dimethyl ether

Journal Pre-proof Formation and nanoscale-characteristics of soot from pyrolysis of ethylene blended with ethanol/dimethyl ether Lijie Zhang, Kaixuan Yang, Rui Zhao, Yaoyao Ying, Dong Liu PII:

S1743-9671(19)30863-3

DOI:

https://doi.org/10.1016/j.joei.2019.11.012

Reference:

JOEI 668

To appear in:

Journal of the Energy Institute

Received Date: 27 June 2019 Revised Date:

21 November 2019

Accepted Date: 25 November 2019

Please cite this article as: L. Zhang, K. Yang, R. Zhao, Y. Ying, D. Liu, Formation and nanoscalecharacteristics of soot from pyrolysis of ethylene blended with ethanol/dimethyl ether, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.11.012. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd on behalf of Energy Institute.

1

Formation and nanoscale-characteristics of soot from

2

pyrolysis of ethylene blended with ethanol/dimethyl ether

3

Lijie Zhang1,2, Kaixuan Yang1,2, Rui Zhao1,2, Yaoyao Ying1,2*, Dong Liu1,2*

4 5

1 MIIT Key Laboratory of Thermal Control of Electronic Equipment, School of

6

Energy and Power Engineering, Nanjing University of Science and Technology,

7

Nanjing 210094, P. R. China.

8

2 Advanced Combustion Laboratory, School of Energy and Power Engineering,

9

Nanjing University of Science and Technology, Nanjing 210094, P. R. China.

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

*Corresponding Authors.

22

E-mail: [email protected] (D. Liu), [email protected] (Y.Y. Ying). 1

23

Abstract

24

Ethanol and dimethyl ether (DME) have been considered to be two of the most

25

potential additives for conventional hydrocarbon fuels. This paper focused on the

26

nanoscale characteristics of soot from ethylene pyrolysis with ethanol and DME

27

additions. The pyrolysis experiments were conducted in a α-alumina tube flow reactor

28

at 1273K, 1373K and 1473K, with the replacement of 0%, 50% and 100% (mole

29

fraction) ethylene by the two oxygenated fuels. The gas-phase kinetic modeling was

30

also performed to explore and understand the soot formation process. The main

31

pathways and some key soot precursors in the pyrolysis have been obtained. Soot

32

samples were characterized by high resolution transmission electron microscopy

33

(HRTEM), X-ray diffraction (XRD) and thermogravimetric analysis (TGA) to acquire

34

their internal structure and oxidation reactivity. Results showed that the mass of

35

collected soot diminished with the increase of the replacement of ethylene by

36

ethanol/DME. The effects of DME to inhibit the formation of soot were more obvious.

37

The least amount of soot was collected in the pyrolysis of pure DME. Peak mole

38

fraction of C2H2, C4H2, C4H4 and C5H5 also decreased with the increase of

39

replacement of ethylene by ethanol/DME, displaying the same tendency with the

40

variation trend of soot mass in the different pyrolysis conditions. According to TEM

41

and HRTEM results, the additions of ethanol and DME could decrease the growth rate

42

of soot contrasted with the pyrolysis of pure ethylene. Soot collected from the

43

pyrolysis of pure DME at 1273K and 1373K showed a typical amorphous structure

44

with short, highly-curved and turbulent fringe. With the reduction of the replacement 2

45

of ethylene by DME, mature soot with longer and more ordered fringe formed at

46

1373K and 1473K. The sequence of the mean fringe tortuosity of soot samples was

47

100% ethylene<50% DME<100% ethanol<50% ethanol< 100% DME. The order was

48

the same as the sequence of oxidation reactivity. Furthermore, with the increase of

49

temperature, the mass of soot increased. More mature soot with higher degree of

50

graphization, longer fringe length, smaller fringe tortuosity and lower oxidation

51

reactivity was obtained. High correlation between soot nanostructure and soot

52

oxidation reactivity was found.

53 54

Keyword: Soot; Pyrolysis; Ethanol; Dimethyl ether; Nanostructure; Reactivity

55

3

56

1. Introduction

57

Soot particles released from many combustion systems such as furnace and internal

58

engine have caused many environmental problems and could do great harm to human

59

health [1-4]. Facing to the increasingly harsh soot emission regulations, many

60

researchers began to focus on the investigations aiming to reduce soot production

61

including seeking for some alternatives to take the place of traditional fossil fuels.

62

Oxygenated biofuels such as alcohols, esters and ethers have attracted more and more

63

attentions as substitutes of conventional fuels due to their potential to reduce the

64

emission of soot and greenhouse gases [5-11].

65

Ethanol and dimethyl ether (DME) stand out prominently serving as the role of

66

substitutes of hydrocarbon fuels such as diesel and gasoline among the oxygenated

67

fuels [12-15]. Due to their different molecular structures, the two isomer oxygenated

68

fuels present various properties. More and more scholars have performed some

69

studies to obtain the combustion and pyrolysis features about the two fuels.

70

Research about the effects of ethanol or/and DME additions on the combustion and

71

pyrolysis fundamental characteristics, including intermediate species, reaction

72

pathways, gas products, ignition delay time and the flame speed, has been carried out

73

experimentally and numerically [16-23]. Barraza-botet et al. [16] performed the

74

ignition experiments of iso-octane and ethanol blends to acquire the ignition delay

75

time. Furthermore, speciation experiments were performed at 10 atm. Eight stable

76

intermediate species were measured by fast-gas sampling, gas chromatography and

77

mass spectrometry to depict the reaction pathway during the ignition. In the same year, 4

78

Hashemi et al. [17] studied the pyrolysis and oxidation experiments of ethanol in a

79

laminar flow reactor at the pressure of 50 bar and temperatures of 600-900 K. A

80

detailed chemical kinetic model was developed to predict ignition delay time and

81

flame speed of ethanol in literature. Geng et al. [18] studied the ignition delay time of

82

n-butanol/dimethyl ether mixtures in the shock tube. Wang et al. [19] focused on the

83

isomer influences on the composition of reaction intermediates in propene flames

84

with dimethyl ether or ethanol additions. The completed series of propene flames

85

blended with ethanol/DME were further analyzed by Frassoldati et al. [20] with a

86

kinetic model. Liu [21] performed the comparative study to get the chemical effects of

87

CO2 additions on ethanol/O2/Ar flame and DME/O2/Ar flame using a detailed

88

chemical mechanism. Paul et al. [22] illustrated the emission performance of a single

89

cylinder engine utilizing the blends of Diesel-DME(DEE) and Diesel-DME-ethanol.

