State of the art of multi-fuel reformers for fuel cell vehicles: problem identification and research needs

International Journal of Hydrogen Energy 26 (2001) 243–264

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State of the art of multi-fuel reformers for fuel cell vehicles: problem identi cation and research needs Lars J. Petterssona; ∗ , Roger Westerholmb a KTH-Royal

Institute of Technology, Department of Chemical Engineering and Technology, Chemical Technology, SE-100 44 Stockholm, Sweden b Department of Analytical Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden

Abstract This review is focused on discussions about multi-fuel reformer technology for fuel cell vehicles where techniques for onboard hydrogen generation and gas clean-up processes, as well as fuel considerations and emissions are included. Our conclusion is that the potential for developing a highly ecient, durable and reliable reformer system for automotive applications is considerably higher if dedicated fuel reformers are used instead of applications where all types of fuels ranging from natural gas to heavy diesel fuels can be used. The authors propose that petroleum-derived fuels should be designed for potential use in mobile fuel cell applications. The present literature review together with site visit discussions has led to the conclusion that there are relatively low emissions from fuel cell engines compared to internal combustion engines. However, the major research work on reformers=fuel cells have been performed during steady-state operation. Emissions during start-up, shutdown and transient operation are basically unknown and must be investigated in more detail. ? 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. Keywords: Reformer; Multi-fuel; Fuel processing; Hydrogen; Fuel cell; Vehicle; Emissions

1. Introduction The introduction of new types of fuels aimed at the transportation market is associated with very large costs. Due to the infrastructure needed for distribution and supply of fuel it is more common that alternative fuels are tested in such vehicle eets since public transport companies often have a lling station at the bus garage. In the Stockholm Transport eet there are at present 228 buses running on bio-ethanol and the number of such buses is expanding [1]. Another way to circumvent the distribution diculties is to use exible fuel vehicles (FFV) utilizing fuels such as

∗

Corresponding author. Tel.: +46-8-790-8259; fax: +46-8108-579. E-mail address: [email protected] (L.J. Pettersson).

gasoline=methanol blends. A FFV can in principle run on neat gasoline to neat methanol or any mixture in between which indeed makes it exible with respect to range. This

exibility is an advantage with respect to infrastructure costs. However, important factors to consider are emissions generated by vehicles since they may contribute to air pollution, potential environmental and health problems and need to be substantially reduced in the near future. A future vehicle concept that have a potential to substantially reduce exhaust emissions in automotive applications is the use of fuel cells. Using the estimation of Biedermann and co-workers [2] shown in Table 1 it seems, with respect to energy and emissions, bene cial to use PEM fuel cells. Hydrogen, which is the preferred fuel for an automotive fuel cell, is an environmentally friendly resource, which throughout history has played an important part in a vast number of industrial and commercial applications, ranging

0360-3199/01/$ 20.00 ? 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 0 0 ) 0 0 0 7 3 - 2

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Table 1 Energy consumption and emissions of carbon dioxide (CO2 ), nitrogen dioxide (NO2 ), sulphur dioxide (SO2 ), carbon monoxide (CO), and volatile organic compounds (VOC) from passenger cars with dierent propulsion systems calculated in the ECE driving cycle [2]. Energy and emission per 100 km driving distance Propulsion system

Energy

Gasoline engine 79 Diesel engine 64 Electric motor PEM, using methanol 34

CO2 (kg)

NO2 (g)

SO2 (g)

CO (g)

VOC (g)

20 17

52 89

8 19

180 83

130 21

7

3

1

1

2

Fig. 1. Onboard hydrogen generation by the use of a multi-fuel reformer.

from small-scale production of pharmaceuticals to large bulk production of ammonia. The usage of hydrogen in fuel cell applications is a valuable method for converting chemical energy directly into electrical energy. The space program of the USA has successfully used such fuel cell technology for electricity production [3]. The application most frequently used in the space program is the alkaline fuel cell, which is a very ecient fuel cell. However, it has some drawbacks regarding economy as well as some diculties to operate due to a high sensitivity for poisoning [4]. The handling of hydrogen may be risky, as the formation of explosive mixtures of hydrogen and air may cause severe damage. The formation of “Oxy-Hydrogen gas” takes place with ease whenever hydrogen in con ned volumes comes into contact with air and the hydrogen concentration supersedes the ammability limit. Onboard reforming of liquid fuels is however a way to solve the problem of handling hydrogen (See Fig. 1). By the use of an on-board multi-fuel reformer for the production of hydrogen gas, to be used in the fuel cell, both good driving range and low emissions are expected. Using a process simulation software package based on chemical equilibria Amphlett et al. [5] predicted emission factors for unburned methanol, formaldehyde and carbon monoxide to be less than 1 g=km and NOx less than 0.03 g=km. Although, the predicted emission factors are low, transients were not included as chemical equilibria was simulated.

If fuels such as gasoline and diesel are to be used in fuel cell applications already available infrastructure will be taken advantage of and this will reduce the costs for future investments. This will also speed up the introduction of automotive fuel cell applications to the market. Daimler Benz has developed a car which uses a fuel cell for propulsion of the vehicle (NECAR) [6]. However, as far as known, the vehicle runs on pure hydrogen gas and is not equipped with a reformer allowing multi-fuels to be used. Cooper and Feasey [7] recently reported emission data from NECAR III using the FTP72 driving cycle. From this, CO and NOx emissions were not detectable, while total hydrocarbon emissions were 0.005 g=mile. It is important to point out, however, that the test conditions did not meet the FTP72 requirements, since a cold start was not included. Podolski [8] reports that General Motors (GM) has demonstrated the feasibility of a 30 kW fuel cell system in a methanol-fuelled Opel Za ra minivan with very low steady-state emissions far below the ULEV standards. This car was also equipped with a nickel-metal hydride battery. The reported eciency was, however, lower than predicted. The maximum eciency was approximately 30% compared to the predicted 38%. Consequently, due to the still early development phase of fuel cells, this report includes very few data concerning mobile systems. Between these companies there also exist alliances and joint ventures, such as DaimlerChrysler and Ballard [9 –11]. Ecostar Electric Drives, Ballard Power Systems, and dbb fuel cell engines each own 1=3 of Ballard automotive, which was formed to market fuel cell cars [7]. dbb fuel cell engines GmbH is owned by DaimlerChrysler (51%), Ballard Power Systems (27%), and Ford Motor Co. (22%). Ecostar Electric Drives is controlled by Ford Motor Co. (62%), Ballard Power Systems (21%), and DaimlerChrysler (17%). DaimlerChrysler owns 20% of Ballard Power Systems and Ford Motor Co. owns 15% of the same company [7]. Furthermore, alliances exist between Shell and dbb fuel cell engines, as well as between Ford Motor Co. and Mobil. A recent and comprehensive review concerning the status of fuel cells for automotive applications was published by Kalhammer et al. [12]. We refer the interested reader to this report for technical details concerning fuel cells and reformer data. The review made by the Fuel Cell Technical Advisory Panel is based on an ambitious survey that include all the major companies active in fuel cell development. The focus is on the development in the USA. In Table 2 we have assembled a list of reformer developments worldwide. The purpose of the work presented in this review was to collect available information about the chemical composition of the product gas from fuel reformers to be used in automotive fuel cell applications. An important part of this paper is focused on pinpointing the technical problems that exist in mobile fuel cell systems and to suggest recommendations for future research. A special section is devoted to a discussion of this subject.

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Table 2 Fuel reformer development worldwide [8,12–14] Company Epyx

Corpa

International Fuel Cells Hydrogen Burner Technology Wellman CJB Catalytica Advanced Technologies McDermott Technology Johnson Matthey DaimlerChryslerb Ballard Power Systemsb Ford Motor Co General Motors Delphi Energy & Engine Management Systems Toyota Nissan Energy Research Corp. EER Praxair Shell Haldor Topsoe Exxon

National laboratory

University

Argonne National Laboratory

Royal Military College, Canada Royal Institute of Technology, Sweden

Los Alamos National Laboratory Forschungszentrum Julich

a Formed b Via

by Arthur D. Little. dbb fuel cell engines.

2. The fuel cell principle The fuel cell converts the chemical energy of a fuel to electrical energy. Fuel cells operate at high eciencies and normally emit very low amounts of hazardous compounds. The rst fuel cell was demonstrated by Sir William Grove (see Figs. 2 and 3) at the Royal Society of Chemistry in London in 1839 [3]. The catalysis pioneer Wilhelm Ostwald (1853–1932) was also involved in the early days of fuel cell development. Other important work in the late 19th century was performed by Walther Nernst (1864 –1941) and Fritz Haber (1868–1934). All of these three German scientists were later together with Carl Bosch (1874 –1940) involved in the development of the ammonia process, which would revolutionize the chemical industry in general and large-scale high pressure manufacture of chemicals in particular. The rst practical alkaline fuel cell was developed by Francis T. Bacon (1904 –1992) and co-workers at Cambridge University. There is a signi cant dierence between a battery and a fuel cell. A battery has to store its chemical energy and the output will be depleted over time until eventually, the battery is totally discharged. If it is a rechargeable (secondary) battery it can be used again after charging, otherwise it has to be exchanged by a new battery (primary battery). A fuel cell, on the other hand, is continuously fed with a fuel, e.g. hydrogen which is stored outside of the cell. The process

Fig. 2. William Grove (1811–1896), the fuel cell pioneer.

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2.1. PEM fuel cells

Fig. 3. William Grove’s fuel cell [15].

A cell of this sort is built around an ion-conducting membrane such as Na on [18] (DuPont trademark for a per uorosulfonic acid membrane). The electrodes are made from carbon coated with catalyst, and several construction alignments are feasible. The ion exchange membrane fuel cells use the membrane as electrolyte. Usually a proton exchange membrane is used and therefore the cells are referred to as proton exchange membrane fuel cells (PEMFC). The water transport is dicult to control [16] and the membrane may thus easily dry up. This means that water has to be fed since the membrane otherwise will collapse. The electrodes usually contain platinum as catalyst and are pressed on both sides of the membrane. The fuel cell consists of several stacks of electrodes. The fuel must be almost carbon monoxide-free — otherwise the catalyst will be poi◦ soned at ca 80 C. The eciency of a fuel cell is high compared to an internal combustion engine and is not dependent on the Carnot cycle [19]. In the latter a fuel is burned at high temperatures and gas is expanded to transfer chemical energy into mechanical energy. The fuel cell converts chemical energy directly into electrical energy. Because of this fundamental characteristic, fuel cells may convert fuels to useful energy at an eciency as high as 60%, whereas the internal combustion engine is limited to eciencies of near 40% or less as in the diesel engine, or around 30% or less as in the spark-ignition (SI) engine. However, it should be pointed out that gasoline direct injection (GDI) engines exhibit higher eciencies than SI engines because of the lean combustion.

Fig. 4. The operating principle of a fuel cell [17].

3. Fuels producing the electrical current continues for as long as there is a supply of reactants. This is because unlike in a regular battery, the electrodes and electrolyte of a fuel cell are designed to remain unchanged by the chemical reactions. The basic principle of a fuel cell is as follows [16]: Hydrogen enters the pores of the anode and reaches the reaction zone, where the gas, the liquid or solid electrolyte meet (see Fig. 4). The hydrogen gas diuses to the electrochemically active site where it is adsorbed and dissolved by the electrolyte. Subsequently, the hydrogen molecule dissociates to 2H+ at the anode. Below, the anode and cathode reactions are given. Anode: 2H2 → 4H+ + 4e− : Cathode: 4H+ O2 + 4e− → 2H2 O: The overall reaction is: 2H2 + O2 → 2H2 O:

A fuel cell requires hydrogen gas to work properly. This means that a fuel cell can run on pure hydrogen or some other type of fuel that may be reformed to generate hydrogen gas prior to the fuel cell. Depending on the fuel properties, such as chemical composition, a large variation in fuel parameters is generated which will have an impact on the optimal working conditions for the fuel reformer. The most commonly used liquid fuel in fuel cell applications is methanol. A by-product gas formed in the fuel reformer is carbon monoxide (CO) which reduces the power economy for the fuel cell. The level of CO tolerable is less than 20 ppm in the fuel reformer product gas [20]. This is, however, dependent on the fuel cell design. The reduction of fuel cell conversion eciency by CO is, however, partly reversible. Amphlett et al. [20] summarized PEM fuel cell anode gas feed speci cations: Hydrogen 50 –100%; CO 10 –100 ppm, dependent on fuel cell design; nitrogen and carbon dioxide relatively inert; water, variably dependent on fuel cell design; methane is relatively inert, but should be as low as possible to avoid decreasing fuel eciency; formic acid is

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a severe irreversible poison which should be eliminated; methanol, formaldehyde and methyl formate may give a reversible performance loss at 5000 ppm. Some desirable qualities of the fuel is that it has to be easy to handle, non-toxic and, from an economical point of view, competitive [21]. Today, there is no fuel available which can meet all of these requirements, but there are some fuels which come close to the ideal. More experimental veri cation is however needed. The US Department of Energy (DOE) label themselves as fuel independent and has not put one fuel before another. DOE is thus supporting research involving many types of fuels. The California Energy Commission has identi ed four dierent fuels for the near-term, mid-term and long-term [22]: • • • •

gasoline, methanol, hydrogen, natural gas.

