A Novel Method Based on Hydrodynamic Cavitation for Improving Nitric Oxide Removal Performance of NaClO₂

Abstract

In the removal of nitric oxide (NO) by sodium chlorite (NaClO₂), the NaClO₂ concentration is usually increased, and an alkaline absorbent is added to improve the NO removal efficiency. However, this increases the cost of denitrification. This study is the first to use hydrodynamic cavitation (HC) combined with NaClO₂ for wet denitrification. Under optimal experimental conditions, when 3.0 L of NaClO₂ with a concentration of 1.00 mmol/L was used to treat NO (concentration: 1000 ppmv and flow rate: 1.0 L/min), 100% of nitrogen oxides (NO_x) could be removed in 8.22 min. Furthermore, the NO removal efficiency remained at 100% over the next 6.92 min. Furthermore, the formation of ClO₂ by NaClO₂ is affected by pH. The initial NO_x removal efficiency was 84.8–54.8% for initial pH = 4.00–7.00. The initial NO_x removal efficiency increases as the initial pH decreases. When the initial pH was 3.50, the initial NO_x removal efficiency reached 100% under the synergistic effect of HC. Therefore, this method enhances the oxidation capacity of NaClO₂ through HC, realizes high-efficiency denitrification with low NaClO₂ concentration (1.00 mmol/L), and has better practicability for the treatment of NO_x from ships.

1. Introduction

Over 80% of global trade transport is through ships [1]. Furthermore, it is estimated that the average annual growth rate of international maritime transport trade will be 3.5% from 2019 to 2024 [2]. However, there are increasing concerns about the environmental problems caused by the ships’ exhaust. The ships’ exhaust mainly contains particulate matter (PM), nitrogen oxides (NO_x), carbon dioxide (CO₂), sulfur oxides (SO_x), and other substances hazardous to human health and the environment [3,4,5,6]. Additionally, NO_x is the most difficult to remove [7,8]. Selective catalytic reduction (SCR) and exhaust gas recirculation (EGR) can remove NO from ship exhaust. SCR uses catalysts and ammonia to reduce NO_x emissions. However, the removal of NO by SCR is greatly affected by temperature [9], and SO_x and water in the exhaust gas from ships can cause catalyst poisoning [2]. EGR can reduce NO production at the source. However, increasing the EGR rate will lead to incomplete combustion, causing an increase in the PM [10] and reducing fuel economy [11]. SCR and EGR can only deal with NO, not SO₂ and PM simultaneously. Wet scrubbing technology has the advantage of treating multiple pollutants simultaneously and has attracted extensive attention from scholars [12]. The NO_x emissions from ships consist of 90–95% NO. Moreover, NO is difficult to dissolve in water [13]. In wet removal of NO, it is oxidized with oxidants, and then NO_x removal is promoted with absorbents [14]. High-potential oxidants include H₂O₂ [15], Fenton-like reagents [16,17], persulfate salts [18,19], NaClO₂ [20], and KMnO₄ [21]. The absorbents include sodium humate (HA-Na) [22,23], NaSO₃ [24], and Ca(OH)₂ [25]. Hao et al. [26] compared the performance of different advanced oxidation processes (AOPs) such as UV/H₂O₂, UV/NaClO, UV/Na₂S₂O₈, and UV/NaClO₂ in terms of NO removal. UV/NaClO₂ has the best NO conversion performance among these AOPs. Additionally, secondary pollution to the ocean can be avoided when NaClO₂ is used as the oxidant, since it can be converted into sodium and chloride ions [27]. Scholars have studied the removal of NO by NaClO₂ using different reactors. Deshwal et al. [28] used a bubble reactor to remove NO. When the concentration of NaClO₂ was 200.00 mmol/L, the NO_x removal efficiency reached 81%. However, the bubble reactor has problems such as increasing the exhaust back pressure of the diesel engine and poor gas–liquid mass transfer. Han et al. [29] used a cyclic scrubbing system containing a fine-droplet spray nozzle to remove NO. When the concentration of NaClO₂ is 10.00 mmol/L, the NO_x removal efficiency could reach 87.7%. When dealing with actual ship flue gas, particulates may tend to clog the nozzles, and their operation requires high energy consumption. Hao et al. [30] used an vaporization device to evaporate the composite oxidant containing NaClO₂ to pre-oxidize NO and then absorb NO_x through an absorption device containing a HA-Na solution. When the concentration of NaClO₂ is 80.00 mmol, the NO_x removal efficiency can reach 92.0%. This two-stage system can effectively remove NO_x, but the complex device and the use of various drugs increases the cost. The HC reactor has a higher gas–liquid mass transfer coefficient [31], and the special reaction conditions generated by cavitation can also promote the removal of NO_x. In addition, the HC reactor system is simple and flexible. It can be arranged in groups, making efficient use of the limited space on the ship while simultaneously desulfurizing, denitrifying, and removing PM without clogging. In this study, we combined HC with 1.00 mmol/L NaClO₂ to remove NO, thereby increasing the NO_x removal efficiency and achieving effective denitrification in a single reactor.

