Transformation of coke carbon textures and minerals during

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Transformation of coke carbon textures and minerals during gasification investigated by SEM/EDS Hao Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00849 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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Energy & Fuels

1

Transformation of Coke Carbon Textures and

2

Minerals

3

SEM/EDS

4

Hao Zhang*

5

School of Mechanical, Materials, Mechatronic and Biomedical Engineering, University of

6

Wollongong, Wollongong, New South Wales 2522, Australia

7

ABSTRACT: The carbon textures, minerals and their interactions of a metallurgical coke during

8

gasification reaction were investigated using scanning electron microscopy/energy dispersive

9

spectroscopy (SEM/EDS) by analyzing exactly the same sites of coke surface before and after

10

reaction. The results indicate that most inert maceral-derived component (IMDC) had a greater

11

propensity to react with CO2 than reactive maceral-derived component (RMDC). A portion of

12

the carbon in mineral-carrying IMDC was removed as a result of the reaction with CO2 leaving

13

minerals, primarily alumino-silicates, more pronounced. However, the reactivity of IMDC was

14

found to be similar to RMDC in some cases. The physical features of the alumino-silicates in the

15

coke were barely altered but bound with alkalis during the reaction. The Fe-containing phases

16

were highly active and had strong interactions with the carbon matrix. Voids were formed in the

17

carbon matrix associated with the Fe-containing phases as a result of the catalytic carbon-gas

18

reactions. Iron played a major role in the reactions. The Mg and Ca-containing phases reacted

during

Gasification

Investigated

by

ACS Paragon Plus Environment

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with the surrounding alumino-silicates and transformed into slag globules. Metallic iron

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dispersed in the voids or attached on the slag globules was formed as a result of the reduction of

21

iron sulphate or sulphide.

22

1. INTRODUCTION

23

Coke is a key material for the blast furnace (BF) operation, acting as a heat supplier, reducing

24

agent and mechanical support for the permeability through the burden column. Coke is subjected

25

to significant degradation in the BF due to the thermal and mechanical stresses as well as

26

chemical reactions. Major reactions relevant to coke in the BF include gasification,

27

graphitization and carburization, etc. Among all the reactions, gasification has drawn the most

28

attention due to its significant impact on coke degradation and fines generation in the BF.1

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To reduce the consumption of expensive coke, supplementary fuels such as pulverized coal,

30

natural gas and oil are injected into the raceways to substitute for a portion of the coke.2-4 At high

31

injection rates, coke quality becomes more critical as less coke is available to fulfil its roles.

32

Reactivity and strength are the most significant parameters that characterize the coke quality,

33

which in turn depend on the organic carbon-based components and inorganic matter (minerals)

34

inherited from the parent coals and serve to determine the coke performance in the BF.5

35

Due to the strong impact of the carbon-based components on coke strength and reactivity, the

36

degradation of coke in the BF is highly related to the modification of its carbon textures caused

37

by gasification and exposure to high temperatures. Thus, a great interest has been paid to the

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carbon textures of coke by many researchers who have developed their own terminology and

39

classification schemes in their studies.6-10 Generally, coke textures can be divided into two

40

components, i.e., the inert maceral-derived component (IMDC) and the reactive maceral-derived


2

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component (RMDC). IMDC is formed from infusible macerals and RMDC is from fusible

42

macerals during carbonization.8 IMDC and RMDC have been found to behave differently in

43

terms of strength, reactivity, and graphitization.11 IMDC is often regarded to be more reactive

44

than RMDC in many references.12,13 However, RMDC was found to react more easily with

45

CO2 than IMDC in the BF due to the catalytic effect of alkalis in a recent study.14

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Due to the close relationship between coke carbon and mineral matter, coke minerals could

47

affect the strength and reactivity of coke in many ways such as the formation of cracks and weak

48

spots in the coke matrix and the catalyzing reactions by catalytic mineral phases.15-18 The

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production and utilization of highly reactive catalyst-loaded coke to increase the reaction

50

efficiency and decrease the reducing agent rate in the BF have been extensively reported.19-22

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Additionally, the BF environment such as high temperatures and recirculating alkalis could also

52

affect coke gasification. Therefore, it is crucial to gain a better understanding of the coke

