Promoting Effect of H⁺ and Other Factors on NO Removal by Using Acidic NaClO₂ Solution

Abstract

In this study, NaClO₂ was selected as a denitration oxidant. In order to clarify the mechanism of NaClO₂ as an oxidation agent for NO removal efficiency, the effects of H⁺ and other factors (NaClO₂ concentration, temperature, and the other gas) on the NO removal efficiency were investigated. NaClO₂ showed a promotional ability on NO removal, whose efficiency increased with the increase of NaClO₂ concentration. One hundred percent removal efficiency of NO could be achieved when the NaClO₂ concentration was 0.014 mol/L. Furthermore, raising the reaction temperature benefited the removal of NO. The lower the pH, the better the NO removal efficiency. The promoting effect of H⁺ on the NO removal was studied by the Nernst equation, ionic polarization, and the generation of ClO₂. Under the optimal conditions, the best removal efficiency of NO was 100%. Based on the experimental results, the reaction mechanism was finally speculated.

1. Introduction

The use of solid waste, coal, and fuel oil has produced a large number of harmful gases such as NO_x, resulting in a series of serious environmental pollution problems [1,2,3]. In the past few decades, various technologies have been used to reduce NO emissions, which can be grossly divided into two categories: The combustion process optimization, which can only achieve a 50–70% reduction, far from meeting gas emission standards [4], and the other are post-combustion control methods. Among the post-combustion control methods, SCR (selective catalytic reduction) is a developed technology that can remove NO_x with a high efficiency, and has been widely applied to industry [2,5,6,7], but there are still some shortcomings such as expensive investment, high operation costs, easy sulfur poisoning, and easy NH₃ slip [8,9]. In the thermal power plant industry, the flue gas composition is relatively stable, so the SCR units are usually placed before desulfurization units, which can lead to easy sulfur poisoning. However, due to the violent fluctuations of the flue gas composition and content, SCR units are arranged after desulfurization units in cement plants, glass plants, and some steels plants, which make the gas temperature lower than the temperature window. To some extent, these factors limit the application scope of SCR. Another post-combustion control method is selective non-catalytic reduction (SNCR). SNCR units are installed at the flue gas outlet of the furnace, whose denitration process requires no catalysts, but need higher temperatures of about 800–1000 °C and four times as much ammonia as SCR, so its removal efficiency can only reach 60–80% using excess ammonia. When the temperature is higher than the temperature window, the oxidation reaction of NH₃ begins to play a dominant role, resulting in the generation of NO [10]. For power plants, the temperature varies with changeable coal quality and the frequent load variation of units, resulting in the decrease of NO removal efficiency. In addition, the efficiency of SNCR technology applied to a large boiler is relatively low. Therefore, SNCR can only play an auxiliary role in denitration.

From July 2014, Chinese thermal power plants executed ultra-low emission standards of NO_x, with concentrations not exceeding exceed 50 mg/m³. By the end of 2018, most power plants had met the ultra-low emission standards by installing a selective catalytic reduction (SCR) system [11]. However, some steels plants and most cement plants still failed to meet the standards due to various restrictions such as high operational costs and a variety of flue gas conditions. Hence, advanced technology, which is applicable for low-load conditions, should be developed for the deep removal of NO_x.