HC is widely used in sewage treatment and pretreatment of wood cellulose [32,33], but there are few studies on the use of HC for the denitrification of ship exhaust gas. Song et al. [34] applied the HC reactor in combination with chlorine dioxide (ClO₂) to remove NO. When the ClO₂ solution concentration was 1.00 mg/L, NO_x removal efficiency exceeded 90% for 100 s, and the outlet NO₂ concentration was very low (17 ppmv). Yang et al. [35] studied the effects of pressure difference (ΔP) in the HC reactor, solution temperature, NO concentration, gas flow rate, solution pH, and ClO₂ concentration on denitrification, determined the optimal denitrification conditions, and discussed the reaction pathway. Then, Yang et al. [36] used HC and ClO₂ to conduct non-circulation desulfurization and denitrification research. When ClO₂ with a concentration of 600.00 mg/L was added at a rate of 6.67 mL/min, the removal rate of SO₂ was close to 100%, and the removal rate of NO_x was 95.0%. NO₂ is produced during NO removal by wet oxidation. However, the concentration of NO₂ generated when HC removes NO is low. Therefore, Song et al. [37] carried out research on the removal of NO₂ by H₂O₂, NaS₂O₈, NaClO₂, and ClO₂ under HC conditions, clarified the enhanced removal mechanism of HC on NO₂, and found that NaClO₂ with too high of a concentration (10.00 mmol/L) was not conducive to the removal of NO₂. Hydrodynamic cavitation combined with ClO₂ for NO removal poses the issue of ClO₂ evaporation. This decreases the drug’s utilization rate and is not conducive to long-term NO removal. By adjusting the solution’s pH and other conditions, it is possible to regulate the rate at which the proper concentration NaClO₂ produces ClO₂. This can reduce the escape of ClO₂ and prolong the time for the solution to remove NO. In addition, the cost of NaClO₂ is lower than that of ClO₂, so this paper studied hydrodynamic cavitation combined with NaClO₂ for denitrification.

This study investigated the effects of differential pressure ∆P, initial pH, reaction temperature, and NaClO₂ concentration on NO removal under HC. Under optimal experimental conditions, using a 1.00 mmol/L NaClO₂ solution with 3.0 L volume to remove NO (concentration: 1000 ppmv, flow rate: 1.0 L/min), NO_x could be completely removed in 8.22 min. Meanwhile, the duration of 100% NO removal was as long as 15.10 min. Furthermore, this method significantly improved the NO_x removal efficiency and reduced the NaClO₂ concentration compared to previously reported studies [20,22,38,39]. This method provides a novel possibility for the future treatment of ship exhaust gases.

2. Experimental

2.1. Reagents and Materials

The HC reactor was acquired from Mazzei Injector Company in Bakersfield, CA, USA; the Model 287 Venturi was used in the experiments. The cavitation chamber is constructed from glass-filled polypropylene. Structure and dimensions are depicted in detail in Figure S1 of the Supplementary Materials. This experiment also utilized a flue gas analyzer, a high-speed camera, a dryer, and a water purification system. The equipment used in the experiment is shown in

Table 1. The experiment instruments.

Equipment	Equipment Type	Manufacturer
Flue gas analyzer	Gasboard-3000UV	Cubic-Ruiyi Co., Ltd., Wuhan, China
High-speed camera	FASTCAM Mini UX50	Photron, San Diego, CA, USA
pH meter	S210	Mettler-Toledo Instruments Co., Ltd., Columbus, OH, USA
Dryer	XX100A-03	Suzhou Xiaoxiong Electric Co., Ltd., Suzhou, China
HC reactor	Model 287	Mazzei Injector Company, LLC, Bakersfield, CA, USA
Milli-Q Plus water purification system	Master-Q15	Millipore, Burlington, MA, USA

.

The reagents used in the experiment are shown in

Table 2. Experimental reagents.

Reagent	Purity (Concentration)	Manufacturer
NaClO2	AR	Sinopharm Chemical Reagent Co., Shanghai, China
HCl	36–38%	Sinopharm Chemical Reagent Co.
N2	99.999%	Dalian Special Gases Co., Ltd., Baotou, China
NO/N2	1000 ppmv	Dalian Special Gases Co., Ltd.

.

2.2. Experimental

As shown in Figure 1, the experimental setup was made of simulated gas, the HC reactor, the NaClO₂ solution, and the flue gas analyzer. The numbers 1 and 2 represent gas cylinders, and the numbers 3–8 represent valves. Different colors are used to depict the solution or gas in different states. The black, blue, red, green, and pink lines represent the simulated gas, NaClO₂ solution, gas–liquid mixture solution, treated exhaust, and reacted solution, respectively. Mass flow controllers regulated the flow rate of the simulated gas. The temperature of the NaClO₂ solution (3.0 L) was controlled by the thermostat bath. The differential pressure ∆P was regulated by valves 4 and 6.

In this study, the NaClO₂ solution was drawn from the thermostat bath through the pump. When the NaClO₂ solution flowed through the HC reactor at high speed, a low-pressure suction was created at the throat of the HC reactor, drawing the NO mixture. The gas–liquid mixture solution was separated by a gas–liquid separator. After being dried, the treated gas entered the flue gas analyzer for measurement. Simultaneously, the reacted solution flowed back into the thermostat bath through valve 7.