Three of these fuels have to be reformed before use. The strongest argument in favour of gasoline-type fuels is the existence of a global infrastructure for their distribution and supply [23]. The biggest drawback is that it is highly probable that gasoline used for PEM fuel cells will have to be reformulated into a fuel to an extent which is not known at the moment. The price of such a fuel is dicult to predict, but it is quite clear that it will be substantially higher than for today’s fuels. Gasoline is a blend of dierent kinds of petroleum-derived compounds, such as parans, cyclic hydrocarbons, ole ns, aromatics, naphthenes, oxygenates, and the carbon chain and ring structures makes it dicult to convert to hydrogen and carbon dioxide [24]. The risk of producing carbon on the surface of the catalyst is obvious and this would ultimately lead to catalyst deactivation and, consequently, an impaired vehicle performance. One obvious drawback with a multi-fuel reformer system is that the reformer will not be optimized for any fuel. Methanol reforming, for example, is carried out at considerably lower temperature than diesel reforming. A diesel oil reformer system requires more components and is therefore likely to have slower response dynamics and require more maintenance than a methanol reformer [25]. The hydrogen=carbon ratio is also unfavourable in diesel fuel. Moreover, the vaporization of diesel is an energy-consuming process, which slows down start-up and response times. Examples of potential fuels to be used as feedstocks for multi-fuel reformers for the production of hydrogen are: methanol [24,26,27], ethanol [25,28–31], compressed natural gas (CNG) [26,32–34], methane and biogas (mainly methane) [35,36], naphtha and gasoline [29,37– 40], diesel [25,41– 43], kerosene [44], aviation fuels (JP-8, etc.) [45], marine fuels [40], various hydrocarbons (pentane, hexane, octane, acetylene) [35,46,47], propane [29,48], butane [48], dimethyl ether [49], ammonia [21].

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Compressing gaseous fuels to high pressures consumes a substantial amount of energy. About 15% of the lower heating value for hydrogen and about 5% for methane is, for example, consumed when compressing the gases ideally to 35 MPa [50]. For a real compression the corresponding percentages are 30 and 10%, respectively. If “higher boiling” compounds are present in the fuel (as in the case of gasoline and diesel fuels) a possible carbonization may occur within the fuel reformer. Ledje-Hey et al. [51] report that the solid carbon build-up consists of three dierent forms, i.e. amorphous carbon, soot or graphite, depending on operating conditions. The carbon build-up takes place in the gas phase and emanates from acetylene, diole ns, and aromatics possibly forming Polycyclic Aromatic Hydrocarbons (PAH). These authors, furthermore, discuss possible growth mechanisms, such as decomposition of hydrocarbons on carbon particles, agglomeration of small carbon particles, and polymerization of hydrocarbons for the carbon build-up. They also state that soot particles determined in the gas phase are spherical in shape with a diameter of approximately 0:8 m. Depending on to what extent this carbonization process has proceeded the fuel reformer eciency decreases in an irreversible manner. To partly regenerate the fuel reformer a temperature increase is necessary, an increase which may also cause the deterioration of the catalyst material within the reformer. According to Dicks [52], the decomposition of carbon monoxide may also result in carbon formation in nickel steam reforming catalysts. He stated that by external pre-reforming of the fuel high molecular weight hydrocarbons would be removed from the fuel, which would reduce potential cracking and carbon formation. An advantage of fuel pre-reforming is that the designed fuels may be expected to simplify and speed up the development of fuel cells. A requirement for fuels to be used in the fuel reformer is that their sulphur content is low or can be removed prior to the fuel cell reformer (see section on desulphurization below). For military fuel cell applications fuels with sulphur levels as high as 1% may be of interest for use. It may be stated that the more complex the fuel used in the fuel reformer, the more complex a product gas is formed. To obtain a clean product gas, this requires a higher operating temperature of the fuel reformer. Wang et al. [46] describe hydrogen production from biomass. The conversion of biomass to hydrogen is made by catalytic steam reforming and shift conversion of fractions emanating from pyrolysis and aqueous=steam processes of biomass. Catalyst formulation used for reforming of naphthas is used “eectively” also for producing hydrogen from bio-oil. A problem addressed is the strategy to minimize coke formation on the catalyst used. A palladium membrane reactor (PMR) was used by Birdsell et al. [35] to produce hydrogen from reforming octane, methane, ethanol and methanol. The temperature ◦ needed for the reforming of octane was 600 C, and the corresponding temperatures for methane, ethanol and methanol

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Table 3 Fuel exible fuel processing technical targets [13,57] Characteristic

2000 Target

2004 Target

Energy eciency Power density Speci c power Cost Start-up to full power

40%a

80% 750 W=dm3 Not given $10=kW ¡ 1 min

250 W=dm3 250 W=kg $150=kW ◦ 5 min (−40 C) ◦ 2 min (20 C) 20 s Tier II Not given 2000 h

Transient response (10 –90% power) Emissions CO content (transient) Durability (¡ 5% power degradation) a 25%

10 s ¡Tier II 100 ppm Not given

power.

◦

reforming were 450, 525 and 300 C, respectively. If the ◦ temperature was increased to 350 C by using methanol as fuel the hydrogen production decreased while the methane production increased. The authors stated that a PMR would be a viable alternative to conventional reforming systems for hydrogen production in fuel cell applications. Inexpensive Pd membranes and pumping systems are, however, needed to make the PMR cost-eective. 3.1. Designed fuels Diesel fuel does not work well as a fuel for reformers because of problems arising in the evaporation process and too low a H=C ratio. Thus, only limited laboratory experiments have been performed. Gasoline on the other hand, seems to be a better alternative. To optimize the fuel cell concept for automobile applications a designed fuel should be developed. This designed fuel cell fuel should not contain sulphur, aromatics or other compounds with a low H=C ratio. However, the speci cation of this designed fuel cell fuel requires further research and development. This includes matching of new catalysts and reformers. A fuel where high sulphur contents are expected is the fuel used in military applications. As high as 1 wt% sulphur is acceptable, for example, in NATO fuel. A drawback is that the specially formulated fuels would require separate distribution and storage at the retail station [53]. Ahmed et al. [37] sketch a system where petroleum derivatives could be speci cally formulated to optimize reforming. There is extensive worldwide research going on regarding the production of fuels from natural gas. Via Fischer-Tropsch synthesis or methanol synthesis a lot of dierent methane-derived fuels can be produced [54]. 4. System requirements and costs Partnership for a New Generation of Vehicles (PNGV) has established the following project goals for the vehicles

of the near future [55]: • • • •

full-sized vehicle, 80 miles=gallon (“tripling of fuel economy”), no decrease in vehicle performance, use gasoline — capable of transitioning to alternative fuels (ethanol, methanol, natural gas), • prototype power systems by 2000, • commercial vehicles by 2004. The time goals seem very optimistic for fuel cell vehicles and it remains an open question whether commercial vehicles are suciently ecient and have the proper endurance at this stage. Estimated costs for a conventional internal combustion engine for automobile propulsion is USD 20 –30=kW, i.e. 1000 to 1500$ for a 50 kW engine. At present the cost for a fuel cell for automobile application is estimated to be in the range of 4000$=kW. For a medium-size passenger car of about 50 kW an approximate cost at present for a fuel cell concept is 200,000$ (ca 1,6MSEK). A target for the PNGV is 10$=kW. The hydrogen requirement in the vehicle is roughly 1 Nm3 hydrogen=h for every kWe [56]. For a well-functioning system this gure is normally a little bit lower. 5. Techniques for onboard hydrogen generation 5.1. Multi-fuel reformers The American approach for fuel cell vehicles is based on multiple fuel operation capability. The US Department of Energy (DOE) has presented targets for the development of multi-fuel processors, which are shown in Table 3. According to Davis [57] the DOE strategy accepts the constraints of the marketplace, while at the same time encouraging a transition to sustainable, renewable fuels. Using this strategy DOE currently places their main emphasis on gasoline, methanol, ethanol and natural gas. The three primary

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Fig. 5. Catalytic gasoline reformer [37].

driving forces for DOE are energy security, economic independence and reduced emissions of hazardous compounds. However, when using the multi-fuel approach both reformer trade-os and system trade-os are necessary. The advantage is the freedom of choice of fuel and availability of the fuel. The world’s rst gasoline to fuel cell power unit was demonstrated in October 1997 [8]. In the following sections we will discuss the dierent options for onboard hydrogen generation. Dierent fuels lay dierent constraints on the reformer design, the catalysts and on the operating conditions (see the section on fuels). Criteria for evaluating various on-board reformer options are speci c to transportation applications, such as rapid start-up from cold start, good transient performance of reformer, compact, light-weight construction, and low cost. The latter can be achieved by mass production of components. A very low CO (or any other detrimental compound) content in reformer product gas is crucial. An example of a catalytic gasoline reformer can be seen in Fig. 5. A way of producing hydrogen from commercial fuels such as gasoline is by steam reforming [21]: CH1:75 + H2 O → 1:875H2 + CO: Another option is to use diesel fuels. The problems associated with diesel fuel processing are primarily a low H=C ratio (risk for carbon precipitation), high energy consumption in fuel evaporation, and a high temperature at fuel reforming. In the beginning of the seventies there was research going on in the USA, Germany, Japan and Sweden, in trying to decrease the emissions from gasoline engines. The rst eorts were made to produce hydrogen in a reactor onboard the vehicle. Hydrogen permits lean operation and this in turn leads to decreased emissions of CO, hydrocarbons and NOx . Important work was for example done at the Jet Propulsion Laboratory in the USA and at Siemens Research Laborato-

249

ries in Germany. These research eorts are summarized in a review on methanol decomposition for motor fuel use by Pettersson and Sjostrom [58]. In an early investigation on fuel cell applications Callahan [59] studied various hydrocarbon fuels for onboard hydrogen generation. The author tested combat gasoline (C7:36 H13:5 ), Sunoco 190 gasoline (C7:32 H15:15 ), jet fuel (C8:35 H17:59 ), and diesel fuel (C15:10 H24:76 ). The catalytic reformer operated ◦ at temperatures between 850 and 1200 C. The data indicated that two reactors with internal volumes of 675 cm3 each could produce about 0:36 m3 =h of hydrogen from 0.23 kg of a typical hydrocarbon fuel, such as combat gasoline. According to Callahan this would be sucient to run a 500 W acid electrolyte fuel cell stack. Khan et al. [60] studied steam reforming from the commercially available fuels gasoline and JP-4 jet fuel, respectively. The sulphur content in the gasoline was 146 ppm, the ole n content was zero and the Reid vapor pressure was 12. A high steam=hydrocarbon ratio (4.46) had to be used to avoid carbon deposition. The ◦ reformer operated at about 500 C in the gasoline case. Furthermore, also hexane reforming was studied. Ahmed et al. [33] presented a catalytic fuel exible bench-scale reformer which yields 40 dm3 H2 =min (3 kWe) using a 1:7 dm3 reactor when gasoline is fed to the reactor. The catalyst was stable after 40 h on stream when using n-decane, steam and oxygen as feed for operating the partial ◦ oxidation reactor at 800 C. The dry and nitrogen-free concentration of hydrogen was above 60% during the whole experimental period. 5.2. Evaporators An important part of the energy needed for the process is consumed in the vaporization step. This is a fact which is often overlooked and needs to be addressed in the system design. Because of the high boiling point interval of the fuel diesel evaporation is an energy-consuming process where high temperatures are needed. If heavy feedstocks are used the temperature required for evaporation can be so high that a part of the fuel will crack. This can then lead to soot precipitation and clogging of the evaporator. The heat needed for vaporization is distributed by the catalytic afterburner where the anode o-gas is burnt by utilizing heat from the combustion of excess hydrogen. The afterburner is sometimes also used for combustion of fuel during start-up for providing heat required for the evaporation [32]. 5.3. Partial oxidation The process is based on extreme rich fuel combustion (low air=fuel ratios), where the reactions given below may occur: Cx Hy Oz + (x − z=2)O2 → y=2H2 + x CO2 :

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Table 4 Concentration of hydrogen in the product gas obtained with dierent hydrocarbons [37] Fuel

H2 concentration, % (dry) Theoretical

Experimental

H2 selectivitya (%)

Temperature ◦ ( C)

Methanol Ethanol i-Octane Cyclohexane 2-Pentene Toluene

70 71 68 67 67 61

64 62 60 61 58 50

91 88 88 91 88 82

450 580 630 700 670 660

a Selectivity

= (%H2 in product gas) ×100=(%H2 theoretically possible). For example, with C8 H18 and oxygen-to-fuel ratio=4, the theoretical hydrogen percentage is 68%. Thus, 60% hydrogen in the product is equal to a selectivity of 88%.

