Enhancing the Reactivity of Petroleum Coke in CO2 via Co

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Enhancing the reactivity of petroleum coke in CO2 via co-processing with selected carbonaceous materials Ashak Mahmud PARVEZ, Yu Hong, Edward Henry Lester, and Tao Wu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02000 • Publication Date (Web): 02 Jan 2017 Downloaded from http://pubs.acs.org on January 10, 2017

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Energy & Fuels

1

Enhancing the reactivity of petroleum coke in CO2 via co-

2

processing with selected carbonaceous materials Ashak Mahmud Parvez a, Yu Hong a, Edward Lester b, Tao Wu a,*

3 4 5 6 7 8 9

a

Ningbo Municipal Key Laboratory of Clean Energy Conversion Technologies, The University of Nottingham Ningbo China, Ningbo 315100, China

b

Department of Chemical and Environmental Engineering, The University of Nottingham, Nottingham NG7 2RD, U.K.

Abstract

10

This work presents a novel approach to enhance the gasification reactivity of highly

11

unreactive petroleum coke by purposely promote its synergistic interactions with selected

12

carbonaceous materials. To achieve this, an Australian coal and gum wood were chosen for

13

co-processing with petroleum coke. It was found that the addition of gum wood resulted in

14

more significant enhancement in the gasification reactivity of petroleum coke, which was

15

attributed to the combined influence of the catalytic effect of alkali and alkaline earth metals

16

(AAEM) in gum wood and the unique features of bio-char, such as high surface area, more

17

active sites, low crystalline index etc. It confirmed that interactions between an unreactive

18

fuel, such as petroleum coke, and selected carbonaceous materials could be capitalized on to

19

enhance overall reactivity of the blends. This approach helps improve the conversion

20

efficiency of unreactive fuels such as petroleum coke and therefore promotes their large-scale

21

utilization.

22

Key words: Petroleum Coke; Gasification; CO2 Gasification Reactivity; Interactions;

23

Synergy.

24

1 Introduction

25

With the increasing consumption of crude oil, the production of petroleum coke increases as a

26

by-product from oil refining industry 1-3. Due to its inert nature, it is a big challenge to utilize

27

petroleum coke in large scale and in a sustainable manner. In the past decades, considerable

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research has been carried out on the utilization of petroleum coke via combustion, pollutants

29

emission, and agglomeration

30

biomasses has also been investigated extensively

31

coke en route to energy and chemicals production is considered as one of the attractive

32

methods for its utilization in large scale 3, 7, 11, 12. It is projected that the amount of petroleum

33

coke to be utilized will rise in the near future as more petroleum coke processing units are

34

being added to meet the increasing fuel demand

35

coal or biomass is a promising option for its large-scale utilization in terms of capital

36

investment, greenhouse gas emission and energy security 1, 14, 15.

37

CO2 can be used as a gasifying agent, the use of which can result in less net CO2 emission and

38

lower steam consumption

39

adjust syngas composition for various downstream applications, and contributes to greater

40

environmental and economical benefits for the entire process

41

been made to investigate biomass gasification in CO2 atmosphere

22-24

42

16, 23, 24

. The effects of operating

43

parameters, such as temperature, pressure and heating rate, on the CO2 gasification of

44

petroleum coke, coal and biomass had also been reported

45

very difficult to gasify due to its low reactivity. This is particularly a technical problem in

46

CO2 gasification since CO2 is a much weaker oxidizing agent. To date, not much research on

47

CO2 gasification of petroleum coke mixed with biomass/coal has been reported

48

of the huge potential of CO2 as a gasifying agent, not much work has been conducted to study

49

the synergistic effects under CO2 gasification and how reactivity of a poor fuel could be

50

enhanced.

51

In this study, the objective was to enhance the gasification reactivity of a poor fuel, i.e.,

52

petroleum coke, via the promoting its synergistic interactions with other carbonaceous

4-7

. The co-firing of petroleum coke with coal and different 8-10

. Normally, gasification of petroleum

2, 3, 13

. Gasification of petroleum coke with

16-19

. More importantly, the use of CO2 offers the flexibility to

, gasification reactivity

12, 18, 25

, and characteristics

19-21

. Much effort has therefore , such as kinetic study

12, 26, 27

2, 11, 12


. Usually, petroleum coke is

13, 15

. In spite

2

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materials, such as coal and biomass. CO2 gasification was carried out using a

54

Thermogravimetric Analyser (TGA) under isothermal conditions. Moreover, co-gasification

55

of a suite of samples with air/CO2 was also conducted to demonstrate how to select proper

56

carbonaceous sample to achieve the maximum reactivity of petroleum coke under CO2

57

gasification conditions.

58

2 Materials and method

59

2.1 Materials

60

The raw materials used in this research were petroleum coke (SINOPEC, China), Australian

61

coal (NSW, Australia) and gum wood (Huzhou, China). All the samples were prepared

62

following British Standard for sample preparation to ensure representativeness of the samples

63

28

64

for future use. In order to show significant interactions between individual components, a

65

blending ratio of 1:1 were adopted in this study. Ash samples of these materials were also

66

prepared based on British Standard 29.

