Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
THE FLORIDA STATE UNIVERSITY
FAMU–FSU COLLEGE OF ENGINEERING
ANALYSIS AND CONTROL OF AN IN SITU HYDROGEN GENERATION
AND FUEL CELL POWER SYSTEM FOR AUTOMOTIVE APPLICATIONS
PANINI K. KOLAVENNU
A Dissertation submitted to the
Department of Chemical Engineering
in partial fulﬁllment of the
requirements for the degree of
Doctor of Philosophy
Spring Semester, 2006
The members of the Committee approve the dissertation of Panini K. Kolavennu defended
on December 8, 2005.
Professor Directing Dissertation
Outside Committee Member
John C. Telotte
Bruce R. Locke
The Oﬃce of Graduate Studies has veriﬁed and approved the above named committee members.
I would like to express my deep sense of gratitude to my advisor Dr. Srinivas Palanki for
his guidance, help and encouragement throughout the course of this research. I am extremely
thankful to Dr. John Telotte for his help and invaluable suggestions and inspiring me with
his thoughtful insights into my research. I am indebted to Dr David Cartes who introduced
me to the adaptive control technique. I extend my heartfelt gratitude to Dr. Bruce R. Locke
and Dr. Ravindran Chella for their suggestions and continuing interest in my research. A
very very special thanks to my brother Dr. Soumitri Kolavennu who introduced me to the
concept of fuel cells and process control. He has been and will continue to be my guru and
a role model whose footprints have been my guiding lights.
A special thanks to Charmane Caldwell and Dr. Jyothy Vemuri for their help during
various stages of this research. I also thank my colleagues and friends in the department for
their help and constant support. A special thanks to all my roommates Nirup, Sasi, Vijay
and Sarma for their constant support and surviving my awe(some!)ful cooking.
I am grateful to my parents for their support and encouragement and instilling the
research spirit in me. I am thankful to my loving sister who helped me a lot right from
my childhood and thanks a lot for patiently listening to all the complaints my teachers had
about my mischief ( also for hiding them from my parents). A special thanks to Ananth
Ravi and Neelima. Last but not the least I would like to thank Supriya for her wonderful
love and support and for being there for me always and making my graduate life a pleasant
LIST OF TABLES
1.1 Hydrogen production based on the type of fuel . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2 Pertinent physical properties of transportation fuels . . . . . . . . . . . . . . . . . . . . . 5
2.1 Salient features of the diﬀerent types of fuel cells . . . . . . . . . . . . . . . . . . . . . . . 22
3.1 Standard heat of formations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.2 Standard heat of reactions and type of reaction . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.3 Heat duty calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.4 Heat from the anode tail gas for diﬀerent initial ﬂow rates into the reformer. . . 38
3.5 Heat from the combustor when methane is fed at 25%, 30%, 35% in excess to
that fed to the reformer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.6 Eﬀect of varying the steam to carbon ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.7 Kinetic parameters for the three reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.8 Parameters to calculate the equilibrium constant for the water gas shift reactor 49
3.9 Volume required for 90% conversion of CO in LTS reactor for diﬀerent
temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
4.1 Regression ﬁt data obtained from the Pukrushpan model . . . . . . . . . . . . . . . . . 66
4.2 Eﬀect of varying the methane ﬂow rate on the power output . . . . . . . . . . . . . . 72
5.1 Zeigler-Nichols Controller Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
5.2 Average ITAE error in kW obtained for the UDDS and US06-HWY proﬁles . . 84
5.3 ITAE error for the Adaptive controller with the derivative action designed for
the UDDS proﬁle and also implemented on the USHWY06 . . . . . . . . . . . . . . . . 89
5.4 Performance of MRAC on diﬀerent road proﬁles . . . . . . . . . . . . . . . . . . . . . . . . 94
5.5 Moles of methane required for a driving range of 300 and 400 miles for the
diﬀerent cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
5.6 Average ITAE for the PAFC for a step pulse and band limited white noise input104
5.7 Steady State Average ITAE for the PAFC including the deadzone . . . . . . . . . . 104
LIST OF FIGURES
1.1 Schematic diagram of the fuel cell system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1 Cross section of a polymer electrolyte membrane fuel cell . . . . . . . . . . . . . . . . 19
3.1 Eﬀect of operating temperature and oxygen excess ratio on heat duty . . . . . . . 39
3.2 PFD of Fuel Processing Subsystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.3 The concentration proﬁles obtained as a function of the reactor volume (a)
CHEMCAD results (b)MATLAB results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.4 Concentration proﬁles along the volume of the reformer. . . . . . . . . . . . . . . . . . . 55
3.5 Conversion of CO inside the WGS reactor along the volume of the reactor for
diﬀerent temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
3.6 Volume required for 90% conversion of CO inside the low temperature WGS
reactor for diﬀerent temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
3.7 Eﬀect of change in methane ﬂow rate on the hydrogen production . . . . . . . . . . 58
4.1 Methane feed Vs Power produced . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
4.2 Representative fuel cell performance curve at 25 o
C and 1 atm . . . . . . . . . . . . . 62
4.3 Eﬀect of relative humidity on the fuel cell polarization curve. . . . . . . . . . . . . . . 68
4.4 Pressure dependence of the fuel cell polarization curve. . . . . . . . . . . . . . . . . . . . 69
4.5 Polarization curve for a fuel cell operating at 353 K, pressure 5 bar and relative
humidity 100%. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.6 Power density vs. current density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
4.7 Eﬀect of Methane Flow on Power Generated . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
5.1 Model Reference Adaptive Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
5.2 Implementation of Model Reference Adaptive Control . . . . . . . . . . . . . . . . . . . . 77
5.3 PID controller Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
5.4 System Identiﬁcation using a step input in current . . . . . . . . . . . . . . . . . . . . . . 81
5.5 Speed Vs time proﬁle and Force Vs time proﬁle for UDDS . . . . . . . . . . . . . . . . 82
5.6 Power Vs time proﬁle for UDDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
5.7 Simulink diagram of the adaptive controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
5.8 Error obtained(kW) for the PID and Adaptive controllers implemented on the
nonlinear model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
5.9 Speed and Power proﬁles for the US06-HWY driving cycle . . . . . . . . . . . . . . . 85
5.10 Adaptive controller with derivative action . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
5.11 Error Vs time plot for the adaptive controller with derivative action imple-
mented on the UDDS power proﬁle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
5.12 Error Vs time plot for the Adaptive controller with derivative action imple-
mented on the US HWY-06 power Proﬁle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
5.13 FTP Cycle: Speed Vs time and Power Vs time proﬁles . . . . . . . . . . . . . . . . . . . 91
5.14 FTP Cycle: Error Vs time plot for the Adaptive controller with derivative action. 92
5.15 US06 Cycle: Speed Vs time and Power Vs time proﬁles. . . . . . . . . . . . . . . . . . . 93
5.16 US06 Cycle: Error Vs time plot for the Adaptive controller with derivative
action. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
5.17 HFET Cycle: Speed Vs time and Power Vs time proﬁles. . . . . . . . . . . . . . . . . . 95
5.18 HFET Cycle: Error Vs time plot for the Adaptive controller with derivative
action. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
5.19 EUDC Cycle: Speed Vs time and Power Vs time proﬁles. . . . . . . . . . . . . . . . . . 97
5.20 EUDC Cycle: Error Vs time plot for the Adaptive controller with derivative
action. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
5.21 EUDC-LOW Cycle: Speed Vs time and Power Vs time proﬁles. . . . . . . . . . . . . 99
5.22 EUDC-LOW Cycle: Error Vs time plot for the Adaptive controller with
derivative action. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
5.23 IHP Cycle: Speed Vs time and Power Vs time proﬁles. . . . . . . . . . . . . . . . . . . . 101
5.24 IHP Cycle: Error Vs time plot for the Adaptive controller with derivative action.102
5.25 Error Vs time plot for the PID controller for a step of 100. . . . . . . . . . . . . . . . . 103
5.26 (a), (b) Errors for the adaptive controller for a white band noise of magnitude
of 100 and 1000. (c), (d) Errors for the PID controller at magnitudes of 100
and 1000 respectively . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
5.27 (a) Error without dead zone for white noise of a magnitude 1000, (b) error in
the presence of dead zone for white noise of a magnitude 1000. . . . . . . . . . . . . . 105
5.28 (a) Steady state error for PID controller with pulse load (b) steady state error
for adaptive controller with deadband. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
6.1 Power Requested, Fuel cell Power, Battery power proﬁles for a step increase
and decrease in Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
6.2 Speed proﬁle for the Urban Dynamometer Driving Schedule (UDDS) . . . . . . . . 112
6.3 Power proﬁle for the UDDS schedule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
6.4 State of Charge variation for diﬀerent initial SOC. . . . . . . . . . . . . . . . . . . . . . . 114
D.1 Simulink diagram to simulate the switching controller design . . . . . . . . . . . . . . 138
A new future in automotive transportation is approaching where vehicles are powered by
new, clean and eﬃcient energy sources. While diﬀerent technologies will contribute to this
future, many see fuel cells as the leading long term candidate for becoming the power source
for emissions-free, mass produced light vehicles.
The development of emissions-free vehicles, which run directly on hydrogen, is the true
long term goal. However signiﬁcant diﬃculties exist in developing these vehicles, due to
hydrogen storage problems. For automotive applications, it is desirable to use a carbon-based
hydrogenous fuel. The focus of this research was to analyze a fuel cell system for automotive
applications, which generated hydrogen in situ using methane as a fuel source. This system
consists of four parts: (1) an in situ hydrogen generation subsystem, (2) a power generation
subsystem, (3) a thermal management subsystem and (4) a switching control subsystem.
The novelty of this research lies in the fact that the entire system was considered from a
systems engineering viewpoint with realistic constraints.
A fuel processor subsystem was designed and its volume optimized to less than 100 liters.
A relationship between the fuel fed into the fuel processor and the hydrogen coming out of
it was developed. Using a fuel cell model an overall relationship between the fuel feed rate
and the power output was established.