53

minerals and their interactions with coke carbon matrix under BF conditions. Current

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understandings of the mineral matter in coke are largely based on the oxide analysis of the coke

55

ash, which has not taken into account the fact that coke minerals are present in various sizes,

56

distributions, and associations within the carbon matrix.23 The X-ray diffraction (XRD) can only

57

identify crystalline phases. However, more than 60% of the inorganic matter of the original

58

cokes is present in glassy or amorphous phases.1

59

Microscopic approaches have also been applied to the investigations of coke textures and

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minerals.1,23-25 However, previous microscopic analyses on the changes in coke textures and

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minerals were mainly conducted by observing different areas of different samples before and

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after reactions, which are difficult to interpret due to the heterogeneous nature of cokes. In this

63

study, the carbon texture and minerals of coke were investigated using scanning electron


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microscopy/energy dispersive spectroscopy (SEM/EDS) by analyzing exactly the same sites of

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coke surface before and after reaction. The objective is to determine the transformation of carbon

66

textures (IMDC and RMDC) and minerals during coke gasification.

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2. EXPERIMENTAL SECTION

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2.1. Coke Preparation. The cylindrical coke pellets used in this study were prepared by drilling

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cores from metallurgical coke lumps and facing the cylindrical ends. The pellet size was 8-mm

70

diameter by 10-mm height. The raw metallurgical coke lumps were produced in a coke oven

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battery in Australia. The proximate and ultimate analyses of the raw sample are provided in

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Table 1. The ash analysis is provided in Table 2.

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The top circular surface of the unreacted pellets was polished on a polishing machine (Struers

74

Tegram) before being examined by SEM/EDS. It should be noted that the pellets were not

75

mounted by resin because they needed to be reacted in the furnace so that the same sites of the

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top surface can be examined by SEM/EDS again after reaction and compared with the unreacted

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ones.

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Table 1. Proximate and Ultimate Analyses of Coke

Proximate analysis (wt%, ad)

79

Ultimate analysis (wt%, daf)

Moisture

Volatile matter

Ash

Fixed carbon

C

H

N

S

O

0.9

1.4

13.0

84.7

96.6

0.4

1.6

0.5

0.9

ad: air dry; daf: dry ash-free.


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Energy & Fuels

Table 2. Ash Analysis of Coke (wt%)

SiO2

Al2O3

Fe2O3

CaO

MgO

Na2O

K2O

TiO2

55.7

28.1

7.40

1.82

0.59

0.54

1.01

1.23

Mn3O4

P2O5

SO3

BaO

SrO

ZnO

V2O5

0.09

1.02

0.37

0.07

0.07

0.02

0.05

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2.2. Gasification Experiment. As illustrated in Figure 1, the coke pellet was put in an alumina

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reactor inserted into a vertical furnace for gasification reaction. The reactor consists of a gas

83

ducting tube and a sheath. The pellet was located at the bottom of the gas ducting tube on an

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alumina plinth to support the pellet. A type B thermocouple was inserted through the gas ducting

85

tube with the tip located above the pellet so that the temperature of the top surface of the pellet

86

can be monitored. Prior to heating, the reactor was purged with N2 for 20 min. Then, the sample

87

was heated in a pure N2 atmosphere to 1200 oC with a heating rate of 10 oC/min. After the

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temperature was stabilized, N2 was replaced by the CO2-CO-N2 gas mixture with a total flow rate

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of 1 L/min. The gasification was run for 15 min to achieve a reaction extent of about 10%, after

90

which the sample was cooled to ambient temperature in pure N2. The reaction extent was limited

91

to 10% in order to maintain the basic morphology of the coke surface so that the reacted sample

92

could be compared with the unreacted one. The gas composition used in the study (10 vol% of

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CO2, 40 vol% of CO, and 50 vol% of N2) was obtained by controlling the flow rates of

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individual gases with mass flow controllers (Aalborg DFC). The temperature and gas

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composition were selected to simulate the gasification conditions in the cohesive zone of the BF.


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Figure 1. Schematic of the experimental setup.

