Experimental and Modeling Investigations on Soot Formation of

Subscriber access provided by Queen Mary, University of London

Article

Experimental and Modelling Investigations on Soot Formation of Ethanol, N-butanol, 2,5-Dimethylfuran and Biodiesel in Diesel Engine Xinlei Liu, Hu Wang, and Mingfa Yao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01622 • Publication Date (Web): 02 Oct 2017 Downloaded from http://pubs.acs.org on October 3, 2017

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 free 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 accessible to all readers and 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.

Energy & Fuels 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 31

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

Energy & Fuels

1

Experimental and Modelling Investigations on Soot Formation of Ethanol, N-butanol,

2

2,5-Dimethylfuran and Biodiesel in Diesel Engine

3 Xinlei Liu1, Hu Wang1,*, Mingfa Yao1

4 5

1

State Key Laboratory of Engines, Tianjin University, No.92 Weijin Road, Nankai District, Tianjin, 300072, P. R. China

6 7 8

Abstract

9

The soot formation in the combustion of four oxygenated fuels on a single-cylinder engine has been

10

investigated experimentally and numerically. To accomplish this objective, a reduced combustion

11

mechanism was proposed for the modelling studies, which has been extensively validated. Then direct

12

injection compression ignition experiments fueled with diesel, biodiesel and its blends with 20% volume

13

fraction of ethanol (E20), n-butanol (B20) and 2,5-dimethylfuran (D20) have been conducted. In the three

14

dimensional (3-D) modelling studies, the reduced mechanism can well predict the experimental

15

combustion and soot emission results. In contrary to the combustion phasing, the soot emissions for the

16

five fuels were sequenced as diesel > biodiesel > B20 > D20 > E20, which was mainly due to the different

17

oxygen content and fuel reactivity in the spray-combustion processes. Furthermore, 0-D modelling

18

investigations were conducted as well to clarify the effects of the different oxygenated structures on the

19

polycyclic aromatic hydrocarbons (PAHs) formation under homogeneous condition. It was shown that for

20

the five pure fuel cases, the sooting tendencies were sequenced as ethanol < n-butanol < methyl-decanoate

21

< n-heptane < 2,5-dimethylfuran. According to the reaction pathway analyses, oxygenated fuels with

22

longer carbon chain molecular structure could produce more intermediate species, which favor the PAHs

23

and soot formation. However, much more phenol and cyclopentadienyl radical can be produced due to the 1

ACS Paragon Plus Environment

Energy & Fuels

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

Page 2 of 31

24

special cyclic structure of 2,5-dimethylfuran, resulting in the highest soot formation among these

25

investigated fuels.

26

Keywords: Soot; Oxygenated Fuels; Biodiesel; Alcohol; PAH; 2,5-Dimethylfuran

27

1. Introduction

28

Due to concerns on the fossil fuels and global greenhouse gas emissions, more stringent fuel

29

consumption and emission regulations have been enacted to improve the fuel economy and harmful gas

30

emissions. Oxygenated bio-fuels, including alcohols 1, ethers 2 and esters 3, have obtained much attention

31

worldwide, known as the alternative fuels of petroleum. Therefore, it is of great interest to investigate the

32

combustion and emission characteristics of these oxygenated fuels in internal combustion engines (ICEs).

33

Compared with spark ignition engine, compression ignition (CI) engine typically gets better fuel

34

economy for the higher compression ratio, which has been widely applied in areas like transportation,

35

power plant and many other areas. However, due to the diffused combustion process, conventional CI

36

engines fueled with diesel usually encounter the problems of high NOx and soot emissions. To refrain the

37

NOx emission, exhaust gas recirculation (EGR) was frequently used to reduce the overall combustion

38

temperature, mainly attributed to the thermal and dilution effects. However, there exists a trade-off

39

relationship between NOx and soot emissions with the variation of EGR. Installation of after-treatment

40

devices may be advisable, but that would increase the cost. Given the oxygen content in the molecule,

41

oxygenated fuels could be a better solution to simultaneously reduce the NOx and soot emissions in CI

42

engines. Thus, many researches have been conducted on the combustion of different oxygenated fuels in

43

CI engines 4.

44 45

Miyamoto et al.

4

reported that engine emissions and thermal efficiency could be simultaneously

improved, and the engine performance largely depended on the oxygen content regardless of the type of 2


Page 3 of 31

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

Energy & Fuels

46

oxygenated agent. However, according to the investigation of Chen et al. 5, the brake specific fuel

47

consumption increased with the addition of oxygenate (ethanol) into diesel, although soot, NOx and HC

48

emissions were reduced. Meanwhile, Klein-Douwel et al.

49

oxygenated fuels by using the high-speed imaging and spectroscopy methods, with the oxygen content

50

fixed at 10% (weight fraction). It was concluded that for non-cyclic oxygenates, the soot incandescence

51

was comparable, but that of the cyclic oxygenate was the lowest.

6

compared the sooting behavior of different

52

Many researches have been conducted to investigate the sooting behaviors and the inherent

53

mechanisms for the oxygenated fuels. Notably, a great deal of works have been accomplished by McEnally

54

and Pfefferle 7. They compared the sooting tendencies of 186 oxygenates in methane/air non-premixed

55

flames, with the oxygenates at 1000 ppm, which can well serve as a database for understanding the soot

56

phenomenon. Major findings revealed that the sooting tendencies of n-alcohols were similar to the

57

corresponding n-alkanes, while those of secondary alcohols were slightly higher. More recently, Eveleigh

58

et al. 8 investigated the soot formation of ethanol, propanol, n-pentanol, cyclo-pentanol, ethyl-acetate, and

59

toluene on a laminar tube reactor by using 13C labelling technique. The advanced technology found out that

60

about 68% of soot was from the methyl moiety of ethanol and the ester group contributed to zero soot

61

formation, which experimentally proved that different oxygen-containing functional groups affected the

62

soot formation significantly. These above fundamental findings can provide helpful guidance for the

63

engine researchers to seek for a cleaner and more energy-efficient biofuel. Table 1. Fuel properties of diesel, biodiesel, n-heptane, ethanol, n-butanol, DMF25 and MD. Parameters

Diesel

Biodiesel N-heptane Ethanol

n-Butanol DMF25 MD

Molecular formula

C12-C25

C18.8H35O2 C7H16

C2H6O

C4H10O

C6H8O

C11H22O2

Cetane number

55

51

56

11

17

-

47

Octane number

-

-

-

108

96

119

-

Oxygen content (wt.%)

0

10

0

34.8

21.6

16.7

17.2

3

Density (g/cm ) 20°C

0.820

0.887

0.688

0.790

0.810

0.890

0.871

Viscosity (mm2/s) 40°C

1.9-4.0

4.0

0.59a

1.08

2.22

0.65

1.72 3


Energy & Fuels

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

Boiling point (°C)

180-370

Latent heating (kJ/kg) 25°C 270-301 Lower heating value (MJ/kg) a

42.5

262-359

98.5

78.4

Page 4 of 31

117

92-94 a

200

317

904

585

333

37.5

44.93

26.8

32.01

33.7

223 355 36.5

denotes the parameter is measured at 20°C.

64

Among these oxygenated fuels, there have been considerable engine combustion researches

65

conducted on the combustion of ethanol 9, n-butanol 10, 2,5-dimethylfuran (DMF25) 11 and biodiesel 12,13.

