CO2 Gasification of Municipal Solid Waste in a Drop-Tube Reactor

Mar 6, 2018 - Shanghai Urban Construction Design and Research Institute, Shanghai ... municipal solid waste (MSW) was studied in a drop-tube reactor...
0 downloads 0 Views 734KB Size
Subscriber access provided by - Access paid by the | UCSB Libraries

Environmental and Carbon Dioxide Issues

CO2-gasification of municipal solid waste (MSW) in a drop tube reactor: Experimental study, Thermodynamic analysis of syngas Xiaoyuan Zheng, Zhi Ying, Bo Wang, and Chong Chen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b04133 • Publication Date (Web): 06 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 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

1

CO2-gasification of municipal solid waste (MSW) in a drop tube

2

reactor: Experimental study, Thermodynamic analysis of syngas

3 4 5 6 7

Xiaoyuan Zheng1, Zhi Ying1*, Bo Wang1, Chong Chen2 1. School of Energy and Power Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China 2. Shanghai Urban Construction Design and Research Institute, Shanghai 200125, China *Corresponding author. [email protected]

8 9

Abstract

10

Gasification-based waste-to-energy technique has been considered as a

11

promising alternative to direct incineration. With the potential benefits of reducing the

12

greenhouse gas emission and producing the syngas, CO2-gaisification of municipal

13

solid waste (MSW) was studied in a drop tube reactor. Process parameters including

14

temperature and CO2/MSW mass ratio were investigated. Based on the experimental

15

results, energy and exergy analyses were conducted to evaluate the thermodynamic

16

quality. Results indicated that temperature had a significant impact on the syngas

17

composition, while the effects of CO2/MSW mass ratio were not so profound. The

18

tendency of the syngas composition revealed that Boudouard reaction, water gas

19

reaction and free radical combination reactions were the most influential reactions in

20

the gasification process. Energy and exergy analyses showed that the total energy and

21

exergy values of syngas increased with rising temperature, whereas they declined

22

initially and then rose with the increase in CO2/MSW mass ratio. The detailed energy

23

and exergy distributions of syngas component were different at various temperatures

24

and CO2/MSW mass ratios. Due to the remarkable difference between physical energy

25

and exergy of sensible heat, the exergy value of syngas was much lower than its 1

ACS Paragon Plus Environment

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

26

energy value. It followed well with the energy value.

27

Keywords

28

Gasification; Carbon dioxide; Municipal solid waste; Energy; Exergy

29

1 Introduction

30

The economy development and the consequent improvement in living standards

31

in the cities of China have resulted in an increasing production of MSW. Its annual

32

production in China was as high as 191 million tons in 2015 [1] and is predicted to be

33

510 million tons by 2025 [2]. The representative components of Chinese MSW

34

included food residue, plastics, paper, textiles, wood waste, rubber and

35

non-combustibles [3]. Therefore, thermal treatment has been proven to be a promising

36

waste-to-energy approach, which has been adopted by various countries and regions

37

[4]. For example, MSW incineration was widely used with the advantages of volume

38

reduction and energy recovery by the thorough destruction of combustible

39

components [5]. However, toxic substances such as dioxins and heavy metals, as well

40

as harmful residues had adverse impacts on the environment and human health [6].

41

On the other hand, a MSW incineration plant only had the energy efficiency of ca.

42

20%, which was much lower than that of a coal-fired power plant. As a consequence,

43

more and more people are against the MSW incineration.

44

In recent years, MSW gasification has emerged as a promising alternative for its

45

treatment, which was defined as the thermochemical conversion of MSW to useful

46

products (i.e., syngas, char and tar) [7-9]. Due to the reducing environment of

47

gasification process, it could reduce dioxins and NOx emissions. The gasifying agents 2

ACS Paragon Plus Environment

Page 2 of 30

Page 3 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

48

included air, steam, carbon dioxide (CO2), and their mixtures. The air gasification

49

produced a syngas with relatively low heating value due to the dilution effect of

50

nitrogen. Yet nitrogen-free syngas was preferable for synthesis of liquid fuel and

51

chemicals [10]. The steam gasification provided an effective way for hydrogen-rich

52

syngas production [11]. But its overall thermal efficiency was low because of the heat

53

loss of unreacted steam [12]. Introducing CO2 as gasifying agent resulted in a higher

54

thermal efficiency through the substantial rise in the final mass conversion by the

55

Boudouard reaction and the tar cracking reactions [13]. What’s more, using CO2 as

56

gasifying agent could provide a reliable way for converting CO2 into clean fuel and

57

favored reducing the emission of greenhouse gas to the atmosphere [14].

