Energy Fuels 2009, 23, 5276–5283
Published on Web 08/27/2009
: DOI:10.1021/ef900444d
Use of CaMn0.875Ti0.125O3 as Oxygen Carrier in Chemical-Looping with Oxygen
Uncoupling
Henrik Leion*
Department of Environmental Inorganic Chemistry, Chalmers University of Technology, S-412 96 Goteborg, Sweden
Yngve Larring, Egil Bakken, and Rune Bredesen
Department of Energy Conversion and Materials, SINTEF Materials and Chemistry, Oslo, Norway
Tobias Mattisson and Anders Lyngfelt
Department of Energy and Environment, Chalmers University of Technology, S-412 96 Goteborg, Sweden
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Publication Date (Web): August 27, 2009 | doi: 10.1021/ef900444d
Received May 12, 2009. Revised Manuscript Received July 23, 2009
Chemical-looping with oxygen uncoupling (CLOU) is a novel method to burn fuels in gas-phase oxygen
without the need for an energy-intensive air separation unit. The carbon dioxide from the combustion is
obtained separated from the nitrogen in the combustion air. The technique is based on chemical-looping
combustion (CLC) but does not involve any direct reaction between the fuel and oxygen carrier. Instead,
the CLOU process uses three steps in two reactors, one air reactor where a metal oxide captures oxygen
from the combustion air (step 1), and a fuel reactor where the metal oxide releases oxygen (step 2) and
where this oxygen reacts with a fuel (step 3). This means that the fuel burns directly with gaseous O2. In this
work CaMn0.875Ti0.125O3 will be used as oxygen carrier. Experiments were first performed with a
thermogravimetric analyzer (TGA). Here the sintering temperature, and thereby the porosity, for the
produced granulates was varied and optimized. The substitution of Ti on Mn sites in CaMnO3 was chosen
since this material showed no coke formation even in dry CH4 at high temperatures. This was followed by
fluidized bed experiments with both methane and petroleum coke as fuel. The CaMn0.875Ti0.125O3 particles
showed promising results both for the tests performed in TGA and in fluidized bed experiments.
CaMn0.875Ti0.125O3 released O2 both in inert and reducing atmosphere, making it a possible candidate
as oxygen carrier in CLOU.
costs of electricity with respect to a conventional power plant
differs between the CO2 capture techniques.3,4 In chemicallooping combustion (CLC) CO2 is inherently separated from
the other flue gas components, that is, N2 and unused O2.
Thus, no energy input is expended for the gas separation,
whereas other CO2 capture techniques, such as oxyfuel, pre- or
postcombustion involves a energy consuming, and thereby
costly, gas separation step.5
The idea of CLC originates from a patent for CO2 production by Lewis and Gilliland6 in 1954. An oxygen carrier is
circulated between two fluidized bed reactors, an air and a fuel
reactor. The fuel is introduced to the fuel reactor where it
reacts with an oxygen carrier (MexOy) to CO2 and H2O,
reaction 1. The reduced oxygen carrier is transported to
Introduction
It is today believed that the increase in greenhouse gas
concentrations has caused a rise in global temperature that in
turn has caused changes in today’s climate.1 Therefore a
reduction in emissions of greenhouse gases, and in particular
CO2, is necessary. It is today possible to store CO2 in depleted
oil- and natural gas fields as well as in deep coal beds.2 Also
storage in aquifers, which are geologically sealed formations
often filled with saline water, provides a large potential for
storage of CO2. This is today practiced in the North Sea
outside Norway at the Utsira Formation.2
In order to store CO2 produced by combustion, it first needs
to be separated from the combustion gases. Costs and efficiency losses due to storing and transport of CO2 to the storage
site can be assumed to be equal for all combustion processes.
However, the way of capturing CO2 differs between the
technologies. The loss in efficiency and increased production
(3) Davidson, J.; Thambimuthu, K. In Technologies for Capture of
Carbon Dioxide; 7th International Conference on Greenhouse Gas Control
Technologies, Vancover, Canada, 2004.
(4) Herzog, H. J. In CO2 Capture and Storage: Cost and Market
Potential; 7th International Conference on Greenhouse Gas Control Technologies, Vancover, Canada 2004.
(5) Metz, B.; Davidson, O.; de Coninck, H. C.; Loos, M.; Meyer, L. A.
IPCC Special Report on Carbon Dioxide Capture and Storage; Prepared
by Working Group III of the Intergovernmental Panel on Climate Change. In
IPCC, Cambridge University Press: Cambridge, United Kingdom and New
York, USA, 2005.
(6) Lewis, W. K.; Gilliland, E. R. Production of Pure Carbon Dioxide.
U.S. Patent 2665972, 1954.
*To whom correspondence should be addressed. Telephone: þ46-317722886. Fax: þ46-31-7722853. E-mail: [email protected].
(1) Rosenzweig, C.; Karoly, D.; Vicarelli, M.; Neofotis, P.; Wu, Q.;
Casassa, G.; Menzel, A.; Root, T. L.; Estrella, N.; Seguin, B.;
Tryjanowski, P.; Liu, C.; Rawlins, S.; Imeson, A. Nature 2008, 453
(15), 353–357.
(2) IPPC, I. P. o. C. C. Carbon Dioxide Capture and Storage; 2005.
r 2009 American Chemical Society
5276
pubs.acs.org/EF
Energy Fuels 2009, 23, 5276–5283
: DOI:10.1021/ef900444d
Leion et al.
the air reactor where it is oxidized back to its original state by
air, reaction 2.
