Removal of NOx and SO2 in the Coal-fired Flue Gas Using the

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Catalysis and Kinetics

Removal of NOx and SO2 in the Coal-fired Flue Gas Using the Rotating Packed Bed Pilot Reactor with Peroxymonosulfate activated by Fe(II) and Heating. Xiaojiao Chen, and Xiaomin Hu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00881 • Publication Date (Web): 09 Jun 2019 Downloaded from http://pubs.acs.org on June 10, 2019

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Energy & Fuels

1

Removal of NOx and SO2 in the Coal-fired Flue Gas Using the Rotating Packed Bed Pilot Reactor

2

with Peroxymonosulfate activated by Fe(II) and Heating.

3

Xiaojiao Chen, Xiaomin Hu *

4 5

School of Resources & Civil Engineering, Northeastern University, Shenyang 110819, PR China;

6 7

*Corresponding author:

8

Xiaomin Hu, e-mail: [email protected]

9 10 11 12 13 14 15 16 17 18 19 20 21 22

1

ACS Paragon Plus Environment

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Abstract

24

An advanced oxidation processes (AOPs) involving the combined Fe(II) ions and heating activation of

25

peroxymonosulfate (PMS) for NOx and SO2 removal in coal-fired flue gas was conducted for the first

26

time in a rotating packed bed pilot reactor (RPB). The major influencing factors, radical species,

27

reaction mechanisms and products, mass transfer process and kinetic of simultaneous removal NOx and

28

SO2 were investigated. The results indicated that NO removal efficiency reached above 70%, while the

29

SO2 and NO2 were almost completely removed. As the temperature increased, the NO removal

30

efficiency was increased. Moreover, the intensity of SO•4 - , •OH and

31

activated PMS and

32

temperature by the electron spin-resonance spectroscopy (ESR) test. The increased liquid flow led to

33

higher NO removal efficiency and increased gas flow played the opposite role. Increased rotational

34

speed first resulted in an increase in the NO removal efficiency and then decrease. Furthermore, the

35

mass transfer coefficient obtained in this RPB was significantly higher than that in the conventional

36

bubbling reactor. Finally, NO removal process in the PMS/Fe(II)/RPB systems was proved to be a

37

pseudo-first-order reactions, and be a fast reaction completed in the liquid film base on the investigate

38

of Hatta number, strengthening factors and liquid-phase reaction utilization efficiency. The pilot

39

experiment is to further study the main parameters affecting the chemical reaction in a certain scale

40

device, to solve the problems that the laboratory cannot solve or discover, and it is the necessary link

41

for the transformation of scientific and technological achievements.

•NO

1

O2 radicals generated by the

converted by radical-radical reaction were enhanced with increasing

42 43 44

2


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Energy & Fuels

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TOC

47

1. Introduction.

48

Emission of NOx (mainly NO~90%, NO2~10%) and SO2 during fossil fuels and solid waste

49

combustion cause serious damage to the environment (haze, acid rain and eutrophication) and human

50

health (cardiovascular and respiratory diseases).1-4 Generally, SO2 has been effectively controlled using

51

wet flue gas desulfurization (WFGD) process, however, there is no effective measure to control the

52

NOx emissions due to the inferior solubility.5 Currently, selective catalytic reduction (SCR) and

53

selective non-catalytic reduction (SNCR) denitrification technologies have achieved large-scale

54

commercial applications in the field of coal-fired power plants. However, due to the huge economic

55

burden, large area, chemical treatment hazards and other shortcomings, they cannot be widely used,

56

especially small and medium industrial furnaces and boilers.6-8 Therefore, the contamination of NOx in

57

air environment has become a great concern in recent years. And considerable researchers in this filed

58

have been attracted to explore more effective and innovative simultaneous removal of SO2 and NOx

59

technologies. Fortunately, the researchers have found that the liquid oxidant can significantly increase

60

the solubility of NO in the liquid, thereby improving the removal efficiency of the gaseous pollutant.9-13

61

Currently, the liquid oxidants commonly employed are PMS, KMnO4, persulfate (PS), H2O2, 3


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NaClO and NaClO2 etc..14-21 Among these oxidation absorbents, PMS seems to be more attractive.

63

PMS is a cost-effective and environmentally friendly oxidant base on some excellent properties such as

64

strong oxidative, non-selective behavior, relatively stability at room temperature and lower energy

65

input.22-26 In general, heating, UV or transition metal ions can effectively activate PMS to create SO•4 - ,

66

•OH

67

strong oxidation-reduction potential which will enhance the oxidation performance thereby ultimately

68

elevating contaminants decontamination efficiency. Liu et al.32 developed the absorption-oxidation

69

process of NO using PMS induced by ultrasound and Mn2+/Fe2+ in a traditional bubble reactor and

70

investigated NO absorption rate and the process parameters. Recently, we reported the removal of NO

71

using aqueous PMS with synergic activation of Fe2+ ions and high temperature in the bubble reactor.33

72

In the meanwhile, various research groups have used various reactor configurations to study the

73

reaction of NO with strong liquid oxidant at different experimental conditions. Some studies12,26

74

indicated that the absorption rate was strongly affected by the gas-liquid mass transfer resistance of

75

oxidation system in these traditional reactors such as stirred cell and bubble column. As mentioned

76

above, the performance of PMS activated by heating and Fe(II) ions investigated by our group is

77

generally restricted by the mass transfer rate, especially for inferior solubility NO treatment. To

78

overcome this obstacle, we designed a rotating packed bed with PMS/ Fe(II) oxidation system to

