Reaction Rate Rules for High and Low Temperature...

Lawrence Livermore National Laboratory

S. Mani Sarathy M. Mehl, C.K. Westbrook, W.J. Pitz

LLNL-PRES-502049

Lawrence Livermore National Laboratory, P. O. Box 808, Livermore, CA 94551!This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344

Reaction Rate Rules for High and Low Temperature Oxidation of Lightly Methylated Alkanes

July 2011

2 S.M. Sarathy, [email protected]!


Petroleum diesel fuel surrogate palette

n-alkanes branched alkanes cycloalkanes aromatics

tetralin

1-methylnaphthalene

1,2,4-trimethylbenzene

decalin

n-dodecylcyclohexane

n-hexadecane

n-dodecane

2-methylpentadecane

3-methyldodecane

2,9-dimethyldecane

1. Pitz and Mueller, Prog. Energy Comb. Sci. (2010)



Renewable and Fischer-Tropsch diesel fuel palette

n-alkanes

branched alkanes n-hexadecane

n-dodecane

2-methylpentadecane

3-methyldodecane

2,9-dimethyldecane Deoxygenation

Selective Cracking/Isomerization

Synthetic Renewable Diesel

Oils and Fats

Gasification-FT

Biomass

BTL HVO



12

34

56

7

1 2-methylheptane

a

a

bc

de

fg

12

3

45

67

3

3-methylheptane

a

cb

z

de

fg

12

3

1 2,5-dimethylhexane

a

a

bc

1a

1a

2b

3c

12

34

43

2

1n-octane

Surrogate molecules for larger alkanes



Motivation for current work

LLNL mechanisms have been developed for a wide variety of fuels and been successfully applied to practical simulations.

BUT • Continuous improvement is needed to be consistent with fundamental and theoretical kinetic studies. • Incorporate new reaction classes and develop new rate rules. • Our models generally tend to be “RIGHT”, but we want to be “RIGHT” for the “RIGHT” reasons!



LLNL reaction classes for oxidation chemistry

High temperature mechanism

1: Unimolecular fuel decomposition

2: H atom abstractions from fuel

3: Alkyl radical decomposition

4: Alkyl radical isomerization

5: H atom abstraction from alkenes

6: Addition of radical species to alkenes

7: Alkenyl radical decomposition

8: Alkene decomposition 9: Retroene decomposition reactions



LLNL reaction classes for oxidation chemistry Low temperature mechanism 10: Alkyl radical addition to O2 (R + O2 = RO2) 12,13, and 14: R + R’O2 = RO + R’O 15: Alkylperoxy radical isomerization (RO2=QOOH) 16: Concerted eliminations (RO2 = alkene + HO2) 17: RO2 + HO2 = ROOH + O2 18: RO2 + H2O2 = ROOH + HO2 19: RO2 + CH3O2 = RO + CH3O +O2 20: RO2 + R’O2 = RO + R’O + O2 21: ROOH = RO + OH 22: RO decomposition 23: QOOH = cyclic ether + OH (cyclic ether formation) 24: QOOH = alkene+ HO2 (radical beta to OOH) 25: QOOH = alkene + carbonyl + OH (radical gamma to OOH) 26: Addition of QOOH to molecular oxygen O2 (QOOH+O2=O2QOOH) 27: O2QOOH isomerization to carbonylhydroperoxide + OH 28: Carbonylhydroperoxide decomposition 29: Reactions of cyclic ethers with OH and HO2



Chemical kinetic mechanism for n-alkanes and 2-methylakanes

Includes all n-alkanes upto C16 and 2-methylalkanes up to C20, which covers the entire distillation range for gasoline and diesel fuels

Complete Mechanism

7,200 species

31,400 reactions

Built with a consistent set of reaction classes and reaction rate rules

C8 High T Mechanism

714 species

3397 reactions

Sarathy et al., Combust. Flame (2011)



Jet Stirred Reactors

•  Idealized chemically reacting flow systems with/without simplified transport phenomenon

Non Premixed Flames

Premixed Laminar Flames

Experimental data used to validate the chemical kinetic mechanism

Shock tube

Rapid Compression Machines

2-methylheptane

Fuel used for model validation



Some important reaction classes and rate rules

High temperature mechanism 2: H atom abstractions from fuel (specifically by HO2) 4: Alkyl radical isomerization Low temperature mechanism 15: Alkylperoxy radical isomerization (RO2=QOOH) 16: Concerted eliminations (RO2 = alkene + HO2) 23: QOOH = cyclic ether + OH (cyclic ether formation) 27: O2QOOH isomerization to carbonylhydroperoxide + OH



H-atom abstraction from fuel: Uncertainty in HO2+fuel rate Class 2

CSM rates are a 2-4x faster than KIT-NUI rate calculations for n-butane/iso-butane systems.

Aguilera-Iparraguirre et al., J. Phys. Chem. A (2008)

Carstensen et al., Proc. Combust. Inst. (2007)

KIT-NUI rate calculations appear to be in better agreement with experimental work.



Uncertainty in HO2+fuel rate: Effect on shock tube ignition delay time Class 2

LLNL uses KIT-NUI rate calculations as a rule for large branched alkanes.

12

34

56

7

1 2-methylheptane

a

a

bc

de

fg

12

3

45

67

3

3-methylheptane

a

cb

z

de

fg

12

3


a

a

bc

1a

1a

2b

3c

12

34

43

2

1n-octane

Applying CSM estimates would improve intermediate temperature predictions.



Alkyl radical isomerization: New rules and estimates Class 4

LLNL rate constants are based on recent experimental/theoretical work from NIST.