58

However, most of recent gasification investigations involving CO2 focused on

59

biomass, coal or blending of them [15-17]. Some studies aimed at the kinetic

60

behaviors of gasification under CO2 atmosphere, which were conducted with char

61

samples [18, 19]. To our knowledge, less study can be found about MSW gasification

62

under single CO2 atmosphere. Couto et al. adopted a developed and validated

63

numerical model to investigate MSW gasification with air-CO2 mixture in a

64

semi-industrial fluidized bed gasifier [20]. At thermogravimetry (TG) scale, Gu et al.

65

studied the effect of CO2 on the thermal degradation process of five representative

66

components of combustible solid waste [21]. The reactivities of char samples derived

67

from MSW components to CO2 were characterized in a fixed-bed reactor by Hla et al.,

68

indicating that gasification rates of wood waste and garden waste chars were

69

comparable to those from other biomass while the least reactive chars were produced 3

ACS Paragon Plus Environment

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

70

from textiles and printed paper [22]. Furthermore, it has been reported that there

71

existed the interactions among MSW components during their pyrolysis, which is the

72

initial step of gasification [23]. Therefore, in light of the composition differences

73

among biomass, coal and MSW, as well as to bridge the knowledge gap on CO2

74

gasification of real MSW, it is of significance to investigate the gasification

75

performance of practical MSW under single CO2 atmosphere without combining the

76

effect of other gasifying agents.

77

Besides, to better reflex the performance of waste gasification, energy and

78

exergy analyses were well-proven and effective thermodynamic methods [24]. Energy

79

analysis was based on the first law of thermodynamics, taking the energy value of all

80

input and output streams into considerations. It took thermal efficiency as the basic

81

index of energy assessments. However, energy was not only about volume, but it also

82

had a discrepancy of quality. Thus, thermal efficiency could only reflect the amount of

83

energy, instead of an all-around energy utilization status. With the realization of this

84

fact, the exergy theory was proposed in this study to incorporate both quantity and

85

quality of energy. Exergy was defined as the maximum amount of work that could be

86

obtained from a system when it comes to equilibrium with its reference environment

87

[25]. Exergy analysis was on the basis of the first and the second law of

88

thermodynamics. It could provide a reasonable, scientific and efficient method for

89

energy utilization and overcome the limitations of conventional energy analysis and

90

describes efficiency and performance of energy utilization [26].

91

So far, energy and exergy analyses have been successfully performed to evaluate 4

ACS Paragon Plus Environment

Page 4 of 30

Page 5 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

92

the thermodynamic performance of MSW management systems or to improve the

93

system efficiency. Kaushik et al. critically reviewed the application of energy and

94

exergy analyses to thermal power plants, revealing that energy and exergy analyses

95

were favorable to understand the performance of thermal systems [27]. However, the

96

evaluation of MSW gasification process, especially under sole CO2 atmosphere, is

97

quite lacking [28-31]. Therefore, it is meaningful to carry on a thorough energy and

98

exergy assessment and to better understand the thermodynamic performance of CO2

99

gasification-based MSW to the energy system.

100

In terms of the above-mentioned literature review and the growing interest of

101

using CO2 as a gasifying agent, the present study undertook the sole CO2 gasification

102

of real MSW in a drop tube reactor without the compounding effects of other

103

gasifying agents like steam or air, as well as conduct comprehensive energy and

104

exergy analyses on its thermodynamic performance. The effect of temperature and

105

CO2/MSW mass ratio on MSW gasification performance and the most influential

106

reactions dominating the chemistry of gasification process were addressed. The

107

detailed energy and exergy of syngas from CO2-gasification of MSW and the effects

108

of temperature and CO2/MSW mass ratio on the energy and exergy of syngas were

109

studied. The relationship between energy and exergy of syngas was determined. These

110

knowledge can assist in evaluating the potential of CO2 gasification as a

111

waste-to-energy pathway for energy recovery from MSW and provide scientific

112

experience to facilitate the future development of MSW treatment.

113

2 Materials and methods 5

ACS Paragon Plus Environment

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

114

Page 6 of 30

2.1 Experiments

115

The MSW sample used in this study was collected from a local city in China. Its

116

physical component included waste paper, plastics, woods, textiles, rubbers, food

117

residue and so on. Its properties are summarized in Table 1. The sample was dried,

118

crushed, milled and sieved to the size in the range of less than 0.110 inch.