66

Ethanol has received much attention for the high industrial production. However, it may meet problems of

67

cold start and poor miscibility with diesel. Thus additives or other combustion methods (fumigation and

68

double injection)

69

production of n-butanol and DMF25 have been reported 15,16, indicating the possibly massive production in

70

the future. On the other hand, for the higher energy densities and lower latent heating values compared

71

with ethanol, as seen in Table 1, n-butanol and DMF25 may have a promising future for engine

72

applications. Biodiesel is another promising biofuel, which can be directly produced from vegetable oils

73

and animal fat

74

biodiesel has been investigated globally as a relatively ideal alternative fuel for diesel

75

investigations have demonstrated that biodiesel could be efficient in the reduction of the hydrocarbon,

76

carbon monoxide and soot emissions in CI engine combustion.19,20

77

17

14

should be adopted for practical applications. Recent years, new findings of the

, with good miscibility with diesel, and higher cetane number as well as viscosity, thus 18

. Previous

Many fundamental experimental and modelling investigations on the fundamental combustion

78

chemistries of different oxygenates have also been performed 7. Westbrook et al.

21

79

suppression mechanism of different kinds of oxygenates (alcohols, ethers and esters) when blended with

80

n-heptane by using detailed kinetic mechanisms. But only modelling results of soot precursor species were

81

analyzed to describe the soot formation. On the other hand, for applications it is difficult and expensive to

82

adopt detailed combustion mechanisms directly in the three-dimensional (3-D) computational fluid

investigated the soot

4


Page 5 of 31

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

Energy & Fuels

83

dynamics modelling investigations. As a consequence, reduced combustion mechanisms with a compact

84

size but reasonable predictability of the experimental results are important to describe the combustion

85

details occurring in the combustion chamber 22.

86

Though there have been many works about the combustion of different oxygenated fuels, most of

87

these works are mainly focused on only the experimental studies in engine combustion or the fundamental

88

combustion chemistries under homogeneous combustion. This paper tried to clarify the different

89

combustion and soot formation characteristics of four potential oxygenated fuels (ethanol, n-butanol,

90

DMF25 and biodiesel) by simultaneously adopting the experimental, 3-D modelling and 0-D modelling

91

methods to elucidate the results. Besides, a combined reduced combustion mechanism has been proposed

92

for that purpose.

93

Given that biodiesel (compositions seen in Table S1) is a promising alternative biofuel for diesel,

94

direct injection compression ignition (DICI) experiments fueled with biodiesel and its blends with ethanol,

95

n-butanol and DMF25 at 20% volume fraction (vol.) have been performed, and the pure diesel combustion

96

experiment has also been conducted for comparisons. Besides, the experimental combustion and soot

97

emission results have been collected for 3-D simulations. 0-D modelling studies and reaction pathway

98

analyses have also been conducted to further clarify the PAH and soot formation mechanisms of these

99

fuels.

100

2. Mechanism development

101

2.1 Construction of the reduced mechanism

102

To investigate the PAHs and soot formation processes of the oxygenated fuels, a combined reduced

103

toluene reference fuel (TRF)/ethanol/n-butanol/DMF25/methyl-decanoate (MD)/PAH combustion

104

mechanism has been developed, based the reduced TRF/DMF25/PAH mechanism developed by Liu et al. 5


Energy & Fuels

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

Page 6 of 31

105

23

106

sub-mechanism developed by Wang et al. 25. Given that these reduced mechanisms were all developed by

107

the authors’ research group in a hierarchical structure by using a semi-decoupling method 26, the reduced

108

TRF/DMF25/PAH mechanism was taken as the base mechanism directly, and then the upper-class

109

sub-mechanisms of n-butanol and MD were incorporated together to form a combined reduced mechanism.

110

Meanwhile, as described in ref. 27, the ethanol sub-mechanism has already been included in the base C0-C2

111

sub-mechanism, which was reduced from the detailed C1-C2 hydrocarbon mechanism developed by

112

Metcalfe et al. 28 (AramcoMech 1.3).

, the reduced PRF/butanol isomers mechanisms developed by Wang et al.

24

, and the reduced MD

113

In the current study, the original reduced MD sub-mechanism from Wang et al. 25 was further reduced

114

before merging to improve the computational efficiency. During this reduction process, rate of production

115

analysis was used to identify the important species and reactions. Since the initial H-abstraction reactions

116

of MD converted to MD2J radical mostly, and that reaction rate constants forming MD3J, MD6J and

117

MD8J were the same 25, only MD2J and MD6J were retained in the sub-mechanism of MD, with the other

118

two species (MD3J and MD8J) and the corresponding related reactions removed from the mechanism. This

119

MD sub-mechanism was then included in the base mechanism.

120

As explained earlier, the reduced TRF/DMF25/PAH and n-butanol sub-mechanisms were developed 26

121

based on the same C0-C2 sub-mechanism in a hierarchical manner

122

done for the reduced TRF/PAH, ethanol, n-butanol and DMF25 sub-mechanisms. But further adjustment

123

was necessary for the reduced MD sub-mechanism to better predict the experimental results, based on

124

sensitivity analysis of ignition delays. Detailed description about this optimization method can be

125

referenced from the work of Ra and Reitz 22.

126

, thus no more adjustment has been

In brief, the optimization method was implemented by perturbation of the pre-exponential factor

22

,

6


Page 7 of 31

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

Energy & Fuels

127

based on the evaluation of the ignition delay sensitivity for specified reactions. The sensitivity coefficient

128

was defined as,

129

. 1 .

(1)

130

where is the sensitivity coefficient, . is the ignition delay with the pre-exponential factor multiplied

131

by 2, and

. is the original ignition delay. As indicated by the definition, a negative value of can

132

increase the system reactivity and thus reduce the ignition delay.

133 134

Figure 1. Sensitivity analyses of ignition delay for MD sub-mechanism at φ = 1.0, temperatures of 670 K and 1250 K, and

135

pressure of 16 atm.

136

Figure 1 shows the most sensitive reactions to ignition delay in the MD sub-mechanism at φ = 1.0,

137

temperatures of 670 K and 1250 K, and pressure of 16 atm. Besides, the sensitivity at low temperature has

138

been highlighted in blue color, while red color represents the high temperature result. By comparison, it

139

can be seen that the reactions sensitive to ignition delays under lower temperature (670 K) exhibit

140

significantly different behavior under higher temperature (1250 K). The low temperature prefers the

141

isomerization reactions of MD2O2 and MD6O2 (R1 and R5), hydrogen abstraction reaction from the sixth

142

carbon of MD (R2), and the decomposition reactions of MDKET68 and MDKET24 (R3 and R4), which all

143

enhance the system reactivity. However, these reactions exhibit negligible sensitivity at high temperature

144

condition except the reaction R2, which inhibits the system reactivity. In contrast, the high temperature

7


Energy & Fuels

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

Page 8 of 31

145

prefers the decomposition reactions (R12 and R13), and the hydrogen abstraction reactions by O2 and HO2

146

from MD and MP2D (R6, R7 and R8). But the hydrogen abstraction reactions by H from MD inhibit the

147

system reactivity at high temperature. Conversely, these reactions exhibited much lower sensitivity at low

148

temperature condition.

