Denitrification utilizing a vaporized enhanced-Fenton reagent: kinetics and feasibility

Abstract

This paper proposes a novel integrative process for NO removal, in which NO was initially oxidized by a vaporized enhanced-Fenton reagent (EF), composed of hydrogen peroxide, ferrous and peroxyacetic acid (PAA), then absorbed by Ca(OH)₂. The effects of EF constitution, the reaction temperature, the pH of EF solution and the SO₂ concentration on NO removal were systematically investigated, and the experimental results indicated that both FeSO₄ and PAA can significantly promote the oxidation rate of NO; the decreasing of pH and increasing of temperature played a key role in enhancing NO removal. The NO depletion exhibited a pseudo-first order kinetics pattern in 1–2 half-lives, based on the macrokinetics of NO oxidation. In addition, the rate constants determined in the temperature range of 60 to 120 °C were well fitted to the Arrhenius equation, yielding the apparent activation energy of 14.1 kJ mol⁻¹. The mechanism of NO oxidation was also speculated.

---

1 Introduction

The SO₂ and NO present in coal combustion gases emitted from thermal power stations have brought significant harm to human health and the ecosystem. Therefore, in recent years, both of them have caused considerable concerns. Because China is the largest coal-fired country in the world, Chinese government has tried the best effort to control the air pollution, and a large number of wet flue gas desulfurization systems (WFGD) and selective catalytic reduction systems (SCR) have been installed for the flue gas treatment of coal-fired power plants. However, the SCR-WFGD lay-out has large and complex systems, as well as high capital and operating costs: thus the simultaneous removal technology that has the characteristics of simplified equipment, smaller occupying areas and lower operating cost has good future prospects for development and applications, in which several advanced flue gas treatment technologies, such as electrochemistry,^1,2 gas solid phase adsorption,^3,4 gas solid phase catalysis,^5,6 liquid phase absorption^7,8 and liquid phase oxidation,^9–17 can be attempted for the simultaneous removal of SO₂ and NO. However, because of the high costs or technical limitations, these simultaneous removal technologies still cannot completely replace the combination of WFGD and SCR technologies. Therefore, developing new flue gas treatment technologies has become one of the major development trends in the field of coal-fired flue gas control.

Compared with other developing methods, the oxidation method appears to be one of the promising ways for the simultaneous removal of SO₂ and NO; the core principle of the method is to rapidly oxidize the insoluble NO to NO₂, which is then absorbed by the following flue gas circulation fluid bed (CFB) or WFGDs. The potential reagents that can be used in the oxidation process include O₃, KMnO₄, NaClO₂, NaClO, K₂S₂O₈, KFeO₄ and H₂O₂.^{7–10,18–20} However, some of these classical oxidants either have lower economical efficiencies or may release several hazardous byproducts that can adversely affect the environment. For example, sodium chlorite is considered to be one of the most effective reagents, but it is estimated that about 1.38 pounds of NaClO₂ is needed to remove 1 pound of NO_x; therefore, its cost is prohibitive. In addition, a high removal efficiency of NO can also be obtained by using permanganate; however, its cost is too high to be applied in the industry. Moreover, a large number of heavy metals, manganese and chlorine species that can cause secondary pollution will remain as the byproducts. Ozone is an environmentally benign and effective gas oxidant, but the energy consumption required for its generation is too excessive. Therefore, the development of innovative absorbent for NO removal is urgent, and the research focuses on the following three aspects: relative lower cost, high removal efficiency and less secondary environmental impact.

As mentioned in the above requirements, H₂O₂ appears to be the most suitable basic oxidant because of its environmental friendliness and lower price. However, its weak oxidizability creates difficulties to effectively and completely oxidize NO. Therefore, in this paper, we selected peroxyacetic acetic (PAA) and ferrous as the additives to enhance the oxidizing ability of H₂O₂, and then a H₂O₂-based complex oxidant (CO) was prepared. In addition, a novel flue gas cleaning process is proposed, in which NO was initially oxidized to NO₂ by vaporized CO and then absorbed by Ca(OH)₂. To our knowledge, there are no reports in the field of NO removal based on the usage of a vaporized CO as the novel approach.

The main objective of our research is to evaluate the macrokinetics and technical feasibility of the novel process in removing NO; thus the bench scale kinetics tests were conducted to determine reaction order and apparent activation energy with respect to NO. Factors affecting the treatment, including the CO constitution, the pH of the CO solution, the reaction temperature and the SO₂ concentration, were also assessed.

