LICENTIATE THESIS IN CHEMICAL ENGINEERING

Hydrogen Generation from Dimethyl Ether by Autothermal Reforming

MARITA NILSSON

Department of Chemical Engineering and Technology

School of Chemical Science and Engineering KTH – Royal Institute of Technology

Stockholm, Sweden 2007

Hydrogen Generation from Dimethyl Ether by Autothermal Reforming MARITA NILSSON TRITA-CHE-Report 2007:34 ISSN 1654-1081 ISBN 978-91-7178-673-9 Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan framlägges till offentlig granskning för avläggande av teknologie licentiatexamen i kemiteknik, torsdagen den 7 juni 2007 klockan 13.00 i Seminarierum 591, Kemisk Teknologi, Teknikringen 42, Stockholm. © Marita Nilsson 2007 Tryck: Universitetsservice US AB

ABSTRACT Heavy-duty trucks are in idle operation during long periods of time, providing the vehicles with electricity via the alternator at standstill. Idling trucks contribute to large amounts of emissions and high fuel consumption as a result of the low efficiency from fuel to electricity. Truck manufacturers are working to develop equipment using auxiliary power units to supply the trucks with electricity, which operate independently of the main engine. Fuel cell-based auxiliary power units could offer high efficiencies and low noise and vibrations. The hydrogen required for the fuel cell can be generated in an onboard fuel reformer. This thesis is devoted to hydrogen generation from dimethyl ether, DME, by autothermal reforming focusing on the application of fuel cell auxiliary power units. In the search for alternative fuels, DME has lately been identified as a promising diesel substitute. The first part of the thesis gives an introduction to the field of DME reforming with a literature survey of recent studies within the area. Included are also results from thermodynamic equilibrium calculations. In the following parts of the thesis, experimental studies on autothermal reforming of DME are presented. A reformer constructed to generate hydrogen to feed a 5 kWe polymer electrolyte fuel cell is evaluated with emphasis on trying to work close to a practically viable process, i.e. without external heating and using gas mixtures resembling real conditions. Additional experiments have been conducted to investigate the use of catalytic oxidation of dimethyl ether as a heat source during startup. The results of these studies are presented in Paper I. In the second experimental study of this thesis, which is presented in Paper II, Pd-based monolithic catalysts are evaluated at small scale for use in autothermal reforming of DME. A screening of various catalyst materials has been performed followed by a study of the influence on the product composition of varying operating parameters such as oxygen-to-DME ratio, steam-to-DME ratio, and temperature. Keywords: autothermal reforming, auxiliary power unit, dimethyl ether, fuel cell, hydrogen, palladium, reforming catalyst, temperature-programmed reduction, truck idling, X-ray diffraction, zinc

SAMMANFATTNING Tomgångskörning av tunga lastbilar blir allt vanligare för att via generatorn förse fordonets komfortsystem med elektricitet. Den låga verkningsgraden från bränsle till elektricitet under tomgång leder till stora mängder emissioner och hög bränsleförbrukning. Fordonstillverkarna arbetar med utveckling av hjälpkraftaggregat, sk APU-system, som kan förse lastbilen med elektricitet oberoende av förbränningsmotorn. Bränslecellsbaserade APU-system skulle kunna ge hög verkningsgrad samt låg ljud- och vibrationsnivå. Vätgasen till bränslecellen kan genereras ombord genom reformering av bränslet. Denna avhandling behandlar vätgasgenerering från dimetyleter, DME, genom autoterm reformering med fokus på bränslecellsbaserade APU-system. DME är ett bränsle som på senare tid har pekats ut som ett lovande diesel-substitut. I den första delen av avhandlingen ges en introduktion till området DME- reformering med en litteraturundersökning av den senaste forskningen inom området. Resultat från termodynamiska jämviktsberäkningar är också inkluderade. I de efterföljande delarna av avhandlingen presenteras experimentella studier av autoterm DME-reformering. En reformer som konstruerats för att generera vätgas till en 5 kWe polymerelektrolytbränslecell utvärderas med avsikt att försöka komma nära en praktiskt genomförbar process, dvs utan extern uppvärmning och med realistiska gasblandningar. Experiment har också genomförts för att utvärdera användningen av katalytisk oxidation av DME som en värmekälla under uppstart. Resultaten är presenterade i Artikel I. I den andra experimentella studien i avhandlingen, som är presenterad i Artikel II, utvärderas Pd-baserade katalysatorer belagda på monoliter för användning vid autoterm reformering av DME. En undersökning av olika katalysatormaterial har genomförts åtföljd av en studie av inverkan av driftsparametrar såsom syre-till-DME-förhållande, ånga-till-DME-förhållande och temperatur. Nyckelord: autoterm reformering, bränslecell, dimetyleter, hjälpkraftaggregat, palladium, reformeringskatalysator, röntgendiffraktion, temperatur-programmerad reduktion, tomgångskörning, vätgas

List of publications and presentations Publications referred to in this thesis

I. M. Nilsson, L.J. Pettersson, B. Lindström, “Hydrogen generation from dimethyl ether for fuel cell auxiliary power units”, Energy & Fuels 20 (2006) 2164.

II. M. Nilsson, P. Jozsa, L.J. Pettersson, “Evaluation of Pd-based catalysts

and the influence of operating conditions for autothermal reforming of dimethyl ether”, accepted for publication in Applied Catalysis B: Environmental.

Other publications and conference/seminar contributions

i. S. Nassos, E. Elm Svensson, M. Nilsson, M. Boutonnet, S. Järås, “Microemulsion-prepared Ni catalysts supported on cerium-lanthanum oxide for the selective catalytic oxidation of ammonia in gasified biomass”, Appl. Catal. B: Environ. 64 (2006) 96.

ii. S. Eriksson, M Nilsson, M. Boutonnet, S. Järås, “Partial oxidation of

methane over rhodium catalysts for power generation applications”, Catal. Today 100 (2005) 447.

iii. M. Nilsson, L.J. Pettersson, B. Lindström, P. Ekdunge, F. von Corswant,

C. Villa, “Evaluation of diesel fuel reformer for PEFC-APU”, presented at 30th Fuel Cell Seminar, Honolulu, November 13-17, 2006.

iv. M. Nilsson, L.J Pettersson, “DME – The sustainable hydrogen carrier

alternative for automotive applications”, presented at the pre-conference of TOCAT 5: Catalysis for a sustainable society, Tokyo, July 22, 2006.

v. M. Nilsson, L.J. Pettersson, B. Lindström, “Reforming of dimethyl ether

over Cu and Pd catalysts”, presented at TOCAT 5 – Fifth Tokyo Conference on Advanced Catalytic Science and Technology, July 23-28, 2006.

vi. M. Nilsson, B. Lindström, L.J. Pettersson, “Hydrogen from commercial

diesel fuel using Rh catalysts: Performance of kilowatt-scale reformer”, presented at the 12th Nordic Symposium on Catalysis, Trondheim, May 28-30, 2006.

vii. L.J. Pettersson, M. Nilsson, P. Jozsa, L. Andersson, “Catalytic hydrogen generation from DME – Strategies for emission reduction in heavy-duty trucks”, presented at the 2nd International DME Conference, London, May 15-17, 2006.

viii. P. Jozsa, L. Andersson, S. Erkfeldt, L.J. Pettersson, M. Nilsson,

“Reforming and deNOx for DME heavy-duty applications”, presented at the 2nd International DME Conference, London, May 15-17, 2006.

ix. B. Lindström, M. Petersson, S.K. Wirawan, D. Creaser, M. Nilsson, L.J.

Pettersson, J. Lindmark, J. Hedlund, “Innovative fuel reformer for heavy-duty truck APU”, presented at the 9th Grove Fuel Cell Symposium, London, October 4-6, 2005.

x. L.J. Pettersson, M. Nilsson, J. Nyström, B. Lindström, P. Ekdunge, “Fuel

upgrading by catalytic reforming for vehicle applications”, presented at Europacat VII, Sofia, Aug 28-Sep 1, 2005.

xi. M. Nilsson, L.J. Pettersson, B. Lindström, P. Ekdunge, “Hydrogen

production from autothermal reforming of diesel fuel for fuel cell auxiliary power units”, presented at Europacat VII, Sofia, Aug 28-Sep 1, 2005.

xii. M. Nilsson, L.J. Pettersson, B. Lindström, P. Ekdunge, “DME reforming

for fuel cell APU”, presented at the 4th International Conference on Environmental Catalysis, Heidelberg, June 5-8, 2005.

xiii. M. Nilsson, B. Lindström, L.J. Pettersson, P. Ekdunge, “DME - the

sustainable fuel for the future. A novel approach for supplying hydrogen onboard heavy-duty trucks”, presented at the 19th North American Catalysis Society Meeting, Philadelphia, May 22-27, 2005.

xiv. M. Nilsson, L.J. Pettersson, B. Lindström, P. Ekdunge, “Diesel fuel

processing for fuel cell auxiliary power units in heavy-duty trucks”, presented at the 19th North American Catalysis Society Meeting, Philadelphia, May 22-27, 2005.

xv. M. Nilsson, B. Lindström, L.J. Pettersson, P. Ekdunge, “Reforming of

diesel fuel for fuel cell-based auxiliary power units in heavy-duty trucks”, presented at the 11th Nordic Symposium on Catalysis, Oulu, May 23-25 2004.

The author’s contribution The author had an active part in all stages of the work presented in this thesis. In Paper I, the author was responsible for planning as well as conducting the experiments and writing the paper. The catalysts were prepared by diploma worker Jonas Nyström under the author’s supervision; he also participated in the ATR catalyst activity measurements. In Paper II, the preparation of the catalysts and all of the experiments were done by the author. The parameter study was performed together with Peter Jozsa at Volvo Technology. The author had the main responsibility in the writing of the paper.

