Effect of temperature on the sulfur fate during hydrothermal carbonization of sewage sludge

Journal Pre-proof Effect of temperature on the sulfur fate during hydrothermal carbonization of sewage sludge Zhexian Wang, Yunbo Zhai, Tengfei Wang, Chuan Peng, Shanhong Li, Bei Wang, Xiangmin Liu, Caiting Li PII:

S0269-7491(19)35524-1

DOI:

https://doi.org/10.1016/j.envpol.2020.114067

Reference:

ENPO 114067

To appear in:

Environmental Pollution

Received Date: 25 September 2019 Revised Date:

15 January 2020

Accepted Date: 23 January 2020

Please cite this article as: Wang, Z., Zhai, Y., Wang, T., Peng, C., Li, S., Wang, B., Liu, X., Li, C., Effect of temperature on the sulfur fate during hydrothermal carbonization of sewage sludge, Environmental Pollution (2020), doi: https://doi.org/10.1016/j.envpol.2020.114067. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

*Corresponding Author. Tel.+86 731 8882 2829, Fax. +86 731 8882 2829. E-mail Address: [email protected](Y.B. ZHAI)

1

Effect of temperature on the sulfur fate during hydrothermal

2

carbonization of sewage sludge

3

Zhexian Wanga,b,c,,Yunbo Zhaia,b,* , Tengfei Wanga,b, Chuan Penga,b, Shanhong Lia,b,

4

Bei Wanga,b，Xiangmin Liua,b,Caiting Lia,b

5

a

6

410082, P. R. China

7

b

8

Ministry of Education, Changsha 410082, P. R. China

9

c

College of Environmental Science and Engineering, Hunan University, Changsha

Key Laboratory of Environmental Biology and Pollution Control (Hunan University),

State Key Laboratory of Petroleum Pollution Control, Beijing 102206, P.R. China

10

*Corresponding Author. Tel.+86 731 8882 2829,Fax. +86 731 8882 2829.

11

E-mail Address: [email protected](Y.B. ZHAI)

12

a,b,c contributed equally to this work and should be considered co-first Affiliation.

13

ABSTRACT

14

To understand the effect of reaction temperature on sulfur during hydrothermal

15

carbonization (HTC) of sewage sludge (SS), seven group of temperature (180-300 ℃)

16

were chosen to investigate the distributions and evolution of sulfur-containing

17

compounds in hydrochar and the liquid products. Elemental analysis, X-ray

18

photoelectron spectroscopy (XPS), and X-Ray powder diffraction (XRD) were used

19

to characterize the distribution of sulfur in hydrochar. The concentrations of sulfate

20

ions and sulfide were determined in the liquid sample. The experimental results

21

showed that as the temperature increased, the O/C ratio decreased because of the

22

improved carbonization degree of SS. After hydrothermal carbonization, 90% of the

23

sulfur in SS remained in hydrochar. As the temperature increased, the amount of

24

sulfur in the liquid, mainly in the form of sulfate ions, tended to decrease. However,

25

the experimental results for the gas phase were the opposite of the liquid phase.

26

Capsule abstract：This manuscript discusses the evolution and transformation of

27

sulfur species in sewage sludge for different hydrothermal carbonization temperature.

28

Keywords: Hydrothermal carbonization; Sewage sludge; Sulfur balance; Evolution

29

mechanism.

30

1. Introduction

31

Recently, sewage sludge (SS) as a by-product from sewage treatment plants has

32

aroused extensive public concern. SS is composed of wastewater, mineral particles,

33

organic debris, and bacteria (Zhai et al., 2016). Approximately 300 million tons per

34

year of dry sludge is produced in China (Zhang et al., 2017), and only a small amount

35

of this sludge can be reutilized. Thermal conversion technologies, such as pyrolysis,

36

gasification, and combustion (Samolada and Zabaniotou, 2014; Yaman, 2004; Zhao et

37

al., 2013), have been applied to SS. However, potential risks from high moisture

38

content, heavy metals, and hazardous and toxic organic components limit the further

39

utilization of SS. Therefore, effective treatment technology is critical.

