Evaluation of spray dried oxygen carriers based on manganese ore and
Ca(OH)2 for chemical-looping with oxygen uncoupling
Sebastian Sundqvist†,*, Tobias Mattisson‡, Jasper van Noyen§, Henrik Leion†,
Anders Lyngfelt‡.
† Department of Chemical and Biological Engineering, Division of Environmental Inorganic
Chemistry, Chalmers University of Technology, S-412 96 Goteborg, Sweden
‡ Department of Energy and Environment, Division of Energy Technology, Chalmers University of
Technology, S-412 96 Goteborg, Sweden
§
Flemish Institute for Technological Research (VITO), Unit Materials, B-2400 Mol, Belgium
*Corresponding author
Abstract
Chemical Looping Combustion (CLC) is an innovative method of combustion with inherent CO2
capture. By using two fluidized bed reactors with an oxygen carrier, typically a metal oxide, to
transport oxygen between the reactors. One reactor, the air reactor (AR), has oxidizing atmosphere
with air and the other, the fuel reactor (FR), has reductive atmosphere with fuel. The oxygen carrier is
oxidized in the AR and then transported to the FR where it reacts with the fuel to produce a product
stream of CO2 and H2O. The water can then be easily removed through cooling and the pure CO2
stream can then be compressed and stored.
CaMnO3 has earlier been found to be a highly interesting oxygen carrier, and it can be produced from
Ca(OH)2 and Mn3O4. However, in order to reduce costs it would be highly advantageous if this
material can be produced using natural minerals or ores. In this work five different oxygen carriers of
CaMnO3 have been produced utilizing different manganese ores as starting material. The manganese
ores were mixed with commercial Ca(OH)2 and spray-dried to obtain particles of feasible size. The
experiments to investigate the oxygen carriers produced were carried out in a fluidized batch reactor at
temperatures ranging from 850°C to 1050°C. Both the uncoupling behavior as well as the reactivity
with methane was investigated in this unit. The particles exhibited CLOU properties, something which
the raw ores did not do, thus suggesting that the perovskite material had been formed. The methane
conversion varied considerably depending upon the material and the temperature used, with CO2
yields ranging from 5% to 90%. The effect of high temperatures (up to 1050°C) on the particles
activity was also investigated.
Introduction
Chemical Looping Combustion (CLC) is a innovative method for CO2 capture [1] which originates
from a patents in the fifties concerning production of pure CO2 [2]. A CLC system consists of two
reactors where solid oxygen carrier particles circulate between reducing conditions in a fuel reactor
and oxidizing conditions in an air reactor, thus transferring oxygen for fuel conversion from air, see
Figure 1.
Figure 1 Illustration of Chemical-Looping Combustion
The oxygen carrier is typically a metal oxide, here referred to as MeOy (oxidized state) and MeOy-1
(reduced state), which can react directly with gaseous fuels through solid-gas reactions and with gas
phase O2 from the air, respectively. Recently a new method of accomplishing this process has emerged,
chemical-looping with oxygen uncoupling (CLOU) [3]. This is a method were the oxygen carrier can
release gaseous oxygen in the fuel reactor, reaction (1) below. The oxygen released can then directly
react with the fuel via combustion, reaction (2). This is in contrast to normal CLC, where the fuel or
fuel intermediates react with the oxygen carrier particles directly [4].
(1)
⁄
(2)
The suitability of an oxygen carrier in chemical-looping combustion (CLC) or chemical-looping with
oxygen uncoupling (CLOU) is determined by its lifetime and reactivity. Both are factors relates to the
cost of the oxygen carrier in relation to the amount of carbon dioxide captured. There are likely losses
of oxygen carrier from attrition in continuous operation and in the case with solid fuels there are
additional losses expected together with the ash extraction. Hence there is a need to produce
inexpensive oxygen carriers which requires the use of cheap raw materials, such as ores or industrial
waste materials. This would have advantages with respect to cost for oxygen carrier production.
One type of oxygen carrier that has been shown to have excellent reactivity with gaseous fuels is the
perovskite structured CaMnO3, and an example of such a particle was studied in batch reactor system
in [3][5] and in a continuous fluidized bed system in [6]. Källén et al., studied an oxygen carrier of
CaMn0.9Mg0.1O3 in a continuous operation for 55 h with natural gas operation, and was able to achieve
full methane conversion to CO2 [7]. The oxygen carrier has oxygen uncoupling properties, that is, it
releases oxygen in the gas phase, the amount which is governed by the partial pressure of oxygen in
the surroundings. In this project, the aim was to produce a CaMnO3 particle from cheap raw materials
such as manganese ore and commercial Ca(OH)2.