Table of contents 1 Introduction.................................................................................................... 1 1.1 Setting the scene............................................................................................ 1 1.2 Structure of the thesis.................................................................................... 2 1.3 Scope of the work ......................................................................................... 2 2 Background..................................................................................................... 5 2.1 DME as a future fuel alternative ................................................................... 5 2.2 Truck idling and auxiliary power units......................................................... 8 2.3 Fuel cells for auxiliary power units ............................................................ 10 3 Fuel processing of DME .............................................................................. 13 3.1 Hydrogen from DME.................................................................................. 13 3.2 Gas cleanup ................................................................................................. 23 4 Evaluation of a kilowatt-scale DME reforming system (Paper I) ........... 25 4.1 Full-scale experiments ................................................................................ 25 4.2 Startup of the reformer................................................................................ 28 4.3 Autothermal reforming tests ....................................................................... 29 4.4 Summary ..................................................................................................... 31 5 Catalytic materials and operating conditions for DME autothermal reforming (Paper II) ....................................................................................... 33 5.1 Small-scale experiments ............................................................................. 33 5.2 Screening of Pd-based monolithic catalysts ............................................... 37 5.3 Catalyst characterization: Effect of Zn on CO2 selectivity......................... 41 5.4 Influence of operating parameters on reformer performance ..................... 44 5.5 Summary ..................................................................................................... 47 6 Conclusions ................................................................................................... 49 7 Final remarks and future perspectives ...................................................... 51 Acknowledgements.......................................................................................... 53 Nomenclature .................................................................................................. 55 References ........................................................................................................ 57 Appendices: Papers I-II

Introduction

1 Introduction

1.1 Setting the scene It is widely accepted that oil is a finite resource. Within a short period of the human history, the world supplies of fossil fuels, that took nature hundreds of millions of years to create, will become exhausted. This period of time is often referred to as the “Golden age of oil” during which the welfare in the developed countries of the world was escalating and a dependence on oil for heating, power generation, and transportation was created. Oil will never run out completely, but how much that will be accessible in the future is very difficult to estimate. This depends both on technologies available for extraction of oil but also on political, economical, and environmental circumstances. A continuous decline in the global oil production, commonly referred to as “Peak Oil”, is expected to start before 2010 [1, 2]. Today, 5 % of the global population consumes 25 % of the oil produced [3]. In the rich countries, continued increasing oil consumption is expected. At the same time, the energy needs in fast-growing countries such as China and India are rapidly increasing. Existing oil reservoirs may not be capable of meeting this increasing world demand and technology development does not keep pace with the surging demand. Moreover, fossil fuel usage has led to rising levels of pollution, a growing instability in natural ecosystems, and an increasing gap between rich and poor. A radical improvement is needed by means of a more efficient energy use. The energy used for transport in Sweden in 2005 corresponded to approximately 25 % of the total [4]. A transformation is needed in the transport sector through the introduction of renewable fuels and new vehicle propulsion systems in order to achieve a sustainable transport system. The total use of gasoline and diesel in the transport sector in Sweden corresponds to around 85 TWh [4] and there is potential for a part of this to be replaced with biomass-derived fuels [5] making use of for example pulp mill-integrated black liquor gasification [6]. The Swedish Commission on Oil Independence has proposed a reduction of petroleum-based fuels in the transport sector with 40-50 % by 2020 [7]. This is a challenging target. The phasing out of fossil fuels is suggested to be accomplished by using more energy-efficient transport, increasing the production of renewable fuels and by changing over to a fleet of vehicles that are not dependent on fossil fuels. The commission calls for carbon dioxide

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based vehicle and fuel taxes, preferential taxation of fuel-efficient cars, and a contribution from the Government to pilot and demo plants for production of bio-based fuels. Dimethyl ether (DME) has been proposed as one of several promising energy sources for the future. In a future outlook, a possibility for dimethyl ether to be implemented as a diesel fuel alternative can be identified. The opportunities to provide low emission levels from trucks will be several, including for example the use of DME as a reducing agent for nitrogen oxides (NOx) and as a hydrogen carrier for generating fuel cell feeds in auxiliary power units. Auxiliary power units (APUs) are systems providing electricity to trucks operating independently of the main engine. By using fuel cells as the power source in an auxiliary power unit, commercialization of fuel cells in the near future could be possible.

1.2 Structure of the thesis This thesis is based on two appended papers focusing on hydrogen generation from dimethyl ether by autothermal reforming. The structure of the thesis is the following: Chapter 2 provides an introduction to the area of using dimethyl ether as an alternative fuel and to fuel cell auxiliary power units. Chapter 3 gives an overview on the topic “hydrogen generation from dimethyl ether”. Different technology options are described and recent research on catalyst development is outlined. In addition, results from thermodynamic equilibrium calculations are included. Chapters 4 and 5 summarize the results from the experimental work in Paper I and Paper II on DME autothermal reforming. Finally, in Chapters 6 and 7, respectively, conclusions and final remarks are given.

1.3 Scope of the work The work described in the present thesis is part of the project “Aftertreatment and fuel upgrading system for DME-fueled diesel engines”. The vision has been to develop a system meeting the future demands on alternative and sustainable transport solutions in Sweden. The system combines lean combustion of DME in a compression ignition (CI) engine with lean nitrogen oxide aftertreatment where the nitrogen oxides are reduced, either by hydrogen produced in a reformer or by DME directly. The hydrogen from the reformer can also be used in a fuel cell auxiliary power unit. Such a system

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Introduction

would create a vehicle with low emissions of NOx, hydrocarbons, and particulate matter (PM) compared to existing technology. The objective of the work presented here has been to investigate the potential to reform dimethyl ether into a hydrogen-rich gas for use in fuel cell auxiliary power units. Especially, mapping of the operating conditions of the reformer has been important in order to locate regions of operation with high concentration of hydrogen associated with low concentration of carbon monoxide in the product gas. In Paper I, a kilowatt-scale autothermal reforming system was evaluated for generating hydrogen from dimethyl ether. Special emphasis was placed on using realistic gas mixtures and avoiding the use of an external heat source. In addition, a start-up sequence was studied, where dimethyl ether was catalytically ignited and combusted in air. In Paper II, a small-scale reactor set-up was used for screening of various palladium-based monolithic catalysts in order to identify materials suitable for use in DME autothermal reforming. Additionally, a parameter study was performed for evaluating the influence of varying operating conditions.

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4

Background

2 Background

2.1 DME as a future fuel alternative 2.1.1 Properties of DME DME, with the chemical formula H3C-O-CH3, is a colourless, gaseous ether at room temperature and atmospheric pressure. It is biologically degradable and non-hazardous from a health point of view. At high concentrations it has an anaesthetic effect. The key physical properties and combustion characteristics of DME are summarized in Table 2.1 and compared to diesel fuel. The possible application areas for DME are several and make it attractive in a long-term perspective. Today, the major application of DME is as an aerosol propellant, for example in hairsprays and paintsprays, where it has replaced the formerly used ozone-destroying chlorofluorocarbons (CFCs). Other possibilities are to use DME for power generation (in turbines or stationary fuel cells), as a cooking gas (substitute or blend for liquid petroleum gas, LPG), for industrial use (as a chemical feedstock, solvent or refrigerant), and for transportation applications (in compression ignition engines or fuel cell vehicles). Table 2.1 Key properties of DME compared to diesel [8]

an-cetane

DME Diesel

Chemical formula CH3OCH3 ~ C14H26 Molecular weight [g/mol] 46.1 ~ 194 Boiling point @ 1 bar [°C] -24.8 ~ 150 – 380 Vapor pressure @ 20 °C [bar] 5.1 – Liquid density @ 20 °C [kg/m3] 666 800 – 840 Relative density, gaseous (air=1) 1.59 – Liquid viscosity @ 25 °C [kg/m⋅s] 0.12 – 0.15 2 – 4 Lower heating value [MJ/kg] 29 42 Autoignition temperature [°C] 235 – 350 206a Flammability limits in air [vol %] 3 – 17 0.6 – 6.5 Cetane number 55-68b 40 – 55 Sulfur content [wt ppm] 0 0 – 500

bReference [9]

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2.1.2 Production of DME The first use of DME in transportation applications was as an ignition improver for methanol in compression ignition engines [10, 11]. But rather than regarding DME as improving the methanol combustion characteristics it may have been more correct to look on methanol as a contaminant deteriorating the ignition of DME. DME was at that time synthesized from an expensive process through methanol dehydration and was therefore not considered as a potential fuel. In the 90’s, the use of DME as a fuel achieved renewed attention. A low cost synthesis route was developed [12]. DME could now be produced directly from synthesis gas, instead of via a first methanol synthesis step. In the direct DME synthesis process, CO and H2 react according to equation 2.1 or 2.2 to form methanol that subsequently forms DME in the same reactor [12, 13].

2332 COOCHCHH3CO3 +→+ (2.1)

OHOCHCHH4CO2 2332 +→+ (2.2)

The direct DME synthesis route requires a catalyst, or alternatively a physical mixture of catalysts, capable of producing methanol from synthesis gas that is then further reacted into DME. The catalyst can be either a bifunctional catalyst [14, 15] or consist of a mechanical mixture of two catalysts [16, 17]. Cu/ZnO is an expected component in a syngas-to-methanol catalyst and has been widely investigated for this reaction. The methanol-forming step is limited by chemical equilibrium, but when dehydration of methanol occurs simultaneously, a high synthesis gas conversion can be achieved. Solid acid catalysts are reported to be active for dehydration of methanol in DME production [18, 19]. DME can be produced from numerous feedstocks including natural gas, coal, and biomass, at lower energy use and greenhouse gas emissions than other GTL (gas-to-liquid) or BTL (biomass-to-liquid) fuels [20]. Thus, DME offers high energy security and could qualify as a renewable fuel. 2.1.3 DME as a diesel substitute In the search for alternative solutions in the transport sector, available and novel technologies are competing in terms of well-to-wheels greenhouse gas emissions and energy use. Other factors of importance are versatility, infrastructure, safety, and economics as well as the use of non-fossil feedstocks. In Figure 2.1, liquid fuel pathways from different resources for

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Background

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Crude oil Natural gas Coal Wood Sugar-rich plants Vegetable oils

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Figure 2.1 Liquid transportation fuel pathways for internal combustion engine, hybrid, and fuel cell vehicle technology internal combustion engines (spark ignition – SI and compression ignition direct injection – CIDI), and fuel cell or hybrid vehicles are shown. As a compression ignition engine fuel, DME could offer high efficiency from well to wheels [20]. It is an interesting diesel fuel candidate from several points of view. The cetane number is very high, which means that the ability to autoignite is good. Swedish diesel fuel (Environmental Class 1) has a cetane number of around 50 [21] while for DME it has been cited as being above 65 [9]. A high cetane number is desirable since it gives rise to shorter ignition delays, which is advantageous as the NOx emissions can be reduced. It has also been shown that DME can be operated with high exhaust gas recirculation (EGR) allowing for further lowering of nitrogen oxides [22, 23]. No carbon-carbon bonds and high oxygen content (35 wt %) decrease the tendencies of producing particulates during combustion and therefore, the NOx/particulate trade-off problem in a diesel engine will not exist for DME. Furthermore, DME can be used as a NOx-reducing agent with high conversion over a wide range of temperatures (275 - 450°C) [24]. To be able to run a diesel engine on neat DME, the injection system has to be adapted. The lower heating value of DME is 29 MJ/kg (compared to 42 MJ/kg for diesel [25]) and the density is 666 kg/m3, meaning that it is necessary to

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inject a larger volume of fuel into the engine. This leads to long injection periods requiring advanced timing. A common-rail system can be used, operating at an injection pressure significantly lower than that of a diesel engine [22]. The low operating pressure offers low levels of noise. DME can be stored in LPG tanks, with a special fuel feed system. The physical characteristics are similar to those of LPG, a proven commercial product stored and transported globally. But a dedicated infrastructure has to be developed which is a disadvantage. Other problems arise from the low lubricity and low viscosity of DME, leading to problems with leakages and with lubrication of moving parts. DME is also a good organic solvent, which could lead to problems with gaskets and sealing. Only fluorine-based polymers can be used. In addition, due to the lower energy content, lower liquid density and the need for a vapor space above the fuel in the tank, running a compression ignition engine on DME will require a tank twice the size to drive the same distance compared to diesel fuel [26]. 2.1.4 DME as a hydrogen carrier for fuel cells DME is a also a candidate as a hydrogen carrier for fuel cells, requiring a lower reforming temperature compared to conventional fuels such as diesel and gasoline and with the possibility to obtain high hydrogen yields. One application of fuel cells is auxiliary power units for heavy-duty trucks, described in the following sections.