The following reactions will also occur: Cx Hy Oz + (x=2 − z=2)O2 → y=2H2 + x CO; Cx Hy Oz + (x + y=4 − z=2)O2 → y=2H2 O + x CO2 : The process can be performed in both a catalytic and a non-catalytic manner [61]. If a catalytic system is used the reformer can be operated at a much lower temperature and the heat can be supplied directly into the catalyst bed. An advantage with this process is that it is rather insensitive to contaminants and that it is rather independent of fuel. The biggest drawback is the risk for carbon precipitation and a subsequent fouling of the system. If a catalytic system is used there is a risk for catalyst deactivation. The Boudouard reaction, the methane decomposition, as well as the CO and CO2 hydrogenation reaction must also be controlled: 2CO(g) CO2 (g) + C(s); CH4 (g) 2 H2 (g) + C(s); CO(g) + H2 (g) H2 O(g) + C(s); CO2 (g) + 2 H2 (g) 2 H2 O(g) + C(s): Methane cracking is an endothermic reaction, while the others are exothermic. Carbon formation will only occur if the gas composition exceeds the equilibrium constant for the above reactions [62]. When the product gas is cooled the thermodynamics will favor carbon formation. However, counteracting this are the kinetics which will decrease the reaction rate at lower temperatures. In mobile applications rapid start-up and good transient behaviour are two important factors. Furthermore, no additional energy is required to maintain hydrogen generation reactions as in steam reforming. Partial oxidation can be followed by steam reforming. The heat generated during the partial oxidation can be used to supply the energy necessary for the steam reforming reaction.

Cu–Zn has been investigated as a catalyst in several papers. Either unsupported or supported on -Al2 O3 . Huang and Wang [63] and Alejo et al. [64] have investigated hydrogen generation via partial oxidation of methanol. Cu–ZnO is the active form, which means that to avoid deactivation nitrogen has to be used during purge when the system is shut down. If the catalyst (Fe–Cr-oxide, Cu– Zn-oxide) is oxidized by air during shutdown it can be reduced by hydrogen in situ in order to recreate its initial activity. Care must be taken not to superheat the catalyst during reduction, since the process is exothermic. Excessive heating can cause the copper particles to sinter and a loss of metal surface area will thus occur. One possibility is to use low concentrations of hydrogen in nitrogen (5 –10%) during the reduction process. In a paper presented by Woods et al. [61] the authors claim that after the multi-fuel partial oxidation reformer the product gas has the following composition: ¿ 35% H2 in mixed gas, ¡ 10 ppm CO at steady state, ¡100 ppm CO at transients,¡1 ppm H2 S; ¡1 ppm NH3 , and no particulates. Results on catalytic partial oxidation with methanol as a fuel has been reported by Konig et al. [65], and Pettersson and Sjostrom [66,67]. All three papers present investigations of xed bed reactors. In Table 4, Ahmed et al. [37] have calculated the hydrogen concentration which may be attained when using dierent fuels and then compared these values with experimental results. 5.4. Steam reforming Cx Hy Oz + (2x − z) H2 O → (y=2 + 2x − z)H2 + x CO2 : The biggest advantage with steam reforming is the high concentrations of hydrogen which may be achieved. When using methanol as feedstock (see below) 75 vol% hydrogen is achieved if total conversion is obtained at stoichiometric conditions. This means a high system eciency. When using nickel-based catalysts carbon deposition can, at certain operating conditions, be a problem. Steam reforming is an endothermic process, but an important part of the heat

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consumed is the heat of vaporization (see reactions below). This is valid both for methanol and ethanol [68]. The use of steam as reactant increases the energy demand of the process. However, the addition of oxygen to the steam reforming process speeds up the conversion, due to the occurrence of exothermic oxidation reactions. CH3 OH(l) + H2 O(l) 3H2 (g) + CO2 (g); ◦ kJ
Hr = 131 (1 atm; 298 K) mol  ◦ kJ
Hvap (CH3 OH) = 37:4 ; mol  ◦ kJ
Hvap (H2 O) = 44:0 ; mol

0:12 pH−0:27 rdec = kdec pCH 3 OH 2

(4) When unreacted methanol is present the rate of the water-gas-shift reaction is negligible meaning that the reaction rates above can be used to predict the gas composition in the reformer outlet [74]. As the conversion of methanol is approaching 100% the rate of the water-gas-shift reaction (see below) becomes signi cant and the carbon monoxide concentration in the reformer product gas approaches equilibrium. The value can then be calculated according to Eq. (5) below. The equilibrium constant, Kp , for example, ◦ is 39 at 300 C. CO + H2 O CO2 + H2 ;

kJ
Hr = 348 (1 atm; 298 K) mol   ◦ kJ
Hvap (C2 H5 OH) = 42:3 : mol

Kp =

A background to the subject of catalytic steam reforming can be found in Rostrup–Nielsen [69] and in Twigg [70]. The latter publication contains equilibrium constants at various temperatures. Teagan [71] proposes the use of precious metal catalysts for steam reforming in fuel cell applications instead of the conventional base metal materials. A catalyst study was performed by Amphlett et al. [72]. These authors tested commercial catalysts as well as catalysts prepared in-house. G66B (Girdler), C18HC (Catalysts & Chemicals International), K3-110 (BASF), K33-5 (ICI) and K53-1 (ICI). High-pressure catalytic steam reforming of methanol was investigated by Peppley et al. [73] using a commercial low-temperature shift catalyst, BASF K3-110. Amphlett et al. [74] used copper-based catalysts for steam reforming of methanol. They concluded that the initial activity of a Cu=ZnO=Al2 O3 catalyst for hydrogen production varies with the method of reduction used. The authors found that reduction with CO — or reduction with steam-methanol mixture — resulted in better initial activity than reduction with hydrogen. The authors developed a kinetic model for the commercial Cu=ZnO=Al2 O3 C18HC catalyst (see also Ref. [75]). It is sold by United Catalysts as a low temperature-shift catalyst. Below the reaction rates for steam reforming of methanol (SR) and decomposition of methanol (dec) are shown. T is the temperature in K and p is the partial pressure in bar. 0:62 rSR = kSR pCH pH−0:66 3 OH 2

(mol=s kg);

  −108; 000 kSR = 6:5 × 10 exp (mol bar 0:04 =s kg); 8:314 T

(1)

9

(2)

(3)

  −136; 000 kdec = 3:2 × 1010 exp (mol bar 0:15 =s kg): 8:314 T

C2 H5 OH(l) + 3H2 O(l) 2CO2 (g) + 6H2 (g); ◦

(mol=s kg);

251

pH2 pCO2 : pH2 O pCO

(5)

Copper-based catalysts are some of the strong candidates for this type of reaction when ethanol is used as fuel. Amphlett et al. [76] found CuO=ZnO; CuO=SiO2 ; CuO= Cr 2 O3 and CuO=NiO=SiO2 to be the best catalysts. Compared to methanol steam reforming catalysts deactivation presents a possible problem, due to the higher reaction temperature. CuO=ZnO=Al2 O3 , where the alumina is added for thermal stabilization, is a good catalyst for methanol steam reforming. This support can, however, be a problem in ethanol reforming due to by-product formation. The ◦ methanol process operates at 250–260 C with a CuO=ZnO catalyst. Ethanol requires operating temperatures above ◦ 300 C, which decreases activity. Garca and Laborde [77], Vasudeva et al. [78], Freni et al. [79], and Fishtik et al. [80] have made theoretical thermodynamic analyses of steam reforming of ethanol for hydrogen production. Pettersson and Lindstrom [68] have made a theoretical comparison between ethanol and methanol as fuels for fuel cell cars when using various hydrogen generation options. Cheng et al. [81] performed a thermodynamic analysis of equilibrium products and heat requirements for octane, methanol, ethanol and propane. Lwin et al. [82] made a thermodynamic analysis based on equilibrium concentrations of steam reforming of methanol. Chan and Wang [83] made a thermodynamic analysis of equilibrium products for natural gas fuel processing under simultaneous partial oxidation and steam reforming processes. Peppley et al. [84,85] made an investigation of the reaction network, and developed a kinetic model, for steam reforming of methanol. They point out that although the decomposition reaction is much slower than the steam-reforming reaction it must be included in the kinetic model. The authors show that to be able to explain the complete range of observed product compositions, rate expressions for all three reactions (methanol steam reforming, water-gas shift and methanol decomposition) must be

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included in the kinetic analysis. Asprey et al. [86] have performed kinetic studies of steam reforming of methanol using a temperature-scanning method to increase the rate of data acquisition. The authors claim that their method is much faster than the conventional isothermal methods of reactor operation normally used. In all three studies a commercial Cu=ZnO=Al2 O3 catalyst (BASF K3-110) was used. In a recent investigation Amphlett et al. [87] presented a study of Cu=Zn=Cr catalysts, where preparation procedures and characterization were included. 5.5. Autothermal reforming Cx Hy Oz + r H2 O + s O2 → t H2 + u CO2


Hr ≈ 0:

The reaction can in theory be totally heat balanced (
Hr = 0), but in reality a certain excess of air is added to compensate for heat losses and to obtain a reformer with a rapid response. For methanol, an example of an almost heat balanced reaction can be seen below. 2 CH3 OH(l) + H2 O(l) + 1=2 O2 → 5 H2 + 2 CO2
Hr = −24 kJ=mol In the literature reforming processes including both steam and oxygen are often designated as autothermal. A more correct labelling of the process would be combinatorial reforming, since the reforming process only can be autothermal in one point of operation. 5.6. Catalytic decomposition This method works best with compounds such as methanol and has to be coupled with ecient water-gas shift steps to convert the carbon monoxide. The process is strongly endothermic. An extensive review on hydrogen generation by methanol decomposition can be found in Pettersson and Sjostrom [58]. CH3 OH(l) → 2 H2 + CO


Hr = 128 kJ=mol:

