Study on the Pyrolysis Characteristics of a Typical Low Rank Coal with

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Study on the Pyrolysis Characteristics of a Typical Low Rank Coal with Hydrothermal Pretreatment Hui Chang, Zhuangzhuang Zhang, Luyao Qiang, Ting Gao, Tingwei Lan, Ming Sun, Long Xu, and Xiaoxun Ma Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04312 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 2, 2019

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Energy & Fuels

1

Study on the Pyrolysis Characteristics of a Typical Low Rank Coal with

2

Hydrothermal Pretreatment

3 4

Hui Chang, Zhuangzhuang Zhang, Luyao Qiang, Ting Gao, Tingwei Lan, Ming Sun, Long Xu, Xiaoxun Ma



5

School of Chemical Engineering, Northwest University, International Scientific and

6

Technological Cooperation Base of the Ministry of Science and Technology (MOST)

7

for Clean Utilization of Hydrocarbon Resources, Chemical Engineering Research

8

Center of the Ministry of Education (MOE) for Advanced Use Technology of Shanbei

9

Energy, Shaanxi Research Center of Engineering Technology for Clean Coal

10

Conversion, Collaborative Innovation Center for Development of Energy and

11

Chemical Industry in Northern Shaanxi, Xi'an, 710069, PR China

12

ABSTRACT: Coal pyrolysis characteristics are closely related to its structure and

13

composition, and revealing the relationship between them is essential for deep

14

understanding the pyrolysis mechanism. Therefore, in this work, Shendong coal

15

(SDC) was treated by hydrothermal pretreatment (HTP) to change its structure and

16

composition, and the influence of the changes on pyrolysis characteristics were

17

investigated. The changes in physicochemical structure and composition of treated

18

coal samples were characterized by N2 isothermal adsorption, Fourier transform

19

infrared (FTIR) spectroscopy,

20

diffraction (XRD), and inductively coupled plasma optical emission spectrometry

21

(ICP-OES). The pyrolysis experiments of raw and treated coal samples were carried

13C

nuclear magnetic resonance (NMR), X-ray

 Corresponding author. E-mail address: [email protected]

1

ACS Paragon Plus Environment

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22

out in a fluidized bed reactor. Then, the pyrolysis product distribution, the gas

23

composition, the tar chemical composition and properties, and the char gasification

24

behaviors were comprehensively studied. The experimental results showed that the

25

HTP was effective in surface area and pore volume increases, oxygen removal and

26

transfer, hydrogen radical introduction, and water soluble inorganic metals removal

27

for SDC. Correspondingly, the distribution and composition of pyrolysis products of

28

treated coal samples changed. The pyrolysis gas quality was improved with more

29

formations of H2 and CH4. The tar yield and N-hexane insoluble component

30

increased. More oxygen was enriched in macromolecular tar instead of gas and char

31

products, enhancing the char gasification activity. In addition, the structure-activity

32

relationship between the changes in coal structure and composition and the pyrolysis

33

characteristics was revealed in this work.

34

KEYWORDS: Hydrothermal pretreatment; Structure; Coal pyrolysis; Product

35

distribution; Pyrolysis mechanism

36

1. INTRODUCTION

37

Though efforts have been made to transform energy structure and reduce carbon

38

dioxide emission, the proportion of coal in China’s energy consumption is still up to

39

60.4 % in 2017.1 Low rank coals are and will be a master energy source in the

40

foreseeable future due to its abundance in China. The efficient and clean utilization of

41

low rank coals is imminent. Coal pyrolysis has attracted widespread attentions in

42

recent years since it is the initial step in gasification, liquefaction, coking, and

43

combustion process.2,3 Middle or low temperature pyrolysis is the most optimum 2


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Energy & Fuels

44

method for low rank coals utilization, obtaining fuel gases, semi-coke, liquid fuels,

45

and other high value-added chemicals.4 Considerable studies have shown that the coal

46

pyrolysis characteristics are influenced by the coal types and operating conditions,

47

such as pyrolysis final temperatures, reaction atmospheres, heating rates, residence

48

times, and different reactor types.5-9 Basically, the coal pyrolysis characteristics are

49

principally governed by its structure and composition, and a good knowledge of the

50

relationship between them is essential for deep understanding the pyrolysis

51

mechanism. Pretreatment is confirmed to be a promising method for causing changes

52

in physicochemical structure and composition of low rank coals,10-13 which

53

correspondingly influences the thermal conversion behaviors.

54

So far, several pretreatments for low rank coals have been adopted,10 mainly

55

including organic solvent thermal pretreatment, thermal pretreatment, and

56

hydrothermal pretreatment (HTP). Organic solvent thermal pretreatment was effective

57

in dewatering and deoxygenation of brown coals,11,12 which advanced the

58

hydro-liquefaction reactivity, but reduced the pyrolysis reactivity. Thermal

59

pretreatment was effective in deoxygenation but increased the cross-linking degree of

60

brown coals, resulting in the decrease of pyrolysis reactivity.11

61

HTP is another widely used pretreatment method for low rank coals.13 Compared

62

to organic solvent pretreatments, the hydrothermal pretreatment is simpler, and the

63

separation of water from coal sample is easier. After HTP, water can be removed in

64

the liquid form irreversibly due to the decomposition of oxygen-containing functional

65

groups. Zhundong coal was effectively upgraded with the increases in surface area 3


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66

and pore volume under 300 oC.14 Amounts of carboxyl, alcoholic hydroxyl, ether, and

67

carbonyl groups decreased in coal, whereas no obvious changes in phenolic hydroxyl

68

groups were observed owing to its high bonding energy.15,16 Some heavy metals, such

69

as Hg, Se, and As, can also be removed by the strong solubility of sub-critical

70

water.17,18 The HTP greatly improved the coal combustion characteristics, especially

71

above 250 oC of the pretreatment temperatures.19 The slurrying ability and rheological

72

behaviors of low rank coals were also enhanced,20 as well as the liquefaction

73

behaviors, particularly at the pretreatment temperature of 250 oC.21,22

74

Previous studies mainly focused on how the HTP changed the physicochemical

75

structure and composition of low rank coals,13-18 and further enhanced the coal

76

slurrying ability, liquefaction and combustion behaviors.19-22 A few investigations

77

have focused on the effect of HTP on coal pyrolysis characteristics. Ge et al.

