Development of a methanol reformer for
fuel cell vehicles
Bård Lindström
KTH-Kungliga Tekniska Högskolan
Department of Chemical Engineering and Technology
Chemical Technology
Stockholm 2003
PhD Thesis
TRITA-KET R172
ISSN 1104-3466
ISRN KTH/KET/R-172-SE
ISBN 91-7283-406-4
To Amanda and my parents
Abstract
Vehicles powered by fuel cells are from an environmental aspect superior to the
traditional automobile using internal combustion of gasoline. Power systems
which are based upon fuel cell technology require hydrogen for operation. The
ideal fuel cell vehicle would operate on pure hydrogen stored on-board.
However, storing hydrogen on-board the vehicle is currently not feasible for
technical reasons. The hydrogen can be generated on-board using a liquid
hydrogen carrier such as methanol and gasoline. The objective of the work
presented in this thesis was to develop a catalytic hydrogen generator for
automotive applications using methanol as the hydrogen carrier.
The first part of this work gives an introduction to the field of methanol
reforming and the properties of a fuel cell based power system. Paper I reviews
the catalytic materials and processes available for producing hydrogen from
methanol.
The second part of this thesis consists of an experimental investigation of the
influence of the catalyst composition, materials and process parameters on the
activity and selectivity for the production of hydrogen from methanol. In Papers
II-IV the influence of the support, carrier and operational parameters is studied.
In Paper V an investigation of the catalytic properties is performed in an attempt
to correlate material properties with performance of different catalysts.
In the third part of the thesis an investigation is performed to elucidate whether
it is possible to utilize oxidation of liquid methanol as a heat source for an
automotive reformer. In the study which is presented in Paper VI a large series
of catalytic materials are tested and we were able to minimize the noble metal
content making the system more cost efficient.
In the final part of this thesis the reformer prototype developed in the project is
evaluated. The reformer which was constructed for serving a 5 kWe fuel cell
had a high performance with near 100 % methanol conversion and CO
concentrations below 1 vol% in the product stream. The results of this part are
presented in Paper VII.
Keywords: methanol, fuel cell, vehicle, catalyst, copper, hydrogen, on-board,
steam reforming, partial oxidation, combined reforming, oxidative steam
reforming, auto-thermal reforming, zinc, zirconium, chromium, aluminium
oxide, manganese, characterization, temperature programmed reduction, X-ray
diffraction, chemisorption, carbon monoxide, poisoning, reformer.
Sammanfattning
Fordon med bränsleceller som drivkälla överträffar ur miljösynpunkt
traditionella bilar utrustade med bensindrivna förbränningsmotorer.
Elgenererande system baserade på bränslecellteknik behöver väte för driften.
Det ideala bränslecellfordonet utnyttjar rent väte som lagras ombord. Emellertid
är inte lagring ombord på fordonet utförbar på grund av tekniska skäl. Väte kan
framställas ombord genom att använda en vätebärare som metanol och bensin.
Målet för arbetet som presenteras i denna avhandling var att utveckla en
katalytisk vätgasgenerator för mobila tillämpningar genom att använda metanol
som vätebärare.
Den första delen av arbetet ger en introduktion till området metanolreformering
och egenskaperna hos ett bränslecellbaserat kraftsystem. Artikel I ger en kritisk
översikt av katalytiska material och processer som är tillgängliga för att
producera väte från metanol.
Andra delen av denna avhandling består av en experimentell undersökning av
inflytandet av katalysatorsammansättningen, material- och processparametrar på
aktiviteten och selektiviteten för produktion av väte från metanol. I artikel II-IV
studeras inflytandet av bärare, substrat och driftparametrar. I artikel V
genomförs en undersökning av de katalytiska egenskaperna i ett försök att
korrelera materialegenskaper med prestanda för olika katalysatorer.
I den tredje delen av avhandlingen genomförs en undersökning för att belysa om
det är möjligt att utnyttja oxidation av vätskeformig metanol som värmekälla för
en automotiv reformer. I studien, som presenteras i artikel VI, testas en stor
serie av katalytiska material och vi kunde minimera ädelmetallinnehållet vilket
gör systemet mer kostnadseffektivt.
I den sista delen av avhandlingen utvärderades reformerprototypen som
utvecklats i projektet. Reformern som konstruerats för att betjäna en 5 kWe
bränslecell hade hög prestanda med nära 100 % omsättning av metanol och COkoncentrationer under 1 vol% i produktgasen. Resultaten från denna del är
presenterade i artikel VII.
Nyckelord: methanol, fuel cell, vehicle, catalyst, copper, hydrogen, on-board,
steam reforming, partial oxidation, combined reforming, oxidative steam
reforming, auto-thermal reforming, zinc, zirconium, chromium, aluminium
oxide, manganese, characterization, temperature programmed reduction, X-ray
diffraction, chemisorption, carbon monoxide, poisoning, reformer.
An expert is a person who has
made all the mistakes that can
be made in a very narrow field.
Niels Bohr
Publications referred to in this thesis
The work presented in this thesis is based upon the following publications,
referred to by their Roman numerals. The papers are appended at the end of the
thesis
I.
Agrell, J., Lindström, B., Pettersson, L.J., and Järås, S.G. (2002).
Catalytic hydrogen generation from methanol, in Spivey, J.J. (Ed.),
Catalysis – Specialist Periodical Reports, Royal Society of Chemistry,
Cambridge, Vol. 16, pp. 67-132.
II.
Lindström, B. and Pettersson, L.J. (2001). Hydrogen generation by steam
reforming of methanol over copper-based catalysts for fuel cell
applications. Int. J. Hydrogen Energy 26, 923-33.
III.
Lindström, B., Agrell, J., and Pettersson, L.J. (2002). Combined
reforming of methanol for hydrogen generation over monolithic catalysts.
Chemical Engineering Journal (in press).
IV.
Lindström, B. and Pettersson, L.J. (2002). Steam reforming of methanol
over copper-based monoliths: The effects of zirconia doping. J. Power
Sources 106, 264-273.
V.
Lindström, B., Pettersson, L.J., and Menon, P.G. (2002). Activity and
characterization of Cu/Zn, Cu/Cr and Cu/Zr on γ-alumina for methanol
reforming for fuel cell vehicles. Appl. Catal. A 234, 111-125.
VI.
Lindström, B. and Pettersson, L.J. (2003). Catalytic oxidation of liquid
methanol as a heat source for an automotive reformer. Accepted for
publication in Chemical Engineering and Technology.
VII. Lindström, B. and Pettersson, L.J. (2003). Design and development of an
auto-thermal reformer for fuel cell applications. Accepted for publication
in Journal of Power Sources.
Other publications
Other publications and conference papers on methanol reforming
1.
Lindström, B. and Pettersson, L.J. (2001). Deactivation of copper-based
catalysts for fuel cell applications. Catal. Lett. 74, 27-30.
2.
Lindström, B. and Pettersson, L.J. (2000). Steam reforming of methanol
for fuel cell applications. Proc. 9th Nordic Symposium on Catalysis, June
4-6, Lidingö, Sweden, p. 101-102.
3.
Lindström, B. and Pettersson, L.J. (2000). A study of ethanol and
methanol as a fuel for onboard hydrogen generation by steam reforming
on copper-based catalysts. Proc. XIII International Symposium on
Alcohol Fuels, Stockholm, July 3-6, 2000.
4.
Lindström, B. and Pettersson, L.J. (2000). Steam reforming of methanol
for automotive applications. Proc. 2000 Fuel Cell Seminar, Portland,
Oregon, October 2000, pp. 325-328.
5.
Lindström, B., Agrell, J., and Pettersson, L.J. (2001). Combinatorial
Reforming of Methanol for Hydrogen Generation over Monolithic
Catalysts. Proc. 17th North American Catalysis Society Meeting,
Toronto, June 3-8, 2001, p. 140.
6.
Lindström, B. and Pettersson, L.J. (2001). Catalytic steam reforming of
methanol for automotive fuel cell applications. Proc. 5th European
Congress on Catalysis (EUROPACAT 5), Limerick, Ireland, September
2-7, 2001.
7.
Lindström, B. and Pettersson, L.J. (2001). Steam reforming of methanol
over copper-based monoliths: The effects of zirconia doping. Proc. 7th
Grove Fuel Cell Symposium, London, September 11-13, 2001.
8.
Lindström, B., Pettersson, L.J. and Menon, P.G. (2002). Influence of the
operating conditions on the performance of a methanol reformer. Proc.
10th Nordic Symposium on Catalysis, Helsingør, Denmark, June 2-4,
2002.
9.
Pettersson, L.J. and Lindström, B. (2002). Catalytic fuel processing for
fuel cell cars. Proc. Fourth International Tokyo Conference on Advanced
Catalytic Science and Technology, Tokyo, July 14-19, 2002
10.
Lindström, B. and Pettersson, L.J. (2002). Development of a methanol
fuelled reformer for fuel cell applications. Proc. Fuel Cells - Science and
Technology 2002, Scientific Advances in Fuel Cell Systems, Amsterdam,
25-26 September, 2002.
11.
Lindström, B. and Pettersson, L.J. (2002). Strategies for optimizing a
methanol reformer for fuel cell vehicles. Proc. XIV International
Symposium on Alcohol Fuels, Phuket, Thailand, 12-15 November 2002.
