Diesel Reforming for Fuel Cell Auxiliary Power Units

Diesel Reforming for Fuel Cell Auxiliary Power Units SECA Core Technology Program Review Tampa, Fl, Jan 27, 2005

Rod Borup, W. Jerry Parkinson, Michael Inbody, Eric L. Brosha, and Dennis R. Guidry Los Alamos National Laboratory DOE Program Managers SECA – Magda Rivera/Norman Holcombe/Wayne Surdoval

POC: Rod Borup:[email protected] - (505) 667 - 2823

Hydrogen and Fuel Cell Institute

Applications of Diesel Reformers in Transportation Systems Fuel Tank

Anode Recycle

SOFC

Engine EGR

Diesel Engine Reductant POx/SR Lean NOx Catalyst

Particulate Trap

Diesel Exhaust

Reforming of diesel fuel can have simultaneous vehicle applications: • SECA application: reforming of diesel fuel for Transportation SOFC / APU • Reductant to catalyze NOx reduction, regeneration of particulate traps • Hydrogen addition for high engine EGR • Fast light-off of catalytic convertor Our goal is to provide kinetics, carbon formation analysis, operating considerations, catalyst characterization and evaluation, design and models to SECA developers.


Diesel Fuel Processing for APUs Technical Issues ¾ Diesel fuel is prone to pyrolysis upon vaporization • Fuel/Air/Steam mixing • Direct fuel injection

¾ Diesel fuel is difficult to reform • Reforming kinetics slow • Catalyst deactivation – – – –

Fuel sulfur content Minimal hydrocarbon slip Carbon formation and deposition High temperatures lead to catalyst sintering

¾ Water availability is minimal for transportation APUs • Operation is dictated by system integration and water content – water suppresses carbon formation Hydrogen and Fuel Cell Institute

Diesel Reforming Objectives and Approach ¾ Objectives: Develop technology suitable for onboard reforming of diesel • Research fundamentals (kinetics, reaction rates, models, fuel mixing) • Quantify operation (recycle ratio, catalyst sintering, carbon formation) ¾ Approach: Examine catalytic partial oxidation and steam reforming • Modeling – Carbon formation equilibrium – Reformer operation with anode recycle • Experimental – Carbon formation – Adiabatic reformer operation • • • •

Anode recycle simulation Direct diesel fuel injection, SOFC anode and air mixing Catalyst temperature profiles, evaluation, durability Hydrocarbon breakthrough

– Isothermal reforming and carbon formation measurements • Catalyst evaluation, activity measurements • Carbon formation rate development


Diesel Reforming Measurements and Modeling Adiabatic Reactor with nozzle

Window for Catalyst Reaction Zone Observation Windows for laser diagnostics

Air / anode recycle Nozzle

Catalyst (Pt/Rh)

Furnace Iso-thermal system • Measure kinetics • Steam reforming / POx • Light-off • Carbon formation

Iso-thermal Microcatalyst Hydrogen and Fuel Cell Institute

Modeling Equilibrium Kinetic Composition

¾ To avoid carbon formation during vaporization requires direct fuel injection ¾ Directly inject fuel to reforming catalyst • Commercial nozzle, control fuel pressure for fuel flow (~ 80 psi) • Air / anode recycle (H2 / N2) distribute in annulus around fuel line / nozzle

Relative Carbon Formation During Fuel Refluxing

Direct Injection Fuel Nozzle Operation 0.1

0.06

0.04

0.02

0 Diesel (commercial)

– Minimum fuel flow dictated by fuel distribution from nozzle

• Requires control of fuel/air preheat, limiting preheat (~ < 180 oC)

Diesel (low - S)

Fuel

Kerosene (low-S)

Hexadecane

Dodecane

P

Air

Simulated Anode – Prevents fuel vaporization/particulate Exhaust Recycle formation (H2, CO, CO2, N2, H2O, HCs)


Kerosene (commercial)

-0.02

¾ Experimental results • Operated successfully at steady state

Carbon formed during vaporization

0.08

Reticulated foam Supported Catalyst

Nozzle

POx/SR

SOFC Anode Recycle Modeling Fuel Tank

Anode Recycle Stream

Air

Exhaust SOFC

POx/SR

S/C

940 Outlet Temperature / oC

1.2

Outlet Temp at O/C = 1

1

Reformer Outlet Flowrate Fractional Increase

920

0.8

900

0.6

880 860

0.4

840 0.2

820 800

0 0

10

20

30 Recycle Rate / %

40


50

60

S/C and Reformer Outlet Flowrate Fractional Increase

960

Reformate Recycling of 50% SOFC Anode Flow, S/C = 0.7 Model assumes 50% anode fuel conversion

