Using Chlorine Dioxide to Remove NO x in Low-Temperature Flue Gas

Abstract

Wet scrubbing is an economical and effective denitration method in low-temperature flue gas. ClO₂ is a highly efficient oxidant and is economical. Using ClO₂ to remove NO can be expected to become a more ideal method. A detailed study of using ClO₂ to remove nitrogen oxides (NO_x) in low-temperature flue gas is performed, and meaningful results have been obtained. With the increase of the molar ratio of ClO₂:NO, from 0.5 to 1.0, the oxidation efficiency of NO increases linearly, from 55% to 90%. With the increase of moisture concentration, from 0.5% to 9%, the NO oxidation efficiency increases slowly, from 75% to 90%. With the increase of flue gas temperature, NO oxidation efficiency and the denitration efficiency decrease slowly. The presence of SO₂ improves the denitration efficiency, from 75% to 80% with 20 ppm SO₂. The acidic absorbent is beneficial to denitration, but will also aggravate the escape of HCl, ∼20 ppm. NO_x is converted into nitrate in the sorbent when the supply of ClO₂ is sufficient, otherwise nitrite will be produced. The NO_x in the outlet gas is almost NO₂. Through experimental research, a new idea for denitration by ClO₂ is proposed, using acid sorbent to oxidize NO and alkali sorbent to absorb NO₂. This study is of great help to the subsequent engineering application.

Introduction

As one of the main air pollutants, nitrogen oxides (NO_x) can form acid rain and acid fog to destroy the ecological environment and combine with volatile organic compounds to form photochemical smog, damaging the ozone layer and causing global warming (Gan et al, 2018; Liang et al, 2021; Tan et al, 2021). To control NO_x emission, great efforts have been made and several technologies have been developed, including selective noncatalytic reduction (Cai et al, 2021; Masera and Hossain, 2021), selective catalytic reduction (SCR) (Lanza et al, 2021; Wu et al, 2020), wet scrubbing (Yang et al, 2018; Zhou and Wang, 2020), adsorption (Liu et al, 2021c; Xie et al, 2019), and electron beam irradiation and so on (Chmielewski et al, 2018; Zwolinska et al, 2020). However, the denitration in low-temperature flue (<200℃) gas is still a difficult problem.

SCR technology is the current industrial denitration technology around the world. However, its operating temperature is ∼350℃, and it is only effective when in contact with hot flue gas. The SCR reactor is usually located behind the economizer and in front of the high-efficiency dust collector and the flue gas desulfurization system (Xu et al, 2017b). The alkali metal in the dust and dust itself, as well as the heavy metal and SO₂, will poison and inactivate the catalyst and greatly shorten its service life (Ma et al, 2021; Wang et al, 2019).

In actual industrial conditions, many boiler flue gases are low temperature (<200℃). For example, the temperature of sintering flue gas is usually 120–180℃. Moreover, due to the limited temperature that can be tolerated by dust remover bag, the flue gas in the process of bag dust collection before desulfurization and denitrification shall be controlled <200℃ (Govindan et al, 2012; Wang et al, 2019).

Wet scrubbing is a very economical and effective denitration method in low-temperature flue gas (Fang et al, 2011). In the previous study (Li et al, 2019; Xia et al, 2015), the desulfurization and denitrification were performed at the same time by wet scrubbing. The desulfurization efficiency was ∼100%. But, the inert nature of NO has posed a persistent problem. NO accounts for >90% of NO_x in the flue gas. The low solubility of NO in the aqueous solution appreciably increases the liquid phase resistance to the mass transfer.

It is very important to oxidize the relatively insoluble NO to the soluble NO₂. Strong oxidants, such as O₃, KMnO₄, Na₂S₂O₈, H₂O₂, and NaClO₂, have been used to oxidize NO (Hu et al, 2018; Li et al, 2019; Liu et al, 2021a; Lourens et al, 2016; Zhao et al, 2020). The cost of these oxidants, as well as ClO₂, is listed in Table 1. Notably, the required molar quantity of oxidants in Table 1 is only the theoretical value. The usage in practical application is always higher than the theoretical value. For example, to obtain a satisfactory denitrification efficiency, the molar ratio of O₃ to NO is usually up to 1.5:1–2:1 in practical engineering applications (Ma et al, 2016). As shown in Table 1, ClO₂ and H₂O₂ have obvious economic advantages. However, the oxidation efficiency of H₂O₂ is low, only ∼30% (Han and Zhong, 2011). In terms of economy, ClO₂ is a more promising NO oxidant.

Table 1.

The Economy of ClO₂ Compared with Other Oxidants

Chemical	Purity (%)	Industrial price ($/kg)	Pure chemical price ($/kg)	Molar quantity of chemical required (mol/mol NO)	Consumption (kg/kg NO)	Cost ($/kg NO)
ClO₂	99	1.53	1.55	0.4	0.027	0.042
KMnO₄	99	2.37	2.39	0.67	0.11	0.252
Na₂S₂O₈	99	1.00	1.01	0.50	0.12	0.121
H₂O₂	27.5	0.42	1.52	1.00	0.12	0.052
NaClO₂	80	0.98	1.22	0.75	0.08	0.082
O₃		1.39	1.39	1.00	0.05	0.070

Note: the price of O₃ is calculated based on 10 kWh of industrial power per kilogram.