2.2 Truck idling and auxiliary power units There are clear economic and environmental incentives for developing more efficient solutions to providing trucks with electricity during driver rest periods, replacing the common practice of engine idling. American studies suggest that heavy-duty trucks operate in idling mode during 20-40 % of the time the engine is running [27]. This is a consequence of an increased utilization of just-in-time production increasing the time the drivers spend in the truck. At the same time, the demands on driver comfort systems are growing. Truck drivers idle their engines to cool or heat the cab, to keep the fuel and engine warm during cold weather and to power electrical equipment in the sleeper. An average of 6 hours of idling per day has been reported [27]. A significant quantity of fuel is consumed this way and considerable amounts of nitrogen oxides, hydrocarbons, carbon oxides, and particulates are emitted [28]. Furthermore, the energy efficiency when generating electricity via the alternator at standstill can be as low as a few percent [29].

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Background

An alternative to idling of the truck engine is to install an auxiliary power unit. Auxiliary power units, APUs, are systems providing electricity to trucks operating independently of the main engine. APUs consisting of a small internal combustion engine are commercially available today but have drawbacks in terms of weight and noise [28]. By using fuel cells in heavy-duty truck auxiliary power units, commercialization of fuel cells could be possible in the near term. They provide lower noise and less vibration compared to existing technology. In addition, higher electrical efficiencies are expected compared to an APU based on a small combustion engine. For a fuel cell APU including fuel processing of diesel, an efficiency of 34-38 % has been reported [30]. Using fuel cell APUs, truck drivers could meet impending legislation concerning idling and emission levels [31, 32]. At the same time, the fuel economy can be improved and the need for vehicle maintenance lowered. Figure 2.2 shows a CAD model of the packaging of a fuel cell APU [33]. The packaging and integration of the APU is important considering durability and safety as well as customer acceptance. An effective packaging will increase the efficiency and minimize the weight and volume of the APU. The package is to be placed on the frame rails of the truck. System development targets for truck fuel cell APUs based on a polymer electrolyte fuel cell (PEFC) fuelled with diesel reformate are given in Table 2.2 [34].

Figure 2.2 CAD model of an auxiliary power unit for a heavy-duty truck (Used by kind permission of Volvo PowerCell)

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Table 2.2 Development targets for truck fuel cell APU [34] Peak power 3-6 [kW] System cost 500-1000 [$/kW] Specific power 50-100 [W/kg] Volumetric power density 30-50 [W/dm3] Lifetime 10 000 [h] System efficiency >35 [%] Ambient temperature - 25 to + 45 [°C]Number of cold starts 2000

2.3 Fuel cells for auxiliary power units A fuel cell converts chemical energy into electrical energy by electrochemical oxidation of hydrogen into water [35]. The principle of a polymer electrolyte fuel cell is shown in Figure 2.3. The cell basically consists of an electrolyte medium between two electrodes. Hydrogen is fed to the negative anode, where it is ionized, creating electrons and protons (Eq. 2.3). The protons migrate through the membrane from the anode to the cathode, while the electrons are forced through an external circuit, generating an electric current on their way to the cathode. Oxygen is fed to the positive cathode, where it reacts with the protons and electrons forming water and heat (Eq. 2.4), thereby completing the total reaction (Eq. 2.5). Anode reaction: (2.3) −+ +→ e4H4H2 2

Cathode reaction: (2.4) OH2Oe4H4 22 →++ −+

Total reaction: (2.5) OHOH2 222 →+

The electrodes, electrode catalysts, and membrane together form the membrane electrode assembly, MEA. Bipolar plates are placed on each side of the MEA, allowing for a good electrical contact with the surface of the electrodes and providing channels for distribution of the gases. The ideal potential for the H2 – O2 reaction is 1.23 V [35]. However, due to losses in the MEA, a single cell usually operates at 0.5-0.8 V. By connecting several cells in series (Figure 2.4 [33]), higher voltages can be achieved. Through the direct conversion of fuel to electrical energy, fuel cells offer high efficiency compared to internal combustion engines, especially at part load.

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Background

ElectrolyteAnode

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Figure 2.3 Fuel cell principle The choice of electrolyte material governs the operating temperature of the fuel cell. For use in auxiliary power units, either low-temperature (80-90 °C) polymer electrolyte fuel cells (PEFC) or high-temperature (>600 °C) solid oxide fuel cells (SOFC) could be appropriate. The SOFC is tolerant to poisons and does not need noble metal catalysts. Negative aspects are long startup times, low response, and low power density. PEFC systems can be made light and compact. The shorter startup times and good response to transient operation are advantageous and make them more suitable in automotive applications. However, they need expensive platinum metal as catalyst, and CO in the feed will preferentially adsorb on the Pt sites at low temperatures, blocking sites active for the anode reaction. The impact of CO on the power output of a PEFC increases with the concentration and will become significant already at ppm levels [36, 37]. Non-inert effects of CO2 have also been reported lately [38]. PEFCs operating on neat H2 at optimal conditions show good performance over a wide range of loads [35]. However, the volumetric energy density of hydrogen is very low and the problem with storage is obvious for automotive applications. Pressurized gas cylinders, liquid hydrogen tanks or metal hydrides could be used but occupy volume, increase weight and decrease fuel efficiency. Storage in carbon nanofibers is an alternative although the production and distribution of the gas is difficult. As a result of this, the most feasible solution at present is probably to generate hydrogen onboard the vehicle by conversion

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of the same fuel that is used for propulsion. The fuel can be converted into hydrogen by catalytic reforming. The fuel cell feed will then contain CO, CO2, H2O, and N2 apart from H2. The CO has to be removed in order not to poison the fuel cell anode catalyst. Using DME as the source of hydrogen is advantageous in that it can be converted at relatively low temperature (300-400 °C) while generating fairly low concentrations of CO. This will make the subsequent CO cleanup system less complex. Lately, there has been enhanced interest in operating PEFCs at temperatures >100 °C [39]. This would minimize the effect of CO impurities in the feed gas since the adsorption is less favoured at higher temperatures [40]. In addition, a higher temperature will enhance reaction rates and simplify water management. Since the need for CO cleanup would decrease, the cost of the fuel processing system could be lowered.

Fuel cell Fuel cell stack

Membrane electrode assembly

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Figure 2.4 Principle of fuel cell stack construction (Used by kind permission of Volvo PowerCell)

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Fuel processing of DME

3 Fuel processing of DME

3.1 Hydrogen from DME 3.1 Hydrogen from DME 3.1.1 Reforming technology options 3.1.1 Reforming technology options There are three major technology options for producing hydrogen-rich fuel cell feeds from dimethyl ether: steam reforming (SR), partial oxidation (POX), and autothermal reforming (ATR). During autothermal reforming, thermally neutral conditions are obtained by combining the endothermic steam reforming and the exothermic partial oxidation reaction.

There are three major technology options for producing hydrogen-rich fuel cell feeds from dimethyl ether: steam reforming (SR), partial oxidation (POX), and autothermal reforming (ATR). During autothermal reforming, thermally neutral conditions are obtained by combining the endothermic steam reforming and the exothermic partial oxidation reaction. SR: SR: 22233 H6CO2)g(OH3OCHCH 22233 H6CO2)g(OH3OCHCH +→+ (3.1)

POX: ( ) 222233 N88.1H3CO2N76.3O21OCHCH ++→++ (3.2)

Moving from right to left in Figure 3.1, i.e. from pure steam reforming conditions to complete oxidation, the hydrogen yield decreases. Maximum energy efficiency for the fuel processing system can be obtained during autothermal reforming, where the heat needed for the steam reforming is supplied by partial oxidation. In practice, it is common that autothermal systems require the addition of a small amount of heat and are therefore operated at slightly exothermic conditions. The term autothermal is used more generally for processes where steam reforming is combined with partial oxidation and can also be referred to as oxidative steam reforming or combined reforming. Complete oxidation is not an option when hydrogen is the desired product but could be used to generate heat, for example during startup of the reformer. The highest amount of CO is obtained during partial oxidation conditions.

AutothermalPartial oxidationOxidation Steam reforming

EndothermicExothermic

H2 min H2 maxCO max

Thermally neutral

AutothermalPartial oxidationOxidation Steam reforming

EndothermicExothermic

H2 min H2 maxCO max

Thermally neutral

Figure 3.1 Reforming technology options

13

Marita Nilsson

The current top fuel cell candidate for automotive applications, PEFC, has been described in section 2.3. In order to achieve as high performance of the fuel cell as possible, it is of great importance to reach optimal operating conditions in the reformer [38]. This means the hydrogen concentration should be as high as possible and the carbon monoxide concentration as low as possible. Steam reforming can yield high concentrations of hydrogen but the main drawbacks are that the reaction is slow and needs an external heat supply. Partial oxidation is fast but the amount of hydrogen generated is lower and there is a risk of hot spot formation in the catalyst due to the exothermicity of the reaction. Automotive fuel processors have to be able to operate during both transient and steady-state operation and with frequent startups and shutdowns. The more dynamic and energy-efficient ATR process is advantageous over SR and POX for transportation applications [41]. 3.1.2 Thermodynamic analysis of the DME ATR process When combining steam reforming and partial oxidation the inlet ratios of steam, air, and DME, that will result in optimal performance of the system under consideration, have to be determined. Thermodynamic equilibrium calculations can be used for guidance to locate regions of operation where the required conditions are likely to occur [42, 43]. However, they do not take the kinetics of the reactions into account. The reactions are assumed to be fast enough to reach chemical equilibrium at the end of the reactor. Furthermore, the type of catalyst and reactor used can be ignored. Equilibrium concentrations have been calculated by the minimization of Gibbs free energy as described in Perry’s Chemical Engineers’ Handbook [44]. Six species were included in the calculations, CH3OCH3, CO, CO2, H2, N2, and O2, giving rise to six equilibrium equations,

∑∑

=+=k

ikk

ii

ii,f 0aRTn

nln

RTG λΔ

(3.3)

ispeciesofkelementforatomsofnumberamultiplierLagrange

icompoundofmolesofnumbernttanconsgasR

ispeciesofformationofenergyGibbsdardtansG

ik

k

i

i,f

====

=

λ

Δ

14

Fuel processing of DME

The equilibrium equations were subject to the following constraints, resulting in another five equations, ∑ =i

kiki Aan (3.4)

1n

ny

ii

i

ii ==

∑∑ (3.5)

ispeciesoffractionmoleykelementofmasstotalA

i

k

==

The reactants were assumed to be ideal gases, the reaction rates were assumed infinite and the process isothermal. Given the reforming temperature and pressure, the equilibrium composition was analyzed by changing the oxygen and steam feed rate. Figure 3.2 shows thermodynamic equilibrium concentrations of hydrogen at varying reactant gas compositions. The oxygen-to-DME ratio has a considerable impact on the hydrogen yield. The hydrogen concentration decreases linearly with the O2:DME for a constant steam-to-DME ratio. Maximum hydrogen yield will be obtained during pure steam reforming conditions, i.e. at an O2:DME equal to zero. Instead, varying the steam-to-DME ratio while keeping the oxygen-to-DME constant, the hydrogen concentration is fairly steady between 1 and 2 and after that starts to decrease slightly. Changing the temperature was not significant for the hydrogen yield. The decrease in the hydrogen concentration at higher temperatures is mainly an effect of the water-gas shift equilibrium.