The fuel cell car must be fully operational within a minute or so of a cold-start and must
respond to rapidly varying loads. Signiﬁcant load transitions occur frequently as a result
of changes in driving conditions. These engineering constraints were addressed by coupling
a battery to the fuel cell. A switching controller was designed and it was validated using
realistic power proﬁles. Finally, a model reference adaptive controller was designed to handle
nonlinearities and load transitions. The adaptive controller performance was enhanced by
adding dead zone compensation and derivative action. The enhanced adaptive controller
was validated for diﬀerent power proﬁles.
The beginning of the 19th century marked the advent of the modern automobile systems.
Fueled by hydrocarbons, automobiles utilizing the internal combustion engine technology
changed the way we travel. At the dawn of a new century, we are at the threshold of a new
future in automobile technology, where the emphasis is on clean and eﬃcient energy sources.
While diﬀerent technologies will contribute to this future, many see fuel cells as the leading
long term candidate for becoming the power source for emissions-free, mass produced light
For automotive applications, it is desirable to use a carbon-based hydrogenous fuel such as
methane or gasoline. Such fuels are particularly desirable as they are easy to store onboard a
vehicle and a nationwide infrastructure of service stations that supply this fuel already exists.
There are several important technological breakthroughs that are necessary to make a fuel
cell based automobile commercially viable. In the past, there has been signiﬁcant research
eﬀort in the development of new fuel cell membranes and catalysts . However, it has only
recently been recognized  that for this technology to compete favorably with the internal
combustion engine technology, it is necessary to design and optimize the performance of the
entire operation in the face of dynamic constraints and uncertainty. Fuel cell power systems
for automotive applications are usually rated at 50 kW electrical power output. A power
plant of this size faces several performance constraints in an automotive environment. The
system must be fully operational within a minute or so of a cold-start and must be able to
respond rapidly to varying loads. Signiﬁcant load transitions occur frequently as a result
of changes in driving conditions (e.g. acceleration while passing another vehicle, driving in
hilly conditions, highway vs. city driving etc.). These engineering constraints have to be
addressed properly for successful design of a vehicle powered by a fuel cell.
In this research, a virtual prototype of an integrated in situ hydrogen production and
fuel cell power system for automotive applications is developed and analyzed. This system
consists of four parts:
• An in situ hydrogen generation subsystem where the hydrocarbon fuel is converted to
• A power generation subsystem where the hydrogen is converted to electrical energy via
a fuel cell.
• A thermal management subsystem that maintains the various subsystems at the desired
optimal temperature proﬁles.
• A switching control subsystem that switches between the fuel cell and a battery backup
depending on the power requirements of the vehicle.
For developing a commercially viable system, each of the above subsystems has to be properly
designed and evaluated. In this research, an overall systems level analysis, which is a key
component for making this technology feasible is proposed.
The novelty of this research lies in the fact that the entire system is being considered from
a systems engineering viewpoint with realistic constraints. Past work has typically focussed
on only one subsystem and the interaction between systems has been ignored. At the end of
this research, a virtual prototype of an integrated in situ hydrogen production and fuel cell
system that is capable of powering a small car will be developed. This research is a crucial
step for experimentally building a prototype vehicle.
The results of the proposed research will provide a key technology for developing an
economical fuel cell based automobile that provides a viable alternative to the conventional
automobiles based on an internal combustion engine. At present, automobiles based on
fuel cell technology promise the best opportunity to achieve near zero emissions of air
pollutants and greenhouse gases. Proper engineering design and optimization can result
in the development of a fuel cell system that is small enough to ﬁt in a car, cheap enough to
be aﬀordable by the masses and have suﬃcient driving range to replace conventional vehicles
based on the internal combustion engine. Most importantly, the use of an integrated fuel
cell system for an application as complex and demanding as an automobile would portend
a major paradigm shift in global energy consumption and supply. The potential would
exist to create new industries and allow people throughout the global community to enjoy
the beneﬁts of access to an eﬃcient, cost-eﬀective, and reliable new technology . The
virtual prototype developed in this research project will provide the key systems integration
parameters necessary for building a prototype vehicle. We expect that the development of
a viable virtual prototype will provide the necessary proof-of-concept for attracting research
and development investment in this important technological area from automotive and
mobile power generation companies and this will lead to a hydrogen economy.
1.2 Hydrogen Economy
Hydrogen is the most abundant element in the universe. However, not much is available
in pure form on earth and is available either as water (when combined with oxygen) or as
a hydrocarbon (when combined with carbon). For this reason, hydrogen is only an energy
carrier and not a primary energy source. While hydrogen is a very clean fuel and burning it
results in no greenhouse emissions or undesirable carbon compounds, its production ma still
have a considerable carbon footprint. It seems logical to produce hydrogen via electrolysis
of water. However, the electrolysis process is highly energy intensive and the electricity
needed for this process needs to be generated at another site. Most of the electricity in the
United States is currently produced by burning coal or oil which contributes signiﬁcantly
to the greenhouse emissions. This scenario is unlikely to change in the near future and
considerable advances in alternative sources of energy (e.g. solar energy or nuclear energy)
are needed to change the scenario. Another alternative to produce hydrogen is to extract
it from a hydrocarbon source which also results in emission of greenhouse gases. For this
reason, the switch to the hydrogen economy is expected to occur gradually in the next several
decades rather than suddenly in the next few years.
Hydrogen production is a large, modern industry with commercial roots reaching back
more than a hundred years. Globally, hydrogen is widely used for two purposes. The ﬁrst is
in the fertilizer industry where hydrogen has long been used for ammonia synthesis (NH3).
The second area is in oil reﬁneries where hydrogen has been used for hydro-formulation,
or high-pressure hydro-treating, of petroleum in reﬁneries. In the United States alone
hydrogen production is currently about 8 billion kg, roughly 90 billion Normal cubic meters
. Global annual production is about 45 billion kg or 500 billion Nm3
is extracted from diﬀerent sources as shown in Table 1.1 which breaks down the annual
hydrogen production depending on the fuel type and the main method of production.
Table 1.1. Hydrogen production based on the type of fuel
Fuel Amount percentage Method of
(billions of Nm3
Natural Gas 240 48% Steam Reforming
Oil 150 30% Partial Oxidation Reforming
Coal 90 18% Coal gasiﬁcation
Water 20 4% Electrolysis
Methane or natural gas is the fuel of choice and almost 50% of industrial hydrogen
production uses methane as a fuel. Steam reforming, which will be discussed in detail in the
next chapter, is generally used to obtain hydrogen from natural gas. For higher hydrocarbons
like gasoline or diesel partial oxidation reforming is generally used. During recent years
a combination of steam reforming and partial oxidation reforming known as autothermal
reforming is increasingly being employed. Hydrogen can also be obtained from gasiﬁcation
of coal and currently 18% of the world’s hydrogen is produced from coal. Currently only 4% of
the total hydrogen production is produced via electrolysis of water and is generally used when
high purity hydrogen is required. This method can be made more environmentally friendly
by using renewable energy sources such as hydroelectric power systems, wind energy systems,
ocean thermal energy conversion systems, geothermal resources, and a host of direct solar
energy conversion systems including biomass production, photovoltaic energy conversion,
solar thermal systems, etc. However,at present the cost per kilowatt of producing energy
through these techniques is very high making the cost of producing hydrogen using these
energy sources prohibitive.
From the above analysis it is clear that in the current environment, the most practical
source for generating hydrogen is a hydrocarbon source. Some of the hydrocarbon fuels
Table 1.2. Pertinent physical properties of transportation fuels
Fuel H/C Density Heating value Energy density CO2 CO2 emission
considered atom of fuel of fuel of fuel emissions relative
- ratio kg/m3
kg/kJ to CH4
gasoline 2.03 99.79 42.95 32781 69.18 1.4
diesel 1.63 117.73 40.65 36612 72.06 1.46
propane 2.67 77.52 46.46 27539 59.89 1.21
methanol 4 105.71 19.92 16105 63.29 1.28
methane 4 56.70 50.163 21746 58.08 1
hydrogen - 9.45 120.16 8681 - -
and their salient features  are given in Table 1.2. Fuels like gasoline and diesel have a
low hydrogen to carbon ratio whereas methane and methanol have a higher ratio. Even
though lower hydrocarbons have higher heating value(MJ/kg), higher hydrocarbons have
greater densities and hence have greater energy densities. Table 1.2 compares the amount of
CO2 produced for diﬀerent fuels per unit energy production (kJ). Of all the hydrocarbons,
methane produces the least amount of CO2 and the relative ratio of CO2 production with
respect to methane is given in Table 1.2. For the same amount of energy using any other
hydrocarbon will produce at least 20 % more CO2 than methane. It is observed that methane
or natural gas would be the most suitable hydrocarbon for onboard hydrogen production.
1.2.1 Fueling Infrastructure
A pure hydrogen economy will require a sea change in the fueling infrastructure that has
been built over the past century to service the automobiles based on internal combustion
engine. Two key issues will determine the nature of that infrastructure: (1) where the
hydrogen is produced and (2) in what form is it stored on board the hydrogen vehicle.
Hydrogen could be produced at fueling stations located in cities and on highways by
reforming fossil fuels. This is sometimes called forecourt production. Hydrogen could also
be produced at centralized facilities nearer to potential fuel sources, such as coal plants or
windmills and transported to the fueling stations.
