Simultaneous Removal of NO x and SO2 by MgO Combined with O3 Oxidation: The Influencing Factors and O3 Consumption Distributions.

Simultaneous removal of NO x and SO2 by MgO combined with O3 oxidation was studied. The effects of the O3/NO molar ratio, oxidation temperature, and oxidation residence time on N2O5 decomposition and O3 consumption distributions were systematically illustrated, which is of great significance for improving NO x removal efficiency and reducing O3 consumption in practical application. When the O3/NO molar ratio was greater than 1.0, the highest N2O5 yield was achieved at 90 °C. The NO x removal efficiency reached 96.5% at an O3/NO molar ratio of 1.8. The oxidation temperature increased from 90 to 130 °C, resulting in the decrease of N2O5 yield, the improvement of O3-ICC (O3 invalid cycle consumption) caused by N2O5 decomposition, and the decrease of NO x removal efficiency from 96.5 to 76%. Besides, the effects of pH, SO2 concentration, and MgSO3 addition on NO x removal efficiency were also investigated. The results showed that the removal efficiency of NO x decreased with the increase of SO2 concentration, while MgSO3 addition into MgO slurry could promote the absorption of NO2 due to the reaction between NO2 and SO3 2-.

Introduction

Sulfur dioxide (SO2) and nitrogen oxides (NO) are recognized as the major pollutants causing environmental problems,[1,2] such as acid rain[3] and photochemical smog. With the NO emission standards becoming more and more strict, numerous methods have been developed for NO removal,[4] such as selective catalytic reduction (SCR),[5−8] activated carbon method,[9−12] and ozone oxidation–absorption method.[13−15] Among them, the ozone oxidation–absorption method has attracted great interest because of its advantages of simultaneous removal of SO2 and NO with high efficiency and low cost.

Huge emissions of iron ore sintering flue gas from steel industry with high gas flow amounts and low temperature (120–180 °C) are a great concern, and many efforts have been devoted to the development of denitrification technologies. In comparison to other denitrification strategies such as SCR and activated carbon method, ozone oxidation–absorption technology is more suitable because of its application temperature and operation convenience. In the ozone oxidation combined with wet flue gas desulfurization (WFGD),[16,17] NO is oxidized into NO2 or N2O5 by injecting O3 in front of the wet spray tower. Subsequently, NO and SO2 in the spray tower are simultaneously removed.[18,19]

The WFGD[20−23] include limestone–gypsum,[24−26] ammonia,[27−29] MgO,[30−32] and double-alkali methods.[33] The magnesium base WFGD has been reported to possess various advantages, such as low investment, high removal efficiency of SO2, and high comprehensive utilization value of byproducts. Additionally, it has been reported[34,35] that MgO slurry reacts with SO2 in the spray tower to produce MgSO3, which promotes NO removal. Accordingly, it would be feasible to use magnesium base WFGD combined with ozone oxidation to accomplish the simultaneous removal of NO and SO2 in flue gas.

In the ozone oxidation–absorption technology, the removal efficiency of NO highly relies on the composition of NO (solubility in water:[4] NO, 0.032 g/dm3; NO2, 213.0 g/dm3; N2O5, 500.0 g/dm3).[36] Compared with NO2, N2O5 is easier to be removed in the spraying system but needs more O3 for its generation.[37,38] Wang et al.[39] reported the reaction parameters and kinetic mechanism for the generation of NO2 and N2O5 during the NO oxidation process. Sun et al.[40] studied the effects of the pH value, initial SO2 concentration, MgO concentration, and other operating parameters on the NO2 removal efficiency. Shen et al.[41] presented a novel magnesium-based WFGD process in which sodium thiosulfate was used to inhibit the oxidation of the desulfurization byproduct. However, these reports paid little attention to the decomposition of N2O5 at high temperatures and the consumption distribution of O3 during the whole process. To study the decomposition of N2O5 and the consumption distribution of O3 in the oxidation process, it is beneficial to improve NO removal efficiency and reduce O3 consumption.