119

Table 1 Properties of MSW (* ad represents air dried basis.) Items Proximate analysis (ad*, %) Moisture content Volatile matter Fixed carbon Ash content Ultimate analysis (ad*, %) C H O N S Lower heating value/ MJ•kg-1)

Values 5.06 59.34 8.36 27.24

(ad*,

46.15 5.71 11.86 3.67 0.31 18.59

120

As shown in Fig.1, a quartz drop tube reactor (DTR), which was 1.8m long,

121

25.4mm outer diameter and 19mm inner diameter, was used. An insulation collar at

122

the top and bottom of the furnace was assembled in order to block heat transfer from

123

the exposed end positions and to secure the quartz tube. Gasification temperatures

124

were achieved using a split-hinged vertical furnace. The MSW sample was

125

continuously introduced into the DTR using a screw feeder with feeding rate of

126

0.78g/min for all experiments in this study. Due to the limitation of the screw feeder,

127

the inert quartz sands were added into the sample with the mass ratio of sample to

128

quartz sand of 1:7 in order to alleviate the adhesiveness of pure sample and maintain 6

ACS Paragon Plus Environment

Page 7 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

129

continuous feeding.

130

All gases used in this study were of ultrahigh purity and supplied from the

131

cylinders. Their flow rates were controlled using mass flow controllers (MFC).

132

During the experiments, the flow rate of the produced gas could be controlled through

133

a set of a flow meter and a vacuum pump. After the system achieved its stability,

134

produced gas composition was continuously analyzed by a micro-GC. All the

135

experiment conditions were repeated in three times. The presented data in this paper

136

was the average values of the results.

Solid Screw Feeder

MFC

MFC

Electric Furnace

Micro-GC

Vacuum Pump

Dryer/ Filter CO2

N2 Char Collector

137 138

Fig.1 Schematic diagram of the experimental system

139

2.2 Theoretical methods

140

2.2.1 Energy analysis of syngas

141 142

Flow Meter

The total energy of a flow syngas can be described as the sum of various kinds of energy values [32]. 7

ACS Paragon Plus Environment

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 8 of 30

143

 =  +  +  + 

144

Where En is the total energy of syngas (kJ/kg);  ,  ,  , and 

145

represent the kinetic energy, potential energy, physical energy and chemical energy of

146

syngas (kJ/kg), respectively.

(1)

147

In general, kinetic energy and potential energy are small enough and, thus, can

148

be neglected in the analysis [33]. Therefore, the total energy of syngas can be

149

determined by its chemical composition (chemical energy, ) and physical state

150

(physical energy,  ). Eq. (1) is simplified as

151

 =  + 

152

(2)

The syngas produced from MSW CO2-gasification is a mixture of H2, CO, CO2,

153

CH4, N2 and so on. The physical energy of syngas is calculated as [34]

154

 = ∑  ℎ

155

Where xi and hi are the molar yield of syngas component i (mol/kg-msw) and the

156

specific enthalpy of syngas component i (kJ/kmol), respectively.

(3)

157

According to the specific enthalpy of syngas components at the environment

158

state (dead state) (temperature T0 = 298.15K and pressure P0 = 1 atm) specified in

159

Table 2, the specific enthalpy of syngas components under a given condition can be

160

calculated with the following equation [35]

161

ℎ = ℎ +   

162

where

163

 =  +  +   +  

164

Where h and h0 are the specific enthalpy of syngas components under a given



(4)



(5)

8

ACS Paragon Plus Environment

Page 9 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

165

condition and under the dead state (kJ/kmol), respectively. T is the temperature of

166

syngas components under a given condition. cP is the constant pressure specific heat

167

capacity (kJ/kmol·K). a, b, c, d represent the coefficients of constant pressure specific

168

heat capacity and are presented in Table 2.

169

The chemical energy of syngas is [32]

170

 = ∑  

171

Where HHVi is the higher heating value of syngas component i (kJ/kmol) and are

172

listed in Table 2.

173

2.2.2 Exergy analysis of syngas

174

(6)

The total exergy of a flow syngas can be expressed as the sum of all kinds of

175

exergy values [32].

176

 =   +   +   + 

177

Where Ex is the total energy of syngas (kJ/kg).   ,   ,   , and 

178

represent the kinetic exergy, potential exergy, physical exergy and chemical exergy of

179

syngas (kJ/kg), respectively.