Cn H2m þ ð2n þ mÞMex Oy T nCO2 þ mH2 O
þ ð2n þ mÞMex Oy -1
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Publication Date (Web): August 27, 2009 | doi: 10.1021/ef900444d
O2 þ2Mex Oy -1 T 2Mex Oy
ð1Þ
ð2Þ
This gives one stream of oxygen-depleted air leaving the air
reactor and one stream of combustion gases, which mainly
consists of CO2 and H2O, leaving the fuel reactor. The water is
easily condensed and the CO2 can, after compression, be
transported to an underground storage location. Because
the fuel never meets the air, CO2 is inherently separated
without any direct loss in efficiency.
The CLC process has been successfully demonstrated using
gaseous fuel with different oxygen carriers in several prototype units based on interconnected fluidized beds.7-17 An
overview of literature concerning CLC is given by Lyngfelt
et al.18 or Hossain and de Lasa.19 The major part of the work
concerning CLC has so far been with gaseous fuel, such as
natural gas or methane, but recent research is aiming to adapt
the technology to solid fuels.20-22 One way of doing this is to
introduce the coal directly to the fuel reactor where the
gasification of the coal and subsequent reactions with the
Figure 1. Schematic picture of the CLOU-process. Two interconnected fluidized bed reactors, one air and one fuel reactor, with
circulating oxygen carrying particles. The fuel is assumed to be pure
carbon, C.
metal oxide particles will occur simultaneously.23-29 Another
way is to use an oxygen carrier that releases O2 in the fuel
reactor and actually burns the fuel with gas-phase oxygen.30-33
The last alternative is referred to as chemical-looping with
oxygen uncoupling (CLOU)
CLOU involves three steps in two reactors, as shown in
Figure 1. In the air reactor, an oxygen carrier captures oxygen
from the combustion air (step 1), according to reaction 2. The
oxygen carrier is transported to the fuel reactor, where it
releases oxygen (step 2) according to the reverse reaction:
(7) Lyngfelt, A.; Thunman, H. Construction and 100 h of operational
experience of a 10-kW chemical-looping combustor. Carbon Dioxide
Capture for Storage in Deep Geologic Formations;Results from the CO2
Capture Project 2005, 1, 625–645.
(8) Lyngfelt, A.; Kronberger, B.; Adanez, J.; Morin, J.-X.; Hurst, P.
In The GRACE project. Development of oxygen carrier particles for
chemical-looping combustion. Design and operation of a 10 kW chemicallooping combustor; The 7th International Conference on Greenhouse Gas
Control Technologies Vancouver, Canada, 2004.
(9) Ryu, H.-J.; Jin, G.-T.; Bae, D.-H.; Yi, C.-K. Continuous Operation
of a 50 kWth Chemical-Looping Combustor: Long-Term Operation with
Ni- and Co-Based Oxygen Carrier Particles; Presented at the 5th ChinaKorea Joint Workshop on Clean Energy Technology, October 25-28,
Qingdao University, China, 2004; pp 221-230.
(10) Abad, A.; Mattisson, T.; Lyngfelt, A.; Ryden, M. Fuel 2006, 85
(9), 1174–1185.
(11) Johansson, E.; Mattisson, T.; Lyngfelt, A.; Thunman, H. Fuel
2006, 85 (10-11), 1428–1438.
(12) de Diego, L. F.; Garcia-Labiano, F.; Gayan, P.; Celaya, J.;
Palacios, J. M.; Adanez, J. Fuel 2007, 86 (7-8), 1036–1045.
(13) Adanez, J.; Gayan, P.; Celaya, J.; de Diego, L. F.; GarciaLabiano, F.; Abad, A. Ind. Eng. Chem. Res. 2006, 45 (17), 6075–6080.
(14) Johansson, E.; Mattisson, T.; Lyngfelt, A.; Thunman, H. Chem.
Eng. Res. Des. 2006, 84 (A9), 819–827.
(15) Abad, A.; Mattisson, T.; Lyngfelt, A.; Johansson, M. Fuel 2007,
86 (7-8), 1021–1035.
(16) Linderholm, C.; Abad, A.; Mattisson, T.; Lyngfelt, A. Int. J.
Greenhouse Gas Control 2008, 2 (4), 520–530.
(17) Proll, T.; Kolbitsch, P.; Bolhar-Nordenkampf, J.; Hofbauer, H.
In A Dual Circulating Fluidized Bed (DCFB) System for Chemical
Looping Processes; AIChE Annual Meeting, Philadelphia, USA, November
16-21, 2008.
(18) Lyngfelt, A.; Johansson, M.; Mattisson, T. In Chemical-looping
combustion - Status of development; 9th International Conference on
Circulating Fluidized Bed (CFB-9), Hamburg, Germany, May 13 -16, 2008.
(19) Hossain, M. M.; de Lasa, H. I. Chem. Eng. Sci. 2008, 63, 4433–
4451.
(20) Berguerand, N.; Lyngfelt, A. Fuel 2008, 87, 2713–2726.
(21) Berguerand, N.; Lyngfelt, A. Int. J. Greenhouse Gas Control 2008,
2 (2), 169–179.