Both the methods mentioned above require signiﬁcant economic investment and also
pose some technical problems. First, in order for a car to run on pure hydrogen, it must
be able to safely, compactly, and cost-eﬀectively store hydrogen on board, which is a major
technical challenge. As seen from Table 1.2 hydrogen has a far lower energy to volume
ratio than hydrocarbon fuels such as methane, methanol, propane and octane. That is,
hydrogen contains much less energy per gallon than other fuels at the same pressure. At
room temperature and pressure, hydrogen takes up approximately 3,000 times more space
than gasoline containing an equivalent amount of energy. Hydrogen does have an exceptional
energy content per unit mass (120.16 MJ/kg), nearly triple that of gasoline (42.9 MJ/kg),
but the storage equipment on a car ﬁtted for hydrogen use, such as pressurized tanks, adds
signiﬁcant weight to the system and negates this advantage. Secondly, hydrogen storage
systems need to enable a vehicle to travel 300 to 400 miles and ﬁt in an envelope that does not
compromise either passenger space or storage space. Current energy storage technologies are
insuﬃcient to gain market acceptance because they do not meet these criteria. The driving
range requirement will probably require a tank holding about 5 kg of hydrogen or more,
depending on the size and weight of the vehicle. At the same time, the vehicle needs to be
fueled in a short time, with a storage system that is safe, leak proof, and also is lightweight
and aﬀordable. These constraints have to be overcome before automobiles that operate on
pure hydrogen become commonly available.
The near term solution is onboard hydrogen generation using fossil fuels. Hydrogen could
be generated on the car or truck itself, most likely by a methane or gasoline reformer. If
onboard reforming of gasoline proves to be practical, the existing infrastructure of gasoline
can be used to power fuel cell based automobiles. This will provide a solid platform to launch
a more advanced hydrogen based car. As a starting point for this research a methane based
fuel reformer that can produce a hydrogen rich stream is studied.
1.3 Schematic Diagram
In this research, the primary components of an automotive fuel cell system are analyzed.
Fundamental chemical engineering principles are utilized to assess the role of thermody-
namics, heat transport, mass transport and reaction kinetics. In addition to the methane
reforming unit we need a CO removal section to protect the catalyst in the fuel cell from
CO poisoning. Furthermore, a fuel cell system should be designed such that it delivers upto
50 kW of power that is suitable for automotive applications. A power plant of this size
faces several performance constraints in an automotive environment. The system must be
fully operational within a minute or so of a cold-start and must respond to rapidly varying
loads. To address these issues, in addition to the fuel cell stack, the power generation
subsystem should also include a battery backup. A suitable thermal management control
system as well as a switching control system are proposed based on the dynamics of the
system. The dynamic analysis requires advanced tools from numerical methods and nonlinear
analysis. Furthermore, the control systems design and analysis is based on modern advanced
optimization tools and systems engineering approaches. A schematic of the fuel cell system
under consideration is shown in Fig. 1.1.
Figure 1.1. Schematic diagram of the fuel cell system
The fuel cell system is divided into the following four subsystems:
1. Fuel processing subsystem
2. Power generation subsystem
3. Thermal management subsystem
4. Switching control subsystem
The fuel processing subsystem consists of three packed bed reactors:
• Steam Reformer (SR): In this reactor, the hydrocarbon fuel is converted to hydrogen
and carbon monoxide. The methane reacts with steam to form three moles of hydrogen
and a mole of carbon monoxide as given by Eq. 1.1. Part of the carbon monoxide
reacts with water to produce carbon dioxide and hydrogen as shown in Eq. 1.2. A
side reaction in which four moles of hydrogen are produced also takes place as shown
in Eq. 1.3.
CH4 + H2O CO + 3H2; ∆Ho
298 = 205.81kJ/mol (1.1)
CO + H2O CO2 + H2; ∆Ho
298 = −41.16kJ/mol (1.2)
CH4 + 2H2O CO2 + 4H2; ∆Ho
298 = 164.64kJ/mol (1.3)
• Water Gas Shift Reactor (WGS): In this reactor, most of the remaining carbon
monoxide is converted to carbon dioxide via the water gas shift reaction given by
CO + H2O CO2 + H2; ∆H0
298 = −41.1kJ/mol (1.4)
• Preferential Oxidation Reactor (PROX): In this reactor, the feed from the WGS is
reacted with air to reduce the carbon monoxide concentration to less than 100 ppm to
avoid damage to the fuel cell membrane. Some of the hydrogen reacts with the oxygen
to produce water.
O2 → CO2; ∆Ho
298 = −283kJ/mol (1.5)
O2 → H2O; ∆Ho
298 = −242kJ/mol (1.6)
The power generation system consists of a Polymer Electrolyte Membrane (PEM) fuel
cell that utilizes the hydrogen coming from the fuel processing subsystem and converts it
into electricity that is used to power an electric motor for the automobile. In addition to the
fuel cell, there is a battery backup that the electric motor switches to when the fuel cell is
unable to deliver the necessary power.
The reforming reactions are endothermic while the water gas shift reaction and the
preferential oxidation reactor is exothermic. Furthermore, each reactor in the fuel processing
subsystem may have a diﬀerent optimal temperature proﬁle. Thus, it is necessary to design
an eﬃcient thermal management system to eﬀectively utilize the system energy and to
improve fuel economy. Furthermore, signiﬁcant load transitions occur frequently as a result
of changes in driving conditions (e.g. acceleration while passing another vehicle, driving in
hilly conditions, highway vs. city driving etc.). For this reason, it is necessary to have a
battery backup that the electric motor has the option to switch to when the fuel processing
subsystem is unable to provide the necessary hydrogen to generate the necessary power. Since
the size of the battery in an automotive application is limited, it is necessary to develop an
eﬀective switching control system that switches between the power generation system and
the battery backup depending on the supply and demand of hydrogen.
1.4 Thesis Overview
In this dissertation a fuel cell power system for automotive applications will be analyzed.
A block ﬂow diagram of the system under consideration is shown in Fig. 1.1. Speciﬁcally,
the following issues will be addressed:
1. Assess the thermodynamic feasibility of the system.
2. Design and analyze the reactors necessary for the fuel processor subsystem.
3. Design and analyze the fuel cell system.
4. Develop a switching control system for eﬀectively running the power generation sub-
system and the battery backup.
5. Develop an adaptive control algorithm to control reactant ﬂow rate into the fuel cell
system to follow the power trajectory.
6. Implement the controllers on realistic power proﬁles.
In Chapter 2, methods for onboard hydrogen storage and various methods for reforming
hydrocarbons are reviewed. Kinetic models for the three reactors in the fuel processing
system are presented. The working principle, various areas of application and types of fuel
cells are discussed. A review of fuel cell models is presented. An introduction to process
control techniques, PID controller design and tuning, and adaptive control techniques is
In Chapter 3, the three diﬀerent reactors of the fuel processing system are designed.
Operating parameters such as steam to carbon ratio, operating temperatures and pressures
and feed stream composition are established. A relationship between the methane ﬂow rate
and the hydrogen output is obtained by varying the feed rate of methane.
In Chapter 4, the power generation subsystem is designed. Two diﬀerent models for the
fuel cell are presented. The size of the polymer electrolyte fuel cell stack is calculated.
Chapter 5 discusses the adaptive control technique introduced in Chapter 2 in more
detail. Two diﬀerent fuel cells the PEM fuel cell and the phosphoric acid fuel cell (PAFC)
models are studied. In the case of the PEM fuel cell the adaptive controller is implemented
on the nonlinear model and its performance is compared to that of a PID controller by
implementing the controllers on realistic power proﬁles.
In Chapter 6 a battery model suitable for control purposes is presented and a switching
controller is designed which eﬀectively switches back and forth between the fuel cell and
battery. Finally, in Chapter 7 the main results of this dissertation are summarized and the
direction of future work is proposed in Chapter 8.
The process ﬂow diagram presented in Fig. 1.1 has many similarities to those in the
chemical process industry. The entire system can be divided into subsystems and most
research papers typically focus on the steady state analysis of a speciﬁc component of the
overall system. However, a review of the literature indicates that research on overall dynamic
behavior of fuel cell systems is sparse. The literature in this area can be classiﬁed as follows:
2.1 Fuel Processing System
Fuel cells need hydrogen and oxygen for operation while oxygen can be obtained from air
we need to develop a strategy to supply hydrogen. As discussed in Section 1.2 hydrogen is not
a primary fuel and it has to be extracted from hydrogen rich fuels. Hydrogen can be extracted
from these fuels at centralized plants and then distributed to the local fueling stations. In
such a scenario we need to develop an onboard hydrogen storage system which supplies the
hydrogen. Otherwise we can miniaturize the centralized plant to produce hydrogen through
in situ generation and then supply this hydrogen on an “as needed” basis.
2.1.1 Onboard Hydrogen Storage
It is challenging to store hydrogen safely in an automobile. The energy to volume ratio of
hydrogen is very low and if hydrogen is stored as a gas, a very large fuel tank is needed for a
relatively limited driving range. Hence, there is a lot of ongoing research on developing novel
methods for hydrogen storage. The success of these methods will depend on which method
is portable, aﬀordable, can give the maximum driving range, can occupy a smaller volume
and is adjustable to ﬂuctuations of the hydrogen demand. The ﬁve diﬀerent methods often
quoted in the literature through which hydrogen can be stored are as follows :
• Metallic hydrides
• Complex hydrides
Hydrogen can be stored in a pressurized cylinder with pressures up to 20 MPa, but the
energy density is too low to satisfy the fuel demand of current driving practice. Storing
hydrogen onboard in compressed gas cylinders has been investigated by Hwang et al. 
and they have successfully test run an experimental vehicle, but the range of the vehicle
is very low and needs refueling for every 100 miles. About four times higher pressure is
needed to meet the driving purpose; however, such high pressure cylinders are not available
commercially. Liquid hydrogen is widely used today for storing and transporting hydrogen
. This method faces two signiﬁcant challenges: (1) the eﬃciency of the liquefaction process
and (2) the boil-oﬀ of the liquid hydrogen.
Hydrogen can be adsorbed onto certain materials like nanotubes and the adsorbed gas
can be released reversibly. Zhang et al.  and Service  proposed storing hydrogen in
nanotubes or nanoballs and this has been a hot topic for research (, , ). Indications
are that hydrogen may be stored in nanotubes in quantities exceeding that of metal hydrides
and at a lower weight penalty , but no designs exist yet.