2.3. Nomenclature and Calculation of Removal Efficiency

The nomenclature and notation used in this study are shown in

Table 3. Nomenclature and Notation.

HC	Hydrodynamic cavitation	TηNOx,100%	The time of ηNOx = 100% (min)
∆P	Differential pressure (bar)	ηNOx	The removal efficiency of NOx (%)
ηNO	The removal efficiency of NO (%)	η●	The removal efficiency of NOx when NO is initially detected by the flue gas analyzer (%)
ηNOx initial	The initial removal efficiency of NOx with an initial pH of 4 − 7 (%)	ηNOx max	The maximum removal efficiency of NOx with an initial pH of 4 − 7 (%)
TηNO,100%	The time of ηNO = 100% (min)	Cv	Cavitation number

.

NO_x concentration in the treated gas is calculated as follows:

$${\mathrm{C}}_{{\mathrm{NO}}_{\mathrm{x}}, \mathrm{out}}{=\mathrm{C}}_{\mathrm{NO}, \mathrm{out}}{+\mathrm{C}}_{{\mathrm{NO}}_2, \mathrm{out}}$$

(1)

The removal efficiencies of NO and NO_x can be calculated by the following equations:

$${\eta }_{\mathrm{NO}}=\frac{{\mathrm{C}}_{\mathrm{in}}{ - \mathrm{C}}_{\mathrm{NO}, \mathrm{out}}}{{\mathrm{C}}_{\mathrm{in}}}$$

(2)

$${\eta }_{{\mathrm{NO}}_{\mathrm{x}}}=\frac{{\mathrm{C}}_{\mathrm{in}} {- \mathrm{C}}_{{\mathrm{NO}}_{\mathrm{x}}, \mathrm{out}}}{{\mathrm{C}}_{\mathrm{in}}}$$

(3)

where ${\mathrm{C}}_{\mathrm{in}}$ is the concentration of NO in simulated gas. ${\mathrm{C}}_{\mathrm{NO}, \mathrm{out}}$ and ${\mathrm{C}}_{{\mathrm{NO}}_{\mathrm{x}}, \mathrm{out}}$ are the concentrations of NO and NO_x, respectively.

2.4. Measurement of Gas Concentration and pH

First, high-purity N₂ is used to clean the oxygen (O₂) in the experiment. The experiment started when the O₂ content dropped to 0.00%. Next, the pH meter and flue gas analyzer record the data regularly, with the counting interval uniformly set to 5 s. When valve 5 is opened, the simulated NO gas is introduced into the system, and data recording is initiated.

3. Results and Discussion

3.1. NO Removal Enhanced by HC Mechanism

3.1.1. Effect of HC on NO Removal

Cavitation is the generation, growth, and collapse of cavities when the local pressure in the liquid is lesser than the saturated vapor pressure at the local temperature. As illustrated in Figure 2a, the NaClO₂ solution moving at high speed enters the HC reactor from A and forms a low suction pressure. The NaClO₂ solution with dissolved NO mixture forms local cavities at low pressure. According to Gogate’s research, the pressure at the moment of cavitation is generally lower than the saturated vapor pressure at the corresponding temperature [40]. Under low pressure, the cavitation liquid film tends to evaporate inward, thereby balancing the pressure difference between the interior and exterior of the cavities. As the pressure decreases further, the bubble expands rapidly. The cavity is continuously filled with molecules of gas evaporated from the liquid film. At X₂–X₃ in Figure 2a, when the NaClO₂ solution flows through the throat of the HC reactor, the flow area becomes more significant, and the pressure on the NaClO₂ solution can recover rapidly. As shown in Figure 2b, the volume of cavities decreases continuously under the restoring pressure. Since the compression process of the cavity is extremely short, it can be considered an adiabatic compression process. Rapid compression raises the temperature of the cavities sharply. It forms hot spots with high local temperature and pressure of 5000–15,000 K [41,42,43,44,45] and 100–500 MPa [46,47,48], respectively, ultimately leading the cavity to collapse. As shown in Figure 2c, the collapse of the cavity results in the formation of many tiny bubbles and microjets. The cavitation process promotes chemical reactions through mechanical, thermal, and chemical effects, strengthening NO_x removal.

As shown in Figure 2d, under the cavitation conditions, ^•OH and ^•H radicals are produced by the pyrolysis of water molecules [49] (as given by Equation (4)).

$${\mathrm{H}}_2\mathrm{O} \to {}^{•}\mathrm{OH}+{}^{•}\mathrm{H}$$

(4)

The ^•OH radicals have a strong oxidation capacity with a redox potential of 2.80 eV [50]. Additionally, NO or NO₂ may react with ^•OH either inside or on the surface of the cavities, finally oxidizing to nitric acids (HNO₃) and nitrous (HNO₂) [34] (as given by Equations (5)–(7)).

$$\mathrm{NO}+{}^{•}\mathrm{OH} \to { \mathrm{HNO}}_2$$

(5)

$$\mathrm{NO}+{}^{•}\mathrm{OH} \to { \mathrm{NO}}_2+{}^{•}\mathrm{H}$$

(6)

$${\mathrm{NO}}_2+{}^{•}\mathrm{OH} \to { \mathrm{HNO}}_3$$

(7)