90

However, only quite limited references focusing on the sooting behavior and

91

characteristics of ethanol and DME in different lab-scale combustion configurations

92

could be found. Kang et al. [24] studied the effects of DME additions on the soot

93

formation characteristics in ethylene premixed burner-stabilized stagnation (BBS)

94

flame and found that the soot formation rate decreased with DME additions in the

95

BBS flame. The synergistic influence of DME additions on the soot formations was

96

not observed. Luo et al. [25] carried out the experimental research about the effects of

97

dimethyl ether addition on soot formation, evolution and characteristics in flame-wall

98

interactions. Moreover, few number of studies on the sooting behavior of ethanol and

99

DME during the laboratory pyrolysis have been performed. Esarte et al. [26-28] 5

100

conducted some investigations about the pyrolysis experiments with ethanol or/and

101

DME additions. Esarte et al. [26] carried out the experimental study about the

102

pyrolysis of acetylene-ethanol and acetylene-DME blends. Results showed that both

103

of the two oxygenated fuels could inhibit soot formation. The concentration of the rest

104

gas products in the outlet stream such as CO and CO2 had some relationship with the

105

various pathways during the pyrolysis process. But Esarte et al. focused on the

106

formation of soot and gas products like CO, CO2, CH4 and C6H6 and did not pay

107

attention to the soot characteristics. And there were no detailed discussions about the

108

relationships for the key species with soot formations. Furthermore, it has been

109

identified and validated that soot structure can affect its oxidation reactivity [11, 25,

110

28]. However, there are no available studies which are devoted to explore nanoscale

111

characteristics of soot in the flow reactor pyrolysis with the two different isomer

112

oxygenated fuels additions. Pyrolysis of the fuels is the first step in the whole

113

combustion process, which plays a pretty important role. It could have great effects on

114

formation of soot. The main aim of the present work was to study the effects of

115

ethanol and DME additions on nanostructure and oxidation reactivity of soot from

116

ethylene pyrolysis, and provide deep analyses about the pyrolysis performance of

117

these two isomer oxygenated fuels as fuel additives.

118

In this study, ethylene was chosen as the primary fuel because it was one of the

119

most important compositions among hydrocarbon fuels and has been widely used in

120

the research which concentrated on the soot formation. The pyrolysis experiments

121

with 0%, 50% and 100% replacement of ethylene by ethanol/DME at 1273K, 1373K 6

122

and 1473K were conducted. The three temperatures can reflect the three different

123

stages in the pyrolysis process, which was the starting point of soot production, the

124

lower soot production and the higher soot production. The gas-phase kinetic modeling

125

was performed to catch some key soot precursors. And main reaction pathways

126

analyses of ethylene, ethanol and DME during the different pyrolysis conditions were

127

also carried out to better comprehend soot formation process. To acquire nanoscale

128

characteristics of soot, high revolution transmission electron microscopy (HRTEM)

129

and X-ray diffraction (XRD) were utilized. A thermogravimetric analyzer (TGA) was

130

employed to get the soot oxidation reactivity.

131 132

2. Experimental and kinetic modeling

133

2.1 Experimental set up

134

The atmosphere pyrolysis of ethylene blended with ethanol/DME took place in a

135

α-alumina tube flow reactor at 1273K, 1373K and 1473K in Ar atmosphere, as shown

136

in Fig.1. Ethylene, Ar and DME were introduced to the mixing vessel from gas

137

cylinder by mass flow controllers (Sevenstar, CS200A). Liquid ethanol was delivered

138

to the evaporator by a Harvard PHD2000 syringe pump. All the pipelines were

139

wrapped with heating bands to prevent the gas ethanol liquefying. The α-alumina tube

140

with 700mm in length and 45mm internal diameter was placed inside an electric

141

furnace. Two furnace plugs were set in the entrance and exit of the tube separately to

142

avoid heat loss. The temperature profiles along the flow reactor were measured by

143

moving the K-type thermocouple, as shown in Fig.2. The step distance was 2 cm. 7

144

There existed a constant-temperature area from the location of 26cm to 34cm. C2H4 Ar

Exhaust Emissions

Mass Flow Controller

Particle Filter One-way Valve Mixing Vessel

145 146

Syringe Pump Evaporator

Tube Furnace

Fig.1. Schematic of the experimental set up.

147 148

Fig.2. Temperature profiles along the center of the α-alumina tube.

149 150

To study the effects of different carbon sources on soot formation and

151

characteristics, the mole amount of reacting carbon was kept the same in all the

152

pyrolysis conditions. Ar for carrying ethanol were kept 2% of the total gas in volume

153

fraction. Ar for dilution was introduced to obtain a total flow rate of 1000 ml/min

154

(STP, standard temperature and pressure),which leaded to the gas residence time

155

dependent on the reaction temperature, t(s)= 2605/T. C/O ratio is an important

156

parameter in this research. So two sets of the experiments were designed and 8

157

conducted. When C/O ratio equals 2, it means half-mole carbon in the C2H4 is

158

replaced by DME/ethanol. Pure DME/ethanol pyrolysis takes place when C/O ratio is

159

4. For any pyrolysis condition, experiments under three temperatures were performed.

160

Each experiment lasted for 1 hour. The detailed experimental conditions were shown

161

in Table 1. Table 1. Experimental conditions in the pyrolysis.