In this paper, the effects of the molar ratio of O3/NO, oxidation temperature, oxidation residence time, and adsorption operation parameters (pH value, initial concentration of SO2, and MgSO3 addition) on N2O5 yields and denitrification efficiencies were studied. Additionally, the distribution of O3 consumption during the whole process was particularly concerned.

Results and Discussions

N2O5 Yield and O3-ICC Ratio

Factors Influencing the N2O5 Yield

The solubility of N2O5 is much higher than those of NO and NO2, and the yield of N2O5 greatly influences the absorption efficiency of NO. The molar ratio of O3/NO, the oxidation temperature, and the oxidation residence time are the key factors to determine the N2O5 yield. Considering the practical working conditions of the sintering process, the oxidation residence time was chosen to be 1.2 s. The influence of the O3/NO molar ratio and oxidation temperature on N2O5 yield was studied with the oxidation residence time of 1.2 s, and the results were illustrated as Figure .

Figure 1

Yield of N2O5 under different O3/NO molar ratios and temperatures (conditions: [NO], 200 ppm; [SO2], 500 ppm; [O2], 16%; gas flow rate Q, 6 L/min; and oxidation residence time = 1.2 s).

It can be seen that N2O5 was not formed when O3/NO ≤ 1, and NO is gradually oxidized to NO2, as shown by R1. The reaction of R1 is very fast, and it is the main reaction step when O3/NO ≤ 1.[36,37] Further increase of the O3/NO molar ratio led to the generation N2O5 yield, which was mainly ascribed to reactions as R2 and R3.

For the oxidation temperature, a similar phenomenon was observed in our previous research.[38] The results showed that the N2O5 yield first increased and then decreased with the increase of oxidation temperature, reaching the highest value at 90 °C for all O3/NO > 1. When the temperature of gas was higher than 90 °C, the N2O5 yield decreased very rapidly. Especially at 180 °C, N2O5 generation was not detected. The decrease of N2O5 at the high-temperature region (110–180 °C) was mainly ascribed to reactions of R4 and R5. Moreover, our previous research[38] showed that O3 had strong oxidation selectivity for NO, the addition of SO2 had a little effect on N2O5 yield and O3 consumption.

Oxidation residence time is also an important factor influencing the N2O5 yield in oxidation reaction. Herein, the oxidation residence time was tested between 0.5 and 6.7 s to investigate its correlation with N2O5 yield. Considering the yield of N2O5, the operating cost, the O3 escape, and the O3/NO molar ratio was suggested to be within the range of 1.5–2.0. Therefore, the O3/NO molar ratio of 1.8 was chosen. In Wang et al.’s research,[39] the O3/NO molar ratio also chose a similar value (1.75). As shown in Figure , for all five oxidation residence times, the N2O5 yield decreased with increasing oxidation temperature. In addition, the oxidation residence time had different effects on the N2O5 yield at different oxidation temperatures. When the oxidation temperature was 90 and 110 °C, the N2O5 yield increased first and then decreased with the extension of oxidation residence time, and the N2O5 yield reached the highest value at 2.5 s. When the oxidation temperature was 130 and 150 °C, the N2O5 yield decreased as the oxidation residence time was prolonged. These results indicated that choosing a reasonable oxidation residence time at different oxidation temperatures is beneficial to promote the formation of N2O5.

Figure 2

Yield of N2O5 under different oxidation residence times and temperatures (conditions: [NO], 200 ppm; [SO2], 500 ppm; [O3], 360 ppm; [O2], 16%; gas flow rate Q, 6 L/min).

Factors Influencing O3 Invalid Cyclical Consumption

During the oxidation process, extra O3 consuming processes include N2O5 decomposition, O3 decomposition, and reaction with SO2. In our previous research,[38] it was found that O3 hardly decomposed when the oxidation temperature ranged from 90 to 150 °C, and SO2 hardly consumed O3. Therefore, in this temperature range, the research was mainly focused on the O3 extra consumption caused by the decomposition of N2O5, regardless of the decomposition of O3 and the O3 consumed by the reaction with SO2.