2 Materials and experiments

2.1 Reagents and preparation of CO solution

The reagents used were analytical grade (Kermel Company, Tianjin). 30% (w/w) of H₂O₂, 16% (w/w) of PAA and 99% (w/w) of FeSO₄·7H₂O were used to prepare the CO solution, in which the fresh solutions of FeSO₄, PAA and H₂O₂ were added to a beaker in turn by using pipettes (10–1000 μL and 1–5 mL) and then mildly shaken. Its pH was adjusted by 1 mol L⁻¹ of H₂SO₄ and 1 mol L⁻¹ of NaOH. In addition, Ca(OH)₂ was employed as the absorbent for absorbing the reaction products of NO, and anhydrous CaCl₂ was used as the dryer to avoid damaging the flue gas analyzer.

2.2 Experimental apparatus and procedures

The bench scale experiments were carried out through a self-designed experimental system, which mainly consisted of the simulated flue gas generation, the vaporization of CO solution, the integration of preoxidation and absorption, and tail gas detection, as shown in Fig. 1. The simulated flue gas was generated from N₂, SO₂, NO, O₂ and CO₂ provided in the compressed cylinders (1–5) (North special gas company, Baoding). A peristaltic pump (12) (BT100-1F, Longerpump, Baoding) was employed to pump CO solution (11) into the vaporization device of CO solution (9) that was heated by a thermally controlled electric heater (10) (ZDHW, Zhongxingweiye company, Beijing). The reactor was a U-type quartz tube (14) having a length of 30 cm and an inner diameter of 2.5 cm, heated by a thermostat oil bath (16) (DC-RB, Duchuang technology company, Beijing). The temperatures of the vaporization device (9) and reactor were detected by thermocouples. The inlet and outlet flue gas were detected by a flue gas analyzer (18) (ECOM-J2KN, RBR Company, Germany), which can detect various gases such as O₂ in a range of 0–21% (±0.01%), CO in a range of 0–10 000 ppm (±10 ppm), NO in a range of 0–5000 ppm (±1 ppm), NO₂ in a range of 0–1000 ppm (±1 ppm) and SO₂ in a range of 0–5000 ppm (±1 ppm), in a working temperature range of 25–85 °C. The pH of the CO solution was detected by a pH meter (PHS-3C, Youke company, Shanghai).

Fig. 1 Schematic diagram of the experimental apparatus of a fix-bed. 1–5 represent CO₂, N₂, SO₂, NO and O₂ gas cylinders; 6-flowmeters; 7-buffer bottle; 8-tee joint; 9-vaporization device; 10-thermally controlled electric heater; 11-CO solution; 12-peristaltic pump; 13-thermocouple; 14-reactor; 15-Ca(OH)₂; 16-thermostat oil bath; 17-dryer; 18-flue gas analyzer.

During the experiments, NO and N₂ were monitored through a mass flow controller (6) and mixed in a buffer bottle (7), in which NO and the other coexistence gases were diluted by N₂ to the desired concentrations, from this the simulated flue gas was formed. Then, the CO solution (11) was pumped by peristaltic pump (12) into the vaporization device (9), where it was immediately vaporized. Simultaneously, the vaporized CO carried by the simulated flue gas oxidized the NO in the reactor (14). Finally, the unreacted oxidants, the iron precipitates and NO_x were absorbed by Ca(OH)₂ supported on a glass wool. The method for controlling NO oxidation time was designed as follows: based on the volumes of the reactor and vaporization device, and inlet flue gas flow, the time (Δt) required by NO/N₂ to fully fill the reactor and vaporization device was calculated and determined as a duration required for CO addition. During the experiments, NO/N₂ was fed until the gas flow was constant. Simultaneously, CO was pumped into the vaporization device for a duration of Δt. The flue gas was then switched to bypass in order to control the reaction time of NO oxidation. Subsequently, the main path was turned on to remove the reacted flue gas for detecting the concentrations of NO and NO₂.

