Fuel Cell Program

Diesel Reforming for Fuel Cell Auxiliary Power UnitsSECA Core Technology Program Review

Boston, Ma, May 11-13

Rod Borup, W. Jerry Parkinson, Michael Inbody, Jose Tafoya, and Dennis R. Guidry

Los Alamos National Laboratory

DOE Program ManagersSECA – Norman Holcombe/Wayne Surdoval/David Berry

POC: Rod Borup:Borup@lanl.gov - (505) 667 - 2823

Fuel Cell Program

Applications of Diesel Reformers in Transportation Systems

Reforming of diesel fuel can have simultaneous vehicle applications:• SECA application: reforming of diesel fuel for 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 Exhaust

POx/SR

Lean NOxCatalyst

Fuel Tank

ReductantDiesel Engine

SOFCEngineEGR

ParticulateTrap

AnodeRecycle

Fuel Cell Program

Diesel Fuel Processing for APUsTechnical Issues

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

– Nozzle turndown and atomization qualityDiesel 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 - reformer start-up an issue

Fuel Cell Program

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

Fuel Cell Program

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

Fuel Cell Program

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

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0

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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

Fuel Cell Program

Water Addition for Steam Reforming→SOFC Anode Recycle to Reformer

Water required for:• steam reforming of fuel• carbon suppression

Methods for water introduction and availability:• Separate water tank (tank, freezing, refilling)• Anode water recovery by condensation (heat ex., cond., tank, pump freezing)• Anode recycle to reformer (blower)

Preferred systems are water neutralSimplest method is anode recycle to reformer

POx/SR

Fuel Tank

SOFCAirReformate

Anode Recycle Stream Exhaust

Fuel Cell Program

SOFC Anode Recycle Modeling

Green – Fractional increase in flow caused by increasing gas volume due to recycle ratio, leads to larger reformer

800

820

840

860

880

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920

940

960

0 10 20 30 40 50 60

Recycle Rate / %

Out

let T

empe

ratu

re /

o C

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0.2

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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

Most data presentedsimulates 35% recycle

Anode Recycling Model AssumptionsFuel - Diesel (C12H26)Power - LHV Fuel In 16O/C = 1 1SOFC Conversion 50%

Fuel Cell Program

Reforming of Diesel with SOFC RecycleTemperature and Hydrogen / CO production

Pt / Rh supported catalystResidence time ~ 20 msecAnode recycle simulated with

H2, N2, H2O

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0.8 0.9 1 1.1 1.2 1.3 1.4

Oxygen / Carbon Ratio

Con

cent

ratio

n / %

H2 (40% Recycle)CO (40% Recycle)H2 + CO (40% Recycle)H2 (30% Recycle)CO (30% Recycle)H2 + CO (30% Recycle)

CO

H2

H2 + CO

30 % Recycle

30 % Recycle

30 % Recycle

40 % Recycle

40 % Recycle

40 % Recycle

760

780

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940

0.8 0.9 1 1.1 1.2 1.3 1.4

Oxygen / carbon ratio

Ref

orm

er O

utle

t Tem

pera

ture

/ o C

40 % Recycle30 % Recycle

• Higher recycle reduces operating temperature

• Operation with recycle < 30 % difficult due to high operating temperatures and catalyst sintering

Fuel Cell Program

Axial Temperature Profiles during Diesel Reforming

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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)

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

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

Fuel Cell Program

Reactor Axial Profile with Time(Commercial diesel fuel)

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0 10 20 30 40 50 60 70Reactor axial length

Tem

pera

ture

/ o C

T = 0 hrsT = 1 hrsT = 2 hrsT = 3 hrsT = 4 hrsT = 5 hrsT = 5.5 hrs

After shut-down/restart

Time = 0 hrs

Time = 1 - 5 hrs

Initial temperature profile flattens out at ~ 800 °CSubsequent temperature profiles peak at > 850 °C and then decrease to outletTemperature (oxidation) profile shifts downstream following shutdown/restart cycle