(22) Andrus, H. E., Jr.; Chiu, J. H.; Liljedahl, G. N.; Stromberg, P. T.;
Thibeault, P. R.; Jain, S. C. ALSTOM’s hybrid combustion-gasification
chemical looping technology development; Proceedings - 22nd Annual
International Pittsburgh Coal Conference, 2005; pp 122/1-122/20.
(23) Scott, S. A.; Dennis, J. S.; Hayhurst, A. N.; Brown, T. AIChE J.
2006, 52 (9), 3325–3328.
(24) Lyon, R. K.; Cole, J. A. Combust. Flame 2000, 121, 249–261.
(25) Dennis, J. S.; Scott, S. A.; Hayhurst, A. N. J. Energy Inst. 2006,
79 (3), 187–190.
(26) Rubel, A.; Liu, K.; Neatherya, J.; Taulbee, D. Fuel 2009, 88, 5.
2My Ox T2My Ox -1 þ O2 ðgÞ
ð3Þ
The released oxygen reacts with a fuel (step 3), in this case
assumed to be only carbon C, according to normal combustion:
C þ O2 ðgÞ f CO2
ð4Þ
The reduced oxygen carrier is then recirculated to the air
reactor to be regenerated, that is, step 1. Since reactions 2 and
3 cancel each other, the net reaction over the CLOU system is
simply reaction 4, that is, normal combustion. This means that
the total heat release over the fuel and air reactor is the same as
for conventional combustion or regular CLC. The added
advantage compared to regular CLC is that the slow gasification step when employing solid fuels is eliminated. This has
the implication that much less oxygen carrier material is
needed in the system, which will also reduce the reactor size
and thereby associated costs.
(27) Gao, Z.; Shen, L.; Xiao, J. J. Chem. Ind. Eng. 2008, 59 (4), 1242–
1250.
(28) Leion, H.; Mattisson, T.; Lyngfelt, A. CO2 Capture from Direct
Combustion of Solid Fuels with Chemical-Looping Combustion; In 33rd
International Technical Conference on Coal Utilization & Fuel Systems,
Clearwater Florida, USA, 2008.
(29) Leion, H.; Mattisson, T.; Lyngfelt, A. Effects of steam and CO2 in
the fluidizing gas when using bituminous coal in Chemical-Looping
Combustion; In The 20th International Conference on Fluidized Bed Combustion, Xian, China, 2009.
(30) Mattisson, T.; Lyngfelt, A.; Leion, H. Int. J. Greenhouse Gas
Control 2009, 3 (1), 11–19.
(31) Mattisson, T.; Leion, H.; Lyngfelt, A. Fuel 2009, 88 (4), 683–690.
(32) Leion, H.; Mattisson, T.; Lyngfelt, A. Combustion of a German
Lignite Using Chemical-Looping with Oxygen Uncoupling (CLOU); In
33rd International Technical Conference on Coal Utilization & Fuel Systems,
Clearwater Florida, USA, 2008.
(33) Leion, H.; Mattisson, T.; Lyngfelt, A. Using Chemical-Looping
with Oxygen Uncoupling (CLOU) for Combustion Six Different of Solid
Fuels; In 9th International Conference on Greenhouse Gas Control Technologies, Washington D.C, USA, 2008.
5277
Energy Fuels 2009, 23, 5276–5283
: DOI:10.1021/ef900444d
Leion et al.
In previously published work the oxygen carrier used in
CLOU has been Cu-based.30-32 In this work CaMn0.875
Ti0.125O3 will be used as oxygen carrier. Bakken et al.34 have
showed that CaMnO3 material has a continuous loss of
oxygen when reducing the partial pressure of O2 stepwise
from 100 kPa to approximately 1 kPa O2(g) by reduction of
Mn4þ ions to Mn3þ. Previous tests of this material shows that
it has a tendency to decompose at high temperature into
Ca2MnO4 and CaMn2O4,34 and this might restrict the full
reoxidation of this material when cycling between reducing
and oxidizing conditions. The material has therefore been
stabilized by Ti in order to restrict this type of decomposition.
Experiments were first performed with a thermogravimetric
analyzer (TGA) in order to verify the improved stability.
Further, the reactivity at different sintering temperatures were
investigated and optimized. This was followed by fluidized bed
experiments with both petroleum coke and methane as fuel.
Figure 2. Conical fluidized-bed reactor of quartz.
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Publication Date (Web): August 27, 2009 | doi: 10.1021/ef900444d
Experimental Section
the bed by 0.7%, was added from the top of the reactor, falling
into the bed. During the reduction with petroleum coke, the bed
was fluidized with 50% steam in N2 at a flow of 600 mLn/min.
The petroleum coke had a size of 0.125-0.180 mm, that is,
somewhat larger than the oxygen carrier particles. An elementary analysis of the petroleum coke is presented in Table 1. In the
experiment where methane was used as fuel, a flow of 450 mLn/min
consisting of pure CH4 was added from the bottom of the reactor.
Between each oxidizing and reducing period, inert nitrogen with
a flow of 450 mL/min gas was introduced for 60 s for both the
methane and petroleum coke experiments.
The height of the bed was 30 mm, in both reactors, when the
bed was not fluidized. The temperature was measured 5 mm
under and 10 mm above the porous quartz plate, using Pentronic CrAl/NiAl thermocouples enclosed inconel-600 in quartz
shells. All temperature references for the fluidized bed experiments in this paper refer to the upper thermocouple in the actual
bed. The steam was delivered by a steam generator (Cellkraft,
Precision Evaporator E-1000).
