Study on the Kinetics of NO Removal from Simulated Flue Gas by a

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Study on the Kinetics of NO Removal from Simulated Flue Gas by a Wet Ultraviolet/H2O2 Advanced Oxidation Process Yangxian Liu, Jun Zhang,* and Changdong Sheng School of Energy and Environment, Southeast University, Nanjing 210096, People’s Republic of China ABSTRACT: On the basis of the steady-state approximation theory and the two-film theory, the kinetic model of NO removal by a wet ultraviolet (UV)/H2O2 advanced oxidation process (AOP) was established. The mass-transfer reaction kinetic process of NO removal was also analyzed preliminarily. The results showed that the NO removal process by the wet UV/H2O2 AOP was a pseudofirst-order reaction for NO. The NO absorption process in the wet UV/H2O2 AOP system belonged to the fast reaction kinetic region. The chemical reaction process could be completed in the liquid film. The NO absorption rate mainly depended upon the chemical reaction rate, the diffusion rate, and the NO partial pressure, but it was not affected by the liquid-phase mass-transfer coefficient. Therefore, the NO absorption rate could be increased by improving the chemical reaction conditions, increasing the gas
liquid contact area, and raising the NO partial pressure. The comparison of experimental values and model values of pseudofirst-order reaction rate constants showed that the mass-transfer reaction kinetic model deduced had good reliability.

1. INTRODUCTION During the coal-burning process, a large number of pollutants, including SO2, NOx, trace elements, and volatile organic compounds (VOCs), are released. These pollutants have brought great harm to human health and the environment.1,2 Although the wet limestone
gypsum flue gas desulfurization (Ca
WFGD) and the ammonia selective catalytic reduction (NH3
SCR) processes have achieved large-scale commercial application, neither of them can achieve alone the simultaneous removal of multiple pollutants from flue gas. The combined process of the Ca
WFGD and NH3
SCR can simultaneously remove SO2 and NOx, but the large and complex systems and the high capital and operating costs limit its wide use in the developing world. Therefore, developing more effective flue gas purification technologies is one of the major research interests in the flue gas purification field.1,2 Advanced oxidation processes (AOPs) can produce •OH free radicals with strong oxidation to simultaneously oxidize and remove multiple pollutants from flue gas. Therefore, a lot of attention has been paid to this field in recent years. Some AOPs, mainly including plasma oxidation,3,4 photochemical oxidation,5,6 sonochemical oxidation,7,8 and Fenton oxidation,9,10 have been developed and applied widely. Although none of them can substitute for the combined process of the Ca
WFGD and NH3
SCR yet, thus far, they show big potential because the multiple pollutants from flue gas can be removed simultaneously in one reactor, potentially leading to the decrease of capital costs. Therefore, developing new and more effective advanced oxidation flue gas purification technologies (AOFGPTs) is one of major research interests in the field of energy and environment at present. Many results11
14 show that the ultraviolet (UV)/H2O2 AOP can produce •OH free radicals by photolysis of H2O2 to oxidize and remove various pollutants. This method has strong oxidation ability and a simple and secure process, and it has no secondary pollution simultaneously.11 Therefore, the UV/H2O2 AOP has r 2011 American Chemical Society