1. Introduction .......................................................................................................3
1.1 Background..................................................................................................3
1.2 Fuel cell technology ....................................................................................5
1.3 Fuel strategies for mobile fuel cell applications..........................................6
1.4 Methanol production and application........................................................10
1.4.1 Manufacturing processes.....................................................................11
1.4.2 Synthesis gas sources ..........................................................................12
1.4.3 Methanol applications .........................................................................12
1.4.3 Health aspects......................................................................................13
1.5 Catalytic fuel processing ...........................................................................13
1.5.1 Decomposition ....................................................................................14
1.5.2 Steam reforming..................................................................................14
1.5.3 Partial oxidation ..................................................................................14
1.5.4 Combined reforming ...........................................................................15
1.6 Scope of the Thesis....................................................................................16
2. Properties of the system components of the methanol fuel cell propulsion
system..................................................................................................................18
2.1 Introduction ...............................................................................................18
2.2 Fuel processor............................................................................................18
2.2.1 Catalytic reformer ...............................................................................19
2.2.2 Catalytic burner...................................................................................21
2.2.3 Evaporator ...........................................................................................21
2.3 Fuel cell .....................................................................................................21
2.4 Gas clean-up module .................................................................................23
2.4.1 Water-gas shift reaction ......................................................................23
2.4.2 Preferential oxidation ..........................................................................24
2.4.3 Other clean-up processes ....................................................................24
2.4.4 Evaluation of the clean-up processes..................................................25
2.5 Air module.................................................................................................25
3. Materials and processes for catalytic hydrogen generation from methanol
(Paper I)...............................................................................................................26
3.1 Steam reforming ........................................................................................26
3.1.1 Catalytic materials for steam reforming of methanol .........................27
3.2 Partial oxidation.........................................................................................28
3.2.1. Catalytic materials for partial oxidation of methanol ........................29
3.3 Combined reforming .................................................................................30
3.4 Summary....................................................................................................31
4. Reforming catalyst and process optimisation (Papers II, III, IV and V .........32
4.1 Introduction ...............................................................................................32
4.2 Laboratory experimental set-up.................................................................32
4.3 Copper based materials for steam reforming of methanol (Paper II)........35
4.3.1 Background .........................................................................................35
1
4.3.2 Influence of the promoter on the activity and selectivity ...................35
4.4 Influence of the steam-to-methanol ratio on the catalytic activity of the
steam reforming of methanol...........................................................................36
4.5 Copper-based materials for the combined reforming process (Paper III).38
4.5.1 Background .........................................................................................38
4.5.2 Influence of the promoter material on the combined methanol
reforming process.........................................................................................39
4.6 Zirconia-doped catalysts for the steam reforming process (Paper IV) .....40
4.6.1 Background .........................................................................................40
4.6.2 Influence of zirconia-doping on the formation of CO for the steam
reforming of methanol..................................................................................41
4.7 Characterization of copper-based catalysts for methanol reforming (Paper
V) .....................................................................................................................42
4.7.1 Background .........................................................................................42
4.7.2 Influence of the surface properties on the catalytic activity ...............43
4.8 Summary....................................................................................................46
5. Catalytic oxidation of liquid methanol (paper VI) .........................................47
5.1 Introduction ...............................................................................................47
5.2 Experimental lab-scale set-up....................................................................47
5.3 Influence of catalyst material on the activity for methanol oxidation ......48
5.4 Summary....................................................................................................52
6. Development and evaluation of a catalytic reforming system (Paper VII) ....53
6.1 Introduction ...............................................................................................53
6.2 Experimental set-up...................................................................................53
6.3 Evaluation of laboratory tests....................................................................54
6.4 Industrial evaluation and optimisation of reformer prototype ..................58
6.5 Summary....................................................................................................59
7. Conclusions.....................................................................................................61
Acknowledgements.............................................................................................63
Nomenclature ......................................................................................................64
References ...........................................................................................................66
2
1. Introduction
1.1 Background
Man has been walking the earth for more than 30 000 years and despite his
intelligence and ingenuity he continued to do so for the first 25 000 years. The
first forms of transportation which were constructed using the basic laws of
mechanics can be traced back to the Sumerian civilization around 3500 B.C.
[1,2]. The Sumerians developed simple forms of carriages that consisted of flat
structures mounted on wheels that were pulled by either horses or men.
The principles developed by the Sumerians were later employed by all major
civilizations for more than 5000 years, and it was not until the middle of the
eighteenth century that the first means of power driven transportation was
constructed by the French engineer Nicolas Cugnot [2-4]. The original steam
powered vehicle that Cugnot constructed in 1769, was developed for the French
army to transport artillery (see Figure 1). Cugnot, however modified the vehicle
one year later to carry passengers. Hence he created the first self-propelled road
vehicle. Cugnot, however, ran out of money and his project was abandoned.
Several attempts were made around the world to continue the development of
the steam wagon. The low performance of the steam powered vehicle prevented
it from being used commercially [4,5].
Figure 1: Cugnot´s steam powered road vehicle
3
During the nineteenth century several efforts were made to develop new fuels
and engines for powering automobiles. The first real significant breakthrough
came in 1876 when Nikolaus Otto and Eugen Langen invented the four-stroke
engine (the Otto engine). It was, however, not until 1885 when Gottlieb Daimler
invented the modern combustion engine, operating on gasoline injected through
a carburettor, which was mounted on the first four-wheeled automobile [3,6].
The years that followed were characterized by the development and
improvements of the automobile which culminated in 1908 when Henry Ford
produced the first serially produced automobile, the T-Ford (see figure 2). The
actual expansion of the auto market came, however, in the post World War 2
era, when the serially produced automobiles hit the European markets [6-8].
Figure 2: T-Ford
The environmental impacts of having an automobile in every home were never
anticipated by the automakers in the early days of automotive manufacturing.
There are a wide range of toxic substances emitted, from vehicles powered by
the internal combustion engine, with different impacts on the environment. The
pollutants emitted from automobiles today can be classified into two different
categories: (i) substances emitted as unburned fuel pass through the engine and
(ii) substances formed as by-products of the combustion [9,10].
The toxic substances which are inherent from the petroleum, such as benzene
and non-aromatic hydrocarbons, are carcinogenic and have a direct impact on
the health of humans when exposed to these substances. The pollutants that are
produced in the internal combustion engine, such as nitrogen oxides (NOx),
sulphur oxides (SOx) and carbon dioxide (CO2) have a long-term impact on the
environment through acid rain and global warming [9,10].
4
The environment first came into focus in the early sixties when Rachel Carson’s
“Silent spring” [11] sparked off the first real environmental debate. Demands
from consumers and legislators prompted the automotive industry to reduce the
pollutants emitted from their vehicles by changing the operating conditions in
the combustion chamber and implementing exhaust gas catalysts.
The actual impact that automobiles have on the environment has only started to
become clear during the last decades as the result of serious smog related
incidences, witnessed in cities with heavy traffic (especially Los Angeles and
London), and observed global warming. The environmental effects of the
modern automobile have impelled lawmakers around the world, particularly in
California and the EU, to create emission regulations for automobiles [12].
The structure of the environmental regulations was constructed so that the
limitations on the allowed emissions would increase over time. It is, however,
not possible to reach the zero emission demands by modification of the internal
combustion engine. This has prompted the automotive industry to search for
viable replacements for the internal combustion engine. Automakers and
legislators generally consider fuel cell based automobiles today as the most
realistic alternative for replacing the internal combustion engine [13,14].
1.2 Fuel cell technology
Humphry Davy initially demonstrated the principal concept of the fuel cell in
1802, when he successfully showed that a galvanic cell could be used to
produce oxygen and hydrogen from water [15]. William Grove deduced that the
reverse reaction should result in the production of electricity, and in 1839 Grove
constructed the world’s first fuel cell. Grove’s fuel cell operated on pure oxygen
and hydrogen with sulphuric acid functioning as the electrolyte (see Figure 3).
[16].
Figure 3: Groves’s fuel cell
5
The path from Grove’s crude acidic fuel cell to modern polymer and solid oxide
fuel cells is long, and a wide variety of fuel cells have been tested on the way.
During the sixties the fuel cell concept received much attention, when alkaline
fuel cells were successfully used in the Apollo space flights and scientists were
predicting that the fuel cell would solve the energy problems of the world.
Today fuel cells are seen as a realistic replacement for the internal combustion
engine, with scientific evidence predicting the reduction of greenhouse gases by
up to 68 % [17]. The type of fuel cell most likely to be used in automotive
applications is the Polymer Electrolyte Fuel Cell (PEFC). The PEFC uses a
Nafion™ (perfluorosulphonic acid polymer) type electrolyte and operates
ideally at temperatures, 80-90 °C. The main drawbacks of the PEFC are that it
has a low tolerance towards carbon monoxide (CO), is sensitive to temperature
and expensive [18, 19].
The fuel cell operates by combining oxygen and hydrogen to form water while
at the same time generating electricity and the most challenging task in mobile
applications is how to provide the hydrogen. There are two main options
available that both have their advantages and disadvantages: (i) Storing pure
hydrogen in tanks or (ii) extracting the hydrogen on-board from an alternate
hydrogen carrier. The available options are discussed in detail in the next
section.
1.3 Fuel strategies for mobile fuel cell applications
Fuel cell-based power systems require only hydrogen and oxygen (obtained
from air) for operation, and the size and complexity of the system is directly
dependent on the source and technology used for providing the hydrogen.
For automotive applications the hydrogen can either be stored in pressurized
tanks or generated on-board using a liquid hydrogen carrier such as methanol or
gasoline [20-22]. The use of solid hydrogen carriers (metal hydrides) has also
been proposed as an alternate storage form for hydrogen in automotive
applications [23]. The physical properties of selected hydrogen carriers are
listed in Table 1.
From a technical aspect the storage of hydrogen in pressurized tanks is the most
feasible solution as no intermediate step is required between the storage unit and
the fuel cell. The use of pure hydrogen also eliminates the need for a clean-up
unit required to remove by-products of a hydrogen generation process.
6
The use of pressurized hydrogen is however, currently not viable in automotive
applications due to certain technological limitations such as:
•
•
•
•
•
Limited driving range
The absence of an infrastructure for gaseous fuels
Logistic problems associated with refuelling
Hydrogen is highly flammable
Safety concerns
When using a hydrogen carrier, the hydrogen must be extracted from the fuel
on-board by means of a suitable conversion process. The use of metal hydrides
is currently unrealistic, as a fuel system based upon this technology would
exceed the weight limitations of a fuel cell vehicle. Hydride systems also
require a gaseous hydrogen refuelling system.
Liquid fuels such as primary alcohols, gasoline and diesel are today favoured by
the automotive industry as the most suitable hydrogen carriers for fuel cell
vehicles. The two top contenders for the fuel cell market today are methanol and
gasoline. From a technological point of view methanol is by far the most
desirable fuel for several reasons:
•
•
•
•
•
•
Conversion takes place at relatively low temperatures (250-300 °C)
High hydrogen to carbon ratio (4:1)
No C-C bond thus minimizing the risk of soot formation
Low formation of by-products (especially CO)
High hydrogen content in product stream (up to 75%)
No aromatics or sulphur in the fuel
The fuel cell is highly sensitive to impurities (especially CO) and for a system
using gasoline in the feed a desulphurisation unit is required and the size of the
gas clean-up module would also be significantly larger than for a system using
methanol.
7
802.5
241.8
77-253 °C
28.8
Liquid density
(kg/m3)
Specific heat at
25 °C
(J mol-1 K-1)
0.90
Heat of
vaporization at bp
(kJ/mol)
35.7
423-162 °C
8.19
-161.5
Boiling point (°C) -252.9
Lower heating
value at 25 °C
(kJ/mol)
-182.4
-259.3
Melting point
(°C)
8
35.1
682-33.4 °C
23.3
316.3
-33.4
-77.7
Table 1: Fuel properties (adapted from Dicks [23] and Lide [24])
Methane
Ammonia
Hydrogen
CH4
NH3
H2
Molecular weight 2.02
16.04
17.03
81.1
79120 °C
35.2
638.5
64.6
-97.6
Methanol
CH3OH
32.04
112.3
78920 °C
38.6
1275.9
78.5
-114.1
Ethanol
C2H5OH
46.07
239.1
69025 °C
34.4
5512.0
99.2
-107
Gasolinea
C8H18
114.2
4 - 77
4 - 16
16 – 27
6 – 50
3 – 19
1–6
9
Auto ignition
571
632
651
470
365
417
temperature in air
(°C)
a
Gasoline is a blend of hydrocarbons that varies with origin, producer, application and season. iso-octane (2,2,4trimethyl pentane) is a reasonable representative of the physical properties.