Additionally, ^•H radicals have an extremely strong reduction capacity and may react with NO or NO₂ (as given by Equations (8) and (9)).

$${}^{•}\mathrm{H}+\mathrm{NO} \to {}^{•}\mathrm{N}+{}^{•}\mathrm{OH}$$

(8)

$${\mathrm{NO}}_{2\left(\mathrm{aq}\right)}+{}^{•}\mathrm{H} \to \mathrm{NO}+{}^{•}\mathrm{OH}$$

(9)

In a previous study, the size of bubbles produced at the outlet of the HC and bubbling reactors were compared. It was found that the size of the bubbles in the HC reactor (0.62 mm) was far smaller than those in the bubbling reactor (23.19 mm) [34]. As shown in Figure 2a, the low suction pressure is generated at the throat of the HC reactor. the HC reactor creates low suction pressure in the throat, drawing NO from B. Consequently, the flowing NO was cut by the NaClO₂ solution flowing at high speed and forming many gas-filled bubbles. The gas-filled bubbles are formed at low pressure, and when the HC reactor’s restoring pressure compresses them, their volumes become smaller (0.50–1.50 mm). Furthermore, since the gas-filled bubbles are slowly compressed, they cannot collapse. However, the small space inside them increases the collision between NO and ^•OH or ClO₂, which is conducive to the gas-phase chemical reaction. Additionally, compression of the gas-filled bubbles increases their temperature. Consequently, it increases the impact speed and frequency of NO molecules on the surface of the gas-filled bubbles, enhancing the gas–liquid mass transfer [51,52].

3.1.2. Effect of ∆P on NO Removal

Furthermore, the differential pressure ∆P was adjusted to promote the occurrence of cavitation. The cavitation number C_v decreases when ∆P increases. As C_v decreases, the cavitation intensity increases. Meanwhile, more reactive radicals may be generated by a higher cavitation intensity which is conducive to NO_x removal. Additionally, C_v is defined as follows:

$${\mathrm{C}}_{\mathrm{v}}=\frac{2\left({\mathrm{P}}_3-\mathrm{P}\right)}{{\mathsf{\rho }\mathrm{V}}^2}$$

(10)

where, ${\mathrm{P}}_3$, P, V, and ρ denote the outlet pressure of the HC reactor, the vapor pressure of the liquid at saturation temperatures, the velocity of the liquid at the HC reactor throat, and the liquid density, respectively. Furthermore, ideally, cavitation occurs at C_v ≤ 1. However, since the introduction of the NO mixture in this study causes the solution to contain dissolved gas, cavitation occurs at C_v > 1 [17,53].

A transparent acrylic tube (Length: 300.00 mm, Outside diameter: 30.00 mm) was connected to the HC reactor to observe the gas-filled bubbles at the outlet, as shown in Figure 3a. Then a high-speed camera was used to capture the gas-filled bubbles in the 40.00 mm area of the acrylic tube. As illustrated in Figure 3b, as ∆P increases, the size of the gas-filled bubbles at the outlet decreases, and they are gathered more densely. As shown in Figure 3c, the diameters of the gas-filled bubbles were about 0.62 mm with ∆P = 3.00 bar, while they were 0.53 mm with ∆P = 5.00 bar. The surface area and the volume of gas-filled bubbles with ∆P = 5.00 bar were 4.60 and 0.36 times the amount of their equivalents with 3.00 bar. A higher ∆P promotes mixing gas and liquid to increase the contact area. Therefore, the chemical reaction rate accelerates with increasing ∆P for a certain time. A higher ∆P also increases liquid velocity, reducing the overall chemical reaction time. As shown in Figure 3c, when ∆P increased from 3.00 bar to 6.00 bar, the velocity of the gas-filled bubbles increased by 0.20 m/s, and the contact time between gas and liquid reduced by 0.20 s.

Furthermore, the increase in ∆P leads to an increased rate of chemical reaction and shortened reaction time, and this competitive effect affects the duration of the NO_x removal efficiency, η_NOx. As illustrated in Figure 3d, as ∆P increases, the time of η_NOx = 100%, Tη_NOx,100%, first increases and then decreases. When ∆P was 3.00 bar, Tη_NOx,100% was 3.92 min. Tη_NOx,100% was maximum (8.22 min) and minimum (1.92 min) when ∆P was 5.00 bar and 6.00 bar, respectively. Therefore, the competitive effect was balanced when ∆P was 5.00 bar.

