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Investigation on removal characteristics of SO3 acid mist during limestone-gypsum desulfurization process Danping Pan, Dongping Zhang, and Wendi Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03041 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on October 29, 2018

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Energy & Fuels

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Investigation on removal characteristics of SO3 acid mist during limestone-gypsum

2

desulfurization process

3

Pan Danping *, Zhang Dongping, Zhang Wendi

4

School of Environmental Engineering, Nanjing Institute of Technology, No.1 Hongjing Road, Nanjing,

5

211167, China

6

ABSTRACT: With the increasing attention on SO3 emission in coal-fired power plants, the removal of

7

SO3 acid mist in limestone-gypsum wet flue gas desulfurization (WFGD) system was investigated. The

8

generation properties of SO3 acid mist were analyzed with an experimental system and the effect of

9

flue gas properties and technologies for ultra-low emission on the SO3 removal in WFGD systems were

10

investigated with a pilot plant. The results revealed that the sizes of SO3 acid aerosols generated by

11

homogeneous nucleation were principally less than 0.1μm and it had positive impact on the coagulation

12

and growth of fly ash particles via the collision. When the gas temperature at the inlet of the scrubber

13

went up, the removal efficiency of WFGD system on SO3 acid mist was decreased and the decline was

14

more obvious as the temperature fell below dew point of the acid. Higher concentrations of SO3 and fly

15

ash before desulfurization were beneficial for the improvement of SO3 removal. Furthermore,

16

technologies for ultra-low emission improved the removal during the desulfurization process. The

17

removal efficiencies in different WFGD systems were increased by 18% to 28% and the enhancement

18

effect of these technologies was shown in order as the addition of tower component＜single-scrubber

19

with dual-cycle＜dual-scrubber with dual-cycle.

20

Key words: SO3 acid mist; Wet flue gas desulfurization; Generation; Removal; Ultra-low emission

1

ACS Paragon Plus Environment

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1、Introduction

22

With the higher demands of national regulations on environmental problems, more and more

23

attention has been paid to the control of pollutant emissions in coal-fired power plants. SO3, among

24

these pollutants, has been recognized as an important one in the coal-fired flue gas. Generally,

25

0.5-2.0% of SO2 was oxidized into SO3 during the coal combustion process

26

generated with the SO2 oxidation during the subsequent process of selective catalytic reduction (SCR)

27

denitration

28

was decreased. Correspondingly, SO3 acid mist was generated when the gas temperature fell below

29

dew point of the acid. The existence of SO3 acid mist gave rise to harmful influences on the equipment

30

operation and the environment such as the corrosion of devices and pipelines, visible plumes and the

31

generation of secondary aerosols [5-6].

[2-3].

[1]

and more SO3 was

The SO3 then turned into H2SO4 in the air preheater (APH) [4] where the temperature

32

When the coal-fired flue gas came into the desulfurization scrubber, the temperature was quickly

33

fell below dew point of the acid and the generation of SO3 acid mist easily occurred [7-8]. Chang Jingcai

34

et al

35

several to dozens of microns. Schaber K et al

36

formation of SO3 acid mist during the cleaning process of the flue gas and it was concluded that

37

submicron SO3 acid aerosols were generated. Hence the further investigations on this aspect were

38

needed. Meanwhile, fly ash particles from the combustion process were in the coal-fired flue gas while

39

the relationship between the fly ash particles and SO3 was barely taken into consideration in current

40

investigations. Furthermore, the SO3 acid mist was partly removed via the scrubbing of desulfurization

41

slurry in the WFGD system. According to the current experimental and computational investigations

[9]

found that the sizes of SO3 acid mist formed during the desulfurization process ranged from [10-12]

experimentally and numerically studied the

2


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[13-17],

43

made site measurements of different coal-fired utility boilers and found that the removal efficiencies of

44

desulfurization systems on SO3 acid mist were related to the flue gas components. Pan D et al

45

investigated the effect of different coals on the removal of SO3 by the desulfurization system and the