Using this decomposition reaction with hydrocarbons is not suitable due to the risk of coking. 6. Reformer design considerations Kumar and Ahmed [88] suggested that the multi-fuel reformer should be thermally independent from the rest of the fuel cell system, although thermal integration with the fuel cell stack would be desirable for periods of steady-state driving. They also proposed that the fuel reformer should use direct heat transfer partial oxidation reforming because of the rapid start-up and quick response capabilities of this technique. Kumar et al. [89] presented a catalytic partial-oxidation reformer for methanol based on direct heat transfer to the catalyst. Methanol and water were injected at the top of the reactor by using an ultrasonic nozzle, which generated a ne spray of 20 m droplets. A copper-zinc

oxide catalyst both in pellet and honeycomb structures were tested. Pettersson and Sjostrom [66,67] used palladium and platinum catalysts in a xed bed where methanol was partially oxidized to supply heat for the hydrogen generation reactions. A catalytic reactor con guration based on a two-bed system was rst presented by Jenkins [90]. A mixture of methanol and air is fed to the upstream zone of the reactor and permeates initially through the downstream zone where the air=fuel is ignited by a mixture of a 3 wt% copper on silica catalyst and a 5 wt% palladium on silica catalyst. The temperature in the system rises and the partial oxidation of methanol takes place in the copper=silica bed in the upstream zone and hydrogen is produced. Jenkins and Shutt [91] later tested this con guration with methane, propane, n-hexane, petroleum ether, lead-free gasoline and diesel fuel. From cold start-up the reaction took 25 –30 min to reach a steady-state. The temperature within the reactor ◦ was then constant at ca 600 C, and the hydrogen concentration in the reformate stabilised at 41%. This HotSpot TM fuel processor concept was later developed with a focus on methanol operation and it is now based on a modular design. The volume of each cylindrical reactor is 245 cm3 and it produces 750 dm3 H2 =h [23,92]. This is equivalent to a power density of 3 kW=dm3 , assuming that 1 m3 H2 =h is needed to produce 1 kW of power. The authors claim that when vaporized liquid feed and air were introduced into cold reactors, 100% output was achieved in 50 s. To attain this, the air ow was increased by 20% during the start-up period. Schmidt et al. [93], Hohlein et al. [94], and Emonts et al. [14,95] have presented work on methanol reformer development. The rst reformer was based on a Cu=ZnO=Al2 O3 catalyst, where the objective was to achieve a hydrogen yield of 2–3 Nm3 H2 =(h dm3 catalyst) at a methanol conversion of more than 95%. In the following a xed bed reactor has been used where the aim was to reach 12 Nm3 H2 =(h dm3 catalyst). In the latter phase a pellet catalyst from Haldor Topsoe was used which is designated MDK-20. The catalytic burner is incorporated in the reformer to reach maximal compactness and to provide the energy required for the reforming reaction. The burner is a ceramic hollow cylinder to which fuel gas premixed with air is internally supplied. On the outer cylinder surface, a wire mesh coated with precious metal is placed where the catalytic combustion reaction takes place [95]. The burner has a density of 311 kg=m3 and the amount of catalyst on the wire mesh is 20 g=m2 . The thickness of the active catalyst layer is 0:26 mm. Geyer et al. [96] developed a dynamic reformer model. Response times are projected to achieve 50 –100% of the steady-state methanol conversion for two dierent catalyst tube diameters. A system with high thermal capacity can have time constants of the order of several seconds to many minutes. Heat transfer eects limit the rapid response of PEMFC systems. Startup of the reformer is a great disadvantage of a steam-reformed PEMFC system. The overall

L.J. Pettersson, R. Westerholm / International Journal of Hydrogen Energy 26 (2001) 243–264

eciency can be de ned as overall =

Pout top (Pout =sys )top + Estart

(6)

where Pout is the output power, top is the operational time, sys is the steady-state system eciency, and Estart is the startup energy. According to Geyer et al. [96] the startup energy can be estimated according to the following formula: Estart =

WPout Cp
T Fsys LHV

(7)

where W=F is the ratio between amount of catalyst in kg and the ow rate of fuel given in mol=s. Cp is the heat capacity of the catalyst bed and
T is the temperature rise to design conditions. LHV is the lower heating value of the fuel. To avoid pyrolysis and the resulting carbon deposition in the reformer it is important to keep the temperature as low as possible. Hydrocarbons can for example be cracked according to the following scheme: CH4 (g) → C(s) + 2 H2 (g) Dams et al. [97] have described a reactor made of aluminium with a plate and n heat exchanger design. After oxidation of the aluminium surface the thin washcoat layer has been applied directly on the reactor material. A big advantage with the design is that heat transfer characteristics are favourable. This means that the heat needed for steam reforming can be eciently supplied to the catalyst surface where it is required. A problem with their current design is however that rather little catalyst can be coated in the reactor. The monolithic design is appealing and it is probable that several research groups eventually will work in that direction. 6.1. Fixed bed or monolith The literature contains mostly information on experiments with xed bed reactors. Fixed beds are susceptible to vibrations and mechanical attrition. The most important drawback with xed bed reactors is the pressure drop. However, an interesting approach is to use monolithic reactors for hydrogen generation [98]. Monolithic reactors can be either ceramic or metallic. The most common ceramic type is made of cordierite (2 MgO · 5 SiO2 · 2 Al2 O3 ). Metallic monoliths are preferably made of special steels with high aluminium content, e.g. FeCrAlloy. The major advantage with monoliths is the low pressure drop and robustness of the structure, while it is a drawback that there exists no mass transfer between channels. Another disadvantage is the laminar ow in the channels, which also slows down heat transfer. 7. Gas clean-up The clean-up system is an important part of a fuel cell car since it eliminates compounds which are detrimental to the fuel cell. Reforming of gasoline or diesel to produce hydrogen for a PEM fuel cell is analogous to the industrial

253

ammonia synthesis process where hydrogen is produced from either natural gas, naphtha, heavy oil or coal. The product gas is then cleaned in several steps until the resulting gas used for the ammonia synthesis is obtained. This gas contains very low levels of both CO and CO2 , which both are severe poisons for the iron sponge catalyst. Also the sulphur level is very low. This means that there is a considerable volume which is used for the clean-up processes. 7.1. General trends and impurities Dams et al. [99] have demonstrated fuel cell systems running on unleaded gasoline, diesel, reformulated gasoline (RFG) and lique ed petroleum gas (LPG). The process included desulphurization, pre-reforming, reforming, high and low temperature shift and selective oxidation. They have also developed a reactor design of their own utilizing corrugated aluminium plates which were coated with active material. The basic concept is to use methanol reforming catalysts and combustion catalysts on either side of a metal substrate. Primary contaminants are CO, CO2 and CH3 OH. The maximum level of CO that can be tolerated in the fuel cell determines fuel stream pre-treatment requirements. Dusterwald et al. [100] conducted an experimental investigation regarding axial temperature, concentration gradient and catalyst ageing in order to establish the basis for a design of a methanol reformer. They used a CuO=ZnO=Al2 O3 catalyst in a xed bed. They concluded that the higher the methanol conversion the higher was the selectivity for CO formation. According to Amphlett et al. [74] a maximum methanol content of 5000 ppm in the anode gas is permissible without loss in performance. A general trend is that the higher the temperature is, the higher the carbon monoxide concentration will be. The reason for this is that the water-gas shift reaction equilibrium is favoured by low temperatures. CO + H2 O CO2 + H2 : A drawback with membranes is that high pressures are needed to achieve a good separation of CO [72]. According to the authors it is possible to reach CO levels below 30 ppm at temperatures consistent with a methanol reformer at ◦ ◦ approximately 200 C and the fuel cell at about 80 C. Weisbrod and Vanderborgh [101] reported that up to 1 mol% of methanol in the gas phase reduced the current density by less than 10%. According to Weisbrod and Vanderborgh [101] catalytic poisoning may be reduced or eliminated by: • higher operating temperatures, such as in PAFC technology, • development of tolerant electrocatalysts, • air injection as part of the anode feed. Reversible power losses can be caused by methanol, methyl formate, and formaldehyde which caused only reversible

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power losses in a Ballard fuel cell stack [25]. Formic acid caused irreversible power losses at levels of 250 and 5000 ppm. Sulphur, chloride and higher alcohols in the methanol feed to a reformer can aect the performance of Cu=ZnO catalysts. For example hydrogen sulphide may react with copper oxide and zinc oxide to form copper (I) sulphide and zinc sulphide.

the aromatic content of the fuel (HDA). This lowers the risk for carbon precipitation in the catalyst bed. Amphlett et al. [103] used a steady-state electrochemical fuel cell model to perform a process simulation of an ethanol-steam reforming system. They concluded that the gas clean-up system aects the overall heat balance signi cantly.

7.2. Desulphurization

7.3. High-temperature and low-temperature shift steps

Depending on the type of fuel used the desulphurization unit has to be adapted to the level of sulphur concentration. High-sulphur fuels, such as naval distillates or NATO fuels, require a high volume desulphurization unit. The most common method for gas-phase desulphurization is to use a xed bed of high-surface area ZnO. The oxide will be converted into zinc sulphide according to the following reaction: ZnO(s) + H2 S(g) → ZnS(s) + H2 O(g): The zinc sulphide formed must be discarded or regenerated. The total operating time of the sulphur trap depends primarily on the sulphur content of the fuel. According to King [102], 2–3 kg of ZnO should be sucient for one year of operation. Normally, sulphur in the fuel is converted to hydrogen sulphide in the fuel processor, since an excess of hydrogen is present there. It is also possible that carbonyl sulphide, COS, will be formed. To use zinc oxide is the most common way of dealing with hydrogen sulphide emissions in the reformer. Zinc oxide is converted to zinc sulphide which ultimately has to be disposed. Another possibility is to use liquid fuel desulphurization onboard the vehicle. In a joint eort by McDermott Technology and Catalytica Advanced Technologies Flynn et al. [40] are aiming to demonstrate the desulphurization of gasoline containing 30 – 40 ppm sulphur. The desulphurized gasoline should contain maximally 1–2 ppm sulphur. The authors claim that they will achieve a signi cant reduction in size and weight compared to in the gas-phase treatment. They will evaluate several approaches based on both adsorption and chemical reaction. Their system approach is based on a stand-alone unit which may be removed if methanol is used or alternatively reduced in size if low-sulphur gasoline is utilized. Flynn et al. do not expect that the liquid desulphurizer will remove 100% of the sulphur in the fuel. Therefore, it may be necessary to include a ZnO bed between the high- and low-temperature shift steps. The authors estimate that assuming a concentration of 5 ppm H2 S in the reformate gas and an annual driving of 15,000 mile, approximately 225 g of ZnO would be required. Logistic fuels, diesel fuel (DF-2) and jet fuel (JP-8) have been tested [45]. Sulphur may be present up to 0.5 wt%. The desulphurization step is normally based on hydrotreatment technology to convert sulphur compounds to hydrogen sulphide. The H2 S formed is then removed by adsorption in a ZnO bed. In addition, the hydrotreatment step also lowers

CO + H2 O CO2 + H2 : This equilibrium reaction is exothermic and is independent of pressure. Since it is an exothermic reaction the heat evolved will force the equilibrium in the direction of the left and, hence, CO formation will increase. After the high-temperature step with intermediate cooling a low-temperature shift step is normally installed. The high-temperature shift step normally uses an iron and ◦ chromium oxide catalyst which is active in the 330 –530 C temperature interval [62]. Normally this step is operated at ◦ 350 – 450 C. The temperature rise in the bed is dependent on the inlet CO concentration. The low-temperature shift is normally equipped with a catalyst consisting of copper and zinc oxide supported on alumina. This catalyst is extremely sensible to sulphur and chlorine and often a desulphurization step is included to prevent catalyst deactivation by poisoning (see above). During reduction, start-up and shutdown procedures, the catalyst requires special care due to the risk of oxidation of copper and its subsequent deactivation. The CO concentration after these two steps should maximally be 10 ppm. Since this level of CO cannot be reached by these methods a CO clean-up step must be added (see below). Colsman [104] has performed an experimental investigation of a CO conversion system using a CuO –ZnO=Al2 O3 catalyst in a tubular reactor. The catalyst was prereduced in a gas stream containing 2% H2 in N2 . CuO + H2 → Cu + H2 O: The results from the CO conversion experiments show that the CO concentration was lowered from 2 to 0.7% at a tem◦ perature of 180 C and a space velocity of 300 h−1 . The author calculated that for a 60 kWe fuel cell system 60 dm3 catalyst is required to lower the CO concentration in the reformer gas from 2 to 1 vol%. This calculation is based on a hydrogen ow of 40; 000 Ndm3 =h and a fuel cell eciency of 50%. Colsman has supposed a hydrogen content of 66.7 vol%, which leads to a total out ow from the reformer of 60; 000 Ndm3 =h. A volumetric hourly space velocity of 1000 h−1 is then attained. 7.4. CO clean-up If more CO-tolerant Pt-anode catalysts are developed the requirements on the CO clean-up system will decrease.

