Experimental Investigation of Simultaneous Removal of SO 2 and NO x Using a Heteropoly Compound

Aiming at developing a simple, feasible method for SO 2 and NO x removal, in this work, various doping amounts of heteropolyacid and heteropolyacid salts were prepared and used in wet absorption for simultaneous removal of SO 2 and NO. The desulfurization and denitrification performance was evaluated by investigating dynamic SO 2 and NO absorption in a simulated reactor containing a heteropoly compound (HPC) solution along with various factors influencing the reaction. The results demonstrate that this system had a high removal efficiency of SO 2 and modest NO removal. The amount of Wolfram (W) doping and the pH of the solution affecting the removal performance was studied, and a stable SO 2 removal efficiency higher than 95% was obtained for HPW 1 Mo at a pH value of 5.5. In addition, the desulfurization performance of NaPWMo with a 50% removal efficiency was higher than that of the HPWMo solution (20%) after a 60 min reaction time. A high concentration of the NaPWMo solution and the presence of O 2 were beneficial to desulfurization at room temperature. In addition, the removal efficiency of SO 2 or NO by NaPWMo was only affected by its own gas concentration rather than their mutual influences on each other. This work should be beneficial to the optimization leading to the development of simultaneous desulfurization and denitrification systems in practical applications.


INTRODUCTION
All over the world, a large amount of flue gases from coal-fired power plants, oil refining and gas processing, and smelting are emitted into the atmosphere every day. Massive amounts of SO 2 and NO, as the main gas pollutants, not only do much harm to human health, but also are commonly recognized as the major contributor to acid rain and haze formation (Mclinden et al,. 2016;Oberschelp et al., 2019;Pi et al., 2020). At present, this phenomenon is particularly prominent in China because China is the largest coal producer and consumer in the world. On the other hand, the new emission standards adopted by governments at the global scale have put forward stricter requirements for limiting SO 2 and NO emissions. Thus, SO 2 and NO removal is a great concern that needs to be resolved as soon as possible. However, the development of new, efficient desulfurization and denitrification technologies is still necessary.
Commonly used SO 2 removal methods for flue gas include dry absorption and wet absorption. Activated carbon, activated coke, and zeolite have been widely used in dry absorption due to several advantages, including low cost and high operational flexibility (Karatepe et al., 2008;Yan et al., 2013;Rezaei et al., 2015;Xu et al., 2016), but the low sulfur loading process still needs further improvement. Wet flue-gas desulfurization technology (WFGD) is regarded as the most industrialized process, especially the lime/limestone process, because of its high efficiency and stability (Liu et al., 2010;Yan et al., 2013). However, the main drawbacks (high costs, complex equipment, and the formation of secondary pollutants) restrict its wider applications (Chen et al., 2015;Hao et al., 2017). Several denitrification technologies, including selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR), are available for NO x removal (Zhang et al., 2016). Among them, SCR with NH 3 as the reducing agent is the most widely applied method in stationary source combustion units owing to its high efficiency and selectivity (Forzatti et al., 2009;Imanaka and Masui 2012). However, the presence of H 2 O and SO 2 in power plant flue gas and vehicle exhaust gas causes the deposition of (NH 4 ) 2 SO 4 /NH 4 HSO 4 and metal sulfates, in turn poisoning the catalyst (Han et al., 2019). The separate removal of SO 2 and NO x results in many problems; thus, a great deal of attention has been paid to developing a system for simultaneous removal of SO 2 and NO x .
Recently, the oxidation-absorption process has been shown to have great advantages in many processes for simultaneous removal of NO x and SO 2 . Some common oxidants, including H 2 O 2 (Liu et al., 2011;Zhao et al., 2017), Na 2 S 2 O 8 (Khan and Adewuyi, 2010;Xi et al., 2019), O 3 , and NaClO (Mondal and Chelluboyana, 2013), show better NO x removal efficiency using the wet method. Liu et al. (2019) studied the simultaneous removal of SO 2 and NO x through a two-stage process composed of catalytic ozonation and the subsequent absorption of reaction gases by NH 3 /(NH 4 ) 2 S 2 O 8 . The main products were ammonium sulfate and ammonium nitrate, which can be used as fertilizers. Activated circulating fluidized bed boiler (CFB) fly ash used as a catalyst has been applied in the simultaneous removal of NO x and SO 2 , which can achieve 99% SO 2 and 92% NO x removal efficiency at 140°C with 2 mol L -1 and 5 mol L -1 of H 2 O 2 , respectively (Cui et al., 2020). Zhou et al. (2020) proposed a novel process that can achieve a higher NO x and SO 2 removal efficiency and a lower nitrate concentration by using urea peroxide (CO(NH 2 ) 2 ·H 2 O 2 ) combined with a countercurrent wet scrubber. The synergistic desulfurization and denitrification effect of the catalyst and the effect of the corona discharge on the activity of the catalyst were studied, where the desulfurization and denitrification rates on the La-Ce-V-Cu-ZSM-5 catalyst reached 97% and 83%, respectively (Qi et al., 2020). In these studies, the simultaneous removal of SO2 and NO was achieved by adding oxidants or composites. However, it is worth attempting to develop a simple, feasible method that can accomplish this goal without an oxidizing agent. The heteropoly compound (HPC) has several advantages, including versatility, favorable acidity, oxidation-reduction flexibility, and a pseudo-liquid phase feature, that have attracted significant scholarly attention and have been widely used in oil desulfurization, where it exhibits excellent activity (Zhang and Wang, 2018;Dou and Wang, 2019). In our previous work, various solid HPCs were prepared and studied initially as adsorbents used for capturing NOx Zhang et al., 2020).
In this study, various doped heteropoly acids and heteropoly acid salts were prepared and used in wet absorption for simultaneous removal of SO 2 and NO. The removal performance was investigated in a simulated reactor contained an HPC solution. The factors influencing the removal performance, such as the doping amount, the reaction temperature, the HPC concentration, and the NO/SO 2 concentrations, were investigated. This work should be beneficial to demonstrating the optimization potential of simultaneous desulfurization and denitrification systems for practical application.

