Title Page Influences of in-furnace kaolin addition on the formation

Subscriber access provided by University of Sussex Library

Remediation and Control Technologies

Influences of in-furnace kaolin addition on the formation and emission characteristics of PM2.5 in a 1000 MW coal-fired power station Yishu Xu, Xiaowei Liu, Hao Wang, Xianpeng Zeng, Yufeng Zhang, Jinke Han, Minghou Xu, and Siwei Pan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02251 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 9, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24

Environmental Science & Technology

1

Title Page

2

Influences of in-furnace kaolin addition on the formation

3

and emission characteristics of PM2.5 in a 1000 MW

4

coal-fired power station

5

Yishu Xu a, Xiaowei Liu a, *, Hao Wang a, Xianpeng Zeng a, Yufeng Zhang a, Jinke Han a, Minghou Xu a, *,

6

Siwei Pan b

7 8

a

9 10

State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Luoyu Road 1037, Wuhan 430074, China.

b

Electric Power Research Institute of Guangdong Power Grid Corporation, Guangzhou 510080, China

11 12

*Corresponding Author.

13

E-mail: [email protected]; Tel: +86-27-87542417; Fax: +86-27-87545526

14

E-mail: [email protected]; Tel: +86-27-87546631; Fax: +86-27-87545526

15 16 17 18 19 20 21

** Submitted to “Environmental Science & Technology” *** A file as Supporting Information is attached to this manuscript. 1 ACS Paragon Plus Environment


22

Page 2 of 24

Abstract Art (TOC)

23 24 25

ABSTRACT: The impacts of in-furnace kaolin addition on the formation and emission characteristics

26

of PM2.5 from a 1000 MW coal-fired utility boiler equipped with electrostatic precipitators (ESPs) are

27

investigated for the first time ever in this contribution. Detailed characterization of the chemical

28

composition, micromorphology, melting characteristics of the fine PM, total fly ash and/or bottom ash

29

samples were carried out using the X-ray fluorescence probe, the field emission scanning electron

30

microscope coupled with an energy dispersive X-ray detector, the ash fusion analyser and the dust

31

specific resistivity analyzer. The results showed that the formation of fine PM was reduced when kaolin

32

was added, and the mass concentrations of the particulate matter with the aerodynamic diameters of ≤0.3

33

and 2.5 µm (PM0.3 and PM2.5) were reduced by 55.97% and 5.48% respectively. As expected, kaolin

34

reacted with the volatile mineral vapors (e.g., Ca, Na) and inhibited their partitioning into ultrafine PM.

35

It was interesting to find that the added kaolin modified the ash melting behaviour, and promoted the

36

capture of the ultrafine PM onto the coarse particles. What is more, the added kaolin reduced the

37

specific resistivity of the fly ash and improved their capture efficiency in the ESPs. Finally, the above

38

combined effects brought about the emission reductions of 41.27% and 36.72% for PM0.3 and PM2.5

39

after the ESPs. These results provided a direct confirmation on the feasibility of in-furnace kaolin

40

addition on the PM reduction in the realistic combustion conditions.

41

KEYWORDS: Particulate matter; Coal combustion; Kaolin; de-PM additive; Electrostatic precipitator. 2 ACS Paragon Plus Environment

Page 3 of 24

42


1. Introduction

43

PM2.5 (particulate matter with the aerodynamic diameters of ≤2.5 µm), which is enriched in

44

hazardous components and harmful to human beings, has become one of the most concerned air

45

pollutants

46

primary PM emission sources

47

electrostatic precipitators (ESPs) are equipped downstream of the boilers to dedust the flue gas

48

However, these dust collectors cannot effectively remove the fine and ultrafine PM

49

troublesome PM, various methods improving the performance of dust collectors have been tried, yet

50

they are generally costly

51

additive(s) has been proposed and rapidly developed 11-21. The “de-PM” additives are designed to reduce

52

the formation of fine/ultrafine PM during the combustion process and thereby make up for the

53

deficiencies of the dust collectors. In this way, this technique could targetedly strengthen the emission

54

control of the fine PM, and help the power stations meet emission requirements at lower costs.

55

1-3

. China is suffering a severe PM2.5 pollution and coal-fired power stations are one of the

8-10

4,5

. Currently, various PM removal devices/technologies such as

5-8

3,5

.

. To handle these

. In recent years, a novel in-furnace PM reduction technology based on

Existed studies in this area are mainly carried out on the bench/pilot scale reactors and focused on 15,16,20

14,18-21

56

the screening of additives

57

combustion conditions (e.g., temperature, atmosphere, fuel properties, etc.)