46

results showed that the physical properties of SO3 and fly ash at the inlet of the scrubber were

47

connected with the removal efficiency. In addition, in order to improve the removal of SO2 and

48

particles in the WFGD system, technologies for ultra-low emissions have been used in industry in

49

recent years, such as the addition of tower component, single-scrubber with dual-cycle desulfurization

50

system, dual-scrubber with dual-cycle desulfurization system and so on. Pan D et al

51

investigations on the SO3 removal efficiencies by different desulfurization systems and the results

52

revealed that the dual-scrubber with dual-cycle desulfurization system could achieve higher removal

53

efficiency than the conventional system. But the current investigations on these technologies were

54

mainly focused on the achievement of efficient SO2 removal. Overall, to reduce the emission of SO3

55

acid mist, it was essential to improve the removal of SO3 acid mist in the WFGD system. However, this

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aspect has not been investigated extensively. As a consequence, it is of great importance to further

57

investigate the removal properties of SO3 acid mist during the desulfurization process.

the removal efficiency was generally inefficient which was mainly below 50%. Cao Y et al

[20]

[18]

[19]

made

58

In this paper, armed with an experimental system, the generation properties of SO3 acid mist were

59

analyzed. Moreover, the effect of flue gas properties and technologies for ultra-low emission on the

60

SO3 removal in WFGD system were investigated with a pilot plant. The aim is to further reveal the

61

removal properties of SO3 acid mist in WFGD system and provide the optimization conditions in

62

desulfurization systems for SO3 reduction. 3


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2、Experimental details

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2.1 Experimental system

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The experimental system, as shown in Fig. 1(a), was set up to study the generation properties of SO3

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acid mist. SO3 was generated via the SO3 generator, which applied the reaction between SO2 and O3.

67

The water and fly ash particles from certain coal-fired power plant could be added into the reactor to

68

adjust the humidity and particle concentrations. The simulated flue gas was heated up to 150℃ at the

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inlet of the reactor and then it was cooled with the water jacket. SO3 acid mist was generated in the

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reactor as the temperature fell below dew point of the acid. The size distributions of fine particles in the

71

simulated flue gas were measured during the experiments.

72

The schematic diagram of the pilot plant was illustrated in Fig. 1(b). The system contained a

73

coal-fired boiler, a buffer vessel, an ESP system and a WFGD system. The volume flux of simulated

74

flue gas was up to 350 Nm3·h-1 from the coal-fired boiler, and the buffer vessel was used to guarantee

75

the uniformity of particle concentrations. The gas temperature at the inlet of the WFGD system was

76

adjusted by the heat exchanger. And the additions of fly ash and SO3 were used to adjust the

77

concentrations of particles and SO3 before desulfurization, respectively. The fly ash was collected from

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flue gas after ESP in the certain power plant. The WFGD system adopted sprayed scrubber with the

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technology of limestone-gypsum desulfurization whose diameter and height were 200mm and 6500mm,

80

respectively. Meanwhile, reconstructions of technologies for ultra-low emission were carried out with

81

this system. The measurement locations were set at the inlet and outlet of the spary scrubber.

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2.2 Measurement technique

4


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The concentration and size distribution of fine particles were measured in real time by means of the electrical low-pressure impactor (ELPI, Dekati Ltd, Finland) which had a measurement range from 0.023μm to 9.314μm. The mass concentrations of fly ash were measured with the WJ-60B automatic smoke sampler. According to the national standard (GB/T21508-2008), the sampling process of the SO3 acid mist was illustrated in Fig. 2. A heated sampling tube was used to extract the flue gas and the temperature was heated up to 260℃. Then the SO3 acid mist was captured in the next spiral condensers whose temperature was around 65℃. The ion chromatograph (ICS-2100, Dionex Ltd, USA) was used to measure the content of SO42- and then the concentration of the SO3 acid mist was determined. The removal efficiency of SO3 acid mist was calculated via the formula as follows.



c1  c2 100% c1

(1)