Furthermore, when η_NOx is in the range of η_●−99.9%, only NO₂ is detected in the treated exhaust. Additionally, the oxidation capacity of the NaClO₂ solution still keeps 100% NO removal efficiency. The highest NO₂ concentration in the treated gas is reached when η_NOx is η_●. When ∆P was 3.00 bar, 4.00 bar, 5.00 bar, and 6.00 bar, η_● was equal to 89.6%, 86.9%, 86.5%, and 82.1%, and the maximum NO₂ concentration was equal to 104 ppmv, 131 ppmv, 135 ppmv, and 179 ppmv, as shown in Figure 3d,f, respectively. The maximum NO₂ concentration at ∆P = 5.00 bar was 135 ppmv, which is higher than that at ∆P = 3.00 bar. When the NO₂ concentration reaches the maximum, η_NO decreases from 100%. At that moment, the NaClO₂ solution cannot oxidize NO completely. At ∆P = 5.00 bar, Tη_NO,100% (15.14 min) was 5.58 min longer than Tη_NO,100% (9.42 min) at ∆P = 3.00 bar, as shown in Figure 3c. When the NO₂ concentration reaches the maximum, the NaClO₂ consumption at ∆P = 5.00 bar was larger than that at ∆P = 3.00 bar. Therefore, the maximum concentration of NO₂ increased. When ∆P = 6.00 bar, Tη_NOx,100% was only 1.92 min, and the maximum concentration of NO₂ (179 ppmv) was reached at the 16th min. Therefore, there was a significant increase in the maximum NO₂ concentration.

As illustrated in Figure 3e, Tη_NO,100% was the longest for ∆P = 6.00 bar. The primary reason for the increase in the NO₂ concentration was that large amounts of ClO₂ escape due to a high ∆P. The high ∆P results in lower suction and pressure of the gas-filled bubbles, which is more conducive for vaporizing the liquid into the bubbles. NaClO₂ generates adequate ClO₂ rapidly for initial pH of 3.50 (as given in Equation (11)). Therefore, at the high ∆P, ClO₂ in the liquid phase is more likely to be vaporized into and discharged together with the gas-filled bubbles.

$${5\mathrm{ClO}}_2^{-}{+\mathrm{H}}^{+} \to { 4\mathrm{ClO}}_2{+\mathrm{Cl}}^{-}{+2\mathrm{H}}_2\mathrm{O}$$

(11)

Additionally, a high ∆P shortens the reaction time. Furthermore, the absorption of NO₂ becomes insufficient due to the short contact time between gas and liquid. NO₂ requires time to be converted to N₂O₃ and N₂O₄ (as given in Equations (12) and (13)), which were dissolved by the liquid phase (as given in Equations (14) and (15)) [20,54,55]. It is generally accepted that the increase in nitrogen valency increases the solubility of gaseous nitrogen in the aqueous phase [56]. Therefore, the short reaction time would inhibit this process, and the absorption of NO₂ would become insufficient.

$${\mathrm{NO}+\mathrm{NO}}_2 \to { \mathrm{N}}_2{\mathrm{O}}_3$$

(12)

$${2\mathrm{NO}}_2 \to { \mathrm{N}}_2{\mathrm{O}}_4$$

(13)

$${\mathrm{N}}_2{\mathrm{O}}_3{+\mathrm{H}}_2\mathrm{O} \to { 2\mathrm{HNO}}_2$$

(14)

$${\mathrm{N}}_2{\mathrm{O}}_4{+\mathrm{H}}_2\mathrm{O} \to { \mathrm{HNO}}_2{+\mathrm{HNO}}_3$$

(15)

Summing up, for ∆P = 5.00 bar, the influence of increased reaction rate and shortened reaction time reached a good balance. Furthermore, the maximum NO₂ concentration was only 135 ppmv, and Tη_NOx,100% was the maximum (8.22 min). Therefore, ∆P = 5.00 bar was used as the experimental optimal ∆P.

3.2. Effect of Initial pH of NaClO₂ Solution on NO Removal

According to the Nernst equation, the reduction potential of NaClO₂ decreases as the pH increases. However, since pH affects the generation of NaClO₂ to ClO₂, there is an optimal pH for NO_x removal [13,57,58]. Yang et al. [59] and Adewuyi et al. [58] suggested removing NO_x by NaClO₂ in neutral or slightly acidic conditions. Therefore, the experiments were first performed at an initial pH of 4.00−7.00 in this study. As the initial pH decreases, the η_{NOx initial} increases. As illustrated in Figure 4a,b, for the initial pH range of the solution of 4.00−7.00, the initial NO_x removal efficiency η_{NOx initial} is 84.8−54.8%. Since, at this time, the amount of ClO₂ generated by NaClO₂ was not enough to oxidize NO_x completely, the η_{NOx initial} could not reach 100%. Furthermore, as the solution absorbs more NO_x, its pH gradually decreases, and η_NOx reaches its maximum value η_{NOx max}. When the initial pH was 4.00, 5.00, 6.00, and 7.00, the values of η_{NOx max} were 87.4%, 71.9%, 65.4%, and 64.4%. The increase in η_{NOx max} is by 2.6%, 11.9%, 8.5%, and 9.6%, respectively, compared to η_{NOx initial}. As shown in Figure 4c, the instantaneous pH range of achieving η_{NOx max} is 3.50−3.70, according to the experimental results. The authors of this study were of the opinion that adjusting the initial pH to 3.50−3.70 may improve η_{NOx initial}, so the experiment was carried out for initial pH = 3.50. Subsequently, it was shown that η_{NOx initial} could reach 100% for the initial pH = 3.50. When the initial pH of the NaClO₂ solution is 4.00 − 7.00, the NO_x of treated emissions consists of NO and NO₂. NO was not completely oxidized, and NO₂ was not completely absorbed, resulting in the η_{NOx initial} being less than 100%. The reduction of initial pH could significantly improve the oxidation capacity of the NaClO₂ solution. Gong et al. [60] explained that 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. In addition, when the initial pH of the NaClO₂ solution is 4.00−7.00, due to the reduced amount of ClO₂ generated, NO₂ was not completely absorbed. Song et al. [37] carried out research on the removal of NO₂ by H₂O₂, NaS₂O₈, NaClO₂, and ClO₂ under HC conditions, and reported that ClO₂ has a higher oxidation selectivity for NO₂ compared with NaClO₂. When the initial pH was 4.00−7.00, the ClO₂ generated per unit of time was small [61]. When the experiment was carried out for initial pH = 3.50, the amount of ClO₂ generated per unit of time was more significant [62]. Therefore, η_{NOx initial} could reach 100% for the initial pH = 3.50.