180

(7)

Generally, kinetic exergy and potential exergy values are very small. They are

181

negligible in the analysis [33]. Thus, eq. (7) can be described as

182

 =   + 

183

(8)

The physical exergy of syngas can be calculated as [36,37]

184

  = ∑  ℎ − ℎ −  ! − ! "

185

where [35]

186

s = ! + 

187

Where s and s0 are the specific entropy values of syngas component i under a given

 $ 



(9)



 − % ln 

(10)



9

ACS Paragon Plus Environment

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 10 of 30

188

condition and under dead state (kJ/kmol·K), respectively. P is the pressure of syngas

189

component i under a given condition (Pa). R is the universal gas constant of

190

8.314472J/mol·K.

191

The chemical exergy of syngas is caused by the difference of its chemical

192

composition and concentration with dead state as [39]

193

 = ∑  ()  + % ln ∑ + ,

194

Where )  is the standard chemical exergy of syngas component i as presented in

195

Table 2 (kJ/kmol).

*

(11)

*+

10

ACS Paragon Plus Environment

Page 11 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

Energy & Fuels

196

Table 2 Coefficients of constant pressure specific heat capacitya, higher heating valuesa, standard chemical exergyb, specific enthalpya and

197

specific entropya of some syngas components at dead state Gas

a

b (×10-2)

c (×10-5)

d (×10-9)

temperature range (K)

HHV (kJ/kmol)

exch (kJ/kmol)

h0 (kJ/kmol)

s0 (kJ/kmol·K)

N2

28.90

-0.157

0.808

-2.873

273-1800

0

720

8669

191.502

H2

29.11

-0.192

0.400

-0.870

273-1800

285,840

236,100

8468

130.574

CO

28.16

0.168

0.533

-2.222

273-1800

282,990

275,100

8669

197.543

CO2 22.26

5.981

-3.501

7.469

273-1800

0

19,870

9364

213.685

CH4 19.89

5.024

1.269

-11.010

273-1500

890,360

831,650

-

-

a

Coefficient data, HHV, h0 and s0 for syngas components are obtained from ref [36].

components are obtained from ref [37].

11

ACS Paragon Plus Environment

b

The standard chemical exergy data of syngas

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

198

3 Results and Discussion

199

3.1 Effect of temperature on gasification performance

Page 12 of 30

200

In this section, effect of temperature on CO2-gasification was analyzed at

201

CO2/MSW mass ratio of 0.66. The total flow rate of N2 and CO2 kept constant at

202

1L/min.

203

Fig.2 shows averaged values of the main syngas components over the entire

204

temperature ranges in the steady state. The variations of their molar yields with

205

temperature demonstrated that temperature had a noticeable impact on the syngas

206

composition. Various reasons contributed to these changes, such as (1) influence on

207

the production in the initial pyrolysis step whose rate was faster at increasing

208

temperature [40]. The pyrolysis step contributed to the CO2, CO, CH4 and lower yield

209

of H2, which were derived from the primary decomposition of MSW and secondary

210

reactions of the volatiles. (2) The change in syngas components via the endothermic

211

char gasification, favored by higher temperatures. The molar yield tendency of each

212

syngas component was examined to postulate the possible dominant reactions, which

213

was discussed in detail below.

214

MSW gasification was a complex process. The main reactions (R1 – R9)

215

involving in this process are listed in Table 3 [15]. They occurred at the same time and

216

usually competed with each other.

217

Table 3 Main MSW gasification reactions Reaction no. R1 R2

Reaction name

Reaction chemistry

water gas Boudouard

C +  . ↔ 0. +  C + 0. ↔ 20. 12

ACS Paragon Plus Environment

∆H (MJ/kmol) +131 +172

Page 13 of 30

R3 R4 R5 R6 R7 R8 R9

methane steam reforming water gas shift methanation reactions

combustion combustion

02 +  . ↔ 0. + 3

+206

CO +  . ↔ 0. +  C + 2 ↔ 02 CO + 3 ↔ 02 +  . 2CO + 2 ↔ 02 + 0. C + 0.5. ↔ 0. CO + 0.5. ↔ 0.

-41 -75 -205 -247 -111 -283

22 o

o

900 C 1000 C o o 1100 C 1200 C CO2/MSW mass ratio = 0.66

20

Molar yield of syngas (mol/kg-msw)

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

18 16 14 12 10 8 6 4 2 0

218

H2

CO

CO2

Syngas composition

CH4

219

Fig.2 Effect of temperature on syngas composition (N2 is excluded; Error bars

220

represent standard error and they are very small to see clearly in some data points.)

