Pyrolysis of municipal solid waste with iron-based additives: A study on the kinetic, product distribution and catalytic mechanisms

Journal Pre-proof Pyrolysis of municipal solid waste with iron-based additives: A study on the kinetic, product distribution and catalytic mechanisms Qiang Song, Hongyu Zhao, Jinwei Jia, Li Yang, Wen Lv, Jiuwen Bao, Xinqian Shu, Qiuxiang Gu, Peng Zhang PII:

S0959-6526(20)30729-0

DOI:

https://doi.org/10.1016/j.jclepro.2020.120682

Reference:

JCLP 120682

To appear in:

Journal of Cleaner Production

Received Date: 22 September 2019 Revised Date:

20 November 2019

Accepted Date: 18 February 2020

Please cite this article as: Song Q, Zhao H, Jia J, Yang L, Lv W, Bao J, Shu X, Gu Q, Zhang P, Pyrolysis of municipal solid waste with iron-based additives: A study on the kinetic, product distribution and catalytic mechanisms, Journal of Cleaner Production (2020), doi: https://doi.org/10.1016/ j.jclepro.2020.120682. 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.

Qiang Song: Writing - Original Draft Hongyu Zhao: Resources Jinwei Jia: Investigation Li Yang: Formal analysis Wen Lv: Validation Jiuwen Bao: Methodology Xinqian Shu: Data Curation Qiuxiang Gu: Funding acquisition Peng Zhang: Conceptualization Ideas

Pyrolysis of municipal solid waste with iron-based additives: a study on the kinetic, product distribution and catalytic mechanisms Qiang Song1, Hongyu Zhao2, Jinwei Jia3, Li Yang4, Wen Lv3, Jiuwen Bao1, Xinqian Shu5, Qiuxiang Gu6, Peng Zhang1* 1

Center for Durability & Sustainability Studies of Shandong Province, Qingdao

University of Technology, 266033 Qingdao, P.R. China 2

School of Civil and Resource Engineering, University of Science & Technology

Beijing, 100083 Beijing, P.R. China 3

Central Research Institute of Building and Construction Co., Ltd., MCC Group,

Beijing, 100088 Beijing, P.R. China 4

Chinese Research Academy of Environmental Science, Beijing 100012, P.R. China

5

School of Chemical and Environmental Engineering, China University of Mining &

Technology, Beijing, 100083 Beijing, P.R. China 6

Key Laboratory of Coal Resources Exploration and Comprehensive Utilization,

Ministry of Land and Resources, 710000 Xi'an, P.R. China

Qiang Song1, E-mail: [email protected] Hongyu Zhao2, E-mail: [email protected] Jinwei Jia3, E-mail: [email protected] Li Yang4, E-mail: [email protected] Wen Lv3, E-mail: [email protected]

Jiuwen Bao1, E-mail: [email protected] Xinqian Shu5, E-mail: [email protected] Qiuxiang Gu6, E-mail: [email protected] Peng Zhang1*, [email protected], Tel.: +86 0532-85071202; Conflicts of interest There are no conflicts to declare.

1

Pyrolysis of municipal solid waste with iron-based additives:

2

a study on the kinetic, product distribution and catalytic

3

mechanisms

4

Qiang Song1, Hongyu Zhao2, Jinwei Jia3, Li Yang4, Wen Lv3, Jiuwen Bao1, Xinqian

5

Shu5, Qiuxiang Gu6, Peng Zhang1*

6

1

7

University of Technology, 266033 Qingdao, P.R. China

8

2

9

Beijing, 100083 Beijing, P.R. China

Center for Durability & Sustainability Studies of Shandong Province, Qingdao

School of Civil and Resource Engineering, University of Science & Technology

10

3

11

Beijing, 100088 Beijing, P.R. China

12

4

Chinese Research Academy of Environmental Science, Beijing 100012, P.R. China

13

5

School of Chemical and Environmental Engineering, China University of Mining &

14

Technology, Beijing, 100083 Beijing, P.R. China

15

6

16

Ministry of Land and Resources, 710000 Xi'an, P.R. China

17

* Corresponding author: Zhang Peng. Tel.: +86 0532-85071202;

18

E-mail address: [email protected]

19

Abstract

20

To realize highly efficient and environmentally friendly utilization of municipal solid

21

waste (MSW) and iron ore, we proposed a novel method for combining MSW

Central Research Institute of Building and Construction Co., Ltd., MCC Group,

Key Laboratory of Coal Resources Exploration and Comprehensive Utilization,

22

pyrolysis and iron ore reduction. The effects of two iron-based additives (iron ore

23

and iron oxide) on the pyrolysis characteristics of MSW were first investigated by

24

using TGA, and the kinetic results illustrated that the average activation energy of

25

MSW pyrolysis was 180.32 kJ/mol. By adding iron ore and iron oxide, the activation

26

energy decreased to 151.76 and 150.18 kJ/mol, respectively. Then, the product yield

27

and product composition of MSW were analyzed by a fixed-bed reactor, GC-MS and

28

GC. The fixed-bed reactor experiments of MSW pyrolysis indicated that the iron ore

29

and iron oxide acted as catalysts to change the yield and composition of pyrolysis

30

gas and tar, thereby promoting thermal cracking of MSW and showing a high

31

conversion rate for MSW pyrolysis (55.81 and 55.05%). The GC-MS and GC

32

analyses demonstrated that the two additives could significantly reduce the

33

heteroatomic compounds of pyrolysis tar and increase H2, CO and CO2 production.

34

Furthermore, the reduction of iron ore and the catalytic mechanism were analyzed by

35

H2-TPR, XPS and BET. The H2-TPR results showed that compared with the peak of

36

iron oxide, the characteristic peaks of iron ore shifted to a high temperature due to

37

being suppressed by minerals in the iron ore. XPS suggested that the MSW volatiles

38

led to an increase in the binding energy of Fe 2p3/2 and Fe 2p1/2 and a decrease in

39

the binding energy of O 1s during the reduction of iron ore. BET analysis indicated

40

that the high activity of the catalyst might be attributed to its high surface area.

41

Key words: Pyrolysis; Municipal solid waste; Iron-based additives Nomenclature MSW Municipal solid waste MSW+1 MSW with iron ore MSW+2 MSW with iron oxide

XRD H2-TPR TGA GC GC-MS HRTEM BET α k EA A R T β m m1 m2 m1 Y1 Y2 Y3 Y4 Y5 ANOVA I ALH ALO PHE PAH HAC MAH ARO 42

X-ray diffraction H2-Temperature programme reduction Thermogravimetric analysis Gas chromatography Gas chromatography-mass spectrometer High resolution transmission electron microscope Brunauer-emmett-teller Fractional conversion Rate constant Activation energy Pre-exponential factor Universal gas constant Temperature Heating rate (K/min) Mass of feedstock in fixed-bed reactor Mass of char Mass of tar Mass of water The yield of char The yield of tar The yield of water The yield of gas The pyrolysis conversion rate Variance analysis Addition rate Aliphatic hydrocarbon Aliphatic oxygenates Phenols Polycyclic aromatic hydrocarbon Heteratomic compounds Monocyclic aromatic hydrocarbon Aromatic oxygenates

1 Introduction

43

The amount of municipal solid waste (MSW) reached 2.15×108 tons in 2017

44

according to the China National Bureau of Statistics. The accumulation of MSW in

45

the open air causes health and environmental issues, such as pollution in the air, land

46

and water, due to bacteria and insects, so it is urgent to develop treatment,

47

management and disposal technologies for MSW (Chen et al. 2015). In general, the

48

disposal methods of MSW are landfill, compost and incineration. The landfill disposal

49

of MSW accounts for more than 90% of the total MSW in Beijing (Yao et al. 2019);

50

however, it has multiple problems, such as the presence of hazardous organic

51

compounds in the leachate and landfill gas (Fang et al. 2017; Bejgarn et al. 2015).

