Energy Conversion and Management: Xiaoyuan Zheng, Chong Chen, Zhi Ying, Bo Wang
Energy Conversion and Management: Xiaoyuan Zheng, Chong Chen, Zhi Ying, Bo Wang
Energy Conversion and Management: Xiaoyuan Zheng, Chong Chen, Zhi Ying, Bo Wang
a r t i c l e
i n f o
Article history:
Received 16 December 2015
Received in revised form 16 March 2016
Accepted 17 March 2016
Available online 22 March 2016
Keywords:
MSW
Gasification
Fixed bed reactor
Hydrogen
Calcium oxide
a b s t r a c t
Gasification performance of key components including polyethylene (PE) and bamboo of municipal solid
waste (MSW) was examined in a bench-scale fixed bed. Effects of equivalence ratio, gasification temperature, steam/feedstock ratio, and calcium oxide (CaO) presence on syngas composition and lower heating
value (LHV) were investigated. As equivalence ratio increased, both combustible gas components and
LHV of syngas from bamboo and PE gasification decreased while the yield of CO2 increased generally.
Higher gasification temperature favored improving H2 and CO production and lowering the yield of
CO2 from PE gasification while an optimal temperature of 700 C existed for the best syngas quality
and the highest LHV of syngas from bamboo gasification. Different variations of CO2 between bamboo
and PE were observed as steam/feedstock ratio increased. CaO was more effective to increase the yields
of H2, CO, and CH4 and lower the yield of CO2 from bamboo and PE gasification under both air and steam
atmosphere, excluding the syngas composition of PE steam gasification. The work described here favors
us understand the real MSW gasification process and thus facilitates the industrial application of gasification technology.
2016 Elsevier Ltd. All rights reserved.
1. Introduction
Municipal solid waste (MSW) treatment, management and disposal have been common concerns in every country. The conventional landfilling method is encountering some problems
including land shortage, underground water pollution, air pollution, and leachate disposal [1]. In recent years, incineration technology has been widely used with the advantages of substantial
and immediate reduction of MSW volume. However, toxic dioxins
emissions derived from waste incineration have been observed [2].
Therefore, both pyrolysis and gasification are regarded as feasible
alternative ways to incineration for MSW disposal, due to the
improved energy extraction by co-firing of syngas in large power
plants or combustion of syngas in the combined cycle gas turbine,
as well as better pollution control including the reduction of some
pollutants as dioxins, furans and NOx [35].
Extensive investigations of MSW pyrolysis by thermogravimetry (TGA) under inert atmosphere were reported in literature. Fang
et al. studied the co-pyrolysis characteristics of MSW, paper sludge
and their blends at N2 atmosphere. Meanwhile their kinetics were
Corresponding author.
E-mail address: xyzheng@usst.edu.cn (X. Zheng).
http://dx.doi.org/10.1016/j.enconman.2016.03.044
0196-8904/ 2016 Elsevier Ltd. All rights reserved.
studied as well [6]. TGA study was performed under inert nitrogen
atmosphere by Velghe et al. to get information on the potential of
MSW pyrolysis [7]. Chen et al. investigated the pyrolysis and gasification characteristics of the most common components of MSW
using TGA and analyzed their decomposed characteristics in N2
and CO2 atmosphere [8]. However, gasification studies under other
atmosphere were rare. Gasification performance of MSW depends
on many factors, like feedstock properties, reactor configurations,
and reaction conditions. Investigations focused on the reaction
conditions such as temperature, pressure, heating rate, and catalysts have been performed. The effect of catalyst and reactor temperature on the yield and product composition of MSW steam
catalytic gasification was investigated by He et al. [9]. Hu et al.
studied the effects of moisture content, [Ca]/[C], and reactor temperature on H2 yield and gas composition, when an in-situ MSW
steam gasification method was proposed using CaO as catalyst
and CO2 sorbent [10]. In order to produce tar-free fuel gas from
MSW steam catalytic gasification, Guan et al. investigated the
effect of catalyst, temperature, steam on the tar content, dry gas
yield and composition, and carbon conversion efficiency using
the gasifier composed of gasification reactor and catalytic reactor
[11]. Feedstock properties have great influences on gasification
performance as well. Researches have been conducted to
394
Carbonization:
10
MSW devolatilization:
Cn Hm DH0298
>0
Boudouard reaction:
Table 1
Properties of bamboo and PE.
