burning modes and oxidation rates of soot: relevance to ... · burning modes and oxidation rates of...
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Burning Modes and Oxidation Rates of Soot:Relevance to Diesel Particulate Traps
Randy L. Vander Wal(*)(1), Aleksey Yezerets(2), Krishna Kamasamudram(2), Neal W. Currier(2), Do Heui Kim(3),Chong Min Wang(3)
(1) - USRA at the NASA-Glenn Research Center Cleveland, OH44135, USA
(2) - Cummins Inc., Columbus, IN, 47201, USA (3) - Pacific Northwest National Laboratory, Richland, WA, USA
DEER 2007 Conference, Detroit, MI Aug. 13th - 16th
Acknowledgements:Support through contract IND61957 with Cummins Inc.Soot samples supplied by Cummins Inc. and Cabot Corp.
Outline
I. Current Oxidation Status
II. Alternative Burning Modes via HRTEM
III. Nanostructure Quantification via Image Analyses
IV. Current Efforts
V. Conclusions
ic
What is Diesel Particulate Matter ?
• Composition:–“Dry carbon”
• turbostratic graphite • initial evidence of fulleren
structures in some cases –Adsorbed HCs–Inorganic materials
• Lube oil ash, H2SO4, HNO3, H2O
• Nanostructure - Amorphous, fullerenic, graphitic
• Morphology: –Primary particles: ~20-40 nm –Agglomerates: 0.1-1 micron
DieselNet
Dieselnet.com
Reactivity evolution over a life cycle of a soot particle
Printex
A: High reactivity due toambient agingB: Steady-state oxidationC: Steep increase
Diesel Soot
A: High reactivity due to: – Adsorbed HC; ambient aging
0.00E+00
2.00E-04
4.00E-04
6.00E-04
8.00E-04
1.00E-03
1.20E-03
0 20 40 60 80 100
soot oxidized, %
Norm
aliz
ed r
ate
[m
in-1
] Printex,380°C,10%O2
0.000
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
0.009
0.010
0 10 20 30 40 50 60 70 80 90 100
soot consumed, %
No
rmal
ized
rat
e, [m
in -1
] Diesel soot 450°C,10%O2
A B
C
B
C A
B: “Steady-state” oxidation C: Increased reactivity at later stages of oxidation
r = A · exp(-Ea/RT) · [C]a · [O2] b · [H2O]c
Progressive oxidation
Reactivity is increasing with degree of oxidation:
-9
-8
-7
-6
-5
-4
-3
-2
1.2 1.25 1.3 1.35 1.4 1.45 1.5 1.55
1000/T (K-1)
Ln
(K,
min
-1)
8.4-19.4%19.4-28.7%28.7-34.6%34.6-42.9%43.3-49.6%49.6-56.5%67.4-72.8%72.8-78.1%88.2-92.2%Linear (8.4-19.4%)Linear (19.4-28.7%)Linear (43.3-49.6%)Linear (28.7-34.6%)Linear (34.6-42.9%)Linear (49.6-56.5%)Linear (67.4-72.8%)Linear (72.8-78.1%)Linear (88.2-92.2%)
IncreasingOxidation
– No measurable changes of Ea, or reaction order in O2* Reaction chemistry appears to be independent of the degree of
carbon oxidation.* Number (density) of reactive sites (A) appears to be near constant!
Specific Surface Area*
200
300
400
500
600
700
800
0 20 40 60 80Degree of carbon oxidation, %
Sp
ecific
S.A
. m
2/g
Printex
Soot A
Soot B
A. Yezerets, et al. Applied catalysis B: 61 (2005), 134-143.
–BET surface area measured in-situ at different stages of oxidation–samples pre-treated by thermal desorption
* Development of the reactivity does not appear to correlate directly with thespecific surface area
* Need a different parameter which would correlate with the number of active sites *Courtesy: Dr. Do Heui Kim, (PNNL)
Puzzles (thus far):
* Comparative changes in reactivity
* Comparative evolution of surface areas
Advantages of electron microscopy (HRTEM)
* Direct observation without property assumptions
* Potential to reveal changes in nanostructure (during oxidation)
* Correlate oxidation characteristics (rate) with nanostructure
………What changes in nanostructure?
Nanostructure and Implications: Reactivity
0
3
6
9
12
15
18
21
0 0.93 1.9 2.8 3.7 4.7 5.6 6.5
% o
f F
rin
ge
s
Fringe Length (nm)
Acetylene Derived Soot
0
3
6
9
12
15
18
21
0 0.93 1.9 2.8 3.7 4.7 5.6 6.5
% o
f F
rin
ge
s
Fringe Length (nm)
Benzene Derived Soot
0
3
6
9
12
15
18
21
0 0.93 1.9 2.8 3.7 4.7 5.6 6.5
% o
f F
rin
ge
s
Fringe Length (nm)
Ethanol Derived Soot
Amorphous Fullerenic Graphitic
(g/c
m-s
) x 1
0
Soot Burnout Rates
0
0.5
1
1.5
2
2.5 Bu
rnou
t Rat
e 2
-5
Ethanol Benzene AcetyleneNSC
w
0 5 10 15 20 25 30 35
Percent (%) Excess Oxygen
ENG-A - Original from Trap
ENG-A - Post partial Oxidation (TGA, 50%)
-15 degree tilt
No tilt ENG-A 75% Burnout
Hollow particlesviewed thoughSuccessive tilts
+15 degree tilt
Eng-A: Oxidized at 450 °C
Eng-A: As-received
“Solid particles” “Hollow particles”
PNNL
Printex - as supplied
Printex - Post Partial Oxidation50% Burnout, via TGA
Summary of Results
Sample
designation
Ash contents,
wt %
SOF contents,
wt%
Observations
ENG(A) 6.5 ± 0.5% 9 ± 1% No shells/capsules
Printex-U™ <0.5% 4 ± 1% No shells
Does Internal burning correlate with either ash or soluble organicfraction (SOF) content??
A Carbon Black
Statistical Properties Extracted FromHRTEM Images (of soot nanostructure)
A T = L/A
* Other inputs –Maximum join distance –Minimum fringe length
PositionPosition
LengthLength
FringeFringeSeparationSeparation
OrientationOrientation
TortuosityTortuosity
FringeFringeDensityDensity
Fringe Length (nm)
Tortuosity
Interpretation(s)
A. Densification - a pseudonym for graphitization
1. Thermally induced densificationCreation of radicals by thermal evolution of volatiles or loss of H-atoms permits lamella growth
2. Oxidation Creation of radical sites by oxidative removal of amorphouscarbon or loss of H-atoms
B. Internal Burning - disordered carbon and/or trapped volatiles preferentially burnout
Trap conditions could promote both densification and/or volatileevolution.
Phenols
XPS-Characterization of Carbon Nanostructure
C-C sp2
CarbonylsCarboxylic
C-C sp3
Conclusions * Burning mode dependent upon nanostructure and oxidation conditions
(ash and SOF are not unique predictors)* Diesel soot and Printex U exhibit nearly identical activation energies
and burning rates and even similar active site numbers BUT vastly different surface area evolution! * Measures other than surface area are needed for modeling
burning mode and rate.(A key feature will link the distribution of active sites to the
nanostructure)* Convolved with initial nanostructure are the change(s) enabled by
oxidation.
Implications:* Latter stage burnout will strongly depend upon burnout mode* Soot burning mode(s) could affect regeneration efficiency and models.* DPF regeneration costs fuel and each cycle limits lifetime.
Costs money!