58

regarded as the most practically feasible one in the future industrial application. As reported by Chen et

59

al. 19 and Si et al. 18, adding 5% kaolin into coal could reduce the emission of PM1 and PM2.5 by 33~58%

60

and 35% during the combustion. It is generally recognized that fine PM is reduced via two pathways. On

61

one hand, the added kaolin would react with the gaseous PM precursors (e.g., Na-contained mineral

62

vapor, etc.) and inhibit their partitioning into the fine PM 14,16,19,21. On the other hand, some products of

63

kaolin would melt during the high-temperature combustion process and form liquidus substance, which

64

captures the PM via collision and aggregation 18,20. The performance of kaolin on reducing the formation

65

of fine PM is of the highest level of all the reported additives; however, it would change with the

, their PM capture mechanism

and performance under different 17-19

. Clay mineral kaolin is

3 ACS Paragon Plus Environment


Page 4 of 24

66

specific work conditions. It is reported that its PM capture efficiency would significantly decrease when

67

the temperature exceeds 1373 K because of the melting/sintering phenomenon, indicating a high

68

sensitivity to temperature

69

more complicated. Studies of Chen et al.

70

better performance in the O2/CO2 atmosphere. Nevertheless, studies of Si et al. 18 at a temperature (1773

71

K) higher than Chen et al. showed some contrary results. The above disparities seem to be resulted from

72

the different formation behaviors of the liquidus substance on the additive/products at the different

73

experimental conditions. What is more, our latest study showed that the combustion-derived species

74

such as H2O and HCl would also affect the PM capture characteristics of kaolin 17. To sum up, the PM

75

reduction performance and process of kaolin are strongly related to the combustion conditions, which

76

makes its performance and feasibility in the realistic industrial applications of great uncertainty and

77

concerns.

14,19

. The effects of combustion atmosphere on the performance of kaolin are 19

at temperature of ≤1573 K showed that kaolin exhibited a

78

In the full-scale boiler’s furnace, there are much more complicated temperature/atmosphere

79

profiles and turbulent flow conditions compared with the simplified ones in the lab, which may lead to

80

different behaviors and performances of kaolin. However, there are still no reports on the PM reduction

81

performance of kaolin in those real furnaces. So, the PM reduction efficiency of kaolin, its controlling

82

mechanism as well as the possible disparities with that in the lab studies are still unknown. What is more,

83

to evaluate the feasibility of the de-PM additive in the large scale power plants, the potential effects of

84

additives addition on the whole unit system should also be considered. In-furnace kaolin addition might

85

also change the physical/chemical properties of the fly ash (e.g., composition, resistivity, etc.) out of the

86

furnace besides of the concentration, which might affect their capture characteristics in the downstream

87

dust collectors, especially the ESPs. Therefore, the impacts of kaolin addition on the PM capture

88

efficiency of ESPs and the final PM emission are also required to be clarified.


Page 5 of 24


89

To address these gaps, kaolin was mixed into the coal at a 1000 MW utility boiler and burned

90

together with the pulverized coal in the furnace. Systemic field samplings and measurements were

91

performed at the inlets and outlets of the ESPs, and the ash/slag hoppers below the furnace. Systematic

92

information on the mass yield, size distribution, composition and morphology, melting and specific

93

resistivity characteristics of the PM, total fly ash and/or bottom ash/slag was obtained. Finally,

94

comprehensive discussions on the influence of kaolin on the formation of PM in the furnace, the PM

95

removal performance of ESPs and the emission of PM after the ESPs were performed. This is the first

96

ever study clarifying the influences of in-furnace kaolin addition on the formation and emission

97

characteristics of PM2.5 on the full scale coal-fired power stations world widely, which is desired to

98

complement the lab studies and makes great contributions to the development of the de-PM additive

99

based in-furnace PM reduction technique.

100

2. Experimental methods

101

2.1. Material

102

The field study was carried out on a 1000 MW ultra-supercritical coal-fired power station unit

103

located in Guangdong province in China (sketched in Fig. 1). Briefly, the unit is equipped with a dry-

104

bottom-furnace boiler, two ammonia-spraying SCR DeNOx reactors, two three-chamber four-electric-

105

field ESPs and a limestone-gypsum WFGD reactor. The boiler has six groups of swirling burners (i.e.,

106

B/F, C/E and D/A) located oppositely on the front and back walls in the furnace. Coal is prepared in a

107

direct-fired pulverizing system equipped with six HP type medium-speed mills. Bottom ash/slag from

108

the furnace is treated in a dry-type slag extractor and then stored in the hopper. Detailed information on

109

this unit is provided in Table S1 in the Supporting Information.



Page 6 of 24

110 111

Fig. 1. Sketch of the 1000 MW power station unit with the kaolin adding system and the distribution of

112

sampling sites.

113

Blends of a lignite and a bituminous coal (denoted as YN and SH respectively) were burned and

114

their properties are listed in Tables S2 and S3. As can be seen, the feed coal has relatively high contents

115

of Ca, S and Na. Natural kaolin powder was selected as the de-PM additive. Its chemical composition,

116

size distribution and mineral composition are presented in Table S4 and Fig. S1. Most importantly, the

117

kaolin is mainly composed of kaolinite with a little content of quartz. The volume-moment mean

118

diameter D[4, 3] of the kaolin is ~30.77 µm, which indicates that most of the kaolin are coarse particles.

