Removal of nitric oxide from simulated flue gas using aqueous persulfate with activation of ferrous ethylenediaminetetraacetate in the rotating packed bed

Abstract

Nitric oxide being a major gas pollutant has attracted much attention and various technologies have been developed to reduce NO emission to preserve the environment. Advanced persulfate oxidation technology is a workable and effective choice for wet flue gas denitrification due to its high efficiency and green advantages. However, NO absorption rate is limited and affected by mass transfer limitation of NO and aqueous persulfate in traditional reactors. In this study, a rotating packed bed (RPB) was employed as a gas–liquid absorption device to elevate the NO removal efficiency (η_NO) by aqueous persulfate ((NH₄)₂S₂O₈) activated by ferrous ethylenediaminetetraacetate (Fe²⁺-EDTA). The experimental results regarding the NO absorption were obtained by investigating the effect of various operating parameters on the removal efficiency of NO in RPB. Increasing the concentration of (NH₄)₂S₂O₈ and liquid–gas ratio could promoted the oxidation and absorption of NO while theη_NO decreased with the increase of the gas flow and NO concentration. In addition, improving the high gravity factor increased the η_NO and the total volumetric mass transfer coefficient (K_Gα) which raise the η_NO up to more than 75% under the investigated system. These observations proved that the RPB can enhance the gas–liquid mass transfer process in NO absorption. The correlation formula between K_Gα and the influencing factors was determined by regression calculation, which is used to guide the industrial scale-up application of the system in NO removal. The presence of O₂ also had a negative effect on the NO removal process and through electron spin resonance spectrometer detection and product analysis, it was revealed that Fe²⁺-EDTA activated (NH₄)₂S₂O₈ to produce •SO₄^–, •OH and •O₂^–, played a leading role in the oxidation of NO, to produce NO₃^– as the final product. The obtained results demonstrated a good applicable potential of RPB/PS/Fe²⁺-EDTA in the removal of NO from flue gases.

Key words： rotating packed bed; Fe²⁺-EDTA; sulfate radical; hydroxyl radical; NO removal efficiency

1 Introduction

Nitrogen oxides (NO_x, NO ~90% or more, NO₂ ~10%) and SO₂ are the main pollutants, produced by burning coal in the iron, steel, coking, industrial kilns and boilers and cause adverse effect on environment (photochemical smog, acid rain and water eutrophication) and human health (respiratory system and cardiovascular disease) [1–3]. The state has promulgated the ultra-low emission standards of SO₂ and NO_x for steel and other enterprises, and are controlled at 35 and 50 mg·m^–3 respectively [4]. The strict emission regulations require further development of coal-fired flue gas emission control technology. For the reduction of SO₂ and NO_x emission, the common technologies can be organized in two categories: the one is combustion control method and the other is post-combustion control method [5]. At present, wet flue gas desulfurization and selective catalytic reduction or selective non-catalytic reduction methods can effectively reduce SO₂ and NO_x emissions, but due to the huge system, the investment cost, large area, high operating temperature and secondary pollution [6,7], it is difficult to apply these methods to sintering plants and other small and medium-sized boilers. Wet scrubbing is recognized as a kind of promising flue gas purification technology due to its simple process [8]. Reduction, complex formation and oxidation are the three most common pathways for wet washing and simultaneous desulfurization and denitrification process [9–12]. However, high costs or technical problems hinder the practical application of reduction and complex absorption technologies.

Oxidation absorption technologies are to oxidize insoluble NO into higher NO_x, which is the most widely studied denitrification process. Therefore, screening and developing the cost-effective oxidants to improve the solubility of NO in water is the key to oxidative denitrification process. Hence, the use of NaClO [13], NaClO₂ [14], KMnO₄ [15], H₂O₂ [16], peroxymonosulfate (PMS) [17] and persulfate (PS) [18], etc. for this purpose are the current research hotspots. In contrast, PS is a cost-effective and environmentally friendly oxidant with standard oxidation−reduction potential to be E₀ = +2.01 V, which shows its strong oxidizing properties. It is also more stable than H₂O₂ and is easy to store and transport. Some studies have shown that the •SO₄^– and •OH produced by PS activation bears high redox potential and strong oxidizing ability, hence high removal efficiency for the pollutants can be achieved [19,20]. At the same time, heat, ultraviolet radiation, ultrasound and transition metal ions can also effectively activate the •SO₄^– and •OH produced by PS [21]. The use of Fe²⁺ homogeneous activation of PS in the activation of transition metals has attracted wide attention [22,23], because of its low price, easy activation process, high efficiency, and no requirement for additional energy. However, Fe²⁺ is easy to inactivate and the large accumulation of Fe³⁺ in the system will cause the decreased reaction rate [24]. The chelating agent ethylenediaminetetraacetic acid (EDTA) is often added to form a complex with Fe²⁺ to adjust and maintain the concentration of Fe²⁺ to reduce unnecessary losses [25], while promoting Fe²⁺ regeneration and reducing Fe³⁺ accumulation. Adewuyi et al. [26] also showed that the removal efficiency of NO in the Fe²⁺-EDTA system is much higher than that of achieved in the Fe²⁺ activation system, and revealed that Fe³⁺-EDTA and Fe²⁺-EDTA (NO) can be converted into Fe²⁺-EDTA. Meanwhile, some studies [26–28] also showed that the gas–liquid mass transfer resistance during the oxidation of NO had a great influence on NO removal efficiency (η_NO) in traditional reactors such as bubbling towers. Liu et al. [29] designed an impinging stream reactor and investigated its removal performance for NO and SO₂ using PMS with synergic activation of Cu²⁺/Fe³⁺ at high temperature, and found that the NO absorption rate was increased by increasing the gas phase mass transfer coefficient and interfacial area in the reactor structure. Chen and Hu [30] had developed a multi-stage bubble reactor combined with an agitator, which increased the NO content in liquid and promote the gas–liquid mass transfer. Thus, selecting a reactor with high mass transfer efficiency to enhance gas–liquid mass transfer is the key task to improve η_NO and reduce reactor volume during this research.

