Simultaneous Removal of NO_x and SO₂ through a Simple Process Using a Composite Absorbent

Abstract

In this work, the feasibility of the simultaneous removal of NO_x and SO₂ through a simple process using a composite absorbent (NaClO₂/Na₂S₂O₈) was evaluated. Factors affecting the removal of NO_x and SO₂, such as NaClO₂ and Na₂S₂O₈ concentrations, solution temperature, the initial pH of solution, gas flow rate, and SO₂, NO, and O₂ concentrations were studied, with a special attention to NO_x removal. Results indicate that a synergistic effect on NO_x removal has been obtained through combination of NaClO₂ and Na₂S₂O₈. NaClO₂ in the solution played a more important role than did Na₂S₂O₈ for the removal of NO_x. The above factors had an important impact on the removal of NO_x, 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 NO_x removal of more than 80% was obtained. Meanwhile, tandem double column absorption experiments were conducted, and a NO_x removal efficiency of more than 90% was reached, using NaOH solution as an absorbant in the second reactor. A preliminary reaction mechanism for NO_x 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.

1. Introduction

NO_x (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 NO_x, 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 NO_x 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 NO_x control technology is developed, and it is better to develop an economical, and simplified method to achieve synergistic removal NO_x and SO₂.

The subject of multi-pollutant (NO_x, 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 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 (K₂FeO₄) [13], sodium chlorite (NaClO₂) [14], sodium hypochlorite (NaClO) [15], UV/H₂O₂ [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 Na₂S₂O₈ 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, NO_x, 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 (H₂O₂/Na₂S₂O₈) 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. Na₂S₂O₈ 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 NO_x. Hence, in this study, a composite absorbent consisting of NaClO₂ and Na₂S₂O₈ was utilized to simultaneously remove SO₂ and NO_x. The main purpose of this study is to evaluate the feasibility of simultaneous removal of SO₂ and NO_x by using a dual oxidant solution. Meanwhile, factors affecting the removal of NO_x and SO₂, such as NaClO₂ and Na₂S₂O₈ concentrations, solution temperature, the initial pH of solution, gas flow rate, and SO₂, NO, and O₂ concentrations were studied, with a special attention to NO_x removal. The preliminary reaction mechanisms of SO₂ and NO_x 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 Na₂S₂O₈ (98%, AR). CO(NH₂)₂ (99%, AR), Ca(OH)₂ (95%, AR), Na₂S₂O₈ (98%, AR), and NaOH (96%, AR), were used in the second absorber when needed (Figure 1). The solution pH was 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.

2.2. Equipment

The experimental apparatus consisted of a simulated flue gas generation system, a wet scrubbing system, an online monitoring system and an exhaust gas adsorption system, as shown in Figure 1.

SO₂, NO, O₂, and N₂ gases were supplied by the compressed cylinders (Gas Co., Ltd. of Zhuo Zheng, Guangzhou, China), and metered by the mass flow controllers (Beijing Sevenstar Flow Co., Ltd., Beijing, China). Various gases flowed into the Mixer 1 to produce the flue gas, where the total flue gas flow (Q) was kept at 2 L/min. The two absorbers (4 cm i.d. × 65 cm length) were all made of borosilicate glass. The first absorber was filled with ceramic Raschig rings (1.25 cm i.d. × 1.25 cm length) of 20 cm height, heated by a temperature-controlled heating belt, and the second absorber was an empty column. The volume of the solution in the absorber was 1 L. The simulated flue gas was introduced into the first absorber through a gas distribution pipe fitted at the bottom of the absorber. When the tandem double column absorption experiments were conducted, the simulated flue gas flowed through two absorbers in sequence. The purified flue gas was then dried by anhydrous CaCl₂ and then it was introduced into the detection system. In this work, SO₂ concentration range was 0–2100 ppm, NO concentration range was 0–1000 ppm, O₂ concentration range was 1–15% (v/v). The time for each set of experiments was 1 h. The experimental conditions were summarized and shown in

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, [Na2S2O8] = 0.5 wt.%, pH = 12
3	Effect of Na2S2O8 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, [Na2S2O8] = 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%, [Na2S2O8] = 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, [Na2S2O8] = 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, [Na2S2O8] = 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, [Na2S2O8] = 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, [Na2S2O8] = 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, [Na2S2O8] = 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, [Na2S2O8] = 0.5 wt.%, [NaClO2] = 0.2 wt.%, pH = 12