been widely studied and applied for the degradation and discoloration of organic pollutants in the water treatment field.11
14 Recently, some results15
19 show that the UV/H2O2 AOP also can be used for effectively purifying the multiple pollutants from flue gas. Cooper et al.15 first used a semi-dry UV/H2O2 AOP to oxidize and remove NO from simulated flue gas by radiating UV and spraying H2O2 in flue. The results showed that the NO removal efficiency was markedly increased in comparison to the single H2O2 oxidation but this semi-dry UV/H2O2 AOP was not further developed because of the large self-decomposition consumption of H2O2 in high-temperature flue. Jeong et al.16 used plasma radiation in combination with UV radiation to remove Hg0 and NO in simulated flue gas. The results showed that the addition of UV radiation could greatly increase NO and Hg0 removal efficiencies. Ma et al.17 used a wet UV/H2O2 AOP to remove NOx and SO2 from simulated flue gas using a UV lamp to radiate the surface of the H2O2-containing bubble column reactor. The results showed that NOx and SO2 achieved high removal efficiencies and, especially, the use rate of H2O2 was improved significantly. Recently, Liu et al.18,19 developed a new and more applicable wet UV/H2O2 AOP to remove NO from simulated flue gas by setting a UV lamp in the bubble column reactor. Several main process parameters were optimized, and the reaction mechanism was also studied in depth. The results showed that the removal process of NO was markedly affected by the H2O2 initial concentration, UV lamp power, NO partial pressure, and gas flow rate. NO was mainly removed by the oxidation of •OH free radicals and the oxidation of H2O2, and the final reaction product was the usable HNO3 solution. In summary, the wet UV/H2O2 AOP is a promising new AOFGPT, which can achieve the simultaneous removal of multiple pollutants from flue gas. Received: January 16, 2011 Revised: March 17, 2011 Published: March 29, 2011 1547

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However, thus far, there have been few studies concerned with the mass-transfer reaction kinetics of NO removal by the wet UV/H2O2 AOP. To obtain more knowledge about the absorption kinetic process of NO in the wet UV/H2O2 AOP system, here, on the basis of our previous experimental results,18,19 the kinetic model of NO removal by wet UV/ H2O2 AOP was established by applying the steady-state approximation theory and the two-film theory. Furthermore, the mass-transfer reaction kinetic process of NO removal was also analyzed preliminarily. The results will provide some useful foundations for strengthening the NO absorption process and the amplification and model selection of the reactor in future industrial applications.

radicals and the oxidative removal of NO by H2O2. Thus, the total reaction rate of NO removal by the wet UV/H2O2 AOP, rtotal,NO, can be expressed as the sum of the oxidative removal rate of NO by •OH free radicals, r•OH,NO, and the oxidative removal rate of NO by H2O2, rH2O2,NO rtotal, NO ¼


k1 ðλ < 300 nmÞ

H2 O2 þ hνs f 2•OH

k2

NO þ •OH f HNO2

k2 ¼ 5:5  1014 M
1 s
1

k3

NO þ •OH f NO2 þ •H k3 ¼ 108
1012 M
1 s
1

ð2Þ ð3Þ

NO2 and HNO2 formed can be further transformed into the more stable product, HNO3, through the following eqs 4 and 5, which has been confirmed by ion chromatography (IC) analysis of the liquid product and conservation calculation of nitrogen in NO:17,18,21 k4

NO2 þ •OH f HNO3

k4 ¼ 3:0  1010 M
1 s
1

k5

HNO2 þ •OH f HNO3 þ •H k5 ¼ 1:5  109 M
1 s
1

ð5Þ

k6

k7

•OH þ •OH f H2 O2

k7 ¼ 5:5  109 M
1 s
1

ð6Þ ð7Þ

22
24

show that NO also can be directly In addition, many results oxidized into HNO3 by the oxidation of H2O2 according to the following eq 8:

rH2 O2 ¼ k8 CmH2 O2 CnNO

ð11Þ

where m is the reaction order for H2O2 and n is the reaction order for NO. Baveje et al.24 systematically studied the kinetics of NO absorption in H2O2 solution. The results showed that the absorption process of NO in H2O2 solution was a two-order reaction, which was a first-order reaction for H2O2 and NO, respectively. Thus, eq 11 also can be briefly rewritten as the following eq 12: rH2 O2 , NO ¼ k8 CH2 O2 CNO

