Development of Methane Oxidation Catalysts

for Different Gas Turbine Combustor Concepts

Sara Eriksson

Licentiate Thesis 2005

KTH - The Royal Institute of Technology

Department of Chemical Engineering and Technology Chemical Technology

SE-100 44 Stockholm, Sweden

TRITA-KET R211 ISSN 1104-3466 ISRN KTH/KET/R--211--SE

ABSTRACT

Due to continuously stricter regulations regarding emissions from power

generation processes, development of existing gas turbine combustors is

essential. A promising alternative to conventional flame combustion in gas

turbines is catalytic combustion, which can result in ultra low emission levels

of NOx, CO and unburned hydrocarbons. The work presented in this thesis

concerns the development of methane oxidation catalysts for gas turbine

combustors. The application of catalytic combustion to different combustor

concepts is addressed in particular.

The first part of the thesis (Paper I) reports on catalyst development for fuel-

lean methane combustion. The effect on catalytic activity of diluting the

reaction mixture with water and carbon dioxide was studied in order to

simulate a combustion process with exhaust gas recirculation. Palladium-based

catalysts were found to exhibit the highest activity for methane oxidation under

fuel-lean conditions. However, the catalytic activity was significantly

decreased by adding water and CO2, resulting in unacceptably high ignition

temperatures of the fuel.

In the second part of this thesis (Paper II), the development of rhodium

catalysts for fuel-rich methane combustion is addressed. The effect of water

addition on the methane conversion and the product gas composition was

studied. A significant influence of the support material and Rh loading on the

catalytic behavior was found. The addition of water influenced both the low-

temperature activity and the product gas composition.

Keywords: catalytic combustion, methane oxidation, ceria, palladium,

platinum, rhodium, TPO

SAMMANFATTNING

Eftersom utsläppskraven för energiproducerande anläggningar kontinuerligt

skärps krävs utveckling av dagens förbränningsteknik. Katalytisk förbränning

är ett attraktivt alternativ till konventionell flamförbränning då

utsläppsnivåerna av NOx, CO och oförbrända kolväten kan minskas avsevärt.

Arbetet som presenteras i denna avhandling behandlar utveckling av

katalysatorer för förbränning av metan. Tillämpningen av katalytisk

förbränning i olika förbränningskoncept för gasturbiner har särskilt undersökts.

Första delen av avhandlingen (artikel I) behandlar katalysatorutveckling för

förbränning av metan i luftöverskott. Inverkan på metanomsättningen av att

späda ut reaktionsblandningen med vatten och koldioxid studerades i syfte att

simulera en förbränningsprocess med recirkulation av förbränningsprodukter.

Palladiumbaserade katalysatorer uppvisade den högsta aktiviteten vid

luftöverskott. Emellertid minskade aktiviteten drastiskt då vatten och koldioxid

tillsattes.

Den andra delen av avhandlingen (artikel II) rapporterar om utvecklingen av

rodiumkatalysatorer för metanförbränning i bränsleöverskott. Effekten av att

tillsätta vatten till reaktionsblandningen undersöktes. Signifikanta skillnader för

metanomsättning och sammansättning av produktgas upptäcktes för

katalysatorer bestående av olika bärarmaterial och innehållandes olika halter av

rodium. Tillsatsen av vatten påverkade metanomsättningen särskilt vid låga

temperaturer liksom sammansättningen av produktgasen.

Nyckelord: katalytisk förbränning, oxidation av metan, ceriumoxid, palladium,

platina, rodium, TPO

PUBLICATIONS REFERRRED TO IN THE THESIS

The work presented in this thesis is based on the following publications,

referred to in the text by their Roman numerals. The papers are appended at the

end of the thesis.

I. Catalytic combustion of methane in steam and carbon dioxide-diluted

reaction mixtures

S. Eriksson, M. Boutonnet and S. Järås, submitted to Appl. Catal. A.

II. Partial oxidation of methane over rhodium catalysts for power generation

applications

S. Eriksson, M. Nilsson, M. Boutonnet and S. Järås, Catal. Today (2005), in

press.

OTHER PUBLICATIONS

1. Microemulsion: an alternative route to preparing supported catalysts

S. Rojas, S. Eriksson, M. Boutonnet, Catal. Special. Period. Rep. R. Soc.

Chem. 17 (2004) 258.

2. Preparation of catalysts from microemulsions and their applications in

heterogeneous catalysis

S. Eriksson, U. Nylen, S. Rojas, M. Boutonnet, Appl. Catal. A 265 (2004)

207.

3. Catalytic combustion of methane over bimetallic catalysts

A. Ersson, H. Kusar, S. Eriksson, M. Boutonnet, S. Järås, presented at the

5th International Workshop on Catalytic Combustion, Seoul, Korea, 2002.

4. Stability tests of catalysts for methane combustion

S. Eriksson, K. Persson, M. Boutonnet and S. Järås, presented at the 10th

Nordic Symposium on Catalysis, Helsingør, Denmark, 2002.

5. Catalytic combustion of methane in humid gas turbine cycles

K. Persson, S. Eriksson, P.O. Thevenin, S.G. Järås, presented at the 10th

Nordic Symposium on Catalysis, Helsingør, Denmark, 2002.

6. Stability tests of catalysts for methane combustion

S. Eriksson, K. Persson, M. Boutonnet and S. Järås, presented at the 2nd

EFCATS School on Catalysis, Tihany, Hungary, 2002.

7. Combustion of methane over ceria based catalysts

S. Eriksson, M. Boutonnet and S. Järås, presented at the 6th EuropaCat,

Innsbruck, Austria, 2003.

8. Partial oxidation of methane over rhodium catalysts for power generation

applications

S. Eriksson, M. Nilsson, M. Boutonnet and S. Järås, presented at the 11th

Nordic Symposium on Catalysis, Oulu, Finland, 2004.