Where  is the removal efficiency, c1 is the inlet mass concentration and c2 is the outlet mass concentration. 3、Results and discussion 3.1 Generation properties of SO3 acid mist Armed with the experimental system, the generation properties of SO3 acid mist were studied. During the experiments, the gas temperatures at the inlet and outlet of the reactor were controlled at 150℃ and 50℃, respectively. The size distributions and concentrations of fine particles were obtained by the ELPI with the results as shown in Fig. 3 and Fig. 4. Fig. 3 illustrated the size distributions of SO3 acid droplets which were generated by the homogeneous nucleation during the cooling process. As can be seen in Fig. 3, number concentrations were obviously decreased with the increase of the sizes 5


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103

and the majority of SO3 acid droplets were of sizes less than 0.1μm. According to the current

104

investigations

105

nucleation of H2O-H2SO4 system and the reactions can be expressed as follows.

[21],

the homogeneous nucleation of SO3 acid aerosols was caused by the binary

106

SO3  H 2 O  H 2 SO4

(2)

107

H 2 SO4  H 2 O  H 2 SO4  H 2 O

(3)

108

H 2 SO4  H 2O  H 2O  H 2 SO4  2 H 2O

(4)

H 2 SO4  nH 2 O  H 2 O  H 2 SO4  (n  1) H 2 O

109

(5)

110

Where

111

process, the high supersaturation degree and rapid nucleation rate were achieved[22-23], leading to the

112

generation of submicron SO3 acid droplets. Meanwhile, as particles of fly ash existed in the flue gas,

113

the effect of SO3 on the particle properties was investigated. The fly ash was taken from flue gas after

114

ESP in the certain power plant and the molar ratio of fly ash and SO3 was adjusted at 2.0 during the

115

experiment. The particle concentration and size distribution in the simulated flue gas before and after

116

the addition of SO3 were shown in Fig. 4. As can be seen in Fig. 4(a), the number concentration was

117

significantly increased from 3.2×106 cm-3 to 4.5×106 cm-3 after the addition of SO3, which can be

118

attributed to the formation of SO3 acid droplets. The results in Fig. 4(b) showed that number

119

concentration of particles with size less than 0.2μm was increased, while that of micron particles was

120

decreased. During the desulfurization process, the existence of SO3 had influence on the particle size

121

distributions from the two following aspects: (1) SO3 acid aerosols generated by homogeneous

122

reactions and heterogeneous reactions; (2) the positive impact on the coagulation of fly ash via the

123

collision. As can be seen in Fig. 3, the SO3 acid aerosols generated by homogeneous reactions were

n

is the water molecule number of one hydrate. Based on the conditions of the desulfurization

6


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principally less than 0.1μm, leading to the increase of submicron particle concentrations. Meanwhile,

125

SO3 has higher possibility to adsorb on larger fly ash particles [24], giving rise to the coagulation of

126

larger particles. With the growth of these fly ash particles, the number concentration of micron particles

127

was correspondingly decreased.

128

3.2 Effect of flue gas properties on the removal of SO3 acid mist

129

3.2.1 Effect of flue gas temperature before desulfurization

130

Generally, the flue gas temperature before desulfurization ranged from 120 ℃ to 140 ℃ . With the

131

application of low-low ESP in industry, the inlet flue gas temperature would be decreased to about 90

132

℃. Since the temperature of the flue gas played an important role on the formation of SO3 acid mist,

133

armed with the pilot plant, the effect of inlet flue gas temperature on the removal performance of SO3

134

acid mist in WFGD system was studied. The experiments were conducted with temperature ranging

135

from 80 ℃ to 140 ℃ and the liquid-gas ratio was 15Lm-3 with results shown in Fig. 5. With the

136

temperature went up from 80 ℃ to 140 ℃ , the removal efficiency of SO3 acid mist was decreased

137

correspondingly from 43.8% to 38.2% and the decline was more obvious when the temperature fell