149

Given that species like MD3J, MD6J and MD8J have been removed from the original MD

150

sub-mechanism, the low temperature combustion chemistry would have been weakened consequently. As a

151

result, the pre-exponential factors of the reactions which have the negative sensitivity to ignition delay

152

were enhanced accordingly to better predict the experimental results, including the reactions R1, R6, R8,

153

R9 and R12. Detailed adjustments for these reactions have been summarized in Table S2.

154

After the adjustment, a combined reduced TRF/ethanol/n-butanol/DMF25/MD/PAH combustion

155

mechanism composed of 171 species and 766 reactions was developed. The thermodynamic and transport

156

data were referenced from the corresponding original sub-mechanisms. The reduced combustion

157

mechanism in CHEMKIN format is available and provided in the Supplemental Material. However, it

158

should be mentioned that, due to the fact that there is no transport data available for the detailed MD

159

mechanism 29, transport data of the MD sub-mechanism in the reduced mechanism was not included.

160

2.2 Mechanism validations

161

To ensure the reliability of the reduced mechanism so that it could be used for the following

162

investigations of the soot formation processes for the different oxygenated fuels, the reduced mechanism

163

has been validated extensively against the experimental ignition delays, species concentration profiles in

164

flames and pyrolysis, and laminar flame speeds. The corresponding experimental results were collected

165

from literatures, and the experimental conditions have been summarized in Tables S3-S6 of the

166

Supplemental Material. 8


Page 9 of 31

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

Energy & Fuels

167

The CHEMKIN package was used for further validations 30. The PREMIX code was used to simulate

168

the premixed flame species concentrations and laminar flame speeds in burners. The SENKIN code was

169

used to simulate the ignition delays in shock tubes and rapid compression machines, and pyrolysis species

170

concentration profiles in flow reactors, while the AURORA code was used to simulate the pyrolysis

171

species concentration profiles in jet stirred reactors.

172

2.2.1 Ignition delays

173

Ignition delay is very important, especially in engine combustion, which can influence the mixture

174

distribution, combustion and emissions significantly, because this parameter decides the time available for

175

the pre-mixed process before combustion. Experimental ignition delays of n-heptane, toluene, ethanol,

176

n-butanol, DMF25 and MD at high pressures from literatures 31-37 have been collected for validations. The

177

corresponding experimental conditions have been listed in Table S3 of the Supplemental Material.

178

Figure S1 shows comparisons of the experimental and predicted ignition delays of n-heptane, toluene,

179

ethanol, n-butanol, DMF25 and MD, respectively. Overall, the reduced mechanism could well capture the

180

ignition delays of these fuels with the variation of temperature. Ignition delays of ethanol at lower

181

temperature region (< 1000 K) were over-predicted by a factor of about 70%, although the reduced ethanol

182

sub-mechanism exhibited the similar predictive performance compared to the detailed mechanism

183

(AramcoMech 1.3)

184

should be conducted.

185

2.2.2 Species concentration profiles

186

38

, indicating that more investigations on the low temperature combustion of ethanol

Experimental species concentration profiles in both premixed flames and pyrolysis were collected to

187

further validate the reduced mechanism

188

summarized in Tables S4 and S5.

39-43

. The corresponding experimental conditions have been

9


Energy & Fuels

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

189

Castaldi et al.

39

Page 10 of 31

investigated the PAHs formation in a premixed ethene/oxygen/argon flame, at

190

pressure of 1 bar and φ = 3.06. Experimental concentration profiles of benzene (A1), naphthalene (A2),

191

phenanthrene (A3) and pyrene (A4) were used for validation of the PAHs sub-mechanism in the reduced

192

mechanism, as illustrated in Figure S2. It can be seen that, generally, the PAHs can be reasonably predicted,

193

especially the peak values, although A2 formation in the post-flame region were over-predicted by a factor

194

of about 2.0, in agreement with the predicted results from ref. 44.

195

From Figure S3, it can be observed that major species mole fraction profiles of n-heptane, toluene,

196

n-butanol and DMF25 can all be well predicted, including the fuel molecules, oxygen (O2), carbon oxide

197

(CO), carbon dioxide (CO2), hydrogen (H2) and water (H2O). But the largest discrepancy (a factor of about

198

2.5) existed for the prediction of H2O in the n-butanol combustion, which could be attributed to the

199

perturbation effects of the nozzle

200

sub-mechanisms of these species in this reduced mechanism should be reliable.

27

. These validations indicate that the high temperature combustion

201

While premixed flame experiments in burners are usually conducted at low pressures, different from

202

the real engine combustion conditions (> 40 atm). As a result, pyrolysis data of ethanol, n-butanol, DMF25

203

and MD at higher pressures have been collected to further validate the reduced combustion mechanism

204

35,45-47

. The experimental conditions have been listed in Table S5.

205

Comparisons of the experimental and predicted mole fraction profiles for ethanol, n-butanol, DMF25

206

and MD in pyrolysis have been depicted in Figure S4. Overall, it is observed that fuel consumption and

207

some major combustion intermediates (H2O, CO, CO2) with the variation of temperature for four

208

oxygenated fuels could be well captured by the reduced mechanism. Meanwhile, intermediate species,

209

such as methane (CH4), acetylene (C2H2), ethene (C2H4), ethane (C2H6) et al., could also be reasonably

210

predicted. The largest discrepancies of the corresponding peak values for each fuel are 60% for C2H4 10


Page 11 of 31

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

Energy & Fuels

211

(ethanol), 119% for C2H2 (n-butanol), 65% for A1 (DMF25) and 45% for C2H4 (MD), respectively.

212

2.2.3 Laminar flame speed

213

Laminar flame speed is another important combustion characteristic in fuel combustion, especially for

214

the characterization of flame propagation in spark ignition engines. Experimental laminar velocities of

215

n-heptane, toluene, ethanol, n-butanol and DMF25 from literatures have been used for further validations

216

of the reduced combustion mechanism, with the experimental conditions summarized in Table S6 35,48-51.

217

Since transport data of the MD sub-mechanism was not included in the reduced mechanism, validation

218

against the experimental laminar flame speed of MD has not been performed.

219

Figure S5 compares the experimental and predicted laminar flame velocities for n-heptane, toluene,

220

ethanol, n-butanol and DMF25. Overall, good agreement with the experimental results at different

221

equivalence ratios has been achieved by the reduced combustion mechanism. Although the reduced

222

combustion mechanism over-predicted the laminar velocities of ethanol at φ = 0.8 by a factor of 25%, it

223

exhibited similar predictive performance as compared to the detailed mechanism 38.

224

Above all, these validations prove that the reduced TRF/ethanol/n-butanol/DMF25/MD/PAH

225

mechanism developed in this work can reasonably predict the formation of PAHs and the major

226

combustion characteristics of n-heptane, toluene, ethanol, n-butanol, DMF25 and MD. Therefore, in the

227

following section, the reduced mechanism has been coupled with CFD code for 3-D spray-combustion and

228

0-D combustion simulations.

229

3. Experimental and modelling methods

230

3.1 Experimental setup

11


Energy & Fuels

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

Page 12 of 31

231 232

Figure 2. Schematic of the single-cylinder engine setup. 1. Electric supercharger. 2. One-way valve. 3. Airflow meter. 4.