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2.3. Microscopic Analyses. The microscopic analyses on the unreacted and reacted coke pellets

99

were conducted by a JOEL JSM-6490LV SEM equipped with an EDS (Oxford Instruments X-

100

Max). Secondary electron (SE) mode was used to observe the topography of the carbon matrix,

101

and backscattered electron (BSE) mode was used with EDS to examine the coke minerals and

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their interactions with carbon. Polarizing optical microscope (Leica DMRM) was used to

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recognize IMDC and RMDC on the top surface of the unreacted coke pellets.

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3. RESULTS AND DISCUSSION

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3.1. Transformation of Coke Carbon Textures during Gasification. According to the

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analyses on over 30 sites on the coke pellets, most IMDC was shown to have a greater propensity

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to react with CO2 than RMDC, but exceptions existed. Two typical sites, Site 1 and 2, were

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selected here to demonstrate the two different cases.

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Figure 2a illustrates the SE image on Site 1 of the unreacted coke. IMDC was marked as the area

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within the dotted border line, and the surrounding area was RMDC. The carbon matrix of the


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unreacted coke was relatively compact with a continuous surface of IMDC and RMDC. Minerals

112

contained within the IMDC grain appeared as discontinuous laminar bodies (originally under

113

depositional control) while the mineral bodies within the RMDC were distributed irregularly.

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After reaction, as shown in Figure 2b, the surface was etched to different extents. The depth of

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penetration of the reaction on IMDC was greater than that on RMDC. After a portion of carbon

116

was reacted, the laminated structure of the minerals within the IMDC body became more

117

pronounced. For the RMDC, the texture of the surface suggests penetration along sub-domain

118

boundaries controlled by the mosaic leaflet size associated with a mosaic size up to 5 µ and the

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other fine microtextural types incorporated in the fused mass.

(a)

(b)

120 121

Figure 2. SE images on Site 1 of (a) the unreacted coke and (b) the reacted coke.

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Figure 3a illustrates the EDS mapping patterns of C, Al and Si on Site 1 of the unreacted coke.

123

The abundance distributions of Al and Si indicate that the mineral phases within the IMDC body

124

were alumino-silicates. As presented in Figure 3b, a larger proportion of carbon was removed

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from IMDC than RMDC after reaction, although a quantitative analysis on IMDC and RMDC

126

respectively is difficult. This is consistent with the greater depth of etching on IMDC than


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RMDC shown in Figure 2b. The Al and Si within the IMDC body appeared to be more abundant

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than those before reaction, which might be because that the electron beam interaction volume

129

encompassed a higher proportion of minerals due to the enhanced removal of carbon.

(a)

130

(b)

131 132

Figure 3. EDS patterns of C, Al and Si on Site 1 of (a) the unreacted coke and (b) the reacted

133

coke.

134

Figure 4a illustrates the SE images on Site 2 of the unreacted coke. Like Site 1, IMDC was

135

marked as the area within the dotted border line. The unreacted coke exhibited a compact surface

136

with a continuous surface of IMDC and RMDC. After reaction, as shown in Figure 4b, the

137

surface became etched. Unlike the above-mentioned trend on Site 1, the severities of reaction on

138

IMDC and RMDC were similar.


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Energy & Fuels

(b)

(a)

139 140

Figure 4. SE images on Site 2 of (a) the unreacted coke and (b) the reacted coke.

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Figure 5a illustrates the EDS mapping patterns of C, Al and Si on Site 2 of the unreacted coke. A

142

small portion of carbon was removed from both IMDC and RMDC after reaction (Figure 5b).

143

The abundance of Al and Si in the IMDC body did not change significantly before and after

144

reaction, which was different from Site 1.

145

From Figure 2b and Figure 4b, the grain boundaries (mosaic unit boundaries within RMDC,

146

boundaries between carbons and minerals, and generally between IMDC and RMDC) gave a

147

high definition to the position of reaction. Apart from carbon textures, the reactivity of coke

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carbon is affected by other factors such as carbon orders, minerals and pores. Grigore et al.13


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proposed that the major factors that make the IMDC more reactive than RMDC are the

150

concentration of catalytic iron phases and micropores. In this study, as the IMDC bodies on Site

151

1 and 2 were similar in terms of minerals and pores, the difference in their reactivity might be

152

attributed to different carbon orders.