52

Composting is recognized as a biological decomposition and stabilization technology

53

for organic MSW, but it also suffers from a long production turnaround time and

54

occupation of land (Doña-Grimaldi et al. 2019; Sánchez et al. 2017). Furthermore,

55

composting MSW presents a large risk of greenhouse gas (e.g., CH4, CO and N2O)

56

emissions (Ermolaev et al. 2019). Although MSW incineration significantly decreases

57

the content of MSW, it also has the risk of releasing dioxin and acidic gas, and the ash

58

from MSW incineration is classified as hazardous for its high content of heavy metals

59

(e.g., Pb, Zn, Cu, Cd and Hg) (Alorro et al. 2008).

60

MSW pyrolysis is taken as a simple and easily conducted technology (Fu et al.

61

2018). MSW is a continual feedstock that provides season-wide usability for pyrolysis

62

production. Because of the versatile composition of MSW, energy is stored in the

63

form of chemical bonds between C, H and O, and cracking those bonds releases

64

energy to produce chemicals and fuels (e.g., gas, liquid and char). It has been reported

65

that the heating value of MSW production is approximately 20 MJ/kg (Sipra et al.

66

2018), which can be used in power generation, transportation and petrochemical

67

industries, and pyrolysis of MSW has higher energy recovery efficiency than those of

68

other thermochemical reactions. In addition, MSW pyrolysis has the characteristics of

69

a low emission of heavy metals, nitrogen oxides and sulfur oxides due to its reductive

70

atmosphere (Shen et al. 2018; Younan et al. 2016). Hence, easy availability, high

71

energy recovery efficiency and environmental friendliness are the advantages of

72

MSW as a feedstock for pyrolysis.

73

To date, studies have been extensively conducted on MSW pyrolysis

74

characteristics and their products, while other studies have focused on the effect of the

75

experimental conditions and feedstock components on MSW pyrolysis (Williams et al.

76

2013;Ramos et al. 2018 Lin et al. 2016). For example, Ma et al. (2019) investigated

77

the pyrolysis characteristics of five typical MSW components and the interaction of a

78

mixture of those components; Xue et al. (2015) studied the effect of pyrolysis

79

temperature on product yield, light gases and cracking of pyrolysis-oil. As an effective

80

pyrolysis method, catalytic pyrolysis of MSW has recently been increasing. For

81

example, Ates et al. (2013) observed the pyrolysis characteristics of MSW in the

82

absence of catalysts (e.g., Y-zeolite, β-zeolite, HZSM-5 and Al(OH)3), and the results

83

showed that the added catalysts promoted the volatile matter of MSW and converted

84

aliphatic hydrocarbons to aromatic; Sebestyen et al. (2017) demonstrated that the

85

decomposition temperature of plastic decreased markedly in the absence of an

86

HZSM-5 catalyst; Fang et al. (2016) compared the pyrolysis characteristics and

87

activation energy of MSW with and without three additives (MgO, Al2O3 and ZnO),

88

and a decreasing trend in the initial temperature and activation energy of MSW

89

pyrolysis was found. In addition to the above catalysts, the addition of biomass and

90

char (biochar and MSW char) to the pyrolysis of MSW has received attention for its

91

easy availability and catalytic reforming (Wang et al. 2017a; Zhao et al. 2017a).

92

Although studies on the catalytic pyrolysis of MSW have been conducted, the above

93

catalysts have suffered from various problems, such as being costly or easily

94

deactivated. The unpaired electrons and spare orbits in Fe cause the absorption of

95

active groups leading to pyrolysis conversion, thus, Fe-based catalysts are widely

96

recognized as an efficient and economic catalyst for the pyrolysis of coal, biomass,

97

sludge and so on. (Xu et al. 2018; Liu et al. 2017; Yu et al. 2018; Xing et al. 2017).

98

For example, Xu et al. (1989) compared the effect of four metal oxides on the volatile

99

cracking of coal. The results revealed that the abilities of metal oxides to crack

100

aliphatic

and

aromatic

hydrocarbons

were

in

the

following

order:

101

Fe2O3 >Al2O3 >CaO >SiO2. Shao et al. (2010) investigated the catalytic effects of

102

metal oxides on the pyrolysis of sewage sludge, and the results proved that the

103

additives accelerated the initial decomposition of sludge samples, and the decreasing

104

order of additives on the weight loss rate of the samples was as follows: Fe2O3 > CaO.

105

Liu et al. (2016) found that Fe2O3 facilitated the decomposition of MSW. Although

106

the application of Fe2O3 in promoting the conversion of coal, biomass and sludge has

107

been sufficiently investigated, the above studies of metal oxides on the effect of

108

sample pyrolysis were mainly focused on the kinetics, and the application of Fe or

109

Fe-based additions on the pyrolysis of MSW needs to be further investigated.

110

Moreover, the application of iron ore on the thermal cracking behaviors and product

111

distribution of MSW has rarely been reported. Iron ore, especially hematite, is widely

112

known to have high availability and low utilization in China due to its weak magnetic

113

strength and low grade (Li et al. 2010a). The disposal of MSW with iron-based

114

additives not only promotes the pyrolysis conversion of MSW by the catalytic effect

115

of iron but also the iron is reduced by the pyrolysis gas.

116

The aim of this paper is to investigate the effect of iron-based additives on the

117

thermal cracking behavior and product composition of MSW. MSW with pure iron

118

oxide and typical iron ore are systemically and comprehensively compared. First, the

119

effects of pyrolysis temperature, heating rate, residence time and rate of catalyst

120

addition on the conversion rate of MSW pyrolysis are conducted on a fixed bed

121

reactor to determine the optimum conditions. It is remarkable that MSW is a similar

122

concept to a supply chain (Hao et al. 2018; Awasthi et al.2019; Sayyadi et al. 2018;

123

Rabbani et al. 2019; Gharaei, et al. 2019a; Sayyadi et al. 2019; Tsao et al. (2015). In

124

this paper, a response surface methodology was proposed to optimize MSW pyrolysis

125

with iron ore. Then, the kinetic parameters, gas and tar composition with iron ore and

126

iron oxide were investigated by TGA, GC and GC-MS. Furthermore, the reduction

127

rules of the two iron-based additives were tested by XRD, H2-TPR and TEM. Finally,

128

the mechanisms of MSW with iron ore and iron oxide were investigated by XPS and

129

BET. The concept of utilizing disposal waste (MSW pyrolysis) with waste (iron ore)

130

as a means of recycling is developed for the high conversion of organic solid waste.

131

The disposal of MSW with iron-based additives can be helpful for obtaining fuels

132

(e.g., pyrolysis tar and syngas) and recycling weak magnetic iron ore with a low

133

grade.