L.J. Pettersson, R. Westerholm / International Journal of Hydrogen Energy 26 (2001) 243–264

Present technology is normally referred to a limit of 10 ppm in the reformate gas. A problem in the fuel cell stack is the competitive adsorption of H2 and CO on the platinum sites. CO is preferentially adsorbed on the platinum electrocatalyst in the PEFC, thus blocking the access of hydrogen to the surface of the catalyst and resulting in the degradation of the fuel cell performance. Common solutions are catalytic preferential oxidation, anode air bleed or a combination of these two. These methods may reduce carbon monoxide concentrations to trace levels, but their use in a dynamically varying system is problematic. Various methods for reducing the carbon monoxide content in the reformer product gas have been proposed. CO clean-up can be performed by either chemical or physical techniques. We will begin by describing some of the chemical methods available. 7.4.1. Selective oxidation Selective oxidation of CO (sometimes called preferential oxidation) can be achieved by using a suitable catalyst at low temperatures. A small amount of oxygen is added to the fuel gas stream. CO + 12 O2 → CO2 ; H2 + 12 O2 → H2 O: Both of these reactions are highly exothermic with reaction enthalpies of 283 kJ=mol and 242 kJ=mol, respectively. Hence, it is crucial to be able to remove heat from the reactor and the temperature can be kept at a low level by using a multi-bed system with intermediate cooling. Platinum is a catalyst which is active for CO oxidation. The adsorption of CO on Pt is also strong and since CO binds stronger to the Pt surface than hydrogen does this can be utilized for selective oxidation. The selectivity for CO oxidation compared to H2 oxidation is reduced at higher temperatures. However, a loss of hydrogen will occur and, as a consequence a drop in system eciency. It is also necessary to keep track of the inlet concentration of CO. If the concentration of CO is so high that full surface coverage of CO is attained the losses of hydrogen are rather small. When most of the CO in the system is consumed the hydrogen losses will increase. Thus, a system like this is rather complicated. A catalyst with high surface area and low platinum loading on -alumina is commonly used. This is because of its low tendency for hot spots. Apart from Pt, precious metals such as Pd, Rh, or Ru are strong candidates as catalysts. Furthermore, Cu=Co, Cu=Cr, Ni=Co=Fe and Ag, Cr, Fe and Mn have also been proposed [105]. Not only the hydrogen in the reformer gas but also the CO2 adsorption will aect the CO oxidation. The authors claim that when 100% conversion of CO is reached approx. 20% of the hydrogen is lost. Okada et al. [106] performed laboratory tests with various alumina-supported precious metal catalysts (Pt, Pd, Rh and Ru). The ruthenium catalyst was active over a wide tem-

255

perature range, but the hydrogen losses were higher above ◦ 200 C compared with the other catalysts. Amphlett et al. [107] present the following kinetic expression for CO oxidation over Pt on alumina catalysts, where rCO is the reaction rate for CO oxidation, CCO is the concentration of CO and T is the temperature in K. rCO = −kCO CCO (mol=s kg catalyst);

(8)

where

  1000 kCO = 0:0226 exp − [m3 =s kg catalyst]: T

(9)

If isothermal plug ow is assumed this rate expression can be used directly in the design equation for a plug ow reactor which gives the following equation for the required catalyst bed weight: dX (10) = −rCO dW where FCO; 0 is the molar ow of CO entering the reactor, X is the conversion of CO and W is the catalyst weight [108]. If the equation is integrated over the total catalyst weight, while assuming a negligible volume change during the reaction, we obtain FCO; 0

CCO = CCO; 0 (1 − XCO ); W =−

FCO; 0 ln(1 − XCO )(kg catalyst): CCO; 0 kCO

(11) (12)

Since the conversion of CO by necessity must be very high, the logarithmic term will have a high negative value. Three dierent options for decreasing the catalyst weight are available [107]: (1) To reduce the CO concentration in the reformer outlet gas (more ecient water–gas shift steps). (2) To increase the concentration (i.e., the partial pressure) of CO, without increasing the molar ow. (3) Increase the rate constant, kCO , by increasing the temperature or replacing the catalyst by a more active one. However, when using this method there will always be a loss of hydrogen. To control the CO clean-up reactor is also dicult, since there is no simple, reasonably priced and compact CO sensor available. 7.4.2. Methanation Another approach is to use the methanation procedure according to the reaction below: CO + 3 H2 CH4 + H2 O: This means that 3 mol of hydrogen are lost for every mole of CO treated. Methane is however a powerful greenhouse gas which it is undesirable to emit. The system is however fairly simple, since it is only necessary to control the temperature. A disadvantage is the hydrogen penalty and the emissions of methane. Methane will act as an inert, or dilutant, and will thus not react in the fuel cell. Baker et al. [109] used

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Fig. 6. Simpli ed scheme of a fuel cell system with fuel reforming (adopted from [113]). Water recovery is not included.

a ruthenium or a rhodium on an alumina support catalyst to selectively hydrogenate carbon monoxide in a gas mixture containing high concentrations of carbon dioxide, where the reaction below had to be avoided. The temperature in the ◦ catalytic bed was kept at 100 –220 C. CO2 + 4 H2 CH4 + 2 H2 O: Oshiro et al. [110] evaluated three commercially available Ni=Al2 O3 catalysts. They concluded that the 10 ppm target value of CO was possible to achieve at steady-state condi◦ tions at 250 C and a space velocity of 300 h−1 . Cuzens et al. [111] claim that the transient emissions of CO will be below 20 ppm with a system based on methanation. 7.4.3. Other methods (internal preferential oxidation, regenerative adsorption, membrane technology) An alternative method to those given above is internal preferential oxidation. In this case a small amount of oxygen is added to the reformate gas at the inlet of the anode. The major disadvantage of this method is that it appears to cause sintering of the electrocatalyst of the anode and thereby leads to a deterioration of the fuel cell performance [107]. Examples of physical puri cation methods are pressure swing adsorption (PSA), high temperature diusion through a metal membrane, low temperature diusion through a polymer membrane and solvent absorption of CO and CO2 [107]. Other possible ways of removing CO is by adsorption or by use of membranes [99,104]. Lee et al. [112] developed a sorption process based on the reversible reactions of complex-forming and dissociation reactions of CO with Cu(I). They studied two types of porous, high-surface-area, solid supports on which CuCl was dispersed or reacted. The system was designed to treat reformer gas of 0.5 –1% CO after the water–gas shift step. In Fig. 6 a scheme of the reformer system is presented where the most important compounds in each section are in-

dicated. S; Cx Hy ; NH3 ; Na+ ; Ca2+ ; Fe2+ and Fe3+ are irreversible accumulation poisons for the fuel cell. Reversible accumulation occurs with CO. To avoid problems it is very important that the correct temperatures are attained before the fuel is introduced into the reformer. In each section (reformer, fuel cell, afterburner) the residence time is approximately 1 s, which amounts to a total residence time of approximately 3 to 4 s. A future target for the complete system would be about 0.5 s.

8. Emissions A complete life cycle emission analysis is not included in the present report. The review is focused on the emissions generated during the following operating conditions: Start-up, transient operation, steady-state operation and shutdown. As may be seen in Fig. 6 above, the emissions generated by a PEM fuel cell originates in the reformer and the afterburner device. This is why they may be seen as individually contributing to the overall emissions. 8.1. Start-up emissions Depending on the type of fuel used and ambient temperatures several operating parameters pertaining to the conditions for the fuel cell need to be optimized for the reduction of emissions. It is assumed that, depending on shorter start-up times, the energy input demand to the fuel cell reformer will increase depending on the fuel to be used in the PEM fuel cell. If a more chemically complex fuel is to be used, the result will most likely be that higher temperatures will be needed in the fuel reformer compared to with a chemically less complex fuel. A means to partly decrease the energy need in the fuel reformer is to start the PEM fuel cell with pure hydrogen and then switch to the main fuel, when the required temperature is reached in the fuel

L.J. Pettersson, R. Westerholm / International Journal of Hydrogen Energy 26 (2001) 243–264

reformer. It is also of importance to optimize the reformer catalyst with regards to the main fuel to be used in the fuel reformer. Several problems however need to be solved before a PEM fuel cell meets the criteria for automotive multi-fuel operation. A problem pinpointed by Kalhammer et al. [12] is to develop a rapid start-up mode which is considered to be very dicult and a requirement not yet met. 8.2. Transient emissions In the general transient operation of the PEM fuel cell it will generate increased emissions compared to in the steady-state operation. This is due to the fact that the energy needed varies with time. To reduce transient emissions it is necessary to develop more sophisticated control systems which are adapted to the fuel being used. At present there is a shortage of emission data available from transient operation of PEM fuel cells. In a recent report by Kalhammer et al. [12] it is stated that the development of fuel processors is on the level of proving the principle. 8.3. Steady-state emissions At steady-state operation there is a constant energy output from the PEM fuel cell. This is the simplest operating mode which is also the one which is easiest to control. In the literature there is relatively more emission data presented from this operating mode than the others (see below). 8.4. Shutdown emissions It is important that, during the shutdown process, the fuel cell is prepared for the start-up phase to come. It is proposed that during the shutdown period the reformer catalyst must be purged with nitrogen to prevent the catalyst from being oxidized and, hence, thereby become deactivated. This is especially important if Cu=ZnO catalysts are used. From our site visits we conclude that only limited work has been performed regarding a general chemical characterization of emissions from PEM fuel cells. A potential problem discussed regarding the use of diesel or gasoline in fuel cell applications was the possibility for the formation of carbon from heavier compounds in the fuel. 9. Sampling and analysis of product gas and emissions from PEM fuel cells Within the PEM fuel cell two types of emissions may be expected from the PEM fuel cell concept; product gas and afterburner emissions. It is important to chemically characterize the reformer product gas resulting from dierent operating conditions. In this way it may be possible to optimise the system with respect to eciency and emissions. Wasmus et al. [114] uses a mass spectrometer for the on-line

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analysis of by-products. These were however generated by methanol oxidation in a direct methanol fuel cell. In general, if transient operating conditions are to be studied, there is a demand for short analysis time, preferably from on-line real time determinations. Another analysis method used at some of the sites visited and in the literature [51] is gas chromatography (GC). Only steady-state operation may, however, be studied by means of such equipment due to the relative longer analysis time required. Compounds determined in product gas in a study by Ledje-Hey et al. [51] using this analytical set-up were hydrogen, methane, ethyne, ethane, propane, nitrogen and oxygen. Steady-state emission analysis from a proton exchange membrane fuel cell operated on California Phase II gasoline was performed by Mitchell et al. [115] with a GC equipped with a Thermal Conductivity Detector (TCD). In this case the compounds determined were hydrogen, nitrogen, methane, ethyne, ethene, carbon dioxide and carbon monoxide. Additional analyses of carbon monoxide and carbon dioxide by Non-Dispersive Infra-Red (NDIR) analyser, nitrogen oxides by chemiluminescence detector (CLD), hydrocarbons by Flame Ionization Detector (FID) were also reported [116]. Within the automobile industry there has been an increased need for shorter analysis time in the study of exhaust emissions from transient engine operation at a relative increased time resolution. A measurement technique meeting this demand is based on mass spectrometry combined with dierent ion source energies (REFX). A possible method is to use such a sample set-up and adapt it to the characterization of emissions from PEM fuel cells. Relative short time resolutions (¡ 1 Hz) may be obtained depending on the total number of compounds studied. Another advantage is the possible use of the mass spectrometer for the identi cation of unknown compounds potentially present in the emissions.

10. Emission tests on vehicles equipped with fuel cells or laboratory tests with fuel cells A publication by Wimmer et al. [117] present transient and steady-state emission test results from a hybrid fuel cell bus. The bus was run on a methanol=water fuel mixture and a 50 kW phosphoric acid fuel cell equipped with an extra 100 kW battery for additional power whenever this is needed in transient operation. A comparison between the dierent fuels investigated in the report is summarized in Table 5. This was conducted among buses operating in the transient central business district driving cycle [118]. The authors explain the relatively large emissions of HC and CO from the phosphoric acid fuel cell by variations in neat methanol and air ow in the reformer burner. They suggest that this problem will be solved in the next generation of fuel cell powered buses.

