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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
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
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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
32
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
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
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[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].
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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,
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more and more people are against the MSW incineration.
44
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
67
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
69
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
88
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].
91
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
95
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
99
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).
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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
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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
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= +
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
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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)
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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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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
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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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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
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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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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
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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
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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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syngas
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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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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
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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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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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The authors appreciate the financial supports from National Natural Science
430
Foundation of China (nos. 51706144, 51606128), Natural Science Foundation of
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Shanghai (nos. 17ZR1419500, 16ZR1422900) and Shanghai Municipal Education
432
Commission (ZZslg16001).
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