67

2.2 Sample characterization

68

Proximate analysis was carried out using a TGA (NETZSCH STA 339 F3) following the

69

procedures adopted elsewhere

70

contents were measured using a PE 2400 Series II CHNS/O Elemental Analyser (Perkin

71

Elmer, USA). In each test, approximately 3 mg (accuracy up to 0.01 mg) of dried fine powder

72

of the sample was used, which was manually grinded further prior to testing to eliminate the

73

influence of particle size on diffusion. Each test was repeated at least three times to minimize

74

the experimental error. The C, H, N and S contents of samples were measured directly, while

75

O content was calculated by difference. Calorific values of individual samples were

76

determined using a calorimeter (IKA C 200, Germany), which was calibrated prior to testing

77

using benzoic acid to achieve a relative standard deviation less than 0.2%. As for ash

. Approximately 1.0 kg of each sample was grinded into a size range of smaller than 106 µm

30

. Carbon (C), hydrogen (H), nitrogen (N) and sulphur (S)


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composition, each sample was analysed three times using an X-Ray Diffraction (XRD)

79

(Bruker D8 Advance, Germany) to minimize experimental error to be within +/- 1.5%. The

80

stacking height (LC) and interplanar spacing (d002) of char samples were calculated following

81

the formula described elsewhere 11, 31. The surface properties such as BET (Brunauer-Emmett-

82

Teller) surface area and pore volume of each sample were measured by accelerated surface

83

area porosimetry instrument (ASAP 2020, Micromeritics Instrument, Inc., USA) with N2

84

adsorption.

85

CO2 chemisorption test of char samples was conducted in a TGA by adopting similar

86

experimental procedure reported elsewhere 32, 33. At the beginning of each test, approximately

87

15 mg of the sample was heated to 850 °C (20 K/min) in pure N2 (99.99%). After 30 min

88

outgassing process at 850 °C, the sample was cooled down to 300 °C and then stabilized for

89

another 30 min. CO2 chemisorption was started when N2 was switched to CO2 and was held

90

for 30 min. The change in weight was continuously monitored. Finally, the CO2 was switched

91

back to N2, followed by outgassing for 30 min to remove weakly chemisorbed molecules from

92

the surface. The flow rates of both N2 and CO2 in this process were set as 120 mL/min. From

93

the test, two sets of chemisorption data were obtained for each sample. The first one gave the

94

total chemisorbed CO2 volume (strong and weak chemisorption), denoted by Ctot, while the

95

second one was the strong chemisorption of CO2 (Cstr) at 300 oC, which was still absorbed on

96

the char surface even after the experiment was finished. The weak chemisorption of CO2

97

(Cwea) was calculated through the desorbed volume from the char surface during the

98

outgassing stage.

99

2.3 Combustion and gasification characteristics

100

Combustion and gasification experiments were carried out using the TGA. Prior to each test,

101

the sample was further grounded manually in a mortar to a size even smaller than 106

102

microns. During non-isothermal test, the sample prepared was heated at a heating rate of 20 ACS Paragon Plus Environment

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K/min from the ambient temperature to 1200 °C under the presence of air or CO2 (40

104

ml/min). In isothermal gasification test, the sample was heated to 1200 °C in N2 (30 ml/min),

105

switched to CO2 (40 ml/min) and kept isothermal until no evident of mass loss was detected.

106

Each test was repeated three times to minimize experimental error.

107

2.4 Reactivity indexes

108

In this study, several parameters, such as ignition temperature (Tig), peak temperature (PT, Tp)

109

and burnout temperature (BT, Tb), were adopted as indicators to assess the combustion

110

behaviours of the blends

111

substance continues to burn without the need for external heat supply. Peak temperature is the

112

temperature at which the combustion rate reaches the maximum, while the temperature under

113

which combustion rate falls below 1 wt%/min is defined as the burnout temperature. Ignition

114

index (Di), as expressed in Eq. (1), was also used to measure ignition performance of fuels 34.

115

Di =

116

where, Rmax (%/min) and tmax represent the maximum combustion rate and corresponding time

117

(min), respectively, while tig stands for ignition time.

118

Combustion index (Sc) is determined by 36:

119

Sc =

120

where, Rmax, Rmean, Tig and Tb are the maximum mass loss rate, average mass loss rate,

121

ignition temperature and burnout temperature, respectively. Normally, greater values of Di

122

and Sc suggest better ignition and combustion performance, respectively.

123

Based on the TGA data, carbon conversion (Xc) and gasification reaction rate (RG) of the

124

studied samples can be calculated using Eq. (3) and Eq. (4), respectively 11.

34, 35

. Ignition temperature is the minimum temperature at which a

Rmax

(1)

tmax ⋅tig

Rmax ⋅Rmean

(2)

Tig2 ⋅Tb


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125

XC =

m0 − m m0 − mash

(3)

126

RG =

dX C dt

(4)

127

where m is the mass at a particular time, mo is the initial mass and mash is ash content, t

128

represents the time of gasification reaction.

129 130

2.5 Interactions indexes

131

Thermal characteristics of petroleum coke, Australian coal, gum wood and their blends were

132

extracted from experimental DTG profiles in air/CO2. The theoretical DTG curves of the

133

blends were calculated using Eq. (5) based on the mass loss rates of each sample assuming

134

additive property applies. Any deviation between experimental and calculated DTG curves

135

was used as indication for the interactions between samples, where the higher the value, the

136

greater the interactions 1, 37, 38.

137

dm  dm   dm  = x SP1   + x SP 2   dt  dt  SP1  dt  SP 2

138

 dm   dm  where  are the mass loss rates (%/min) of individual samples while  and    dt  SP1  dt  SP 2

139

x SP1 and xSP2 are the corresponding mass fractions in the blends, respectively. The Root

140

Mean Square Interactions Index (RMSII) was used to measure the interactions between

141

components in the blend, which compares the deviation of calculated values with

142

experimental values. Normally, a greater RMSII value indicates a stronger interaction

143

between the samples. The RMSII can be calculated using the Eq. (6) 39.