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Table 5 Average emission factors of hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx ) and particulates (PM) [117]

1996 New Flyer Diesel 1996 New Flyer natural gas 1992 TMS ethanol 1992 Flexible methanol Fuel Cell

Buses tested

HC (g=mile)

CO (g/mile)

NOx (g=mile)

PM (g=mile)

8 10 5 5 1

0.13 15.8 8.9 37.5 0.23

4.9 9 37.1 25.1 7.32

30.1 20.8 13.4 11.6 0.01

0.28 0.02 0.63 0.39 ¡ 0:01

Emission tests have also been performed on a fuel cell running on gasoline (California phase II reformulated gasoline) as fuel [115]. Only steady-state operation emissions at 25 kW fuel cell input were included. However, assuming a Federal Urban Driving cycle with 12.5 kW average power the tailpipe emission factors reported were as follows: carbon monoxide 0.013 g=mile, hydrocarbons 0.017 g=mile and NOx 0.003 g=mile. Start-up emissions were not reported in this article. The start-up time reported is 10 min. This needs to be shortened in order to reduce emissions and maximize the system eciency of the driving cycle. It was stated that in addition to gasoline the fuel cell has been operated on ethanol, methanol and natural gas as fuels for a time period in excess of 600 h. 11. Problem identiÿcation and recommendations for future research Fuel cells for automotive applications have potential bene ts such as reduced exhaust emissions, reduced noise emissions and improved energy eciency compared to combustion engines. The Fuel Cell Technical Advisory Panel stated in mid-1998 [12] that it focused on the proton exchange membrane cell since this potentially has the greatest bene ts for automotive applications. These bene ts include engine performance, durability and relatively low cost. Moreover, the panel believed that hydrogen will not become a fuel available to private automobiles in the near future. By using petroleum-based or alternative fuels in fuel cells a breakthrough for automotive applications would occur. Based on the present literature review and our site visits with discussions with prominent technology developers we have drawn the conclusion that there are several important issues that still need to be addressed. In the following discussion we have focused mainly on the reformer. For a full estimate of the environmental impact of a fuel cell vehicle a complete life cycle assessment would be needed. This is, however, not within the scope of the present paper. There is a potential for developing ecient and environmentally favourable fuel cell systems for automotive applications. The obvious advantage with multi-fuel systems is the fuel independency, since an available fuel infra-structure is used. However, the multi-fuel approach is questionable

from an eciency point-of-view and it adds complexity to the reformer system. The motor fuels presently available on the market, such as gasoline with high aromatic content, diesel and various high-sulphur fuels, are not suitable for onboard hydrogen generation in passenger cars. On the other hand, if eciency is not an important issue and if desulphurization, vaporization and reactor design are substantially improved, the situation can be changed. There is still a substantial amount of work left to be done before an eective and reliable system can be put on the market. There is high-quality research and development activities going on in several laboratories and some of them have come up with ingenious solutions for dicult problems. A key question is the various catalysts being used in the dierent reactor modules. Another important issue is the reactor design where an ecient heat transfer to the catalytic surface is crucial for fast start-up and transient operation. To reduce costs the dierent parts must also be suited for mass production. An interesting aspect of the reformer is whether designer fuels should be developed or not. Re nery processes to increase aromatic content, such as catalytic reforming or platforming, are not bene cial for a hydrogen generation unit in a fuel cell car. This calls for another tuning of the re nery, where certain parts of the crude oil are undesirable. More hydrotreating steps have to be added in the re neries, which means that hydrogen consumption in the re neries will increase. Designer fuels will be more expensive than the types of motor fuels which are on the market today. This issue is one of the topics for future research (see below). To stimulate production of designed fuels a tax incentive is expected to be used. The question of the optimal fuel for fuel cell cars is an interesting one. Strictly from the automobile eciency point-of-view it seems that methanol would presently be the best choice. An important aspect to consider here is the relative ease with which the fuel may be utilized to produce hydrogen without giving rise to by-products, such as soot deposition at low temperatures. An advantage is that methanol can be made from abundant resources of natural gas as well as from renewable energy sources. Gasoline as currently produced from crude oil is not sustainable, either from a pollution=greenhouse gas perspective or from a resource perspective [119].

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To what extent multi-fuel reformer cars will one day come to be used on a large scale is dependent on many dierent factors. The automobile market is very competitive and it is not likely that fuel cell cars will be introduced on a short-term basis if legislative measures are not taken by authorities and governments to promote such a step. Subsidising low-emitting vehicles may in the short term be a way of introducing fuel cell cars on the market. For this a strong governmental commitment must however be at hand which can give the market a push in the right direction. Only environmentally conscious consumers will not suce. The move has to be followed by economic incentives on a relatively long-term basis to increase market share and to create a persistent market. The following listing of important research issues are based strictly on the personal ndings of the authors and do not necessarily re ect the opinions of either the authorities or the companies=institutes visited. What is listed below are important issues for the development of multi-fuel reformer systems. 11.1. Catalyst development for fuel processing The catalyst must be active, durable, resistant to vibrations, and to attrition by ow and variations in temperature and pressure. It must also be resistant to poisoning. The catalyst should convert the fuel into hydrogen without creating devastating amounts of by-products. Since the fuel cell is very sensitive to poisoning by carbon monoxide the catalyst should not produce high amounts of this compound. A more ecient reformer catalyst means that the following shift and gas clean-up steps can be minimized. Here, price is also an important issue. In this context it is important to point out that more active water gas shift catalysts must be developed. Conventional shift catalysts will not meet volume requirements. 11.2. Catalyst deactivation studies Some of the steam reforming catalysts used are poisoned by oxygen, which is strongly adsorbed on the catalyst surface. This means that care must be taken to keep air out of the system. Extensive catalyst deactivation studies should be undertaken to address this and other problems associated with catalyst malfunctioning due to poisoning. If the catalyst is deactivated by sintering, poisoning or fouling, such as carbon precipitation, the activity can decrease to critical values [120]. This means that the fuel cell will not operate properly and vehicle performance will drop. When using hydrocarbon fuels such as diesel or gasoline there is always a great risk of carbon precipitation. Some types of reformer catalysts are sensible to air and their metallic forms will be oxidized to their less active oxide forms. This can be detrimental for the performance of the reformer system. The shift catalysts are highly sensible to

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temperature uctuations and, especially, the copper-based low-temperature shift catalysts may be destroyed. 11.3. Reactor development The design of the reactor is crucial for the performance of the fuel cell car. The reformer must be constructed by light-weight components and its volume must be minimized to t in an automotive application. The use of monolithic type reactors should be considered as an alternative to the common xed bed reactors. One big advantage with monolithic reactors is their low pressure drop. Fast warm-up is another requirement which has to be taken into account when designing automotive reformers. The residence time in the reactor must be minimized in order to achieve a quick response in the system. When using endothermic reactions for hydrogen generation it is thus crucial to transport the heat to the catalytic surface with as small losses as possible. This includes reactor development where both the design itself and the materials used are important. The heat capacity of the reactor must also be minimized in order to achieve a rapid response system. Materials with high heat conductivity are then advantageous. Especially in the USA there is an urge to develop a fuel cell system in which dierent types of fuels may be used. This calls for a large eort to develop catalytic systems that can convert fuels into hydrogen from a range of dierent feedstocks. 11.4. Designed fuels Multi-fuel reformers are designed to operate on almost any motor fuel which is available on the market. Presently, both gasoline and diesel are very complex fuels, which both contain compounds such as aromatics that are hard to reform. According to Chrysler, in the future “fuels will be designed, not re ned” [121]. From the present study it seems obvious that designed fuels need to be chemically well-de ned. Some important fuel parameters for fuel cell applications (in random order) are: Sulphur content, boiling point interval, aromatic content, hydrogen-to-carbon ratio, carbon chain length, energy content, ammability, ease of storage and handling. Furthermore, cost and availability are two very important issues. 11.5. Catalyst development for CO clean-up Selectivity is the keyword for CO abatement. All available systems suer from a hydrogen penalty, which means a decrease in eciency. The development of catalysts goes hand in hand with the development of reactors with an ecient heat transfer. When using platinum catalysts it is important to perform the oxidation of carbon monoxide at low temperatures to avoid simultaneous excessive losses of hydrogen.

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11.6. Emission characterization Our study has shown that there are still data missing before we arrive at a more complete and unbiased chemical characterization both of compounds in the product gas from the reformer and in the exhaust from a complete fuel cell system. A possible explanation to this lack of data is that the development of the reformer=fuel cell has not yet reached beyond the prototype stage. Especially interesting operating points are start-up, transients and shutdown. Another important issue is cold-start emissions at low (sub-zero) ambient temperatures, which in winter is common for example in Sweden and in several other countries in the Northern hemisphere. Consequently, we propose that a study should be initiated where both the product gas from the reformer and the emissions from the entire fuel cell system are characterized at various temperatures. 11.7. Vehicular adaptation of reformer and gas clean-up parts The price of the reformer, including all extra devices, is crucial for the success of the fuel cell car. The catalyst is of course here an important factor, as well as the various reformer and heat exchanger parts. Already at the drawing board developers need to think in terms of actual vehicle applications. Some chemical engineering solutions may not be suitable for mobile applications, and vibrational and mechanical shock considerations may often be neglected in the chemical industry. Moreover, the reactor design has to be rustic and the materials should be reasonably priced and, furthermore, have a potential availability on the market even if demand increases considerably. 11.8. Start-up (cold start) improvement The start-up performance is very important when fuel cells are used in automobiles. The customer will not accept start-up times considerably higher than for diesel-fueled cars. At low ambient winter temperatures this is critical in the Nordic countries of Europe, in Canada and in the Northern states of the USA. Consequently, care must be taken to ensure that the parts used are easy to heat up (low heat capacity) and that the heat conductivity is high. 11.9. Improvement of transient response characteristics of reformers The reformer system must be able to withstand transient periods when hydrogen production either has to be increased (acceleration) or decreased (deceleration). Such periods are also very important to consider regarding the performance of the CO clean-up system. The fuel cell may withstand short spikes of high CO concentrations, but this should not be allowed to exceed a couple of hundred ppm. Another way of tackling the problem is to develop more CO-resistant

fuel cell catalysts and important work is currently going on in this direction. That is however beyond the scope of this paper. 11.10. System design and energy integration optimisation When using steam reforming there is a need for an ecient transfer of heat needed for the endothermic reaction. Normally the energy will be taken from the anode o-gas which contains unconverted hydrogen. If the reformer works in the partial oxidation mode the heat must be transported away from the reactor to ensure that no superheating takes place. Hot spots can severely deactivate the reformer catalyst. Fuel vaporization is another process which consumes energy. Eorts are under way to model fuel cell systems in order to examine the in uence of design parameters on system eciency and performance, as well as the size of the individual components. Kumar et al. [122] developed a comprehensive model of a PEMFC power system for automotive propulsion called GCtool. The model can be used to perform parameter sweeps, constrained optimizations, and time integrations for dynamic simulations. Mousavi [123] recently presented a study where a fuel cell system including a methanol reformer was modelled. By using this model, the behaviour of the fuel cell system and parameters such as fuel consumption and system eciency can be studied.

12. Conclusions At present it does not seem feasible to develop an ecient and reliable multi-fuel reformer for automotive applications, i.e. a reformer where all types of fuels ranging from natural gas to heavy diesel fuels can be used. The potential for developing a safe and reliable system is however considerably higher if dedicated fuel reformers are to be used. Furthermore, if petroleum-derived fuels are used, these should be designed. It is still uncertain how these are to be designed, but it may be predicted that these fuels should not be chemically complex. It is likely that standard diesel fuels will mostly be of interest in military propulsion applications. Based on the site visit discussions we conclude that the cost for a fuel cell vehicle equipped with a multi-fuel reformer is very high given the level of technology available today. A rough estimate yields a cost exceeding 100,000$ for a fuel cell vehicle. To reduce the very high costs of the fuel cell and the reformer system a major research eort within an international co-operative program is necessary. Such a program should be scienti cally interdisciplinary involving the petroleum industry, automotive industry, catalyst industry, research institutes, and universities, as well as governmental and legislative agencies.

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In accordance with the present literature survey and the site visit discussions we conducted there are relatively low emissions associated with fuel cell engines compared to internal combustion engines. However, the major research work on reformers=fuel cells has been made during steady-state operation. Emissions during start-up, shutdown and transient operation are basically unknown and must thus be investigated in more detail. Due to the expected low emissions only a limited number of investigations have been devoted to environmental and health aspects associated with the use of fuel cell vehicles. Acknowledgements The authors would like to acknowledge the following persons and companies=institutes for fruitful discussions and rewarding site visits: David King at Catalytica Advanced Technologies, Richard Woods and John Cuzens at Hydrogen Burner Technology Inc., Alfred Meyer and Joseph King at International Fuel Cells, Peter Teagan at Arthur D. Little Inc., Romesh Kumar and Shabir Ahmed at Argonne National Laboratory, Robert Dams, Stephen Moore and Paul Hayter at Wellman CJB. The authors would also like to acknowledge Per Ekdunge, Volvo Technological Development Corporation, for valuable information concerning automotive fuel cells. Financial support given by the Swedish Transport and Communications Research Board (KFB) is gratefully acknowledged.