Rotating packed bed (RPB), as the main device of high gravity technology, can enhance the gas–liquid mass transfer process in a high centrifugal force field. In the RPB, when the fluid passes through the filler, the high-speed rotating filler generates strong shear force and converts it into droplets, liquid film and liquid mist, which greatly increase the interface and surface renewal rate of the transfer between phases [31]. This intensifies the mass transfer process and micro-mixing of the gas–liquid two-phase, increasing the mass transfer coefficient to 1–3 orders of magnitude to the traditional packed bed [32]. RPB is also widely used in the field of gas purification due to its small size of equipment, stable operation, convenient startup/shutdown, low energy consumption and low cost [33,34].

Based on this theory, we used a RPB as the absorption device and investigated its removal performance for NO using PS with activation of Fe²⁺-EDTA. The main purpose of this article was to study the influence of operating parameters such as gas flow, gas–liquid ratio, high gravity factor, ammonium PS concentration, NO inlet concentration and O₂ exist on system performance. Identification of active species and products was carried out by ion chromatography and electron paramagnetic resonance spectrometer which revealed the NO removal pathway. The strengthening mechanism of RPB for NO removal was studied and the results also provided the theoretical guidance for the industrial applications of NO oxidation absorption.

2 Experimental

2.1 Experimental materials

NO (5%), O₂ (99%) and N₂ (99.9%) were used as the source of gas stream (they were purchased in Taineng Gas Co., Ltd.). (NH₄)₂S₂O₈ (99.0%), FeSO₄·H₂O and disodium ethylenediaminetetraacetate were all of analytical reagents. All solutions were prepared using the deionized water.

2.2 Experimental setup

The experimental setup was mainly composed of a flue gas preparation system, a gas−liquid reaction system, a detection system and an exhaust gas treatment system as shown in Fig.1 while the RPB was schematically shown in Fig.2. The simulated flue gas in this experiment was composed of NO, N₂ and O₂ completely mixed in the buffer tank (5) and the concentration and flow rate were controlled by a gas flow meter which flowed from the buffer tank to the gas inlet (7.3) of the RPB (7) in the form of a continuous phase. Under the action of pressure, the flue gas entered the rotating packing layer from the bottom of the packing (7.2) along the axis, and passed through the packing layer after cross-flow contact with the absorbing liquid in the packing. The PS/Fe²⁺-EDTA solution was introduced from solution storage tank (9) and was injected into the RPB by the liquid distributor (7.9). The liquid was evenly sprayed to the inner edge of the packing by the liquid distributor. Under the action of centrifugal force, it was spread and split into liquid elements like micro-scale droplets, thread or film, and moved along the inner edge of the packing to the outer edge. The high-speed liquid elements contacted cross-current with the flowing gas to activate the removal process of NO in flue gas, and then are discharged through the gas outlet (7.8) and liquid outlet (7.6), respectively. The inner and outer diameters of the rotor (7.1) was respectively 30 and 120 mm while the axial height was 70 mm. And the packing used in this study was a stainless steel wire mesh having a diameter of 0.25 mm and was wound around in the rotor with 95% porosity. The installed rotating packing was in a fixed cabinet (7.7) and was rotated at an adjustable speed. Data was recorded when the flue gas flowed through the flue gas analyzer (10) before and after the reaction. The exhaust gas could be further processed through the exhaust gas absorber (13).

