NO Removal from Simulated Flue Gas with a NaClO 2 Mist Generated Using the Ultrasonic Atomization Method

In order to enhance the mass transfer efficiency between gas–liquid interfaces, NaClO2 mist generated by an ultrasonic humidifier was used to remove NO from simulated flue gas. The effects of some key parameters (the gas flow rate, the NaClO2 concentration in the solution, the inlet NO concentration, the NaClO2 solution pH) on NO removal efficiency were investigated preliminarily. The results showed that NaClO2 mist could oxidize NO with a much higher efficiency compared with other mists containing either NaClO or H2O2 as oxidants. With an increase in the gas flow rate from 1.5 to 3.0 L·min−1, the atomizing rate of the NaClO2 solution increased almost linearly from 0.38 to 0.85 mL·min−1. When the gas flow rate was 2.0 L·min−1, a complete removal of NO had been reached. NO removal efficiency increased obviously with an increase in the NaClO2 concentration in the solution. With an increase in the inlet NO concentration, the ratio of NO in the flue gas and NaClO2 in the mist increased almost linearly. Furthermore, the NaClO2 mist exhibited a relatively stable and high NOx removal efficiency in a wide pH range (4–11) of NaClO2 solutions. The reason for the high NO removal efficiency was mainly ascribed to both the strong oxidative ability of NaClO2 and the improved mass transfer at the gas-liquid interface.


Introduction
A great deal of air pollutants are emitted from the combustion of fossil fuels in stationary sources and mobile sources every year, which results in serious damage to the ecological environment [1,2].During the past decades, numerous efforts have been made to effectively remove sulfur oxides (SO x ), nitrogen oxides (NO x ), particle matters (PMs), and other air pollutants, from the waste gas [3].Comparatively speaking, it is easy to decrease the emission of SO x and PMs with a high efficiency by adopting a wet scrubbing method [4,5].As to NO x , NO accounts for more than 90% of the total makeup and it is insoluble in water, so NO x cannot be removed effectively within the desulfurization scrubbers [6].At present, a lot of NO x emission control technologies have been developed, in which selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR) are commercially available.SCR can remove NO x with an efficiency of 80-95% and it has been successfully applied in power plants, heavy-duty vehicles, and ocean-going ships [7,8].But there are still some challenges for SCR to deal with.It requires a large installation space and has a high investment cost [9].When SO 2 and NO x in flue gas are treated step-by-step through an integrated system, ammonium bisulfite salt formed in flue gas may deactivate the SCR catalyst.In addition, SCR requires a complicated control Energies 2018, 11, 1043 2 of 15 system to avoid ammonia slip.For SNCR, its NO x removal efficiency is obviously lower than that of SCR.Furthermore, a very high reaction temperature of 850-1100 • C is essential for SNCR to facilitate the reduction reaction between NO x and the reductants [10].Since both SCR and SNCR belong to the dry methods for NO removal, they have to be combined in series with a wet scrubbing desulfurization process for the simultaneous removal of NO x and SO 2 .Such an integrated system suffers from some obvious drawbacks, such as the high cost, large size, complicated operation, and so on.To a great extent, these factors limit the application of the integrated system in some industrial areas.
In recent years, more and more researchers have been interested in developing new methods for the simultaneous removal of NO x and SO 2 based on wet scrubbing processes [11].In such cases, appropriate additives are required to improve the water solubility of NO x .A number of reports have suggested NO removal with the help of an iron chelating agent such as Fe 2+ -EDTA (Ethylene Diamine Tetraacetic Acid) because of its fast absorption rate for NO and high absorption capacity.However, the chelating agent could be easily oxidized by NO, NO 2 , and O 2 in flue gas to form Fe 3+ -EDTA that is not capable of binding with NO [12].It also needs to overcome other main drawbacks such as the high cost of EDTA, and the need to remove the N-S complex [13].Another possible way is to oxidize the insoluble NO into soluble NO 2 , and then the NO x molecules can be absorbed through wet scrubbing using the proper absorbents [14].Although both nonthermal plasma and ozone can be used as excellent oxidants to oxidize NO effectively, they usually require expensive equipment and a large energy supply [15,16].
Since oxidants in a solution can oxidize NO effectively and economically, more and more interest is being focused on various inorganic reagents, such as KMnO 4 [17], H 2 O 2 [18], oxone [19], Na 2 S 2 O 8 [20,21], NaClO [22][23][24], NaClO 2 [25][26][27], ClO 2 [28] and so on.Among these oxidants, NaClO 2 is found to be one of the most promising chemicals for the oxidation and absorption of NO.Some studies on the simultaneous removal of NO and SO 2 using a NaClO 2 solution have been publishes in the past decades [29,30].Most of the work done till now has concentrated more or less on chemical scrubbing experimentation.However, the traditional chemical scrubbing method requires a large volume of reactors, expensive pipes, and powerful fans.In order to improve the economic feasibility of chemical scrubbing, a novel wet reaction system based on the ultrasonic atomization process has been proposed by Park et al. [31,32].A very fine NaClO 2 mist generated by the ultrasonic atomization method was used to absorb NO and SO 2 simultaneously, and then the formed aerosol particles were collected in an electrostatic precipitator.Since the size of the ultrasonic mist was much smaller than that of the spraying droplets, it was beneficial to increase the gas-liquid contact area between the reactants and waste gas.Park et al. had preliminarily investigated the simultaneous removal performance and kinetics for NO and SO 2 using NaClO 2 mist.But more research is still necessary to further explore the feasibility of the process of NO removal with NaClO 2 mist.In this work, a NaClO 2 mist generated by an ultrasonic humidifier was used to remove NO from simulated flue gas, and the effects of some key operating parameters (the different oxidants, the gas flow rate, the concentration of the NaClO 2 solution, the inlet NO concentration, the initial solution pH) on the NO removal efficiency were investigated experimentally; the possible reaction pathways are also discussed.

Experimental Apparatus
The experimental system is shown in Figure 1.Two kinds of gases, pure N 2 gas (99.999%) and NO span gas (10.04%NO with N 2 as a balance gas), were used to prepare the simulated flue gas.A custom-made ultrasonic humidifier with an ultrasonic atomizer (16.8 W, 1.67 MHz) situated inside was used to produce NaClO 2 mist.A column reactor was located at the top of the ultrasonic humidifier.Both the ultrasonic humidifier and reactor column were made of polymeric methyl methacrylate (PMMA).The height and inner diameter of the column reactor were 300 mm and 50 mm, respectively.An electric condenser was used to remove moisture from the flue gas before it entered the gas analyzer.A gas analyzer (MGA5, MRU) with non-dispersive infrared (NDIR) sensors was used to measure the concentrations of multi-pollutants such as NO, NO 2 , and NO x .Here the concentration of NO x in the experiments referred to the total sum of the NO and NO 2 concentrations.The measurement accuracy of the analyzer was ±2% of full scale.Since the change of liquid level in humidifier will affect the atomizing rate of the mist to some extent, a peristaltic pump (BT100-2J, Longer, Baoding, China) was used to continuously feed NaClO 2 solution into the ultrasonic humidifier to keep the liquid level constant during the atomization process.
ultrasonic humidifier to keep the liquid level constant during the atomization process.