Some metals and alloys absorb hydrogen and form hydrides. Hydrogen diﬀused into
appropriate metal ions can achieve storage densities greater than that of liquid hydrogen.
There are two classes of hydrides, metallic hydrides and complex hydrides. The main
diﬀerence between them is the transition of metals to ionic or covalent compounds for the
complex hydrides upon absorbing hydrogen. Toyota  has been working on developing
high density metal alloys. Some of the metallic hydrides of interest for storage purpose
are listed in . Group I, II, and III elements, (e.g. Li, Mg, B, Al) form a large variety
of metal−hydrogen complexes. NaAlH4 , LiBH4  and NaBH4  can reversibly
absorb/desorb hydrogen at moderate temperatures. While complex hydrides are a promising
solution of the hydrogen storage problem, the mass storage densities are still less than
10% of those of conventional fuels , making this method doubtful for economical mobile
applications. All the technologies listed above are still in their nascent stage of development
and require a lot of research work before they become commercial products. Even after
the hydrogen storage problem is solved, it is necessary to establish a hydrogen distribution
system which will take a lot more time and money.
2.1.2 In situ Hydrogen Generation
Hydrogen is a very diﬃcult fuel to store onboard and there is a lack of infrastructure
for distribution of hydrogen. To make these cars commercially viable it is necessary to use
fuels like gasoline, diesel and natural gas as they already have a wide distribution network.
Hence, we need a reformer which can produce the required hydrogen onboard from these
hydrocarbons. When we use fuels like gasoline and diesel they have to be ﬁrst broken down
to smaller molecules like methane. So as a starting point we chose methane as the fuel of
choice. There are diﬀerent methods by which we can produce the hydrogen from methane
as described below
Steam Reforming (SR)
This is the process that is being used to produce hydrogen industrially. In this method
methane reacts with steam to produce CO and H2 as shown in Eq. 2.1. This is often
accompanied by a water gas shift reaction given by Eq. 2.2, in which CO and H2O react to
form CO2 and H2. In addition to this a side reaction also takes place where for each mole of
natural gas four moles of hydrogen are obtained as shown in Eq. 2.3. The overall reaction
is endothermic requiring an external heat source.
SR Initial Reaction
CH4 + H2O → CO + 3H2; ∆Ho
298 = 205.81kJ/mol (2.1)
Water Gas Shift Reaction
CO + H2O → CO2 + H2; ∆Ho
298 = −41.16kJ/mol (2.2)
SR Side Reaction
CH4 + 2H2O → CO2 + 4H2; ∆Ho
298 = 164.64kJ/mol (2.3)
All the reactions occur at high temperature and the reacting temperature can be reduced
by the addition of a catalyst. Nickel, chromium-promoted iron oxide, copper, zinc catalysts
supported on alumina are the catalysts generally used. If the fuel is being supplied to a
polymer electrolyte fuel cell stack, further puriﬁcation is required to reduce the concentration
of CO to less than 100 ppm .
Partial Oxidation Reforming (POX)
In partial oxidation reforming the feed consists of methane and oxygen. In the reformer
methane is partially oxidized to H2 and CO. The reaction is given in Eq. 2.4. The reaction
is exothermic and takes place at very high temperatures (> 1200o
POX Initial Oxidation Reaction:
CH4 + 0.5O2 → CO + 2H2; ∆H = −36kJ/mol (2.4)
The water gas shift reaction which is also seen in the SR method also takes pace converting
some of the CO to CO2. If catalysts are used the reaction temperature is reduced and the
process is known as catalytic POX. SR is more eﬃcient than POX because for every mole of
methane more amount of hydrogen is produced in SR compared to POX method.
Autothermal Reforming (ATR)
This method is a combination of both the POX and SR methods. In this method the heat
generated by the POX method (exothermic) is used to supply the heat needed for the SR
reaction (endothermic). Since no external heat source is required it is called an autothermal
reformer. When the ratio of number of moles of CH4 reformed by SR to POX is n:m, the
total ATR reaction can be expressed as
ATR Total Reaction
(n + m)CH4 + (1/2m))O2 + (2n + m)H2O → (n + m)CO2 + (4n + 3m)H2 (2.5)
2.1.3 Development of Kinetic Models
Steam reforming of hydrocarbons for hydrogen production has been studied for several
decades, mainly for applications in ammonia synthesis, methanol synthesis and for substitute
natural gas applications. In areas where natural gas is available in large quantities,
interest centered around steam reforming of methane and methane reforming technology
was pioneered by BASF in the ﬁrst quarter of the 20th century . Steam reforming of
higher hydrocarbons has been the focus in countries such as Japan and the U.S. where natural
gas is not as abundant. Rostrup-Nielson , Tottrup  and Christensen  used heptane
as a model feed for the investigation of steam reforming of higher hydrocarbons and they
found out that these reactors are too large to ﬁt under the hood of a car. Xu and Froment
 developed a detailed reaction scheme for the steam reforming of methane, accompanied
by water gas shift reaction on a Ni/MgAl2O4 catalyst. Based on this reaction scheme they
developed Hougen-Watson-type equations for the reaction rates given by Eq. 2.6, Eq. 2.7
and Eq. 2.8.
PCH4 PH2O −
(1 + KCOPCO + KH2 PH2 + KCH4 PCH4 + KH2OPH2O/PH2 )2
(1 + KCOPCO + KH2 PH2 + KCH4 PCH4 + KH2OPH2O/PH2 )2
(1 + KCOPCO + KH2 PH2 + KCH4 PCH4 + KH2OPH2O/PH2 )2
where r is the reaction rate, k is the Arrhenius rate constant, Px and Kx stands for the
partial pressure and adsorption coeﬃcients of a component x. To avoid carbon formation
that poisons the catalyst, a high steam-to-carbon ratio in the range of 2-5 is commonly used
. While industrial ﬁxed-bed reactors operate at relatively high pressures (∼ 30 bar), fuel
cell applications typically operate at about 2-5 bar. To avoid excessive pressure drop, large
catalyst particles are used which result in an extremely low eﬀectiveness factor in the range
The water gas shift reaction, is an industrially important reaction that was ﬁrst com-
mercialized for the manufacture of ammonia. Typically, iron based catalysts are used in
this process and a second catalyst based on copper is also used in order to achieve higher
conversion of carbon monoxide to carbon dioxide. Choi and Stenger  developed the
kinetic rate expressions for the water gas shift reaction based on a Cu/ZnO/Al2O3 catalyst.
They proposed an empirical rate expression for the amount of CO consumed as shown in
rCO = kPCOPH2O(1 − β) (2.9)
where β is the reversible factor given by
where Keq is the equilibrium constant which can be obtained from thermodynamic
The water gas shift reaction results in a stream that is approximately 0.3% carbon
monoxide. However, it is necessary to reduce the carbon monoxide concentration in the
hydrogen stream to about 100 ppm before it can be sent to the fuel cell to avoid poisoning the
catalyst in the fuel cell membrane. The preferential oxidation (PROX) of carbon monoxide in
a hydrogen-rich atmosphere has long been of technical interest for puriﬁcation of hydrogen. In
order to keep the overall energy conversion process as eﬃcient as possible, the CO oxidation
has to be highly selective. Catalyst formulations for this reaction typically comprise of
platinum on alumina. Copper catalysts on alternative supports are also being developed
. Kahlich et al. developed a rate expression for selective CO oxidation based on a
platinum catalyst given by Eq. 2.10.
rCO = k1P0.42
where λ is a process parameter which represents the oxygen in excess with respect to the
amount of oxygen required for the oxidation of CO to CO2 as given by Eq. 2.11.
The process parameter λ accounts for the amount of oxygen that is consumed by the
oxidation of hydrogen. A more detailed discussion of the kinetic rate expressions are
presented in the next chapter.
2.2 Fuel Cell
2.2.1 Working Principle
A fuel cell is an electrochemical device which combines a fuel (e.g. hydrogen, methanol)
and oxygen to produce a direct current. Unlike storage batteries fuel cells can be continuously
fed with a fuel so that the electrical power output is sustained for a longer period of time.
The fuel used is generally hydrogen which produces electrical energy and heat through the
reaction of hydrogen and oxygen to form water. The process is that of electrolysis in reverse.
The anode and cathode reactions are given in Eq. 2.12 and Eq. 2.13 respectively and the
overall cell reaction is given by Eq. 2.14.
H2 → 2H+
O2 → H2O (2.13)
O2 → H2O (2.14)
The hydrogen comes in at the anode where it splits into hydrogen ions and electrons
in the presence of a catalyst. The hydrogen ions pass through the electrolyte towards the
cathode. The electrons which cannot pass through the electrolyte, pass through an external
circuit from the anode to the cathode thereby producing a current. At the cathode the
oxygen combines with the electrons and hydrogen ions in the presence of a catalyst to form
Figure 2.1. Cross section of a polymer electrolyte membrane fuel cell
water. Fig. 2.1  shows a cross-sectional diagram of a single cell polymer electrolyte fuel
2.2.2 Types of Fuel Cells
There are numerous applications for fuel cells today and depending on the speciﬁc
application, diﬀerent types of fuel cells are now available in the market. They diﬀer mainly
in the type of membrane used, operating temperature, oxidant composition, reforming
technology etc. Some of the most common fuel cells are listed below and salient features of
the diﬀerent types of fuel cells are listed in Table 2.1.
Phosphoric Acid Fuel Cell (PAFC)
As the name suggests the electrolyte in a phosphoric acid fuel cell is phosphoric acid.
PAFC is tolerant to carbon monoxide poisoning. The operating temperature is around 190
C. These fuel cells are very sensitive to temperature changes. At lower temperatures the
water evolved by the fuel cell reaction is dissolved in the electrolyte thereby diluting the
electrolyte and reducing the eﬃciency of the fuel cell drastically. At higher temperatures the
phosphoric acid starts to decompose which also signiﬁcantly decreases the eﬃciency of the
fuel cell. PAFC require nobel metal catalysts. Platinum and silicon carbide are generally
used as catalysts.