2.3 Investigations of various influencing factors

To investigate the influences of ferrous and PAA on NO removal, the experiments with different CO constitutions, such as H₂O₂, H₂O₂/PAA and H₂O₂/PAA/FeSO₄, were carried out. Because the generation of hydroxyl radicals mainly depends on the ferrous concentration, the effects of the ferrous concentrations in CO solutions, 3 × 10⁻³, 5 × 10⁻³, 7 × 10⁻³ and 9 × 10⁻³ mol L⁻¹, were studied. In addition, a series of experiments based on the pH of CO solution, 0.53, 1.02, 2.03, 2.98 and 4.04, were conducted to investigate the effect of pH. Based on the actual temperature conditions of a typical coal-fired power plant, seven temperature points, 60, 70, 80, 90, 100, 110 and 120 °C and four SO₂ contents, 520, 1050, 1560 and 2100 mg m⁻³, were selected to evaluate the effects of reaction temperature and SO₂ on NO removal. Based on the data of temperature dependence, the apparent activation energy of NO removal was calculated using the Arrhenius equation.

3 Results and discussion

3.1 NO oxidation

First, the reaction mechanism of NO oxidation by vaporized CO was analyzed. The components in CO are H₂O₂, PAA and ferrous, in which the former two are oxidants, and the latter is a catalyst. It has been known that the combination of H₂O₂ and ferrous can generate hydroxyl radicals (HO˙) (eqn (1)), which is an active oxidant produced from ferrous catalyzing H₂O₂. Moreover, a similar catalytic action between PAA and ferrous can occur, from which the oxidation potential of PAA is significantly enhanced in the acidic condition;²¹ however, the product are still the hydroxyl radicals (eqn (2)). In regards to H₂O₂ and PAA, their synergy in the acidic condition has been previously revealed, in which acetic acid resulting from the reduction or the decomposition of PAA (eqn (3)) can participate in the reaction between H₂O₂ and PAA (eqn (4)), leading to an inhibition of H₂O₂ decomposition and maintaining the concentration of PAA. Therefore, all of the three components in CO can improve the performance of vaporized CO oxidizing NO.

According to the abovementioned analysis, the main oxidation species in CO are concluded to be H₂O₂, PAA and HO˙, in particular, HO˙ because of the high reaction rates between HO˙ and NO, which are 5.5 × 10¹⁴ M⁻¹ s⁻¹ or 10⁸–10¹² M⁻¹ s⁻¹.²² Furthermore, from the electrochemical perspective, the standard electrode potentials of H₂O₂ (1.770 V), HO˙ (2.800 V) and PAA (1.960 V) are significantly higher than those of NO₂/NO (1.049 V), NO₃⁻/NO (0.957 V), NO₂⁻/NO (0.460 V) and NO₃⁻/NO₂⁻ (0.835 V), which also shows the feasibility of NO oxidation by vaporized CO (eqn (5)–(8)).^23–29 After oxidation, the generated oxidation products are rapidly absorbed by Ca(OH)₂, producing Ca(NO₃)₂ and Ca(NO₂)₂ (eqn (9)–(11)).

Fe²⁺ + H₂O₂ → Fe³⁺ + HO˙ + OH⁻ (1)

Fe²⁺ + CH₃COOOH → Fe³⁺ + CH₃COO⁻ + HO˙ (2)

M + CH₃COOO⁻ → MO + CH₃COO⁻ (3)

H₂O₂ + CH₃COO⁻ → H₂O + CH₃COOO⁻ (4)

NO + HO˙ → NO₂ + H˙ (5)

NO₂ + HO˙ → NO⁻₃ + H⁺ (6)

2NO + 3H₂O₂ → 2NO⁻₃ + 2H⁺ + 2H₂O (7)

NO + CH₃COOO⁻ → NO₂ + CH₃COO⁻ (8)

Ca(OH)₂ + 2NO₂ → Ca(NO₂)₂ + H₂O (9)

Ca(OH)₂ + 2HNO₂ → Ca(NO₂)₂ + 2H₂O (10)

Ca(OH)₂ + 2HNO₃ → Ca(NO₃)₂ + 2H₂O (11)

3.2 Reaction order

The NO removal by vaporized CO as a function of reaction time is shown in Fig. 2. It can be observed that the typical NO depletion appears to be exponential to the reaction time, indicating that the depletion conforms to the pseudo-first-order kinetics pattern in a rapid depletion zone (1–2 half-lives) with respect to NO. Obviously, the oxidation rate of NO dominated the NO depletion rate. Therefore, the overall rate of NO depletion can be expressed as follows (eqn (12) and (13)):

k_obs = k₁[˙OH] + k₂[H₂O₂] + k₃[PAA] (13)

where k_obs is the pseudo-first-order rate constant that represents an overall rate of NO removal by a variety of oxidizing agents (e.g., H₂O₂, PAA, and HO˙) produced in the system; [H₂O₂], [PAA] and [HO˙] are the concentrations of H₂O₂, PAA, and HO˙, mmol m⁻³; [NO] is the concentration of NO at any time, mg m⁻³.