Awan et al., J. Phys. Chem. A (2010)

McGivern et al., J. Phys. Chem. A (2010)

Analogies were made to estimate rate of isomerizations involving 6- and 7-membered transition states.

12

34

56

7

1 2-methylheptane

a

a

bc

de

fg

12

3

45

67

3

3-methylheptane

a

cb

z

de

fg

12

3


a

a

bc

1a

1a

2b

3c

12

34

43

2

1n-octane



Alkyl radical isomerization: Chemistry in diffusion flames Class 4

Sarathy et al., Combust. Flame (2011)

Correct prediction of straight and branched alkenes requires accurate isomerization rate constants.

0

1000

2000

3000

4000

5000

6000

7000

2 4 6 8 10

MO

LA

R C

ON

CE

NT

RA

TIO

N (

PP

M)

DISTANCE FROM FUEL PORT (mm)

C2H

2

C2H

4

CH4

10%

16%

16%

2-methylheptane

iso-butene

20%

+ +

99%

99%

5,2-H atom shift

2-methyl-5-hexene

+ CH3

+

97%

+

CH4

propene9%

1-butene

40%

10%

40%

60%5,1-H atom shift

6,2-H atom shift



Concerted elimination (RO2 = alkene + HO2)

Class 16

Current rules only consider the nature of the C-H bond broken and not the nature of the C-OO bond.

DeSain et al., J. Phys. Chem. A (2003)

LLNL rules based on n-propylperoxy+O2 and iso-propylperoxy+O2 experimental and theoretical studies. Tertiary rate is a crude estimate.

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QOOH = cyclic ether + OH Uncertainty in cyclic ether formation Class 23

LLNL rates for tetrahydrofuran formation are above 10x faster at low-to-intermediate temperatures.

Wijaya et al., J. Phys. Chem. A (2003)

MIT prescribes rates according to the nature of the radical carbon site (i.e., primary, secondary, or tertiary).

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1

.OOH

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Cyclic ether formation rate: Effect on shock tube ignition delay time Class 23

LLNL uses rules that are considerably different than MIT, but changing the rate rules does not alter ignition delay times significantly.

12

34

56

7

1 2-methylheptane

a

a

bc

de

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12

3

45

67

3

3-methylheptane

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1n-octane

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Class 23

LLNL rules provide better agreement with JSR speciation data for THF, but room for significant improvement exists.

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7

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a

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3

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Cyclic ether formation rate: Effect on jet stirred reactor species profiles

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3-methyl-5-propylTHFc8h16o1-4-2

Tetrahydrofuran species profiles in the JSR P=10 atm, Φ=1, τ=0.7s

Adjusting LLNL rules to consider nature of the radical carbon site could improve predictions.



Alkylperoxy isomerization (RO2=QOOH) O2QOOH isomerization

LLNL uses estimates based on successful reproduction of ignition delay times for a wide variety of hydrocarbons.

Sharma et al., J. Phys. Chem. A (2010)

Theoretical calculations are 10-20x faster for 6-membered ring 7-membered ring iomserizations

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Class 15 and Class 27

Zhang et al., J. Phys. Chem. A (2011)



Class 15

Class 27

LLNL uses rules that are considerably different than MIT, and changing the rate rules does appear to decrease ignition delay times.

12

34

56

7

1 2-methylheptane

a

a

bc

de

fg

12

3

45

67

3

3-methylheptane

a

cb

z

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12

3


a

a

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1a

1a

2b

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34

43

2

1n-octane

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RO2=QOOH, O2QOOH=ketohydroperoxide+OH: Effect on shock tube ignition delay time



Conclusions from Present Analysis A novel chemical kinetic mechanism for 2-methylalkanes was effectively used to test specific reaction rate rules for lightly branched alkanes • Discrepancies in radical abstraction reaction rate calculations can significantly alter ignition delay predictions. • Studies that combine both experimental and theoretical work appear to provide the best platform for developing rate rules. • Kinetic rate calculations appear to vary amongst theoreticians. Mechanism developers need to critically assess the data to develop a rate rule.

Future Work

LLNL and CSM working to implement rate rules from ab initio calculations using a consistent set of methods for all reaction classes.

Verification of thermochemistry also needs to be critically assessed in LLNL models.


Questions and Constructive Comments

CAUTION: Harsh criticisms, insults, obscenities, and rotten tomatoes are welcome, but may be responded to with brute force. Proceed with caution.

Publication: Comprehensive chemical kinetic modeling of the oxidation of 2-methylalkanes from C7 to C20 S.M. Sarathy, C.K. Westbrook, M. Mehl, W.J. Pitz, C. Togbe, P. Dagaut, H. Wang, M.A. Oehlschlaeger, U. Niemann, K. Seshadri, P.S. Veloo, C. Ji, F.N. Egolfopoulos, T. Lu Combust. Flame (2011), doi:10.1016/j.combustflame.2011.05.007



Class 27

LLNL mechanisms assume O2QOOH abstracts H atom for carbon bonded to OOH group.

This assumption cannot be applied to tertiary carbons, so alternate pathways must be included to avoid “dead-end” pathways.

O2QOOH=ketohydroperoxide+OH: Alternate routes for tertiary C-H sites

1 2

3

4

56

7

1 .OOH

OOH

O

OOH

+ OH

6m, sEa = 17.45 kcal/mol

1 2 3

45 6

7

1

OOH

.OO H

1 2 34

1

OOH

O + OH

+

6m, sEa = 20.45 kcal/mol

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