Experimental Procedures
The NaClO2 solution was prepared using NaClO2 powder (80%, Aladdin) and deionized water.The initial pH value of the NaClO2 solution was adjusted by adding 1 mol/L HCl or 1 mol/L of NaOH solution, and measured with a pH meter (S210, Mettler-Toledo, Zurich, Switzerland).The flow rates of the pure N2 gas and NO span gas were metered through mass flow controllers (MFCs, D07-19B, Sevenstar, Beijing, China).At first, the N2 was introduced into the ultrasonic humidifier to activate the generated NaClO2 mist.NO span gas was injected into the pipe that connected the humidifier and the column reactor.An extra electric condenser was used to remove the mist from the flue gas before it entered the flue gas analyzer.Each part of the experiment was conducted at room temperature (20 °C).The concentrations of NO, NO2, and NOx were recorded using the gas analyzer at an interval of 10 s.

Data Processing
Before the ultrasonic atomization of NaClO2 solution, the concentrations of NO and NOx in the simulated flue gas were measured as inlet concentrations.At the beginning of the atomization process, the NOx concentrations decreased and stabilized within tens of seconds.Then, the atomization process lasted for another 10 min.The averaged concentrations of NO and NOx measured in a stable state were used as the outlet concentrations.Thus, the removal efficiencies of the pollutants could be calculated by the following equation: where η is the removal efficiency of the targeted pollutant (%), Cin and Cout are the inlet and outlet concentrations of the targeted pollutants (ppm), respectively.

Experimental Procedures
The NaClO 2 solution was prepared using NaClO 2 powder (80%, Aladdin) and deionized water.The initial pH value of the NaClO 2 solution was adjusted by adding 1 mol/L HCl or 1 mol/L of NaOH solution, and measured with a pH meter (S210, Mettler-Toledo, Zurich, Switzerland).The flow rates of the pure N 2 gas and NO span gas were metered through mass flow controllers (MFCs, D07-19B, Sevenstar, Beijing, China).At first, the N 2 was introduced into the ultrasonic humidifier to activate the generated NaClO 2 mist.NO span gas was injected into the pipe that connected the humidifier and the column reactor.An extra electric condenser was used to remove the mist from the flue gas before it entered the flue gas analyzer.Each part of the experiment was conducted at room temperature (20 • C).The concentrations of NO, NO 2 , and NO x were recorded using the gas analyzer at an interval of 10 s.

Data Processing
Before the ultrasonic atomization of NaClO 2 solution, the concentrations of NO and NO x in the simulated flue gas were measured as inlet concentrations.At the beginning of the atomization process, the NO x concentrations decreased and stabilized within tens of seconds.Then, the atomization process lasted for another 10 min.The averaged concentrations of NO and NO x measured in a stable state were used as the outlet concentrations.Thus, the removal efficiencies of the pollutants could be calculated by the following equation: where η is the removal efficiency of the targeted pollutant (%), C in and C out are the inlet and outlet concentrations of the targeted pollutants (ppm), respectively.

Comparison of Different Oxidants in Mist
NaClO 2 , NaClO, and H 2 O 2 are common oxidants that are used to remove NO from flue gas through wet scrubbing.Here they are chosen to combine with the ultrasonic atomization process in order to investigate the effect of various oxidants on NO x removal efficiency; the results are shown in Figure 2. The concentration of oxidants in the solution was 0.02 mol•L −1 .The flow rate of the simulated flue gas was 2 L/min.The inlet concentrations of NO and NO 2 were 500 and 0 ppm, respectively.The initial pH values of oxidant solution were adjusted to 4, 7, and 10, respectively.The atomization rate of the oxidant solution was measured to be about 0.6 mL/min.