134

2 Materials and Methods

135

2.1 Materials

136

In this paper, MSW from the Datun transfer station in Beijing of China was

137

obtained. The inorganic part of MSW was sorted out and then dried at 105 °C for 10

138

hours by a vacuum dryer (Zhongxingweiye, DF-6020). Organic components of four

139

samples were homogeneously mixed and broken by a crusher (Tiandishouhe, FS-100),

140

then the samples were sieved by passing through a 100-mesh standard screen. The

141

composition, proximate and element analyses of MSW were performed according to a

142

previous paper (Song et al. 2018), and the results are shown in Table 1. Iron oxide

143

(Fe2O3) was purchased from Sinopharm Chemical Reagent Co., Ltd. (CAS number:

144

309-37-1), and iron ore (hematite) was collected from Tulufan, Xinjiang Uygur

145

Autonomous Region of China. The chemical composition analysis of iron ore was

146

based on the national standard GB/T 6730, and the results are shown in Table 1. The

147

pyrolysis experiments of MSW with iron ore and iron oxide are termed MSW+1 and

148

MSW+2, respectively.

149

2.2 Experiment devices and methods

150

2.2.1. Thermogravimetric experiment

151

In this paper, a thermogravimetric analysis (Mettler Toledo, TGA2-SF) was used

152

to investigate the pyrolysis characteristics of MSW. Before the TG analysis, the MSW

153

was homogeneously mixed with iron ore and iron oxide, and the additive amount of

154

iron ore and iron oxide was both 5%. During the pyrolysis process, approximately 15

155

mg of samples was measured with a high-purity N2 flow of 40 mL/min, and the final

156

temperature was set at 900 °C with a heating rate of 20 °C /min.

157

In this paper, the Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS)

158

methods were applied to obtain the parameters of MSW pyrolysis. The details of the

159

pyrolysis kinetic model were based on a previous study (Fong et al. 2019; Singh et al.

160

2020; Zhao et al. 2018). Equations (1) and (2) represent the FWO and KAS methods.

161

In Equation (2), the activation energy of FWO was obtained by ln(β) against 1/T, and

162

the activation of KAS was obtained by ln(β/T2) against 1/T. A pre-exponential value

163

(A) was obtained from the intercept. The results of the kinetic analysis are shown in

164

Table. 4.

ln(β ) = ln(

ln(

β T

2

0.0048 AEA E ) −1.502 A Rg (a) RT

) = ln

AR E − A EA g (a) RT

(1)

(2)

165

In the above equations, α, β, T are the pyrolysis conversion rate of MSW, heating

166

rate and thermodynamic temperature (K), respectively; A and EA represent the

167

frequency and pyrolysis activation energy, respectively; and R is the universal gas

168

constant 8.314 J/mol•K.

169

2.2.2. Fixed-bed pyrolysis experiments

170

To investigate the effect of iron ore on the yield of MSW and product

171

composition, a laboratory fixed-bed device (Fig. 1) was designed and used, and the

172

parameters of the device can be seen in our previous research (Song et al. 2019a).

173

Before each test, high-purity N2 with a flow rate of 40 ml/min  was injected into the

174

quartz tube to maintain an inert atmosphere for reactions. The reaction temperature

175

was set at 300, 375, 450, 525, and 600 °C, respectively, with a heating rate of

176

10-30 °C /min. The pyrolysis samples were incubated for 5-45 min after the final

177

temperature was reached and then cooled down with N2. Char products were collected

178

and weighed after being cooled to room temperature. Pyrolysis tar was collected with

179

dichloromethane and then distilled by a rotary evaporator and analyzed by GC-MS

180

(Thermo Fisher Orbitrap

181

analyzed by GC (Shjinmi GC112A). The detailed parameters of GC-MS and GC can

182

be seen in our previous research (Zhao et al. 2019a). The product yield was calculated

183

as follows:

TM

). The gas products were collected in a gas bag and then

m1 × 100% m m Y2 = 2 × 100% m m Y3 = 3 × 100% m Y1 =

Y4 =100-Y1 −Y2 −Y3 Y5 =100-Y1

(3) (4) (5) (6) (7)

184

Where Y1, Y2, Y3 and Y4 are the yield of pyrolysis char, tar, water and gas,

185

respectively; Y5 is the pyrolysis conversion rate; and m1, m2, m3 and m represent the

186

mass of char, tar, water and feedstock, respectively.

187

2.2.3. Analysis of iron ore and iron oxide characteristics

188

To analyze the mechanistic iron catalysis and its reduction pathway, the crystal

189

structures of the samples were analyzed by X-ray diffraction (XRD, PANalytical

190

X’Pert PRO). To investigate the redox properties of the samples, H2-TPR

191

(Micromeritics, Auto Chem Ⅱ 2920) was conducted in this paper. In addition,

192

transmission electron microscopy (TEM, JEM 2100F) and X-ray photoelectron

193

spectroscopy (XPS, Escalab 250Xi) analyses were used to investigate the surface

194

microstructure and elemental speciation of the samples. In Figs. 8-10 and Table 5, the

195

iron ore and iron oxide before and after the reaction were termed iron ore-0, iron ore-1

196

and iron oxide-0, iron oxide-1, respectively.

197

3 Results and Discussion

198

3.1 Effect of pyrolysis conditions on MSW conversion

199

3.1.1. Fix-bed reactor experiments of MSW pyrolysis

200

Fig. 2(a), (b) and (c) shows the conversion rate as a function of temperature,

201

heating rate, and holding time during MSW pyrolysis. In Fig. 2(a), the pyrolysis

202

temperature was 300, 375, 450, 525 and 600 °C, the heating rate was 10 °C/min, and

203

the residence time was 25 min. As shown in Fig. 2(a), the conversion rate increased as

204

the temperature increased to 525 °C (46.32%). A decrease in the conversion rate was

205

observed after 525 °C owing to a secondary cracking of organic volatiles (Yue et al.

206

2019b; Zhao et al. 2017b). In Fig. 2(b), the conversion rate of pyrolysis initially

207

increased with increasing heating rate (<20 °C/min), reaching a maximum of 49.32%

208

at 20 °C/min. In Fig. 2(c), the conversion rate of pyrolysis started to increase with

209

increasing holding time (<35 min) because promoting the holding time is conducive

210

to the release of volatile matter and thus to increasing the conversion rate (Yu et al.

211

2018); the maximum conversion rate of MSW pyrolysis reached 49.83% at 35 min.

212

The effects of iron ore and iron oxide on the conversion rate of MSW pyrolysis are

213

shown in Fig. 2(d). The results in Fig. 2(d) revealed that both iron ore and iron oxide

214

could promote the conversion rate of MSW pyrolysis. With the addition of up to 7.5%

215

iron ore and iron oxide, the conversion rates of MSW+1 and MSW+2 increased to

216

55.81 and 55.05%, respectively. According to previous research by Fu et al. (2016),

217

the addition of iron oxide significantly influenced thermal cracking of the feedstock,

218

which in turn enhanced its reactivity. In addition, the conversion rate of MSW showed

219

a difference with the same amount of iron ore and iron oxide, which might be

220

attributed to the difference in chemical composition and physical structure between

221

iron ore and iron oxide. As shown in the figure, the promotion effect of iron oxide was

222

greater than that of iron ore under a low concentration of additive, but it was opposite

223

under a high concentration of additive.

