Biodiesel Kinetics and Flame Chemistry

Yiguang Ju, Princeton University

On behalf of CEFRC: Biodiesel Thrust and Flame Chemistry Working Group

Sept. 17-20, 2012, MACCCR

•Mechanisms of butanols •Automatic mechanism

generation

•Flow reactor experiments •Reactivity and species history •H2, CO, small HC chemistry •DME, small oxygenates

•Shock tube/Laser diagnostics

• Ignition/Species histories •Rate constants

•Flame species by Synchrotron MS

•Burner-stabilized flames •Ab initio methods •Thermochemical kinetics •Ester chemistry

•Potential energy surfaces •Rate constants with

tunneling

•Potential energy surfaces •Reaction rate constants •High pressure theories

W. H. Green

N. Hansen

H. Wang

F. L. Dryer

R. K. Hanson

C. J. Sung

E. A. Carter

D. G. Truhlar

S. J. Klippenstein

Synergistic

2D Research

Structure

Three Thrusts

Unite the Team

Alcohols Biodiesel

Foundation Fuels

•Rapid compression machine

•High-pressure ignition •Thermometry and species

C. K. Law

F. N. Egolfopoulos

S. B. Pope

Y. Ju

J. H. Chen

•Flame chemistry •Biodiesel kinetics •Model reduction & multi-scale modeling

•Laminar flame speeds,

extinction and ignition, pollutants

•DNS of HCCI/SACI combustion •DNS data for model validation •High pressure turbulence/

chemistry interaction

•LES/PDF/ISAT turbulent combustion

•Turbulence/chemistry

•Low-T combustion engines •HCCI and RCCI CFD modeling • Interface with DERC

consortium

R. D. Reitz •Soot •Small HC chemistry •Transport properties •UQ methods

•High-pressure flames •Turbulent flames •Droplet processes

Motivation

Biodiesel:

• Produced from vegetable oils, animal fats, & waste materials

• Energy density much higher than ethanol

• 28 billion gallons of biodiesel produced in 2010 worldwide

• Large molecules: C16-C18 with ester functional group

• Different combustion chemistry/emissions from hydrocarbons

• Large disparities in alkyl chain length and structures

Biodiesel Trans-esterification

O

O

R1

R2

Sooting Propensity of Diesel Surrogate and Large Ester Flames

Diesel surrogate: 70% n-C10H22 + 30% 1-methyl naphthalene Dagaut and coworkers (2010)

Diesel Biodiesel

(Law, Princeton)

Scientific Questions?

How to address the knowledge gaps in

kinetics of large, oxygenated fuel molecules?

How can we use quantum chemistry and

kinetic experiments to provide a better,

predictive model?

How to address the transport and chemistry

interaction in flames?

Research Objectives

Advance the understanding of combustion

kinetics of methyl esters

Develop a validated kinetic methyl ester kinetic

mechanism to model oxidation with quantum

chemistry calculations

Advance understanding of chemistry/transport

interaction

+ =

Methyl Butanoate

(C4+1)

Alkane

(C14) methyl stearate (C18+1)

Decomposition

1. Biodiesel Kinetics: Hypothesis

O

O

O

O

O

O

O

O

O

O

Methyl Formate Methyl Acetate Methyl Popanoate Methyl Butanoate

Methyl Decanoate

Similarity between Small/Large Esters?

Biodesel

Methyl Propanoate

1A. Small Methyl Ester Pyrolysis in Shock Tube Stanford University

0.60 0.65 0.70 0.75 0.800.0

0.2

0.4

0.6

0.8

1.0

C

O2 F

ractional Y

ield

1000/T [K-1]

1428K 1250K

1666K

MA

MB MP

2% Methyl Ester/Argon

1.5 atm, Yield at 1 ms

The reactivity is strongly affected by the alkyl chain length

1B. Comparison of Premixed Flame Speeds

of Small Methyl-Esters/Air (C1-C4: 1 atm)

Egolfopoulos et al. •Methyl formate has the highest reactivity

•Methyl propanoate is the second

0

100

200

300

400

500

0.5 1 1.5 2

Exti

nc

tio

n s

tra

in r

ate

aE

[1/s

]

Transport-Weighted Enthalpy [cal/cm3]