. An ECOM-J2KN flue gas analyzer (RBR Company, Germany) was utilized to detect online the inlet and outlet SO₂, NO₂, NO, and O₂ concentrations. An MP511 pH Detector (Shanghai Precision Instruments Co., Ltd., Shanghai, China) was used to detect the solution pH. Concentrations of NO₃⁻, Cl⁻, ClO₂⁻, and ClO₃⁻ were measured with an ion chromatography system (IC, Thermo ICS-1100, America). The NO_x and SO₂ removal were calculated by Equation (1):

$$\mathsf{\eta }=\frac{{\mathrm{C}}_{\mathrm{inlet}}-{\mathrm{C}}_{\mathrm{outlet}}}{{\mathrm{C}}_{\mathrm{inlet}}}\times 100\%$$

(1)

where η is the removal efficiency of NO_x or SO₂; C_inlet and C_outlet are the inlet and outlet concentrations of NO_x or SO₂, respectively.

3. Results and Discussion

3.1. Simultaneous Removal of SO₂ and NO_x with Different Oxidant Solutions

Based on the literature and on our previous works [12,20,22,23], NaClO₂, KMnO₄, and Na₂S₂O₈ were considered as better NO oxidants. Thus, in this study, the contrast experiments for different oxidants on SO₂ and NO_x removal were investigated, and the results are displayed in Figure 2 and Figure 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 Na₂S₂O₈ + 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 NO_x removal, and the capacity on the NO_x removal of three oxidant solutions was NaClO₂ > KMnO₄ > Na₂S₂O₈, with the average NO_x removal efficiencies being at 48.69%, 48.13%, and 23.98%, respectively. Although the average efficiency of NO_x removal by KMnO₄ solution was close to that of the NaClO₂ solution, NO_x removal efficiency by the KMnO₄ solution decreased, with the reaction time increasing. Thus, it can be speculated that the NO_x removal efficiency by the NaClO₂ solution is significantly better than that of KMnO₄ solution with the increase of reaction time. However, a NO_x 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 (0.2 wt.%) by adding a certain amount of Na₂S₂O₈ (0.1 wt.%), and the results are illustrated in Figure 2 and Figure 3. The results show that SO₂ and NO_x could be efficiently absorbed, and nearly 100% of the SO₂ and 73.51% of the NO_x were simultaneously removed. NO_x removal using a dual oxidant solution was much better than removal by using a single oxidant solution. SO₂ removal remained constant at about 100% in all tests, but the NO_x removal was affected by many factors, so the emphasis was placed on the discussion of NO_x removal by using a NaClO₂ + Na₂S₂O₈ solution, in subsequent experiments.

3.2. Effect of NaClO₂ Concentration

Several sets of experiments at various NaClO₂ concentrations (0, 0.05, 0.1, 0.15, 0.20, and 0.30 wt.%) were carried out to study the effect of NaClO₂ concentration on NO_x removal efficiency at the same persulfate level (0.5 wt.%). Figure 4 shows that the NO_x removal efficiency sharply increased with the increment of NaClO₂ concentration at first, and then it gradually increased; when the NaClO₂ concentration increased to 0.20 wt.%, a maximum NO_x removal efficiency of 80.61% was obtained, and thereafter, NO_x 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 NO to nitrate (Equation (4)) [20]. So, even the introduction of 0.05 wt.% of NaClO₂ had a very pronounced effect, where the NO_x removal efficiency went increased from 29.16% without any NaClO₂ to 54.68% with NaClO₂. Moreover, Na₂S₂O₈ in the solution could be activated to generate sulfate-free radicals (${\mathrm{SO}}_4{}^{•-}$) and hydroxyl radicals (${\mathrm{OH}}^{•}$), and a large number of chlorine-free radicals (${\mathrm{Cl}}^{•}$,${\mathrm{Cl}}_2^{•-}$, ${\mathrm{ClOH}}^{•-}$) could be produced through complex reactions via the reactions of Equations (5)–(8) [23,24,25]. The chlorine-free radicals could react with dissolved NO to form nitrates and nitrites (Equations (9)–(11)). Therefore, the addition of Na₂S₂O₈ could increase the total number of reactive radical species, and enhance the NO_x 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 NO_x 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.%.

$$2\mathrm{NO}\left(\mathrm{l}\right)+{\mathrm{ClO}}_2^{-}\left(\mathrm{aq}\right)\to 2{\mathrm{NO}}_2\left(\mathrm{l}\right)+{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)$$