Fig.1 Schematic diagram of experimental setup (1. Steel cylinder for NO; 2. Steel cylinder for N₂; 3. Steel cylinder for O₂; 4. Valve unit; 5. Gas buffer tank; 6. Gas flow meter; 7. Homemade RPB; 8. Motor; 9. Solution storage tank; 10. Flue gas analyzer; 11. Liquid flow meter; 12. Pump; 13. Tail gas absorber).

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Fig.2 Schematic diagram of RPB (7.1-rotor, 7.2-packing, 7.3-gas inlet, 7.4-shaft, 7.5-shaft seal, 7.6-liquid outlet, 7.7-cabinet, 7.8-gas outlet, 7.9-liquid distributor, 7.10-liquid inlet).

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2.3 Analytical method

The flue gas analyzer (KM9106) was used to determine the import and export concentrations of NO. Iron species (Fe²⁺ and Fe³⁺) were determined spectrophotometrically using DRA-2500 spectrophotometer while the ion chromatography (ICS-1100) was used to determine the NH₄⁺, SO₄^2–, NO₂^– and NO₃^– in solution. The electron paramagnetic resonance spectrometer (EPR, MS5000X) was used to detected the •SO₄^–, •OH and •O₂^– by combining with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) (> 99%) as a spin trapping agent to trap inorganic radicals. The pH meter (PHS-3C) monitored the initial and final pH values of each experiment adjusted by the addition of NaOH and HCl.

2.4 Data-processing method

The experimental results evaluated the NO removal effect of PS/Fe²⁺-EDTA/RPB system by the change of NO concentration in the gas phase. The η_NO in the simulated flue gas can be calculated with the following formula:

(1){\eta }_{\mathrm{N}\mathrm{O}}=\frac{C_{\mathrm{N}\mathrm{O},\mathrm{i}\mathrm{n}}-C_{\mathrm{N}\mathrm{O},\mathrm{o}\mathrm{u}\mathrm{t}}}{C_{\mathrm{N}\mathrm{O},\mathrm{i}\mathrm{n}}}\times 100\mathrm{\%},

where C_NO,in is the inlet concentration of NO in the simulated flue gas, ppm; C_NO,out is the outlet concentration of NO in the simulated flue gas, ppm (10⁻⁶); η_NO is the removal efficiency. The total volumetric mass transfer coefficient (K_Gα, s^–1) of RPB was calculated as follows [35]:

(2)K_{\mathrm{G}}\alpha =\frac{Q_{\mathrm{G}}}{\text{π}h(r_2^2-r_1^2)}NTU,

where Q_G is gas flow, m³·h^–1; h is axial height of packing, m; r₁, r₂ are the inner and outer radius of the filler, respectively, m; NTU is the number of mass transfer units [36] and the calculation formula is as follows:

(3)NTU={\int }_{C_{\mathrm{o}\mathrm{u}\mathrm{t}}}^{C_{\mathrm{i}\mathrm{n}}}\frac{1}{C-C^{\ast }}\mathrm{d}C,

where C* represents the equilibrium concentration of gas at the gas–liquid interface and the studies have shown that the absorption and oxidation process of NO in the PS system is a quasi-first-order rapid reaction [37], so C* can usually be assumed to be 0. Therefore, the K_Gα can be written as:

(4)K_{\mathrm{G}}\alpha =\frac{Q_{\mathrm{G}}}{\text{π}h(r_2^2-r_1^2)}\mathrm{l}\mathrm{n}\frac{C_{\mathrm{i}\mathrm{n}}}{C_{\mathrm{o}\mathrm{u}\mathrm{t}}}.

3 Results and discussion

3.1 The influence of gas flow on η_NO

As shown in Fig.3, it was obvious that the η_NO showed a steady decline with the increase in the flue gas flow. When the gas flow increased from 1 to 10 m³·h^–1, the η_NO dropped from 57.3% to 32.8%. Increasing the gas flow would have increased the NO content, which reduced the molar ratio of oxidants to NO. At the same time, the speed of gas entering the packing was accelerated, which shortened the gas–liquid contact time, and the reaction could not be fully carried out. Only a part of NO reacted with the oxidizing substance, and most of the NO was blown out without participating in the reaction process, which was not conducive to NO removal. On the other hand, increasing the gas flow would have enhanced the turbulence of the gas phase which constantly updated the surface of liquid droplets and firms [38], and was beneficial to the mass transfer between gas and liquid. Thus the K_Gα increased with the increase of gas flow. However, the effect of shortening the contact time and insufficient reaction was far greater than the effect of enhancement of mass transfer by increasing the gas flow, which reduced the removal efficiency of NO.

Fig.3 The influence of gas flow on η_NO (conditions: C_A (NO concentration) = 500 ppm, C_B ((NH₄)₂S₂O₈ concentration) = 0.1 mol·L^–1, C (Fe²⁺-EDTA) = 0.01 mol·L^–1, pH = 4.50, Q_L (liquid flow) = 60 L·h^–1, β (high gravity factor) = 68.21, T = 293 K).