233

Surge tank. 5. Cooler. 6. DI Fuel tank. 7. ECU. 8. Pressure transducer. 9. DI Injector. 10. Smoke meter. 11. Exhaust gas

234

analyzer. 12. Crank angle encoder. 13. Eddy current dynamometer. 14. Charge amplifier. 15. Computers. Table 2. Engine specifications. Displacement (L)

1.08

Number of valves

4

Bore/stroke (mm)

105/125

Connecting rod length (mm)

210

Compression ratio Intake valve close timing (CA ATDC) Exhaust valve open timing (CA ATDC) Common rail injector

16:1 -133 125 8 holes, 0.15 mm

235 Table 3. Engine operating conditions. Engine speed (rpm)

1500±1.0

Injected mass (mg/cyc)

60±0.5

Injection pressure (MPa)

80±0.8

SOI (CA ATDC)

-10±0.5

Intake air pressure (bar)

2.0±0.01

Intake air temperature (K)

308±0.5 0, 30, 50±0.15

EGR ratio (%)

236

DICI experiments were performed on a single-cylinder CI engine, which was modified from a

237

6-cylinder engine with the other five cylinders unchanged. The engine specifications have been listed in

238

Table 2, and the experimental setup schematic has been depicted in Figure 2. Details of the engine setup

239

can be found in ref. 52. Briefly, an electric air compressor was used to simulate the turbocharger, in order to

12


Page 13 of 31

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

Energy & Fuels

240

obtain the required intake pressures for combustion. Meanwhile, EGR can be realized by adjusting the

241

EGR valve and a backpressure valve, with the latter valve to regulate the pressure difference between the

242

intake pressure and back pressure. The in-cylinder pressure was detected by a piezoelectric transducer

243

(Kistler 6125A). Then the detected signals were amplified and converted to voltage signals, which were

244

further processed to obtain the pressure data. For each case, 50 consecutive cycles have been averaged,

245

considering the stable combustion quality during the experiment. The gas emissions and EGR ratio were

246

measured by a Horiba emission detector (MEXA-7100DEGR), and the smoke emission was measured by

247

an AVL415S smoke meter.

248

Four oxygenated fuels have been tested in the experiment, including pure biodiesel and its blends

249

with ethanol (E20 cases), n-butanol (B20 cases) and DMF25 (D20 cases) at the 20% (vol.) blending ratio.

250

Meanwhile, engine combustion fueled with pure diesel was taken as the base case for comparisons. The

251

engine operating conditions have been summarized in Table 3.

252

In the experiment, the engine was running at 1500 rpm. The intake temperature and pressure were

253

maintained at 308 K and 2.0 bar, respectively. The DI injection pressure was set at 80 MPa, and the start of

254

injection (SOI) was set at -10 crank angle after top dead center (CA ATDC). The injected fuel mass per

255

cycle for the oxygenated cases have been adjusted to have an equivalent energy of 60 mg diesel. Besides,

256

three different EGR ratios (0%, 30% and 50%) have been swept to investigate the EGR effects on the

257

combustion and soot emissions.

258

3.2. Modelling method

259

In this work, 3-D modelling investigations have been performed to study the effects of different 53

260

oxygenated structures on the combustion and soot formation processes. KIVA3V code

261

CHEMKIN chemistry solver was used for that purpose. This code has incorporated some updated models

coupled with

13


Energy & Fuels

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

262

54−60

263

occurring in the combustion chamber, as listed in Table 4.

Page 14 of 31

to well simulate the turbulence, heat transfer, spray, atomization, and spray-wall interaction processes

Table 4. Sub-models used in the KIVA code 54−60. Description

Model

Turbulence

Renormalized (RNG) k-ε 54

Heat transfer

Han and Reitz 55

Spray breakup

KH-RT 56

Droplet collision

Radius of influence (ROI) 57

Near nozzle flow

Gas-jet 58

Spray wall interaction

Han and Xu et al. 59 Discrete multi-component fuel (DMC) 60

Evaporation

264

Specially for the heat transfer model, which is very important to predict the in-cylinder pressure

265

profile, Han and Reitz 55 derived a new temperature/wall function to describe the variable-density turbulent

266

flows, which have been commonly found in engines. The increase of the turbulent Prandtl number in the

267

boundary layer has also been considered in the heat transfer model. By assumptions that the turbulent

268

effect is dominant and the chemical heat release is negligible in the boundary layer, the wall heat flux can

269

be given as,

270

∗ ⁄ . .

,

! ∗ ⁄"

(2)

271

where ρ is density, $% is specific heat at constant pressure, ! ∗ is friction velocity, & and & are local

272

temperature and wall temperature, respectively, and " is kinematic viscosity. Satisfactory agreement has

273

been achieved between the measured and predicted heat fluxes by using this model 55.

274 275

Figure 3. Computational mesh. 14


Page 15 of 31

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

Energy & Fuels

276

For the 3-D spray-combustion modelling studies, a 45°-sector computational mesh (9560 grid points)

277

was used considering the axisymmetric combustion chamber and the 8-hole injector structure, as shown in

278

Figure 3. The modelling investigations started from -133 CA ATDC and ended at 125 CA ATDC,

279

corresponding to the intake valve closing timing and exhaust valve opening timing, respectively. The initial

280

mixture and temperature conditions were considered homogeneous. The initial pressure was slightly

281

adjusted (within 0.05 bar) to capture the in-cylinder pressure profile before combustion, but it has been

282

kept at the same value for different fuel cases. Besides, to model the EGR effects, CO2 and H2O were

283

considered as the typical EGR species, as referenced from ref. 44.

284

The reduced TRF/ethanol/n-butanol/DMF25/MD/PAH mechanism developed in this work was

285

coupled with the KIVA code to describe the fuel combustion chemistry. Diesel and biodiesel oxidations

286

were represented by the combustion chemistries of n-heptane 44 and MD 25, and the physical properties of

287

n-tetradecane and MD, respectively, given that n-heptane has the similar cetane number with diesel and

288

MD can reasonably model the combustion and emissions of biodiesel 61. While the other involved species

289

(ethanol, n-butanol and DMF25) were represented by their corresponding combustion chemistries and

290

physical properties. Meanwhile, the soot formation and oxidation processes were predicted using a

291

multi-step phenomenological soot model with A4 as the precursor

292

oxidation processes are summarized as follows:

293

(a) Soot inception: A4 → 16C(s) + 5H2

294

(b) Surface growth 1: C(s) + C2H2 → 3C(s) + H2

295

(c) Surface growth 2: C(s) + PAHk,j → C(s+k) + 0.5jH2

296

(d) Soot coagulation: nC(s) → C( s)n

297

(e) Soot oxidation process 1: C(s) + 0.5O2 → CO

62

. The brief soot formation and

15


Energy & Fuels

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

298 299 300

301

302

Page 16 of 31

(f) Soot oxidation process 2: C(s) + OH → CO + 0.5H2 An equation was adopted to account for the two additional transport equations that are solved in the CFD code, in order to calculate the soot species density and number density, '( ')

/

(

/ ∇

∇+, + ∇ .01 ∇ . 2 3 + 4+ 2

4 0.75/.1 +

67 8

5 3 + (

3, 9 0.9

(3)

(4)

303

where + stands for either soot species density in g-cm-3, or soot number density in particles cm-3. < is

304

5 the Schmidt number, μ is the viscosity, > is the density, T is the temperature, and ( represents the

305

source terms, which can be derived from the above reaction rates. Details about the multi-step

306

phenomenological soot model and the related reaction rates can be found in ref. 62,63.