Fig. 2 Depletion of NO as a function of time and inset that determination of reaction order with respect to NO. Simulated flue gas inlet velocity is 4.0 L min⁻¹, adding rate of CO is 200 μL min⁻¹ CO solution pH is 0.7, reaction temperature is 363 K, NO concentration is 550 mg m⁻³.

The reaction order of NO oxidation could be calculated using the initial rate method. A series of curves of the NO depletion versus reaction time under various initial NO concentrations was obtained, from which the initial rates (r₀) were obtained via reaction (15). The reaction order then could be calculated by integrating the differential form of reaction (16). Reaction (17) expresses the relation of lg(−d_c/d_t) and lg c, in which n is the reaction order with respect to NO. As shown in Fig. 2, the determined reaction order of NO is 1.119; R² is 0.998, indicating that the reaction order can be considered as of pseudo-first-order kinetics.

c = f(t) (14)

where c and c₀ are the NO concentrations at each time and at t = 0, respectively, mg m⁻³; r₀ is the rate of NO depletion at t = 0, ppm⁻¹ s⁻¹; k′ is the rate that represents an overall rate of the NO removal by a variety of oxidizing agents (e.g., H₂O₂, PAA, and HO˙) produced in the system, ppm¹⁻ⁿ s⁻¹; n is the NO reaction order.

3.3 Effect of the CO constitution

The effect of CO constitution on NO removal was investigated. The experimental conditions are shown in Table 1. It can be seen from Fig. 3 that for all particular constitutions, the NO depletion exhibits a pseudo-first-order pattern (R² = 0.99). When H₂O₂ is used as the oxidant, the pseudo-first-order rate constant is 0.01093 s⁻¹, which is the minimum among the three tests. When the CO is made up of PAA and H₂O₂, the rate constant increases to 0.01486 s⁻¹. However, the highest rate constant of 0.02234 s⁻¹ is obtained, when the CO consists of PAA, H₂O₂ and ferrous. Therefore, the additions of ferrous and PAA can significantly accelerate NO oxidation rate. In regards to the efficiency, a similar trend is also observed in Fig. 3, the efficiencies are 59.3%, 79.7% and 87.2% for the CO constitutions of H₂O₂, H₂O₂/PAA and H₂O₂/PAA/ferrous, respectively. Therefore, the effects of ferrous and PAA on the rate and the efficiency of NO oxidation was demonstrated; the functional mechanism due to ferrous and PAA is shown in Section 3.1.

Table 1 Pseudo-first-order kinetic parameters of NO removal at various CO constitutions

Test	CO constitution			t1/2, s	Rate constant (k, s^−1)	Denitrification efficiency	Correlation coefficient, R^2
	H2O2, mol L^−1	PAA, mol L^−1	Fe^2+, mmol L^−1
1	4	0	0	63.4	0.01093	59.3%	0.98993
2	4	1	0	46.6	0.01486	79.7%	0.99725
3	4	1	5	31	0.02234	87.2%	0.99301
4	4	1	3	62.6	0.01107	81.4%	0.98083
5	4	1	7	51.4	0.02348	82.5%	0.98227
6	4	1	9	25.6	0.02706	77.1%	0.95897

---

Fig. 3 Pseudo-first-order depletion of NO at various CO constitutions and inset that effect of CO constitutions on NO removal. Simulated flue gas inlet velocity is 4.0 L min⁻¹, adding rate of CO is 200 μL min⁻¹; CO solution pH is 0.7, reaction temperature is 363 K, NO concentration is 550 mg m⁻³.