138

below dew point of the acid. According to the A.G.Okkes Equation[25], the dew point of the acid of

139

coal-fired flue gas was generally below 120℃, thus the temperature was increased across the path of

140

dew point of the acid. When the temperature fell below dew point of the acid, SO3 turned into SO3 acid

141

mist and partly absorbed onto fly ash which became larger via the coagulation. It was further cooled in

142

the spray scrubber. Then the SO3 acid droplets and fly ash were partly trapping into the desulfurization

143

slurry. The decrease of flue gas temperature before desulfurization was beneficial for the capture of

144

SO3 acid mist. Meanwhile, when the temperature was above dew point of the acid, the SO3 acid mist 7


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was generated in the spray scrubber. As the inlet temperature went up, the degree of supersaturation

146

was higher, resulting to the smaller SO3 acid droplets. Hence the removal efficiency was decreased.

147

3.2.2 Effect of SO3 concentrations before desulfurization

148

As sulfur contents were discrepant in different coals and different catalysts were used in the SCR

149

system, the SO3 concentration was varied before desulfurization. Fig. 6 illustrated the removal

150

efficiency of SO3 acid mist by WFGD system at different inlet SO3 concentrations. The SO3

151

concentration was adjusted by the SO3 generator and the temperature of the inlet flue gas was around

152

125℃. As can be seen in Fig. 6, the removal efficiency was increased from 32.2% to 46.2% when the

153

inlet SO3 concentration went up from 10 mg·m-3 to 70 mg·m-3. As the generated SO3 acid droplets were

154

principally less than 0.1μm, the removal of these droplets during desulfurization was inefficient. With

155

the increase of SO3 concentration, the number concentration did not change considerably while the SO3

156

acid droplets turned to be larger

157

desulfurization system. Therefore, the removal of SO3 acid mist in WFGD system was improved with

158

higher SO3 concentrations before desulfurization. However, the outlet SO3 concentration was

159

correspondingly increased and more SO3 acid mist was emitted into the atmosphere. As a consequence,

160

to effectively control the SO3 acid mist emission after desulfurization, it was appropriate to enlarge the

161

SO3 acid droplets in the WFGD system and reduce the SO3 concentration before desulfurization.

162

3.2.3 Effect of fly ash concentrations before desulfurization

[12],

which was to the benefit of the capture of SO3 acid mist in the

163

As the ash contents of coals and operating conditions of ESP were different, the inlet

164

concentrations of fly ash before desulfurization were varied. The fly ash concentration was adjusted by 8


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means of the solid aerosol generator and the temperature of inlet flue gas was around 125 ℃. Fig. 7

166

illustrated the removal efficiency of SO3 acid mist by WFGD system at different inlet concentrations of

167

fly ash. When the concentration of fly ash went up from 40 mg·m-3 to 100 mg·m-3, the removal

168

efficiency of SO3 acid mist was increased from 37.6% to 44.3%. When the concentration of fly ash

169

became higher, the particle quantity rose, giving rise to greater contact probability between SO3 and fly

170

ash particles. Thus more SO3 was absorbed onto fly ash particles which was to the benefit of the

171

coagulation and growth of these particles. With the capture of these particles due to the desulfurization

172

slurry scrubbing, the SO3 acid mist was removed from the flue gas simultaneously.

173

3.3 Effect of technologies for ultra-low emission on the removal of SO3 acid mist

174

The technologies for ultra-low emission attracted increasing attention in recent years and

175

reconstructions of traditional desulfurization systems were performed in coal-fired power plants, such

176

as the addition of tower component, single-scrubber with dual-cycle and dual-scrubber with dual-cycle.