Comparison of Different Oxidants in Mist
NaClO2, NaClO, and H2O2 are common oxidants that are used to remove NO from flue gas through wet scrubbing.Here they are chosen to combine with the ultrasonic atomization process in order to investigate the effect of various oxidants on NOx removal efficiency; the results are shown in Figure 2. The concentration of oxidants in the solution was 0.02 mol•L −1 .The flow rate of the simulated flue gas was 2 L/min.The inlet concentrations of NO and NO2 were 500 and 0 ppm, respectively.The initial pH values of oxidant solution were adjusted to 4, 7, and 10, respectively.The atomization rate of the oxidant solution was measured to be about 0.6 mL/min.It can be seen from Figure 2 that NOx removal efficiencies changed greatly for the various oxidants in the mist.For the H2O2 mist, NO removal efficiencies were below 1%.That is due to the low oxidative ability of H2O2 compared with other oxidants.Some enhancement techniques are usually required for H2O2 oxidant to improve its NO removal efficiency [33][34][35].
For the NaClO mist, NO removal efficiency increased slightly from 21.9% to 23.8% and NOx removal efficiency increased from 5.8% to 7.9% with NaClO solution pH increasing from 4 to 7. As the solution pH was increased from 7 to 10, NO removal efficiency increased obviously up to 31.6% and NOx removal efficiency increased up to 18.4%.The variation trend of NO removal efficiency is the same as that for NOx removal efficiency.Since a part of the generated NO2 had not been absorbed by the scrubbing solution, NOx removal efficiency was obviously lower than the corresponding NO removal efficiency.It was reported in a previous study that an acidic or a basic condition was favorable for NO removal by wet scrubbing using a NaClO solution, because HClO was considered as the effective oxidant among the active chlorine species [36,37].However, for the ultrasonic atomization process, the effect of the initial pH of NaClO solution on the NO removal efficiency was quite different from that for the wet scrubbing process.Here HClO is still considered as the effective composition in the NaClO mist.NOx removal efficiency for pH 10 NaClO mist is obviously higher than pH 7 NaClO mist.The reason might be that on one hand, the pH of NaClO mist generated from the pH 10 NaClO solution may be different from the pH 10 NaClO solution.It is possible that the former is a little lower than the latter.On the other hand, the size of the NaClO mist is much finer than the spraying droplets.The size of the ultrasonic mist has been measured using a Laser Particle Sizer (DP-02, OMEC, Zhuhai, China), and it is in the range of 2-5 μm.The size of the spraying droplets are usually in the range of 100-300 μm.This means that the fine mist can be acidized easily during the NO absorption process, as a result the NaClO mist pH decreases to below 7 quickly in the reactor.Thus, it leads to a high NOx removal efficiency for the NaClO mist It can be seen from Figure 2 that NO x removal efficiencies changed greatly for the various oxidants in the mist.For the H 2 O 2 mist, NO removal efficiencies were below 1%.That is due to the low oxidative ability of H 2 O 2 compared with other oxidants.Some enhancement techniques are usually required for H 2 O 2 oxidant to improve its NO removal efficiency [33][34][35].
For the NaClO mist, NO removal efficiency increased slightly from 21.9% to 23.8% and NO x removal efficiency increased from 5.8% to 7.9% with NaClO solution pH increasing from 4 to 7. As the solution pH was increased from 7 to 10, NO removal efficiency increased obviously up to 31.6% and NO x removal efficiency increased up to 18.4%.The variation trend of NO removal efficiency is the same as that for NO x removal efficiency.Since a part of the generated NO 2 had not been absorbed by the scrubbing solution, NO x removal efficiency was obviously lower than the corresponding NO removal efficiency.It was reported in a previous study that an acidic or a basic condition was favorable for NO removal by wet scrubbing using a NaClO solution, because HClO was considered as the effective oxidant among the active chlorine species [36,37].However, for the ultrasonic atomization process, the effect of the initial pH of NaClO solution on the NO removal efficiency was quite different from that for the wet scrubbing process.Here HClO is still considered as the effective composition in the NaClO mist.NO x removal efficiency for pH 10 NaClO mist is obviously higher than pH 7 NaClO mist.The reason might be that on one hand, the pH of NaClO mist generated from the pH 10 NaClO solution may be different from the pH 10 NaClO solution.It is possible that the former is a little lower than the latter.On the other hand, the size of the NaClO mist is much finer than the spraying droplets.The size of the ultrasonic mist has been measured using a Laser Particle Sizer (DP-02, OMEC, Zhuhai, China), and it is in the range of 2-5 µm.The size of the spraying droplets are usually in the range of 100-300 µm.This means that the fine mist can be acidized easily during the NO absorption process, as a result the NaClO mist pH decreases to below 7 quickly in the reactor.Thus, it leads to a high NO x removal efficiency for the NaClO mist ultrasonically generated from a pH 10 NaClO solution.Moreover, it is worth noting that the molar ratio of the NO concentration in flue gas and the NaClO concentration in the solution is approximately 3.45:1, suggesting that the molar ratio of reactants of NO and NaClO in the reactor is nearly 1:1.The result showed that the utilization of the NaClO oxidant in the mist is high.
As shown in Figure 2, when the NaClO 2 oxidant was used to oxidize NO, the NO removal efficiency decreased slightly from 57.8% to 53.6% and the NO x removal efficiency decreased from 33.4% to 31.5% with the NaClO 2 solution pH increasing from 4 to 10.With the change of solution pH, the variation trend of the NO x removal efficiency for the NaClO 2 mist was different from that for the NaClO mist.At first, the oxidation power of NaClO 2 is much higher than NaClO.A high and stable NO x removal efficiency can be reached by wet scrubbing using NaClO 2 solution in a wide pH range of 3-10.So it is understandable that one could get a high and stable NO x removal efficiency by using NaClO 2 mist ultrasonically generated from a NaClO 2 solution with a wide pH range of 4-11.Note that the active components in the NaClO 2 solution are different from those in the NaClO solution.HClO 2 in the NaClO 2 solution is considered as the effective composition for NO oxidation.We think that a very small fractional composition of HClO 2 can oxidize NO efficiently, so it is normal for NaClO 2 mist to obtain an excellent NO removal performance.As the fractional composition of HClO 2 in NaClO 2 mist decreases gradually with the increase of solution pH, it can explain the slight decrease in NO x removal efficiency for the NaClO 2 mist when the initial solution pH increases from 4 to 10.The molar ratio of NO concentration in flue gas and NaClO 2 concentration in the solution is 3.45:1, but NO removal efficiency for the NaClO 2 mist is approximately twice compared with that for the NaClO mist.It implies that the molar ratio of reactants of NO and NaClO 2 in the reactor increased up to 2:1.One could deduce that the oxidation and absorption process of NO by the NaClO 2 mist might involve the reactions below [26,38]: It is known that the oxidative ability of NaClO 2 is much higher than those of NaClO and H 2 O 2 .The results demonstrate that the NO removal performance for the oxidants in mist is in the order of NaClO 2 > NaClO >> H 2 O 2 .As both NO removal efficiency and the utilization of the NaClO 2 oxidant in mist are much higher than those for NaClO and H 2 O 2 , NaClO 2 is chosen as the oxidant in this study.

Effect of Gas Flow Rate
During the ultrasonic atomization process, with the increase in the gas flow rate, the amount of ultrasonic mist will increase accordingly.The effect of the flow rate of the simulated flue gas on the atomizing rate of the NaClO 2 solution was investigated, and the results are shown in Figure 3.The concentration of NaClO 2 in the solution was 0.04 mol•L −1 , and the solution pH was adjusted to 7. The flow rate of the simulated flue gas was in the range of 1.5-3.0L•min −1 .It can be seen from Figure 3 that with the increase in gas flow from 1.5 to 3.0 L•min −1 , the atomizing rate increased almost linearly from 0.38 to 0.85 mL•min −1 .A fitted line was obtained with a high correlation coefficient of 0.99891 and a slope of 0.28894.Since the intercept of the fitted line was 0, the slope of the fitted line was equal to the ratio of the atomizing rate and gas flow, namely the liquid-gas ratio in the column reactor.Thus, the liquid-gas ratio in this study could be considered as constant, which was much smaller than that for the spraying droplets.
Energies 2018, 11, x 6 of 15 to the ratio of the atomizing rate and gas flow, namely the liquid-gas ratio in the column reactor.Thus, the liquid-gas ratio in this study could be considered as constant, which was much smaller than that for the spraying droplets.4, the NO removal efficiency increased from 78.3% to 100% as the gas flow increased from 1.5 to 2.0 L•min −1 .Accordingly, the NOx removal efficiency increased from 46.2% to 61.5%.However, on further increasing the gas flow from 2.0 to 3.0 L•min −1 , both the NO and NOx removal efficiencies dropped slowly.Generally, the atomizing rate of the ultrasonic humidifier is mainly affected by the solution surface tension, the liquid height, the ultrasonic power, and gas flow.The liquid height and the ultrasonic power were kept constant in our experiments.As the concentration of the oxidant in the solution was comparatively low, the effect of the oxidant additive on the surface tension of the deionized water solution was neglected.Thus, the flow rate of the simulated flue gas became the major factor that determined the atomizing rate of the NaClO2 solution.On one hand, the gas disturbance in the ultrasonic humidifier would be enhanced with the increase of gas flow, which increased the probabilities of inelastic collision and aggregation between the mist droplets after they left the liquid surface.This might adversely increase the diameter of the mist droplets.On the other hand, the mist droplets could quickly leave the atomization zone when the gas flow increased.To some extent, it was helpful to decrease the probabilities of inelastic collision and aggregation, resulting in more mist droplets flowing out together with the flue gas as the gas flow increased.This might be the reason for the change in the NOx removal efficiency as the gas flow increased from 1.5 to 2.0 L•min −1 .When the gas flow increased from 2.0 to 3.0 L•min −1 , the residence time in the column reactor decreased obviously, though the atomizing rate of the NaClO2 solution had increased proportionally.The less the residence time was, the lower the NOx removal efficiency was.Therefore, a gas flow of 2 L•min −1 was chosen for the subsequent experiments in order to achieve a relatively high NOx removal efficiency.4, the NO removal efficiency increased from 78.3% to 100% as the gas flow increased from 1.5 to 2.0 L•min −1 .Accordingly, the NO x removal efficiency increased from 46.2% to 61.5%.However, on further increasing the gas flow from 2.0 to 3.0 L•min −1 , both the NO and NO x removal efficiencies dropped slowly.Generally, the atomizing rate of the ultrasonic humidifier is mainly affected by the solution surface tension, the liquid height, the ultrasonic power, and gas flow.The liquid height and the ultrasonic power were kept constant in our experiments.As the concentration of the oxidant in the solution was comparatively low, the effect of the oxidant additive on the surface tension of the deionized water solution was neglected.Thus, the flow rate of the simulated flue gas became the major factor that determined the atomizing rate of the NaClO 2 solution.On one hand, the gas disturbance in the ultrasonic humidifier would be enhanced with the increase of gas flow, which increased the probabilities of inelastic collision and aggregation between the mist droplets after they left the liquid surface.This might adversely increase the diameter of the mist droplets.On the other hand, the mist droplets could quickly leave the atomization zone when the gas flow increased.To some extent, it was helpful to decrease the probabilities of inelastic collision and aggregation, resulting in more mist droplets flowing out together with the flue gas as the gas flow increased.This might be the reason for the change in the NO x removal efficiency as the gas flow increased from 1.5 to 2.0 L•min −1 .When the gas flow increased from 2.0 to 3.0 L•min −1 , the residence time in the column reactor decreased obviously, though the atomizing rate of the NaClO 2 solution had increased proportionally.The less the residence time was, the lower the NO x removal efficiency was.Therefore, a gas flow of 2 L•min −1 was chosen for the subsequent experiments in order to achieve a relatively high NO x removal efficiency.As shown in Figure 5, when the NaClO2 mist was introduced into the reactor at the beginning, the NO concentration in outlet gas dropped quickly.The higher the NaClO2 concentration in the solution was, the lower the outlet NO concentration was.When the NaClO2 concentration in the solution was 0.08 mol/L, NO had been removed completely.The change in the NOx removal efficiency and the outlet NO2 concentration in the flue gas with the NaClO2 concentration in the solution are shown in Figure 6.NO removal efficiency increased greatly from 18.4% to 85.0% with the NaClO2 concentration in the solution increasing from 0.01 to 0.04 mol•L −1 .Accordingly, the NO2 concentration in outlet gas increased from 37 ppm to 215 ppm,  As shown in Figure 5, when the NaClO 2 mist was introduced into the reactor at the beginning, the NO concentration in outlet gas dropped quickly.The higher the NaClO 2 concentration in the solution was, the lower the outlet NO concentration was.When the NaClO 2 concentration in the solution was 0.08 mol/L, NO had been removed completely.