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3.2 The effect of liquid–gas ratio on η_NO

Liquid–gas ratio is an important factor in absorption equipment, pumps, pipelines and investment, which directly affects the operation and cost of the system, and it is also considered as an effective method to adjust absorption performance. In this experiment, the influence of liquid–gas ratio on η_NO was investigated by changing the absorbent inlet flow.

As could be seen from Fig.4, with the increase of liquid–gas ratio, η_NO and K_Gα greatly increased. When liquid–gas ratio increased from 0.016 to 0.033, η_NO increased from 39.3% to 70.4%, and K_Gα increased greatly from 0.0435 to 0.09996. When the gas flow was constant, the increasing of the liquid–gas ratio meant that the liquid flow increased. That made the circulation amount of PS, •SO₄^– and •OH through the reactor per unit time increase, which increased the relative molar ratio of oxidants to NO, hence the NO removal process was enhanced. On the other hand, the increase of the liquid volume not only increased the content of the oxidant, but also increased the wetting degree of the filler, the effective contact area of the gas–liquid and the turbulence of the liquid phase, thereby improving the mass transfer rate of NO in the gas–liquid phases and was beneficial for the removal process of NO. Therefore, under the condition that the gas flow remained unchanged, an appropriate increase in the liquid flow could effectively improve η_NO and K_Gα. However, the increase of the liquid flow would also have increased the energy consumption, so it was more appropriate to choose a gas–liquid value of about 0.033 in this experiment.

Fig.4 The effect of liquid–gas ratio on η_NO (conditions: C_A = 500 ppm, C_B = 0.1 mol·L^–1, pH = 4.50, C (Fe²⁺-EDTA) = 0.01 mol·L^–1, Q_G = 1 m³·h^–1, β = 68.21, T = 293 K).

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3.3 The influence of β on η_NO

According to the research methods of chemical engineering, dimensionless parameters are usually used to express the strength of the high gravity field, and that is the β, which is convenient for comparison of RPBs of different sizes and different speeds. It is defined as the ratio of centrifugal acceleration to gravitational acceleration at any place (or any point) in the high gravity field, and the Eq. (5) is [39]:

(5)\beta =\frac{{\omega }^{2}r}{\mathrm{g}}=\frac{N^{2}r}{900},

where ω is the rotor angular speed, s^–1; r is the rotor radius, m; N is the rotor speed, r·min^–1, g is the gravitational acceleration, m·s^–2.

It can be seen from Eq. (5) that when the speed is constant, the β increases linearly with the rotor radius. Due to the different high gravity factors at different radius, the average β is generally used to describe the strength of the high gravity field as shown by Eq. (6):

(6)\overline{\beta }=\frac{{\displaystyle {\int }_{r_1}^{r_2}\beta \cdot 2\text{π}r\text{d}r}}{{\displaystyle {\int }_{r_1}^{r_2}2\text{π}r\text{d}r}}=\frac{2N^2\left(r_1^2+r_1{r}_2+r_2^2\right)}{3\times 900\left(r_1+r_2\right)}.

Fig.5 shows the variation of K_Gα and η_NO with β. The K_Gα and η_NO obviously increased with an increase in β ranging from 4.85 to 77.61. When β exceeded 77.61, K_Gα and η_NO still increased gradually, but the changes tended to be gentle, indicating that the positive effect of β on the η_NO began to decrease as well. When β increased, the speed of the filler would also increase, which would generate greater shear force, increase the cutting effect on the liquid, and accelerate the formation of smaller droplets and liquid film and the rate of surface renewal [31]. As a result, the mass transfer area of •SO₄^– and •OH–NO was increased, and the possibility of NO–•SO₄^–(•OH) reaction on the liquid film surface was improved. When β was further increased, the liquid element further decreased, and the residence time of the element in the packing would have become shorter with the increase in the rotating speed, and the gas–liquid contact time decreased, which was not conducive to the chemical absorption process of NO and S₂O₈^2–, •SO₄^– and •OH. Hence the change of η_NO and K_Gα tended to be flattened. The experimental results showed that the enhancement of mass transfer by increasing β was obviously higher than the adverse effect of residence time, but it also caused the increase of energy consumption. Thus, β of 77.61 was the best choice according to this study.

Fig.5 The influence of β on η_NO (conditions: C_A = 500 ppm, C_B = 0.1 mol·L^–1, pH = 4.50, C (Fe²⁺-EDTA) = 0.01 mol·L^–1, Q_G = 1 m³·h^–1, L = 0.033, T = 293 K).