119

2.2. Kaolin adding strategy

120

During the measurements, kaolin was added into coal at a constant ratio of 2.5 g-kaolin/100 g-coal.

121

The coal/kaolin blends were loaded into coal bunkers corresponding to the bottom and middle burners

122

(i.e., burner groups B/F and C/E in Fig. 1) to provide kaolin a sufficient residence time in the furnace.

123

Particularly, a kaolin adding system was designed and embedded into the coal conveying system. As

124

shown in Fig. 1, the adding system was positioned at the inlets of the standby coal feeder in the coal

125

yard, and consisted of additive storage silos, feeders and speed controllers. When loading the coal

126

bunkers, kaolin and coal fell onto the conveyer belt at the set ratio, preliminarily mixed in the coal


Page 7 of 24


127

crushers and then stored in the bunkers. Then, the crushed coal and kaolin were further mixed during the

128

coal grinding.

129

The actual work performance of the kaolin adding strategy was verified by the composition of the

130

total fly ash collected at the ESPs’ inlets, and the detailed results were provided in Fig. S2. What is more,

131

the power load, coal feeding rate and O2 content in the flue gas during the measurements were

132

monitored and exhibited in Fig. S3. As can be seen, the boiler load was stabilized at ~90% of its full

133

load and the other key operating parameters were also stable, which facilitated the study to focus on the

134

influences of kaolin addition.

135

2.3. PM sampling and analysis

136

PM samplings were performed at the inlets and outlets of the ESPs (see Fig. 1) with two DLPI 22-24

137

sampling systems, following the procedures described elsewhere

138

system consists of a cyclone (SAC-10, Dekati), a Dekati low pressure impactor (DLPI), a pressure

139

gauge and a vacuum pump. During the sampling, a stream of flue gas (10 L/min) was isokinetically

140

extracted from the center of the flue duct via a probe and introduced into the cyclone to separate the

141

particles with the aerodynamic diameters of >10 µm (PM>10). Then, the flue gas passed through the

142

DLPI in which the left particles ≤10 µm (PM10) were separated and collected into 13 stages according to

143

the particle size. All the sampling probe, cyclone, DLPI and the tubes connecting them were heated to

144

408 K, which is above the condensing temperature of SO3 (~383K) in the flue gas, to avoid the

145

condensation of acidic gases 10, 22, 23. Each PM sampling at the inlets and outlets of the ESPs lasted for

146

1.5 minutes and 90 minutes respectively to collect suitable amount of PM sample. Moreover, PM

147

samplings at the ESPs’ inlets and outlets at each experimental condition were repeated for 4 times and 3

148

times respectively, with the averages and error bars presented in the results.

. Briefly, the DLPI sampling

149

Aluminum foils and polycarbonate membranes were used in the DLPI. Aluminum foils, before and

150

after each sampling, were weighed on a microbalance (MSA6.6S-0CE-DM, Sartorius Co., accuracy of 7 ACS Paragon Plus Environment


Page 8 of 24

151

0.001 mg) to obtain the mass yield of PM. A high temperature grease (Apiezon-H) was coated on the

152

aluminum foils used. Meanwhile, PM samples collected on the polycarbonate membranes were analyzed

153

via an X-ray fluorescence probe (XRF, EAGLE III, EDAX Inc.) and a field emission scanning electron

154

microscope coupled with energy dispersive X-ray analyzer (FESEM-EDX, Sirion 200, FEI Inc.) to

155

characterize their micromorphology and chemical composition characteristics. Prior to the observation

156

under FESEM, a thin layer of platinum (Pt) was coated onto the samples to improve their conductivity 16.

157

To ensure the reproducibility of the data, three parallel runs were conducted under each condition, with

158

the average values and error bars being reported.

159

2.4. Total fly ash and bottom ash/slag

160

Samples of the total fly ash in the flue gas into the ESPs were also collected via a commercial dust 22

161

collector (Laoying 3012H smoke/gas analyzer), following the procedures described elsewhere

162

gas was extracted out of the flue duct via a probe and the entrained ash particles were collected in the

163

glass fiber filter cartridge embedded in it. The chemical composition and specific resistivity of the ash

164

samples were determined via the XRF and a dust specific resistivity analyzer (DR-3, North China

165

Electric Power University). Unfortunately, due to the malfunction of the dust collector, the mass

166

concentrations of the total fly ash were not successfully obtained in the experiments. However, the

167

measured chemical composition (see Fig. S2) agreed well with the calculated one with the factor “R

168

adding ratio”

169

fly ash, which would lead to the increase of the total fly ash emitted out of the furnace.