Axial Temperature Profiles during Diesel Reforming Low-S Swedish diesel fuel

Adjusted O/C for similar operating temperatures

1000 900 20 %

Temperature / oC

800

40 %

700

30 % (O/C = 0.7 - .75)

600 500 30% Recycle - O/C = .65

400

30% Recycle - O/C = .75 30% Recycle - O/C = .7

300


30 %

200

40% Recycle - O/C = 1

(O/C = 0.65)


100

20% Recycle - O/C = .55

0 0

10

20

30

40

Reactor Axial Distance / mm

50

60

70

Pt / Rh supported catalyst Residence time ~ 50 msec Anode recycle simulated with H2, N2, H2O

Higher recycle ratios move oxidation downstream in reformer Lower recycle ratios require low O/C for similar adiabatic temperature rise Hydrogen and Fuel Cell Institute

BET Surface Area Distribution

Catalyst Surface Area

Adiabatic Reformer Catalyst Surface Area Axial and Radial Profile 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0

0.5

1

1.5

2

Reactor Axial Distance

2.0 -.75

0

.75

0 Original Surface Area ~ 4.3 Hydrogen and Fuel Cell Institute

1.0

Catalyst Sintering Measurements Catalyst surface area with exposure at 900 oC (inert atmosphere) 14

Surface Area

12 10 8 6 4 2 0 0

100

200

300

400

500

Time / hrs

Initial catalyst sintering tests to determine effects on catalyst surface area loss (temperature, chemical environment, poisoning) Hydrogen and Fuel Cell Institute

Carbon Formation Issues ¾ Avoid fuel processor degradation due to carbon formation • • • •

Carbon formation can reduce catalyst activity, system pressure drop Operation in non-equilibrium carbon formation regions Low water content available for transportation diesel reforming Rich operation - Cannot avoid favorable carbon equilibrium regions

¾ Catalysts • Various catalysts more/less prone to carbon formation

¾ Diesel fuels • Carbon formation due to pyrolysis upon vaporization


Carbon Formation from Low-Sulfur Swedish and Commercial Diesel Fuel (iso-thermal) AutoThermal Reforming

C arb on Fo rm ation

0.16 0.14

Steam Reforming 0.45

Low - S Commercial

0.4

Carbon form ation

0.18

0.35

0.12 0.1

Low - S Commercial

0.3

0.25

0.08 0.06

0.2

0.15

0.04 0.02

0.05

0

500

0.1

550

600

650

700

750

800

850

Reformer Setpoint Temperature / oC

O/C = 1.0 S/C = 0.34

0

500

550

600

650

700

o

Reformer Temperature / C

S/C = 1.34

Increased carbon formation with increasing temperature during both ATR and SR Hydrogen and Fuel Cell Institute

750

800

850

Adiabatic Reactor Carbon Formation Measurements AutoThermal Reforming 0.250

• Simulates 35% SOFC anode recycle

Carbon Formation

Low S Diesel 0.200

– S/C ~ 0.34

Comm. Diesel

0.150

• Average 3x higher carbon with commercial fuel than Low-S

0.100

• Carbon formation increases with increasing air (T) for commercial • Carbon formation decreases with increasing air flow (T) for Low–S

0.050 0.000 0.75

1.25

1.75

Air / Fuel Ratio Air (SLPM) / Fuel (ml/min)


2.25

Carbon Formation vs. recycle ratio (adiabatic ATR reforming) 0.05 0.045

Carbon Formation

0.04 0.035 0.03 0.025 0.02 0.015 0.01 0.005 0 0%

10%

20%

30%

40%

50%

Recycle Ratio

Constant air/fuel ratio: O/C = 0.7 Fuel: Swedish Diesel Fuel Hydrogen and Fuel Cell Institute

60%

Sulfur Effect on Diesel Reforming Sufur content in tested fuels Odorless Kerosene Commercial Diesel Swedish Diesel Kerosene

N.D. 314 ± 17 N.D. 149 ± 16

Added 300 ppm S (by wt% S) From Thiophene and DiBenzoThiophene (DBT) to LowSulfur Swedish diesel and dodecane to examine effect on reforming fuel conversion and carbon formation. Hydrogen and Fuel Cell Institute

Sulfur compounds Thiophene Dibenzothiophene S

S

Sulfur effect on Carbon formation (Adiabatic)

(Iso-thermal)

0.6

0.06 0.05

Carbon Formation

Carbon Formation

0.5 0.4 0.3 0.2

0.04 0.03 0.02 0.01

0.1

0

0 Dodecane

Dodecance + Dodecance + 300 ppm S 300 ppm S Thiophene DBT

Swedish

Swedish + 300 ppm S Thiophene

Swedish + 300 ppm S DBT

Dodecane - Dodecane - Dodecane 0 ppm S 300 ppm S - 300 ppm S Thiophene DBT

Swedish Diesel - 0 ppm S

Swedish Swedish Diesel - 300 Diesel - 300 ppm S ppm S DBT Thiophene

• Addition of Sulfur compounds (thiophene and DBT) does not increase carbon formation • Higher carbon formation from pure dodecane than from Swedish diesel • No detectable carbon (by XRF) in carbon samples regardless of sulfur content in fuel (Dodecane and Low-S Swedish Diesel Fuel)


Carbon Formation Analysis and Location (TGA) Thermal Gravimetric Analysis of catalyst after carbon formation measurements in isothermal reactor 0.25

99.95

0.2

99.9

0.15

Catalyst Weight Gain

100

99.85

Wt %

Catalyst weight change after carbon formation measurements in the isothermal reactor

99.8 99.75 99.7 99.65 99.6

0.1 0.05 0 600 -0.05

650

700

750

800

-0.1 -0.15

99.55

-0.2

99.5 0

200

400

600

800

1000

1200

Temperature / C

-0.25

Temperature / C

Carbon removal is about 0.4 % catalyst weight

Carbon is not typically ‘bound’ to catalyst surface (for noble metal catalysts / with oxide supports) Hydrogen and Fuel Cell Institute

850

900

Carbon Formation Rate Activation energy for carbon formation: rcarbon = k exp(-Ea/RT) Isothermal steam reforming (S/C = 1.0) commercial diesel low-S diesel

86.8 kJ/mol 134.2 kJ/mol

Isothermal ATR (O/C = 1.0, S/C = 0.34) (Simulating 35% recycle) commercial diesel low-S diesel

97.9 kJ/mol 72.4 kJ/mol

Literature values for carbon formation of 118 kJ/mol (CO2 reforming of CH4 over Ni/Al2O3 catalysts) Wang, S., Lu, G., Energy & Fuels 1998, 12, 1235.


Carbon from fuel that ends up as carbon particulate Iso-thermal ATR 0.13% Low-S Diesel 0.12% Commercial Diesel Iso-thermal SR 0.22% Low-S Diesel 0.21% Commercial Diesel Adiabatic ATR 0.03% Low-S Diesel 0.09% Commercial Diesel

Low –S Diesel ATR scales to 3.1 kg Carbon (10,000 hrs) 12.4 kg Carbon (40,000 hrs)

Nanocomposite Ni Catalyst Work • Initial success usingNi/YSZ and Ni/ZrO2 nanocomposite catalysts (separate project) • Freeze-drying process to prepare nanocomposites: • Ultrasonic nozzle makes aerosol of liquid droplets • Liquid droplets frozen in LN2 and collected • Solvent removed by sublimation • Obtain low density reactive precursor powder • Catalyst activated by calcining and reduction Particle Sizes by XRD 42 nm. Ni/ZrO2 Black Ni/ZrO2 141 Å Grey Ni/ZrO2 206 Å Ni/YSZ 60 Å

After activation 0.10 g/ml

Initial Catalyst Precursor 0.018 g/ml

Initial development of this work funded by LANL LDRD Hydrogen and Fuel Cell Institute

Ni Nano-composite Stability During CH4 Reforming Activity of Ni/ZrO2 nanocomposite is comparable to that of Ni Nanocomposite Ni/ZrO2 is stable over time during CH4 Reforming Ni degraded rapidly at 800 oC due to carbon deposition

Nanocomposite nickel catalysts showed better durability than nickel during CH4 Reforming Hydrogen and Fuel Cell Institute

Carbon Formation During Diesel Reforming over Nickel Catalysts Carbon Formation

35

16

30

12

25

C a r b o n F o r m a t io n

14

10

Diesel Fuel Conversion

20

8 15

6 10

4 2

5

0

0

Nano-Composite Ni/ZrO2 (141Å)

Nano-Composite Ni/ZrO2 (206Å)

Non-promoted Ni

Promoted Ni

Nano Ni/ZrO2 Nano Ni/ZrO2 Promoted Ni Non-promoted (141Å) (206Å) Ni

Nanocomposite nickel catalysts (Ni/ZrO2) show worse carbon formation • Nanocomposite nickel catalysts (Ni/ZrO2) do not show good reforming activity with diesel fuel • Examine nano - Ni/YSZ composite • Potentially better application for catalyst is SOFC anode for CH4 Hydrogen and Fuel Cell Institute

Pt/Rh

Amorphous Carbon Formation Modeling Carbon Gibb’s Free Energies Computed data with Least Squares fit for C2* amorphous carbon DGf (Ref 2)

DGf (predicited)

DGf (Ref 1)

6000

6000

5000

5000

D G f (cal/g-m ole)

D G f (cal/g-m ole)

DGf (Ref 1)

Computed data with Least Squares fit for C1* amorphous carbon

4000 3000 2000 1000 0 -1000600

700

800

900

1000

1100

Temperature (K)

1200

DGf (Ref 2)

DGf (predicited)

4000 3000 2000 1000 0 -1000500

600

700

800

900

1000

1100

1200

1300

Temperature (K)

The Gibb’s Free Energies Plotted were computed from measured Kvalues from carbon formation, with the definition: ∆G = -RTln(K) Hydrogen and Fuel Cell Institute

Carbon Heat Capacity Determination Heat capacities for C1* amorphous carbon from Gibb’s Free Energy data

Heat capacities for C2* amorphous carbon from Gibb’s Free Energy data

500

0 -2000 0

200

400

600

800

1000

1200

1400

-4000 -6000 -8000 -10000 -12000 -14000

Cp (amorphous)

Cp (graphite)

0 -500

0

200

400

600

800

1000

1200

1400

-1000 -1500 -2000 -2500 -3000

Temperature (K)

∆G = ∆H – T∆S Cp = a + bT + cT2 + d/T2

Cp (cal/(g-mole- K))

Cp (cal/(g-m ole - K))

2000

Cp (amorphous)

Cp (graphite)

Temperature (K)

Cp ∆H = ∫ CpdT − and −∆S = ∫ dT T

∆b 2 ∆c 3 ∆d ∆G = A + BT − ∆aT ln T − T − T − 2 6 2T


Carbon Enthalpy with Temperature Enthalpy (cal/g-mole)

18000 16000 14000

HC (graphite)

12000

Enthalpy for C2* carbon, referenced to graphite, with 0 enthalpy at 648 K

DH + HC

10000 8000 6000 4000

H

2000 0 0

200

400

600

800

1000

1200

E nthalpy (B TU /pound)

Temperature (K)

1400 1200 1000 800

hl (BTU/#)

600

hv (BTU/#)

HVaporization

Enthalpy-Temperature diagram for liquid water to steam.

400 200 0 0

100

200

300

400

500

Temperature (F)

600

• Carbon Enthalpies show carbon thermodynamics not consistent • Different thermodynamic carbon species are formed Hydrogen and Fuel Cell Institute

Summary/Findings ¾ Direct fuel injection via fuel nozzle • Control of fuel temperature critical (Prevent fuel vaporization, fuel pyrolysis) • Turndown can be limited by the nozzle fuel distribution

¾ Reformer operation with SOFC anode recycle • High adiabatic temperatures at low recycle rates (Leads to catalyst sintering) • Increasing recycle rates moves oxidation downstream in reformer • Operation at 30 – 40 % recycle rate has shown most reasonable results

¾ Nanocomposite nickel catalysts • Showed promising results during CH4 reforming • Ni/ZrO2 not as promising for diesel reforming

¾ Carbon Formation • Addition of Sulfur (thiophene and DBT) do no increase carbon formation • Carbon formation modeling shows at least two different thermodynamic types of carbon • Higher carbon formation with commercial diesel than low-S diesel (adiabatic) • Carbon formation primarily not adherent to catalyst surface

¾ Catalyst Durability • Catalyst loss in surface area during reforming and with temperature


Future Activities ¾ Carbon formation • Define diesel components contributing to high carbon formation rates • Examine additive effects on carbon formation (EtOH) • Stand-alone startup & consideration to avoid C formation • Develop carbon removal/catalyst regeneration schemes

¾ Catalyst sintering and deactivation • Characterize durability – catalyst sintering • Develop reformer operational profiles that limit catalyst sintering • Stabilize active catalyst particles

¾ Durability and hydrocarbon breakthrough on SOFC ¾ Modeling (Improve carbon formation model) – Improve robustness of code, develop ‘user-friendly’ interface

• Examine system effects of anode recycle Hydrogen and Fuel Cell Institute