79

improve mass transfer efficiency and thus improve NO removal efficiency because the rotating packed

80

bed is an efficient mass transfer device. When the fluid flows through the filler, it will be smashed into

81

rich ultrafine droplets by strong shear forces, which increases the interface area of the reaction, thus

82

greatly improving the mass transfer process and micro-mixing between the gas-liquid phase.34,35

83

Simultaneously, it has stable operation, small equipment, low energy consumption and investment,

and

1

O2 radicals.27,28 Some results28-31 have shown that the three kinds of radicals display the

4


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Energy & Fuels

84

which have shown their unique advantages in exhaust gas absorption treatment.36-38 Excessively high

85

rotational speed may damage the liquid seal, which will cause to the flue gas pass through reactor

86

without reaction or a reaction in reduced contact time. This will cause NO removal efficiency decrease

87

and high energy consumption, so the relatively low speed rotating packed bad pilot reactor is designed

88

and used by our team.

89

To the best of our knowledge, this article should be considered an innovative and effective study

90

integrating a rotating packed bad pilot reactor, strong oxidants, catalysis and high temperature to

91

systematically investigate NO absorption removal process in coal-fired flue gas. The objectives of the

92

study are to (1) calculate mass transfer coefficient of NO removal in the PMS/Fe(II)/RPB system; (2)

93

evaluate the mass transfer performance of the RPB with traditional bubble reactor; (3) investigate the

94

NO removal efficiency in the PMS/Fe(II)/RPB system at the different reaction conditions; (4) study the

95

absorption kinetics of NO removal, in term of the reaction order Hatta number, strengthening factors

96

and liquid-phase reaction utilization efficiency.

97

2. Materials and methods.

98 5


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99

Fig.1. Schematic design of the rotating packed bed pilot reactor used for removing NO from coal-fired

100

flue gas.

101

2.1. Experimental apparatus.

102

The experimental apparatus mainly includes following parts: (1) the coal-fired flue gas; (2) relief

103

valve unit; (3) flow velocity meter; (4) rotating packed bed pilot reactor; (5) rotor; (6) motor; (7)

104

solution storage tank; (8) pump; (9) heater; (10) solid dryer; (11) flue gas analyzer of outlet; (12) flue

105

gas analyzer of inlet; (13) tail gas absorber, which has been described in Fig. 1.

106

2.2. Experimental methods.

107

The experimental setup for NO removal was schematically shown in Fig. 1. The apparatus was

108

mainly composed of a flue gas supply system, a gas-liquid reaction system, a measurement system and

109

an exhaust gas treatment system.

110

The flue gas for experiment was coal-fired flue gas from the small boiler. The main gas composition

111

were NO, NO2, SO2, O2, CO and CO2, and the concentrations measured by the flue gas analyzer were

112

NO-330~350 mg/m3, NO2-40~50 mg/m3, SO2-2000~2200 mg/m3 and O2 with a volume fraction of

113

8%-10%, respectively. The flue gas flow rate was 0.112-0.255 m3/s, and the concentrations and flows

114

were regulated by the flow velocity meter. The key part of reactor was a vertical rotating packed bed

115

with a height of 1.0 m, an inner diameter of 0.9 m, which was packed the filler. The filler used in this

116

study was a stainless steel wire mesh having a diameter of 0.2 mm and was wound around in the rotor

117

of the RPB. And the pilot reactor with the rotating speed varying from 500 to 1000r/min, and the

118

rotating speed was measured using a Dc motor speed measuring instrument. The rotor was installed

119

throughout the reactor and connected to the motor.

120

The PMS/Fe(II) (the molar ratio of 1:1; Sinopharm Reagent Co., Ltd.; Analytical reagent) solution

6


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Energy & Fuels

121

was introduced from the solution storage tank through a pump at a flow rate of 0.98 L/s ~ 2.06 L/s and

122

injected into the inside of the packed bed from the liquid distributor. Then, the liquid stream

123

radioactively moved in the packed bed and was smashed into rich ultrafine droplets by the strong shear

124

forces due to centrifugal force. Finally, the liquid stream exited from the lateral export, and flowed

125

back to solution storage tank. Simultaneously, the coal-fired flue gas flowed into the RPB pilot reactor

126

as well. The gas stream flowed inward from the outside of the packed bed by the pressure driving force,

127

and reacted with the PMS/Fe(II) oxidation liquid, then, was discharged through the gas outlet.

128

Oxidation liquid for experiments was carried out in the 22-80℃ range, and temperature setting was

129

maintained by the heater until the temperature reached desired level and stabilizing.

130

The exit gas from the reactor passed through a solid dryer prior to analysis to remove moisture. The

131

data was recorded when the flue gas after the reaction flowed through the outlet flue gas analyzer.

132

Exhaust gas could be further processed by the exhaust gas absorber. The pH probe was used for pH

133

measurement, and pH was regulated by NaOH and HCl (Sinopharm Reagent Co., Ltd.; Analytical

134

reagent).

135

2.2. Analytical method.

136

The flue gas analyzers (ZR-3710D) are used to measure the inlet and outlet concentrations of gas.

137

The ion chromatography (IC, CIC-300) is used to measure NO3- , NO2- and

138

PMS/Fe(II)/RPB system. The pH meter (SIN-pH120) is used to measure the initial and final pH value.