The gas from the reactor was led to an electric cooler, where
the steam was removed, and then to a gas analyzer (Rosemount
NGA-2000) where the concentrations of CO2, CO, CH4, and O2
were measured in addition to the volumetric gas flow. For the
experiments with petroleum coke, a small flow of N2 was added
in the top of the reactor, partly to sweep the fuel down into the
reactor and partly to serve as carrier gas after the steam had been
removed in the cooler. A schematic layout of the laboratory
setup is presented in Figure 3.
Data Evaluation. The gas yield (γ) is used to quantify the
conversion of gas in the gaseous fuel fluidized bed experiments.
γ is the fraction of CO2 in the outgoing gas divided with the sum
of the fractions of carbon-containing gases in the outgoing gas.
Hence, a γ of 1 corresponds to total conversion of the fuel to
CO2. With CH4 as reducing gas this gives:
Production of Particles. The initial activities were to optimize
the material composition, that is, the Ti concentration in
CaMn1-xTixO3, and optimising the sintering temperature of
the freeze granulated materials. Small batches of Ca0.97Mn1-xTixO3 (x = 0.1, 0.125, 0.175, and 0.25) were synthesized by
spray pyrolysis and by freeze granulation before the capacities
and kinetics were measured in a TGA.
Powders of CaMn1-xTixO3 were synthesized by spray pyrolysis using standardized aqueous solutions of 2 M Ca(NO3)2,
2 M Mn(NO3)2, and 0.85 M Ti-isopropoxide citric acid complex. The solutions were mixed to the desired stoichiometry. The
solutions were sprayed with a two-phase nozzle at a rate of 8 L/h
directly into a rotating furnace at 875 °C. The outlet temperature
was 560 °C. The as-prepared powders were calcined at 800 °C for
6 h. The calcined powders were ball milled (YSZ balls) in water
for 18 h, dried, ground, and sieved to a fine size.
Porous granulates of proper size of CaMn1-xTixO3 were
made by freeze granulation and freeze-drying. The powder from
spray pyrolysis was mixed with water, dispersant, binder, and
antifoam additive and then ball milled. The obtained slurry was
pumped through a nozzle and sprayed into liquid N2. This gave
frozen granulates that contained water. The water was removed
by freeze-drying. Granulates were finally sintered and sieved to
obtain the desired properties and size fractions.
Fluidized Bed Experiments, Setup. The solid fuel experiments
were conducted in a fluidized-bed reactor of quartz presented in
Figure 2. In order to achieve good solids mixing between fuel
and oxygen carrier, the reactor was conically shaped just above
the distributor plate to enhance mixing in the bed. The reactor
had a total length of 870 mm with a porous quartz plate placed
370 mm from the bottom of the reactor. The same reactor
has previously been used in CLC and CLOU experiments at
Chalmers.33,35
The fluidized bed experiments with methane were made with a
straight reactor with inner diameter of 22 mm. The reactor had
the same length as the conical reactor, and the porous quartz
plate was located at the same position as in the conical reactor,
that is, 370 mm from the bottom.
A total of 15 g of oxygen carrier of size 0.09-0.125 mm was
placed on the porous plate and tested under alternating oxidizing and reducing atmospheres. During the oxidation, a flow of
1000 mLn/min (0 °C, 100 kPa) with 5.5% of O2 in N2 was
added from the bottom of the reactor. During reduction with
petroleum coke, 0.05 g of fuel, equivalent to a mass reduction of
γ ¼
xCO2
xCO2 þ xCO þ xCH4
ð5Þ
where xi is the fraction of component i in the outgoing gas flow.
All gas measurements were made on dry gases downstream of
the cooler. However, eq 5 is independent of whether the gas is
wet or dry.
The degree of mass-based conversion, ω, is used to describe
the oxygen uptake of the oxygen carrier. ω is defined as the mass
of the oxygen carrier divided with the mass of the oxygen carrier
in its most oxidized state:
m
ω ¼
ð6Þ
mox
(34) Bakken, E.; Norby, T.; Stoelen, S. Solid State Ionics 2004, 176
(1-2), 217–223.
(35) Leion, H.; Mattisson, T.; Lyngfelt, A. Fuel 2007, 86 (12-13),
1947–1958.
where m is the mass of the oxygen carrier at any given time, and
mox is the mass of the oxygen carrier at the end of the oxidation.
5278
Energy Fuels 2009, 23, 5276–5283
: DOI:10.1021/ef900444d
Leion et al.
Table 1. Fuel Analysis of Petroleum Coke
proximate [wt %, as received]
[wt% dry ash free]
ultimate [wt %, dry ash free]
Hi [MJ/kg](as received)
moisture
ash
combustibles
volatiles
C
H
N
S
O
30.9
8.0
0.5
91.5
10.9
88.8
3.1
1.0
6.6
0.5
Table 2. The Capacity of CaMn1-xTixO3 (wt % O2), for reduction
with 10% CH4 and 25% CO2 and Oxidation with Air at Three
Different Temperaturesa
CaMn1-xTixO3
800 °C
900 °C
1000 °C
x = 0.1
x = 0.125
x = 0.175
x = 0.25
9.4
9.4
9.0
9.4
9.4
9.1
8.6
8.6
8.3
9.1
9.2
8.9
a
Values for capacity of CaMn1-xTixO3 (wt % O2) using 5% H2 and
25% CO2 gives nearly the same results (deviation less than 0.1 wt %).