224

3.1.2 Response surface methodology experiments of MSW pyrolysis

225

In this paper, a central composite method (by Design Expert 10 software) was

226

applied to evaluate the interactions of the parameters according to previous research

227

(Sulaiman et al. 2018; Yuan et al. 2019). The pyrolysis temperature (T), heating rate

228

(β) and addition rate (I) were explored according to the results of Fig. 2. Table 2

229

shows the experimental design and result of MSW+1 and MSW+2 pyrolysis. As

230

shown in Table 2, the three pyrolysis parameters were categorized into four levels, i.e.,

231

temperature (483, 500, 525 and 550 °C), and 40 experiments were carried out

232

according to the experimental design for MSW+1 and MSW+2.

233

Table 3 is the variance (ANOVA) analysis of the data based on Table 2, and the

234

3D response surface image for conversion rate is shown in Fig. 3. As shown in the

235

table, for MSW+1, the F value of 23.81 revealed that the model fits well, with a

236

P-value Prob>F (P value) of less than 0.0001. The P value>F illustrated that the

237

T-temperature and I-addition rate are significant factors (<0.05), while the β-heating

238

rate is not significant. The P value analysis of Tβ, TI and βI showed that the

239

descending order of combination effects was TI (0.03256), Tβ (0.21558) and βI

240

(0.65963), and the interaction between the T and I factors was significant. For

241

MSW+2, the P value illustrated that T-temperature was a significant factor, while

242

β-heating rate and I-addition were insignificant. Based on the data in Table 2, two

243

quadratic models were proposed as follows: MSW + 1_Conversion rate = 55.71 + 0.97 ∗ T − 0.042 ∗ β + 0.81 ∗ I

244

+0.25 ∗ Tβ − 0.47 ∗ TI + 0.086 ∗ βI − 1.42 ∗ T ! − 0.52 ∗ β! − 0.85 ∗ I! (8) MSW + 2_Conversion rate = 55.79 + 0.85 ∗ T + 0.12 ∗ β − 0092 ∗ I

245

+0.28 ∗ Tβ + 0.29 ∗ TI − 0.034 ∗ βI − 1.34 ∗ T ! − 0.8 ∗ β! − 0.8 ∗ I! (9)

246

The R2 (MSW1, 0.9554 and MSW+2, 0.9572) proved that the quadratic model

247

agreed well with the experimental data (Noordin et al. 2004). Fig. 3 shows the 3D

248

response surface image for the conversion rate, Fig. 3(a)-(c) correspond to MSW+1,

249

and Fig. 3(d)-(f) correspond to MSW+2. The optimal level and interaction of the

250

parameters can be concluded from the image. The convex surface of the 3D response

251

surface image indicated a maximum value for the variable (Gao et al. 2010). The

252

figure suggested that for MSW+1, the interaction between factor T and β was

253

significant, while the interaction between β and I, and between T and I were almost

254

insignificant. According to previous research, the maximum conversion rate could be

255

obtained by the surface confined in the smallest ellipse (Tanyildizi et al. 2005), and

256

the predicted maximum value of the model reached 56.01% (MSW+1) at 532.05 °C

257

with a heating rate of 20.15 °C/min and an addition of 8.01%. The maximum value of

258

the model reached 55.94% (MSW+2) at 533.25 °C with a heating rate of

259

20.33 °C/min and an addition of 7.5%. Then, verification experiments were

260

performed, and the maximum conversion rate of MSW+1 and MSW+2 that could be

261

reached were 56 and 55.9% with the above conditions, which indicated that the

262

experimental data was close to the theoretical data and the quadratic model fit well.

263

3.2 Thermal degradation of MSW and its products

264

3.2.1. TG analysis of MSW pyrolysis

265

The thermal cracking behaviors of MSW with and without the presence of iron

266

ore and iron oxide were tested by TGA, and Fig. 4 shows the TG curves of the

267

samples at heating rates of 10, 20 and 30 °C/min. According to Fig. 4, the pyrolysis

268

process of MSW could be divided into three stages: volatility of small molecular

269

gases and moisture (from room temperature to 200 °C), decomposition of

270

macromolecular organic compounds (from 200 to 600 °C) and decomposition of

271

minerals (from 600 to 800 °C) in MSW. The TG curves of the three samples were the

272

same as the temperature increase. In the first and last stages of MSW pyrolysis, a

273

slight weight loss was observed, while samples exhibited a major weight loss in the

274

second stage. In the first stage, the weight loss of MSW, MSW+1 and MSW+2 were

275

4.11-5.19%, 3.51-5.44% and 1.80-5.51%, respectively. The added iron ore and iron

276

oxide had no significant influence on the vaporization of moisture in MSW. In the

277

second stage, as the temperature reached 600 °C, the weight losses of MSW, MSW+1

278

and MSW+2 were 58.22-59.34%, 59.98-66.75% and 59.98-64.94%, respectively. The

279

above results proved that iron oxide and iron ore could enhance the pyrolysis

280

conversion of macromolecular organic compounds in MSW. Fig. 4(b) and (c) also

281

illustrated that the weight loss of MSW+1 was higher than that of MSW+2, indicating

282

that Fe2O3 had a greater impact than that of the other minerals in iron ore. In addition,

283

a right shift of the TG curve was observed when the heating rate increased from 10 to

284

30 °C/min, which was consistent with previous research (Singh et al. 2020; Song et al.

285

2019b). This might be due to the influence of heat transfer at various heating rates that

286

led to a delayed decomposition (Wu et al. 2019a).

287

The kinetics of MSW were investigated by using two typical kinetic methods

288

(FWO and KAS), and the results are shown in Fig. 5 and Table 4. According to the

289

results of KAS (Table 4), the average EA of MSW was 179.62 kJ/mol, while with the

290

presence of iron oxide and iron ore, the average EA of MSW was 154.56 and 153.99

291

kJ/mol, respectively. The average EA of MSW, MSW+1 and MSW+2 were 180.32,

292

151.76 and 150.18 kJ/mol, respectively (by FWO). These results illustrated that the

293

presence of the two iron-based additives reduced the activation energy of MSW

294

pyrolysis, and the decrease of the activation energy indicated the catalytic influence

295

in the pyrolysis process and the potential for transforming MSW into chemicals and

296

fuels. The value of A represents the degree of collision of the MSW pyrolysis

297

reaction in each minute. As shown in Table 4, for individual MSW pyrolysis, the

298

average A values were 1.01×1019 and 1.60×1019S-1, from the KAS and FWO kinetic

299

models, respectively; the results of MSW with the presence of iron oxide were

300

3.47×1018 and 2.41×1018 S-1, respectively, and the A values of MSW with the

301

presence of iron ore were 1.77×1018 and 9.05×1017 S-1, respectively. A low A value

302

indicated that less heat was required for molecular collision. As shown in Table 4,

303

the correlation coefficients (R2) had a good linear plot between lnβ (and lnβ/T2) and

304

1/T at various conversions from 0.1-0.9, and the average R2 of KWO was higher

305

than that of KAS, which indicated that the FWO model gave the best prediction of

306

MSW pyrolysis.

307

3.2.2. Composition of MSW gas

308

The releasing characteristics of pyrolysis gas are shown in Fig. 6. As shown in the

309

figure, H2, CH4, CO and CO2 were the main components of MSW pyrolysis gases. It

310

has been reported that unpaired electrons and spare orbits in iron cause an absorption

311

of active groups, leading to a release of free radicals and gas (Chareonpanich et al.