CONTENTS

1 INTRODUCTION 1

1.1 The Advanced Zero Emission Power (AZEP) concept 2

1.2 Scope of the work 2

2 GAS TURBINE COMBUSTION 5

2.1 Gas turbine configuration 5

2.2 Emissions from combustion 5

2.3 Zero emission combustion concepts 8

2.3.1 Post-combustion capture 9

2.3.2 Oxy-fuel combustion 10

2.3.3 Pre-combustion decarbonization 11

3 CATALYTIC COMBUSTION 13

3.1. Introduction 13

3.2. Catalytic combustor design 14

3.2.1 Fully catalytic design 14

3.2.2 Hybrid design 15

3.3 Commercial status of catalytic combustors 16

4 CATALYST DEVELOPMENT 17

4.1 Introduction 17

4.2 Fuel-lean catalytic combustion (Paper I) 17

4.2.1 Experimental set-up and testing conditions 17

4.2.2 Metal oxide catalysts 18

4.2.3 Noble metal catalysts 20

4.2.4 Temperature programmed oxidation (TPO) experiments 22

4.3 Fuel-rich catalytic combustion (Paper II) 22

4.3.1 Experimental set-up and testing conditions 23

4.3.2 Rhodium catalysts 24

4.4 The influence of combustion products on the catalyst performance

(Papers I and II) 26

4.4.1 Palladium and platinum catalysts 26

4.4.2 Rhodium catalysts 27

5 CONCLUSIONS 29

ACKNOWLEDGEMENTS 31

CONTRIBUTIONS TO THE PAPERS 32

REFERENCES 33

APPENDICES: Papers I and II

1 INTRODUCTION

Gas turbines that burn fuel to generate heat are widely used for the production

of electricity. Natural gas is commonly used as fuel for stationary gas turbines

due to low levels of impurities and long-term availability. However, the

combustion process produces emissions that are harmful for the environment,

i.e. oxides of nitrogen (NOx), unburned hydrocarbons (UHCs) and carbon

monoxide (CO). The effect of these emissions was noticed in the 1950s when

phenomena such as smog and acid rain appeared. This awareness resulted in

strict regulations regarding emissions of pollutants being issued. Today, the

control of emissions is one of the most important factors when designing a gas

turbine.

Catalytic combustion is an attractive alternative to conventional flame

combustion from an environmental point of view. The main advantages of this

technique are the opportunity to perform the combustion reaction over a wide

range of fuel-to-air ratios and the ability to operate at lower temperatures than

for conventional flame combustion. These features provide the possibility to

reduce emission levels of pollutants such as NOx, CO, UHC and soot

considerably. Since the concept of catalytically stabilized combustion was

introduced in the 1970s, extensive research on finding suitable catalyst

materials has been conducted.

Furthermore, the environmental effect of carbon dioxide (CO2), which is one of

the major combustion products, has been recognized. Carbon dioxide is a

greenhouse gas and contributes to 50 % of the global warming. The production

of CO2 cannot be avoided in combustion processes. However, a process can be

modified in order to facilitate the separation of carbon dioxide from the

exhaust. The separated CO2 could potentially be stored in geological reservoirs

preventing air pollution.

1

1.1 The Advanced Zero Emission Power (AZEP) concept

The Advanced Zero Emission Power (AZEP) concept has been a research

project within the European Union with the aim of developing a zero emission,

gas turbine-based power generation process. The AZEP concept enables NOx

elimination as well as cost reduction of CO2 separation compared to

conventional techniques (post-combustion capture). These targets can be

achieved by i) combusting natural gas in pure oxygen produced by a mixed-

conductive membrane in which O2 is separated from air and, ii) dilution of the

fuel/oxygen mixture with combustion products, i.e. water and carbon dioxide,

instead of nitrogen.

The division of Chemical Technology at KTH has been involved in the design

of a combustor for the AZEP project together with ALSTOM Power and Paul

Scherrer Institut. At an early stage, a catalytically stabilized combustion

process was found to be an attractive option. Therefore, our work has been

focused on the development of catalysts for AZEP mixtures, which contain

large amounts of water and carbon dioxide.

1.2 Scope of the work

The objective of the present thesis is to describe the development of catalysts

for combustion of methane in gas turbine applications. Special attention was

given to the influence of combustion products, i.e. water and carbon dioxide,

on the catalytic performance. Furthermore, different combustion concepts were

investigated.

Fuel-lean catalytic combustion of methane was studied in Paper I. A variety of

catalysts were prepared, with emphasis on palladium-based materials, and the

activity in both air and exhaust gas-diluted mixtures was investigated.

2

In Paper II, a fuel-rich catalytic combustion concept was studied. The

performance of rhodium-based catalysts was investigated with focus on the

effect of support material, Rh loading and presence of water vapor on the

methane conversion efficiency and the product gas composition.

3

4

2 GAS TURBINE COMBUSTION

2.1 Gas turbine configuration

The main components of a gas turbine are a compressor, a combustion chamber

and a turbine [1]. The configuration of a simple gas turbine system is presented

in Figure 1. The compressed air is fed to the combustion chamber together with

the fuel. The temperature increases during combustion resulting in an outlet

temperature of approximately 1800 ºC. Before entering the turbine, the

temperature of the exhaust must be reduced to avoid turbine material damage.

Part of the compressed air is therefore bypassed the combustor and used for

exhaust gas cooling. Power is then generated by expansion of the hot exhaust

gas in the turbine. The main factors influencing the performance of gas turbines

are the component efficiencies and the turbine working temperature.

Air

FuelCombustor

Compressor Turbine

ExhaustBypass air350 °C

1800 °C

1400 °C

Air

FuelCombustor

Compressor Turbine

ExhaustBypass air350 °C

1800 °C

1400 °C

Figure 1. Flow diagram of a conventional gas turbine combustor

2.2 Emissions from combustion

The major compounds formed in combustion reactions, using air or oxygen as

oxidant, are water and carbon dioxide. Furthermore, small amounts of nitrogen

oxides (NOx), unburned hydrocarbons (UHCs), sulfurous oxides (SOx) and

5

carbon monoxide (CO) can be produced [2]. Even though the formation of

these substances is limited, the high throughput rates in gas turbines will result

in substantial emission levels. The emission levels of UHCs, CO and NOx

depend on operating conditions, especially the air/fuel ratio as presented in

Figure 2. Since the negative environmental effects of these compounds were

noticed in the 1950s, continuously stricter regulations regarding emissions of

pollutants have emerged.

Figure 2. Dependence of emission levels of pollutants on air/fuel ratio. Adapted from

Thevenin [3]

When the combustion process is incomplete, only partial oxidation of the fuel

occurs resulting in the formation of UHCs and CO. The formation of CO is

favored by increasing the flame temperature (at constant air/fuel ratio),

reducing the amount of H2 in the fuel and a rapid exhaust gas cooling rate. The

main environmental concern regarding carbon monoxide is related to human

health since it is highly toxic and can cause suffocation. The formation of UHC

emissions is controlled by the same factors as CO and, therefore, they tend to

follow the same trends.

6

Sulfurous oxides (SOx) may be formed when combusting a sulfur-containing

fuel. This is generally the case for fuels such as heavy oils and coal. However,

gaseous fuels such as natural gas contain very small amounts of sulfur. The

main sulfurous oxide formed under combustion conditions is SO2. The primary

environmental effects of SOx emissions include acid rain and human

suffocation at high enough concentrations.

The minimization of NOx emissions is at present one of the major concerns of

gas turbine combustion. Environmental effects such as ozone depletion, acid

rain and smog formation have been identified.

Nitrogen oxides include nitric oxide (NO) and nitrogen dioxide (NO2) with NO

being the predominant form in combustion emissions. Nitric oxide can be

produced according to three different mechanisms: fuel, prompt and thermal.