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3.4 The influence of NO concentration on its removal efficiency

The influence of NO concentration on removal efficiency of NO was studied and the experimental results are shown in Fig.6. With the increase of NO concentration, the η_NO and K_Gα showed a downward trend. When NO concentration increased from 200 to 1000 ppm, the η_NO decreased from 75.1% to 65.1%, and the K_Gα decreased from 0.1167 to 0.0971 s^–1. As the NO concentration increased, the gas-phase driving force became larger, which was conducive to gas-phase mass transfer. But increasing the NO concentration was equivalent to reducing the concentration of strong oxidizing groups per unit volume to remove NO, causing the slower liquid phase oxidation reaction rate and hindrance in the liquid phase mass transfer. The interaction caused the total mass transfer coefficient a gradual decrease, which further showed that the removal of NO by the advanced oxidation of (NH₄)₂S₂O₈ was a mass transfer control process.

Fig.6 Effect of NO inlet concentration on η_NO (conditions: C_B = 0.1 mol·L^–1, pH = 4.50, Q_G = 1 m³·h^–1, L = 0.033, C (Fe²⁺-EDTA) = 0.01 mol·L^–1, β = 77.61, T = 293 K).

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3.5 The influence of (NH₄)₂S₂O₈ concentration on η_NO

PS concentration often has a large effect on free radical generation and pollutant removal and the influence of (NH₄)₂S₂O₈ concentration on η_NO was investigated, and the results are shown in Fig.7. K_Gα and η_NO increased with the increase of (NH₄)₂S₂O₈ concentration. The increase in the concentration of (NH₄)₂S₂O₈ meant that the content of (NH₄)₂S₂O₈ per unit liquid volume was increased, and the chances of contact with NO were also increased, which effectively reduced the equilibrium partial pressure of the liquid solute and raised the driving force for mass transfer. Besides, Fe²⁺ effectively activated the PS to produce •SO₄^–, which also could react with H₂O to produce •OH according to the Eqs. (7) and (8) [40]. As the C_B increased, the above (7) and (8) would accelerate and the yields of •SO₄^– and •OH would increase, thereby being able to enhance removal of NO (Eqs. (9)–(11)) [37]. When the concentration of (NH₄)₂S₂O₈ increased from 0.05 to 0.1 mol·L^–1, the change range of η_NO was 4.8% and when the concentration was increased from 0.1 to 0.25 mol·L^–1, the change range of η_NO was only 2.4%. This was because part of •SO₄^– were scavenged by Fe²⁺, and high level •SO₄^– could also be consumed by itself and S₂O₈^2– (Eqs. (12) and (14)) [41], causing a slight decrease in the increase of η_NO.

Fig.7 Effect of (NH₄)₂S₂O₈ concentration on η_NO (conditions: C_A = 500 ppm, pH = 4.50, Q_G = 1 m³·h^–1, β = 77.61, C (Fe²⁺-EDTA) = 0.01 mol·L^–1, L = 0.033, T = 293 K).

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(7){\mathrm{S}}_2{\mathrm{O}}_8^{2-}+\mathrm{F}{\mathrm{e}}^{2+}\stackrel{k_1=1.7-2.7\mathrm{o}\mathrm{r}2.0\times 10\mathrm{L}\cdot \mathrm{m}\mathrm{o}{\mathrm{l}}^{-1}\cdot {\mathrm{s}}^{-1}({70}^{\circ }\mathrm{C})}{\to }{\mathrm{S}\mathrm{O}}_4^{2-}+\cdot {\mathrm{S}\mathrm{O}}_4^{-}+\mathrm{F}{\mathrm{e}}^{3+}

(8)\cdot {\text{SO}}_4^{-}+{\text{H}}_{\text{2}}\text{O}\stackrel{k_2=6.6\times {10}^2{\text{s}}^{-1}}{\to }{\text{HSO}}_4^{-}+\text{OH}\cdot

(9)\cdot {\text{SO}}_4^{-}+\text{NO}+{\text{H}}_{\text{2}}\text{O}\stackrel{k_{\text{sr}}}{\to }{\text{HSO}}_4^{-}+{\text{NO}}_2^{-}+{\text{H}}^{+}

(10)\cdot \text{OH}+\text{NO}\stackrel{k_3=2\times {10}^{10}\text{L}\cdot \text{mo}{\text{l}}^{-1}\cdot {\text{s}}^{-1}}{\to }{\text{H}}^{+}+{\text{NO}}_2^{-}

(11){\text{NO}}_2^{-}+\cdot {\text{SO}}_4^{-}\stackrel{k_4=9.8\times {10}^8\text{L}\cdot \text{mo}{\text{l}}^{-1}\cdot {\text{s}}^{-1}}{\to }{\text{SO}}_4^{2-}+\text{N}{\text{O}}_2

(12)\cdot {\text{SO}}_4^{-}+\text{F}{\text{e}}^{2+}\to {\text{SO}}_4^{2-}+\text{F}{\text{e}}^{3+}