312

1995). As shown in Fig. 6(a), both the addition of iron ore and iron oxide promoted

313

the yield of H2, which was consistent with that of previous research (Sato et al. 1989).

314

Furthermore, the addition of iron oxide had a greater effect on promoting H2 yield

315

than that of iron ore, which indicated that Fe2O3 was more active on promoting the

316

polycondensation of aromatic rings than that of the other minerals in iron ore. The

317

maximum H2 yield of MSW+1 and MSW+2 reached 10.41 and 11.32% at 10% iron

318

ore and 10% iron oxide, respectively. The CH4 of the pyrolysis gas mainly came from

319

the cracking of the aliphatic side chain (Wu et al. 2019b), and the addition of iron ore

320

and iron oxide had an insignificant influence on the CH4 yield.

321

The addition of iron ore and iron oxide significantly enhanced the yield of CO

322

and CO2. As the addition rate of iron ore and iron oxide reached 10 and 7.5%,

323

respectively, the CO yields of MSW+1 and MSW+2 reached maximum values of

324

25.88 and 24.84%, respectively. For CO2, the maximum values of MSW+1 (with 10%

325

iron ore) and MSW+2 (with 10% iron oxide) reached 22.77 and 21.03%, respectively.

326

CO and CO2 mainly came from the decomposition of oxygen-containing functional

327

groups, and the yield of CO and CO2 was higher than that of H2 and CH4, which

328

indicated that MSW contained a relatively high content of oxygen. In addition, the

329

changing trend of gas production was not in accordance with the conversion rate of

330

MSW in Fig. 2(d), which illustrated that the gas of MSW pyrolysis came not only

331

from the primary pyrolysis process but also from the secondary thermal cracking of

332

the volatiles.

333

3.2.3. Chemical composition of MSW tar

334

The products of MSW, MSW+1 and MSW+2 with better pyrolysis conversion

335

rates were selected for further analysis by GC-MS and GC. Fig. 7 shows the GC-MS

336

analysis of MSW pyrolysis tar, and the chemical compounds of MSW, MSW+1 and

337

MSW+2 are shown in Appendix A. Concentration information from GC-MS is given

338

with the help of pure standards. In Fig. 7(b), the chemical compounds of MSW

339

pyrolysis tar were divided into seven kinds: aliphatic hydrocarbon (ALH), monocyclic

340

aromatic hydrocarbon (MAH), polycyclic aromatic hydrocarbon (PAH), phenols

341

(PHE), heteroatomic compounds (HAC), aliphatic oxygenates (ALO) and aromatic

342

oxygenates (ARO). As shown in Fig. 7(b), the tar mainly consisted of ALH, MAH and

343

HAC (nitrogen-containing compounds and sulfur-containing compounds). The

344

addition of iron ore and iron oxide both enhanced the concentration of ALH and MAH

345

and decreased the concentration of HAC, indicating that the addition of iron ore and

346

iron oxide were beneficial to the decomposition of HAC and that HAC decomposed

347

to form ALH and MAH. Virginie et al. (2010) found that iron could break the C-C and

348

C-H bonds, and Kandel et al. (2014) reported that short-chain aliphatic hydrocarbons

349

underwent cyclization and aromatization to form aromatics with the presence of iron.

350

Therefore, the reaction pathway of HAC, ALH and MAH might be that the addition of

351

iron ore and iron oxide increased the cracking of the C-C and C-H of HAC, which

352

caused the enhancement of ALH and MAH. Thus, the cyclization and aromatization

353

of ALH contributed to the increase of MAH (Fig. 7d). From Fig. 7(b), we can also

354

conclude that the addition of iron ore decreased the concentrations of PHE, ALO and

355

ARO. The concentrations of PHE, ALO and ARO in MSW were 2.39, 5.14 and 2.96

356

×107, respectively, while the concentrations of PHE, ALO and ARO in MSW+2 were

357

0.20, 1.82 and 0×107, respectively. The decrease of the above three oxygen-containing

358

compounds might be caused by direct deoxygenation or hydrogenation and then

359

deoxygenation over iron oxide (Bunch et al. 2002; Zeng et al. 2018). As shown in Fig.

360

7(b), a low concentration of PAH was detected in MSW+2, while no PAH was

361

detected in MSW and MSW+1, indicating that the iron oxide had an effect on

362

promoting the condensation of MAH, thus accelerating PAH performance (Zhang et

363

al. 2019b). For ALO and ARO, the addition of iron oxide enhanced the concentration,

364

which can be attributed to the cracking reaction of HAC over iron oxide (Zhang et al.

365

2019c). Fig. 7(c) shows that the relative concentration of carbon chains with less than

366

5, 5-10 and higher than 10 carbons in MSW was 40.44, 22.17 and 5.77×107,

367

respectively. MSW exhibited a higher fraction of carbon chains with less than five

368

carbons compared to those of MSW+1 and MSW+2. Compounds with carbon chains

369

with less than 5 carbons in MSW were mainly composed of acethydrazide (C2H6N2O),

370

and the compounds with carbon chains with higher than 5 and less than 10 carbons in

371

MSW+1 and MSW+2 were mainly composed of ALH and MAH, which indicated that

372

the breaking of acethydrazide contributed to the production of ALH and MAH with

373

the presence of iron ore and iron oxide.

374

3. 3 Mechanism analysis

375

3.3.1 Phase transformation of iron ore and iron oxide

376

XRD and TPR were applied in this part to investigate the effect of pyrolysis

377

volatiles on the phase composition of the additives; the results are shown in Fig. 8. In

378

Fig. 8(a), the iron ore and iron oxide before and after the reaction were termed iron

379

ore-0, iron ore-1, and iron oxide-0, iron oxide-1, respectively. The XRD results

380

demonstrated minerals consisting of SiO2 and Fe2O3 in the iron ore. The iron-based

381

oxides underwent reduction with the presence of reductive gases (e.g., H2, CO and

382

CH4), and the resulting phase transformation caused the appearance of Fe3O4 and Fe;

383

thus, the reduction process of Fe2O3 was in the sequence of Fe2O3→Fe3O4 →FeO

384

→Fe (Wang et al. 2017b). From Fig. 8(a), we can learn that during the synergistic

385

disposal of MSW and the iron-based catalyst, the Fe3O4 and Fe intensity of iron oxide

386

was higher than that of iron ore, which revealed that a deeper reduction of iron oxide

387

occurred than that of iron ore by the reductive gases. To understand the redox

388

properties of the samples, H2-TPR was conducted in Fig. 8(b). As shown in the figure,

389

the pure iron oxide sample had reduction peaks belonging to Fe2O3 (Kim et al. 2010).

390

Two obvious peaks at 341 and 548 °C occurred in the H2-TPR, and they represented

391

the reduction of Fe2O3 into Fe3O4 and the reduction of Fe3O4 → FeO→ Fe,

392

respectively (Zhou et al. 2017). Compared with the peak of iron oxide, the

393

characteristic peaks of iron ore shifted to a high temperature. The reduction of Fe2O3

394

into Fe3O4 and the reduction of Fe3O4 → FeO→ Fe were at 577 and 877 °C,

395

respectively, which indicated that the minerals (e.g., CaO, Al2O3 and SiO2) in iron ore

396

inhibited the reduction of Fe2O3. In this paper, the pyrolysis temperature was set at

397

approximately 550 °C, so the reduction of Fe2O3 into Fe3O4 and the reduction of

398

Fe3O4 → FeO→ Fe occurred in iron oxide, while only the reduction of Fe2O3 into

399

Fe3O4 occurred in iron oxide, so this might explain the occurrence of Fe in the iron

400

oxide by XRD (Fig. 8a).