Table 3. Maximum Reaction Rate in the CH4-Containing Gas
Mixture at Different Temperatures As a Function of Composition As
Obtained in the TGA (-dω/dt (g/kg s))
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Figure 3. Schematic layout of the laboratory setup.
Note that due to the different oxygen uptake at different
temperatures of the oxygen carrier used in this work, mox is
dependent on the temperature of the oxygen carrying particles.
In TGA m is directly measured, but since this is not possible in
the fluidized bed experiments ω was calculated as the time
integral of exhaust gas concentrations through eq 7,
Rt
_ O
ωi ¼ ωi -1 - t01 nM
ð7Þ
mox ð4xCO2 þ 3xCO - xH2 Þ dt
CaMn1-xTixO3
800 °C
900 °C
1000 °C
x = 0.125
x = 0.175
x = 0.25
0.47
1.00
1.49
0.51
1.04
1.47
0.70
1.19
1.57
0.67
1.12
1.47
Table 4. Maximum Reaction Rate in 10% CH4 and 25% CO2 at
Different Temperatures As a Function of Temperature Treatment for
Powder Prepared by Spray Pyrolysis (-dω/dt (g/kg s))
where n_ is the total molar flow rate, and MO is the molar mass of
oxygen.
In CLC-literature, the degree of conversion of the oxygen
carrier, X, is often used to quantify the conversion of the oxygen
carrier. X can easily be converted to ω by:
ω ¼ 1 þ RO ðX -1Þ
x = 0.1
CaMn0.875Ti0.125O3
800 °C
900 °C
1000 °C
ð8Þ
1050 °C (125-180
μm)
1200 °C
(powder)
1250 °C (125-180
μm)
2.2
3.1
3.9
1.4
2.3
2.9
1.4
2.0
2.4
Table 5. Maximum Reaction Rate in 5% H2 and 25% CO2 at
Different Temperatures As a Function of Temperature Treatment
(Powder Prepared by Spray Pyrolysis) (-dω/dt (g/kg s))
where RO is the oxygen transfer capacity, that is, the fraction of
available oxygen in the oxygen carrier. Again note that also RO
will be dependent on temperature for the oxygen carrier used in
this work.
In solid fuel experiments the rate of conversion for the
petroleum coke is given as an average rate, calculated from:
1 mt
rAve ¼
ð9Þ
mtot t
CaMn0.875Ti0.125O3
800 °C
900 °C
1000 °C
1050 °C (125-180
μm)
1200 °C
(powder)
1250 °C (125-180
μm)
1.0
1.0
1.0
0.4
0.7
0.7
0.3
0.6
0.6
content up to a Ti fraction of 0.125, after which it started
to decrease. The mechanism behind this behavior is not
completely understood, but the same trend was seen for all
operating temperatures. Since a Ti content of 0.125 showed
the best performance, this composition was chosen for
further development and testing.
Reactivity of Oxygen Carrier. The sintering profile of
CaMn0.875Ti0.125O3 was determined by dilatometry, and
the reactivity of samples annealed at different temperatures
was measured by TGA. Tables 4 and 5 show the reactivity of
granulates (125-180 μm) sintered at 1050 and 1250 °C and
the reactivity of powder that was annealed at 1200 °C. The
maximum rate is given as -dω/dt. It is clear that the reaction
rate decreases with increasing sintering temperature. In this
work the appropriate sintering conditions for sufficient
attrition strength of the particles were determined to be 3 h
at 1200 °C.
Available Oxygen in Tested Particles. The fraction of
available oxygen, RO, in the oxygen carriers of CaMn0.875Ti0.125O3 was obtained when samples were reduced in diluted
hydrogen, 5% H2, 25% CO2, in Ar for 30 min, and in diluted
where t is the time elapsed since the start of the cycle, mtot is the
total mass of carbon converted during the entire reducing
period, and mt is the mass of carbon converted up until time t.
The total carbon in the CLC solid fuel experiments is determined
from the integration of the outgoing CO and CO2 concentrations. The rate used in this paper, is the average rate for a mass
conversion of the fuel of 95%, identical to the rate used in
previously published work.35
Results
TGA Experiments. Optimizing Ti Content. Only small
differences in the capacity and the reactivity were observed
for oxygen carriers with different Ti fractions. Tables 2 and 3
show the oxygen capacities and the maximum reaction rates,
respectively, in reducing atmosphere for 10% CH4 and 25%
CO2 in Ar and the milder reduction of 5% H2 and 25% CO2
in Ar. The material was in the form of a fine powder, and the
tests were made at temperatures of 800, 900, and 1000 °C.
Not surprisingly, the reaction rate increased with increasing
temperature. The reactivity increased with increasing Ti
5279
Energy Fuels 2009, 23, 5276–5283
: DOI:10.1021/ef900444d
Leion et al.
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Publication Date (Web): August 27, 2009 | doi: 10.1021/ef900444d
Figure 4. Oxygen release as a function of time for CaMn0.875Ti0.125O3 under reducing conditions. Capacity as a function of time
using 5 min Ar(g) and then 30 min of Ar(g) with 5% H2(g) and 25%
CO2(g).
Figure 6. Experiments with CaMn0.875Ti0.125O3 in the TGA using
particles of size 1.4-4.0 mm at 950 °C for 45 cycles with: 5 min of
10% CH4, 1 min of Ar(g), and 5 min of 20% O2.