In recent years, more and more researchers have focused on developing wet scrubbing technology due to its low cost and simple process. As is widely-known, NO gas, which accounts for the vast majority of NO_x in exhaust gas, is extremely insoluble in water [12]. Therefore, the choke point to wet scrubbing denitration is to increase the solubility of NO. The main methods include oxidation absorption, chelating absorption, reduction absorption, and alkali absorption. Some researchers have reported that Fe²⁺-EDTA (ethylenediaminetetraacetic acid) showed a fast absorption rate and high absorption capacity, which has received much attention for its potential advantages [13,14,15]. However, traditional coal-fired flue gas contains a certain amount of oxygen, which will oxidize Fe²⁺-EDTA to Fe³⁺-EDTA, which is not capable of binding with NO. Another way is to add some additives to first oxidize the relatively water insoluble NO to fairly soluble NO₂. Numerous oxidant agents such as KMnO₄, ozone, H₂O₂, NaClO, S₂O₈²⁺, and NaClO₂ have been studied [16,17,18,19]. Compared with other oxidants, NaClO₂ has proven to be one of the most promising oxidants [20]. Many articles regarding the removal of NO by NaClO₂ have been published. Yang et al. [21] introduced ultraviolet irradiated sodium chlorite to remove NO_x. NaClO₂ with UV treatment could generate ClO₂, and the results showed that the NO_x removal efficiency increased from 28 to 77% as the UV irradiation time increased from 0 to 600 s. Lee et al. [22] investigated the simultaneous removal of NO and SO₂ in a wetted wall column where the results showed that the solution pH did not affect the DeSO_x and DeNO_x efficiency, and the excess of NaClO₂ did not enhance NO_x removal efficiency. The maximum SO_x and NO_x removal efficiencies achieved at the typical operating conditions of commercialized FGD (flue gas desulfurization) processes are about 100 and 67%, respectively. Chien et al. [23] used NaClO₂/NaOH solution as the additive/absorbent to determine the extent of NO_x removal in a wet scrubbing system. Results showed that the major parameters affecting NO_x removal efficiencies were the L/G ratio and the dosage of the additive. The maximum NO_x removal efficiency was in the range of 36–72%. Although a lot of work has been done, more research is essential to further develop the practicability of the process of NO removal with NaClO₂.

To our best knowledge, almost all research on NO removal using NaClO₂ has studied the effect of pH, whose influence is of vital importance. However, few studies have reported on the mechanism of the effect of H⁺ on NO removal. Therefore, this paper will focus on the effect of H⁺ on NO removal.

The effect of H⁺ on NO removal was deeply investigated from three points: The Nernst equation, ionic polarization, and the formation of ClO₂, which possesses a stronger oxidation capacity. The effect of other parameters (NaClO₂ concentration, temperature, and coexist gas) were also investigated in a lab scale bubbling reactor. The oxidation—reduction potential (ORP) of the solutions were measured at different temperatures and pH values. The reaction pathways and the mechanism of NO removal by NaClO₂ were proposed. Although, the pH value of the solution should not be too low in order to prevent equipment corrosion in industrial applications, the theoretical study on NO removal in acidic conditions is still very necessary.

2. Materials and Methods

2.1. Materials

NaClO₂ (99%) as an oxidant was obtained from the Aladdin Industrial Corporation; HCl (Beijing Chemical Works, Beijing, China) was used to adjust the pH value of the solutions; the standard gases of NO (1% NO with the N₂ as the balance gas), SO₂ (1% SO₂ with the N₂ as the balance gas), O₂ (purity > 99.999%) and N₂ (purity > 99.999%) were provided by Beijing ZG Special Gases Science & Technology Co. Ltd., China. All chemicals were analytical grade reagents.

2.2. Apparatus

Figure 1 shows the schematic diagram of the experimental apparatus for NO removal by wet scrubbing, which consists of a simulated flue gas supply system, oxidation bubble-column reactor, heating unit, pH meter, flue gas analyzer, and tail gas absorption device. Four kinds of gases (SO₂, NO, O₂, and N₂) were used to prepare the simulated flue gas in a gas mixing tank using mass flow controllers (D07-7, Beijing Sevenstar Electronics Co. Ltd., Beijing, China). The removal processes were carried out in a flask with three necks, of which the volume was 500 mL. The deionized water was poured into the reactor through the feeding hole. For each run of this experiment, when the deionized water reached the reaction temperature based on the experimental need (20 °C, 30 °C, 40 °C, 50 °C, 60 °C, 70 °C, 80 °C, or 90 °C), the oxidant was poured into the bubble column reactor and HCl was injected into the reactor to adjust to the desired pH value. The total volume of solution was constant for each experiment at 400 mL. When the concentrations of mixed gas and solution temperature reached the required values and were stable, flue gas was bubbled into the liquid. Then, the concentrations of outlet gas were continuously measured by a flue gas analyzer (Testo 350, Testo AG Company, Freiburg, Germany). Readings from the flue gas analyzer were recorded at an interval of 1 s. During the whole experimental process, the inlet velocity of the simulated gas flow was 1.5 L/min and the NO inlet concentration was 500 ppm.

