Fuel Cell Systems: an Introduction for the Chemical Engineer
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1 Fuel Cell Systems: an Introduction for the Chemical Engineer Professor Donald J. Chmielewski Center for Electrochemical Science and Engineering Illinois Institute of Technology Presented to the Chicago Section of the AIChE September 14 th, 005 Illinois Institute of Technology
2 What is a Fuel Cell???? Illinois Institute of Technology
3 Applications Stationary (00 kw) Mobile (50 kw) International Fuel Cells Toyota Illinois Institute of Technology 3
4 What is a Fuel Cell? Fuel Cell Air O Answer: An electrochemical device that converts a fuel directly to electrical power Electric Power Illinois Institute of Technology 4
5 Where Does the Energy Come From? Air Fuel Cell O Electric Power Illinois Institute of Technology 5
6 Where Does the Energy Come From? Fuel Cell Air O Answer: The enthalpy released by the reaction: + ½ O O Electric Power ( H ~ 58 kcal/mole ) Illinois Institute of Technology 6
7 The Fuel Cell Reactor? Air Fuel Cell O??? Illinois Institute of Technology 7
8 The Fuel Cell Reactor? Fuel Cell Air O Problem: Heat is released but not electric power Heat Illinois Institute of Technology 8
9 The Fuel Cell Reactor Air Fuel Cell O Solution: Two reactors separated by an electrolyte membrane. Illinois Institute of Technology 9
10 The Fuel Cell Reactor Air Fuel Cell E cell Current Voltage * Current = Electric Power O Solution: Two reactors separated by an electrolyte membrane. This allows for manipulation of electrons Illinois Institute of Technology 10
11 Electrolyte Types Polymer Membrane: Solid Oxide Membrane: e - e - e - e - H + O N O - O N H + N O O N O - N O O N N O N O H + O N O - O N H + O O - O Anode Electrolyte Cathode Anode Electrolyte Cathode Illinois Institute of Technology 11
12 Polymer Electrolyte Membrane Fuel Cell (PEMFC) e - e - Electrolyte conducts ions (H + ), but not electrons (e - ). H + O N Anode H + H + H + Electrolyte N O N O O Cathode O N N O Electrodes (Anode and Cathode) conduct electrons (e - ), but not ions (H + ). Illinois Institute of Technology 1
13 Solid Oxide Fuel Cell (SOFC) e - e - Electrolyte conducts ions (O = ), but not electrons (e - ). O - O N Anode O - O - O - Electrolyte N O N O O Cathode O N N O Electrodes (Anode and Cathode) conduct electrons (e - ), but not ions (O = ). Illinois Institute of Technology 13
14 Where Does the Water Go? Polymer Membrane: Solid Oxide Membrane: e - e - e - e - H + O N O - O N H + N O O N O - N O O N N O N O H + O N O - O N H + O O - O Anode Electrolyte Cathode Anode Electrolyte Cathode Illinois Institute of Technology 14
15 Where Does the Water Go? Polymer Membrane: Solid Oxide Membrane: e - e - e - e - O H + H + H + H + O N O N O O O O O O O N N O N O O O O O O O O O - O - O - O - O N O N O O N O N N O Anode Electrolyte Cathode Anode Electrolyte Cathode Illinois Institute of Technology 15
16 Where Do the Electrons Go? Illinois Institute of Technology 16
17 Where Do the Electrons Go? SOFC Configuration: Fuel Interconnect/Bipolar plate: (La,Sr)CrO 3 or High Temp Alloy Anode: Ni - (Zr,Y)O - cermet Electrolyte: (Zr,Y)O - Air Cathode: (La,Sr)MnO 3 Illinois Institute of Technology 17
18 Fuel Cell Stack Illinois Institute of Technology 18
19 Current Collectors SOFC: O Anode Electrolyte Cathode e - e - e - O = e - O Illinois Institute of Technology 19
20 The Electrode Electrolyte Assembly O Anode Electrolyte Cathode e - e - O = O Anode Grains e - e - Electrolyte O= e - O Illinois Institute of Technology 0