40

Hydrothermal carbonization (HTC) is a promising thermochemical technology

41

that has received attention because this process can directly utilize the moisture of raw

42

materials as a reaction medium (Chiang et al., 2001; Yoshikawa, 2012). This process

43

generally occurs at a low temperature range (180-250 ℃) in high-pressure vessels

44

(Wang et al., 2018b). Moreover, the hydrothermal carbonization process can be

45

affected by reaction conditions, such as temperature, residence time, solid-to-liquid

46

ratio, and raw material selection (Danso-Boateng et al., 2015; Parshetti et al., 2013),

47

although temperature is the most important of these factors.

48

With increasing applications of hydrochar, concerns are also increasing regarding

49

the environment owing to the release of sulfur-containing gases when biochar is

50

combusted (Meng et al., 2016). The sulfur content in sludge varies from 0.2 to 9.5%

51

(Anna et al., 2007; Chen et al., 2015; He et al., 2013; Ros et al., 2006). Therefore, a

52

thorough investigation of the sulfur species in SS is essential. Generally, the sulfur

53

species in SS are divided into two major categories: organic and inorganic sulfur.

54

Organic sulfur species include sulfide, sulfone, sulfoxide, thiophene. and mercaptan;

55

inorganic sulfur species are typically sulfate and sulfite (Zhong et al., 2018). Until

56

now, only a handful of studies had been conducted on sulfur species transformation of

57

SS during pyrolysis process. The transformation of sulfur species in sludge had been

58

previously investigated for both microwave pyrolysis and pyrolysis at several

59

temperatures (Zhang et al., 2017). Their results indicated that aliphatic sulfur

60

compounds (e.g., mercaptan) were likely to degrade below 500

61

broke the C-S bond in these compounds. However, stable aromatic sulfur required a

62

higher temperature in the range of 500-800

63

Furthermore, the yield of H2S tended to increase with increasing (microwave)

64

pyrolysis temperature. Chiang and coworkers claimed that the sulfur concentration in

65

petrochemical sludge increased with increasing pyrolysis temperature (Chiang et al.,

66

2001). Because only a few researchers have estimated the sulfur evolution and

when pyrolysis

before degradation (Zhang et al., 2017).

67

distribution in SS during HTC, investigating the effect of HTC temperature on sulfur

68

evolution is essential and meaningful for the further application of SS.

69

This is the first study to discuss the effect of HTC temperature on sulfur species

70

in SS. The main objectives of this study are: (1) to investigate the forms of sulfur in

71

different phases; (2) to understand the effect of HTC temperature on the

72

transformation of sulfur species.

73

2. Materials and methods

74

2.1. Materials

75

The sludge used in this study was obtained from the secondary clarifier of a

76

petrochemical wastewater treatment plant (Henan, China). Sludge was black and

77

sticky, and its composition was summarized in Table 1. Prepared sludge was stored in

78

plastic bags at 4

79

weeks.

80

2.2. HTC experiments

. Before every experiment, sludge was naturally dried for two

81

SS hydrochar was collected from the HTC experimental apparatus (a 500 mL

82

316 stainless steel enclosed reactor). 66.66 g SS particles was added to 100 mL

83

deionized water and mixed. The HTC temperature ranged from 180 to 300

84

intervals. The heating rate and stirring rate were 4

85

respectively. The residence time was 30 min. After that, reaction apparatus was

86

naturally cooled to room temperature. The hydrochar and liquid sample were

87

collected and separated by vacuum filtration. The char was then dried at 104

88

the dried hydrochar was grinded to fine particles (less than 0.25 mm).

in 20

/min and 150 rpm/min,

, and

89

In this study, gaseous products were collected for a limited amount, and will be

90

investigated in our future research. Residual hydrochar on the inner wall of

91

stainless-steel reactor was washed into a beaker to avoid mass loss. Hydrochar was

92

labeled based on the reaction temperature, such as SS-180 or SS-300. Hydrochar yield

93

was calculated by the following equation:

94


ℎ    % =

95

Where  represents the hydrochar mass and  is the weight of SS.

96

        % = S1× S2,

97

Where S1 represents the relative proportion of sulfur species and S2 is hydrochar

98

yield.

99

2.3. Analysis of solid and liquid phases.

 

,

(1)

(2)

100

The basic properties of SS (hydrochar) are presented in Table 1. Proximate

101

analysis was detected according to the Chinese standard (GB/T 28731-2012), and

102

elemental analysis was performed using the Perkin-Elmer 2400 Series CHNS/O

103

Elemental Analyzer. The higher heating value (HHV) was calculated by the equation:

104

HHV=0.339C+1.443(H−0.125O)−0.0224(9H)+0.0093S+0.001464N (Wang et al.,

105

2018a).