78

researched the effect of HTP on pyrolysis characteristics of low rank coals by

79

TG-FTIR and found that the pyrolysis activity declined but more phenol and CH4

80

were formed.23 Liu et al. found that the tar yield of treated coal samples increased,24

81

while higher pretreatment temperatures (> 260

82

decreasing.25,26 However, the investigation on the relationship between the coal

83

pyrolysis characteristics and the changes in its structure and composition caused by

84

HTP was insufficiently conducted.

oC)

resulted in the tar yield

85

In this work, a comprehensive study of the effect of HTP on Shendong coal

86

(SDC) pyrolysis characteristics was conducted. The HTP of SDC at different

87

pretreatment temperatures and water to coal (W/C) ratios was performed from 4


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Energy & Fuels

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200-300 oC. The changes in physicochemical structure and composition of treated

89

coal samples were characterized by BET, FTIR, 13C NMR, XRD, and ICP-OES. The

90

pyrolysis experiments of SDC and treated coal samples were carried out at 600 oC in a

91

fluidized bed reactor. Then, the pyrolysis product distribution, the gases composition,

92

the tar properties, and the char gasification reactivity were analyzed in detail. Element

93

analyzer, synchronous UV-fluorescence spectroscopy, and GC-MS were employed to

94

characterize the tar properties. Additionally, the char gasification reactivity was

95

evaluated. Finally, the structure-activity relationship between the changes in structure

96

and composition and the pyrolysis characteristics was discussed.

97

2. EXPERIMENTAL METHODS

98

2.1. Experimental Materials

99

Shendong coal (SDC), a typical low rank coal, from China was crushed and

100

ground to 40-100 μm, vacuum dried at 105 oC for 12 h and sealed in cabinet filled

101

with N2.

102

The HTP was performed in a sealed autoclave (500 mL). In each run, 30 g SDC

103

and 90 mL deionized water were placed in the autoclave, and purged with 1.0 MPa of

104

N2 (purity, >0.9999) for three times to remove air residues. Then, the pressure was

105

reduced to 0.1 MPa of N2. Subsequently, the reactor was heated at a rate of 3.5 oC/min

106

and a stirring rate of 300 r/min, from ambient temperature to designed temperature

107

(200, 220, 240, 260, and 300 oC), holding for 1 h, respectively. After the autoclave

108

cooling down to ambient temperature, the residues were separated by using filtration

109

method, dried in a vacuum oven at 105 oC for 12 h, and preserved in cabinet filled 5


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110

with N2. All treated coals were marked with HTP-200, HTP-220, HTP-240, HTP-260,

111

and HTP-300, respectively, based on the pretreatment temperatures. Different water

112

to coal ratios (W/C, volume of deionized water to mass of coal, mL/g) at 240 oC were

113

performed.

114

2.2. Pyrolysis Experiment

115

Pyrolysis experiments were implemented in a powder-particle fluidized bed

116

reactor (in Figure 1), with silica sand (180-360 nm) as the bed material. The minimum

117

fluidization velocity and carrying velocity of bed material were 0.17 and 2.84 m/s,

118

respectively. The particle size of feedstock was 40-100 nm, with the calculated

119

carrying velocity of 0.16 m/s. Then, 0.20 m/s was designed as the operating

120

superficial gas velocity, and the fluidizing bed height was 13.5 cm. Briefly, 4.00±0.05

121

g of the sample pyrolyzed at 600 oC. N2 gas was employed as pyrolysis atmosphere,

122

consisting of feeding gas and fluidizing gas, with a flow rate of 1.20 L/min and 0.6

123

L/min, respectively. When the specified temperature was reached, the coal sample

124

was carried by feeding gas entering into the reactor. Then, the volatiles and char were

125

carried out of reactor and passed through a cyclone, after which the char was gathered

126

in collecting bottles. Then, the tar was collected via cold traps (alcohol condensation

127

bath at -40 oC). Followed by, the uncondensed gases were quantitatively monitored by

128

a GC device (Agilent Micro 3000), equipped with two conductivity (TCD) detectors,

129

a capillary column (PLOT-Q), and 5A molecular sieve, and the gas yield was

130

calculated by difference. To ensure that the results are accurate, all pyrolysis

131

experiments were repeated more than three times and the experimental results were 6


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132 133

Energy & Fuels

repeatable and deterministic. The coal pyrolysis products yields were obtained by the formulas (1)-(4) below. 𝑚𝑡𝑎𝑟 𝑎𝑛𝑑 𝑤𝑎𝑡𝑒𝑟 ― 𝐶𝑤 × 𝑚𝑙

(1)

134

𝑊𝑡𝑎𝑟 =

135

𝑊𝑤𝑎𝑡𝑒𝑟 =

136

𝑊𝑐ℎ𝑎𝑟 = 𝑚𝑐𝑜𝑎𝑙 × 100%

(3)

137

𝑊𝑔𝑎𝑠 = (1 ― 𝑊𝑡𝑎𝑟 ― 𝑊𝑤𝑎𝑡𝑒𝑟 ― 𝑊𝑐ℎ𝑎𝑟) × 100%

(4)

138

where mtar and water is the mass of liquid product, g; ml, is the mass of dichloromethane

139

solution of liquid product, g; cw is the water content of dichloromethane solution, g,

140

which was measured by micro-moisture meter based on K-F Coulomb Method; mcoal

141

and mchar are the mass of coal and char on dry and ash-free basis, respectively, g.

