Co-absorption and Reduction Mechanism of SO 2 and NO 2 from Flue Gas Using a Na 2 SO 3 Solution with an Oxidation Inhibitor

Abstract

Herein, a novel Na₂SO₃ process enhanced with an oxidation inhibitor was proposed for the synergic removal of SO₂ and NO₂ from nonferrous smelting flue gas. The co-absorption and reduction characteristics and mechanism of SO₂ and NO₂ were investigated. Results showed that when 80.59, 161.17, and 241.76 mM Na₂S₂O₃ was added into 2% of Na₂SO₃ solution, NO₂ absorption capacity increased to 264%, 300%, and 330%, and that of SO₂ increased to 162%, 198%, and 232%, respectively. The inhibition action of Na₂S₂O₃ was achieved by reacting $S_{2} O_{3}^{2 -}$ with $∙ {S O}_{3}^{-}$ to form an inert substance polythionate that prevents $∙ {S O}_{3}^{-}$ from reacting with dissolved oxygen. The co-absorption and reduction characteristics of SO₂ and NO₂ revealed that ∼42.5% of NO₂(N) was reduced to N₂, 21.5% of NO₂(N) combined with ${S O}_{3}^{2 -}$ to form ${O N S O}_{3}^{-}$ compounds, 5% of NO₂(N) formed ${N O}_{3}^{-}$ in solution, and 22.6% of NO₂(N) was reduced to NO into flue gas. Therefore, the enhanced Na₂SO₃ process shows potential for synergistic desulfurization and NO₂ removal from the nonferrous metal smelting industry.

Introduction

At present, haze pollution is persistent in the northern area of China. Detrimental effects of particulate matter (PM_2.5) (Huang, 2015; Zhao et al., 2019) produced through haze pollution is a serious concern, and recently, it has attracted considerable research attention. Nitrate and sulfate salts are the main secondary inorganic chemical components causing the increase of PM_2.5 (Hao and Liu, 2016; Peng et al., 2016). The gaseous SO₂ and NO_X emitted during this nucleation are oxidized to form sulfates and nitrates. Therefore, integrated control of various primary pollutants such as SO₂ and NO_X is pertinent to solving the problems of haze pollution (Li et al., 2014; Guo et al., 2016).

The nonferrous metal smelting flue gas contains different concentrations of SO₂, NOx, and various heavy metals owing to the use of sulfide ore and coal as a raw material in the nonferrous metal smelting industry (Liu et al., 2019; Ma et al., 2021). At present, the selective catalytic reaction (SCR), selective noncatalytic reaction (SNCR), and ozone oxidation are widely used to remove NO_X from the flue gas (Jo et al., 2016; Vinciguerra et al., 2017). However, the high concentrations of SO₂ and multiple heavy metals in nonferrous metal smelting flue gas can poison the SCR catalyst. The work temperature and NH₃ reductant of the SNCR are not in an acceptable range for the particular parameter in smelting furnaces. Moreover, O₃ oxidation process is frequently used to convert NO to NO₂ for denitration from nonferrous metal smelting flue gas. However, the NO₂ still needs to be removed. At present, the NaOH desulfurization process is used to absorb NO₂ in most nonferrous metal smelters, but the removal efficiency of this desulfurization technique is not ideal (Zheng et al., 2014; Hao et al., 2017). Consequently, this study focuses on the removal of NO₂ after NO oxidation.

Desulfurization technologies of the nonferrous smelting flue gas primarily involve sodium hydroxide, the organic amine process, the ionic liquid process, and the cyclic sodium sulfite process (Wu et al., 2004b; Bal et al., 2019). However, therein, NaOH is limited because the desulfurization by-product Na₂SO₄ is easily crystallized to block the pipe, difficult to recycle into Na₂SO₃ (Meikap et al., 2002; Ma et al., 2018). Although the ionic liquid can recycle gaseous SO₂, sodium and heavy metal ions easily accumulate during adsorption, thereby wasting ionic liquid and incurring a high cost. The organic amine method is restricted by its low regeneration efficiency and high operating cost. The cyclic sodium sulfite process is deemed a resource recovery process with good application prospects; therein, Na₂SO₃ absorbs SO₂ to form NaHSO₃, followed by desorbing using heat to re-release recoverable SO₂. Equations of the corresponding reactions are as given in Equations (1) and (2) (Wu et al., 2004a; Safe et al., 2008). $N a_{2} S O_{3} + S O_{2} + H_{2} O \to 2 N a H S O_{3}$ (1)