(a)

153

(b)

154 155

Figure 5. EDS patterns of C, Al and Si on Site 2 of (a) the unreacted coke and (b) the reacted

156

coke.

157

3.2. Transformation of Coke Minerals during Gasification. In this study, the majority of the

158

coke minerals were found to be alumino-silicates. The minerals existed in a wide variety of sizes,

159

distributions and associations. Figure 6a illustrates a typical large mineral aggregate surrounded

160

by small fines disseminated within the carbon matrix on Site 3. Figure 7a illustrates another

161

mineral aggregate and some fine minerals included in pores on Site 4. Generally, the aggregate

162

minerals were composed of alumino-silicates or silica. Silica in this coke was normally present

163

as quartz. The fine minerals included in pores were normally metallic iron or iron oxides. The


10

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physical features of the alumino-silicates and silica in the reacted coke (Figure 6b and Figure 7b)

165

were marginally changed in comparison with the unreacted coke (Figure 6a and Figure 7a).

(a)

(b)

166 167

Figure 6. BSE image of an alumino-silicates aggregate on Site 3 of the (a) unreacted coke and (b)

168

reacted coke.


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(a)

Page 12 of 21

(b)

169 170

Figure 7. BSE image of an alumino-silicates aggregate on Site 4 of the (a) unreacted coke and (b)

171

reacted coke.

172

The spectra from EDS point analyses presented in Figure 6a and Figure 7a show that the

173

alumino-silicates on Site 3 and 4 were with low Si/Al ratios (~1) and combined with little alkali

174

elements (0.1wt% K and 0.1wt% Na). This indicates that the alumino-silicates were derived from

175

kaolinite clay in the parent coals. Kaolinite was dehydrated in the coking process to form the

176

amorphous meta-kaolinite.1 Figure 8a and Figure 9a illustrate the EDS mapping patterns of Al,

177

Si, Na and K on Site 3 and 4. Although the meta alumino-silicates are regarded as less reactive

178

phases, it can be seen that the chemical compositions of them were slightly modified after

179

reaction in terms of their alkali levels (Figure 8b and Figure 9b). K and Na appeared to be mobile

180

and bound with the meta alumino-silicates during the reaction. The alkalization of meta alumino-


12

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silicates can also be demonstrated by the spectra from EDS point analyses presented in Figure 6b

182

(1.0wt% K and 0.6wt% Na) and Figure 7b (2.1wt% K and 0.5wt% Na).

(a)

183

(b)

184 185

Figure 8. EDS patterns of Al, Si, Na and K on Site 3 of (a) the unreacted coke and (b) the

186

reacted coke.

(a)

187

(b)

188 189

Figure 9. EDS patterns of Al, Si, Na and K on Site 4 of (a) the unreacted coke and (b) the

190

reacted coke.


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191

The alkali reacted phase of silicates in the BF coke has been reported by Kerkkonen26. In the BF,

192

the meta alumino-silicates in the coke react with alkali recirculation in the reducing atmosphere.

193

Then the alkali silicates are gradually dissolved into the slag and removed from the furnace.

194

The Fe-containing phases appeared to be the most active phases in the coke and had strong

195

interactions with the carbon matrix. Fe-containing phases were observed above the IMDC body

196

on Site 2 (Figure 4a). The EDS spectrum presented in Figure 4a suggests that they might be

197

sulphates or sulphides. As shown in Figure 4b, a void was formed after reaction on the carbons

198

associated with the Fe-containing phases as a result of the catalytic carbon-gas reactions. Iron in

199

the forms of sulphate or sulphide was reduced to metallic iron dispersed in the void (Figure 4b).

200

This means that iron played a major role in the reactions. In contrast, the alumino-silicate phase

201

on the left of this site was marginally modified after reaction and showed little interaction with

202

the carbon matrix.