Effect of NaClO2 Concentration
Figure 5 shows the effect of NaClO2 concentration in the solution on the change of the outlet concentration of NO in the flue gas during the denitrification process.The gas flow of the simulated flue gas was 2 L/min.The inlet concentrations of NO and NO2 were 700 and 0 ppm, respectively.NaClO2 concentrations in the solutions were 0.01, 0.02, 0.04, and 0.08 mol•L −1 .The corresponding pH values of the NaClO2 solutions without adjustment were 10.1, 10.3, 10.5, and 10.7, respectively.It can be seen that when there was no addition of HCl or NaOH to adjust the initial solution pH, the pH values of the NaClO2 solution increased almost linearly with the increment of the NaClO2 oxidant.As shown in Figure 5, when the NaClO2 mist was introduced into the reactor at the beginning, the NO concentration in outlet gas dropped quickly.The higher the NaClO2 concentration in the solution was, the lower the outlet NO concentration was.When the NaClO2 concentration in the solution was 0.08 mol/L, NO had been removed completely.The change in the NOx removal efficiency and the outlet NO2 concentration in the flue gas with the NaClO2 concentration in the solution are shown in Figure 6.NO removal efficiency increased greatly from 18.4% to 85.0% with the NaClO2 concentration in the solution increasing from 0.01 to 0.04 mol•L −1 .Accordingly, the NO2 concentration in outlet gas increased from 37 ppm to 215 ppm, The change in the NO x removal efficiency and the outlet NO 2 concentration in the flue gas with the NaClO 2 concentration in the solution are shown in Figure 6.NO removal efficiency increased greatly from 18.4% to 85.0% with the NaClO 2 concentration in the solution increasing from 0.01 to Energies 2018, 11, 1043 8 of 15 0.04 mol•L −1 .Accordingly, the NO 2 concentration in outlet gas increased from 37 ppm to 215 ppm, and the NO x removal efficiency increased from 13.1% to 42.5%.The result implied that the NaClO 2 mist could oxidize NO efficiently.When the NaClO 2 concentration in the solution was 0.04 mol•L −1 , the molar ratio of NO in the flue gas and NaClO 2 in the mist was about 2.38:1.The results suggest that the reaction between NO and NaClO 2 possibly occurred as described in Equation ( 2).It also demonstrates that the generated NO 2 could be effectively absorbed by the mist.
Energies 2018, 11, x 8 of 15 and the NOx removal efficiency increased from 13.1% to 42.5%.The result implied that the NaClO2 mist could oxidize NO efficiently.When the NaClO2 concentration in the solution was 0.04 mol•L −1 , the molar ratio of NO in the flue gas and NaClO2 in the mist was about 2.38:1.The results suggest that the reaction between NO and NaClO2 possibly occurred as described in Equation ( 2).It also demonstrates that the generated NO2 could be effectively absorbed by the mist.

Effect of NO Concentration
The effect of the inlet NO concentration on the NO removal efficiency was investigated, and the results are shown in Figure 7.The flow rate of the simulated flue gas was 2 L/min.The inlet NO concentrations were in the range of 100-900 ppm, and the inlet NO2 concentrations were 0 ppm.NaClO2 concentrations in the solutions were 0.01, 0.02, 0.04, and 0.08 mol•L −1 , and the initial solution pH values were 10.1, 10.3, 10.5, and 10.7 accordingly.As shown in Figure 7, when the NaClO2 concentration in the solution was 0.01 mol•L −1 , the NO removal efficiency decreased from 50.5% to 12.7% with the increase of the inlet NO concentration from 100 to 900 ppm. Figure 8 presents the change of the molar ratio between NO in the flue gas and NaClO2 in the mist with the inlet NO concentration.It can be seen that with the inlet NO concentration increasing from 100 to 900 ppm, the molar ratio between NO in the flue gas and NaClO2 in the mist increased from 1.6 to 13.0.Generally, the increase of molar ratio would promote the mass-transfer driving force of NO, which improves the oxidation and absorption of NO.However, NO concentration in the gas was relatively much higher than that of the NaClO2 mist, it seemed to be reasonable to obtain a declining NO removal rate [39].As shown in Figure 7, when the NO concentration changed from 100 ppm to 300 ppm, the NO removal efficiency for 0.01 mol•L −1 NaClO2 concentration decreased from 50.5% to 41.5%, but the NO removal efficiency for 0.02 mol•L −1 NaClO2 concentration increased from 72.1% to 78.8%.That is because the NaClO2 concentration of 0.01 mol•L −1 was a little low in this experiment, and so the reaction rate between NaClO2 and NO played a dominant role at that moment.Thus, there was a decline trend in the NO removal efficiency for 0.01 mol•L −1 NaClO2 concentration.When the NaClO2 concentration in the solution increased from 0.01 mol•L −1 to 0.02 mol•L −1 , the increase of the NaClO2 concentration will enhance the mass transfer rate at the liquid-gas interface to some extent.Therefore, the NO removal efficiency for 0.02 mol•L −1 NaClO2 concentration increased at the beginning, and then decreased slowly after further increasing the inlet NO concentration from 300 ppm to 900 ppm.