2 N a H S O_{3} =Δ N a_{2} S O_{3} + S O_{2} + H_{2} O

(2)

SO₂ and NO₂ can be simultaneously absorbed during the sodium sulfite absorption process (Guo et al., 2014). However, NO₂ is absorbed to form nitrite ( ${N O}_{2}^{-}$ ) and nitrate ( ${N O}_{3}^{-}$ ). The accumulation of ${N O}_{2}^{-}$ and ${N O}_{3}^{-}$ in the desulfurization solution causes two types of hazards: the absorption efficiency of NO₂ is limited and the treatment of high-concentration nitrogen-containing wastewater becomes difficult. In previous research, denitration was achieved by the reduction of nitrite to N₂ using a reducing agent (Okubo et al., 2006). Therefore, achieving rapid co-absorption of NO₂ and SO₂ in the Na₂SO₃ desulfurization process forms the basis for multipollutant synergistic control; thus, strengthening the rapid absorption of NO₂ and reduction of ${N O}_{2}^{-}$ in the desulfurization system is the key to determining the level of denitration. However, in the presence of oxygen in the flue gas, Na₂SO₃ is easy to be oxidized, which leads to poor absorption capacity of SO₂ and NO₂, so the problem of oxidation of Na₂SO₃ needs to be solved primarily. In the absorption process, once the ${N O}_{3}^{-}$ is formed, it is difficult to be reduced and removed. Therefore, the morphological changes of NOx need be clarified to promote the rapid and maximized formation of N₂ escaping the system and realize real denitration.

In this study, the characteristics of synergistic desulfurization and denitration were studied by investigating the co-absorption of SO₂ and NO₂ under different flue gas components and absorption conditions. The morphological changes of NOx in the absorption system were also studied to clarify the mechanism of flue gas synergistic desulfurization and denitration. Through this research, the synergistic desulfurization and denitration effect of the Na₂SO₃-enhanced process can be further promoted and the control theory and technology of multipollutants from the nonferrous smelting flue gas can be developed.

Experimental Apparatus and Methods

Experimental apparatus

Supplementary Figure S1 shows a schematic of the experimental apparatus (as shown in Supplementary Data) comprising four parts: the simulated flue gas distribution system, the absorption reaction system, the online monitoring system, and the exhaust gas treatment system. The simulated flue gas flow rate was 1.0 L/min and it is supplied by several cylinders. The flue gas contained 2000–5000 mg/m³ SO₂, 200–600 mg/m³ NO₂, 4–8% O₂, and the balance gas was N₂. All gases were controlled using mass flow meters (Sevenstar D07, China). The SO₂ absorption experiment was performed in a three-port bubbling reactor with 50 mL of 2–4% Na₂SO₃ solution. The initial temperatures of flue gas and absorption solution were 353 K and 293 K, respectively. The flue gas from the outlet was continuously absorbed by 5% NaOH and 5% KMnO₄ solution such that the tail gas was purified to discharge.

The concentrations of SO₂ and NO_X in the inlet and outlet of the reactor were monitored online using a flue gas analyzer (Testo–350; Germany). The nitrite and nitrate concentration was determined using an ultraviolet (UV)-visible spectrophotometer (TU-1810; China).

Absorption capacities of SO₂ and NO₂

The absorption capacities of SO₂ and NO₂ in the Na₂SO₃ solution were obtained by integrating the absorption curve; the results are as given in Equations (3) and (4). $τ_{S O_{2}} = \frac{1}{V} \int_{t_{2}}^{t_{1}} (C_{S O_{2 (i n)}} - C_{S O_{2 (o u t)}}) \times f \times d t$ (3)

τ_{N O_{2}} = \frac{1}{V} \int_{t_{2}}^{t_{1}} (C_{N O_{2 (i n)}} - C_{N O_{2 (o u t)}}) \times f \times d t

(4)

where $τ_{S O_{2}}$ and $τ_{N O_{2}}$ represent the absorption capacities of SO₂ and NO₂ in g/L respectively; $C_{S O_{2 (i n)}}$ and $C_{S O_{2 (o u t)}}$ represent the concentrations of the SO₂ inlet and outlet in mg/m³, respectively; $C_{N O_{2 (i n)}}$ and $C_{N O_{2 (o u t)}}$ represent the concentrations of the NO₂ inlet and outlet measured in mg/m³, respectively; v represents the volume of Na₂SO₃ solution in mL; t₁ and t₂ represent the absorption time (min) of SO₂ and NO₂ on starting and ending, respectively; and, f represents the flue gas flow measured in L/min.

