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Comprehensive study of Fe2O3/Al2O3 reduction with ultra low concentration methane under conditions pertinent to chemical looping combustion Yongxing Zhang, Elham Doroodchi, and Behdad Moghtaderi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b00080 • Publication Date (Web): 23 Feb 2015 Downloaded from http://pubs.acs.org on February 24, 2015
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Energy & Fuels
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Comprehensive study of Fe2O3/Al2O3 reduction with ultra low
3
concentration methane under conditions pertinent to chemical looping
4
combustion
5
Yongxing Zhanga,*, Elham Doroodchic, Behdad Moghtaderib
6 7 8
a
National Engineering Laboratory for Pipeline Safety/Beijing Key Laboratory of Urban Oil & Gas Distribution Technology, China University of Petroleum，Beijing, China b
9 10
c
Centre for Frontier Energy Technologies,
Priority Research Centre for Advanced Particle Processing & Transport
11
Chemical Engineering, School of Engineering,
12
Faculty of Engineering & Built Environment, The University of Newcastle, Australia
13
*Corresponding Author,
[email protected] 14 15
Abstract
16
An experimental study was conducted to identify the most suitable alumina supported
17
iron-based oxygen carrier for the abatement of ultra low concentration methane using a
18
chemical looping approach. This was done by evaluating the performance characteristics
19
such as reactivity, cyclic stability and gas conversion. The experiments were carried out
20
in a thermogravimetric analyser and a fixed bed reactor setup under the desired
21
conditions. Thermodynamics analysis was carried out using the commercially available
22
software-ASPENPLUS. The analysis suggested that the favorable iron-based oxygen
23
carriers were those with the weight content of Fe2O3 less than 50 wt%. Three
24
Fe2O3/Al2O3 samples were therefore prepared with the metal oxide contents in the range
25
of 10-45 wt%, i.e., Fe10Al, Fe25Al and Fe45Al. The TGA experimental results showed
26
that the reduction reactivity and stability were improved with the additive of support 1 ACS Paragon Plus Environment
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material compared with unsupported Fe2O3. Moreover, the reduction reactivity varied
2
with the solid conversion range and the weight content of the parent material. For full
3
reduction of Fe2O3 to Fe3O4, the sample Fe10Al showed the highest reduction reactivity.
4
But in terms of the rate of oxygen transport (which considers the combined effects of the
5
oxygen transfer capacity and reactivity), the highest value was achieved by the Fe45Al
6
sample. The gas conversion of CH4 to CO2 was also quite dependent on the weight
7
content of Fe2O3. Essentially Fe45Al delivered the longest duration on high level
8
conversion (i.e., complete conversion of CH4 to CO2). In summary Fe45Al was found to
9
be the most suitable oxygen carrier candidate in this application. The effect of operational
10
parameters was further examined with various reaction temperatures (873-1073 K),
11
methane concentrations (0.1-1.5 vol%) and CO2 compositions (0-50 vol%).
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1. INTRODUCTION
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With the increasing concern on the greenhouse gas (GHG) emissions and global warming
3
issue, more strict carbon emission policies tend to be evolved and implemented by
4
governments to reduce the emissions of CH4 and CO2, which are regarded as the main
5
anthropogenic contributor to the greenhouse effect due to the large quantity of annual
6
emission and long lifetime in the atmosphere [1]. Methane is reported to be a potent
7
greenhouse gas: 25 times more powerful than CO2 over a 100-year time period. As a
8
result, reductions in methane emissions could be an effective option to stabilise the
9
climate in the near term, buying time for longer term energy technology solutions to be
10
implemented [2]. Approximately 25% of anthropogenic methane emissions come from
11
energy and resources sectors, such as coalmining and natural gas and oil recovery
12
activities, in the form of ultra low concentration methane. In a previous study chemical
13
looping combustion technology was proposed to reduce the ultra low concentration
14
methane [3].