(5)

6

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 (dm / dt ) iEXP − (dm / dt ) iCAL  ∑ (dm / dt ) iCAL i =1  N N

  

2

144

RMSII =

145

where ( dm / dt ) EXP and (dm / dt) CAL denote experimental and calculated mass loss rates,

146

respectively. N represents the number of points undertaken.

147

3 Results and discussion

148

3.1 Characteristics of samples

149

Results of proximate, ultimate and ash analyses as well as calorific value of the investigated

150

samples are listed in Table 1. It can be seen that gum wood contained the highest amount of

151

volatiles, over twice and eight times as much as those of Australian coal and petroleum coke,

152

respectively, but significantly less amount of fixed carbon (11.8 wt%) and ash (0.1 wt%). In

153

addition, gum wood also had the lowest content of sulphur among the three samples. From

154

Table 1, petroleum coke (36.4 MJ/kg daf) had the highest calorific value, comparable to that

155

of Australian coal (35.5 MJ/kg daf). Hence, co-processing of coal with petroleum coke might

156

not result in considerable decrease in combustion/gasification temperature in utility boilers or

157

gasifiers. In contrast, gum wood (14.9 MJ/kg daf) had the lowest calorific value, which would

158

result in a lower processing temperature if co-processed with coal.

159

Table 1: Characteristics of samples.

(6)

PC

AC

GW

36.4

35.5

14.9

Moisture

0.8

0.7

2.1

Volatile matter

10.9

34.6

86.0

Fixed carbon

87.1

48.2

11.8

Ash

1.2

16.5

0.1

Net calorific value a,b (MJ/kg) Proximate analysis c (wt %)

Ultimate analysis a, b (wt %)


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C

91.1

81.3

47.1

H

3.8

4.9

6.3

Od

2.6

9.7

43.5

N

1.3

1.9

2.1

S

1.2

2.2

1.0

SiO2

36.8

42.6

52.5

SO3

6.1

1.4

1.2

CaO

3.4

4.2

7.6

Na2O

6.4

2.3

2.9

Fe2O3

9.5

7.4

7.3

MgO

2.3

2.2

6.8

Al2O3

33.9

39.1

17.2

K2O

1.6

0.8

4.5

Ash analysis (wt %)

a

Dry basis. bAsh free basis. cAs-received basis. cBy difference.

160

3.2 CO2 gasification reactivity

161

Fig. 1 presents mass loss during pyrolysis stage (prior to 1000 ºC) and CO2 gasification stage

162

(isothermal at 1000 ºC) of different samples. In general, gum wood had the highest mass loss

163

rate in both pyrolysis and gasification stages. The reasons behind this fastest reactivity can be

164

attributed to its high volatile and oxygen content as well as thermal degradation properties of

165

the sample’s constituents, i.e., lignin, cellulose, hemi-cellulose

166

fundamental aspect of gasification showed that the concentration of active sites in the char is

167

directly related to the oxygen-containing functional groups of the parent sample

168

was significantly higher in gum wood than in petroleum coke. Furthermore, AAEMs in gum

169

wood might also have positive impacts on gasification reactivity as normally AAEMs are

170

catalysts to both combustion and gasification processes

171

Australian coal and petroleum coke samples showed much lower reactivity, which can be

172

explained by their lower volatile and oxygen content and higher fixed carbon content. For

173

petroleum coke, the highest amount of final residue (about 65 wt%) was found. This could be

35, 37, 40, 41

. Research on

37, 40

, which

42, 43

. Compared with gum wood,


8

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44-46

174

attributed to its aromatic nature and high ordered carbon fraction

175

that aromatic and graphitic (high ordered) carbon atoms had lower reactivity than aliphatic

176

and the disordered carbon, respectively

177

amount of carbon while the oxygen content was the lowest. It can be seen in Fig. 1 that

178

complete conversion of gum wood was achieved, the conversion of Australian coal was yet

179

not complete, while only about 35 wt% of petroleum coke was converted. Therefore, the

180

testing of petroleum coke was conducted at higher temperatures.

. It is important to note

46

. Moreover, petroleum coke contained the highest

100

1200

PC

Temp 1000

800 60 600 40 400

Temperature (oC)

80

Mass (%)

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

AC

20

200

GW 0

0 0

20

40

60

80

100

120

Time (min)

181 182

Figure 1: Thermal behaviours of individual samples upon heating

183

The comparison of CO2 gasification profiles of petroleum coke at 1000 ºC, 1100 ºC and 1200

184

ºC under isothermal conditions is shown in Fig. 2. The results demonstrated that compared

185

with other carbonaceous materials, to raise the CO2-gasification rate to a reasonable level, a

186

much higher operating temperature must be adopted, which is in accordance with the analyses

187

of previous investigation

188

has to be conducted at a temperature higher than 1200 ºC. Thus, in order to compare

189

gasification reactivity of different samples, experiments were performed at 1200 ºC, the

190

results of which are shown in Fig. 3a.

2, 25

. To achieve a high conversion of petroleum coke, gasification


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Energy & Fuels

100

1400 1200 oC

1200 1100 oC 1000 oC

1000

60

800

PC (1000 oC)

40

PC (1100

600

oC)

Temperature (oC)

80

Mass (%)

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

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400 20 200

PC (1200 oC) 0

0 0

20

40

60

80

100

120

191

Time (min)

192

Figure 2: CO2 gasification profiles of petroleum coke at different temperatures.