References [1] Stenback E. Stockholm transport. Personal communication, 26 January 1999. [2] Biedermann P, Dusterwald H, Hohlein B, Stimming U, Birkle S, Kircher R, Nolcher C, Voigt H, Drenckhahn W. Energy conversion chains and legally restricted emissions for road trac in Germany. Proceedings of 7th ISATA Conference, 1994. p. 2–15. [3] Lindstrom O. That incredible fuel cell. CHEMTECH 1988;17:490. [4] Nowell GP. The promise of methanol fuel cell vehicles. American Methanol Institute, Washington, DC, 1998. [5] Amphlett JC, Mann RF, Peppley BA. Predicted emissions from a methanol-fueled “electrochemical automobile engine” based on a PEM fuel cell. SAE Paper 952374, 1995. [6] Panik F. Fuel cells for vehicle applications in cars — bringing the future closer. J Power Sources 1998;71:36 – 8. [7] Cooper R, Feasey G. Summary of dbb’s MeOH fuel cell engine development activities. Proceedings of Fuel Cell Reformer Conference, 1998, South Coast Air Quality Management District, Diamond Bar, CA, 1999. p. 116 –25. [8] Podolski WF. Reformer technology challenges. Proceedings of Fuel Cell Reformer Conference, 1998, South Coast Air Quality Management District, Diamond Bar, CA, 1999. p. 70 – 6.

261

[9] Kost R. DOE perspective on fuel cells in transportation. U.S. Department of Energy, Technical Report, 1996. p. 198–232. [10] Maruo K. Strategic alliances for the development of fuel cell vehicles. KFB — Swedish Transport and Communications Research Board, Stockholm. KFB Report No 1998:37, 1998. [11] Dircks K. Recent advances in fuel cells for transportation applications. SAE Paper 1999-01-0534, 1999. [12] Kalhammer FR, Prokopius PR, Roan V, Voecks, GE. Status and prospects of fuel cells as automobile engines. A Report of the Fuel Cell Technical Advisory Panel. State of California Air Resources Board, Sacramento, California, USA, 1998. [13] Chalk SG, Milliken J, Miller JF, Venkateswaran SR. The US Department of Energy-investing in clean transport. J Power Sources 1998;71:26 –35. [14] Emonts B, Bogild Hansen J, Schmidt H, Grube T, Hohlein B, Peters R, Tschauder A. Fuel cell drive system with hydrogen generation in test. Project presentation, Forschungszentrum Julich, Institute for Materials and Processes in Energy Systems (IWV-3), Germany, 1999. [15] Grove WR. Philos. Mag. 1842;21:417–20. [16] Kordesch KV, de Oliveira JCT. Fuel cells. In: Elvers B, Hawkins S, Ravenscroft M, Rounsaville JF, Schulz G, editors. Ullmann’s encyclopedia of industrial chemistry, vol. A12. Weinheim, Germany: VCH Verlagsgesellschaft, 1989. p. 55 – 83. [17] Dams RAJ, Hayter PR, Moore SC. Wellman CJB Limited. Personal communication, September 23, 1999. [18] Thomas S, Zalbowitz M. Fuel cells — Green Power. Los Alamos National Laboratory, New Mexico, Report No LA-UR-99-3231, 1999. [19] Schmidt VM, Stimming U. Fuel cell systems for vehicle applications. In: Barsukov V, Beck F, editors. New promising electrochemical systems for rechargeable batteries. Deventer, The Netherlands: Kluwer Academic Publishers, 1996. p. 233– 46. [20] Amphlett JC, Mann RF, Peppley BA. On board hydrogen puri cation for steam reformer=PEM fuel cell vehicle power plants. In: Hydrogen energy progress X, Proceedings of the World Hydrogen Energy Conference, vol. 3, 1994. p. 1681–90. [21] Prigent M. On board hydrogen generation for fuel cell powered electric cars. A review of various available techniques. Rev Ins Francais Petrole 1997;52:349–59. [22] Blackburn B. Addressing California’s infrastructure needs for fuel cell vehicles. Proceedings of Fuel Cell Reformer Conference, 1998, South Coast Air Quality Management District, Diamond Bar, CA, 1999. p. 159 – 62. [23] Edwards N, Ellis SR, Frost JC, Golunski SE, van Keulen ANJ, Lindewald NG, Reinkingh JG. On-board hydrogen generation for transport applications: the HotSpot TM methanol processor. J Power Sources 1998;71:123– 8. [24] Ahmed S, Doshi R, Lee SHD, Kumar R, Krumpelt M. Partial oxidation reformer development for fuel cell vehicles. Proceedings of 32nd Intersociety Energy Conversion Engineering Conference, 1997. p. 843– 6. [25] Amphlett JC, Klassen RD, Mann RF, Peppley BA. Methanol, diesel oil and ethanol as liquid sources of hydrogen for PEM fuel cells. Proceedings of 28th Intersociety Energy Conversion Engineering Conference, vol. 1, 1993. p. 1221–26. [26] Prater KB. Polymer electrolyte fuel cells: a review of recent developments. J Power Sources 1994;51:129– 44.

262

L.J. Pettersson, R. Westerholm / International Journal of Hydrogen Energy 26 (2001) 243–264

[27] Moser T, Rao J, Grecksch E. PEM fuel cells: technology and application in electrically driven vehicles. Proceedings of 13th International Electrical Vehicle Symposium, 1996. p. 680 –5. [28] Block G, Mitchell WL. Evaluation of hydrated ethanol in an advanced fuel processor for fuel cell vehicles. SAE Paper 971646, 1997. [29] Teagan WP, Bentley J, Barnett B. Cost reductions of fuel cells for transport applications: fuel processing options. J Power Sources 1998;71:80–5. [30] Luengo CA, Ciampi G, Cencig MO, Steckelberg C, Laborde MA. A novel catalyst system for ethanol gasi cation. Int J Hydrogen Energy 1992;17:677– 81. [31] Lindstrom B, Pettersson LJ. A study of ethanol and methanol as a fuel for onboard hydrogen generation by steam reforming on copper-based catalysts. Proceedings of XIII International Symposium on Alcohol Fuels, Stockholm, Sweden, July 3–7, 2000. [32] Carpenter I, Edwards N, Ellis S, Frost J, Golunski S, van Keulen N, Petch M, Pignon J, Reinkingh J. On-board hydrogen generation for PEM fuel cells in automotive applications. SAE Paper 1999-01-1320, 1999. [33] Ahmed S, Wilkenhoener R, Lee SHD, Carter JD, Kumar R, Krumpelt M. Fuel Processing for fuel cell powered vehicles. Presented at the 195th Meeting of the Electrochemical Society, Seattle, WA, May 2– 6, 1999. [34] Meyer AP, King JM, Kelly D. Progress in development and application of fuel cell power plants for automobiles and buses. SAE Paper 1999-01-0533, 1999. [35] Birdsell SA, Willms RS, Dye RC. Pure hydrogen production from octane, ethanol, methanol, and methane reforming using a palladium membrane reactor. Proceedings of 32nd Intersociety Energy Conversion Engineering Conference, 1997. p. 1942– 6. [36] Recupero V, Pino L, Di Leonardo R, Lagana M, Maggio G. Hydrogen generator, via catalytic partial oxidation of methane for fuel cells. J Power Sources 1998;71:208–14. [37] Ahmed S, Doshi R, Kumar R, Krumpelt M. Gasoline to hydrogen — a new route for fuel cells. Electric & Hybrid Vehicle Technology ’97, 1997. p. 77– 80. [38] Ahmed S, Krumpelt M, Kumar R, Lee SHD, Carter JD, Wilkenhoener R, Marshall C. Catalytic partial oxidation reforming of hydrocarbon fuels. Proceedings of 1998 Fuel Cell Seminar, Palm Springs, CA, 1998. p. 242–5. [39] Ralph T, Hards G. Powering the cars and homes of tomorrow. Chem Ind 1998;(9)337– 42. [40] Flynn TJ, Privette RM, Perna MA, Kneidel KE, King DL, Cooper M. Compact fuel processor for fuel cell-powered vehicles. SAE Paper 1999-01-0536, 1999. [41] O’Hara JB, Roszkowski TR, Kurpit SS, Starkovich EG. Hydrogen-rich fuel gas from diesel fuels. In: Proceedings of 21st Intersociety Energy Conversion Engineering Conference, vol. 2, 1986. p. 1118–22. [42] Steinfeld G, Skaanderup-Larsen J, Kurpit SS. Diesel fuel processing for phosphoric acid fuel cells. Proceedings of 21st Intersociety Energy Conversion Engineering Conference, vol. 2, 1986. p. 1092– 6. [43] Amphlett JC, Mann RF, Peppley BA, Roberge PR, Rodrigues A, Salvador JP. Simulation of a 250 kW diesel fuel processor=PEM fuel cell system. J Power Sources 1998;71:179– 84.

[44] Suzuki T, Iwanami H-i, Yoshinari T. Steam reforming of kerosene on Ru=Al2 O3 catalyst to yield hydrogen. Int J Hydrogen Energy 2000;25:119–26. [45] Abens SG, Steinfeld G. Direct carbonate fuel cell power plant operating with logistic fuels. Proceedings of 32nd Intersociety Energy Conversion Engineering Conference, 1997. p. 851– 6. [46] Wang D, Czernik S, Montane D, Mann M, Chornet E. Hydrogen production via catalytic steam reforming of fast pyrolysis oil fractions. Making a Business from Biomass, ISBN 0-08-042996-3, 1997. p. 845 –54. [47] Muradov NZ. CO2 -free production of hydrogen by catalytic pyrolysis of hydrocarbon fuel. Energy Fuels 1998;12:41– 8. [48] Hashimoto N, Kudo H, Adachi J, Shinagawa M, Yamaga N, Igarashi A. Development of reforming technology for a portable generator. Proceedings of 32nd Intersociety Energy Conversion Engineering Conference, 1997. p. 847–50. [49] Dams RAJ, Hayter PR, Moore SC. The processing of alcohols, hydrocarbons and ethers to produce hydrogen for a PEMFC for transportation applications. Proceedings of 32nd Intersociety Energy Conversion Engineering Conference, 1997. p. 837– 42. [50] Roan VP. An overview of fuels and fuel issues for automotive fuel cell systems. Proceedings of Fuel Cell Reformer Conference, 1998, South Coast Air Quality Management District, Diamond Bar, CA, 1999. p. 22–7. [51] Ledje-Hey K, Formanski V, Kalk Th, Roes J. Compact hydrogen production systems for solid polymer fuel cells. J Power Sources 1998;71:199–207. [52] Dicks A. Advances in catalysts for internal reforming in high temperature fuel cells. J Power Sources 1998;71:111–22. [53] Lewis RA, Dolan GA. Looking beyond the internal combustion engine: the promise of methanol fuel cell vehicles. SAE Paper 1999-01-0531, 1999. [54] Vanderborgh NE. On-board fuel processing issues. Proceedings of Fuel Cell Reformer Conference, 1998, South Coast Air Quality Management District, Diamond Bar, CA, 1999. p. 59 – 67. [55] Prabhu SK. Fuel processors for fuel cell power systems. Proceedings of Fuel Cell Reformer Conference, 1998, South Coast Air Quality Management District, Diamond Bar, CA, 1999. p. 79 – 89. [56] Ekdunge P. Volvo Technological Development Corporation. Personal communication, March 9, 1999. [57] Davis P. The Department of Energy Fuel Cells for Transportation Program. Fuels & Fuel Policy. Proceedings of Fuel Cell Reformer Conference, 1998, South Coast Air Quality Management District, Diamond Bar, CA, 1999. p. 54 – 6. [58] Pettersson L, Sjostrom K. Decomposed methanol as a fuel — A review. Combust Sci Technol 1991;80:265–303. [59] Callahan MA. Thermo-catalytic hydrogen generation from hydrocarbon fuels. Electrocatal Fuel Cells 1972;189 –204. [60] Khan AR, Meek J, Baker BS. Hydrogen from light distillates for fuel cells. Chem Engng Prog, Symp Ser 1967;63:31– 40. [61] Woods RR, Cuzens J, Mauzey J. Fuel- exible, fuel processor subsystem development — an update. Proceedings of Fuel Cell Reformer Conference, 1998, South Coast Air Quality Management District, Diamond Bar, CA, 1999. p. 92–106. [62] Pietrogrande P, Bezzeccheri M. Fuel processing. In: Blomen, LJMJ, Mugerwa MN, editors. Fuel cell systems. New York: Plenum Press, 1993. p. 121–56.