15
Chemical looping combustion (CLC), as an advanced CCS technology (Carbon capture
16
and storage), represented a temporary solution to stabilise CO2 concentration in
17
atmosphere due to the inherent ability of producing a concentrated CO2 stream, which is
18
ready to be transported and stored in instead of released into the atmosphere directly [4].
19
A common step to most of advanced low emission technologies, CO2 separation from the
20
flue gas, can be removed as a result. In a CLC process (as shown in Figure 1), the
21
reduction/oxidation (redox) reactions occur between two connected reactors by
22
circulating metal oxide particles as the oxygen transfer medium (see reactions "a" to "c").
23
The oxidation of fuel takes place in the Fuel Reactor (FR) while metal oxide particles (i.e.
24
oxygen carriers) are reduced to a lower valence state. The reduced oxygen carriers are
25
then transferred to the Air Reactor (AR) to react with air and oxidised to their original
26
oxidation state and a cycle is finished. This is commonly referred to as regeneration. The
27
main products from the fuel reactor are carbon dioxide and steam although minute
28
quantities of CO, CH4, H2 and other hydrocarbons can also be found in the exhaust
29
stream. The product gas from air reactor primarily consists of N2 and excess oxygen if
30
there is no leakage between the FR and AR. As noted, CO2 and N2 do not mix in the CLC 3 ACS Paragon Plus Environment
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1
process and as such the step for separation of CO2 from N2 can be eliminated. As a result,
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the technical and economic efficiency are both improved dramatically compared with
3
conventional combustion.
4
Reduction: 4MaOb + c CH4 →4MaOb-c + c CO2 + 2c H2O
(a)
5
Oxidation: 2MaOb-c + c O2→2MaOb
(b)
6
Overall reaction: CH4 + 2O2→CO2 + 2H2O
(c)
7 8
Figure 1: Scheme of Chemical Looping Combustion.
9
Chemical looping combustion was initially proposed to secure higher energy conversion
10
efficiency due to its lower irreversibility loss. It was then found to possess the ability of
11
separating CO2 from N2 inherently, a step causing large efficiency penalties to obtain a
12
concentrated stream of CO2. Therefore, it was recommended to use in the field of fuel
13
combustion for power generation due to the CO2 emissions policies enforced by Kyoto
14
protocol in 1997. The theoretical analysis revealed that the CLC system with gaseous
15
fuels or solid fuels delivers a higher thermal efficiency than the conventional power
16
plants either with or without CCS. Ishida [5] evaluated the performance of a chemical
17
looping combustion system with Fe-based oxygen carrier (Fe2O3-FeO) and methane as
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fuel. The graphic exergy analysis showed that the thermal efficiency was as high as 50.2%
19
(LHV) due to the less exergy loss during chemical reactions. The similar results were
20
also obtained by other researchers. 4 ACS Paragon Plus Environment
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As to its practicability, much experimental work [6-14] has been carried out to determine
2
the most suitable oxygen carriers candidates and their viability for use in different CLC
3
systems since the concept was first proposed [15]. In terms of the cyclic chemical and
4
mechanical stability, Nickel-, iron-, copper-, manganese-, and calcium- based metal
5
oxides are the most attractive candidates due to their excellent chemical and mechanical
6
performance [16, 17]. Iron oxide, as a nature abundant and cheap material (this is
7
extremely important for the application of CLC with a low heating value fuel), was
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believed to deliver some advantages over its challengers and was widely used in various
9
chemical looping combustion processes due to the nature of multiple oxidative states.