10

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100

1500

PC

(a)

Temp 1200 Temperature (oC)

80

Mass (%)

AC 60

900

40

600

GW 20

300

0

0 0

20

40

60

80

100

120

Time (min)

193 1

(b)

AC-Iso

1200 oC

0.8 Conversion (XC)

GW-Iso

0.6

PC-Iso 0.4

0.2

0 0

194

15

30 Time (min)

45

60

0.8

1200 oC (c)

Gasification rate (RG)

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

GW-Iso

0.6

0.4

PC-Iso

0.2

AC-Iso

0 0

0.2

0.4

0.6

0.8

1

Conversion (XC)

195 196

Figure 3: Pyrolysis (a, b) and CO2 gasification (c) profiles of individual samples.


11

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197

The thermal profiles at 1200 ºC were comparable to those found in 1000 ºC but high

198

conversion was observed for Australian coal and petroleum coke. The profiles of carbon

199

conversion versus gasification time and gasification rate of the tested samples using

200

isothermal method are presented in Fig. 3b and Fig. 3c, respectively. It can be seen in Fig. 3b

201

that gum wood and Australian coal had 75 wt% and 98 wt% of conversion in 3 min and 9

202

min, respectively, demonstrating their high reactivity in CO2 atmosphere. In contrast, the

203

conversion of petroleum coke experienced an exponential rise over the entire period of

204

testing. This gradual increase in conversion confirmed the sluggish onset of the Boudourd

205

reaction. This was due to the low reactivity of petroleum coke as mentioned earlier.

206

The gasification rate of gum wood increased initially and then decreased after reaching a peak

207

value as seen in Fig. 3c. It remained a constant thereafter. The conversion corresponding to

208

the maximal gasification rate was around 0.25. The presence of maximum gasification rate

209

could be explained by the evolution of pore structure of the solid fuel, leading to a higher

210

surface area and eventually a higher number of active carbon sites per unit weight

211

development of porosity in the solid fuel is closely associated with the formation of pore

212

clusters and the continuously consumption of carbon materials. During the gasification

213

process, the increase in accessible porosity takes place on the available solid sites at the

214

boundary of a pore cluster. Consequently, the rise of gasification rate with the conversion was

215

observed due to the enlargement of accessible surface area. Thereafter, the pores collapsed

216

and resulted in a reduction of reaction surface area, which subsequently led to the drop of

217

gasification rate. For Australian coal, a comparable pattern was obtained which could be

218

explained by the evolution of pore structure. However, the maximum gasification rate found

219

to be lower than that of gum wood. This was due to the fact that char derived from coal is

220

normally less reactive than that derived from biomass. It was also found that gasification rate

221

of petroleum coke gradually decreased during testing and the rate was the lowest among all


2, 47

. The

12

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Energy & Fuels

11, 48

222

samples. These properties are closely associated with graphitization phenomenon

. In

223

comparison with chars derived from both coal and biomass, petroleum coke is generally a

224

part-graphitized carbon material with higher crystallinity and higher degree of order 2. It was

225

reported that the degree of graphitization in petroleum coke was greater than that of chars

226

derived from coal at high temperature pyrolysis

227

petroleum coke was graphitized in CO2 at higher temperatures (above 800 oC), which resulted

228

in a well-structured char and the decrease of active site of the sample 11, 49. Consequently, the

229

rate of petroleum coke gasification gradually decreased.

230

The average gasification rate R0.5 ( R0.5 = 0.5 / t 0.5 ) was used to indicate gasification reactivity

231

of individual samples due to the variations of their properties, where t0.5 denotes the duration

232

of the half conversion

233

order of gum wood (0.38 /min) > Australian coal (0.21 /min) > petroleum coke (0.04 /min).

234

Normally, gasification reactivity is largely influenced by both specific surface area (SBET) and

235

crystalline structure of char

236

possesses a higher gasification reactivity

237

proportional to the ratio of stacking height of the carbon crystal to interlayer spacing (Lc/d002)

238

value

239

reactivity

240

were identified as important parameters for the evaluation of active sites of char, the higher

241

the values of Ctot and Cstr, the greater the gasification reactivity. In order to confirm the order

242

of reactivity, these parameters were measured and listed in Table 2. Results showed that the

243

relationship between CO2 gasification reactivity with BET surface area, crystalline structure

244

and CO2 chemisorbed volumes followed the same trend as the average gasification rate,

245

which proved that these parameters could be used for both quantitative and qualitative

246

evaluation of average gasification rate.

11

. Another research also showed that

50

. The gasification reactivity of the three samples was ranked in the

2, 11, 12

. In general, char with a larger BET specific surface area 51

while gasification reactivity is inversely

12

. The number of active sites of char also shows strong correlation with gasification 32, 33, 52

. Therefore, total chemisorbed (Ctot) and strong chemisorbed (Cstr) volumes


13

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Table 2: BET surface area, crystalline structure and CO2 chemisorption features of samples.

Sample

SBET (m2/g)

Lc/d002

Ctot (mg/g)

Cstr (mg/g)

Cwea (mg/g)

Gum wood

6.44

2.18

2.08

1.65

0.43

Australian coal

2.29

2.73

0.75

0.54

0.21

Petroleum coke

0.49

5.23

0.59

0.44

0.15

248

Therefore, it can be concluded that apart from operating temperature, the gasification

249

reactivity was highly influenced by four major factors, i.e. specific pore surface area, the

250

number of active sites, crystalline structure and AAEM content of the char. The graphitization

251

of petroleum coke that occurred in the presence of CO2 and at higher temperatures was

252

believed to be the main reason for the gradual decrement of gasification rate. Volatile matter,

253

oxygen content and the composition of mineral matters also played a key role in the overall

254

reactivity. Hence, it is normal that CO2 gasification rate of petroleum coke was significantly

255

lower than those of gum wood and Australian coal. However, thermal processing petroleum

256

coke at temperatures higher than 1200 ºC and for a longer period of time could make a near

257

complete conversion become possible.