Removal efficiencies of SO₂ and NO₂

The removal efficiencies of SO₂ and NO₂ were calculated using Equations (5) and (6). $η_{S O_{2}} = \frac{C_{S O_{2 (i n)}} - C_{S O_{2 (o u t)}}}{C_{S O_{2 (i n)}}} \times 100 %$ (5)

η_{N O_{2}} = \frac{C_{N O_{2 (i n)}} - C_{N O_{2 (o u t)}}}{C_{N O_{2 (i n)}}} \times 100 %

(6)

where $η_{S O_{2}}$ and $η_{N O_{2}}$ represent the removal efficiencies of SO₂ and NO₂, respectively; $C_{S O_{2 (i n)}}$ and $C_{S O_{2 (o u t)}}$ represent the concentrations (mg/m³) of the SO₂ inlet and outlet, respectively; and $C_{N O_{2 (i n)}}$ and $C_{N O_{2 (o u t)}}$ represent the concentrations (mg/m³) of the NO₂ inlet and outlet, respectively.

Calculation of N mass balance

The mass of ${N O}_{2}^{-}$ , ${N O}_{3}^{-}$ , and TN was measured from the sample in the solution, and they are represented as $M_{{N O}_{2}^{-}}$ , $M_{{N O}_{3}^{-}}$ and $M_{T N}$ , respectively.

The mass of NO and NO₂ were calculated using the following Equations (7) and (8). $M_{N O} = \int_{t_{2}}^{t_{1}} C_{N O} \times f \times d t$ (7)

M_{N O_{2}} = \int_{t_{2}}^{t_{1}} C_{N O_{2}} \times f \times d t

(8)

where M_NO and $M_{N O_{2}}$ represent the mass of NO and NO₂ in the tail gas (mg), respectively; C_NO and $C_{N O_{2}}$ represent the concentrations (mg/m³) of NO and NO₂ in the tail gas, respectively; t₁ and t₂ represent the starting and ending time (min) of the absorption experiment, respectively; and f represents the gas flow rate measured in L/min.

The mass of sulfur nitrogen compound ( $M_{S N C}$ ) was calculated using Equation (9). The mass of N₂ ( $M_{N_{2}}$ ) escaped from the absorption solution was calculated using Equation (10). $M_{S N C} = M_{T N} - M_{{N O}_{2}^{-}} - M_{{N O}_{3}^{-}}$ (9)

M_{N_{2}} = C_{N {O_{2}}_{(i n)}} \times f \times (t_{2} - t_{1}) - M_{{N O}_{2}^{-}} - M_{{N O}_{3}^{-}} - M_{S N C}

(10)

Results and Discussion

Absorption effect of NO₂ under different conditions

NO₂ absorption is affected by various factors, such as the parameters in absorption solution and flue gas. As given in Fig. 1, the curves a and b indicate that the absorption capacity of NO₂ is significantly stronger in the Na₂SO₃ solution than in pure water. This is because the absorption of NO₂ by water involves simple dissolution with small dissolution equilibrium, whereas Na₂SO₃ reacts with NO₂ to promote absorption of NO₂, as given in Equations (11) and (12) (Jia et al., 2013; Sun et al., 2015; Wu et al., 2018). Therefore, the synergetic desulfurization and denitration from the smelting flue gas by Na₂SO₃ are matched. Curves b and c show that when the oxygen concentration increases from 4% to 8% in the flue gas, the absorption capacity of NO₂ decreases from 0.33 to 0.22 g/L in the Na₂SO₃ solution. This observation may be attributed to the side reaction that converts ${S O}_{3}^{2 -}$ to ${S O}_{4}^{2 -}$ , which no longer absorbs NO₂, as given in Equations (13) and (14) (Littlejohn et al., 1993). Therefore, inhibiting the oxidation of ${S O}_{3}^{2 -}$ in the absorption system is crucial.

FIG. 1.

Effects of main parameters of sodium sulfite solution on NO₂ absorption a: 6% O₂, water, 600 mg/m³ NO₂; b: 6% O₂, 2% Na₂SO₃, 600 mg/m³ NO₂, pH = 10; c: 8% O₂, 2% Na₂SO₃, 600 mg/m³ NO₂, pH = 10; d: 6% O₂, 2% Na₂SO₃, 600 mg/m³ NO₂, pH = 8; e: 6% O₂, 4% Na₂SO₃, 600 mg/m³ NO₂, pH = 10; f: 6% O₂, 2% Na₂SO₃, 200 mg/m³ NO₂, pH = 10; g: 4% O₂, 2% Na₂SO₃, 600 mg/m³ NO₂, pH = 10; h: 6% O₂, 2% Na₂SO₃, 600 mg/m³ NO₂, pH = 6; i: 6% O₂, 2% Na₂SO₃, 400 mg/m³ NO₂, pH = 10; j: 6% O2, 3% Na₂SO₃, 600 mg/m³ NO₂, pH = 10.