Fuel NOx is produced by oxidation of fuel-bound nitrogen whereas prompt

NOx is formed by high-speed reactions of nitrogen, oxygen and hydrocarbon

radicals. Prompt NOx is usually formed in combustion chambers operating with

hydrocarbon fuel-rich flames. Thermal NOx is formed by oxidation of

atmospheric nitrogen and is generally the main mechanism above 1100 Cº. The

formation rate of thermal NOx has been found to increase exponentially with

temperature making a reduction of the flame temperature the key factor for

achieving low NOx emissions. For gas turbine systems, this means that a

decrease of the adiabatic flame temperature is required.

Emissions of nitrogen oxides can be drastically reduced by water injection,

exhaust gas clean up by selective catalytic reduction (SCR) or dry low NOx

technologies. The injection of water, or steam, results in a decrease of the flame

temperature and, therefore, lower levels of NOx emissions. In dry low NOx

technologies, combustor designs resulting in a lower flame temperature are

used. A common approach is to use a lean premixed combustor. However, this

can result in problems such as poor combustion efficiency and flame instability

7

and, therefore, higher emission levels of CO and UHC. An example of dry low

NOx technologies is catalytic combustion, which allows a stable combustion

reaction to take place at significantly lower temperatures than for conventional

flame combustion. Therefore, ultra-low levels of NOx emissions (< 3 ppm) can

be achieved without the formation of UHC and CO.

2.3 Zero emission combustion concepts

More recently, the environmental effect of carbon dioxide has been noticed.

Although some controversy exists, most agree that emissions of CO2 are

connected to global warming. The high CO2 levels present today are related to

both fossil-fuel combustion and forest depletion. The production of CO2 cannot

be avoided for combustion processes since it is a natural by-product. However,

the process can be modified in order to facilitate the separation of carbon

dioxide from the exhaust. The captured CO2 can potentially be stored in oceans

or geological reservoirs, i.e. depleted oil and gas reservoirs or deep saline

aquifers. Another alternative is to use carbon dioxide for enhanced oil

recovery, which is already occurring today.

To facilitate the reduction of greenhouse gases, the Kyoto Protocol was

adopted in 1997 [4]. The aim of the protocol is to prevent dangerous

anthropogenic interference with the climate system and contains legally

binding emission targets for developed countries for the post-2000 period. A

total reduction of greenhouse gas emissions from 1990 levels by 5 % over the

period 2008-2012 should be achieved. Within the European Community, an

emission trade system for CO2, according to the Kyoto Protocol, started in

February 2005.

8

Technologies for carbon dioxide capture from fossil fuel combustion processes

can be divided into the following main groups [5]:

• Post-combustion capture

• Oxy-fuel combustion

• Pre-combustion decarbonization

2.3.1 Post-combustion capture

In the post-combustion capture concept, CO2 is separated from the exhaust gas

without modifying the gas turbine system as presented in Figure 3.

Air

Fuel

Gas Turbine Steam Generator

Steam Turbine

CO2 for storage

To atmosphere(N2, H2O, O2)

CO2capture

Air

Fuel

Gas TurbineAir

Fuel

Gas Turbine Steam Generator

Steam Turbine

CO2 for storageCO2 for storage

To atmosphere(N2, H2O, O2)

CO2capture

Figure 3. Post-combustion capture of CO2

Several different techniques can be employed to capture the carbon dioxide

depending on gas composition and flow rates. Absorption into an alkaline

solution, such as an amine, is the most suitable technique for fossil fuel power

plants where the partial pressure of CO2 normally is in the range of

0.15-0.3 bar.

Problems related to the use of this technology for gas turbines are high oxygen

content in the flue gas, which could cause oxidation of amines to carboxylic

9

acids, and the presence of NO2 resulting in formation of amine salts. Both

processes result in amine losses and corrosion problems. Another important

parameter to consider is the cost of installation and operation of this type of

system.

2.3.2 Oxy-fuel combustion

In oxy-fuel combustion, the fuel is combusted in CO2-diluted reaction mixtures

instead of nitrogen. A unit separating air into O2 and N2 is required prior to the

combustion process as depicted in Figure 4. Recycling of CO2 is necessary in

order to lower the flame temperature to values similar to conventional

combustors. The steam produced in the combustion reaction can easily be

separated from the exhaust gas by condensation.

Air

Fuel

Gas Turbine

Air separation

N2

O2

CO2

Condenser H2O

CO2 for storage

Combustor

Air

Fuel

Gas Turbine

Air separation

Air separation

N2

O2

CO2

Condenser H2O

CO2 for storage

Combustor

Figure 4. Oxy-fuel combustion with CO2 capture

The main advantage of this combustion technique is a reduced cost of CO2

separation due to the absence of nitrogen. Furthermore, the formation of NOx

emissions is avoided. A challenge with this process is to separate oxygen from

air in a cost-effective way. Pure oxygen could be produced by cryogenic air

separation or membrane technology.

10

An alternative approach is to recycle both CO2 and water. This concept is

applied in the Advanced Zero Emission Power (AZEP) process together with

membrane technology for air separation [6, 7]. Here, the exhaust gas mixture is

recycled to a unit consisting of an air separation membrane and a combustor.

This unit is described in Figure 5 and is here referred to as MCM (mixed

conductive membrane) reactor. A major advantage with the AZEP concept is

that conventional gas turbine equipment can be utilized and the installation of a

CO2 turbine can be avoided. A challenge with this concept is to achieve a

stable combustion process with the high amounts of water present. Preliminary

investigations have shown that catalytic combustion may be an attractive

alternative.

Air

Fuel

Air compressor

CH4 + O2 CO2 + H2O

O2 O2Q Q---------------------------------

Air turbine

CO2 and H2O to separation unit

MCM reactor

Air

Fuel

Air compressor

CH4 + O2 CO2 + H2O

O2 O2Q Q

CH4 + O2 CO2 + H2O

O2 O2Q Q---------------------------------

Air turbine

CO2 and H2O to separation unit

MCM reactor

Figure 5. Description of the AZEP concept with the MCM reactor unit

2.3.3 Pre-combustion decarbonization

The pre-combustion decarbonization process is described in Figure 6. The fuel

is first converted to syngas, i.e. H2 and CO, by partial oxidation and/or steam

reforming [8]. Partial oxidation is preferable when combusting coal, heavy oils

and high molecular weight hydrocarbons, which are difficult to convert by

steam reforming. The partial oxidation process uses air from the gas turbine

compressor.

11

In the steam reforming route, approximately 25 % of the exhaust gas is

recycled and used in order to form CO and H2. Thereafter, CO2 is formed by

the water gas shift reaction, CO + H2O → CO2 + H2, and separated from the

hydrogen. Separation of carbon dioxide by physical absorption is preferred

under the prevailing conditions.

Challenges with this combustion concept are related to the combustion of

hydrogen mixtures. Modifications of existing burners might be necessary in

order to obtain a reliable combustion process.