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The couple of Fe2O3/Fe3O4 is preferred as oxygen mediator due to good reactivity with
11
gaseous fuels, high gas conversion and high melting temperature although the oxygen
12
transfer capacity (OTC) is not good (0.033). It was indicated that the transition of Fe2O3
13
to Fe3O4 was able to fully convert CH4 or syngas to CO2 and H2O at 1073 K from
14
thermodynamic aspects of view [18]. Basically, the couple of Fe2O3/FeO shows
15
improvement on the oxygen transfer capacity (0.11) while the reduction reactivity with
16
fuel is not as good as Fe2O3/Fe3O4. Mattisson [19] believed that the reduction rate of
17
Fe2O3 (dX/dt) with methane was a function of the solid conversion range (∆X), being 11%
18
and 33% for the reduction of Fe2O3 to Fe3O4 and Fe2O3 to FeO.
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Various support materials were adapted to improve the chemical and mechanical
20
performance of iron oxides with CH4. Cho [20] investigated the reactivity of Fe2O3/Al2O3
21
with CH4 at 1223 K and identified the reduced phases of iron oxide as Fe3O4 and/or
22
FeAl2O4. Adanez [6] thought that Fe2O3 supported by Al2O3 or ZrO2 showed high
23
reactivity with CH4 for the transition phase of Fe2O3 to FeO compared with being
24
supported by SiO2, TiO2 and sepiolite. Ishida [21] claimed that Fe2O3/Al2O3 composite
25
particles containing corundum was a suitable looping material in terms of long-term
26
operation. Gayan [22] revealed that a Fe-based impregnated oxygen carrier, 15 wt%
27
Fe2O3/Al2O3, showed improved reactivity with CH4 within the conversion of Fe2O3 to
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FeAl2O4 compared with other Fe-based material found in the literature. Mattisson [23]
29
suggested that Fe2O3 supported with MgAl2O4, ZrO2 or Al2O3 exhibited good reactivity
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with CH4. Johansson [24] highlighted that 60 wt% Fe2O3/MgAl2O4 was the best oxygen
31
carrier considering together crushing strength and reactivity for the reduction of Fe2O3 to 5 ACS Paragon Plus Environment
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Fe3O4 with CH4. Zafar [25] studied the reduction reactivity of Fe2O3 supported by SiO2
2
or MgAl2O4 with CH4 at temperature of 1073-1273 K. It seemed that Fe2O3/SiO2 maybe
3
not a suitable oxygen carrier due to the formation of silicate at high temperature. Corbella
4
[26] tested the titania supported iron oxide as oxygen carrier in CLC of CH4. The
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developed metal oxide showed acceptable reactivity and durability but low oxygen
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capacity due to the formation of irreversible FeTiO3 after the first cycle.
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Nevertheless, studies related to the reactivity of metal oxides under ultra low methane
8
concentrations (i.e. 0.1-1 vol%) are generally scarce. The purpose for the current study is
9
to identify the most suitable alumina supported iron-based oxygen carrier for chemical
10
looping combustion of ultra low concentration methane. There are indeed some specific
11
reasons for choosing the alumina as support material. First of all, it is nature abundant
12
and thus inexpensive. Secondly, it has the perfect adsorption capability due to the large
13
pore volume, ensuring that the iron oxide particles are able to well-distributed on the
14
surface. Thirdly, its melting temperature is as high as 2000oC. Finally, it is hard to be
15
cracked due to good mechanical strength. With various Fe2O3 loading contents (10-45
16
wt%), a set of experiments were designated to identify the Fe2O3/Al2O3 with the best
17
performance in reactivity and methane conversion, conducting in TGA and fixed bed
18
reactor.
19
2. EXPERIMENTAL SECTION
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2.1. TGA Experiments
21
The pure iron oxide sample is prepared by direct thermal decomposition of the ferric
22
nitrate, which is first heated at 873 K for 3 h and further heated at 1073 K for 6 h. The
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supported particle samples, Fe2O3/Al2O3, are prepared by dry impregnation method [27]
24
and calcined at 873 K for 3 h in a muffle furnace in air and sintered further at 1223 K for
25
6 h. The precursory iron nitrates and binder α-phase alumina were purchased from
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Sigma-Aldrich. The weight contents of Fe2O3 on Al2O3 are about 10 wt% (Fe10Al), 25
27
wt% (Fe25Al) and 45 wt% (Fe45Al). TGA experiments were conducted in a
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thermogravimetric analyser (TA Q50, refer to Figure 2) under isothermal conditions. The
29
furnace temperature is ramp up to the desired reaction temperatures (873-1073 K) at a 6 ACS Paragon Plus Environment
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constant heating rate of 20 K ·min-1 under the inert gas flow of N2. During the reduction
2
period, a mixture of CH4 with N2 is introduced into the furnace and passed over the
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sample when the preset temperature is reached. The detailed experimental procedure can
4
be found in our previous study [28].
