THE FLORIDA STATE UNIVERSITY
FAMU–FSU COLLEGE OF ENGINEERING
ANALYSIS AND CONTROL OF AN IN SITU HYDROGEN GENERATION
AND FU...
The members of the Committee approve the dissertation of Panini K. Kolavennu defended
on December 8, 2005.
Srinivas Palank...
To My Grandparents . . .
iii
ACKNOWLEDGEMENTS
I would like to express my deep sense of gratitude to my advisor Dr. Srinivas Palanki for
his guidance, h...
TABLE OF CONTENTS
List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ....
4. FUEL CELL DESIGN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
4.1 Design ...
LIST OF TABLES
1.1 Hydrogen production based on the type of fuel . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2 P...
LIST OF FIGURES
1.1 Schematic diagram of the fuel cell system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
...
5.9 Speed and Power profiles for the US06-HWY driving cycle . . . . . . . . . . . . . . . 85
5.10 Adaptive controller with ...
D.1 Simulink diagram to simulate the switching controller design . . . . . . . . . . . . . . 138
x
ABSTRACT
A new future in automotive transportation is approaching where vehicles are powered by
new, clean and efficient ene...
CHAPTER 1
INTRODUCTION
1.1 Overview
The beginning of the 19th century marked the advent of the modern automobile systems.
...
hilly conditions, highway vs. city driving etc.). These engineering constraints have to be
addressed properly for successf...
based on the internal combustion engine. Most importantly, the use of an integrated fuel
cell system for an application as...
The second area is in oil refineries where hydrogen has been used for hydro-formulation,
or high-pressure hydro-treating, o...
Table 1.2. Pertinent physical properties of transportation fuels
Fuel H/C Density Heating value Energy density CO2 CO2 emi...
be able to safely, compactly, and cost-effectively store hydrogen on board, which is a major
technical challenge. As seen f...
CO poisoning. Furthermore, a fuel cell system should be designed such that it delivers upto
50 kW of power that is suitabl...
The fuel cell system is divided into the following four subsystems:
1. Fuel processing subsystem
2. Power generation subsy...
to produce water.
CO +
1
2
O2 → CO2; ∆Ho
298 = −283kJ/mol (1.5)
H2 +
1
2
O2 → H2O; ∆Ho
298 = −242kJ/mol (1.6)
The power ge...
1. Assess the thermodynamic feasibility of the system.
2. Design and analyze the reactors necessary for the fuel processor...
battery. Finally, in Chapter 7 the main results of this dissertation are summarized and the
direction of future work is pr...
CHAPTER 2
LITERATURE SURVEY
The process flow diagram presented in Fig. 1.1 has many similarities to those in the
chemical p...
• Compression
• Liquefaction
• Physisorption
• Metallic hydrides
• Complex hydrides
Hydrogen can be stored in a pressurize...
absorb/desorb hydrogen at moderate temperatures. While complex hydrides are a promising
solution of the hydrogen storage p...
SR Side Reaction
CH4 + 2H2O → CO2 + 4H2; ∆Ho
298 = 164.64kJ/mol (2.3)
All the reactions occur at high temperature and the ...
2.1.3 Development of Kinetic Models
Steam reforming of hydrocarbons for hydrogen production has been studied for several
d...
[25]. While industrial fixed-bed reactors operate at relatively high pressures (∼ 30 bar), fuel
cell applications typically...
where λ is a process parameter which represents the oxygen in excess with respect to the
amount of oxygen required for the...
Figure 2.1. Cross section of a polymer electrolyte membrane fuel cell
water. Fig. 2.1 [29] shows a cross-sectional diagram...
Phosphoric Acid Fuel Cell (PAFC)
As the name suggests the electrolyte in a phosphoric acid fuel cell is phosphoric acid.
P...
(2) side chains which connect the molecular backbone to the ionic part and (3) the ion
clusters consisting of sulfonic aci...
Fuel Cell Stack
The maximum voltage of a single fuel cell at 100 % efficiency is 1.23 V . As most
applications require highe...
and weaknesses. There are three basic market segments for fuel cells: portable/battery
substitution, transportation and ut...
and a number of other major manufacturers are gearing up to do the same. Nissan leased
its first FCV in 2004, Dihatsu, Ford...
on the performance of the fuel cell. Motupally et al. [38] and Sridhar et al. [39] studied the
effect of water diffusion on ...
the feasibility of fuel cells and hence all the models that came out in the 1990s were steady
state models which were used...
steam reformer using a diesel fuel and found out that both incomplete reaction and heat
transfer losses reduce the efficienc...
2.5 Controller Design and Power Distribution System
For a fuel cell vehicle it is necessary to design a control system tha...
that made the implementation of complex controllers feasible contributed to an increased
interest in applications of adapt...
automobile. To improve robustness a discontinuous dead zone and derivative action will be
added. The adaptability of the c...