139

The SO•4 - , •OH,

140

(DMPO), 2,2,6,6-Tetramethyl-1-piperidinyloxy (TEMPO) and imidazole nitroxide (IN), and are

141

detected by the electron spin-resonance spectroscopy (Bruker ESP-A300). The measurements were

142

carried out at center field of 3570 GS, microwave frequency of 9.93 GHz, sweep width of 100 GS and

O2 and •NO radicals are trapped with

1

SO23 - , and SO24 - in

5,5-Dimethy l-1- Pyrrolidine N-Oxide

7


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143

modulation frequency of 100 GHz.

144

3 Results and discussions.

145

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The main reaction pathways of NO removal were listed as follows (Eqs.(1)-(13)). 1,9,10,24,26,39

146

Fe2 + +HSO5- →Fe3 + +SO•4 - +OH -

(1)

147

NO + SO•4 - + H2O→HSO4- +NO2- + H +

(2)

148

NO + •OH NO2- + H +

(3)

149

2NO + HSO5- + H2O→HSO4- +2NO2- +2H +

150

1

151

SO2 + H2O↔ HSO3- + H +

(6)

152

HSO3- ↔ SO23 - + H +

(7)

153

SO23 - + •OH→SO•3 - +OH -

(8)

154

SO23 - +2SO•4 - + H2O→2HSO4- +SO24 -

(9)

155

SO23 - +HSO5- →2SO24 - + H +

(10)

156

3NO2 + H2O→2H + +2NO3- +NO

(11)

157

NO2 + •OH→H + +NO3-

(12)

158

NO2 + SO•4 - + H2O→NO3- +2HSO4-

(13)

(4) (5)

O2 +2NO→2NO2

159

Furthermore, the Fe(Ⅲ) ions converted from Fe(Ⅱ) in the reaction system activated HSO5- to

160

regenerate Fe(Ⅱ) ions. The catalyst regeneration allowed the catalyzed and reaction could be cycled

161

until the PMS was completely consumed afforded a sufficient reaction time (Eqs. (14)- (15)). 24

162

Fe3 + +HSO5- →Fe2 + +SO•4 - +OH -

(14)

163

Fe3 + +HSO5- →Fe2 + +SO•5 - +OH +

(15)

164

3.1 Mass transfer coefficient of gas in solution.

8


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Energy & Fuels

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In order to quantitatively characterize the mass transfer process of NO removal in the

166

PMS/Fe(II)/RPB system, the gas–liquid mass transfer coefficient is evaluated as the basis of the

167

kinetics for investigation. The NO gas absorption and oxidation by the PMS liquid undergo a

168

moderately fast pseudo-first-order reaction process (the hypothesis will be proved in the back).

169

Therefore the classical Danckwerts absorption system method can be employed to derive the mass

170

transfer coefficients in the gas phase and liquid phase through the conversion the CO2 system to NO

171

system as follows (16) - (18):40,41

172

kNO,G = kCO2 ,G (DCO

173

kNO,L = kCO2 ,L

174

aCO2 = aNO

175

Where, kNO ― G and kNO ― L are the gas phase and liquid phase mass transfer coefficients of NO

176

respectively, (m/s); kCO2 ― G and kCO2 ― L are the gas phase and liquid phase mass transfer coefficients

177

of CO2 respectively, (m/s); DNO - N2 and

178

NO respectively, (m2/s); DCO2 - N2 and DCO2 - H2O are the diffusion coefficients in N2 and water of

179

CO2 respectively, (m2/s); in general m=0.5~1.0, Sada et al think m=2/3, take 2/3in this paper;42,43 aCO2

180

and aNO are the specific interfacial areas of CO2 and NO, respectively, m-1.

181

DCO2 - N2, and DNO - N2 can be obtained by the equation of champman-enskog (19): 40,41

182

DA,B = 0.00266PM0.5

183

Where, MA and MB stand for the molecular weights, MAB =

[ [

DNO - N2 2

]

m

(16)

) -N 2

DNO -

H2O

DCO2

- H2O

]

n

(17) (18)

DNO - H2O are the diffusion coefficients in N2 and water of

T1.5

(19)

2 AB σABΩD

2 1 MA

1

+M

, g/mol. T is the temperature in

B

σA + σB

184

Kelvin , K; P is the pressure (atm); σAB stands for the characteristic length, σAB =

185

= 3.941, σNO = 3.492, σN2 = 3.798),44 and ΩD stands for the collision integral which can be

186

calculated by the Eqs. (S1)-(S3) in the SI.

9


2

(σCO2

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Page 10 of 30

187

DCO2 - H2O, and DNO - H2O can be estimated by the following Eq. (20): 40,41

188

Dgas,

189

Here, VNO and VCO2 stand for the molar volume of NO and CO2, which are 23.6 and 34, respectively,

190

cm3/mol; Х stands for the association factor, хH2O=2.6, other non-association factors х = 1; νH2O

191

stands for the kinetic Newtonian viscosities of water, Pa • s; MH2O stands for the molecular weights of

192

H2O.

T(хH2OMH2O)

-12 H2O = 7.4 × 10

0.5

(20)

νH2OV0.6 gas

193

In a conventional reactor, as bubbler reactor and spraying reactor frequently used in the field of

194

advanced oxidation, the liquid phase mass transfer coefficient played a key role in the removal of

195

gaseous pollutants. According to some reports45,46 the liquid phase mass transfer coefficient of the NO

196

removal reaction was approximately 10 -5~10 -4 m/s, such as Liu et al obtained the result was around

197

1.69 × 10 -4 m/s in a bubbler reactor.26 In this research, the liquid phase mass transfer coefficient of the

198

denitrification process with the RPB pilot reactor measured was 8.612 × 10 -3 m/s (Table 1). Base on

199

this result, we could conclude that compared to the conventional bubbler reactor, the mass transfer

200

coefficient of the liquid phase in the RPB was increased by about one order of magnitude, which

201

indicated that the RPB showed a very obvious strengthening effect on the absorption process. Some

202

studies47,48 have shown that the mass transfer process was the key parameter for controlling the NO

203

removal process in the oxidation system, and the NO removal efficiency can be increased with the

204

increase of the mass transfer coefficient. Therefore, according to the previous comparison results, the

205

RPB of this research could improve the performance of mass transfer process. Thus we used the

206

PMS/Fe(II)/RPB oxidation system to further optimize the process of removing NO in the coal-fired

207

exhaust gas, thereby improving the NO removal efficiency.