It is important to investigate the effect of ferrous concentration on NO removal because the amount of HO˙ generation is directly affected by the ferrous addition. Therefore, a series of experiments with four different ferrous concentrations were conducted at a fixed NO concentration of 550 mg m⁻³. It can be seen from Table 1 and Fig. 4 that the rate constants are 0.01107, 0.02234, 0.02348 and 0.02706 s⁻¹ with respect to the ferrous concentrations of 3, 5, 7 and 9 mmol L⁻¹, indicating that the oxidation rate is evidently increased with the ferrous concentration. Fig. 4 also shows the dependence of removal efficiency on ferrous concentration. It can be observed that the removal efficiency increases in the ferrous concentration range of 3 to 5 mmol L⁻¹; however, declines in the concentration range of 5 to 9 mmol L⁻¹. The experimental phenomenon revealed that more amount of ferrous was beneficial for promoting the oxidation rate rather than the removal efficiency because the more amount of ferrous could generate more hydroxyl radicals in a short time, resulting in an increase of oxidation rate; however, the ferrous-induced side reactions²⁵ resulting from the excess amount of ferrous, such as the quenching of hydroxyl radicals (eqn (18)) and the decomposition of H₂O₂ (eqn (19)), would consume lots of effective oxidants and decrease the oxidizability of CO, which will lead to a decline in the NO removal efficiency. Not only the generated OH⁻ via the reaction 18 may destroy the catalytic function of ferrous because of the ferrous precipitation, simultaneously, the H₂O₂ decomposition also would be further aggravated (eqn (18), (20) and (21)).

Fe²⁺ + HO˙ → HO⁻ + Fe³⁺ (18)

FeHO²⁺₂ + H₂O₂ → Fe(OH)(HO₂)⁺ + H⁺ (19)

H₂O₂ → HO⁻₂ + H⁺ (20)

H₂O₂ + HO⁻₂ → OH⁻ + O₂ + H₂O (21)

Fig. 4 Pseudo-first-order depletion of NO at various ferrous concentrations and inset that effect of ferrous concentration on NO removal. Simulated flue gas inlet velocity is 4.0 L min⁻¹, adding rate of CO is 200 μL min⁻¹; CO solution pH is 0.7, reaction temperature is 363 K, NO concentration is 550 mg m⁻³.

3.4 Effect of reaction temperature

The effect of reaction temperature on NO removal was investigated. The pseudo-first-order kinetics rate constants under various reaction temperatures are shown in Table 2. It can be seen from Fig. 5 that as the temperature increases from 333 K to 393 K, the rate constant stepwise increases from 0.00946 to 0.01980 s⁻¹. Fig. 5 also shows the dependence of removal efficiency on the reaction temperature. It can be observed that the lower temperature in the range of 333–363 K creates an obvious promotion in the NO removal efficiency, while the higher temperature in the range of 363–393 K plays an inhibition role. Therefore, the optimal temperature was 363 K, which was consistent with the actual temperature conditions of the ESP outlet, indicating that the vaporized CO had a potential to couple with the ESP system to detect the simultaneous removal of SO₂ and NO. During the experiments, some phenomenon of temperature dependence occurred as the temperature was varied, i.e. the elevated temperature in the low temperature range could promote the vaporization rate of CO, the diffusion of reactants and the chemical reaction rate; however, the excessive increase of temperature was unfavorable for the chemical reaction because of the intense decomposition of the reactants and the increase of mass transfer resistance between the oxidation products and Ca(OH)₂.

Table 2 Pseudo-first-order kinetic parameters of NO removal under various reaction conditions

	Reaction conditions			t1/2, s	Rate constant (k, s^−1)	Removal efficiency	Correlation coefficient, R^2
	Temperature (K)	pH	NO/SO2 (mg m^−3)
1	333	0.7	550	73.3	0.00946	83.2	0.99264
2	343	0.7	550	68.9	0.01006	84.3	0.98695
3	353	0.7	550	60	0.01156	86.7	0.99432
4	363	0.7	550	51.5	0.01346	88.9	0.99765
5	373	0.7	550	45.7	0.01517	85.3	0.99266
6	383	0.7	550	36.6	0.01895	84.9	0.99738
7	393	0.7	550	35	0.01980	85.4	0.99967
8	363	0.5	550	48.7	0.01423	90.3	0.98734
9	363	1.0	550	76.2	0.00909	81.1	0.99673
10	363	2.0	550	99	0.00698	68.7	0.99785
11	363	3.0	550	140	0.00496	48.7	0.96773
12	363	4.0	550	202	0.00343	42.1	0.99213
13	363	0.7	550/520	45.9	0.01511	88.6	0.95714
14	363	0.7	550/1050	51.6	0.01344	88.3	0.95388
15	363	0.7	550/1560	53	0.01308	84.7	0.94676
16	363	0.7	550/2100	48.5	0.01429	85.5	0.96450

---

Fig. 5 Pseudo-first-order depletion of NO under various reaction temperatures and inset that effect of reaction temperature on NO removal. Simulated flue gas inlet velocity is 4.0 L min⁻¹, adding rate of CO is 200 μL min⁻¹; CO solution pH is 0.7, NO concentration is 550 mg m⁻³.

