Diesel Reforming for Fuel Cell Auxiliary Power Units...Hydrogen and Fuel Cell Institute Diesel Reforming for Fuel Cell Auxiliary Power Units SECA Core Technology Program Review Tampa,

Hydrogen and Fuel Cell Institute

Diesel Reforming for Fuel Cell Auxiliary Power UnitsSECA 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 ManagersSECA – Magda Rivera/Norman Holcombe/Wayne Surdoval

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


Applications of Diesel Reformers in Transportation Systems

Diesel Exhaust

POx/SR

Lean NOxCatalyst

Fuel Tank

ReductantDiesel Engine

SOFCEngineEGR

ParticulateTrap

AnodeRecycle

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 APUsTechnical Issues

Diesel fuel is prone to pyrolysis upon vaporization• Fuel/Air/Steam mixing• Direct fuel injectionDiesel 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

Water availability is minimal for transportation APUs• Operation is dictated by system integration and water content

– water suppresses carbon formation


Diesel Reforming Objectives and Approach

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)

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

ModelingEquilibrium

KineticComposition

Iso-thermal Microcatalyst

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

Adiabatic Reactor with nozzle Window for Catalyst Reaction Zone Observation

Windows forlaser diagnostics

NozzleAir / anode recycle Catalyst(Pt/Rh)

Furnace

Direct Injection Fuel Nozzle Operation


Simulated Anode Exhaust Recycle

(H2, CO, CO2, N2, H2O, HCs)

POx/SR

Reticulated foam Supported

Catalyst

Fuel

Air

P

Nozzle

To avoid carbon formation during vaporization requires direct fuel injectionDirectly 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

Experimental results• Operated successfully at steady state

– Minimum fuel flow dictated by fuel distribution from nozzle

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

– Prevents fuel vaporization/particulate formation

-0.02

0

0.02

0.04

0.06

0.08

0.1

Diesel(commercial)

Diesel (low - S)

Kerosene(commercial)

Kerosene(low-S)

Hexadecane Dodecane

Rel

ativ

e C

arbo

n Fo

rmat

ion

Dur

ing

Fuel

Ref

luxi

ng

Carbon formedduring vaporization


SOFC Anode Recycle Modeling

800

820

840

860

880

900

920

940

960

0 10 20 30 40 50 60

Recycle Rate / %

Out

let T

empe

ratu

re /

o C

0

0.2

0.4

0.6

0.8

1

1.2

S/C

and

Ref

orm

er O

utle

t Flo

wra

te

Frac

tiona

l Inc

reas

e

Outlet Temp at O/C = 1

S/C

Reformer Outlet FlowrateFractional Increase

Recycling of 50% SOFC Anode Flow, S/C = 0.7

Model assumes 50% anode fuel conversion

POx/SR

Fuel Tank

SOFC

Air

Reformate

Anode Recycle Stream Exhaust


Axial Temperature Profiles during Diesel Reforming

0

100

200

300

400

500

600

700

800

900

1000

0 10 20 30 40 50 60 70

Reactor Axial Distance / mm

Tem

pera

ture

/ o C

30% Recycle - O/C = .6530% Recycle - O/C = .7530% Recycle - O/C = .740% Recycle - O/C = .940% Recycle - O/C = .9540% Recycle - O/C = 120% Recycle - O/C = .6520% Recycle - O/C = .620% Recycle - O/C = .55

20 %

40 %30 %

(O/C = 0.7 - .75)

30 % (O/C = 0.65)

Adjusted O/C for similar operating temperatures

Low-S Swedish diesel fuel

Pt / Rh supported catalystResidence time ~ 50 msecAnode recycle simulated with H2, N2, H2O

Higher recycle ratios move oxidation downstream in reformerLower recycle ratios require low O/C for similar adiabatic temperature rise


Adiabatic Reformer Catalyst Surface AreaAxial and Radial Profile

BET Surface Area Distribution

0

0

2.0

1.0-.75 .75

00.10.20.30.40.50.60.70.80.9

0 0.5 1 1.5 2

Reactor Axial Distance

Cat

alys

t Sur

face

Are

a

Original Surface Area ~ 4.3


Catalyst Sintering Measurements

0

2

4

6

8

10

12

14

0 100 200 300 400 500

Time / hrs

Surf

ace

Are

a

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

Initial catalyst sintering tests to determine effects on catalyst surface area loss (temperature, chemical environment, poisoning)