AirGas Turbine

H2

Combustor

N2, H2O

Fuel

CO2separation

CO2 for storage

COshift

Partial oxidation and/or reforming

Gas turbine exhaust(steam reforming)

Compressed air(partial oxidation)

AirGas Turbine

H2

Combustor

N2, H2O

Fuel

CO2separation

CO2 for storage

COshift

Partial oxidation and/or reforming

Gas turbine exhaust(steam reforming)

Compressed air(partial oxidation)

Figure 6. Pre-combustion decarbonization

12

3 CATALYTIC COMBUSTION

3.1. Introduction

Catalytic combustion can be divided into primary and secondary combustion

processes. In primary processes, the objective is to produce heat while limiting

the amount of pollution formation. Secondary processes typically deal with

abatement of volatile organic compounds (VOC). Secondary processes are also

called low-temperature catalytic combustion since they occur at temperatures

below 400 ºC. In primary processes, higher concentrations of hydrocarbons are

present and, therefore, considerably higher temperatures (> 1000 ºC) can be

achieved. Only catalytic combustion for gas turbine applications, which of

course is a primary process, will be discussed here.

Catalytic combustion for gas turbines is a promising alternative to conventional

flame combustion that has been extensively investigated for more than 30 years

[9, 10]. This technique utilizes a catalyst, which is placed in the combustion

chamber as presented in Figure 7. The fuel is oxidized at the catalyst surface

without requiring a flame. Since complete combustion can occur outside the

flammability limits of the fuel/oxygen mixture, lower combustor outlet

temperatures are achieved and the need for by pass air cooling can be avoided

[11].

The main advantages of this technique are the opportunity to perform the

combustion reaction over a wide range of fuel-to-air ratios and the ability to

operate at lower temperatures than possible for conventional flame combustion.

These qualities provide the possibility to reduce emission levels of pollutants

such as NOx, CO, soot and unburned hydrocarbons significantly.

13

Air

Fuel

Catalyticcombustor

Compressor Turbine

Exhaust350 °C

<1400 °C

Air

Fuel

Catalyticcombustor

Compressor Turbine

Exhaust350 °C

<1400 °C

Figure 7. Flow diagram of catalytic gas turbine combustor

The major challenge with this combustion method is to find a catalytic material

that can withstand the harsh combustor environment, i.e. temperatures up to

1400 ºC, high gas velocities (10-40 m/s) and the possibility of thermal shocks,

for longer periods of time. The activity of catalysts for gas turbine combustors

should also be stable and last for at least one year of operation without

problems.

3.2. Catalytic combustor design

Ideally, a catalyst with both a high activity and long-term stability under all

conditions relevant for gas turbine combustors, i.e. over a broad power load

range, is preferred. In reality, it is a great challenge to obtain such a catalyst.

Several combustor designs have been proposed in order to overcome this

problem. Some of these configurations will be summarized here. More detailed

information on this subject can be found elsewhere [12].

3.2.1 Fully catalytic design

In this design, all of the fuel and compressed air are mixed before entering the

catalytic combustor. This configuration relies on a highly active ignition

catalyst at the entrance and thermally stable catalysts further down stream. The

14

catalytic end segment should be active to some extent but, most importantly,

stable at temperatures up to 1300 ºC, which is very difficult to obtain.

3.2.2 Hybrid design

This two-stage combustion configuration consists of a catalytic segment

followed by homogeneous gas phase combustion, i.e. flame combustion. Thus,

a high temperature catalytic end segment can be avoided. The fuel is partly

combusted over the catalyst, resulting in a moderate temperature increase, and

complete conversion is obtained in the homogeneous zone. The catalyst outlet

temperature is generally below 1000 ºC. Several different designs resulting in

partial conversion of the fuel have been proposed.

A catalyst of monolith structure with alternately coated channels can be utilized

to control the catalyst temperature. The fuel/air mixture entering the inactive

channels without catalyst coating will not be combusted before the high

temperature homogeneous zone is reached. An advantage with this catalyst

configuration is that the heat formed by the combustion reaction in the active

channels is transported to the inactive channels. Thus, the formation of hot

spots, which can damage the catalyst, can be avoided.

If a fully coated catalyst structure is used, secondary fuel or secondary air

configurations can be used to limit the catalyst temperature. In the secondary

fuel configuration, part of the fuel is completely combusted over the catalyst

and the remaining fuel is introduced directly to the subsequent homogeneous

zone. In the secondary air concept, fuel-rich catalytic combustion occurs in the

first stage producing hydrogen and carbon monoxide in addition to products of

complete combustion. Additional air is added after the catalyst to fully oxidize

the partial oxidation products, i.e. H2 and CO, in the homogeneous zone.

15

3.3 Commercial status of catalytic combustors

The first commercial catalytic combustor for natural gas-fired gas turbines was

recently introduced. In 2002, the XONON cool combustion system, which

utilizes a two-stage lean premix preburner configuration, was commercialized

by Catalytica Energy Systems [13, 14]. Since then, a 1.4 MW Kawasaki M1A-

13X gas turbine equipped with a XONON combustor has been operating in

California. The combustor has a guaranteed life of 8000 h with emission levels

of NOx < 3 ppm and CO and UHC < 6 ppm.

Development of hybrid catalytic combustors has been carried out by Siemens

Westinghouse Power Corp. for large industrial engines (> 200 MW) and by

Solar Turbines for smaller engines (< 20 MW) [15].

However, further development of catalytic combustors is required to improve

this technique and expand the field of application. The development of catalytic

combustors has until now mainly been focused on fuel-lean methane

combustion. This configuration usually requires a pre-burner in order to

achieve low-temperature ignition, which result in higher levels of NOx

emissions. Also, the flexibility of the combustor should be improved to meet

variations in load, ambient conditions and gas turbine size.

Moreover, other combustion concepts have emerged lately resulting in the need

for different catalytic systems. The combustor concept utilizing secondary air

resulting in fuel-rich catalytic combustion is one example. Benefits such as

low-temperature ignition have been observed [16]. Therefore, pre-heaters could

potentially be avoided. The development of catalytic materials for this concept

together with testing under gas turbine conditions are of interest. Another

example is the zero emission combustion concept utilizing exhaust gas

recirculation as discussed previously.

16

4 CATALYST DEVELOPMENT

4.1 Introduction

In general, combustion catalysts consist of a substrate, a washcoat (or support)

and an active phase. The substrate, which can be metallic or ceramic, is usually

of monolith structure. The washcoat is typically a metal oxide of relatively high

surface area facilitating a high dispersion of the active phase.

Several important factors should be considered when designing a catalyst. For

example, the support material can play an important role. Common support

materials for oxidation catalysts are Al2O3, ZrO2, SiO2 and mixed metal oxides.

Properties such as surface area, acidity, crystal structure and support-noble

metal interaction can influence the activity significantly. Aspects to consider

related to the active phase include dispersion, particle size and oxidation state.

In this chapter, results regarding catalysts for both fuel-lean (Paper I) and fuel-

rich (Paper II) combustion of methane are summarized. Issues related to

combustion in AZEP mixtures, i.e. high amounts of water and CO2, were

addressed in particular.