The temperature of the solution was controlled by a thermostatic water bath (DF-101S, Jinyi Instrument Technology Co. Ltd., Zhengzhou, China), and the pH value was detected by a pH meter (starter 3100, Ohaus Instrument Technology Co. Ltd., Shanghai, China). The oxidation—reduction potential value of the solution was measured at different temperatures and pH values through a portable conductivity detector (multi 3420, WTW Company. Munich, Germany). A TU-1900 Dual-beam UV-visible spectrophotometer (Beijing Purkinje General Instrument Company, Beijing, China) was used to detect the concentration of ClO₂ in the liquid phase. Each experiment lasted at least 60 min to make the data stable. All experiments were repeated 2–3 times and the average values of these data were used for further calculation.

The removal efficiency of NO were calculated with the following formula [24,25]:

$$\mathsf{\eta }=\frac{{\mathrm{C}}_{\mathrm{in}}-{\mathrm{C}}_{\mathrm{out}}}{{\mathrm{C}}_{\mathrm{in}}}\times 100\%$$

(1)

where η is the removal efficiency with a unit of % and C_in and C_out are the inlet and outlet concentrations (ppm), respectively.

3. Results

3.1. Effect of NaClO₂ Concentration on the NO Removal Efficiency

First, the effect of the NaClO₂ concentration on the NO removal was investigated. As shown in Figure 2, when the NaClO₂ concentration increased from 0.002 to 0.014 mol/L, the NO removal efficiency increased from 41 to 100%. As the NaClO₂ increased, more NO oxidized [26], which resulted in an increase in the NO removal efficiency. In addition, a high concentration of NaClO₂ enhanced the gas–liquid mass transfer, which can be expressed as the following formula [27]:

$${\mathrm{N}}_{\mathrm{NO}}={\mathrm{K}}_{\mathrm{L},\mathrm{NO}}\left({\mathrm{c}}_{\mathrm{NOi}}-{\mathrm{c}}_{\mathrm{NO}}\right)$$

(2)

where N_NO is the absorption rate of NO (mol/m²/s); k_L,NO is the liquid-phase mass transfer coefficients of NO (m/s); and c_NO and c_NOi are the NO concentration in the liquid phase (mol/m³) and the NO concentration in the phase interface (mol/m³), respectively. An increase in the NaClO₂ concentration could reduce the NO concentration in the liquid phase, which could enhance the mass-transfer driving force of NO, thereby increasing the NO removal efficiency.

3.2. Effect of Temperature on the NO Removal Efficiency

The influences of temperature on chemical reaction and gas–liquid mass transfer are very significant, thus a series of experiments with different temperatures were carried out, and the results are shown in Figure 3. Obviously, the reaction temperature had a significant effect on the removal of NO. As the temperature increased from 20 to 80 °C, the NO removal efficiency increased from approximately 50 to 100%. When the temperature was above 80 °C, the NO removal efficiency could achieve 100%. Hence, the temperature between 30 and 80 °C was favorable for the removal of NO. The reasons for the above phenomenon can be summarized from three aspects: (1) the high temperature could accelerate the oxidation reaction, which oxidized more NO in the same gas residence time; (2) the higher temperature increased the redox potential of sodium chlorite as shown in Figure 3; and (3) a high temperature was favorable for producing ClO₂ gas through Equations (3) and (4), which has a stronger oxidation ability than ClO₂⁻ [28]. Although higher temperatures would reduce the solubility of NO in solution to some degree, the increment of the reaction rate and more ClO₂ generation compensated for the decrease in NO solubility. Thereafter, relatively high temperatures promoted the removal of NO.