162 Pyrolysis

Flow rate (ml/min)

C/O

T(K)

conditions

Ethylene

Ethanol

DME

100% ethylene

20

0

0

/

50% ethanol

10

0.02623

0

4

1273

50% DME

10

0

9.8

4

1373

100% ethanol

0

0.05246

0

2

1473

100% DME

0

0

19.6

2

Dilution Ar

960

Carrier Ar

20

163 164

Soot was collected in the tail of the α-alumina tube by a piece of filtration

165

membrane. The filtration membrane with a diameter of 50 mm and aperture of 0.45

166

µm was put in front of the furnace plug. The quartz fiber cotton with a pore light of

167

lower than 5 µm was placed after the furnace plug in the end of the tube to prohibit

168

the uncollected soot blocking the back pipeline. The mass of the membrane was

169

weighed before the reaction and after the reaction separately by a high precision

170

balance in each pyrolysis condition. Soot mass was acquired by the difference. To

171

reduce measurement error, the operations of the measurements was conducted three

172

times.

173 174 175

2.2 Soot characterizations To get the morphology of the soot samples, an FEI Tecnai G2 F30 S-SWIN 9

176

transmission electron microscopy (TEM) was used. By magnifying the TEM pictures,

177

HRTEM pictures could be obtained to study the nanostructure of the soot particles.

178

The soot samples were firstly dispersed in ethanol ultrasonically for 90 min. Then,

179

three drops of the suspension were dropped to the carbon film (200 mesh) to do the

180

TEM and HRTEM tests. More than three locations of each grid were chosen to

181

guarantee the accuracy of the results. To make quantitative analysis, the homemade

182

MATLAB software [29-31] using the algorithms by Yehliu et al. [32, 33] was applied

183

to get the fringe parameters such as length and tortuosity.

184

A D8 Advance X-ray diffractometer with Cu Kα radiation was employed to analyze

185

the graphitization degree of soot samples. The scan range was 10°-100° and the scan

186

step size and speed were 0.05 and 0.2s/step respectively.

187

To obtain the oxidation reactivity of soot collected from different pyrolysis

188

conditions, an STA 449 F3 thermogravimetric analyzer was employed. Each soot

189

sample was weighed 5±0.3 mg at first and put into a quartz crucible. Soot sample was

190

heated in pure Ar atmosphere with a flow rate of 100 ml/min from 50℃ to 300℃ for

191

an hour to remove the volatile compounds. Afterwards, soot sample was heated from

192

300℃ to 500℃ continually. Then the temperature was kept to 500℃ for 150 minutes

193

in a mixture flow (78% Ar and 22% O2). The total flow rate was maintained to 100

194

ml/min in the whole process. The normalized oxidation reactivity curve of each soot

195

sample could be obtained through the calculation of the mass loss in the entire process.

196

The uncertainty of the experiments was ±4.7% error with 95% confidence [30].

197 10

198

2.3 Kinetic modeling

199

To further supplement our study and qualitative acquire the additional information, we

200

performed the chemical kinetic modelling. A detailed chemical kinetic mechanism

201

(AramcoMech 3.0) was employed in the kinetic simulation for the pyrolysis of

202

ethylene blended with ethanol/DME. This mechanism containing 581 species and

203

3037 reactions has been widely utilized and validated against the experimental

204

measurements [34]. Considering to complexity of the multi-dimension, we simplified

205

the process and adopted Plug Flow Reactor (PFR) to carry out the modelling, which

206

was consistent with many pyrolysis model [38]. The measured temperature profile

207

was used as input temperature parameter to make the simulation keep consistent with

208

the experiments. The running step distance was 0.2 cm in the modeling. The mole

209

fraction of fuels and gas products at every specific position (every 0.2 cm) could be

210

acquired during the pyrolysis process.

211 212

3. Results and discussion

213

3.1 Soot formation analysis

214

3.1.1 Soot production and analysis

215

Fig.3 illustrated soot mass at different pyrolysis conditions. Soot amount decreased

216

with the increase of the replacement of ethylene by ethanol or DME at a certain

217

temperature. Moreover, it was obvious that less soot formed when replacing ethylene

218

with DME instead of ethanol at the same ethylene replacement (50% or 100%).

219

Contrasted to ethylene, ethanol and DME possessed lower C/H ratio. Furthermore, the

11

220

existence of O in ethanol and DME may lead to the oxidation reaction and could

221

affect soot formation reaction to some degree. This could give some reasons that soot

222

amount reduced when ethanol and DME were mixed into ethylene. It was found that

223

C-C bond may have some relationship with soot formation [26]. Compared with

224

ethanol, C-C bond was not found in DME. Instead, two C atoms connected with O

225

atom. Although they have the same chemical formula, different functional groups (-O-

226

and -OH) in DME and ethanol showed their various abilities to produce soot. No soot

227

can be collected from pyrolysis of pure DME at 1273K. When the blending ratios of

228

the oxygenated fuels were fixed, more soot produced as the temperature increased. It

229

implied that the soot formation process enhanced at high temperatures, which was

230

consistent with the previous pyrolysis studies [11, 17, 35].

231 232

Fig.3. Mass of soot from pyrolysis of ethylene blended with 0%/50%/100%

233

ethanol/DME at 1273/1373/1473K

234 235

Reaction pathway analysis at lower temperature (1273K) and higher temperature 12

236

(1473K) during the pyrolysis was performed separately in order to comprehend the

237

concrete chemical reactions and soot formation process. To choose proper locations to

238

carry out reaction pathway analysis, mole fraction profiles of ethylene, ethanol and

239

DME along the flow reactor at 1273K and 1473K were plotted, as shown in Fig.4.

240

With 50% ethanol additions, the mole fraction of ethylene increased to a peak at first

241

and then decreased to a certain value. The peak mole fraction of ethylene was

242

acquired when ethanol was entirely consumed, indicating that ethanol may produce

243

ethylene. Not all the ethylene was consumed at 1273K with 0% and 50%

244

ethanol/DME additions. When the temperature was 1473K, almost all the ethylene

245

was depleted. As shown in Fig.3, soot mass increased with the raise of temperature. It

246

could be inferred that at higher temperatures, the conversion of ethylene promoted

247

and more ethylene may take part in the reactions which were related to soot formation.