221

The molar yield of CO in the syngas increased significantly over the entire

222

temperature range. Its value ranged from 3.02 mol/kg-msw at 900 oC to 18.88

223

mol/kg-msw at 1200 oC. Therefore, it was inferred that the activity of the reactions

224

involving CO production was promoted by increasing temperature. This description

225

fitted well for the reactions R1 and R2. At high gasification temperature (>700oC),

226

water gas reaction (R1) and Boudouard reaction (R2) were responsible for the

227

observed CO yield. It occurred due to the adsorption of any available oxygen either

228

fed into the gasifier as CO2 or arose out of the MSW structure itself by the highly

229

porous char, formed in the initial step of gasification [41]. These trapped oxygen

230

entities could react with char to form CO, which was released by subsequent heating 13

ACS Paragon Plus Environment

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

231

of the char in the gasification step. This continuously liberated CO with increasing

232

temperature accounted for the rise in its yield that appears in Fig.2. Additionally, some

233

O2 released from the decomposition of oxygenated materials in MSW bring about the

234

occurrence of oxidation reaction R8 [15].

235

The H2 trend also showed a significant increase. Its value ranged from 1.45

236

mol/kg-msw at 900 oC to 9.87 mol/kg-msw at 1200 oC. This further consolidated the

237

above idea that the water gas reaction R1 was a very dominant reaction since high

238

yield of hydrogen was produced.

239

What’s more, free radicals, formed after depolymerization and condensation

240

reactions, were coated on the char surface. A study has deduced the formation

241

mechanism of hydrogen free radicals in the pyrolysis step [42]. They were generally

242

extracted by other free radicals. However, the hydrogen free radical presented a

243

greater tendency to form molecular hydrogen in the temperature range between 700

244

o

245

the increase in H2 yield. However, above 1100 oC, molecular H2 might break down,

246

making the increase in H2 yield slow. As shown in Fig. 2, the yield of H2 increased

247

slightly at 1200 oC.

C and 1100 oC. Contribution from this combination of hydrogen free radicals caused

248

The CH4 yield increased slightly from 900 oC to 1100 oC and then decreased at

249

1200 oC. Its yield stayed at low levels under the experimental conditions. Guizani et al.

250

reported a slight rise in CH4 concentration during biomass pyrolysis at 850 oC when

251

20% CO2 was introduced into the N2 environment [43]. This little “extra” CH4 was

252

probably consumed via the methane dry reforming reaction as the reverse reaction R7, 14

ACS Paragon Plus Environment

Page 14 of 30

Page 15 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

253

which was thermodynamically possible only above 640 oC and was active under our

254

experimental conditions. Two molecules each of CO and H2 were produced through

255

this reaction. Therefore, it was favorable to the increase in the yields of CO and H2

256

over 900 oC.

257

The molar yield of CO2 showed a decline trend as the temperature rose. However,

258

it had some difficulties to discriminate at the output position that whether the CO2 was

259

produced by the reactions such as devolatilization and water gas shift or it was the

260

unreacted part of the injected CO2 as gasifying agent. Therefore, the carbon dioxide

261

conversion efficiency 89:; was defined [20]

262

89:; =

263

Negative value of 89:; implied net production of CO2 in the gasification process

264

while positive one meant that the CO2 consumed from the input stream was higher

265

than that generated from the gasification reactions. As presented in Fig.3, the positive

266

value of 89:; showed an increasing tendency, further consolidating the increasing

267

activity of R2 with rising temperature. Furthermore, the CO/CO2 ratio, which could

268

give indication into the extent of Boudouard reaction, increased as the temperature

269

rose. More CO2 was converted into CO via R2.

? @ 9:; * AB?@C*A BA>*DE? @ 9:; * FGH >G AB? ? @ 9:; * AB?@C*A BA>*D

15

ACS Paragon Plus Environment

(12)

Energy & Fuels

60

3.0

CO/CO2 ratio

XCO2

2.5

50

2.0

40

1.5

30

1.0

20

0.5

10

0

0.0 900

270 271 272 273 274 275

XCO2 /%

H2/CO ratio

H2/CO ratio, CO/CO2 ratio

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

1000

o

1100

1200

Temperature/ C

Fig.3 Effect of temperature on H2/CO ratio, CO/CO2 ratio and XCO2 Three different efficiency indicators are shown in Fig.3 and Fig.4 at different temperatures: (1) The carbon conversion efficiency (CCE) (Eq. (13)) gives an indication on gasification efficiency. 9BFJ* * ?C*AB? KCO2>H2. From 1000oC to 1200oC, the rank

363

turned into CO> (N2 and H2) >CH4>CO2. Under different CO2/MSW mass ratios, the

364

distribution of H2>CO>N2>CH4>CO2 was obtained at 0.17 and 0.33 while the

365

distribution turned into CO>H2>N2>CH4>CO2 from 0.50 to 0.83.