Molten Carbonate Fuel Cell (MCFC)
MCFC operate ata very high temperature of 650 o
C. At these high operating temper-
atures the fuel cell acts as its own reformer. The electrolyte here is molten carbonate salt.
These fuel cells require carbon dioxide in the oxidant stream to regenerate the carbonate. The
main application areas of these fuel cells are large scale and stationary electricity production
for utility power generation. These cannot be used for transportation purposes because of
their bulk, thermal cycling, diﬃcult start-up and complex control requirements.
Solid Oxide Fuel Cell (SOFC)
Solid oxide fuel cells are very useful when natural gas is used as a fuel because they are
very tolerant to sulphur and also they have better operating lives than the other fuel cells.
Operating temperature is around 1000 o
C. Internal reforming is one of the main advantage of
using SOFC. High operating temperature causes slow start up and also start up/shut down
cycles are stressful to cell integrity. SOFC use nickel as a catalyst and have very narrow
operating temperature range.
Polymer Electrolyte Membrane Fuel Cell (PEM)
The electrolyte in a PEM fuel cell is a solid, organic polymer and is usually referred
to as a membrane. It consists of three parts: (1) the Teﬂon like, ﬂuorocarbon backbone,
(2) side chains which connect the molecular backbone to the ionic part and (3) the ion
clusters consisting of sulfonic acid ions. In the presence of water the negative ions in the
membrane are held within the structure, but the positive ions (H+
ions) are mobile and
are free to carry positive charge through the membrane. The other important property of
the polymer electrolyte membrane is that the electrons cannot pass through them. Hence,
the electrons produced at the cathode pass through an external circuit thereby producing
current. Another advantage of these membranes is they act as eﬀective gas separators. So
that the gases at the anode and cathode do not mix. The most popular PEM membrane is
Naﬁon 117 .
The reactions taking place at the anode and cathode given by Eq. 2.12 and Eq. 2.13
respectively. These reactions are normally very slow but in the presence of a catalyst like
platinum the reactions become fast. Platinum is costly and lowering the platinum catalyst
levels is an ongoing research eﬀort . Each electrode consists of porous carbon to which
small platinum particles are bonded. The combination of electrodes and membrane is called
the Membrane Electrode Assembly (MEA). The MEA is very thin (around 0.2 - 0.5mm in
thickness) and is generally sold as a single unit.
The MEA are enclosed in backing layers, ﬂow ﬁelds and current collectors which are
designed to maximize the current that can be obtained from a MEA. Backing layers are
placed next to the anode and cathode. They are usually made of a porous carbon paper or
carbon cloth. Carbon conducts the electrons exiting the anode and entering the cathode and
the porous nature ensures eﬀective diﬀusion of each reactant gas to the membrane electrode
assembly. The backing layers also help in water management by supplying the right amount
of water vapor to the membrane to prevent drying or ﬂooding of the membrane. Adjacent
to the backing layers is a plate which serves the dual purpose of a ﬂow ﬁeld and a current
collector. The ﬂow ﬁelds are used to carry reactant gas from the point it enters the fuel cell
to the point at which the gas exits. The plates also serve as current collectors. Electrons
produced at the anode pass through the backing layers and through the plate before exiting
the cell. After passing through an external circuit the electrons re-enter the fuel cell through
the cathode plate.
Fuel Cell Stack
The maximum voltage of a single fuel cell at 100 % eﬃciency is 1.23 V . As most
applications require higher voltages than this, the required voltage is obtained by connecting
individual fuel cells in series to form a fuel cell stack. To decrease the overall volume and
weight of the stack instead of two current collectors (one for the anode and one for the
cathode), a single plate is used with a ﬂow ﬁeld cut into each side of the plate. This type of
plate is called a bipolar plate.
For automotive applications it is desirable to have a fuel cell system with a low operating
temperature. MCFC operate at a very high temperature hence they are not used in this
application. One of the ﬁrst fuel cell vehicles were developed using PAFC technology. PAFCs
have good designs as they had a lot of funding over the past 20 years, because they were
judged most tolerant of reformed hydrocarbon fuels. The operating temperature window is
small for PAFC systems and this is the major drawback. PEM fuel cells, because of their low
cost, ease of operation, lower operating temperature and higher energy density, are gaining
preference to PAFC systems. Many of the leading automotive manufactures have come up
with hybrid fuel cell cars using the PEM fuel cell. For a PEM fuel cell car a continuous
supply of hydrogen is neede which can be obtained by reforming of methane as discussed in
Table 2.1. Salient features of the diﬀerent types of fuel cells
Property PAFC PEMFC MCFC SOFC
Electrolyte Phosphoric acid Polymer Molten carbonate salt Ceramic
Operating Temperature 190 o
C 80 o
C 650 o
C 1000 o
Fuels H2 H2 H2/CO H2/CO/CH4/
Reforming External External External/Internal External/Internal
Oxidant O2/air O2/air O2/air CO2/O2/air
Fuel cells have many applications today and the list is growing fast. The development of
the various technologies is application dependent with each fuel cell type having strengths
and weaknesses. There are three basic market segments for fuel cells: portable/battery
substitution, transportation and utility power.
Portable and Battery Substitution
Portable power is one of the areas where the ﬁrst widespread application of fuel cell
technology is expected. Fuel cells as battery chargers are expected to be commercially viable
in the near future. Another exciting area is the world of consumer portable electronics.
Laptops, mobile phones, PDAs and many other electronic devices have shown better
performance and longer run times with fuel cells powering them in place of batteries.
Research is still in progress and issues such as heat management and space constraints
have to be resolved. There is a lot of interest shown by military over the use of fuel cell
battery packs. With the increase of sophisticated electronic equipment used by the military
a battery which runs for longer time while oﬀering portability will be a good option. Many
novel applications such as powering small cycles and scooters have also been proposed and
are under development. All the major electronics companies such as Canon, Casi, Fujitsu,
Hitachi, Sanyo, Sharp, Sonyand Toshiba have ongoing research in this ﬁeld.
In terms of size, value and environmental impact, automotive markets represent the
biggest prize for fuel cells. Fuel cells were ﬁrst used to power vehicles over forty years ago.
For many years development work was insigniﬁcant, and as a result until the mid nineties
only a handful of vehicles were developed. Fuel cell vehicles are now available in the light
and heavy duty vehicles category. The most successful area so far has been fuel cell buses
(FCBs). In 1993 Ballard powered the ﬁrst fuel cell bus in the world. Recently in 2003
the Evobuses were introduced in Iceland and are being operated under the ECTOS project
. In the U.S. the California Fuel Cell Partnership  is coordinating the deployment of
several FCBs at a number of californian transit agencies. Most of these buses run on direct
methanol fuel cells  which run directly on methanol instead of hydrogen.
In light duty vehicles such as cars and vans all the major automotive manufacturers have
shown interest, Honda and Toyota have already delivered vehicles to customers in California
and Japan. In 2004 Daimler Chrysler has also begun to deliver FCVs for limited ﬂeet trials
and a number of other major manufacturers are gearing up to do the same. Nissan leased
its ﬁrst FCV in 2004, Dihatsu, Ford and Hyundai are all expected to follow suit later this
The stationary applications can be divided into two groups small stationary power plants
(0.5-10 kW) and large stationary plants or utility generation (> 10 kW). In the small
stationary market main areas of focus over the past few years has been residential, UPS
or backup sector. A growing number of market segments including telecommunications,
emergency services such as hospitals and the banking industry have started to take an active
interest in fuel cell technology. Companies like Ballard, Plug Power, Fuji Electric, Kyocera
and ReliOn have limited commercialization of the 1kW PEM.
Large stationary power was one of the ﬁrst applications for fuel cells. Most of the early
fuel cells were based on phosphoric acid and molten carbonate fuel cells. There are also a
number of companies developing SOFC and PEM fuel cells. In 2004 itself, more than 50
large stationary units were installed across the world, with North America and Japan leading
the way with the highest number of installations. UTC and Fuji Electric are the leaders
in terms of the total systems sold and they are based on phosphoric acid technology. Fuel
cell Energy and MTU CFC solutions have developed molten carbonate fuel cells. General
Motors, Siemens Westinghouse, Rolls Royce and Mitsubishi Electric have developed systems
based on SOFC and PEM fuel cells.
2.3 Fuel Cell Modeling
The PEM fuel cell is the most promising system currently available because of the
simplicity of its design and the low temperature of operation (around 80 o
C). For this
reason, there have been several experimental and theoretical attempts in the past decade
to characterize the operation of PEM fuel cells. Rho et al.  utilized diﬀerent mixtures
of oxygen and inert gases and studied mass transport phenomena across the PEM fuel cell
system. Beattie et al.  studied the eﬀect of temperature and pressure on oxygen reduction
at the platinum and Naﬁon interfaces. Jordan et al.  studied the eﬀect of diﬀusion layer
on the performance of the fuel cell. Motupally et al.  and Sridhar et al.  studied the
eﬀect of water diﬀusion on these membrane reactors. Theoretical modeling of transport and
reaction in fuel cells is challenging due to the numerous design and operating parameters that
can inﬂuence its performance. The transport of water and ions in a PEM fuel cell has been
modeled at various levels of complexity by many groups. Mass transport of gas and water
was also studied with both one dimensional , , ,  and two dimensional models
, , . Verbrugge and Hill  developed a steady state fuel cell model to study
the transport properties of perﬂuorosulfonic acid membranes under electrolyte supported
conditions. Bernardi and Verbrugge  developed a one-dimensional steady state model
to study the eﬀects of transport of gases in gas diﬀusion electrodes on the performance of
PEM fuel cells. Springer et al.  developed an isothermal, one-dimensional steady state
model for a complete polymer electrolyte fuel cell. Their model also predicted the net water
ﬂow per proton through the membrane and the increase in membrane resistance due to the
membrane water content.