4.2 Fuel-lean catalytic combustion (Paper I)

The catalytic activity of a variety of materials has been investigated for

methane combustion under fuel-lean conditions [17]. Noble metal catalysts are

generally found to be suitable ignition catalysts whereas metal oxide catalysts

are active at higher temperatures.

4.2.1 Experimental set-up and testing conditions

The activity tests were carried out in a laboratory-scale reactor consisting of a

quartz tube placed in a cylindrical furnace as presented in Figure 8.

17

Thermocouples were inserted at the catalyst inlet and inside the furnace to

monitor the temperature. Reactants and products were analyzed by

paramagnetic and IR-spectroscopy techniques. Catalysts of monolith structure,

each containing about 120 mg of catalyst material, were placed in the test rig.

Exhaust

Pump Gas analyser

Evaporator

Furnace

Thermocouples

H2O trap

CH4

H2O

air/N2

CatalystQuartz-glasreactor

CO2

Exhaust

Pump Gas analyser

Evaporator

Furnace

Thermocouples

H2O trap

CH4

H2O

air/N2

CatalystQuartz-glasreactor

CO2 Figure 8. Experimental set-up

The experiments were carried out at atmospheric pressure under fuel-lean

conditions (2.6 vol.% CH4 in air) at a gas hourly space velocity (GHSV) of

110 000 h-1. Repeated heating and cooling cycles in the temperature range

300-900 ˚C were performed for each catalyst whilst measuring the activity.

4.2.2 Metal oxide catalysts

The catalytic properties of ceria-based materials have been widely investigated

mainly due to the important role they play in the three-way catalysts (TWCs)

used for pollution abatement but these materials are also of interest for gas

turbine combustors. The properties that make ceria a promising material for use

in catalysis are mainly (i) the ability to shift easily between reduced and

oxidized state (i.e. Ce3+/Ce4+) and (ii) a high oxygen transport capacity [18].

18

However, pure cerium oxide suffers from poor thermal resistance and inferior

low-temperature activity for combustion applications. The catalytic

performance of cerium oxide can be improved by addition of another metal

oxide or a noble metal [19, 20]. In this section, the catalytic behavior of La or

Zr doped cerium oxide is addressed.

The methane conversion of doped cerium oxide materials is presented in

Figure 9. A strong dependence of catalytic activity on catalyst inlet temperature

can be observed. The mixed metal oxide containing lanthanum showed a higher

activity in the entire temperature range investigated compared to the Zr-doped

material. Ignition temperatures of 750 °C and 840 °C were obtained for

Ce0.9La0.1O2 and Ce0.9Zr0.1O2, respectively. Here, the ignition temperature (or

light-off temperature) is defined as the temperature required for 10 %

conversion of methane. In contrast to the Zr-doped catalyst, complete

conversion could be obtained for the La-doped catalyst at approximately

850 °C.

600 650 700 750 800 850 9000

20

40

60

80

100

CH

4 con

vers

ion

(%)

Catalyst inlet temperature (°C) Figure 9. Methane conversion versus catalyst inlet temperature for Ce0.9La0.1O2 (■) and

Ce0.9Zr0.1O2 (●)

19

Since the catalysts are active in the temperature range 750-900 °C, they are not

suitable as light-off catalysts. For this application ignition should occur around

400 °C. However, materials of this type could be utilized as mid-temperature

catalysts in a multiple-stage catalytic combustor.

4.2.3 Noble metal catalysts

The high activity of palladium-based catalysts for methane oxidation has been

well recognized for years [21]. In general, Pd is considered to be the most

active ignition catalyst for fuel-lean combustion of methane. Platinum is also

an efficient oxidation catalyst, especially for higher alkanes.

In order to increase the activity of the La-doped ceria catalyst, 5 wt.% Pd or Pt

was deposited on Ce0.9La0.1O2. The methane conversion was significantly

improved in the entire temperature range as shown in Figure 10. A Pd/ZrO2

catalyst was also investigated to elucidate the influence of the support. The

lowest ignition temperature was obtained for the supported Pd catalysts with

light-off occurring at 400 °C. Significant differences in activity could be

observed for different support materials. Both Pd catalysts showed similar

activities in the low-temperature range; however, an improvement in

conversion can be observed at higher temperatures when using Ce0.9La0.1O2.

The active phase of palladium catalysts in fuel-lean methane oxidation is still

an issue of debate. However, PdO is generally considered to be more active

than metallic Pd [22-24]. The phase transformation PdO → Pd0 occurs during

heating in the temperature range 680-800 °C depending on catalyst

composition and reaction conditions [25, 26].

20

300 400 500 600 700 800 9000

20

40

60

80

100

CH

4 con

vers

ion

(%)

Catalyst inlet temperature (°C)

Figure 10. Methane conversion versus catalyst inlet temperature during heating (closed

symbols) and cooling (open symbols) for 5 % Pd/Ce0.9La0.1O2 (■ □), 5 % Pd/ZrO2 (▲ ∆) and

5 % Pt/Ce0.9La0.1O2 (● ○) in air

A decrease in methane conversion of the palladium catalysts at higher

temperatures related to this phase transformation can be observed in Figure 10.

The activity is recovered upon cooling since re-oxidation of the metallic

palladium occurs. The re-oxidation occurs at a lower temperature resulting in a

PdO decomposition-reformation hysteresis.

The activity of the supported platinum catalyst was considerably lower than for

the Pd catalysts with light-off occurring at 660 °C. The activation of the C-H

bond in methane is the first critical step in the combustion reaction. This

activation occurs according to different mechanisms for Pd and Pt [27]. For Pd,

a completely oxidized surface is generally preferred whereas a partially

oxidized surface results in the highest activity for Pt. Since fuel-lean reaction

conditions produce completely oxidized surfaces, this explains why the activity

of the palladium catalysts is significantly higher than for the platinum catalyst

in this study.

21

4.2.4 Temperature programmed oxidation (TPO) experiments

In order to study the redox behavior of palladium deposited on different

support materials more in detail, TPO experiments were performed. The results

are presented in Figure 11. The decomposition of PdO occurs in approximately

the same temperature range for both catalysts during heating. A temperature

gap between reduction and re-oxidation of palladium can be observed. The re-

oxidation of palladium takes place at a higher temperature for the Ce0.9La0.1O2-

supported catalyst, which is probably related to the high oxygen mobility of the

support, resulting in a stabilization of the PdO phase. These observations are in

agreement with activity test results assuming that PdO is the active phase of the

catalyst.

300 400 500 600 700 800 900

O2 c

once

ntra

tion

(a.u

.)

Temperature (°C)

5 % Pd/Ce0.9La0.1O2

5 % Pd/ZrO2

Figure 11. TPO profiles for the palladium catalysts

4.3 Fuel-rich catalytic combustion (Paper II)

In the hybrid combustor design utilizing secondary air as described previously,

a catalytic fuel-rich stage where partial oxidation of the fuel occurs is followed

by lean homogeneous combustion. Syngas, i.e. H2 and CO, is produced under

fuel-rich conditions, resulting in a hydrogen-stabilized second stage

22

combustion process. Only limited studies of this combustion concept have been

published in the past. Lyubovsky et al. investigated methane combustion under

various fuel-to-air ratios for supported Pd, Pt and Rh catalysts demonstrating

benefits such as low light-off temperature during fuel-rich operation [16].