Experimental
Several oxygen carriers of CaMnO3 have been produced utilizing different manganese ores as starting
material, listed in Table 1 also note that Colormax is not an ore but a refined product (color pigment).
The manganese ores were mixed with commercial Ca(OH)2 from Nordkalk (SL-KÖ) and spray-dried
at VITO in Belgium. Each material (excluding CHR1) was calcined at both 1200°C and 1275°C for 4h
dwell time. In addition, CHR5 was also calcined at 1300, 1325 and 1350C for further investigation.
All particles were sieved to obtain particles in the size range of 125 - 180 µm. The elemental
composition of the individual ores is given in Table 2. As can be seen, the raw ores contain more than
just the manganese (Mn), such as iron (Fe) and silica (Si). There are other elements as well in the ores
but in small amounts.
Table 1 Materials made from the ores and Ca(OH)2 and content of each.
Material Name
Ore
Ore Content (wt%) Ca(OH)2 Content (wt%)
CHR1
S. African A
62.6
37.4
CHR2
S. African B
60.2
39.8
CHR3
Egyptian
66.2
33.8
CHR4
Gabon
53.9
46.1
CHR5
Colormax
47.4
52.6
Table 2 Elemental composition of the ores which were used to produce the particles, given in wt %.
Element Colormax Egyptian
Si
Al
Ca
Fe
K
Mg
Mn
Na
P
Ti
0.09
0.23
0.09
6.39
0.06
0.02
82.3
0.05
1.07
0.01
2.13
0.58
10.15
15.95
0.10
0.93
37.8
0.41
0.06
0.06
Gabon
5.19
3.26
0.37
4.92
0.37
0.01
49.11
0.05
0.50
0.10
South
African A
0.84
0.20
4.12
18.61
0.07
0.47
44.30
0.06
0.04
0.02
South
African B
2.63
0.17
5.42
13.08
0.04
0.65
49.03
0.04
0.03
0.01
Both apparent density and crushing strength (CS) was measured for all the particles. Apparent density
was determined by filling a volume of 5 ml with particles and weighing the particles. The sample
container was tapped a multitude of times in order to improve the packing of the particles. The
crushing strength (CS) was measured using a digital force gauge on which the particles were placed,
the particles were then subjected individually to a manual force from straight above until the particle
was crushed to yield the force required for fracture.
The reactivity experiments were conducted in a fluidized batch reactor of quartz, where the particles
were placed on a porous quartz plate. The plate was placed 370 mm from the bottom of the 870 mm
long reactor and the reactor had a constant inner diameter above the plate of 22 mm and inner diameter
of 10 mm below the plate. The temperature in the experiments was measured 10 mm above the porous
quartz plate using a Pentronic CrAl/NiAl thermocouples enclosed inconel-600 in quartz shells.
During the experiments the reactor was placed in an oven and connected to magnetic valves which
were used to alternate the atmosphere inside the reactor, Figure 2. The pressure drop over the reactor
was measured to verify fluidization throughout the experiments. The flue gases exiting from the
reactor were led to a cooler in order to remove any steam from the gas stream before reaching the
analyzer (Rosemount NGA-2000) which measured the volume fraction of the common products on
dry-basis.
Figure 2 Schematic drawing of the system including valves, reactor, oven, pressure measurement, cooler and gas
analyzer.
Experiments were conducted in the batch reactor with the aim of i) studying the oxygen uncoupling
effect, or CLOU properties, and ii) reactivity with methane and syngas. In all experiments 15 g of
oxygen carrier were used in the size range of 125-180 µm. The experimental procedure is outlined in
Table 3. The first two inert cycles were performed to determine initial state of the particles and
whether the particles had any CLOU effect, e.g. could release oxygen to the gas phase. Additionally
the longer inert cycle was used to gauge the oxygen available for CLOU. During cycle 4 the
temperature is increase during the inert phase in order to investigate the increased oxygen release due
to temperature increase, while carrying out the oxidation at a lower temperature. The particles are then
exposed to several cycles of methane and syngas at 950C with a flow of 450 ml/min of the relevant
gas. After the fuel cycles some inert cycles were done in order to determine whether there has been
any degradation of the uncoupling properties of the particles due to reaction with the fuel. The sample
was then collected in an oxidized state after having been oxidized at 900°C.
