Surrogate Fuels for Transportation Fuels

Charles WestbrookLawrence Livermore National Laboratory

December 5, 2007

SERDP MeetingWashington, DC

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The fuel situation in 1922 looks pretty familiar

Thomas Midgley, Chief of Fuels Section for General Motors, 1922US Geological Survey -- 20 years left of petroleum reservesProduction of 5 billion gallons of fuel in 1921

Potential new sources of petroleumOil shaleOils from coalAlcohol fuels from biomass

Higher efficiency a high priority for conservation reasonsPeople will not buy a car “lacking in acceleration and hill climbing”Solution is higher compression ratio, then at about 4.25 : 1Obstacle is engine knock, whose origin is unknownResult was development of TEL as antiknockPhenomenological picture with no fundamental understanding

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Explanation of engine knock, ON, antiknocks, diesel ignition, and HCCI ignition came in the 1990’s from DOE/BES theoretical chemistry and supercomputing and EERE knock working group

OO

H

H

OO

H

H

R

Reactant Transitionstate

HC

OO

H

H

R

R

HC

OO

H

H

R R

RR

RR

Most work has been done for alkane fuels, and many questions remain for aromatics, cyclic paraffins, large olefins

+ O2

OH +

+ OH

O

O

O

OH

OOH

O

O

O

O

OH

O

O

Low temperature chain branching pathsAlkylperoxy radical isomerization rates are differentin paraffin and cyclic paraffin hydrocarbons

4

0

50

100

150

200

250

330 340 350 360 370 380 390Crank Angle

iso-Octane

PRF80

• Heat release rates in HCCI combustion of two fuels, iso-octane with no low T heat release, and PRF-80 with two stage heat release

Results from experiments of Sjöberg and Dec, SNL 2006

• We are seeing researchers debating which makes the better HCCI fuel. Both debaters have completely acceptedthe existence and source of the low T reactivity.

Chemistry of alkylperoxy radical isomerization has reached street-level awareness

This is serious,black-beltfuel chemistry and computationalchemistry

Low temperatureheat release

5

Recent revolution in understanding of diesel engine combustion processes

As recently as 1990, the entire basis of diesel combustion was poorly understood

Fuel evaporating from a Droplets being shed from aliquid jet liquid core and then burning

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Approach: Investigate the processes in the cylinder of an operating diesel engine using advanced optical diagnostics

Modified heavy-duty truck engine provides good optical access while maintaining the basic combustion characteristics of a production engine.

Data from multiple advanced laser diagnostics have substantially improved our understanding of diesel combustion and emissions formation.

Heavy-Duty Diesel Engine Research

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New conceptual picture developed 1990 - 1997

Extended role of multiple advanced laser diagnostic techniquesdeveloped under BES program, used by EERE program

Team led by John Dec, SNL

Explains 2 stages in diesel burningignition and cetanesooting logic

This is serious, black-beltoptical physics science

Lots still unknown,soot chemistry, spray dynamicsfuel effects, etc.

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Homogeneous Charge Compression Ignition (HCCI) engine delivers high efficiency, and low particulate and NOx emissions:

Technical challenges:

engine controlmulti-cylinder balancingstartabilitylow power outputhigh HC and CO emissions

Advantages:

low NOxlow particulate matterhigh efficiency

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Have we have come a long way since 1922?

We still are looking to oil shale, oil sands and biomass for thefutureHowever, our understanding of knocking, antiknocks and low T chemistry has grown enormouslye.g., Current engine designers debate how much low T heat release is

best, and take its sources for grantedConceptual model for diesel combustion has led to breakthroughse.g., Understanding of anti-sooting action of oxygenates

Entirely new concept engine (HCCI) is being developed Great majority of this progress is due to basic science

understanding, e.g., optical diagnostics, quantum chemistry and electronic structure theory, high performance computing, etc.