5 6
Figure 2: Scheme of TGA experiments.
7
The reactions (reaction d-f) listed below are considered when calculating the weight
8
changes for the reduction of Fe2O3 stoichiometrically:
9
12Fe2O3 + CH4 + Al2O3 → 8Fe3O4 + CO2 + 2H2O + Al2O3
(d)
10
4Fe3O4 + CH4 + 12Al2O3 → 12FeAl2O4 + CO2 + 2H2O
(e)
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4FeAl2O4+ CH4 → 4Fe+ 4Al2O3+ CO2+ 2H2O
(f)
12
The fractional conversion of solid samples, Xred, is employed to analyse the reactivity and
13
expressed as
14
X red =
15
where Mox is the weight of metal oxide in its oxidation state; Mred the sample weight in
16
reduction state and M the instantaneous weight of the sample. The conversion X=1 was
17
for the transformation set of Fe2O3/Fe3O4.and the higher values for the further conversion
18
to Fe2+ and/or Fe.
M ox − M M ox − M red
(1)
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The plot of fractional conversion X vs time t is fitted to obtain the polynomial regression
2
equation. The reaction rates (dX/dt) at different fractional conversions (X) are calculated
3
by differentiating a fifth-order polynomial equation.
4
2.2. Fixed Bed Reactor Experiments
5
The CLC process is simulated in a fixed bed reactor setup (as shown in Figure 3). It
6
mainly comprises a gas control unit, reactor and furnace, condenser and gas analyser. The
7
reactor is a cylindrical fused-silica tube with the length of 800 mm and diameter of 7 mm.
8
The oxygen carrier particles (i.e., Fe2O3/Al2O3) are placed in the middle area (400 mm
9
length) while the both sides are loaded with quartz wool to reduce the residence time of
10
gases in the reactor as well as preventing the solid materials from moving in the reactor.
11
The reaction temperature is controlled and stabilised by the furnace and measured using
12
an enclosed Pt/Rh thermocouple. The high purity reactant gases, CH4 and air, are diluted
13
by N2 through mass flow controllers and lead to two four-way valves, which are able to
14
direct a stream of gas to the reactor while the others to the atmosphere. In this way, it is
15
possible to control the amount of time exposed to reducing, purge and oxidising
16
atmosphere. The metal oxides are initially exposed to air until the desired reaction
17
temperature is reached. After then it is exposed to the reducing and oxidising
18
environment alternatively, between them the inert gas N2 is introduced to avoid the
19
mixture of these two gases. The product gas stream is led to a gas analyser (i.e. Agilent
20
Micro-GC 4900) where the concentrations of CH4, CO2, CO, H2 and O2 are measured in
21
real time. The exhaust is eventually ventilated to the atmosphere. Every set of experiment
22
is repeated in five cycles to ensure that the data was reproducible and the data for the fifth
23
cycle is saved for determining the results.
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Figure 3: Schematic of the fixed bed reactor rig.