248

Ethanol and DME could be completely consumed at both 1273K and 1473K in a

249

pretty short distance no matter the ethylene replacement was 50% or 100%. Moreover,

250

it was apparent that ethanol and DME participate in the consumption reactions ahead

251

of ethylene. At 1473K, starting and ending points of ethylene, ethanol and DME

252

consumption moved forward compared with those at 1273K. This could result from

253

the different temperature profiles as shown in Fig.2.

254

Reaction pathway analysis was conducted at 1273K and 1473K by selecting proper

255

positions where around 70% fuel was consumed and most intermediates had high

256

mole fractions [36, 37]. The detailed related parameters were exhibited in Table 2.

257

Table 2. The related parameters of the fuels. 13

Temperature

Conditions

Distance

Ethylene

Ethanol

DME

(cm)

conversion

conversion

conversion

100% ethylene

38

68.8%

/

/

50% ethanol

38

60.3%

100%

/

50% DME

38

70.5%

/

100%

100% ethanol

21.4

/

77.8%

/

100% DME

21.4

/

/

65.4%

100% ethylene

24.8

73.9%

/

/

50% ethanol

24.8

68%

100%

/

(K)

1272K

1473K

50% DME

24.8

76.2%

/

100%

100% ethanol

18.8

/

77.2%

/

100% DME

18.8

/

/

63.8%

258

259 260

Fig.4. Mole fraction profiles of ethylene, ethanol and DME along the flow reactor for

261

different pyrolysis conditions. (a) (b) (c):1273K, (d) (e) (f):1473K. 14

262 263

Figs.5 and 6 exhibited the consumption pathways of ethylene at 1273K and 1473K

264

respectively with the replacement of ethylene by 0%/50% ethanol/DME. Through the

265

comparison of Figs.5 and 6, it could be easily found that the reaction pathways of

266

ethylene were simpler and the formation of soot precursors was more direct at 1473K.

267

The formation of soot precursors such as C2H2, C3H3 and C6H6 went through fewer

268

steps at 1473K. As shown in Figs.5 and 6, the most significant unimolecular

269

decomposition way of ethylene was the consumption pathway to produce C2H3 by the

270

attack of H radical both at 1273K and 1473K. The C2H3 radical could convert to C2H2

271

through R2 and R3. It was obvious that the contribution of the production of C2H3 and

272

C2H2 could be greater at 1473K. As seen from Fig.3, soot amount increased with the

273

raise of the temperature. It could be inferred that at higher temperature, the reactions

274

benefiting to the production of soot precursors were enhanced, which could result in

275

the promotion of soot production. C2H4+H＝C2H3+H2

(R1)

C2H3(+M)＝C2H2+H(+M)

(R2)

C2H3+H＝C2H2+H2

(R3)

276

In Figs.5 and 6, C3H3 could be found after a series of intermediate reactions. C3H3

277

could contributed to the production of C6H6 and FULVENE through self-combination,

278

as shown in R4 and R5. Then C6H5 and FULVENE could convert to C6H6 through R6

279

and R7. C3H3+C3H3＝C6H6 15

(R4)

C3H3+C3H3＝FULVENE

(R5)

C6H5+H2＝C6H6+H

(R6)

FULVENE+H＝C6H6+H

(R7)

280

As one of the most important precursors of large monocyclic aromatic hydrocarbons

281

(MAHs) and polycyclic aromatic hydrocarbons (PAHs), C6H6 could take part in the

282

reactions related to the aromatics growth through hydrogen abstraction carbon

283

addition (HACA) and so on [38]. However, the reaction pathways of ethylene related

284

to the formation of C6H6 and its precursors in the pyrolysis pathways were similar

285

with the replacement of 50% ethylene by ethanol/DME. So it was hard to distinguish

286

the accurate chemical effects of ethanol and DME through Figs.5 and 6. To better

287

understand the soot formation process, the pathways of ethanol and DME at different

288

locations were depicted in Figs. 7 and 8 separately.

289 16

290

Fig.5. Reaction network of ethylene at 1273K at the location of 38cm. Percentages

291

given in parentheses are corresponding to the results of 100% ethylene. Percentages

292

given in square bracket are corresponding to the results of 50% ethanol. Percentages

293

given in angle bracket are corresponding to the results of 50% DME.

294 295

Fig.6. Reaction network of ethylene at 1473K at the location of 24.8cm. Percentages

296

given in parentheses are corresponding to the results of 100% ethylene. Percentages

297

given in square bracket are corresponding to the results of 50% ethanol. Percentages

298

given in angle bracket are corresponding to the results of 50% DME.

299 300

Fig.7 showed the reaction network of C2H5OH with the replacement of 100%

301

ethylene by ethanol at 1273K/1473K at the location of 21.4cm/18.8cm where

302

77.8%/77.2% of the initial ethanol was exhausted respectively. As seen from Fig.7,

303

The consumption of ethanol primarily involved two pathways. One route was through

304

the dehydration reaction to produce ethylene. This was consistent with the previous

305

results in Fig.4. 17

C2H5OH＝C2H4+H2O 306 307

308 309

(R8)

The other pathway was to form SC2H5OH by the attack of H, OH and CH3, contributing 49.1%/40.2% together at 1273K and 1473K individually. C2H5OH +H＝SC2H4OH+H2

(R9)

C2H5OH +OH＝SC2H4OH +H2O

(R10)

C2H5OH +CH3＝SC2H4OH +CH4

(R11)

Great majority of SC2H5OH was consumed to bring about the formation of CH3CHO→CH3CO→CO through R12-R15. SC2H4OH＝CH3CHO+H

(R12)

CH3CHO+H＝CH3CO+H2

(R13)

CH3CHO+CH3＝CH3CO+CH4

(R14)

CH3CO(+M)＝CH3+CO(+M)

(R15)

310

As observed in Fig.4, ethanol could take part in the reactions ahead of ethylene.