Nguyen and White  developed a two dimensional steady state model to describe the
heat transfer and mass transfer in the fuel cell. They also investigated the eﬀectiveness of
various humidiﬁcation designs. Thampan et al.  developed a steady state analytical
transport-reaction model by drawing parallels with membrane reactors. Fuller and Newman
 examined the water, thermal and reactant utilization of the fuel cell by developing a
two dimensional mass transport model of the membrane electrode assembly. Van Zee et
al.  presented a three dimensional numerical model that predicts the mass ﬂow between
the cathode and anode channels. Several publications , , ,  have focused on
fuel cell polarization curves and identiﬁcation of the various fuel cell resistances that are
encountered at diﬀerent operating conditions. The resistances are then used to predict the
fuel cell voltage-current characteristics or the fuel cell polarization curves.
The steady state models focus on developing the complex electrochemical, thermody-
namic and ﬂuid mechanics principles and include spatial variations. These models are very
useful in designing the various components inside individual fuel cells like membrane electrode
assemblies, backing layers, ﬂow ﬁelds etc. Design of these components is essential to establish
the feasibility of fuel cells and hence all the models that came out in the 1990s were steady
state models which were used to design the various components of the fuel cell. Once the
commercial viability of the fuel cells was realized, focus shifted from steady state models to
performance models which focus on the eﬃciency of the fuel cell under diﬀerent operating
conditions. As the research became more application oriented the focus was on identifying
the current voltage characteristics which were useful in calculating the number of cells and
the area of cell depending upon the power demand, current required, operating voltage etc.
A single fuel cell cannot produce enough voltage and generally a group of cells are put
together and this arrangement is also known as the fuel cell stack. Several models were
developed to represent the behavior of fuel cell stacks , . These models were used to
determine the operating conﬁgurations for the stack and for the stack ﬂow ﬁeld design. The
equal distribution of the gases to the various cells inside the stack is very critical for proper
functioning of the fuel cell.
In this dissertation analytical models of the fuel cell polarization curve will be used to
establish a good operating point for the fuel cell operation. Using the maximum power
demand the number of fuel cells in the stack, the cross sectional area of each fuel cell will
be calculated. Based on the results obtained by Nyugen and White  a linear model will
be developed and used to calculate the number of cells and operating points. Using these
values as initial guesses a more thorough estimate will be obtained using nonlinear model
given by Pukrushpan et al.  in Chapter 4.
2.4 Thermal Management System
The in situ hydrogen generation subsystem consists of a combination of exothermic and
endothermic reactions. The steam reforming reactions are endothermic and these reactions
take place at very high temperatures. Suﬃcient heat has to be supplied to the fuel and
steam to heat them to the reactor temperature and also maintain the reactor temperature.
Most of the literature on steam reforming thermodynamics is based on the large steam
reformers used industrially . Lutz et al.  did a thermodynamic analysis of a compact
steam reformer using a diesel fuel and found out that both incomplete reaction and heat
transfer losses reduce the eﬃciency of the process. The gases leaving the reformer have to
be cooled to the operating temperature of the water gas shift reactor. The WGS reaction
is exothermic so there is excess heat available which can be redistributed to the reactors
requiring heat. The preferential oxidation reaction is also exothermic and produces heat.
Furthermore, the gases from the PROX coming out of the fuel processing subsystem have
to be cooled to the temperature at which the power generation subsystem operate. For
this reason, it is necessary to develop an eﬃcient thermal management system for optimized
operation. This fact has been recognized in recent feasibility studies by Zalc and Loﬄer
 where the heat requirements for each reactor system were calculated based on overall
energy balances. Godat and Marechal  developed a model of a system including a proton
exchange membrane (PEM) fuel cell and its fuel processing section. They investigated the
process conﬁgurations to identify optimal operating conditions and optimal process structure
of the system by applying modeling and process integration techniques. They used pinch
technology techniques to model the integrated heat exchange system to get an estimate
of the net energy requirement for a PEM fuel system. Sorin and Paris  applied pinch
technology to the thermodynamic analysis of a process through the exergy load distribution
method. The focus of this study was on feasibility of operation, rather than on the dynamic
heat load of the operation, which is important from a control standpoint. In addition to the
fuel processing system we may need a combustor to provide the necessary heat for the steam
reformer. A thermal management system should be designed which can distribute the heat
among the diﬀerent reactors. The design should take into consideration the dynamic eﬀects
of the diﬀerent processes.
In this dissertation the overall heat duty requirements for the three reactors in series will
be calculated for diﬀerent ﬂow rates of methane. Even though the WGS and PROX reactor
produce heat they operate at a lower temperature compared to SR and a heat source which
operates at a temperature higher than SR is needed. Hence a combustor which operates at
a higher temperature than SR will be designed. The thermal system design will be based on
steady state modeling.
2.5 Controller Design and Power Distribution System
For a fuel cell vehicle it is necessary to design a control system that can track the power
demand from the fuel cell. The reference for this control system is the power demand
of the automobiles, which changes with road conditions as well as driving characteristics.
Reference tracking problems are conventionally handled by PID controllers which are the
most commonly used controllers in the process industry. However, since the power demand
proﬁle is not known a priori, a PID controller that is tuned to one set of conditions (e.g.
highway driving) may not work well under a diﬀerent set of conditions (e.g. city driving).
It is necessary to design a controller that adapts to varying driving and road conditions.
2.5.1 Adaptive Controller
Interest in adaptive control techniques ﬁrst started during the early 1950s when it was
used for the design of autopilots for high performance aircraft. This motivated an intense
research activity in adaptive control. High performance aircraft undergo drastic changes in
their dynamics when they ﬂy from one operating point to another that cannot be handled by
constant-gain feedback control. A sophisticated controller, such as an adaptive controller,
that could learn and accommodate changes in the aircraft dynamics was needed. Model
Reference Adaptive Control (MRAC) was suggested by Whitaker et al.  to solve the
autopilot control problem. The sensitivity method and the MIT rule was used to design the
adaptive laws of the various proposed adaptive control schemes. An adaptive pole placement
scheme based on the optimal linear quadratic problem was suggested by Kalman . During
1960s development of control theory and adaptive control in particular was facilitated by
the introduction of state space techniques and stability theory based on Lyapunov theory.
Developments in system identiﬁcation and parameter estimation lead to the reformulation
and redesign of adaptive control techniques. The MIT rule-based adaptive laws used in the
MRAC schemes of the 1950s were redesigned by applying the Lyapunov design approach.
During this time the adaptive controllers designed were applicable only to a special class
of linear time invariant plants but nevertheless this provided a nice platform for further
rigorous stability proofs in adaptive control for more general classes of plant models. On
the other hand, the simultaneous development and progress in computers and electronics
that made the implementation of complex controllers feasible contributed to an increased
interest in applications of adaptive control. The 1970s witnessed several breakthrough results
in the design of adaptive control. MRAC schemes using the Lyapunov design approach
were improved. The concepts of positivity and hyperstability were used to develop a
wide class of MRAC schemes with well established stability properties . At the same
time parallel eﬀorts for discrete-time plants in a deterministic and stochastic environment
produced several classes of adaptive control schemes with rigorous stability proofs (,
). The non-robust behavior of adaptive control became very controversial in the early
1980s when more examples of instabilities were published demonstrating lack of robustness
in the presence of unmodeled dynamics or bounded disturbances , . This stimulated
many researchers, whose objective was to understand the mechanisms of instabilities and
ﬁnd ways to counteract them. By the mid 1980s, several new redesigns and modiﬁcations
were proposed and analyzed, leading to a body of work known as robust adaptive control.
An adaptive controller is deﬁned to be robust if it guarantees signal boundedness in the
presence of reasonable classes of unmodeled dynamics and bounded disturbances as well as
performance error bounds that are of the order of the modeling error .
The solution of the robustness problem in adaptive control led to the solution of the long
standing problem of controlling a linear plant whose parameters are unknown and changing
with time. By the end of the 1980s several breakthrough results were published in the area
of adaptive control for linear time-varying plants . The focus of adaptive control research
in the late 1980s to early 1990s was on performance properties and on extending the results
of the 1980s to certain classes of nonlinear plants with unknown parameters. These eﬀorts
led to new classes of adaptive schemes, motivated from nonlinear system theory  as well
as to adaptive control schemes with improved transient and steady state performance,
. Adaptive control has a rich literature full with diﬀerent techniques for design, analysis,
performance and applications. Several survey papers ,  and books  have already
In this dissertation a model reference adaptive controller will be designed using the
Lyapunov method for tracking a time varying power proﬁle in the fuel cell powered
automobile. To improve robustness a discontinuous dead zone and derivative action will be
added. The adaptability of the controller will be tested by implementing the controller on
diﬀerent power proﬁles which simulate actual power requirement of diﬀerent road conditions.
The performance of the adaptive controller is compared with a conventional PID controller
and the adaptive controller is shown to perform better than the PID.
2.5.2 Switching Controller
The fuel cell system requires time for the diﬀerent reactors in the fuel processing system to
heat upto their respective optimum operating conditions. The fuel can be directly sent to the
combustor to produce the suﬃcient heat for this process. Nevertheless this may take several
minutes and thus an auxiliary power source is needed to supply the power in the meantime.
An auxiliary power source is also needed when the instantaneous power demand exceeds
the power supplied by the fuel cell. For the fuel cell to provide more power it is necessary
to process a higher ﬂow rate of hydrocarbon fuel which results ina time lag in producing
the desired power. During this lag time the automobile has to operate on auxiliary power.
Instead of the lead-acid battery which has a energy density of 20-35 Wh/kg a Lithium-ion
battery can be used as it has a higher energy density of 100-200 Wh/kg . Newman 
considered high power batteries for hybrid vehicles and developed a model for a lithium-ion
battery. A simpler model was developed by He et al.  who also were investigating battery
performance for a hybrid vehicle. Lee et al.  conducted experiments to study the eﬀect
of load increase on a battery backup system and showed that it was necessary to have a
control system to switch eﬀectively to the battery. Gokdere et al.  computed the power
requirements for rapid acceleration and deceleration to study the dynamics of the battery in
a hybrid electric car.