3
3. RESULTS AND DISCUSSION
4
3.1. Reduction of Pure Fe2O3 with CH4
5
Pure metal oxides have been proved to be incompetent as oxygen carriers due to instable
6
redox reactivity by a large number of research works [29-31]. The test conducted in this
7
section represents a baseline result in comparison with the results demonstrated in the
8
next section. A five-cycle test for pure Fe2O3 is conducted in TGA at different reaction
9
temperatures (isothermal at 973, 1023 and 1073 K) and each cycle is in 10 minutes time
10
scale as shown in Figure 4 (noted that the inert time is not shown). As can be seen, the
11
reactivity of pure Fe2O3 is in extremely poor stability even at a low temperature of 973 K,
12
which decreases gradually except for the first two cycles. It is also observed that an
13
increase in the temperature leads to a higher reaction rate but a worse reactivity stability.
14
To obtain an in-depth vision a 30-cycles test is further carried out at 1023 K. Not
15
surprisingly the reactivity is decreased with cycles except the first two cycles.
16
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1.00
1.00
0.99 0.99 0.98 0.97
ω
ω
0.98
0.97
0.96 0.95
0.96
0.94
(a)
40
50
60
70
80
90
100
110
(b)
0.95
120
40
50
60
70
t (min)
1
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90
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120
t (min) 1.00
1.00
0.99 0.99
ω
0.98
ω
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0.98
0.97 0.97
0.96 (d)
(c)
0.96
2
40
50
60
70
80
90
100
0.95
110
100
200
300
400
500
t (min)
t (min)
3
Figure 4: Reduction of pure Fe2O3 with 1 vol% CH4 at (a) 1073 K, (b) 1023 K and (c)
4
973 K in 5 cycles and (d) 1023 K in 30 cycles
5
3.2. Reduction of Alumina-supported Fe2O3 with CH4
6
The findings in the literature indicate that the optimum metal oxide content should be
7
between 40-60 wt% for an iron-based chemical looping combustion process [24]. In the
8
case of chemical looping combustion of ultra low concentration methane, however, a
9
suitable oxygen carrier should be selected more carefully. In a chemical looping
10
combustion process, the reactions in the air reactor are exothermic while endothermic in
11
the fuel reactor. Extra energy source is required to maintain the temperature in the FR if
12
no measurements are taken to transfer the heat energy from the AR side to the FR side.
13
The oxygen carrier particles, circulating between the two connected reactors, could act as
14
the heat transfer medium. To raise the temperature in the fuel reactor to a higher level, 10 ACS Paragon Plus Environment
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more energy is needed to be transferred. This can be partially solved by adjusting the
2
weight content of support material. It has been confirmed by the thermodynamics
3
calculation using the ASPENPLUS software. The used oxygen carrier couple is
4
Fe2O3/Fe3O4 and the methane concentration is set to be 1 vol%. The temperature in the
5
air reactor is set to be the typical values, i.e., 1173 K and 1273 K and the fuel reactor
6
operates in autothermal condition. The variations of the fuel reactor temperature with the
7
content of alumina are plotted in Figure 5. As shown by the curves the temperature is
8
increased with the decrease in the content of Fe2O3 (5-100 wt%). It is also found that the
9
curves are steeper at the higher Al2O3 content. In fact, the temperature increases by 230 K
10
and 270 K with the Al2O3 content increasing from 50 wt% to 90 wt%, as double as that
11
from 0 to 50 wt%. The combination of experimental results and calculation results
12
indicate that the oxygen carriers with Fe2O3 content lower than 50% are more suitable for
13
chemical looping combustion of ultra low concentration methane.
14
Given the above background, three alumina-supported iron-based oxygen carriers are
15
prepared and the weight contents of Fe2O3 are 10, 25 and 45 wt% respectively. The
16
reactivity for the three alumina-supported iron oxides sample is investigated in 30 cycles
17
and compared with that for the pure iron oxide. As shown in Figure 6, the stability of the
18
reduction reactivity is improved by the additive of alumina. Specifically, the deviations
19
on mass loss (defined as ω = m/mox, mox is the weight of sample in its oxidation state) are
20
1.88%, 7.79% and 2.51% for Fe10Al, Fe25Al and Fe45Al respectively whereas it is
21
28.82% for unsupported iron oxide. It can be also found in this figure that lower metal
22
oxide content generally leads to a higher ω and thereby lower oxygen transfer capacities
23
(OTC defined as OTC = (mox-m)/mox, which is found to be 0.048, 0.027, 0.0185 and 0.012
24
for the metal oxide content of 100, 45, 25 and 10 wt%).