(2)

$$4{\mathrm{NO}}_2\left(\mathrm{l}\right)+{\mathrm{ClO}}_2^{-}\left(\mathrm{aq}\right)+4{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)\to 4{\mathrm{NO}}_3^{-}\left(\mathrm{aq}\right)+{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$$

(3)

$$4\mathrm{NO}\left(\mathrm{l}\right)+3{\mathrm{ClO}}_2^{-}\left(\mathrm{aq}\right)+4{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)\to 4{\mathrm{NO}}_3^{-}\left(\mathrm{aq}\right)+3{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$$

(4)

$${\mathrm{SO}}_4{}^{•-}\left(\mathrm{aq}\right)+{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)\leftrightarrow {\mathrm{SO}}_4{}^{2-}\left(\mathrm{aq}\right)+{\mathrm{Cl}}^{•}\left(\mathrm{aq}\right)$$

(5)

$${\mathrm{Cl}}^{•}\left(\mathrm{aq}\right)+{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)\leftrightarrow {\mathrm{Cl}}_2^{•-}\left(\mathrm{aq}\right)$$

(6)

$${\mathrm{OH}}^{•}\left(\mathrm{aq}\right)+{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)\leftrightarrow {\mathrm{ClOH}}^{•-}\left(\mathrm{aq}\right)$$

(7)

$${\mathrm{ClOH}}^{•-}\left(\mathrm{aq}\right)+{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)\to {\mathrm{Cl}}_2^{•-}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)$$

(8)

$${\mathrm{ClOH}}^{•-}\left(\mathrm{aq}\right)+\mathrm{NO}\left(\mathrm{l}\right)\to {\mathrm{HNO}}_2\left(\mathrm{aq}\right)+{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)$$

(9)

$${\mathrm{Cl}}_2^{•-}\left(\mathrm{aq}\right)+{\mathrm{NO}}_2^{-}\left(\mathrm{aq}\right)\to {\mathrm{NO}}_2\left(\mathrm{l}\right)+2{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)$$

(10)

$${\mathrm{ClOH}}^{•-}\left(\mathrm{aq}\right)+{\mathrm{NO}}_2\left(\mathrm{l}\right)\to {\mathrm{HNO}}_3\left(\mathrm{aq}\right)+{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)$$

(11)

$$8{\mathrm{H}}^{+}\left(\mathrm{aq}\right)+8{\mathrm{ClO}}_2^{-}\left(\mathrm{aq}\right)\to 6{\mathrm{ClO}}_2\left(\mathrm{l}\right)+{\mathrm{Cl}}_2\left(\mathrm{l}\right)+4{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$$

(12)

$$4{\mathrm{H}}^{+}\left(\mathrm{aq}\right)+5{\mathrm{ClO}}_2^{-}\left(\mathrm{aq}\right)\to 4{\mathrm{ClO}}_2\left(\mathrm{l}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)$$

(13)

$$4{\mathrm{ClO}}_2{}^{-}\left(\mathrm{aq}\right)+2{\mathrm{H}}^{+}\left(\mathrm{aq}\right)\to {\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)+2{\mathrm{ClO}}_2\left(\mathrm{l}\right)+{\mathrm{ClO}}_3{}^{-}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$$

(14)

$$5\mathrm{NO}\left(\mathrm{l}\right)+2{\mathrm{ClO}}_2\left(\mathrm{l}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to 5{\mathrm{NO}}_2\left(\mathrm{l}\right)+2\mathrm{HCl}\left(\mathrm{aq}\right)$$

(15)

$$5{\mathrm{NO}}_2\left(\mathrm{l}\right)+{\mathrm{ClO}}_2\left(\mathrm{l}\right)+3{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to 5{\mathrm{NO}}_3^{-}\left(\mathrm{aq}\right)+{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)+6{\mathrm{H}}^{+}\left(\mathrm{aq}\right)$$

(16)

3.3. Effect of Na₂S₂O₈ Concentration

The results in Figure 5 illustrate the effect of Na₂S₂O₈ concentration on NO_x removal efficiency at a constant NaClO₂ concentration of 0.2 wt.% and at 50 °C. The addition of Na₂S₂O₈ could effectively enhance the NO_x removal efficiency, and Na₂S₂O₈ concentration had an important influence on the absorption of NO_x. Figure 5 shows that when the Na₂S₂O₈ concentration in the solution increased from 0.00 wt.% to 0.1 wt.%, the NO_x removal efficiency rapidly increased from 48.69% to 72.34%, and then it slightly increased from 72.34% to 80.61% in the Na₂S₂O₈ concentration range of 0.1 wt.% to 0.5 wt.%. However, further increases in Na₂S₂O₈ concentration resulted in a slight drop in NO_x removal efficiency, which then maintained stability. For example, the NO_x removal efficiencies were 78.96% and 79.42% when the Na₂S₂O₈ concentrations were 0.7 wt.% and 0.9 wt.%, respectively.