ð12Þ

In summary, the total chemical reaction rate equation of NO removal by the wet UV/H2O2 AOP can be expressed as the following eq 13: rtotal, NO ¼


dCNO dt

¼ ðk2 þ k3 ÞC•OH CNO þ k8 CH2 O2 CNO ¼ ððk2 þ k3 ÞC•OH þ k8 CH2 O2 ÞCNO

ð13Þ

The •OH free radical has a very low concentration because of its very short lifetime; therefore, the concentration of the •OH free radical can be approximately regarded as a constant based on the steady-state approximation theory.25
28 Furthermore, the potential intermediates, HNO2 and NO2, also have very low concentrations and very short lifetimes;17,18,21 therefore, the steady-state approximation theory can also be used for the determination of their concentrations. Thus, the ordinary differential equations referred to •OH, NO2, and HNO2 can be expressed as the following eqs 14, 15, and 16, respectively: dC•OH ¼ k1 φIa CH2 O2
k2 CNO C•OH
k3 CNO C•OH dt
k4 CNO2 C•OH
k5 CHNO2 C•OH
k6 CH2 O2 C•OH
k7 C •OH C•OH ¼ 0 ð14Þ

k8

2NO þ 3H2 O2 f 2HNO3 þ 2H2 O k8 ¼ 5:7  102 M
1 s
1

ð10Þ

Furthermore, rH2O2,NO can be expressed as the following chemical reaction empirical rate eq 11:25

ð4Þ

When the H2O2 concentration is excessive, because of the reducibility, H2O2 is also an effective scavenger of •OH free radicals and can consume a large amount of •OH free radicals through the following several side reactions simultaneously:12
14 H2 O2 þ •OH f HO2 • þ H2 O k6 ¼ 2:7  107 M
1 s
1

r•OH, NO ¼ ðk2 þ k3 ÞC•OH CNO

ð1Þ

The •OH free radical has extremely strong oxidation (its redox potential is 2.80 eV, being next to 2.87 eV of fluorine). Its two-order reaction rate constants reacting with most organic matter usually reach 108
1010 mol L
1 s
1,12
14 and especially, that reacting with NO is as high as 5.5  1014 mol L
1 s
1.2 The reaction process of NO removal can be expressed by the following eqs 2 and 3:2,7,8,20,21

ð9Þ

where rtotal,NO is the total reaction rate of NO removal by the wet UV/H2O2 AOP, r•OH,NO is the oxidative removal rate of NO by •OH free radicals, and rH2O2,NO is the oxidative removal rate of NO by H2O2. On the basis of the law of mass action,25 r•OH,NO can be expressed as the following eq 10:

2. MECHANISM ANALYSIS Under UV radiation with a certain wavelength, 1 mol of H2O2 can decompose into 2 mol of •OH free radicals according to the following reaction (eq 1):12
14

dCNO ¼ r•OH, NO þ rH2 O2 , NO dt

ð8Þ

dCNO2 ¼ k3 CNO C•OH
k4 CNO2 C•OH ¼ 0 dt

3. KINETIC ANALYSIS The results17,18,21 show that the removal process of NO from simulated flue gas by the wet UV/H2O2 AOP mainly includes two reaction pathways: the oxidative removal of NO by •OH free

dCHNO2 ¼ k2 CNO C•OH
k5 CHNO2 C•OH ¼ 0 dt 1548

ð15Þ ð16Þ

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According to eqs 14
16, eq 17 expressing the concentration of the •OH free radical can be solved C•OH ¼

k1 φIa CH2 O2 ð2k2 þ 2k3 ÞCNO þ k6 CH2 O2 þ k7 C•OH

ð17Þ

where φ is the UV light quantum yield and Ia is the transmitted UV light intensity (mW/cm2). On the basis of the steady-state approximation theory, the concentration of the •OH free radical can be considered as a constant under steady state; therefore, eq 17 can also be approximately rewritten as the following eq 18, in which this approximation treatment method has been used widely:29
32 C•OH

k1 φIa C0H2 O2 ¼ ð2k2 þ 2k3 ÞC0NO þ k6 C0H2 O2 þ k7 C•OH

ð19Þ

Simultaneously, we place eqs 18 and 19 into eq 13; therefore, the total chemical reaction rate equation of NO removal can be finally obtained rtotal, NO ¼


dCNO ¼ dt

k1 ðk2 þ k3 ÞφIa C0H2 O2 2ðk2 þ k3 ÞC0NO þ k6 C0H2 O2 þ k7 C•OH !