As shown in Fig. 6, obviously, ln k_obs linearly decreases with 1/T, which can be fitted to an Arrhenius model (eqn (22)). After fitting, the apparent activation energy was observed to be 14.1 kJ mol⁻¹ and the ln A was 0.58. Compared with some other apparent activation energies of NO oxidation obtained in the previous researches, such as the 42.5 kJ mol⁻¹ for NaClO₂ (ref. 30) and the 27.8 kJ mol⁻¹ for H₂O₂/NaOH,³¹ the apparent activation energy of vaporized CO oxidizing NO was lower and the following reasons may account for the phenomena: compared with the wet bubble reaction system, the proposed novel method had an advantage of larger contact area between the oxidants and NO resulting from the vaporization process. In addition to that, the oxidation potential and reactivity of ˙OH are far higher than ClO₂, ClO₂⁻ and HO₂⁻ employed by predecessors. Therefore, the reaction barrier of NO oxidation in our reaction system was considerably lower.

Fig. 6 Arrhenius plots for the removal of NO. Simulated flue gas inlet velocity is 4.0 L min⁻¹, adding rate of CO is 200 μL min⁻¹, pH of CO solution is 0.7, NO concentration is 550 mg m⁻³.

Then, the log-type was deduced as follows:

ln k = ln A − Ea/RT (23)

where A is the pre-exponential factor, Ea is the apparent activation energy, R is the universal gas constant, and T is the absolute temperature.

3.5 Effect of pH

The pH has a significant influence on the oxidation potentials of H₂O₂ and PAA and the stability of ferrous. Therefore, the effect of the pH of CO on NO removal was investigated, as shown in Fig. 7 and Table 2. It can be seen that the rate constants are 0.01423, 0.00909, 0.00698, 0.00496 and 0.00343 s⁻¹, corresponding to the pH of 0.5, 1.0, 2.0, 3.0 and 4.0. Apparently, the lower pH is favorable for increasing the oxidation rate. Similarly, as shown in Fig. 7, the removal efficiency is also sharply decreased from 90.3% to 42.1% with the same variation of pH, which may be due to the following reasons: (1) at a higher solution pH, H₂O₂ was rapidly decomposed as HO₂⁻ (eqn (20)),^32,33 which could adversely accelerate the H₂O₂ decomposition via reaction (21) (ref. 34) and consume ˙OH (eqn (24)); thus the oxidizability of the reaction system was decreased with the increase in solution pH. 2) Similarly, the elevated pH also produced a great inhibition on the activities of PAA and ferrous, as discussed in the Section 3.1 and Section 3.3.

HO˙ + HO⁻₂ → OH⁻ + HO˙₂ (24)

Fig. 7 Pseudo-first-order depletion of NO under various pH and inset that effect of CO pH on NO removal. Simulated flue gas inlet velocity is 4.0 L min⁻¹, adding rate of CO is 200 μL min⁻¹; reaction temperature is 363 K, NO concentration is 550 mg m⁻³.

3.6 Effect of SO₂ concentration

SO₂ is a coexistence gas in the coal-fired flue gas and has a potential to affect NO removal. Therefore, the experiments with four different SO₂ concentrations, 520, 1050, 1560 and 2100 mg m⁻³, were conducted to investigate the effect of SO₂ on NO removal. It can be seen from Fig. 8 and Table 2 that SO₂ has no significant influence on the oxidation rate, but the higher SO₂ concentration in the range of 1050 to 2100 mg m⁻³ produces a slight inhibition on the NO removal efficiency, which may be a consequence of a competition reaction between SO₂ and NO for the limited oxidants. Generally, the vaporized CO exhibited a good performance on the adaption of SO₂ variation. Therefore, the proposed method can be adaptive to the various types of coal and the working conditions of boiler.

Fig. 8 Pseudo-first-order depletion of NO under various SO₂ contents and inset that effect of SO₂ content on NO removal. Simulated flue gas inlet velocity is 4.0 L min⁻¹, adding rate of CO is 200 μL min⁻¹; reaction temperature is 363 K, NO concentration is 550 mg m⁻³.