15

Marita Nilsson

0.4 0.5 0.6 0.7 0.8 0.930323436384042444648505254

H2 c

once

ntra

tion

[%]

O2:DME

T=350 ºC T=375 ºC T=400 ºC

1.0 1.5 2.0 2.5 3.030323436384042444648505254

H2 c

once

ntra

tion

[%]

H2O:DME

T=350 ºC T=375 ºC T=400 ºC

Figure 3.2 Thermodynamic equilibrium concentrations of hydrogen for autothermal reforming of dimethyl ether at varying O2:DME (H2O:DME=2.5) and H2O:DME (O2:DME=0.7) Conversely, the CO concentration is mainly a function of the steam content. Figure 3.3 shows how the concentration of CO is affected by changing oxygen-to-DME and steam-to-DME, respectively. CO can be generated via partial oxidation, partial steam reforming, and the reverse water-gas shift reaction. A surplus of steam is often used in fuel cell applications in order to induce the water-gas shift reaction where CO is reacted with steam producing CO2 and additional H2. For an increased oxygen-to-DME ratio, the amount of CO decreases due to a higher degree of complete oxidation of DME. For increased steam-to-DME ratios and decreased temperatures, the water-gas shift reaction will be shifted further towards hydrogen and carbon dioxide. The effects from increases in the oxygen-to-DME and steam-to-DME will also be due to the dilution with nitrogen via air as well as with steam.

16

Fuel processing of DME

0.4 0.5 0.6 0.7 0.8 0.90123456789

101112

CO

con

cent

ratio

n [%

]

O2:DME

T=350 ºC T=375 ºC T=400 ºC

1.0 1.5 2.0 2.5 3.00123456789

101112

CO

con

cent

ratio

n [%

]

H2O:DME

T=350 ºC T=375 ºC T=400 ºC

Figure 3.3 Thermodynamic equilibrium concentrations of carbon monoxide for autothermal reforming of dimethyl ether at varying O2:DME (H2O:DME=2.5) and H2O:DME (O2:DME=0.7) It is apparent from the calculations that dimethyl ether autothermal reforming thermodynamically can generate high concentrations of hydrogen (~ 40-50 %) in combination with low CO concentrations (< 5 %) over a wide range of conditions. The heat integration will be unique for every system and has to be experimentally evaluated. Plotting the equilibrium conversion of DME vs. temperature for varying oxygen-to-DME and steam-to-DME ratios (Figure 3.4), the conversion is found not to be thermodynamically limited at temperatures above 200 °C operating at a H2O:DME ratio of at least 1.5 and an O2:DME ratio of 0.4.

17

Marita Nilsson

100 125 150 175 200 225 250 275

80

85

90

95

100

DM

E co

nver

sion

[%]

Temperature [ºC]

O2:DME=0.4 O2:DME=0.5 O2:DME=0.6 O2:DME=0.7 O2:DME=0.8 O2:DME=0.9

100 125 150 175 200 225 250

80

85

90

95

100

DM

E co

nver

sion

[%]

Temperature [ºC]

H2O:DME=1 H2O:DME=1.5 H2O:DME=2 H2O:DME=2.5 H2O:DME=3

Figure 3.4 Thermodynamic equilibrium conversion of DME for autothermal reforming; effect of varying O2:DME at a constant H2O:DME of 1.5 and of varying H2O:DME at a constant O2:DME of 0.4

3.1.3 DME-reforming catalysts An optimized catalyst is essential to achieve the desired conversion and product selectivity in dimethyl ether reforming. For autothermal reforming, a catalyst active for both steam reforming and partial oxidation is needed. A possible solution could be to use a mechanical mixture of two catalysts [45]. Steam reforming of DME is considered to proceed via two succeeding reactions [46-49]. The first step is hydrolysis of DME into methanol (Eq. 3.6) and the second step is steam reforming of methanol (Eq. 3.7).

18

Fuel processing of DME

OHCH2OHOCHCH 3233 →+ (3.6)

2223 COH3OHOHCH +→+ (3.7) The common approach is to use a solid acid catalyst for the hydrolysis of DME to methanol, e.g. zeolites, γ-Al2O3, and ZrO2 combined with a methanol reforming catalyst such as copper. It could be either a physical mixture of a DME hydrolysis catalyst and a methanol steam reforming catalyst or a single dual-function catalyst. DME hydrolysis is thermodynamically limited [42] but when methanol steam reforming occurs simultaneously, high DME conversions can be achieved. The most commonly used catalyst for hydrolysis of DME is γ-Al2O3. Acid site density, strength of the acid sites, and hydrophobicity have been suggested to influence the activity of the hydrolysis catalyst [50, 51]. Zeolites may generate long-chain hydrocarbons at temperatures above 300 °C through the methanol-to-gasoline reaction [50-52]. ZrO2 has been tested but did not reach predicted equilibrium conversions of DME during experiments on DME hydrolysis [42]. Copper catalysts are widely known to have high activity for steam reforming of methanol. This reaction occurs at temperatures of 200-300 °C but the hydrolysis of DME to methanol occurs in the temperature range of 300-400 ºC [50] and therefore the durability of copper will be a concern. Copper catalysts are prone to sintering at temperatures above approximately 300-350 ºC [53] leading to loss of activity. Therefore, they must be made thermally stable for use in reforming of DME. Copper-manganese interactions in spinel oxides have been reported to suppress sintering of copper at temperatures up to 400 ºC during steam reforming of DME [54]. Palladium catalysts have also been shown to exhibit high activity for DME steam reforming, however associated with the formation of large amounts of CO [47, 48]. This is not a surprise since Pd is known to be selective for methanol decomposition (Eq. 3.8) [55, 56].

23 H2COOHCH +→ (3.8) For use in methanol steam reforming, zinc can be incorporated with the palladium to promote the selectivity to CO2 and H2 [57-59]. Partial oxidation of DME has been studied over various metal catalysts supported on Al2O3 and over nickel on different supports [45, 60]. The most active and selective catalysts were Ni supported on LaGaO3 and rhodium (Rh) supported on Al2O3. The reaction has been suggested to proceed through the

19

Marita Nilsson

oxidation of methyl (-CH3) or decomposition of methoxy species (-O-CH3) formed by dissociative adsorption of DME [60]. Several studies have been performed lately studying catalysts for hydrogen generation from DME. A selection of those is presented in Table 3.1. It is worth mentioning that none of the reviewed journal papers were considering autothermal reforming of DME.

20

21

Fuel processing of DME

Tab

le 3

.1 A

sele

ctio

n of

refe

renc

es o

n hy

drog

en g

ener

atio

n fr

om d

imet

hyl e

ther

Cat

alys

tM

etho

dH

2O:D

ME

O2:D

ME

T [°

C]

GH

SV

Des

crip

tion

Ref

.Pt

/Al 2O

3 +

Ni-M

gOPO

X-

1.1

600-

750

8800

ml g

cat-1

h-1

Hyb

rid c

ompa

red

to d

ual b

ed c

atal

yst

[45]

Effe

ct o

f tem

pera

ture

and

resi

denc

e tim

e

H-m

orde

nite

+ C

u/C

eO2

SR3.

3-

200-

300

4450

ml g

cat-1

h-1

Dea

ctiv

atio

n of

the

cata

lyst

by

coke

form

atio

n[4

7]H

-mor

deni

te +

Pd/

CeO

2on

the

copp

er sp

ecie

s

CuZ

n/A

l 2O3

SR3

-20

0-50

0no

t sho

wn

Sol-g

el c

atal

ysts

[48]

CuP

d/A

l 2O3

Act

ivity

of P

d at

low

tem

pera

ture

Ga 8

Al 2O

15SR

3-

200-

400

1000

0-20

000

h-1Ef

fect

of G

a 2O

3; so

l-gel

cat

alys

ts

[49,

64]

Effe

ct o

f add

ing

Cu

to G

a 2O

3-A

l 2O3

CuF

e 2O

4 + A

l 2O3

SR3

-20

0-45

022

00 h

-1C

ompo

site

cat

alys

t of s

olid

aci

d +

spin

el o

xide

[51]

Rol

e of

the

solid

aci

d

CuZ

nO/A

l 2O3

+ ZS

M-5

SR3

-20

0-40

024

00 m

l gca

t-1 h

-1H

omog

eneo

us p

reci

pita

tion

cata

lyst

[52]

Dea

ctiv

atio

n of

Cu

by c

oke

form

atio

n

CuM

nFe

+ A

l 2O3

SR3

-25

0-50

060

00 h

-1Th

erm

al st

abili

ty o

f Cu-

base

d sp

inel

oxi

des

[54]

Ni,

Rh,

Co,

Ru,

Fe,

Pt,

Ag

POX

-0.

535

0-65

030

000

ml g

cat-1

h-1

Effe

ct o

f tem

pera

ture

, O2:D

ME

and

cont

act

[62]

on v

ario

us su

ppor

tstim

e. R

eact

ion

path

way

22

Marita Nilsson

Tab

le 3

.1 (c

ontd

.)

Cat

alys

tM

etho

dH

2O:D

ME

O2:

DM

ET

[°C

]G

HSV

D

escr

iptio

nR

ef.

WO

3/ZrO

2-C

uO/C

eO2

SR3.

5-

200-

300

not s

how

nA

ctiv

e st

ate

of tu

ngst

en o

xide

s[6

1,62

]D

urab

ility

test

s for

100

h

CuZ

n +

solid

aci

dSR

3-

125-

400

3600

h-1

Dua

l fun

ctio

n ca

taly

st[6

3]C

ompa

rison

of v

ario

us so

lid a

cids

Fuel processing of DME

3.2 Gas cleanup Water-gas shift, WGS (Eq. 3.9) and preferential oxidation, PrOx (Eq. 3.10) are the two main processes used for removing impurities from the reformer product gas. Water-gas shift is thermodynamically favored at low temperatures and is therefore often separated in two steps with cooling in between, one operating at 300-400 °C (HTS - high-temperature shift) and another operating at 200-300 °C (LTS - low-temperature shift). The concentration of CO after the shift steps is typically 0.5-1 % [64]. The gas can then be fed to a PrOx reactor, where CO is selectively oxidized resulting in an expected CO concentration below 50 ppm [65]. The main drawbacks with this process are that hydrogen oxidation will be a competing reaction and that the addition of nitrogen via air will further dilute the fuel cell feed gas. An example of a fuel processor including an autothermal reformer, a water-gas shift step and a preferential oxidizer is shown in Figure 3.5. WGS: 222 HCOOHCO +↔+ (3.9)

PrOx: 22 COO21CO →+ (3.10)

FC

Reformer WGS PrOx

Afterburner

Water

Fuel

Air

Figure 3.5 Schematic of a fuel processor using autothermal reforming, water-gas shift (WGS) and preferential oxidation (PrOx)

23

Marita Nilsson

24

Evaluation of a kilowatt-scale DME reforming system

4 Evaluation of a kilowatt-scale DME reforming system (Paper I)

4.1 Full-scale experiments 4.1.1 Introduction In Paper I, a reforming system designed to generate hydrogen from DME for a heavy-duty truck fuel cell auxiliary power unit was evaluated at full-scale. Hydrogen generation by catalytic autothermal reforming was combined with catalytic oxidation of DME for startup. The reactor setup is described in the following section.