𝑚𝑐𝑜𝑎𝑙 𝐶𝑤 × 𝑚𝑙 𝑚𝑐𝑜𝑎𝑙

× 100%

(2)

× 100%

𝑚𝑐ℎ𝑎𝑟

5

7

5

6 9

11 12

10

4

Gas

8

3

3

F

2

Agilent Micro 3000

2

1

F P

13

N2

P

1.Valve of N2 2.Pressure meter 3.Gas mass flowmeter 4.Coal feeding device 5. Temperature controller 6.Pyrolysis furnace 7. Heat tracing belt

142 143 144 145 146 147

8.Powder-particle fluidized bed 9.Cyclone separator 10.Semi coke bottle 11.Condensation collecting bottle 12.Wet flowmeter 13.GC analyzer

Figure 1. The scheme of powder-particle fluidized bed equipment for pyrolysis. 2.3. Analysis Method The ultimate and proximate analysis of each sample were measured by SDTGA5000a (Sundy Ltd., China) and EL-Ⅲ analyzer (Vario Ltd., Germany). The minerals in coal samples were analyzed by A D/MAX-3C diffractometer 7


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148

(Rigaku Ltd., Japan) with Cu Ka radiation from 5o to 70o, with a step size of 0.02o.

149

The ICP-OES (PerkinElmer Company, America) was applied to determine the content

150

of designated metals.

151

The pore structure parameters of each sample were determined by ASAP2460

152

physisorption apparatus (Micromeritics Ltd., America). Brunauer–Emmet–Teller

153

(BET) equations and Barrett–Joyner–Halenda (BJH) method were respectively used

154

to determine the specific surface area and the pore volume of each sample.

155

Main organic functional groups of each sample were detected by Vertex 70

156

infrared spectrum analyzer (Bruker Company, Germany) and solid-static

157

DP/MAS NMR spectrometer (Bruker AVANCE III 600, Germany).

13C

NMR

158

The composition of tar samples was analyzed by Shimadzu GC-MS-QP 2010

159

Plus equipment. The flow rate was 1.0 mL/min, with a split ratio of 10:1. The oven

160

temperature program was set as follows: began at 40 oC for 4 min, heated to 70 oC at

161

4 oC/min (holding for 2 min), heated to 200 oC at 10 oC/min (holding for 3 min), and

162

finally heated to 300 oC at 4 oC/min (holding for 5 min). The injection and ion-source

163

temperatures were set at 300 and 230oC respectively. The mass spectrometer was set

164

in the electron ionization (EI) mode at 70 eV with m/z from 50 to 500. The

165

compounds were identified in the light of NIST08 and NIST08s library datas via

166

probability matching method. The relative content of compounds was calculated using

167

the peak area normalization method.

168

The aromatic ring condensation degree of tar samples was examined by

169

Synchronous UV-fluorescence spectroscopy (RF-5301) with the initial excitation 8


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Energy & Fuels

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wavelength of 220 nm, and the synchronous emission spectra was within 300~700

171

nm.

172

The char gasification was performed in a fixed bed reactor coupled with infrared

173

gas analyzer (SIELINS Ltd., China). 300 mg of each sample was placed in the silicon

174

tube and heated from 30 to 900 oC at a heating rate of 30 oC/min, holding for 90 min

175

at 900 oC. The gasification agent CO2 and carrier gas N2 were at a flow rate of 30

176

mL/min and 40 mL/min, respectively. Then, the CO, CO2, H2, and CH4 were detected

177

by infrared gas analyzer. The char gasification yield was obtained by the formula (5)27

178

as follows:

179

W𝑔𝑎𝑠𝑖𝑓𝑖𝑐𝑎𝑡𝑖𝑜𝑛 𝑦𝑖𝑒𝑙𝑑 =

180

where mchar and mresidue are the mass of the char sample and residue after gasification,

181

respectively, g; 𝑀𝑎𝑑 and 𝐴𝑎𝑑 are the moisture and ash content of char sample on the

182

air dry base, g.

𝑚𝑐ℎ𝑎𝑟 ― 𝑚𝑟𝑒𝑠𝑖𝑑𝑢𝑒

(5)

𝑚𝑐ℎ𝑎𝑟 × (1 ― 𝑀𝑎𝑑 ― 𝐴𝑎𝑑)

The gasification reactivity was evaluated by the formula (6)28 as follows:

183

0.5

(6)

184

R𝑖 = 𝜏0.5

185

where 𝜏0.5 is the time that the fixed carbon conversion in char samples reached 50 %,

186

min.

187

3. RESULTS AND DISCUSSION

188

3.1. Influence of HTP on Coal Composition

189 190 191

9


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Table 1. Proximate and ultimate analyses of SDC and treated coal samples.

192

Samples

SDC

HTP-200

HTP-220

HTP-240

W/C ratio

0:1

3:1

3:1

1:1

2:1

3:1

HTP-260

HTP-300

4:1

3:1

3:1

Proximate analysis (wt%) Mad

15.3

7.1

7.5

7.6

8.1

7.8

7.48

6.6

6.2

Ad

9.5

7.8

7.5

7.7

7.6

7.58

7.58

7.3

7.1

Vdaf

35.6

35.4

35.4

35.1

35.1

35.1

35.1

34.6

34.2

FCdaf*

64.4

64.6

64.6

64.7

64.8

64.9

64.9

65.5

65.8

Ultimate analysis (wt%, daf) C

77.8

78.9

79.3

79.2

79.3

79.2

79.4

79.5

79.9

H

4.7

4.8

4.9

5.0

4.9

5.0

4.9

4.8

4.9

N

1.0

1.3

1.2

1.2

1.1

1.2

1.2

1.2

1.2

S

0.4

0.7

0.6

0.6

0.6

0.7

0.6

0.6

0.5

O*

16.1

14.3

14.0

14.0

14.1

13.9

13.9

13.9

13.5

Atomic ratio and calorific value

193

*:

AO/C

0.16

0.14

0.13

0.13

0.13

0.13

0.13

0.13

0.13

AH/C

0.73

0.72

0.75

0.76

0.74

0.76

0.74

0.73

0.73

Qb,ad(MJ/kg)

27.3

30.6

30.6

30.5

30.4

30.5

30.7

31.0

31.2

By difference.