4 Conclusions

This study made an effort to explore the macrokinetics of NO oxidation by a vaporized H₂O₂-based complex oxidant (CO). The reaction order was 1.119 with respect to NO, which could be considered to be the pseudo-first-order kinetics in a rapid depletion zone (1–2 half-lives). The effects of the CO constitution, the ferrous concentration, the reaction temperature, the pH of CO and the SO₂ concentration on NO removal were experimentally investigated. The rate constants of temperature dependence were well fitted to an Arrhenius model, from which the apparent activation energy of NO oxidation was calculated to be 14.1 kJ mol⁻¹. The experimental results demonstrated that the novel process can rapidly and effectively remove NO, which provided a viable alternative option to reduce the NO_x emission from coal-fired power plants.

Acknowledgements

The authors appreciate the financial support by a grant from the National High Technology Research and Development Program of China (863 Program, no. 2013AA065403), Hebei provincial Natural Science Foundation, P. R. China (B2011502027), Program for Changjiang Scholars and Innovative Research Team in University (IRT1127) and Zhejiang Provincial Engineering Research Center of Industrial Boiler & Furnace Flue Gas Pollution Control, Hangzhou, P. R. China (311202).

References

1. S. J. Chung, K. C. Pillai and I. S. Moon, A sustainable environmentally friendly NO_x removal process using Ag(II)/Ag(I)-mediated electrochemical oxidation, Sep. Purif. Technol., 2009, 65, 156–163.
2. M. Govindan, S. J. Chung and Il. S. Moon, Simple Technical Approach for Perpetual Use of Electro generated Ag(II) at Semi pilot Scale: Removal of NO and SO₂ as a Model System, Ind. Eng. Chem. Res., 2012, 51, 2697–2703.
3. P. Davini, SO₂ and NO_x adsorption properties of activated carbons obtained from a pitch containing iron derivatives, Carbon, 2001, 39, 2173–2179.
4. Q. Tang, Z. G. Zhang, W. P. Zhu and Z. D. Cao, SO₂ and NO selective adsorption properties of coal-based activated carbons, Fuel, 2005, 84, 461–465.
5. K. Krishna and M. Makkee, Coke formation over zeolite and CeO₂-zeolite and its influence on selective catalytic reduction of NO_x, Appl. Catal., B, 2005, 62, 35–44.
6. M. J. Sang and D. K. Sang, Removal of NO_x and SO₂ by CuO/γ-Al₂O₃ sorbent/catalyst in a fluidized-bed reactor, Ind. Eng. Chem. Res., 2000, 39, 1911–1916.
7. L. Wang, W. R. Zhao and Z. B. Wu, Simultaneous absorption of NO and SO₂ by FeII EDTA combined with Na₂SO₃ solution, J. Chem. Eng., 2007, 132, 227–232.
8. X. L. Long, W. D. Xiao and W. K. Yuan, Kinetics of gas–liquid reaction between NO and Co(NH₃)₆²⁺, J. Hazard. Mater., 2005, 123, 210–216.
9. H. Chu, T. W. Chien and B. W. Twu, The absorption kinetics of NO in NaClO₂/NaOH solutions, J. Hazard. Mater., 2001, 84, 241–252.
10. J. C. Wei, Y. B. Luo and B. Yu, Removal of NO from flue gas by wet scrubbing with NaClO₂/(NH₂)₂CO solutions, J. Ind. Eng. Chem., 2009, 15, 16–22.
11. Y. Zhao, T. X. Guo, Z. Y. Chen and Y. R. Du, Simultaneous removal of SO₂ and NO using M/NaClO₂ complex absorbent, Chem. Eng. J., 2010, 160, 42–47.
12. Y. Zhao, Y. H. Han, T. Z. Ma and T. X. Guo, Simultaneous Desulfurization and Denitrification from Flue Gas by Ferrate (VI), Environ. Sci. Technol., 2011, 45, 4060–4065.
13. C. Brogren, H. T. Karlsson and I. Bjerle, Absorption of NO in an alkaline solution of KMnO₄, Chem. Eng. Technol., 1997, 20, 396–402.