þ k8 C0H2 O2 CNO ¼ kov1 CNO

kov1 ¼

ð20Þ

k1 ðk2 þ k3 ÞφIa C0H2 O2 þ k8 C0H2 O2 2ðk2 þ k3 ÞC0NO þ k6 C0H2 O2 þ k7 C•OH

!

ð21Þ where kov1 is the pseudo-first-order reaction rate constant (s
1), CH2O20 is the initial concentration of H2O2 (mol/L), and C0NO is the initial concentration of NO (mol/L). Considering the complexity, eqs 20 and 21 need to be further simplified to facilitate the analysis and verification of the kinetic model. Because (2k2 þ 2k3)C0NO ≈ 1011 . k10CH2O20 ≈ 107 and k11C•OH ≈ 101
10
4, k10CH2O20 and k11C•OH in eqs 20 and 21 can be neglected approximatively on the basis of the order analysis method.29
32 Thus, eqs 20 and 21 can be further simplified into the following eqs 22 and 23, respectively: ! k1 ðk2 þ k3 ÞφIa C0H2 O2 dCNO 0 ¼ þ k8 CH2 O2 rNO ¼
dt 2ðk2 þ k3 ÞC0NO ! k1 φIa C0H2 O2 0 CNO ¼ þ k8 CH2 O2 CNO ¼ kov1 CNO ð22Þ 2C0NO where kov1 ¼

k1 φIa C0H2 O2 þ k8 C0H2 O2 2C0NO

ð24Þ where DNO,H2O2 is the diffusion coefficient of NO in H2O2 solution (m2/s) and x is the differential element thickness in liquid film (cm). Equation 24 can be further simplified to the following ordinary differential eq 25: DNO, H2 O2

ð18Þ

In addition, because the initial concentration of H2O2, CH2O20 ≈ 100
101, is far larger than that of NO, C0NO ≈ 10
6
10
7, on the basis of the classical isolation method or the excessive concentration method,25,29,30,32 it can also be approximately regarded as a constant in the short term. Thus, eq 12 can be further changed into the following eq 19: rH2 O2 , NO ¼ k8 C0H2 O2 CNO

balance of NO, the following diffusion-reaction eq 24 can be established:33 " # dCNO dCNO d2 CNO ¼
DNO, H2 O2 þ
DNO, H2 O2 dx þ rtotal, NO dx dx dx dx2

! ð23Þ

Furthermore, on the basis of the two-film theory and the material

d2 CNO ¼ rtotal, NO dx2

ð25Þ

Equation 26 can be obtained by taking the intrinsic chemical reaction rate eq 20 into the diffusion reaction eq 2533 DNO, H2 O2

d2 CNO ¼ kov1 CNO dx2

ð26Þ

with the boundary conditions: x = 0, CNO = CNO,i; x = xH2O2,L, CNO = 0 (here, to facilitate the solution of eq 26, it is first supposed that the NO absorption process in the wet UV/H2O2 AOP system belongs to the fast reaction region; therefore, the NO concentration in the liquid body can be considered as 0; CNO = 0). Thus, the concentration distribution of NO can be obtained by solving the ordinary differential eq 26.33 Then, on the basis of Fick’s law,34 the absorption rate eq 27 of NO removal by the wet UV/H2O2 AOP can be finally obtained   dCNO NNO ¼
DNO, H2 O2 dx x ¼ 0 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi CNO, i DNO, H2 O2 kov1 ¼ ð27Þ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi! DNO, H2 O2 kov1 th kNO, L where kNO,L is the liquid-phase mass-transfer coefficient (m/s) and CNO,i is the phase interface concentration (mol/L). According to the two-film theory, when kov1 is very large, the expression, th((DNO,H2O2kov1)1/2/kNO,L) f 1, can be met approximately;33,34 therefore, the NO absorption rate eq 27 can be finally simplified to the following eq 28: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð28Þ NNO ¼ CNO, i DNO, H2 O2 kov1 where kov1 ¼