194

The proximate and ultimate analyses of samples are shown in Table 1.

195

Obviously, SDC had higher moisture and oxygen contents. After HTP, the calorific

196

value increased from 27.33 MJ/kg of SDC to 31.19 MJ/kg of HTP-300. The oxygen

197

to carbon (O/C) ratio decreased, while the hydrogen to carbon (H/C) ratios of

198

HTP-220 and HTP-240 increased. Thus indicated that the HTP decomposed the

199

oxygen-containing functional groups and introduced hydrogen radical to coal matrix

200

with decreasing cross-linking degree.11,29 The volatile matter content changed slightly 10


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201

below 260 oC of the pretreatment temperatures and decreased obviously above 260

202

oC,

203

HTP.13,30 The solid yields (dry and ash-free base) after the HTP were 98.8 %

204

(HTP-200), 98.5 % (HTP-220), 97.7 % (HTP-240), 97.3 % (HTP-260), and 96.4 %

205

(HTP-300), respectively. And the hydrogen content of HTP-240 was the highest.

206

Therefore, 240

207

pretreatments of different W/C ratios at 240 oC were conducted and no obvious

208

discrepancy was observed on the composition of treated coal samples, especially

209

above 3:1. Hence, the W/C ratio in this paper was mainly based on 3:1.

due to the cleavage of aliphatic hydrocarbons side chains above 260 oC by

oC

was the optimum pretreatment temperature. Additionally,

c b

a

b a

a:quartz, b:kaolinite, c:muscovite, d:siderite, e:xanthophyllite

c d ea a

a a ab

Intensity (a.u.)

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

a

SDC HTP-200 HTP-220 HTP-240 HTP-260 HTP-300

10

210 211

20

30

40 50 2 (°)

60

70

80

90

Figure 2. XRD spectra of SDC and treated coal samples.

212

The ash content of treated coal samples decreased due to the dissolution of the

213

minerals by HTP.14,31-33 As the XRD spectra displayed in Figure 2, the intensities of

214

kaolinite, muscovite, and siderite peak in treated coal samples decreased,

215

corresponding to the composition of designated metals in each sample (in Table 2).

216

Therefore, the cross-linking points between carboxyl and AAEM species in treated

217

coal samples declined.34,35 Furthermore, the concentration of Hg also decreased, from

218

8.2 × 10-3 mg/gcoal,daf (SDC) to 0.1 × 10-3 mg/gcoal,daf (HTP-300), as well as As from 11


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23.1 × 10-3 mg/gcoal,daf (SDC) to 10.7 × 10-3 mg/gcoal,daf (HTP-300).

220

Table 2. Composition of designated metals in SDC and treated coal samples by

221

ICP-OES. Content (mg/gcoal,daf)

Samples

Content ( × 10 ―3mg/gcoal,daf )

Na

K

Mg

Ca

Fe

Al

Hg

As

SDC

1.5

1.9

0.5

7.4

3.8

8.7

8.2

23.1

HTP-200

1.1

1.3

0.5

6.6

3.7

7.7

5.6

22.7

HTP-220

0.8

1.1

0.4

5.9

2.9

7.5

4.5

22.0

HTP-240

0.9

1.3

0.5

6.4

3.2

7.3

2.6

19.9

HTP-260

0.5

1.0

0.4

5.4

3.0

6.2

1.4

15.7

HTP-300

0.6

1.1

0.4

5.7

3.0

6.3

0.1

10.7

Table 3. Composition of released gases during HTP.

222

Composition( × 10-3mmol/gcoal,daf)

Samples H2

CH4

CO

CO2

C2+C3

HTP-200

0.06

0.02

1.39

8.82

0.01

HTP-220

0.06

0.04

1.67

10.13

0.01

HTP-240

0.06

0.04

1.82

10.76

0.02

HTP-260

0.07

0.08

2.16

11.74

0.02

HTP-300

0.09

0.09

2.71

12.89

0.04

223

Table 3 shows the composition of released gases during HTP. With the rising of

224

pretreatment temperatures, the CO2 relative evolution increased from 8.82 × 10-3

225

mmol/gcoal,daf (HTP-200) to 12.89 × 10-3 mmol/gcoal,daf (HTP-300), due to the cleavage

226

of carboxyl functional groups. The fracture of methoxy and aldehyde functional

227

groups weakly bonded to the matrix led to the increase of CO relative evolution from

228

1.39 × 10-3 mmol/gcoal,daf (HTP-200) to 2.71 × 10-3 mmol/gcoal,daf (HTP-300).35 In 12


Page 13 of 30 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

229

addition, the relative evolution of C1~C3 increased along with the rising of

230

pretreatment temperatures because of the thermal rupture of weak covalent bonds,

231

such as aliphatic side chains and bridges.