Flammability
limits in air (%)
Gasoline on the other hand, requires no modification of the infrastructure,
although the gasoline used in fuel cell applications requires higher standards
with respect to impurities (especially sulphur). Gasoline can also be used in both
vehicles powered by the combustion engine as well as fuel cells, thus
eliminating complications during a transition phase.
The conversion of gasoline is complex and yields several by-products, which
are harmful to the fuel cell and must be removed on-board increasing the cost of
the fuel cell vehicle. By using methanol in the feed the emissions of NOx, CO,
hydrocarbons and CO2 are reduced compared both to the Super Ultra Low
Emission Vehicle (SULEV) standard set by the state of California and low
sulphur gasoline fed fuel cell vehicles [25, 26].
The problems related with aromatics and sulphur in gasoline-based systems can
be solved by using designed fuels manufactured by the Fischer-Tropsch
synthesis. SASOL in South Africa is currently utilizing this method for
manufacturing gasoline and diesel fuels.
The concept of a multi-fuel reformer was discussed during the initial stage of
the project, but was abandoned, as a system that is fuel flexible could never be
optimised for all types of fuels. The fuel flexible system would as a result
generate higher CO concentrations, which would require a larger gas-cleanup
module, and thus increasing the cost and complexity of the system. The
advantages of a fuel flexible system is that no changes in the current
infrastructure would be required and the transition from internal combustion to
fuel systems would be inexpensive
We have therefore chosen methanol as the hydrogen carrier as it is the most
suitable fuel from both a technological and environmental perspective. The
performance of the reforming system is dependent on the process used to extract
the hydrogen from the methanol and the process alternatives and their properties
are presented in section 1.5. Details of the commercial production and
applications of methanol are presented in the next section.
1.4 Methanol production and application
Methanol (Methyl alcohol) is a primary alcohol that can be manufactured from
a wide variety of sources and has an equally wide variety of applications.
Methanol is found as a colourless liquid at room temperature and is recognized
by its mild characteristic alcohol odour [27, 28]. Methanol has traditionally been
used as a solvent or feedstock for bulk chemicals (especially formaldehyde and
10
MTBE). With the growing interest in fuel cell applications the role of methanol
is however expected to change from only serving as a bulk chemical to a
commodity product.
1.4.1 Manufacturing processes
Methanol was originally known as wood alcohol as it was originally produced
by the dry distillation of wood. Today methanol is mainly produced from
synthesis gas, a mixture of CO and H2. A brief discussion of the available
manufacturing methods will be presented here:
Extraction from natural sources
Methanol can be extracted from several types of plants and woods, which
served as the only commercial source of methanol production until the
introduction of the synthetic process in 1923 [29, 30].
The extraction process is based upon thermal decomposition (i.e. pyrolysis) of
hardwoods, such as birch and oak, in the absence of air at temperatures between
160-450 °C. The earliest reference to this process is found in Boyle´s
“Sceptical Chymist” from 1661 [31]. The decomposition generates a large
mixture of products, in addition to methanol significant quantities of charcoal,
tar, methanoic acid and acetone. The methanol yield is also low, (1-2 %) for
hardwoods and even lower for softwoods. Therefore this process is unsuitable
for commercial applications [29, 30].
Fermentation
The production of methanol in plants and living organisms takes place by a
process known as fermentation. Fermentation can be described as the
decomposition of complex organic materials by bacteria to methanol. This
process is similar to the fermentation of carbohydrates to ethanol. There are
currently no solutions available for commercialisation of this technology [29].
Synthetic production
The realization that methanol could be produced from inorganic compounds
came first in 1905, when the French scientist Paul Sabatier proposed that
methanol could be synthesized by hydrogenation of CO. The first commercial
process was established in 1923 by BASF in Leuna, Germany [29, 32, 33]. The
process which used a zinc-chromium catalyst was developed upon the basic
11
principals of the ammonia synthesis. The low activity of the catalyst implied
that the process had to be operated at elevated temperature and pressures
(T=320-450 °C, P=250-350 atm). The thermodynamics are also favoured by
operation at high pressures. The high pressure process dominated the market
until 1960 when ICI developed a highly active copper-zinc catalyst which could
operate at pressures below 100 atm. The copper-zinc based catalysts are still
today dominant for the production of methanol from synthesis gas (syngas) [29,
34].
1.4.2 Synthesis gas sources
All hydrocarbons that can be converted into synthesis gas either through steam
reforming (equation 1) or gasification with oxygen (equation 2) are potential
sources for commercial production of methanol. [27, 28, 32]
CnHm + n H2O Æ n CO + (n + m/2) H2
(1)
CnHm + (n/2) O2 Æ n CO + m/2 H2
(2)
Steam reforming of natural gas today accounts for more than 80 % of the
world’s production of methanol. The steam reforming reaction is usually
performed over a nickel based catalyst in an endothermic reactor. The methanol
synthesis catalysts are sensitive to sulphur-based compounds which have to be
removed prior to the reforming, thus methanol produced from steam reforming
is redundant of sulphur and thereby the potential of methanol as a fuel for fuel
cell-based applications is increased. [32,35,36]
1.4.3 Methanol applications
The world’s production capacity of methanol today exceeds more than
32,575,000 metric tons with production facilities in more than 30 countries (see
figure 4). [37]
12
Capacity (Metric tonnes)
6750000, 19%
7791000, 22%
North America
South America
Europe
Middle East/Africa
7798000, 22%
Asia/Pacific
7391000, 21%
5545000, 16%
Figure 4: World wide methanol production capacity
Methanol is today mainly used as a solvent and as a bulk chemical for the
production of formaldehyde and MTBE. [37]
1.4.3 Health aspects
The most common question raised today regarding the suitability of methanol as
a fuel, is how toxic methanol really is. The answer to this question is simple:
Yes, methanol is poisonous and as with many other fuels it is highly toxic if
taken orally. Caution should be taken, however there is no need to increase level
of caution used for petroleum today. It should however be noted that the body
can metabolise low concentrations of methanol with no ill effects. [38]
1.5 Catalytic fuel processing
The generation of hydrogen from methanol is possible through several process
alternatives: decomposition, steam reforming, partial oxidation and a
combination of steam reforming and partial oxidation. The combined reforming
process is often referred to as auto-thermal reforming when operated under
thermally neutral conditions [39].
13
1.5.1 Decomposition
Decomposition (equation 3) is usually ruled out when discussing hydrogen
generation for fuel cell applications since carbon monoxide (CO) is one of the
main products of the process. The process can theoretically deliver a product
stream containing up to 67 vol% hydrogen (H2) [40].
∆H0 = 91 kJ/mol (3)
CH3OH (g) Æ 2 H2 + CO
1.5.2 Steam reforming
Steam reforming of methanol (equation 4) is a highly developed and thoroughly
studied process [41-46]. Steam reforming of methanol can yield a product gas
containing up to 75 % hydrogen while maintaining a high selectivity towards
carbon dioxide. The main drawback of the steam reforming process is that it is
slow and endothermic.
∆H0 = 49 kJ/mol (4)
CH3OH (g) + H2O (g) Æ 3 H2 + CO2
The steam reforming of methanol is often operated using 30 % excess steam in
the feed stream (equation 5) in order to lower the CO concentrations [41-42].
The maximum theoretical hydrogen concentration in the product stream is
however consequently lowered to 70 % as a result of the dilution.
CH3OH (g) + 1.3 H2O (g) Æ 3 H2 + CO2 + 0.3 H2O (g)
∆H0 = 49 kJ/mol (5)
1.5.3 Partial oxidation
Partial oxidation is a highly exothermic process (equation 6), which can be used
to construct highly dynamic and fast reforming systems [43, 47-49]. The
formation of hot-spots is one of the main drawbacks from using the partial
oxidation process as the formation of these hot-zones in the catalyst can result in
sintering thus lowering the catalyst activity.
∆H0 = -192 kJ/mol
CH3OH (g) + ½ O2 Æ 2 H2 + CO2
(6)
The partial oxidation process can theoretically at complete conversion generate
a product stream containing up to 67 % hydrogen. However for automobile
solutions the oxygen will most likely be supplied using compressed air
(equation 7) which results in a dilution of the product stream with nitrogen and
14
subsequently lowering the maximum hydrogen concentration to 41 %. The
performance of the fuel cell is dependent on the hydrogen concentration and
therefore operating the reformer with only partial oxidation may not be suitable
[19].
CH3OH (g) + ½ O2 + 1,88 N2 Æ 2 H2 + CO2 + 1,88 N2
∆H0 = -192 kJ/mol (7)
1.5.4 Combined reforming
By combining partial oxidation and steam reforming (equation 8) it is possible
to obtain a system which is dynamic while generating relatively high hydrogen
concentrations [17,43,50-53] as well as avoiding the formation of hot-spots in
the catalyst bed.
When operating the combined process under stoichiometric conditions the sum
of the molar coefficients of water (x) and oxygen (y) equals the coefficient of
methanol (x + y =1).
CH3OH (g) + x H2O (g) + ½ y O2 Æ (3x +2y) H2 + CO2 ∆H0=49x-192y kJ/mol
(8)
The maximum theoretical hydrogen concentration is dependent on the oxygen
to methanol ratio, OMR (mole oxygen / mole methanol), as shown in Figure 5.
When operating under stoichiometric conditions x + y = 1. The thermal nature
of the process is also dependent on the OMR as illustrated in Figure 5.
For automotive solutions the oxygen would even for the combined process be
supplied as compressed air and the system would also operate using 30 %
excess water to lower the formation of carbon monoxide (Equation 9) [41, 42].
CH3OH + 1,3x H2O + ½ y O2 + 1,88y N2 Æ (3x +2y) H2 + CO2 + 1,88y N2 +
(9)
0,3x H2O
The dependence of the hydrogen concentration on the OMR for the combined
process using air in the feed is illustrated in Figure 5.
15
Figure 5: Influence of oxygen-to-methanol ratio on hydrogen concentrations
1.6 Scope of the Thesis
The present study was part of the activities within the framework of the Swedish
Energy Agency programme “Energy Systems for Road Vehicles” Project no
P10792-2 (1998-2003). The work at the Royal Institute of Technology focused
on the development of catalysts and catalytic systems for automotive reforming.
Laboratory tests were performed in collaboration with the Volvo Technology
Corporation in order to ensure that the reformer met the industrial standards of
an automotive reformer.
The purpose of the work presented here was to develop and test catalysts for
catalytic hydrogen generation from methanol, as well as to develop a methanol
reformer capable of serving a 5 kWe fuel cell. Emphasis was placed on
designing a self-sustaining reformer (avoiding electrical heating), with low
start-up times and low concentrations of carbon monoxide. A catalytic system
for igniting liquid methanol for an internal heat exchange system for the
reformer was also developed.