! k1 φIa þ k8 C0H2 O2 2C0NO

4. DATA PROCESSING 4.1. Physical Property Parameters. The solubility coefficients of NO in H2O2 solution were calculated by the Wilke
Chang empirical equation.33,34 The diffusion coefficients of NO in H2O2 solution were calculated by the Nan
Krevelen and Hoftizer empirical equations.33,34 Furthermore, the gas
liquid mass-transfer parameters of NO were determined by the classic CO2 chemical absorption method,34,35 including the absorption process of CO2 in NaClO
Na2CO3/NaHCO3 solution and the 1549

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Table 1. Solubility Coefficients, Diffusion Coefficients, and Mass-Transfer Parameters HNO,H2O2 (10
8, mol L
1 Pa
1)

DNO,H2O2 (10
9, m2/s)

kNO,L (10
4, m/s)

kNO,G (10
6, mol s
1 m
2 Pa
1)

aNO,L (m
1)

1.82

2.49

10.99

1.47

19.37

absorption process of CO2 in NaOH solution, and the Danckwerts plot theory.35,36 This classic method is often regarded as the standard method, and it is unnecessary to be verified by others.35 The related parameters under solution temperature of 298 K, initial pH value of 3.2, UV lamp power of 36 W, and gas flow of 600 mL/min were summarized in Table 1. 4.2. NO Removal Efficiency. The NO removal efficiency, ηNO, can be calculated by the following eq 29: ηNO ¼

CNO, in
CNO, out  100% CNO, in

ð29Þ

where CNO,in is the inlet concentration of NO (mg/m3) and CNO,out is the outlet concentration of NO (mg/m3). 4.3. NO Absorption Rate. On the basis of the material balance of NO, the NO absorption rate, NNO, can be calculated by the following eq 30:33 NNO ¼

ηNO CNO, in QG  10
3 60MNO aNO, L VL

ð31Þ

The absorption rate equation of NO in gas film can be expressed as the following eq 32:33 NNO ¼ kNO, G ðpNO, G
pNO, i Þ

ð32Þ

Therefore, the phase interface concentration of NO, CNO,i, can be finally obtained by the joint solution of eqs 31 and 32 CNO, i ¼ HNO, H2 O2 ðpNO, G
NNO =kNO, G Þ

Table 2. Kinetic Parameters under Different H2O2 Concentrations

ð30Þ

where MNO is the molecular weight of NO (g/mol), VL is the volume of H2O2 solution (mL), QG is the gas flow (mL/min), and aNO,L is the gas
liquid specific surface area (m
1). 4.4. Phase Interface Concentration of NO. According to Henry’s law,33 the following equilibrium relation (eq 31) in the gas
liquid phase interface can be met: CNO, i ¼ HNO, H2 O2 PNO, i

Figure 1. Effects of NO phase interface concentrations on (a) NO absorption rate and (b) the Hatta coefficient.

ð33Þ

where HNO,H2O2 is the solubility coefficient of NO in H2O2 solution (mol L
1 Pa
1), pG is the NO partial pressure in the phase body (Pa), pi is the NO partial pressure in the phase interface (Pa), and kNO,G is the gas-phase mass-transfer coefficient (mol s
1 m
2 Pa
1).

5. RESULTS AND DISCUSSION 5.1. Kinetic Region. To identify the mass-transfer reaction kinetic region and then simplify the kinetic analysis process, the Hatta coefficient, Ha, is calculated by the following eq 34:33,34 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DNO, H2 O2 kov1 Ha ¼ ð34Þ kNO, L

On the basis of the two-film theory, when the Hatta coefficient Ha > 3.0, the absorption process belongs to the fast reaction kinetic region. The effects of NO phase interface concentrations and H2O2 concentrations on the Hatta coefficient, Ha, are shown in

CH2O2 (mol/L)

kov1 (104, s
1)

0.2

1.32

5.2

81.3

1.0

5.43

10.6

437.6

Ha

1/2EH2O2,i (103)