Figure 7. Concentration profile of oxygen for the inert phase
between two oxidation phases for a number of different temperatures as measured in the fluidized bed reactor using the CaMn0.875Ti0.125O3 oxygen carrier.
Figure 5. Oxygen released a function of time CaMn0.875Ti0.125O3
under reducing conditions. Capacity as a function of time using 5
min of Ar(g) and then 30 min of Ar(g) with 10% CH4(g) and 25%
CO2(g).
methane, 10% CH4, 25% CO2, in Ar for 30 min, at 800, 900,
and 1000 °C. The results are presented in Figures 4 and 5 and
are in the range of 8-9 wt % of the sample mass with the
lowest values at the highest temperature.
Mechanical Strength and Chemical Stability. Crushing
strength is the average force (N) it takes to crush one particle.
As a very general rule, particles used in a fluidized bed should
have a crushing strength of at least 1 N. The crushing
strength of CaMn0.875Ti0.125O3 particles sintered at 1050,
1150, and 1250 °C were measured. From these results, as well
as the reactivity investigations in TGA, a larger batch of
particles was sintered at 1200 °C since this was considered to
give particles with sufficient strength for fluidized bed experiments. Table 6 presents the crushing strength of all
sintered particles with a size range of 125-180 μm. Note
that crushing strength values found in the literature are
generally made on particles in the size range 180-250
μm,36 this must be considered when comparing the results
in this work to other published results.
In general, the reactivity falls with rising sintering temperature since the surface and porosity of the particles
decreases, Tables 4 and 5. However, the mechanical properties become more favorable at higher sintering temperature.
Hence the optimal sintering temperature is a compromise
between reactivity and mechanical strength.37
Further TGA experiments with the batch sintered at
1200 °C included 45 cycles at 950 °C using 10% CH4 in Ar
during a 5 min reduction and 20% O2 during a 5 min
oxidation, see Figure 6. In these tests CO2 was omitted,
and the conditions were thus rather severe during the reducing gas steps. The oxygen loss and uptake in CaMn0.875Ti0.125O3 was reversible through the whole test, and no coke
on the particles was found during any of the reductions.
Fluidized Bed Experiments with Gas. A CLOU oxygen
carrier releases O2 as long as the partial pressure of oxygen
around the particles is lower than the thermodynamic partial
pressure for that particular material. To evaluate the magnitude of O2 release, a few cycles were made at different
temperatures but with only inert N2 instead of fuel. Figure 7
gives the O2 concentration as a function of time. The inlet gas
is switch from 5.5% O2 in nitrogen to pure nitrogen after 30 s.
After 390 s, the flow is switched back to 5.5% O2. This means
that any oxygen in the outgoing flow between 30 and 390 s is
(36) Johansson, M.; Mattisson, T.; Lyngfelt, A. Therm. Sci. 2006, 10
(3), 93–107.
(37) Mattisson, T.; Johansson, M.; Lyngfelt, A. Fuel 2006, 85 (5-6),
736–747.
Table 6. Crushing Strength (in N) for a Particle of 0.125-0.180 mm at
Different Sintering Temperatures
sintering temperature (°C)
crushing strength (N)
1050
0.6
1150
1.09
1200
1.25
1250
2.48
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Figure 8. The O2 concentration (solid curve) variation with temperature (dashed curve) during a fluidized bed experiment. Inlet O2
concentration is 5.5%.
Figure 10. Concentration profile for a reducing cycle for CaMn0.875Ti0.125O3 sintered at 1200 °C. The inlet gas is pure CH4, and the
temperature is 900 °C.
Figure 9. The O2 concentration (solid curve) variation with temperature (dashed curve) during a fluidized bed experiment. Inlet O2
concentration is 5.5%.
released by the particles. As also seen in Figure 7, higher
oxygen release at higher temperatures also gives a higher
oxygen uptake in the following oxidation. This is promising
since it indicates that the material can be fully recovered, in
terms of oxygen content, at all temperatures. However, there
is no apparent difference between 900 and 950 °C. Also note
the very low O2 level at 630 °C.
Apart from the O2 release phenomena presented in
Figure 7, which is dependent on the surrounding partial
pressure of oxygen, the amount of oxygen that the
CaMn0.875Ti0.125O3 particles can hold during the fluidized
bed experiments also seemed to be dependent on the temperature of the particles. Fully oxidized particles were heated
and cooled down while being exposed to a fluidizing gas
containing 5.5% O2. Figures 8 and 9 give the O2 concentration in the outgoing flow during heating and cooling of
CaMn0.875Ti0.125O3. As seen, the particles release oxygen
when the temperature rises and take up oxygen when the
temperature falls. This means that the available amount of
oxygen in the particles is dependent on the temperature of the
material. This is in line with the lower RO value obtained at
higher temperatures in Figures 4 and 5.
Figure 10 shows the outlet dry gas concentrations as a
function of time for the third reducing period with 15 g of
CaMn0.875Ti0.125O3 sintered at 1200 °C. The CH4 is turned
on at time 0, but the residence time in the system delays the
response with around 20 s before the CO2 rapidly increases.
After roughly 20 s the CO2 concentration reaches a maximum. The CH4 is then turned off and replaced by inert N2.
O2 is released during the inert phase as well as during the
Figure 11. Gas yield, γ, as a function of ω for experiments
with CaMn0.875Ti0.125O3 and CH4 as fuel in the fluidized bed
reactor.
beginning of the reduction phase. This gives complete conversion of the incoming CH4 to CO2 up to a point where the
particles are unable to release sufficient amount of O2,
resulting in a decreased O2 concentration and increasing
amounts of CH4. Note that no CO at all is detected throughout the whole cycle.