4.1.2 Laboratory setup A schematic of the setup used in the experiments is shown in Figure 4.1. DME (Chemical Dimethyl Ether 99.9 %, Gerling Holz & Co) and air were fed to the two startup combustors where the fuel was catalytically ignited and combusted at short contact time. The inlet air was preheated to resemble compressor outlet air in a truck. The hot product gases from the combustion were directly heat-exchanged with the reformer catalyst bed trying to achieve a rapid startup of the reformer. When the required operating temperature was achieved, the reforming reactants were fed to the reformer and the combustors were shut down. The reactors consist of stainless steel tubular units designed for a 1-5 kWe fuel cell-based auxiliary power unit (combustor inner diameter=56 mm, combustor length=180 mm, reformer inner diameter=80 mm, reformer length=220 mm). The temperature was measured at three different points in each reacting unit (T1-T10 in Figure 4.1) using K-type thermocouples and evaluated in LabVIEW®. The product gases were analyzed by a Varian 3800 gas chromatograph (GC), equipped with Hayesep and MS 5A columns and with thermal conductivity and flame ionization detectors. The GC was calibrated for quantification of CH4, CO, CO2, DME, H2, N2, and O2. Both the oxidation and the reforming catalysts consisted of spherical γ-Al2O3 pellets (Sasol Germany GmbH, 2.5 mm diameter) impregnated with various metals using the wet impregnation method, in which aqueous precursors are deposited onto the oxide support. Wet impregnation means an excess of solution is used, that after the impregnation step is separated from the solid [67]. Important parameters are the concentration of the impregnating solution, the pore volume of the support and the binding of the precursor onto the

25

Marita Nilsson

support [67]. After impregnation, the supported precursor has been thermally stabilized and activated by means of drying, calcination and reduction. Both noble metal (Pt), base metal oxide (Fe) catalysts and mixtures thereof were tested in the startup combustors. For the reforming reaction γ-alumina-supported copper-based catalysts were evaluated: CuFe, CuMn, CuZn, and CuZn + 0.5 % Pd. Further details on the catalyst preparation and characterization are outlined in Paper I.

26

27

Evaluation of a kilowatt-scale DME reforming syste

LabV

iew

Com

pute

r

Ref

orm

er

Pro

duct

gas

GC

Com

pute

r

Ther

moc

oupl

e m

odul

e

Pre

heat

er

Gas

C

hrom

atog

raph

Air

Ste

am

Gen

erat

orT8T9T1

0

Com

bust

or 1

Com

bust

or 2

T4T5

T1

Wat

erM

eter

ing

valv

e

T6 T2

DM

E

T7 T3

MFC

MFC

MFC

m

Figu

re 4

.1 S

chem

atic

pic

ture

of t

he fu

ll-sc

ale

expe

rimen

tal s

etup

(MFC

– m

ass f

low

con

trolle

r; Tx

– th

erm

ocou

ple)

Marita Nilsson

4.2 Startup of the reformer The startup of a fuel processor for automotive applications is a challenging task. Major issues are startup time, fuel efficiency, and system complexity. A method for generating heat for the reforming reaction is combustion of the existing truck fuel. This can be done in a conventional burner or, like here, in a catalytic process. The choice of fuel will have a significant effect on the magnitude of energy needed for startup. DME is advantageous in that it is a gas at atmospheric pressure and has low tendency for carbon formation [68]. DME will also have lower demands on startup energy compared to other hydrocarbon fuels and oxygenates, such as diesel, gasoline, and ethanol [68, 69]. Startup without stored water onboard the truck is highly desirable to decrease the complexity of the system. When the steam generated in the catalytic combustors is fed to the reformer together with fuel and air, the process can be gradually changed into autothermal reforming. In this way, steam can be generated directly from the fuel, which removes the need for storing water for the startup sequence. In the startup experiments, two combustors were used as a means of avoiding excessive heating of the oxidation catalyst. The tests were designed to heat a reformer that could be used in a 2.5 kWe PEFC system. Using Pt/γ-Al2O3 catalysts, light-off was easily achieved during fuel-rich conditions. The time-on-stream to ignition decreased with the lambda value (λ=actual-to-stoichiometric air/fuel ratio) with an optimum at λ=0.3-0.5. Following ignition, the combustor was operated in excess air, at a point where the temperature could be controlled at around 600 °C (λ=4-5). The temperature development after start of fuel injection in the combustor and reformer is shown in Figure 4.2. Reformer operating temperature was attained after 4 min 45 s. During this time the consumption of fuel corresponded to 132 g (0.2 dm3) or 3807 kJ. The startup time can be decreased by means of better heat integration of combustor with reformer as well as with the other parts in the fuel processor. Using a 10 % Fe/γ-Al2O3 catalyst, the time-on-stream to ignition was longer, approximately 30 seconds after start of fuel injection. The temperature increase following ignition was also comparatively slower than when using Pt. Part of the Pt catalyst bed could therefore be replaced by Fe catalyst to get a more even temperature distribution in the bed and avoid the formation of hot spots.

28

Evaluation of a kilowatt-scale DME reforming system

0 50 100 150 200 250 3000

100

200

300

400

500

600

Combustor temperature

Reformer temperatureTem

pera

ture

[ºC

]

Time [s] Figure 4.2 Startup of the reformer by catalytic oxidation of DME using a 1 % Pt/Al2O3 catalyst; catalyst ignition at λ=0.5 and temperature stabilization at λ=5

4.3 Autothermal reforming tests The focus in the autothermal reforming tests was to investigate whether hydrogen production from DME could be a feasible solution to generate fuel cell anode feeds in auxiliary power units. Experimental results presented in literature on autothermal reforming of dimethyl ether were at this point not available and therefore it was considered important to investigate the feasibility of the process. In the experiments, the major objective was trying to obtain a system working close to a practically viable process. This was addressed by several means: 1. The experiments were conducted at full-scale. Gas flows corresponding to a

hydrogen yield that theoretically would give 2.5 kW electricity in a PEFC were used.

2. Gas mixtures simulating realistic operating conditions were used. No dilution with inert gases was used.

3. No external heat source was used for heating of the reformer. The temperature was controlled by means of the self-sustaining autothermal reforming reaction.

DME and steam followed by air were fed to the reactor at an oxygen-to-DME ratio of 0.25 and a steam-to-DME ratio of 2.5. A temperature profile as function of time after start of fuel injection is shown in Figure 4.3.

29

Marita Nilsson

10 20 30 40 50 600

100

200

300

400

500

T at L=130 mm T at L=175 mm

Tem

pera

ture

[ºC

]

Time [min] Figure 4.3 Temperature profile in the reformer catalyst bed at L=130 mm and L=175 mm (Lreactor=220 mm, catalyst bed from L=100 mm to L= 200 mm) It was found that hydrogen-rich gases, of almost 60 % H2 (wet gas), could be obtained through autothermal reforming of DME. Furthermore, due to the low concentration of CO (< 3 %) in the product gas, few cleanup steps will be needed downstream the reformer. A Pd-doped CuZn/γ-Al2O3 catalyst was shown to exhibit high selectivity for hydrogen production from dimethyl ether. It was found that the reaction was more temperature-dependent for this catalyst, reaching a maximum value at 350 °C (Figure 4.4). The major drawback was the dynamics of the reformer. The response to transient operation was slow and the temperature was hard to control upon changes in oxygen-to-DME ratio. These problems were ascribed to the high thermal mass of the pellets, leading to slow response. This is an important and often overlooked issue. The dynamic system response is a critical factor in the overall design process in order to satisfy rapid load change demand. Not only the reformer has to be considered but also the CO cleanup steps, startup system, heat exchangers, and balance of plant (pumps, valves, regulators etc.), which will affect the response of the fuel processor. It will be necessary to take into account both thermal integration, such as the use of waste heat, and chemical integration, for example by providing steam needed for the fuel reforming from the water produced by the electrochemical cell reaction. Optimal reactor design will be of importance to achieve dynamics as well as compactness and efficiency. Very efficient systems are often more complex and hence more expensive than simpler, less efficient systems. A problem is also that dedicated

30

Evaluation of a kilowatt-scale DME reforming system

220 240 260 280 300 320 340 360 380 4000

10

20

30

40

50

60

Hyd

roge

n co

ncen

tratio

n [%

]

Temperature [ºC]

CuZn CuZn/Pd CuFe CuMn

Figure 4.4 Hydrogen concentration vs. temperature for the different catalysts components for fuel cell fuel processors are hard to find, which could lead to a higher cost and increased parasitic power.

4.4 Summary This study demonstrated that autothermal reforming of DME can be used to generate hydrogen-rich fuel cell anode feeds. The performance of the reformer was not good enough considering the dynamics, a problem that is strongly influenced by the extent of thermal integration of the reformer; most likely the dynamics could be improved by using thin-wall monoliths instead of a fixed bed of pellets. It was further shown that dimethyl ether can be catalytically combusted in order to generate heat for an autothermal DME reformer. Fe/γ-Al2O3 can be used as a combustion catalyst but platinum is needed to achieve a short ignition time. Better heat management will decrease the time needed for startup.

31

Marita Nilsson

32

Catalytic materials and operating conditions for DME autothermal reforming

5 Catalytic materials and operating conditions for DME autothermal reforming (Paper II)

5.1 Small-scale experiments 5.1.1 Introduction In Paper II, Pd-based materials deposited on cordierite monolith substrates were evaluated for use in autothermal reforming of DME. The objectives were to find a catalyst that could generate a high concentration of hydrogen accompanied by high selectivity to carbon dioxide and then to investigate the effect on product gas composition of varying operating parameters for this catalyst. Two different laboratory setups were used: one for screening of Pd-based catalysts with different supports, Pd loading, and preparation methods and another setup for studying the operating characteristics of the process. Schematic drawings of the reactor setups are shown in Figures 5.1 and 5.2, and described in the following sections. 5.1.2 Reactor setup 1: Catalyst screening In the screening study, the reformer consisted of a stainless steel tubular reactor with 23.7 mm inner diameter. Air, DME, and balance nitrogen (GHSV=15 000 h-1) were fed by separate mass flow controllers at an oxygen-to-DME ratio of 0.4 and mixed with steam, at a steam-to-DME ratio of 2.5, in the tubing prior to the reformer. The temperature of the tubing was kept at approximately 170 °C using a heating cable. The reformer was heated by Watlow MI band heaters controlled with FGH S2000 temperature regulators. The reformer contained a preheating zone and a reactor zone where the catalyst sample was placed using high temperature-insulation tape for sealing against the reactor wall. The catalysts consisted of monoliths coated with different Pd-based materials (Monolith dimensions: length=35 mm, diameter=23 mm). Analysis was performed using a Varian 3800 gas chromatograph equipped with Porapak Q and MS 5A columns for separation and with thermal conductivity and flame ionization detectors for analysis of the components. CH4, CH3OH, CO, CO2, DME, H2, N2, and O2 could be quantified in the GC analysis. It was possible to analyze a sample every ten minutes. The temperature was measured using thermocouples type K placed prior to and after the monolith and evaluated in LabVIEW®.

