Evaluation of novel ceria-supported
materials as oxygen carriers for chemical-
looping combustion
Master’s Thesis in the Master Degree Program, Innovative and Sustainable
Energy Engineering
ALI HEDAYATI
Department of Energy and Environment
Division of Energy Technology
CHALMERS UNIVERSITY OF TECHNOLOGY
Götebory, Sweden 2011
Master’s Thesis T2011-354
REPORT NO. T2011/354
Evaluation of novel ceria-supported materials
as oxygen carriers for chemical-looping
combustion
ALI HEDAYATI
Department of Energy and Environment
Division of Energy Technology
CHALMERS UNIVERSITY OF TECHNOLOGY
Göteborg, Sweden 2011
Evaluation of novel ceria-supported materials as oxygen carriers
for chemical-looping combustion
Ali Hedayati
Supervisors: Dr. Henrik Leion, Prof. Abdul-Majeed Azad
Examiner: Dr. Tobias Mattisson
Department of Energy and Environment
Division of Energy Technology
Chalmers University of Technology
Abstract:
According to the IPCC, increasing concentration of greenhouse gases in the atmosphere is the main
reason of climate change and global warming. Resulting mainly from burning of fossil fuels, CO2 has the most apparent global warming potential. Increasing rate of energy consumption by the society and
high dependency of energy production on fossil fuels along with the obvious negative environmental
consequences of climate change, oblige human to concern about the atmospheric CO2 concentration.
Thus, quick and efficient techniques are necessary to be applied for CO2 sequestration to prevent it released to the atmosphere.
Chemical-looping combustion (CLC) is one new technology to capture CO2. CLC consists of two interconnected fluidized bed reactors i.e. air reactor and fuel reactor. In the fuel reactor, fuel reacts
with an oxygen carrier in absence of nitrogen and is converted to CO2 and H2O. Then the reduced
oxygen carrier is transferred to the air reactor to be re-oxidized back to its original oxidized state. So, the oxygen is carried from the air reactor to the fuel rector. After condensation of water vapor in the
outflow gas coming from the fuel reactor, a highly pure stream of CO2 is obtained.
This thesis investigates the reactivity and performance of some oxygen carrier particles supported on ceria and gadolinia doped-ceria (GDC) for chemical-looping combustion. The oxygen carriers were
oxides of copper, manganese and iron supported on ceria and GDC. Oxygen carriers were synthesized
via extrusion technique and tested for successive oxidation and reduction cycles using methane and syngas (50% CO and 50% H2) as fuel. Tests were performed using fluidized bed batch reactors made
of quartz. Reduction cycles were performed at 950°C for iron and manganese containing oxygen
carriers and at 900 and 925°C for copper oxide. The reactivity during reduction and oxidation cycles, fluidization and agglomeration properties, oxygen release characteristics (CLOU effect) and phase
analysis were evaluated for all the tested particles.
The results showed that in general GDC supported particles were more reactive compared to ceria supported ones during reduction cycles. Methane was totally converted by copper oxide supported on
GDC and the particles showed very high oxygen release potential as well, qualifying it as a CLOU
material. Syngas was fully converted to CO2 and H2O by all the oxygen carriers synthesized and tested in this work. Very good fluidization properties and low attrition and agglomeration were
observed for all the particles. Manganese oxide containing particles showed very low conversion
during methane cycles that was somehow expected according to previous reports on this material.
Keywords: carbon dioxide (CO2), chemical-looping combustion (CLC), oxygen carrier, ceria,
gadolinia doped ceria (GDC), copper, iron, manganese, oxygen release
0
Table of contents:
1 Introduction 1
1.1 Greenhouse gases 1
1.2 Global warming 1
1.3 Concerns of high atmospheric CO2 concentration 2
1.4 Anthropogenic CO2 emission reduction 2
1.4.1 Pre-combustion carbon capture 2
1.4.2 Oxyfuel combustion carbon capture 3
1.4.3 Post-combustion carbon capture (PPC) 3
1.5 Chemical-Looping Combustion (CLC) 3
1.5.1 CLC with solid fuels 4
1.6 Oxygen carriers 5
1.7 Objective 7
2 Experimental 8
2.1 Synthesis and fabrication of oxygen carriers 8
2.2 Experimental set up 10
2.3 Data analysis 12
3 Results and discussion 13
3.1 Phase analysis of the oxygen carriers 13
3.2 Reactivity test results 13
3.2.1 Pure ceria 13
3.2.2 Fuel conversion 14
3.2.2.1 Methane conversion 15
3.2.2.2 Syngas conversion 19
3.2.2.3 Phase analysis 19
3.2.2.4 Fluidization and agglomeration characteristics 20
3.2.3 Oxygen release 21
3.2.4 Oxidation phase 22
3.2.5 Temperature variation during oxidation and reduction cycles 24
4 Conclusion 26
5 Acknowledgements 27
6 References 28
1
1. Introduction
Energy conversion is a key factor for the development of human society1. Living without
energy supply – mainly electricity- is not possible in the modern world. Thus, as development
continues with an aim to eradicate poverty and enhance quality of life, increase in energy
production by all sectors is unavoidable2. A rapid shift toward efficient and cost effective
sources of energy is obviously required. Apart from the energy supply issues, environmental
concerns have also emerged. In a few decades, the energy demand may be double in
comparison to today which presents challenge to the preservation of environment due to the
concomitant increase in emissions caused by the power generation processes3. Consequently,
clean technologies for energy production have attracted greater attention in order to reduce
the emission of pollutants to the atmosphere, soil and water.
1.1 Greenhouse gases
When sunlight passes through the atmosphere and reaches the earth surface, a part of this
light is radiated back in longer wavelengths to the atmosphere. Certain atmospheric gases
called greenhouse gases absorb these wavelengths and radiate them back to the surface of the
earth resulting in an increase in ground temperature. This naturally occurring phenomenon is
called the greenhouse effect4 caused by greenhouse gases where the most important ones are
H2O, CO2, CH4, N2O and halocarbons. Water vapor is accounted for 60% of total greenhouse
gas effect but human activities do not play any direct role in the balance of this substance in
the atmosphere5. Moreover, the lifetime of water vapors in the atmosphere is about 9 days
compared to hundreds of years for carbon dioxide. According to IPCC, human activities have
today resulted in a significant increase in atmospheric concentrations of carbon dioxide,
methane and nitrous oxide compared to 17506. This increase has caused a change in the
energy balance toward warming up the atmosphere. CO2 is the most important anthropogenic
greenhouse gas which contributes most towards the rise of atmospheric temperature; its main
source is fossil fuel burning7. Accordingly, CO2 is at the center of attention with regard to
climate change and global warming concerns.
1.2 Global warming
Svante Arrhenius was one of the first who mentioned global warming probability due to the
increasing concentration of CO2 in the atmosphere7. He suggested that the mean temperature
of the earth will probably increase due to emission of carbon dioxide originating from human
activities. Now it is a fact that the atmospheric temperature has risen over the last decades and
it is believed to be a result of increased concentration of greenhouse gases in the atmosphere
contributing to an intensified greenhouse effect8. The mean temperature increase is estimated
to between 0.4 and 0.8oC during the last century and this has resulted in having 10 of the
warmest years among the past 15 past years9. Greenhouse gas emissions have risen by 70%
between 1970 and 2004 of which the larger part has come from energy production10
. CO2 has
the strongest effect on global warming and 77% of total greenhouse gas emission in 2004 has
been CO2.
2
1.3 Concerns of high atmospheric CO2 concentration
There are several natural reservoirs of carbon which interact with one another according to
the carbon cycle. Because natural processes which can sequester carbon are rather slow, CO2
from human activities will accumulate and last for a long time in the atmosphere11
.
Atmospheric concentrations of CO2 have increased from natural mean value of 280 ppm to
379 ppm in 20056 and it is estimated to exceed 400 ppm by 2030
12 mainly due to the burning
of fossil fuels. About 80% of world’s primary energy in 2004 came from fossil fuels which
released 26.4 Gt CO2 to the atmosphere. It is expected that the worldwide energy
consumption will increasing rapidly so that the yearly CO2 emissions from energy production
will reach 33.8 Gt in 2020 and 42.4 Gt in 203513
. These statistics show that carbon dioxide
emission will have a severe effect in coming decades. Thus it is necessary to apply methods
to prevent or at least decrease the emissions to the atmosphere otherwise harsh consequences
of global warming would be inevitable.