106

Different types of sulfur-containing compounds were confirmed by X-ray

107

photoelectron spectroscopy (XPS). XPS spectra was obtained by a Kratos Axis Ultra

108

DLD spectrometer using non-monochromatic Al Kα radiation and a system pressure

109

of 1×10−8 Pa. Moreover, the mineral compositions of SS and hydrochar were

110

determined using X-Ray powder diffraction (XRD, XPert3 Powder, Malvern

111

Panalytical). The scanned area ranged from 3° to 80° in the 2θ range with a 0.02° step

112

size and 0.02 s step time.

113

The concentration of sulfate ions in the liquid phase was quantified with barium

114

chromate spectrophotometry. Before the test, a 1 mL liquid sample was diluted to 50

115

mL to measure the concentration of sulfate ions in the diluted sample. Sulfide was

116

measured by the methylene blue method. In this study, the experiments were carried

117

out thrice, and the data was averaged. Organic compounds in bio-oil were detected by

118

a gas chromatograph equipped with a mass selective detector (QP2010, Shimadzu).

119

The ion source was 250

120

temperature was 40

121

of 4

122

2013).

123

3. Results and discussion

124

3.1 Properties of SS and hydrochar

/min.

, and the carrier gas was helium. The initial oven

with a 4 min residence time, and then raised to 300

at a rate

This method was described in detail in a previous study (Tian et al.,

125

The properties of SS and hydrochar are presented in Table 1. As shown in Table

126

1, hydrochar had lower O/C ratios than those of SS, which resulted from the loss of

127

oxygen-containing functional groups and irreversible dewatering (Mursito et al.,

128

2010). Similar results can be found in previous research. Previous studies claimed that

129

the H/C and O/C ratios decreased continuously when the HTC temperature was raised

130

from 150

131

SS and hydrochar remained at similar levels or decreased slightly because a small

132

amount of carbon transferred into the liquid phase or was released as carbon oxides

to 380

(Mursito et al., 2010; Wu et al., 2015a). The carbon contents in

133

(Berge et al., 2011). As shown in Table 1, when temperature increased from 180

134

300

135

of volatile matter (VM) decreased by 7.1-14.8% , indicating that the increase in HTC

136

temperature caused severe carbonization and devolatilization. Notably, the hydrochar

137

yield declined after HTC as well. This decreased yield is related to dehydration,

138

decarboxylation, and devitalization reaction (Zhai et al., 2017). Additionally, the HHV

139

and sulfur content of hydrochar increased to their maximum value of 3.02% and

140

32.09%, respectively.

141

Table 1

142

Elementary composition and fuel properties of SS and hydrochar.

Sample

, the percentage of fixed carbon (FC) increased to 530% (300

Elemental analysis (%) C

H

SS

47.01

SS-180

a

), whereas that

Proximate analysis (%)

H/C

O/C

b

FC

Ash

to

c

HHV

Yield

(MJ/kg)

(%)

O

N

S

6.25

10.50

1.21

2.68

0.13

0.22

0.70

35.03

64.27

22.21

/

46.95

6.06

5.76

0.93

2.96

0.13

0.12

0.43

40.30

59.27

22.38

82.42

SS-200

46.41

6.52

8.02

1.03

2.96

0.14

0.17

2.27

38.02

59.71

22.54

85.11

SS-220

47.69

6.17

4.40

0.86

2.85

0.13

0.09

1.79

40.88

57.33

22.88

84.78

SS-240

47.46

4.98

6.59

0.70

2.47

0.12

0.14

2.95

40.27

56.78

17.98

84.91

SS-260

45.43

5.62

6.95

0.84

2.99

0.12

0.15

4.09

41.16

54.75

21.19

80.76

SS-280

47.28

5.82

3.14

0.85

2.97

0.12

0.07

2.30

42.91

54.79

22.43

72.94

SS-300

44.35

6.12

8.14

0.86

3.54

0.14

0.18

4.41

40.53

55.06

21.35

69.26

143

a

Calculated by deference: O%=100%−(C%+H%+N%+ S%)−ash%.

144

b

FC is fixed carbon.

145

c

VM is volatile matter.

146

d

HHVs=0.339C+1.443(H−0.125O)−0.0224(9H)+0.0093S+0.001464N.