Furthermore, the reduction of initial pH could significantly improve the NO_x removal efficiency. Experiments with an initial pH of 2.00−3.50 were carried out in this study to explore further the influence of initial pH on removing NO based on HC combined with NaClO₂, so an acidic oxidation mechanism was followed between NO and NaClO₂ [63]. Therefore, NO is removed by reacting with ClO₂⁻ (as given by Equation (16)) or ClO₂ (as given by Equation (17)). In addition, ^•OH (as given by Equations (5) and (6)) and ^•H (as given by Equation (8)) will also promote the removal of NO. As shown in Figure 4d, when the initial pH was 2.00, 2.50, 3.00, and 3.50, η_{NOx initial} reached 100% and maintained this value for more than 8 min. Furthermore, this depends on the rapid decomposition of NaClO₂ to generate ClO₂ in acidic conditions (as given in Equation (11)). A significantly low value of pH shortens Tη_NOx,100%. When the initial pH was 2.00, 2.50, and 3.50, Tη_NOx,100% was 8.50 min, 9.22 min, and 9.43 min, respectively, as shown in Figure 4d. The reason for this phenomenon may be the escape of excess ClO₂ from the liquid phase [64]. The reaction rate of Equation (11) may be influenced by the ClO₂⁻ and H⁺ concentrations. The reaction rate of Equation (11) is faster for higher ClO₂⁻ and H⁺ concentrations, and more ClO₂ is produced per unit of time. Tη_NOx,100% was the maximum for initial pH = 3.00, and the production of ClO₂ was sufficient for NO removal (1000 ppmv, 1.0 L/min) in a unit of time. However, ClO₂ was overproduced for initial pH of 2.00 and 2.50. The excessive ClO₂ vaporized into gas-filled bubbles and discharged together with them. Thus, NaClO₂ consumption was accelerated, leading to a reduction of Tη_NOx,100%.

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

(16)

$${2\mathrm{ClO}}_2{+5\mathrm{NO}+\mathrm{H}}_2\mathrm{O} \to { 5\mathrm{NO}}_2{+2\mathrm{H}}^{+}{+2\mathrm{Cl}}^{-}$$

(17)

When η_NOx was in the interval of η_●−99.9%, the NaClO₂ solution concentration decreased, and the reaction rate of Equation (11) became slow. In this case, the influence of ClO₂ escaped on the duration of the interval of η_NOx became smaller, and the remaining NaClO₂ in the solution had a more significant influence on the duration of the interval. The amount of NaClO₂ remaining in the solution became lesser as the duration of the interval lengthened. When the initial pH was 2.00, 2.50, 3.00, and 3.50, the duration of the interval of η_NOx was 4.21 min, 4.37 min, 5.64 min, and 6.92 min, respectively, as shown in Figure 4d. Simultaneously, the remaining NaClO₂ in the solution also affected the maximum NO₂ concentration in the treated gas. As shown in Figure 4f, the maximum NO₂ concentration generally declines. When the initial pH was 2.00, 2.50, 3.00, and 3.50, the maximum NO₂ concentration was 205 ppmv, 182 ppmv, 195 ppmv, and 135 ppmv, respectively. Chin et al. [65] and Brogren et al. [66] explained that 60−80% of the NO₂ generated in the reaction can be removed by Equations (18)–(22).

Additionally, ^•OH (Equation (7)) and ^•ClO (Equations (23)–(25)) also promoted NO₂ absorption [37]. Therefore, this may be the reason for the complete NO₂ removal when η_NOx was 100%. When η_NOx was in the interval of η_●−99.9%, the NaClO₂ solution concentration decreased, which inhibited NO₂ removal by Equations ((22)–(25)). In this case, the generated NO₂ may be removed by the hydrolysis of NO₂ (as given by Equations (18) and (19)), oxidative absorption of ClO₂⁻ (as given by Equations (20) and (21)), and ^•OH (as given by Equation (7)). Therefore, this could also explain that, as NaClO₂ in solution decreased, the maximum NO₂ concentration was only 205 ppmv.