In this dissertation the simpliﬁed battery model proposed by He et al. will be used and a
switching controller will be designed which eﬀectively switches between the fuel cell and the
battery. The factors to be considered in designing this switching controller are (a) ensure
power demand at all times, (b) ensure that the battery is not completely discharged and (c)
distribute excess power produced by the fuel cell to battery backup.
DESIGN OF FUEL PREPROCESSOR
3.1 Thermodynamic Analysis
The fuel processor system designed should be small enough to ﬁt under the hood of a
car and quick enough to produce the required hydrogen on an “as needed basis” to meet
the power demand. The steam reforming and the water gas shift reactions which take place
in the fuel processing subsystem are reversible reactions. The design of processes involving
reversible reactions, generally begins with a feasibility study or a thermodynamic analysis.
The thermodynamic analysis does not specify the sizes of the reactor or information about
how fast the reaction occurs, but it provides a theoretical limit on the conversion possible
based on the equilibrium conditions. This analysis is also useful in identifying whether the
overall process produces heat or requires heat. The fuel cell system schematic diagram as
shown in Fig. 1.1, consists of 3 packed bed reactors, a PEM fuel cell and a combustor. There
are diﬀerent reactions that are taking place in the fuel processor and it is ﬁrst necessary to
identify the reactions that require heat (endothermic) and the reactions that produce heat
(exothermic). This can be calculated easily based on the standard heat of reaction. If the
standard heat of reaction is positive then the reaction is said to be endothermic and if it
is negative the reaction is exothermic. The standard heat of reaction can be obtained from
the standard heat of formation of the individual species involved in the reaction by using
the Hess’s Law. The standard heat of formation shown in Table 3.1 were obtained from the
NIST Chemistry Webbook . For oxygen and hydrogen the standard heat of formation
can be assumed to be zero .
Table 3.1. Standard heat of formations
Species Standard heat of formation (kJ/mol)
Using the heat of formation data, the heat of reaction can be computed using Hess’s law.
Depending on the sign of the standard heat of reaction We can tell whether a reaction is
exothermic or endothermic.
Table 3.2. Standard heat of reactions and type of reaction
Reaction Standard heat of reaction Type
CH4 + H2O 3H2 + CO 205 endothermic
CO + H2O CO2 + H2 -41 exothermic
CH4 + 2H2O 4H2 + CO2 164 endothermic
CO + (1/2)O2 CO2 -283 exothermic
H2 + (1/2)O2 H2O -242 exothermic
Table 3.2 indicates that the reforming reactions are endothermic while the water gas shift
reaction, preferential oxidation reactions are exothermic. It is necessary to design a heat
distribution system which will distribute the heat produced by the exothermic reactions to
the endothermic reactors. The heat produced by the exothermic reactors may or may not be
suﬃcient to provide the necessary heat to the endothermic reactors. A preliminary analysis
 for a fuel cell system powered by methane indicates that the methane feed stream does
not provide suﬃcient heat for high ﬂow rates and it may be necessary to feed approximately
35% more methane than that required for the power generation subsystem to account for
the heat necessary for the fuel processor subsystem. Hence a combustor is needed to meet
the required heat demand.
3.1.1 Feed Stream Composition
To calculate the exact amount of hydrogen that is required inside the fuel cell, a
relationship between the hydrogen going into the fuel cell and the power produced by the
fuel cell is needed. This requires a fuel cell model which will be discussed in detail in the next
chapter. Once the amount of hydrogen required is known, the amount of methane to be fed
to the reformer can be calculated if a relationship between the methane fed to the reformer
and the hydrogen coming out of the series of reactors is known. As a starting guess it is
assumed that all the methane fed to the reactor is reacting by the main reformer reaction
given by Eq. 2.1, in which 3 moles of hydrogen are produced for every mole of methane.
A rough estimate of the hydrogen required to produce 50 kW (67 hp) of power is required.
The power produced by the fuel cell is given by Eq. 3.1.
P = IV (3.1)
Where P is the power (W) and I is the current (A) and V is the voltage (V). For every
molecule of hydrogen that reacts within a fuel cell, two electrons are liberated at the fuel cell
anode. This is most easily seen in the PAFC and PEM fuel cells, because of the simplicity
of the anode reaction given by Eq. 3.2.
H2 → 2H+
+ 2e− (3.2)
One equivalence of electrons is 1 mol of electrons or 6.022 × 1023
number). This quantity of electrons has a charge of 96,487 C (Faraday’s constant). One
ampere of current is deﬁned as 1 C/sec. Using the above information the moles of hydrogen
(nH2 ) needed to generate 1 A current can be calculated using Eq. 3.3.
nH2 = 1.0A
) = 3.1 × 10−4
The maximum theoretical voltage is 1.23 V. If it is assumed that the cell is operating at 50%
eﬃciency, a voltage of approximately 0.7 V results. The current required inside the fuel cell
to have a power output of 50kW is given in Eq. 3.4.
= 71.43kA (3.4)
From Eq. 3.3 it can be seen that 3.1 × 10−4
mol/min of H2 are required to produce 1A.
Using this factor the amount of fuel that must be provided to supply a desired fuel cell power
output can be determined. Not all the hydrogen that is sent into the fuel cell reacts and
some of the hydrogen comes out unreacted. The ratio of hydrogen reacted to the hydrogen
fed into the reactor is known as hydrogen utilization (U). If an 80% utilization is assumed,
then the hydrogen ﬂow rate is given by Eq. 3.5
3.1 × 10−4
mol/min × 71.43 × 103
∼= 30mol/min (3.5)
where nH2t is the total amount of hydrogen required. Hence around 30 mol/min of H2 is
needed to get a power output of 50 kW. This is the maximum amount of hydrogen needed
as this corresponds to the maximum power. Assuming that the SR main reaction in which
3 moles of hydrogen is produced is the only reaction taking place, the maximum amount of
methane ﬂow rate can be estimated to be 10 mol/min.
3.1.2 Overall Heat Duty
The fuel processing subsystem consists of the reformer, the water gas shift reactor and
the preferential oxidation reactor.
The reactions taking place in the reformer are
CH4 + H2O → CO + 3H2; ∆Ho
298 = 205.81kJ/mol (3.6)
CO + H2O → CO2 + H2; ∆Ho
298 = −41.16kJ/mol (3.7)
CH4 + 2H2O → CO2 + 4H2; ∆Ho
298 = 164.64kJ/mol (3.8)
The reaction represented by Eq. 3.6 is the main reaction in which the methane reacts
with steam to give 3 moles of H2 and a mole of CO. This reaction is endothermic. In
addition to this reaction CO2 is also produced by a side reaction shown in Eq. 3.8, which is
also endothermic. Small amount of the CO produced in the main reaction reacts with steam
to form CO2 and H2 as shown in Eq. 3.7. This reaction is known as the water gas shift
reaction which is an exothermic reaction. A preliminary analysis was done to ﬁnd the heat
requirements of the reformer. The amount of hydrogen required for the maximum power
output (50 kW) is around 30 mol/min. From the amount of hydrogen the approximate
amount of methane required is calculated by assuming that one mole of methane gives
approximately 3 moles of hydrogen, i.e., all the methane entering is reacting via Eq. 3.6
this corresponds to a maximum methane ﬂow rate of 10 mol/min. To avoid the formation
of coke, the steam to methane ratio is maintained at 3:1 ratio.
Water Gas Shift Reactor (WGS)
The water gas shift reaction represented by Eq. 3.9 is an exothermic reaction.
CO + H2O −→ CO2 + H2; ∆Ho
298 = −41.16kJ/mol (3.9)
This reactor is generally divided into two parts the high temperature shift reactor (HTS)
which is operated at a temperature of 700 K and the low temperature shift reactor (LTS)
which is operated at 490 K (The kinetic details and the details about how to obtain the
optimum temperature are discussed in the next section). The exhaust from the reformer is
sent as feed to the WGS reactor. The amount of heat liberated from the WGS reactor for
the diﬀerent ﬂow rates of methane into the reformer is calculated.
Preferential Oxidation Reactor(PROX)
The CO concentration is brought down to less than 100 ppm by preferentially oxidizing
CO with oxygen in air. The amount of oxygen present in air should be at least twice the
amount of CO present in the WGS exhaust. This reaction is also exothermic and the heat
liberated is calculated for an isothermal case of 473 K.
The overall heat duty was calculated for the three reactors. The heat requirement for
diﬀerent ﬂow rates of methane, where the methane ﬂow rate is varied from 1 to 10 mol/min,
was calculated and is shown in Table 3.3. As can be seen from the heat duty calculation
we still need to supply some heat to the reactors and also we need a source of heat at
a temperature greater than 1000 K( i.e. the operating temperature of the reformer). To
supply this heat we added a combustor which can utilize any heat left in the anode tail gas.
If that heat is not suﬃcient more heat is supplied by feeding some methane directly to the
combustor. If the heat requirement is known for a given methane ﬂow into the reformer,
the amount of methane to be fed to the combustor can be calculated as a percentage of the
methane being fed to the reformer.
Table 3.3. Heat duty calculation
Methane ﬂow rate Overall heat
into reformer required
3.1.3 Combustor Calculations
The steam reformer is an exothermic reactor operating at a high temperature of 1000 K
and it needs a heat source which can supply the necessary heat. Some of the heat generated
in the water gas shift reactor and preferential oxidation reactor can be utilized. However,
both these reactors operate at a lower temperature than that of the steam reformer. The
combustor which has to be operated at a temperature higher than the operating temperature
of the reformer is used to supply the heat required. The reactions taking place inside the
combustor are the oxidation of, carbon monoxide as shown in Eq. 3.10, unreacted hydrogen
and methane as shown in Eq. 3.11, Eq. 3.12 respectively.