21

ACS Paragon Plus Environment

Energy & Fuels

14000

CH4

Energy (kJ/kg-msw)

CO2

CO

H2

N2

CO2/MSW mass ratio = 0.66

12000

10000

8000

6000

4000

2000

0 900

366 367

1000

o

1100

1200

Temperature/ C

Fig.8 Energy values of syngas components at different temperatures 14000

CH4

CO2

CO

H2

N2

o

Temperature=1100 C

12000

10000

Energy (kJ/kg-msw)

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 22 of 30

8000

6000

4000

2000

0 0.1

368 369

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

CO2/MSW mass ratio

Fig.9 Energy values of syngas components at different CO2/MSW mass ratios

370 371

Fig.10 Effect of temperature and CO2/MSW mass ratio on the total energy value of 22

ACS Paragon Plus Environment

Page 23 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

372

syngas

373

Table 4 Energy distribution of syngas components Temperature/oC 900 1000 1100 1200 1100

374

CO2/MSW mass ratio 0.66

0.17 0.33 0.50 0.66 0.83

Energy distribution N2>CO>CH4>CO2>H2 CO>N2>H2>CH4>CO2 CO>H2>N2>CH4>CO2 CO>H2>N2>CH4>CO2 H2>CO>N2>CH4>CO2 H2>CO>N2>CH4>CO2 CO>H2>N2>CH4>CO2 CO>H2>N2>CH4>CO2 CO>H2>N2>CH4>CO2

3.4 Exergy analysis of syngas

375

Fig.11 and Fig.12 present the exergy value of syngas components as a function

376

of temperature and CO2/MSW mass ratio, separately. Comparing with energy analysis,

377

similar variation tendency of exergy value with the temperature and CO2/MSW mass

378

ratio has been observed. Nevertheless, the exergy value of each syngas component

379

was less than its corresponding energy value, since exergy was defined as the

380

available part of energy. The most obvious difference between energy and exergy

381

analyses appeared to be the physical energy and exergy values of sensible heat,

382

particularly for incombustible gas components, such as N2 and CO2, because sensible

383

energy degraded significantly in exergy analysis. The similar trend of total exergy

384

value of syngas in comparison to energy analysis under various temperatures and

385

CO2/MSW mass ratios can be found in Fig. 13. The total exergy value of syngas

386

ranged from 3689.05 kJ/ kg-msw to 11065.77 kJ/ kg-msw as the temperature

387

increased. The exergy of combustible gas components accounted for approximately

388

49.83-80.93% in the total exergy of the syngas. As the CO2/MSW mass ratio 23

ACS Paragon Plus Environment

Energy & Fuels

389

augmented from 0.17 to 0.50, the total exergy value of syngas initially declined from

390

10152.50 to 7579.29 kJ/ kg-msw while it increased to 10158.29 kJ/ kg-msw as the

391

CO2/MSW mass ratio further rose to 0.83.

392

Table 5 summarizes the detailed exergy distributions of syngas components. At

393

900 oC, the overall distribution of N2>CO>CO2>CH4>H2 was observed. From 1000

394

o

395

different CO2/MSW mass ratios, at 0.17, the exergy of syngas components was in the

396

rank of H2>CO>N2>CH4>CO2, whereas the rank turned into CO>H2>N2>CH4>CO2

397

from 0.33 to 0.83. It was worth noting that the exergy distribution of syngas followed

398

well with their energy distributions.

C to 1200 oC, the overall distribution turned into CO>H2>N2>CH4>CO2. Under

14000

CH4 12000

CO2

CO

H2

N2

CO2/MSW mass ratio = 0.66

10000

Exergy (kJ/kg-msw)

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

Page 24 of 30

8000

6000

4000

2000

0

399 400

900

1000

o

1100

1200

Temperature/ C

Fig.11 Exergy values of syngas components at different temperatures

24

ACS Paragon Plus Environment

Page 25 of 30

12000

CH4

CO2

CO

H2

N2

o

Temperature=1100 C

10000

Exergy (kJ/kg-msw)