Effect of NO Concentration
The effect of the inlet NO concentration on the NO removal efficiency was investigated, and the results are shown in Figure 7.The flow rate of the simulated flue gas was 2 L/min.The inlet NO concentrations were in the range of 100-900 ppm, and the inlet NO 2 concentrations were 0 ppm.NaClO 2 concentrations in the solutions were 0.01, 0.02, 0.04, and 0.08 mol•L −1 , and the initial solution pH values were 10.1, 10.3, 10.5, and 10.7 accordingly.As shown in Figure 7, when the NaClO 2 concentration in the solution was 0.01 mol•L −1 , the NO removal efficiency decreased from 50.5% to 12.7% with the increase of the inlet NO concentration from 100 to 900 ppm. Figure 8 presents the change of the molar ratio between NO in the flue gas and NaClO 2 in the mist with the inlet NO concentration.It can be seen that with the inlet NO concentration increasing from 100 to 900 ppm, the molar ratio between NO in the flue gas and NaClO 2 in the mist increased from 1.6 to 13.0.Generally, the increase of molar ratio would promote the mass-transfer driving force of NO, which improves the oxidation and absorption of NO.However, NO concentration in the gas was relatively much higher than that of the NaClO 2 mist, it seemed to be reasonable to obtain a declining NO removal rate [39].As shown in Figure 7, when the NO concentration changed from 100 ppm to 300 ppm, the NO removal efficiency for 0.01 mol•L −1 NaClO 2 concentration decreased from 50.5% to 41.5%, but the NO removal efficiency for 0.02 mol•L −1 NaClO 2 concentration increased from 72.1% to 78.8%.That is because the NaClO 2 concentration of 0.01 mol•L −1 was a little low in this experiment, and so the reaction rate between NaClO 2 and NO played a dominant role at that moment.Thus, there was a decline trend in the NO removal efficiency for 0.01 mol•L −1 NaClO 2 concentration.When the NaClO 2 concentration in the solution increased from 0.01 mol•L −1 to 0.02 mol•L −1 , the increase of the NaClO 2 concentration will enhance the mass transfer rate at the liquid-gas interface to some extent.Therefore, the NO removal efficiency for 0.02 mol•L −1 NaClO 2 concentration increased at the beginning, and then decreased slowly after further increasing the inlet NO concentration from 300 ppm to 900 ppm.The change of the NOx removal efficiency with inlet NO concentration is shown in Figure 9.It can be seen that when the NaClO2 concentrations in the solution were ≤0.02 mol•L −1 , the NOx removal efficiency increased as the inlet NO concentration increased from 100 ppm to 300 ppm, and then decreased gradually on further increasing inlet NO concentration from 300 ppm to 900 ppm.When NaClO2 concentrations in the solution were in the range of 0.04-0.08mol•L −1 , the NOx removal efficiency increased as the inlet NO concentration increased from 100 ppm to 500 ppm, and then decreased gradually on further increasing inlet NO concentration from 500 ppm to 900 ppm.This indicates that the mass transfer between NaClO2 in the mist and NO in the flue gas imposed a great impact on the NOx removal efficiency.At the beginning, the increase of the NaClO2 concentration in mist and the NO concentration in the flue gas will enhances the mass transfer rate, resulting in an obvious increase in the NOx removal efficiency.However, with the increase of the NaClO2 concentration in the mist and NO concentration in flue gas, the reaction rate at the liquid-gas   The change of the NOx removal efficiency with inlet NO concentration is shown in Figure 9.It can be seen that when the NaClO2 concentrations in the solution were ≤0.02 mol•L −1 , the NOx removal efficiency increased as the inlet NO concentration increased from 100 ppm to 300 ppm, and then decreased gradually on further increasing inlet NO concentration from 300 ppm to 900 ppm.When NaClO2 concentrations in the solution were in the range of 0.04-0.08mol•L −1 , the NOx removal efficiency increased as the inlet NO concentration increased from 100 ppm to 500 ppm, and then decreased gradually on further increasing inlet NO concentration from 500 ppm to 900 ppm.This indicates that the mass transfer between NaClO2 in the mist and NO in the flue gas imposed a great impact on the NOx removal efficiency.At the beginning, the increase of the NaClO2 concentration in mist and the NO concentration in the flue gas will enhances the mass transfer rate, resulting in an obvious increase in the NOx removal efficiency.However, with the increase of the NaClO2 concentration in the mist and NO concentration in flue gas, the reaction rate at the liquid-gas The change of the NO x removal efficiency with inlet NO concentration is shown in Figure 9.It can be seen that when the NaClO 2 concentrations in the solution were ≤0.02 mol•L −1 , the NO x removal efficiency increased as the inlet NO concentration increased from 100 ppm to 300 ppm, and then decreased gradually on further increasing inlet NO concentration from 300 ppm to 900 ppm.When NaClO 2 concentrations in the solution were in the range of 0.04-0.08mol•L −1 , the NO x removal efficiency increased as the inlet NO concentration increased from 100 ppm to 500 ppm, and then decreased gradually on further increasing inlet NO concentration from 500 ppm to 900 ppm.This indicates that the mass transfer between NaClO 2 in the mist and NO in the flue gas imposed a great impact on the NO x removal efficiency.At the beginning, the increase of the NaClO 2 concentration in mist and the NO concentration in the flue gas will enhances the mass transfer rate, resulting in an obvious increase in the NO x removal efficiency.However, with the increase of the NaClO 2 concentration in the mist and NO concentration in flue gas, the reaction rate at the liquid-gas interface becomes dominant.Thus, NO x removal efficiency decreased slowly on further increasing the inlet NO concentration.
Energies 2018, 11, x 10 of 15 interface becomes dominant.Thus, NOx removal efficiency decreased slowly on further increasing the inlet NO concentration.When the NaClO2 concentration in the solution was higher than 0.04 mol•L −1 and the molar ratio of NO and NaClO2 was below 2, a complete removal of NO could be obtained, indicating that all of NO had been oxidized by the NaClO2 mist.But when the NaClO2 concentration in the solution was lower than 0.02 mol•L −1 , it was difficult to remove NO completely from the flue gas even if the molar ratio of NO and NaClO2 was below 2. It could be ascribed to the mass-transfer between NO and NaClO2.When the NaClO2 concentration in the mist was relatively low, it imposed a negative effect on the mass-transfer rate [40].The lower the NaClO2 concentration in mist was, the more obvious the adverse effect was.When the NaClO2 concentration in the solution was 0.02 mol•L −1 , with the inlet NO concentration increasing from 100 to 300 ppm, NO removal efficiency increased from 72.1% to 78.8%.However, on further increasing the inlet NO concentration, the NO removal efficiency began to decrease gradually.This also resulted from the change of the mass transfer efficiency between NO in gas phase and NaClO2 in liquid phase.