Fuel Cell Program

Fuel Effect on Reactor Temperature Profile

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900

0 10 20 30 40 50 60 70 80 90Reactor Axial Distance / mm

Tem

pera

ture

/ C

Low - S Swedish

Iso-Octane

Commercial Diesel

RFG1 Gasoline

Fuel composition affects the reactor front end light-offSulfur content and aromatic content highest in Diesel > Gasoline > Swedish Diesel > Iso-Octane

35% recycle ratioAdjusted O/C for similar reformer outlet temperature

Fuel Cell Program

Adiabatic Reformer Catalyst Surface AreaAxial and Radial Profile

BET Surface Area Distribution

Original Surface Area ~ 4.3

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2.0

1.0-.75 .75

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0.40.5

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0.70.8

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0 0.5 1 1.5 2

Reactor Axial Distane

Cat

alys

t Sur

face

Are

a

Fuel Cell Program

Isothermal Reformer Catalyst Surface Area

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

500 550 600 650 700 750 800 850

Temperature / oC

Cat

alys

t sur

face

are

a / %

fres

h .

Low Sulfur DieselCommercial Diesel

Greater catalyst surface area loss after testing with commercial diesel fuel

Isothermal Reactor BET Catalyst Surface Area

Large catalyst surface area loss after testing, mostly independent of temperature during isothermal diesel steam reforming

Fuel Cell Program

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 start-up - Cannot avoid favorable carbon equilibrium regions

• Water-less (Water not expected to be available at start-up)Catalysts

• Various catalysts more/less prone to carbon formationDiesel fuels

• Carbon formation due to pyrolysis upon vaporizationCarbon Formation Reactions

2CO ⇔ C + CO2 (Boudart Reaction)CH4 ⇔ C + 2H2 (CH4 Decomposition)

CnH2n → Cn + nH2

Fuel pyrolysis → aromatics → PAH → C

Fuel Cell Program

Carbon Formation Equilibrium Modeling

Various forms of carbon exist • Different carbon forms have different thermodynamic properties

Developed chemical equilibrium code to analyze conditions for carbon formation

• Includes 3 types of amorphous carbon – Operation of model in isothermal modes (adding adiabatic)

• C++ code operates on Windows PCInput:

• Isothermal /Adiabatic (needs improvement for amorphous Carbon)• Gas phase components & concentrations• Equilibrium temperature, pressure, types of solid phase

Output yields (code works where carbon formation is observed)• Gas phase concentration, solid phase quantities• (Delta H reaction, outlet temperature – for adiabatic case)

Model is (will be / maybe??) available • no-cost, non-exclusive license

Fuel Cell Program

Modeling Carbon Formation Dependence for SOFC APU Recycle Ratio

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1300

0 10 20 30 40 50 60Recycle Rate / %

Car

bon

Dis

appe

aran

ce T

empe

ratu

re /

o C

Temperature for disappearance of all types of amorphous carbon as a function of SOFC anode recycle ratio

100.0

200.0

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500.0

600.0

700.0

0.5 1 1.5 2 2.5 3 3.5Steam to Carbon Ratio

Car

bon

Dis

appe

aran

ce T

empe

ratu

re

O/C = 0.6 (Cetane = 50, P = 30)

O/C = 0.8 (Cetane = 50, P = 30)

O/C = 1.0 (Cetane = 50, P = 30)

O/C = 1.2 (Cetane = 50, P = 30)

O/C = 0.6 (Cetane = 50, P = 14.7)

O/C = 0.6 (Gasoline, P = 30 psi)

O/C = 0.8 (Gasoline, P = 30 psi)

O/C = 1.0 (Gasoline, P = 30 psi)

O/C = 1.2 (Gasoline, P = 30 psi)

O/C = 0.8 (Gasoline, P = 14.7 psi)Carbon disappearance temperature as a function of steam to carbon ratio

Fuel Cell Program

• Quantitative carbon measurements indicate carbon made during start-up for all fuels.• Water during start-up suppresses some carbon formation, but carbon is still formed, in

smaller quantities.• Ethanol suppresses carbon formation, while aromatics show higher carbon formation.

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0.35

Iso-

Oct

ane

Iso-

Oct

ane

/ 20%

EtO

H

Iso-

Oct

ane

/ 20%

xyl

ene

Nap

tha

RFG

RFG

/ 20

% E

tOH

Iso-

Oc t

ane/

20%

Dod

ecan

e

Car

bon

form

ed d

urin

g re

acto

rlig

ht-o

ff / g

ram

s ca

rbon S/C = 0

S/C = 0.5

Carbon Formation during light-off:Quantitative carbon measurements

Fuel Cell Program

Isothermal Reactor Carbon Formation Measurements

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600 650 700 750 800 850Temperature Setpoint / C

Car

bon

Form

atio

n

SR Low Sulfur Diesel

SR Commercial Diesel0

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0.25

600 650 700 750 800 850

Temperature Setpoint / CC

arbo

n Fo

rmat

ion

Low S Diesel

Commercial Diesel

Steam Reforming AutoThermal Reforming

O/C = 0.0, S/C = 1.0Peak carbon formation ~ 650 – 700 oCEquilibrium and kinetics effects

O/C = 1.0, S/C = .34(35% Anode Recycle)Carbon formation increases with temperature

5 hour operation

Fuel Cell Program

Adiabatic Reactor Carbon Formation Measurements

AutoThermal Reforming

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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

• Carbon Formed (% fuel flow):

Air (SLPM) / Fuel (ml/min)

Fuel Cell Program

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)

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Temperature / CC

atal

yst W

eigh

t Gai

n

Catalyst weight change after carbon formation measurements in the isothermal reactor

99

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99.4

99.6

99.8

100

100.2

0 200 400 600 800 1000 1200

Temperature / oC

Carbon removal is about 0.4 % catalyst weight

Fuel Cell Program

Carbon Formation RateActivation energy

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

Iso-thermal steam reforming (S/C = 1.0)commercial diesel 86.8 kJ/mollow-S diesel fuel 134.2 kJ/mol.

Iso-thermal ATR (O/C = 1.0, S/C = 0.34)(Simulating 35% recycle)

commercial diesel 97.9 kJ/mollow-S diesel fuel 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

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

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

Fuel Cell Program

Direct fuel injection via fuel nozzle• Control of fuel temperature critical

– Prevent fuel vaporization, fuel pyrolysis / clogging of nozzle• 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– Limits light-off of reformer

• Increasing recycle rates moves oxidation downstream in reformer• High recycle increases reformer size, parasitic losses• Operation at 30 – 40 % recycle rate

Carbon Formation• Equilibrium carbon formation modeling• Carbon formation measurements show kinetic and equilibrium effects• Higher carbon formation during adiabatic operation with commercial diesel

compared with low-S diesel• Carbon formation primarily not adherent to catalyst surface

Summary/Findings

Fuel Cell Program

Future ActivitiesExperimental

Carbon formation• Quantify as a function of catalyst, recycle ratio• 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 SOFC• Incorporate SOFC ‘button’ cell operating on reformate

Sulfur effect on reforming kinetics and carbon formation

Fuel Cell Program

Future ActivitiesModeling & Technology Transfer

Modeling• Improve carbon formation model

– Incorporate enthalpies of other carbon species (CH0.2) and sulfur– Improve robustness of code– Develop ‘user-friendly’ interface

• Examine system effects of anode recycle – Efficiency and parasitics

Technology Transfer• Dissemination of results via publications and presentations

– AIChE, ACS, SECA meetings and reports• Make carbon formation model available for SECA teams

– (effort ongoing for 6 months)