Various metals, such as Rh, Pd, Pt, Ru, Ni, Co and Fe, have been investigated

as catalysts for partial oxidation of methane to syngas [28]:

CH4 + 1/2 O2 = CO + 2 H2 (1)

Different reaction mechanisms for partial oxidation of methane have been

proposed [29-34]. The reaction can either proceed according to i) complete

combustion of some methane forming CO2 and H2O (reaction 2, below),

followed by methane reforming by steam (reaction 3) and CO2 (reaction 4)

yielding syngas or ii) direct partial oxidation forming CO and H2 as primary

products (reaction 1).

CH4 + 2 O2 = CO2 + 2 H2O (2)

CH4 + H2O = CO + 3 H2 (3)

CH4 + CO2 = 2 CO + 2 H2 (4)

Several studies have reported that especially Rh-based catalysts are suitable for

partial oxidation of methane due to their high activity, syngas selectivity and

resistance to carbon deposition [35, 36]. Here, the behavior of supported

rhodium catalysts for combustion applications is discussed.

4.3.1 Experimental set-up and testing conditions

The experimental set up presented in Figure 8 was also used for investigating

the fuel-rich combustion concept. The experiments were carried out at

atmospheric pressure with nitrogen diluted fuel-rich gas mixtures (3.3 vol.%

CH4, 1.7 vol.% O2, balance N2) at a gas hourly space velocity (GHSV) of

99 000 h-1. Repeated heating and cooling cycles in the temperature range

200-750 ˚C were performed for each catalyst whilst measuring the activity.

23

4.3.2 Rhodium catalysts

The catalytic behavior of various supported rhodium catalysts is presented in

Figure 12. It is evident that both the support material and Rh loading can

influence the catalytic performance significantly.

200 300 400 500 600 700 8000

20

40

60

80

100

Con

vers

ion

(%)

Temperature catalyst inlet (°C) Figure 12. Conversion of methane (closed symbols) and oxygen (open symbols) in N2-diluted

air versus catalyst inlet temperature for 0.5 % Rh/Ce-ZrO2 (■ □), 0.5 % Rh/Ce0.9La0.1O2 (

), 0.2 % Rh/ZrO2 (▲ ∆), 0.5 % Rh/ZrO2 (▼ ) and 1 % Rh/ZrO2 (● ○)

The presence of ceria in the support improved the activity, especially at higher

temperatures, and complete conversion could be obtained. This behavior is

probably related to the high oxygen mobility of ceria. Previous studies have

shown that the addition of Ce to ZrO2 increases the activity and stability of Pt

and Ni catalysts for partial oxidation of methane due to an increased CH4

dissociation and enhanced carbon elimination from the surface [37, 38].

The activity can be significantly enhanced by increasing the Rh loading. Here,

increasing the amount of rhodium from 0.2 to 1 wt% resulted in a decrease of

the ignition temperature by 160 °C. The high-temperature activity was also

considerably improved when increasing the Rh loading. These results show

that the noble metal loading is an important factor to consider when preparing

24

combustion catalysts. If the amount of noble metal is too high, the dispersion

can decrease resulting in fewer active sites. Therefore, an optimal metal

loading should be found.

For the fuel-rich combustion concept, the absolute values of the product

concentrations after the catalyst are of importance in order to evaluate the

stability of the subsequent homogeneous combustion stage. Simultaneous H2,

CO and CO2 production can be observed as shown in Figure 13 (closed

symbols). A reaction mechanism consisting of complete oxidation followed by

steam and dry reforming is implied since formation of CO2 occurs mainly at

lower temperatures whereas CO is produced at higher temperatures.

200 300 400 500 600 700 8000

1

2

3

4

5

6

7

8

H2 p

rodu

ctio

n (v

ol.%

)

Temperature catalyst inlet (°C)200 300 400 500 600 700 800

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

CO

pro

duct

ion

(vol

.%)

Temperature catalyst inlet (°C)

200 300 400 500 600 700 8000,0

0,5

1,0

1,5

2,0

CO

2 pro

duct

ion

(vol

.%)

Temperature catalyst inlet (°C)

a b

c

200 300 400 500 600 700 8000

1

2

3

4

5

6

7

8

H2 p

rodu

ctio

n (v

ol.%

)

Temperature catalyst inlet (°C)200 300 400 500 600 700 800

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

CO

pro

duct

ion

(vol

.%)

Temperature catalyst inlet (°C)

200 300 400 500 600 700 8000,0

0,5

1,0

1,5

2,0

CO

2 pro

duct

ion

(vol

.%)

Temperature catalyst inlet (°C)

200 300 400 500 600 700 8000

1

2

3

4

5

6

7

8

H2 p

rodu

ctio

n (v

ol.%

)

Temperature catalyst inlet (°C)200 300 400 500 600 700 800

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

CO

pro

duct

ion

(vol

.%)

Temperature catalyst inlet (°C)200 300 400 500 600 700 800

0

1

2

3

4

5

6

7

8

H2 p

rodu

ctio

n (v

ol.%

)

Temperature catalyst inlet (°C)200 300 400 500 600 700 800

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

CO

pro

duct

ion

(vol

.%)

Temperature catalyst inlet (°C)

200 300 400 500 600 700 8000,0

0,5

1,0

1,5

2,0

CO

2 pro

duct

ion

(vol

.%)

Temperature catalyst inlet (°C)

a b

c

Figure 13. a) H2 production, b) CO production and c) CO2 production in N2-diluted air

(closed symbols) and in 10 % H2O/N2-air (open symbols) versus catalyst inlet temperature for

0.5 % Rh/Ce-ZrO2 (■ □), 0.5 % Rh/Ce0.9La0.1O2 ( ), 0.2 % Rh/ZrO2 (▲ ), 0.5 % Rh/ZrO2

(▼) and 1 % Rh/ZrO2 (● ○)

25

4.4 The influence of combustion products on the catalyst performance

(Papers I and II)

The combustion products, i.e. water and carbon dioxide, may influence the

reaction significantly depending on catalyst composition and reaction

conditions. The product gas composition can be altered, which can be an

advantage in some cases. A major drawback of large amounts of water is that it

may promote sintering of the catalyst at high temperatures resulting in a

decreased dispersion of the active phase on the catalyst surface.

In this section, results from Papers I and II regarding the influence of

combustion products, i.e. water and CO2, on the catalytic activity are discussed.