Effect of Solution pH
The effect of the NaClO2 solution pH on the NO removal performance was investigated, and the results are shown in Figure 10.The flow rate of the simulated flue gas was 2 L/min.The NO and NO2 concentrations in the inlet gas were 500 and 0 ppm, respectively.The NaClO2 concentrations in the solution were 0.02 mol•L −1 .The initial solution pH values were adjusted to be in the range of 4-12.As shown in Figure 10, both the NO and NOx removal efficiencies were kept almost stable when the initial NaClO2 solution pH was changed in the range of 4-11.However, when the solution pH increased from 11 to 12, the NO removal efficiency decreased sharply from 55.3% to 4.4%.It indicated that a strong alkaline medium greatly suppresses the oxidative ability of NaClO2 in the mist [41].The results illustrated that for the ultrasonic atomization process, NaClO2 could reach a relatively stable and high NOx removal efficiency in a wide range of pH, which could be very favorable for industrial application.When the NaClO 2 concentration in the solution was higher than 0.04 mol•L −1 and the molar ratio of NO and NaClO 2 was below 2, a complete removal of NO could be obtained, indicating that all of NO had been oxidized by the NaClO 2 mist.But when the NaClO 2 concentration in the solution was lower than 0.02 mol•L −1 , it was difficult to remove NO completely from the flue gas even if the molar ratio of NO and NaClO 2 was below 2. It could be ascribed to the mass-transfer between NO and NaClO 2 .When the NaClO 2 concentration in the mist was relatively low, it imposed a negative effect on the mass-transfer rate [40].The lower the NaClO 2 concentration in mist was, the more obvious the adverse effect was.When the NaClO 2 concentration in the solution was 0.02 mol•L −1 , with the inlet NO concentration increasing from 100 to 300 ppm, NO removal efficiency increased from 72.1% to 78.8%.However, on further increasing the inlet NO concentration, the NO removal efficiency began to decrease gradually.This also resulted from the change of the mass transfer efficiency between NO in gas phase and NaClO 2 in liquid phase.

Effect of Solution pH
The effect of the NaClO 2 solution pH on the NO removal performance was investigated, and the results are shown in Figure 10.The flow rate of the simulated flue gas was 2 L/min.The NO and NO 2 concentrations in the inlet gas were 500 and 0 ppm, respectively.The NaClO 2 concentrations in the solution were 0.02 mol•L −1 .The initial solution pH values were adjusted to be in the range of 4-12.As shown in Figure 10, both the NO and NO x removal efficiencies were kept almost stable when the initial NaClO 2 solution pH was changed in the range of 4-11.However, when the solution pH increased from 11 to 12, the NO removal efficiency decreased sharply from 55.3% to 4.4%.It indicated that a strong alkaline medium greatly suppresses the oxidative ability of NaClO 2 in the mist [41].The results illustrated that for the ultrasonic atomization process, NaClO 2 could reach a relatively stable and high NO x removal efficiency in a wide range of pH, which could be very favorable for industrial application.

Parallel Tests and Reaction Chemistry
A group of parallel tests were carried out to investigate the repeatability and reproducibility of the NO removal efficiency by ultrasonic atomizing NaClO2 solution, and the results are shown in Figure 11.The flow rate of the simulated flue gas was 2 L/min.NO and NO2 concentrations in the inlet gas were 700 and 0 ppm, respectively.The NaClO2 concentrations in the solution were 0.04 mol•L −1 with the corresponding pH value of 10.5.As shown in Figure 11, the minimum and maximum NO removal efficiencies were 78.2% and 82.1%, respectively.The average NO removal efficiency was 80.3%.The minimum and maximum NOx removal efficiencies were 47.95% and 51.01%, respectively.The averaged NOx removal efficiency was 49.75%.The results demonstrate that the NO removal process based on the ultrasonic atomization method possesses an excellent repeatability and stability as was shown in our experiments.Compared with traditional wet scrubbing modes, such as spraying, bubbling, and packing, the liquid-gas ratio for the ultrasonic atomization process is much lower.For example, the liquid-gas ratio for our experiment was only ~0.3.In addition, the NO removal efficiency reached almost 100%

Parallel Tests and Reaction Chemistry
A group of parallel tests were carried out to investigate the repeatability and reproducibility of the NO removal efficiency by ultrasonic atomizing NaClO 2 solution, and the results are shown in Figure 11.The flow rate of the simulated flue gas was 2 L/min.NO and NO 2 concentrations in the inlet gas were 700 and 0 ppm, respectively.The NaClO 2 concentrations in the solution were 0.04 mol•L −1 with the corresponding pH value of 10.5.As shown in Figure 11, the minimum and maximum NO removal efficiencies were 78.2% and 82.1%, respectively.The average NO removal efficiency was 80.3%.The minimum and maximum NO x removal efficiencies were 47.95% and 51.01%, respectively.The averaged NO x removal efficiency was 49.75%.The results demonstrate that the NO removal process based on the ultrasonic atomization method possesses an excellent repeatability and stability as was shown in our experiments.

Parallel Tests and Reaction Chemistry
A group of parallel tests were carried out to investigate the repeatability and reproducibility of the NO removal efficiency by ultrasonic atomizing NaClO2 solution, and the results are shown in Figure 11.The flow rate of the simulated flue gas was 2 L/min.NO and NO2 concentrations in the inlet gas were 700 and 0 ppm, respectively.The NaClO2 concentrations in the solution were 0.04 mol•L −1 with the corresponding pH value of 10.5.As shown in Figure 11, the minimum and maximum NO removal efficiencies were 78.2% and 82.1%, respectively.The average NO removal efficiency was 80.3%.The minimum and maximum NOx removal efficiencies were 47.95% and 51.01%, respectively.The averaged NOx removal efficiency was 49.75%.The results demonstrate that the NO removal process based on the ultrasonic atomization method possesses an excellent repeatability and stability as was shown in our experiments.Compared with traditional wet scrubbing modes, such as spraying, bubbling, and packing, the liquid-gas ratio for the ultrasonic atomization process is much lower.For example, the liquid-gas ratio for our experiment was only ~0.3.In addition, the NO removal efficiency reached almost 100% Compared with traditional wet scrubbing modes, such as spraying, bubbling, and packing, the liquid-gas ratio for the ultrasonic atomization process is much lower.For example, the liquid-gas ratio for our experiment was only ~0.3.In addition, the NO removal efficiency reached almost 100% when the molar ratio of NO in the flue gas and NaClO 2 in the mist was below 2. It implied that the reaction between NO in the flue gas and NaClO 2 in the mist is extremely efficient, and the utilization of NaClO 2 is very high.This is mainly ascribed to the gas-mist reaction mode, in which the diameters of the NaClO 2 mist generated by ultrasonic atomization are very small.Thus, the gas-liquid contact area is much larger than that found in traditional wet scrubbing processes, and it enhances the mass-transfer rate at the gas-liquid interface to a great extent [31].Here the diameters of the NaClO 2 mist generated by ultrasonic atomization could be approximately calculated according to Lang's relation [42,43]: where d is the mist diameter (m), σ is the surface tension coefficient (7.275 × 10 −2 N•m −1 ), is the liquid density (1.0 × 10 3 kg•m −3 ), and F is the forcing sound frequency (1.67 × 10 6 Hz).The diameter of the NaClO 2 mist in this study is calculated to be ~2.95µm, which is much smaller than those of the spraying droplets.It is favorable for increasing the contact area between the gas phase and liquid phase.Thus, the ultrasonic atomization process is favorable when attempting to achieve a high NO removal efficiency when compared with traditional wet scrubbing methods.
According to the experimental results mentioned above, one can deduce that NO was effectively oxidized into NO 2 by the NaClO 2 mist during the ultrasonic atomization process.The possible reaction pathways are summarized in Figure 12: Energies 2018, 11, x 12 of 15 when the molar ratio of NO in the flue gas and NaClO2 in the mist was below 2. It implied that the reaction between NO in the flue gas and NaClO2 in the mist is extremely efficient, and the utilization of NaClO2 is very high.This is mainly ascribed to the gas-mist reaction mode, in which the diameters of the NaClO2 mist generated by ultrasonic atomization are very small.Thus, the gasliquid contact area is much larger than that found in traditional wet scrubbing processes, and it enhances the mass-transfer rate at the gas-liquid interface to a great extent [31].Here the diameters of the NaClO2 mist generated by ultrasonic atomization could be approximately calculated according to Lang's relation [42,43]: where d is the mist diameter (m), σ is the surface tension coefficient (7.275 × 10 −2 N•m −1 ), ρ is the liquid density (1.0 × 10 3 kg•m −3 ), and F is the forcing sound frequency (1.67 × 10 6 Hz).The diameter of the NaClO2 mist in this study is calculated to be ~2.95μm, which is much smaller than those of the spraying droplets.It is favorable for increasing the contact area between the gas phase and liquid phase.Thus, the ultrasonic atomization process is favorable when attempting to achieve a high NO removal efficiency when compared with traditional wet scrubbing methods.
According to the experimental results mentioned above, one can deduce that NO was effectively oxidized into NO2 by the NaClO2 mist during the ultrasonic atomization process.The possible reaction pathways are summarized in Figure 12:

Conclusions
NaClO2 mist generated by the ultrasonic atomization process was used to remove NO from simulated flue gas, and the effects of various operating parameters on the NO removal efficiency were investigated preliminarily.Compared with other oxidants such as NaClO and H2O2, NaClO2 mist could achieve a much higher NO removal efficiency because of its strong oxidative ability.With the increase in gas flow from 1.5 to 3.0 L•min −1 , the atomizing rate increased almost linearly from 0.38 to 0.85 mL•min −1 .However, a complete NO removal efficiency was only achieved at the gas flow rate of 2.0 L•min −1 .NO removal efficiency increased obviously with the increasing of the NaClO2 concentration in the solution from 0.01 to 0.08 mol•L −1 .The increase of the inlet NO concentration resulted in a decrease in the molar ratio between NO in the flue gas and NaClO2 in the mist.When the NaClO2 concentration in the solution was higher than 0.04 mol•L −1 and the molar ratio of NO and NaClO2 was below 2, a complete removal of NO could be obtained.When the initial pH values of the NaClO2 solution were in the range of 4-11, NO removal efficiency for the NaClO2 mist was relatively stable.However, it decreased sharply to 4.4% when the solution pH increased up to 12; this was because a strong alkaline medium greatly suppressed the oxidative ability of NaClO2 in the mist.The parallel tests indicated that the ultrasonic atomization process possessed excellent repeatability and stability for NO removal applications.The possible reaction pathways were also discussed.

Conclusions
NaClO 2 mist generated by the ultrasonic atomization process was used to remove NO from simulated flue gas, and the effects of various operating parameters on the NO removal efficiency were investigated preliminarily.Compared with other oxidants such as NaClO and H 2 O 2 , NaClO 2 mist could achieve a much higher NO removal efficiency because of its strong oxidative ability.With the increase in gas flow from 1.5 to 3.0 L•min −1 , the atomizing rate increased almost linearly from 0.38 to 0.85 mL•min −1 .However, a complete NO removal efficiency was only achieved at the gas flow rate of 2.0 L•min −1 .NO removal efficiency increased obviously with the increasing of the NaClO 2 concentration in the solution from 0.01 to 0.08 mol•L −1 .The increase of the inlet NO concentration resulted in a decrease in the molar ratio between NO in the flue gas and NaClO 2 in the mist.When the NaClO 2 concentration in the solution was higher than 0.04 mol•L −1 and the molar ratio of NO and NaClO 2 was below 2, a complete removal of NO could be obtained.When the initial pH values of the NaClO 2 solution were in the range of 4-11, NO removal efficiency for the NaClO 2 mist was relatively stable.However, it decreased sharply to 4.4% when the solution pH increased up to 12; this was because a strong alkaline medium greatly suppressed the oxidative ability of NaClO 2 in the mist.The parallel tests indicated that the ultrasonic atomization process possessed excellent repeatability and stability for NO removal applications.The possible reaction pathways were also discussed.

Figure 2 .
Figure 2. The change of NO removal efficiencies with different oxidants in mist.

Figure 2 .
Figure 2. The change of NO removal efficiencies with different oxidants in mist.

Figure 3 .
Figure 3.The change of atomizing rate of the NaClO2 solution with gas flow.

Figure 4
Figure4presents the effect of gas flow on NOx removal efficiency.The concentration of the NaClO2 in the solution was 0.04 mol•L −1 , and the initial pH value was 10.4.The inlet concentrations of NO and NO2 were 500 and 0 ppm, respectively.The flow rate of the simulated flue gas was in the range of 1.5-3.0L•min −1 .As shown in Figure4, the NO removal efficiency increased from 78.3% to 100% as the gas flow increased from 1.5 to 2.0 L•min −1 .Accordingly, the NOx removal efficiency increased from 46.2% to 61.5%.However, on further increasing the gas flow from 2.0 to 3.0 L•min −1 , both the NO and NOx removal efficiencies dropped slowly.Generally, the atomizing rate of the ultrasonic humidifier is mainly affected by the solution surface tension, the liquid height, the ultrasonic power, and gas flow.The liquid height and the ultrasonic power were kept constant in our experiments.As the concentration of the oxidant in the solution was comparatively low, the effect of the oxidant additive on the surface tension of the deionized water solution was neglected.Thus, the flow rate of the simulated flue gas became the major factor that determined the atomizing rate of the NaClO2 solution.On one hand, the gas disturbance in the ultrasonic humidifier would be enhanced with the increase of gas flow, which increased the probabilities of inelastic collision and aggregation between the mist droplets after they left the liquid surface.This might adversely increase the diameter of the mist droplets.On the other hand, the mist droplets could quickly leave the atomization zone when the gas flow increased.To some extent, it was helpful to decrease the probabilities of inelastic collision and aggregation, resulting in more mist droplets flowing out together with the flue gas as the gas flow increased.This might be the reason for the change in the NOx removal efficiency as the gas flow increased from 1.5 to 2.0 L•min −1 .When the gas flow increased from 2.0 to 3.0 L•min −1 , the residence time in the column reactor decreased obviously, though the atomizing rate of the NaClO2 solution had increased proportionally.The less the residence time was, the lower the NOx removal efficiency was.Therefore, a gas flow of 2 L•min −1 was chosen for the subsequent experiments in order to achieve a relatively high NOx removal efficiency.