147

3.2. Analysis of the liquid phase

148

3.2.1 Sulfate ion and sulfide in aqueous fraction

149

VM

d

The sulfur-containing species in the aqueous phase mainly sulfate ion and sulfide,

150

and their concentrations at different HTC temperatures are summarized in Fig. 1. The

151

sulfide concentration fluctuated in the range from 1.15 to 3.40 mg/L. When the

152

reaction temperature was increased to 200

153

increased sulfide possibly originated from the decomposition of organic sulfur in solid

154

phase. As the temperature increased from 220 to 300

155

general decreased trend. A possible cause for this trend was the formation of a

156

precipitate between sulfide and the metal iron. Similarly, the decreased sulfate

157

concentration at higher temperatures was also related to the formation of a sulfate

158

precipitate (such as CaSO4) on the hydrochar surface (Fontboté et al., 2017; Zhuang et

159

al., 2018). Sulfate ion in the liquid phase originated from three main sources. First,

160

sulfur dioxide produced by HTC could dissolve into the liquid phase (Wu et al.,

161

2015b). Second, SO2 was also produced by decomposition of sulfates (e.g. FeSO4)

162

(Wu et al., 2015b).

163

liquid phase and sulfate

, the sulfide concentration increased .The

, the sulfide content showed a

Lastly, soluble sulfate from hydrochar was transferred into the ion was released (Wu et al., 2015b).

Concentration (mg/L)

2000

180 220 260 300 1000

200 240 280

2000 1800 1600 1400 1200 1000 800 600 400

4

4

3

3

2

2

1

1

0

Sulfate ion

Sulfide

0

164 165 166

Fig.1. Sulfate ion and sulfide concentrations in the liquid fraction at different HTC

167

temperatures.

168

3.2.2 Composition analysis of bio-oil

169

Bio-oil was extracted by acetone on solid surfaces and dichloromethane in the

170

aqueous phase. Detailed results for bio-oil are summarized in Fig. S1. In this figure,

171

the main components of the solid phase were hydrocarbons. In addition, bio-oil also

172

contained a small amount of alcohol, ester, and ketone groups. After HTC, the

173

hydrocarbon content increased, whereas the alcohol content decreased. However, the

174

composition of bio-oil in the aqueous phase was different from that of the solid phase.

175

In the aqueous phase, nitrogen-containing species were more prevalent, whereas the

176

hydrocarbon content decreased compared to that of the solid phase. Furthermore, the

177

ketone and alcohol contents in the aqueous phase were higher than those of the solid

178

phase. Bio-oil from both solid and aqueous phases did not contain detectable organic

179

sulfur species, which indicated that the sulfur species in the raw materials were

180

converted into inorganic substances after HTC.

181

3.3. The effect of HTC temperature on the sulfur species of hydrochar

182

Sulfur-containing compounds in the tested SS included sulfone, sulfoxide,

183

thiophene, mercaptan, sulfide, and sulfate. Except for sulfate, the other compounds

184

were organic sulfur. The S2p spectra lines of several representative samples are shown

185

in Fig. 2. The binding energies at 162.2 eV, 163.3 eV, 164.1 eV, 166 eV, 168 eV, and

186

170 eV corresponded to mercaptan (S1), sulfide (S2), thiophene (S3), sulfoxide (S4),

187

sulfone (S5), and sulfate (S6), respectively (Li et al., 2009; Liu et al., 2012; Zhong et

188

al., 2018). The chemical structures of SS and hydrochar were uniform inside and

189

outside, and XPS could achieve sufficient sample testing depth. The proportion of

190

each peak area of the total area indicates the relative amount of the different

191

sulfur-containing compounds. Table 2 summarizes the percentage of sulfur-containing

192

species in hydrochar. The sulfur evolution trend for different temperatures is

193

presented in Fig. 3a.

194 195

Fig. 2. Distribution of sulfur species from XPS analysis.