ClO₂ is a highly efficient oxidant with 1.511 V standard electrode potential. ClO₂ has been widely used as a bleaching agent in paper-making industry (Kaur et al, 2017; Song et al, 2019). Researchers studied NO removal with ClO₂ solution in a bubble reactor, and achieved a denitration efficiency of ∼60% (Deshwal and Hyung-Keun, 2009; Jin et al, 2006). While the spray tower is more suitable for industrial application, the research of denitration by using ClO₂ in spray tower is still few. To provide reference for engineering application, a detailed study for using ClO₂ to remove NO_x in low-temperature flue gas is carried out, including the kinetic simulation of NO oxidation and the experimental study in a spray tower.

Simulation Details and Experimental Setup

Simulation details

A detailed model consisting of 106 chemical reactions is established for kinetic simulation (Supplementary Table S1). The kinetic and thermodynamics parameters are obtained from the CHEMKIN database and the National Institute of Standards and Technology chemical species database (C.K.D.o.t.).

According to the SENKIN module of CHEMKIN package (Kee et al, 1996), the kinetic simulation and sensitivity analysis are carried out. The key elementary reactions are as follows: $C l O_{2} + N O = N O_{2} + C l O R 1$ $C l O_{2} + C l = C l O + C l O R 2$

C l O + C l O = C l_{2} + O_{2} R 7

C l O + N O = N O_{2} + C l R 8

C l O + S O_{2} = C l + S O_{3} R 9

The reactions R1, R2, R7, and R8 are the key reactions affecting the oxidation of NO. The reactions R7, R8, and R9 are the key reactions affecting the oxidation of SO₂. The reaction R3 (ClO₂ + SO₂ = ClO + SO₃) has less influence on the oxidation of SO₂ because of its relatively slow reaction rate.

To verify and compare with the experimental result in the literature, the PSR reactor in CHEMKIN package (Kee et al, 1996) is used for kinetic simulation, and the diffusion mixing correction is carried out, with the correction coefficient of 0.2. For studying NO oxidation, referring to the experimental study of Yang Di (Yang, 2016), the same conditions (initial concentration of NO is 100 ppm, water content is 0.45%, O₂ content is 6%, residence time is 5 s) are selected.

Comparison results (Fig. 1) show that, when ClO₂:NO molar ratio is 0.5 and 0.7, the calculated results of NO oxidation rate are slightly higher than the experimental results. And when ClO₂:NO molar ratio is 0.9, the calculated rate of NO oxidation is slightly lower than the experimental result. The simulation results are basically in agreement with the experimental results.

FIG. 1.

Kinetic simulation and literature experimental results comparison. (a) NO oxidation, (b) simultaneous oxidation of NO and SO₂.

In view of the simultaneous oxidation of NO and SO₂, the experiment of Mimi et al. is used as a reference (Mi, 2014); the same conditions (initial concentration of NO is 300 ppm, initial concentration of SO₂ is 1,000 ppm, water content is 8%, O₂ content is 6%, and gas temperature is 70℃) are selected. The simulation results are also in good agreement with the experimental results.

Experimental setup

The experimental platform (Fig. 2) mainly consists of gas simulating system, spray absorption system, and analysis system.

FIG. 2.

Schematic diagram of the experimental apparatus.

Gas simulating system

NO is used to simulate the NO_x in the actual flue gas. N₂ is the equilibrium gas. All gases are supplied by cylinder gas. The temperature of flue gas and the concentration of each gas component can be controlled accurately. All the gases are metered through mass flowmeters (MFC, Beijing seven-star electronics Co., Ltd., China).

Spray absorption system

The absorption of NO is conducted in a spray empty absorption tower with 400 mm length and 76 mm internal diameter. The simulated gas continuously flows through the system. The absorption liquid is collected under the tower in a cylindrical tank. And the absorption liquid is continuously pumped to the top of the tower from the cylindrical tank. A pH-electrode (PHSJ-4A PH meter; LeiCi Instruments) is immersed into the liquid to measure the pH value. The liquid storage tank is equipped with temperature control device, which can control the temperature of absorption solution. The spray flow rate can be controlled by peristaltic pump to adjust the liquid–gas ratio.

Analysis system

The system includes flue gas analyzer and ion chromatography. The concentrations of NO, NO₂, SO₂, and other flue gas components are analyzed by continuous emissions monitors (Rosemount Analytical NGA2000; Emerson Process Management Co., Ltd.). And the ion chromatography (883 basic Ion chromatography analysis, Metrohm measuring instruments Co., Ltd.) is used to analyze the concentration of nitrate and sulfate in absorbent solution.