Na₂S₂O₈ is a strong and nonselective oxidant, and it can be activated by heat and alkaline [21], to produce an intermediate sulfate free radical (${\mathrm{SO}}_4{}^{•-}$) (Equation (17)) [23,26], then ${\mathrm{SO}}_4{}^{•-}$ reacts with hydroxide ions or water molecules to produce a sulfate ion and hydroxyl radicals (${\mathrm{OH}}^{•}$) (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 ${\mathrm{SO}}_4{}^{•-}$ to ${\mathrm{OH}}^{•}$ via Equation (19) becomes more important under alkaline conditions [23]. Both ${\mathrm{SO}}_4{}^{•-}$ and ${\mathrm{OH}}^{•}$ 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 ${\mathrm{SO}}_4{}^{•-}$ generation in the solution increased with the increase of Na₂S₂O₈ concentration at the constant reaction temperature. Therefore, the NO_x removal efficiency increased as the Na₂S₂O₈ concentration increased. However, a further increase of Na₂S₂O₈ concentration could lead to the production of a large amount of ${\mathrm{SO}}_4{}^{•-}$. 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 NO_x removal reactions to consume a large amount of free radicals, finally leading to the decrease of NO_x removal efficiency [23,26,27]. Thus, considering the NO_x removal and economic costs, the optimum Na₂S₂O₈ concentration range of 0.3–0.5 wt.% and the optimal concentration of 0.5 wt.% were selected.

$${\mathrm{S}}_2{\mathrm{O}}_8{}^{2-}\left(\mathrm{aq}\right)\stackrel{\mathsf{\Delta }/\mathrm{alkaline}}{\to }2{\mathrm{SO}}_4{}^{•-}\left(\mathrm{aq}\right)$$

(17)

$${\mathrm{SO}}_4{}^{•-}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to {\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{SO}}_4{}^{2-}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{•}\left(\mathrm{aq}\right)$$

(18)

$${\mathrm{SO}}_4{}^{•-}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)\to {\mathrm{SO}}_4{}^{2-}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{•}\left(\mathrm{aq}\right)$$

(19)

$${\mathrm{OH}}^{•}\left(\mathrm{aq}\right)+\mathrm{NO}\left(\mathrm{l}\right)\to {\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{NO}}_2^{-}\left(\mathrm{aq}\right)$$

(20)

$${\mathrm{SO}}_4{}^{•-}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+\mathrm{NO}\left(\mathrm{l}\right)\to {\mathrm{HSO}}_4{}^{-}\left(\mathrm{aq}\right)+{\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{NO}}_2^{-}\left(\mathrm{aq}\right)$$

(21)

$${\mathrm{S}}_2{\mathrm{O}}_8{}^{2-}\left(\mathrm{aq}\right)+{\mathrm{NO}}_2^{-}\left(\mathrm{aq}\right)\to {\mathrm{SO}}_4{}^{2-}\left(\mathrm{aq}\right)+{\mathrm{SO}}_4{}^{•-}\left(\mathrm{aq}\right)+{\mathrm{NO}}_2\left(\mathrm{l}\right)$$

(22)

$${\mathrm{SO}}_4{}^{•-}\left(\mathrm{aq}\right)+{\mathrm{NO}}_2^{-}\left(\mathrm{aq}\right)\to {\mathrm{SO}}_4{}^{2-}\left(\mathrm{aq}\right)+{\mathrm{NO}}_2\left(\mathrm{l}\right)$$

(23)

$${\mathrm{NO}}_2\left(\mathrm{l}\right)+{\mathrm{OH}}^{•}\left(\mathrm{aq}\right)\to {\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{NO}}_3^{-}\left(\mathrm{aq}\right)$$

(24)

$${\mathrm{S}}_2{\mathrm{O}}_8{}^{2-}\left(\mathrm{aq}\right)+\mathrm{NO}\left(\mathrm{l}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to 2{\mathrm{HSO}}_4{}^{-}\left(\mathrm{aq}\right)+{\mathrm{NO}}_2\left(\mathrm{l}\right)$$