307

In the following section, the average total soot formation, soot oxidized by O2 and OH, and the net

308

soot formation have been used to compare the different soot formation characteristics for the different fuels.

309

These parameters were calculated by integrating each cell value over the whole computational domain,

310

which followed the relationship as,

311

Initial soot formation = Net soot formation + Soot oxidized by O2 + Soot oxidized by OH

(5)

Table 5. Modelling conditions for the O10 cases. Parameters

Ethanol

N-butanol

DMF25

MD

Mol. (%)

47

54

62

43

312

On the other hand, to further clarify the chemical effects of the different oxygenated structures in

313

ethanol, n-butanol, DMF25 and MD on soot formation, 0-D modelling investigations of five pure fuel

314

cases (n-heptane, ethanol, n-butanol, DMF25 and MD) have been performed. Besides, reaction pathway

315

analyses have been conducted to analyze the different PAH and soot formation mechanisms for these fuels.

316

Four other cases with the oxygenates blended with n-heptane at the oxygen content at 10% (wt.) (O10)

317

were also simulated to further explore the soot suppression effects. The blending ratios have been listed in 16


Page 17 of 31

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

Energy & Fuels

318

Table 5.

319

4. Results and discussions

320

4.1. 3-D modelling investigations

321 322

Figure 4. Comparisons of the experimental and modelling in-cylinder pressure and heat release rate profiles.

323

In this section, the experimental combustion and soot emission results have been collected for the 3-D

324

modelling validations. Combustion details of the different oxygenated fuels derived from the modelling

325

investigations have also been presented and analyzed.

326

Figure 4 compares the experimental and modelling in-cylinder pressure and heat release rate profiles

327

for the combustion of diesel, biodiesel, B20, D20 and E20 cases, at three different EGR ratios (0%, 30%

328

and 50%). Generally, the experimental combustion results have been well predicted by the reduced

329

combustion mechanism. It is obvious that the combustion was postponed with the premixed combustion

330

enhanced at the higher EGR ratio, mainly due to the thermal and dilution effects

331

observed in this figure, the combustion phasing at the same EGR ratio can be sequenced as Diesel
Biodiesel > B20 > E20 > D20. However, ethanol has a much higher vaporization heat (about

338

904 kJ/kg at 25 ℃, ref. 68) when evaporating compared with DMF25 (about 333 kJ/kg at 20 ℃, ref. 69),

339

which can reduce the regional temperature and further postpone the combustion in the spray-combustion

340

processes. As a consequence, combustion phasing of E20 case was the latest.

341 342

Figure 5. Comparisons of the experimental and predicted soot emissions.

343 344

Figure 6. Comparisons of the experimental and predicted CA10 (EGR 50%).

345 346

Figure 7. Equivalence ratio, temperature and soot distribution contours (EGR 50%). Phi represents equivalence ratio. T

347

represents temperature.

18


Page 19 of 31

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

Energy & Fuels

348

Figure 5 compares the experimental and predicted soot emissions at various EGR ratios (0%, 30%

349

and 50%). Overall, the experimental soot emissions have been well predicted by the modelling results.

350

Obviously, with the increase of EGR ratio, soot emissions for each fuel increased accordingly, particularly

351

from the EGR ratio of 30% to 50%. By comparisons, soot emissions of the five cases can be sequenced as

352

Diesel > Biodiesel > B20 > D20 > E20, in contrary to the corresponding combustion phasing. As seen in

353

Figure 6, the model can well predicted the experimental ignition timings (CA10, the ignition timing when

354

10% of the total heat has been released) for different cases, and they can be sequenced as Diesel
2.0 regions at CA10 (EGR 50%).

360

Figure 7 illustrates the equivalence ratio, temperature and soot distribution contours for the

361

combustion of diesel, biodiesel, B20, D20 and E20 cases at the EGR ratio of 50%. CA50 and CA90

362

represent the ignition timings when 50% and 90% of the total heat have been released, respectively. It can

363

be observed that compared to diesel, the equivalence ratios of the oxygenated fuels were much lower at

364

CA10 while the combustion temperatures at CA50 were higher mainly owing to the oxygen content, which 19


Energy & Fuels

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

Page 20 of 31

365

is beneficial for the reduction of soot 7,70. Consequently, soot formation of the oxygenated fuels were much

366

lower than diesel.

367

Figure 8 shows the predicted mass distributions of equivalence ratio and Figure 9 shows the cell

368

number of φ 2.0 regions for the five cases at CA10. It can be seen that E20 had the leanest

369

mixture distribution, with the highest cell number of φ < 2.0 region, followed by D20, B20, biodiesel and

370

diesel. Given that soot is mainly formed in high equivalence ratio regions (φ > 2.0) 71, its formation would

371

be lower with the leaner mixture. Therefore, soot formation for the four oxygenated fuels was sequenced as

372

Biodiesel > B20 > D20 > E20.

373

To further get a deeper insight into the soot formation process in diesel engine, time evolutions of

374

important soot precursors (C2H2, A1, and A4) and net soot formation averaged over the combustion

375

chamber have been illustrated in Figure 10. Overall, it can be seen that C2H2 was firstly produced in the

376

main combustion period, followed by A1, A4 and soot, respectively, in agreement with the soot growth

377

process. Besides, for different fuels, peaks of the soot precursors like C2H2 and A1 exhibited same trend as

378

that of soot. However, the peak A4 value was higher for D20 than B20 although the final soot formation for

379

the former blending fuel was lower. The higher A4 formation was due to the specific chemical structure of

380

DMF, which will be discussed in detail in the next section, while the lower final soot formation can be

381

attributed to the leaner mixture distribution and thus the more intensive soot oxidation processes by OH

382

and O2 44.

20


Page 21 of 31

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

Energy & Fuels

383 384

Figure 10. Time evolutions of C2H2, A1, A4 and net soot formation (EGR 50%).

385 386

Figure 11. Time evolutions of initial soot formation, soot oxidized by OH and O2, and net soot formation (EGR 50%).

387 21


Energy & Fuels

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

388

Page 22 of 31

Figure 12. Time evolutions of oxidized soot fractions (EGR 50%).

389

Figure 11 depicts the initial soot formation, the soot amount oxidized by O2 and OH, and the net soot

390

formation for different fuels. Comparatively, as seen from the initial soot formation figure, the oxygenated

391

fuels had much lower sooting tendency than diesel. The large discrepancies between the initial soot

392

formation of diesel and oxygenated fuels can mainly be attributed to the oxygen content of the base

393

biodiesel fuel. Meanwhile, the oxidized soot amount had a positive correlation with the initial soot

394

formation, but the discrepancies between diesel and the oxygenated fuels were lower than those of the

395

initial soot formation, mainly due to the enhanced premixed combustion processes of the oxygenated fuels

396

for the lower reactivity, which is beneficial for the soot oxidization.

397

Time evolutions of the oxidized soot fractions for different fuels have also been illustrated in Figure

398

12. The net soot would peak at the 50% oxidized soot fraction and then decrease after that. It can be seen

399

that the soot formation contribution exhibited the similar importance for these fuels before the peak values,

400

which means that the soot suppression effects of the oxygenated fuels can mainly account for the lower net

401

soot formation than diesel. While after the peak values, the oxidation effects played the more important

402

role in the soot reduction. Eventually, owing to the soot oxidization processes by OH and O2, a certain

403

amount of soot has been oxidized during the main combustion period, and the final net soot production

404

was much lower than the initial soot formation. On the other hand, it can also be observed that the D20

405

blend exhibited the higher oxidization capability than the B20 blend owing to the leaner mixture

406

distribution despite of the higher initial soot formation. As a result, D20 had the lower net soot production

407

than B20.