Table 3 Experimental scheme for gaseous fuel experiments. The abbreviation ox and red stand for oxidation and
reduction respectively. Flows are calculated to correspond to 1 bar and 25°C.
No of
Cycles
2
1
1
3
3
2
1
Gas
stream
Nitrogen
Nitrogen
Nitrogen
Methane
Syngas
Nitrogen
Nitrogen
Fox
(mL/min)
900
900
900
900
900
900
900
Finert
(mL/min)
600
600
600
600
600
600
600
tinert (s)
360
720
360
60
60
360
720
Fred
(mL/min)
450
450
-
tred (s)
Tox (°C)
20
80
-
900
900
900
950
950
900
900
Tred/TCLOU
(°C)
900
900
900-1000
950
950
900
900
The oxygen carriers were also evaluated at five different temperatures (850°C, 900°C, 950°C, 1000°C,
1050°C) under inert conditions to measure the oxygen release (CLOU properties) and conversion of
CH4. The first of the inert cycles was 360 s long and the second 720 s long and two CH4 cycles were
done at each temperature. The reason for conducting the experiment with the long inert period was to
gauge the amount of available oxygen for CLOU. The experiments were conducted according to the
parameters listed in Table 4. After the last cycle with methane the particles weren’t oxidized again but
instead the system was shut off and cooled down during inert phase. The samples were then collected
in a reduced state and analyzed with XRD.
Table 4 Experimental scheme for gaseous fuel experiments at different temperatures.
No of
Cycles
2
2
2
2
Gas
stream
Nitrogen
Methane
Nitrogen
Methane
Fox
(mL/min)
900
900
900
900
Finert
(mL/min)
600
600
600
600
tinert (s)
(1st)360 (2nd) 720
60
(1st)360 (2nd) 720
60
Fred
(mL/min)
450
450
tred
(s)
20
20
Tox
(°C)
850
850
900
900
Tred/TCLOU
(°C)
850
850
900
900
2
2
2
2
2
2
1
1
Nitrogen
Methane
Nitrogen
Methane
Nitrogen
Methane
Nitrogen
Methane
900
900
900
900
900
900
900
900
600
600
600
600
600
600
600
600
(1st)360 (2nd) 720
60
(1st)360 (2nd) 720
60
(1st)360 (2nd) 720
60
360
60
450
450
450
450
20
20
20
20
950
950
1000
1000
1050
1050
900
900
950
950
1000
1000
1050
1050
900
900
Results
The crushing strength and the apparent density of the particles were determined and are listed in Table
5. It is clear that most of the particles have low crushing strength while apparent density is between 12 grams/cm3. The density measured increases with higher sintering temperature.
Table 5 The results of crushing strength test and the apparent density of the particles.
Particle
CHR1
CHR2
CHR2
CHR3
CHR3
CHR4
CHR4
CHR5
CHR5
CHR5
CHR5
CHR5
Sintering
Temperature
1200
1200
1275
1200
1275
1200
1275
1200
1275
1300
1325
1350
Crushing
Strength (N)
0.5
0.8
0.8
0.4
0.5
0.4
0.5
0.5
1.4
1.4
1.9
2.3
Density
(g/cm3)
1.0
1.0
1.3
1.0
1.4
0.9
1.1
1.4
1.7
1.8
1.9
1.9
In Figure 3 a typical inert cycle (left) and a reduction cycle with methane (right) can be seen for the
material CHR5 sintered at 1200°C. From the left-hand figure it is clear that as the oxidation gas is
switched to inert, the O2 volume fraction decreases gradually as a function of time. However, the
oxygen concentration doesn’t reach zero since the oxygen carrier releases oxygen throughout the cycle.
After 360 s the atmosphere is changed to 5% O2 and the particles are oxidized again. For the reduction
cycle seen in the right hand graph, the particles are first oxidized with 5% O2, after which there is a 60
s long inert period. After this the reduction period begins and fuel is introduced. At this point all O2 is
consumed and methane (CH4) is converted to CO2.
Figure 3 An outlet O2 profile for a 360 s inert cycle 950°C (left) and an outlet gas profile for a reduction cycle 950°C
(right) for the material CHR5 sintered at 1200°C.