311

When almost all the ethanol was consumed, ethylene just started to react. When 50%

312

or 100% ethanol was added to substitute ethylene at 1273K or 1473K, part of ethanol

313

could seize some radicals like H, OH and CH3 to take part in the formation of CO

314

rather than the yield of some vital soot precursors, resulting in the diminution of soot

315

production compared to pure pyrolysis of ethylene with the same mole amount, as

316

shown in Fig.3. Moreover, with the increase of the replacement of ethylene by ethanol

317

at a fixed temperature, more mole amount of C took part in the formation of CO,

318

which would could cause the reduction of soot amount.

319

Moreover, at 1473K, the consumption pathway of ethanol through the attack of H, 18

320

OH and CH3 to generate PC2H5OH could contribute 15.4% totally. Part of PC2H5OH

321

could convert to CO finally through the similar routes as R13-R15. However,

322

PC2H5OH could also participate in the reaction to produce ethylene at 1473K, which

323

gave the contribution of 71.3%. Then ethylene could join in the process to the

324

formation of soot through the pathway in Fig.6. So the effects of ethanol to suppress

325

soot production were much more obvious at 1273K. This was consistent with the

326

results shown in Fig.3. PC2H4OH＝C2H4+OH

(R16)

327

328 329

Fig.7. Reaction network of C2H5OH when 100% ethanol was introduced at 1273K at

330

the location of 21.4cm. Percentages given in big parentheses are corresponding to the

331

results of 100% ethanol pyrolysis at 1473K at the location of 18.8cm.

332 19

333

Fig.8 presented the consumption pathway of DME when 100% DME was

334

introduced at 1273K/1473K at the location of 21.4cm/18.8cm where 65.4%/63.8% of

335

the initial DME was exhausted individually. The most vital unimolecular

336

decomposition of DME was the process to form CH3OCH2 by the attack of H,

337

contributing around 68% of DME consumption. Another reaction, which were the

338

production of CH3OCH2 and CH4 by the pelt of CH3, contributed 17.6%.

339

CH3OCH3+H＝CH3OCH2+H2

(R17)

CH3OCH3+CH3＝CH3OCH2+CH4

(R18)

Subsequently, CH3OCH2 could converted to CH3 and CH2O through R19. CH3OCH2＝CH3+CH2O

(R19)

340

Moreover, another reaction, which was the formation of CH3 and CH3O through the

341

decomposition of CH3OCH3, could occur. CH3OCH3(+M)＝CH3+CH3O(+M)

(R20)

342

Then, by the successive H abstraction, CO appeared through the pathway of

343

CH2O→HCO →CO.

344

Through Fig.8 as well as the reactions above, it was clear that DME was mainly

345

consumed to form CO, which indicated that this large part of C did not take part in the

346

formation of aromatics and soot. This could explain why soot production was lower

347

with the replacement of 50% or 100% ethylene by DME compared with soot amount

348

from co-pyrolysis of 50% ethylene and 50% ethanol, pure ethylene pyrolysis and pure

349

ethanol pyrolysis at the same temperature.

350

During the pyrolysis process, CH3 could take part in the formation of CH4 through 20

351

352

R21 and R22. CH3+H2＝CH4+H

(R21)

CH2O+CH3＝HCO+CH4

(R22)

Moreover, C2H6 could form through the recombination of CH3. CH3+CH3(+M)＝C2H6(+M)

353

(R23)

C2H6 could convert to C2H4 through R24-R26. C2H6+H＝C2H5+H2

(R24)

C2H6+CH3＝C2H5+CH4

(R25)

C2H5(+M)＝C2H4+H(+M)

(R26)

354

As shown in Figs.5 and 6, the pathways of C2H4 at lower and higher temperatures

355

were different. The abilities of C2H4 to form soot was stronger at 1473K. This could

356

give some evidence that more soot was collected at 1473K during the pyrolysis of

357

pure DME, although the consumption of DME at 1273K and 1473K followed the

358

similar pathway.

21

359 360

Fig.8. Reaction network of DME when 100% DME was introduced at 1273K at the

361

location of 21.4cm. Percentages given in big parentheses are corresponding to the

362

results of 100% DME at 1473K at the location of 18.8cm.

363

3.1.2 Relationship between soot amount and products involving soot formation

364

To get the relationship between soot amount and some species related to soot

365

formation, the simulated key mole fraction profiles of C2H2, C3H3, C4H2, C4H4, C5H5

366

and C6H6, which appeared in the previous pathways, were plotted during the pyrolysis

367

process at 1273K and 1473K separately, as presented in Figs.9 and 10.

368

It was clear to notice that the peak mole fraction of C2H2, C4H2, C4H4 and C5H5

369

decreased with the increase of the replacement of ethylene by ethanol or DME both at

370

1273K and 1473K. The peak mole fraction reduction of the four species was more

371

prominent with the replacement of ethylene by DME. For a fixed temperature, the

372

sequence of the peak mole fraction values of C2H2, C4H2, C4H4 or C5H5 kept the same,

373

which was 100% ethylene>50% ethanol>50% DME>100% ethanol>100% DME. 22

374

This was in the same tendency with the trend of soot amount. It could be deduced that

375

there existed a high correlation between soot amount and production of some

376

hydrocarbon species. During the pyrolysis of ethylene blended with ethanol/DME,

377

soot production had positive correlation with the peak mole fraction values of C2H2,

378

C4H2, C4H4 and C5H5.