232

3.2. Influence of HTP on Coal Structure

233

Table 4. Specific surface area and pore structure parameters of coal samples. Surface Area

Pore Volume

Average Pore Diameter

(m2/g)

(cm3/g)

(nm)

SDC

2.01

9.91 × 10-3

19.73

HTP-200

3.81

8.84 × 10-3

9.30

HTP-240

7.31

1.20 × 10-2

6.58

HTP-300

7.81

1.22 × 10-2

6.33

Samples

234

The specific surface area and the pore structure parameters of coal samples are

235

shown in Table 4. The surface area and pore volume of treated coal samples increased

236

with the rising of pretreatment temperatures, whereas the average pore diameters

237

sharply decreased. According to the pore size distribution of the SDC and treated coal

238

samples (in Figure S1 and S2), smaller mesopores in the treated coals were evolved

239

probably from hitherto closed pores and micropores, and more distribution peaks

240

appeared from 2 to 20 nm compared with SDC. This could be attributed to gases

241

evolution during HTP caused by the cleavage of oxygen-containing functional groups

242

and aliphatic hydrocarbon side chains. The disintegration of macropore was attributed

243

to the shrinkage forces caused by the hydrothermal treatment.

244

The FTIR analysis of all samples are depicted in Figure 3. Each infrared

245

spectrum can be divided into four sections as reported in previous literatures:37-39

246

3600-3000 cm-1 (-OH groups stretching vibration), 3000-2800 cm-1 (aliphatic C-H 13


Energy & Fuels

247

groups stretching vibration), 1800-1000 cm-1 (oxygen-containing functional groups

248

stretching vibration) and 900-700 cm-1 (aromatic C-H groups out-of-plane bending

249

vibration). Taking SDC as an instance, the curve-fitted FTIR spectra of four sections

250

by Gaussian method with PeakFit® (Version 4.12) were exhibited in Figure 4. The

251

same method was adopted for treated coal samples, which was not showed here.

HTP

Absorbance (a.u.)

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 14 of 30

252 253

-2 0 0

SDC HTP

-2 2 HTP 0 -2 H TP 40 -2 6 0 HTP -3 0 0

4000 3500 3000 2500 2000 1500 1000 -1 Wavenumbers (cm )

500

Figure 3. Infrared spectra of SDC and treated coal samples.

254

Several parameters19,24,38,39 related to different functional groups were calculated

255

via the areas of FTIR spectra fitted peaks. The ratio of I1 (3000-2800 cm-1 / 1613

256

cm-1) represents the relative abundance of aliphatic bonds. The CH2/CH3 (2921 cm-1 /

257

2959 cm-1) ratio measures the length or branching degree of aliphatic side chains and

258

bridge bonds. The ratio of I2 (1700-1640 cm-1 / 1630-1500 cm-1) corresponds to the

259

carbonyl or carbonyl groups to aromatic carbon groups. The ratios of I3 (1310 cm-1 +

260

1178 cm-1 + 1150 cm-1 / 1613 cm-1) and I4 (1231 cm-1/1613 cm-1) refer to the richness

261

of phenolic hydroxyl and aryl ethers, respectively. The ratio of I5 (873 cm-1 + 849

262

cm-1 + 818 cm-1 / 873 cm-1 + 849 cm-1 + 818 cm-1 + 799 cm-1 + 776 cm-1 + 751 cm-1)

263

is related to the substitution degree of aromatic ring system. Thus, the chemical

264

structural parameters of coal samples derived from FTIR analysis are presented in 14


Page 15 of 30

265

Table 5. 0.05

Experimental curve Fitted curve Fitted peak

(a) -OH

(b) Aliphatic CH

0.032

Absorbance

Absorbance

0.04 0.03 0.02

0.05 0.04

3500

3400 3300 3200 -1 Wavenumber (cm )

(c) O-containing

3100

Experimental curve Fitted curve FItted peak

0.016

0.0032

2950 2900 2850 -1 Wavenumber (cm )

(d) Aromatic CH

2800


0.0024

0.03 0.02

0.0016

0.0008

0.01

267

0.024

0.000 3000

3000

Absorbance

0.00 3600

266


0.008

0.01

Absorbance

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

0.00 1800 1700 1600 1500 1400 1300 1200 1100 1000 -1 Wavenumber (cm )

0.0000 900 880 860 840 820 800 780 760 740 720 700 -1 Wavenumber (cm )

268

Figure 4. Curve fittings of SDC infrared spectrum in different wavenumber bands, (a)

269

3600-3000 cm-1, (b) 3000-2800 cm-1, (c) 1800-1000 cm-1, (d) 900-700 cm-1.

270

As shown in Table 5, the values of I1, CH2/CH3, and I5 for treated coal samples

271

decreased, due to the cleavage of weak covalent aliphatic side chains, as well as

272

longer aliphatic side chains cracking into shorter ones during HTP. All those can be

273

responsible for the gases release of C1~C3. The I2 ratio of treated coal samples

274

decreased, further indicating that the carbonyl and carboxyl groups were decomposed

275

with the evolution of CO2 and CO by HTP. With the addition of water, abundant

276

hydroxyl and hydrogen radicals are possibly cross-linked to the benzene or phenoxy

277

of matrix to generate phenolic hydroxyl groups during HTP.40 Correspondingly, the

278

value of I3 increased from 1.15 (SDC) to 1.30 (HTP-300). However, the ratio of I4 15


Energy & Fuels

279

decreased from 0.52 (SDC) to 0.46 (HTP-300). The aryl ethers are not easily

280

decomposed at low temperatures.41 It can be speculated that some aryl ether oxygen

281

transferred to phenolic hydroxyl oxygen by aryl ether hydrolysis reactions during

282

HTP. Table 5. Parameters derived from FTIR analysis of SDC and treated coal samples. I3

I4

I5

CH2/CH3

SDC

0.93

0.41

1.15

0.52

0.43

2.92

HTP-200

0.81

0.39

1.21

0.51

0.40

2.83

HTP-220

0.78

0.40

1.19

0.48

0.39

2.76

HTP-240

0.76

0.38

1.29

0.47

0.34

2.62

HTP-260

0.71

0.35

1.29

0.45

0.33

2.51

HTP-300

0.65

0.35

1.30

0.46

0.31

2.49

Aro C

O-alk

Alk C

I2

COO

I1

0.65

1.33 0.17

1.0

1.91

3.61

0.53

1.10 0.20

1.0

1.67

2.56

C=O

Samples

O-aro

283

Intensity (a.u.)