Materials
All chemicals were of analytical reagent grade and used without further purification. H 3 PMo 12 O 40 , H 3 PW 12 O 40 , H 3 SiW 12 O 40 were purchased from Linghu Fine Chemical Factory. Na 2 WO 4 ·2H 2 O, NaH 2 PO 4 ·2H 2 O, and HCl were purchased from Sinopharm Chemical Reagent Co., Ltd., Na 2 MoO 4 ·2H 2 O was purchased from Tianjin chemical reagent factory, China.  (Misono et al., 1982). For example, the preparation of H 3 PW 1 Mo 11 O 40 was as follows: 26.51 g Na 2 MoO 4 ·2H 2 O and 3.29 g Na 2 WO 4 ·2H 2 O were dissolved in 100 mL of deionized water and heated to boil under magnetic stirring, after which 1.56 g of NaH 2 PO 4 ·2H 2 O was added. After 30 min, a 6 mol L −1 HCl solution was added dropwise to reach a pH value of 1. After 5 h, the solution was cooled to approximately 60°C, ice bathed, and transferred to a separatory funnel. Small amount of 1:1 sulfuric acid and ether were added, and the solution was shaken. The solution was divided into 3 layers after standing, and the lower layer of light green oily matter was mixed polyacid etherate. The residuary ether from the obtained mixed polyacid etherate was removed in a fume hood, and a small amount of water was added. The final product was obtained after drying at 60°C overnight. The three samples were denoted as HPW1Mo, HPW 3 Mo, and HPW 6 Mo, respectively.

HPC Preparation
The Na 9 [PW 5 Mo 4 O 34 ]·2H 2 O was prepared as follows: 16.49 g Na 2 WO 4 ·2H 2 O, 9.97 g Na 2 MoO 4 ·2H 2 O and 1.56 g NaH 2 PO 4 ·2H 2 O were dissolved in 50 mL deionized water under magnetic stirring. 12 mol L -1 HCl solution was added dropwise until pH = 6. After 15 min, 25 mL acetone was added, and a light yellow oil was formed. The oil was washed with acetone several times until a powder was formed. The powder was filtered and recrystallized to obtain the product (denoted as NaPWMo).