N2O5 decomposition mainly generates NO2, especially at high temperatures. The part of NO2 reacts with O3 to reproduce N2O5 again, leading to invalid cyclical consumption of O3. In this process, the main reaction is as R2, R3, R4, and R5. Hence, it is meaningful to investigate the proportion of O3 extra wasted by invalid cyclical consumption (O3-ICC), which was caused by N2O5 decomposition. Figure showed the ratios of O3-ICC at different temperatures, which were calculated based on the concentrations of NO2, N2O5, and unreacted O3.

Figure 3

O3-ICC ratio under different O3/NO molar ratios and temperatures (conditions: [NO], 200 ppm; [SO2], 500 ppm; [O2], 16%; gas flow rate Q, 6 L/min; and oxidation time = 1.2 s).

As shown in Figure , it was found that when O3/NO ≤ 1, no O3-ICC appeared. The ratio of O3-ICC increased with improvement of temperature and O3/NO molar ratio when O3/NO > 1. For an O3/NO molar ratio of 1.8, the oxidation temperature changed from 60 to 180 °C, and the O3-ICC ratio was increased from 0 to 38%. The results indicate that O3 was all consumed to generate NO2, and no O3 was wasted when O3/NO ≤ 1; the decomposition rate of N2O5 increased and consumed more O3 with increasing temperature when O3/NO > 1. Moreover, when the oxidation temperature was 180 °C, the decomposition of O3 also consumed part of O3. These results indicated that a lower oxidation temperature was beneficial to decrease O3-ICC.

Subsequently, the effect of oxidation residence time on the consumption ratio of O3-ICC in the oxidation process was also studied with the O3/NO ratio of 1.8 at 90, 110, 130, and 150 °C, respectively. As summarized in Figure , the O3-ICC ratio increased with improvement of oxidation residence time and temperature. When the oxidation residence time was 0.5 or 1.2 s, O3-ICC was not observed at 90 °C. Further increase of the oxidation temperature led to the dramatic increase of the O3-ICC ratio. When the reaction temperature was 130 °C, the O3-ICC ratio increased from 10 to 40% with oxidation time extension from 0.5 to 6.7 s. The experimental results showed that longer oxidation residence time led to more N2O5 decomposition and higher O3-ICC ratio. This phenomenon is more pronounced especially at high temperatures.

Figure 4

O3-ICC ratio under different oxidation residence times and temperatures (conditions: [NO], 200 ppm; [SO2], 500 ppm; [O2], 16%; [O3], 360 ppm; gas flow rate Q, 6 L/min).

Removal Efficiency of NO and the O3 Consumption Distributions

Removal Efficiency of NO

After the oxidation process, the flue gas was introduced into the spraying tower for absorption. As shown in Figure , for O3/NO ≤ 1.0, the removal efficiency of NO increased with the improvement of the molar ratio of O3/NO, and different oxidation temperatures led to little change of the NO removal efficiency. When O3/NO was >1.0, the NO removal efficiency increased with the increase of the molar ratio of O3/NO and decreased with the improvement of the oxidation temperature. NO removal efficiency increased from 43 to 97% with the improvement of the O3/NO molar ratio from 1.0 to 1.8 at 90 °C. The NO removal efficiency increased from 43 to 76% when the molar ratio of O3/NO was changed from 1.0 to 1.8 at 130 °C.

Figure 5

NO removal efficiencies under different O3/NO molar ratios and temperatures (conditions: [NO], 200 ppm; [SO2], 500 ppm; [O2], 16%; MgO slurry concentration C, 0.05 mol/L; initial pH of slurry, 9.25; gas flow rate Q, 6 L/min; and oxidation time = 1.2 s).

Studies revealed that when O3/NO ≤ 1.0, the absorbed NO were NO and NO2, and NO2 was more easily absorbed than NO. Therefore, the removal efficiency of NO increased with the improvement of O3/NO molar ratio. The oxidation products were NO2 and N2O5 when O3/NO > 1.0. Considering N2O5 is more easily absorbed than NO2, removal efficiency of NO increases with the increase of the concentration of N2O5. With the improvement of oxidation temperature, the decomposition of N2O5 was accelerated, and the N2O5 yield decreased, and NO removal efficiency accordingly decreased.