14. P. Fang, C. P. Cen, Z. X. Tang, P. Y. Zhong, D. S. Chen and Z. H. Chen, Simultaneous removal of SO₂ and NO_x by wet scrubbing using urea solution, Chem. Eng. J., 2011, 168, 52–59.
15. Y. X. Liu, J. Zhang, C. D. Sheng, Y. C. Zhang and L. Zhao, Simultaneous removal of NO and SO₂ from coal-fired flue gas by UV/H₂O₂ advanced oxidation process, Chem. Eng. J., 2010, 162, 1006–1011.
16. Y. X. Liu and J. Zhang, Photochemical Oxidation Removal of NO and SO₂ from Simulated Flue Gas of Coal-Fired Power Plants by Wet Scrubbing Using UV/H₂O₂ Advanced Oxidation Process, Ind. Eng. Chem. Res., 2011, 50, 3836–3841.
17. C. D. Cooper, C. A. Clausen and L. Pettey, Investigation of UV enhanced H₂O₂ oxidation of NO_x emissions, Environ. Eng., 2002, 128, 68–72.
18. Y. S. Mok, Absorption–reduction technique assisted by ozone injection and sodium sulfide for NO_x removal from exhaust gas, Chem. Eng. J., 2006, 118, 63–67.
19. P. Fang, C. P. Cen, X. M. Wang, Z. J. Tang, Z. X. Tang, D. S. Chen and Z. H. Chen, Simultaneous removal of SO₂, NO and Hg⁰ by wet scrubbing using urea+KMnO₄ solution, Fuel Process. Technol., 2013, 106, 645–653.
20. X. H. Xua, Q. F. Ye, T. M. Tang and D. H. Wang, Hg⁰ oxidative absorption by K₂S₂O₈ solution catalyzed by Ag⁺ and Cu²⁺, J. Hazard. Mater., 2008, 158, 410–416.
21. E. S. Huyser and G. W. Hawkins, Ferrous Ion Catalyzed Oxidations of 2-Propanol with Peroxyacetic Acid, J. Org. Chem., 1983, 48, 1705–1708.
22. Y. X. Liu, J. Zhang and C. D. Sheng, Study on the Kinetics of NO Removal from Simulated Flue Gas by a Wet Ultraviolet/H₂O₂ Advanced Oxidation Process, Energy Fuels, 2011, 25, 1547–1552.
23. E. S. Hwang, J. N. Cash and M. J. Zabik, Ozone and Hydrogen Peroxyacetic Acid Treatment To Reduce or Remove EBDCs and ETU Residues in a Solution, J. Agric. Food Chem., 2001, 49, 5689–5694.
24. E. S. Huyser and G. W. Hawkins, Ferrous ion catalyzed oxidations of 2-propanol with peroxyacetic acid, J. Org. Chem., 1983, 48, 1705–1708.
25. B. Ensing, F. Buda and E. J. Baerends, Fenton-like Chemistry in Water: Oxidation Catalysis by Fe(III) and H₂O₂, J. Phys. Chem. A, 2003, 107, 5722–5731.
26. M. Kitis, Disinfection of wastewater with peracetic acid: a review, Environ. Int., 2004, 30, 47–55.
27. J. C. Farrand and D. Johnson, Peroxyacetic acid oxidation of 4-methylphenols and their methyl ethers, J. Org. Chem., 1971, 36, 3606–3612.
28. G. B. Payne and P. H. Williams, Notes-reaction of mesityl oxide with peroxyacetic acid, J. Org. Chem., 1959, 24, 284–286.
29. J. D. Laat and T. G. Le, Kinetics and Modeling of the Fe(III)/H₂O₂ System in the Presence of Sulfate in Acidic Aqueous Solutions, Environ. Sci. Technol., 2005, 39, 1811–1818.
30. F. Liu, Experimental and Mechanism Study on Simultaneous Removal of SO₂ and NO_x using jet bubble reactor, J. North China Electr. Power Univ., 2009, 87–92.
31. T. X. Guo, Experimental Investigation on Simultaneous Removal SO₂ and NO_x in Liquid Phase by New-type Complex Absorbent, J. North China Electr. Power Univ., 2011, 69–82.
32. Q. H. Hu, C. L. Zhang, Z. R. Wang, Y. Chen, K. H. Mao, X. Q. Zhang, Y. L. Xiong and M. J. Zhu, Photodegradation of methyl tertbutyl ether (MTBE) by UV/H₂O₂ and UV/TiO₂, J. Hazard. Mater., 2008, 154, 795–803.
33. F. Yuan, C. Hu, X. X. Hu, J. H. Qu and M. Yang, Degradation of selected pharmaceuticals in aqueous solution with UV and UV/H₂O₂, Water Res., 2009, 43, 1766–1774.
34. W. M. Wang, J. Song and X. Han, Schwertmannite as a new Fenton-like catalyst in the oxidation of phenol by H₂O₂, J. Hazard. Mater., 2013, 262, 412–419.

---