33

Marita Nilsson

5.1.3 Reactor setup 2: Parameter study For the parameter study, a stainless steel tubular reactor was used. The catalyst samples were placed in a sample holder with 20 mm inner diameter allowing the reactant gases to flow through an area with diameter 18 mm (Monolith dimensions: length=35 mm, diameter=20 mm). The inlet gas flows were regulated by mass flow controllers and heated by a capillary preheater. The reactor wall was heated by metallic heating coils. A porous quartz plate was placed at the reactor inlet to distribute the gases over the monolith channels. After the reformer, the product gas was analyzed by a Maihak Modular System S710 equipped with NDIR and TCD analyzers and a Gasmet Cr-2000 FTIR Instrument. CH4, CH3OH, CO, CO2, DME, HCHO, H2, and H2O could be continuously quantified. The product gas was analyzed every second. The temperature was measured with K-type thermocouples placed at the catalyst inlet and outlet centers and at the reactor wall. The supply of gases and regulation of the temperature was controlled by computer in a program developed at Volvo Technology.

34

35

Catalytic materials and operating conditions for DME autothermal reforming

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Marita Nilsson

36

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Catalytic materials and operating conditions for DME autothermal reforming

5.2 Screening of Pd-based monolithic catalysts 5.2.1 Catalyst materials Nine different Pd-based materials, shown in Table 5.1, were tested for autothermal reforming of DME; γ-Al2O3, ZSM-5, and a Zn-Al hydrotalcite-like compound were compared as supports for Pd. The addition of Zn and of Pt to Pd was evaluated and the effect of changing the Pd content and Pd:Zn ratio was tested for the Zn-containing catalyst. Two different preparation methods were used, incipient wetness (IW) impregnation and coprecipitation. Those are methods commonly used for synthesis of heterogeneous catalysts [67]. In impregnation techniques, a solution of a metal salt is added onto a support. Incipient wetness indicates that the volume of the solution should be just enough to fill up the pores of the support. When the catalyst is dried and calcined the metal will be deposited on the surface of the catalyst. Coprecipitation ans the simultaneous precipitation of more than one component, in this case Pd, Zn, and Al, which could make possible a closer interaction between the various metals. A “hybrid” catalyst was also prepared, by physically m g alumina and coprecipitated Pd and Zn before coating of the monolith, trying to separate the sites active for DME hydrolysis from the ones active for reforming. Prior to reaction, the catalysts were activated in a flow of hydrogen. More details on the preparation procedure for the catalysts in Table 5.1 can be found in Paper II. Table 5.1 Catalyst m rials evaluated in the screening experiments (wt %)

Catalyst composition Preparation method BET SA [m2/g]

me

ixin

ate

Pd(1)/γ-Al2O3 IW impregnation 168 Pd(1)/ZSM-5 IW impregnation 281 Pd(1)/HTlca IW impregnation 118 Pd(1)Zn(9)/γ-Al2 IW impregnation 134 Pd(2)Zn(9)/ γ-Al IW impregnation 137 Pd(5)Zn(9)/γ-Al2 IW impregnation 139 Pd(1)Zn(9)-Al2O Coprecipitation 310 Pd(1)Zn(9) + γ-A 3

b Coprecipitation 137 Pd(1)Pt(0.5)/γ-Al IW impregnation 155

O3 2O3 O3 3 l2O2O3

aHydrotalcite-like c ound bHybrid catalyst IW – Incipient wetn

omp

ess

37

Marita Nilsson

5.2.2 Monoliths as catalyst substrates Catalysts used in automotive applications must be able to withstand mechanical

s and frequent changes in load and operating onditions. The pellet catalysts used in the experiments in Paper I may

he powders described in section 5.2.1 have been deposited onto cordierite MgO⋅5SiO2⋅2Al2O3) monolith samples by means of dip-coating.

onolith substrates. Left: Cordierite honeycomb structure. Right: ately flat and corrugated metal foils.

stress caused by vibrationctherefore not be suitable in automotive environments since they can be easily damaged by means of attrition. Furthermore, it was found in Paper I that the higher thermal mass of the pellets caused problems with dynamic operation of the reformer. Catalyst monoliths offer the robustness required in vehicular applications. Monoliths are extruded blocks of ceramics or metals containing straight parallel channels (Greek. mono=single, lithos=stone). Figure 5.3 shows examples of monolith substrates. The catalyst powder is deposited onto the walls of the channels. A well-known application of monoliths is the three-way catalyst used for reduction of emissions in exhaust from spark-ignition engines. The major advantage with monoliths is the high open frontal area providing a low pressure drop. A significantly lower Reynold’s number compared to a fixed bed will lead to decreased mass and heat transfer, which could be a drawback. Furthermore, since there is no mass transfer in the radial direction, the conversion could be negatively affected. It is therefore of great importance to obtain a high washcoat loading and a uniform distribution of active material on the walls of the monolith. T(2

igure 5.3 CataF lyst mStructure based on altern

38

Catalytic materials and operating conditions for DME autothermal reforming

5.2.3 Evaluation of the catalysts All Pd-based catalyst materials tested in the experimental study were active for DME autothermal reforming for an O2:DME of 0.4 and a H2O:DME of 2.5. The catalysts were compared in terms of hydrogen concentration and CO2 selectivity in the product gas. Table 5.2 summarizes the results from the screening study at a temperature of 400 °C. The following equations (Eqs. 5.1-5.3) were used for evaluation of the data with S corresponding to the selectivity and ηref to the reformer efficiency (LHVH2=242 kJ/mol,

HV =1310 kJ/mol). L DME(g)

100F

FF(%)conversionDME

in,DME

out,DMEin,DME ⋅−

= (5.1)

100FF

F(%)S

CO2CO

2CO2CO ⋅

+= (5.2)

100LHVF

LHVF(%)

)g(DMEin,DME

2Hproduced,2Href ⋅

⋅

⋅=η (5.3)

Al2O3 was found to be the most suitable support considering activity and hydrogen generation. The zeolite and the Zn-Al hydrotalcite exhibited selectivity for the MTG reaction (methanol-to-gasoline), i.e. higher molecular weight compounds were formed. Hydrocarbons were observed in the

able 5.2 Results from the screening of various Pd catalysts: DME conversion, product gas concentration (H2O and N2 not shown), reformer efficiency, and CO2 selectivity

Catalyst DME conv. [%]

Product gas concentration [%]

ηref [%]

CO2 selectivity [%]

condensate obtained in the cold trap as well as in the gas analysis. The MTG T

H2 CO CO2 CH4 Pd(1)/γ-Al2O3 80 40 14 2.8 1.4 36 17

aNot detected

Pd(1)/ZSM-5 44 8 3.0 5.1 2.2 5 63 Pd(1)/HTlc 40 20 n.d.a 9.5 2.8 14 - Pd(1)Zn(9)/γ-Al2O3 76 46 3.8 17 0.24 59 82

γ-Al2O3 78 43 6.0 15 0.08 58 71 d(5)Zn(9)/γ-Al2O3 94 49 9 15 0.5 66 63

d(1)Pt(0.5)/γ-Al2O3 58 38 14 3.1 3.2 36 18

Pd(2)Zn(9)/ PPd(1)Zn(9)-Al2O3 85 42 20 2.3 1.7 40 10 Pd(1)Zn(9)+γ-Al2O3

57 31 12 2.6 1.9 26 18 P

39

Marita Nilsson

reaction has been shown to be favored by strong acid sites on catalyst surfaces t temperatures above 300 °C [70, 71].

++→ (5.5) Incorporation of zinc oxide to Pd promotes the methanol steam reforming reaction, producing mainly CO and H . Pd/ZnO catalysts have been investigated by several res g of methanol and been shown to have high selectivity towards CO2 [59-61]. In this study, Zn was added to Pd with the objective comparable trend during autothermal reforming of DME over alumina-supported catalysts. The ifference from methanol steam reforming is not only the reactants but also the

lectivity for the same amount of Zn. This topic is further elucidated in section .3.

selective d to the copreci o p by a

large fraction of micropores in the coprecipitated catalyst (shown by BET liquid nitrogen adsorption, BET surface areas presented in Table 5.1), leading

encapsulation of sites cessar reach high selectivity. The hybrid catalyst activity ich cat at Pd din ou e lower

et loading this atalyst. This would lead to lower DME due to therm nami it s e h olysis reaction

ethan eform ac y.

to Pd ulted in a higher concentration of CH ociated centr n, su tin at t M de posed over Pt , and

a The Pd(1)/γ-Al2O3 catalyst generated large amounts of carbon monoxide. Pd is known to be active for decomposition of methanol (Eq. 5.4) [57, 58], which is generated via the hydrolysis of DME but could also be formed through the direct decomposition of dimethyl ether (Eq. 5.5).

23 H2COOHCH +→ (5.4) C 2462 HCOCHOH

2 2

earchers lately for steam reformin

to examine if there was a

dfact that alumina is supposed to act both as a support and to participate in the reactions by converting DME to methanol. It was found that the addition of Zn to Pd increased the selectivity for the DME reforming reactions. For the Pd(1)Zn(9)/γ-Al2O3 catalyst, a CO2 selectivity of 82 % was achieved. Further increasing the amount of Pd resulted in increased activity but decreased CO2 se5 The catalyst prepared using the impregnation technique was morecompare pitated ne, which possibly could be ex lained

to ne y toexhibited a low , wh indi es th the loa g c ld bthan the targ for cconversion ody c lim ation of th DME ydras a result of low m ol-r ing tivit The addition of Pt res 4 asswith a high CO congenerating CH , CO

atio gges g th he D E is com4

H2 (Eq. 5.5).