1.4 Anthropogenic CO2 emission reduction
Quick and effective actions are needed to reduce the atmospheric concentration of CO2.
Actions are necessary both in power production plants which are mainly based on fossil fuels
and in commercial operations. There are some protocols and treaties like Kyoto protocol to
decrease the emission of CO2 but these are not enough and the main question remains
unanswered of how to reduce the emission14
. The first solution would be the substitution of
fossil fuel-based energy generation units by other technologies such as nuclear power plants,
biomass, solar arrays and wind farms. But these technologies are not without challenges and
limitations, such as the high cost per unit energy produced by these devices and non-
feasibility of rapid transition toward these technologies due to the required mammoth
infrastructure changes15
. So the tendency is toward faster and more reliable existing methods
to mitigate the emission of CO2. One such method is carbon capture and storage. There are
three main techniques to capture CO2 i.e. pre-combustion carbon capture, oxyfuel combustion
and post-combustion carbon capture. Captured CO2 is then transferred to the storage sites
where it is stored in deep geological formations or under sea beds.
1.4.1 Pre-combustion carbon capture (Pre-C3)
Pre-combustion carbon capture refers to chemical processes where a fuel – mainly solid fuel -
is converted to hydrogen and carbon dioxide and the latter is separated by physical (pressure
swing adsorption, PSA) or chemical (amine adsorption) methods. Hence, the carbon in the
fuel is completely removed. The process consists of two steps. First, the fuel reacts with
oxygen/air/steam to produce carbon monoxide and hydrogen. Carbon monoxide undergoes a
catalyzed water-gas-shift (WGS) reaction wherein carbon monoxide reacts with steam,
generating more hydrogen plus carbon dioxide16
. The main advantage of this method is to
produce a clean carbonless fuel which can be used in variety of industrial applications, but the
drawback is the high operating cost of the process.
3
1.4.2 Oxyfuel combustion carbon capture (Oxy-C3)
In oxyfuel combustion, oxygen is used instead of air, resulting in a nitrogen-free combustion
with high concentrations of water vapor and CO2 in the flue gas stream. The concentration of
carbon dioxide in flue gases is nearly 80% which simplifies the separation processes16
. The
flame temperature in the case of pure oxygen would be very high so a portion of the CO2-rich
flue gases are recirculated both to control the temperature and to reach the required gas flow.
Advantages of oxyfuel combustion are: (1) easy CO2 separation, (2) smaller flue gas volume,
(3) better desulfurization and, (4) prevention of NOx formation. Disadvantages of the oxyfuel
combustion are the high cost and electrical demands for oxygen production.
1.4.3 Post-combustion carbon capture (Post-C3)
Post combustion carbon capture is the most used technology for CO2 capture. The flue gases
coming from the combustion of fossil fuels are treated and CO2 is separated mainly by a
chemical sorbent4. One of the disadvantages of post combustion carbon is large amount of
flue gases resulting in low concentration of CO2 followed by high temperature of flue gases
making it necessary to use powerful solvents16
. There are different technologies for
absorption of carbon dioxide like chemical and amine absorption, cryogenic purification,
membrane separation and also algal bio-fixation11
.
1.5 Chemical-Looping Combustion (CLC)
Chemical-looping combustion (CLC) is one of the emerging technologies which makes it
possible to have a nitrogen free fuel conversion without the need for costly and energy
consuming processes for oxygen production and purification of the exhaust. In CLC oxygen
is provided by oxygen carrier particles so that direct contact between fuel and combustion air
is avoided4, 11, 17
. The main benefit of CLC is that a high concentration of CO2 mixed with
water vapor is obtained. Water vapor is condensed and a highly pure stream of CO2 (nearly
100%) is ready for sequestration. Thus, there is no need for CO2 separation units. Besides,
there is no NOx emission and the heat released from the combined oxidation and reduction
process is equal to that in conventional combustion. One of the main challenges regarding
CLC technology is the economical availability of oxygen carriers with required properties.
Oxygen carrier particles, which are mainly solid oxides18
, are discussed in details later in this
thesis.
Chemical looping combustion consists of two interconnected reactors, namely, the air reactor
and the fuel reactor. In the air reactor, oxygen carries particles are exposed to an air flow and
are oxidized according to reaction (1):
O2 (g) + 2 MexOy-1 ↔ 2 MexOy (1)
The fully oxidized particles are transported to the fuel rector. If the fuel is gaseous, it reacts
with oxygen carrier particles, reducing them according to the reaction (2):
(2n + m)MexOy + CnH2m ↔ (2n + m) MexOy-1 + mH2O + nCO2 (2)
The reduced particles are transferred back again to the air reactor and the cycle is repeated
again. Thus, the fuel reacts with the oxygen in the carrier while no nitrogen exists in the fuel
reactor.
4
Reaction 1 is highly exothermic while reaction 2 can be exothermic or endothermic
depending on oxygen carrier characteristics and type of fuel. Flue gases are mainly composed
of high concentration of carbon dioxide and some water vapor which is condensed and
separated from gaseous CO2 4,17
.
In Figure 1 a scheme of CLC unit introduced by Lyngfelt et al.19
is shown.
Figure 1 – layout of a chemical-looping combustion process: 1) air reactor and riser, 2) cyclone and 3) fuel reactor
19.
There are two fluidized bed reactors connected with each other through loop-seals.
Fluidization results in very effective and close mixing of particles and air or fuel gas,
respectively; this design is very similar to fluidized bed systems designed for solid fuels.
Oxygen carrier particles are separated from the air stream in the cyclone and drop down to the
fuel reactor by gravity. There are also particle locks in place to prevent mixing of air from the
air reactor and gases from the fuel reactor4.
1.5.1 CLC with solid fuels
It is beneficial to utilize solid fuels in CLC systems since they are cheaper and more abundant
compared to the gaseous fuels like natural gas. It is possible to gasify the solid fuel mainly via
gasification process in the presence of steam or CO2 followed by converting the gasified
products namely, CO, H2 and CH4. To avoid the slow gasification reactions, the oxygen
carrier must be capable of releasing molecular oxygen so that the solid fuel reacts directly
with gas phase oxygen as in the case of normal combustion. This strategy is called chemical-
looping combustion with oxygen uncoupling (CLOU) and the oxygen release behavior of the
oxygen carrier is known as the CLOU effect 17
. The CLOU reaction during which oxygen is
released in the gas phase can be represented as follow:
MexOy ↔ MexOy-2 + O2 (g) (3)
The reduced oxygen carrier is re-oxidized back to its original state in the air reactor via the
following reaction:
MexOy-2 + O2 ↔ MexOy (4)
5
As an example, copper oxide is well known for its CLOU effect17
. This effect will be shown
and discussed later in this work. Copper oxide releases gas phase oxygen through the
following decomposition reaction:
4 CuO → 2 Cu2O + O2 (5)
The molecular oxygen released reacts with the solid fuel. However, the focus of this thesis
would be on gaseous fuels i.e. methane and syngas.
1.6 Oxygen carriers
Oxygen carrier particles are a central part of the CLC system and play the main role of
transporting oxygen from the air to the fuel reactor. Therefore, the properties of these
particles are important for investigation and improvement of the chemical-looping
combustion technology 4, 17
. According to Jerndal et al. relevant properties of particles are:
sufficient rate of oxidation and reduction, ability to perform high conversion of fuel to CO2
and H2O, resistance against attrition and fragmentation, and, being cheap and
environmentally sound20
. In addition, there are other criteria that are of relevance in the
selection of suitable carrier, such as melting temperature of the oxygen carrier, oxygen ratio
which is the maximum transported mass of oxygen for a given mass flow of particles20
.
The most commonly used active materials in oxygen carriers are the oxides of nickel, copper,
manganese or iron. According to Leion, a general comparison of the reactivity of metal
oxides with methane shows their propensity in the following descending order:
NiO>CuO>Mn2O3>Fe2O317
. Copper oxide, iron oxide and manganese oxide react with
methane as follows21
:
CuO (at 800°C):
CH4 +4 CuO CO2 +2 H2O + 4 Cu (fuel oxidation) (6)
4 Cu + 2 O2 4 CuO (carrier regeneration) (7)
Fe2O3 (at 800°C):
CH4 + 12 Fe2O3 CO2 + 2 H2O + 8 Fe3O4 (fuel oxidation) (8)
8 Fe3O4 + 2O2 12 Fe2O3 (carrier regeneration) (9)
CH4 + 4Fe2O3 CO2 + 2 H2O + 8 FeO (fuel oxidation) (10)
8 FeO + 2 O2 4 Fe2O3 (carrier regeneration) (11)
Mn3O4 (at 950°C):
CH4 + 4 Mn3O4 CO2 + 2 H2O + 12 MnO (fuel oxidation) (12)
12 MnO + 2 O2 4 Mn3O4 (carrier regeneration) (13)
Reduction and oxidation (regeneration) reactions are continuously done in the fuel reactor
and air reactor, respectively.
6
Copper oxide is a very promising oxygen carrier with advantages 21
, such as, being very
reactive during oxidation and reduction cycles, full conversion of gaseous hydrocarbon fuels
like methane, exothermic nature of the oxidation and reduction reactions and, reasonable
price of the material. The challenges of copper oxide are its decomposition (it can be
advantageous in terms of oxygen release via CLOU effect) at rather low temperature as well
as the low melting point of elemental copper. Even then, due to its obvious advantages,
copper oxide has been investigated a great deal as a potential oxygen carrier. For example,
Gayan et al. investigated the effect of different supports on the behavior of copper oxide-
based oxygen carriers22
.
Iron oxide has also been extensively investigated as an oxygen carrier. Its natural abundance
together with favorable thermodynamic properties makes it quite attractive for CLC
applications. Activity of iron oxide supported by alumina, silica and titanium dioxide has
been tested by various researchers and it has been shown that transformation of hematite to
magnetite is the main chemical reaction during the process21
. Abad et al. tested iron oxide
supported on alumina at different temperatures and reported 10-94% conversion of
methane23
.
Johansson et al. have investigated the reactivity of manganese oxides produced by different
methods and reported poor reactivity and evidence of agglomerations24
. Literature shows that
manganese oxides react with the supports made up of Al2O3, SiO2 and TiO2 resulting in lower
activities. Johansson et al. investigated manganese oxides on ZrO2 supports stabilized with
CaO, MgO or CeO2 and reported good activities during the reduction cycles with methane24
.
Several other oxygen carrier systems have also been studied in the literature21, 25
.
Oxygen carriers are generally supported on inert materials. The supports are usually porous
materials used for maintaining the mechanical structure of particle during the process and
porosity increases the surface area of particle and the reactivity as well4. A number of
supports have been used. However, Al2O3, SiO2, TiO2, ZrO2, NiAl2O4, and MgAl2O4 are
among the most tested and reported supports26
. A comprehensive literature survey of various
oxygen carriers was done by Lyngfelt et al. 27
. Abad et al. developed a model to investigate
the behaviour of oxygen carriers in the fuel reactor and they used CuO-based oxygen carries
as a validation model28
.
As mentioned above, the major efforts have concentrated on investigating various oxygen
carriers supported on inert materials. So, it would be innovative to utilize supports that are
participating and thus can act as a minor but additional oxygen carrier or as a facilitating
oxidizing catalyst during CLC operation. Thereby exploiting the synergy of the composites
made up of the support and the oxygen carrier. One of these materials is cerium dioxide
(CeO2) also known as ceria, which is in use extensively in three-way catalysts (TWC) for
oxidizing carbon monoxide and unburnt hydrocarbons and reducing nitrogen oxides in the
exhaust stream of automobiles before they are released to the environment. Ceria as an
oxygen carrier supported on alumina has been investigated by Wei et al. for partial oxidation
of methane29
. They investigated different compositions of ceria and alumina at different
temperatures and showed that 10% ceria on alumina had the highest methane conversion up
to 80% at 925°C. Xing et al. prepared ceria supported on Fe2O3 to reform methane to
hydrogen and syngas in the presence of steam at 850°C30
. They found out that CeO2-Fe2O3 is
a suitable oxygen carrier for methane conversion and pure hydrogen production, where they
reported that CeFeO3 was formed under harsh reductive environment of the test that could
help the durability and performance of the oxygen carrier during successive cycles.
7
These data show that investigation of ceria for reforming applications is not a new concept.
However, utilization of ceria as a support for common oxygen carriers for direct application
in CLC systems for combustion of carbonaceous fuels is a new and promising idea in the
light of the known behavior of ceria. The favorable properties of ceria supported oxygen
carriers would be higher activity and lower cost of production due to larger fraction of the
active materials in terms of mass and volume.
1.7 Objective
The objective of this study is to fabricate oxygen carriers containing oxides of copper, iron
and manganese supported either on pure ceria or gadolinia-doped ceria (GDC, Gd0.1Ce0.9O1.9)
and investigate their performance and activity in chemical-looping combustion processes
using the fuel of syngas and methane in a fluidized bed batch reactor.
8
2. Experimental
2.1 Synthesis and fabrication of oxygen carriers
CuO, Mn2O3 and Fe2O3 were chosen as active phases due to their known properties as CLC
materials and also for the sake of comparison of the synthesis method adopted in this work
with spray drying and freeze granulation techniques. The selected metal oxides were mixed
with ceria or GDC for the production of oxygen carriers. In each case, a given metal oxide
was mixed with ceria or GDC in the weight ratio of 60:40 to make 170g batch. The dry
powders were transferred to a pear-shaped distillation flask; 400g of water was added and the
mixture was homogenized using a Buchi R-110 rotary evaporator equipped with a Buchi
vacuum pump, pressure controller and a chiller. The water bath was maintained at 60°C.
After 2/3rd
of the water was removed by distillation, the thoroughly homogenized mixture
(now reduced to a thick slurry) was dried in an air oven at 150oC. The rotary evaporator set-
up used for this purpose is shown in Figure 2.
Figure 2 – rotary evaporator set-up used for oxygen carrier synthesis
The ideal formulation of the ceramic dough that is ready for extrusion is dependent on several
factors, such as the particle size and particle size distribution, viscosity and rheology of the
mix as well as the choice of solvent, dispersant, binder and the plasticizer, each of which
plays an important role to impart the right property to the dough that needs to be extruded. In
our case, the following components were employed:
Quaternary ammonium compound as dispersant
Ammonium hydroxide as peptizing agent
PVA as a binder
Water soluble starch as auxiliary binder
Water both as binder/solvent
Dispersant is added to improve separation of particles and to prevent settling or clamping; its
role is akin to that of a surfactant (Surface Active reagent). Plasticizers are additives with low
molecular weight that reduce the deformation temperature of the binder to room temperature
or lower. It acts as an internal lubricant to aid in densification. The choice of plasticizer is
important, the most important criterion being that the plasticizer must be soluble in the
binder. PEG is an effective plasticizer for PVA. The relative concentration of the two must be
selected and adjusted properly by experimental trials for optimum results. In our case, use of
PEG was not required.
9
A binder glues together the particles of a ceramic body to give it strength after forming. PVA
is a classic binder. In the case of aqueous solutions (using water as solvent), using PVA as
binder is advantageous because of its good binding property, low viscosity, appreciable
pseudoplasticity and easy removal during burnout.
In some cases, other high-polymer compounds such as cellulose or polysaccharides (sugars)
can act both as plasticizers as well as binders, in high-shear forming techniques (with viscous
mixtures).
After obtaining a suitable viscosity of blended materials, they were extruded using a hand-
held single-screw manual extruder. Extrudates were dried on a stainless steel of aluminium
plate at 220°C overnight. Calcination of the dry extrudates was carried at 950°C or 1050°C
for 6 or 12 hours depending on the materials, using a well-conceived heating schedule for the
binder burn-out and consolidation of the carrier material; the firing schedule is shown in
Figure 3:
Figure 3 - Binder burn-out and calcination schedule adopted in this work for the calcinations of
the extruded oxygen carriers
Calcined oxygen carriers were sieved into size ranges of 125-180 μm and 180-250 μm. In the
case of relatively hard materials, occasional dry ball-milling (using alumina jar and alumina
milling balls) was resorted to. In every case, crushing strength and apparent density of the
oxygen carriers were measured before the fluidized bed experiment. To perform the phase
analysis, X-Ray diffraction (XRD) was done for both fully oxidized and fully reduced
particles of all the tested oxygen carriers. Results of XRD tests will be discussed in a later
section. Production data and properties of the tested oxygen carriers are present in Table 1.
RT
350°C/2h
500°C/2h
½°/min.
1°/min.
T°C/xh
5°/min.
RT
5°/min.
10
System tested Composition
ratio (wt%)
ID Calcination
temperature
(°C)/duration (h)
Size range
μm
Crushing
strength (N)
Apparent
density (kg/m3)
CuO-CeO2 60-40 COC-ÅF 950/6h 180-250 0.33 2380
CuO-CeO2 60-40 COC1 950/6h 125-180 0.42 3200
CuO-CeO2* 60-40 COC2 950/12h 125-180 <0.2 2284
CuO-GDC* 60-40 COGDC1 950/6h 125-180 0.64 2980
CuO-GDC 60-40 COGDC2 950/12h 180-250 0.33 3366
Fe2O3- CeO2 60-40 FOC1 950/6h 125-180 1.21 2610
Fe2O3- CeO2 60-40 FOC2 950/12h 180-250 0.84 2410
Fe2O3-GDC 60-40 FOGDC 950/6h 125-180 0.49 1946
Mn2O3- CeO2 60-40 MOC 1100/6h 180-250 0.85 3165
Table 1 – synthesis data and properties of prepared oxygen carriers
Formulations identified with an asterix (*) are the one on which tests could not be completed
due to fluidization problems that made it impossible to proceed with the test. These will not
be considered or discussed further.
2.2 Experimental set up
Experiments were carried out in a quartz fluidized-bed reactor, 870 mm long and 22 mm in
inner diameter. A porous quartz plate was placed at a height of 370 mm from the bottom and
the reactor temperature was measured with chromel-alumel (type K) thermocouples sheathed
in inconel-600 clad located about 5 mm below and 25 mm above the plate. Honeywell
pressure transducers with a frequency of 20 Hz were used to measure pressure drop over the
bed. This kind of setup has been used in previous works4.
15 g of the oxygen carrier were placed on the porous plate and was then exposed to
alternating oxidizing and reducing conditions. The experiment was initiated by heating the
reactor to 900°C in an ambient of 5%-O2 in N2 to ensure full oxidation of the carrier prior to
the experiments. Use of 5% partial pressure of oxygen is to simulate the expected conditions
at the air reactor’s outlet and also to obviate large temperature increase during the exothermic
oxidation.
When the desired temperature reached, particles were exposed to successive reduction and
oxidation periods to investigate the performance of the particles. To prevent the mixing of
reducing and oxidation gases, pure nitrogen was injected for 60 seconds between every
reduction and oxidation period. Syngas (50% hydrogen and 50% carbon monoxide) and
methane were used as the fuel. Methane was used since the largest portion of the natural gas
is methane, and syngas of specified composition was used to mimic the composition of solid
fuels gasification product.
The flow rate for different gases was different depending on the density of the oxygen carriers
and the flow rate needed for adequate fluidization. Higher density of ceria compared to the
commonly used supports (such as alumina, magnesium aluminate or silica) made it necessary
to increase the flow rate to obtain better fluidization conditions and prevent defluidization
during reduction phases.
The time for reduction for each oxygen carrier was calculated according to the maximum
oxygen available in each case while reacting with methane or syngas and adjusted
accordingly. The oxidation phases were long enough to allow full oxidation of the oxygen
carriers.
11
Before and after the fuel cycles, after full oxidation of the particles, nitrogen was purged for
360s to monitor the oxygen release behavior of the oxygen carriers, CLOU behavior31
, which
is important in the case of solid fuels even though the chemical-looping combustion of solid
fuels was not considered in this work. Experimental conditions used for testing and various
carriers fabricated in this work are presented in Table 2. Syngas was not used for testing
CuO-based carriers. The operational temperature during reduction cycles was 950°C for iron
and manganese oxide containing particles and 900 or 925°C for copper oxide.
Oxygen
carrier
Inert (nitrogen)
gas
Methane Syngas Oxidation
Flow rate (ml/min)
Duration (s)
Flow rate (ml/min)
Duration (s)
Flow rate (ml/min)
Duration (s)
Flow rate (ml/min)
Duration (s)
COC-ÅF 1200 60 900 12 - - 1200 Until fully
oxidized
MOC 600 60 450 20 450 80 900 Until fully oxidized
FOC1 600 60 450 20 450 80 900 Until fully oxidized
FOC2 600 60 450 20 450 80 900 Until fully
oxidized FOGDC 600 60 450 20 450 80 900 Until fully
oxidized COGDC2 600 60 900 12 - - 1000 Until fully
oxidized
Table 2 – Test conditions employed for evaluating the oxygen carriers developed in this work.
Nitrogen gas was used as an intermediate inert purge for 60 s in between oxidation and
reduction. The exit gas stream from the reactor was led into a condenser to remove water
from the fuel gas. Composition of the dry gas was analyzed by a Rosemount NGA-2000
analyzer to compute the concentrations of O2, CO2, CO and unconverted CH4. Inlet flow rates
of 450, 900 and 600 mLN/min were used during reduction, oxidation and inert purge,
respectively. The experimental set-up used for the evaluation of the reactivity characteristics
of the oxygen carriers is shown in Figure 4.
12
Figure 4- Scheme of the experimental setup used in this investigation.
2.3 Data analysis
The reactivity of a given oxygen carrier is quantified in terms of gas yield or conversion
efficiency (), and is defined as the fraction of fully oxidized fuel divided by the carbon
contain gases in outlet stream, in this work CO2, CO and CH4.
(14)
Here denotes the composition of the respective gas, obtained from measured concentration
in the gas analyzer and the mass flow rate indicated by the flow meter.
The theoretical oxygen capacity of a given carrier is defined in terms of oxygen ratio (Ro), as
the maximum mass change of oxygen in the oxygen carrier as follows:
(15)
Where and are the mass of the oxygen carrier in respectively fully oxidized and
reduced state.
The conversion of the oxygen carrier ( ) is defined as:
(16)
Where is the actual mass of the oxygen carrier.
Equations (17) and (18) are employed for calculating as a function of time during reduction
period from the measured concentrations of various gaseous species in the gas analyzer for
methane and syngas cycles, respectively:
(17)
(18)
Where is the instantaneous conversion at time , is the conversion in the preceding
instant, and are the initial and final time of measurement, is the molecular weight of
oxygen, is the molar flow rates of the gas at outlet of the reactor after water removal.
13
3 Results and Discussion
3.1 Phase analysis of the oxygen carriers
For all the materials, the XRD results of fresh samples were in agreement with the phases
present in the initial raw materials used. No new phases were formed nor were any phase
transformation observed in the oxygen carriers calcined at different temperatures for different
durations. The phase analysis on fresh samples is summarized in Table 3.
Oxygen carrier COC FOC MOC FOGDC COGDC
Phases detected by
XRD
CuO,
CeO2
Fe2O3,
CeO2
Mn3O4,
CeO2
Fe2O3,
Ce0.9Gd0.1O1.9
CuO,
Ce0.9Gd0.1O1.9
Table 3 – phases detected by XRD in fresh samples
3.2 Reactivity test results
In order to present the data in a concise and effective way, the analysis is divided into two
broad categories with respect to the nature of the support. Thus, the discussion will be on one
group consisting of COC, FOC and MOC carriers that are ceria supported and the other group
would consist of COGDC and FOGDC where the active carriers are supported on gadolinia-
doped ceria supports. Tests on MOGDC analogue are underway.
3.2.1 Pure ceria
To gain an insight into the predicted contribution of ceria towards the reactivity of the
prepared oxygen carriers, pure ceria was first tested under identical experimental conditions.
Ceria is known to perform as an oxygen-contributing catalyst in many common catalytic
processes. The results, however, showed that, ceria did not release oxygen during nitrogen
purge. Nevertheless, the results from fuel injection cycles were interesting. At temperatures
higher than 900oC, pure ceria showed propensity to convert syngas into about 50% of CO2
and H2O. As seen from Figures 5a and b, a portion of syngas was converted and during
oxidation subsequent to the reduction cycle, ceria consumed oxygen in the oxidation gas
stream (5% O2) for 105s at 900 and for 130s at 950°C.
14
Figure 5 – Time dependence of gas composition in the exit stream for syngas cycles with ceria at: (a) 900°C and (b) 950°C.
These observations, particularly, the uptake of oxygen during oxidation cycle subsequent to
reduction in fuel, show that ceria is active at the tested temperatures. This is an attribute that
could be favourably exploited in combustion applications. It is also possible that ceria could
be used for chemical-looping reforming processes, where natural gas is transformed to
hydrogen and carbon monoxide i.e. syngas. Furthermore, no carbon deposition was detected
during the fuel cycle, and the activity of ceria was not affected by successive redox cycles,
which is a testimony to its much desired robustness.
3.2.2 Fuel conversion
As one the most important and basic criteria of evaluating the potential of the oxygen carriers
developed in this work, their fuel conversion potential was investigated. In this work,
methane and syngas were used as fuels and the conversion performance was calculated using
the parameters introduced above, namely, the gas yield (γ) and degree of mass-based
conversion (ω). The gas yield as a function of degree of mass-based conversion was plotted
for all the formulations.
05
101520253035404550
0 50 100 150 200 250
con
cen
trat
ion
of
gase
s %
a time (sec)
Time dependence of gas composition in the exit stream for ceria at 900°C using syngas as fuel
oxygen
CO2
CO
05
10152025303540455055
0 50 100 150 200 250 300
con
cen
tra
tio
n o
fga
ses
%
b time (sec)
Time dependence of gas composition in the exit stream for ceria at 900°C using syngas as fuel
oxygen
CO2
CO
15
3.2.2.1 Methane conversion
Methane, as the main component of natural gas, is an important fuel which is used
extensively in power plants and industrial applications. Due to the importance of methane as
one of the most commonly used fuels, and its rather complicated conversion process in
comparison to syngas, large efforts have been made to investigate the performance of oxygen
carriers with regard to methane conversion. In this respect, the performance of oxygen
carriers towards methane combustion has become a standard yardstick in their evaluation and
acceptance or rejection. Generally, if an oxygen carrier can convert methane to CO2 and H2O
quantitatively, it is deemed a promising material for future commercial applications, provided
it also possesses the other necessary properties given above. For instance, copper oxide has
been known to convert methane completely21
.
In this work, three methane cycles were run at given temperatures (900 or 925 or 950°C) with
every oxygen carrier to ascertain reproducible and reliable performance of the material used.
For example, the CuO-based carriers were tested in methane cycles at 900°C and at 925°C,
three times each. Temperatures higher than 925°C were not considered in this case, because
of the thermodynamic limitations of the phase equilibria in the Cu-O system. On the other
hand, the iron- and manganese-based oxygen carriers were tested in methane cycles at a
standard temperature of 950°C. An example of the gas composition obtained from the
reactivity tests is illustrated in figure 6 for reduction cycle of COGDC using methane as fuel.
It is clear that initially, prior to fuel addition, there is some release of oxygen. This is due to
the decomposition of CuO via the CLOU reaction. When the fuel is added the CH4 is
converted to CO2 only, with no CO detected. Also there is O2 released from the reactor,
indicating that there is a rapid release from the particles. The increase seen is likely due to a
combination of a temperature increase as well as the concentrations are measured downstream
the cooler, i.e. without steam. Hence the actual concentrations in the reactor would be lower.
Figure 6 – outlet gas composition by time for COGDC during reduction period using methane as fuel
at 925°C
0
10
20
30
40
50
60
70
80
60 90 120 150
con
cen
tra
tio
ns
of
gase
s (%
)
time (sec)
Time dependence of gas composition in the exit stream for COGDC at 925°C using methane as fuel
O2
CO2
CO
CH4
CO2
O2
16
The conversion of methane for all the tested particles, for ω>0.99 is presented in table 4.
Oxygen
carrier
COC FOC1 FOC2 MOC FOGDC COGDC
Temperature
(°C)
900 925 950 950 950 950 900 925
Methane
conversion
(%)
88
95
68
74
< 10
88
100
100
Table 4 – Summary of methane conversion by the oxygen carriers for ω>0.99
Gas yield of methane versus degree of mass-based conversion for the ceria-based oxygen
carriers is plotted in Figure 7.
Figure 7 - Gas yield () vs. degree of mass-based conversion () for ceria-supported carriers using methane as fuel
As seen from Figure 7, the reactivity of COC at both 900 and 925°C is very high. Also, with
time, the conversion of methane is slightly increased, up to about 95% at 900oC and 99% at
925oC. Thus, as expected, methane conversion is higher at higher temperature, such that at
925oC when the conversion is nearly complete. in order to maintain good fluidization
conditions and prevent agglomeration of materials in the reactor bed considerably higher flow
rate of methane (900 ml/min) was necessary during the test due to the high density of COC
(and COGDC as well) formulation. However, the high flow rate of the fuel seems to be the
reason why methane was not fully converted by the copper oxide containing oxygen carrier.
The somewhat lower conversion of methane at 900°C compared to that at 925°C could be a
function of temperature as well as the high flow rate. In summary, it could be concluded that
ceria- based copper oxide is a promising and favourable oxygen carrier.
In the case of FOC particles, methane conversion was complete in the very beginning of the
methane cycle. With time, however, the fuel conversion decreased which could be due to the
depletion of oxygen in the carrier particles. Generally, iron particles are not expected to cause
full conversion of methane with this setup.
00.10.20.30.40.50.60.70.80.9
1
0.970.980.991
γ
ω
γ vs. ω for ceria-supported oxygen carriers with CH4
COC at T=925
COC at T=900
FOC2 at T=950
FOC1 at T=950
MOC at T=950
17
Cho et al. tested iron oxides particles supported on alumina in the experimental setup and
conditions similar to this work and have reported similar results32. Very high conversion of
methane was observed at the beginning, followed by a decrease due possibly to the oxygen
depletion. Abad et al. investigated, in detail, the use of iron oxide for its application in CLC
systems23. They used both syngas and natural gas in a 300 W fluidized bed unit to study the
fuel conversion performance of iron oxides. They reported low conversion (~70%) when
methane was used at 800°C; the conversion increased up to 94% at 850°C by lowering the
fuel flow rate.
In the case of MOC, very low methane conversion was observed at 950°C. In order to
investigate the reason for this apparent anomaly, MOC was tested at 800, 850 and 900oCas
well. The Ellingham diagram for the Mn-O system is shown in Figure 8. The test results
showed that at none of the temperatures employed for testing (viz., 800-950°C), methane
conversion improved for MOC samples. The XRD signatures collected on fresh and fully
oxidized samples showed no trace of Mn2O3; all samples contained hausmannite (Mn3O4) and
ceria (CeO2) only; in the fully reduced samples (reduced by syngas) the phases detected were
Mn3O4, MnO and CeO2. This can be explained by the predicted equilibrium of this system, as
seen in the Ellingham diagram in Figure 8. At 5% O2 it is clear that the phase change between
Mn2O3 and Mn3O4 occurs below 800oC, hence Mn3O4 will be the stable phase in the
experiments conducted here. During the reducing phase the equilibrium product will be MnO.
Figure 8 – Ellingham diagram of manganese-oxygen system33
Zhu et al. investigated the cerium-manganese mixed oxides in details for the oxidation of
methane and n-butane 34 . They presumably synthesized and used Mn-substituted ceria
(Ce0.5Mn0.5O1.75 and Ce0.8Mn0.2O1.9) but observed the presence of separate phases of
manganese oxide during the test. They mentioned the clear observation of transmission from
MnO to Mn3O4 during the oxidation cycle. It is worth pointing out that when they tried to
oxidize Mn3O4 to Mn2O3 in pure oxygen at 700°C, it was not successful. Also, the partial
pressure of oxygen for the MnO-Mn3O4 equilibrium was shifted to lower values due to the
interaction between ceria and manganese oxide. It may be possible that a similar interaction
mechanism is operative in our testing protocol with MOC.
18
From the foregoing discussion it appears that the Mn2O3-CeO2 system requires a thorough,
systematic and comprehensive investigation. According to Johansson et al. similar results
have been reported by many researchers24
but Adanez et al. reported methane conversions
higher than 80% using a TGA system25
.
Gas yield of methane versus degree of mass-based conversion for the GDC-based oxygen
carriers is plotted in Figure 9.
Figure 9 – Gas yield () vs. degree of mass-based conversion () for GDC-supported carriers using methane as fuel
As shown in Figure 9, COGDC converted methane fully (100%) both at 900 and 925°C and
the conversion remained stable with time. The performance is comparable to the results
obtained with the COC system, discussed above. For both particles, tests were done at 900
and 925oC with a 900 ml/min flow rate of methane. As discussed above, some unconverted
methane was detected in the outflow stream of the COC tests, while it was fully converted in
the case of COGDC under identical experimental conditions. It, therefore, appears that the
contribution of pure ceria towards the reactivity of copper oxide is somewhat lower than that
of GDC. It should be noted that the conversion rate of methane by COC increased with time
(Figure 7), even though methane was not purged for more than 12s, because further reduction
of copper(I) oxide to elemental copper was not desired.
The approximate duration of methane purge was calculated based on the flow rate so that the
formation of metallic copper was prevented, although from a thermodynamic point of view,
the formation of some small amounts of copper is inevitable.
In comparison to the reactivity of FOC particles (discussed above), FOGDC oxygen carrier
interestingly showed higher conversion of methane, lasting for longer time, with higher
activity as the test progressed. Cho et al. reported a decreasing conversion of methane with
time for iron oxides32
. Also, Johansson et al. tested some iron oxides sintered at different
temperatures and supported on various materials like silica, alumina, zirconia and magnesium
aluminate35. Large amount of unconverted methane was detected in the flue gas stream,
signifying rather low reactivity with respect to conversion of methane in the presence of 50%
0
0.2
0.4
0.6
0.8
1
1.2
0.970.980.991
γ
ω
γ vs. ω for GDC-supported oxygen carriers with CH4
COGDC T=925
COGDC T=900
FOGDC T=950
FOGDC
19
steam. These results indicate that FOGDC could be a promising oxygen carrier for large scale
CLC applications.
The preliminary test results show that the contribution of GDC towards the reactivity of
oxygen carriers examined in this work is higher and more favourable than ceria. Full
conversion of methane by COGDC and around 90% conversion by FOGDC materials
confirm the potential of GDC as an effective participating support and warrant the need for
more systematic investigation of the reactivity and performance of these materials in the
future, to unequivocally establish their benign contribution.
3.2.2.2 Syngas conversion
Syngas conversion was investigated to assess the performance of the developed oxygen
carriers with the gaseous products derived from the solid fuel gasification process. The
syngas conversion for all the materials tested in this work was over 99%. The gas yield (γ) vs.
the degree of mass-based conversion (ω) is plotted in Figure 10. For the case of MOC, the
conversion for most of the process is over 99% and the decreasing rate is probably due to the
oxygen depletion in the materials.
Figure 10 - Gas yield () vs. degree of mass-based conversion () for ceria-supported carriers using syngas as fuel
It is concluded that ceria and GDC supported oxygen carriers can fully convert the syngas.
3.2.2.3 Phase analysis
XRD signatures were collected on all the oxygen carriers tested in this work, in order to
examine the phase change(s), if any. The X-ray diffraction patterns were obtained on each
sample in fully oxidized as well as in fully reduced state. Particles in fully oxidized state were
obtained by flowing 5% O2 in a N2 stream after the last reduction phase. Enough time was
0.97
0.975
0.98
0.985
0.99
0.995
1
1.005
0.960.970.980.991
γ
ω
γ vs. ω for ceria-supported oxygen carriers with syngas at 950oC
FOC2
FOC1
MOC
FOC1
FOC2
MOC
20
allowed for the particles to get fully oxidized. The samples were cooled to room temperature
in the oxidizing environment. Samples in fully reduced state were obtained by cooling the
carrier in the fuel reactor in a dynamic flow of high purity nitrogen gas after the last reduction
cycle was completed. The X-ray diffraction results are summarized in Table 5. The reducing
gas in the case of CuO-based carriers was methane, while syngas was the fuel for the Fe2O3-
and Mn2O3-based particles due to better and complete reactions.
Oxygen
carrier
COC FOC1 FOC2 MOC FOGDC COGDC
Reducing gas Methane Syngas Syngas Syngas Syngas Methane
Phases identified in
reduced sample
Cu2O CuO
CeO2
Fe3O4 CeO2
Fe3O4 CeO2
MnO Mn3O4
CeO2
Fe3O4 Ce0.9Gd0.1O1.9-x
Cu2O Cu
Ce0.9Gd0.1O1.9-x
Phases identified in
fully oxidized
sample
CuO CeO2
Fe2O3 CeO2
Fe2O3 CeO2
Mn3O4 CeO2
Fe2O3 Ce0.9Gd0.1O1.9
CuO Ce0.9Gd0.1O1.9
Table 5 – Phase analysis summary in reduced and oxidized oxygen carriers after testing
The XRD results presented in Table 5 conform to the phases expected in these carriers in both
oxidized and reduced conditions. Moreover, there was no evidence of the formation of new
compound or solid solutions between the carriers and the support in any of these cases. As
stated earlier, Mn3O4 was seen rather than Mn2O3 even in the fully oxidized sample of MOC
which is reasonable according to the phase diagram in figure 8. However, the behavior of
FOC and FODGC carriers was as expected and the phases in the XRD patterns are as
expected. In these cases, according to the phase analysis, further reduction of Fe3O4 to FeO
did not occur, thereby satisfying the theoretical phase equilibria in the iron-based systems
during successive redox cycles, which makes both these materials promising as oxygen
carriers, especially FOGDC with high methane conversion.
Copper oxide particles supported on ceria and GDC both showed excellent performance,
detection of elemental copper in the case of COGDC particles is a result of longer reduction
period. Existence of CuO in the XRD analysis of COC shows the presence of unreacted CuO
particles even in the reduced sample, due possibly to the fact that there is 60 wt% CuO in the
sample. Since testing with ceria (discussed above) showed that ceria has the propensity to
oxidize the fuel at its own, a similar behavior could be speculated (tests underway) with DGC
as well. However, XRD is not the right tool to reveal if nonstochiometric ceria (CeO2-x; x <
0.5) or GDC (Ce0.9Gd0.1O2-x; x < 0.5) were formed. X-ray photoelectron spectroscopy (XPS)
or extended X-ray absorption fine structure (EXAFS) technique would be adequate tools for
such investigation in future studies.
3.2.2.4 Fluidization and agglomeration characteristics
Defluidization incidence was monitored by pressure difference measurements over the bed of
the reactor. All the oxygen carriers tested in this study showed very good fluidization
properties during the successive oxidation and reduction cycles. No defluidization was seen
during the tests except for FOC1 particles after reduction with syngas. Defluidization after
reduction is not unusual and is expected for oxygen carriers with high reactivity during a fuel
21
cycle. This defluidization could be due to the formation of a more reduced phase during
reduction, in the case of iron oxide-based carriers this could be FeO (wustite).
Agglomeration was seen in the case of FOC2 sample (6040 Fe2O4-CeO2 calcined at
950°C/12h) after the test. Around 15% of the sample was found agglomerated on the quartz
bed; however, the agglomerated particles were very soft and could be broken down easily by
slight tapping of the reactor. Small agglomeration was observed in the case of COGDC
particles as well. According to the XRD results in Table 5, metallic copper exists in the
reduced COGDC sample which could be the reason for the observed agglomeration.
Transformation of copper oxide to metallic copper makes particles stick together and thus
agglomerate at high temperature. For other oxygen carriers, no sign of agglomeration was
observed. This is a distinct advantage for the carrier materials supported on ceria or GDC,
whereas severe cases of agglomerations have been reported in the case of other supports such
as alumina and zirconia.
In the case of COC particles (made by freeze-granulation and calcined at 950°C/6h), large
amount of dust was seen on the top part and along the walls of the reactor. This is a sign of
high attrition under the test conditions in the fluidized bed reactor. Around 10% of the
materials in the bed got stuck to the top part of walls of the reactor and could not be recovered
after the test. This could be attributed to the disintegration of the granulated (somewhat
hollow) particles under the combined force of fluidization and reaction.
3.2.3 Oxygen release
Oxygen release aspect of oxygen carriers is one the interesting properties to investigate. If a
particle can release oxygen to the gas phase in an inert atmosphere- such as nitrogen- it is
deemed a promising carrier material for application in solid fuel conversions via chemical-
looping with oxygen uncoupling31
. Solid fuels cannot penetrate to the surface of the oxygen
carriers so it is necessary that oxygen be available on the surface to react with the solids. In
this work, the only particles that released oxygen during inert cycles were COC and COGDC.
This is not surprising, since CuO is well known to decompose to Cu2O at these temperatures,
with subsequent release of gas phase oxygen. Spontaneous oxygen release was not seen for
either the iron- or manganese-based particles. The behaviour of ceria and GDC-supported
CuO particles during inert cycles is shown in Figure 11. Pure nitrogen was purged for 360s at
constant temperature.
22
Figure 11 – oxygen release of: (a) ceria- and (b) GDC-based oxygen carriers during inert cycles.
3.2.4 Oxidation phase
The reactivity of oxygen carriers during oxidation - especially after reduction cycles by fuel-
is sometimes followed in order to estimate the residence time needed by the reduced particles
in the air reactor to get fully oxidized. Figure 12 shows the oxidation behaviour of ceria and
GDC-based oxygen carriers after reduction by methane or syngas.
0
1
2
3
4
5
6
0 100 200 300 400
oxy
gen
co
nce
ntr
ati
on
%
a time (sec)
oxygen release characteristics of ceria-based carriers
MOC T=900
FOC1 T=900
FOC2 T=900
COC T=875
COC COC T=875
0
1
2
3
4
5
6
0 100 200 300
oxy
gen
co
nce
ntr
ati
on
%
b time (sec)
oxygen release characteristics of GDC-based carriers
COGDC T=900
COGDC T=925
FOGDC at T=900
23
Figure 12 – Oxygen uptake characteristics of the: (a) ceria-based and (b) GDC-based oxygen
carriers during oxidation in 5% O2-N2 stream after their reduction by fuel
It appears that the behavior could be divided into two distinct patterns. They either oxidized
gradually over a long period of time in oxidizing stream, or they consumed the available
oxygen quickly after a short induction period.
FOC2 and FOGDC both consumed oxygen quickly and fully to reach the original chemical
state. MOC and FOC1, on the contrary, took longer and were gradual in reaching the original
fully oxidized state. This oxidation behaviour after reduction by syngas demonstrates the
higher activity of these oxygen carriers in syngas compared to methane. Generally, in the case
of iron- and manganese-based oxygen carriers, the oxidation phase subsequent to the syngas
reduction cycle is longer than that for the methane cycle due to the higher reactivity of the
carrier with syngas, which led to higher conversion as well. In the case of FOGDC on the
other hand, the particle was oxidized for more than 300s after reduction by methane which
signifies higher reactivity leading to higher degree of phase change to a reduced form of the
carriers.
Copper oxide particles changed to the fully oxidized state gradually which is commensurate
with the theoretically predicted thermodynamic considerations at and above 900°C.
In summary, all the particles got oxidized to the desired forms after the reduction periods.
00.5
11.5
22.5
33.5
44.5
55.5
0 100 200 300 400 500 600 700
oxy
gen
co
nce
ntr
ati
on
%
a time (sec)
oxygen uptake by the reduced ceria-based oxygen carriers MOC after syngasFOC1 after methaneFOC1 after syngasFOC2 after methaneFOC2 after syngasCOC T=900
00.5
11.5
22.5
33.5
44.5
55.5
0 100 200 300 400 500 600 700 800 900 1000
oxy
gen
co
nce
ntr
ati
on
%
b time (sec)
oxygen uptake by the reduced GDC-based oxygen carriers
COGDC after methane T=900
COGDC after methane T=925
FOGDC after methane T=950
FOGDC after syngas T=950
24
3.2.5 Temperature variation during oxidation and reduction cycles
As stated in the introduction section, oxidation reactions are exothermic and reduction
reactions could be either exothermic or endothermic. For example, for copper oxide-based
carriers, it is advantageous that both the reduction and oxidation reactions are exothermic. In
Tables 6 and 7, changes in temperature during the oxidation and reduction reactions for the
tested oxygen carriers are presented. The positive and negative signs refer to exothermic and
endothermic reactions, respectively. However, this is only the observed temperature change
which should not be confused with the actual enthalpy changes. But it gives a relative
direction and size of the enthalpy change.
Reduction by
methane
Oxidation after
reduction by
methane
Reduction by
syngas
Oxidation after
reduction by syngas
FOC1 -14 +18 +2 +22
FOC2 -14 +18 +2 +22
MOC +2 +10 +18 +25
FOGDC -14 +15 +2 +18
Table 6 – Temperature variation for the Fe2O3- and Mn2O3-based oxygen carriers.
Reduction by
methane at
T=900OC
Oxidation after
reduction by
methane at
T=900OC
Reduction by
methane at
T=925O
C
Oxidation after
reduction by
methane at
T=925OC
COC +22 +6 +18 +4
COGDC +27 +9 +25 +7
Table 7 – Temperature variation for the CuO-based oxygen carriers.
As can be seen, all the Fe2O3-based oxygen carriers showed nearly identical behaviour with
regard to temperature variations during the fuel and the oxidation cycles after reduction,
though the oxidation of the FOC series is a little more exothermic than for FOGDC but the
difference is only minimal. Also, the reduction reaction with syngas is exothermic while it is
endothermic with methane for the iron-based particles. On the other hand, the copper oxide-
based particles showed an exothermic trend for the reduction as well as the oxidation
reactions during the all tests. The trend in the temperature variation for the copper oxide-
based carriers is interesting. For example, the exothermicity of COGDC is higher than that of
the COC particles. This could be attributed to the slightly higher reactivity of COGDC
carriers than of COC. The temperature variation data presented in Tables 6 and 7 shows the
superiority of the GDC-supported materials over those supported on ceria. This could be
interrelated to the better oxygen transport capability of the GDC support due to the oxygen
ion vacancies in it by virtue of doping.
Again, the behavior of MOC is different. First, both reduction and oxidation reactions are
exothermic by a large amount (in terms of temperature changes). It is worth mentioning that
methane conversion with MOC was below 10%, but the temperature increased by 2° during
methane cycle and by 10° during the post-reduction oxidation phase purge. Also, the
temperature increase is significantly higher than those observed with iron oxide-based
samples during syngas and oxidation cycles.
25
It may be recalled that the XRD results showed the presence of MnO produced via Mn3O4 ↔
MnO and not Mn2O3 (Mn2O3 ↔ Mn3O4). The theoretical oxidation enthalpies of MnO and
Mn3O4 at 950oC are as below
21, 34:
6 MnO + O2 2 Mn3O4 ΔH = -451.4 kJ/mol (19)
4 Mn3O4 + O2 6 Mn2O3 ΔH = -189.5 kJ/mol (20)
The enthalpy change for the Mn3O4 ↔ MnO reaction is much higher than that for the Mn2O3
↔ Mn3O4 reaction, thus, a large temperature increase is expected during the oxidation
process. Thus, by virtue of the presence of MnO in the reduced sample of MOC, the observed
temperature increase is not unexpected or abnormal. Same argument could be made for the
high temperature increase seen in the case of reaction of MOC with syngas.
26
4 Conclusion
Samples of copper oxide, iron oxide and manganese oxide oxygen carriers supported on ceria
and gadolinia-doped ceria (GDC) were fabricated by extrusion and their behavior was
investigated by fluidized bed reactivity tests. Relevant parameter of significance to the CLC
process, such as fuel conversion, oxygen release measurement, fluidization properties and,
temperature variations during fuel and oxidation cycles, were examined for all the oxygen
carriers made in this work. In the light of the obtained results, it is concluded that ceria and
GDC-supported oxygen carriers hold promise for CLC applications. All the oxygen carriers
showed very good fluidization properties during the tests without any agglomeration. Copper
oxide-based oxygen carriers showed nearly full conversion of methane with high oxygen
release, high temperature increase during oxidation and no sign of defluidization. FOGDC
showed very high and improved conversion of methane together with favourable reactivity
during oxidation periods after the fuel cycles.
The performance of FOGDC and COGDC materials was the most promising in terms of their
reactivity behavior. However, the behaviour of MOC was somewhat different. Some
explanation has been offered in this thesis for the observed behavior of MOC in the light of
thermodynamic consideration of the phases involved and the XRD results. Nevertheless,
MOC system needs to be more comprehensively and systematically investigated in the future.
Based on the preliminary results obtained in this work, there are opportunities to make great
improvements in the performance of copper oxide and iron oxide-based carriers with ceria
and GDC supports. Finally, the GDC supported oxygen carriers showed better performance
than their ceria counterparts.
27
5. Acknowledgements
I would like to express my greatest thanks to my supervisors Henrik Leion and Abdul-Majeed Azad
for their close, friendly and precise supervision and help during my work. I am thankful to Anders Lyngfelt for providing me with the opportunity to work with the CLC group. I also owe lots of my
knowledge and work to Abdul-Majeed Azad who taught me a lot both scientifically and ethically. I
thank the members of the CLC group in the chemical engineering department – Erik Jerndal, Golnar Azimi, Peter Hallberg, Dazheng Jing and Mehdi Arjmand – who helped me a lot and were my
teachers as well. Special thanks to my examiners Tobias Mattisson and Magnus Ryden for very
informative and useful discussions. Last but not least, I owe my gratitude to my parents who
supported me emotionally and financially from thousands of miles away.
28
6. References
1 United Nations Development Program, Energy for Development, Human development report, 2007
2 United Nations Development Program, Human Development Report, 20th Anniversary Edition,
2010
3 IPCC, technical paper I: technologies, policies and measures for mitigating climate change, 1996
4 Erik Jerndal, investigation of nickel and iron based oxygen carriers for chemical looping
combustion, Doctoral Thesis, Chalmers university of technology, Gothenburg, Sweden, 2010
5 J. T. Kiehl and Kevin E. Trenberth, Earth’s Annual Global Mean Energy Budget, National Center for
Atmospheric Research, Boulder, Colorado, 1997
6 IPCC, Summary for Policymakers, Contribution of Working Group I to the Fourth Assessment
Report of the Intergovernmental Panel on Climate Change, 2007
7 Vante Arrhenius, On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground,
Philosophical Magazine, 1896, Vol 41, 237-276
8 Bo Nordell, Thermal pollution causes global warming, Global and Planetary Change 38, 305– 312,
2003
9 EPA, United States Environmental Protection Agency, 2001, available at
www.epa.gov/globalwarming, as accessed 2 February 2011
10 IPCC, Summary for Policymakers. In: Climate Change 2007: Mitigation. Contribution of Working
Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007
11 Golnar Azimi, Experimental evaluation and modeling of steam gasification and hydrogen inhibition
in CLC with solid fuel, master of Science thesis, Chalmers University of Technology, Gothenburg,
Sweden, 2010
12 IPCC, Implications of Proposed CO2 Emissions Limitations, IPCC Technical Paper 4, 1997
13 EIA, International Energy Outlook 2010, U.S. Department of Energy DOE, 2010
14 Sujata Gupta and Preety M Bhandari, An effective allocation criterion for CO2 emissions, Energy
Policy, Vol 27, 727-736, 1999
15 Kourosh E. Zanganeh and Ahmed Shafeen, A novel process integration, optimization and design
approach for large-scale implementation of oxy-fired coal power plants with CO2 capture,
International Journal of Greenhouse gas Control, Vol 1, 4 7 – 54, 2007
16 Abass A. Olajire, CO2 capture and separation technologies for end-of-pipe applications - A review,
Energy, Vol 35, 2610-2628, 2010
17 Henrik Leion, capture of CO2 from solid fuels using chemical-looping combustion and chemical
looping combustion uncoupling, Doctoral thesis, Chalmers University of Technology, Gothenburg, Sweden, 2008
18 Tobias Pröll et al, Natural minerals as oxygen carriers for chemical looping combustion in a dual
circulating fluidized bed system, energy Procedia, Vol 1, 27-34, 2009
19 Anders Lyngfelt et al, A Fluidized-bed combustion process with inherent CO2 separation;
application of chemical-looping combustion, Chemical Engineering Science, Vol 56, 3101–3113,
2001
20 E. Jerndal, T. Mattisson and A. Lyngfelt, Thermal analysis of chemical looping combustion,
chemical engineering research and design, Vol 84, 795-806, 2006
21 Mohammad M. Hossain, Hugo I. de lasa, Chemical-looping combustion (CLC) for inherent CO2
separation-a Review, Chemical Engineering Science, Vol 63, pp 4433-4451, 2008
29
22
Pilar Gay, Carmen R. Forero, Alberto Abad, Luis F. de Diego, Francisco Garc-Labiano, and Juan
Adnez, Effect of support on the behavior of Cu-based oxygen carriers during long-term CLC operation at temperatures above 1073 K, Energy and Fuel, Vol 25, pp 1316–1326, 2011
23 A. Abad, T. Mattisson, A. Lyngfelt, M. Johansson, The use of iron oxide as oxygen carrier in a
chemical-looping reactor, Fuel, Vol 86, pp 1021-1035, 2007
24 M. Johansson, T. Mattisson and A. Lyngfelt, Investigation of Mn3O4 with stabilized ZrO2 for
chemical-looping combustion, ICHEME, 2006
25 J. Ada´nez, L. F. de Diego, F. Garcı´a-Labiano, P. Gaya´n, and A. Abad, Selection of Oxygen
Carriers for Chemical-Looping Combustion, Energy & Fuels, Vol 18, pp371-37, 2004
26 He Fang, Li Haibin, and Zhao Zengli, Advancements in Development of Chemical-Looping
Combustion: A Review, International Journal of Chemical Engineering, Volume 2009, 2009
27 A. Lyngfelt, M. Johansson, and T. Mattisson, Chemical-looping combustion -Status of
development, 9th International Conference on Circulating Fluidized Beds (CFB-9), 2008, Hamburg,
Germany
28 Alberto Abad, Juan Adanez, Francisco Garcia-Labiano, Luis F. de Diego, Pilar Gayan, Modeling of
the chemical-looping combustion of methane using a Cu-based oxygen-carrier, Combustion and Flame, Vol 157, pp602–615, 2010
29 Yonggang Wei, Hua Wang, Fang He, Xianquan Ao, and Chiyuan Zhang, CeO2 as the Oxygen
Carrier for Partial Oxidation of Methane to Synthesis Gas in Molten Salts: Thermodynamic Analysis and Experimental Investigation, Journal of Natural Gas Chemistry, Vol 16, pp6-11, 2007
30 ZHU Xing, WANG Hua, WEI Yonggang, LI Kongzhai, CHENG Xianming, Hydrogen and syngas
production from two-step steam reforming of methane over CeO2-Fe2O3 oxygen carrier, Journal of rare eraths, Vol. 28, 2010
31 Tobias Mattisson, Anders Lyngfelt and Henrik Leion, Chemical-looping with oxygen uncoupling
for combustion of solid fuels, International Journal of Greenhouse Gas Control, Vol3, pp11-19, 2008
32 P. Cho, T. Mattisson, A. Lyngfelt, Defluidization Conditions for Fluidized-Bed of Iron, Nickel, and
Manganese oxide-Containing Oxygen-Carriers for Chemical-Looping Combustion. Industrial and
Engineering Chemistry Research 2006, 45, (3), 968-977
33 Collaboration: Authors and editors of the volumes III/17G-41D: Mn2O3: phase diagram, crystal
structure, lattice parameters of high temperature phase. Madelung, O., Rössler, U., Schulz, M. (ed.).
SpringerMaterials - The Landolt-Börnstein Database (http://www.springermaterials.com). DOI:
10.1007/10681735_467
34 Gong Zhu, Parag R. Shah, Raymont J. Gorte, A Study of Cerium–Manganese Mixed Oxides for
Oxidation Catalysis, Catalysis Letters, Vol 120, pp191-197
35 Marcus Johansson, Tobias Mattisson, Anders Lyngfelt, comparison of oxygen carriers for chemical
looping combustion, Thermal science, Vol10, pp93-107, 2006