$${\mathrm{ClO}}_2^{-}+{\mathrm{H}}^{+}\to {\mathrm{HClO}}_2.$$

(3)

$$8{\mathrm{HClO}}_2\to 6{\mathrm{ClO}}_2+{\mathrm{Cl}}_2+4{\mathrm{H}}_2\mathrm{O}$$

(4)

Furthermore, it could also be found that when temperature further increased from 80 to 90 °C, the NO removal efficiency descended from 100 to 98.9%, but the ORP value increased from 715 to 726 mv. This is mainly because the high temperature enhanced the oxidation ability of the solution, but the solubility of NO in solution declined sharply at excessive high temperatures [29], resulting in less NO reacting with NaClO₂ in solution. Thus, the NO oxidation reaction rate decreased, which can be expressed as Equation (5). The increase in the oxidizing ability of the solution was not enough to compensate for the decrease in the NO solubility; Another reason is that the higher temperature led to a decrease in the yield of ClO₂ and NaClO₂ decomposed into NaClO₃ (less oxidative) instead of ClO₂ (more oxidative), expressed as Equation (6) [30]. Therefore, temperatures over 80 °C should be avoided.

$$\mathrm{r}=\mathrm{k}\left[{\mathrm{NaClO}}_2{]}^{\mathrm{a}}\right[\mathrm{NO}{]}^{\mathrm{b}}$$

(5)

where r is the rate of reaction (mol/L/s); k is the reaction rate coefficient; [NaClO₂] and [NO] are the concentrations of NaClO₂ and NO, respectively; and a and b are the reaction order of NaClO₂ and NO, respectively.

$$4{\mathrm{NaClO}}_2\to 2\mathrm{NaCl}+2{\mathrm{NaClO}}_3+{\mathrm{O}}_2$$

(6)

3.3. Effect of Initial pH on the NO Removal Efficiency

Considering that the pH of the solution has a significant influence on NO removal efficiency [31], the effects of the initial pH on NO removal were investigated. Before studying the effect of pH on NO removal, research on the relationship between pH and OPR should be done first. As shown in Figure 4, as the pH value decreased from 8.6 to 4.0 and 1.5, the ORP increased from 357 mV to 540 mV and 673 mV. The results showed that ORP was apparently influenced by the pH value. The stronger the acidity of the solution, the higher the OPR, and the stronger the oxidizing ability. As a whole, the ORP of NaClO₂ had a negative correlation with the pH value.

The NO removal efficiency results are shown in Figure 5. The NO removal efficiency increased with the decrease of pH. A 100% removal efficiency of NO could be achieved when the pH was below 2.5. We can also see from Figure 6 that when the initial pH was 6, 7.5, and 9, the outlet NO concentration was unstable, which decreased from 248, 270, and 260 to 170, 200, and 175 ppm in 50 min, respectively. This is because the ORP would increase in the process of oxidation reaction (shown in Figure 7), resulting in the increase of oxidizing ability, which is beneficial to NO removal.

As shown in Figure 7, the ORP value increased from 525 to 618, and the pH decreased from 8.67 to 3.65 in 60 min in the process of oxidation reaction. The reason for this phenomenon is that the removal process of NO generated a lot of H⁺ in solution (Equations (9)–(11)). Thus, the reason for the influence of H⁺ on the removal performance deserves further investigation. This can be summarized as follows:

The oxidizing ability of ClO₂⁻ is affected by the H⁺ concentration. First, it can be explained at the molecular level. As shown in Figure 8, the electron cloud between the oxygen and chlorine atoms binds them closely together. However, when H⁺ accumulates around the ClO₂⁻, induced polarization will more or less lead to the deformation of the electron cloud and deviate from the original distribution under the action of the H⁺ electric field. With the increase in the ionic polarization degree between H⁺ and ClO₂⁻, the covalent bond between the oxygen atom and central atom chlorine gradually transits to the ionic bond, resulting in the covalent bond being easily broken, and then the released oxygen atom from ClO₂⁻ easily bonds with the nitrogen atom to form NO₂. Thus, ClO₂⁻ could be provided with a higher oxidation performance with the help of H⁺.