Tf = 500 K, Tox = 298 K

Methyl Formate

Methyl Ethanoate

Methyl Propanoate

Methyl Butanoate

Methyl Pentanoate

Methyl Hexanoate

Methyl Octanoate

Methyl Decanoate

1C. Comparison of Extinction Limits of Methyl Esters (C1-C10)

•Uniqueness of small methyl esters: methyl formate & methyl propanoate

•Similarity of large methyl esters

Extinction limit vs. Transport weighted enthalpy (TWE)

MRSDCI /cc-pV∞Z // B3LYP

CBS-QB3-Isodesmic*

1D: BDEs (D298 ) (kcal/mol) in Biodiesel Methyl Butanoate (MB)

* Osmont et al. J. Phys. Chem. A, 111, 3727 (2007)

• Weakest bonds: dissociated radicals are resonance stabilized.

• C-C bonds are weaker than C-H bonds: alkyl fragments allow more

structural relaxation than H.

C O C C C C

H O H H H

H H H H

H H

98.0

98.9

95.4

93.5

92.9

94.2

96.8

98.7

98.6

101.1

83.1

84.4

85.8

89.1

MB

101.2

101.3

Oyeyemi, V. B.; Pavone, M.; Keith, J. A.; Carter, E. A.

in preparation, (2012).

Seshadri et al. : 80.8 kcal/mol, 2009

C8-C10

MB Extension

C1-C7

H2/O2

C1-C7: n-heptane model Curran et al., 2008, 2010

MB: Ester functional group Dooley et al., 2008

1E. Kinetic Mechanism Development

(Ester-MECH: C2-C11 methyl esters)

H2/O2: PU hydrogen model

Dievart et al., 34th Symposium on Combustion on Comb., 2012

Dievart et al. Combustion and Flame, 2012, Vol.159 , pp. 1795-1803.

1F. Model Validation: Ignition Delay Time

Ignition delays from Hanson’s group (Aerosol Shock

Tube, very lean mixtures, diluted in argon, ~7.5 atm)

Present model in good

agreement (35%), whereas

literature models overestimate

MD oxidation rate (50 to 80%)

Bond dissociation energy

affects strongly fuel

decomposition pathway

Present model, Seshadri et al’s model:

Metathesis reactions: 95% Fuel Decomposition: 5%

Seshadri et al’s model:

Metathesis reactions: 55% Fuel Decomposition: 45%

Model validation: JSR & Flame speeds

methyl decanoate

• high temperature kinetics • speciation profiles, flame speeds

0.6 0.8 1.0 1.2 1.4 1.6

20

30

40

50

60

70

Experimental data

Reduced model (529 species)

Reduced model (228 species)

Seshadri et al.'s model [14]

Luo et al's model [20]

Fla

me

sp

ee

d [

cm

.s-1]

Equivalence Ratio

0

5000

10000

15000

20000

25000

0

500

1000

1500

2000

2500

3000

500 600 700 800 900 1000 1100

CO

and

CO

2M

ole

Fraction

[pp

m]

MD

an

d C

2H

4M

ole

Fra

ctio

n [

pp

m]

Temperature [K]

Jet-Stirred reactor (Glaude et al., C&F 157, 2010)

P = 1atm, τ = 1.5 s Laminar Flame Speeds

(Wang et al., C&F 158, 2011) P = 1atm, T = 403 K

14 Glaude et al., CF, 2010.

Model comparison in diffusion flame: MD

Model validation: Diffusion flame extinction

Methyl formate Methyl ethanoate Methyl propanoate Methyl butanoate

0

100

200

300

400

500

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35

Extinc

tion

Str

ain r

ate

, aE[s

-1]

Fuel Mole Fraction, Xf

C2-C11 esters

Methyl pentanoate Methyl hexanoate Methyl octanoate Methyl decanoate

Dievart et al., 34th symposium on combustion, 2012

Model validation: Species time history

17

0.0E+00

5.0E-13

1.0E-12

1.5E-12

2.0E-12

2.5E-12

3.0E-12

0.E+00 5.E-04 1.E-03 2.E-03 2.E-03

[H] , [m

ol.c

m-3]

Time [s]

T = 1333 K P = 0.51 atm Xfuel = 239.1 ppm

Methyl formate

H abstraction reactions by OH and H: Methyl Formate

CH3OCHO + OH = CH3OCO + H2O

Large deviations between the rate constants calculated by the Carter’s group (J. Phys. Chem. A, 2012) and the previous estimates or calculations.