Besides, O3 is also an atmospheric pollutant, and whether there is O3 escaping after absorption is a concern in the industrial applications. Hence, the amount of O3 escape after absorption was analyzed at different temperatures and molar ratios. As shown in Figure , O3 escape gradually appeared in the absorbed gas with the increase of the molar ratio. On the contrary, O3 escape decreased with the improvement of the oxidation temperature. The oxidation temperature at 90 °C gave the highest O3 escape value. When the oxidation temperature was 180 °C, no O3 escape was detected within the full range of the O3/NO molar ratio tested.

Figure 6

O3 escape under different O3/NO molar ratios and temperatures (conditions: [NO], 200 ppm; [SO2], 500 ppm; [O2], 16%; MgO slurry concentration C, 0.05 mol/L; initial pH of slurry, 9.25; gas flow rate Q, 6 L/min; and oxidation residence time = 1.2 s).

The reason for the O3 escape was that O3 was not completely consumed in the oxidation reaction tube and the spray tower. As the oxidation temperature increases, O3 is consumed more in the oxidation reactor. Therefore, the O3 entering the spray tower was reduced and the O3 escape was reduced.

The oxidation residence time has a significant effect on the N2O5 yield and thus on NO removal efficiency. Hence, the corresponding research was conducted with an oxidation temperature at 130 °C. As shown in Figure , extension of the oxidation residence time led to the decrease of NO removal efficiency when O3/NO > 1.0. At O3/NO = 1.8, the removal efficiencies of NO were 96, 76, and 60% with the oxidation residence time being 0.5, 1.2, and 2.5 s, respectively.

Figure 7

NO removal efficiencies under different O3/NO molar ratios and oxidation residence times (conditions: [NO], 200 ppm; [SO2], 500 ppm; [O2], 16%; MgO slurry concentration C, 0.05 mol/L; initial pH of slurry, 9.25; gas flow rate Q, 6 L/min; and gas temperature T, 130 °C).

Meanwhile, reducing the oxidation residence time will lead to increasing the amount of O3 entering the spraying tower, thus affecting the amount of O3 escape. Hence, O3 escape after absorption was studied at different oxidation residence times and O3/NO molar ratios. As shown in Figure , when the oxidation residence time was 0.5, 1.2, and 2.5 s, O3 began to escape with the O3/NO molar ratio increasing to 1.5, 2.0, and 2.5, respectively. The results revealed that when the oxidation temperature was 130 °C, shorter oxidation residence time led to less O3-ICC, suggesting more O3 entering the spraying tower. In the spraying tower, some O3 would be consumed, and the remaining unreacted O3 would escape from the spraying tower.

Figure 8

O3 escape under different O3/NO molar ratios and oxidation residence times (conditions: [NO], 200 ppm; [SO2], 500 ppm; [O2], 16%; MgO slurry concentration C, 0.05 mol/L; initial pH of slurry, 9.25; gas flow rate Q, 6 L/min; and gas temperature T, 130 °C).

O3 Consumption Distributions

In addition to the performance of desulfurization and denitration, optimization of O3 consumption is also of great importance in the ozone oxidation–absorption method. Great energy is consumed during the O3 production process, and decreasing O3 consumption is a main strategy to reduce the operation cost of the denitration device. In order to reduce O3 consumption, it is necessary to establish an analytical method for O3 consumption distributions under different conditions and to verify the influencing factors.

Based on the flue gas analysis after oxidation and absorption, the O3 balance was calculated, and the O3 distributions were classified into five parts (Figure ): after oxidation, the product was generated as NO2 and N2O5, which could be analyzed at the outlet of the oxidation reactor. The oxidation product NO2 corresponds to equimolar O3 consumption named as Part A. The oxidation product N2O5 corresponds to the O3 consumption classified as Part B (excluding O3 consumption caused by decomposition of N2O5). As described above, some O3 is extra wasted in the invalid cyclical consumption (O3-ICC), which was caused by N2O5 decomposition. Herein, O3-ICC was classified as Part C. The remaining O3 enters into the spraying tower, and many reactions occurred in the tower through complicated steps, such as further oxidation of NO2 to generate N2O5, the oxidation of SO32– to produce SO42–, the oxidation NO2– to produce NO3–, the oxidation of NO which was released during the absorption of NO2, and so forth. It is very difficult to analyze every single small part of O3 consumption for every reaction. Hence, the O3 consumption in the spraying tower is defined as Part D. At last, the O3 escaping from the spraying tower was classified as Part E.