The manganese ores themselves showed no or little CLOU properties when investigated by Arjmand
et al. [8]. However, the particles produced from the ores and Ca(OH)2 seems to all have CLOU
properties and release oxygen to the gas phase, as is shown in Figure 4. This suggests that the calcium
manganite was indeed formed during production. In Figure 4, the outlet O2 volume fraction after 360
and 720 s under inert conditions is shown as a function of reaction temperature, for particles calcined
at 1275°C. It can be seen that the oxygen release varies depending upon the material investigated. For
CHR3 there is a clear increase in O2 release as a function of temperature, while CHR2 and CHR4
show a relatively constant release tendency at the higher temperature. The CHR5 has the highest
amount of oxygen released of all samples, but the oxygen released clearly decreases at higher
temperatures, as is seen in the figure. It should be mentioned that the CHR5 was produced with a
refined manganese product, Colormax, and is not a natural ore.
Figure 4 Oxygen concentration measured at the outlet of the reactor for particles sintered at 1275°C after 360 s (left)
and 720 s (right) at different temperatures.
In Figure 5, the average γ CH4, or CO2 yield, when methane (CH4) is used as fuel, is shown at
temperatures ranging from 850°C to 1050°C. Here, the CO2 yield is defined as (3).
(3)
The CO2 yield is the fraction of the carbon containing gases converted to CO2 and the average
methane yield here is taken under the entire reduction period. γ varies among the particles and across
the temperatures range from 5% to 95%. Initially CHR1, CHR3 and CHR4 have low yield of 10%
CO2 and reaches a maximum around 80% at 1000°C which is maintained at 1050°C. CHR2 has a CO2
yield of around 25% at 850°C, which increases steadily to around 85% CO2 yield at 1000°C. CHR5
has an initial yield similar to CHR2 at 850°C and reached the highest CO2 yield of any of the particles
at 1050°C.
Figure 5 CO2 yield when CH4 is used as fuel for particles sintered at 1200°C (left) and 1275°C (right) at different
temperatures.
Figure 6 compares the average CO2 yield for CH4 before and after all of the CH4 cycles at 950°C,
1000°C and 1050°C, i.e. to determine if the high temperature cycle had any detrimental effect on the
reactivity, see Table 4. It can be seen that CHR1, CHR3 and CHR4 show an increase in reactivity after
testing, although the degree of increase varies. CHR2 also shows an increase but only for the particle
sintered at 1200°C. CHR4 has the greatest increase from around 15% to close to 45%. The reactivity
of CHR2 sintered at 1275°C remains approximately the same and seems to not have suffered any
detrimental effects from the high temperature cycles. However all CHR5 materials except for the one
sintered at 1200°C have decreased CO2 yield after the CH4 cycles.
Figure 6 γ (CO2 yield) with CH4 (methane) as fuel at 900 °C before and after increasing the temperature to 1050 °C.
It was quite clear from the initial study presented in Fig.4 and 5 that the CHR5 material had the most
interesting properties with respect to reactivity and uncoupling properties. So although it is produced
from a more refined Mn-product, it was decided to explore this material further. Figure 7 shows the
oxygen release and the conversion of CH4 at different temperatures for these particles sintered at five
different temperatures. From the figure it can be seen that the results are very similar and show the
same behavior but are lower for the materials sintered at a higher temperature. The oxygen
concentration obtained is higher for the materials sintered at a lower temperature and the trend can
clearly be seen in the figure. Higher sintering temperature results in less oxygen concentration and less
methane conversion as can be seen in Figure 7. The resulting conversion of CH4 at 1050°C ranges
from 70% for CHR5 sintered at 1350°C to 95% for CHR5 sintered at 1200°C. It can be seen that at
1050°C the oxygen concentration is similar among all the CHR5 materials but the CH4 conversion
varies to certain degree.
Figure 7 Outlet oxygen concentration (left) and conversion of CH4 (right) at different temperatures for the CHR5
particles sintered at five different temperatures.