{S O}_{3}^{2 -} + 2 N O_{2} + H_{2} O \to 2 {N O}_{2}^{-} + {S O}_{4}^{2 -} + 2 H^{+}

(11)

{H S O}_{3}^{-} + 2 N O_{2} + H_{2} O \to 2 {N O}_{2}^{-} + {S O}_{4}^{2 -} + 3 H^{+}

(12)

2 {S O}_{3}^{2 -} + O_{2} \to 2 {S O}_{4}^{2 -}

(13)

2 {H S O}_{3}^{-} + O_{2} \to 2 {S O}_{4}^{2 -} + 2 H^{+}

(14)

Curves b and d show that the absorption capacity of NO₂ decreases in the absorption solution with lower pH. Comparing curves b, e, and f reveals that reducing the initial NO₂ concentration or increasing the Na₂SO₃ concentration can improve the absorption efficiency of NO₂, whereas the NO₂ absorption capacity per-unit absorption solution remains unchanged.

Effects of Na₂S₂O₃ as an inhibitor on NO₂ absorption

To ensure the preferable performances of denitration by Na₂SO₃, appropriate inhibitors must be added into the absorption solution to inhibit the oxidation of Na₂SO₃. The effects of ethanol, ethylene glycol, and Na₂S₂O₃ as inhibitors on NO₂ absorption were investigated herein, and the obtained results are given in Fig. 2. When the initial NO₂ concentration was 600 mg/m³ and that of O₂ was 6% in the simulated flue gas at a flow rate of 1.0 L/min, results show that the absorption effect of NO₂ when 161.17 mM ethanol or ethylene glycol was added is similar to that obtained when no inhibitor was added into 2% Na₂SO₃ absorption solution. However, the Na₂SO₃ solution with additional 161.17 mM Na₂S₂O₃ exhibited excellent performance in terms of NO₂ absorption, and the absorption capacity increased from 0.29 to 2.97 g/L. Therefore, Na₂S₂O₃ was added into the Na₂SO₃ absorption solution as a compatible inhibitor to improve the NO₂ absorption performance.

FIG. 2.

Effects of different inhibitors on NO₂ absorption.

The effect of various Na₂S₂O₃ concentrations on the NO₂ absorption performance is given in Fig. 3, wherein the NO₂ absorption time was 38 min and the absorption capacity was 0.29 g/L in unmixed 2% Na₂SO₃ solution. When 80.59, 161.17, and 241.76 mM Na₂S₂O₃ was added to 2% Na₂SO₃ solution, the NO₂ absorption time was extended to 252, 330, and 400 min, and the NO₂ absorption capacity was increased to 2.09, 2.97, and 3.78 g/L, respectively. In addition, the NO₂ absorption capacity was poor in the single 2% Na₂S₂O₃ solution, revealing that NO₂ absorption by the addition of Na₂S₂O₃ is not particularly responsible for the significant increase in the absorption capacity of NO₂ in the Na₂SO₃ solution.

FIG. 3.

Effects of various Na₂S₂O₃ concentration on NO₂ absorption.

Therefore, the enhancement of the NO₂ absorption performance by Na₂S₂O₃ may occur through the two mechanisms described hereunder. First, the chain reactions of the oxidizing ${S O}_{3}^{2 -}$ are cut off by polythionates generated by the reactions presented as Equations (15)–(18) (Shen et al., 2013). Polythionates protect the· ${S O}_{3}^{2 -}$ radicals from O₂ oxidation. Furthermore, polythionates can be easily decomposed into $S_{2} O_{3}^{2 -}$ and ${S O}_{3}^{2 -}$ in aqueous solution to ensure the continuous absorption of NO₂ and SO₂ from the flue gas; hence, reducing the oxidation rate of ${S O}_{3}^{2 -}$ increases NO₂ absorption. Second, the valences of the S in Na₂S₂O₃ are +6 and −2; thus, Na₂S₂O₃ is reducible because of S²⁻. In the absorption solution, the reaction between Na₂S₂O₃ and NO₂ promotes the absorption of NO₂ from the flue gas, as given in Equation (19); however, the absorption effect is relatively weak. $∙ {S O}_{3}^{-} + S_{2} O_{3}^{2 -} \to ∙ S_{2} O_{3}^{-} + {S O}_{3}^{2 -}$ (15)