408

4.2 0-D modelling investigations

22


Page 23 of 31

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

Energy & Fuels

409 410 411

Figure 13. Comparisons of the A4 formation for ethanol, n-butanol, MD, DMF25 and n-heptane in 0-D simulations.

Soot formation in the spray-combustion processes is a complex issue, depending on both the 62

412

gas-phase reactions and physical processes

413

only factor affecting the soot formation. Since A4 is one of the most important soot precursors and its

414

formation followed the similar trend as soot formation, it was used to represent the soot formation

415

tendency in this work, as referenced from ref. 23.

, but in homogeneous combustion, chemical kinetics is the

416

In this section, 0-D modelling investigations on the PAH formation of five pure fuel cases (ethanol,

417

n-butanol, MD, DMF25 and n-heptane) and four O10 cases (Table 5) have been performed using SENKIN

418

code at initial pressure of 84.5 atm, with the temperature ranging from 1000 K to 3000 K, φ ranging from

419

1.0 to 6.0. The retention time was set at 1.5 ms, corresponding to the main combustion duration time in

420

representative CI engines. Meanwhile, for each case, the mass fraction of A4 formation has been

421

normalized by the same peak value for comparison. Furthermore, reaction pathway analyses of n-heptane,

422

ethanol, n-butanol, DMF25 and MD at φ = 5.0 and temperature of 1800 K have been conducted to clarify

423

the soot formation pathways of various fuels.

23


Energy & Fuels

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

Page 24 of 31

424 425

Figure 14. Reaction pathway analyses of n-heptane, ethanol and n-butanol at φ = 5.0, temperature of 1800 K and pressure of

426

84.5 atm.

427 428

Figure 15. Reaction pathway analysis of MD at φ = 5.0, temperature of 1800 K and pressure of 84.5 atm.

24


Page 25 of 31

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

Energy & Fuels

429 430

Figure 16. Reaction pathway analysis of DMF25 at φ = 5.0, temperature of 1800 K and pressure of 84.5 atm.

431

Figure 13 compares the A4 formation regions for the five pure fuel combustion cases. Comparatively,

432

it is seen that DMF25 exhibited the highest sooting tendency, while ethanol produced the least A4. The

433

sooting tendencies for the four oxygenate fuels could be sequenced as ethanol < n-butnaol < MD < DMF25,

434

which was in agreement with the oxygen content in their molecules. However, DMF25 produced the

435

highest A4 among all these tested fuels, including non-oxygenated n-heptane. Therefore, it is desirable to

436

further explore the A4 formation reaction pathways of various fuels to explain the current phenomenon.

437

Figures 14 to 16 show the reaction pathways of n-heptane, ethanol, n-butanol, MD and DMF25 at φ =

438

5.0 and temperature of 1800 K, with the rate of production integrated over the whole computational time.

439

It can be seen in Figure 14 that with longer carbon chain in the n-heptane molecule, it tended to produce

440

more intermediate small hydrocarbons like propene (C3H6) and ethene (C2H4), which favor the soot

441

precursor propargyl and acetylene (C3H3 and C2H2) formation, and thus PAHs as well as soot formation

442

consequently.

443

By comparisons, only a small amount of ethanol could produce C2H4, while a certain amount of

444

n-butanol converted to C3H6 and C2H4. Eventually, C3H6 and C2H4 converted to C3H3 and C2H2,

445

respectively, both of which are important PAHs precursors. Therefore, n-butanol exhibited the higher 25


Energy & Fuels

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

Page 26 of 31

446

sooting tendency. Given that MD had longer carbon chain moiety in the molecule, it had the higher chance

447

to produce soot precursors, as seen in Figure 15. Besides, the two oxygen atoms in the MD molecule were

448

connected to the same carbon atom, which was not as effective as that of alcohols for soot reduction

449

As a result, MD exhibited the higher sooting tendency than n-butanol. On the other hand, although

450

n-heptane had a shorter carbon chain length than that of MD, there was no oxygen in the molecule and thus

451

almost all of its reaction pathways would lead to the formation of C3H3 and C2H2. Consequently, the

452

sooting tendency of n-heptane was even higher than MD. However, despite of the 16.7% oxygen content in

453

the DMF25 molecule, it could directly produce cyclic intermediates, including phenol (C6H5OH) and

454

cyclopentadienyl (C5H5), as seen in Figure 16, which considerably enhanced the PAHs formation

455

Therefore, DMF25 exhibited the highest sooting tendency.

8,21

27

.

.

456

While for practical applications, it is not homogeneous in engine combustion, and soot formation

457

should be depended on both the mixing process and the combustion chemistry. Therefore, although

458

DMF25 exhibited the highest sooting tendency in homogeneous condition, less soot was produced in the

459

spray-combustion processes, owing to the lower reactivity and the consequently leaner mixture distribution,

460

which is beneficial for soot oxidation processes, and this is the main reason for the lower net soot

461

formation of D20 than B20 in the 3-D modelling studies.

26


Page 27 of 31

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

Energy & Fuels

462 463

Figure 17. Comparisons of the A4 formation for the four O10 and pure n-heptane combustion cases.

464

To further explore the potential of the soot suppression effects for the four oxygenates in n-heptane,

465

four O10 cases have been performed as well. The blending ratios have been listed in Table 5. Figure 17

466

compares the A4 formation for n-heptane and four O10 cases. Generally, the A4 formation and sooting

467

tendencies can be sequenced as ethanol < n-butanol ≈ MD < n-heptane < DMF25. It is known that for

468

homogeneous combustion both chemical and dilution effects affect soot formation

469

Figure 13, it is seen that discrepancies among O10 cases were not as significant as those in the pure fuel

470

cases, which should be mainly attributed to the dilution effects of the oxygenated fuels when blended with

471

n-heptane.

21

. Combined with

472

Since ethanol exhibited the lowest sooting tendency, A4 formation for its O10 blend could be mainly

473

attributed to n-heptane. While both n-butanol and MD blends exhibited higher sooting tendencies

474

compared to the ethanol blend, which can be attributed to the fact that n-butanol and MD themselves could

475

produce much more A4 than ethanol, as seen in Figure 13. On the other hand, although more n-heptane

476

(57%) is needed to maintain the 10% oxygen content in MD/n-heptane blend than that in

477

n-butanol/n-heptane blend (46%), the MD/n-heptane blend got higher mass fraction of nonyl radical 27


Energy & Fuels

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

Page 28 of 31

478

moiety in MD (40%) than that of the propyl radical moiety in n-butanol (27%) for the n-butanol/n-heptane

479

blend. As a result, the two factors together contributed to the similar sooting tendencies for the MD and

480

n-butanol blends. Still, for the special cyclic ring structure of DMF25 as explained above, the

481

DMF25/n-heptane blend exhibited the highest sooting tendency despite of the same 10% oxygen content.

482

Therefore, care should be taken when using DMF25 as the fuel in engine applications 27.