196 197

Table 2 Percentage of sulfur species in SS and hydrochar. S1

S2

S3

S4

S5

S6

162.2 eV

163.3 eV

164.1 eV

166 eV

168 eV

170 eV

SS

24.19

39.44

22.19

0.06

14.05

0.07

SS-180

17.69

22.13

24.04

9.65

12.26

14.23

SS-200

16.33

17.11

29.42

7.02

12.27

17.85

SS-220

20.15

20.63

25.09

5.84

10.93

17.37

Sample

SS-240

20.86

18.81

22.94

4.80

12.84

19.75

SS-260

16.74

20.89

26.88

5.24

11.92

18.32

SS-280

15.02

19.65

33.49

2.04

10.31

19.48

SS-300

13.04

17.14

37.48

3.06

8.31

20.96

198 199

The main sulfur species in SS (Fig. 3a) were sulfide (~39%), mercaptan (~24%),

200

thiophene (~22%), and sulfone (~14%). There was a dramatic increase in sulfoxide

201

(9.65 %) and sulfate (14.23 %) when the HTC temperature was increased to 180

202

whereas the amounts of sulfide and mercaptan decreased rapidly. A speculative

203

explanation for this result is that HTC had a noticeable effect on aliphatic sulfur

204

compounds (mercaptan) and sulfide, transforming these compounds into sulfoxide

205

and sulfate. Zhang and coworkers reported that sulfate originated from the partial

206

oxidation of sulfide by oxygen and hydroxyl on the SS surface after thermal

207

treatment(Zhang et al., 2017). As the reaction temperature increased, thiophene

208

increased from 22.19 % (SS) to 37.48 % (HTC-300), and the sulfate content also

209

increased to 20.96 %. More vulnerable sulfur compounds (mercaptan, sulfide, sulfone,

210

and sulfoxide) were transformed into stable sulfur-containing compounds (Li et al.,

211

2015). The increase of thiophene sulfur was due to poly-condensation between sulfide,

212

sulfoxide, and sulfone (Wang et al., 2016; Zhuang et al., 2018). Another reason for the

213

increase in sulfate content was that sulfate and metal ions formed a precipitate in the

214

liquid phase, causing the amount of sulfate ion in liquid fraction to decrease (Fig. 1).

,

215 216

Fig.3. (a) Sulfur species in SS and hydrochar and (b) percentage of organic and

217

inorganic sulfur at different HTC temperature.

218

The proportion of organic and inorganic sulfur can be calculated on the basis of

219

Fig. 3a. Inorganic sulfur only included sulfate and the remaining sulfur species were

220

classified as organic sulfur (Fig. 3b). The organic sulfur content declined continuously

221

from 99.9 % (SS) to 79.0 % (HTC-300) with increasing temperature, which indicated

222

that the higher HTC temperature broke organic sulfur functional groups, and the

223

resulting soluble intermediate could form inorganic sulfur (Chen et al., 2016; Lin et

224

al., 2015). There was also a process where decomposition and transformation of

225

organic sulfur species occurred. The inorganic sulfur content (sulfate) had a large

226

increase at 180

227

slowly increase from 180

228

(mercaptan and sulfide) were more likely to form sulfate after HTC. The increased

229

sulfate content (i.e. CaSO4) was also proven by the XRD results in Fig. S2. Ultimately,

230

the sulfate fraction achieved its maximum value at 300

231

3.4. Sulfur mass balance

compared to that of SS. Subsequently, the sulfate content tended to to 300

, revealing that unstable organic sulfur species

.

232

Figure. 4 shows the sulfur mass distribution of SS in the solid, liquid, and gas

233

phases. Sulfur loss was mainly from the transfer process of solids and liquids products.

234

The difference between the amount of total sulfur and the amounts of sulfur in the

235

liquid and solid phases were considered the sum of sulfur in the gas phase and loss.

236

Sulfur content of the gas phase was not directly measured and was a topic of

237

investigation in our future work. The amount of sulfur in the solid, liquid and gas

238

phases varied with temperature. The amount of sulfur in the solid and liquid phases

239

steadily diminished with increasing temperature, whereas the amount of sulfur in the

gas phase showed the opposite trend. Sulfur species were possibly converted into the

241

gaseous phase with an increase of HTC temperature. Sulfur in the liquid phase was

242

mainly sulfate ions, and the decrease in the amount of sulfur in the liquid phase was

243

caused by the combination of sulfur with metal ion. In contrast, 90-95% of sulfur

244

remained as hydrochar, indicating that HTC only transferred a part of sulfur. The

245

majority of sulfur was preserved in the solid matrix.