(25)

$${\mathrm{SO}}_4{}^{•-}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{•}\left(\mathrm{aq}\right)+\mathrm{NO}\left(\mathrm{l}\right)\to {\mathrm{HSO}}_4{}^{-}\left(\mathrm{aq}\right)+{\mathrm{NO}}_2\left(\mathrm{l}\right)$$

(26)

$${\mathrm{OH}}^{•}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{•}\left(\mathrm{aq}\right)\to {\mathrm{H}}_2{\mathrm{O}}_2\left(\mathrm{aq}\right)$$

(27)

$${\mathrm{SO}}_4{}^{•-}\left(\mathrm{aq}\right)+{\mathrm{SO}}_4{}^{•-}\left(\mathrm{aq}\right)\to {\mathrm{S}}_2{\mathrm{O}}_8{}^{2-}\left(\mathrm{aq}\right)$$

(28)

$${\mathrm{SO}}_4{}^{•-}\left(\mathrm{aq}\right)+{\mathrm{S}}_2{\mathrm{O}}_8{}^{2-}\left(\mathrm{aq}\right)\to {\mathrm{S}}_2{\mathrm{O}}_8{}^{•-}\left(\mathrm{aq}\right)+{\mathrm{SO}}_4{}^{2-}\left(\mathrm{aq}\right)$$

(29)

$${\mathrm{OH}}^{•}\left(\mathrm{aq}\right)+{\mathrm{S}}_2{\mathrm{O}}_8{}^{2-}\left(\mathrm{aq}\right)\to {\mathrm{S}}_2{\mathrm{O}}_8{}^{•-}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)$$

(30)

3.4. Effect of Initial pH

The effect of the solution initial pH ranging from 6.0−12.0 on NO_x removal was studied. Figure 6 illustrates that NO_x removal efficiency was much higher under alkaline rather than acidic conditions, and the NO_x 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 NO_x 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 S₂O₈²⁻ to form SO₃ and HSO₄⁻ [23,27]. Hence, increasing the H⁺ concentration would promote the decomposition of S₂O₈²⁻, and inhibit the generation of free radicals, finally resulting in a decrease in the amount of reactive radicals produced, and the decrease of NO_x removal. Meanwhile, H⁺ can also catalyze the decomposition of ClO₂⁻ to generate strongly oxidizing ClO₂ (Equations (12)–(14)) [20,28]. When the initial pH decreased from 12.0 to 6.0, the solution color varied from colorless to yellowish green, and then its color gradually deepened. The result indicated that large amounts of ClO₂ were being produced and dissolving into the solution under low pH conditions. Dissolved ClO₂ could react with dissolved NO to preferentially form NO₂ (Equation (15)) [29]. Although the NO₂ produced could be further oxidized to nitrate, it was more likely to escape from the solution under low pH conditions [20], leading to the increase of NO₂ concentration and the decrease of NO concentration in the exhaust with the decrease of solution pH [20]. When the initial pH of the solution increased, the NO_x removal increased. This is because more free reactive radicals were generated (especially ${\mathrm{OH}}^{•}$) through the inhibition of the decomposition of S₂O₈²⁻ and ClO₂⁻ under alkaline conditions. In particular, alkaline conditions could promote the conversion of ${\mathrm{SO}}_4{}^{•-}$ to ${\mathrm{OH}}^{•}$ by Equation (19). ${\mathrm{OH}}^{•}$ is more reactive than ${\mathrm{SO}}_4{}^{•-}$ [23]. Meanwhile, the escape amount of NO₂ decreased significantly under alkaline conditions. So, NO_x removal increased with the increase of initial pH. The study indicates that this new process is more suitable for operation under alkaline conditions, so in this study, the best pH was determined to be 12.0.

3.5. Effect of Solution Temperature

Several experiments were carried out at various temperatures (25, 40, 50, 60, and 70 °C) to study the effects of solution temperature on NO_x removal, and the results are shown in Figure 7. As depicted in Figure 7, NO_x removal efficiency decreased slowly at first, then it decreased rapidly as the temperature increased, with the turning point of the solution temperature being 50 °C. The NO_x removal efficiencies were 91.77%, 84.48%, 80.61%, 66.29%, and 49.94% when the solution temperatures were 25, 40, 50, 60, and 70 °C, respectively. The research also found that NO concentration decreased and NO₂ concentration increased in the exhaust with the increase of the solution temperature. The average emission concentrations of NO were 33.72, 31.24, 20.39, 0.00, and 0.00 ppm, and the average emission concentrations of NO₂ were 7.43, 46.36, 87.18, 168.55, and 250.30 ppm, when the solution temperature changed from 25 to 70 °C, respectively. The fractional 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 ${\mathrm{OH}}^{•}$. 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 NO_x removal efficiency is decreased. The influence of solution temperature on NO_x removal is ultimately determined by the above two aspects. The results indicate that the increase of absorption temperature is not conducive to the NO_x removal. Thus, it shows that NaClO₂ in the dual oxidant solution plays a more important role than Na₂S₂O₈ for the NO_x removal. The temperature of the absorption liquid of WFGD is usually approximately 40–50 °C. Considering the actual situation, in this study, the solution temperatures of the new process were all selected as 50 °C.

3.6. Effect of Gas Flow Rate

The effect of gas flow rate on NO_x removal is shown in Figure 8. Results indicate that the gas flow rate had an important effect on NO_x removal, and NO_x removal decreased with the increasing of the gas flow rate. When the gas flow rate increased from 1.0 to 3.0 L·min⁻¹, the NO_x removal efficiencies were 88.28%, 83.30%, 80.61%, 72.07%, and 56.24%, respectively. Zhao et al. [2] also found that the removal efficiencies of SO₂, NO₂, and NO decreased as the gas flow rate increased. Meanwhile, it was also found that the average NO concentration increased sharply in the exhaust from 10.67 ppm to 195.56 ppm with the increase of gas flow rate from 1.0 to 3.0 L·min⁻¹. The gas–liquid contact time decreased with the increase in the gas flow rate. On the one hand, NO molecules in the gas could not have the necessary time to dissolve into the solution. On the other hand, the NO₂ generated by the reactions was taken out of the system without further reaction to form nitrates. Therefore, the NO_x removal efficiency was reduced with an increase in the gas flow rate.

3.7. Effect of SO₂ Concentration

The effect of SO₂ concentration on NO_x 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 NO_x removal; the NO_x 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 NO_x rapidly increased from 62.05% to 83.37%, then with the SO₂ concentration further increasing from 500 ppm to 2100 ppm, the NO_x removal efficiency decreased from 83.37% to 70.66%. Results indicate that the removal of NO_x could be promoted at low SO₂ concentration conditions, but higher concentrations of SO₂ inhibited NO_x 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)₂²⁻ and ONSO₃⁻ (Equations (31)–(33)) [30,31,32]. Meanwhile, dissolved NO can also directly react with SO₃²⁻ 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, finally promoting NO removal. On the other hand, HSO₃⁻ and SO₃²⁻ 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 NO_x 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 NO_x 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 NO_x removal. Meanwhile, the pH of the solution drops sharply with the large amount of SO₂ dissolved, which will promote the decomposition of S₂O₈²⁻ [23,27], and this is not conducive to NO_x removal. In addition, the dissolved SO₂ also can react with ${\mathrm{NO}}_3^{-}$ to form NO (Equation (41)); the NO can escape from the solution, leading to the decrease of NO_x removal [20].

$${\mathrm{NO}}_2^{-}\left(\mathrm{aq}\right)+{\mathrm{H}}^{+}\left(\mathrm{aq}\right)+2{\mathrm{HSO}}_3^{-}\left(\mathrm{aq}\right)\leftrightarrow \mathrm{HON}{\left({\mathrm{SO}}_3\right)}_2^{2-}\left(\mathrm{aq}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$$

(31)

$${\mathrm{NO}}_2^{-}\left(\mathrm{aq}\right)+{\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{HSO}}_3^{-}\left(\mathrm{aq}\right)\leftrightarrow {\mathrm{ONSO}}_3^{-}\left(\mathrm{aq}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$$

(32)

$${\mathrm{ONSO}}_3^{-}\left(\mathrm{aq}\right)+{\mathrm{HSO}}_3^{-}\left(\mathrm{aq}\right)\to \mathrm{HON}{\left({\mathrm{SO}}_3\right)}_2^{2-}\left(\mathrm{aq}\right)$$

(33)

$$\mathrm{NO}\left(\mathrm{aq}\right)+{\mathrm{SO}}_3^{2-}\left(\mathrm{aq}\right)\to \mathrm{ON}{\left({\mathrm{SO}}_3\right)}^{2-}\left(\mathrm{aq}\right)$$

(34)

$$\mathrm{NO}\left(\mathrm{aq}\right)+\mathrm{ON}{\left({\mathrm{SO}}_3\right)}^{2-}\left(\mathrm{aq}\right)\to {}_{}{}^{-}\mathrm{O}\mathrm{N}\left(\mathrm{NO}\right){\mathrm{SO}}_3^{-}\left(\mathrm{aq}\right)$$

(35)

$$2{\mathrm{NO}}_2\left(\mathrm{aq}\right)+{\mathrm{HSO}}_3^{-}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to 2{\mathrm{NO}}_2^{-}\mathrm{aq}+{\mathrm{SO}}_4^{2-}\left(\mathrm{aq}\right)+3{\mathrm{H}}^{+}\left(\mathrm{aq}\right)$$

(36)

$$2{\mathrm{NO}}_2\left(\mathrm{aq}\right)+{\mathrm{SO}}_3^{2-}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to 2{\mathrm{NO}}_2^{-}\mathrm{aq}+{\mathrm{SO}}_4^{2-}\left(\mathrm{aq}\right)+2{\mathrm{H}}^{+}\left(\mathrm{aq}\right)$$

(37)

$${\mathrm{S}}_2{\mathrm{O}}_8{}^{2-}\left(\mathrm{aq}\right)+{\mathrm{SO}}_2\left(\mathrm{l}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to 2{\mathrm{HSO}}_4^{-}\left(\mathrm{aq}\right)+{\mathrm{H}}_2{\mathrm{SO}}_4^{-}\left(\mathrm{aq}\right)$$

(38)

$${\mathrm{SO}}_3^{2-}\left(\mathrm{aq}\right)+2{\mathrm{OH}}^{•}\to {\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+{\mathrm{SO}}_4^{2-}\left(\mathrm{aq}\right)$$

(39)

$$2{\mathrm{SO}}_2\left(\mathrm{l}\right)+{\mathrm{ClO}}_2^{-}\left(\mathrm{aq}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to 4{\mathrm{H}}^{+}\left(\mathrm{aq}\right)+2{\mathrm{SO}}_4^{2-}\left(\mathrm{aq}\right)+{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)$$

(40)

$$3{\mathrm{SO}}_2\left(\mathrm{l}\right)+2{\mathrm{NO}}_3^{-}\left(\mathrm{aq}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to 2\mathrm{NO}\left(\mathrm{l}\right)+3{\mathrm{SO}}_4^{2-}\left(\mathrm{aq}\right)+4{\mathrm{H}}^{+}\left(\mathrm{aq}\right)$$

(41)

3.8. Effect of NO Concentration

Figure 10 indicates that the effect of NO concentration on NO_x removal. The results show that NO_x removal was slightly affected by NO concentration. NO_x removal increased slowly from 78.33% to 80.61%, with an increase of NO concentration from 100 to 500 ppm, and then it slowly decreased from 80.61% to 74.04%, with the NO concentration changing from 500 to 1000 ppm. The key step in the removal of NO by using oxidation absorption method is to promote the NO dissolution. The driving force of the mass transfer of the NO absorption increases with NO concentration in flue gas increasing. Hence, the amount of dissolved NO increases with an increase of NO concentration in flue gas. Because the amount of oxidants and free radicals were relatively sufficient when the NO concentration was less than 500 ppm, therefore the NO_x removal slightly increased with an increase of NO concentration. However, when the NO concentration in the flue gas exceeded 500 ppm, although the amount of dissolved NO increased, but more amount of NO was not absorbed by the solution and run out of the reactor with the flue gas due to the limited gas-liquid contact time. Meanwhile, the consumption rates of the oxidants and free radicals increased with NO concentration increasing, and the pH value of the absorbent solution also dropped very quickly. These reasons all are not conducive for the NO removal. Therefore, the combined result is that the absolute amount of NO removal increased, but the NO removal efficiency slightly reduced. For example, when the NO concentrations in flue gas were 500 ppm and 1000 ppm, the NO_x removal efficiencies were 80.61% and 74.04%, and the absolute amount of NO removal were 86.85 mg and 159.54 mg, respectively.

3.9. Effect of O₂ Concentration

The influence of O₂ concentration on NO_x 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 NO_x removal. When the O₂ concentration increased from 1 to 15% (v/v), the NO_x 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 NO_x removal increased slowly with an increase of O₂ concentration.