m
H2 nCO2 DH0298 > 0
Cn Hm 2nH2 O ! 2n
2
a
b
Proximate analysis
Bambooa
PEa
Moisture
Volatile matter
Fixed carbonb
Ash
7.14
74.35
17.02
1.49
0.65
98.87
0.17
0.31
Ultimate analysis
Carbon
Hydrogen
Oxygenb
Nitrogen
Sulfur
LHV (MJ/kg)
44.83
5.96
40.08
0.35
0.15
18.32
83.62
13.56
1.31
0.55
0
43.83
The fixed bed reactor was built with 0Cr25Ni20 stainless steel
and surrounded by electric heater, by which the reactor temperature could be adjusted from room temperature to 1000 C. The
electric heater was covered with an insulation layer outside. The
reactor possessed an inner diameter of 100 mm and a height of
1400 mm. A screw feeder was used to continuously feed powder
MSW components into the reactor from the top. The steam or air
was introduced from the bottom of the reactor.
Before the experiments, the electric heater was turned on.
After the desired value of reactor temperature was achieved, the
steam generator or blower began to work and the steam or air
could be introduced into the reactor. The steam flow rate was
kept constant at 3 kg/h. Its temperature was kept at 160 C. The
flow rate of air at room temperature was adjusted according to
the equivalence ratio. They passed from the bottom to the top
for 10 min. Then the screw feeder was turned on. The blended
CaO and powder MSW components with mass ratio of 1:1 were
injected into the reactor from the top to the bottom with a setting
rate. The produced gas in the hot reactor flown out from the reactor, passed through the condenser and air pump, and combusted
by exhaust burner. When the sampling of produced gas was
required, the gas passed through the following condenser and filter units. A condenser in series was used to cool the syngas and
capture tar. A cold-gas filter in series was used to clean the syngas. The collected gas was analyzed by a gas analyzer (GS-101M
Gas Chromatography). All the experiments were run at atmospheric pressure.
In general, the stable state could be achieved after 20 min to
ensure the reliability of test data. Then the gas was sampled every
10 min in triplicate. The averaged values were reported in this
study. And the data variability was within 5%. Reproducibility
was tested in this study as well.
395
396
(A) 100
Bamboo, T=800 oC
60
H2
CO
CH 4
40
CO 2
20
0
0.2
0.3
0.4
0.5
Equivalence ratio
(B)
80
o
PE, T=800 C
H2
70
Syngas composition/mol%
CO
60
CH 4
CO 2
50
40
30
(A) 100
20
90
H2
80
CO
10
0
0.2
0.3
0.4
0.5
Equivalence ratio
Fig. 2. Effect of equivalence ratio on syngas composition (T = 800 C).
10
CH 4
70
CO 2
60
50
40
30
20
T=800 oC
Bamboo
10
PE
0
600
700
800
Gasification temperature/ o C
(B)
80
70
4
3
2
1
0
0.2
0.3
0.4
0.5
Equivalence ratio
Syngas composition/mol%
LHV (MJ/Nm3)
Syngas composition/mol%
Syngas composition/mol%
80
60
PE, equivalence ratio=0.4
50
H2
CO
40
30
CH 4
CO 2
20
10
0
600
700
800
Gasification temperature/ oC
Fig. 4. Effect of gasification temperature on syngas composition (Equivalence
ratio = 0.4).
397
(A) 60
7
6
Syngas composition/mol%
50
LHV (MJ/Nm3)
5
4
3
2
1
Equivalence ratio=0.4
Bamboo
o
Bamboo, T=800 C
H2
CO
40
CH4
CO2
30
20
10
PE
0
600
700
0.4
800
0.5
(B) 50
0.7
0.8
0.9
1.0
PE, T=800 oC
CH4
CO
H2
Syngas composition/mol%
0.6
Steam/feedstock ratio
Gasification temperature/ oC
CO2
C2 H 4
40
30
20
10
0
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Steam/feedstock ratio
Fig. 6. Effect of steam/feedstock ratio on syngas composition (T = 800 C).