21 The Three Phase Region Anode Grains e - O Electrolyte O= Ni YSZ YSZ O YSZ YSZ Ni Ni Ni YSZ Ni Ni Ni Ni YSZ YSZ Ni O = e - Illinois Institute of Technology 1
22 Design Issues SOFC: O Anode Electrolyte Cathode e - e - e - O = e - O Illinois Institute of Technology
23 How Much Fuel Does a Fuel Cell Use? Air Fuel Cell O E cell Current Voltage * Current = Electric Power Illinois Institute of Technology 3
24 How Much Fuel Does a Fuel Cell Use? Air Fuel Cell E cell Current Voltage * Current = Electric Power O Reaction rate is proportional to current density r H j n F Current j* Area Illinois Institute of Technology 4
25 How Do We Calculate Current Density? Illinois Institute of Technology 5
26 Load How Do We Calculate Current Density? Fuel Cell DC R int E o I=j*A cell E cell A fuel cell looks like a battery to the electrical world. Current output depends on the load. Illinois Institute of Technology 6
27 Review of Circuits 101 Battery I R int E load R load DC E o Illinois Institute of Technology 7
28 Review of Circuits 101 Battery R int I Equation #1: E load = E o - I*R int E load R load Equation #: DC E o E load = I*R load Illinois Institute of Technology 8
29 Review of Circuits 101 E load E o E load = E o - R int *I Equation #1: E load = E o - I*R int -R int I Illinois Institute of Technology 9
30 Review of Circuits 101 E load E o E load = E o - R int *I Equation #1: E load = E o - I*R int -R int Equation #: E load = R load *I R load I E load = I*R load Illinois Institute of Technology 30
31 Review of Circuits 101 E load E o E load = E o - R int *I Equation #1: E load = E o - I*R int -R int Equation #: E load = R load *I R load I E load = I*R load Illinois Institute of Technology 31
32 How Much Fuel Does a Fuel Cell Use? r H n I / A F E load E o E load = E o - R int *I -R int moles of H sec m E load = R load *I R load I Illinois Institute of Technology 3
33 Fuel Used is Proportional to Current r H I / n moles of A F sec m cell H Air Fuel Cell E cell Current Voltage * Current = Electric Power O Illinois Institute of Technology 33
34 Changing the Reaction Rate E load E load = E o - R int *I r H n I / A F E o Battery I -R int R int E load R load E load = R load *I I DC E o Illinois Institute of Technology 34
35 Load Circuit Perspective of the SOFC Solid Oxide Fuel Cell I=j*A cell R int E cell DC E o Illinois Institute of Technology 35
36 Load Circuit Perspective of the SOFC Solid Oxide Fuel Cell I=j*A cell R int (T cell ) E cell DC E o (P H,P O, P HO ) Illinois Institute of Technology 36
37 Resistance in the SOFC Zirconia Electrolyte Fuel Air Cathode (~30 μm) Electrolyte (10-00 μm) Anode ( up to 1 mm) Bipolar Plate (3-10 mm) R int = r (T ) * ( thickness / Area ) Illinois Institute of Technology 37
38 Circuit Perspective of the SOFC E load E load = E o - R int *I E o Lower T R load I Illinois Institute of Technology 38
39 Load Circuit Perspective of the SOFC Solid Oxide Fuel Cell I=j*A cell R int (T cell ) E cell DC E o (P H,P O, P HO ) Illinois Institute of Technology 39
40 Equilibrium Voltage Air O Fuel Cell E o E o g RT F F m log P H P H P 1/ O O Illinois Institute of Technology 40
41 Circuit Perspective of the SOFC E load E load = E o - R int *I E o Lower P R load I Illinois Institute of Technology 41
42 Load Circuit Perspective of the PEMFC PEM Fuel Cell I=j*A cell R int (T cell, j ) E cell DC E o (P H,P O, P HO, j ) Illinois Institute of Technology 4
43 PEMFC Polarization Curve Illinois Institute of Technology 43
44 PEMFC Polarization Curve Illinois Institute of Technology 44