L.J. Pettersson, R. Westerholm / International Journal of Hydrogen Energy 26 (2001) 243–264 [63] Huang T-J, Wang S-W. Hydrogen production via partial oxidation of methanol over copper–zinc catalysts. Appl Catal A 1986;24:287– 97. [64] Alejo L, Lago R, Pena MA, Fierro JLG. Partial oxidation of methanol to produce hydrogen over Cu–Zn based catalysts. Appl Catal A 1997;162:281– 97. [65] Konig A, Ellinger K-W, Korbel K. Engine operation on partially dissociated methanol. SAE Paper 850573, 1985. [66] Pettersson L, Sjostrom K. Decomposed methanol — An excellent cold start fuel for spark ignition engines. Proceedings of VII International Symposium on Alcohol Fuels, Paris, Complementary Publication, 1986. p. 77– 82. [67] Pettersson L, Sjostrom K. Onboard hydrogen generation by methanol decomposition for the cold start of neat methanol engines. Int J Hydrogen Energy 1991;16:671–6. [68] Pettersson LJ, Lindstrom, B. A comparison between ethanol and methanol as feedstocks in the onboard hydrogen generation for fuel cell automobiles. KTH-Royal Institute of Technology, Department of Chemical Engineering and Technology, Chemical Technology, Stockholm, Sweden. Report No KTR 98-33, 1998. [69] Rostrup-Nielsen JR. Catalytic steam reforming. In: Anderson JR, Boudart M, editors. Catalysis, science and technology. Berlin: Springer, 1984. [70] Twigg MV, editor. Catalyst handbook, 2nd ed. London: Wolfe Publishing Co., 1989. [71] Teagan WP. PMG opportunities in fuel cell systems. Presented at 22nd International Precious Metals Conference, Toronto, Canada, June 14 –17, 1998. [72] Amphlett JC, Mann RF, Peppley BA, Stokes DM. Some design considerations for a catalytic methanol steam reformer for a PEM fuel cell power generating system. Proceedings of 26th Intersociety Energy Conversion Engineering Conference, 1991. p. 642– 9. [73] Peppley BA, Amphlett JC, Kearns LM, Mann RF, Roberge PR. Hydrogen generation for fuel-cell power systems by high-pressure catalytic methanol-steam reforming. Proceedings of 32nd Intersociety Energy Conversion Engineering Conference, 1997. p. 831– 6. [74] Amphlett JC, Mann RF, Peppley BA. Performance and operating characteristics of methanol steam-reforming catalysts for on-board fuel-cell hydrogen production. Proceedings of 11th World Hydrogen Energy Conference, 1996, Stuttgart, Germany, 1996. p. 1737– 43. [75] Amphlett JC, Creber KAM, Davis JM, Mann RF, Peppley BA, Stokes DM. Hydrogen production by steam reforming of methanol for polymer electrolyte fuel cells. Int J Hydrogen Energy 1994;19:131–7. [76] Amphlett JC, Leclerc S, Mann RF, Peppley BA, Roberge PR. Fuel cell hydrogen production by catalytic ethanol-steam reforming. Proceedings of 33rd Intersociety Energy Conversion Engineering Conference, 1998. p. 269= 1–269=7. [77] Garca EY, Laborde MA. Hydrogen production by the steam reforming of ethanol: Thermodynamic analysis. Int J Hydrogen Energy 1991;16:307–12. [78] Vasudeva K, Mitra N, Umasankar P, Dhingra C. Steam reforming of ethanol for hydrogen production: Thermodynamic analysis. Int J Hydrogen Energy 1996;21:13– 8.

263

[79] Freni S, Maggio G, Cavallaro S. Ethanol steam reforming in a molten carbonate fuel cell: a thermodynamic approach. J Power Sources 1996;62:67–73. [80] Fishtik I, Alexander A, Datta R, Geana D. A thermodynamic analysis of hydrogen production by steam reforming of ethanol via response reactions. Int J Hydrogen Energy 2000;25:31– 45. [81] Cheng Y-L, Chen L-D, Seaba JP. Thermodynamic analysis of fuel processing. SAE Paper 1999-01-0538, 1999. [82] Lwin Y, Daud WRW, Mohamad AB, Yaakob Z. Hydrogen production from steam-methanol reforming: thermodynamic analysis. Int J Hydrogen Energy 2000;25:47–53. [83] Chan SH, Wang HM. Thermodynamic analysis of natural-gas fuel processing for fuel cell applications. Int J Hydrogen Energy 2000;25:441– 9. [84] Peppley BA, Amphlett JC, Kearns LM, Mann RF. Methanol-steam reforming on Cu=ZnO=Al2 O3 . Part 1. The reaction network. Appl Catal A 1999;179:21– 9. [85] Peppley BA, Amphlett JC, Kearns LM, Mann RF. Methanol-steam reforming on Cu=ZnO=Al2 O3 . Part 2. A comprehensive kinetic model. Appl Catal A 1999; 179:31– 49. [86] Asprey SP, Wojciechowski BW, Peppley BA. Kinetic studies using temperature-scanning: the steam-reforming of methanol. Appl Catal A 1999;179:51–70. [87] Amphlett JC, Kearns LM, Mann RF, Peppley BA. Preparation and characterisation of a Cu=Zn=Cr catalyst optimised for methanol-steam reforming. Proceedings of 16th Canadian Symposium on Catalysis, 2000, Ban, Alberta, Canada, 2000. p. 116. [88] Kumar R, Ahmed S. Multi-fuel reformer R & D. Proceedings of Annual Automotive Technology Development Contractors’ Meeting, SAE Special Publication P-256, 1992. [89] Kumar R, Ahmed S, Krumpelt M. Rapid-start reformer for methanol in fuel-cell vehicles. Electric & Hybrid Vehicle Technology ’96, 1996. p. 123–7. [90] Jenkins JW. Catalytic hydrogen generator for use with methanol. US Patent No 4,789,540 1988. [91] Jenkins JW, Shutt E. The Hot Spot TM reactor. Hydrogen generation using a novel concept. Platinum Met Rev 1989;33:118–27. [92] Golunski S. HotSpot TM Fuel Processor. Advancing the case for fuel cell powered cars. Platinum Met Rev 1998;42:2–7. [93] Schmidt VM, Brockerho P, Hohlein B, Menzer R, Stimming U. Utilization of methanol for polymer electrolyte fuel cells in mobile systems. J Power Sources 1994;49:299–313. [94] Hohlein B, Boe M, Bogild-Hansen J, Brockerho P, Colsman G, Emonts B, Menzer R, Riedel E. Hydrogen from methanol for fuel cells in mobile systems: development of a compact reformer. J Power Sources 1996;61:143–7. [95] Emonts B, Bogild Hansen J, Lgsgaard Jorgensen S, Johlein B, Peters R. Compact methanol reformer test for fuel-cell powered light-duty vehicles. J Power Sources 1998;71: 288– 93. [96] Geyer HK, Ahluwalia RK, Kumar R. Dynamic response of steam-reformed, methanol-fueled, polymer electrolyte fuel cell systems. Proceedings of 31st Intersociety Energy Conversion Engineering Conference, 1996. p. 1101– 6. [97] Dams RAJ, Hayter PR, Moore SC. Progress in fuel processing for PEMFC systems for transport applications.

264

[98]

[99]

[100] [101]

[102] [103]

[104] [105] [106]

[107] [108] [109]

[110]

L.J. Pettersson, R. Westerholm / International Journal of Hydrogen Energy 26 (2001) 243–264 Proceedings of 33rd Intersociety Energy Conversion Engineering Conference, 1998. p. 236=1– 6. Voecks GE. Unconventional utilization of monolithic catalysts for gas-phase reactions. In: Cybulski A, Moulijn JA editors. Structured catalysts and reactors. New York: Marcel Dekker, 1998. p. 179 –208. Dams RAJ, Hayter PR, Moore SC. Advances in fuel processing for PEMFC systems for transport. Proceedings of 1998 Fuel Cell Seminar, Palm Springs, CA, 1998. p. 238– 41. Dusterwald HG, Hohlein B, Kraut H, Meusinger J, Peters R, Stimming U. Methanol steam-reforming in a catalytic xed bed reactor. Chem Engng Technol 1997;20:617–23. Weisbrod KR, Vanderborgh NE. Eect of operating parameters and anode gas impurities upon polymer electrolyte fuel cells. Proceedings of 29th Intersociety Energy Conversion Engineering Conference, 1994. p. 855 – 60. King DL. Catalytica Advanced Technologies. Personal communication, May 3, 1999. Amphlett JC, Leclerc S, Mann RF, Peppley BA, Roberge PR. Simulation and system analysis of an ethanol fuel processor=PEM fuel cell power plant. Proceedings of 33rd Intersociety Energy Conversion Engineering Conference, 1998. p. 270=1– 6. Colsman G. Industrial processing optimization of fuel gas production for fuel cells. Technical Report No JUEL-3127, Forschungszentrum Julich (IEV), Julich, Germany, 1995. Oh SH, Sinkevitch RM. Carbon monoxide removal from hydrogen-rich fuel cell feedstreams by selective catalytic oxidation. J Catal 1993;142:254–62. Okada S, Takatani Y, Terada S, Ohtani S, Tsuchiyama S. Development of a selective oxidation CO removal reactor for methanol reformate gas. Proceedings of 1996 Fuel Cell Seminar, Orlando, FL, 1996. p. 315 – 8. Amphlett JC, Mann RF, Peppley BA. On board hydrogen puri cation for steam reformation=PEM fuel cell vehicle power plants. Int J Hydrogen Energy 1996;21:673– 8. Fogler HS. Elements of chemical reaction engineering, 3rd ed. Upper Saddle River, NJ: Prentice-Hall, 1999. p. 619 –20. Baker BS, Huebler J, Linden HR, Meek J. Process for selective removal by methanation of carbon monoxide from a mixture of gases containing carbon dioxide. US Patent No 3,615,164, 1971. Oshiro H, Nagaya K, Mitani K, Tsuchiyama S. Performance of catalysts for CO removal by methanation. Proceedings of 1996 Fuel Cell Seminar, Orlando, FL, 1996. p. 319 –22.

[111] Cuzens JE, Mauzey J, Woods RR. Fuel- exible fuel processor. Proceedings of 1998 Fuel Cell Seminar, Palm Springs, CA, 1998. p. 234 –7. [112] Lee SHD, Kumar R, Krumpelt M. Removal of CO from reformate for PEFC application. Proceedings of 1998 Fuel Cell Seminar, Palm Springs, CA, 1998, p. 578– 81. [113] Kumar, R. Argonne National Laboratory. Personal communication, May 10, 1999. [114] Wasmus S, Wang J-T, Savinell RF. Real-time mass spectrometric investigation of the methanol oxidation in a direct methanol fuel cell. J Electrochem Soc 1995;142: 3825–33. [115] Mitchell WL, Hagan M, Prabhu SK. Gasoline fuel cell power systems for transportation applications: A bridge to the future of energy. SAE Paper 1999-01-0535, 1999. [116] Mitchell WL. Development of an integrated reformer=fuel cell power system for petroleum and alcohol fueled vehicles. Part 1 — Brassboard demonstration. Proceedings of 1998 Fuel Cell Seminar, Palm Springs, CA, 1998. p. 230 –3. [117] Wimmer R, Fletcher J, Clark N, McCain D, Lyons D. Emissions testing of a hybrid fuel cell bus. SAE Paper 980680, 1998. [118] SAE J1376. Fuel economy measurement test (Engineering type) for trucks and buses. SAE Handbook, vol. 4. Warrendale, USA, 1993. [119] Thomas CE, James BD, Lomax Jr FD, Kuhn Jr IF. Fuel options for the fuel cell vehicle: hydrogen, methanol or gasoline? Int J Hydrogen Energy 2000;25:551– 67. [120] Kendall K, Williams D. Catalysts for Butane Reforming in Zirconia Fuel Cells. Platinum Met Rev 1998;42:164 –7. [121] Borroni-Bird CE. Fuel cells and other clean power sources. Proceedings of Fuel Cell Reformer Conference, 1998, South Coast Air Quality Management District, Diamond Bar, CA, 1999. p. 127–52. [122] Kumar R, Ahluwalia R, Doss ED, Geyer HK, Krumpelt M. Design, integration and trade-o analyses of gasoline-fueled polymer electrolyte fuel cell systems for transportation. Proceedings of 1998 Fuel Cell Seminar, Palm Springs, CA, 1998. p. 226 – 9. [123] Mousavi H. Dynamic modelling of a fuel cell system. MSc thesis, KTU 00-17, KTH -Royal Institute of Technology, Department of Chemical Engineering and Technology, Chemical Technology, Stockholm, Sweden, 2000.