∙ {S O}_{3}^{-} + ∙ S_{2} O_{3}^{-} \to p o l y t h i o n a t e s

(16)

∙ S_{2} O_{3}^{-} + ∙ S_{2} O_{3}^{-} \to p o l y t h i o n a t e s

(17)

p o l y t h i o n a t e s + H_{2} O \to S_{2} O_{3}^{2 -} + {S O}_{3}^{2 -} + 2 H^{+}

(18)

8 N O_{2} + S_{2} O_{3}^{2 -} + 5 H_{2} O \to 8 {N O}_{2}^{-} + 2 {S O}_{4}^{2 -} + 10 H^{+}

(19)

Effects of SO₂ concentration on SO₂ and NO₂ co-absorption

In practice, nonferrous metal smelting flue gas contains a relatively high concentration of SO₂, and the Na₂SO₃ absorption process is used for SO₂ removal from the flue gas; thus, considering the effect of SO₂ and NO₂ co-absorption in the Na₂SO₃ absorption solution is necessary. The 2000, 3000, and 5000 mg/m³ SO₂ was, respectively, introduced into flue gas containing the 600 mg/m³ NO₂ to investigate the effect of SO₂ concentration on SO₂ and NO₂ co-absorption in 2% Na₂SO₃ solution. As given in Fig. 4, Na₂SO₃ had a good co-absorption effect on SO₂ and NO₂. The lifetime of SO₂ absorption decreased from 46 to 34 min, but the SO₂ absorption capacity increased from 1.51 to 2.76 g/L with increasing the initial SO₂ concentration from 2000 to 5000 mg/m³. This is mainly because of the improvement in the partial pressure of SO₂ and the driving force of mass transfer.

FIG. 4.

Effects of SO₂ concentration on SO₂ and NO₂ co-absorption.

The maximum absorption efficiency of NO₂ was reduced from 93% to 87% following the increase of the initial SO₂ concentration in flue gas, which is because more ${S O}_{3}^{2 -}$ was consumed by high concentration of SO₂. However, the lifetime and capacity of NO₂ absorption was slightly weakened because that the ${S O}_{3}^{2 -}$ consumed in the original Na₂SO₃ solution by the absorption of SO₂ was supplemented and simultaneously generated ${H S O}_{3}^{-}$ during the SO₂ absorption. The ${S O}_{3}^{2 -}$ and ${H S O}_{3}^{-}$ could reacted with NO₂ as given in Equations (11) and (12), which indicates that the presence of SO₂ has no obvious effects. However, both the lifetime of NO₂ and SO₂ co-absorption were shorter as opposed to the NO₂ or SO₂ absorption alone, which was attributed to the gradual decrease of the pH with the co-absorption of SO₂ and NO₂.

Effects of NO₂ and Na₂SO₃ concentration on SO₂ and NO₂ co-absorption

When SO₂ concentration was maintained at 5000 mg/m³ and that of Na₂SO₃ was 2%, the effects of different NO₂ concentrations on SO₂ and NO₂ co-absorption were investigated and the results are given in Fig. 5. When the NO₂ concentration increased from 200 to 600 mg/m³, the lifetime and capacity of SO₂ absorption remained almost constant as 33 min and 2.80 g/L, respectively, indicating that NO₂ concentration had a weak effect on SO₂ absorption. However, the absorption capacity of NO₂ significantly increased from 0.09 to 0.22 g/L, and NO₂ removal efficiency decreased from 95% to 87%. That may be owing to the increase in the partial pressure of NO₂ at the gas–liquid interface with increasing NO₂ concentration, which is favorable for NO₂ absorption. Meanwhile, preceding the reaction between NO₂ and ${S O}_{3}^{2 -}$ , some unreacted NO₂ escaped from the absorption solution, thereby reducing the NO₂ removal efficiency.

FIG. 5.

Effects of NO₂ concentration on SO₂ and NO₂ co-absorption.