401

The microstructure of the four samples was tested using a high-resolution

402

transmission electron microscope (HRTEM), and the results are shown in Fig. 9. Fig.

403

9(a) and Fig. 9(b) revealed that nanoparticles of pure iron oxide and iron ore before

404

the reaction had similar shapes and strong lattice fringes, indicating high crystallinity

405

of the formed Fe2O3. The interplanar spacings of the (311) and (220) planes of iron

406

oxide-0 and iron ore-0 were 0.258 and 0.294 mm, respectively. In Fig. 9(c) and Fig.

407

9(d), the interplanar spacings of the (311) and (220) planes of iron oxide-1 and iron

408

ore-1 were 0.268 and 0.305 mm, respectively. The interplanar spacings of the (311)

409

and (220) planes were in accordance with those of previous research results (Yue et al.

410

2019a). The XRD analysis of the iron oxide-1 and iron ore-1 showed that the main

411

contributor of Fe-bearing phases was Fe3O4, and the similarity between the XRD

412

patterns of Fe2O3 and Fe3O4 indicated a similar crystal structure. The (440) planes of

413

iron ore-1 were observed with interplanar spacings of 0.158 nm in Fig. 9(d).

414

Compared with those of iron oxide-0 and iron ore-0, iron oxide-1 and iron ore-1

415

showed irregular surface lattice fringes, which indicated that the reduction of surface

416

FexOy-bearing phases occurred with the help of CO, CH4 and H2.

417

3.3.2 XPS analysis of iron ore and iron oxide

418

XPS was applied in this section to investigate the surface chemical state and

419

bonds of Fe and O; the results are shown in Fig. 10. The results of the Fe 2p spectra

420

show that the peaks of Fe 2p3/2 in iron ore-0, iron ore-1, iron oxide-0 and iron

421

oxide-1 are 710.70, 710.65, 710.58, and 710.56 eV, respectively. Compared with those

422

of iron ore-0 and iron oxide-0, iron ore-1 and iron oxide-1 have low intensity and low

423

binding energy. The decrease of intensity in iron ore-1 and iron oxide-1 may be due to

424

an absence of Fe on the surface of the samples and a valence state change of Fe. It has

425

been widely accepted that lower valence states of Fe have lower binding energies.

426

Therefore, the decreased binding energy of iron ore-1 and iron oxide-1 indicated that

427

a reduction reaction occurred in the iron oxide and iron ore. It is noteworthy that the

428

satellite peaks of Fe 2p3/2 and Fe 2p1/2 disappear in iron ore-1 and iron oxide-1, and

429

this may account for the counteraction of the satellite peak of an Fe2+O octahedron at

430

approximately 717 eV. The reduction of iron ore and iron oxide contributed to the

431

formation of Fe3O4, and the XPS analysis showed that iron ore-1 and iron oxide-1 are

432

composed of an Fe2+O octahedron, Fe3+O octahedron, and Fe3+O tetrahedron, while

433

iron ore-1 and iron oxide-1 consist of an Fe3+O octahedron and Fe3+O tetrahedron

434

(Yue et al. 2019a). The O 1s spectra are shown in Fig. 10(b). As shown in the figure,

435

the peaks of O 1s in iron ore-0 and iron oxide-0 were both 531.51 eV, and the peaks of

436

O 1s in iron ore-1 and iron oxide-1 were 531.90 eV. It has been reported that a shift of

437

O 1s peaks is associated with the content of Fe2O3 and Fe3O4 in the samples.

438

According to Fig. 10(b), the spectrum of the samples was symmetrical except for iron

439

ore-0, which indicated that goethite (FeHO2) existed with a lower electron density on

440

the surface of the iron ore (Rodríguez-Padrón et al. 2019).

441

3.3.3 BET analysis of iron ore and iron oxide

442

Table 5 shows the surface area and average pore diameter of iron ore and iron

443

oxide. The surface areas of iron oxide-0 and iron ore-0 were calculated as 23.466 and

444

9.677 m2/g, respectively, while the surface areas of iron oxide-1 and iron ore-1 were

445

calculated as 79.671 and 61.932 m2/g, respectively. From the results of the BET

446

analysis, we could infer that the high reactivity of iron oxide on promoting the

447

conversion of MSW might be attributed to its high surface area. The enhancement of

448

the surface area in iron ore and iron oxide was due to a carbon deposition on the

449

catalytic surface, and the carbon deposition on the surface of iron ore reduced the

450

active sites, thus reducing its reactivity (Loy et al. 2018). Table 5 also showed that the

451

reaction of MSW volatile matter with the catalysts reduced the average pore diameter,

452

and the decrease of average pore diameter indicated that the internal micropores of the

453

iron oxide and iron ore were blocked by carbon. From the above results, we could

454

conclude that the reaction of volatile matter with the catalysts not only produced a

455

carbon deposition on the catalytic surface but also in the micropores. The change of

456

surface area and pore diameter eventually reduced the activity of the catalysts.

457

MSW is a continual feedstock that provides season-wide usability for pyrolysis

458

production. The above results imply the potential reliability of MSW pyrolysis with

459

iron-based additives to produce high quality liquid fuel, high calorific value gas and

460

high-grade iron ore. Thus, MSW could be an alternative renewable resource to take

461

over traditional feedstocks for obtaining fuel. However, it is notable that a highly

462

effective utilization of MSW with iron-based additives desires to establish an

463

optimized supply chain model that can not only reduce uncertain feedstock supplies

464

but also ensure a low price and environmentally friendly treatment process. Although

465

an MSW supply chain has been extensively investigated in the past ten years (Dubey

466

et al. 2015; Duan et al. 2018; Gharaei et al. 2018; Kazemi et al. 2018; Rabbani et al.

467

2019), it also faces many problems. For example, for the MSW supply chain, the

468

challenge is a high cost, especially in areas where long distance transportation is

469

needed (Alizadeh et al. 2019; Gharaei et al. 2019b; Gharaei et al. 2019c; Giri et al

470

2014; Giri et al.2018; Sarkar et al. 2019; Shah et al. 2018; Shekarabi et al. 2019; Yin

471

et al. 2016), and the environmental impacts of MSW have also been the subject of

472

increasing attention. Therefore, further study will take into account all of the above

473

problems and extend the current scope of MSW pyrolysis.

474

4 Conclusions

475

The cooperative disposal of MSW with iron-based additives is reported in this

476

paper to realize highly efficient pyrolysis of MSW. The weight loss of MSW is

477

increased by the addition of iron ore and iron oxide, and the kinetic analysis indicates

478

that the FWO model gives the best prediction of MSW pyrolysis; the iron oxide

479

(151.76 kJ/mol) has a higher catalytic effect on MSW pyrolysis compared to that of

480

iron ore (150.18 kJ/mol). The response surface methodology experiments predicted

481

that the maximum value of pyrolysis conversion can reach 56.01% (with an addition

482

of 7.5% iron ore). Moreover, the added iron ore and iron oxide significantly promote

483

the concentration of aliphatic hydrocarbons and monocyclic aromatic hydrocarbons.