NO_x concentration is measured at the inlet and outlet of the gas. Three-way valves regulate the gas stream, which can be measured. And the system runs steadily for 10 min before measuring. The average value within 8 min after stabilization is taken as the measured value. Through measurement and calculation, the error of using this method to measure the concentration of NO_x in the gas is ∼2%. The removal efficiency of NO_x is defined as $E_{f} = \frac{C_{i n} - C_{o u t}}{C_{i n}} \times 100 %,$

where E _f is the NO_x removal efficiency with the unit of %; C _in is the inlet concentration of NO_x with the unit of ppm; C _out is the outlet concentration of NO_x. In this experiment, the inlet NO_x is almost NO, and the outlet NO_x is almost NO₂.

Results and Discussion

Kinetic simulation of NO oxidation by ClO₂

Based on the detailed reaction mechanism, the kinetic simulation is performed to provide reference and guidance for the subsequent experiments. The influences of temperature, ClO₂:NO molar ratio, residence time, and other factors are investigated. The working conditions of kinetic simulation are summarized in Table 2. Simulation results are illustrated in Figs. 3 and 4, and briefly analyzed as follows:

FIG. 3.

The effect of temperature (kinetic simulation).

FIG. 4.

Results of kinetic simulation of NO oxidation by chlorine dioxide. (a) The effect of ClO₂:NO molar ratio, (b) the effect of residence time, (c) the effect of SO₂ presence, (d) the effect of moisture concentration.

Table 2.

Working Condition of Kinetic Simulation

Influencing factors	NO (ppm)	ClO₂ (ppm)	ClO₂:NO	H₂O (%)	O₂ (%)	SO₂ (ppm)	Residence time (s)	Flue gas temperature (℃)	Solution temperature (℃)
Temperature	200	100	0.5	0.45	6	—	5	50	40
		120	0.6					70
		140	0.7					90
		160	0.8					110
		180	0.9					130
		200	1.0					150
		200	1.0					170
ClO₂:NO molar ratio	200	100	0.5	0.45	6	—	5	—	40
		120	0.6					70
		140	0.7					90
		160	0.8					110
		180	0.9					130
		200	1.0					—
Residence time	200	140	0.7	0.45	6	—	0–10 s (interval: 0.01 s)	90	40
								110
								130
								150
SO₂ presence	200	140	0.7	0.45	6	0	5	70	40
						20
						60
						80
						120
						140
						160
						180
Moisture content	200	140	0.7	2.5	6	60	5	70	40
				4.5
				6.5
				8.5

(1) Temperature: The temperature range of 50–170℃ basically covers the industrial low-temperature flue gas that may be used. As shown in Fig. 3, with the increase of flue gas temperature, the NO oxidation efficiency decreases slowly. When the temperature rises to 170℃, the NO oxidation rate decreases considerably. High temperature is not conducive to the oxidation of NO by ClO₂, which is basically consistent with the experimental results in the literature (Cheng, 2015; Xu et al, 2017a).

(2) ClO₂:NO molar ratio: As shown in Fig. 4a, with the increase of the molar ratio of ClO₂:NO, the oxidation rate of NO increases linearly. When the molar ratio of ClO₂:NO increases from 0.5 to 1.0, the efficiency of NO oxidation increases from 55% to 90%. To obtain 70–80% oxidation efficiency, 0.7–0.8 molar ratio of ClO₂:NO is required.

(3) Residence time: As shown in Fig. 4b, the reaction rate of NO oxidation by ClO₂ is faster, and the reaction residence time of 2–3 s can make NO oxidation reach stable saturation state.

(4) SO₂ presence: As shown in Fig. 4c, the effect of SO₂ is twofold: (1) a small amount of SO₂ promotes the oxidation of NO by ClO₂, and 20 ppm of SO₂ increases the oxidation efficiency of NO from 75% to 80%; (2) more SO₂ inhibits the oxidation, ClO₂ has a certain oxidation ability to SO₂, the oxidation efficiency of SO₂ is 30–50%. With the increase of SO₂ concentration, the oxidation efficiency of NO decreases gradually. And the oxidation efficiency of SO₂ also decreases gradually. This can be attributed to the gradual decrease of the molar ratio of Cl:(NO+SO₂).

(5) Moisture content: As shown in Fig. 4d, the oxidation efficiency of NO increases slowly with the increase of moisture concentration. The reason may be that part of NO₂ is consumed by reacting with H₂O to produce HNO₂ and HNO₃. And the oxidation efficiency of SO₂ increases greatly with the increase of water content. When water content is 9%, the oxidation efficiency of SO₂ reaches 90%. The reason may be that SO₃ is easier to be absorbed by water than NO₂, and the rapid consumption of SO₃ accelerates the oxidation of SO₂ by ClO₂.

Experimental study on the denitration by ClO₂

On the platform of spray empty tower, experimental research on spraying and removal of NO by ClO₂ is carried out. The experiment investigates the effects of flue gas temperature, residence time, liquid–gas ratio and the concentration, the temperature and pH value of ClO₂ solution. This experimental research aims to explore better working conditions.

