diesel reforming and carbon formation
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Fuel Cell Program
Diesel Reforming and Carbon Formation
Measurements
2002 SECA Core Technology Meeting
June 18, 19 Pittsburg, PA
Rod Borup, Jerry Parkinson, Mike Inbody, Troy
Semelsberger and Jose Tafoya
Los Alamos National Laboratory
DOE Program Managers
SECA – Norman Holcombe/Wayne Surdoval(OAAT – JoAnne Milliken/Pete Devlin)
CIDI – Kathi Epping (NOx reduction)
POC: Rod Borup:[email protected] - (505) 667 - 2823
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Fuel Cell Program
Technical IssuesFuel Processing for Fuel Cells – Hydrogen Production
• Fuel cells operate on hydrogen rich mixtures best
• Hydrogen rich leads to highest power density in fuel cell• Cost of SOFC is high, lower cost SOFC system is potentially highest power density
SOFC stack
• Partial Oxidation / steam reforming
• Steam reforming requires water • Partial oxidation less efficient that SR
• Combination of POx/SR leads to ATR (autothermal reforming)
• Durability
• Carbon formation
• Sulfur tolerance
• Repeated cycling
• Operational hours (400,000(?))
• Cost
• Understand parameters that affect fuel processor lifetime and durability.
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Fuel Cell Program
Fuel Processing Systems
Autothermal
Reformer
High
Temperature
Shift Reactor
Low
Temperature
Shift Reactor
CO Clean-
upPEM Stack
> 700 oC ~ 450 oC ~ 220 oC ~ 150 oC ~ 80 oC
Autothermal
Reformer
> 700 oC
SOFC
Stack
> 700 oC
Catalysts:noble metal / Ni
40% H2
10% CO
0.4 - 1% CH4
Catalysts:Pt, Fe2O3, FeCr
4% CO
Catalysts:Pt, Cu/ZnO
1% CO
Catalysts:Noble metal
10 ppm CO
Catalysts:Pt
Sulfur
Trap
PEM Fuel Cell System
SOFC Fuel Cell System
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Fuel Cell Program
ATR Reactions
Autothermal
Reformer
700-900 oC
Fuel
Air
H2
O
40% H2
10% CO
13% CO2
37% N2
Partial Oxidation (POx) - Rich fuel burn (exothermic):oxygen / carbon ratio (oxygen from air only) (O/C = 1)
CnH(2n+2) + (n/2)O2 --> nCO + (n+1)H2
(O/C < 1)
CnH(2n+2) + (m/2)O2 --> mCO + C(n-m)H2(n-m) + H2
Steam Reforming (endothermic):
Steam / carbon ratio (S/C = 1)CnH(2n+2) + nH2O nCO + (2n+1)H2
Couple POx (exotherm) and Steam Reforming (endotherm) for ATR
(Autothermal reforming)
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Fuel Cell Program
Potential Applications of Diesel Refomers in
Transportation Systems
DieselExhaust
POx/SR
Lean NOx
Catalyst
Fuel Tank
Reduced
NOx Exhaust
NOx Reductant
Diesel Engine
SOFC
Engine
EGR
The reforming of diesel fuel potentially has simultaneous on-board vehicle applications:
• fuel for SOFC / APU• reductant for lean-burn engines catalyzing NOx reduction
• Hydrogen addition to the fuel charge allowing high engine EGR
• fast light-off and heating of engine / catalytic convertor
Incorporation into vehicles may require reforming to be suitable for all of the concurrentapplications even though the requirements and applications can be significantly different.
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Fuel Cell Program
Objectives and Tasks
Objectives:• Develop technology for reforming of diesel fuel for APU applications.
• Fuel/air mixing, catalytic partial oxidation / steam reforming
• Understand parameters that affect fuel processor lifetime and durability.
• Catalyst durability
• Carbon formation and system durability
Tasks:
• Measurement of Carbon Formation in Diesel Fuel Processing• Equilibrium and component modeling
• Experimental carbon formation measurement
• Fuel Mixing
•Vaporization / Fuel atomization• Direct liquid injection
• ‘Waterless’ Partial Oxidation of Diesel Fuel
• Start-up
• SOFC anode recycle (water addition to reformer)
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Fuel Cell Program
Carbon Formation Issues
• Avoid Fuel Processor Degradation due to Carbon Formation• Operation in non-equilibrium Carbon formation regions
• High temperatures / Steam Content – limits efficiency (80 %)
• Promoted catalysts• Operation for maximum efficiency
• minimization of O/C and S/C as possible (CH4, C limits)
• 100 % fuel conversion
• Start-up• Rich start-up
• Cannot avoid favorable carbon equilibrium regions
• Water-less (Water not expected to be available at start-up)• Diesel fuels
• carbon formation due to pyrolysis upon vaporization
• pre-ignition of fuel
• Transient operation & fuel processor control
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Fuel Cell Program
Modeling of Carbon Formation Disappearance
for Different Fuel Compositions
100.0
200.0
300.0
400.0
500.0
600.0
700.0
0.5 1 1.5 2 2.5 3 3.5
Steam to Carbon Ratio
C
a r b o n D i s a p p e a
r a n c e T e m p e r a
t u r e
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 formation varies greatly with steam content, only slightly with pressure and cetane #.
