after the reaction were 620.78 and 626.74 mg/L, respectively. The results indicate that the Cl element was maintained with mass conservation during the reaction. It was also found that there existed a large amount of NO3 − , but no NO2 − , in the solution. This result is mainly attributed to the strong oxidizing properties of the system. Due to the presence of large amounts of free radicals and oxidants in the system, the NO2 − formed during the reaction are easily oxidized to NO3 − : 2ClO3 − (aq) + SO3 2− (aq) + 2H+ (aq) → SO4 2− (aq) + 2ClO2 (l) + H2 O(l)

(42)

As listed in Table 2, NO3 − was measured in the absorption solution with a concentration of 142.58 mg/L. The average initial concentration of NO in the flue gas was 516 ppm (about 691 mg/m3 ). The NOx removal efficiency was calculated to be 83.20%, according to the NO3 − concentration in the solution, which was close to the actual NOx removal efficiency of 81.12%. One of the disadvantages of the liquid absorption method is the disposal of the wastewater. Table 2 indicates that a large amount of Cl− is present in the absorption solution. The presence of large amounts of Cl− in the solution will increase the difficulty of wastewater treatment. Fortunately, the Friedel’s salt precipitation method has been developed to effectively remove Cl− from wastewater. Meanwhile the method can effectively synergistic remove SO4 2− , F− and heavy metal ions. The purified wastewater can be reused to reduce the consumption of water and alkali, or discharged into the enterprise waste water treatment system. The precipitated solids can be used to replace part of alkali or landfilled [34]. Therefore, through the effective treatment of wastewater, the ecological risk caused by the use of absorbent can be avoided. Figure 13 shows the NO and NO2 emissions in the outlet flue gas at the optimal conditions. As the reaction progressed, the NO concentration sharply decreased to nearly 0 ppm, and this was maintained for nearly 25 min at first, then it slowly increased to about 25 ppm at the end of the experiment. The concentration of NO2 increased sharply to about 85 ppm at first, then it decreased slowly to about 75 ppm. This is because the NaClO2 and Na2 S2 O8 concentrations in the solution decreased as the reaction proceeded, while the solution pH also decreased rapidly with the reaction time increasing. In addition, due to the strongly alkaline pH of the solution, the SO2 concentration decreased drastically to 0 ppm, and this was maintained until the end of the reaction. Based on experimental results and the literature, the mechanism of simultaneous removal of NOx and SO2 using NaClO2 /Na2 S2 O8 solution was deduced. The mechanism of SO2 removal is clear and relatively simple; SO2 is eventually converted to SO4 2− via absorption, acid–base neutralization, and oxidation. The removal mechanism of NOx is very complicated. Equations (2)–(42) can be used to explain the mechanism of NOx removal; finally, NO is partially converted into NO2 , while most of the NO is converted into NO3 − , which exists in the solution. Finally, SO2 and NOx are removed efficiently by the dual oxidant solution.

Sustainability 2018, 10, 4350 Sustainability 2018, 10, x FOR PEER REVIEW

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Figure 13. NO and NO2 emissions in the outlet flue gas. Figure 13. NO and NO2 emissions in the outlet flue gas.

5. Conclusions Based on experimental results and the literature, the mechanism of simultaneous removal of this simultaneous removal of NOx and SO2 through a simple process a composite NOx In and SOwork, 2 using NaClO2/Na2S2O8 solution was deduced. The mechanism of SOusing 2 removal is clear absorbent (NaClO /Na S O ) has been studied in a bubble column reactor. In view of the high SO2 2− 2 2 2 8 and relatively simple; SO2 is eventually converted to SO4 via absorption, acid–base neutralization, removal efficiency, factors affecting the NO removal were systematically investigated. The following and oxidation. The removal mechanism ofx NOx is very complicated. Equations (2)–(42) can be used conclusions can be obtained on experimental results: to explain the mechanism of based NOx removal; finally, NO is partially converted into NO2, while most of − the NO is converted into NO 3 , which exists in the solution. Finally, SO2 and NOx are removed (1) NaClO2 in the solution played a more important role than Na2 S2 O8 for NOx removal. NaClO2 and efficiently dual oxidant solution. Na S by O the concentrations, solution temperature, the initial pH of the solution, the gas flow rate, 2 2

8

and SO2 , NO, and O2 concentrations all had a certain impact on the NOx removal efficiency. 5. Conclusions Among them, solution temperature, the initial pH of the solution, and the oxidant concentrations had significant effects on the NOx removal In this work, simultaneous removal of NOx efficiency. and SO2 through a simple process using a composite absorbent (NaClOthe 2/Na 2S2x Oremoval 8) has been studiedand in aitsbubble column In view of the high SOx2 (2) Considering NO efficiency economic costs,reactor. the optimal conditions for NO ◦ C, the initial removal efficiency, factors affecting NOthe x removal were systematically investigated. The following removal were determined to bethe when solution temperature was 50 solution pH − 1 conclusions experimental results: was 12,can the be gasobtained flow ratebased was 2on L·min , and NaClO 2 and Na2 S2 O8 concentrations were 0.2 wt.% and 0.5 wt.%, respectively. A NOx removal efficiency of more than 80% could be obtained at (1) NaClO2 in the solution played a more important role than Na2S2O8 for NOx removal. NaClO2 optimal conditions. When using a NaOH solution as an absorbent in the second absorber, the NO and Na2S2O8 concentrations, solution temperature, the initial pH of the solution, the gas flowx removal efficiency could reach more than 90%. rate, and SO2, NO, and O2 concentrations all had a certain impact on the NOx removal efficiency. (3) Among A preliminary reactiontemperature, mechanism for simultaneous removal and of NO and SO2concentrations was deduced, them, solution thethe initial pH of the solution, thex oxidant based on experimental results. The dual oxidant (NaClO /Na S O ) solution can effectively 2 2 2 8 had significant effects on the NOx removal efficiency. remove multi-pollutants and, thus, it has the potential to be applied in the wet desulfurization (2) Considering the NOx removal efficiency and its economic costs, the optimal conditions for NOx and denitration process to realize the synergistic removal of multi-pollutants. removal were determined to be when the solution temperature was 50 °C, the initial solution

pH was 12, the gas flow rate was 2 L·min−1, and NaClO2 and Na2S2O8 concentrations were 0.2 Author Contributions: Data curation: P.F. and Z.T.; formal analysis: P.Z. and X.C.; wt.% and 0.5 wt.%, respectively. A NO x removal efficiency of more thanJ.H.; 80%methodology: could be obtained project administration: P.F.; supervision: C.C.; validation: Z.T.; writing—original draft: P.F.

at optimal conditions. When using a NaOH solution as an absorbent in the second absorber, the

Funding: research was supported by the National Key R and D Program of China (2017YFC0210704), NOx This removal efficiency could reach more than 90%. National Natural Science Foundation of China (NSFC-51778264), the Project of Science and Technology Program (3)Guangdong A preliminary reaction mechanism the simultaneous ofTalent NOx and SOSupport 2 was deduced, of Province (2016B020241002 andfor 2017B020237002), Youthremoval Top-notch Special Program based onProvince experimental results. and ThePearl dual oxidant (NaClO 2/Na2S2O solution can effectively of Guangdong (2016TQ03Z576) River S and T Nova Program of8)Guangzhou (201610010150).

remove multi-pollutants thus, it has the Conflicts of Interest: The authorsand, declare no conflicts of potential interest. to be applied in the wet desulfurization and denitration process to realize the synergistic removal of multi-pollutants. References

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