The effect of Na₂SO₃ concentration on the co-absorption of NO₂ and SO₂ was also measured under the operating conditions of 600 mg/m³ NO₂ and 5000 mg/m³ SO₂ in flue gas and 2–4% Na₂SO₃ in solution; the corresponding results are given in Supplementary Fig. S2. NO₂ absorption capacity increased from 0.22 to 0.44 g/L, and SO₂ absorption capacity increased from 2.76 to 4.49 g/L with the increase in Na₂SO₃ concentration from 2% to 4%. The lifetime and of SO₂ and NO₂ co-absorption significantly extended with increasing Na₂SO₃ concentration, which was consistent with the trend observed when SO₂ or NO₂ were exclusively absorbed by the Na₂SO₃ solution.

Effect of Na₂S₂O₃ concentration on SO₂ and NO₂ co-absorption

As observed in the previous experiments conducted herein, Na₂SO₃ was easily oxidized in the presence of oxygen, thereby negatively influencing SO₂ or NO₂ absorption. However, adding an inhibitor (Na₂S₂O₃) into the Na₂SO₃ solution could effectively solve this problem; therefore, the effects of various concentrations of Na₂S₂O₃ on SO₂ and NO₂ co-absorption were investigated. The experimental conditions were 600 mg/m³ NO₂ and 5000 mg/m³ SO₂ in flue gas with 2% Na₂SO₃ in solution. Based on Figs. 5 and 6, comparative examination of the absorption curve in the absence of Na₂S₂O₃ revealed that the lifetime of NO₂ absorption was extended by 45, 53, and 65 min, and that of SO₂ absorption was extended by 48, 63, and 78 min. However, when Na₂S₂O₃ was added in the concentrations of 80.59, 161.17, and 241.76 mM to the 2% Na₂SO₃ solution, NO₂ absorption capacity increased to 264%, 300%, and 330%, and that of SO₂ increased to 162%, 198%, and 232%, respectively.

FIG. 6.

Effects of Na₂S₂O₃ concentration on SO₂ and NO₂ co-absorption.

Based on Fig. 6, the SO₂ and NO₂ co-absorption capacity slightly increased when Na₂S₂O₃ was added in various concentrations; therefore, the inefficiency of Na₂S₂O₃ to particularly absorb NO₂ and SO₂ is evident. The main function of Na₂S₂O₃ is to inhibit the oxidation of ${S O}_{3}^{2 -}$ , thus ensuring the presence of more ${S O}_{3}^{2 -}$ in the solution to absorb NO₂ and SO₂, which is consistent with the experimental results and the mechanism described in Section “Effects of Na₂S₂O₃ as an inhibitor on NO₂ absorption.”

Absorption and reduction of NOx in the co-absorption solution

${N O}_{2}^{-}$ and ${N O}_{3}^{-}$ could be generated when NO₂ was absorbed by Na₂SO₃ from the flue gas. To investigate the speciation transformation and distribution of nitrogen, experiments on the absorption of only 600 mg/m³ NO₂ in flue gas by 2% Na₂SO₃ solution were performed and samples were collected at various absorption stages. ${N O}_{2}^{-}$ and ${N O}_{3}^{-}$ concentrations were measured using a UV-visible spectrophotometer. As given in Fig. 7, the concentrations of ${N O}_{2}^{-}$ and ${N O}_{3}^{-}$ in solution generated by the absorption of NO₂ were rapidly accumulated to 60.57 and 17.03 mg/L in 0 to 25 min, followed by a gradual increase to the end of the absorption in 25 min. This is consistent with the characteristic of NO₂ absorption given in Fig. 1b wherein a turning point is observed at ∼25 min on the curve. According to Equations (11) and (22), NO₂ was absorbed by ${S O}_{3}^{2 -}$ to generate ${N O}_{2}^{-}$ , and little ${N O}_{2}^{-}$ was oxidized to ${N O}_{3}^{-}$ by oxygen in the solution. The calculation from the data obtained in Figs. 1b and 7 shows that the amount of NO₂ absorbed was 4.12 mg (N), and the amount of ${N O}_{2}^{-}$ and ${N O}_{3}^{-}$ in the solution was 3.03 mg (N) and 0.55 mg (N) at 25 min, respectively; thus, NO₂ was absorbed and generated the major ${N O}_{2}^{-}$ and minor ${N O}_{3}^{-}$ in the Na₂SO₃ solution.

FIG. 7.

Variations of ${N O}_{2}^{-}$ and ${N O}_{3}^{-}$ concentration in the NO₂ absorption solution.