4.4.1 Palladium and platinum catalysts

The influence of water and carbon dioxide on palladium catalysts has been

investigated in several studies [39-42]. Most authors agree that water has a

negative effect on methane conversion whereas different opinions regarding the

influence of CO2 can be found. A reaction order of approximately –1 has been

identified for water. Both zero order and strong inhibition of order –2 have

been reported for CO2. Furthermore, the effect of water was found to be

dominant when both combustion products were present in the feed. Water may

affect the activity according to the following reaction [43]:

PdO + H2O = Pd(OH)2

Most studies have investigated the effect of relatively low amounts of water on

the activity of palladium catalysts. However, the results presented in this work

have been obtained in reaction mixtures consisting of 1.2 vol.% CH4, 2.4 vol.%

O2, 11.3 vol.% CO2, 27.3 vol.% H2O, 57.8 vol.% N2 at a gas hourly space

velocity (GHSV) of 110 000 h–1. The high concentration of combustion

products has been investigated since these conditions are relevant in exhaust

gas-diluted combustion processes such as the AZEP concept.

26

The results presented in Figure 14 show a drastic decrease in methane

conversion for the Pd-based catalysts when water and CO2 are present in the

feed, see Figure 10 for comparison. An influence of the support material on the

activity can be observed with Ce0.9La0.1O2 exhibiting a significantly higher

activity than ZrO2. Ciuparu et al. [44] propose that support materials with high

oxygen mobility, such as ceria, will reduce the effect of water during methane

combustion by oxygen transfer from the bulk to the PdO particles. This process

will reduce the formation of surface hydroxyls, which probably prevent

methane activation resulting in a decreased reaction rate. The activity of the

supported platinum catalyst was only slightly decreased in the presence of

water and CO2.

300 400 500 600 700 800 9000

20

40

60

80

100

CH

4 con

vers

ion

(%)

Catalyst inlet temperature (°C)

Figure 14. Methane conversion versus catalyst inlet temperature during heating (closed

symbols) and cooling (open symbols) for 5 % Pd/Ce0.9La0.1O2 (■ □), 5 % Pd/ZrO2 (▲ ∆) and

5 % Pt/Ce0.9La0.1O2 (● ○) in exhaust gas-diluted mixtures

4.4.2 Rhodium catalysts

The effect of water (10 vol.%) on supported rhodium catalysts for fuel-rich

methane combustion was studied. The results presented in Figure 15 show that

water has a negative effect on the catalytic performance in the low-temperature

range with increased light-off temperatures, see Figure 12 for comparison. The

27

delay in ignition is presumably due to water adsorbed on the catalyst surface

preventing methane C-H bond activation. However, advantages of adding

water could be observed at higher temperatures. For example, the conversion of

methane increased in this temperature range for all ZrO2-supported catalysts.

200 300 400 500 600 700 8000

20

40

60

80

100

Con

vers

ion

(%)

Temperature catalyst inlet (°C)

Figure 15. Conversion of methane (closed symbols) and oxygen (open symbols) in 10 vol.%

H2O/N2-air versus catalyst inlet temperature for 0.5 % Rh/Ce-ZrO2 (■ □), 0.5 %

Rh/Ce0.9La0.1O2 ( ), 0.2 % Rh/ZrO2 (▲ ∆), 0.5 % Rh/ZrO2 (▼ ) and 1 % Rh/ZrO2 (● ○)

Furthermore, the product gas composition is altered when water vapor is added

to the reaction mixture as observed in Figure 13. Higher amounts of CO2 are

formed in the entire temperature range whereas less CO is formed. Also, at

higher temperatures, the formation of H2 was enhanced by adding water

resulting in H2/CO ratios higher than 2. The product gas composition is altered

due to the water gas shift reaction taking place (reaction 5). The large amount

of water vapor in the reaction mixture shifts the equilibrium favoring the

products H2 and CO2.

CO + H2O = CO2 + H2 (5)

28

5 CONCLUSIONS

Two different catalytic combustion concepts have been investigated, i.e. fuel-

lean and fuel-rich combustion of methane. In order to simulate a combustion

process utilizing exhaust gas recirculation, the effect of water and CO2 on the

catalyst performance was studied in particular. Significant differences in

activity for methane oxidation and product gas composition could be detected

for the different configurations.

Palladium-based catalysts were found to exhibit the highest activity for

methane oxidation under fuel-lean conditions. In fuel-air reaction mixtures,

both supported Pd catalysts, i.e. Pd/Ce0.9La0.1O2 and Pd/ZrO2, exhibited similar

light-off behaviour. The high temperature activity was however improved when

using Ce0.9La0.1O2. The catalytic activity was significantly decreased by adding

water and CO2 resulting in unacceptably high ignition temperatures of the fuel.

Furthermore, the high-temperature activity was decreased due to the

PdO → Pd0 phase transition.

Even though the activity of the platinum catalyst was only slightly affected by

the addition of combustion products, the initial activity was significantly lower

than for Pd catalysts making this catalyst unsuitable for the first light-off stage.

A variety of supported rhodium catalysts were prepared and tested for the fuel-

rich combustion concept. The results from the activity tests revealed a

significant importance of the support material and Rh loading on the catalytic

behavior. The catalysts containing ceria in the support material showed the

highest activity and production of H2 and CO.

The addition of water was found to have both positive and negative effects on

the catalytic performance of rhodium under fuel-rich conditions. A negative

effect on the activity in the low-temperature range with increased light-off

29

temperatures could be observed. However, the formation of hydrogen was

increased in the presence of water at higher temperatures, which is favorable

for a combustion configuration with a homogeneous final stage.

The results presented here show that a hybrid combustor design consisting of a

first fuel-rich catalytic stage followed by H2-stabilized homogeneous

combustion is an attractive alternative to the more traditional fuel-lean catalytic

combustion approach. The fuel-rich concept is particularly of interest for oxy-

fuel combustion processes utilizing exhaust gas recirculation.

30

ACKNOWLEDGEMENTS

I would like to thank Prof. Sven Järås for giving me the opportunity to carry

out my graduate studies at the Department of Chemical Engineering and

Technology at KTH. I also thank Ass. Prof. Magali Boutonnet for her

supervision.

I want to thank my colleagues, former and present, at Chemical Technology

and Chemical Reaction Engineering for providing a positive working

environment and good friendship. Special thanks to Katarina, Ulf, Erik, Sergio,

Anders and Stelios for stimulating collaboration. Also thanks to Marita and

Bård for being good room-mates.

I am grateful to Christina Hörnell for improving the linguistic quality of this

thesis and Inga Groth for assistance with BET and XRD analysis.

I would also like to thank the undergraduate students who I had the opportunity

to work with, Marita Nilsson and Andrea Scarabello, for doing an excellent

job.

The financial support from the European Commission in FP5, contract number

ENK5-CT-2001-00514, and the Swiss Government (BBW01.0054-1) is

gratefully acknowledged. I want to thank the partners in the AZEP project with

whom I had the pleasure to work with, especially Markus Wolf and Raffaella

Villa at ALSTOM Power and Ioannis Mantzaras and Adrian Schneider at Paul

Scherrer Institut.

Last, but not least, I would like to thank my family and Jonas for support and

encouragement during the years.

