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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
Hydrogen and Fuel Cell Institute
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.
Hydrogen and Fuel Cell Institute
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
Hydrogen and Fuel Cell Institute
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
Hydrogen and Fuel Cell Institute
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
Hydrogen and Fuel Cell Institute
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
0.02
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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
Hydrogen and Fuel Cell Institute
SOFC Anode Recycle Modeling
800
820
840
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920
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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
Hydrogen and Fuel Cell Institute
Axial Temperature Profiles during Diesel Reforming
0
100
200
300
400
500
600
700
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900
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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)
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
Hydrogen and Fuel Cell Institute
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
Hydrogen and Fuel Cell Institute
Catalyst Sintering Measurements
0
2
4
6
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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
Hydrogen and Fuel Cell Institute
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
Hydrogen and Fuel Cell Institute
Carbon Formation from Low-Sulfur Swedish and Commercial Diesel Fuel (iso-thermal)
0
0.02
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0.12
0.14
0.16
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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
Hydrogen and Fuel Cell Institute
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)
Hydrogen and Fuel Cell Institute
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
Hydrogen and Fuel Cell Institute
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
Hydrogen and Fuel Cell Institute
Sulfur effect on Carbon formation
0
0.1
0.2
0.3
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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
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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
Hydrogen and Fuel Cell Institute
(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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0.1
0.15
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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
Hydrogen and Fuel Cell Institute
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.
Hydrogen and Fuel Cell Institute
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
Hydrogen and Fuel Cell Institute
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
Hydrogen and Fuel Cell Institute
Carbon Formation During Diesel Reforming over Nickel Catalysts
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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
Hydrogen and Fuel Cell Institute
Amorphous Carbon Formation ModelingCarbon Gibb’s Free Energies
Computed data with Least Squares fit for C2
* amorphous carbon
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0
1000
2000
3000
4000
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600 700 800 900 1000 1100 1200
Temperature (K)
DG
f (ca
l/g-m
ole)
DGf (Ref 1) DGf (Ref 2) DGf (predicited)
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0
1000
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3000
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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)
Hydrogen and Fuel Cell Institute
Carbon Heat Capacity DeterminationHeat capacities for C2
* amorphous carbon from
Gibb’s Free Energy data
-14000
-12000
-10000
-8000
-6000
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-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
Hydrogen and Fuel Cell Institute
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
Hydrogen and Fuel Cell Institute
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
Hydrogen and Fuel Cell Institute
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