We have used basic science advances to make big jumps in understanding, but we are back to trial and error in many cases

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Octane numbers ofheptanes are due exclusively to their different molecularstructures

This was recognizedin 1920’s but no explanation in fundamental terms had been provided prior to our work

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Engine knock is an undesirable thermal ignition

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Kinetic features of engine knock

• History of octane numbers and empirical observations

• End gas self-ignition prior to flame arrival

• Actual ignition driven by H2O2 decomposition at ~ 900K

• Kinetic influence of molecular size and structure

• Effects of additives, both promoters and inhibitors

• Reduced models must retain H2O2 decomposition reaction to

describe ignition

• Issue of real SI engine fuel being complex mixture of components

• Lots of kinetics research still needed (aromatics, cyclics, etc.)

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US Auto/Oil program was an essential advance in emissions research

Joint project between auto manufacturers and oil companies in 1980’s and 1990’sEngine emissions shown to be a combination of unburned fuel and products of incomplete combustionBoth depend on the specific molecular structure of the fuel moleculesTherefore, prediction of emissions requires a detailed knowledge of fuel compositionSingle-component representations of practical fuels are therefore completely inadequate, and a thorough picture of the fuel components is essential

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Most transportation fuels consist of complex mixtures of many chemical species

Natural gasGasolineDieselJet fuelRocket fuelThese fuels contain too many components for detailed mechanisms

Gasoline, diesel and jet fuel have hundreds of components (even natural gas)

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Classes of compounds in practical fuels

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Gasoline has many branched alkanes

Gasoline is lower incycloalkanes

Jet fuel has the highestn-alkane

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Diesel fuel and a surrogate

• Diesel fuel is made up of straight-chain alkanes, branched-chain

alkanes,cyclic alkanes, simple aromatics, alkylated aromatics,

polycyclic aromatics and others

• Example test: Surrogate diesel:

n-heptane: cetane no. of 56

branched chain component: iso-octane

cyclic alkane component: cyclohexane or methyl cyclohexane

aromatic component: toluene

Most common surrogate is 100% n-heptane

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Fuel Surrogate Palette for Diesel

n-alkanebranched alkanecycloalkanesaromaticsothers

butylcyclohexanedecalin

hepta-methyl-nonane

n-decyl-benzenealpha-methyl-naphthalene

n-dodecanen-tridecanen-tetradecanen-pentadecanen-hexadecane

tetralin

Surrogate Fuel ComponentSelection

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We have greatly extended the components in the palette that can be modeled in the high molecular weight range:

n-octane (n-C8H18)n-nonane (n-C9H20)n-decane (n-C10H22)n-undecane (n-C11H24)n-dodecane (n-C12H26)n-tridecane (n-C13H28)n-tetradecane (n-C14H30)n-pentadecane (n-C15H32)n-hexadecane (n-C16H34)

Surrogate Fuel ComponentSelection

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Fuel components that have highermolecular weights are needed

Diesel fuel has mostly C14 to C24 components centered around C16

Amou

nt

Molecular Weight

Real Diesel

C16C10 C24

Surrogate Fuel ComponentSelection

(SAE 2007-01-0201Presentation)

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To span the cetane number scale, easily ignitable components (e.g. n-hexadecane) and less-ignitable components (aromatics, iso-alkanes) are needed

Recommended components from Diesel Surrogate Fuel Working Group (SAE 2007-01-0201):

n-hexadecaneheptamethylnonanen-decylbenzene1-methylnapthalene

Surrogate FuelFormulation

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Chemical kinetic mechanism for nC8-nC16 surrogate components:

2116 species8130 reactionsLow and high temperature chemistry => can use to investigate low temperature combustion strategiesSame reaction rate rules as highly validated n-heptane mechanismTailor the mechanism to fit specific fuels for computational efficiency

Detailed ChemicalKinetics for Components

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Includes high and low temperature ignition chemistry: Important for predicting low temperature combustion regimes