40

Catalytic materials and operating conditions for DME autothermal reforming

41

at this value does not take the heat tegration of the system into account but is a value indicating the amount of

account the concentration of ydrogen in the product gas and the selectivity to carbon dioxide, a

PdZn/γ-Al2O3 catalyst prepared by incipient wetness impregnation, was lected for use in the study of the operating parameters, outlined in section

5.4.

d/γ-Al2O3 catalyst, the CO2 selectivity was increased from 17 to 82 %.

n as a function of Pd:Zn ratio and Pd loading. perating conditions: T=400 °C, O2:DME=0.4, H2O:DME=2.5

Table 5.2 also shows the reformer efficiency obtained using the various catalysts. It should be pointed out thinfuel converted into hydrogen. Side reactions, such as methane formation and complete combustion of the fuel cause a decrease in efficiency. Based on the catalyst screening results, taking intoh

se

5.3 Catalyst characterization: Effect of Zn on CO2 selectivity 5.3.1 Background As described in the previous section, incorporation of Zn to the Pd/γ-Al2O3 catalyst increased the selectivity to CO2 during autothermal reforming of DME. This feature was improved by preconditioning of the catalyst in hydrogen. During steam reforming of methanol, the formation of Pd-Zn species at reduction temperatures above 350 °C has been implicated as the source for high CO2 selectivity over Pd/ZnO catalysts [72]. The effect of the Pd:Zn ratio on the CO2 selectivity and of the Pd loading on the activity during autothermal reforming of DME is illustrated in Figure 5.4. By adding 9 % of Zn to the 1 % P

Figure 5.4 CO2 selectivity and DME conversioO

1:9 2:9 5:9 1:00

10

20

30

40

50

60

70

80

90

100

[mol

%]

CO2 selectivity DME conversion

Pd:Zn ratio [wt:wt]

Marita Nilsson

The activity was found to increase with the Pd loading but since the concentration of Zn was kept constant, the CO2 selectivity decreased. This phenomenon could be attributed to the presence of a larger amount of metallic Pd, catalyzing decomposition reactions and leading to poorer reforming activity. It is assumed that the interaction of Zn with Pd on the alumina surface hanges the selectivity towards carbon dioxide. This hypothesis was evaluated

-rays scattered by atoms in an ordered lattice interfere in directions given by

the ttice plane and n is an integer called the order of reflection [73]. When the

tal, 2d⋅sin θ, is a multiple of the wavelength, constructive terference occurs and diffracted intensity is obtained. The angles of

s ontaining low it

of the instrume ans of XRD.

cby means of X-ray diffraction (XRD) and temperature-programmed reduction (TPR). 5.3.2 X-ray diffraction XBragg’s law, illustrated in Figure 5.5; λ is the wavelength, d is the distance between two lattice planes, θ is the angle between the incoming X-rays andlapathlength in the crysinmaximum intensity are used to calculate the distance between the lattice planes. In this way, crystalline bulk phases in a sample can be identified [73]. X-ray diffractograms were obtained for the PdZn/γ-Al2O3 powder samples using a Siemens 2000 diffractometer in which a movable detector scans the intensity of the diffracted radiation as a function of the angle 2θ. Figure 5.6 shows diffraction patterns for the Pd(5)Zn(9)/γ-Al2O3 catalyst where a fresh and a reduced sample were compared. No metallic Pd could be identified in the diffraction patterns due to overlapping of the other peaks. For the samplec er amounts of Pd, the Pd content was below the detection lim

nt and could therefore not be analyzed by me

Figure 5.5 Illustration of Bragg’s law

dθ

Incoming X-rays Reflected X-rays

λ

n⋅λ=2d⋅sinθ

Incoming X-rays Reflected X-rays

λ

dθ dθ

Incoming X-rays Reflected X-rays

λ

n⋅λ=2d⋅sinθ

42

Catalytic materials and operating conditions for DME autothermal reforming

10 20 30 40 50 60 70 80 90

Reduced

Fresh PdOPdOPdO

PdO

PdZnZnO

ZnO

Inte

nsity

[a.u

.]

Al2O3

2 theta [º] Figure 5.6 X-ray diffraction patterns for PdZn/γ-Al2O3 powder samples, both unreduced and following reduction in 5 % H2 in N2 at 400 °C for 1 h Therefore it could not be concluded whether the higher Pd:Zn ratios gave rise to a larger amount of metallic Pd. It was however found that reduction of the sample at 400 °C resulted in the formation of Pd-Zn species. 5.3.3 Temperature-programmed reduction Temperature-programmed reduction provides information about the temperature necessary for complete reduction of a catalyst. TPR of bimetallic catalysts can reveal whether the two metals are in contact or not. The reduction of a metal oxide can be described by equation 5.6.

OnHMnHMO 22n +→+ (5.6) In TPR experiments, the sample is placed in a furnace, where the temperature an be linearly controlled with time. As the catalyst consumes hydrogen, the hange in thermal conductivity of the gas is measured using a TCD.

he experiments were performed using a Micromeritics Autochem 2910 strument. A Pd/γ-Al2O3 catalyst was compared to its Zn-containing

ounterpart. The lower reduction temperature and higher degree of hydrogen onsumption for the PdZn/γ-Al2O3 (Figure 5.7) catalyst suggest that the Pd and n interact. ZnO is not an easily reducible oxide, but in the presence of Pd, ydrogen spillover can reduce the ZnO at lower temperatures [72, 74]. This

the selectivity to CO2 during autothermal eforming of dimethyl ether over PdZn/γ-Al2O3 catalysts.

cc TinccZhfeature is believed to influencer

43

Marita Nilsson

0 100 200 300 400 500 600

Pd(1)/Al2O

3

Pd(1)Zn(9)/Al2O

3

H2 c

onsu

mpt

ion

[a.u

.]

Temperature [ºC]

Figure 5.7 TPR profiles comparing Pd(1)/γ-Al2O3 and Pd(1)Zn(9)/γ-Al2O3

5.4 Influence of operating parameters on reformer performance

parameter study was performed using a Pd(3)Zn(27)/γ-Al2O3 catalyst. The ating conditions affected the

Considering the oxygen-to-DME ratio, it is important to have enough oxygen the feed to generate heat for the steam reforming reaction but too much

5.4.1 Introduction Aobjective was to investigate how different oper

erformance during autothermal reforming of DME. The influence of oxygen-pto-DME ratio and steam-to-DME ratio on the selectivity to hydrogen and carbon dioxide was evaluated. 5.4.2 Influence of oxygen-to-DME ratio

inoxygen will result in loss of efficiency due to complete combustion of the fuel as well as dilution with nitrogen. The O2:DME ratio was varied from 0.4 to 0.9 with the temperature ranging from 350 to 400 °C. The results are shown in

igure 5.8. The conversion of DME was found to increase at increasing oxygen Ffeed as a result of the higher reaction rate of the oxidation compared to the steam reforming and the increase in temperature. Considering the hydrogen concentration an optimum could be identified at O2:DME values between 0.5 and 0.7 for a temperature of 400 °C. At an O2:DME of 0.7, the concentration of hydrogen amounted to 48 %. At a further increase of the oxygen feed, the hydrogen concentration decreased accompanied by a slight increase in the CO2 and H2O (not shown) concentrations.

44

Catalytic materials and operating conditions for DME autothermal reforming

45

igure 5.8 CO2 selectivity, DME conversion, and H2 concentration as a function of O2:DME

the Pd(3)Zn(27)/γ-Al2O3 catalyst were omparable to those of the Pd(1)Zn(9)/γ-Al2O3 catalyst tested in the screening

e lowest O2:DME ratio, since this is where the onversion is affected the most. It is evident from this study that an oxygen-to-

the catalyst screening, will be ecessary in order to supply sufficient heat for the steam reforming reaction.

cially the case when using fuels requiring high temperatures to be converted (e.g. diesel or gasoline).

0.4 0.5 0.6 0.7 0.8 0.9

30

40

50

60

70

80

90

100 DME conversion

CO2 selectivity

H2 concentration

Tin=350 ºCTin=375 ºCTin=400 ºC

[mol

%]

O2:DME [mol:mol]

Ffor catalyst inlet temperatures of 350, 375, and 400 °C (H2O:DME=2.5) The values of CO2 selectivity for cstudy but the activity was higher at increased Pd loading. That is, the selectivity for PdZn/γ-Al2O3 catalysts is mainly dependent on the Pd:Zn ratio whereas the conversion can be increased by increasing the Pd content. The CO2 selectivity was not considerably affected by changes in the oxygen feed. The influence of the temperature was larger at thcDME ratio higher than 0.4, which was used inn 5.4.3 Influence of steam-to-DME ratio Regarding the steam-to-DME ratio, there are two options in a fuel processor. Either, steam is added in several steps, first in the reformer and then in the water-gas shift units. Alternatively, all of the steam is added directly to the reformer. A surplus of steam in the reformer is advantageous for suppression of the CO concentration but at the expense of the cost to superheat a large amount of steam. Considering the volume and weight of the vaporizer and the condenser recovering the water from the fuel cell, a low H2O:DME ratio is preferable. Furthermore, additional water could be needed to lower the temperature in the WGS steps, why it is unfavourable to add all of the steam directly into the reformer. This is espe

Marita Nilsson

Figure 5.9 CO selectivity, DME conversion, and H concentration as a function of 5.9 CO selectivity, DME conversion, and H concentration as a function of

1.0 1.5 2.0 2.5 3.0

30

40

50

60

70

80

90

100

1.0 1.5 2.0 2.5 3.0

30

40

50

60

70

80

90

100

H2 concentration

CO2 selectivity

DME conversion

[mol

%]

H

Tin=350 ºC Tin=375 ºCTin=400 ºC

2O:DME [mol:mol]

2 2

H2O:DME for catalyst inlet temperatures of 350, 375, and 400 °C (O2:DME=0.7)

2 2

H2O:DME for catalyst inlet temperatures of 350, 375, and 400 °C (O2:DME=0.7) TThe H2O:DME ratio mainly affected the CO2 selectivity, while the impact on the hydrogen yield was small. The H2O:DME ratio was varied from 1 to 3 at an oxygen-to-DME ratio of 0.7 and a catalyst inlet temperature of 400 °C (Figure 5.9). A H2O:DME ratio of 1 gave a DME conversion of 97 % but more steam will be needed to induce the water-gas shift reaction and suppress carbon formation. The highest CO2 selectivity was found at the highest steam-to-DME ratio. However, at higher H2O:DME ratios, the diluting effect of the steam will come into play, decreasing the concentration of hydrogen. The effect of steam on the CO2 selectivity was more pronounced at the lower H2O:DME ratios. A low temperature will be favourable for the water-gas shift reaction, resulting in higher selectivity to CO2.

he H2O:DME ratio mainly affected the CO2 selectivity, while the impact on the hydrogen yield was small. The H2O:DME ratio was varied from 1 to 3 at an oxygen-to-DME ratio of 0.7 and a catalyst inlet temperature of 400 °C (Figure 5.9). A H2O:DME ratio of 1 gave a DME conversion of 97 % but more steam will be needed to induce the water-gas shift reaction and suppress carbon formation. The highest CO2 selectivity was found at the highest steam-to-DME ratio. However, at higher H2O:DME ratios, the diluting effect of the steam will come into play, decreasing the concentration of hydrogen. The effect of steam on the CO2 selectivity was more pronounced at the lower H2O:DME ratios. A low temperature will be favourable for the water-gas shift reaction, resulting in higher selectivity to CO2. 5.4.4 Product gas composition 5.4.4 Product gas composition Figure 5.10 shows the product gas composition during autothermal reforming of dimethyl ether for an O2:DME of 0.7 and a H2O:DME of 2.5 at 350-400 °C. At a temperature of 400 °C the concentrations amounted to 48 % H2, 17 % CO2, 4.6 % CO, 15 % H2O, 15 % N2, 2100 ppm CH4, and 60 ppm CH3OH. HCHO was never observed during the experiments. It should finally be emphasized that a reformer in a fuel cell system has to operate at complete conversion of the fuel for optimization in terms of efficiency as well as economics. However by working below total conversion in this study, the influence on the conversion by changes in the operating conditions could be evaluated.

Figure 5.10 shows the product gas composition during autothermal reforming of dimethyl ether for an O2:DME of 0.7 and a H2O:DME of 2.5 at 350-400 °C. At a temperature of 400 °C the concentrations amounted to 48 % H2, 17 % CO2, 4.6 % CO, 15 % H2O, 15 % N2, 2100 ppm CH4, and 60 ppm CH3OH. HCHO was never observed during the experiments. It should finally be emphasized that a reformer in a fuel cell system has to operate at complete conversion of the fuel for optimization in terms of efficiency as well as economics. However by working below total conversion in this study, the influence on the conversion by changes in the operating conditions could be evaluated.