For the reforming system pellet-based catalysts and monolithic catalysts were
tested. The γ-alumina pellets used in all experiments were provided by SASOL
Germany and the cordierite monoliths were from DOW Corning. The catalysts
were impregnated with various base metals supported on γ-alumina (γ-Al2O3).
The influences of the operating parameters were also studied in great detail in
order to obtain a system that has a high activity and selectivity. For the
oxidation system a mixture of base metal oxides and noble metals were studied.
16
We have also used various characterization techniques in order to improve the
understanding of the structural and chemical properties of the catalysts. The
characterization methods used in this study were: Temperature Programmed
Reduction (TPR), Temperature Programmed Oxidation (TPO), Pulse
Chemisorption, X-ray diffraction, BET-Surface area analysis and Scanning
Electron Microscopy (SEM).
The thesis consists of 7 papers and a main section where the results from the
papers have been summarized in four chapters: “Materials and processes for
catalytic hydrogen generation from methanol”, “Reforming catalyst and process
optimisation”, “catalytic oxidation of liquid methanol” and “Development and
evaluation of a catalytic reforming system” Specifics regarding the preparation
methods, characterization techniques and operating parameters are described in
detail in the papers.
Paper I is a review describing the various methods and materials used for
catalytic hydrogen generation from methanol. The paper presents detailed
descriptions of the materials used as well as a detailed account of proposed
mechanisms for the various processes available for converting methanol into
hydrogen. Included in the paper is also an analysis of the current industrial
activities in the field of automotive reforming.
Papers II-V present investigations of the influence of the catalytic material and
operating conditions on the activity and selectivity for methanol reforming. The
influence of applying a promoter with the active phase as well as the influence
of the substrate properties is also studied.
Paper VI deals with the problems of igniting liquid methanol at room
temperature. In this paper a novel reactor system designed at the Royal Institute
of Technology is presented. The paper also includes a catalytic study, where
various materials are tested.
Paper VII presents the reformer prototype developed in our laboratory, focusing
on reducing start-up times and CO concentrations.
17
2. Properties of the system components of the methanol
fuel cell propulsion system
2.1 Introduction
The fuel cell-based engine is a complete power solution that can easily be
incorporated into a wide range of vehicle and stationary platforms.
When describing the methanol fuel cell (MFC) propulsion system it is
convenient to divide the system into a number of interacting functional
modules, where the overall performance of the system is dependent on the
compatibility and performance of the individual modules. The main modules of
the MFC propulsion system are:
•
•
•
•
Fuel processor
Gas clean-up
Air module
Fuel cell module
When designing the individual modules there are several criteria that have to be
met with respect to cost and performance. The US Department of Energy (DOE)
and the automotive industry have together developed a series of criteria for all
of the modules in a fuel cell vehicle (FCV). These criteria were considered
when we designed the fuel processor [54].
The properties and functions of the individual modules will now be discussed in
detail. The main focus of this chapter is placed on the fuel processor, as the
objective of my thesis was to develop a compact methanol reformer for fuel cell
vehicles. A brief schematic of the MFC propulsion system is shown in Figure 6.
2.2 Fuel processor
The role of the fuel processor is to extract clean hydrogen from a hydrogen
carrier (in our case methanol). When designing an on-board fuel processor there
are several key requirements which have to be fulfilled [54, 55]
• The processor must be compact and light, due to the limited space in
automobiles where the weight of the vehicle sets the power requirements
18
• The reformed gas must contain low concentrations of by-products as the
fuel cell has a low tolerance to impurities and the size and weight of the
clean-up module is proportional to the by-product concentration
• The system must have a short start-up time
• The system must be highly responsive to changes in the hydrogen
demand
The fuel processing module consists of three separate units: catalytic reformer,
catalytic burner, and evaporator. Detailed descriptions of the units are outlined
in the following sections.
2.2.1 Catalytic reformer
The size and weight of the fuel cell system is ultimately dependent on the
performance of the fuel processor and it is therefore vital that every aspect of
the fuel processor is optimised. The fuel processing system is usually designed
around the catalytic reformer, translating the performance demands of the fuel
processor onto the reforming catalyst [54, 55].
Reforming catalyst requirements:
•
•
•
•
•
•
•
High activity in order to obtain a compact and light reformer
High selectivity to minimize the size of the clean-up module
Low temperature operation to minimize start-up time
Robust –The catalyst must withstand mechanical stress
Long life
Thermally resistant
Low cost
The development of reforming catalysts is described in detail in Chapter 4 and
Papers II-V.
19
20
Figure 6: MFC propulsion system
There are several process alternatives available for converting methanol into
hydrogen that can be implemented in an automotive reformer. The choice of
process and operating conditions greatly influences the performance of the fuel
cell system and thus process optimisation is vital for the construction of a
functional FCV. Detailed descriptions of the process alternatives are presented
in Chapter 3.
2.2.2 Catalytic burner
The exploitable energy surplus in a fuel cell vehicle is low and therefore one of
the most important tasks is to provide the heat required for the evaporation and
conversion of methanol and water. The most viable solution is to provide the
energy through indirect heat transfer with a catalytic burner.
The catalytic burner must be designed to consume methanol during start-up and
unreacted hydrogen during normal operation, as there is no energy surplus
available for electrical heating during start-up. These harsh operational criteria
increases the performance demands of the catalytic burner as it must therefore
be designed to handle oxidation and evaporation of liquid methanol as well as of
hydrogen. The performance criteria described for the reforming catalysts are
also applicable to the oxidation catalyst.
The catalytic burner and the oxidation catalysts developed in this project are
described in detail in Chapter 5 and Paper VI.
2.2.3 Evaporator
The evaporator utilizes indirect heat exchange with the catalytic burner and
simultaneously vaporises methanol and water. A detailed description of the
vaporising unit developed in this project can be found in Chapter 6.
2.3 Fuel cell
The fuel cell operates by electrochemical oxidation of hydrogen to form water
while generating electricity (see Figure 7) [23]. The electrochemical reactions
which take place on the catalytic surfaces of the anode and cathode can be
illustrated by the following reactions:
21
Anode: 2 H2 Æ 4 H+ + 4e-
(10)
Cathode: O2 + 4e- + 4H+ Æ 2 H2O
(11)
Figure 7: Fuel cell schematic
At the anode the hydrogen is ionised, releasing electrons while generating
protons. The hydrogen ions are transported through the membrane and at the
cathode oxygen reacts with the electrons and protons (produced at the anode) to
form water. The cathodic reaction is exothermic, releasing energy which has to
be removed by a coolant (see Figure 6).
The polymer electrolyte fuel cell (PEFC) used in automotive applications is
highly sensitive to poisoning especially by carbon monoxide, which may
significantly lower the performance of the fuel cell at concentrations above 50
ppm [18, 19].
The performance of the system is also dependent on the hydrogen concentration
of the reformed gas and thus it is favourable to select a hydrogen generation
process with a high hydrogen production capacity.
22
2.4 Gas clean-up module
There are two main processes which are considered for removing by-products
from the reformed gas in methanol fuel cell vehicles: Water-gas shift and
preferential oxidation. The processes are usually combined in series for optimal
results with respect to activity and selectivity.
2.4.1 Water-gas shift reaction
The water-gas shift (WGS) reaction (Equation 12) is an important aspect of
automotive reforming as it involves the removal of carbon monoxide (CO) from
the product stream [36]. The shift step can either be used as a separate clean-up
step or as part of the reforming process which is the main reason for running the
steam reforming and the combined reforming steps with excess steam.
The maximum theoretical hydrogen concentration for the steam reforming and
combined reforming processes is also dependent on the formation of carbon
monoxide (CO), as the decomposition process yields lower H2 concentrations
(67 %) than the combined and steam reforming processes
The WGS reaction, shown in Equation 12 below, is slightly exothermic and
hence its equilibrium constant decreases with temperature (see Figure 8) and as
a result the equilibrium conversion decreases with increasing temperature.
∆H0 = -41 kJ/mol (12)
CO + H2O (g) Æ H2 + CO2
The efficiency of the WGS process can be improved by operating the reforming
process with excess steam [36].
23
Figure 8: Variation of the equilibrium constant for the WGS reaction with temperature
(adapted from Twigg [36])
2.4.2 Preferential oxidation
Selective catalytic oxidation is a promising technique for reducing the CO
concentration, in the reformed gas, to 10 ppm or less. The main problem with
this method is that hydrogen oxidation competes with the CO oxidation and also
further dilution of the reformed gas by nitrogen and hence leading to a loss of
system efficiency. In order to reduce the loss of hydrogen highly selective
catalysts must be developed.
Desired reaction: CO + ½ O2 Æ CO2
(13)
Undesired reaction: H2 + ½ O2 Æ H2O
(14)
2.4.3 Other clean-up processes
There are other clean-up processes currently being considered as alternatives for
stationary and mobile fuel cell applications. The main contenders are
methanation and membrane separation [23]. The methanation reaction
(Equation 15) operates by forming methane from CO and H2. The process does
not require additional air, which is an advantage as one can avoid diluting the
system with nitrogen. However for each mole of CO removed three moles of H2
24
is consumed making the process unsuitable for automotive fuel cell
applications.
∆H0 = -206 kJ/mol (15)
CO + 3 H2 Æ CH4 + H2O (g)
Membrane technology is a method which can separate CO from H2 without
decreasing the H2 concentration in the stream. The technology requires high
pressure which is energy consuming and currently the membranes are based
upon Pd making the system expensive and not feasible for automotive
applications.
2.4.4 Evaluation of the clean-up processes
The WGS reactor utilizes excess water from the reformer to convert CO to CO2
while generating H2, and therefore increasing the overall performance of the
system. The PROX reactor removes CO by oxidation, however the process is
not 100 % selective and so the hydrogen content is reduced during this clean-up
step. The PROX process also utilizes air to supply the required oxygen which
implies that the reformed gas is diluted with nitrogen and thus lowering the
overall performance of the system. In order to obtain optimal results from the
fuel cell system, utilization of the WGS reaction should be maximized in order
to avoid lowering of the hydrogen content during the gas clean-up.
2.5 Air module
Oxygen is required for the operation of the fuel cell system and has to be
transported all around the MFC system. Compressing ambient air to the
operating pressure of the fuel cell provides the oxygen. Some of the air is
required for the operation of the preferential oxidizer and the combustor and is
compressed further in a secondary compressor. It is also vital that impurities are
separated by filters and that each module is fed with exact concentrations. The
modules that require oxygen are:
•
•
•
•
Fuel cell
PROX reactor
Catalytic burner
Reformer
25
3. Materials and processes for catalytic hydrogen
generation from methanol (Paper I)
Catalytic production of hydrogen from methanol has been studied for the last
30-40 years, and decomposition of methanol to carbon monoxide (CO) and
hydrogen (H2) has been utilized commercially by the steel industry for decades
as a method for providing carbon monoxide for the carbonisation of steel.