177

These technologies were beneficial for the removal of SO2 and fly ash particles. According to structural

178

designs of these technologies, the corresponding parts and devices were produced. Then the

179

reconstructions mentioned above were performed based on the pilot plant. Then investigations on the

180

removal efficiencies of SO3 acid mist by different WFGD systems were conducted. In an attempt to

181

ensure the accuracy, the coal quality, the boiler load and the flue gas temperature were kept the same

182

during the experiments. The inlet temperature of the flue gas and the liquid-gas ratio were 125℃ was

183

15Lm-3, respectively. As can be seen in Fig. 8, compared with the traditional desulfurization system

184

with a spray scrubber, these technologies improved the removal of SO3 acid mist. The scrubber

185

equipped with trays leaded to the trend of uniform distribution of flow field and due to the disturbance 9


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186

of flow field, the contact possibility between particles in the flue gas and droplets of desulfurization

187

slurry was getting higher. Thus the removal efficiency of SO3 acid mist was increased from 40.2% to

188

58.1% compared with the traditional WFGD system. The single-scrubber with dual-cycle had an

189

oxidation zone and an absorption zone, which were controlled separately. The pH values of

190

desulfurization slurry in oxidation zone and absorption zone were around 4.5 and 6.5, respectively. The

191

higher pH value was to the benefit of the removal of SO3 and the removal efficiency of SO3 acid mist

192

was increased to 60.4%. Similarly, the dual-scrubber with dual-cycle had two scrubbers with pH values

193

of desulfurization slurry around 4.5 and 6.5, respectively. As this WFGD system had longer residence

194

time and more spray layers, the removal efficiency was further increased to 68.3%. In general,

195

compared with the traditional WFGD system, the technologies for ultra-low emission were beneficial

196

for the removal of SO3 acid mist and the removal efficiencies were increased by 18% to 28%. The

197

enhancement effect of these technologies was shown in order as the addition of tower component ＜

198

single-scrubber with dual-cycle＜dual-scrubber with dual-cycle.

199

4、Conclusions

200

In an attempt to further study the removal characteristic of SO3 acid mist during

201

limestone-gypsum desulfurization process, the formation properties of SO3 acid mist were analyzed

202

with the experimental system and the effect of flue gas properties and technologies for ultra-low

203

emission on the SO3 removal in WFGD system were investigated with the pilot plant. The following

204

conclusions were drawn.

205

(1) Due to the high supersaturation degree and rapid nucleation rate, the sizes of SO3 acid mist

206

generated by homogeneous nucleation were principally less than 0.1μm. Meanwhile, the SO3 acid mist 10


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had positive impact on the coagulation and growth of fly ash particles due to the collision.

208

(2) When the gas temperature at the inlet of the scrubber went up, the removal efficiency of

209

WFGD system on SO3 acid mist was decreased and the decline was more obvious when the

210

temperature fell below dew point of the acid. Higher concentrations of SO3 and fly ash before

211

desulfurization were beneficial for the improvement of SO3 removal.

212

(3) Technologies for ultra-low emission improved the removal during the desulfurization process.

213

The removal efficiencies of SO3 acid mist by different WFGD systems were increased by 18% to 28%

214

and the enhancement effect of these technologies was shown in order as the addition of tower

215

component＜single-scrubber with dual-cycle＜dual-scrubber with dual-cycle.

216

Acknowledgments

217

The authors thank the Jiangsu Provincial Natural Science Foundation of China (BK20181023) and

218

Research Program Foundation of Nanjing Institute of Technology (YKJ201733) for their financial

219

support.

220

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wet flue gas desulfurization system on fine particles and SO3 acid mist from coal-fired power

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plants. Proceedings of the CSEE, 2016, 36(16), 4356-4362.

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[20] Pan, D. P; Yang, L. J.; Wu, H.; Huang, R. T. Removal characteristics of sulfuric acid aerosols from coal-fired power plants. J. Air Waste Manage. 2017, 67(3), 352-357. [21] Seinfeld, J. H.; Pandis, S. N.; Noone, K. Atmospheric Chemistry and Physics: From Air Pollution 13


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to Climate Change. Environ. Sci. Policy Sustain. Dev. 1998, 40 (7), 26-26. [22] Mirabel, P.; Katz, J. L. Binary homogeneous nucleation as a mechanism for the formation of aerosols. J. Chem. Phys. 1974, 60(60), 1138-1144.