379

However, at 1273K and 1473K, the tendency of the peak mole fraction values of

380

C3H3 and C6H6 did not follow the same trend as 100% ethylene>50% ethanol>50%

381

DME>100% ethanol>100% DME. At 1273K, both of the peak mole fraction values of

382

C3H3 and C6H6 from pure DME pyrolysis were the smallest among the five pyrolysis

383

conditions. This was the same with the results of soot amount, which showed that the

384

least amount of soot was collected from pyrolysis of pure DME. The peak values of

385

C3H3 from pure ethanol pyrolysis, as well as co-pyrolysis of 50% ethylene and 50%

386

ethanol/DME were almost the same. With the elevation of ethylene replacement by

387

DME, the peak mole fraction of C6H6 decreased, which was coincident with the

388

results of soot amount. But compared with pure ethylene pyrolysis, the peak mole

389

fraction value of C6H6 was larger when 50% ethanol was added. As mentioned in

390

previous discussion, C3H3 has closely relationship with the formation of C6H6, as

391

showed in R4-R7. It could be found through the reaction pathways of ethylene and

392

ethanol that pyrolysis of ethanol was easier to produce C3. So the concentration of

393

C3H3 could increase slightly compared to pure ethylene pyrolysis. This phenomenon

394

was more obvious when the temperature was higher. The enhancement of

395

concentration of C3H3 could directly lead to the increase of the concentration of C6H6. 23

396

In Fig.3, higher amount of soot was collected in the pyrolysis of pure ethylene at a

397

fixed temperature. Soot mass decreased with the addition of 50% ethanol. The joint

398

influence of diverse soot precursors could cause such results. It could be clearly

399

noticed that the mole fraction of other important soot precursors reduced and the

400

reduction range was higher in comparison to C6H6. Therefore, it could be deduced

401

that soot formation was dominated by the co-contribution of the other various soot

402

precursors, which led to the reduction of soot amount. At 1473K, the smallest values

403

of the peak mole fraction of C3H3 and C6H6 were obtained from pure ethylene

404

pyrolysis because that much more C3H3 and C6H6 take part in the formation of PAHs

405

at higher temperatures. Moreover, as shown in Fig.6, when 50% oxygenated fuels

406

were added, the primary consumption of C2H2 caused the formation of

407

C3H4-P→C3H3→C6H6. During the pyrolysis of pure ethylene, the main deception of

408

C2H2 leaded to the production of C4H4 and C4H2, which may also result in the lower

409

values of the peak mole fraction of C3H3 and C6H6.

410

For a fixed ethylene replacement, the peak mole fraction of C2H2, C3H3, C4H2,

411

C4H4, C5H5 and C6H6 increased as the temperature increased. Positive correlation was

412

found between temperature and the peak mole fraction values of these species.

24

413 414

Fig.9. Simulated mole fraction information of C2H2, C3H3, C4H2, C4H4, C5H5, C6H6

415

during the pyrolysis process at 1273K. Mole fraction profiles: (A)C2H2, (B)C3H3,

416

(C)C4H2, (D)C4H4, (E)C5H5, (F)C6H6. Peak mole fraction with different replacement

417

of ethylene: (a)C2H2, (b)C3H3, (c)C4H2, (d)C4H4, (e)C5H5, (f)C6H6.

25

418 419

Fig.10. Simulated mole fraction information of C2H2, C3H3, C4H2, C4H4, C5H5, C6H6

420

during the whole pyrolysis process at 1473K. Mole fraction profiles: (A)C2H2,

421

(B)C3H3, (C)C4H2, (D)C4H4, (E)C5H5, (F)C6H6. Peak mole fraction with different

422

replacement of ethylene: (a)C2H2, (b)C3H3, (c)C4H2, (d)C4H4, (e)C5H5, (f)C6H6.

423 424

3.2 Soot characteristics analysis

425

3.2.1 Soot morphology and nanostructure analysis 26

426

Fig. 11 showed the typical TEM and HRTEM images of soot collected from

427

pyrolysis of ethylene blended with ethanol/DME at 1273K. As presented in Fig.

428

11(e)-(h), soot appeared to be liquid-like materials, which may result from the

429

chemical condensation of heavy PAHs at low temperature. These soot particles were

430

similar to young and nascent soot as presented in many previous research [29, 39, 40].

431

It was hard to obviously distinguish the independent individual particles and clarify

432

the borderline among particles because of the irregular shapes. However, a detailed

433

observation of soot particles could discover that particle-like protrusions or shaped

434

knobs formed under the cover of the film-like materials in Fig. 11(e)-(h). These

435

protrusions and knobs may develop from liquid-like materials [39]. No obvious soot

436

could be collected from pyrolysis of 100% DME at 1273K. The sections in the

437

specific yellow circles in Fig. 11(e)-(h) were magnified to acquire the HRTEM images

438

of soot, as described in Fig. 11(i)-(l). Fig. 11(m)-(p) were skeleton images extracted

439

from Fig. 11(i)-(l) for further inspections. These images showed short, highly-curved

440

and random fringe, presenting typical amorphous structures.

441

Fig. 12 gave the representative TEM and HRTEM images of soot collected from

442

pyrolysis of ethylene blended with ethanol/DME at 1373K. It was easy to perceive

443

many approximately rounded particles in the fashion of loosely chain-like way [17,

444

37], as presented in Fig. 12(f)-(i). More mature soot was found compared with soot in

445

Fig. 11. Fig. 12(j) exhibited morphology of soot from pyrolysis of 100% DME, which

446

was similar to young soot morphology mentioned in Fig. 11. The arrows in the yellow

447

circles marked the core structure which consisted of randomly arranged carbon fringe. 27

448

Outside the cores, planar carbon layer structures were found in Fig. 12(p)-(s). The

449

outer shell was comprised of some planar crystallites. Longer and relatively more

450

organized fringes could be seen obviously in parallel orientation. However, as 100%

451

DME took the place of 100% ethylene, soot sample showed contained mainly

452

turbostratic short and curve lattice fringes, as displayed in Fig. 12(t). It could be

453

inferred that soot collected from pyrolysis of 100% DME might be more reactive.