258

Although high temperature and longer processing time favour high conversion efficiency for

259

petroleum coke, this approach is not always practical. Since AAEMs have catalytic effects for

260

gasification process, AAEMs in carbonaceous materials, such as coal and biomass, could

261

potentially catalyse CO2 gasification of petroleum coke. Gasification rate of petroleum coke at

262

relatively low temperatures could therefore be enhanced via co-processing with coal/biomass.

263

3.3 Combustion and gasification under non-isothermal conditions

264

The TGA profiles of petroleum coke, Australian coal and gum wood samples under air and

265

CO2 atmospheres are presented in Fig. 4. Gum wood were the most reactive sample in both

266

combustion and CO2 gasification. As seen in Fig. 4, the first and second main mass losses

267

occurred at temperature ranges of 200 - 400 ºC and 400 - 1000 ºC, respectively. The ACS Paragon Plus Environment

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Page 15 of 29

268

temperature of the first stage (devolatilization) was found to be narrow as the decomposition

269

of hydrocarbons occurred rapidly, which could complete within 20 - 30 ms. The maximum

270

mass loss rate of the devolatilization stage in both cases was similar (about 10% deviation),

271

which suggested that oxidizing agent did not have significant impacts on devolatilization. The

272

second stage (char gasification) took place at a wide temperature range due to the low

273

reactivity of char and the heterogeneous nature of the reaction. Since air is a much stronger

274

oxidizing agent than CO2, consequently, the second maximum mass loss in air occurred at a

275

relatively low temperature (480 ºC) and a higher rate (6.3 wt%/min) whereas a higher

276

temperature (965 ºC) and a lower rate (2.7 wt%/min) were observed in CO2 atmosphere.

1200

100

(a)

PC (CO2)

AC (CO2)

800 60 600 40 400

GW (CO2)

20

AC (Air)

Temperature (oC)

1000

80

Mass (%)

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

200

GW (Air)

0

0 0

277 278

10

20

30

40

50

60

Time (min)

(b)


15

Energy & Fuels

20 GW (Air)

16 PC (Air)

- DTG (%/min)

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 16 of 29

12

AC (Air)

8

PC (CO2) AC (CO2) GW (CO2)

4

0 0

200

400

800

1000

1200

Temperature (oC)

279 280 281

600

Figure 4: Non-isothermal combustion and gasification of individual samples under air and CO2 atmosphere.

282

3.4 Combustion characteristics

283

Table 3 summarises the combustion characteristics of individual samples. It is obvious that

284

gum wood showed the lowest ignition temperature (282 ºC), which was mainly attributed to

285

its high volatile content. In contrast, petroleum coke had the highest ignition temperature (494

286

ºC) followed by coal (444 ºC). Under the same experimental conditions, gum wood was easy

287

to be ignited while the ignition of petroleum coke and Australian coal was much more

288

difficult compared with gum wood.

289

Table 3: Combustion characteristics of the samples. DTGmax

tmax

Di x10 -2 Sc x10 -7

Sample

Tig (ºC)

Tp (ºC)

PC

494

556

15.7

641

25.3

2.8

7.1

AC

444

533

10.9

633

24.2

2.3

4.8

GW

282

329/474

16.9/6.2

540

14.0

10.0

33.0

(wt%/min)

Tb (ºC)

(min)

290

In Table 3 it can be seen that gum wood had two peak temperatures (Tp), representing the two

291

stages of mass losses. In contrast, only one peak temperature was noticed for petroleum coke

292

and Australian coal. Additionally, the peak temperature for gum wood was lower than the ACS Paragon Plus Environment

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293

other two samples. Burnout temperature (Tb) of all the samples followed similar trend as seen

294

in Table 3. These characteristics indicated that gum wood had the highest reactivity whereas

295

petroleum coke demonstrated the lowest reactivity. However, the differences of Tp and Tb

296

between petroleum coke and Australian coal were insignificant (23 ºC and 8 ºC, respectively),

297

which suggested that the decomposition and combustion of these samples occurred at a

298

similar temperature range.

299

Table 3 showed that the values of ignition index and combustion index of the tested samples

300

followed almost the same pattern as the ignition temperatures. Therefore, gum wood is

301

considered a good fuel due to its low ignition temperature (282 ºC), short ignition time (14

302

min), low burnout temperature (540 ºC), and a high maximum burning rate (16.9 wt%/min).

303

Despite Australian coal had lower ignition temperature (444 oC) and shorter tmax (20 min), its

304

ignition index value was only 17% smaller than those of petroleum coke which was due to the

305

occurrence of higher value of maximum burning rate (15.7 wt%/min) in petroleum coke.

306

These findings suggested that besides ignition temperature, other combustion parameters,

307

such as maximum burning rate, also had significant impacts on fuel performance.

308

3.5 CO2 gasification characteristics of the samples

309

Table 4 shows the gasification characteristics of individual samples under CO2 atmosphere.