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 30

SDC

HTP-240

200

284 285

150 100 50 Chemical shift (ppm)

0

Figure 5. Solid-state 13C NMR spectra of SDC and treated coal samples.

286

Figure 5 shows the 13C NMR spectra of SDC and HTP-240. The spectrums can

287

be divided into six sections,15,42 nonpolar alkyls (alk C, 0-50 ppm), O-alkyls (O-alk,

288

50-90 ppm), aromatics (aro C, 90-150 ppm), aromatic C-O (O-aro, 150-165 ppm),

289

carboxyls (COO, 165-190 ppm), and carbonyls (C=O, 190-220 ppm). As based on the

290

area of aromatics at 90-150 ppm, the ratios of alk C to aro C, COO to aro C, C=O to

291

aro C, and O-alk to aro C for HTP-240 decreased, indicating that the cleavage of 16


Page 17 of 30 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

292

some weak aliphatic bonds, carbonyl and carboxyl groups occurred during HTP, as

293

well as aliphatic alcoholic hydroxyl and methoxy. Additionally, the ratio of O-aro to

294

aro C for HTP-240 was significantly higher than that of SDC, suggesting that more

295

phenolic hydroxyl oxygen were formed during HTP.

296

3.3. Influence of HTP on Coal Pyrolysis Table 6. Pyrolysis products distribution of SDC and treated coal samples.

297

Yield %)

(wt

SDC

HTP-200

HTP-220

HTP-240

HTP-260

HTP-300

W/C ratio

--

3:1

3:1

1:1

2:1

3:1

4:1

3:1

3:1

Tar

9.2

9.9

10.1

10.2

10.4

10.5

10.5

9.9

9.7

Char

68.4

70.3

70.6

70.7

71.0

71.3

71.3

72.7

73.7

Gas

19.6

17.1

16.5

16.5

16.1

15.7

15.7

14.8

14.2

Water

3.0

2.7

2.8

2.6

2.5

2.5

2.5

2.6

2.4

298

The pyrolysis products yields are given in Table 6. Both pyrolysis gas and water

299

yields decreased, while the char yield increased with the rising of pretreatment

300

temperatures. The tar yield increased first and then decreased. The products yields of

301

treated coal samples under different W/C ratios were basically the same. Thence,

302

pretreatment temperature is the main influencing factor for HTP.43

303

3.3.1. Gas Products

304

The composition of gas products is shown in Table 7. The CO2 and CO of treated

305

coal samples decreased with the rising of pretreatment temperatures, due to the

306

decomposition of carboxyl, aldehyde groups and methoxy during HTP. And, part of

307

CO was achieved by the fracture of aryl ethers at higher temperatures during pyrolysis

308

process.36 Some aryl ether oxygen transferred to phenolic hydroxyl oxygen during 17


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 18 of 30

309

HTP, resulting in the CO evolution decrease during pyrolysis process. Secondly, the

310

absence of water soluble inorganic metals suppressed the tar precursors cracking into

311

small molecular gases in pyrolysis.44 The H2 formation of HTP-240, HTP-260, and

312

HTP-300 increased, which was attributed to the reduction of hydrogen radical

313

consumption during pyrolysis process.45 The hydrogen radical was introduced and

314

longer aliphatic side chains were cracked to short ones during HTP, resulting in the

315

increase of CH4 but decrease of C2~C3 for treated coal samples, compared to that of

316

SDC. However, with the rising pretreatment temperatures, more CH4 was evolved

317

during HTP, leading to the decrease of CH4 (HTP-260, and HTP-300) during

318

pyrolysis.

319

Table 7. Composition of gas products of SDC and treated coal samples. Composition (mmol/gcoal,daf)

Samples

320

H2

CO

CO2

CH4

C2H4

C2H6

C3H6

C3H8

SDC

0.63

1.36

0.72

1.35

0.29

0.10

0.16

0.03

HTP-200

0.59

1.22

0.50

1.43

0.26

0.10

0.10

0.03

HTP-220

0.61

1.04

0.44

1.43

0.25

0.10

0.09

0.03

HTP-240

0.65

0.92

0.43

1.47

0.16

0.09

0.08

0.03

HTP-260

0.66

0.87

0.42

1.40

0.15

0.09

0.07

0.03

HTP-300

0.70

0.82

0.40

1.39

0.14

0.08

0.06

0.03

3.3.2. Tar Products

321

The tar yield and composition are principally governed by coal structure and

322

composition. Its generation consists of the formation and rearrangement of free

323

radical fragments. As reported in previous studies,46-48 the formation of tar precursors

324

was hindered by the existence of AAEM metals due to their cross-linking effects. The 18


Page 19 of 30

325

AAEM metals in treated coal samples were efficiently removed and hydrogen radical

326

was introduced, which are beneficial to the formation and stabilization of free radical

327

fragments in time during pyrolysis process. Moreover, the increase of smaller

328

mesopores (2-20 nm) in treated coal samples reduced the mass transfer resistance for

329

volatiles evolution (e.g., the molecular diameter of naphthalene is 0.8 nm). Hence, the

330

tar yield increased from 9.2 % (SDC) to 10.5 % (HTP-240). Conversely, the higher

331

pretreatment temperatures promoted the cleavage of weak covalent bonds, leading to

332

lower tar yield of 9.7 % (HTP-300).

50

Tar compositions (area %)

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

40

334

HTP-200 HTP-240 HTP-300

30

20

10

0

333

SDC HTP-220 HTP-260

s s s s atics -ring Ar. -ring Ar. PAH mpound mpound mpound 2 1 Aliph co co r co olic ated Othe Phen Oxygen

Figure 6. Composition of tar products of SDC and treated coal samples.

