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

6 days ago - (44) (3) The cold gas efficiency (CGE) is calculated here for CO, H2, and CH4 (eq 14, in which Vsyngas is the syngas flow rate in Nm3/h, ...
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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

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Energy & Fuels

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

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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

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

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

12

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

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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

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

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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

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

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and exergy distributions of syngas component were different at various temperatures

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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

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energy value. It followed well with the energy value.

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Keywords

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

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1 Introduction

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The economy development and the consequent improvement in living standards

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

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

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

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

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

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

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

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reduction and energy recovery by the thorough destruction of combustible

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

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

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

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

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more and more people are against the MSW incineration.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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However, most of recent gasification investigations involving CO2 focused on

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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performance of practical MSW under single CO2 atmosphere without combining the

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effect of other gasifying agents.

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

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

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

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

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

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

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

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

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

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

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

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

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energy utilization and overcome the limitations of conventional energy analysis and

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describes efficiency and performance of energy utilization [26].

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

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the thermodynamic performance of MSW management systems or to improve the

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

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

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

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

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

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exergy assessment and to better understand the thermodynamic performance of CO2

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gasification-based MSW to the energy system.

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

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

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

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

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

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

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reactions dominating the chemistry of gasification process were addressed. The

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

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

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

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

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

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experience to facilitate the future development of MSW treatment.

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2 Materials and methods 5

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2.1 Experiments

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

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

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

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

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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

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

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

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

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

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

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

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

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

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

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continuous feeding.

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

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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

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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

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 =  +  +  + 

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

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(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)

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According to the specific enthalpy of syngas components at the environment

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

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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

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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

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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

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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].

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b

The standard chemical exergy data of syngas

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3 Results and Discussion

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3.1 Effect of temperature on gasification performance

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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

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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

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∆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)

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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

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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

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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

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(12)

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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

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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.

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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

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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

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Energy & Fuels

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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

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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

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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

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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

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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

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Energy & Fuels

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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).

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