310

The initial devolatilization temperature of gum wood (305 ºC) was clearly the lowest,

311

followed by that of Australian coal (422 ºC). Two stages of mass losses took place resulting in

312

two maximum mass loss rates as illustrated in Fig. 4b. The first mass loss can be attributed to

313

the release of volatiles whereas char gasification contributed to the second mass loss. It is

314

important to note that unlike combustion process, the heterogeneous reaction between coal-

315

derived char and CO2 was much slower. This phenomenon can be explained by Boudouard

316

reaction (C(s) + CO2 → 2CO). Normally, at atmospheric condition, the Boudouard reaction is

317

thermodynamically favourable at temperatures above 900 ºC whilst the combustion of char ACS Paragon Plus Environment

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Page 18 of 29

318

occurrs at significantly lower temperatures (below 630 ºC). This is because air is a much

319

stronger oxidizing agent compared with CO2. On the other hand, for petroleum coke its

320

devolatilization started at a much higher temperature (Ti=1110 ºC) due to its low volatile

321

content and high heavy aromatic-to-aliphatic ratios (aromatic carbon atoms are less reactive

322

than aliphatic atoms

323

delayed and occurred at higher temperatures. In addition, more ordered carbon crystalline

324

structure existed in petroleum coke also contributed to its low reactivity

325

from Table 4 that a higher maximum mass loss rate and corresponding temperature followed

326

the order of gum wood > Australian coal > petroleum coke. Generally, the reactivity of the

327

samples is directly proportional to the rate of maximum mass loss and inversely proportional

328

to the maximum temperature 1, 37, which is also dependent on the type of oxidizing agent used.

329

Hence, petroleum coke required a much higher temperature (1142 ºC) and a longer time (54.6

330

min) its gasification reaction under CO2 atmosphere to complete. In order to raise the

331

reactivity of petroleum coke, its co-processing with reactive fuels, such as gum wood, could

332

be one of the viable options in utility boilers since co-processing is normally associated with

333

interactions (synergistic effects) between fuels.

334

335

45

). Consequently, devolatilization of petroleum coke was considerably

11, 12

. It can be seen

Table 4: Gasification characteristics of the samples. DTGmax

Sample

Ti

Tp (ºC)

tmax (min)

PC

1110

1142

54.6

5.7

AC

422

460/1123

20.5/53.7

6.0

GW

305

361/961

15.6/45.5

14.4

(wt%/min)

Ti = Initial devolatilization temperature

336

3.6 Characteristics of co-processing

337

Fig. 5 shows the combustion profile of petroleum coke, Australian coal, gum wood and their

338

blends. It can be noticed in Fig. 5a that the peak temperature of petroleum coke/coal blend

339

(1:1) decreased slightly (by 6 ºC) compared with that of petroleum coke. In addition, 13% ACS Paragon Plus Environment

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Energy & Fuels

340

reduction of maximum mass loss rate of the blend was also witnessed as the reactivity of coal

341

found to be lower compared with that of petroleum coke. As shown in Fig. 5b, gum wood had

342

a significantly lower ignition temperature (Table 2) so that it started to burn at a temperature

343

lower than that of petroleum coke which resulted in the first mass loss stage of petroleum

344

coke/gum wood blend (1:1). Therefore, the decomposition of petroleum coke started when

345

devolatilization of gum wood had completed. The first and the second peak temperatures of

346

the blend represented the first peak temperature of gum wood and petroleum coke,

347

respectively. It is also found that the maximum mass loss rate decreased by 54% in the

348

devolatilization stage due to a small amount of volatiles being added from gum wood.

349

Meanwhile, the increase of maximum mass loss rate by 100% in char gasification stage was

350

also observed, which was mainly caused by the increasing amount of char derived from

351

petroleum coke.


19

Energy & Fuels

(a)

20

PT for PC

- DTG (%/min)

15

PT for PC:AC

PT for AC 10

5

0 0

352 353

200

400

600

800

1000

1200

Temperature (oC)

(b) 20

PT for GW PT for PC 15 PT for PC:GW - DTG (%/min)

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 29

1st PT for PC:GW 10

5

0 0

200

400

600

800

1000

1200

Temperature (oC)

354 355

Figure 5: Combustion profile of petroleum coke and its blends with (a) Australian coal and (b) gum wood.

356

Fig. 6 illustrates the CO2 gasification profiles of petroleum coke, coal, biomass and their

357

blends. In comparison with combustion process, similar thermal profiles were observed for

358

petroleum coke/Australian coal blend. However, the blend of petroleum coke/gum wood

359

exhibited a different trend than that of petroleum coke, where its second peak temperature

360

shifted to a lower temperature by 35 ºC. This suggested the presence of interactions during co-

361

gasification.


20

Page 21 of 29

(a)

8

PT for AC

- DTG (%/min)

6

PT for PC

1st PT for PC:AC

4

2

0 0

362 363

200

400

600

800

1000

1200

Temperature (oC)

(b) 20

PT for GW 16 - DTG (%/min)

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

1st PT for PC:GW 12 PT for PC 8 PT for PC:GW 4

0 0

200

400

600

800

1000

1200

Temperature (oC)

364 365 366 367

Figure 6: Gasification profiles of petroleum coke and its blends with (a) Australian coal and (b) gum wood.

368

3.7 Enhanced reactivity via co-processing

369

Interactions between fuels during co-processing are mainly attributed to the high volatile

370

content and AAEMs in biomass 15, 38, 53. These interactions could then be purposely utilized to

371

enhance the reactivity of petroleum coke, a fuel with low reactivity. In this study, an

372

Australian coal and gum wood were chosen to improve reactivity of petroleum coke by taking

373

advantages of the synergistic effects of co-processing.