1.5

8.61

13.3

690.0

2.0 2.5

11.72 14.29

15.6 17.2

941.6 1207.8

Figure 1b and Table 2. It can be seen that, with the increase of NO phase interface concentrations, the Ha decreases but, under all experimental conditions, the Hatta coefficient, Ha, is always kept at greater than 3.0, Ha > 3.0. Furthermore, with the increase of H2O2 concentrations, the Ha increases and is also kept at greater than 3.0 under all H2O2 concentrations. The results show that the NO absorption process in the wet UV/H2O2 AOP system belongs to the fast reaction kinetic region. The chemical reaction process can be completed in liquid film. The NO absorption rate mainly depends upon the chemical reaction rate, the diffusion rate, and the NO partial pressure, but is not affected by the liquid-phase mass-transfer coefficient. Thus, the NO absorption rate can be increased by improving the chemical reaction conditions, increasing the gas
liquid contact area, and raising the NO partial pressure. Intensifying the convection and turbulence in the liquid-phase body cannot increase the NO absorption rate.33,34 5.2. Reaction Order. As shown in Figure 1a, with the increase of the NO phase interface concentration, the NO absorption rate increases and is kept an approximate linear relationship with the NO phase interface concentration (the correlation coefficient reached 0.990). The results show that the NO removal process by the wet UV/H2O2 AOP is a first-order reaction for NO. Furthermore, according to the two-film theory, a second-order reaction can be regarded as a pseudo-first-order reaction only when the interrelation of the Ha and the instantaneous enhancement factor, EUV/H2O2,i, can meet the following discriminant eq 35:33,34 Ha ,

EUV=H2 O2 , i 2

ð35Þ

where EUV/H2O2,i is the instantaneous enhancement factor of UV/ H2O2 AOP oxidation. 1550

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Figure 2. Comparison of experimental values and model values of pseudo-first-order reaction rate constants under different H2O2 concentrations.

According to the results by Liu et al.,18,21 the removal process of NO by the wet UV/H2O2 AOP mainly includes two reaction pathways: the oxidative removal of •OH and the oxidative removal of H2O2. Thus, the instantaneous enhancement factor, EUV/H2O2,i, can be decomposed into the sum of the instantaneous enhancement factor of the •OH oxidation, E•OH,i, and the instantaneous enhancement factor of the H2O2 oxidation, EH2O2,i. Thus, the discriminant eq 35 can be further expressed as the following discriminant eq 36: Ha ,

EUV=H2 O2 , i ðEH2 O2 , i þ E•OH, i Þ ¼ 2 2

DH2 O2 , H2 O2 CH2 O2 D•OH, H2 O2 C•OH 1 ¼ þ 1þ νNO, H2 O2 DNO, H2 O2 CNO, i νNO, •OH D•OH, H2 O2 CNO, i 2

!

ð36Þ where E•OH,i is the instantaneous enhancement factor of the •OH oxidation, EH2O2,i is the instantaneous enhancement factor of the H2O2 oxidation, DH2O2,H2O2 is the diffusion coefficient of H2O2 in H2O2 solution (m2/s), D•OH,H2O2 is the diffusion coefficient of •OH in H2O2 solution (m2/s), νNO,•OH is the chemical stoichiometric coefficient of NO reacting with •OH, and νNO,H2O2 is the chemical stoichiometric coefficient of NO reacting with H2O2. However, considering the high complexity in determinating the related physical property parameters of •OH in H2O2 solutions, it is very hard to directly calculate the value of EUV/ H2O2,i; therefore, the discriminant eq 36 needs to be further simplified. According to the discriminant eq 36, if the following new discriminant eq 37 is met, the discriminant eq 36 can also be met simultaneously. Therefore, here, it only needs to verify the following new discriminant eq 37: ! E H2 O2 , i DH 2 O 2 , H 2 O 2 C H 2 O 2 1 1þ ¼ ð37Þ Ha , 2 2 νNO, H2 O2 DNO, H2 O2 CNO, i where νNO,H2O2 = 1.5 and DH2O2,H2O2/DNO,H2O2 ≈ 1.0.33,34 As shown in Table 2, it can be seen that the Hatta coefficients Ha were far less than the instantaneous enhancement factors of the H2O2 oxidation, EH2O2,i, under all H2O2 concentrations. Therefore, the second-order reaction process of NO absorption in the wet UV/H2O2 AOP system can be regarded as a pseudofirst-order reaction for NO. 5.3. Model Test. According to the NO absorption rate eq 28, the H2O2 concentrations should keep a linear relationship with the pseudo-first-order rate constants. As shown in Figure 2, it can be seen that the H2O2 concentrations can keep a good linear relationship with the pseudo-first-order rate constants and the