. Flue

being set as 2.5%. This suggested that most of the added kaolin powder migrated into the total

170

The bottom ash/slag samples were collected from the slag extractor below the furnace. Ash/slag

171

samples were put into a mortar grinder (RM200, Retsch) and grinded with identical settings (e.g., speed,

172

pressure and time). The ash/slag powders were sieved into 6 size fractions (0.3 µm

182

contents via XRF) revealed that the ultrafine PM had high contents of the volatile inorganic species such

183

as Ca, S and Na. Consistent with the previous studies

184

the volatile mineral matter in coal via the vaporization-nucleation process. It is interesting that the

185

ultrafine PM had a high Ca content, possibly due to two reasons. First, Ca in the lignite/bituminous

186

blends primarily occurred as organic-bound and/or included minerals, which underwent higher

187

combustion temperature and stronger reducing atmosphere 28,29. Thereby it vaporized and migrated into

188

the ultrafine PM at a great extent. Second, coal and mineral particles experienced a much higher

189

temperature and more violent fragmentation in the real furnace than in the bench/pilot scale reactors,

190

which would also promote the vaporization of mineral matter. Refractory elements such as Si and Al

191

were also observed in the ultrafine PM, which was resulted from the reduction-vaporization-nucleation

192

mechanism 3,30.

25

. Results in Figs. 2(a) and S4 (detailed results on elemental

3,26,27

, the ultrafine PM was mainly derived from



Page 10 of 24

193 194

Fig. 2. (a) PSDs of the PM at the ESPs’ inlets, with inserted figure giving composition of the PM0.08-0.15

195

(particles collected on the 3rd stage of DLPI); (b) PM reduction efficiency of kaolin.

196

For investigating the impacts of kaolin addition on the formation of PM2.5, the size-fractionated PM

197

reduction efficiencies of kaolin were further calculated according to Eq. (1), with the results being

198

presented in Fig. 2(b). Moreover, the mass concentrations of PM0.3, PM1 (particulate matter with the

199

aerodynamic diameters of ≤ 1 µm) and PM2.5 were calculated and listed in Table 1. The contents of

200

some typical components (e.g. Na, Ca), micromorphology and composition of PM, and the composition

201

of the bottom ash/slag were also provided in Figs. 3~5 respectively. At least three important findings

202

can be made from these data.

203

η, , , / , 100%

(1)

204 205


Page 11 of 24

206


Table 1 Mass concentrations of PM0.3, PM1, and PM2.5 without

with

Efficiency

kaolin

kaolin

of kaolin, %

PM concentration, mg/Nm3 ESP inlet PM0.3

17.56±2.21

7.73±0.89

55.97±9.56

PM1

45.88±2.65

32.86±1.61

28.38±2.15

PM2.5 307.39±22.35 290.53±25.93

5.48±0.63

ESP outlet PM0.3

0.63±0.05

0.37±0.05

41.27±5.88

PM1

2.04±0.09

1.32±0.11

35.29±3.24

PM2.5

9.15±0.19

5.79±0.37

36.72±2.44

Efficiency of ESP, % PM0.3

96.41±13.87

95.21±16.14

–

PM1

95.55±6.91

95.58±9.07

–

PM2.5

97.02±7.33

98.01±10.71

–

207

First, the generated PM was still of bimodal distribution after kaolin was added. However, as

208

expected, the addition of kaolin significantly reduced the yield of the ultrafine PM. As listed in Table 1,

209

the mass concentration of PM0.3 reduced from 17.56 mg/Nm3 to 7.73 mg/Nm3 under the experimental

210

conditions, corresponding to a reduction efficiency of 55.97%. Comparatively, the influence of kaolin

211

on the yield of coarse PM was not so significant. As can be seen, the concentrations of PM1 and PM2.5



Page 12 of 24

212

were reduced from 45.88 and 307.39 mg/Nm3 to 32.86 and 290.53 mg/Nm3 when kaolin was added,

213

corresponding to reduction efficiencies of 28.38% and 5.48%.

214

Second, the formation of the ultrafine PM appeared to be reduced by kaolin via two pathways. On

215

one hand, as shown in Fig. 3, contents of Na and Ca in the ultrafine PM were remarkably reduced after

216

kaolin was added. These phenomena indicated that kaolin and the derived meta-kaolin reacted with the

217

Na-/Ca-contained vapors and fixed them onto the additive particles, inhibiting their nucleation into

218

nuclei and thereby reducing the formation of ultrafine PM 14,17.

219 220 221

Fig. 3. Na2O and CaO contents in PM0.08-0.15, PM1.04-1.77 and PM2.65-4.45 (particles collected on stages 3, 8 and 10 of DLPI).

222

On the other hand, as shown in Figs. 4(a-d), much more ultrafine and fine particles were adhered

223

on the surface of coarse particles after kaolin was added. Further composition characterization of the

224

aggregates [shown in Figs. 4(b1-b2)] confirmed that the aggregates were exactly the clusters of the

225

above Ca-enriched ultrafine PM [Fig. 2(a)]. These direct morphology observations verified that adding

226

kaolin promoted the liquidus-substance capture of the ultrafine PM by the melting coarse PM. It’s worth

227

noting that the high content of Ca in the coal and the generated PM in this study contributed to the

228

ultrafine PM reduction in the above liquidus capture mechanism

18,20,31

. As clarified by Ninomiya et al. 12

ACS Paragon Plus Environment

Page 13 of 24


229

20

230

addition ratios. These results explained the above morphology results, and further confirmed that the

231

added kaolin promoted the formation of liquidus substance (i.e., alkali silicates, alkali aluminosilicate,

232

etc.), finally enhancing the capture of the formed ultrafine PM by collision and aggregation.

and Wei et al.