21

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374

Figures 7 and 8 illustrate the curves of co-combustion and CO2 co-gasification processes,

375

respectively. Fig. 7a shows the deviations between experimental and calculated DTG curves

376

of petroleum coke/Australian coal blend in the temperature region of 350-650 ºC, which

377

indicated the existence of interactions during co-combustion. However, both experimental and

378

calculated DTG curves matched well at temperatures below 350 ºC and beyond 650 ºC, which

379

suggested that, interactions were minimum in this two temperature ranges, which were

380

dominated by coal devolatilization and char burnout respectively. For petroleum coke/gum

381

wood blend, a significant difference was observed in the temperature range of 400-700 ºC as

382

shown in Fig. 7b, which is a sign of strong interactions in this region. This deviation was due

383

to the presence of highly reactive gum wood-derived char and its high AAEMs’ catalytic

384

effects on gasification. In general, the experimental curves of both blends shifted to the low

385

temperature zone as demonstrated in Fig. 7. This shift depicted that the combustion of blend

386

samples occurred at a lower temperature, which indicated the enhanced reactivity due to

387

interactions between the two components in the blend. However, the deviation in petroleum

388

coke/gum wood blend was more significant and showed stronger interactions, which resulted

389

in the mass loss rate of experimental curve much higher than that of calculated one. These

390

behaviours also suggested that gasification reactivity was enhanced during co-processing.

391

Moreover, the interaction index (RMSII) of the blends was evaluated in order to determine

392

intensity of the interactions. The RMSII values of individual blends were calculated within

393

the previously mentioned temperature region where the deviation occurred. For petroleum

394

coke/gum wood blend, the RMSII value was 0.55, which was 22% higher that that of

395

petroleum coke/Australian coal blend (0.45). This indicated that the overall reactivity of

396

petroleum coke/gum wood was much higher than that of petroleum coke/Australian coal.


22

Page 23 of 29

(a)

16

AC:PC(1:1)-EXP

DTG (%/min)

12

AC:PC(1:1)-CALC 8

4

0 0

397 398

200

400

600

800

Temperature

1000

1200

(oC)

(b) 16

GW:PC(1:1)-EXP 12 DTG (%/min)

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

GW:PC(1:1)-CALC 8

4

0 0

200

400

600 Temperature

399

800

1000

1200

(oC)

400 401

Figure 7: Experimental and calculated DTG curves in co-combustion (a) petroleum coke/Australian coal and (b) petroleum coke/gum wood.

402

Fig. 8 shows the correlation between experimental and calculated curves of the two blends

403

under CO2 atmosphere. In both blends, it can be seen that there was a good correlation up to

404

approximately 900 ºC. However, significant deviation was observed at the temperature range

405

between 1050 and 1200 ºC for petroleum coke/Australian coal blend (Fig. 8a), which suggest

406

the existence of synergistic effects.


23

Energy & Fuels

(a) 8

AC:PC(1:1)-CALC

DTG (%/min)

6

AC:PC(1:1)-EXP

4

2

0 0

407

200

400

600

800

1000

1200

Temperature (oC)

(b) 10 8

DTG (%/min)

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 29

GW:PC(1:1)-EXP

6

4

GW:PC(1:1)-CALC 2

0 0

200

400

600

800

1000

1200

Temperature (oC)

408 409 410

Figure 8: Experimental and calculated DTG curves in CO2 co-gasification (a) petroleum coke/Australian coal and (b) petroleum coke/gum wood.

411

Similarly, the curves of petroleum coke/gum wood (Fig. 8b) also experienced a notable

412

difference at higher temperatures (890 ºC to 1200 ºC). Moreover, the RMSII index indicated

413

that petroleum coke/gum wood blend had more significant interactions (2.82) compared with

414

that of petroleum coke/Australian coal (0.25). This suggested that the reactivity was higher in

415

the former one.

416

The aforementioned results demonstrated that a significant enhancement occurred in both

417

combustion and CO2 gasification when petroleum coke was blended with gum wood,


24

Page 25 of 29

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Energy & Fuels

418

especially at high temperatures. Between the two processes, CO2 co-gasification demonstrated

419

an enhanced performance by showing more significant interactions between individual fuels.

420

These interactions were attributed to chars derived from gum wood which were of high BET

421

surface area and have more active sites, lower crystalline index; and higher AAEMs (and

422

therefore stronger catalytic effect). As seen in Table 2 that BET surface area and crystalline

423

index value of gum wood char were 1.8 times larger and 58% lower than those of Australian

424

coal char, respectively. These properties subsequently contributed to the increased

425

gasification reactivity. Similarly, CO2 chemisorbed volumes (Ctot and Cstr) were found to be in

426

the range of 2.5 to 3.0 times higher than those of Australian coal, which indicated enhanced

427

gasification reactivity. In addition, the reactivity was enhanced due to the presence of high

428

amount AAEMs in ash originated from gum wood (as listed in Table 1) at high temperature 1,

429

38, 53

430

the CaO and K2O in gum wood were found to be 7.6 wt% and 4.5 wt%, respectively. These

431

two species were the main component for catalytic effect. In contrast, ash from Australian

432

coal and petroleum coke contained much lower amount of AAEMs (CaO, K2O and MgO)

433

than those of gum wood. In addition, alumina acted as a deactivator for catalytic effect

434

Since gum wood ash had the lowest percentage of Al2O3 (17.2 wt%), it experienced the least

435

difficulty to undergo the catalytic activities.

436

Therefore, this study confirmed that physiochemical properties of carbonaceous material i.e.,

437

active sites in char, AAEMs content etc., played a vital role in enhancing gasification

438

reactivity in CO2 atmosphere. Due to the significant differences in various carbonaceous

439

feedstock in terms of volatiles, fixed carbon and mineral content, the corresponding char

440

reactivity was influenced

441

CO2 atmosphere for chars derived from the blend of gum wood and petroleum coke was

442

observed, which was a result of the interactions between biomass and petroleum coke. It is

. The mechanism of these interactions were detailed in elsewhere 38. As seen in Table 1,

54

.