31

CONTRIBUTIONS TO THE PAPERS

I. I am the main author of Paper I and the experimental work was performed

by myself.

II. I am the main author of Paper II. Marita Nilsson performed most of the

experimental work under my guidance.

32

REFERENCES

[1] H. Cohen, G.F.C. Rogers, H.I.H. Saravanamuttoo, Gas turbine theory 4th

ed., Addison Wesley Longman Limited, Essex, 1996.

[2] C.E. Baukal, Industrial combustion pollution and control, Marcel Dekker,

New York, 2004.

[3] P. Thevenin, Catalytic combustion of methane, PhD thesis, KTH-

Kungliga Tekniska Högskolan, Department of Chemical Engineering and

Technology, Stockholm, 2002.

[4] European Commission,

http://europa.eu.int/comm/environment/climat/kyoto.htm, accessed

2004-09-06.

[5] Carbon Dioxide Capture and Storage, Report of DTI International

Technology Service Mission to the USA and Canada from 27th October to

7th November 2002.

[6] M. Reinke, R. Carroni, D. Winkler, T. Griffin, Experimental Investigation

of Natural Gas Combustion in Oxygen / Exhaust Gas Mixtures for Zero

Emissions Power Generation, 6th Int. Conf. on Technologies and

Combustion for a Clean Environment, 9-12 July, 2001, Oporto, Portugal,

1:15.

[7] T. Griffin, S.G. Sundkvist, K. Åsen, T. Bruun, Advanced Zero Emissions

Gas Turbine Power Plant, ASME TURBO EXPO, 16-19 June, 2003,

Atlanta, Georgia, USA.

[8] H. Audus, O. Kaarstad, G. Skinner, CO2 Capture by Pre-Combustion

Decarbonisation of Natural Gas, IEA GHG Paper presented at GHGT-4,

Interlaken, Switzerland, 1998.

[9] W.C. Pfefferle, Belgian Patent 814 (1974), to Engelhard Corporation.

[10] W.C. Pfefferle, J. Energy 2 (1978) 142.

[11] R.E. Hayes, S.T. Kolaczkowski, Introduction to catalytic combustion,

Gordan and Breach Science Publishers, Amsterdam, 1997.

33

[12] E.M. Johansson, D. Papadias, P.O. Thevenin, A.G. Ersson, R.

Gabrielsson, P.G. Menon, P.H. Björnbom, S.G. Järås, Catal. Special.

Period. Rep. R. Soc. Chem. 14 (1999) 183.

[13] D.Y. Yee, K. Lundberg, C.K. Weakley, ASME Paper 2000-GT-0088,

2000.

[14] Catalytica Energy Systems,

http://www.catalyticaenergy.com/xonon/index.html, accessed 2005-01-13.

[15] D.B. Fant, G.S. Jackson, H. Karim, D.M. Newburry, P. Dutta, K.O.

Smith, R.W. Dibble, ASME Paper 99-GT-57.

[16] M. Lyubovsky, L.L. Smith, M. Castaldi, H. Karim, B. Nentwick, S.

Etemad, R. LaPierre, W.C. Pfefferle, Catal. Today 83 (2003) 71.

[17] T.V. Choudhary, S. Banerjee, V.R. Choudhary, Appl. Catal. A 234

(2002) 1.

[18] A. Trovarelli, M. Boaro, E. Rocchini, C. de Leitenburg, G. Dolcetti, J.

Alloys Compd. 323-324 (2001) 584.

[19] C. de Leitenburg, A. Trovarelli, J. Llorca, F. Cavani, G. Bini, Appl. Catal.

A 139 (1996) 161.

[20] C. Bozo, N. Guilhaume, E. Garbowski, M. Primet, Catal. Today 59 (2000)

33.

[21] R.B. Anderson, K.C. Stein, J.J. Feenan, L.J.E. Hofer, Ind. Eng. Chem. 53

(1961) 809.

[22] R.J. Farrauto, M.C. Hobson, T. Kennelly, E.M. Waterman, Appl. Catal. A

81 (1992) 227.

[23] R. Burch, F.J. Urbano, Appl. Catal. A 124 (1995) 121.

[24] J.G. McCarty, Catal. Today 26 (1995) 283.

[25] R.J. Farrauto, J.K. Lampert, M.C. Hobson, E.M. Waterman, Appl. Catal.

B 6 (1995) 263.

[26] G. Groppi, C. Cristiani, L. Lietti, P. Forzatti, Stud. Surf. Sci. Catal. 130

(2000) 3801.

[27] R. Burch, D.J. Crittle, M.J. Hayes, Catal. Today 47 (1999) 229.

[28] S.C. Tsang, J.B. Claridge, M.L.H. Green, Catal. Today 23 (1995) 3.

34

[29] E.P.J. Mallens, J.H.B.J. Hoebink, G. B. Marin, J. Catal. 167 (1997) 43.

[30] J. C. Slaa, R.J. Berger, G.B. Marin, Catal. Lett. 43 (1997) 63.

[31] W.Z. Weng, M.S. Chen, Q.G. Yan, T.H. Wu, Z.S. Chao, Y.Y. Liao, H.L.

Wan, Catal. Today 63 (2000) 317.

[32] W.Z. Weng, C.R. Luo, J.J. Huang, Y.Y. Liao, H.L. Wan, Top. Catal. 22

(2003) 87.

[33] O.V. Buyevskaya, D. Wolf, M. Baerns, Catal. Lett. 29 (1994) 249.

[34] O.V. Buyevskaya, K. Walter, D. Wolf, M. Baerns, Catal. Lett. 38 (1996)

81.

[35] K. Heitnes Hofstad, J.H.B.J. Hoebink, A. Holmen, G.B. Marin, Catal.

Today 40 (1998) 157.

[36] F. Basile, G. Fornasari, F. Trifirò, A. Vaccari, Catal. Today 64 (2001) 21.

[37] W-S. Dong, K-W. Jun, H-S. Roh, Z-W. Liu, S-E. Park, Catal. Lett. 78

(2002) 215.

[38] L.V. Mattos, E. Rodino, D.E. Resasco, F.B. Passos, F.B. Noronha, Fuel

Process. Technol. 83 (2003) 147.

[39] R. Burch, F.J. Urbano, P.K. Loader, Appl. Catal. A 140 (1996) 17.

[40] J.C. van Giezen, F.R. van den Berg, J.L. Kleinen, A.J. van Dillen, J.W.

Geus, Catal. Today 47 (1999) 287.

[41] K. Fujimoto, F.H. Ribeiro, M. Avalos-Borja, E. Iglesia, J. Catal. 179

(1998) 431.

[42] D. Ciuparu, N. Katsikis, L. Pfefferle, Appl. Catal. A 216 (2001) 209.

[43] R. Burch, P.K. Loader, F.J. Urbano, Catal. Today 27 (1996) 243.

[44] D. Ciuparu, E. Perkins, L. Pfefferle, Appl. Catal. A 263 (2004) 145.

35