Figure 9

O3 combined with the MgO oxidation–absorption process of the O3 consumption distribution schematic diagram.

After building the analytical method for O3 consumption distributions, it was employed for the experiments conducted at 90, 110, 130, and 150 °C with the oxidation residence of 1.2 s and a O3/NO molar ratio of 1.8. As shown in Figure , we found that the ratio of Part A and Part C increased with increasing oxidation temperature, while the ratios of Part B, Part D, and Part E decreased with increasing oxidation temperature. For the oxidation temperatures of 90 and 110 °C, the largest percentage was Part B. For the oxidation temperatures of 130 and 150 °C, the largest percentage was Part A. The ratio of Part E was the least in all four oxidation temperatures. This indicated that most O3 was consumed, with very few O3 escapes, and even no O3 escape was detected at 130 and 150 °C.

Figure 10

Consumption distributions of O3 in the process of magnesia combined with O3 oxidation (conditions: [NO], 200 ppm; [SO2], 500 ppm; [O2], 16%; [O3], 360 ppm; MgO slurry concentration C, 0.05 mol/L, gas flow rate Q, 6 L/min; and oxidation time = 1.2 s).

With the increase of oxidation temperature, the decomposition rate of N2O5 was accelerated, the yield of N2O5 decreased, and the concentration of NO2 increased. When the oxidation temperature changed from 90 to 150 °C, O3-ICC increased, and O3 entering the spray tower to participate in the reaction decreased. Therefore, the ratio of Part A and Part C increased and that of Part B, Part D, and Part E decreased. These results indicated that oxidation temperature is the key factor affecting O3 consumption distribution.

Effects of Operation Parameters on Desulfurization and Denitrification

Effect of the pH Value

Figure showed the efficiencies in removing NO and SO2 at different pH values when the molar ratios of O3/NO were 1.0 and 1.5. The slurry was continuously circulated in the spraying tower to absorb oxidized simulated flue gas.

Figure 11

NO/SO2 removal efficiencies under different pH values (conditions: [NO], 200 ppm; [SO2], 500 ppm; [O2], 16%; MgO slurry concentration C, 0.05 mol/L, gas flow rate Q, 6 L/min; gas temperature T, 130 °C, and oxidation time, 1.2 s).

It can be seen from Figure that the pH value had little influence on MgO’s absorption of NO, and the removal efficiency fluctuation under different pH values was less than 5%. The trend of NO removal efficiency at different pH values was similar when the molar ratio of O3/NO was 1.0 and 1.5. As the pH changed from 9 to 8, the NO removal efficiency was lowered. However, the pH further decreased from 8 to 4.5, the NO removal efficiency did not change. In addition, as shown in Figure , the removal efficiency of SO2 remains at 100% when the pH value was above 5.5. When the pH dropped to 4.5, the removal rate of SO2 slightly decreased.

Figure 12

NO removal efficiencies under different SO2 initial concentrations (conditions: [NO], 200 ppm; [O2], 16%; MgO slurry concentration C, 0.05 mol/L; initial pH of slurry, 9.25; gas flow rate Q, 6 L/min; gas temperature T, 130 °C, and oxidation time, 1.2 s).

N2O5 is easily absorbed, so the decrease of NO removal efficiency at different pH values is due to the decrease of NO2 removal efficiency. It can be seen from following reaction that the removal efficiency of NO2 was related to the concentration of OH–. The concentration of OH– decreased as the pH decreased and the NO2 removal efficiency decreased. As the pH drops further, SO32– converted to HSO3–. Both SO32– and HSO3– would react with NO2 in the slurry (R7 and R8). In this pH range (8–4.5), the positive effects of HSO3– and the negative effects of OH– cancel each other out, and the NO removal efficiency did not change with pH.

Effect of Initial SO2 Concentration

In the practical industrial applications, the concentration of SO2 contained in flue gas fluctuates greatly. Therefore, it was necessary to study the initial concentration of SO2. Figure studied the effect of different initial SO2 concentrations on the absorption of NO by MgO.

As shown in Figure , when the molar ratio of O3/NO was 0.5, 1.0, and 1.5, the removal efficiency of NO increased with the increase of the initial concentration of SO2, but the degree of increase was different. However, the NO removal efficiency decreased when the molar ratio of O3/NO is 1.8. It was found that SO32– could react with NO2, thus promoting the absorption of NO (R7). The concentration of SO2 in the flue gas component is usually greater than that of NO2. Therefore, the concentration of NO2 largely determines the promoting effect of SO2 on NO absorption. NO2 content in flue gas: O3/NO molar ratio 1.0 > 0.5 > 1.5, SO2 on the NO absorption promotion effect decreased in turn. At the same time, SO32– generated by SO2 will consume O3 in the spraying tower and reduce the reaction between O3 and NO2, thus having a negative effect on NO absorption (R9). When the molar ratio of O3/NO was 1.8, the negative effect was more obvious.

Effect of Magnesium Sulfite

It has been reported[31,35] that SO32– can react with NO2 to promote the absorption of NO. Therefore, MgSO3 with different concentrations was added to the MgO slurry to investigate its effect on the removal efficiency of NO. The results are shown in Figure . It is shown in this figure that the increase of the MgSO3 concentration promoted the removal efficiency of NO2. If there was no MgSO3 in the slurry, the removal efficiency of NO2 was 42.5%. While the MgSO3 concentration increased to 100%, the removal efficiency increased to 79.5%. However, the promotion of MgSO3 cannot be sustained. This is because the SO32– ions in the slurry are consumed by NO2 and O2 (R7 and R10). Therefore, as the concentration of MgSO3 increased, the time for promoting NO removal efficiency was prolonged.

Figure 13

NO removal efficiencies under different MgSO3 initial concentrations (conditions: [NO], 200 ppm; [SO2], 500 ppm; [O2], 16%; [O3], 200 ppm; MgO and MgSO3 slurry concentration C, 0.05 mol/L, gas flow rate Q, 6 L/min; gas temperature T, 130 °C, and oxidation time, 1.2 s).

Conclusions

The factors influencing the denitrification performance of MgO and the consumption distribution of O3 were studied, including the O3/NO molar ratio, oxidation temperature, oxidation residence time, initial concentration of SO2, pH value, and addition of MgSO3. Oxidation temperature and residence time affect N2O5 yield, O3 consumption distribution, and NO removal efficiency. Under the conditions of the O3/NO molar ratio being 1.8 and oxidation residence time being 1.2 s, the highest N2O5 yield was achieved at 90 °C without O3-ICC consumption and the NO removal efficiency was 96.5%. When the oxidation temperature was improved to 130 °C, the O3 consumption percentage corresponding to NO2 was the largest, and the O3-ICC ratio increased to 21.94%, and the NO removal efficiency decreased to 76%. Nevertheless, when the oxidation residence time was shortened to 0.5 s, the O3-ICC ratio decreased to 10%, and the NO removal efficiency increased to 95.5%. When the O3/NO molar ratio was 1.8, improvement of the SO2 concentration led to the decrease of the NO removal efficiency, revealing an inhibition effect. Besides, adding MgSO3 to the MgO slurry can promote the absorption of NO2 due to the reaction of NO2 and SO32–.

Experimental Section

Experimental System

A schematic diagram of the experimental setup for the simultaneous removal of NO and SO2 using MgO slurry-combined ozone oxidation is shown in Figure . The system included an ozone generator, gas supply system, oxidation reactor, spraying tower, and gas analysis system.

Figure 14

Schematic diagram of the experimental apparatus: (1) mass flow controllers, (2) gas mixing heating tube, (3) ozone generator, (4) oxidation reaction tube, (5) spraying tower, (6) the heat-collecting magnetic heating stirrer, (7) pH meter, (8) pump, (9) dehydration device, and (10) infrared gas analyzer.