Carbon Formation Issues


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

Catalysts• Various catalysts more/less prone to carbon formation

Diesel fuels• Carbon formation due to pyrolysis upon vaporization


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

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

500 550 600 650 700 750 800 850

Reformer Setpoint Temperature / oC

Car

bon

Form

atio

n

Low - SCommercial

AutoThermal Reforming

O/C = 1.0S/C = 0.34

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

500 550 600 650 700 750 800 850Reformer Temperature / oC

Car

bon

form

atio

n

Low - SCommercial

Steam Reforming

S/C = 1.34

Increased carbon formation with increasing temperature during both ATR and SR


Adiabatic Reactor Carbon Formation Measurements

AutoThermal Reforming

0.000

0.050

0.100

0.150

0.200

0.250

0.75 1.25 1.75 2.25Air / Fuel Ratio

Car

bon

Form

atio

n

Low S Diesel

Comm. Diesel

• Simulates 35% SOFC anode recycle– S/C ~ 0.34

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

• Carbon formation increases with increasing air (T) for commercial

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

Air (SLPM) / Fuel (ml/min)


Carbon Formation vs. recycle ratio(adiabatic ATR reforming)

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

0% 10% 20% 30% 40% 50% 60%

Recycle Ratio

Car

bon

Form

atio

n

Constant air/fuel ratio: O/C = 0.7Fuel: Swedish Diesel Fuel


Sulfur Effect on Diesel ReformingSufur content in tested fuelsOdorless Kerosene N.D.Commercial Diesel 314 ± 17Swedish Diesel N.D.Kerosene 149 ± 16

Added 300 ppm S (by wt% S)From Thiophene and DiBenzoThiophene (DBT) to Low-Sulfur Swedish diesel and dodecane to examine effect on reforming fuel conversion and carbon formation.

Sulfur compoundsThiophene

S

Dibenzothiophene

S


Sulfur effect on Carbon formation

0

0.1

0.2

0.3

0.4

0.5

0.6

Dodecane Dodecance +300 ppm SThiophene

Dodecance +300 ppm S

DBT

Swedish Swedish +300 ppm SThiophene

Swedish +300 ppm S

DBT

Car

bon

Form

atio

n

• 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)

0

0.01

0.02

0.03

0.04

0.05

0.06

Dodecane -0 ppm S

Dodecane -300 ppm S -Thiophene

Dodecane -300 ppm S -

DBT

SwedishDiesel - 0

ppm S

SwedishDiesel - 300

ppm SThiophene

SwedishDiesel - 300ppm S DBT

Car

bon

Form

atio

n

(Adiabatic)(Iso-thermal)

Carbon Formation Analysis and Location


(TGA) Thermal Gravimetric Analysis of catalyst after carbon formation measurements in isothermal reactor

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

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

600 650 700 750 800 850 900

Temperature / CC

atal

yst W

eigh

t Gai

n

Catalyst weight change after carbon formation measurements in the isothermal reactor

99.5

99.55

99.6

99.65

99.7

99.75

99.8

99.85

99.9

99.95

100

0 200 400 600 800 1000 1200

Temperature / C

Wt %

Carbon removal is about 0.4 % catalyst weight


Carbon Formation RateActivation energy

for carbon formation:rcarbon = k exp(-Ea/RT)

Isothermal steam reforming (S/C = 1.0)commercial diesel 86.8 kJ/mollow-S diesel 134.2 kJ/mol

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

commercial diesel 97.9 kJ/mollow-S diesel 72.4 kJ/mol

Iso-thermal ATR0.13% Low-S Diesel0.12% Commercial Diesel

Iso-thermal SR0.22% Low-S Diesel0.21% Commercial Diesel

Adiabatic ATR0.03% Low-S Diesel0.09% Commercial Diesel

Carbon from fuel that ends up as carbon particulate

Low –S Diesel ATR scales to3.1 kg Carbon (10,000 hrs)12.4 kg Carbon (40,000 hrs)

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.


Nanocomposite Ni Catalyst Work• Initial success usingNi/YSZ and Ni/ZrO2nanocomposite 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 After

activation0.10 g/ml

Initial Catalyst Precursor0.018 g/mlParticle Sizes by XRD

42 nm. Ni/ZrO2Black Ni/ZrO2 141 Å Grey Ni/ZrO2 206 ÅNi/YSZ 60 Å

Initial development of this work funded by LANL LDRD


Ni Nano-composite Stability During CH4 Reforming

Activity of Ni/ZrO2 nanocomposite is comparable to that of NiNanocomposite Ni/ZrO2 is stable over time during CH4 ReformingNi degraded rapidly at 800 oC due to carbon deposition

Nanocomposite nickel catalysts showed better durability than nickel during CH4 Reforming


Carbon Formation During Diesel Reforming over Nickel Catalysts

0

2

4

6

8

10

12

14

16

Nano-CompositeNi/ZrO2 (141Å)

Nano-CompositeNi/ZrO2 (206Å)

Non-promoted Ni Promoted Ni

Car

bon

Form

atio

n

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

0

5

10

15

20

25

30

35

Nano Ni/ZrO2(141Å)

Nano Ni/ZrO2(206Å)

Promoted Ni Non-promotedNi

Pt/Rh

Diesel Fuel ConversionCarbon Formation


Amorphous Carbon Formation ModelingCarbon Gibb’s Free Energies

Computed data with Least Squares fit for C2

* amorphous carbon

-1000

0

1000

2000

3000

4000

5000

6000

600 700 800 900 1000 1100 1200

Temperature (K)

DG

f (ca

l/g-m

ole)

DGf (Ref 1) DGf (Ref 2) DGf (predicited)

-1000

0

1000

2000

3000

4000

5000

6000

500 600 700 800 900 1000 1100 1200 1300

Temperature (K)

DG

f (ca

l/g-m

ole)

DGf (Ref 1) DGf (Ref 2) DGf (predicited)

Computed data with Least Squares fit for C1

* amorphous carbon

The Gibb’s Free Energies Plotted were computed from measured K-values from carbon formation, with the definition:

∆G = -RTln(K)


Carbon Heat Capacity DeterminationHeat capacities for C2

* amorphous carbon from

Gibb’s Free Energy data

-14000

-12000

-10000

-8000

-6000

-4000

-2000

0

2000

0 200 400 600 800 1000 1200 1400

Temperature (K)

Cp

(cal

/(g-m

ole

- K))

Cp (amorphous) Cp (graphite)-3000

-2500

-2000

-1500

-1000

-500

0

500

0 200 400 600 800 1000 1200 1400

Temperature (K)C

p (c

al/(g

-mol

e- K

))Cp (amorphous) Cp (graphite)

Heat capacities for C1* amorphous

carbon fromGibb’s Free Energy data

dTTCpSandCpdTH ∫∫ =∆−−=∆

TdTcTbTaTBTAG

262ln 32 ∆

−∆

−∆

−∆−+=∆

∆G = ∆H – T∆SCp = a + bT + cT2 + d/T2

Carbon Enthalpy with Temperature


02000400060008000

1000012000140001600018000

0 200 400 600 800 1000 1200Temperature (K)

Enth

alpy

(cal

/g-m

ole)

HC (graphite) DH + HC

H

0200400600800

100012001400

0 100 200 300 400 500 600

Temperature (F)

Enth

alpy

(BTU

/pou

nd)

hl (BTU/#) hv (BTU/#)

HVaporization

Enthalpy-Temperature diagram for liquid water to steam.

Enthalpy for C2* carbon,

referenced to graphite, with 0 enthalpy at 648 K

• Carbon Enthalpies show carbon thermodynamics not consistent• Different thermodynamic carbon species are formed

Summary/Findings


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

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

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

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

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

Future Activities


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

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

Durability and hydrocarbon breakthrough on SOFCModeling (Improve carbon formation model)

– Improve robustness of code, develop ‘user-friendly’ interface• Examine system effects of anode recycle

Documents

Diesel Reforming for Fuel Cell Auxiliary Power Units...Hydrogen and Fuel Cell Institute Diesel Reforming for Fuel Cell Auxiliary Power Units SECA Core Technology Program Review Tampa,