398
(A) 60
18
T=800 oC
Bamboo
16
Bamboo
50
Without CaO
PE
With CaO
Yield of syngas/mol%
LHV (MJ/Nm 3)
14
12
10
8
40
30
20
10
6
4
0.4
0.5
0.6
0.7
0.8
0.9
1.0
H2
Steam/feedstock ratio
50
(B) 40
CO2
C2H4
PE
T=800 oC, steam/feedstock ratio=0.4
Bamboo
Without CaO
With CaO
30
Without CaO
Yield of syngas/mol%
Yield of syngas/mol%
60
CH4
Syngas composition
(A) 70
CO
With CaO
40
30
20
10
20
10
0
H2
H2
CO
CH4
CO2
Syngas composition
(B) 50
CO
CH4
CO2
C2 H4
Syngas composition
Fig. 9. Effect of CaO on syngas composition of steam gasification (T = 800 C,
Equivalence ratio = 0.3).
PE
T=800 oC, equivalence ratio=0.3
Without CaO
With CaO
Yield of syngas/mol%
40
30
20
10
0
H2
CO
CH4
CO2
Syngas composition
Fig. 8. Effect of CaO on syngas composition of air gasification (T = 800 C,
Equivalence ratio = 0.3).
And there was little change between 5.43 MJ/Nm3 and 5.84 MJ/
Nm3 in LHV of syngas for bamboo.
3.4. Effect of CaO presence
In the present work, both air and steam gasification in the presence of CaO were conducted at 800 C. Fig. 8(A) and (B) showed the
The yields of H2, CO and CH4 increased while the yield of CO2
decreased significantly. Different gasification performance of PE
was observed in Fig. 9(B), compared with that of bamboo. The
yields of H2, CO and CO2 declined slightly while those of CH4 and
C2H4 rose. Comparing with Fig. 8, the effect of CaO on the syngas
composition was weak. Negative effect was even observed for PE.
The effect of CaO on the syngas composition under the steam
atmosphere could be summarized as follows: (a) CaO played an
adsorbing role in the gasification process and thus reduced the
yield of CO2. It also drove the watergas shift reaction (Eq. (6))
and steam reforming reactions (Eqs. (4) and (7)) in the direction
to enhance combustible gas formation from bamboo gasification
[16]. (b) Excess steam would consume more energy, leading to
the decrease of reactor temperature, and thus lower the syngas
yield [27]. (c) Due to the differing nature of gasified materials,
the effect of excess steam on syngas composition from PE gasification was stronger than that of CaO.
In the presence of CaO, LHV of syngas for bamboo and PE
increased from 5.81 MJ/Nm3 and 10.52 MJ/Nm3 to 17.33 MJ/Nm3
and 12.18 MJ/Nm3, respectively.
4. Conclusions
In this study, comprehensive studies on the effect of equivalence ratio, gasification temperature, steam/feedstock ratio and
CaO presence on syngas composition and LHV of key MSW components including bamboo and PE were performed using a benchscale fixed bed reactor.
As the equivalence ratio increased, combustible gas components and LHV of syngas decreased, while the yield of CO2
increased for both bamboo and PE.
The effect of gasification temperature on syngas composition
and LHV of syngas from bamboo and PE gasification was different.
For bamboo, there existed an optimal temperature of 700 C for the
best syngas quality and the highest LHV value of 6.22 MJ/Nm3.
However, higher temperature was more conducive to increase
the yields of H2 and CO and decrease the yield of CO2 of PE. And
thus the LHV of syngas rose from 3.67 MJ/Nm3 to 5.31 MJ/Nm3.
Different gasification performance between bamboo and PE was
observed as steam/feedstock ratio increased. The yield of CO2 from
bamboo gasification decreased slightly about 1.41% while that
from PE gasification increased sharply about 14.91%.
Due to the adsorption of CO2 via CaO carbonation, combustible
gas components from bamboo and PE gasification under the atmosphere of both air and steam increased while the yield of CO2
declined. However, the yields of H2 and CH4 from PE gasification
decreased while that of CO2 rose slightly under the steam
atmosphere.
The basic data described here favors us understand the real
MSW gasification process and thus facilitates the industrial application of gasification technology.
Acknowledgements
The authors appreciate the financial supports from Science and
Technology Commission of Shanghai Municipality (NO.
13DZ2260900) and Shanghai Municipal Education Commission
(NO. ZZslg15012).
399
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