In this paper, cylinder gas (Beijing Hua Yuan Gas Chemical Co. N2, 99.999%; O2, 99.999%; NO, 1%, balanced with N2; SO2, 2%, balanced with N2) is used as the gas source, and the mass flow meter (Beijing Sevenstar Electronics Co., Ltd.) is used to control the flow of simulated flue gas. The flow rates of NO and SO2 were controlled at 120 and 150 mL/min (273.15 K and 1 atm.), respectively, and the flow rates of N2 and O2 were controlled at 4770 and 960 mL/min (273.15 K and 1 atm.), respectively. The total gas flow was 6 L/min (273.15 K and 1 atm.).

The ozone was produced by the Ozonia Lab2B generator using high-purity oxygen (>99.999%) as the ozone source, which is based on the principle of high-voltage discharge. The simulated flue gas is introduced into the mixing heater and mixed with O3 and enters the reactor for oxidation. The heating temperatures of the mixing heater are consistent with that of the oxidation reactor, which are 60, 90, 110, 130, 150, and 180 °C, respectively.

The MgO slurry spraying tower is employed for absorption, and the countercurrent slurry spraying tower is made of stainless steel with a height of 400 mm and an inner diameter of 72 mm. On the top of the tower, a single spraying nozzle (1/4 inch internal thread, 1.0 mm aperture spraying nozzle, Dongpu Shagu Spray Co., Ltd.) is installed, and the liquid–gas interaction distance is about 200 mm. The simulated flue gas and slurry droplets enter the tower from the bottom and top, respectively. MgO slurry is prepared using MgO (purity 98%, Shanghai Aladdin Industry Co., LTD.) and deionized water, storing in a 2 L three-necked, round-bottomed flask. The slurry is heated and stirred with a heat-collecting magnetic heating agitator (DF-101S, Jiangsu Jinqia Instrument Technology Co., LTD.). MgO slurry is introduced into the spraying tower by a peristaltic Pump (BT100-2J, Longer Precision Pump Co., Ltd.). The slurry is circulated between the spraying tower and the three-necked flask with a concentration of 0.05 mol/L and flow rate of 270 mL/min at 60 °C. In addition, the pH value of the slurry was determined by a pH meter (FE20, METTLER-TOLEDO Co., Ltd.).

The concentration of NO, NO2, SO2, and O3 in front of and behind the spraying tower was continuously detected online by Fourier transform infrared spectrometer (Bruker Co., Ltd., Germany). When the molar ratio of O3/NO is greater than 1, NO2 is further oxidized to N2O5, and two basic reactions occur, which including NO2 is oxidized to NO3 and reaction of NO2 and NO3 to form N2O5, as shown by reaction formulas R2 and R3. According to the National Institute of Standards and Technology, the reaction rate constant K3 of R3 is far greater than that of R2 (K2: 3.5 × 1017, K3: 2.2 × 1030, temperature: 25 °C). The NO in the flue gas is mainly composed of NO2 and N2O5, and the NO3 content is rarely. Therefore, we calculate the concentration of N2O5 by the conservation of nitrogen oxide.

Removal Efficiency

The NO2 yield is calculated by eq , the N2O5 concentration is calculated by eq , and the N2O5 yield is calculated by eq , and [NO] is the reactor inlet NO concentrations; [NO2] and [N2O5] are the reactor outlet NO2 and N2O5 concentrations, respectively.

The NO and SO2 removal efficiencies are calculated by eqs and 5, respectively. [SO2] is the reactor inlet SO2 concentration, [NO] and [SO2] are the spraying tower outlet NO and SO2 concentrations, respectively. The NO in this paper only includes NO, NO2, and N2O5, excluding other NO (such as NO3 and N2O).

O3 Consumption Distributions

The destination of O3 can be divided into five parts. The ratio of O3 consumption distribution can be calculated by eqs –10. Because of the N2O5 decomposition, extra O3 was wasted by invalid cyclical consumption (O3-ICC). [O3] is the reactor inlet O3 concentration, [O3] is the reactor outlet O3 concentration, [O3] is the spraying tower outlet O3 concentration.