Discussion
The calcium manganite system is a very interesting possibility for CLOU, as has been previously
demonstrated [3][5][6][9]. It would be very valuable if these types of material can be produced by
mixing commercial raw materials, such as ores. In this work commercial Ca(OH) 2 and manganese ores
were mixed, spray-dried and calcined. All finished particles showed clear oxygen uncoupling,
indicating that the perovskite material was formed. Further, the gradual release pattern of uncoupled
oxygen is very similar to such perovskites, as shown by Hallberg et al [10]. As the raw manganese
ores themselves have little or no CLOU properties, this further confirms this perovskite formation. Xray powder diffraction was conducted on the fresh and used materials, and the perovskite structure has
been confirmed for three out of the five particles, the other two were inconclusive. Methane
conversion varied in a wide range depending upon temperature, although full conversion was never
reached. However, there seems to be an activation of the particles, and hence it would be of interest to
conduct tests for longer periods of time and for more alternating cycles. The crushing strength of many
of the particles were low which could indicate that’s it’s required to change the production of the
particles to longer sintering time, or increase the sintering temperature, if possible.
Conclusions
A series of materials based on manganese ores and commercial Ca(OH)2 were produced and tested
with respect to parameters important for CLOU. The main results of the study were:

The particles exhibit CLOU properties, indicating that a perovskite type calcium manganite
was formed in all particles.

The particles made from ores didn’t show degradation at 1000°C to 1050°C and show signs of
activation, i.e. increasing reactivity.

The particle made from refined material (CHR5) exhibit the best CLOU properties but
degrades at temperatures from 1000°C to 1050°C.
Acknowledgment
The research leading to these results has received funding from the Swedish Energy Agency and
from European Research Council under the European Union's 7th Framework Programme, ERC grant
agreement n° 291235.
References
[1] M. Ishida, D. Zheng, T. Akehata, Evaluation of a chemical-looping-combustion power-generation system by
graphic exergy analysis. Energy (Oxford, United Kingdom), 1987. 12(2): p. 147-54.
[2] W.K. Lewis, E.R. Gilliland, Production of pure carbon dioxide. 1954: USA.
[3] H. Leion,Y. Larring, E. Bakken, R.Bredesen, T. Mattisson, A. Lyngfelt, Use of CaMn0.875Ti0.125O3 as
Oxygen Carrier in Chemical-Looping with Oxygen Uncoupling. Energy Fuels, 23, 5276–5283, 2009.
[4] T. Mattisson, Materials for Chemical-Looping with Oxygen Uncoupling. ISRN Chemical Engineering,
Volume 2013, 2013
[5] S. Sundqvist, H. Leion. M. Rydén, T. Mattisson, A. Lyngfelt, CaMn0.875Ti0.125O3−δ as an Oxygen Carrier for
Chemical-Looping with Oxygen Uncoupling (CLOU)—Solid-Fuel Testing and Sulfur Interactio. Energy
Technology, Volume 1, Issue 5-6, pages 338–344, June 2013.
[6] M. Rydén, T. Mattisson, A. Lyngfelt, CaMn0.875Ti0.125O3 as oxygen carrier for chemical-looping
combustion with oxygen uncoupling (CLOU)—Experiments in a continuously operating fluidized-bed reactor
system. International Greenhouse Gas Control 5, 356-366, 2011.
[7] M. Källén, M. Ryden, C. Dueso, T. Mattisson, A. Lyngfelt, CaMn0.9Mg0.1O3‑δ as Oxygen Carrier in a GasFired 10 kWth Chemical-Looping Combustion Unit. Industrial and Engineering Chemistry Research , 52 (21), pp
6923–6932, 2013.
[8] M. Arjmand, H. Leion, T. Mattisson, A. Lyngfelt, Investigation of different manganese ores as oxygen
carriers in chemical-looping combustion (CLC) for solid fuels. Applied Energy, in press, 2013.
[9] D. Jing, T. Mattisson, H. Leion, M. Rydén, A. Lyngfelt, Examination of Perovskite Structure CaMnO3δ with MgO Addition as Oxygen Carrier for Chemical Looping with Oxygen Uncoupling Using Methane and
Syngas. International Journal of Chemical Engineering, Volume 2013 (2013).
[10] P. Hallberg, D. Jing, M. Rydén, T. Mattisson, A. Lyngfelt, Chemical Looping Combustion and Chemical
Looping with Oxygen Uncoupling Experiments in a Batch Reactor Using Spray-Dried CaMn1–xMxO3−δ (M =
Ti, Fe, Mg) Particles as Oxygen Carriers. Energy Fuels, 27 (3), pp 1473–1481, 2013.