52

. In this study, significantly enhanced gasification reactivity in


25

Energy & Fuels

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Page 26 of 29

443

proved that gasification reactivity of petroleum coke can be enhanced via co-processing with

444

gum wood. This is a promising approach to enhance gasification performance of fuels of low

445

reactivity such as petroleum coke.

446

4 Conclusions

447

In this study, it is proved that synergistic interactions, which usually occur during the co-

448

processing of two fuels, could be utilized to enhance the combustion/gasification reactivity of

449

unreactive petroleum coke. However, in order to achieve this, the carbonaceous materials that

450

are to be co-processed with petroleum coke must have high volatile content, high AAEMs

451

content, and can form chars of large BET surface area, more active sites and low crystalline

452

index. This novel approach of purposely utilization of synergistic interactions to enhance

453

reactivity of unreactive fuels is of significant importance as it enables the large scale use of

454

poor quality fuels, such as petroleum coke and low rank coals.

455

Acknowledgement

456

Part of this work is sponsored by Ningbo Bureau of Science and Technology under its

457

Innovation Team Scheme (2012B82011). The University of Nottingham Ningbo China is

458

acknowledged for providing scholarships to the first author.

459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475

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6. Zhou, Q.; Zhao, Q.; Zhou, G.; Wang, H.; Xu, T.; Hui, S., Comparison of combustion characteristics of petroleum coke and coal in one-dimensional furnace. Frontiers of Energy and Power Engineering in China 2010, 4, (3), 436-442. 7. Leion, H.; Mattisson, T.; Lyngfelt, A., The use of petroleum coke as fuel in chemicallooping combustion. Fuel 2007, 86, (12–13), 1947-1958. 8. Hower, J. C.; Thomas, G. A.; Mardon, S. M.; Trimble, A. S., Impact of co-combustion of petroleum coke and coal on fly ash quality: Case study of a Western Kentucky power plant. Applied Geochemistry 2005, 20, (7), 1309-1319. 9. Jia, L.; Anthony, E. J.; Lau, I.; Wang, J., Study of coal and coke ignition in fluidized beds. Fuel 2006, 85, (5–6), 635-642. 10. Hong, T.; Zhengzhu, L., CO-COMBUSTION THERMOGRAVIMETRY EXPERIMENT OF BIOMASS AND PETROLEUM COKE. Acta Energiae Solaris Sinica 2015, 2, 002. 11. Wu, Y.; Wu, S.; Gu, J.; Gao, J., Differences in physical properties and CO2 gasification reactivity between coal char and petroleum coke. Process Safety and Environmental Protection 2009, 87, (5), 323-330. 12. Huo, W.; Zhou, Z.; Chen, X.; Dai, Z.; Yu, G., Study on CO2 gasification reactivity and physical characteristics of biomass, petroleum coke and coal chars. Bioresource Technology 2014, 159, (0), 143-149. 13. Zhan, X.; Jia, J.; Zhou, Z.; Wang, F., Influence of blending methods on the cogasification reactivity of petroleum coke and lignite. Energy Conversion and Management 2011, 52, (4), 1810-1814. 14. Van Loo, S.; Koppejan, J., The Handbook of Biomass Combustion and Co-firing. Earthscan LLC: 2012. 15. Edreis, E. M. A.; Luo, G.; Li, A.; Chao, C.; Hu, H.; Zhang, S.; Gui, B.; Xiao, L.; Xu, K.; Zhang, P.; Yao, H., CO2 co-gasification of lower sulphur petroleum coke and sugar cane bagasse via TG–FTIR analysis technique. Bioresource Technology 2013, 136, (0), 595-603. 16. Wang, L.; Sandquist, J.; Varhegyi, G.; Matas Güell, B., CO2 Gasification of Chars Prepared from Wood and Forest Residue: A Kinetic Study. Energy & Fuels 2013, 27, (10), 6098-6107. 17. Skodras, G.; Nenes, G.; Zafeiriou, N., Low rank coal – CO2 gasification: Experimental study, analysis of the kinetic parameters by Weibull distribution and compensation effect. Applied Thermal Engineering 2015, 74, (0), 111-118. 18. Lahijani, P.; Zainal, Z. A.; Mohamed, A. R.; Mohammadi, M., CO2 gasification reactivity of biomass char: Catalytic influence of alkali, alkaline earth and transition metal salts. Bioresource Technology 2013, 144, (0), 288-295. 19. Butterman, H. C. C., M.J., CO2 as a Carbon Neutral Fuel Source via Enhanced Biomass Gasification. Environ. Sci. Technol. 2009, (43), 9030–9037. 20. Yi, Q.; Li, W.; Feng, J.; Xie, K., Carbon cycle in advanced coal chemical engineering. Chemical Society Reviews 2015, 44, (15), 5409-5445. 21. Parvez, A. M.; Mujtaba, I. M.; Wu, T., Energy, exergy and environmental analyses of conventional, steam and CO2-enhanced rice straw gasification. Energy 2016, 94, 579-588. 22. Edreis, E. M. A.; Luo, G.; Yao, H., Investigations of the structure and thermal kinetic analysis of sugarcane bagasse char during non-isothermal CO2 gasification. Journal of Analytical and Applied Pyrolysis 2014, 107, (0), 107-115. 23. Khalil, R.; Várhegyi, G.; Jäschke, S.; Grønli, M. G.; Hustad, J., CO2 gasification of biomass chars: a kinetic study. Energy & Fuels 2008, 23, (1), 94-100. 24. Sircar, I.; Sane, A.; Wang, W.; Gore, J. P., Experimental and modeling study of pinewood char gasification with CO2. Fuel 2014, 119, (0), 38-46.


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