Effect of ClO₂ concentration

In view of the influence of the concentration of ClO₂, two sets of independent working condition tests are carried out. The test (a) is carried out under the normal temperature denitrification condition: the residence time is 2.72 s, NO concentration is ∼100 ppm, O₂ content is 6%, and the liquid–gas ratio is 16 L/m³, the flue gas temperature and spray liquid temperature are 15℃. The test (b) conditions are as follows: the reaction residence time is 2.72 s, NO concentration is 130 ppm, O₂ content is 6%, liquid–gas ratio is 16 L/m³, the flue gas temperature is 70℃, and the liquid temperature is 40℃.

Experimental results are shown in Supplementary Fig. S1. According to the results of the two groups of tests, when the ClO₂ concentration is 0.1–1 g/L, the denitration rate gradually increases from ∼50% to ∼70%. The appropriate ClO₂ concentration ensures sufficient oxidant supply and is conducive to the oxidation of nitric oxide. When the ClO₂ concentration is >1 g/L, the denitration rate remains basically stable and may decrease slightly.

This may be due to the reaction under appropriate conditions, such as reaction residence time, temperature, and other factors. The reaction rate between ClO₂ concentration of appropriate concentration and NO reaches saturation. In acidic environment, excessive ClO₂ concentration may lead to the disproportionation reactions (6ClO₂ + 3H₂O = 5ClO₃⁻ + Cl⁻ + 6H⁺, ClO₂ + Cl = ClO + ClO), so that the removal efficiency of NO slightly decreases. According to test results, it is better to set the concentration of ClO₂ at 0.5 g/L. At this concentration of ClO₂, >70% denitrification rate can be obtained.

Effects of flue gas temperature

For low-temperature flue gas, experimental research on denitrification in the range of 50–130℃ flue gas temperature is conducted. The experimental conditions are as follows: ClO₂ concentration is 0.5 g/L, reaction residence time is 2.72 s, NO concentration is 130 ppm, oxygen content is 6%, liquid–gas ratio is 16 L/m³, and liquid temperature is 40℃. As shown in Fig. 5a, the denitration rate basically decreases slowly with the increase of flue gas temperature. When the flue gas temperature increases from 50℃ to 130℃, the denitration rate decreases from 69.8% to 62.9%, which indicates that the temperature affects the stability of ClO₂. In the spray state, the introduction of high-temperature flue gas makes ClO₂ disproportionate to generate ClO₂⁻ and ClO₃⁻, resulting in a great decrease in ClO₂ stability (Zhu and Chen, 2005).

FIG. 5.

Experimental results of the denitration by chlorine dioxide. (a) The effect of flue gas temperature, (b) the effect of absorption solution temperature, (c) the effect of pH value of absorption solution, (d) the effect of reaction residence time, (e) the effect of liquid–gas ratio, (f) the effect of SO₂ presence.

Effect of sorbent temperature

The sorbent in spray tower can generally be maintained at 40–60℃ through continuous heat exchange of flue gas. Therefore, the denitrification in the absorbent temperature range of 30–70℃ is investigated. The experimental conditions are similar to those observed in the Effects of Flue Gas Temperature section, and the flue gas temperature is 70℃.

As shown in Fig. 5b, the denitrification efficiency remains stable in the range of 30–70℃. The temperature of the absorbent solution has little effect on the denitrification efficiency. Although the temperature affects the stability of ClO₂, compared with the higher flue gas temperature, the temperature of the sorbent at 30–70℃ has little effect. This indicates that the inhibitory effect of high temperature on the stability of ClO₂ mainly occurs in the spray area.

Effect of pH value of sorbent

The denitrification in the range of pH value 5–11 of the sorbent is investigated. The experimental conditions are similar to those observed in the Effect of Sorbent Temperature section. The temperature of the sorbent is 40℃. The pH value is adjusted by adding nitric acid or sodium hydroxide. As shown in Fig. 5c, acidic sorbent is beneficial to ClO₂ denitrification, and its denitrification efficiency is significantly higher than that of alkaline sorbent. When pH value is 5, the denitrification efficiency is ∼67.1%, and when pH values are 9 and 11, the denitrification efficiencies are only 52.4% and 55.0%, respectively.

This means that the oxidation of ClO₂ is related to the acidity and alkalinity of the solution. The stronger the acidity of the solution, the stronger the oxidation ability of ClO₂. This may be because the pH value of the solution seriously affects the stability of ClO₂. The oxidation of ClO₂ is related to the acidity and basicity of the solution, the more acidic the solution the stronger its oxidation ability (Hoikyung et al, 2008). The appropriate concentration of ClO₂ solution disproportionated at different pH values to form ClO₃⁻.

When the ClO₂ solution is in alkaline environment, ClO₂ will be more easily disproportionated to form ClO₂⁻ and ClO₃⁻ (chlorate): 2ClO₂ + 2OH⁻ = ClO₂⁻ + ClO₃⁻ + H₂O, which greatly reduces the stability of ClO₂. When the ClO₂ solution is in acidic environment, the amount of ClO₂ disproportionation in acidic solutions is slower than that observed in alkaline conditions. The denitrification rate decreases only slightly from 67.1% to 66.0% when the pH of the absorption solution increases from 5 to 7. This is due to the fact that ClO₂ solution is relatively most stable at pH of 6 (Liu et al, 2021b).

Considering the corrosion of the sorbent to steel, the pH value of the sorbent should not be too low. Moreover, excessive acidity of absorbent will lead to more HCl escape, which is analyzed in section 3.3 in detail. In addition, when the pH value of the sorbent increases from 5 to 7, the denitrification efficiency decreases slightly; therefore, the sorbent should be weakly acidic (pH: 5–7). Because NO_x and SO₂ are acidic gases, the pH value of the sorbent will gradually decrease with the system operation. It is necessary to maintain the pH value by slowly adding alkaline compounds such as NaOH. Considering that ClO₂ solution itself is acidic, NaOH and other alkaline compounds should not be mixed with ClO₂ solution directly.

Effect of reaction residence time

Reaction residence time directly determines the height and volume of absorption tower, and has an important impact on process cost. By adjusting the height of spray tower, denitrification in the reaction residence time range of 2.72–6.80 s is studied. The specific test conditions are similar to those observed in the Effect of pH Value of Sorbent section. As shown in Fig. 5d, the denitrification efficiency remains basically stable within the residence time range of 2.72–6.80 s. With the extension of residence time, the increase in denitrification efficiency is very low.

The experimental results are basically consistent with the kinetic simulation, which indicates that the reaction residence time of 2.5–3 s is basically enough. This may be due to the use of atomization nozzles in the experimental spray zone, which makes the sprayed absorption solution fill the entire spray atomization zone with very fine and uniform atomized droplets under peristaltic pump spraying. And this makes the gas–liquid contact more effective, and the NO removal reaction more successful.

The influence of liquid–gas ratio

By adjusting spray flow with peristaltic pump, the denitrification efficiency in the range of liquid–gas ratio 8–16 L/m³ is studied. The experimental conditions are similar to those observed in the Effect of Reaction Residence Time section, and the reaction residence time is 2.72 s. As shown in Fig. 5e, the denitration efficiency remains stable within the range of liquid–gas ratio of 8–16 L/m³ (the flow rate of flue gas is 10 L/min, and the liquid flow rates were 80, 100, 120, 140, and 160 mL/min, respectively).

With the increases of the liquid–gas ratio, the denitrification efficiency increases slowly and remains basically stable. A liquid-to-gas ratio >10 L/m³ is sufficient for ClO₂ to absorb NO. This is because the experimental spraying area adopts the atomization nozzle, which makes the spraying absorption solution fill the whole spraying atomization area with very fine and uniform atomized droplets under the peristaltic pump spraying. And this makes the gas–liquid contact more effective, and the reaction between ClO₂ and NO in solution has been fully reacted.

Effect of the presence of SO₂

The influence of the presence of SO₂ on the denitrification of ClO₂ is studied. The specific test conditions are similar to those observed in the Effect of Reaction Residence Time section. NO concentration is maintained at 130 ppm by adding SO₂ of 0 ppm, 50 ppm, and 100 ppm and comparing each other. As shown in Fig. 5f, at the early stage of trial operation, SO₂ is conducive to ClO₂ denitration when the supply of ClO₂ is sufficient. Especially when SO₂ of 50 ppm is added, the denitrification efficiency increases from ∼50% to 55–60%. As the experiment progresses, ClO₂ is gradually consumed, and the denitrification efficiency begins to decrease.

Figure 5f shows that for the blank test without SO₂, the denitrification efficiency begins to decrease at the turning point time of ∼50 min; on the contrary for the test with 50 and 100 ppm SO₂, the denitrification efficiency begins to decrease at the turning point times of ∼40 and 30 min, respectively. This indicates that SO₂ also can be oxidized and absorbed by ClO₂, which has a competitive effect on the denitrification of ClO₂. The existence of SO₂ will increase the consumption of ClO₂. These experimental results are in good agreement with the kinetic simulation and the experimental results in the literature.

In summary, the presence of SO₂ facilitated ClO₂ denitrification and enhanced the denitrification rate in the early part of the test run when ClO₂ is in sufficient supply. It is due to that, in the presence of a small amount of SO₂ (50 ppm), the following reactions may occur in solution: ClO + SO₂ = Cl + SO₃, ClO₂ + Cl = ClO + ClO, ClO + NO = Cl + NO₂ (Liu et al, 2021b). It leads to a certain degree of improvement in NO removal efficiency. However, the presence of large amounts of SO₂ will inevitably compete with the oxidation of NO by ClO₂, resulting in a significant decrease in NO removal efficiency.

In addition, as shown in Fig. 5f, at the beginning of the spray test, with sufficient ClO₂ supply, the SO₂ removal efficiency is ∼90%. This is basically consistent with the kinetic simulation analysis. In a high-humidity spraying environment, SO₂ can react quickly with water, and can be oxidized by ClO₂ to SO₃, and then enters the liquid phase.

Analysis of outlet gas after denitrification

For the denitrification experiment discussed in the Effect of ClO2 Concentration section, flue gas analyzer is used to detect the flue gas components after denitrification. As shown in Fig. 6a, the results show that NO_x in flue gas after denitrification is NO₂, and no NO has been detected. This indicates that NO has been oxidized and absorbed completely. And thus, it is better to set an alkali tower at the tail to further remove NO₂. Using acid sorbent to oxidize NO and alkali sorbent to absorb NO₂ may be a new idea for efficient denitration.

FIG. 6.

(a) Outlet gas composition analysis. (b) HCl escape and absorbent pH value.

It is also found that there is a certain amount of HCl in the flue gas, 1–20 ppm. According to the analysis reported in the literature (Jin et al, 2006), the reaction of HCl formation is as follows: 5NO +3ClO₂ + 4H₂O → 5HNO₃ + 3HCl. If SO₂ exists, the reaction of HCl formation is as follows: 5SO₂ + 2ClO₂ + 6H₂O → 5H₂SO₄ + 2HCl. It is further found that the HCl escape is negatively correlated with the residual NO₂. As shown in Fig. 6a, HCl escape is negatively correlated with ClO₂ denitrification efficiency. Considering that HCl escape may be related to the pH value of sorbent, so the relationship between HCl escape and pH value of sorbent is tested and analyzed. As shown in Fig. 6b, HCl escape decreases rapidly with the increase of pH value. However, with the increasing pH value, there is decrease in denitrification efficiency. It can be concluded that the sorbent acidity is conducive to denitrification, but it also aggravates HCl escape.

Solution ions analysis after denitrification

The concentration of nitrate and sulfate ions in the sorbent after denitrification is measured and analyzed by ion chromatography. For the denitrification experiment discussed in the Effect of the Presence of SO2 section, the concentration of nitrate and sulfate ions in the sorbent is calculated according to the absorption of NO_x and SO₂ in the process of denitrification. The test results and calculation results are shown in Supplementary Fig. S2. It can be seen from Fig. 5f and Supplementary Fig. S2 that the concentration of ions in sorbent after denitrification is basically consistent with the absorption and removal of NO_x and SO₂ during denitrification.

The more the amount of absorption, the longer the absorption time, and the higher the ion concentration in the absorption solution. For NO_x, the calculated value of ion concentration is ∼50% higher than the detection value of nitrate. The calculated value included nitrate and nitrite. It could be speculated that the mass ratio of nitrite: nitrate is ∼1:2, which may be due to the insufficient supply of ClO₂ in the later stage of denitrification test. For SO₂, the calculated value of ion concentration is ∼5–10% higher than the sulfate detection value. This may be because a small amount of SO₂ converts to SO₃²⁻ and HSO₄⁻.

Conclusion

Use of ClO₂ to remove NO_x in low-temperature flue gas requires better working conditions: ClO₂ concentration is 0.5 g/L, residence time is 2.5–3.0 s, pH value of sorbent is 6–7, the temperature of sorbent is ∼60℃, and liquid–gas ratio is 10–12 L/m³. Moreover, some special experimental phenomena and problems are encountered, such as the effect of SO₂ presence, the pH drop under long-term operation, the escape of HCl, and the treatment of outlet gas. From more in-depth analysis, corresponding solutions are put forward. Furthermore, a new idea for denitration by ClO₂ is proposed; using acid sorbent to oxidize NO and alkali sorbent to absorb NO₂. This study is of great help to the subsequent engineering application.

Footnotes

Authors' Contributions

J.J. and Z.W. carried out the experimental study. Z.W. and Q.H. analyzed the experimental result. Z.W. was a major contributor in writing the article. All authors read and approved the final article.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

This research was supported by Zhejiang Provincial key research and development program (grant no. 2020C03084).

Supplementary Material

References

C.K.D.o.t. U.S. Kinetics Database. Available from: https://kinetics.nist.gov/index.php [Last accessed: April 28, 2022].

Cai

, Zheng

, Wang

. Effects of hydrogen peroxide, sodium carbonate, and ethanol additives on the urea-based SNCR process. Sci Total Environ, 2021; 772:145551–145559; doi: 10.1016/j.scitotenv.2021.145551

Cheng

YB.

Experimental Study on Simultaneous Desulfurization and Denitrification by ClO₂ Oxidation with Calcium Based Absorption. Zhejiang University of Technology: Zhejiang, China; 2015 (In Chinese); doi: CNKI:CDMD:2.1015.434971

Chmielewski

, Zwolińska

, Licki

, et al. A hybrid plasma-chemical system for high-NO_x flue gas treatment. Radiat Phys Chem, 2018; 144:1–7; doi: 10.1016/j.radphyschem.2017.11.005

Deshwal

, Hyung-Keun

LEE

. Mass transfer in the absorption of SO₂ and NO_x using aqueous euchlorine scrubbing solution. J Environ Sci (China), 2009; 21(2):155–161; doi: 10.1016/S1001-0742(08)62244-5

Fang

, Cen

, Tang

, et al. Simultaneous removal of SO2 and NOx by wet scrubbing using urea solution. J Chem Eng, 2011; 168(1):52–59; doi: 10.1016/j.cej.2010.12.030

Gan

, Shi

, Li

, et al. Synergistic promotion effect between NO_x and chlorobenzene removal on MnO_x-CeO₂ catalyst. Acs Appl Mater Inter, 2018; 10(36):30426–30432; doi: 10.1021/acsami.8b10636

Govindan

, Chung

, Jang

, et al. Removal of hydrogen sulfide through an electrochemically assisted scrubbing process using an active Co (III) catalyst at low temperatures. Chem Eng J, 2012; 209:601–606; doi: 10.1016/j.cej.2012.08.017

Han

, Zhong

Experimental study on simultaneous desulfurization and denitrification of H₂O₂ solution. J Environ Eng, 2011; 5(12):5; (In Chinese); doi: 10.5194/acp-1-73-2001

10.

Hoikyung

, Youngjee

, et al. Production and stability of chlorine dioxide in organic acid solutions as affected by pH, type of acid, and concentration of sodium chlorite, and its effectiveness in inactivating Bacillus cereus spores. Food Microbiol, 2008; 25(8):964–969; doi: 10.1016/j.fm.2008.05.008

11.

, Huang

, Zhang

, et al. Facile and template-free fabrication of mesoporous 3D nanosphere-like Mn_xCo₃xO₄ as highly effective catalysts for low temperature SCR of NO_x with NH₃. J Mater Chem A, 2018; 6(7):2952–2963; doi: 10.1039/C7TA08000J

12.

Jin

, Deshwal

, Park

, et al. Simultaneous removal of SO₂ and NO by wet scrubbing using aqueous chlorine dioxide solution. J Hazard Mater, 2006; 135(1–3):412–417; doi: 10.1016/j.jhazmat.2005.12.001

13.

Kaur

, Bhardwaj

, Lohchab

. A study on pulping of rice straw and impact of incorporation of chlorine dioxide during bleaching on pulp properties and effluents characteristics. J Clean Prod, 2017; 170(jan.1):174–182; doi: 10.1016/j.jclepro.2017.09.111

14.

Kee

, Dixon-Lewis

, Warnatz

, et al. Chemkin-III: A Fortran chemical kinetics package for the analysis of gas-phase chemical and plasma kinetics. Sand Rep, 1996; 96(3):142–146; doi: 10.2172/481621

15.

Lanza

, Zheng

, Matarrese

, et al. HCl-doping of V/TiO₂-based catalysts reveals the promotion of NH₃-SCR and the rate limiting role of NO oxidative activation. Chem Eng J, 2021; 416:128933–128938. doi: 10.1016/j.cej.2021.128933

16.

, Wang

, Xu W

, et al. Simultaneous removal of SO2 and NOx from flue gas by wet scrubbing using a urea solution. Environ Technol, 2019; 40(20):2620–2632

17.

, Liu

, Wang

, et al. Oxidation and absorption of SO₂ and NO_x by MgO/Na₂S₂O₈ solution at the presence of Cl. Fuel Process Technol, 2019; 194:106125–106133; doi: 10.1016/j.fuproc.2019.106125

18.

Liang

, Tao

, Mi

, et al. Unraveling the boosting low-temperature performance of ordered mesoporous Cu-SSZ-13 catalyst for NOx reduction. Chem Eng J, 2021; 409(13):128238–128245; doi: 10.1016/j.cej.2020.128238

19.

Liu

, Wang

, Zhu

, et al. Simultaneous removal of SO₂ and NO_x with OH from the catalytic decomposition of H₂O₂ over Fe-Mo mixed oxides. J Hazard Mater, 2021a;404:123936–123946; doi: 10.1016/j.fuproc.2019.106125

20.

Liu

, Wu

, Wang

, et al. A review of basic properties and influencing factors of chlorine dioxide stability. Inorg Chem Ind, 2021b;53(5):18–23(In Chinese); doi: 10.11962/1006-4990.2020-0233

21.

Liu

, You

, Li

, et al. NO_x removal with efficient recycling of NO₂ from iron-ore sintering flue gas: A novel cyclic adsorption process. J Hazard Mater, 2021c;407:124380–124391 doi: 10.1016/j.jhazmat.2020.124380

22.

Lourens

ASM

, Butler

, Beukes

, et al. Investigating atmospheric photochemistry in the Johannesburg-Pretoria megacity using a box model. S Afr J Sci, 2016; 112(1–2):01–11; doi: 10.17159/sajs.2016/2015-0169

23.

, Wang

, Lin

, et al. Characteristics of O₃ oxidation for simultaneous desulfurization and denitration with limestone–gypsum wet scrubbing: Application in a carbon black drying kiln furnace. Energy Fuels, 2016; 30(3):2302–2308; doi:10.1021/acs.energyfuels.5b02717

24.

, Wang

, Yuan

, et al. Co-absorption and reduction mechanism of SO2 and NO2 from flue gas using a Na2SO3 solution with an oxidation inhibitor. Environ Eng Sci, 2021; 38(4):277–284; doi: 10.1089/ees.2020.0324

25.

Masera

, Hossain

. Modified selective non-catalytic reduction system to reduce NO_x gas emission in biodiesel powered engines. Fuel, 2021; 298:120826–120835; doi: 10.1016/j.fuel.2021.120826

26.

Experimental and Mechanism Research on Oxidation of NO and SO₂ by ClO₂ and Synergistic Removal of NO_x-SO₂. Shandong University: Shandong, China; 2014 (In Chinese).

27.

Song

, Yang

, Yu

, et al. Ultra-high efficient hydrodynamic cavitation enhanced oxidation of nitric oxide with chlorine dioxide. Chem Eng J, 2019; 373:767–779; doi: 10.1016/j.cej.2019.05.094

28.

Tan

, Ju

, Huang

, et al. Simulation study of ammonia storage characteristics of selective catalytic reduction for diesel engines. Environ Eng Sci, 2021; 38(7):662–675; doi: 10.1089/ees.2020.0197

29.

Wang

, Li

, Xu

, et al. Simultaneous removal of SO₂ and NOx from flue gas by wet scrubbing using a urea solution. Environ Technol, 2019; 40(20):2620–2632; doi: 10.1080/09593330.2018.1454513

30.

Wang

, Nie

, An

, et al. Improvement of SO₂ resistance of low-temperature Mn-based denitration catalysts by Fe doping. ACS Omega, 2019:4(2):3755–3760; doi: 10.1021/acsomega.9b00002

31.

, Yu

, Huang

, et al. MnOx-decorated VO_x/CeO₂ catalysts with preferentially exposed {110} facets for selective catalytic reduction of NO_x by NH₃. Appl Catal B, 2020; 268:118419–118433; doi: 10.1016/j.apcatb.2019.118419

32.

Xia

, Hu

, He

, et al. Simultaneous photocatalytic elimination of gaseous NO and SO₂ in a BiOI/Al₂O₃-padded trickling scrubber under visible light. J Chem Eng, 2015; 279:929–938; doi: 10.1016/j.cej.2015.05.097

33.

Xie

, Luo

, Zhou

, et al. In situ hydrogen uptake and NO_x adsorption on bifunctional heterogeneous Pd/Mn/Ni for a low energy path toward selective catalytic reduction. ACS Omega, 2019; 4(25):21340–21345 doi: 10.1021/acsomega.9b02945

34.

, Wen

, Liu

, et al. Experimental research of simultaneous desulfurization and denitrification by ClO₂ gas phase oxidation combining with CaCO₃ slurry absorption. Environ Pollut Prevent, 2017a;39(5):504–509 (In Chinese).

35.

, Wu

, Liu

, et al. Effect of barium sulfate modification on the SO₂ tolerance of V2O5/TiO2catalyst for NH3-SCR reaction. J Environ Sci, 2017b;57:110–117; doi: 10.1016/j.jes.2016.12.001

36.

Yang

Oxidation of NO in Flue Gas by ClO₂ Produced by Electrolytic. Beijing University: Beijing, China; 2016.

37.

Yang

, Pan

, Han

, et al. Removal of NO_x and SO₂ from simulated ship emissions using wet scrubbing based on seawater electrolysis technology. Chem Eng J, 2018; 331:8–15; doi: 10.1016/j.cej.2017.08.083

38.

Zhao

, Sun

, Chmielewski

, et al. NO oxidation with NaClO, NaClO₂, and NaClO₃ solution using electron beam and a one stage absorption system. Plasma Chem Plasma Process, 2020; 40(1):433–447; doi: 10.1007/S11090-019-10022-9

39.

Zhou

, Wang

. Study on efficient removal of SO_x and NO_x from marine exhaust gas by wet scrubbing method using urea peroxide solution. Chem Eng J, 2020; 390:124567–124575; doi: 10.1016/j.cej.2020.124567

40.

Zhu

, Chen

Study on the stability of pure chlorine dioxide and its storage method under conventional conditions. Chem Standards Metrol Qual, 2005(09):35–38(In Chinese); doi: CNKI:SUN:HGBJ.0.2005-09-009

41.

Zwolinska

, Sun

, Chmielewski

. Electron beam flue gas technology for SO_x and NO_x simultaneous removal: its process and chemistry evolution from power plants to diesel off-gas treatment. Chem Rev, 2020; 36(8):933–945 doi: 10.1515/revce-2018-0055

Supplementary Material

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Using Chlorine Dioxide to Remove NO x in Low-Temperature Flue Gas

Abstract

Introduction

Simulation Details and Experimental Setup

Simulation details

Experimental setup

Gas simulating system

Spray absorption system

Analysis system

Results and Discussion

Kinetic simulation of NO oxidation by ClO2

Experimental study on the denitration by ClO2

Effect of ClO2 concentration

Effects of flue gas temperature

Effect of sorbent temperature

Effect of pH value of sorbent

Effect of reaction residence time

The influence of liquid–gas ratio

Effect of the presence of SO2

Analysis of outlet gas after denitrification

Solution ions analysis after denitrification

Conclusion

Footnotes

Authors' Contributions

Author Disclosure Statement

Funding Information

Supplementary Material

References

Supplementary Material

Kinetic simulation of NO oxidation by ClO₂

Experimental study on the denitration by ClO₂

Effect of ClO₂ concentration

Effect of the presence of SO₂