483

5. Conclusions

484

The soot formation of ethanol, n-butanol, DMF25 and biodiesel have been investigated

485

experimentally and numerically in engine combustion, and a combined reduced combustion mechanism

486

has been proposed for the 3-D and 0-D modelling studies. Experimental combustion and soot emission

487

results of a single-cylinder engine have been collected for the 3-D simulations. Meanwhile, 0-D modelling

488

investigations on the soot formation of ethanol, n-butanol, DMF25 and MD, compared with n-heptane

489

have been performed. Reaction pathway analyses were also conducted to further clarify the different

490

PAH/soot formation pathways. Major conclusions have been summarized as follows:

491

(1) A reduced TRF/ethanol/n-butanol/DMF25/MD/PAH mechanism has been developed based on previous

492

works, which has been validated extensively, and overall reasonable agreement was obtained.

493

(2) The reduced mechanism can well predict the experimental combustion characteristics and soot

494

emissions for different fuels. Apart from the oxygen content, the fuel reactivity of the oxygenated fuels

495

also played an important role in soot reduction. Specially for the higher vaporization heat of ethanol, the

496

E20 blend had the lowest reactivity and net soot formation owing to the leaner mixture distribution. As for

497

D20, although it had the higher initial soot formation than B20, the final soot formation was lower

498

attributed to the more enhanced soot oxidization effect.

499

(3) By analyzing the soot formation and oxidations, it was found out that the soot suppression effects 28


Page 29 of 31

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

Energy & Fuels

500

played an important role for the oxygenated fuels compared with diesel before the peak net soot formation

501

values, while the soot oxidation effects were more significant after that.

502

(4) The reaction pathway analyses revealed that the longer carbon chain in the molecule moiety

503

contributed to the more PAHs formation. While for the specific cyclic ring structure of DMF25, it tended

504

to produce even more PAHs than n-heptane under homogeneous condition, indicating that it was the lower

505

reactivity of the DMF25 structure but not the oxygen content that should account for its lower soot

506

formation under engine combustion condition.

507

AUTHOR INFORMATION

508

*Corresponding author

509

Email: [email protected]

510

Tel: +0086 13820150627

511

ACKNOWLEDGMENT

512

The authors acknowledge financial support provided by the National Natural Science Found of China

513

(NSFC) through its projects of 51506145 and 91541205.

514

ABBREVIATIONS A1 A2

benzene naphthalene

CO2 CVC

A3

phenanthrene

DICI

A4

DMF25

EGR

exhaust gas recirculation

E20

20% volume fraction of ethanol in biodiesel

ICE

internal combustion engine

C2H2 C2H4

pyrene 20% volume fraction of n-butanol in biodiesel crank angle when 10% total heat released crank angle when 50% total heat released crank angle when 90% total heat released acetylene ethene

MD O10

C3H3

propargyl

PAHs

methyl-decanoate 10% fuel oxygen content polycyclic aromatic hydrocarbons

B20 CA10 CA50 CA90

D20

carbon dioxide constant volume chamber Direct injection compression ignition 2,5-dimethylfuran 20% volume fraction of 2,5-dimethylfuran in biodiesel

29


Energy & Fuels

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

C3H6 C5H5 C6H5OH CI CO

propene cyclopentadienyl phenol compression ignition carbon oxide

TRF Vol. 0-D 3-D

φ

Page 30 of 31

toluene reference fuel volume fraction zero-dimensional three-dimensional equivalence ratio

515

REFERENCES

516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553

(1) Mwangi, J. K.; Lee, W.-J.; Chang, Y.-C.; Chen, C.-Y.; Wang, L.-C. Appl. Energy 2015, 159, 214-236. (2) Zhao, Y.; Wang, Y.; Li, D.; Lei, X.; Liu, S. Energy 2014, 72 (7), 608-617. (3) Muralidharan, K.; Vasudevan, D. Appl. Energy 2011, 88 (11), 3959-3968. (4) Miyamoto, N.; Ogawa, H.; Nurun, N. M.; Obata, K.; Arima, T. Sae Technical Paper 1998, 980506. (5) Chen, H.; Wang, J.; Shuai, S.; Chen, W. Fuel 2008, 87 (15-16), 3462-3468. (6) Klein-Douwel, R. J. H.; Donkerbroek, A. J.; van Vliet, A. P.; Boot, M. D.; Somers, L. M. T.; Baert, R. S. G.; Dam, N. J.; ter Meulen, J. J. Proc. Combust. Inst. 2009, 32 (2), 2817-2825. (7) McEnally, C. S.; Pfefferle, L. D. Environ Sci Technol 2011, 45 (6), 2498-503. (8) Eveleigh, A.; Ladommatos, N.; Balachandran, R.; Marca, A. Combust. Flame 2014, 161 (11), 2966-2974. (9) Wang, X.; Cheung, C. S.; Di, Y.; Huang, Z. Fuel 2012, 94, 317-323. (10) Yao, M.; Wang, H.; Zheng, Z.; Yue, Y. Fuel 2010, 89 (9), 2191-2201. (11) Zhang, Q.; Chen, G.; Zheng, Z.; Liu, H.; Xu, J.; Yao, M. Fuel 2013, 103, 730-735. (12) E, J.; Liu, T.; Yang, W. M.; Li, J.; Gong, J.; Deng, Y. Energy Convers. Manage. 2016, 117, 410-419. (13) E, J.; Liu, T.; Yang, W.; Deng, Y.; Gong, J. Appl. Energy 2016, 181, 322-331. (14) Benajes, J.; Molina, S.; Garcia, A.; Monsalve-Serrano, J. Energy 2015, 90, 1261-1271. (15) Green, E. M. Curr. Opin. Biotechnol. 2011, 22 (3), 337-43. (16) Nishimura, S.; Ikeda, N.; Ebitani, K. Catal. Today 2014, 232 (89-98), 89-98. (17) Agarwal, A. K. Prog. Energy Combust. Sci. 2007, 33 (3), 233-271. (18) Basha, S. A.; Gopal, K. R.; Jebaraj, S. Renew. Sust. Energ. Rev. 2009, 13 (6–7), 1628-1634. (19) Liu, H.; Bi, X.; Huo, M.; Lee, C.-f. F.; Yao, M. Energy Fuels 2012, 26 (3), 1900-1911. (20) Liu, H.; Wang, X.; Zheng, Z.; Gu, J.; Wang, H.; Yao, M. Energy 2014, 74, 741-752. (21) Westbrook, C. K.; Pitz, W. J.; Curran, H. J. J. Phys. Chem. A 2006, 110 (21), 6912-6922. (22) Ra, Y.; Reitz, R. D. Combust. Flame 2011, 158 (1), 69-90. (23) Liu, X.; Wang, H.; Wei, L.; Liu, J.; Reitz, R. D.; Yao, M. Combust. Flame 2016, 165, 453-465. (24) Wang, H.; DelVescovo, D.; Zheng, Z.; Yao, M.; Reitz, R. D. Front. Mech. Eng. 2015, 1, article 3. (25) Wang, X.; Liu, H.; Zheng, Z.; Yao, M. Energy Fuels 2015, 150126152354001. (26) Liu, Y.; Jia, M.; Xie, M.; Pang, B. Energy Fuels 2012, 26 (12), 7069-7083. (27) Liu, X.; Yao, M.; Wang, Y.; Wang, Z.; Jin, H.; Wei, L. Combust. Flame 2015, 162 (12), 4586-4597. (28) Metcalfe, W. K.; Burke, S. M.; Ahmed, S. S.; Curran, H. J. Int. J. Chem. Kinet. 2013, 45 (10), 638-675. (29) Herbinet, O.; Pitz, W. J.; Westbrook, C. K. Combust. Flame 2008, 154 (3), 507-528. (30) Kee, R. J.; Rupley, F. M.; Miller, J. A., CHEMKIN II: A fortran chemical kinetics package for the analysis of gas-phase chemical kinetics, Sandia Laboratories Report, S. 89-8009B, Sandia National Laboratories, 1989. (31) Mehl, M.; Pitz, W. J.; Westbrook, C. K.; Curran, H. J. Proc. Combust. Inst. 2011, 33 (1), 193-200. (32) Shen, H. S.; Vanderover, J.; Oehlschlaeger, M. A. Proc. Combust. Inst. 2009, 32 (1), 165-172. (33) Heufer, K. A.; Olivier, H. Shock Waves 2010, 20 (4), 307-316. (34) Stranic, I.; Chase, D. P.; Harmon, J. T.; Yang, S.; Davidson, D. F.; Hanson, R. K. Combust. Flame 2012, 159 (2), 516-527. (35) Somers, K. P.; Simmie, J. M.; Gillespie, F.; Conroy, C.; Black, G.; Metcalfe, W. K.; Battin-Leclerc, F.; 30