The variations of ${N O}_{2}^{-}$ concentration in the co-absorption solution were investigated when 5000 mg/m³ SO₂ and NO₂ existed in the flue gas; Fig. 8 shows the corresponding results, wherein ${N O}_{2}^{-}$ accumulated to its maximum concentration at 16 min, then began to gradually decline until it almost disappeared at 25 min. The amount of ${N O}_{3}^{-}$ was little in the solution and NO appeared in the flue gas during the whole absorption process, preliminarily demonstrating that ${N O}_{2}^{-}$ can be reduced by SO₂ to form NO thereby escaping from the solution. However, based on the N mass balance calculation, as shown in the curve of 600 mg/m³ NO₂, the amount of NO₂ absorbed was 3.13 mg (N), the total amount of ${N O}_{2}^{-}$ and ${N O}_{3}^{-}$ in the solution was 0.04 mg (N), synchronous NO and NO₂ in the flue gas was 0.12 and 1.40 mg (N) at 25 min; this entails that the N mass was not conserved in the absorption system. Ajdariand and Siddiqi reported that ${N O}_{2}^{-}$ can be reacted with ${S O}_{3}^{2 -}$ to form a sulfur nitrogen compound as an intermediate that is only stable under high S/N ratio and low pH (4–5) (Siddiqi and Petersen, 2001; Ajdari et al., 2015). Xiao et al. reported that ${N O}_{2}^{-}$ can be reduced to N₂ in the reduction system (Xiao et al., 2020). Therefore, the reduction process of ${N O}_{2}^{-}$ and associated products need to be further studied.

FIG. 8.

Variations of ${N O}_{2}^{-}$ concentration in the absorption solution with SO₂ and various concentrations of NO₂ in flue gas.

Mechanism of ${N O}_{2}^{-}$ reduction in the co-absorption solution

In this section, to clarify the mechanism of ${N O}_{2}^{-}$ reduction in the co-absorption solution, SO₂ flue gas was introduced into the solution containing 4600 mg/L ${N O}_{2}^{-}$ ; then, both the SO₂, NO, and NO₂ in the flue gas as well as the ${N O}_{2}^{-}$ , ${N O}_{3}^{-}$ , and TN in the solution were detected. As given in Fig. 9, NO and NO₂ appeared in the tail gas, and their concentrations increased with the increase in the initial concentration of SO₂; in the meantime, the ${N O}_{2}^{-}$ concentration continued to decrease to almost zero and with the ${N O}_{3}^{-}$ maintained a low concentration. It indicated that part of ${N O}_{2}^{-}$ in the solution reacted with SO₂ to form NO and a small amount of NO₂ escaped into the flue gas as given in Equation (20). However, the concentration of TN was greater than the sum of the ${N O}_{2}^{-}$ and ${N O}_{3}^{-}$ concentrations in solution, indicating the generation of ${O N S O}_{3}^{-}$ compounds, as given in Equation (21). The comparison between the amount of ${N O}_{2}^{-}$ in the initial solution, the amount of TN in the solution combined with the NO and NO₂ in the tail gas, revealed that the N mass was not conserved in the absorption system. In addition, N₂ was detected by gas chromatography in the tail gas of SO₂ and Ar. Therefore, it can be concluded that part of ${N O}_{2}^{-}$ in the solution reacted with SO₂ to form N₂ and escaped into the flue gas as given in Equation (23). Table 1 lists the calculated results of each species, revealing that ∼42.5% of NO₂ (N) was reduced to N₂, 21.5% of NO₂(N) combined with ${S O}_{3}^{2 -}$ to form ${O N S O}_{3}^{-}$ compounds and 5% formed ${N O}_{3}^{-}$ in solution, and 22.6% of NO₂(N) was reduced to NO into flue gas, and 5% of NO₂(N) was kept in flue gas. The corresponding reactions are given in Equations (21)–(23).

FIG. 9.

Effects of SO₂ concentration on reduction of ${N O}_{2}^{-}$ in absorption solution (a) the absorption curves of SO₂; (b) the concentration of NO₂ and NO in flue gas.

Table 1.