46

Catalytic materials and operating conditions for DME autothermal reforming

47

2

tion to reforming reactions. A Pd:Zn ratio of 1:9 carbon dioxide. However, both the ratio of ptimized for improved performance of the

atalyst.

found to be O2:DME=0.7 nd H O:DME≥2.5 for a catalyst inlet temperature of 350-400 °C.

350 375 40005

1015202530354045505560

0

500

1000

1500

2000

2500

Con

cent

ratio

n [m

ol %

]

Temperature [ºC]

Left axis: CO CO2 H2 H2ORight axis: CH4 CH3OH

Con

cent

ratio

n [m

ol p

pm]

Figure 5.10 Product gas composition for the Pd(3)Zn(27)/γ-Al2O3 operating at O :DME=0.7 nd H2O:DME=2.5, SV= 15 000 h-1 (N2 not shown) a

5.5 Summary The results presented in Paper II show that Pd-based monolithic catalysts can be used to generate hydrogen-rich fuel cell anode feeds through autothermal reforming of dimethyl ether. Alumina-supported catalysts prepared by impregnation technique were more active for DME ATR than the other catalysts evaluated. The addition of Zn to Pd/γ-Al2O3 catalysts was found to increase the selectivity to carbon dioxide. This characteristic is ascribed to Pd-Zn interactions changing the reaction pathway from decomposiwt/wt resulted in high selectivity to Pd to Zn and the Pd loading can be oc The oxygen concentration in the feed was found to have a significant impact on the hydrogen yield, whereas the concentration of steam mostly had effect on the CO2 selectivity. The optimal operating conditions, considering H2 and CO2 selectivity, for a Pd(3)Zn(27)/γ-Al2O3 catalyst was a 2

Marita Nilsson

48

Conclusions

6 Conclusions Dheavy-duty truck auxiliary power units

imethyl ether has lately been considered as a hydrogen carrier for fuel cells in . Current literature references on

hydrogen generation from DME are mainly devoted to the steam reforming process. In the present work, the feasibility of generating hydrogen by autothermal reforming of DME has been demonstrated. The feasibility of the self-sustaining ATR process, without an external source of heat, was verified at kilowatt-scale. It was further shown that catalytic oxidation of DME can be used during start-up to heat the ATR. The use of a fixed bed of catalyst pellets was found to have a large negative impact on the dynamics of the reformer, shown by slow response to changes in load and oxygen-to-DME ratio. Monolith substrates provided a faster response to transient operation and to changes in operating parameters. Alumina-supported Pd-based catalysts were shown to be active for DME autothermal reforming, yielding hydrogen-rich product gases at temperatures between 350 and 400 °C. PdZn/γ-Al2O3 catalysts were superior among the evaluated Pd catalysts with respect to obtaining high CO2 selectivity, which is desirable considering low-temperature fuel cell applications. Catalyst characterization studies have provided preliminary information on the performance of the catalyst. The increased CO2 selectivity with the addition of Zn was ascribed to interactions between Pd and Zn leading to higher reforming activity and lower activity for decomposition reactions. Results from thermodynamic equilibrium calculations suggest that high concentrations of hydrogen associated with low concentrations of carbon monoxide can be obtained with the DME ATR process for a wide range of oxygen-to-DME and steam-to-DME ratios. The conversion of DME is not thermodynamically limited at the relevant operating temperatures, which is >350 °C. During experiments, the operating conditions were found to have a significant impact on the reformer performance. Increasing the oxygen-to-DME ratio had effect on the reactor temperature as well as on the reformer efficiency. The highest efficiency will be obtained for conditions with sufficient supply of heat to the steam reforming reaction but without loss of hydrogen. The efficiency is strongly dependent on the product gas composition and side reactions such as total combustion and methane formation will influence the hydrogen yield negatively. For the reactor setup used during the experiments described in this

49

Marita Nilsson

thesis, an oxygen-to-DMEchanging the steam-to-DM

ratio of 0.7 is considered necessary. The effect of E ratio was mainly on the carbon dioxide selectivity

with ratios of at least 2.5 being preferable.

50

Final remarks and future perspectives

7 Final remarks and future perspectives Fossil fuel usage in transportation vehicles throughout the world contributes to environmental deterioration and greenhouse gas emissions. Increases in energy usage are driven by population growth and economic development. Consequently, due to the predicted increase in population and economic progress in the future, the world is facing serious environmental issues. In order to use our resources more efficiently, it will not only be necessary to replace fossil fuels; we also need to call attention to the importance of achieving high energy efficiency onboard vehicles. The automotive industry is searching for new solutions. Dimethyl ether could contribute to the expected mix of alternative fuels and vehicle propulsion technologies. Biomass-derived DME offers one route as a future fuel in heavy-duty vehicle applications in society’s effort to achieve future goals of sustainable transportation. Biomass will play a large role in the future and it is important that purposes such as food production and preservation of biodiversity be satisfied. Even though the biomass resources, in the long run, might not be sufficient to supply the total amount of energy needed for transport, the conversion of biomass to liquid fuels represents a step in the right direction towards a more sustainable use of energy in the transport sector. In the project on which this thesis is based, a strategy for establishing ultra-low emission DME trucks (< 0.20 g NOx/bhp-hr, < 0.10 g PM/bhp-hr [8]) has been developed in a joint effort between KTH, Chalmers University of Technology, and Volvo Technology. Hydrogen is produced either to feed an APU based on a polymer electrolyte fuel cell and/or used to regenerate a NOx trap. The NOx aftertreatment system involves reduction of nitrogen oxides either with DME or with hydrogen from the reformer. In combination with the development of a DME-fueled diesel engine this is an approach that presents a large potential to meet upcoming emission regulations. The potential APU market is substantial. Idling trucks contribute to air pollution, greenhouse gas emissions, and excessive fuel consumption. Fuel cell-based APU systems could offer one of the first large market penetration opportunities for fuel cells, and thereby contribute to developing more efficient energy converters. To reach the level of implementation, there are barriers that have to be addressed, including performance (efficiency, reliability, and durability), cost, truck design boundaries (volume, weight), and competing alternatives. It is likely that the acceptance of technologies that require an additional fuel onboard the truck will be lower among the drivers. In that case, the most interesting technology will be to generate hydrogen from the existing

51

Marita Nilsson

truck fuel, as here with DME. A big issue will then be achieving quality reformate. As research goes on, technical barriers are being

fuel cell- removed

y. TEM studies (transmission electron microscopy) will

and idling reduction incentives are being realized that may support the use of APU systems in the future. The work in the present thesis has aided understanding on hydrogen generation from dimethyl ether by autothermal reforming. Knowledge has been built up considering operating characteristics and catalyst materials for this process. In particular, the studies at Chemical Technology, KTH differ from other journal papers on hydrogen generation from DME due to the utilization of combined steam reforming and partial oxidation and the use of monolith catalyst substrates for this process. The experience from the studies will be used in further work with special focus on strategies for fuel processor systems design including reformer technology and catalyst optimization. Emphasis will be placed on the evaluation of startup concepts and system heat management as well as on the integration of a deNOx unit. The design and construction of a laboratory reforming system that includes a CO cleanup step would be valuable for reaching a higher level of proof-of-concept in the project. Furthermore, the effects of space velocity and pressure on the reforming reaction should be investigated. The PdZn/γ-Al2O3 catalyst can be further optimized with respect to activity and selectivitbe used for tailoring of the catalyst.

52

Acknowledgements

Acknowledgements The following organization

pport to this work: The s are gratefully acknowledged for their financial

Swedish Energy Agency, the Swedish Agency for

creative cooperation and to Miroslawa bul-Milh for help with the analyses.

I would also like to thank diploma workers Jonas Nyström and Cristina Peretti whom I had the opportunity to work with during this project. Thanks to Inga Groth for support with the XRD and BET analyses and to Tomas Östberg and Bo Karlsson at the workshop for nice work with the reactor equipment. Finally, my warm thanks to family and friends.

suInnovation Systems, the Swedish Road Administration and the Swedish Environmental Protection Agency. I would like to express my gratitude to my supervisor, Associate Professor Lars J. Pettersson, for continuous support and a never-ending enthusiasm. Mahalo! Thanks to my colleagues and friends at Chemical Technology making it a stimulating and pleasant place to work at. Special thanks to the members of the “Girls’ Knitting Club” and to Sara for being a high-quality room-mate. Thanks also to Monica for spreading warmth throughout the department with her infectious smile and friendly nature. Big thanks to Professor Lars Mattsson. Having a mentor was a very positive experience for me. Further, I would like to thank everybody at Volvo Technology and Volvo PowerCell for the time I got to spend in your laboratories. Special thanks to

eter Jozsa and Bård Lindström for PA

53

Marita Nilsson

54

Nomenclature

Nomenclature aa.u. Number of atoms

Arbitrary unit A mass of element Al2O3 Aluminium oxide (alumina) APU Auxiliary power unit ATR Autothermal reforming BET Brunauer-Emmett-Teller (method for surface area measurement) BTL Biomass-to-liquids CAD Computer-aided design CH3OH Methanol CH3OCH3 Dimethyl ether CH4 Methane CIDI Compression ignition direct injection CNG Compressed natural gas CO Carbon monoxide CO2 Carbon dioxide Cpsi Cells per square inch Cu Copper DME Dimethyl ether EGR Exhaust gas recirculation Eta (η) Efficiency F Molar flow FC Fuel cell Fe Iron FID Flame ionization detector FT diesel Fischer-Tropsch diesel FTIR Fourier transform infrared (spectrometer) Gf Gibbs free energy of formation g/bhp-hr Grams per brake horsepower-hour GC Gas chromatograph GHSV Gas hourly space velocity GTL Gas-to-liquids HCHO Formaldehyde H2 Hydrogen H2O Water H2O:DME Steam-to-DME ratio HTlc Hydrotalcite-like compound HTS High temperature water-gas shift IW Incipient wetness

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Marita Nilsson

L Length Lambda (λ) Air/fuel equivalence ratio

bda (λk) r e

gas

hift bly

frared

el cell membrane

tion

tor med reduction

n

Lam Lagrange multiplieLH Lower heating valuV

G LN Liquefied naturalLPG Liquefied petroleum gas

r-gas sLTS Low temperature wateMEA Membrane electrode assem

MFC Mass flow controller Mn Manganese MTG Methanol-to-gasoline

s n number of moleNDIR Nondispersive inN2 Nitrogen NOx Nitrogen oxides O2 Oxygen

O2:DME Oxygen-to-DME ratioPd Palladium Pt Platinum

ctrolyte fuPEFC Polymer elePEM Proton exchangePM Particulate matter POX Partial oxidation PrOx Preferential oxidaR Gas constant S Selectivity SI Spark ignition SOFC Solid oxide fuel cell SR Steam reforming STY Space-time yield SV Space velocity T Temperature TEM Transmission electron microscopy TCD Thermal conductivity detecTPR Temperature-programTWh Terawatt hour XRD X-ray diffractioWGS Water-gas shift y Molar fraction Zn Zinc ZSM-5 Aluminosilicate zeolite

56

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