335

The classification composition of tar samples by GC-MS analyses (peak area

336

normalization method) is given in Figure 6. Obviously, the relative content of

337

phenolic compounds in tar of treated coal samples was higher than that of SDC.

338

Firstly, according to the FTIR analyses, more phenolic hydroxyl groups were

339

generated in treated coal samples. Secondly, the removal of water soluble organic

340

metals inhibited the conversion of macromolecular phenolic compounds to small

341

molecules.49-51 More phenolic compounds were produced in tar samples.

342

Correspondingly, the relative content of monocyclic aromatic hydrocarbons 19


Energy & Fuels

343

decreased. The relative content of polycyclic aromatic hydrocarbons in tar decreased

344

from 42.1 % of SDC to the average content 30.5 % of treated coal samples. There was

345

a slight decrease in the relative content of aliphatic hydrocarbons from 18.4 % of SD

346

to 14.5 % of HTP-300, due to the fracture of aliphatic side chains by HTP.29 HTP-300

HTP-260 Normalized intensity

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 30

HTP-240

HTP-220

HTP-200

SDC

300

347 348

400 500 600 Wavelength (nm)

700

Figure 7. Normalized UV-F spectra of tar products from SDC and treated coals.

349

The normalized UV-F spectra of tar samples is presented in Figure 7, where the

350

wavelength of the peak shifts from 487 nm of SDC tar to 515 nm of HTP-300 tar.

351

This observation strongly suggested that much more macromolecular tar compounds

352

were generated from macromolecules of treated coal samples.52 In addition, there was

353

no obvious difference among tar composition of treated coal samples, and the tar

354

samples of SDC and HTP-240 were selected for further analysis below.

355

As shown in Table 8, the oxygen content of SDC tar is lower than that of

356

HTP-240 tar, as well as the nitrogen and sulfur contents, indicating that HTP was

357

advantageous for heteroatoms evolution in pyrolysis. Moreover, the O/C and H/C 20


Page 21 of 30

358

ratios of HTP-240 tar increased slightly, which corresponded to the fact that HTP

359

promoted more oxygen enrichment in tar and reduced hydrogen consumption to form

360

water.45,53

361

Table 8. Proximate and ultimate analysis of char and tar from SDC and HTP-240

362

coals. Proximate analysis (wt%)

Ultimate analysis (wt%, daf)

Atomic ratio

Samples Mad

Ad

Vdaf

FCdaf*

C

H

N

S

O*

AO/C

AH/C

SDC

3.7

10.0

20.8

79.2

84.3

3.3

1.5

0.4

10.5

0.09

0.46

HTP-240

3.2

7.7

19.0

81.0

86.5

2.9

1.4

0.3

8.9

0.08

0.40

SDC

--

--

--

--

81.2

7.8

0.9

0.3

9.8

0.09

1.15

HTP-240

--

--

--

--

79.7

7.7

1.1

0.3

11.2

0.11

1.17

Char

Tar

363

by difference.

*:

50

Tar compositions (area %)

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

40 30 20 10 0

364 365

SDC HTP-240

s s s s atics -ring Ar. -ring Ar. PAH mpound mpound mpound 1 2 Aliph co co r co olic ated Othe Phen Oxygen

Figure 8. Composition of N-hexane soluble tar from SDC and HTP-240 coals.

366

The N-hexane extraction rate of SDC tar was 37.7 %, markedly higher than 25.9

367

% of HTP-240 tar, indicating that more macromolecules were formed in tar of treated

368

coal samples. The composition of N-hexane soluble tar samples was analyzed by

369

GC-MS, as depicted in Figure 8. The oxygen-containing compounds content in

370

HTP-240 N-hexane soluble tar decreased. Yet, oxygen content in HPT-240 tar was

371

higher than that in SDC tar. It further suggested that more oxygenated 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

372

macromolecules were formed in tar of treated coal samples.

373

3.3.3. Water Products

374

The pyrolysis water is produced owing to the existence of oxygen-containing

375

functional groups.45 The HTP was beneficial to the removal of aliphatic alcohol

376

hydroxyl beforehand, and more oxygen was enriched in tar. Thus, the water yield

377

decreased from 3.0 % of SDC to 2.4 % of HTP-300.

378

3.3.4. Char Products

379

Some gas components were released during HTP, as shown in Table 3. The

380

higher the pretreatment temperatures, the more gases evolved from coal matrix,

381

resulting in the increase of char yields, from 68.4 % of SDC to 71.3 % of HTP-300.

382

Furthermore, the HTP promoted oxygen enrichment in tar instead of char, as well as

383

nitrogen and sulfur. The average pore diameter of SDC char was 9.39 nm, lower than

384

15.74 nm of HTP-240 char. More macropores were formed in HTP-240 char. The

385

micropore specific surface area of HTP-240 char (1.17 m2/g) was higher than that of

386

SD char (0.25 m2/g).

387

Figure 9 shows the char gasification yield and product distributions. As depicted

388

in Figure 9(a), the char gasification yield from treated coal samples obviously

389

increased. There was no obvious difference in CH4 and H2 evolutions among all char

390

samples, as given in Figure 9(b) and 9(c).

391

Figure 9(d) displays the CO evolution curve during char gasification process. Its

392

evolution began from 600 oC to 900 oC, and reached the maximum at 900 oC. No

393

obvious difference of CO evolution occurred before 700 oC for all samples, verifying 22


Page 22 of 30

Page 23 of 30

394

that the easily decomposable oxygen was fully volatized during pyrolysis process.