45 Ohmic Resistance Electrical Conductivity,, Increases with Humidity Illinois Institute of Technology 45
46 Ohmic Resistance Electrical Conductivity,, Increases with Humidity RH x w P P (T) sat x w = 0.35 Illinois Institute of Technology 46
47 PEMFC Operating Window 80% RH 100% Membrane too dry (Large Ohmic Resistance) Membrane Flooded (Large Mass Transfer Resistance) 0 60 C T air C Illinois Institute of Technology 47
48 CO Poisoning Illinois Institute of Technology 48
49 High Temperature Membranes Electrical Conductivity,, Increases with Humidity RH x w P P (T) sat x w = 0.35 Illinois Institute of Technology 49
50 How Much Heat Does a FC Generate? Q gen ( H ) r f, H O HO P e Illinois Institute of Technology 50
51 Cell Voltage (V) Power Density (watts/cm ) How Much Heat Does a FC Generate? Q gen ( H ) r f, H O HO P e r H j n F E P e cell P j * V e cell Current Density (ma/cm ) Illinois Institute of Technology 51
52 Cell Voltage (V) Power Density (watts/cm ) How Much Heat Does a FC Generate? E P e Current Density (ma/cm ) cell Q gen P e H 0.5 w/ cm f, H O r H 0.4 w/ cm O P e r H 0.4A/ cm F n H f, H OrH O 0.49 w/ cm Illinois Institute of Technology 5
53 Applications Stationary (00 kw) Mobile (50 kw) International Fuel Cells Toyota Illinois Institute of Technology 53
54 The Fuel Cell System Electric Power Conditioner Air Fuel Air Fuel Processor Fuel Cell Stack Spent-Fuel Burner Exhaust O CO Thermal & Water Management Illinois Institute of Technology 54
55 Stationary Applications International Fuel Cells Illinois Institute of Technology 55
56 Flat-Plate Hydrogen Fed SOFC Fuel Cell Stack O H - + O = O +4 e - Anode 4 e - HEAT Solid O = RELEASED Electrolyte V O + 4 e - O = Cathode 4 e - + Anode Flow Cathode Flow O Illinois Institute of Technology 56
57 Plug Flow Reactor Analogy Feed Exhaust Conventional Design Reaction Rate Illinois Institute of Technology 57
58 Thermal Stresses in the Literature Peters et al., state that Large temperature gradients in either direction can cause damage to one or more of the components or interfaces due to thermal stresses Yakabe et al., state that the internal stress would cause cracks or destruction of the electrolytes Figure taken from Selimovic Dissertation, Lund University, (00). Illinois Institute of Technology 58
59 Internal Reforming SOFC CH 4 O CO O Fuel Flow O = Fuel Cell Stack O Air Flow Air Channel Cathode Electrolyte Anode Interconnect Fuel Channel Illinois Institute of Technology 59
60 Internal Reforming H r 1 + O H HO Exothermic CH kref 4 + HO CO + 3H rch kref CCH k ref is very large Endothermic 4 4 CO + H k, f O CO + H r CO k shift, f C H OC Exothermic shift k shift, f is also large CO C H K C eq CO Illinois Institute of Technology 60
61 Plug Flow Reactor Analogy (Internal Reforming) Reforming Reaction Rate Electrochemical Reaction Rate Reforming Heat Generation Electrochemical Heat Generation Combined Heat Generation Illinois Institute of Technology 61
62 Impact of Internal Reforming Figure taken from Selimovic Dissertation, Lund University, (00). Illinois Institute of Technology 6
63 Effective Structure of IR SOFC Methane Steam Pre-Heater CH 4 + H O 3 CO + H O Reforming Section H H + CO + CO H + O H O Electrochemical Section Heat and steam produced not used by reforming. Pre-heating steam is expensive. Steam in the feed lowers hydrogen utilization (reaction rate is a function of hydrogen to steam ratio). Illinois Institute of Technology 63
64 Mobile Applications Toyota Illinois Institute of Technology 64