208

3.2 Influence of temperature.

10


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209

Energy & Fuels

3.2.1 Effect of temperature on removal efficiency.

210

Fig. 2 revealed the effect of PMS liquid temperature on NO removal efficiency. With the increasing

211

temperature from 22 to 80 ℃, NO removal efficiency had a remarkable advance from 23.2% to 78.6%,

212

and the peak was found to occur at the temperature around 70 ℃, then a tendency to stabilize appeared

213

on 70 to 80 ℃. Compared with NO, NO2 and SO2 were almost completely removed at all reaction

214

temperatures because of their very high solubility and relatively high reactive activity in solution. In

215

addition, high temperature could increase the activity of Fe(II) ions to promote the progress of the

216

catalytic reaction, accelerating the generation of free radicals to achieve the effect of increasing the NO

217

removal efficiency.

218

However, the removal efficiency of NO tended to be constant with the temperature increased from

219

70 to 80 ℃. Through additional experiments in the laboratory, we found that when the temperature was

220

higher than 65 ℃, the PMS aqueous solution was susceptible to degradation and decomposition into

221

oxygen and potassium sulfate. This occasion hindered the production of free radicals, resulting in a

222

constant in the target pollutants removal efficiency.

223

Base on the kinetics, when the temperature was from 22 to 80℃, the liquid phase mass transfer

224

coefficient increased (as shown in Table 1). One possibility was the diffusion coefficient that played a

225

key role to the liquid phase mass transfer coefficient was proportional to the temperature, and inversely

226

proportional to the viscosity. Herein, increasing the temperature of the ionic liquid would promote the

227

diffusion coefficient DNO,L of NO in the solution base on the Eq. (25), accompany by an increase of

228

KNO,L. Moreover, the temperature elevated was beneficial to the diffusion process of NO in the

229

PMS/Fe(II) system, which might promote the reduction of the resistance of the gas-liquid mass

230

transfer process. In the end, the liquid phase mass transfer coefficient KNO,L elevated with temperature

11


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231

increase and played a positive role in the NO removal. Therefore, the NO removal efficiency kept

232

rising.

233

Base on the thermodynamics, the NO solubility in the PMS/Fe(II) absorption liquid decreased with

234

increasing temperature. Meanwhile, the NO absorption reaction was an exothermic reaction, and

235

temperature increase caused the chemical equilibrium to move toward the reverse reaction. These were

236

not conducive to the NO removal.40 It could be seen from the experimental results that the effect on the

237

thermodynamics was more obvious after the temperature was raised to 80 ℃, thus the NO removal

238

efficiency did not rise further but tended to be constant in Fig. 2.

239 240

Fig.2. Effect of temperature on NOx and SO2 removal in the coal-fired flue gas using the

241

PMS/Fe(II)/RPB system. Conditions: pH = 3.0; CPMS = 10 mmol/L; QG = 0.17~0.18 m3/s; QL

242

= 1.9~2.0 L/s and RCF =322 g.

243 244

12


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245

Energy & Fuels

Table 1. The mass transfer coefficients of NO in gas and liquid phase at the different parameters. kNO,L × 103

kNO,G × 104

aNO

m/s

mol/(m2•pa•s)

m-1

295

4.314

7.211

3400.45

303

4.899

8.239

3329.17

313

5.701

9.705

3247.10

323

6.571

11.370

3171.96

333

7.502

13.25

3102.32

343

8.612

15.36

3039.27

0.112

6.907

12.142

2625.36

0.152

7.988

14.177

2891.34

QG

0.178

8.611

15.361

3039.27

(m3/s)

0.206

9.200

16.543

3182.87

0.234

9.81

17.648

3313.67

0.255

10.219

18.435

3404.89

Parameters

T(K)

246

3.2.2. Intensity of free radicals changes with temperature.

247

From the obtained data, it was seen that increasing temperature had a significant positive influence

248

on NO removal. Moreover temperature increasing could active PMS solution to dissolve more SO•4 - ,

249

•OH

250

HSO5- +heat→SO•4 - + •OH

(21)

251

HSO5- + SO25 - →HSO4- + SO24 - + 1O2

(22)

and 1O2 radicals (Eqs. (21) and (22)).7,24,28

252

Through the detection of ESR, three kinds of characteristic peaks were captured at 22 ℃ and 70 ℃.

253

At 22 ℃, these constants of hyperfine splitting measured such as aN=14.8 Gs and aH=14.7 Gs

254

maintained good consistency with the data such as aN=15.0 Gs and aH=14.7 Gs from the other

255

literature.49 These parameters were the representative of •OH that reacted with DMPO forming the

256

DMPO-OH. The hyperfine splitting constants aN=13.8 Gs, aH=9.9 Gs, aH=1.47 Gs and aH= 0.75 Gs,

257

which were consistent with the previous literature parameters aN=13.9 Gs, aH=10.2 Gs, aH=1.45 Gs

13


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258

and aH=0.76 Gs.50 These parameters were the representative of SO•4 - that reacted with DMPO

259

forming the DMPO- SO4. Meanwhile, a typical triple ESR spectrum of 1O2 was also monitored and

260

the constants of hyperfine splitting aN=16.5 Gs and g=2.01 Gs were consistent with the previous

261

literature parameters.28 The SO•4 - and

262

intensity increased by about five times according the increase temperature from the ESR spectra in Fig.