395

The hydroxyl groups with high bonding energy and aryl ethers in the residual char

396

links were responsible for the release of CO above 700 oC.36 However, the oxygen

397

content and O/C ratio of HTP-240 char was 8.94 % and 0.078, lower than that of

398

10.29 % and 0.091 of SDC char, respectively. When the gasification temperature was

399

above 700 oC, the CO formation rate and evolution volume for treated coal samples

400

were obviously higher than that of SDC during gasification process. Thus, the CO

401

was generated by the reactions between CO2 and the chars.36 The 𝑅𝑆𝐷 𝑐ℎ𝑎𝑟 was

402

0.0094, and the 𝑅𝐻𝑇𝑃 ― 240 𝑐ℎ𝑎𝑟 was 0.0104. Therefore, the char gasification reactivity

403

of treated coal samples was enhanced. The higher micropores surface area of chars

404

from treated coal samples increased the gasification reacativity, leading to higher CO

405

evolution. Additionally, lower oxygen and sulfur contents in chars also enhanced

406

gasification reactivity. 4

(a) CH4 distribution (mL/g char )

100 Gasification yields (wt.%)

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

90

80

70

60

SDCHTP-200HTP-220HTP-240HTP-260HTP-300

(b)

3 SDC HTP-200 HTP-220 HTP-240 HTP-260 HTP-300

2 1 0 300 600 900 Temperature (C)

407

900C

23


30 60 Time (min)

90

Energy & Fuels

408

50

900C CO distribution (mL/g char )

20 (c) H2 distribution (mL/g char )

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

15 SDC HTP-200 HTP-220 HTP-240 HTP-260 HTP-300

10 5 0 300 600 900 Temperature (C)

30 60 Time (min)

90

Page 24 of 30

(d)

900C SDC HTP-200 HTP-220 HTP-240 HTP-260 HTP-300

40 30 20 10 0

300 600 900 Temperature (C)

30 60 Time (min)

90

409

Figure 9. Gasification yield and gas evolution of char samples, (a) gasification yield,

410

(b) CH4 evolution, (c) H2 evolution, (d) CO evolution.

411

3.4. Possible Mechanism of Coal Pyrolysis with HTP

412

The coal pyrolysis characteristics are closely related to its structure and

413

composition. And the coal pyrolysis mechanism was revised in the previous

414

literature.54 The possible mechanisms during the process of HTP and pyrolysis of

415

treated coal samples were proposed in this work. After HTP, the water soluble

416

inorganic metals and some oxygen-containing functional groups in SDC were

417

removed, and the cross-linking points of carboxyl and AAEM species decreased.

418

Then, the formation of smaller mesopores (2-20 nm) increased from hitherto closed

419

pores and micropores. Moreover, the hydroxyl and hydrogen radicals were introduced

420

into the treated coal samples. The aromatic carboxyl groups first cracked to CO2 and

421

aryl radicals, and then reacted with the hydroxyl radical to form phenolic hydroxyl

422

groups. The aryl methyl ethers first cracked to methyl and aryl ether radicals, and then

423

reacted with hydrogen radical respectively to form CH4 and phenolic hydroxyl

424

groups. Correspondingly, more phenolic radicals and macromolecular tar precursors

425

were formed during pyrolysis process. Meanwhile, the absence of water soluble 24


Page 25 of 30 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

426

inorganic metals inhibited the deoxygenation of tar precursors. Thus, the tar yield

427

increased with more phenolic compounds, as well as the N-hexane insoluble

428

component, but the CO and CO2 evolution decreased. The pyrolysis water yields

429

decreased, while the formation of H2 and CH4 increased. More oxygen was enriched

430

in macromolecular tar instead of gas and char products. Additionally, the char

431

gasification yield and reactivity were enhanced. The schematic diagram of the

432

mechanism of coal pyrolysis with HTP was shown in Figure 10.

433 434 435

Figure 10. Schematic diagram of the mechanism of coal pyrolysis with HTP. CONCLUSION

436

The physicochemical structure and composition of SDC were changed by HTP,

437

resulting in the differences in pyrolysis characteristics between SDC and treated coal

438

samples.

439

1. After HTP, the smaller mesopores (2-20 nm) increased from hitherto closed

440

pores

and

micropores,

the

water

soluble

inorganic

441

oxygen-containing functional groups were removed, and the cross-linking degree of

442

coal matrix was reduced. 25


metals

and

some

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

443

2. The pyrolysis gas yield decreased by the significant reduction in CO and CO2.

444

More H2 and CH4 were formed due to the oxygen removal and hydrogen radical

445

introduction. The tar yield increased because of the more tar precursor formations and

446

the lower mass transfer resistance during pyrolysis. The tar N-hexane extraction rate

447

decreased and more oxygen was enriched in macromolecular tar compounds, which

448

can be attributed to the inhibition of deoxygenation of tar precursors. The water yield

449

decreased due to the removal of aliphatic alcohol hydroxyl beforehand and the oxygen

450

enrichment in tar. Additionally, the oxygen and sulfur contents decreased and more

451

macropores were formed in char samples, improving the char gasification activity.

452

3. The experimental results and mechanism analysis from this work indicated

453

that the hydrogen radical and hydroxyl radical were introduced in treated coal

454

samples, and more phenolic hydroxyl groups were formed by aromatic carboxyl

455

hydrolysis reactions and aryl ether hydrolysis reactions. More evolution of smaller

456

mesopores from the hitherto closed pores and micropores facilitated the escape of

457

volatiles. The absence of water soluble inorganic metals promoted the formation of tar

458

precursors and inhibited the cracking of tar precursors. The decomposition of

459

oxygen-containing functional groups and the introduction of hydrogen radical

460

beforehand improved the gas quality.

461

AUTHOR INFORMATION

462

Corresponding Author

463

* Tel: +86-13772424852. E-mail address: [email protected]

26


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Energy & Fuels

464

Notes

465

The authors declare no competing financial interest.

466

ACKNOWLEGEMENTS

467

This work was supported by the National Natural Science Foundation of China (NO.

468

21536009), and Science and Technology Plan Projects of Shaanxi Province

469

(2017ZDCXL-GY-10-03).

470

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Study on the Pyrolysis Characteristics of a Typical Low Rank Coal with

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