11 ACS Paragon Plus Environment
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1200
Tar=1273K
FR temperature (K)
Tar=1173K
1100
1000
900
800
700 0
20
60
80
100
Al2O3 content (wt%)
1 2
40
Figure 5: the variations of the temperature in fuel reactor with Al2O3 content.
0.99
0.98
0.97
ω
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0.96 pure Fe45Al Fe25Al Fe10Al
0.95
0.94
3 4
0
5
10
15
20
25
30
Cycles Figure 6: Mass loss at 1023 K in 30 cycles for pure Fe2O3, Fe45Al, Fe25Al and Fe10Al.
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4.0 0.06 3.5 pure Fe45Al Fe25Al Fe10Al
0.05 0.04
3.0 2.5 2.0
0.03
X
-1
dX/dt (s )
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1.5 0.02 1.0 0.01 0.00
0.5
0
100
200
300
400
500
600
0.0
t (s)
1 2
Figure 7: Reduction conversion and reaction rate for the 5th cycle at 1023 K for pure (2nd
3
cycle), Fe45Al, Fe25Al and Fe10Al.
4
The conversion and reaction rates during 10 min reduction at 1023 K are illustrated in
5
Figure 7. The results are corresponding to the data of 5th cycle in a 30-cycles test for
6
Fe45Al, Fe25Al and Fe10Al while 2nd cycle for pure iron oxide. It should note that the
7
conversion equal to unity (X=1) represents the full conversion of Fe2O3 to Fe3O4, while
8
X=3 represents the conversion of Fe2O3 to FeAl2O4 as indicated by the ratio of Fe/Al.
9
During reduction period of 10 min, the reduction for the test materials proceeds in two
10
steps. The conversion increases at a fast rate during the first step. Later, a slower
11
reduction step is observed and hence it needs longer time to reach a complete conversion
12
to Fe. As can be observed, the fast step ends at around t=60 sec corresponding to the
13
conversion of X=0.8-2.5 depending on the weight content of Fe2O3. It is then followed by
14
a plateau (i.e. slow step) which represents the conversion of Fe3O4 to FeAl2O4 and/or Fe
15
[21, 32]. This is also reflected in the plot of dX/dt vs. t. The reduction rate climbs to the
16
peak value in a short time around 60 sec and follows by a steep decrease. The sharp
17
points on the reaction rate curves are visible clearly as shown in Figure 7. Besides, the
18
weight content of Fe2O3 put a significant impact on the reduction rate (dX/dt) as well as 13 ACS Paragon Plus Environment
Energy & Fuels
1
the conversion (X). The higher metal oxide content leads to lower reaction rate and lower
2
conversion. It should note that, however, the conversion difference between Fe25Al and
3
Fe45Al is less pronounced than that between Fe45Al and unsupported iron oxide or
4
Fe10Al and Fe25Al. 0.024 0.012
0.020
-1
dX/dt (s )
0.008
-1
dX/dt (s )
0.010
0.006
0.002
0.016
0.012
X=0.5 X=1
0.004
0.008
(a)
0
5
10
15
20
25
30
X=0.5 X=1 X=1.5
(b)
0
5
10
Cycles
5 0.030
0.060
0.024
0.045
-1
0.018
0.012
X=0.5 X=1 X=1.5 X=2
0.006
20
10
15
X=0.5 X=1 X=2 X=2.5 X=3
20
25
30
0
5
Cycles
6
30
0.015
(d)
5
25
0.030
0.000
(c)
0
15
Cycles
dX/dt (s )
-1
dX/dt (s )
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 14 of 26
10
15
20
25
30
Cycles
7
Figure 8: Reactivity variations with cycles at different conversion for (a) pure Fe2O3, (b)
8
Fe45Al, (c) Fe25Al and (d) Fe10Al.