Characterizations of the Catalysts
X-ray diffraction (XRD) patterns were obtained on a Bruker D8 diffractometer using Cu Kα radiation at a scan speed of 5° min −1 . Fourier transform infrared (FTIR) spectroscopy was performed on a Nicolet Avatar 370, and the spectral data were recorded in a range of 400-4000 cm -1 at a 4 cm -1 resolution. The thermogravimetric analysis (TGA) was carried out on a TGA SDT Q 600 thermogravimetric analyzer at a heating rate of 10°C min -1 ranging from 25°C to 600°C under a nitrogen atmosphere.

Performance Measurement
The performance evaluation system for the simultaneous removal of NO and SO 2 using HPC solution is shown in Fig. 1. The system generated the simulated flue gas, and carried out the absorption of NO and SO 2 and detected the outlet gas. The total flow was 500 mL min -1 , and the simulated gases containing SO 2 , NO, 4% O 2 (when used) and balanced N 2 were metered through mass flow controllers. The flue gas was absorbed with a 100 mL HPC solution at room temperature. The inlet and outlet SO 2 and NO concentrations were continually monitored with a TH-990S SO 2 and NO gas analyzer (Wuhan Tianhong, China). The SO 2 and NO removal efficiencies were calculated as follows: Fig. 2(a) shows the XRD pattern of HPW 1 Mo and NaPWMo. The diffraction peaks corresponded with the standard powder diffraction file (PDF) of H 3 PW 12 O 40 ⋅14H 2 O (PDF: 43-0317) and H 3 PW 12 O 40 ⋅14H 2 O (PDF: 50-0656) observed on HPW 1 Mo, confirming a synthesized heteropolyacid with a Keggin type structure. However, for NaPWMo, no regular PDF card corresponding to a single crystalline phase was observed, which may be because NaPWMo consists of several substances.

XRD and FTIR
To further determine the crystal structure of the catalysts, the FTIR spectra of the as-synthesized heteropoly compound were recorded. As shown in Fig. 2(b), HPW 1 Mo showed obvious vibration bands at 785, 872, 965 and 1067 cm −1 , corresponding to the characteristic bands of heteropolyacid with a Keggin structure as reported in the previous literature (Wu et al., 2000(Wu et al., , 2005Cheng and Wang, 2013). Consistent with the XRD results, the FTIR measurement confirmed that the solid Keggin heteropolyacid was successfully prepared. For NaPWMo, similar to HPW 1 Mo, the  characteristic peaks of the Keggin heteropolyacid were also found at corresponding positions, but the peak intensity was significantly reduced.

TGA
Thermogravimetric analyses of HPW 1 Mo and NaPWMo were carried out to investigate their dehydration behavior, for which the results are shown in Fig. 3. The HPW 1 Mo curves in Fig. 3(a) showed three weight loss steps. Physisorbed water was evacuated below 130°C. The second weight loss step was located between 130 and 270°C, which can be regarded as structural water. Additional weight loss was observed above 300°C, which could be attributed to the decomposition of the heteropolyacids into oxides. The NaPWMo shown in Fig. 3(b) also exhibits three weight loss steps, with the second weight loss step located between 135°C and 586°C, illustrating that NaPWMo has higher thermal stability than HPW1Mo.

The Removal Performance of HPMo, HPW, and HSiW
The absorption performance of several commonly used heteropolyacids for SO 2 and NO was evaluated, for which the results are shown in Fig. 4. As shown in Fig. 4(a), several heteropoly  acids had similar desulfurization performance, where it can be seen that as the absorption time increased, the desulfurization performance gradually decreased. HPMo had slightly better activity, with a removal efficiency of 66% at 15 min. Its removal efficiency dropped to 21% after the reaction proceeded for 60 min. The results for NO removal are shown in Fig. 4(b). It can be observed that the three heteropolyacids had relatively stable and similar NO absorption properties, where the removal efficiency remained at approximately 15% even after 60 min of reaction.