correlation coefficient reached 0.998. The comparison of experimental values and model values of the pseudo-first-order rate constants shows that the maximum relative error is less than 12.8% and the average relative error is only 4.2% in the study of the gas
liquid reaction. The results show that the mass-transfer reaction kinetics model (eq 28) deduced had good reliability. In addition, It is worthwhile to note that a typical untreated flue gas includes many and various compositions, such as H2O, O2, SO2, SO3, HCl, CO, Hg, CO2, CO, N2, NOx, H2S, VOCs, etc. Anyone of them may affect the removal of NO. Particulates may absorb and scatter the UV light, also impacting the removal of NO. Thus, these potential effects should be gradually examined in future works involving more complex gas compositions.

6. CONCLUSIONS On the basis of the steady-state approximation theory and the two-film theory, the kinetic model of NO removal by a wet UV/ H2O2 AOP was established. The mass-transfer reaction kinetic process of NO removal was also analyzed preliminarily. Some useful conclusions were obtained as follows: (1) The NO absorption process in the wet UV/H2O2 AOP system belonged to the fast reaction region. The chemical reaction process could be completed in the liquid film. The NO absorption rate mainly depended upon the chemical reaction rate, the diffusion rate, and the NO partial pressure but was not affected by the liquid-phase mass-transfer coefficient. Thus, the NO absorption rate could be effectively increased by improving the chemical reaction conditions, increasing the gas
liquid contact area, and raising the NO partial pressure. However, intensifying the convection and turbulence in the liquid-phase body could not increase the NO absorption rate. (2) The NO absorption rate kept a good linear relationship with the NO phase interface concentration, and the discriminant, Ha , EUV/H2O2,i/2, could be well-met. Thus, the NO removal process by the wet UV/H2O2 AOP system could be regarded as a pseudo-first-order reaction for NO. (3) The H2O2 concentrations kept a good linear relationship with the pseudofirst-order rate constants, and the correlation coefficient reached 0.998. The comparison of experimental values and model values of pseudo-first-order reaction rate constants showed that the maximum relative error was less than 12.8% and the average relative error was only 4.2% in the study of the gas
liquid reaction. The results showed that the mass-transfer reaction kinetic model deduced had good reliability. (4) In addition, a typical untreated flue gas included many and various compositions, such as H2O, O2, SO2, SO3, HCl, CO, Hg, CO2, CO, N2, NOx, H2S, VOCs, etc. Anyone of them may affect the removal of NO. Particulates may absorb and scatter the UV light, also impacting the removal of NO. Thus, these potential effects should be gradually examined in future works involving more complex gas compositions. ’ AUTHOR INFORMATION Corresponding Author

*Telephone: þ86-025-83-79-36-12. Fax: þ86-025-83-79-58-24. E-mail: [email protected].

’ ACKNOWLEDGMENT This study was supported by the International Cooperation Projects of the National Natural Science Foundation of China 1551

dx.doi.org/10.1021/ef200086f |Energy Fuels 2011, 25, 1547–1552

Energy & Fuels (50721140649) and the Foundation of the State Key Laboratory of Coal Combustion (FSKLCC1003) and the Foundation for Excellent Doctorial Dissertation of Southeast University. The authors are grateful for the test support of the Analysis and Testing Center of Southeast University.

ARTICLE

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dx.doi.org/10.1021/ef200086f |Energy Fuels 2011, 25, 1547–1552