9
Table 1: Detailed deviations on reaction rates for different conversion values (%).
pure Fe45Al Fe25Al Fe10Al
X=0.5 46.87 12.90 13.44 23.94
X=1 74.24 11.68 15.33 17.53
X=1.5 / 19.33 25.56 /
X=2 / / 32.93 13.06
10 14 ACS Paragon Plus Environment
X=2.5 / / / 5.20
X=3 / / / 8.89
Page 15 of 26
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Energy & Fuels
1
A detailed comparison in reduction rates for different conversion X during 30 cycles is
2
presented in Figure 8. As shown the reaction rates at X=0.5 and 1 are presented in all
3
figures though the selected conversion values differ for different materials. Experiments
4
reveal that pure iron oxide delivers the least reactivity stability compared with the
5
alumina-supported ones. It is also revealed that the reaction rate stability is rather
6
dependent on the conversion value. For Fe10Al the reaction rate is more stabilised with
7
the increase in conversion while it is not the case for the others. The detailed deviations
8
on reaction rate are summarised in Table 1 for different conversion values. As can be
9
seen, at low conversion X=0.5 and 1 (i.e., conversion of Fe2O3 to Fe3O4) Fe45Al has the
10
best reactivity stability during 30 cycles and follows by Fe25Al, Fe10Al and pure one in
11
sequence. Besides, the figure shows that the reaction rates (dX/dt) vary with the
12
conversion range. The comparative high reaction rate level is located on the conversion
13
range within unity for all of the tested materials as observed.
14
From thermodynamic point of view, for the reduction of iron-based oxygen carriers with
15
CH4, the conversion of Fe2O3 to Fe3O4 and/or FeAl2O4 is of more interest due to the
16
ability to convert CH4 fully to CO2. Regarding to the reactivity, however, Fe2O3/Fe3O4 is
17
in favor due to the comparative high value in most of studied cases. The comparison of
18
reactivity at conversion X=0.5 and 1 for three supported iron oxides is shown in Figure 9.
19
As can be seen it decreases with the increase in the weight content of Fe2O3. However, it
20
requires to take into account the oxygen transport capacity (ROC) together in order to
21
comprehensively evaluate the performance of oxygen carrier candidates [27]. The so-
22
called rate of oxygen transport is therefore applied and defined by
23
ROT = ROC ×
dX dt
(2)
24
where dX/dt is the reaction rate.
25
The reaction rates corresponding to X=0.5 and 1 are used in conjunction with ROC to
26
determine the ROT values for the cases involved as summarised in Figure 9. Interestingly,
27
the case of Fe10Al does show the highest reduction reactivity but it has a much lower
28
ROT than the cases of Fe45Al which show the highest ROT. The above analysis clearly 15 ACS Paragon Plus Environment
Energy & Fuels
1
highlights the shortcomings of just relying on reactivity and conversely the benefits of
2
using ROT, which combines both ROC and reactivity. 0.04 0.06
dX/dt (/s)
0.05
0.03 X=0.5 X=1
0.04
0.02
0.03
ROT (%/s)
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 26
0.02 0.01 0.01 0
3
20
40
Fe2O3 content (%)
4
Figure 9: The effect of loading content on reaction rate and ROT.