CHAPTER 3
DESIGN OF FUEL PREPROCESSOR
3.1 Thermodynamic Analysis
The fuel processor system designed should be small enough...
Table 3.1. Standard heat of formations
Species Standard heat of formation (kJ/mol)
CO -110.53
CO2 -393.51
H2O -242
CH4 -74...
3.1.1 Feed Stream Composition
To calculate the exact amount of hydrogen that is required inside the fuel cell, a
relations...
output can be determined. Not all the hydrogen that is sent into the fuel cell reacts and
some of the hydrogen comes out u...
amount of methane required is calculated by assuming that one mole of methane gives
approximately 3 moles of hydrogen, i.e...
the amount of methane to be fed to the combustor can be calculated as a percentage of the
methane being fed to the reforme...
i
˙Nin
i Hin
i −
i
˙Nout
i Hout
i − ˙Q = 0 (3.13)
where ˙Nin
i , ˙Nout
i are the flow rate of species i coming into the com...
Table 3.4. Heat from the anode tail gas for different initial flow rates into the reformer.
Methane flow rate Overall heat du...
1020 1030 1040 1050 1060 1070 1080 1090 1100
36
37
38
39
40
41
42
43
44
45
TEMPERATURE(K)
Qdot
(kW)
15% excess O2
20% exce...
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 rat...
been chosen for this study.
Table 3.6. Effect of varying the steam to carbon ratio
Steam to carbon H2 from reformer Methane...
time, and long term transients seen in the bed temperature. Bell and Edgar [85] showed
that these effects occur in the time...
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
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Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06
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Analysis and control of an in situ hydrogen generation and fuel cell power system for automotive applications 06

  1. 1. 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 By 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 Degree Awarded: Spring Semester, 2006
  2. 2. The members of the Committee approve the dissertation of Panini K. Kolavennu defended on December 8, 2005. Srinivas Palanki Professor Directing Dissertation David Cartes Outside Committee Member John C. Telotte Committee Member Ravindran Chella Committee Member Bruce R. Locke Committee Member The Office of Graduate Studies has verified and approved the above named committee members. ii
  3. 3. To My Grandparents . . . iii
  4. 4. ACKNOWLEDGEMENTS 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 journey. iv
  5. 5. TABLE OF CONTENTS List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi 1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2 Hydrogen Economy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2.1 Fueling Infrastructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.3 Schematic Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.4 Thesis Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2. LITERATURE SURVEY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.1 Fuel Processing System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.1.1 Onboard Hydrogen Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.1.2 In situ Hydrogen Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.1.3 Development of Kinetic Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.2 Fuel Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.2.1 Working Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.2.2 Types of Fuel Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 2.2.3 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 2.3 Fuel Cell Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 2.4 Thermal Management System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 2.5 Controller Design and Power Distribution System . . . . . . . . . . . . . . . . . . . . . . 28 2.5.1 Adaptive Controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 2.5.2 Switching Controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 3. DESIGN OF FUEL PREPROCESSOR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 3.1 Thermodynamic Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 3.1.1 Feed Stream Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 3.1.2 Overall Heat Duty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 3.1.3 Combustor Calculations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 3.2 Steam to Carbon Ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 3.3 Design of Fuel Processing Subsystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 3.3.1 Kinetics of Steam Reformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 3.3.2 Water Gas Shift Reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 3.3.3 Preferential Oxidation Reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 3.3.4 Varying Feed Rates of Methane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 v
  6. 6. 4. FUEL CELL DESIGN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 4.1 Design of Power Generation Subsystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 4.1.1 Linear Fuel Cell Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 4.1.2 Nonlinear Fuel Cell Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 5. ADAPTIVE CONTROLLER DESIGN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 5.1 Model Reference Adaptive Controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 5.1.1 Design Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 5.1.2 Adaptive Controller with Deadzone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 5.2 PID Controller Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 5.3 Application to PEM Fuel cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 5.3.1 System Identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 5.3.2 Realistic Power Profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 5.3.3 Controller Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 5.3.4 MRAC with Derivative Action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 5.3.5 Design of Fuel Tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 5.4 Application to Phosphoric Acid Fuel Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 6. BATTERY BACKUP MODEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 6.1 Battery Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 6.1.1 State of Charge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 6.1.2 Battery Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 6.2 Switching Controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 6.3 MATLAB implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 7. CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 8. FUTURE WORK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 9. NOMENCLATURE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 APPENDIX A: MATLAB Programs used in Chapter 3 . . . . . . . . . . . . . . . . . 122 APPENDIX B: MATLAB Programs used in Chapter 4 . . . . . . . . . . . . . . . . . . 131 APPENDIX C: MATLAB Programs used in Chapter 5 . . . . . . . . . . . . . . . . . . 134 APPENDIX D: MATLAB Programs used in Chapter 6 . . . . . . . . . . . . . . . . . 137 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 BIOGRAPHICAL SKETCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 vi