O2 → CO2 (3.10)
O2 → H2O (3.11)
CH4 + 2O2 → CO2 + 2H2O (3.12)
The amount of heat liberated can be obtained form a simple energy balance around the
combustor as shown in Eq. 3.13.
i − ˙Q = 0 (3.13)
i , ˙Nout
i are the ﬂow rate of species i coming into the combustor and leaving the
combustor respectively. The enthalpy of these streams is given by Hin
i , Hout
i and ˙Q is the
amount of heat liberated. From the mass balance equation a relationship between the gases
coming into the reactor and leaving the reactor can be established and is given by Eq. 3.14.
i = ˙Nin
νij ˙j (3.14)
where νij is the stoichiometric constant of species i in reaction j (since there are three
reactions taking place j=1, 2, 3) and ˙j is the extent of reaction j. Substituting the mass
balance (Eq. 3.14) into the energy balance (Eq. 3.13) we can obtain an expression for ˙Q as
given by Eq. 3.15.
which can be simpliﬁed as
i − Hout
i ) −
expanding the enthalpy terms we have
is the speciﬁc heat capacity of species i and ∆Hout
j is the heat of reaction, of
reaction j at temperature Tout.
From the heat analysis it is clear that we still need to supply some heat to the reactors.
Some of the heat can be recovered from the gases leaving the anode section of the fuel cell.
If we assume 90% hydrogen utilization inside the fuel cell, the heat available from the rest
of the gases can be calculated assuming total combustion of the anode tail gases.
Table 3.4 lists the amount of heat available from the anode tail gases for diﬀerent ﬂow
rates of methane. It can be seen that this stream does not produce suﬃcient heat and it
Table 3.4. Heat from the anode tail gas for diﬀerent initial ﬂow rates into the reformer.
Methane ﬂow rate Overall heat duty Heat available from
mol/min kW anode tail gas(kW)
1 4.57 1.48
2 9.13 2.97
3 13.30 4.44
4 17.65 5.94
5 22.77 7.45
6 26.64 9.03
7 31.03 10.67
8 35.41 12.41
9 39.57 14.24
10 45.43 16.98
is necessary to supply methane to the combustor. The amount of excess methane required
depends on the available ˙Q which itself depends on the operating temperature and the oxygen
excess ratio. Of the three reactors, the steam reformer operates at the highest temperature of
1000 K. To supply the heat to the reformer the combustor should operate at an even higher
temperature and to avoid pinch zones the combustor should supply heat at a temperature
which is at least 15-20 o
C above that of the reformer temperature. The eﬀect of change in
temperature as it is increased from 1020 to 1100 K on ˙Q is shown in Fig. 3.1. Another
variable is the oxygen supplied in excess to that needed stoichiometrically to ensure complete
combustion. The eﬀect of change in this ratio on ˙Q is also presented in Fig. 3.1. A lower
excess ratio gives a higher ˙Q as less energy is expended in heating up the nitrogen which
comes along with oxygen in air. Lowering the excess ratio may result in the combustion
reactions not going to completion. Based on the above analysis an operating temperature of
1020 K and an oxygen excess ratio of 15 % was selected. The amount of ˙Q for diﬀerent ﬂow
rates when 25%, 30% and 35% of the methane fed to the reformer is fed to the combustor is
given in Table 3.1.3. From Table 3.1.3 it can be seen that35% excess methane is required in
3.2 Steam to Carbon Ratio
The steam to carbon ratio is an important operating parameter which can inﬂuence the
conversion inside the reformer. Steam supplied in the stoichiometric ratio facilitates the main
1020 1030 1040 1050 1060 1070 1080 1090 1100
15% excess O2
20% excess O
25% excess O2
Figure 3.1. Eﬀect of operating temperature and oxygen excess ratio on heat duty
reaction in which 3 moles of hydrogen are produced for every mole of methane. Supplying
steam in excess to the stoichiometric ratio has three advantages. First, in the presence of
excess steam the side reaction in which four moles of hydrogen are produced is favored.
Second, the excess steam pushes the water gas shift reaction equilibrium to the right thereby
producing more hydrogen and also reducing the carbon monoxide levels. Third, a high steam
to carbon ratio reduces the chances of coke formation inside the reformer as steam acts as a
coke inhibitor. In the absence of excess steam the active sites on the catalyst are occupied by
coke forming compounds instead of steam. Coke may be formed by methane decomposition
(Eq. 3.18), Bouduard reaction (Eq. 3.19) or carbon monoxide decomposition (Eq. 3.20).
CH4 C + 2H2 (3.18)
2CO CO2 + C (3.19)
CO + H2 C + H2O (3.20)
Table 3.5. Heat from the combustor when methane is fed at 25%, 30%, 35% in excess to
that fed to the reformer.
CH4 ﬂow rate Overall ˙Q from anode ˙Q for 25% ˙Q for 30% ˙Q for 35%
into reformer Heat duty tail gas excess CH4 excess CH4 excess CH4
(mol/min) (kW) (kW) (kW) (kW) (kW)
1 4.57 1.48 3.66 4.09 4.52
2 9.13 2.97 7.33 8.19 9.14
3 13.30 4.44 10.99 12.27 13.55
4 17.65 5.94 14.67 16.38 18.08
5 22.77 7.45 18.36 20.50 22.64
6 26.64 9.03 22.12 24.69 27.25
7 31.03 10.67 25.96 28.95 31.94
8 35.41 12.41 29.88 33.30 36.72
9 39.57 14.24 33.90 37.74 41.59
10 45.43 16.98 39.74 44.19 48.64
The carbon thus formed decreases the eﬃciency and longevity of the catalyst. Table
3.6 shows the increase in conversion of methane with the increase in steam to carbon ratio.
The volume of the reformer was kept constant at 10 litres and the amount of methane into
the reactor was maintained at the maximum ﬂow rate. The steam ﬂow rate was adjusted
according to the steam to carbon ratio. As seen from Table 3.6 the conversion increases as
the ratio increases. Industrial steam reformers often operate at high steam to carbon ratio.
However, a large steam to carbon ratio requires a large volume of steam or water tank. Since
the total volume available in an a automobile is limited, there is a constraint on the steam to
carbon ratio that is feasible for an automotive application. Moreover from the Table 3.6 it
can be observed that the increase in conversion is accompanied by a lot of unreacted steam.
Thus even though the amount of hydrogen in the reformer exhaust stream increases with
increasing steam to carbon ratio, the quality of hydrogen or the mole fraction of hydrogen
decreases. The steam has to be generated, heated to 1000 K and compressed to 5 atm. The
unreacted steam represents a lot of energy wasted. On the other hand decreasing the ratio
below 3:1 increases the coke formation and thereby reduces the amount of conversion and is
also detrimental to the longevity of the catalyst. Hence a steam to carbon ratio of 3:1 has
been chosen for this study.
Table 3.6. Eﬀect of varying the steam to carbon ratio
Steam to carbon H2 from reformer Methane conversion unreacted
ratio mol/min steam (mol/min)
2:1 20.5 67% 9.3
3:1 24.2 77% 15.9
4:1 26.5 82% 23.2
5:1 28.2 86% 30.8
3.3 Design of Fuel Processing Subsystem
In this section, the design and operation of a fuel cell system for a rating of 50 kW is
considered. This value may seem low (50 kW = 67 hp) when compared to power ratings
of today’s internal combustion engines; yet because electric motors deliver maximum torque
at all rpms while internal combustion engines deliver maximum torque only at an optimal
rpm, internal combustion engines operate at a fraction of their nominal power rating while
electric motors operate at their rated power at all times .
As described in the previous section, the fuel processor subsystem consists of a train
of three tubular reactors. Each reactor is modeled as an isothermal plug-ﬂow reactor. It
is assumed that no axial mixing or axial heat transfer occurs. Furthermore, the transit
times for all ﬂuid elements through the reactor are assumed to be of equal duration. The
automotive application puts a constraint on the total volume of the reactor train since the
entire system has to ﬁt under the hood of the automobile. In this section, it is assumed
that the maximum allowable volume of the fuel processor subsystem is 100 liters. The
initial focus was on the development of detailed dynamic models for each reactor in the
fuel processing subsystem. A time scale analysis of the reactor operations showed that in
the range of operating conditions for an automobile, the dynamic eﬀects of changes to the
inlet conditions would be damped out by the thermal control system. In particular the gas
passing through the reactors had a typical residence time of the order of seconds. Changes
in the inlet feed to such a reactor presents short term responses, based on the residence
time, and long term transients seen in the bed temperature. Bell and Edgar  showed
that these eﬀects occur in the time scale of 30 minutes. During practical vehicle operation,
these long term transients are overcome by the thermal control system. Consequently, it is
only necessary to determine the steady state relation between the methane going into the
steam reformer and the hydrogen coming out of the preferential oxidation reactor. Based
on the kinetic models available the optimum conditions for the reactor operation have been
found for the three packed bed reactors individually.
3.3.1 Kinetics of Steam Reformer
The reactions taking place in the SR are given in Eq. 3.6, 3.8, 3.7. Xu and Froment
 developed intrinsic rate expressions for the steam reforming of methane, accompanied
by the water gas shift reaction on a Ni/MgAl2O3 catalyst. The following reaction rate laws
PCH4 PH2O −
(1 + KCOPCO + KH2 PH2 + KCH4 PCH4 + KH2OPH2O/PH2 )2 (3.21)
(1 + KCOPCO + KH2 PH2 + KCH4 PCH4 + KH2OPH2O/PH2 )2 (3.22)
(1 + KCOPCO + KH2 PH2 + KCH4 PCH4 + KH2OPH2O/PH2 )2 (3.23)
where r1 is the rate of formation of CO for the reaction represented by Eq. 3.6, r2 is
the rate of formation of CO2 for the reaction represented by Eq. 3.7 and r3 is the rate of
formation of CO2 for the reaction represented by Eq. 3.8. Pi are the partial pressures of the
reactants and Ki are the adsorption coeﬃcients. The adsorption coeﬃcients can be found
using the following relations for the respective species:
Ki = A(Ki)exp
, where i = H2, CO, CH4, H20 (3.24)
The rate constants are given by a similar Arrhenius type equation.