Catalytic hydrogen generation by decomposition of methanol received much
attention during the eighties when a significant amount of investigations for
using decomposed methanol for improving efficiency and decreasing emissions
from internal combustion engines as well as improving the cold start of alcohol
engines were performed [40]. The decomposition process is however unsuitable
for fuel cell applications as CO is one of the main products, which has
detrimental effects on the fuel cell.
The processes that are most feasible for on-board production of hydrogen from
methanol for fuel cell applications are: steam reforming, partial oxidation and
combined reforming. In this chapter catalytic materials and processes for
extracting hydrogen from methanol for fuel cell applications will be discussed
in detail.
3.1 Steam reforming
Steam reforming (equation 4) of methanol (SRM) is a well-developed and
commercialised process. The process is usually carried out over copper-based
catalysts [41, 42, 59-70], however there has been some attention given to Group
8-10 metals as well [71-73].
CH3OH + H2O Æ 3 H2 + CO2
(4)
Steam reforming can under favourable conditions generate a product stream
containing 75 % H2 and 25 % CO2 on a dry basis. The high H2 production
capacity of steam reforming is the main argument for choosing steam reforming
over partial oxidation, which can only produce up to 41 % when air is used to
supply the oxygen.
The high-energy requirement of the steam reforming process is a major obstacle
for the implementation of a reformer based upon this process in an automotive
application. There are however several commercial solutions available that are
26
based upon steam reforming [74]. The influence of the catalytic material on the
SRM will now be discussed in detail.
3.1.1 Catalytic materials for steam reforming of methanol
The kinetics and reaction paths for the SRM is dependent on the catalytic
materials used, therefore the discussion is divided into two parts: (i) Traditional
copper-based catalysts supported on γ-alumina and (ii) materials containing
other transition metals supported on various oxides.
Copper-based catalysts
There are two major pathways suggested in the literature for SRM over copperbased catalysts: (i) a decomposition-water-gas shift (WGS) sequence and (ii)
methanol dehydrogenation over methyl formate.
The decomposition-WGS pathway (below) is considered to take place as a
sequence. Initially methanol decomposes to CO and H2 and then the CO reacts
further with water to form CO2 and H2.
CH3OH Æ CO + 2 H2
CO + H2O Æ CO2 + H2
CH3OH + H2O Æ CO2 + 3 H2
This mechanism has been accepted by several authors [42, 60-62, 68] and has
been studied significantly over both commercial and novel catalysts [60-62].
The methyl formate reaction route has been shown to be dependent on the
support, and that no CO was formed and that methyl formate and formic acid
are the only intermediates [41]. The suggested reaction path over γ-alumina was
as follows [63, 68, 71]:
2 CH3OH Æ CH3OCHO + 2 H2
CH3OCHO + H2O Æ HCOOH + CH3OH
HCOOH Æ CO2 + H2
CH3OH + H2O Æ CO2 + 3 H2
The formation of by-products for steam reforming over copper-based catalysts
is generally low. The formation of products such as CO, formic acid and methyl
27
formate (which has been observed by some researchers [63, 67, 71]) is
significant as it poses a threat to the performance of the fuel cell. Operating the
SRM with excess steam and thereby integrating the WGS reaction into the
reformer can minimize the formation of CO.
Other transition metal-based catalysts
The catalytic properties of copper for SRM are significantly different from other
transition metals. Several investigations have been performed on the behaviour
of Group 9-10 transition metals on the conversion of alcohols [70, 71]. Table 2
lists some examples of catalytic materials tested for SRM found in literature
Table 2: Group 9-10 catalytic materials used for SRM
Active phase
Support
Pd
Al2O3
Pt
SiO2
Ni
ZrO2
Rh
ZnO
Support
Cr2O3
La2O3
MnO2
MgO
The major difference between copper and other transition metals is the CO2
selectivity. Several investigations [71-73] showed that CO concentrations up to
25 % could be achieved during SRM, results comparable to decomposition. The
influence of the support was shown to be significant. The high CO
concentrations obtained for these transition metals makes them highly
unsuitable for fuel cell applications.
3.2 Partial oxidation
The production of H2-rich gases with low CO concentrations from methanol
was until the middle of the eighties solely done by steam reforming. The high
endothermic nature of the steam reforming reaction led researchers to search for
alternate methods for generating CO free hydrogen.
Partial oxidation (equation 6) of methanol (POM) offers several advantages
over steam reforming. The reaction is exothermic (i.e. thermodynamically
favourable) and requires only air as an oxidant and displays higher reaction
rates than steam reforming. Theoretically POM can generate a product stream
containing up to 67 % H2, however in automotive applications the oxygen will
most likely be provided from compressed air reducing the theoretical H2
maximum to 41 %.
28
CH3OH + ½ O2 Æ 2 H2 + CO2
(6)
The influence of the catalytic material on the POM reaction will now be
discussed in detail.
3.2.1. Catalytic materials for partial oxidation of methanol
An investigation of the current literature showed that two types of catalysts
were favoured for the POM reaction: copper and palladium. The catalytic
properties of these materials show significant discrepancies with respect to byproduct formation and the effect of oxygen partial pressure. The Cu-based
catalysts display high selectivities for the POM reaction whereas for the Pdbased catalysts the CO concentration is quite significant. The materials will for
these reasons be discussed separately.
Copper-based catalysts
The reaction path for POM over Cu-based catalysts is quite complex. Several
reactions have been observed which are catalysed by copper, e.g. steam
reforming, partial oxidation, decomposition, WGS and total oxidation [47-50,
75-80]. The selectivity for H2 formation over Cu-based catalysts has been
shown to have a strong dependence on the methanol conversion, suggesting that
the oxidation and reforming takes place consecutively [47,48,75-80].
Formaldehyde has been found as an intermediate of the POM reaction over Cubased catalysts and by studying the formation of formaldehyde and the
subsequent decomposition into CO and H2. Investigations have also shown that
when operating POM under fuel-rich conditions a product mixture containing
formaldehyde, CO and H2 was produced. There are several studies that indicate
that the POM reaction involves SRM and decomposition. Huang and Wang [81]
proposed the following mechanism:
CH3OH + ½ O2 Æ H2 + CO + H2O
CH3OH Æ CO + 2 H2
CO + H2O Æ CO2 + H2
Which in essence suggests that partial oxidation consists of all of the reactions
above, while the conventional steam reforming is reported to only consist of
29
decomposition and WGS. Based upon this results we may draw the conclusion
that steam reforming is part of the partial oxidation scheme.
There are several by-products that may be formed in the POM reaction, apart
from unreacted methanol and CO results have shown that dimethyl ether
(DME), formic acid, formaldehyde and methane are possible [81].
The activity and selectivity of the POM is strongly influenced by the reaction
temperature. The conversion of methanol increases with temperature, whereas
the CO2 selectivity decreases. The copper-zinc based catalyst is known to
deactivate quickly during operation, however using an alumina support can
stabilize the material at the cost of lower activity.
Palladium-based catalysts
The activity of Group 10 metals, such as palladium and platinum, are active for
the conversion of methanol. However they are much less selective than the
copper-based catalysts described in the previous section, yielding primary the
decomposition products [71-73]. This catalytic property makes them less
feasible for fuel cell applications. The only exception found was for Pd/ZnO
which showed selectivity close to that of copper [70].
3.3 Combined reforming
The idea of combining steam reforming with partial oxidation was first initiated
by Huang and Wang in 1986 [81] who proposed a new reaction route for
producing H2 from methanol. The combined reforming reaction (equation 8)
provides a method for producing relatively high hydrogen selectivities and low
CO concentrations while maintaining a dynamic system.
CH3OH (g) + x H2O (g) + ½ y O2 Æ (3x +2y) H2 + CO2
(8)
When operating the combined process under stoichiometric conditions the sum
of coefficients of water (x) and oxygen (y) equals the molar coefficient of
methanol (x + y =1).
The combined reforming reaction can be operated under endothermic,
exothermic or thermally neutral conditions, dependent on the chosen oxygen-tomethanol ratio. The number of research groups focusing on combined reforming
has rapidly increased during the past years as well as the number of
publications. [17, 43, 50-53, 81, III] The main by-product for the combined
30
reforming reaction has been shown to be CO, however there has been some
observations of methyl formate and formic acid [50, 81].
The materials used during the combined reforming reaction are mainly the
copper-based materials used for the steam reforming and partial oxidation
reactions and detailed descriptions of these materials are presented in sections
3.1.1 and 3.2.1
3.4 Summary
The literature reviewed in Paper I clearly indicates that methanol reforming
catalysts should contain copper in order to obtain high activity and low CO
concentration in the product stream. The Pt group metals were observed to
generate mainly the decomposition products which make them unsuitable for
methanol reforming.
31
4. Reforming catalyst and process optimisation (Papers II,
III, IV and V)
4.1 Introduction
From the results presented in chapter 3 it is evident that, regardless of the
operating conditions, the catalysts used for catalytic reforming of methanol
should contain copper. The role of the support and promoter as well as the
influence of the structural and surface properties of the catalyst on the catalytic
activity and selectivity is discussed in detail in this chapter. The impact of the
operating conditions on the activity and product distribution is also discussed in
this chapter.
4.2 Laboratory experimental set-up
The activity test was performed in a reactor system developed completely inhouse at our own laboratory. When we constructed the reactor system for the
catalytic screening, several design criteria were set in order to obtain a flexible
reactor system:
(i)
(ii)
(iii)
(iv)
(v)
The catalytic reactor should be multifunctional with respect to the
form and shape of the catalyst (i.e. powders, pellets and monoliths)
The fuels should be completely vaporised and mixed prior to entering
the reactor in order to avoid fuel rich zones which could cause
transient CO levels
Temperature control of the reactants should be maintained in a preheater to enable varying the temperature of the reactants prior to
entering the reactor
Strict temperature control over the entire catalytic reactor
The reactor system must be process flexible in order to enable study of
steam reforming, partial oxidation and combined reforming at various
oxygen-to-methanol ratios and steam-to-methanol ratios.
The reactor system constructed for this project (see Figure 9) utilizes liquid
methanol and water, which are evaporated in separate evaporators prior to
entering the mixing chamber. The reactor was made of stainless steel (ASTM
316) with an inner diameter of 25 mm. The product stream composition was
measured on-line with a gas chromatograph from Varian equipped with both a
thermal conductivity detector (TCD) and a flame ionisation detector (FID). The
32
reactor temperature was measured with thermocouples (type K) connected to a
LabView ™ based measuring system developed at our laboratory.
Prior to the activation tests all of the catalysts used were activated by reducing
the CuO to Cu0 by passing a stream of 10 % H2 in N2 over the catalysts at a
heating rate of 5 °C/min and dwelling at 220 °C for 2 h. The sufficiency of the
reduction process was verified with X-ray diffraction. For a more detailed
description of the reduction process please refer to papers II-V.