5
3.3. Gas analysis for the reduction of alumina-supported Fe2O3 with CH4
6
The variations of outlet product gas concentration with time for Fe10Al, Fe25Al and
7
Fe45Al are plotted in Figure 10(a-c). The experiments are conducted in five redox cycles
8
and the data from the 5th reduction period is applied. The reaction temperature at 1023 K
9
is used in order to compare in an accurate manner because with higher temperature (like
10
1073 K) carbon deposition will be found in 30 min reduction period except for Fe45Al.
11
For Fe10Al, the concentration of CO2 decreases after an initial increase with time and the
12
concentration of CH4 increases. At the same time, some CO and H2 contents are observed
13
because the oxygen in Fe10Al is insufficient and hence CH4 is partially oxidised to CO
14
and H2 other than CO2 and H2O as shown in Figure 10(a). Regarding to the reduction of
15
Fe25Al in Figure 10(b), the product gas has similar profile with that of Fe10Al, and the
16
only difference is that the concentration of CO2 is maintained at very high level between
17
the dramatic increase and decrease. During the reduction of Fe45Al as shown in Figure 16 ACS Paragon Plus Environment
Page 17 of 26
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Energy & Fuels
1
10(c), only CO2 is detected, which indicates that CH4 is completely oxidised to CO2 and
2
H2O.
3
To avoid carbon deposition during the reduction of Fe2O3 with CH4, the best way is to
4
achieve a high conversion of CH4 to CO2 and H2O [33]. Figure 10(d) shows the
5
conversion of methane during the reduction of Fe10Al, Fe25Al and Fe45Al. It is found
6
that Fe10Al has the lowest conversion and never achieves the full conversion to CO2. The
7
peak value is around 94% during the reduction. Methane can be completely oxidised to
8
CO2 by both Fe25Al and Fe45Al but a longer duration time on the full conversion is
9
achieved by the sample of Fe45Al.
10
Based on these results, it can be summarised that the prepared alumina-supported Fe2O3
11
delivers a better reactivity and stability than unsupported materials during cyclic redox
12
experiments. In terms of the reaction rate, it decreases with the increase in Fe2O3 weight
13
content and the sample of Fe10Al possesses the highest reaction rate. To achieve a full
14
conversion of CH4 the solid conversion of Fe2O3 to Fe3O4 is required on the basis of
15
thermodynamics and experimental assessments. Within this conversion range, Fe45Al
16
shows the most stabilised reactivity as well as the highest rate of oxygen transport (ROT).
17
Besides, the gas analysis results reveal that Fe45Al exhibits the best performance on
18
methane conversion. It can be concluded that, therefore, Fe45Al could be the most
19
suitable oxygen carriers for chemical looping combustion of ultra low methane
20
conversion as far as the reactivity associated with gas analysis results are concerned.
21 22
17 ACS Paragon Plus Environment
Energy & Fuels
1.0
(a)
Gas concentration (%)
Gas concentration (%)
1.0
0.8 CO2
0.6
CH4 CO H2
0.4
0.2
0.0
(b)
0.8 CO2 CH4
0.6
CO H2
0.4
0.2
0.0 0
10
20
30
0
10
t (min)
1
20
30
t (min) 1.01
(c)
1.00
0.8
CO2
0.6
CH4 CO H2
0.4
0.2
Methane conversion
Gas concentration (%)
1.0
30 (d)
0.99 20 0.98 0.97 0.96
10
0.95 Methane conversion Duration
0.94
0.0 0
10
20
5
30
10
15
20
25
30
35
40
45
0 50
Fe2O3 content (wt%)
t (min)
2
Duration (min)
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 18 of 26
3
Figure 10: Gas concentration profile for (a) Fe10Al, (b) Fe25Al and (c) Fe45Al with 1
4
vol% CH4 at 1023 K and (d) CH4 conversion and the duration on high conversion.
5
3.4. The effect of reaction temperature, methane concentration and CO2
6
composition
7
In a real chemical looping combustion system for ultra low concentration methane, the
8
reduction occurred in the FR could be influenced by plenty of factors. In the present
9
study, the effects of reaction temperature, methane concentration and CO2 composition
10
are investigated and discussed. The reactivity on conversion of X=0.5, 0.8 and 1 is
11
selected to make comparison, which represented the reactivity during the transformation
12
of Fe2O3 into Fe3O4. The reaction rates at lower fractional conversion (