(13)\cdot {\text{SO}}_4^{-}+\cdot {\text{SO}}_4^{-}\stackrel{k_5=4\times {10}^8\text{L}\cdot \text{mo}{\text{l}}^{-1}\cdot {\text{s}}^{-1}}{\to }{\text{S}}_2{\text{O}}_8^{2-}

(14)\cdot {\text{SO}}_4^{-}+{\text{S}}_{\text{2}}{\text{O}}_8^{2-}\stackrel{k_6=6.1\times {10}^5\text{L}\cdot \text{mo}{\text{l}}^{-1}\cdot {\text{s}}^{-1}}{\to }{\text{SO}}_4^{2-}+\cdot {\text{S}}_{\text{2}}{\text{O}}_8^{-}

3.6 Mass transfer coefficient correlation formula calculation

The studies have shown that the mass transfer rate during the oxidation and absorption of NO by •SO₄^– and •OH were much smaller than the chemical reaction rate, and the process was controlled by mass transfer between gas and liquid [17,42]. Therefore, selecting the optimal conditions to achieve the best mass transfer coefficient was of great significance to NO oxidation and absorption. And in order to further clarify the relationship between the volumetric mass transfer coefficient and various factors, the correlation equation was solved by regression.

The relationship between the K_Gα and influencing factors were as follows:

(15)K_{\mathrm{G}}\alpha =\mathrm{Z}{Q_{\mathrm{G}}}^x{L}^y{\beta }^z{C}_{\mathrm{B}}^m{C}_{\mathrm{A}}^n,

where Z is constant; L is the liquid–gas ratio, m³·m^–3. After the logarithm of the two sides of the formula, the formula (16) was obtained:

(16)\ln \left(K_{\mathrm{G}}\alpha \right)=\ln Z+x\ln Q_{\mathrm{G}}+y\ln L+z\ln \beta +m\ln C_{\mathrm{B}}+n\ln C_{\mathrm{A}}.

When the experimental data was sorted out and brought into the above formula, the parameters were calculated by regression as: Z = 0.397062; x = 0.6437; y = –0.50875; z = 0.19431; m = 0.14044; n = –0.11691.

The correlation formula of the total volume mass transfer coefficient can be expressed as:

(17)K_{\mathrm{G}}\alpha =\frac{0.397062{\beta }^{0.19431}{C}_{\mathrm{B}}^{0.14044}{Q_{\mathrm{G}}}^{0.6437}}{L^{0.50875}{C}_{\mathrm{A}}^{0.11691}}.

The comparison of calculated value of K_Gα and experimental value of K_Gα in RPB is shown in Fig.8. It was clearly shown that the calculated value was linearly related to the experimental value, and the slope of the straight line was 1.14, and the correlation coefficient R² was 0.99. The error of calculated value and experimental value were mostly within 15%. Accordingly, the K_Gα could be well described by the formula (17) and it could found that the order of importance of different factors were gas flow, liquid−gas ratio, high gravity factor, ammonium persulfate concentration and NO concentration, providing guidance and optimization for the system.

Fig.8 Linear fit between experimental value and calculated value.

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3.7 The influence of O₂ on η_NO

The coal-fired flue gas contains certain amount of O₂, and its volume fraction is generally around 3% to 8%. Since Fe²⁺ is easily oxidized to Fe³⁺ by O₂ in the flue gas [43] and the Fe²⁺ concentration that can activate (NH₄)₂S₂O₈ decreases, which has a negative impact on the free radical yield. In the process of Fe²⁺-EDTA activation of (NH₄)₂S₂O₈, O₂ competes with (NH₄)₂S₂O₈ to react with Fe²⁺-EDTA, which affects the purification effect of NO. Fig.9 reflects the change of η_NO in the presence of O₂.

Fig.9 The influence of O₂ on η_NO (conditions: C_A = 500 ppm, C_B = 0.1 mol·L^–1, pH = 4.50, Q_G = 1 m³·h^–1, L = 0.033, C (Fe²⁺-EDTA) = 0.01 mol·L^–1, β = 77.61, T = 293 K).