Calculation of N Mass Balance Under Various Absorption Conditions

N/mg SO₂/mg/m³	TNO₂ (initial, N)/mg	NO(N)/mg	NO₂(N)/mg	TN (solute-on)/mg	${N O}_{2}^{-} (N)$ /mg	${N O}_{3}^{-} (N)$ /mg	${O N S O}_{3}^{-} (N)$ /mg	N₂(N)/mg
2000	70	17.17	6.72	16.82	0.035	3.74	13.05	29.29
3000	70	16.01	6.17	18.59	0.035	3.88	14.68	29.23
5000	70	14.37	4.49	20.32	0.070	2.91	17.34	20.82

3 {N O}_{2}^{-} + S O_{2} \to {S O}_{4}^{2 -} + 2 N O + N O_{2}

(20)

{N O}_{2}^{-} + H^{+} + {H S O}_{3}^{-} \to {O N S O}_{3}^{-} + H_{2}

(21)

2 {N O}_{2}^{-} + O_{2} \to 2 {N O}_{3}^{-}

(22)

2 {N O}_{2}^{-} + 3 S O_{2} + 2 H_{2} O \to {S O}_{4}^{2 -} + 2 H_{2} S O_{4} + N_{2}

(23)

Conclusions

In this study, a novel process of co-absorption and reduction of SO₂ and NO₂ by Na₂SO₃ solution with an oxidation inhibitor was investigated. Results showed that when 80.59, 161.17, and 241.76 mM Na₂S₂O₃ was added into 2% of Na₂SO₃ solution, NO₂ absorption capacity increased to 264%, 300%, and 330%, and that of SO₂ increased to 162%, 198%, and 232%, respectively. However, O₂ in the flue gas decreased the absorption capacity of SO₂ and NO₂ owing to the oxidation of Na₂SO₃. We observed by comparison that the addition of Na₂S₂O₃ as an oxidation inhibitor into Na₂SO₃ solution could significantly improve the lifetime and absorption capacity of SO₂ and NO₂ co-absorption. In the inhibition mechanism of Na₂S₂O₃, $S_{2} O_{3}^{2 -}$ combines with $∙ {S O}_{3}^{-}$ to form an inert substance polythionate that prevents $∙ {S O}_{3}^{-}$ from reacting with dissolved oxygen. The various initial concentrations of SO₂ or NO₂ had a weak effect on the capacity of SO₂ and NO₂ absorption but could decrease the removal efficiency.

Experiments on SO₂ and NO₂ co-absorption indicated that ∼42.5% of NO₂(N) was reduced to N₂, 21.5% of NO₂(N) combined with ${S O}_{3}^{2 -}$ to form ${O N S O}_{3}^{-}$ compounds, 5% of NO₂(N) formed ${N O}_{3}^{-}$ in solution, and 22.6% of NO₂(N) was reduced to NO into flue gas. This work provides new insight and a feasible approach to upgrade the existing Na₂SO₃ flue gas desulfurization process to enhance the co-absorption of SO₂ and NO₂ and reduce NO₂ to N₂, which is expected to be useful in the development and application of the Na₂SO₃ process in the nonferrous metal smelting industry.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

Funding Information

This study was supported by the Scientific and Technological Innovative Talents in Colleges and Universities of Henan Province (21HASTIT012), the Scientific and Technological Project of Henan Province-China (No. 202102310283), the Project of Young-backbone Teacher in Colleges and Universities of Henan Province (No. 2020GGJS125) and the Key Scientific Research Project of Colleges in Henan Province-China (No.20A610011).

Supplementary Material

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Supplementary Material

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Co-absorption and Reduction Mechanism of SO 2 and NO 2 from Flue Gas Using a Na 2 SO 3 Solution with an Oxidation Inhibitor

Abstract

Introduction

Experimental Apparatus and Methods

Experimental apparatus

Absorption capacities of SO2 and NO2

Removal efficiencies of SO2 and NO2

Calculation of N mass balance

Results and Discussion

Absorption effect of NO2 under different conditions

Effects of Na2S2O3 as an inhibitor on NO2 absorption

Effects of SO2 concentration on SO2 and NO2 co-absorption

Effects of NO2 and Na2SO3 concentration on SO2 and NO2 co-absorption

Effect of Na2S2O3 concentration on SO2 and NO2 co-absorption

Absorption and reduction of NOx in the co-absorption solution

Mechanism of N O 2 − reduction in the co-absorption solution

Conclusions

Footnotes

Author Disclosure Statement

Funding Information

Supplementary Material

References

Supplementary Material

Absorption capacities of SO₂ and NO₂

Removal efficiencies of SO₂ and NO₂

Absorption effect of NO₂ under different conditions

Effects of Na₂S₂O₃ as an inhibitor on NO₂ absorption

Effects of SO₂ concentration on SO₂ and NO₂ co-absorption

Effects of NO₂ and Na₂SO₃ concentration on SO₂ and NO₂ co-absorption

Effect of Na₂S₂O₃ concentration on SO₂ and NO₂ co-absorption

Mechanism of ${N O}_{2}^{-}$ reduction in the co-absorption solution