  7. 7. 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 vii
  8. 8. 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 viii
  9. 9. 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 ix
  10. 10. D.1 Simulink diagram to simulate the switching controller design . . . . . . . . . . . . . . 138 x
  11. 11. ABSTRACT 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. xi
  12. 12. CHAPTER 1 INTRODUCTION 1.1 Overview 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 vehicles [1]. 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 [2]. However, it has only recently been recognized [3] 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 1
  13. 13. 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 hydrogen. • 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 2
  14. 14. 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 [1]. 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). 3
  15. 15. 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 Nm3 . Global annual production is about 45 billion kg or 500 billion Nm3 . Hydrogen 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 /year) Production 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 4
  16. 16. 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 MJ/kg MJ/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 [4] 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 5
  17. 17. 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 6
  18. 18. 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 7
  19. 19. 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 Eq. 1.4. 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 8
  20. 20. to produce water. CO + 1 2 O2 → CO2; ∆Ho 298 = −283kJ/mol (1.5) H2 + 1 2 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: 9
  21. 21. 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 presented. 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 10
  22. 22. battery. Finally, in Chapter 7 the main results of this dissertation are summarized and the direction of future work is proposed in Chapter 8. 11
  23. 23. CHAPTER 2 LITERATURE SURVEY 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 [5]: 12
  24. 24. • Compression • Liquefaction • Physisorption • 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. [6] 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 [7]. 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. [8] and Service [9] proposed storing hydrogen in nanotubes or nanoballs and this has been a hot topic for research ([10], [11], [12]). Indications are that hydrogen may be stored in nanotubes in quantities exceeding that of metal hydrides and at a lower weight penalty [13], 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 [14] has been working on developing high density metal alloys. Some of the metallic hydrides of interest for storage purpose are listed in [5]. Group I, II, and III elements, (e.g. Li, Mg, B, Al) form a large variety of metal−hydrogen complexes. NaAlH4 [15], LiBH4 [16] and NaBH4 [17] can reversibly 13
  25. 25. 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 [18], 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) 14
  26. 26. 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 [19]. 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 C). 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) 15
  27. 27. 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 [20]. 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 [21], Tottrup [22] and Christensen [23] 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 [24] 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. r1 = k1 P2.5 H2 PCH4 PH2O − P3 H2 PCO K1 (1 + KCOPCO + KH2 PH2 + KCH4 PCH4 + KH2OPH2O/PH2 )2 (2.6) r2 = k2 PH2 PCOPH2O − PH2 PCO2 K2 (1 + KCOPCO + KH2 PH2 + KCH4 PCH4 + KH2OPH2O/PH2 )2 (2.7) r3 = k3 P3.5 H2 PCH4 P2 H2O − P4 H2 PCO2 K3 (1 + KCOPCO + KH2 PH2 + KCH4 PCH4 + KH2OPH2O/PH2 )2 (2.8) 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 16
  28. 28. [25]. 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 10−2 − 10−3 . 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 [26] 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 Eq. 2.9 rCO = kPCOPH2O(1 − β) (2.9) where β is the reversible factor given by β = PCO2 PH2 PCOPH2OKeq where Keq is the equilibrium constant which can be obtained from thermodynamic properties. 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 [27]. Kahlich et al.[28] developed a rate expression for selective CO oxidation based on a platinum catalyst given by Eq. 2.10. rCO = k1P0.42 O2 λ0.82 (2.10) 17
  29. 29. 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. λ = 2CO2 CCO = 2PO2 PCO (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+ + 2e− (2.12) 2H+ + 2e− + 1 2 O2 → H2O (2.13) H2 + 1 2 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 18
  30. 30. Figure 2.1. Cross section of a polymer electrolyte membrane fuel cell water. Fig. 2.1 [29] shows a cross-sectional diagram of a single cell polymer electrolyte fuel cell. 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. 19
  31. 31. 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 o 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, 20
  32. 32. (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 [30]. 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 [31]. 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. 21
  33. 33. 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 Section 2.1. 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 C 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 2.2.3 Applications 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 22
  34. 34. 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. Transportation 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 [32]. In the U.S. the California Fuel Cell Partnership [33] is coordinating the deployment of several FCBs at a number of californian transit agencies. Most of these buses run on direct methanol fuel cells [34] 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 23