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It could be seen from the Fig.9 that when there was no oxygen in the simulated flue gas, the η_NO remained constant for a long time and the η_NO was higher when β was 77.61 than that when β was 19.40. This was because when the β increased, the high-speed rotating packing generated more shear force. The absorbing liquid was spread and split into finer liquid micro-elements under the action of shear force, which significantly increased the gas–liquid contact area and strengthened the mass transfer. When O₂ existed (O₂ = 8%), the η_NO decreased, but the decreasing trend was different. Because O₂ in the flue gas continuously oxidized Fe²⁺, producing inert Fe³⁺-EDTA by reaction according to Eq. (18) [37,44], the partial deactivation of Fe²⁺ could not continue to participate in the activation process of (NH₄)₂S₂O₈, resulting in the decrease in η_NO. When β was 19.40, the variation of η_NO with time was larger than that when β was 77.61, and the η_NO declined more obviously and this phenomenon could be explained by the research results of Gambardella et al. [45]. When the mass transfer coefficient of the absorption equipment was larger, the absorption selectivity of NO could be improved. Besides, because the solubility of O₂ was smaller than that of NO, the oxidation reaction rate of O₂ and Fe²⁺-EDTA was lower than that of NO and free radicals. When the β increased, the K_Gα of the gas phase increased, which was beneficial to the removal of NO. Therefore, the decreasing trend of NO absorption efficiency was smaller when β was 77.61.

(18)4\mathrm{F}{\mathrm{e}}^{2+}\text{-}\mathrm{E}\mathrm{D}\mathrm{T}\mathrm{A}+{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}\to 4\mathrm{F}{\mathrm{e}}^{3+}\text{-}\mathrm{E}\mathrm{D}\mathrm{T}\mathrm{A}+4\mathrm{O}{\mathrm{H}}^{-}

In order to further clarify the changing trend of iron concentration in the process, the Fe²⁺ and Fe³⁺ in the solution were measured by UV spectrophotometry and the experimental results are shown in Fig.10. It can be seen from Fig.10 that when the oxygen concentration was 0%, Fe²⁺-EDTA could keep the Fe²⁺ concentration in the liquid phase at about 0.004 mg·L^–1 with the activator used to ensure the long-term and efficient activation of (NH₄)₂S₂O₈. The Fe³⁺ concentration was maintained at about 0.002–0.003 mg·L^–1, and it could be seen that large consumption of Fe²⁺ was not observed. When the oxygen concentration was 8%, the Fe²⁺ concentration was around 0.001 mg·L^–1, while the Fe³⁺ concentration showed a slight upward trend and the concentration was higher than that of when the oxygen concentration was 0%. The addition of oxygen caused part of the ferrous iron to react with it to form ferric iron, which was the main reason of reduced removal efficiency of NO.

Fig.10 Changes in the concentration of iron in the solution: (a) Concentration of Fe²⁺, (b) Concentration of Fe³⁺.

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3.8 Reaction pathways of NO removal

In order to further clarify the NO removal process in the reaction system, the components in the liquid phase and the gas phase were detected by an ion chromatograph and a flue gas analyzer as are arranged in Fig. S1 and Tab.1.

Tab.1 Existence form of each substance after absorption^a)

Time/s	Ion concentration/(mg·L–1)
	NH4+	SO42–	NO3–	NO2–	NO2
150	3.32 × 103	0.432 × 102	9.16	–	–
300	3.41 × 103	0.871 × 102	18.22	–	–

a) Conditions: CA = 500 ppm, CB = 0.25 mol·L–1, pH = 4.50, QG = 1 m3·h–1, β = 77.61, C (Fe2+-EDTA) = 0.01 mol·L–1, L = 0.033, T = 293 K.

A lot of ammonium ions were present in the solution, which were produced by the hydrolysis or ionization of (NH₄)₂S₂O₈ in the solution; and sulfate ions were mainly produced by the reaction of •SO₄^– and NO and the ineffective quenching of •SO₄^– (Eqs. (7)–(9), (19) [26]).

(19)\cdot {\mathrm{S}\mathrm{O}}_4^{-}+\cdot \mathrm{O}\mathrm{H}\stackrel{k_7=1.0\times {10}^{10}\text{L}\cdot \text{mo}{\text{l}}^{-1}\cdot {\text{s}}^{-1}}{\to }{\text{HSO}}_4^{-}+\frac{1}{2}{\text{O}}_2

At the same time, the material balance for S was carried out, and it was found that the sulfur content in the sulfate ion was less than the total sulfur content, indicating that the solution contained unreacted PS ion. Nitrate ions were also detected in the reaction solution which were the products of NO oxidation. Based on the date in Tab.1, a simple material balance had been performed and the calculation process and results are displayed in the ESM. As the reaction time increased, the concentration of sulfate ions and nitrate ions increased, indicating that the oxidation process was still going on, and PS could also directly oxidize NO (Eq. (20)) [46]. Meanwhile, nitrite ions and NO₂ were not detected in the reaction liquid and flue gas, because it eventually formed nitrate (Eqs. (21) and (22) [17]), indicating that the byproduct were not produced in the process.