Ratio of sulfur species in hydrothermal products

240

1.04 1.02 1.00 0.98 0.96 0.94 0.92 0.90

Gas+loss

Solid phase 0.039

0.051

Liquid phase

0.073

0.945

0.941

0.915

0.917

0.906

0.053

0.044

0.046

0.032

0.022

180

200

220

240

260

0.086

0.088

0.904

0.903

280

300

0.1

0.0

Temperature (°C) 246 247

Fig.4. Mass distribution of sulfur after HTC

248

3.5. Sulfur evolution route of SS during HTC.

249

Figure. 5 shows the variation of real sulfur species in SS and hydrochar at

250

different HTC temperatures, and the calculation method for this figure was based on a

251

technique in the literature (Zhuang et al., 2018). According to the results in Fig. 5,

252

data above, and relevant literature, a simplified sulfur evolution schematic diagram is

253

summarized in Fig. 6. This scheme is described as follows: As the temperature

increased, stable thiophene was produced by a poly-condensation reaction between

255

sulfide, sulfone, and sulfoxide (Zhuang et al., 2018). Unstable aliphatic compounds

256

(mercaptan and sulfide) in the solid fraction were transformed into sulfate at elevated

257

temperatures. Meanwhile, SO2 was liberated by the decomposition of sulfate, and H2S

258

was generated by the decomposition of sulfide and sulfone (Wu et al., 2015b). A small

259

amount of soluble sulfide combined with metal ions to form a precipitate. Sulfur

260

dioxide was released by HTC, and soluble sulfate on the hydrochar surface dissolved

261

into the liquid phase. Subsequently, sulfate ion was released, and sulfate ions could

262

combine with metal ion to form a precipitate (e.g. CaSO4).

Real content of sulfur species (wt%)

254

Mercaptan Thiophene

100

Sulfone Sulfate

Sulfide Sulfoxide

24.19

80

13.90 14.58 14.05

17.71

13.52

10.91

9.63

17.49

15.97

16.87

21.27

19.48

21.71

14.80

10.96

10.44 9.27

10.10

60

17.08

5.76

14.33

11.87

14.56 18.24

40

9.03

7.52

39.44 25.04 19.81

24.43

25.96

20 22.19

0 raw

11.73

15.19

14.72

16.77

7.95

5.97

4.95

4.08

180

200

220

240

260

14.21

14.52

280

300

263 264

Fig. 5. The variation of the real sulfur species in SS and hydrochar at various HTC

265

temperature.

266 267

Fig.6. Sulfur evolution route of SS after HTC

268

4. Conclusions

269

In this paper, upgraded SS was produced, and the evolution route of sulfur

270

species was summarized. The decrease of the O/C ratio was caused by the reduction

271

of oxygen-containing function groups (-OH). The amount of sulfur species in the

272

liquid fraction (mainly sulfate ions) continuously decreased until a minimum value

273

was achieved at 300

274

The amounts of stable thiophene and sulfate increased with increasing temperature.

275

The organic sulfur content decreased continuously (20.91%) as organic sulfur was

276

transformed into inorganic sulfur. The amount of sulfur in the solid and liquid phases

277

decreased with increasing temperature, whereas the amount of sulfur in the gas phase

278

presented opposite trend.

279

Declaration of competing interest

280

. Moreover, sulfoxide and sulfate were generated above 180

The authors declare that they have no conflict of interests.

.

281

Acknowledgments

282

This research was financially supported by the State Key Laboratory of pollution

283

control and treatment of petroleum and petrochemical industry (PPC2017003), a

284

project of the National Natural Science Foundation of China (No. 51679083), a

285

project of the National a key research and development project of Hunan Province

286

(2018WK2011).

287

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Highlights The sulfur species distribution and evolution in sewage sludge were investigated. Increasing temperature enhanced stable thiophene and sulfate content. Sulfate ion was identified as main sulfur species in liquid fraction Sulfur species depended on HTC temperature

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Corresponding Author. Tel.+86 731 8882 2829, Fax. +86 731 8882 2829.

E-mail Address: [email protected](Y.B. ZHAI)

Zhexian Wang: Writing- Original draft preparation, Methodology, WritingReviewing and Editing. Tengfei Wang: Writing- Reviewing and Editing, Supervision. Yunbo Zhai: Supervision, Validation Chuan Peng: Conceptualization, Investigation. Shanhong Li: Data curation, Investigation. Bei Wang: Visualization, Investigation. Xiangmin Liu: Visualization, Investigation. Caiting Li: Software, Supervision

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: