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

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