33
34
Figure 9: Laboratory reactor system
4.3 Copper based materials for steam reforming of methanol (Paper
II)
4.3.1 Background
Originally we intended to use a commercial catalyst in the reformer in this
project, however, the activity tests over some commercial catalysts quickly
revealed the need for developing new materials for automotive reforming
applications. The goal of the study presented in Paper II was to screen various
copper-based materials for steam reforming of methanol (SRM) and to
investigate the role of the support material for this process.
The formation of CO was particularly interesting at this stage and therefore the
methanol and water was fed at a 1:1 ratio, in order to simulate a worst-case
scenario. The catalysts were prepared by the wet-impregnation method and
deposited onto cylindrical γ-alumina pellets (3.2 mm) from SASOL Germany.
For detailed descriptions on the preparation technique and operating conditions
please refer to Paper II.
A review of the current literature led us to select three different promoter
materials for the screening: chromium (Cr), zinc (Zn) and zirconium (Zr). For
the tests we decided to prepare both binary and ternary mixtures of the materials
with different compositions in order to establish an understanding of the role of
the promoter on the catalytic activity. The catalysts prepared for the tests are
listed in Table 3. The total catalyst loading was 5 wt % of the γ-alumina.
4.3.2 Influence of the promoter on the activity and selectivity
The influence of the promoter material on the activity and selectivity is shown
in Table 3 below. The results indicate that increasing the copper loading
increased the activity for all of the catalysts. The study however only included
catalysts with a high or low copper loading. Therefore the influence of the
copper loading on the dispersion (and thus the activity) that was shown later in
paper V was not apparent in this study.
35
Table 3: Influence of catalyst loading on SRM under stoichiometric conditions
Catalyst Composition H2 200 °C H2 max T 60% H2 Se min Se mean
[vol. %]i [vol. %]ii
[°C]iii
[%]iv
[%]v
[mol %]
Cu/Cr
80/20
50
74
235
89
91
Cu/Zn
80/20
46
65
250
73
87
Cu/Zr
80/20
56
66
210
90
93
Cu/Cr
20/80
38
64
280
78
84
Cu/Zn
20/80
40
66
250
76
86
Cu/Zr
20/80
26
68
260
84
91
Cu/Cr/Zn
60/20/20
40
74
230
90
94
Cu/Cr/Zr
60/20/20
50
72
220
89
93
Cu/Cr/Zn
40/20/40
36
73
238
88
92
Cu/Cr/Zr
40/20/40
37
73
230
88
92
Cu/Cr/Zn
20/40/40
16
71
280
64
86
Cu/Cr/Zr
20/40/40
30
69
260
85
89
Cu/Cr/Zn
20/60/20
17
67
265
80
89
Cu/Cr/Zr
20/60/20
20
68
250
77
90
i
Volumetric hydrogen concentration at 200 °C
ii
Maximum hydrogen concentration obtained for specific catalyst
iii
Temperature at which hydrogen concentration in reformed gas is 60 vol%
iv
Lowest CO2 selectivity (Se = [CO2]/([CO2]+[CO])) obtained for entire
temperature range
v
Average CO2 selectivity
The tests also indicated that for lower temperatures (T<220 °C) the zincsupported catalysts were most active and for temperatures above 220 °C the
chromium-supported catalysts were most active. Zirconium was shown to have
a positive effect on the CO2 selectivity and the highest CO2 selectivity was
obtained for ternary Cu/Cr/Zr catalysts. The Cr promoted catalysts were
generally more active for the steam reforming process under these conditions
and was therefore considered to be the most suitable material.
4.4 Influence of the steam-to-methanol ratio on the catalytic activity
of the steam reforming of methanol
Based upon the results obtained in paper II we concluded that Cr-promoted
catalysts were most suitable for the reforming process. The total CO
concentrations obtained during the experiments were however due to the
reaction conditions too high (CO >2 vol%) and experimental studies in which
the operating conditions were varied were required.
36
Figure 10: Influence of the steam-to-methanol ratio on the hydrogen concentration in
the product stream for Cu/Cr/Al2O3 catalyst.
Cu/Cr/Al2O3 catalysts were therefore prepared and tested for reforming under
various steam-to-methanol ratios. The results (see Figure 10) showed that the
activity severely decreased with increasing steam-to-methanol ratio.
Furthermore the CO concentrations did not significantly decrease at the same
time.
We then therefore decided to test various Cu/Zn/Al2O3 and Cu/Zr/Al2O3
catalysts and discovered that the activity of the Zn-based catalysts was
significantly increased as a result of increasing the steam-to-methanol ratio (see
Figure 11) while producing notably lower CO concentrations (CO <1 %).
The optimal steam-to-methanol ratio for the steam reforming process was found
to be at approximately 1.3-1.4:1. These new results prompted us to switch to
Cu/Zn/Al2O3 as the main catalytic material for the steam reforming process. The
results also showed that the activity of the Zr-based catalysts decreased when
increasing the steam-to-methanol ratio (slightly lower than for Cr-promoted),
however, the CO concentrations were far the lowest for all of the materials
tested.
37
Figure 11: Influence of the steam-to-methanol ratio on the hydrogen concentration in
the product stream for Cu/Zn/Al2O3 catalyst.
4.5 Copper-based materials for the combined reforming process
(Paper III)
4.5.1 Background
The strong influence of the operating conditions that were detected for the
materials for the steam reforming process led us to test all of the promoter
materials used in Paper II for combined reforming. For these tests ceramic
monoliths from Corning (62 cells/cm2) were used as carriers. The ceramic
monoliths were selected, since the γ-alumina pellets that were used in Paper II
may not be suitable for the harsh automotive environment. For this reason we
wanted to investigate the feasibility of using monoliths for the reforming of
methanol, as they have been proven suitable as exhaust gas catalysts. The strong
dependency shown of the activity on the copper loading led us to compare the
different catalysts by wt% of active material and promoter rather than the mol%
used in Paper II.
The ceramic monoliths were initially coated with γ-alumina in order to increase
the surface area before impregnating the copper and promoter. For these tests
38
Table 4: Catalyst composition
Catalyst
CuZn
CuCr
CuZr
Composition
[wt%]
40:60
40:60
40:60
Active material
[wt%]
10.2
10.1
10.3
three different catalysts with the same total metal loading were prepared (see
Table 4).
The catalysts were evaluated for three operating conditions: Steam reforming
(30 % excess steam), combined reforming at near auto-thermal operating
conditions and combined reforming under slightly exothermic conditions. For a
detailed description of the operating conditions and catalyst preparations please
refer to Paper III.
4.5.2 Influence of the promoter material on the combined methanol
reforming process
The combined reforming process was highly efficient for all of the promoters
used in this investigation. The CO concentration was however slightly elevated
when running the process under exothermic conditions compared to steam
reforming. The steam reforming process delivered lower H2 concentrations for
the monolithic carriers compared to the pellet based catalysts, due to bad heat
transfer between the channels in the monolith. This was not a problem for the
combined reforming process as heat was generated at the entrance of the
monolith channels. This problem was resolved in the study described in Paper
IV by increasing the temperature of the reactants.
The results from the experiments is shown in Table 5 below, and it can be seen
that the activity for the catalysts was similar for the combined reforming process
while the highest activity for the steam reforming process was obtained for the
Zn-promoted catalyst. The Cr-promoted catalysts however generated
significantly higher CO concentrations for all of the process alternatives. The
high CO concentration makes Cr unsuitable for promoting copper-based
catalysts for methanol reforming for fuel cell based applications. The Zrpromoted catalysts had the highest CO2 selectivity and may be suitable as part
of a low temperature WGS reactor.
39
Table 5: Comparison of hydrogen generation processes
H2
T 60%
Se
Se
CO
Catalyst Process
H2
210°C
max
H2
min mean
max
i
ii
iii
iv
v
[°C]
[vol%] [vol%]
[%] [%]
[vol%]vi
Cu/Zn
SR
3.6
64
280
97
98
0.77
Cu/Cr
SR
2.3
50
78
90
1.1
Cu/Zr
SR
2.7
52
98 100
0.27
Cu/Zn
CR1
8.3
73
230
94
97
1.1
Cu/Cr
CR1
55
74
220
83
90
2.5
Cu/Zr
CR1
2.1
72
252
86
95
0.84
Cu/Zn
CR2
11
72
220
90
95
1.8
Cu/Cr
CR2
54
72
215
90
78
1.8
Cu/Zr
CR2
2.3
67
230
98
99
0.51
i
Volumetric hydrogen concentration at 210 °C
ii
Maximum hydrogen concentration obtained for specific catalyst
iii
Temperature at which hydrogen concentration in reformed gas is 60 vol%
iv
Lowest CO2 selectivity (Se = [CO2]/([CO2]+[CO])) obtained for entire
temperature range
v
Average CO2 selectivity
vi
Maximum CO concentration in product stream
The results from this study suggests that Zn-promoted catalysts are most
suitable for fuel cell applications for both steam reforming and combined
reforming as they generate product streams with both high hydrogen content
and low CO concentration.
4.6 Zirconia-doped catalysts for the steam reforming process
(Paper IV)
4.6.1 Background
Since we were convinced that the heat transfer limitations in the ceramic
monoliths inhibited the performance of the catalysts we decided to preheat the
reactants to the reactor temperature. While Zr-based catalysts were shown to
have a lower activity than Zn-based catalysts they also, however, generated
lower CO concentrations. We therefore wanted to investigate whether it was
possible to increase the CO2 selectivity of Zn-based catalysts by doping them
with Zr.
40
For this study two sets of catalysts with varying compositions (see table 6) were
prepared using cordierite monoliths in the same manner as those used in paper
III. The Zr loading was for all catalysts 10 wt % of the total metal loading.
Table 6: Catalyst composition
Catalyst
Active material
Cu/Zn
[wt%]
100/0
15.3
80/20
14.9
60/40
15.2
40/60
15.3
20/80
15.2
0/100
15.4
Catalyst
Cu/Zn [Zr]
100/0
80/20
60/40
40/60
20/80
Active material
[wt%]
15.3
15.2
15.3
15.2
15.2
The catalysts were tested for the steam reforming reaction using 30 % excess
steam to minimize the CO concentration in the product stream and the reactants
were for all of the tests fed at the reactor temperature.
4.6.2 Influence of zirconia-doping on the formation of CO for the steam
reforming of methanol
The activity tests performed in this investigation revealed several interesting
details regarding steam reforming of methanol. During these test we were able
to increase the activity of the monolith-supported catalysts by raising the
reactant temperature to the temperature of the reactor and thus eliminating the
heat transfer limitations that were encountered in the study described in paper
III.
Concerning the influence of doping the catalysts with Zr, two significant trends
were apparent:
(i)
(ii)
The Zr-doped catalysts had generally a lower activity than the
catalysts containing only Cu and Zn on γ-alumina.