Carbon-Free Region
Carbon
Formation
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Fuel Cell Program
H O
C
0.5
0.5
0.5
Ternary Diagrams
• Equilibrium Calculations
• Pressure Dependence
• Temperature Dependence
• Fuel composition
• Steam Content
• Air content
• Carbon product
= steam (H2O)
Pure C
Pure H
= diesel fuel
Pure O= diesel, steam, air mixture
(O/C = 1.0, S/C = 2.5)= diesel mix (O/C = 1.0, S/C = 1.0)
= diesel mix (O/C = 1.0, S/C = 0.5)
= diesel mix (O/C = 1.0, S/C = 0.0)
Lower O/C more efficient fuel reformation
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Fuel Cell Program
Carbon Equilibrium (450 oC)
Pressure 14.7 to 30 psi
Temperature 450 oC
--x-- = amorphous carbon--x-- = graphite
Carbon formation
No Carbonformation
H O
C
0.5
0.5
0.5
x x x x x xx x
x x
x
x xx x x
x xx
xx
= (O/C = 1.0, S/C = 2.5)
=(O/C = 1.0, S/C = 1.0)
= (O/C = 1.0, S/C = 0.5)
= (O/C = 1.0, S/C = 0.0)
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Fuel Cell Program
Carbon Equilibrium (270 oC)
Pressure 14.7 to 30 psi
Temperature 270 oC
--x-- = amorphous carbon--x-- = graphite
Carbon formation
No Carbonformation
H O
C
0.5
0.5
0.5
x x x xx x x
x x xx
xx x x x
x
xx x
= (O/C = 1.0, S/C = 2.5)
= (O/C = 1.0, S/C = 1.0)
= (O/C = 1.0, S/C = 0.5)
= (O/C = 1.0, S/C = 0.0)
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Fuel Cell Program
Carbon Formation Monitoring Laser Optics(adiabatic POx/SR reactor)
Laser Chopper
Beam Splitter Reference BeamSignalDetector
Ar Ion Laser
Fluorescence -
Scattering
Reactor
WindowExtinction
Catalyst
Observation
Window
• Laser extinction
measurements monitor on-
set of carbon
• Laser scattering quantifiescarbon formation
• Fluorescence indicates
PAHCs
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Fuel Cell Program
Carbon Formations Diagnostics
MDL ~ 0.6 mg/min.Detection
Electronics
A r
-I onL a s er
PD
POx OutletCross-Section
PD
P D
Reference
Scattering
Extinction
PD = Photodetector
( ) L K I
I ext
t ⋅−== exp
0
τ
≡2~m
= Laser Transmission
( )τ ln1
L
K ext =
It = Transmitted Intensity
Io = Incident IntensityL = Path Length
( )τ π
λ
π
λ ln
2~1~
Im62~
1~Im
6 2
2
2
2
⋅
+
−−=⋅
+
−−=
m
m
L K
m
m f ext v
Complex Index of Refraction
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Fuel Cell Program
Technical Results:Carbon formation measurements
Results• Partial oxidation of
• odorless kerosene• kerosene
• dodecane
• hexadecane
• Carbon formationmonitoring by laser optics
• Carbon formation shown
at low relative O/C ratios
and temperature withkerosene (left)
• Demonstrated start-up
with no water – carbon
formation observed after ~100 hrs of operation
Carbon formation monitoring with laser scattering
Odorless Kerosene; S/C = 1.0
0
5
10
15
20
25
30
35
40
0.2 0.4 0.6 0.8 1
O/C Ratio
R e l a t i v e A b s o r b a n c e
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Fuel Cell Program
Carbon Formation:
Pressure Drop
35
37
39
41
43
45
47
49
51
53
55
0 5000 10000 15000 20000 25000 30000 35000
Time / sec
P r e s s u r e
/ p
s i
Fuel: Kerosene
S/C = 0
Light-off and operation
withoutwater feed to POx
Reactor pressure
drop increases
over time due to
carbon formation
• Carbon formation observed upon vaporization of diesel fuel due to fuel pyrolysis.• Partial oxidation of diesel fuels without water has been demonstrated, however carbon
formation occurs rapidly - in ~7 – 8 hours a prohibitive pressure drop resulted.
• Laser optics being used to observe the onset of carbon formation.
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Fuel Cell Program
Carbon Analysis
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0 100 200 300 400 500 600
Temperature /
o
C
% W e i g h t L o s s
0
5
10
15
20
25
30
0 100 200 300 400 500 600
Temperature /oC
% W e
i g h t L o s s
TGA-thermogravimetric analysis
Carbon formed in SR
Carbon formed after SR 30 % by weight Hydrocarbons
Material is solidified
hydrocarbon compoundsNo initial weight loss
Initial weight loss of 1.7%
Estimate amorphous carbon as:
C1H0.2
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Fuel Cell Program
Catalyst Regeneration:oxidative removal of carbon
• Post-carbon formation experiments:• Regeneration of catalyst/reactor by carbon oxidation
• Air feed to reactor at 500 – 600 oC
• Similar to regenerative particulate filters
• Successful about ~5 times in succession• Control of the reactor temperature could be difficult.
• For large carbon build-up:
• Subsequent oxidation of the carbon yielded high
adiabatic temperatures
• Eventually disables light-off of the partial oxidation stage
• Due to catalyst sintering - loss of catalyst surface area.
• Catalyst regeneration• Potential solution to carbon formation
• Need to control oxidation temperature/rate of oxidation
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Fuel Cell Program
Partial Oxidation Stage Outlet Concentrations(for similar conversions)
0
5
10
15
20
25
30
35
40
Iso-Octane Iso-Octane plus20 % Xylene
Dodecane
% O
u t l e t C o n c e n t r a t i o n
H2+CO
Higher O/C (temperature) required for Dodecane conversion
yields diluted H2/CO fuel mixture
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Fuel Cell Program
Fuel Effect on Auto – Ignition (Pre-Combustion)
Switched Fuel Operation: De-odorized Kerosene to Normal Kerosene
300
400
500
600
700
800
900
1000
18500 18600 18700 18800 18900 19000 19100 19200 19300
Time / sec
T e m p e r a t u r e / o C
Reformer Temperature deg C
Mixing Chamber
Switched
Fuels
Pre –
Auto-ignition Eliminated
Flame
• Diesel fuels show high tendency for pre-combustion
• Show fuel effect on pre-combustion
• Potential design impacts of seasonal diesel fuels
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Fuel Cell Program
Fuel Injection to POx/SR Air
POx
Vaporizer
Fuel
Water
Gasoline
Systemmix
Heat
ExchangerReformate
AirDieselSystem
Diesel fuel components tend pyrolyse upon vaporization
Direct fuel injectionPOxFuel
Water
Heat
ExchangerReformateFuel
nozzle
Our approach is to examine the direct injection of diesel fuel into reactor.
• Fuel nozzle for direct fuel injection
• High pressure / flash vaporization• Reduce residence time before fuel is oxidized
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Fuel Cell Program
• Catalytic oxidation / reforming
• Diesel Fuel Components (Dodecane)• Long chained hydrocarbons require higher residence time for conversion
• aromatics slow and inhibit overall reaction rate
•Pre-combustion
• Diesel fuels much more likely for pre-combustion
• Differences in pre-combustion with fuel components (kerosene)
•Carbon Formation• Equilibrium carbon formation modeling
• Equilibrium varies greatly with air/steam content, slightly with pressure and
cetane #.
• Diesel fuels show high tendency for pyrolysis
• Hysteresis observed after on-set of carbon formation• Greater carbon formation with aromatics
•Regeneration of catalysis for limited number of cycles• Carbon content / oxygen content control to prevent ‘catastropic’ temperature rise
Technical Progress Summary/Findings
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Fuel Cell Program
On-Going / Future Work
• Diesel fuel/air mixing
• Durability testing of fuel processing components
• Carbon formation during start-up and reactor transients
• Carbon formation fundamentals:
• Sulfur effect on carbon formation
• Delineate carbon formation mechanisms
• Kinetic expressions for carbon formation
• Catalyst Regeneration
• Oxidative regeneration of reforming catalysts
• Expand equilibrium modeling to incorporate other carbon species
• carbon with H/C ratio ~ 0.2• PAHC (poly aromatic hydrocarbons)