263

3.

•OH

radicals intensity increased twice, and radical

1

O2

264

According to our previous research, electron transfer occurred between NO and free radicals in

265

PMS/Fe(II) oxidation system to generate nitric oxide radical (•NO). Then it reacted with molecular

266

oxygen dissolved in solution to instantaneously form •NO2 radical via a radical-radical reaction,

267

accompanied by generation of nitrate ions NO3- and nitrite NO2- via the reaction of •NO2 and H2O

268

(Eqs. (23)-(25)).51

269

NO + SO•4 - →•NO + SO24 -

(23)

270

2•NO + O2→2•NO2

(24)

271

2•NO2⇆N2O4

H2O

NO2- +NO3- +2H +

(25)

272

As shown in Fig. 4, it was desirable that the peaks of the •NO radicals were detected by the ESR test

273

in the PMS/Fe(II)/RPB system at the room temperature 22 ℃ and 70 ℃ with spin probe imidazole

274

nitroxide (IN) selected as the spin probe to capture •NO radicals. The iconic nine peaks of the •NO

275

radical were clearly detected base on NO in the liquid in an unsaturated state, and the hyperfine

276

splitting constants were the aN=0.42 mT and g=0.94 mT at the 22 ℃, with the aN=0.4 mT and g=0.9

277

mT at the 70 ℃ respectively, which agreed with the previous literature parameters the aN=0.44 mT and

278

g=0.98 mT.51 The typical nine peaks of •NO in ESR spectrum and two sets of data provided a

279

powerful evidence for a formation of imidazole -•NO compound. This indicated that there was •NO

14


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Energy & Fuels

280

radical production during the reaction process at the different temperature. Meanwhile, the signal

281

intensity of •NO radicals at 70 ℃ was twice that of •NO at 22 ℃, which strongly demonstrated

282

increase temperature not only promoted the mass production of SO•4 - , •OH and 1O2 radicals, but

283

also promoted the output of •NO radicals that had the higher activity than NO gas to react with the

284

radicals. Thus increased temperature was more conducive to the reaction between •NO and free

285

radicals, which fundamentally strengthened the removal reaction of NO. This was one of the reasons

286

why the NO removal efficiency increased with temperature as well.

287 288

Fig. 3. ESR spectra of SO•4 - ,OH• and 1O2 free radicals trapping by DMPO and TEMPO are shown

289

in the PMS/Fe(II)/RPB system at the room temperature 22 ℃ and 70 ℃, respectively (solid

290

quadrilateral represents the DMPO - SO4- , hollow quadrangle represents the DMPO - OH, and

291

circle represents the TEMPO - 1O2).

15


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 30

292 293

Fig. 4. ESR spectra of •NO radical trapping by IN shown in the PMS/Fe(II)/RPB system (a) at the 22 ℃

294

(b) at the 70 ℃.

295

3.2.3. Active energy in NO removal reaction.

296

To better understand the effects of temperature about NO removal reaction, the apparent activation

297

energy was introduced. The apparent activation energy for the NO removal reaction by PMS was

298

calculated based on the rate constants at different temperatures. The Arrhenius equation was a good

299

explanation of the correlation between temperature and reaction rate as follows:

300

lnk = lnA - RT

301

Here, k is reaction rate constant, s -1; T is the absolute temperature, k; A is the pre-exponential factor,

302

for the first order reaction, s -1, the second order reaction, mL/(mol·s); E is the activation energy,

303

kJ/mol; R is the gas constant, 8.314 J/(mol·K).

E

(26)

304

Base on the Eq. (26) and the pseudo-first-order rate constant (in table 2, the calculation formula

305

refers to S4 in SI), calculated the activation energy of NO oxidation removal around 252.4 kJ/mol

306

(R=0.972).

16


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Energy & Fuels

307 308

Fig. 5. The active energy in NO removal reaction base on the experimental data fitted.

309

3.3 Influence of rotating speed.

310

Since the centrifugal force is not only a function of the rotational speed but also a function of the

311

centrifugal radius. Therefore, it is not scientific enough to express centrifugal force only at the

312

rotational speed. In recent years, it has been reasonable to use relative centrifugal force (RCF). The

313

gravitational acceleration equation for rotating equipment is converted to Eq. (27):52

314

g = ω 2r

315

RCF=0.00001118 × r × N2

316

Where, RCF represents the relative centrifugal force, g (g=9.8m/s2); N represents the rotational speed

317

in revolutions per minute, rpm; r represents the centrifugal radius, ie the distance from the bottom end

318

of the centrifuge tube to the axis, cm.