Figure 11 presents the gas yield, γ, as a function of ω for
several particles using methane as fuel. The particles sintered
at 1050 °C have higher reactivity than the particles sintered at
1200 °C. A higher temperature in the bed also gives a slightly
higher reactivity. However, even at the lower bed temperature, particles sintered at 1200 °C have a conversion of CH4
that is higher than any previously investigated manganeseor iron-based particle found in the literature.18,19
Figure 12 shows the concentration of O2 at 900 °C from
Figure 7, which was made on fresh particles, together with an
identical cycle made on the particle that bad been used
during the methane experiments. These two concentration
profiles are so similar that they are actually hard to distinguish. Both show a gradually decreasing release of O2 during
6 min in a flow of inert nitrogen. If there would have been no
release of O2 from the particles, then the O2 concentration
would be expected to decrease to zero within 20 s, such as in
the case of 630 °C in Figure 7. Over 30 reducing cycles with
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Leion et al.
Table 7. Conversion Times and Conversion Rates with Petroleum
Coke at 900 and 950 °C
95% conversion 900 °C
80% conversion 900 °C
95% conversion 950 °C
80% conversion 950 °C
rate [%/min]
7.5
4.2
4.5
2.7
12.8
19.3
21.4
29.6
clear that a higher temperature increases the conversion rate
and decreases the conversion time.
Fluidization Properties. In the fluidized bed experiments,
pressure drop measurements over the bed were used to
determine if the bed was fluidized or not. The CaMn0.875Ti0.125O3 particles sintered at 1050 °C generally showed very
good fluidization properties, and the sample appeared unaffected when removed from the reactor. However, only a
few cycles were performed with these particles, and as the
mechanical strength of the particles was rather low, use in a
continuous system for longer periods of time may not be
feasible.
The CaMn0.875Ti0.125O3 particles sintered at 1200 °C
showed very good fluidization properties for temperatures
up to 900 °C as long as the particles were not reduced too far.
When the reduction of the particles was moderate, the
particles showed good fluidization properties also at higher
temperatures. Further, in the cases where defluidization
occurred due to a high degree of reduction at higher temperatures, the particles could easily be fluidized again by
lowering the temperature to 850 °C or lower. Also, as the
sample was removed from the reactor the particles looked
unaffected by the experiment and no sign of defluidization
could be seen.
X-ray Diffraction Investigations. An XRD measurement
shows that the sample consisted of Ca0.97(Mn,Ti)O3 and very
small amounts of CaMn2O4 and Ca2MnO4 before the TGA
experiment. The XRD pattern of the sample after the TGA
experiments was of rather low quality, which limits the
degree of information obtained. However, it could be seen
that the amount of CaMn2O4 and Ca2MnO4 had increased
compared to Ca0.97(Mn,Ti)O3, indicating that the oxidation
is not 100% complete.
In-situ XRD measurements of Ca097Mn0.875Ti0.125O3
have been performed at 800 °C in diluted hydrogen and air
to check structure stability and recoverability. The sample
was evenly distributed on a Pt strip that was heated to obtain
the desired temperature. The sample was placed in an in situ
cell allowing for various gas mixtures. The gas composition
and gas flows were regulated with mass flow controllers.
In air, before the cycle test started, the main crystal phase is
Ca097Mn0.875Ti0.125O3 with unit cell dimensions a (Å) = 5.351,
b (Å) = 7.554, c (Å) = 5.371, V (Å3) = 217.10 (Vc = 54.28).
After reduction, the main constituent is Ca0.97Mn0.875Ti0.125O2
with unit cell dimensions; a (Å) = 4.679. This shows a full
reduction of the perovskite to an oxide with cubic structure.
The proposed reduction mechanism is:
Ca0:97 Mn0:875 Ti0:125 O3 f Ca097 Mn0:875 Ti0:125 O3 -δ
Figure 12. Concentration profile for the first and last inert phase
between two oxidation phases for CaMn0.875Ti0.125O3 sintered at
1200 °C. The temperature is 900 °C and O2 concentration is 5.5% in
the oxidation phases and 0% in the inert phase inbetween.
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conversion time [min]
Figure 13. Concentration profiles during the reduction with petroleum coke in the fluidized bed reactor. The inlet H2O content is 50%
and the temperature is 950 °C.
CH4 were made at different temperatures between the cycles
presented in Figure 12. Still, the particles did not show any
sign of decreased reactivity, that is, change in the rate and
ability to release and take up O2.
Fluidized Bed Experiments with Petroleum Coke. Figure 13
shows the outlet gas concentrations after condensation of
water as a function of time for a reducing period with
petroleum coke as fuel and at a temperature of 950 °C with
50% steam in the fluidizing gas. The inert period with 100%
N2 in the fluidizing gas starts at time equal to zero, and the
actual reduction starts 1 min later when the petroleum coke is
added and the fluidizing gas is switched to 50% steam in N2.
The initial transient decrease in O2 is due to back mixing, but
after half a minute more-or-less all O2 originates from the
CaMn0.875Ti0.125O3 particles.
When the fuel is added the CO2 concentration increases as
the petroleum coke reacts with the O2 released from the
particles. Small peaks of CO and CH4 are initially detected
and are most likely due to volatiles released from the
petroleum coke that do not have sufficient time to react.