Detailed ChemicalKinetics for Components

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Family of ignition simulations – a valuable analysis tool

n-decane, φ = 1.0, 13 bar pressure

Same approach used by Petersen et al. for propane ignition analyses

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EP ignition

0.01

0.1

1

10

100

6.60 7.60 8.60 9.60

10000/T

igni

tion

dela

EP Umich

A new diagnostic technique for analysis of ignition kinetics

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EP ignition

0.01

0.1

1

10

100

6.60 7.60 8.60 9.60

10000/T

igni

tion

dela

EP Umichmodel

We are familiar with model/experiment comparisons

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EP ignition

0.01

0.1

1

10

100

6.60 7.60 8.60 9.60

10000/T

igni

tion

dela

EP Umichmodel galway

Combining multiple sets of experimental results can provide additional mechanism validation

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Surrogate fuelspast use of n-heptane surrogate for dieselmany similarities between all large n-alkanesn-decane surrogate for kerosene (Dagaut)n-hexadecane surrogate for biodieseln-decane and methyl decanoate similarities role of methyl ester grouppotential of n-cetane + methyl decanoate or smaller methyl ester for biodiesel surrogate

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2,2,4,4,6,8,8-Heptamethyl nonane mechanism

• Primary reference fuel for diesels• Highly branched C16 molecule• Same reaction rate rules as iso-octane

and n-heptane• Low T kinetics only for HMN and not for its

immediate products• Mechanism complete, in validation test

phase

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Cycloalkanes: methyl cyclohexane

• Cycloalkanes are interesting due to oil sands

methylcyclohexane

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Slide courtesy Phil Smith, University of Utah

Oil-sand derived fuels have focused attention on cyclo-alkanes

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• 1.7-2.5 trillion barrels of bitumen in place in the oil sands of Alberta, Canada

• More oil than the known reserves of the Middle East

Graphic: pp. 194, Athabasca Oil Sands – The Karl A. Clark Volume

Composition of Oil Sands

http://www.oilsandsdiscovery.com/oil_sands_story/pdfs/vastresource.pdf

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Asphaltene molecule typical of oil sands

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Cyclic ring structure changes the energy environment of important reactions and requires new kinetic descriptions of ignition

OO

H

H

OO

H

H

R

Reactant Transitionstate

HC

OO

H

H

R

R

HC

OO

H

H

R R

RR

RR

Most work has been done for alkane fuels, and many questions remain for aromatics, cyclic paraffins, large olefins

+ O2

OH +

+ OH

O

O

O

OH

OOH

O

O

O

O

OH

O

O

Low temperature chain branching pathsAlkylperoxy radical isomerization rates are differentin paraffin and cyclic paraffin hydrocarbons

35

Biodiesel fuels

Biodiesel fuels produced from various oleaginous plantsUS: soybean / Europe: rapeseed

triglyceride

methanolmethyl ester glycerol

OO

O

O

O

O

R

R R

+ 3 CH3OHOH

OH

OH

CH3O

O

R

3 +

(R = hydrocarbon chain)

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Composition of Biodiesels

O

O

O

O

O

O

O

O

O

O

m e t h y l p a l m i t a t e

m e t h y l s t e a r a t e

m e t h y l o l e a t e

m e t h y l l i n o l e a t e

m e t h y l l i n o l e n a t e

010203040506070

C16:0 C18:0 C18:1 C18:2 C18:3

%

SoybeanRapeseed

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Comparison with n-Decane Ignition Delay Times

1 .E -0 2

1 .E -0 1

1 .E + 0 0

1 .E + 0 1

1 .E + 0 2

0 .7 0 .8 0 .9 1 .0 1 .1 1 .2 1 .3 1 .4 1 .5 1 .6

1 0 0 0 /T (K -1)

Igni

tion

Del

ay T

ime

(ms)

P = 1 2 a tm

P = 5 0 a tm

Symbols: n-decane experiments (Pfahl et al.)(Pfahl et al.)Line: methyl decanoate mechanism

Ignition delay times very close

n-Alkanes:

cannot reproduce the early formation

of CO2

but

reproduce the reactivity of methyl

esters very well

Equivalence ratio: 1, in air

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Summary

There is a need for kinetic modeling capabilities for practical fuelsSurrogates offer a way to include “real fuel”effectsThere is a continuing need to increase the level of detail in surrogate fuel mixturesThere is a corresponding need to provide kinetic models of fuel components, with steadily growing molecular structure complexity