65 The Fuel Cell System Electric Power Conditioner Air Fuel Air Fuel Processor Fuel Cell Stack Spent-Fuel Burner Exhaust O CO Thermal & Water Management Illinois Institute of Technology 65
66 Fuel Processing Reactors Reformer Water- Gas Shift (WGS) Preferential Oxidation (PrOx) PEMFC Hydrocarbon Feed Large Hydrocarbons Cracked: CO levels down to ~ 10 ppm / CO ratio ~ Most CO converted to CO : ~ 1% CO remaining Illinois Institute of Technology 66
67 Reforming Reactors Steam Reforming Partial Oxidation Autothermal Reforming Illinois Institute of Technology 67
68 Steam Reforming Steam Fuel C m H n + mh O mco + m + n/ ) CO + H + O CO H ( H Illinois Institute of Technology 68
69 Steam Reforming Steam Fuel C m H n + mh O mco + m + n/ ) CO + H + O CO H ( H Heat Illinois Institute of Technology 69
70 Catalytic Partial Oxidation (CPOX) Air C H + ( m + n/ ) O mco + n/ H m n O C m H n + mh O mco + m + n/ ) ( H Fuel CO + H + O CO H Illinois Institute of Technology 70
71 Autothermal Reforming (ATR) Steam Air Fuel C C H + ( m + n/ ) O mco + n/ H m n m H n + mh O mco + m + n/ ) CO + H + O CO H ( H O Illinois Institute of Technology 71
72 Start-up and Regulation of an ATR Water Flow In Air Flow In Fuel Flow In ATR Reformat Flow Out TT TC Illinois Institute of Technology 7
73 The Effect of Water Injection Illinois Institute of Technology 73
74 Closed-loop Water Injection Illinois Institute of Technology 74
75 Slower Water Injection Rate Illinois Institute of Technology 75
76 Fuel Processing Reactors Reformer Water- Gas Shift (WGS) Preferential Oxidation (PrOx) PEMFC Hydrocarbon Feed Large Hydrocarbons Cracked: CO levels down to ~ 10 ppm / CO ratio ~ Most CO converted to CO : ~ 1% CO remaining Illinois Institute of Technology 76
77 Water Gas Shift Reactors CO + H + r O CO H ( ( s) ( s) ( s) ( s) y y y y K ) 3 k3 CO O CO eq High Temp WGS Medium Temp WGS Low Temp WGS Illinois Institute of Technology 77
78 Preferential Oxidation Reactors CO + 1 O CO 1 H + O HO Reformat Air PrOx Illinois Institute of Technology 78
79 Preferential Oxidation Reactors CO O CO H O H O + Reformate Air Air Air Prox Stage o C 100 o C Prox Stage Intercooler Intercooler Prox Stage 3 Illinois Institute of Technology 79
80 Hydrogen Convereted (%) Preferential Oxidation Reactors Stage -Stage 3-Stage Inlet CO Concentration (%) Illinois Institute of Technology 80
81 The Fuel Cell System Electric Power Conditioner Air Fuel Air Fuel Processor Fuel Cell Stack Spent-Fuel Burner Exhaust O CO Thermal & Water Management Illinois Institute of Technology 81
82 Hybrid Fuel Cell - Battery Control i FC i L Fuel Cell DC/DC converter 4-38 V 48V DC/DC converter 1V Load 48V i B Bi-Directional DC/DC Converter Battery Bank Fuel Cell - Battery Hybrid System Illinois Institute of Technology 8
83 Hybrid Controller P L sp e F PI I F Fuel P F +_ Controller Cell + + P L P B sp +_ PI Controller I B Battery P B Illinois Institute of Technology 83
84 Simulated Closed-loop Response Illinois Institute of Technology 84
85 The Fuel Cell System Electric Power Conditioner Air Fuel Air Fuel Processor Fuel Cell Stack Spent-Fuel Burner Exhaust O CO Thermal & Water Management Illinois Institute of Technology 85
86 Acknowledgements IIT Collaborators: Said Al-Hallaj Ali Emadi Herek Clack Argonne National Laboratory: Shabbir Ahmed Rajesh Ahluwalia Students: Kevin Lauzze J. Robert Selman Satish Parulekar Jai Prakash Dennis Papadias Qizhi Zhang Ayman Al-Qattan Funding: Kuwait Institute for Scientific Research Graduate College and Armour College, IIT Argonne National Laboratory Illinois Institute of Technology 86