484

In particular, the addition of iron oxide significantly promotes the breaking of

485

acethydrazide. Furthermore, XRD analysis illustrates that the reduction of iron ore

486

and iron oxide are in the sequence of Fe2O3→Fe3O4 and Fe3O4→FeO→Fe, which has

487

also been verified by TEM, H2-TPR, and XPS analyses.

488

In the future, it is suggested that several types of uncertainties (e.g., seasonal

489

supplies, transportation cost and demand) exist in the disposal of MSW with

490

iron-based additives and should be considered. Therefore, a supply chain concept for

491

MSW should be considered to resolve the above problems.

492

Acknowledgements

493

This work was supported by the National Natural Science Foundation of China

494

(51922052, 51778309, 51704016, 51908307), Natural Science Foundation of

495

Shandong Province (ZR2018JL018), National Key Laboratory of Environmental

496

Protection in the Iron and Steel Industry (Yzc2017Ky03), Key Laboratory of Coal

497

Resources Exploration and Comprehensive Utilization, Ministry of Land and

498

Resources

(KF2019-6)

and

State

Key

Laboratory

of

Hydroscience

and

499

Engineering-Tsinghua University (SKLHSE-2019-C-04).

500

Declaration of interest

501

None

502

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Table 1. The composition, proximate and element analysis of the sample (wt %) MSW Proximate analysis (dry-basis) V A FC Ultimate analysis (daf) C H O* N S Main composition Kitchen waste Plastic Paper Fabric Bamboo Iron ore Total Fe FeO SiO2 Al2O3 CaO MgO MnO K 2O P S

*by difference

55.21 8.85 35.94 45.64 5.92 45.86 2.14 0.44 47.63 25.44 16.13 3.95 6.85

40.48 1.39 20.01 5.82 1.03 0.65 0.10 0.07 0.06 0.08

Table 2. Response surface experimental design and results run 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

T:temperature( ℃) 500 483 550 525 525 525 500 525 525 500 525 500 550 525 550 567 525 525 550 525

β:heating rate（℃/min） 17.5 20 17.5 20 20 20 22.5 20 20 17.5 15.8 22.5 17.5 20 22.5 20 20 24.2 22.5 20

I: addition rate (%) 8.75 7.5 8.75 7.5 7.5 7.5 8.75 5.4 7.5 6.25 7.5 6.25 6.25 9.6 8.75 7.5 7.5 7.5 6.25 7.5

MSW+1_Conversi on rate (%) 52.98 50.24 53.41 55.72 55.67 56 52.83 52.03 56.02 51.01 54.86 50.23 53.04 55.41 53.98 54 55.19 54.43 53.55 55.53

MSW+2-Conversi on rate (%) 51.56 50.45 53.04 56.22 55.88 56.07 51.54 53.76 55.8 52.46 53.92 52.45 52.64 53.77 54 54.04 55.28 53.63 53.86 55.43

Table 3. ANOVA results of MSW+1 and MSW+2 pyrolysis MSW+1

Sum of squares

df

Model T-temperature β-heating rate I-addition rate T2 β2 I2 Tβ TI βI residual Lack of fit Pure erroe Cor tal R2 Adj R-Squared C.V. %

61.91 12.86 0.02 8.95 28.91 3.95 10.43 0.51 1.78 0.06 2.89 2.41 0.48 64.80

9 1 1 1 1 1 1 1 1 1 10 5 5 19 0.9554 0.9153 1.00

6.88 12.86 0.02 8.95 28.91 3.95 10.43 0.51 1.78 0.06 0.29 0.48 0.10

23.81 44.52 0.08 30.97 100.06 13.67 36.09 1.75 6.15 0.21

P-value Prob＞F ＜0.0001 ＜0.0001 0.7788 0.0002 ＜0.0001 0.0041 0.0001 0.21558 0.03256 0.65963

5.00

0.05105

49.03 9.80 0.20 0.11 25.78 9.14 9.22 0.61 0.69 0.01 2.19 1.53 0.66 51.22

9 1 1 1 1 1 1 1 1 1 10 5 5 19 0.9572 0.9187 0.87

5.45 9.80 0.20 0.11 25.78 9.14 9.22 0.61 0.69 0.01 0.22 0.31 0.13

24.85 44.71 0.92 0.52 117.61 41.71 42.08 2.79 3.15 0.04

＜0.0001 ＜0.0001 0.35930 0.48545 ＜0.0001 ＜0.0001 ＜0.0001 0.12608 0.10633 0.84252

2.32

0.18881

MSW+2 Model T-temperature β-heating rate I-addition rate T2 β2 I2 Tβ TI βI residual Lack of fit Pure erroe Cor tal R2 Adj R-Squared C.V. %

Mean square F value

Table 4. Activation energy of sample pyrolysis according to TG analysis Sample

MSW

MSW+1

MSW+2

a 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Average 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Average 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Average

KAS

FWO 8 -1

2

EA(KJ/mol)

A(10 S )

R

103.95 105.78 148.65 175.50 167.81 192.98 205.50 242.98 273.41 179.62 92.77 86.49 138.62 131.72 142.95 159.68 155.82 231.52 251.42 154.56 98.32 98.98 115.14 136.69 154.17 184.24 191.25 204.62 202.49 153.99

1.15×102 2.36×102 4.35×103 9.34×106 4.56×106 6.13×107 5.47×1010 2.07×1011 3.44×1011 1.01×1011 7.36 3.86 7.36×102 4.44×102 2.62×103 8.33×105 6.76×105 9.32×1010 1.15×1011 3.47×1010 9.85 9.92 1.06×102 6.89×102 6.36×105 3.13×107 5.17×107 5.40×1010 5.21×1010 1.77×1010

0.9763 0.9700 0.9772 0.9798 0.9788 0.9885 0.9750 0.9165 0.8930 0.9617 0.9774 0.9381 0.9950 0.9884 0.9823 1.0000 0.9738 0.9467 0.9137 0.9684 0.9995 0.9963 0.9521 0.8916 0.9926 0.9971 0.9992 0.8774 0.9896 0.9662

-1

EA (KJ/mol )

A(108S-1)

R2

104.48 92.20 155.32 172.53 184.08 198.02 201.71 231.63 282.88 180.32 88.14 92.47 134.88 136.38 144.86 166.49 163.63 226.08 212.94 151.76 86.24 105.81 125.40 134.04 152.70 171.41 175.89 195.30 204.82 150.18

1.57×102 6.32 6.19×105 8.62×106 2.78×107 7.32×107 5.15×1010 1.29×1011 7.81×1011 1.60×1011 4.09 6.14 5.51×102 6.78×102 4.10×103 5.15×106 4.12×106 8.17×1010 6.28×1010 2.41×1010 3.22 2.42×102 3.85×102 6.32×102 5.52×105 7.11×106 9.78×106 7.74×107 5.42×1010 9.05×109

0.9906 0.9294 0.9753 0.9975 0.9735 0.9553 0.9975 0.9455 0.9056 0.9634 0.9810 0.9971 0.9895 0.9994 0.9600 0.9749 0.9612 0.9381 0.9457 0.9719 0.9959 0.9991 0.9669 0.9623 0.9978 0.9873 0.9600 0.9999 0.9129 0.9758

Table 5. BET analysis of iron ore and iron oxide Sample

BET surface area (m2/g)