Page 31 of 31

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

554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597

Energy & Fuels

Dirrenberger, P.; Herbinet, O.; Glaude, P. A.; Dagaut, P.; Togbé, C.; Yasunaga, K.; Fernandes, R. X.; Lee, C.; Tripathi, R.; Curran, H. J. Combust. Flame 2013, 160 (11), 2291-2318. (36) Wang, W.; Oehlschlaeger, M. A. Combust. Flame 2012, 159 (2), 476-481. (37) Li, Z.; Wang, W.; Huang, Z.; Oehlschlaeger, M. A. Energy Fuels 2013, 26 (8), 4887–4895. (38) Liu, X.; Wang, H.; Zheng, Z.; Liu, J.; Reitz, R. D.; Yao, M. Energy 2016, 114, 542-558. (39) Castaldi, M. J.; Marinov, N. M.; Melius, C. F.; Huang, J.; Senkan, S. M.; Pit, W. J.; Westbrook, C. K. Symp. (Int.) Combust. 1996, 26 (1), 693-702. (40) Bakali, A. E.; Delfau, J. L.; Vovelle, C. Combust. Sci. Technol. 1998, 140 (1-6), 69-91. (41) Li, Y.; Cai, J.; Zhang, L.; Yuan, T.; Zhang, K.; Qi, F. Proc. Combust. Inst. 2011, 33 (1), 593-600. (42) Oßwald, P.; Güldenberg, H.; Kohse-Höinghaus, K.; Yang, B.; Yuan, T.; Qi, F. Combust. Flame 2011, 158 (1), 2-15. (43) Togbé, C.; Tran, L. S.; Liu, D.; Felsmann, D.; Oßwald, P.; Glaude, P. A.; Sirjean, B.; Fournet, R.; Battin-Leclerc, F.; Kohse-Höinghaus, K. Combust. Flame 2014, 161 (3), 780-797. (44) Wang, H.; Yao, M.; Yue, Z.; Jia, M.; Reitz, R. D. Combust. Flame 2015, 162 (6), 2390-2404. (45) Leplat, N.; Dagaut, P.; Togbé, C.; Vandooren, J. Combust. Flame 2011, 158 (4), 705-725. (46) Dagaut, P.; Sarathy, S. M.; Thomson, M. J. Proc. Combust. Inst. 2009, 32 (1), 229-237. (47) Glaude, P. A.; Herbinet, O.; Bax, S.; Biet, J.; Warth, V.; Battin-Leclerc, F. Combust. Flame 2010, 157 (11), 2035-2050. (48) van Lipzig, J. P. J.; Nilsson, E. J. K.; de Goey, L. P. H.; Konnov, A. A. Fuel 2011, 90 (8), 2773-2781. (49) Davis, S. G.; Wang, H.; Breinsky, K.; Law, C. K. Symp. (Int.) Combust. 1996, 26 (1), 1025-1033. (50) Egolfopoulos, F. N.; Du, D. X.; Law, C. K. Symp. (Int.) Combust. 1992, 24 (1), 833-841. (51) Veloo, P. S.; Egolfopoulos, F. N. Proc. Combust. Inst. 2011, 33 (1), 987-993. (52) Yang, B.; Yao, M.; Cheng, W. K.; Li, Y.; Zheng, Z.; Li, S. Appl. Energy 2014, 113, 722-733. (53) Amsden, A. A., KIVA3V, Rel.2, Improvements to KIVA3V, LA Report, LA-UR-99-915, Los Alamos National Lab., 1999. (54) Han, Z.; Reitz, R. D. Combust. Sci. Technol. 1995, 106 (4-6), 267-295. (55) Han, Z.; Reitz, R. D. Int. J. Heat Mass Transfer 1997, 40 (3), 613-625. (56) Ricart, L. M.; Reitz, R. D.; Dec, J. E. J. Eng. Gas Turbines Power 1999, 122 (4), 588-595. (57) Munnannur, A.; Reitz, R. D. Atomization Sprays 2009, 19 (7), 597-619. (58) Yoshikawa, T.; Reitz, R. D. Int. J. Spray Combust. Dyn. 2009, 1 (3), 283-316. (59) Han, Z.; Xu, Z.; Trigui, N. Int. J. Engine Res. 2000, 1 (1), 127-146. (60) Ra, Y.; Reitz, R. D. Int. J. Multiphase Flow 2009, 35 (2), 101-117. (61) Wang, X.; Yao, M.; Li, S.; Gu, J. SAE Technical Paper 2013, 2013-01-1130. (62) Vishwanathan, G.; Reitz, R. D. J. Eng. Gas Turbines Power 2009, 131 (3), 032801. (63) Vishwanathan, G.; Reitz, R. D. Combustion Science & Technology 2010, 182 (8), 1050-1082. (64) Sjöberg, M.; Dec, J. E.; Hwang, W. SAE Technical Paper 2007, 2007-01-0207. (65) Liu, H.; Xu, J.; Zheng, Z.; Li, S.; Yao, M. Energy 2013, 62, 215-223. (66) Naser, N.; Yang, S. Y.; Kalghatgi, G.; Chung, S. H. Fuel 2017, 187, 117-127. (67) Kalghatgi, G. T. SAE Technical Paper 2005, 2005-01-0239. (68) Liu, H.; Lee, C.-f.; Huo, M.; Yao, M. Energy Fuels 2011, 25 (4), 1837-1846. (69) Zhong, S.; Daniel, R.; Xu, H.; Zhang, J.; Turner, D.; Wyszynski, M. L.; Richards, P. Energy Fuels 2010, 24 (5), 2891-2899. (70) Kitamura, T.; Ito, T.; Senda, J.; Fujimoto, H. Int. J. Engine Res. 2002, 3 (4), 223-248. (71) Akihama, K.; Takatori, Y.; Inagaki, K.; Sasaki, S.; Dean, A. SAE Technical Paper 2001, 2001-01-0655. 31


Experimental and Modeling Investigations on Soot Formation of

Recommend Documents