The Performance of HPWMo
In order to explore the effect of metal doping modifications on the performance of the absorbent, the absorption performance of HPMo doped with different amounts of W as a heteroatom was studied. As shown in Fig. 5(a), HPW 1 Mo exhibits significantly improved desulfurization performance, and the SO 2 removal efficiency was as high as 90% at 15 min. As the number of doped W atoms increased, the SO 2 removal efficiency gradually declined. The order of SO 2 removal efficiency for the W-doped HPMo was as follows: HPW 1 Mo > HPW 3 Mo > HPW 6 Mo. The NO absorption is shown in Fig. 5(b), where it can be observed that the order of NO removal efficiency was the same as that for SO 2 . However, W doping had little effect on the NO removal performance of HPMo.

The Effect of pH
pH is an important factor that affects absorption reactions. Therefore, the effect of pH on the absorption performance of HPWMo was studied with a comparative experiment. In this experiment, the pH of the heteropolyacid solution was adjusted to 5.5 using a NaOH solution. It can be seen in Fig. 6(a) that under a pH value of 5.5, a SO 2 removal rate higher than 95% was found on HPW 1 Mo and remained stable for at least 60 min. HPW 3 Mo and HPW 6 Mo also showed this high removal efficiency, but their stabilization time was shorter than that of HPW 1 Mo, and the absorption efficiency decreased rapidly with reaction time. As the pH value of the absorption solution increased, the desulfurization efficiency also increased. This may have been due to the fact that SO 2 as an acid gas is more easily absorbed by substances with a high pH value. In other words, the added alkaline solution can increase the absorption capacity of SO 2 . On the other hand, it may also have been because the added alkaline solution can undergo an acid-base neutralization reaction with heteropoly acid to form a sodium heteropoly salt, which leads to better absorbability of SO 2 . As shown in Fig. 6(b), the denitrification efficiency was not sensitive enough to the change in the pH value of the solution. A reasonable explanation for this is that due to the low solubility of NO in water, the denitrification efficiency was not as significant as the desulfurization efficiency.

The Effect of Temperature
In the former experiment, it was found that using a NaOH solution to adjust the pH can improve SO 2 absorbability. The absorption performance of the synthesized NaPWMo was studied in order to further explore the essential reason for this. The absorption experiments were carried out at different temperatures. From the Fig. 7(a), it can be found that temperature affected the absorption of SO 2 on NaPWMo solution. When the temperature increased from 25°C to 50°C, the SO 2 absorption curves displayed an obvious downward trend, implying that low temperature is beneficial to the absorption of SO 2 . In addition, as the absorption time was extended, the removal efficiency at each temperature decreased. NaPWMo still had higher absorption of SO 2 at 25°C, and its removal efficiency was approximately 50% even after 60 min. By comparison, it can be seen that the desulfurization performance of NaPWMo was higher than that of the HPWMo solution with a 20% removal efficiency, but less than that of the HPWMo solution in which the pH was adjusted with NaOH. This indicated that the desulfurization performance of heteropolyacid salt may be better than that of parent heteropolyacid. For NO removal, as shown in Fig. 7(b), although NaPWMo had a good absorption capacity at 25°C, the greatest NO removal efficiency was only 13%, which was lower than that of HPWMo.

The Effect of the NaPWMo Concentration
The removal efficiency of SO 2 and NO can be greatly enhanced by an increase in the liquid-gas ratio. The effect of the NaPWMo concentration on SO 2 and NO removal efficiency was further studied, and the results are shown in Fig. 8. It can be seen in Fig. 8(a) that at a concentration of 6 × 10 -3 M for the NaPWMo, the SO 2 removal efficiency achieved 95% within a reaction time of 60 min. When the concentration was 1.2 × 10 -3 M, the desulfurization efficiency maintained above 90% and gradually reduced as the reaction time exceeded 45 min. Furthermore, worse desulfurization efficiency was observed with decreases in the concentration of NaPWMo. This indicated that at a higher concentration of NaPWMo, SO 2 removal efficiency was improved, and the reaction time was longer. As shown in Fig. 8(b), similarly, as the concentration of NaPWMo was increased, the NO removal efficiency also increased slightly. When the concentration of NaPWMo was 6 × 10 -3 M, the removal efficiency was as high as 16% after a reaction time of 60 min.