Average pore diameter (nm)

Iron oxide-0 Iron ore-0 Iron oxide-1 Iron ore-1

23.466 9.677 79.671 61.932

5.01516 10.9736 4.91564 5.48349

N2

Quartz tube

Electric furnace

MSW

GC

Drying tower Catalyst

Ice-water mixture

Liquid nitrogen

Fig. 1. Schematic diagram of the self-design pyrolysis reactor

50

MSW

MSW 50

Conversion rate(%)

Conversion rate(%)

45

40

35

30

48

46

25 44

300

400

500

Temperature(

600

10

)

15

20

25

Heating rate(

(a)

30

/min)

(b) 57

MSW 50

MSW+1 MSW+2

56

Conversion rate(%)

Conversion rate(%)

55

49

48

47

54 53 52 51 50

46

49

0

10

20

30

40

Holding time(min)

(c)

0

2

4

6

8

10

Addition rate(%)

(d)

Fig. 2. Effect of experimental conditions on pyrolysis of MSW (a) temperature, (b) heating rate, (c) holding time, (d) addition rate

β

T

(a)

β

β

I

(b)

T

(d)

T

I

(c)

T

I

(e)

I

β

(f)

Fig. 3. 3D response surface image for conversion rate (a) heating rate and temperature (MSW+1), (b) addition rate and temperature (MSW+1), (c) addition rate and heating rate (MSW+1), (d) heating rate and temperature (MSW+2), (e) addition rate and temperature (MSW+2), (f) addition rate and heating rate (MSW+2)

100

10 20 30

MSW

/min /min /min

Weight loss(%)

80

60

40

20 0

200

400

600

Temperature(

100

800

)

10 20 30

MSW+1

/min /min /min

Weight loss(%)

80

60

40

20 0

200

400

Temperature(

100

600

800

)

10 20 30

MSW+2

/min /min /min

Weight loss(%)

80

60

40

20 0

200

400

Temperature(

600

800

)

Fig. 4. Thermogravimetric analysis of (a) pyrolysis of MSW, (b) pyrolysis of MSW with iron oxide, (c) pyrolysis of MSW with iron ore

-9.0

MSW a=0.1 a=0.2 a=0.3 a=0.4 a=0.5 a=0.6 a=0.7 a=0.8 a=0.9

Ln(β/T2)

-10.0

-10.5

-11.0

MSW

3.4

a=0.1 a=0.2 a=0.3 a=0.4 a=0.5 a=0.6 a=0.7 a=0.8 a=0.9

3.2

3.0

Lnβ

-9.5

2.8

2.6

2.4

2.2

-11.5 0.0012

0.0016

0.0020

0.0010 0.0012 0.0014 0.0016 0.0018 0.0020 0.0022 0.0024

0.0024

1/T(K-1)

-1

1/T(K ) -9.0

MSW+1 a=0.1 a=0.2 a=0.3 a=0.4 a=0.5 a=0.6 a=0.7 a=0.8 a=0.9

Ln(β/T2)

-10.0

-10.5

MSW+1

3.4

a=0.1 a=0.2 a=0.3 a=0.4 a=0.5 a=0.6 a=0.7 a=0.8 a=0.9

3.2

3.0

Lnβ

-9.5

2.8

2.6

-11.0

2.4

2.2

-11.5 0.0012

0.0016

0.0020

0.0010 0.0012 0.0014 0.0016 0.0018 0.0020 0.0022 0.0024

0.0024

1/T(K-1)

-1

1/T(K ) -9.0

MSW+2 a=0.1 a=0.2 a=0.3 a=0.4 a=0.5 a=0.6 a=0.7 a=0.8 a=0.9

Ln(β/T2)

-10.0

-10.5

-11.0

MSW+2

3.4

a=0.1 a=0.2 a=0.3 a=0.4 a=0.5 a=0.6 a=0.7 a=0.8 a=0.9

3.2

3.0

Lnβ

-9.5

2.8

2.6

2.4

2.2

-11.5 0.0012

0.0016

0.0020 -1

1/T(K )

0.0024

0.0010 0.0012 0.0014 0.0016 0.0018 0.0020 0.0022 0.0024

1/T(K-1)

Fig. 5. Kinetic plots of the samples using (a), (b) and (c) KAS, (d), (e) and (f) FWO

12

12

MSW+1 MSW+2

MSW+1 MSW+2

11

11

CH4(mL/g)

H2(mL/g)

10

9

8

10

9

8

7

6

7 0

2

4

6

8

10

0

Addition rate(%)

2

4

6

8

10

Addition rate(%) 24

MSW+1 MSW+2

26

MSW+1 MSW+2 22

24 20

CO2(mL/g)

CO(mL/g)

22

20

18

18

16

14

16

14

12

0

2

4

6

8

Addition rate(%)

10

0

2

4

6

8

Addition rate(%)

Fig. 6. The gas composition of MSW pyrolysis (a) H2, (b) CH4, (c) CO, (d) CO2

10

45

35

80

MSW MSW+1 MSW+2

25 20 15 10

10

15

20

25

Time(min)

30

35

50 40 30 20 10

0 5

MSW MSW+1 MSW+2

60

30

5

0

70

Concentration(107)

40

Concentration (107)

Relative abundance

MSW MSW+1 MSW+2

0 ALH MAH PAH PHE HAC ALO ARO

Chemical compouds

-5

6--10

10-

Carbon atom number

Fig. 7. The GC-MS analysis of samples (a) GC–MS total ion chromatogram, (b) chemical compounds, (c) carbon atom number of samples

1 Fe2O3 3 Fe 5 CaO 2 Fe3O4 4 SiO2 6 Al2O3

Iron oxide-0 Iron ore-0 Iron oxide-1 Iron ore-1

100

1

1

Intensity(a.u.)

Intensity(a.u.)

2

2

3

11

1

4 5

5

4

6

4

548 877

80

2 2

iron oxide-0 iron ore-0

60

341

577

40

20

4

0 20

30

40

50

2θ(°)

60

70

80

200

400

600

800

Temperature( )

Fig.8. Phase analysis of iron oxide and iron ore (a) XRD, (b) H2-TPR

1000

(a)

(b) Fe2O3

(c)

Fe3O4

(311)

0.268nm

(d)

Fig.9. HRTEM images of the samples (a) Iron oxide-0, (b) Iron ore-0, (c) Iron oxide-1, (d) Iron ore-1

50000

Iron ore-0 Iron oxide-0 Iron oxide-1 Iron ore-1

Intensity(a.u.)

40000

2p3/2 2p1/2

35000

2p3/2

30000

2p1/2 satellite

satellite

25000 20000

Iron ore-0 Iron oxide-0 Iron oxide-1 Iron ore-1

80000 70000 60000

Intensity(a.u.)

45000

50000 40000 30000 20000

15000 10000

10000 0

700

710

720

730

740

Binding energy(eV)

Fig.10. XPS spectra of the samples (a) Fe2p, (b) O1s

530

535

Binding energy(eV)

Iron-based additives significantly promote the pyrolysis conversion of MSW MSW with iron oxide shows lower activation energy compared to that with iron ore Iron-based additives lead to a decrease in heteroatomic compounds MSW volatiles lead to an increase in the binding energy of Fe 2p3/2 The high activity of the catalysts may be attributed to their high surface area

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: