integrated micropower generator

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


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


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


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


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

)


0 1 2 3 4 5

0

20

40

60

80

100Deposition percentage (%

)


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)


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


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


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)


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


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


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

)


0

2

4

6

8

10


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


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


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


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 )


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


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)

integrated micropower generator

Documents

co2more oxygen

cell temperature

fuel utilizationoxygen

greater amounts of oxygen

fuel stream andor

main gaseous products

low temperatureanodes

ccarbon deposition