(27)

319

The control experiment was run at a speed of 500 to 1000 rpm to ensure a super-gravity level of

320

125.8 to 503.1 g. As shown in Fig. 6, the highest removal efficiency was 78.6% under the 322 g with a

321

gas flow rate of 0.17-0.18 m3/s. When the rotation speed exceeded 322 g, the NO removal rate started

17


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

322

to decline, indicating that the positive effect of the rotational speed on NO removal efficiency began to

323

decrease as well. This observation was close to that investigated by Pei et al.53 Direct support was

324

available for the conclusions that, the high rotating speeds would accelerate the formation of smaller

325

droplets and thinner films, which resulting in an increase of the gas–liquid mass transfer rate and a

326

continuous rising state in the NO removal efficiency. Afterwards, due to the excessive rotating speed,

327

the liquid seal might be broken such that the flue gas flew through the RPB without reaction or a

328

reaction in reduced contact time, which caused the reduction of NO removal efficiency.53 In addition,

329

NO2 and SO2 achieved a removal rate of above 95% and complete removal, respectively.

330

On the other hand, from the perspective of mass transfer process, the varying NO removal efficiency

331

was analyzed under various rotation velocity. With the increase of rotating speed, the liquid phase

332

shocked on the surface of the filler into lots of finer droplets and the liquid droplets surface was

333

updated faster, which increased gas-liquid contact area to promote the mass transfer process. The above

334

reasons might increase NO removal efficiency.

335 336

Fig. 6. Effect of super-gravity level on NOx and SO2 removal in the coal-fired flue gas using the 18


Page 18 of 30

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Energy & Fuels

system. Conditions: pH = 3.0; CPMS = 10 mmol/L; QG = 0.17~0.18 m3/s; QL

337

PMS/Fe(II)/RPB

338

= 1.9~2.0 L/s and Temperature=70 ℃.

339

3.4 Influence of liquid flow.

340

As shown in Fig. 7, obviously, the NO removal efficiency had a steady rise with the increase of PMS

341

liquid flow. The process of NO2 achieved a higher removal efficiency and SO2 was complete

342

removal. As the liquid flow rate increased, the gas-liquid flow ratio became smaller, causing an

343

increase in the oxidizing liquid distributed per unit volume of gas, which was equivalent to adding the

344

oxidizing power of liquid, thereby promoting the NO removal.52,53 Meanwhile, when the liquid flow

345

rate increased, the sturdiness of the liquid film was enhanced, resulting in a decrease in mass transfer

346

resistance of liquid phase and promoting the NO absorption process. Furthermore, increasing liquid

347

flow would result in the thinning boundary layer, which reduced the mass transfer resistance on the

348

liquid side and promoted the gas-liquid mass transfer, thereby facilitating the increase of KNO,L.52,53

349

As the enhancement of chemical reactions, mass transfer step might gradually start to play a

350

momentous role as well. Therefore, as the liquid flow further increased, the growth of NO removal

351

efficiency gradually flattened.54

19


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

352 353

Fig. 7. Effect of liquid flow on NOx and SO2 removal in the coal-fired flue gas using the

354

PMS/Fe(II)/RPB system. Conditions: pH = 3.0; CPMS = 10 mmol/L; QG = 0.17~0.18 m3/s; RCF =322

355

g and Temperature=70 ℃.

356

3.5. Influence of gas flow.

357

As could be seen from the Fig. 8, NO removal emerged a significantly downward trend with the

358

increase of gas flow from 0.112 to 0.255 m3/s. This could be explained by the fact that an increase in

359

the gas flow rate was equivalent to a decrease in the liquid oxidant and oxidizing power of liquid and

360

gas-liquid contact time. With the increase of gas flow, the liquid phase mass transfer coefficient KNO,L

361

slightly increased due to the surface of liquid droplets and films constantly updated (Table. 1).

362

However, that was not sufficient to counteract the effect of increased gas flow on the NO removal. The

363

NO removal in the PMS solution was a process of liquid phase mass transfer control, and KNO,L

364

played a dominant role in the entire mass transfer process. Therefore, increasing the gas flow had a

365

very little effect on the KNO,L, which not enough to influence the general trend of NO removal.52

366

Moreover, there was no significant reduction in the removal efficiency of NO2, and SO2 could still be

20


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367

Energy & Fuels

completely removed.

368 369

Fig. 8. Effect of gas flow on NOx and SO2 removal in the coal-fired flue gas using the PMS/Fe(II)/RPB

370

system. Conditions: pH = 3.0; CPMS = 10 mmol/L; Temperature=70 ℃; QL = 1.9~2.0 L/s and RCF

371

=322 g.

372

3.6 . The kinetics of NO removal in the PMS/Fe(II)/RPB system.

373

3.6.1. The reaction order.

374

In order to further strengthen the NO removal process, the effect of PMS concentration on NO

375

removal was investigated and the kinetics of absorption reaction was studied. These investigate results

376

would provide more systematic and comprehensive technical parameters for amplification industrial

377

experiments.

378 379 380

The overall reaction equation between PMS and NO can be described by Eq. (28): HSO5- +NO + H2O→SO24 - +NO3- + 3H +

(28)

Base on the Eq. (28), the NO removal chemical reaction rate equation can be expressed as follows:

381

n rNO = KCm NO,iCPMS

382

Where CNO,i is interface concentration of NO, mol/L; m and n are the partial reaction order for NO

(29)

21


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Page 22 of 30

383

and PMS, respectively; H2O as a solvent, its concentration is generally considered to be a constant, so

384

it is not within the scope of investigation in this study.26,55 Establish a mass transfer rate equation based

385

on the double membrane theory：

386

NNO = kNO,L（CNO,i - CNO,L）

387

Where， NNO is the mass transfer rate of NO, mol/(m2s); CNO,L is the concentration of NO in liquid

388

phase, mol/L.