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

(18)

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

(19)

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

(20)

$${\mathrm{NO}}_2{+\mathrm{ClO}}_2^{-} \to { 2\mathrm{NO}}_3^{-}{+\mathrm{ClO}}^{-}$$

(21)

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

(22)

$${\mathrm{ClO}}_{2\left(\mathrm{g}\right)} \stackrel{\mathrm{cavitation}}{\to } {}^{•}\mathrm{ClO}\left(\mathrm{g}\right)+{}^{•}\mathrm{O}\left(\mathrm{g}\right)$$

(23)

$${}^{•}\mathrm{ClO}+{\mathrm{NO}}_2 \to { \mathrm{ClONO}}_2$$

(24)

$${\mathrm{ClONO}}_2{+\mathrm{H}}_2\mathrm{O} \to { \mathrm{HOCl}+\mathrm{HNO}}_3$$

(25)

When the initial pH was 3.50, 100% NO_x removal efficiency was maintained for 8.22 min. Subsequently, NO₂ was detected in the treated gas, but the oxidation capacity of the NaClO₂ solution could still maintain 100% NO removal efficiency for 6.92 min. η_NOx decreased from η_● to 0.0% (η_● = 86.5%), the NO concentration increased from 0 ppmv to 1000 ppmv, and NO₂ concentration rapidly decreased from 135 ppmv to 0 ppmv in the next 3.67 min. Therefore, this indicated that the decreased NaClO₂ solution concentration led to the loss of oxidation capacity for NO removal. Therefore, for the NO_x removal by NaClO₂ solution under acidic conditions, the fundament was the rapid activation of ClO₂, and the increasing Tη_NOx,100% required improved NO₂ absorption. A lower value of initial pH increased Tη_NOx,100%, but a large amount of escaping ClO₂ led to a reduced oxidation capacity and a higher NO₂ concentration in the solution. The lower pH can also be a severe concern for the corrosion of the experimental equipment. Therefore, the optimal initial pH of the solution was taken as 3.50 in this study.

3.3. Effect of Reaction Temperature on NO Removal

The reaction temperature significantly influences the dissolution and diffusion of molecules or ions in the NaClO₂ solution. Additionally, the change in the reaction temperature would affect the change in the saturated vapor pressure of the solution, affecting the cavitation. According to the Arrhenius law, a high temperature promotes ion diffusion and accelerates chemical reactions [10]. The high temperature promotes the thermal decomposition of NaClO₂ to generate ClO₂ (as given in Equation (11)) [67]. As the reaction temperature increased, Tη_NOx,100% first increased and then decreased. When the reaction temperature was 30.0 °C, 35.0 °C, 40.0 °C, 45.0 °C, 50.0 °C, 55.0 °C, and 60.0 °C, Tη_NOx,100% was 5.42 min, 6.75 min, 7.75 min, 8.22 min, 10.10 min, 9.58 min, and 9.33 min, respectively, as shown in Figure 5a. Therefore, the increase in temperature had the same influence on Tη_NO,100%. As shown in Figure 5b, Tη_NO,100% was the shortest (10.60 min) for 30.0 °C reaction temperature. Additionally, Tη_NO,100% was the longest (16.83 min) for 50.0 °C reaction temperature. However, Tη_NO,100% decreased to 14.50 min for 60.0 °C reaction temperature. The decrease in the Tη_NO,100% value indicated that NO could not be fully oxidized. This was because the high temperature accelerated the thermal decomposition of NaClO₂ into ClO₂, which led to the consumption of NaClO₂ in the solution.

However, the high temperature decreases the solubility of NO_x or ClO₂. Additionally, it enhances the mass-transfer resistance between gas and liquid, resulting in a decreased mass transfer of NO from the gas to the liquid phase. Therefore, when the reaction temperature exceeded 50.0 °C, NO_x absorption was inhibited due to the decrease in NO_x solubility. Therefore, as temperature increased, Tη_NO,100% first increased and then decreased. An increase in temperature would also increase the maximum NO₂ concentration in the treated gas. Furthermore, when the temperature increased from 30.0 °C to 60.0 °C, the maximum NO₂ concentration increased from 84 ppmv to 175 ppmv, as shown in Figure 5c. Nitrites of the solution decomposed into NO₂ at higher reaction temperatures (as given in Equation (26)) [67], which may be one of the reasons for an increase in the maximum NO₂ concentration with the increase in temperature.

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

(26)

In addition to this, a temperature change will cause a change in cavitation intensity. The influence of temperature on cavitation intensity is mainly through viscous and thermodynamic effects [32]. As temperature increases and viscosity decreases, the Reynolds number increases proportionally. The generation of turbulence effects increases the intensity of cavitation. Temperature increases the vapor pressure, making it easier for the NaClO₂ solution to evaporate and accelerating the formation of cavities [68]. Therefore, the values of Tη_NOx,100% and Tη_NO,100% keep increasing as the temperature rises from 30 °C to 50 °C. However, too high of a temperature will have a delay effect on cavitation. Brennen quantifies the delays of cavitation with the thermodynamic parameter Σ [69], as follows:

$$\mathsf{\Sigma }=\frac{{\left({\mathsf{\rho }}_{\mathrm{V}} \mathrm{L}\right)}^2}{{\mathsf{\rho }}_{\mathrm{l}}^2{\mathrm{c}}_{\mathrm{p},\mathrm{l}}{\mathrm{T}}_{\infty }\sqrt{{\mathsf{\alpha }}_{\mathrm{l}}}}$$

(27)

where ${\mathrm{T}}_{\infty }$ is the test temperature, ${\mathsf{\rho }}_{\mathrm{V}}$ is the vapor density, ${\mathsf{\rho }}_{\mathrm{l}}$ is the liquid density, L is the evaporative latent heat, ${\mathrm{c}}_{\mathrm{p},\mathrm{l}}$ is the constant pressure specific heat of the liquid, and ${\mathsf{\alpha }}_{\mathrm{l}}$ is the thermal diffusivity of the liquid.