32

, kaolin would react with Ca and facilitate the formation of melt phase at certain

233 234

Fig. 4. (a-d) Micromorphology of the PM1.04-1.77 and PM1.77-2.65 (particles collected on the 8th and 9th

235

stages of DLPI) observed under FESEM and (b1-b2) the chemical composition of the representative

236

particles.

237

Third, as shown in Fig. 2(b), the reduction efficiency of the coarse PM by kaolin was lower than

238

that of the ultrafine PM. The coarse PM was mainly captured by the collision and aggregation with other

239

PM and/or kaolin particles, finally migrating into those large particles and/or the bottom ash/slag. The

240

variation of the chemical composition of the bottom ash/slag testified this to some extent. As can been

241

seen in Fig. 5, higher contents of Al2O3 and CaO were observed in the bottom ash when kaolin was

242

added, indicating that some added kaolin and ultrafine-PM-forming species (e.g., Ca) were migrated into

243

the bottom ash/slag. Yet, as the number concentration of the coarse PM was much lower than the



Page 14 of 24

244

ultrafine PM, the solid-solid collision/aggregation process between the coarse PM underwent at a lower

245

extent and thereby the coarse PM was reduced at lower efficiency than the ultrafine PM.

246 247 248

Fig. 5. Chemical composition of the pulverized bottom ash/slag. 3.2. Impacts of kaolin on the PM removal performance of ESPs

249

Figs. 6(a-b) present the PSDs of the PM at the outlets of the ESPs before and after kaolin addition,

250

coupling with the size-fractionated PM removal efficiencies of the ESPs. Interestingly, the emissions of

251

both ultrafine and coarse PM at the outlets of the ESPs were significantly reduced when kaolin was

252

added. As listed in Table 1, when no kaolin was used, the concentrations of PM0.3 and PM2.5 were

253

reduced from 17.56 and 307.39 mg/Nm3 to 0.63 and 9.15 mg/Nm3 after passing the ESPs, corresponding

254

to the PM removal efficiencies of 96.41% and 97.02% respectively. In comparison, when kaolin was

255

added, concentrations of PM0.3 and PM2.5 at the outlets of the ESPs were further reduced to 0.37 and

256

5.79 mg/Nm3, which meant that the final PM0.3 and PM2.5 emissions at ESPs’ outlets were reduced by

257

41.27% and 36.72% respectively.


Page 15 of 24


258 259

Fig. 6. (a) PSDs of the PM at ESPs’ outlets and (b) fractionated PM removal efficiency of the ESPs with

260

the resistivity of the total fly ash.

261

The size-fractionated PM removal efficiencies of the ESPs in Fig. 6(b) further confirmed the above

262

results. The promoting effects of kaolin addition on the capture behaviour of PM in the ESPs can be

263

attributed to two reasons. On one hand, as expected, the concentration of the hard-captured ultrafine/fine

264

PM at the inlets of the ESPs was reduced (Table 1), therefore less PM would penetrate the ESPs. On the

265

other hand, the specific resistivity of the ash particles into the ESPs was changed. As shown in the

266

inserted figure in Fig. 6(b), the resistivity of the total fly ash into the ESPs was reduced by one order of

267

magnitude. The effects of added kaolin on the resistivity of the fly ash were consistent well with the

268

reduced Ca and increased Si/Al contents (Figs. 3 and S4) in it, which finally enhanced the charging and

269

capture of the PM in the ESPs

270

studies, which gave an important enlightenment that the impacts on the equipment after boiler should

271

also be evaluated when screening suitable in-furnace de-PM additives.

33,34

. Above phenomena has never been observed or realized in previous

272 15 ACS Paragon Plus Environment


273

Page 16 of 24

3.3. Further discussion on the controlling mechanism of ultrafine PM reduction by kaolin

274

The ultrafine PM reduction efficiency (~56%) of the kaolin in this field study is in the range

275

(22~77%) of those reported in the lab studies 16,18,19,35. However, the controlling mechanism of ultrafine

276

PM reduction by kaolin appears to be different to a certain degree.

277

As previously mentioned, kaolin could reduce the formation of the ultrafine PM via two pathways

278

– the vapor capture and the liquidus capture. In the real furnace, there are much more turbulent flow

279

conditions than the bench/pilot scale reactors. Besides, there are higher concentrations of PM in the

280

furnace than that in lab studies, because large air/fuel ratios are normally used in the lab studies to

281

ensure the burnout, which inevitably dilutes the concentration of PM. Both of the distinctive flow

282

conditions and PM concentrations facilitate the collision between the ash particles, which is usually

283

suppressed in the moderate flow/temperature conditions in the lab scale reactors. What is more, the

284

combustion temperature in the coal-fired furnace is reported to be ~2000 K, higher than that in the

285

simulated conditions (1173~1773 K) in the lab studies

286

promote the melting behaviour of the ash/additive particles and therefore more ultrafine PM could be

287

captured by the melting surface when collided together (see Fig. 4). So, under the combustion

288

conditions in the real furnace, the capture of ultrafine PM via the liquidus mechanism is promoted. On

289

the other hand, the enhanced melting behaviour of the kaolin particles would hinder their reactions

290

with those gaseous PM precursors (e.g., Na-contained mineral vapor, etc.), thereby inhibiting the

291

capture of ultrafine PM via the vapor capture mechanism. In the end, the liquidus capture process plays

292

a much more important role in the reduction of ultrafine PM than that in the lab studies.

16,18,19,36

. The increased temperature would

293


Page 17 of 24


294

Supporting Information

295

A file as supporting information is attached to this manuscript. Tables S1−S4 and Figures S1−S4.

296

Disclosures

297

The authors declare no competing financial interest.

298

Acknowledgements

299

The authors acknowledge the financial support of the National Natural Science Foundation of China

300

(51476064), the National Postdoctoral Program for Innovative Talents (BX201700085) and the Project

301

funded by China Postdoctoral Science Foundation (2017M622438). The authors also thank the support

302

of the Analytical and Testing Center at the Huazhong University of Science and Technology and the

303

contributions of co-workers Jiang Cui, Penghui Zhang, Junzhe Guo, Zhiqiang Liao, Ming Mao, Yang

304

Peng, Jingkun Han and Ge Yu in the field sampling. The authors also appreciate the help of Wenqiang

305

Liu in revising the manuscript.

306 307

References

308

(1) Wu, S.; Deng, F.; Wei, H.; Huang, J.; Wang, X.; Hao, Y.; Zheng, C.; Qin, Y.; Lv, H.; Shima, M.;

309

Guo, X., Association of Cardiopulmonary Health Effects with Source-Appointed Ambient Fine

310

Particulate in Beijing, China: A Combined Analysis from the Healthy Volunteer Natural Relocation

311

(HVNR) Study. Environ. Sci. Technol. 2014, 48(6), 3438-3448.

312

(2) Huggins, F. E.; Senior, C. L.; Chu, P.; Ladwig, K.; Huffman, G. P., Selenium and arsenic

313

speciation in fly ash from full-scale coal-burning utility plants. Environ. Sci. Technol. 2007, 41(9),

314

3284-3289.

315 316

(3) Xu, M.; Yu, D.; Yao, H.; Liu, X.; Qiao, Y., Coal combustion-generated aerosols: Formation and properties. P. Combust. Inst. 2011, 33(1), 1681-1697.



317 318 319 320

Page 18 of 24

(4) Liang, C.; Duan, F.; He, K.; Ma, Y., Review on recent progress in observations, source identifications and countermeasures of PM2.5. Environ. Int. 2016, 86, 150-170. (5) Yao, Q.; Li, S. Q.; Xu, H. W.; Zhuo, J. K.; Song, Q., Reprint of: Studies on formation and control of combustion particulate matter in China: A review. Energy 2010, 35(11), 4480-4493.

321

(6) Lind, T.; Hokkinen, J.; Jokiniemi, J. K.; Saarikoski, S.; Hillamo, R., Electrostatic precipitator

322

collection efficiency and trace element emissions from co-combustion of biomass and recovered

323

fuel in fluidized-bed combustion. Environ. Sci. Technol. 2003, 37(12), 2842-2846.

324 325

(7) McElroy, M. W.; Carr, R. C.; Ensor, D. S.; Markowski, G. R., Size distribution of fine particles from coal combustion. Science, 1982, 215(4528), 13-19.

326

(8) Wang, C.; Liu, X.; Li, D.; Si, J.; Zhao, B.; Xu, M., Measurement of particulate matter and trace

327

elements from a coal-fired power plant with electrostatic precipitators equipped the low

328

temperature economizer. P. Combust. Inst. 2015, 35(3), 2793-2800.

329

(9) Gallego-Juárez, J. A.; Riera-Franco De Sarabia, E.; RodrÍguez-Corral, G.; Hoffmann, T. L.;

330

Gálvez-Moraleda, J. C.; RodrÍguez-Maroto, J. J.; Gómez-Moreno, F. J.; Bahillo-Ruiz, A.; MartÍn-

331

Espigares, M.; Acha, M., Application of Acoustic Agglomeration to Reduce Fine Particle

332

Emissions from Coal Combustion Plants. Environ. Sci. Technol. 1999, 33(21), 3843-3849.

333

(10) Xu, Y.; Liu, X.; Cui, J.; Chen, D.; Xu, M.; Pan, S.; Zhang, K.; Gao, X., Field Measurements on the

334

Emission and Removal of PM2.5 from Coal-Fired Power Stations: 4. PM Removal Performance of

335

Wet Electrostatic Precipitators. Energ. Fuel. 2016, 30(9), 7465-7473.

336 337

(11) Mwabe, P. O.; Wendt, J. O., Mechanisms governing trace sodium capture by kaolinite in a downflow combustor. P. Combust. Inst. 1996, 26(2), 2447-2453.

338

(12) Linak, W. P.; Miller, C. A.; Santoianni, D. A.; King, C. J.; Shinagawa, T.; Wendt, J. O.; Yoo, J.;

339

Seo, Y., Formation of fine particles from residual oil combustion: Reducing nuclei through the

340

addition of inorganic sorbent. Korean J. Chem. Eng. 2003, 20(4), 664-669. 18 ACS Paragon Plus Environment

Page 19 of 24

341 342 343 344


(13) Biswas, P.; Wu, C. Y., Control of toxic metal emissions from combustors using sorbents: a review. J. Air Waste Manage. Assoc. 1998, 48(2), 113-127. (14) Gale, T. K.; Wendt, J. O., In-furnace capture of cadmium and other semi-volatile metals by sorbents. P. Combust. Inst. 2005, 30(2), 2999-3007.

345

(15) Zhang, Y.; Liu, X.; Xu, Y.; Sun, W.; Xu, M., Investigation of reducing ultrafine particulate matter

346

formation by adding modified montmorillonite during coal combustion. Fuel Process. Technol.

347

2017, 158, 264-271.

348 349

(16) Xu, Y.; Liu, X.; Zhang, Y.; Sun, W.; Hu, Y.; Xu, M., A novel Ti-based sorbent for reducing ultrafine particulate matter formation during coal combustion. Fuel 2017, 193, 72-80.

350

(17) Chen, D.; Liu, X.; Wang, C.; Xu, Y.; Sun, W.; Cui, J.; Zhang, Y.; Xu, M., Effects of H2O and HCl

351

on particulate matter reduction by kaolin under oxy-coal combustion. Energ. Fuel. 2017, 31(6),

352

6455-6462.

353

(18) Si, J.; Liu, X.; Xu, M.; Sheng, L.; Zhou, Z.; Wang, C.; Zhang, Y.; Seo, Y., Effect of kaolin additive

354

on PM2.5 reduction during pulverized coal combustion: Importance of sodium and its occurrence in

355

coal. Appl. Energy 2014, 114, 434-444.

356 357 358 359 360 361

(19) Chen, J.; Yao, H.; Zhang, P. A.; Xiao, L.; Luo, G.; Xu, M., Control of PM1 by kaolin or limestone during O2/CO2 pulverized coal combustion. P. Combust. Inst. 2011, 33(2), 2837-2843. (20) Ninomiya, Y.; Wang, Q.; Xu, S.; Mizuno, K.; Awaya, I., Effect of Additives on the Reduction of PM2.5 Emissions during Pulverized Coal Combustion. Energ. Fuel. 2009, 23(7), 3412-3417. (21) Gale, T. K.; Wendt, J. O., High-temperature interactions between multiple-metals and kaolinite. Combust. Flame 2002, 131(3), 299-307.

362

(22) Xu, Y.; Liu, X.; Zhang, Y.; Sun, W.; Zhou, Z.; Xu, M.; Pan, S.; Gao, X., Field measurements on

363

the emission and removal of PM2.5 from coal-fired power stations: 3. direct comparison on the PM



Page 20 of 24

364

removal efficiency of electrostatic precipitators and fabric filters. Energ. Fuel. 2016.30(7), 5930-

365

5936.

366

(23) Liu, X.; Xu, Y.; Zeng, X.; Zhang, Y.; Xu, M.; Pan, S.; Zhang, K.; Li, L.; Gao, X., Field

367

measurements on the emission and removal of PM2.5 from coal-fired power stations: 1. a case study

368

for a 1000 MW ultra-supercritical utility boiler. Energ. Fuel. 2016, 30(8), 6547-6554..

369

(24) Xu, Y.; Liu, X.; Zhang, P.; Guo, J.; Han, J.; Zhou, Z.; Xu, M., Role of chlorine in ultrafine

370

particulate matter formation during the combustion of a blend of high-Cl coal and low-Cl coal.

371

Fuel 2016, 184, 185-191.

372 373 374 375

(25) Jia, Y.; Lighty, J. S., Ash Particulate Formation from Pulverized Coal under Oxy-Fuel Combustion Conditions. Environ. Sci. Technol. 2012, 46(9), 5214-5221. (26) Buhre, B. J. P.; Hinkley, J. T.; Gupta, R. P.; Wall, T. F.; Nelson, P. F., Submicron ash formation from coal combustion. Fuel 2005, 84(10), 1206-1214.

376

(27) Zhang, L.; Wang, Q.; Sato, A.; Ninomiya, Y.; Yamashita, T., Interactions among Inherent Minerals

377

during Coal Combustion and Their Impacts on the Emission of PM10. 2. Emission of

378

Submicrometer-Sized Particles. Energ. Fuel. 2007, 21(2), 766-777.

379 380

(28) Gao, Q.; Li, S.; Yuan, Y.; Zhao, Y.; Yao, Q., Role of Minerals in the Evolution of Fine Particulate Matter during Pulverized Coal Combustion. Energ. Fuel. 2016, 30(3), 1815-1821.

381

(29) Gao, X.; Rahim, M. U.; Chen, X.; Wu, H., Significant contribution of organically-bound Mg, Ca,

382

and Fe to inorganic PM10 emission during the combustion of pulverized Victorian brown coal. Fuel

383

2014, 117, 825-832.

384

(30) Xu, Y.; Liu, X.; Zhou, Z.; Sheng, L.; Wang, C.; Xu, M., The role of steam in silica vaporization

385

and ultrafine particulate matter formation during wet oxy-coal combustion. Appl. Energ. 2014, 133,

386

144-151.


Page 21 of 24


387

(31) Ninomiya, Y.; Wang, Q.; Xu, S.; Teramae, T.; Awaya, I., Evaluation of a Mg-Based Additive for

388

Particulate Matter (PM)2.5 Reduction during Pulverized Coal Combustion. Energ. Fuel. 2009, 24(1),

389

199-204.

390

(32) Wei, B.; Wang, X.; Tan, H.; Zhang, L.; Wang, Y., Effect of silicon–aluminum additives on ash

391

fusion and ash mineral conversion of Xinjiang high-sodium coal. Fuel 2016, 181, 1224-1229.

392

(33) Bickelhaupt, R. E., Volume Resistivity-Fly Ash Composition Relationship. Environ. Sci. Technol.

393

1975, 9(4), 336-342.

394

(34) Barranco, R.; Gong, M.; Thompson, A.; Cloke, M.; Hanson, S.; Gibb, W.; Lester, E., The impact of

395

fly ash resistivity and carbon content on electrostatic precipitator performance. Fuel 2007, 86(16),

396

2521-2527.

397 398

(35) Zeng, X.; Yu, D.; Xu, J.; Wu, J.; Fan, B.; Xu, M., Study on the Effect of Kaolin Addition on the PM1 Formation during a Zhundong Coal Combustion. J. Eng. Thermophys., 2015, 36, 2522-2526.

399

(36) Zhou, H.; Mo, G.; Si, D.; Cen, K., Numerical Simulation of the NOx Emissions in a 1000 MW

400

Tangentially Fired Pulverized-Coal Boiler: Influence of the Multi-group Arrangement of the

401

Separated over Fire Air. Energ. Fuel. 2011, 25(5), 2004-2012.

402



Page 22 of 24

403

List of Figures

404

Fig. 1.

405

Sketch of the 1000 MW power station unit with the kaolin adding system and the distribution of

406

sampling sites.

407

Fig. 2.

408

(a) PSDs of the PM at the ESPs’ inlets, with inserted figure giving composition of the PM0.08-0.15

409

(particles collected on the 3rd stage of DLPI); (b) PM reduction efficiency of kaolin.

410

Fig. 3.

411

Na2O and CaO contents in PM0.08-0.15, PM1.04-1.77 and PM2.65-4.45 (particles collected on stages 3, 8 and 10

412

of DLPI).

413

Fig. 4.

414

(a-d) Micromorphology of the PM1.04-1.77 and PM1.77-2.65 (particles collected on the 8th and 9th stages of

415

DLPI) observed under FESEM and (b1-b2) the chemical composition of the representative particles.

416

Fig. 5.

417

Chemical composition of the pulverized bottom ash/slag.

418

Fig. 6.

419

(a) PSDs of the PM at ESPs’ outlets and (b) fractionated PM removal efficiency of the ESPs with the

420

resistivity of the total fly ash.

421


Page 23 of 24


422

List of Supporting Information

423

Tables

424

Table S1.

425

Key parameters of the tested 1000 kW power station unit.

426 427

Table S2.

428

Proximate and ultimate analysis of the feed coal.

429 430

Table S3

431

Ash analysis of the feed coal.

432 433

Table S4

434

Chemical composition and size analysis of the kaolin used.

435



436

Figures

437

Fig. S1.

438

XRD pattern of the kaolin powder.

Page 24 of 24

439 440

Fig. S2.

441

Comparison of the measured chemical compositions of total fly ash with/without kaolin addition and the

442

calculated ones.

443 444

Fig. S3.

445

Load, coal feeding rate and the O2 content in the flue gas during the experiments.

446 447

Fig. S4.

448

Chemical compositions of the PM0.08-0.15, PM1.04-1.77 and PM2.65-4.45 (stages 3, 8 and 10 of DLPI

449

respectively) with/without kaolin addition, determined by XRF.


Title Page Influences of in-furnace kaolin addition on the formation

Recommend Documents