3.10. Tandem Double Column Absorption Experiments

The study found that when using a single absorber for absorption experiments, a maximum NO_x removal of 80.61% could be obtained, and so that the average concentration of NO₂ in the exhaust was about 80 ppm at the optimal experimental conditions. Compared to NO, NO₂ is more soluble, and it is more easily absorbed by the solution. Therefore, in order to increase the NO_x removal, several 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 NO_x from the first absorber, and the results are illustrated in Figure 12. The results show that the NO_x removal efficiency could be significantly increased by using a tandem double column absorption process, compared to using the single column absorption process. The absorption capacities of the four kinds of absorbents were in the order of Na₂S₂O₈ > NaOH > Ca(OH)₂ > CO(NH₂)₂, with NO_x removal efficiencies of 92.41, 91.12, 84.06, and 82.66%, respectively. The NO_x removal efficiency by using Na₂S₂O₈ as an absorbent was higher than that of the other three absorbents. This is because Na₂S₂O₈ as an oxidant has a good efficiency of removal for both NO and NO₂. For alkaline absorbents, the solution pH had a significant effect on NO_x removal. The initial pH values of NaOH, Ca(OH)₂, and CO(NH₂)₂ solution were 13.6, 9.6, and 8.4, respectively. This was because the amount of NO₂ in the composition of NO_x was significantly higher than that of NO. Thus, NO_x removal increased with the solution pH increasing. Considering the NO_x removal and economic costs, NaOH is a better absorbent, due to its high initial pH.

In the study, when using a single column and double column to absorb NO_x, the NO_x removal efficiencies of 80.61% and more than 90% could been obtained, respectively. The NO_x removal efficiencies obtained in this work are significantly higher than that of our previous work (53.05%) [12]. In other methods, a dual oxidant (H₂O₂/Na₂S₂O₈) has been used to remove NO, and the highest NO removal efficiency (82%) was obtained [22], but the concentrations of H₂O₂ and Na₂S₂O₈ were 0.3 mol/L and 0.1 mol/L respectively which were higher than this work. UV/H₂O₂ was also utilized 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 NO_x 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 NO_x 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 [21,22,23], 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. Analysis of ionic components in the absorption solutions (mg/L).

	NO2−	NO3−	Cl−	ClO2−	ClO3−
Initial	—	—	95.888	901.050	119.975
End	—	142.58	145.515	845.950	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⁻:

$$2{\mathrm{ClO}}_3{}^{-}\left(\mathrm{aq}\right)+{\mathrm{SO}}_3{}^{2-}\left(\mathrm{aq}\right)+2{\mathrm{H}}^{+}\left(\mathrm{aq}\right)\to {\mathrm{SO}}_4{}^{2-}\left(\mathrm{aq}\right)+2{\mathrm{ClO}}_2\left(\mathrm{l}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$$

(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 NO_x removal efficiency was calculated to be 83.20%, according to the NO₃⁻ concentration in the solution, which was close to the actual NO_x 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 SO₄²⁻, 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 Na₂S₂O₈ 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 NO_x and SO₂ using NaClO₂/Na₂S₂O₈ solution was deduced. The mechanism of SO₂ removal is clear and relatively simple; SO₂ is eventually converted to SO₄²⁻ via absorption, acid–base neutralization, and oxidation. The removal mechanism of NO_x is very complicated. Equations (2)–(42) can be used to explain the mechanism of NO_x 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 NO_x are removed efficiently by the dual oxidant solution.

5. Conclusions

In this work, simultaneous removal of NO_x and SO₂ through a simple process using a composite absorbent (NaClO₂/Na₂S₂O₈) has been studied in a bubble column reactor. In view of the high SO₂ removal efficiency, factors affecting the NO_x removal were systematically investigated. The following conclusions can be obtained based on experimental results:

(1)

NaClO₂ in the solution played a more important role than Na₂S₂O₈ for NO_x removal. NaClO₂ and Na₂S₂O₈ concentrations, solution temperature, the initial pH of the solution, the gas flow rate, and SO₂, NO, and O₂ concentrations all had a certain impact on the NO_x removal efficiency. Among them, solution temperature, the initial pH of the solution, and the oxidant concentrations had significant effects on the NO_x removal efficiency.

(2)

Considering the NO_x removal efficiency and its economic costs, the optimal conditions for NO_x 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 wt.% and 0.5 wt.%, respectively. A NO_x removal efficiency of more than 80% could be obtained at optimal conditions. When using a NaOH solution as an absorbent in the second absorber, the NO_x removal efficiency could reach more than 90%.

(3)

A preliminary reaction mechanism for the simultaneous removal of NO_x and SO₂ was deduced, based on experimental results. The dual oxidant (NaClO₂/Na₂S₂O₈) solution can effectively remove multi-pollutants and, thus, it has the potential to be applied in the wet desulfurization and denitration process to realize the synergistic removal of multi-pollutants.