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[23] Vehkamäki, H.; Kulmala, M.; Lehtinen, K. E.; Noppel, M. Modelling binary homogeneous

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nucleation of water-sulfuric acid vapours: parameterisation for high temperature emissions.

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Environ. Sci. Technol. 2003, 37(15), 3392-8.

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[24] Zhang, X. H. Studies on synergetic removal of fine particulates and SO3 by an extra cold-side

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electrostatic precipitator. Master Thesis, Master Thesis, Tsinghua University, Beijing, 2015.

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[25] Okkes, A. G. Get acid dew point of flue gas. Hydrocarb. Process, 1987, 66(7).

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Figure captions

279

Fig. 1 Schematic diagram of the experimental system and the pilot plant

280

Fig. 2 Schematic diagram of the SO3 acid mist sampling procedure

281

Fig. 3 Size distributions of SO3 acid droplets generated via homogeneous nucleation

282

Fig. 4 Properties of fine particles before and after the addition of SO3

283

Fig. 5 Effect of flue gas temperature before desulfurization on the removal of SO3 acid mist

284

Fig. 6 Effect of SO3 concentration before desulfurization on the removal of SO3 acid mist

285

Fig. 7 Effect of fly ash concentration before desulfurization on the removal of SO3 acid mist

286

Fig. 8 Effect of technologies for ultra-low emission on the removal of SO3 acid mist

15


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287

Fig. 1 Schematic diagram of the experimental system and the pilot plant

288

(a) Schematic diagram of the experimental system Thermostat water circulator

Water jacket Particles

SO3

150oC

H2O

Gas Heat tracing

289

Blower

290

(b) Schematic diagram of the pilot plant

291

16


Page 17 of 23 1 2 3 292 4 5 6 7 8 9 10 11 12 13 14 293 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

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Fig. 2 Schematic diagram of the SO3 acid mist sampling procedure thermostatic waterbath

Flue sampling gun

filter cartridge

spiral condenser

17


dust parallel sampler

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Fig. 3 Size distributions of SO3 acid droplets generated via homogeneous nucleation

7

-3

10

DN/Dlogdp /cm

1 2 3 294 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 295 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

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6

10

5

10

4

10

3

10

0.01

0.1

1 Dp/um

18


10

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296

Fig. 4 Properties of fine particles before and after the addition of SO3

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(a) Number concentration 6

Number concentration/ cm

-3

8.0x10

before adding SO3 after adding SO3

6

6.0x10

6

4.0x10

6

2.0x10

0.0

0

50

100

298 299

150

T/s

(b) Size distribution

7

before adding SO3

10 -3

after adding SO3 DN/Dlogdp /cm

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

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6

10

5

10

4

10

3

10

0.01

300

0.1

1 Dp/um

19


10

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Fig. 5 Effect of flue gas temperature before desulfurization on the removal of SO3 acid mist 60 50 Removal efficiency /%

1 2 3 301 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 302 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

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40 30 20 10 0

80

100

120

140 o

Inlet flue gas temperature / C

20


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Fig. 6 Effect of SO3 concentration before desulfurization on the removal of SO3 acid mist 60 50 Removal efficiency /%

1 2 3 303 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 304 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

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40 30 20 10 0

0

20

40

60 -3

SO3 concentration /mgm

21


80

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Fig. 7 Effect of fly ash concentration before desulfurization on the removal of SO3 acid mist 60 50 Removal efficiency /%

1 2 3 306 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 307 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

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40 30 20 10 0

40

60 80 -3 Fly ash concentration /mgm

22


100

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Fig. 8 Effect of technologies for ultra-low emission on the removal of SO3 acid mist 100 80 Removal efficiency /%

1 2 3 309 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 310 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

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60 40 20 0 spray scrubber

scrubber single-scrubber dual-scrubber with trays with dual-circle with dual-circle

23


Investigation on removal characteristics of SO3 acid mist during

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