  35. 35. 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 year. Stationary Power 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. [35] utilized different mixtures of oxygen and inert gases and studied mass transport phenomena across the PEM fuel cell system. Beattie et al. [36] studied the effect of temperature and pressure on oxygen reduction at the platinum and Nafion interfaces. Jordan et al. [37] studied the effect of diffusion layer 24
  36. 36. on the performance of the fuel cell. Motupally et al. [38] and Sridhar et al. [39] 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 [40], [41], [42], [43] and two dimensional models [44], [45], [46]. Verbrugge and Hill [47] developed a steady state fuel cell model to study the transport properties of perfluorosulfonic acid membranes under electrolyte supported conditions. Bernardi and Verbrugge [48] 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. [43] 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 [49] 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. [50] developed a steady state analytical transport-reaction model by drawing parallels with membrane reactors. Fuller and Newman [51] 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. [52] presented a three dimensional numerical model that predicts the mass flow between the cathode and anode channels. Several publications [53], [54], [55], [56] 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 25
  37. 37. 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 [57], [58]. 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 [49] 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. [59] 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 [60]. Lutz et al. [61] did a thermodynamic analysis of a compact 26
  38. 38. 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 [3] where the heat requirements for each reactor system were calculated based on overall energy balances. Godat and Marechal [62] 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 [63] 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. 27
  39. 39. 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. [64] 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 [65]. 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 28
  40. 40. 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 [66]. 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 ([67], [68]). 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 [69], [70]. 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 [71]. 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 [72]. 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 [73] as well as to adaptive control schemes with improved transient and steady state performance[74], [71]. Adaptive control has a rich literature full with different techniques for design, analysis, performance and applications. Several survey papers [75], [76] and books [77] have already been published. 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 29
  41. 41. 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 [13]. Newman [78] 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. [79] who also were investigating battery performance for a hybrid vehicle. Lee et al. [80] 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. [81] 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. 30
  42. 42. CHAPTER 3 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 [82]. For oxygen and hydrogen the standard heat of formation can be assumed to be zero [83]. 31
  43. 43. Table 3.1. Standard heat of formations Species Standard heat of formation (kJ/mol) CO -110.53 CO2 -393.51 H2O -242 CH4 -74.5 O2 0 H2 0 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 (kJ/mol) 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 [84] 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. 32
  44. 44. 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 electrons (Avogadro’s 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 1C/sec 1A 1eq. e− 96, 487C ( 1mol H2 2eq. e− )( 60sec 1min ) = 3.1 × 10−4 mol/min (3.3) 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. I = P V = 50kW 0.7V = 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 33
  45. 45. 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 nH2t = nH2 I U = 3.1 × 10−4 mol/min × 71.43 × 103 A 0.8 ∼= 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. Steam Reformer 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 34
  46. 46. 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, 35
  47. 47. 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 (mol/min) (kW) 1 4.57 2 9.13 3 13.30 4 17.65 5 22.77 6 26.64 7 31.03 8 35.41 9 39.57 10 45.43 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. CO + 1 2 O2 → CO2 (3.10) H2 + 1 2 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. 36
  48. 48. i ˙Nin i Hin i − i ˙Nout i Hout i − ˙Q = 0 (3.13) where ˙Nin 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. ˙Nout i = ˙Nin i + j 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. ˙Q = i ˙Nin i Hin i − i ( ˙Nin i + j νij ˙j)Hout i (3.15) which can be simplified as ˙Q = i ˙Nin i (Hin i − Hout i ) − i j νij ˙jHout i (3.16) expanding the enthalpy terms we have ˙Q = i ˙Nin i Tin Tout Cpi dT − j ˙j∆Hout j (3.17) where Cpi 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 37
  49. 49. 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 the combustor. 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 38
  50. 50. 1020 1030 1040 1050 1060 1070 1080 1090 1100 36 37 38 39 40 41 42 43 44 45 TEMPERATURE(K) Qdot (kW) 15% excess O2 20% excess O 2 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) 39
  51. 51. 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 40
  52. 52. 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 [3]. 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 41
  53. 53. time, and long term transients seen in the bed temperature. Bell and Edgar [85] 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 [24] 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 were derived: r1 = k1 P2.5 H2 PCH4 PH2O − P3 H2 PCO K1 (1 + KCOPCO + KH2 PH2 + KCH4 PCH4 + KH2OPH2O/PH2 )2 (3.21) r2 = k2 PH2 PCOPH2O − PH2 PCO2 K2 (1 + KCOPCO + KH2 PH2 + KCH4 PCH4 + KH2OPH2O/PH2 )2 (3.22) r3 = k3 P3.5 H2 PCH4 P2 H2O − P4 H2 PCO2 K3 (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 −∆Ho i RT , where i = H2, CO, CH4, H20 (3.24) The rate constants are given by a similar Arrhenius type equation. 42

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