The Zr-doped catalysts generated lower CO concentrations for all of
the tests
The Zr-doped catalysts were however more catalytically active than the Zrpromoted catalysts tested in the previous study while maintaining significantly
lower CO concentrations than the catalysts only promoted by Zn. The results
from the activity tests are summarized in table 7.
41
Table 7: Effect of zirconium doping
Se min
Se mean
T 60% H2
CO max
Catalyst
H2 210°C H2 max
i
ii
iii
iv
v
[vol%]
[vol%]
[°C]
[%]
[%]
[vol%]vi
Cu100
58
68
225
97
98
0,69
Cu100 [Zr]
44
59
99
100
0,12
Cu80/Zn20
52
62
269
96
97
0,85
Cu80/Zn20 [Zr] 47
61
285
99
100
0,21
Cu60/Zn40
64
75
195
96
97
0,96
Cu60/Zn40 [Zr] 43
60
98
99
0,43
Cu40/Zn60
51
71
238
96
97
1
Cu40/Zn60 [Zr] 37
55
97
98
0,65
Cu20/Zn80
1,8
63
279
69
80
1,2
Cu20/Zn80 [Zr] 1,4
50
95
98
0,85
i
Volumetric hydrogen concentration at 210 °C
ii
Maximum hydrogen concentration obtained for specific catalyst
iii
Temperature at which hydrogen concentration in reformed gas is 60 vol%
iv
Lowest CO2 selectivity (Se = [CO2]/([CO2]+[CO])) obtained for entire
temperature range
v
Average CO2 selectivity
vi
Maximum CO concentration in product stream
4.7 Characterization of copper-based catalysts for methanol
reforming (Paper V)
4.7.1 Background
The results from the activity tests indicate that both the copper loading and
promoter material have a strong influence on the activity and selectivity. In
order to develop an understanding of these results we decided to perform a
study aimed at understanding both the bulk and surface properties of the copperbased catalysts.
For these tests we used spherical γ-alumina pellets (d=2.5 mm) from SASOL
Germany. The catalysts were prepared using the wet-impregnation method and
the total loading was increased to 15 wt % of the γ-alumina pellets for all of the
catalysts used. In the previous experiments we loaded the catalysts with only
10-wt % of the γ-alumina. This change was implemented in order to increase
the activity of the catalysts. For the test three sets of catalysts were prepared
(see table 8) each with different promoter.
42
Table 8: Catalyst sample compositions
Catalyst/Mass
Catalyst/Mass
distribution
distribution
Cu15
Cu12/Cr3
Cu12/Zn3
Cu9/Cr6
Cu9/Zn6
Cu6/Cr9
Cu3/Cr12
Cu6/Zn9
Cu3/Zn12
Cr15
Zn15
Catalyst/Mass
distribution
Cu12/Zr3
Cu9/Zr6
Cu6/Zr9
Cu3/Zr12
Zr15
The bulk and surface properties of the catalysts were characterized by several
techniques including:
(i)
(ii)
(iii)
(iv)
(v)
Temperature programmed reduction (TPR)
Copper surface area measurements by pulse chemisorption of nitrous
oxide
X-ray diffraction (XRD)
Scanning electron microscopy
BET surface area measurements
The catalytic activity of the catalysts was tested for both steam reforming
(operated with 30 % excess steam) and for combined reforming (operated at an
oxygen-to-methanol ratio of 0.15, ∆H° = -23 kJ mol-1).
Detailed description of the operating conditions and characterization techniques
is outlined in paper V)
4.7.2 Influence of the surface properties on the catalytic activity
The metal surface area measurements revealed that with an increase in copper
loading the dispersion increased to a maximum and then decreased with the
loading. The maximum dispersion was obtained when the copper loading was 6
wt % (40 % of the total metal loading) for all of the promoters (see Figure 12
below).
43
a) Zn
b) Cr
c) Zr
Figure 12: Influence of copper loading on copper surface area for various promoters (SCu:
Copper surface area, DCu : Copper dispersion)
The activity of the catalysts could also be correlated to these results, as the
catalysts with the highest copper surface area showed the highest activity (see
figure 13). The significant difference in activity between the Zn- and Crpromoted catalysts can therefore be explained by the large difference in
dispersion obtained from the different promoter materials.
44
Figure 13: Influence of copper surface area on the rate of hydrogen production for
steam reforming (SR) and combined reforming (CR) of methanol
The temperature of the TPR peak maximum was also found to be an indicator of
the catalytic activity. The catalysts with the highest activity yielded the lowest
peak temperature. There was however no correlation found between the BETsurface area and the catalytic activity.
A comparison between the steam reforming and combined reforming process
for the Zn-promoted catalyst loaded with 6 wt% copper was also performed.
The result of this investigation can be seen in Figure 14 below.
The results show that the steam reforming process yields the overall highest
conversions, while the CO concentrations were slightly lower for the combined
process. The energy requirements for the steam reforming reaction is however
higher than for the combined process, which must be considered when selecting
a process.
Figure 14: Influence of hydrogen generation process on methanol conversion and CO
concentration for Zn promoted catalysts
45
The study in Paper V shows that the activity and selectivity is dependent on
several factors including: the copper loading, the copper dispersion as well as
the promoter material.
4.8 Summary
The studies which have been performed in papers II-V clearly show that copperbased materials can be successfully used for catalytic reforming of methanol.
The tests also indicate that the choice of promoter and copper loading has a
notable effect on both catalytic activity and selectivity. Catalysts promoted by
zinc were most active and yielded a high CO2 selectivity. The CO2 selectivity of
Zn-promoted catalysts could be improved by doping the catalysts with Zr,
however, the doping lowers the CO concentrations at the expense of the
catalytic activity.
46
5. Catalytic oxidation of liquid methanol (paper VI)
5.1 Introduction
The exploitable energy surplus in a fuel cell vehicle is low and therefore one of
the most important tasks is to provide the heat required for the evaporation and
conversion of methanol and water. In this study we investigated whether it was
possible to use methanol combustion (equation 16) as an indirect heat source.
CH3OH (l) + 1.5 O2 Æ CO2 + 2 H2O (l)
∆H° =-727 kJ/mol (16)
Combustion of gaseous methanol is a highly developed process, however, as we
are using liquid methanol in the feed, the experiences from traditional
combustion will however not always be applicable for this specific problem.
5.2 Experimental lab-scale set-up
For this study we constructed a unique reactor system (see figure 15) which
utilizes catalysts for both evaporation and oxidation of the methanol. The basic
principle of the reactor system is as follows:
(i)
(ii)
(iii)
Liquid methanol is combined with air in a nebulizer to form a mist of
finely divided droplets
The air/methanol mixture is then transported to the catalyst where the
methanol is evaporated
The vaporised methanol then reacts with oxygen on the catalyst
surface to form CO2 and H2O while generating heat
The catalytic system has been designed for operation with both liquid methanol
and hydrogen in the feed. The multifunctional system was developed so that
unreacted hydrogen from the anode can be used once the system has reached
steady state, in order to increase the fuel efficiency of the system.
For this study we prepared several catalysts (see table 9 on page 50) using both
noble and base metals. The catalysts were prepared using the wet-impregnation
method, and all catalysts were supported on γ-alumina pellets (2.5 mm) from
SASOL Germany.
47
For the tests a gas hourly space velocity (GHSV) of 20 000 was used and an
air/fuel equivalence ratio (λ) of 5 was used to avoid superheating the system.
For a detailed description of the reaction conditions and catalyst preparation
please consult paper VI.
Figure 15: Combustion reactor (1 Nebulizer, 2 Grid separating catalyst chamber from
nebulizer, 3-9 thermocouples, 10 Effluent, 11 catalyst bed)
5.3 Influence of catalyst material on the activity for methanol
oxidation
The general trend of the catalysts tested in this study was that catalysts without
platinum or palladium had low activity and it was only when Pt or Pdcontaining catalysts were used that ignition was possible. The results of all the
catalysts tested are presented in Table 9.
48
The Pd-based catalysts, however, had to be reduced to Pd from PdO in order for
ignition to occur, and they were easily oxidised back to PdO. We therefore
decided to abandon Pd in favour of Pt for the oxidation reaction.
The Pt content was shown to have a great influence on the time-on-stream to
ignition as well as the temperature distribution in the reactor. The risk for hotspot formation was also shown to increase with the Pt loading (see figure 16).
The formation of hot-spots increases the risk for thermal deactivation and
should therefore be avoided.
Further tests showed that it was possible to replace 75 % of the catalyst bed with
a Mn-based catalyst while still maintaining complete conversion. The
temperature distribution was also more even for the system with the mixed
catalysts.
Figure 16: Influence of Pt loading on bed temperature
49
Crystal phase1
Pt
Pt
Pt
Pt
Pt
Pt
Pt
Pt
PdO
PdO
PdO
Pd
Pd
Pd
AgAlO2
AgAlO2
AgAlO2, Pt
AgAlO2, Pt
AgAlO2, Pt
Co3O4
Co3O4
Co3O4, Pt
Co3O4, Pt
Table 9: Activity tests
Catalyst
Pt3
Pt2
Pt1
Pt0.5
Pt0.4
Pt0.3
Pt0.2
Pt0.1
Pd3
Pd2
Pd1
Pd3
Pd2
Pd1
Ag15
Ag10
Ag10Pt0.5
Ag10Pt0.3
Ag10Pt0.1
Co15
Co10
Co10Pt0.5
Co10Pt0.3
50
Maximum methanol
conversion [%]
100
100
100
99
99
98
96
96
22
12
5
95
92
89
11
12
98
89
72
21
18
97
92
Time on stream to max conversion
[s]
80
100
160
180
200
220
240
260
3002
3402
3602
180
220
240
1202
1002
180
200
220
2002
1802
240
260
Co10Pt0.1
Co3O4, Pt
La2O3
La15
La10
La2O3
La10Pt0.5
La2O3, Pt
La2O3, Pt
La10Pt0.3
La10Pt0.1
La2O3, Pt
Mn2O3
Mn15
Mn10
Mn2O3
Mn10Pt0.5
Mn2O3, Pt
Mn10Pt0.3
Mn2O3, Pt
Mn10Pt0.1
Mn2O3, Pt
SrO
Sr15
Sr10
SrO
Sr10Pt0.5
SrO, Pt
SrO, Pt
Sr10Pt0.3
Sr10Pt0.1
SrO, Pt
1
Analyzed by X-ray diffraction before activity testing
2
Time to extinction
51
86
19
16
98
94
90
27
23
100
100
100
9
10
97
91
78
220
2202
2002
180
200
220
2602
2802
120
140
160
1602
1802
200
220
220
5.4 Summary
The results presented in this paper show that it is possible to catalytically
combust liquid methanol at room temperature by using a nebulizer to disperse
methanol and air over the catalyst bed. The results also showed that it is
possible to use base-metal catalysts and that Pt is only required for the initial
ignition of the system.
The success of this study signified that it is possible to design an automotive
reforming system which utilizes the combustion of liquid methanol as a source
of heat for the reformer, thus eliminating the need for electric heating of the
reactor. The reforming system will consequently be smaller as the required
battery power is lowered.