389

(30)

According to the mass transfer rate Eq. (30) and Henry's law, the expression of CNO,i can be derived

390

as follows:

391

CNO,i = HNO,L(PNO,G - kNO,G)

392

Where, HNO,L solubility coefficient of NO in liquid phase can be calculated by the Henry's law,

393

mol/(L·Pa); P NO,G is the gas partial pressure of NO in gas phase body, Pa; Based on the order analysis

394

method, as the

395

Eq. (31) can be simplified to Eq. (32):26,55

396

CNO,i = HNO,LPNO,G

397

NNO

( ) ≈ 10 NNO

(31)

-2

kNO,G

~10

-5

≪ PNO,G ,

NNO kNO,G

can be omitted and deleted approximatively. So the

(32)

To determine the m and n value, the absorption rate of NO in PMS solution can be described as

398

follows on account of double membrane theoretical and reaction rate Eq. (33):26,55

399

NNO,L = (

400

+1 n 1/2 2km,nDNO,LCm NO,i CPMS

m+1

(33)

)

By making the logarithm on both sides of Eq. (34), the relationship between ln NNO,L and ln CNO,i

401

can be obtained.

402

lnNNO,L = 2ln

1

2km,nDNO,LCnPMS

(

m+1

)+

m+1 ln(CNO,i) 2

(34)

403

Based on the data of NO and PMS concentration versus NO removal efficiency and the Eqs. (33)

404

and (34), ln NNO,L and ln CNO,i were fitted to a straight line with the good correlation coefficients of

22


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Energy & Fuels

405

0.99 and 0.975, respectively (Fig. 9 (a)). Two fitted line slopes (mNO/350+1) / 2 = 0.94, and (mNO/500+1)

406

/ 2 = 1.10, This was, mNO/350=0.88≈1.0 and mNO/500=1.2≈1.0. Therefore, NO removal process in the

407

PMS/Fe(II)/RPB system could be regarded as a pseudo first-order reaction under the import NO

408

concentration 350 mg/m3 and 500 mg/m3, respectively.

409

The value of N could be determined according to m≈1, through converting Eq.(34) to Eq.(35), and NNO,L

410

fitting ln CNO,i and ln CPMS.

411

ln CNO,i = 2ln (km,nDNO,L) + 2ln(CPMS)

NNO,L

1

n

(35)

412

As shown in Fig. 9 (b), ln NNO,L/CNO,i and ln CPMS are fitted to a straight line with the good

413

correlation coefficients of 0.976 and 0.982, respectively. Two fitted line slopes were nNO/350/2 = 0.92

414

and nNO/350=1.84 with nNO/500/2 = 1.07 and nNO/500=2.14. Therefore, according to the reaction order

415

of the PMS, it could be considered that the NO absorbing reaction in the PMS/Fe(II)/RPB system was

416

pseudo-1.84-order reaction under the import NO concentration 350 mg/m3, and the reaction of

417

absorbing NO in the PMS/Fe(II)/RPB system was pseudo-2.14-order reaction under the import NO

418

concentration 500 mg/m3, respectively.

419 23


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420

Fig. 9. (a) Logarithmic fit of NNO,L and CNO,i (b) Logarithmic fit of

421

import NO concentration 350 mg/m3 and 500 mg/m3, respectively.

422

3.6.2. Liquid-phase reaction utilization efficiency.

423

Hatta number

Page 24 of 30

NNO,L CNO,i

and CPMS, under the

424

The Hatta number M, represents the ratio of the reaction rate to the mass transfer rate in the liquid

425

phase and is an important parameter for mass transfer kinetics. M can be derived from the following Eq.

426

(36): 41,55

427

M=

428

DNO,LK

(36)

k2NO,L

The value of M calculated and summarized in Table 2.

429

M can be used as a criterion for the rate of chemical reaction. When M ≪ 1, it means that the liquid

430

membrane reaction capacity is much smaller than the transfer ability. At this time, the chemical

431

reaction diffuses into the liquid body, which is considered to be a slow reaction. When M ≫ 1, the

432

reaction rate is much higher than the mass transfer rate, and the reaction proceeds is carried out in the

433

liquid film. In this case, NO removal process occurs in the liquid film, indicating that the NO removal

434

reaction is considered to be a fast reaction. According to Eq. (36) the M has been calculated and

435

summarized in Table 2, it can be judged that the NO removal reaction is a fast reaction.

436

Enhancement factor

437

For an irreversible pseudo-first-order reaction, in line with the double-film theory, the Enhancement

438

factor E of chemical absorption reaction can be estimated by means of the following Eq. (37): 41,55

439

E=

440

Where α is the ratio of liquid to liquid film thickness;

441

M[ M(α - 1) + th M]

(37)

M(α - 1)th M + 1

When M»1, th M tends to 1, and the E = M can be obtained by simplifying the equation (37).55

24


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Energy & Fuels

442

And the values of E were calculated and summarized in Table 2. Base on the effect of the absorption

443

enhancement factor, the proportion of the liquid phase mass transfer resistance was lowered, thereby

444

accelerating the absorption rate.

445

Table 2. The Diffusion Coefficients, reaction rate constant, Hatta number and Enhancement factor at

446

the temperature 343K.

447

DNO,L

K

M

E

5.95 × 10 -9 m2/s

5.81 × 104 S -1

4.67

2.182

3.6.3. Liquid-phase reaction utilization efficiency.

448

Further study of gas-liquid reaction kinetics on the basis of liquid-phase utilization efficiency, and

449

measurement of liquid-phase utilization efficiency can be estimated by means of the following Eq.

450

(38).55

451

η=α

452

where, α is the ratio of liquid to liquid film thickness; M, the dimensionless number (Hatta number),

453

indicates the ratio of the reaction rate to the mass transfer rate in the liquid phase.