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

8000

6000

4000

2000

0 0.1

0.3

0.4

0.5

0.6

0.7

0.8

0.9

CO2/MSW mass ratio

401 402

0.2

Fig.12 Exergy values of syngas components at different CO2/MSW mass ratios

403 404

Fig.13 Effect of temperature and CO2/MSW mass ratio on the total exergy value of

405

syngas

406

Table 5 Exergy distribution of syngas components Temperature/oC 900 1000 1100 1200 1100

CO2/MSW mass ratio 0.66

0.17 0.33 0.50 0.66 0.83

Exergy distribution N2>CO>CO2>CH4>H2 CO>H2>N2>CH4>CO2 CO>H2>N2>CH4>CO2 CO>H2>N2>CH4>CO2 H2>CO>N2>CH4>CO2 CO>H2>N2>CH4>CO2 CO>H2>N2>CH4>CO2 CO>H2>N2>CH4>CO2 CO>H2>N2>CH4>CO2

25

ACS Paragon Plus Environment

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

407 408 409

4 Conclusions In the present work, MSW gasification using CO2 as single gasifying agent was experimentally studied under various temperatures and CO2/MSW mass ratios.

410

Results indicated that the temperature significantly affected the syngas

411

composition through the endothermic gasification reactions. Increasing temperature

412

resulted in higher molar yields of CO and H2. The reactions such as Boudouard

413

reaction, water gas reaction and free radical reactions bringing about the formation of

414

molecular hydrogen dominated the chemistry of gasification process. The increase in

415

the molar yield of CH4 was observed with rising temperature. It could react with CO2

416

via the methane dry reforming reaction, which was thermodynamically possible above

417

640oC. Therefore, significant increase in the yield of CO was observed at 1200 oC.

418

The CO2/MSW mass ratio had an impact on the yields of CO, H2 and CH4. But there

419

was no signification change observed between the CO2/MSW mass ratios of 0.66 and

420

0.83.

421

Besides, the present work also aimed at the thermodynamic performance of

422

MSW CO2-gasification by energy and exergy analyses. It was observed that

423

increasing the temperature from 900 oC to 1200 oC augmented the total energy and

424

exergy values of syngas. The total energy and exergy values initially declined and

425

then rose as the CO2/MSW mass ratio increased. The exergy values of syngas

426

components were lower that their energy values. At the same time, the exergy values

427

of syngas components followed well with their energy values.

428

Acknowledgements 26

ACS Paragon Plus Environment

Page 26 of 30

Page 27 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

429

The authors appreciate the financial supports from National Natural Science

430

Foundation of China (nos. 51706144, 51606128), Natural Science Foundation of

431

Shanghai (nos. 17ZR1419500, 16ZR1422900) and Shanghai Municipal Education

432

Commission (ZZslg16001).

27

ACS Paragon Plus Environment

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

433

References

434

[1] National Bureau of Statistics. China Statistical Yearbook; China Statistics Press:

435

Beijing, 2016.

436

[2] Hoornweg, D.; Bhada-Tata, P.; Kennedy, C. Nature 2013, 202: 615-617.

437

[3] Zhou, H.; Meng, A. H.; Long, Y. Q.; et al. Renew. Sust. Energ. Rev. 2014, 36,

438

107-122.

439

[4] Lombardi, L.; Carnevale, E.; Corti, A. Waste Manage. 2015, 37: 26-44.

440

[5] Aren, U.; Di Gregorio, F. Waste Manage. 2013, 33(5): 1142-2250.

441

[6] Li, H.; Nitivattananon, V.; Li, P. Waste Manage. Res. 2015, 33(5): 401-409.

442

[7] Arena, U. Waste Manage. 2012, 32, 625-639.

443

[8] Dong, J.; Chi, Y.; Tang, Y. J.; et al. Energ. Fuel. 2016, 30(5), 3994-4001.

444

[9] Zheng, X. Y.; Chen, C.; Ying, Z.; et al. Energ. Convers. Manage. 2016, 117,

445

393-399.

446

[10] Bridgwater, A. V. Chem. Eng. J. 2003, 91(2-3), 87-102.

447

[11] Parthasarathy, P.; Narayanan, K. S. Renew. Energ. 2014, 66, 570-579.

448

[12] Prabowo, B.; Umeki, K.; Yan, M.; et al. Appl. Energ. 2014, 113, 670-679.

449

[13] Kwon, E. K.; Cho, S. H.; Kim, S. P. Environ. Sci. Technol. 2015, 49, 5028-5034.

450

[14] Lahijani, P.; Zainal, Z. A.; Mohammadi, M.; et al. Renew. Sust. Energ. Rev. 2015,

451

41, 615-632.