454

Fig. 13 offered the information of morphology and nanostructure of soot collected

455

from pyrolysis of ethylene blended with ethanol/DME at 1473K. The distribution way

456

of the aggregates in Fig. 13(f)-(j) was in common with that in Fig. 12(f)-(i). The

457

particles processed mainly round shape and it was clear to clarify the bounder line

458

among particles [41]. In Fig 13(k)-(o), typical core-shell structures could be observed.

459

The fringes were mainly parallel to each other and arranged in a concentric way,

460

representing the existence of graphitic structures [11, 42]. The irregular inner cores

461

with short fringes were encircled by the ordered long fringes. The outer layer was

462

arranged more neatly in Fig.13(p)-(r) compared with Fig.12, which implied that

463

higher temperature leaded to a more ordered arrangement of carbon layer. Multi-core

464

structure was noticed in Fig. 13(t). The inner cores were surrounded by many

465

paralleled long fringes. Compared to Fig. 13(p)-(s), Fig. 13(t) showed less ordered

466

and shorter fringes.

467

To better understand the morphology of the particles in different pyrolysis

468

conditions, Image J software was adopted to measure the diameter of the soot

469

particles in the TEM pictures. Figure 2.1 showed the mean values of the primary 28

470

particles.

471

Measuring the diameters of the particles was a little hard because the particles were

472

bundled together and it was difficult to identify the clear boundary. So probably the

473

results overestimated the mean diameter of the particles, as mentioned in [29]. For

474

100% DME pyrolysis at 1373K, few particle could be found because that soot just

475

evolve from the liquid-like materials. So the particle sizes at this condition were not

476

performed. It could be found that with the increase of temperature from 1373K to

477

1473K, the mean particles size decreased except 100% ethanol pyrolysis. During this

478

period, the crack between particles took the lead, which resulted in the decrease of the

479

mean diameter of the particles. Moreover, it was interesting that the mean particle

480

diameter decreased with the addition of ethanol/DME at 1373K. However, at 1473K,

481

with the injection of ethanol/50% DME, the average particle size increased. It could

482

be deduced that there presented a suitable temperature range best for soot growth

483

between the low and high temperatures [11]. Different fuel mixtures processed the

484

various temperature range which was most proper for soot growth. At first, film-like

485

substance produced at lower temperature, and then tiny particles emerged and

486

agglomerated with the raise of temperature. The average size of the particles grew

487

bigger. However, at higher temperature, the larger particles broken into smaller ones

488

and made the particles size smaller, limiting the further growth of soot. For 100%

489

ethanol pyrolysis, soot may go through the growth stage from 1373K to 1473K.

490

However, for 50% DME/ethanol pyrolysis, soot may pass the crack and collision

491

process and the mean diameter got smaller from 1373K to 1473K. 29

492

Overall, temperature and fuel mixtures had a great influence in soot morphology

493

and nanostructure as shown in Fig.11-13. As observed in previous research [21],

494

emission of soot may go through different phases in the flame——gas-phase

495

chemistry to produce PAH molecules; inception of PAHs to form primary soot

496

particles; soot mass growth; coagulation and aggregation between particles;

497

carbonization and then oxidation of soot particles. As for pyrolysis processes in

498

present work, although violent oxidation reactions like combustion may not happen,

499

the growth and aggregation of soot were similar. Moreover, with the increase of

500

temperature, from 1273K to 1473K, morphology and nanostructure of soot collected

501

in the outlet of the flow reactor varied. Furthermore, fuel type and composition such

502

as the blended ratio of ethanol/DME could affect the growth rate of soot, the main

503

reaction pathway and the nanostructure of soot [21, 42].

30

504 505

Fig.11. TEM, HRTEM and extracted skeleton images of soot particles collected in

506

different pyrolysis conditions at 1273K.

507

31

508 509

Fig.12. TEM, HRTEM and extracted skeleton images of soot collected in different

510

pyrolysis conditions at 1373K.

32

511 512

Fig.13. TEM, HRTEM and extracted skeleton images of soot particles collected in

513

different pyrolysis conditions at 1473K.

33

514 515

Fig.14. Average sizes of the particles at different pyrolysis conditions.

516

To better understand the nanostructure characteristic of soot collected from

517

pyrolysis of ethylene blended with ethanol/DME, the detailed quantitative analysis

518

was carried out by processing the skeleton images in Fig.11-13. The results were

519

depicted in Fig.14. Fig.15 (a)-(e) conveyed the fringe length information. Fig.15 (f)-(j)

520

described the fringe tortuosity distribution. Visual comparison could clarify that with

521

the increase of temperature, the percentage of fringes whose length was larger than

522

1.5nm was clearly higher. Fringe length less than 1.0 nm had a less proportion. Fringe

523

tortuosity showed the opposite tendency. The percentage of fringe whose tortuosity

524

was bigger than 1.2 was lower as temperature raised. It was obvious that mean fringe

525

length had connections with mean fringe tortuosity. Larger fringe length was

526

correlated with smaller fringe tortuosity. At a fixed temperature, 1373K or 1473K, the

527

mean values of fringe length exhibited the ranking of 100% ethylene>50% DME>100%

528

ethanol>50% ethanol>100% DME. The sequence of mean fringe tortuosity showed

529

the opposite order. Soot with shorter fringe length had the higher reactivity because

530

carbon in the edge sites was more reactive than the basal plane carbon atoms [43]. It 34

531

could be deduced that soot collected from pyrolysis of 100% DME with the shortest

532

fringe length could have the highest reactivity at the fixed temperature.

533 534

Fig.15. Fringe length and tortuosity distribution of soot in different pyrolysis 35

535

conditions at various temperatures.

536 537

3.2.2 XRD analysis for soot

538

XRD patterns of soot collected from various pyrolysis conditions were presented in

539

Fig.16 to get the chemical structure of soot samples [44]. Soot samples collected at

540

1273K were not enough to perform XRD tests and the following TGA examinations,

541

so only the XRD results of soot sampled at 1373K and 1473K were shown here.

542

It was clear to notice that the diffraction peaks were close to the angles of 25° in

543

Fig.16. The existence of 002 band represented the emergence of crystalline graphitic

544

carbon and could assess the graphization degree of soot samples [45, 46]. The peak

545

diffraction angles of soot from different pyrolysis conditions were extracted from

546

Fig.16, as presented in Table 3. The diffraction peak of soot collected from 1373K

547

shifted to the left compared with that at 1473K although at the same ethylene

548

replacement. It showed that the raise of temperature could promote the graphization

549

degree of soot. The distinct differences of soot diffraction angles could be observed

550

when oxygenated fuels was introduced at 1373K or 1473K. With the additions of

551

oxygenated fuels, the graphization degree of soot declined. The results were

552

consistent with previous analysis of TEM and HRTEM results.

553

36

554 555

Fig.16.XRD spectra of soot collected from different pyrolysis conditions: (a) 1373K, (b) 1473K.

556 557 558

Table 3. Peak diffraction angles of soot from different pyrolysis conditions. Pyrolysis condition

1373K

1473K

2θ (degree)

100% ethylene

23.78

50% ethanol

23.43

50% DME

23.60

100% ethylene

24.78

50% ethanol

23.95

100% ethanol

24.41

50% DME

24.60

100% DME

23.81

559 560

3.2.3 Soot oxidation reactivity analysis

561

The oxidation reactivity of soot samples collected from different pyrolysis

562

conditions at 1373K and 1473K was presented in Fig.17. The normalized mass loss

563

curves were acquired from the output results by TGA. With the increase of

564

temperature, the oxidation reactivity of soot was lower. Soot collected at 1273K with

565

the disordered structure owned the higher oxidation reactivity since the disordered

566

carbon was easier to react with oxygen. Soot formed at 1473K with the higher 37

567

graphitization had the lower oxidation reactivity.

From Fig 17(b), it was clear that

568

the curve with the biggest slope belonged to 100% DME pyrolysis, which showed that

569

the oxidation reactivity of soot collected from pyrolysis of 100% DME was the

570

highest at 1473K. The normalized mass curves showed a sequence of oxidation

571

reactivity as 100% ethylene<50% DME<100% ethanol<50% ethanol< 100% DME

572

both at 1373K and 1473K. It fitted well with the sequence of fringe length and

573

tortuosity as well as the XRD results discussed above. With the blending of

574

oxygenated fuels, soot exhibited shorter fringe length, bigger fringe tortuosity, lower

575

graphization degree and lower oxidation reactivity. High relationship between soot

576

nanostructure and soot oxidation reactivity was explored. Soot collected with 50%

577

ethanol addition was more reactive toward O2 than that with 100% ethanol addition

578

because of the less ordered fringe length arrangement, which was in line with the

579

results by Esarte et al. [28]. Although DME has the same formula with ethanol, their

580

sooting characteristic varied.

581

oxidation reactivity than that with 100% DME addition, which was in agreement with

582

the previous HRTEM and XRD results.

583

ethanol, 50% DME or 100% ethanol, the main soot production way was still the

584

decomposition of ethylene, which was similar to the pure ethylene pyrolysis. This

585

may cause the similar oxidation reactivity of the collected soot.

Soot collected with 50% ethanol addition had lower

As shown in Fig. 5-7, when adding 50%

586

38

587 588

Fig.17. TGA curves of soot collected from different pyrolysis conditions: (a)1373K,

589

(b) 1473K.

590

4. Conclusion

591

This pyrolysis experiments have been carried out with the replacement of 0%, 50%

592

and 100% (mole fraction) ethylene of ethylene by ethanol/DME at 1273K, 1373K and

593

1473K respectively to get the effects of these two isomer oxygenated fuels on soot

594

nanostructure and soot oxidation reactivity as substitutes of ethylene. The gas-phase

595

kinetic modeling was also performed to understand the soot formation process. The

596

main conclusions were summarized as follows:

597

(1) With the increase of the replacement of ethylene by ethanol or DME, the mass of

598

soot diminished obviously because of the reduction of C/H ratio and C/O ratio.

599

Moreover, compared with ethanol, DME has stronger potential to reduce soot

600

formation.

601

(2)The peak mole fraction of C2H2, C4H2, C4H4 and C5H5 showed the same tendency

602

with soot mass in the different pyrolysis conditions, which was 100% ethylene>50%

603

ethanol>50% DME>100% ethanol>100% DME. Positive correlation between soot

604

mass and the peak mole fraction of C2H2, C4H2, C4H4 or C5H5 was found. 39

605

(3)Fuel types could affect the growth rate of soot, soot nanostructure and soot

606

oxidation reactivity greatly. The additions of ethanol and DME could decrease the

607

growth rate of soot. The sequence of oxidation reactivity was 100% ethylene<50%

608

DME<100% ethanol<50% ethanol< 100% DME. This order was the same as the

609

ranking of fringe length and peak diffraction angles of soot samples.

610

(4)With the increase of temperature, the mass of soot increased. And positive

611

correlation between temperature and the peak mole fraction of C2H2, C4H2, C4H4 or

612

C5H5 was observed. At higher temperature, more mature soot with higher degree of

613

graphization, longer fringe length, smaller fringe tortuosity and lower soot oxidation

614

reactivity was obtained. At low temperature, soot showed typical amorphous structure

615

and higher oxidation reactivity.

616

(5)The results from HRTEM, XRD and TGA had high correlations. Soot particles

617

with longer fringe length and smaller fringe tortuosity showed more ordered and

618

arranged nanostructure, higher graphitization degree and lower oxidation reactivity.

619 620 621 622

Acknowledgement This work was supported by the National Natural Science Foundation of China (51822605).

623 624

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Highlights: Ranking of soot mass is 100% ethylene>50% ethanol>50% DME>100% ethanol>100% DME. Soot mass is positively correlated with mole fraction of C2H2, C4H2, C4H4 and C5H5. Sequence of reactivity is 100% ethylene<50% DME<100% ethanol<50% ethanol<100% DME. Soot with longer fringe length shows lower oxidation reactivity.