52
6. Development and evaluation of a catalytic reforming
system (Paper VII)
6.1 Introduction
In Paper VI we successfully demonstrated that it was possible to catalytically
ignite liquid methanol at room temperature. Based on this result it was decided
that a compact reforming system, which utilized indirect heat exchange with
combusted methanol, was to be developed. This is in contrast to traditional
reactor systems where the reactants are preheated and the reactor temperature is
controlled by a furnace.
The objective of this current study was to develop and construct a selfsustainable compact methanol fuel processor for serving a 5 kWe PEFC. When
determining the required hydrogen flow rates, calculations were made with the
heating value of hydrogen while assuming that the efficiency of the fuel cell is
50 %. The required hydrogen flow per kWe was then found to be approximately
1000 Ndm3/h. Efforts were also made to optimise the fuel processor with
respect to start-up time and CO2 selectivity.
6.2 Experimental set-up
For this study we developed catalysts for the oxidation and reforming reaction
based upon the results of Paper VI and Papers II-V, respectively. The reforming
catalysts manufactured were copper-based and the oxidation catalyst was
composed of a Pt/MnO2 mixture. The catalysts were all supported on γ-alumina
pellets (2.5 mm) from SASOL Germany and prepared using the wetimpregnation method. For a more detailed description of the preparation
parameters please refer to Paper VII.
The catalytic reforming system constructed for this study (See Figure 17)
consists of four separate modules: (i) catalytic combustor, (ii) vaporizer, (iii)
reformer and (iv) shift reactor.
The basic principles of the reforming system are as follows:
(i)
Initially liquid methanol and air is fed to the catalytic combustor,
which utilizes the catalyst for both evaporation and oxidation
53
(ii)
(iii)
(iv)
(v)
The methanol combustion products are subsequently applied as a
heating medium to raise the temperature in the other modules
Once the desired operating temperature has been reached in the
modules, methanol and water is co-fed to the vaporizer
The gaseous methanol-steam mixture is thereafter combined with the
air in a separate chamber prior to entering the reformer
The reformed gas is then passed through a WGS reactor for CO
removal before analysis
The reformer was operated with CRM with an oxygen-to-methanol ratio (OMR)
of 0.15 (∆H° = -23 kJ/mol). The OMR of 0.15 was chosen in order to have a
process, which was slightly exothermic while maintaining a relatively high
theoretical maximum hydrogen (H2) concentration. The steam-to-methanol ratio
was 1.3 which has been shown in earlier work to be optimal for the CRM with
respect to activity and selectivity. For a more detailed description of the
operating parameters please refer to paper VII.
The exact design of the reforming system and composition of the catalysts
cannot be disclosed due to a proprietary agreement between the Royal Institute
of Technology and the Volvo Technology Corporation.
6.3 Evaluation of laboratory tests
In the laboratory tests it was rapidly determined that the reforming reaction is
strongly dependent on the temperature as seen in Figure 18, where the methanol
conversion is plotted against the average bed temperature.
The conversion exceeds 90 % only at temperatures above 250 °C. The H2
production rate also follows this trend. The CO concentration in the reformed
gas also (see Figure 19) unfortunately increases with temperature and reaches
5000 ppm at 260 °C. The reformer was chosen to operate at 260 °C, in order to
obtain high activity while generating acceptable levels of CO.
54
55
Figure 17: The catalytic reforming system
Figure 18: Influence of temperature on methanol conversion
Figure 19: Influence of temperature on CO concentration
The ability to construct an automotive reformer with a low start-up time is a
decisive factor for success. The start-up time was found to be dependent on the
oxygen-to-methanol ratio in the reformer (see Figure 20).
56
Figure 20: Influence of oxygen-to-methanol ratio on start-up time (CH3OH (g) + x
H2O (g) + ½ y O2 Æ (3x +2y) H2 + CO2)
Shorter start-up time is however obtained at the cost of increasing CO
concentrations in the product stream. The increase in the CO concentration is
most likely attributed to the overall increase in reaction temperature as the
thermodynamics of the WGS reaction is unfavourable at high temperatures [36].
Increasing the CO in the reformed gas implies that the size and cost of the
cleanup reactors will increase and thereby the total cost of the fuel cell system
will increase. In the end one will have to weigh the start-up time against the cost
and decide which factor will be most crucial for the consumer.
The stability of the system was studied for a period of 12 hours, where we see in
Figure 21 that the conversion and product concentrations were stable (within a
the error margin of the analysing equipment).
57
Figure 21: Stability test
6.4 Industrial evaluation and optimisation of reformer prototype
The reformer was also tested and evaluated together with staff at the Volvo
Technology Corporation in Gothenburg. During these tests we were able to
increase the methanol conversion by increasing the oxygen-to-methanol ratio
(see Figure 22).
58
Figure 22: Influence of oxygen-to-methanol ratio on the methanol conversion
The CO concentration was also shown to increase with the increasing
conversion, however we were able to maintain CO concentrations below 1 %
while operating the reformer at maximum capacity.
The start-up time of the reformer was also lowered to 2.3 minutes, by varying
the OMR during start-up. However, this resulted in CO concentrations between
1.2 and 1.3 %. The equipment was not insulated during the experiments.
The product gas was measured using a continuous detection system, consisting
of a TCD detector for H2 and parallel continuous IR-detectors for CO and CO2,
from BOO Instruments.
6.5 Summary
The results from this study shows that it possible to construct a self-sustainable
reformer operated without electrical heating. The CO concentrations obtained
during the experiments calls for implementation of a preferential oxidizer prior
to the fuel cell. The start-up times are currently not acceptable for automotive
applications, however, the start-up times can be reduced by increasing the heat
transfer characteristics of the system.
59
The system showed stability over time as well as over repeated start-up
experiments both before and after 12 hour studies. The start-up time was also
not notably affected by the exposure of the catalyst to the reactants for 12 hours.
60
7. Conclusions
Catalytic hydrogen generation from methanol for fuel cell applications has
received significant attention during the last two decades. The rapid
development of novel catalytic materials during the last decades show a great
potential for methanol reforming as a source for hydrogen production for
automotive fuel cell applications. There is however still much room for
improvement with respect to both the reactor design and the catalytic
performance.
The main conclusion of this work is that zinc-promoted copper-based catalysts
are superior for methanol reforming for fuel cell applications with respect to
activity and selectivity. It has also been shown that the reaction conditions have
a significant impact on the catalytic performance.
When studying the methanol reforming reaction, it was found that the water-tomethanol ratio had a strong influence on both the activity as well as on the
product composition. The optimum reaction condition was found to be when
operating the reformer with 30 % excess steam. The oxygen-to-methanol ratio
was also found to affect the performance of the reformer, especially the start-up
time and carbon monoxide concentration.
It has also been shown that catalytic oxidation of liquid methanol can be utilized
for heating the reformer during start-up. The catalytic material was shown to be
crucial and platinum was the only material with sufficient activity.
The utilization of various catalyst characterization techniques have provided
means for understanding the dissimilar catalytic behaviour of different materials
and compositions as well as providing a better understanding of the catalytic
phenomena.
When evaluating the reformer prototype developed in this project, it was clearly
shown that the start-up time and product composition was strongly dependent
on the feed composition. The start-up time could be reduced significantly by
increasing the oxygen-to-methanol ratio, but this was however done at the
expense of the carbon dioxide selectivity.
This thesis has shown that it is possible to design a self-sustainable methanol
reformer that generates carbon monoxide concentrations below 1 vol% in the
product stream, while still being able to operate at 100 % methanol conversion.
61
There is however the need to improve the start-up times of the reformer before
commercially introducing the technology to automotive fuel cell applications.
62
Acknowledgements
First I would like to start by thanking my supervisor, Lars Pettersson, for his
support and for always taking time to provide me with valuable feedback on my
work. I would also like to thank Per Ekdunge at the Volvo Technology
Corporation (VTEC) for providing valuable insight to the industrial viewpoint
of automotive reforming throughout my project and for scrutinizing my thesis.
I would like to thank the staff at VTEC for creative cooperation and for making
me feel at home in their laboratory. Special appreciation to: Monika RåbergHellsing, Martin Berggren, Ricard Blanc, Lars Johansen, Lars Carlhammar and
Göran Johansson.
I would like to thank my colleagues and friends at Chemical Technology.
Special thanks to Johan Agrell for great cooperation on papers I and III, Inga
Groth for help with the SEM, XRD and BET analyses and to Otto von
Krusenstierna for valuable discussions on the properties of the water-gas shift
reaction. Special thanks are due to Krister Sjöström for reviewing my thesis.
I would also like to thank P. Govind Menon for valuable insight on the various
characterization techniques for copper catalysts and to Joakim Nordlund for
interesting discussions on the properties of fuel cells.
The financial support of the Swedish Energy Agency and Volvo Technology
Corporation is gratefully acknowledged.
Finally I would like to thank my good friends Egil Sjölander, Daniel Hagström
and Roland Gustavsson for our traditional recreation every Thursday at Koppan.
63
Nomenclature
Al2O3
BET
C2H5OH
CH3OH
CH4
CO
CO2
Cr
CR
CRM
Cu
FCV
FID
GHSV
H2
H2O
i-C8H18
La
Lambda (λ)
MFC
Mg
Mn
MTBE
NH3
Ni
N2
NOx
O2
OMR
Pd
PEFC
PO
POM
Aluminium oxide (alumina)
Brunauer-Emmet-Teller (surface area measurements)
Ethanol
Methanol
Methane
Carbon monoxide
Carbon dioxide
Chromium
Combined Reforming
Combined Reforming of Methanol
Copper
Fuel Cell Vehicle
Flame Ionisation Detector
Gas Hourly Space Velocity
Hydrogen
Water
Iso-octane (2,2,4-trimethyl pentane)
Lanthanum
Air/fuel equivalence ratio
Methanol Fuel Cell
Magnesium
Manganese
Methyl tert-buthyl ether
Ammonia
Nickel
Nitrogen
Nitrogen oxides
Oxygen
Oxygen-to-methanol ratio (mol O2/mol CH3OH)
Palladium
Polymer Electrolyte Fuel Cell
Partial Oxidation
Partial Oxidation of Methanol
64
PROX
Pt
Rh
Se
SEM
Si
SOx
SR
SRM
SULEV
TCD
TPO
TPR
WGS
XRD
Zn
Zr
Preferential oxidation
Platinum
Rhodium
CO2 selectivity (Se=[CO2]/([CO2]+[CO]))
Scanning Electron Microscopy
Silicon
Sulphur oxides
Steam Reforming
Steam Reforming of Methanol
Super Ultra Low Emission Vehicle
Thermal Conductivity Detector
Temperature Programmed Oxidation
Temperature Programmed Reduction
Water-Gas Shift reaction
X-ray diffraction
Zinc
Zirconium
65
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