452

[15] Sadhwani, N.; Adhikari, S.; Eden, M. R. Ind. Eng. Chem. Res. 2016, 55,

453

2882-2891.

454

[16] Liang, C.; Zhang, H. X.; Zhu, Z. P.; et al. Fuel 2017, 200, 81-88.

455

[17] Qadi, N. M. N.; Zaini, I. N.; Takahashi, F.; et al. Energ. Fuel. 2017, 31, 28

ACS Paragon Plus Environment

Page 28 of 30

Page 29 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

456

2927-2933.

457

[18] Nilsson, S.; Gómez-Barea, A.; Fuentes-Cano, D.; et al. Fuel 2014, 125, 192-199.

458

[19] Zhang, R.; Wang, Q. H.; Luo, Z. Y.; et al. Energ. Fuel. 2013, 27, 5107-5115.

459

[20] Cuoto, N.; Silva, V.; Rouboa, A. Energy 2016, 104, 42-52.

460

[21] Gu, J.; Wang, Y. Z.; Yuan, H. R.; et al. Energ. Fuel. 2017, 31, 8317-8325.

461

[22] Hla, S. S.; Lopes, R.; Roberts, D. Fuel 2016, 185, 847-854.

462

[23] Zhou, H.; Meng, A. H.; Long, Y. Q.; et al. J. Anal. Appl. Pyrol. 2014, 108, 19-25.

463

[24] Dincer, I. Energ. Policy 2002, 30(2), 137-149.

464

[25] Aviara, N. A.; Onuoha, L. N.; Falola, O. E.; et al. Energy 2014, 73, 809-817.

465

[26] Dincer, I. Int. J. Hydrogen Energ. 2002, 27, 265-285.

466

[27] Kaushik, S.; Reddy, V. S.; Tyagi, S. Renew. Sust. Energ. Rev. 2011, 15(4),

467

1857-1872.

468

[28] Tang, Y. J.; Dong, J.; Chi, Y.; et al. Energ. Fuel. 2016, 30, 7629-7637.

469

[29] Couto, N. D.; Silva, V. B.; Rouboa, A. J. Clean. Prod. 2016, 139, 622-635.

470

[30] Barati, M. R.; Aghbashlo, M.; Ghanavati, H.; et al. Energ. Convers. Manage.

471

2017, 151, 753-763.

472

[31] Couto, N. D.; Silva, V. B.; Monteiro, E.; et al. Energ. Convers. Manage. 2017,

473

134, 235-246.

474

[32] Zhang, Y. N.; Li, B. X.; Li, H. T.; et al. Thermochica Acta 2011, 519(1-2), 65-71.

475

[33] Zhang, Y. N.; Zhao, Y. J.; Gao, X. Y.; et al. J. Clean. Prod. 2015, 95, 273-280.

476

[34] Loha, C.; Chattopadhyay, H.; Chatterjee, P. K. Energy 2011, 36(7), 4063-4071.

477

[35] Lu, Y. J.; Guo, L. J.; Zhang, X. M.; et al. Chem. Eng. J. 2007, 131(1-3), 233-244. 29

ACS Paragon Plus Environment

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

478

[36] Cengel, Y. A.; Boles M. A. Thermodynamics: an engineering approach, Eighth

479

Edition; McGraw-Hill Education: New York, 2015; pp 898-946.

480

[37] Morris, D. R.; Szargut, J. Energy 1986, 11(8), 733-755.

481

[38] Coskun, C.; Oktay, Z.; Dincer, I. J. Clean. Prod. 2012, 32, 71-80.

482

[39] Zhang, Y. N.; Li, B. X.; Li, H. T.; et al. Thermochimica Acta 2012, 538, 21-28.

483

[40] Herguido, J.; Corella, J.; Gonzalez-Saiz, J. Ind. Eng. Chem. Res. 1992, 31,

484

1274-1282.

485

[41] Butterman, H. C.; Castaldi, M. J. Environ. Sci. Technol. 2009, 43, 9030-9037.

486

[42] Demirbaş, A. Energ. Convers. Manage. 2000, 41(6), 633-646.

487

[43] Guizani, C.; Escudero Sanz, F. J.; Salvador, S. Fuel 2014, 116, 310-320.

488

[44] Rauch, R.; Hrbek, J.; Hofbauer, H. WIRes. Energy Environ. 2014, 3, 343-362.

30

ACS Paragon Plus Environment

Page 30 of 30