The above phenomenon can also be explained by the Nernst equation. Equation (7) is the half-cell reaction of ClO₂⁻. According to Equation (7), the Nernst equation can be expressed by Equation (8).

$${\mathrm{ClO}}_2^{-}\to 4{\mathrm{H}}^{+}+4{\mathrm{e}}^{-}\to 2{\mathrm{H}}_2\mathrm{O}+{\mathrm{Cl}}^{-}$$

(7)

$$\mathrm{E}={\mathrm{E}}^{\mathsf{\theta }}+\frac{\mathrm{RT}}{\mathrm{zF}}\ln \frac{\left[{\mathrm{H}}^{+}{]}^4\right[{\mathrm{ClO}}_2^{-}]}{[{\mathrm{Cl}}^{-}]}$$

(8)

where E is the electrode potential; T is the Kelvin thermodynamic temperature; E^θ is the standard electrode potential; R is the ideal gas constant; z is the number of half-reaction transfer electrons; F is the Faraday constant; and [H⁺], [ClO₂⁻], and [Cl⁻] are the concentrations of H⁺, ClO₂⁻, and Cl⁻, respectively.

According to Equation (8), the higher the concentration of H⁺, the higher the potential of the electrode (E), meaning a higher oxidizing ability. That is to say, the solution with a lower H⁺ possesses stronger electric potential (as shown in Figure 4). Thus, it is easy to obtain a high NO removal efficiency in acidic conditions.

In addition, the dominant active species ClO₂ plays an important role in NO removal, therefore, to further understand the mechanisms of NO removal, more experiments were performed to measure the concentration of ClO₂ in solution by a UV–Vis spectrophotometer (shown in Figure 9). The concentration of ClO₂ was determined at a wavelength of 360 nm [21,32], and the height of the peak was directly proportional to the concentration of ClO₂. The peak of 260 nm was assigned to ClO₂⁻ [28]. As we can see from Figure 9, the release of ClO₂ from NaClO₂ was affected by pH; the yield of ClO₂ at pH = 1 was far higher than those at a pH of above 2. The concentration of ClO₂ increased with the decrease of pH. The concentration of ClO₂ agreed well with the NO removal efficiency shown in Figure 5. Thus, the ClO₂ was suggested to make an outstanding contribution to the NO removal.

3.4. Effect of Coexistence Gases on the NO Removal Efficiency

In actual flue gas, the composition as well as the concentration varies with different operating conditions. Thus, the effects of different concentrations of SO₂ and O₂ on NO removal were investigated. The effect of SO₂ on NO removal is shown in Figure 10. It is interesting that different concentrations of SO₂ had a similar effect on NO removal; when the SO₂ concentration was below 400 ppm, the increase in SO₂ promoted NO removal. Interestingly, when the concentration increased to 600 ppm, the NO removal efficiency decreased a little. A small amount of SO₂ enhances the gas–liquid mass transfer rate, which is beneficial to the increment of NO removal efficiency, but excess SO₂ and NO, despite improving the mass transfer efficiency, would react with NaClO₂ in a competitive manner, resulting in a decrease in NO removal efficiency [33].

Figure 10 shows the effect of O₂ on the NO removal. It can be found that 5% O₂ contributes to the removal of NO, and the excess O₂ will not further increase the NO removal efficiency (Equation (9)).

$$4\mathrm{NO}+3{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}\to 4{\mathrm{HNO}}_3$$

(9)

4. Reaction Mechanism

According to the experimental phenomenon, the reaction mechanism of the removal of NO by NaClO₂ can be speculated. The removal process can be sorted into two parts: (1) the oxidation of NO, and (2) the absorption removal of NO. For the oxidation process, ClO₂⁻ and ClO₂ play a major role in the oxidation of NO [34]. The oxidizing ability of the solution was enhanced in an acidic condition (Figure 4), and the yield of ClO₂ increased by H⁺ (Equations (3) and (4)). As mentioned in Section 3.3, the contrapolarization of H⁺ weakened the covalent bond between the oxygen atom and central atom chlorine, resulting in the covalent bond being easily broken, thus, the oxidation performance was increased.

The majority of NO is first converted to NO₂ by ClO₂⁻ in the gas–liquid interface phase (Equation (10)), and then oxidized to NO₃⁻ in the liquid phase (Equation (11)). A small fraction of NO is directly converted to NO₃⁻ in the liquid phase (Equation (12)). The rest of NO was oxidized to NO₂ and by ClO₂ in the liquid phase (Equation (13)).

$$2\mathrm{NO}+{\mathrm{ClO}}_2^{-}\to 2{\mathrm{NO}}_2+{\mathrm{Cl}}^{-}$$

(10)

$$4{\mathrm{NO}}_2+{\mathrm{ClO}}_2^{-}+2{\mathrm{H}}_2\mathrm{O}\to 4{\mathrm{NO}}_3^{-}+4{\mathrm{H}}^{-}+{\mathrm{Cl}}^{-}$$

(11)

$$4\mathrm{NO}+3{\mathrm{ClO}}_2^{-}+2{\mathrm{H}}_2\mathrm{O}\to 4{\mathrm{NO}}_3^{-}+4{\mathrm{H}}^{+}+3{\mathrm{Cl}}^{-}$$

(12)

$$4\mathrm{NO}+2{\mathrm{ClO}}_2\to 4{\mathrm{NO}}_2+{\mathrm{Cl}}_2$$

(13)

During the course of absorption, the absorption paths of NO are mainly reactions (Equations (14)–(18)). NO₂ can directly be absorbed by H₂O (Equations (14) and (15)). A tiny bit of NO directly oxidized and absorbed by ClO₂ and H₂O also takes place (Equations (16) and (17)). Furthermore, NO and NO₂ can be directly absorbed by H₂O without ClO₂ or ClO₂⁻ (Equation (18)).

$$2{\mathrm{NO}}_2+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{HNO}}_3+{\mathrm{HNO}}_2$$

(14)

$$6{\mathrm{NO}}_2+2{\mathrm{H}}_2\mathrm{O}\to 4{\mathrm{HNO}}_3+2\mathrm{NO}$$

(15)

$$5\mathrm{NO}+2{\mathrm{ClO}}_2+{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{HCl}+5{\mathrm{NO}}_2$$

(16)

$$5{\mathrm{NO}}_2+{\mathrm{ClO}}_2+3{\mathrm{H}}_2\mathrm{O}\to 5{\mathrm{HNO}}_3+\mathrm{HCl}$$

(17)

$$\mathrm{NO}+{\mathrm{NO}}_2+{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{NO}}_2^{-}+2{\mathrm{H}}^{+}$$

(18)

5. Conclusions

In this study, a bubble method device was established for the removal of NO by NaClO₂. The effects of various parameters including NaClO₂ concentration, reaction temperature, initial pH value, and other gas on NO removal were investigated. The concentration of NaClO₂ had a significant impact on NO removal. The removal efficiency obviously increased with the increase in the NaClO₂ concentration in the solution from 0.002 to 0.014 mol/L. When the concentration of NaClO₂ was 0.014 mol/L, NO removal efficiency could reach 100% at 25 °C. The removal efficiency increased to 100% with an increase in temperature up to 80 °C. When the temperature exceeded 80 °C, the removal efficiency decreased slightly. The optimal NO conversion temperature was determined as 80 °C. The pH had a great influence on the removal efficiency. In the range of pH 1.5–8.0, the NO removal efficiency dramatically increased with the decrease in pH. When the pH was lower than 2.5, a removal efficiency of 100% could be achieved in a 0.01 mol/L NaClO₂ solution. The ClO₂ generated from NaClO₂ and the contrapolarization of H⁺ in an acidic environment may be the main factor for the improvement of NO removal. Furthermore, the existing gases of O₂ and SO₂ were conducive to NO removal.