(20){\mathrm{S}}_2{\mathrm{O}}_8^{2-}+\mathrm{N}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{N}{\mathrm{O}}_2+2{\mathrm{H}}^{+}+2\mathrm{S}{\mathrm{O}}_4^{2-}

(21)2\cdot \mathrm{S}{\mathrm{O}}_4^{-}+\mathrm{N}{\mathrm{O}}_2^{-}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{N}{\mathrm{O}}_3^{-}+2{\mathrm{H}}^{+}+2\mathrm{S}{\mathrm{O}}_4^{2-}

(22)\text{N}{\text{O}}_2+\cdot \text{OH}\stackrel{k_8=4.5\times {10}^9\text{L}\cdot \text{mo}{\text{l}}^{-1}\cdot {\text{s}}^{-1}}{\to }{\text{H}}^{+}+{\text{NO}}_3^{-}

To verify the existence of free radicals, EPR tests were performed. As shown in Fig. S2, •SO₄^– and •OH were successfully captured by electron spin resonance spectroscopy combining with DMPO. The seven-line peaks in the figure were the typical mixed spectrum shapes of •SO₄^– and •OH radical adducts, in which four circles represented •OH, and three rectangles represented •SO₄^–. Because of the peak overlap problem, the peak for •OH conformed approximately to the characteristic peak of •OH (1:2:2:1), and the characteristic peak of •SO₄^– as these were not obvious [21]. The measuring results demonstrated that both •SO₄^– and •OH were generated in the removal systems. Besides, as the reaction time was extended, •SO₄^– and • OH were still captured, reflecting that Fe²⁺-EDTA could delay the release of free radicals. Meanwhile, a typical sextet EPR spectrum of superoxide free radicals (•O₂^–) was also monitored, which meant that activated PS could also generate •O₂^– (Eq. (23)) [47], which also indirectly acted on the oxidation process of NO.

(23)2{\mathrm{S}}_2{\mathrm{O}}_8^{2-}+2{\mathrm{H}}_2\mathrm{O}\to 3\mathrm{S}{\mathrm{O}}_4^{2-}+\cdot \mathrm{S}{\mathrm{O}}_4^{-}+\cdot {\mathrm{O}}_2^{-}+4{\mathrm{H}}^{+}

Based on the above experimental results and discussion, the mechanism of RPB/PS/Fe²⁺-EDTA removal of NO system is summarized as follows: (1) •SO₄^– , •OH and •O₂^– were generated by Fe²⁺-EDTA activated PS; (2) •SO₄^– and •OH could simultaneously oxidize NO to produce NO₃^–; (3) PS and •O₂^– could also oxidize NO to produce NO₃^–, but not playing a leading role; (4) Fe²⁺-EDTA could delay the release of free radicals to improve free radical utilization; (5) the final products were NH₄NO₃ and (NH₄)₂SO₄; (6) the NO removal reaction had a better mass transfer process in the RPB/PS/Fe²⁺-EDTA system. The above mechanism for NO removal may also be represented by the following Fig. S3.

4 Conclusions

The process and reaction mechanism of NO removal by PS oxidation in high gravity environment are studied and RPB was used as a gas−liquid absorption device to elevate the η_NO by aqueous (NH₄)₂S₂O₈ activated by Fe²⁺-EDTA.

The results indicated that the η_NO increased with the increase of (NH₄)₂S₂O₈ concentration and liquid−gas ratio, but decreased with the increase of the gas flow. Increasing the high gravity factor from 4.85 to 77.61 was beneficial to the enhancement of η_NO. When the high gravity factor was higher than 77.61, the η_NO could reach 75% or higher. The η_NO was also found to decrease with a rise of NO concentration. And an empirical correlation could well predict the K_Gα in this system. The order of importance of different factors were gas flow, liquid−gas ratio, high gravity factor, (NH₄)₂S₂O₈ concentration and NO concentration. In addition, the presence of O₂ in simulated flue gas had shown less effect on the NO removal process at higher speeds in RPB. Finally, according to the related references, determination of free radicals and analyses of removal products, the removal mechanism was speculated, in which, Fe²⁺-EDTA could effectively activate (NH₄)₂S₂O₈ to produce the •SO₄^– and •OH, which were important for the oxidation from NO to NO₃^–, and the final products were NH₄NO₃ and (NH₄)₂SO₄, respectively. They are widely used in agricultural markets and are excellent nitrogen fertilizer. The NH₄NO₃ and (NH₄)₂SO₄ aqueous solutions are still further transformed into recoverable solids by crystallization and evaporation. Thus, it is foreseeable that RPB has great potential for industrial application in NO removal using persulfate as oxidant.

Acknowledgements

The authors thank the National Natural Science Foundation of China International (Regional) Cooperation and Exchange Project (Grant No. 21961160740), the Shanxi Province Applied Basic Research Program (Grant No. 201901D111178) and 2021 Shanxi Postgraduate Innovation Project (Grant No. 2021Y601).

Electronic Supplementary Material

Supplementary material is available in the online version of this article at https://dx.doi.org/10.1007/s11705-022-2224-5 and is accessible for authorized users.

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