203

Similar trends were found on other sites. As shown in Figure 10, a void was observed above the

204

existing pore due to the Fe-containing phases, while the calcium phosphate phase (most likely

205

apatite) on the left of this site was retained after reaction. The Mg and Ca-containing phases

206

reacted with the surrounding alumino-silicates and transformed into a slag silicate globule, while

207

iron in the forms of sulphate or sulphide was reduced to metallic iron dispersed in the void and

208

sulphur was released as vapour (Figure 10b). As shown in Figure 11, a void was also formed

209

after reaction due to the Fe-containing phases while the quartz on the right of this site was

210

retained. The Mg-containing phases also reacted with alumino-silicates and transformed into a

211

slag silicate globule. Microscopic spherules (< 1 µm) of metallic iron attached on the surface of

212

the slag globule were formed as a result of the reduction of iron sulphate or sulphide (Figure

213

11b). Slag silicate and iron spherules in the broken small coke taken from a BF have been


14

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reported by Kerkkonen26 pointing out that the main breakage abrasion has taken place below the

215

cohesive zone while the coke is being washed by metal spherules.

(a)

(b)

216 217

Figure 10. BSE image of disseminated minerals on Site 5 of (a) the unreacted coke and (b) the

218

reacted coke.


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(a)

Page 16 of 21

(b)

219 220

Figure 11. BSE image of disseminated minerals on Site 6 of (a) the unreacted coke and (b) the

221

reacted coke.

222

4. CONCLUSIONS


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The transformation of coke carbon textures and minerals during gasification was studied using

224

SEM/EDS by analyzing the same sites of coke surface before and after reaction. The results

225

show that most IMDC reacted with CO2 more severely than RMDC. After a portion of carbon

226

was removed from IMDC, the remaining unreactive alumino-silicates became more pronounced.

227

However, the reactivity of IMDC was found to be similar to RMDC in some cases. The physical

228

features of the alumino-silicates were barely changed but bound with alkalis during the reaction.

229

The Fe-containing phases were highly active and had strong interactions with the surrounding

230

carbon matrix. Voids were formed in the carbon matrix associated with the Fe-containing phases

231

as a result of the catalytic carbon-gas reactions. The Mg and Ca-containing phases reacted with

232

the surrounding alumino-silicates and transformed into slag globules. Metallic iron dispersed in

233

the voids or attached on the slag globules was formed as a result of the reduction of iron sulphate

234

or sulphide while sulphur was released as vapour. These findings have provided a useful

235

guideline for the coke making industry, i.e., the improvement of coke performance in the BF by

236

selecting coals or coal blends and adjusting coke making conditions to produce coke with better

237

compositions of carbon textures and minerals.

238

AUTHOR INFORMATION

239

Corresponding Author

240

*E-mail: [email protected]. Tel: +61 2 4221 5293.

241

ORCID

242

Hao Zhang: 0000-0001-5710-4327

243

Notes

244

The author declares no competing financial interest.


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245

ACKNOWLEGEMENTS

246

The research was supported by BlueScope Steel, BHP Billiton and Australian Research Council

247

(ARC Linkage Project LP130100701). Australian Government is acknowledged for the Research

248

Training Program Scholarship. The Electron Microscopy Center at the University of Wollongong

249

is appreciated for providing the SEM/EDS instrument. The author would like to thank Dr. Harold

250

Rogers and Dr. Guangqing Zhang for the advice in revising the manuscript.

251

REFERENCES

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(1) Gupta, S.; Dubikova, M.; French, D.; Sahajwalla, V. Effect of CO2 gasification on the

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transformations of coke minerals at high temperatures. Energy Fuels 2007, 21 (2), 1052-1061.

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(2) Du, S.; Yeh, C.; Chen, W.; Tsai, C.; Lucas, J. A. Burning characteristics of pulverized coal

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within blast furnace raceway at various injection operations and ways of oxygen enrichment.

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Fuel 2015, 143, 98-106.

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(3) Suopajarvi, H.; Pongracz, E.; Fabrtius, T. Bioreducer use in Finnish blast furnace ironmaking

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– Analysis of CO2 emission reduction potential and mitigation cost. Appl. Energy 2014, 124 (1),

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82-93.

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technological and economical improvement. Iron Steel Technol. 2013, 10 (3), 63-75.

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(5) Gornostayev, S.; Harkki, J. Graphite crystals in blast furnace coke. Carbon 2007, 45 (6),

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1145-1151.


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(6) Coin, C. D. A. Coke microtextural description: Comparison of nomenclature, classification

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