The CO2 gradually decreases during the whole cycle. The
pulse-like behavior of the CO2 concentration is due to the
irregular feeding of water in the pump in the steam generator.
Small amounts of O2 are visible for a large part of the period,
indicating that the CaMn0.875Ti0.125O3 continues to release
O2. Almost no CO is detected apart from the initial volatiles,
indicating that CaMn0.875Ti0.125O3 is able to completely
convert the fuel to CO2.
Table 7 presents the conversion times and the average
conversion rates for 95 and 80% conversion of the fuel. It is
f Ca097 Mn0:875 Ti0:125 O2
ð10Þ
The reoxidation of Ca097Mn0.875Ti0.125O2 in the XRD
setup was not 100% complete. The main oxidized constituent is most probably the partially oxidized Ca3Mn3O8.
These results indicate that the structures can be slow to
reoxidize, after prolonged reduction, in this case more than
24 h. The perovskite phase has been shown to be stable down
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Leion et al.
to ABO2.3-2.5 for some perovskites, so the phase transfer to a
cubic structure is expected when this material is reduced to
the final state. Under conditions in a real CLC-system the
material will not be reduced so extensively, and this phase
change should be avoided. It can still be noticed that the
material can be reoxidizing back to the original perovskite
structure.
Thermochemical Analysis. The enthalpy of oxidation of
CaMn0.875Ti0.125O3-δ was measured by combined TG-DSC
at 1000 °C when the material is oxidized according to:
!
δ0 -δ
O2 ðgÞ
CaMn0:875 Ti0:125 O3 -δ þ
2
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¼ CaMn0:875 Ti0:125 O3 -δ0
the temperature during oxidation of CaMn0.875Ti0.125O3
seems to be of less importance. Figure 7 show that the particles
release roughly the same amount of oxygen at 900 and 950 °C.
However, Figures 4 and 5 indicate that the amount of available
oxygen is less at higher temperatures.
It is possible that further improvements of the particles can
lead to better fuel conversion, for example the particles
sintered at 1050 °C showed a very high reactivity, but may
be too soft for long-term operation. Doping with other
materials may improve the properties or decrease the cost.
Thus, CaMn0.875Ti0.125O3 showed good fluidization properties and was chemically stable over a large number of cycles
making it an interesting oxygen carrier for CLOU application.
Even if the fuel conversion for CaMn0.875Ti0.125O3 is lower
than for Cu-based particles, the price of CaMn0.875Ti0.125O3 is
expected to be lower than for Cu particles.
ð11Þ
The obtained enthalpy is similar to the enthalpy of oxidation of CaMnO3-δ,34 and when taking the experimental
uncertainties of the TG-DSC into consideration, the experimental values for these two materials were not possible to
distinguish. The measured enthalpy of oxidation is ΔoxHo =
-272 ( 40 kJ/(mol O2(g)). CaMn0.875Ti0.125O3 will thus have
an exothermic reaction both in the fuel and air reactor as
long as the reduction of the material is moderate.
Conclusions
The use of CaMn1-xTixO3 has been investigated as a
possible oxygen carrier for CLOU. The reactivity has been
investigated in both a TGA and fluidized bed reactor using
both gaseous and solid fuel. Results are very promising, with
very high reaction rates, superior to all earlier-investigated
Mn-based oxygen carriers. The most important results found
in the current study are:
• The substitution of Ti on some Mn sites in CaMnO3 was
chosen for investigation since this material showed no
coke formation even in dry CH4. The amount of Ti was
optimized and the composition CaMn0.875Ti0.125O3 was
found most interesting for CLOU applications, releasing oxygen in the gas phase during inert and reducing
conditions.
• CaMn0.875Ti0.125O3 shows promising result both for the
tests performed in TGA and in fluidized bed experiments with very high rates of reaction found for both
petroleum coke and methane.
• No CO was detected during the solid fuel experiments in
the fluidized bed reactor.
• The particles sintered at 1200 °C showed good fluidization properties and were chemically stable over a large
number of cycles.
• The enthalpy of oxidation of CaMn0.875Ti0.125O3-δ when
the material is oxidized is ΔoxHo = -272 ( 40 kJ/(mol
O2(g)) at 1000 °C. Thus, the material will have an
exothermic reaction both in the fuel and air reactor.
Discussion
The concept of CLOU has been demonstrated using
CaMn0.875Ti0.125O3 as oxygen carrier. The rate of conversion
in the solid fuel experiments with petroleum coke is lower than
previously published experiments with a Cu-based oxygen
carrier.31,33 The reason for this lower conversion rate is that
the release of oxygen from the particles is not as fast as in the
case of a Cu-based oxygen carrier. However, compared to
regular CLC at the same temperature, the conversion of the
fuel is faster.35 Also, all fuel in these experiments was fully
converted to CO2, except for some small amounts likely due to
volatile release in the beginning of the cycle. This is in line with
previous CLOU results. And just as with Cu-based CLOU,
the reactions in both the fuel and air reactor are exothermic for
CaMn0.875Ti0.125O3.This reduces the needed particle circulation in an actual system, as compared to a previously
published CLC fuel reactor system.35
One of the main differences for CaMn0.875Ti0.125O3 compared to Cu is the oxidation of particles. Oxidation of Cu is
strongly governed by thermodynamics, and thereby temperature, which limits the possible operation temperatures, whereas
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