integrated micropower generator scott barnett, northwestern university micro- sofc swiss roll...
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Integrated Micropower Generator
Scott Barnett, Northwestern University
Micro-SOFC
Swiss RollCombustor
+
High EfficiencyThermal Management
Integrated MicroPower Generator Review, June 24, 2002
Northwestern University Role
Anode Material Development
• Develop anodes to partially oxidize high energy density liquid hydrocarbon fuels at low temperature
• Anodes must also electrochemically oxidize resulting H2 and CO at low temperature
Approach
• Product gas analysis using differentially pumped mass spectrometry
• Cell testing and impedance spectroscopy measurements• Open-circuit potential measurements compared with
thermodynamic calculations
Integrated MicroPower Generator Review, June 24, 2002
Outline
• Introduction• Thermodynamic equilibrium calculations
– Non-coking conditions
• Mass spectrometer measurements• Single chamber cell tests• Dual chamber cell tests
– Thick GDC electrolyte cells– Anode supported cells– Open circuit voltage
• Conclusions
Integrated MicroPower Generator Review, June 24, 2002
Thermodynamic Calculation
• Determine equilibrium gas composition and whether coking is expected– Used to guide choices of inlet gas composition
• Assumes 10 sccm fuel gas flow– Propane (humidifed)– 5% fuel utilization
• Oxygen added directly to fuel stream and/or via fuel cell operation
• OCV calculation based on effective oxygen partial pressure of equilibrium fuel mixture
Integrated MicroPower Generator Review, June 24, 2002
Equilibrium Calculation: Propane, 800C
• Carbon deposition up to ratio of 1.7
• Main gaseous products: CO and H2
• CO2 and H2O gradually increase with increasing oxygen
0 200 400 600 800 1000 1200
0.0
3.0x10-7
6.0x10-7
9.0x10-7
1.2x10-6
1.5x10-6
1.8x10-6
800oC CO CO2
H2 H2O O2 Carbon
Flo
w/d
epos
ition
rat
e (m
ol/s
)
Current density (mA/cm2)
0 1 2 3 4
0
20
40
60
80
100 Deposition percentage (%
)
O2/C3H8 Ratio
Integrated MicroPower Generator Review, June 24, 2002
Equilibrium Calculation: Propane, 400C
• Carbon deposition up to ratio of 4.7
• Main gaseous products: H2, H2O, and CO2
• More oxygen required to prevent coking than at 800C– Due to greater amounts
of oxygen in equilibrium products
0 400 800 1200 1600
0.0
3.0x10-7
6.0x10-7
9.0x10-7
1.2x10-6
1.5x10-6
1.8x10-6
400oC
CO CO2
H2 H
2O
O2 Carbon
Flo
w/d
epos
ition
rat
e (m
ol/s
)
Current density (mA/cm2)
0 1 2 3 4 5
0
20
40
60
80
100Deposition percentage (%
)
Integrated MicroPower Generator Review, June 24, 2002
Equilibrium Calculation: Propane
• Minimum O2/C3H8 ratio required to avoid coking
• Limit at high T is partial oxidation stoichiometry
• Limit at low T is complete oxidation stoichiometry
400 500 600 700 8001
2
3
4
5
Carbon deposition
No carbon deposition
Critic
al r
atio
Temperature (oC)
Integrated MicroPower Generator Review, June 24, 2002
Equilibrium Calculation Results
• Carbon deposition can be avoided by adding sufficient oxygen– Electrochemical or gas-phase oxygen source
• More oxygen required at lower temperatures– Results from higher oxygen content of equilibrium
products
• Kinetic considerations may be completely different
Integrated MicroPower Generator Review, June 24, 2002
Cell Test / Mass Spectrometer
Current lead
GDC
La0.5Sr0.5CoO3
NiO-GDCC3H8
+O2
+Ar
CO+CO2
+H2
Voltage lead
Alumina tube
Furnace
16 20 24 28 32 36 40 44 480.0
1.0x10-8
2.0x10-8
3.0x10-8
4.0x10-8
Flowmeter: 15.97% C3H
8 - 16.81% O
2 - 67.22% Ar
Mass Spec: 15.94% C3H
8 - 14.35% O
2 - 69.71% Ar
Inte
nsi
ty (
amps)
Mass/Charge
Integrated MicroPower Generator Review, June 24, 2002
Partial Oxidation Reaction
• Mass spec measurement versus cell temperature (no current)
• Ni-YSZ anode support• Inlet mixture: 15.9% propane-
oxygen-Ar• Reforming products vary with T
– CO is main product (Hydrogen sensitivity low: should be larger than CO)
– C3H8 and O2 decrease, but not completely consumed
– H2O, CO2 decrease w/ incr T
– Basic agreement with calculations
550 600 650 700 7500
10
20
30
40
50
60
70 Ar O
2 H
2O H
2 CO CO
2 C
3H
8
Gas
con
tent
(%)
Temperature (oC)
Integrated MicroPower Generator Review, June 24, 2002
Cell Tests
Types of Cells• Thick GDC electrolyte
– Anode: 60% NiO – GDC – Gd0.5Sr0.5CoO3 cathode (similar to SmSrCoO3)
• Anode supported cells– Thin YSZ electrolyte– Ni-YSZ anode– LSM-YSZ cathode
Test Conditions• Standard fuel mixture:
– 10-25% propane, balance Ar-O2(20%)• Temperatures reported are measured at cell
– ~50C higher than furnace temperature
Integrated MicroPower Generator Review, June 24, 2002
Effect of Anode Material
• Ni-GDC thin anodes showed no coking in 15.9% propane mixture
• Ni-YSZ thick anodes showed obvious coking in 15.9% propane mixture– May be related to higher Ni content of thick anode, or Ni-
GDC versus Ni-YSZ
• Both types of anodes coke-free with 10.7% propane
Integrated MicroPower Generator Review, June 24, 2002
Single Chamber: Thick GDC
• Ni–GDC|GDC|Gd0.5Sr0.5CoO3
• 10.7% propane, balance air• Unstable performance
between 511 and 732C• Stable at endpoint
temperatures• OCV ~ 0.5V
– lower than Hibino reports
• Very low current density• No carbon deposition detected
0 5 10 15 20 25 30 35 400.0
0.1
0.2
0.3
0.4
0.5
0.6
Power density (m
W/cm
2)
511oC 561
oC
614oC 663
oC
732oC 654
oC
Ter
min
al v
olta
ge (V
)
Current density (mA/cm2)
0
2
4
6
8
10
Integrated MicroPower Generator Review, June 24, 2002
Dual Chamber: Thick GDC
• Ni–GDC|GDC|Gd0.5Sr0.5CoO3
• 10.7% propane, balance air• Low OCV
– As expected for GDC electrolyte
– But ~0.1V higher than single chamber
• Power density similar to such cells run on hydrogen– Limited by thick 0.5-mm GDC– But much higher power
density than single chamber
0.0 0.3 0.6 0.9 1.2 1.50.0
0.2
0.4
0.6
0.8 827
oC
790oC
Pow
er d
ensi
ty (m
W/c
m2 )
Term
inal
vol
tage
(V)
Current density (A/cm2)
0
50
100
150
200
250
300
Integrated MicroPower Generator Review, June 24, 2002
Dual Chamber: Anode Supported
• NiO-YSZ|YSZ|LSM-YSZ (anode supported)
• 10.7%C3H8–balance air– Propane just below
partial oxidation stoichiometry
• Open circuit voltage = 0.9 to 0.95V
• Power density actually higher than with hydrogen!0.0 0.5 1.0 1.5 2.0
0.0
0.2
0.4
0.6
0.8
1.0
Power density (m
W/cm
2)Term
inal
vol
tage
(V)
Current density (A/cm2)
678oC
728oC
779oC
819oC
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Integrated MicroPower Generator Review, June 24, 2002
Open Circuit Voltage: Propane-Air
• 800oC, dual chamber cell• Experiment:
– Voltage increases from ~0.9 to 1.0V with increasing propane
• Equilibrium calculation– Voltage increases rapidly
from 1.0 to 1.1V with increasing propane to 11%
– Voltage flat for higher propane (solid C present)10 15 20 25 30
0.0
0.3
0.6
0.9
1.2
OCV800 theoretical OCV794 experimental
OC
V (
V)
Propane content
Integrated MicroPower Generator Review, June 24, 2002
OCV and Max Power: Anode Supported
• Dual chamber cell• Two fuels:
– 10.7% propane – balance air
– Humidified hydrogen
• H2 gives higher OCV
• C3H8 gives higher power density
600 650 700 750 8000.0
0.2
0.4
0.6
0.8
1.0
1.2
C3H
8
H2
Ope
n C
ircui
t Vol
tage
(V
)
Temperature (oC)
0.0
0.2
0.4
0.6
Pea
k po
wer
(m
W/c
m2 )
Integrated MicroPower Generator Review, June 24, 2002
Summary
• Thermodynamic calculation shows that more oxygen is required to suppress coking at lower temperature
• Mass spectrometer measurements show expected reforming behavior, agree with calculations
• Single-chamber tests show low voltage and low current density in propane-air
• Dual chamber tests: – High power density for anode supported cells– No coking for propane content < 10.7% in air– More tendency for coking on thick anodes for higher
propane content– Measured open circuit voltages slightly less than equilibrium
calculation
Integrated MicroPower Generator Review, June 24, 2002
Propane OCV
• Humidified propane• Dual-chamber cell• Relatively high OCV due
to low H2O and CO2 partial pressures
• Low T slope resembles H2 fuel operation
• High T slope resembles C partial oxidation
400 500 600 700 8001.10
1.15
1.20
1.25
1.30
Dependence of OCV on temperature for propane based fuel cells
OC
V (
V)
Temperature (oC)