The ∑ parameter depends only upon the liquid’s temperature; thus, various liquids can be compared to each other regarding the thermal delay. Hattori et al. [70] reported that the thermodynamic effect becomes significant when the thermodynamic parameter ∑ = 100 m/s^3/2. For water, the applicable range is 50 °C and 55 °C. When the temperature in this study exceeds 50 °C, the thermodynamic effect significantly retards the development of cavitation. At this point, the increase in vapor pressure tends to evaporate the liquid, causing cavities to merge and reducing the number of individual cavitation structures [68]. The delay in cavitation causes a reduction in cavitation intensity. Therefore, the chemical effect of cavitation will also be weakened, and the production of ^•OH and ^•H (as given in Equation (4)) and ^•ClO (as given in Equation (23)) will be reduced, which is not conducive to the removal of NO_x (as given in Equations (5)–(9) and (24) and (25)).

3.4. Effect of NaClO₂ Concentration on NO Removal

The increased concentration of NaClO₂ enhanced the mass transfer effect between the gas and liquid phases. As illustrated in Figure 6a, when the NaClO₂ concentration was 0.60 mmol/L, η_NOx reached 100%. However, the duration was only 0.50 min. A short duration is not conducive to observing the complete reactive trend of NO_x removal by HC. Furthermore, when the concentration of NaClO₂ increased from 0.60 mmol/L to 1.40 mmol/L, Tη_NOx,100% increased from 0.50 min to 11.50 min. Therefore, Tη_NOx,100% is linear with the NaClO₂ concentration (as given in Equation (28)), and the corresponding slope and the intercept of the straight line are 13.625 ± 1.32 and 7.24 ± 1.37, respectively.

y = (13.625 ± 1.32) x − (7.24 ± 1.37)

(28)

Furthermore, when the NaClO₂ concentration was 1.00 mmol/L, Tη_NOx,100% was 8.22 min, which was 1.84 min higher than the predicted value of 6.38 min in Equation (28). As the concentration of NaClO₂ increases, the amount of ClO₂ produced will also increase. However, when the NaClO₂ concentrations were 0.60 mmol/L, 0.80 mmol/L, 1.20 mmol/L, and 1.40 mmol/L, the corresponding Tη_NOx,100% values were lower than the predicted values, as shown in Figure 6.

In addition to this, as illustrated in Figure 6b, when the concentrations of NaClO₂ were 1.20 mmol/L, and 1.40 mmol/L, Tη_NO,100% values were 15.25 min and 16.25 min, respectively. Furthermore, compared with the Tη_NO,100% value of 15.14 min for NaClO₂ concentration of 1.00 mmol/L, they showed an increase of 0.11 min and 1.11 min, respectively. However, when the concentration exceeded 1.00 mmol/L, Tη_NO,100% values did not increase significantly. Additionally, the increase in NaClO₂ concentration did not significantly reduce the average concentration of NO₂ (135 ± 13 ppmv), as shown in Figure 6c. The maximum concentration of NO₂ was 137 ppmv when the NaClO₂ concentration was 1.20 mmol/L, which was 2 ppmv higher than when the NaClO₂ concentration was 1.00 mmol/L. NO₂ was transported by the escape of ClO₂, resulting in a higher concentration of NO₂ [35]. Therefore, the optimal NaClO₂ concentration was considered as 1.00 mmol/L in this study.

4. Conclusions

Detailed experiments were carried out to study the influence of various parameters on NO removal efficiency, including the ∆P of the HC reactor, the initial pH, the reaction temperature, and the concentration of NaClO₂. The experimental results showed that removing NO from ship exhaust based on HC using NaClO₂ solution was a valid method. The advantages of this novel method were low NaClO₂ concentration and high NO_x removal efficiency. The NO_x removal efficiency reached 100% for the NaClO₂ concentration of 0.60 mmol/L. The HC reactor could generate many gas-filled bubbles with small volumes, which was conducive to enhancing the contact area between liquid and gas to accelerate the reaction rate. The reduction of initial pH could significantly improve the oxidation capacity of the NaClO₂ solution. η_{NOx initial} was below 100% for the initial pH = 4.00–7.00. When the initial pH ≤ 3.50, η_{NOx initial} reached 100% and was maintained for more than 8 min. The fundamentals for NO_x removal by NaClO₂ solution under acidic conditions was the rapid activation of ClO₂, and the increasing Tη_NOx,100% required improved NO₂ absorption. Additionally, ^•OH and ^•ClO produced by HC promoted the NO₂ absorption, which may be one of the reasons for complete NO₂ removal when η_NOx was 100%.