454 455 456

M(α - 1) + th M

(38)

M[(α - 1) Mth M + 1]

According to the above study, the NO removal reaction with PMS solution has a fast reaction and M was much larger than 1, ie th M tend to 1. So Eq. (38) could be simplified to Eq. (39) η=α

1 M

.

(39)

457

Since α stands for the ratio of liquid to liquid film thickness and is much greater than 1 (in general,

458

α was between 100-104)55 and M value is larger than 1. Hence η is a very small value, closes to 0,

459

which further indicates that NO removal reaction using the innovative PMS/Fe(II)/RPB system can be

460

regarded the reaction process is completed in the liquid film, and the average reaction concentration of

461

liquid phase approaches to 0.

462

4. Discussion on the application of this technology 25


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

463

The results above showed that NOx and SO2 could be efficiently removed using the PMS/Fe(II)/RPB

464

advanced oxidation method in the pilot stage. These advantages of small area, low investment and

465

simple process made the removal system more promising for small and medium industrial furnaces and

466

boilers. Moreover, from the previous works and the above measuring results,31-33 it can be seen that

467

NOx and SO2 are oxidized into KNO3 and K2SO4 in the reaction system via absorption

468

oxidation. KNO3 was widely used in agricultural markets, and was a binary compound fertilizer.

469

K2SO4 was also used to prepare other potassium salts, fertilizers, and the like. The KNO3 and K2SO4

470

aqueous solutions can be further converted to recoverable solids by crystallization and evaporation.

471

Obviously, this new process will not cause the secondary pollution. Therefore, simultaneous removal

472

of NOx and SO2 using this PMS/Fe(II)/RPB removal system will be feasible in the future application.

473

5. Conclusions.

474

In this study, using of the AOPs in the PMS/Fe(II)/RPB system for multi-pollutants (NO, NO2 and

475

SO2) reduction in coal-fired flue gas obtained the well removal effect. The results indicated that the NO

476

removal efficiency reached 70% or higher, while the SO2 and NO2 were almost completely removed

477

and were hardly affected by other parameters changes. Both increased temperature and liquid flow led

478

to elevate conversions of NO in the PMS/Fe(II)/RPB system. The increased rotational speed resulted in

479

an increase in NO removal efficiency. When the super-gravity level reached to 322 g, the removal

480

efficiency reached a maximum. As the super-gravity level continued to increase, the removal rate

481

decreased slightly. Nevertheless, gas flow increased resulted in a decrease in NO removal efficiency.

482

Furthermore, the mass transfer coefficient obtained by the classical Danckwerts absorption system

483

method was significantly higher than that of the conventional bubbling reaction, indicating that NO

484

removal reaction had a better mass transfer process in the PMS/Fe(II)/RPB system. Finally, the NO

26


Page 26 of 30

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Energy & Fuels

485

removal processes in the PMS/Fe(II)/RPB oxidation systems studied was accompanied by a

486

pseudo-first-order reaction, and was considered to be a fast reaction of the reaction process completed

487

in the liquid film base on the investigate of Hatta number, strengthening factors and liquid-phase

488

reaction utilization efficiency. In summary, this research conducted a pilot study on the reaction

489

mechanism, gas-liquid mass transfer and kinetics of the NO removal process via PMS absorbing in a

490

rotating super-gravity environment. On this basis, the main reaction parameters were optimized to

491

provide theoretical basis and support for industrial applications. For many years, the pilot test has not

492

received enough attention in China, and the “pilot test gap” phenomenon is more serious. Therefore,

493

our research is more practical and creative.

494

Acknowledgment

495 496 497

This project was supported by the Natural Science Foundation of China (51678118). Supporting Material Calculation methods, equations of physical and mass transfer parameters were added in the

498

supporting material.

499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514

Reference: (1) Liu, Y. X.; Wang, Q.; Pan, J. F. Novel process of simultaneous removal of nitric oxide and sulfur dioxide using a vacuum ultraviolet (VUV)-Activated O2/H2O/H2O2 system in a wet VUV-Spraying reactor. Environ. Sci. Technol. 2016, 50, 12966-12975. (2) Pillai, C. K.; Chung, S. J.; Raju, T.; Moon, I. S. Experimental aspects of combined NOx and SO2 removal from flue-gas mixture in an integrated wet scrubber-electrochemical cell system. Chemosphere 2009, 76, 657-664. (3) Man, C. K.; Gibbins, J. R.; Witkamp, J. G.; Zhang, J. Coal characterization for NOx prediction in air-staged combustion of pulverised coals. Fuel 2005, 84 (17), 2190-2195. (4) Guo, Q. B.; Sun, T. H.; Wang, Y. L.; He, Y. Spray absorption and electrochemical reduction of nitrogen oxides from flue gas. Environ. Sci. Technol. 2013, 47, 9514-9522. (5) Zhao, Y.; Han, Y. H.; Ma, T. Z.; Guo, T. X. Simultaneous desulfurization and denitrification from flue gas by ferrate (VI). Environ. Sci. Technol. 2011, 45, 4060-4065. (6) Hosoi, J.; Hiromitsu, N.; Riechelmann, D.; Fujii, A.; Sato, J. Simple Low NOX combustor technology. IHI Eng. Rev. 2008, 41, 46-50. (7) Adewuyi, Y. G.; Khan, A. M. Nitric oxide removal by combined persulfate and ferrous-EDTA 27


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Removal of NOx and SO2 in the Coal-fired Flue Gas Using the

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