The Effect of Gas Concentration
In order to better understand the ability of NaPWMo to remove SO 2 and NO in complex applications, the removal performance under different gas concentrations was studied. Fig. 9(a) shows the relationship between the desulfurization efficiency and the SO 2 and NO gas ratio. When the SO 2 concentration in the reaction gas was maintained at 630 ppm, as the NO concentration   was increased from 400 ppm to 900 ppm, the SO 2 removal efficiency barely changed, indicating that the NO concentration may have little effect on the desulfurization performance of NaPWMo. When the concentration of NO remained unchanged, the absorption performance decreased significantly with increases in the concentration of SO 2 , which may have been because the high concentration of SO 2 was absorbed more easily, causing the absorption solution to quickly reach saturation and resulting in a decrease in the removal efficiency. For NO removal, as shown in Fig. 9(b), as the NO concentration increased, the NO removal efficiency also showed a downward trend when the SO 2 concentration was maintained at 630 ppm. This means that for NaPWMo, the simultaneous removal of NO and SO 2 was only limited by its own gas concentration, and they did not affect each other.

The Effect of O 2
In practical applications, some oxygen may remain in flue gas because excess air has to be introduced to ensure full combustion of the fuel. An experiment was carried out to investigate the effect of O 2 in the gas flow on desulfurization and denitrification of NaPWMo, for which the results are shown in Fig. 10. In Fig. 10(a), in the presence of 4% O 2 , SO 2 removal efficiency was higher than 95% and was maintained for 45 min. However, in the absence of O 2 , as the reaction time increased, SO 2 removal efficiency gradually decreased, and the overall removal efficiency was lower than that in the presence of O 2 . This implies that O 2 is an important factor affecting the removal of SO 2 by HPC solution. Based on previous reports (Rubio and Izquierdo, 1998;Raymundo-Piñero et al., 2000;Xu et al., 2016), when O 2 is present, SO 2 easily reacts with O 2 to form SO 3 under the action of a catalyst. SO 3 and H 2 O further react to form H 2 SO 4 , which leads to an increase in SO 2 absorption. A similar trend also can be observed in Fig. 10(b), where the NO removal efficiency clearly increased in the presence of O 2 . This may have been due to the fact that NO with low solubility was oxidized to NO 2 and then dissolved into the HPC solution . According to the above experimental results, the optimal experimental conditions for simultaneous desulfurization and denitration of HPC can be summarized as follows: 400 ppm SO 2 , 600 ppm NO, 4% O 2 , T = 25°C, C HPC = 6 × 10 -3 M.
According to the experimental results, the reaction mechanism of simultaneous desulfurization and denitration on HPC could be deduced. First, SO 2 is absorbed by water to form H 2 SO 3 , which can further be oxidized by the HPC to H 2 SO 4 . Then, the reduced HPC is used to reduce NO in the presence of O 2 . The reaction equation can be written as follows (taking sodium phosphomolybdate as an example): In conclusion, HPC can remove SO 2 and NO and at the same time act as a catalytic intermediate following Eq. (6). However, in the reaction process, due to the insufficient reaction described by Eq. (5), the HPC is consistently consumed.
It is worth paying attention to the stability of HPC when attempting simultaneous desulfurization and denitration. Repeated experiments should be conducted to verify the stability and reusable ability of HPC in future work, and the accurate reaction mechanism should further be studied.

CONCLUSION
Heteropoly compounds were employed to simultaneously remove NO and SO 2 from flue gas.
This system had a high SO 2 removal efficiency accompanied by modest NO removal. Heteropolyacids containing P and Mo elements exhibited good removal performance. The amount of doped W and the pH of the solution affected the removal performance, where a stable SO 2 removal efficiency greater than 95% was obtained on HPW 1 Mo at a pH value of 5.5. The desulfurization performance of NaPWMo was higher than that of the HPWMo solution. High concentration of the NaPWMo solution and the presence of O 2 were beneficial to desulfurization at room temperature. The removal efficiency of the SO 2 or NO in NaPWMo was only affected by its own gas concentration rather than their mutual influence on each other.