Figure 3 .
Figure 3.The change of atomizing rate of the NaClO 2 solution with gas flow.

Figure 4
Figure4presents the effect of gas flow on NO x removal efficiency.The concentration of the NaClO 2 in the solution was 0.04 mol•L −1 , and the initial pH value was 10.4.The inlet concentrations of NO and NO 2 were 500 and 0 ppm, respectively.The flow rate of the simulated flue gas was in the range of 1.5-3.0L•min −1 .As shown in Figure4, the NO removal efficiency increased from 78.3% to 100% as the gas flow increased from 1.5 to 2.0 L•min −1 .Accordingly, the NO x removal efficiency increased from 46.2% to 61.5%.However, on further increasing the gas flow from 2.0 to 3.0 L•min −1 , both the NO and NO x removal efficiencies dropped slowly.Generally, the atomizing rate of the ultrasonic humidifier is mainly affected by the solution surface tension, the liquid height, the ultrasonic power, and gas flow.The liquid height and the ultrasonic power were kept constant in our experiments.As the concentration of the oxidant in the solution was comparatively low, the effect of the oxidant additive on the surface tension of the deionized water solution was neglected.Thus, the flow rate of the simulated flue gas became the major factor that determined the atomizing rate of the NaClO 2 solution.On one hand, the gas disturbance in the ultrasonic humidifier would be enhanced with the increase of gas flow, which increased the probabilities of inelastic collision and aggregation between the mist droplets after they left the liquid surface.This might adversely increase the diameter of the mist droplets.On the other hand, the mist droplets could quickly leave the atomization zone when the gas flow increased.To some extent, it was helpful to decrease the probabilities of inelastic collision and aggregation, resulting in more mist droplets flowing out together with the flue gas as the gas flow increased.This might be the reason for the change in the NO x removal efficiency as the gas flow increased from 1.5 to 2.0 L•min −1 .When the gas flow increased from 2.0 to 3.0 L•min −1 , the residence time in the column reactor decreased obviously, though the atomizing rate of the NaClO 2 solution had increased proportionally.The less the residence time was, the lower the NO x removal efficiency was.Therefore, a gas flow of 2 L•min −1 was chosen for the subsequent experiments in order to achieve a relatively high NO x removal efficiency.

Figure 4 .
Figure 4. Effect of gas flow on the NOx removal efficiencies.

Figure 5
Figure 5 shows the effect of NaClO2 concentration in the solution on the change of the outlet concentration of NO in the flue gas during the denitrification process.The gas flow of the simulated flue gas was 2 L/min.The inlet concentrations of NO and NO2 were 700 and 0 ppm, respectively.NaClO2 concentrations in the solutions were 0.01, 0.02, 0.04, and 0.08 mol•L −1 .The corresponding pH values of the NaClO2 solutions without adjustment were 10.1, 10.3, 10.5, and 10.7, respectively.It can be seen that when there was no addition of HCl or NaOH to adjust the initial solution pH, the pH values of the NaClO2 solution increased almost linearly with the increment of the NaClO2 oxidant.As shown in Figure5, when the NaClO2 mist was introduced into the reactor at the beginning, the NO concentration in outlet gas dropped quickly.The higher the NaClO2 concentration in the solution was, the lower the outlet NO concentration was.When the NaClO2 concentration in the solution was 0.08 mol/L, NO had been removed completely.

Figure 5 .
Figure 5.The effect of NaClO2 concentration on the change of NO concentration in outlet gas during the denitrification process.

Figure 4 .
Figure 4. Effect of gas flow on the NOx removal efficiencies.

3. 3 .
Figure 5 shows the effect of NaClO 2 concentration in the solution on the change of the outlet concentration of NO in the flue gas during the denitrification process.The gas flow of the simulated flue gas was 2 L/min.The inlet concentrations of NO and NO 2 were 700 and 0 ppm, respectively.NaClO 2 concentrations in the solutions were 0.01, 0.02, 0.04, and 0.08 mol•L −1 .The corresponding pH values of the NaClO 2 solutions without adjustment were 10.1, 10.3, 10.5, and 10.7, respectively.It can be seen that when there was no addition of HCl or NaOH to adjust the initial solution pH, the pH values of the NaClO 2 solution increased almost linearly with the increment of the NaClO 2 oxidant.As shown in Figure5, when the NaClO 2 mist was introduced into the reactor at the beginning, the NO concentration in outlet gas dropped quickly.The higher the NaClO 2 concentration in the solution was, the lower the outlet NO concentration was.When the NaClO 2 concentration in the solution was 0.08 mol/L, NO had been removed completely.

Figure 4 .
Figure 4. Effect of gas flow on the NOx removal efficiencies.

Figure 5 .
Figure 5.The effect of NaClO2 concentration on the change of NO concentration in outlet gas during the denitrification process.

Figure 5 .
Figure 5.The effect of NaClO 2 concentration on the change of NO concentration in outlet gas during the denitrification process.

Figure 6 .
Figure 6.The change of the NOx removal efficiency and outlet NO2 concentrations with NaClO2 concentration in the solution.

Figure 6 .
Figure 6.The change of the NOx removal efficiency and outlet NO 2 concentrations with NaClO 2 concentration in the solution.

Figure 7 .
Figure 7.The change of the NO removal efficiency with the NO concentration of the inlet gas.

Figure 8 .
Figure 8.The change of the molar ratio of NO and NaClO2 with inlet NO concentration.

Figure 7 .
Figure 7.The change of the NO removal efficiency with the NO concentration of the inlet gas.

Figure 7 .
Figure 7.The change of the NO removal efficiency with the NO concentration of the inlet gas.

Figure 8 .
Figure 8.The change of the molar ratio of NO and NaClO2 with inlet NO concentration.

Figure 8 .
Figure 8.The change of the molar ratio of NO and NaClO 2 with inlet NO concentration.

Figure 9 .
Figure 9.The change of the NOx removal efficiency with the NO concentration of the inlet gas.

Figure 9 .
Figure 9.The change of the NO x removal efficiency with the NO concentration of the inlet gas.

Figure 10 .
Figure 10.Effect of the pH value on the NO and NOx removal efficiencies.

Figure 11 .
Figure 11.The effect of repetition times on NOx removal efficiency.

Figure 10 .
Figure 10.Effect of the pH value on the NO and NO x removal efficiencies.

Energies 2018, 11 , x 11 of 15 Figure 10 .
Figure 10.Effect of the pH value on the NO and NOx removal efficiencies.

Figure 11 .
Figure 11.The effect of repetition times on NOx removal efficiency.

Figure 11 .
Figure 11.The effect of repetition times on NOx removal efficiency.

Figure 12 .
Figure 12.Reaction pathways of NO with the NaClO2 mist.

Figure 12 .
Figure 12.Reaction pathways of NO with the NaClO 2 mist.