CH3OCHO + H = CH3OCO + H2

Good and Francisco, J. Phys. Chem. A, 2002, Vol. 106, pp. 1733-1738 Peukert et al., Combustion and Flame, 2012, Vol. 159, pp. 2312-2323 Akih-Kumgeh and Bergthorson, Comb. Flame, 2011, Vol. 158, pp. 1037-1058 Szilagyi et al., J. Phys. Chem, 2004, Vol. 118, pp. 479-492

1.0E+09

1.0E+10

1.0E+11

1.0E+12

1.0E+13

300 500 700 900 1100 1300 1500 1700 1900

Rate

con

stant

[mol c

m-3.s

-1]

Temperature [K]

Ting Tan (Carter)

Good and Francisco

Current Model

Akih-Kumgeh

Peukert et al. (Argonne)

Peukert et al.1.0E+09

1.0E+10

1.0E+11

1.0E+12

1.0E+13

300 800 1300 1800

Rate

con

stant

[mol c

m-3.s

-1]

Temperature [K]

Ting Tan (Carter)

Good and Francisco

Current Model

Szilagyi et al. (2004)

Decomposition of small methyl ester radicals such as CH3OCO (and C2H5OCO) are key reactions.

Literature: only high pressure limit rate constant with low level PES is available (e.g. BH&HLYP/CC-PVTZ).

Present method: MRACPF/CBS//CASPT2/CC-PVTZ method on PES and VARIFLEX for pressure dependence

L.K. Huynh, A. Violi. J. Org. Chem. 72 (2008) 94-101.

Methyl-Ester Radical Decomposition Reactions (Collaborative work : Carter, Klippenstein and Ju)

1.0E+03

1.0E+05

1.0E+07

1.0E+09

1.0E+11

1.0E+13

0.3 0.8 1.3 1.8

Rate

con

stant

[s-

1]

1000/T [K]

HPL

100 atm

10 atm

1 atm

HPL Huynh and Violi

CH3OCO = CH3 + CO2

Ester-MECH C2-C11 Esters

Carter

• Thermochemistry

H, Cp, S •Rate constants

MF+X , ME+X, MP+X… (OH, H, CH3, HO2)

Yang, Raghu, Ju, Klippenstein

•Rate constants

CH3OCO C2H5OCO

MF, ME, MP… Decomposition

Hanson group

•Rate constants MX+ OH

X=F,A,P,B

•Speciation time history

Egolfopoulos, Ju, Law

•Flame speeds •Flame structure •Extinction •Emissions

Sung and Hanson

•Ignition delay (Shock tube, RCM)

Dryer, Hansen and Ju

•Speciation experiments (Flow tube, flames)

20

Collaborative structure of the Biodiesel

Summary: OH + Methyl Esters Products

• Data agree within 25% with Structure Activity Relationship (SAR) estimated rate constants ( the same rate used in the current model).

21

0.7 0.8 0.9 1.0 1.1 1.2

1E12

1E13

833K909K1000K1111K1250K

MButanoate

MPropanoate

MFormate

MAcetate

Lines: Modified SAR (SAR x 0.75)

kM

eth

yl E

ste

r +

OH [

cm

3 m

ol-1

s-1]

1000/T [1/K]

Methyl Ester + OH = Products

1429K

Methyl Formate Decomposition Kinetics Summary Arrhenius Plot k1: MF → CO + CH3OH

22

Wide T range

Low data scatter

Repeatable

±25%

MBMS/mid-IR with flow reactor/jet stirred reactor

Advanced diagnostics- high pressure reactors

at low and intermediate temperatures

Multipath-IR

Fuel

Pre

he

ate

d a

ir

Hig

h p

res

su

re, h

igh

te

mp

era

ture

ch

am

ber

Mix

ing

JSR

Herriott cell reflections

0

500

1000

1500

2000

2500

0

20

40

60

80

100

120

0 500 1000 1500

Me

asu

red

H2O

[p

pm

]

Me

asu

red

CH

4 [

pp

m]

Calibration mole fraction [ppm]

In air, 60 Torr, 293 KCH4

H2O

500 550 600 650 700 750

0

500

1000

1500

2000

2500

3000

3500

4000

Temperature (K)

H2O

2 c

on

cen

tra

tio

n (

pp

m) MBMS

Modeling

H2O2 Measurements, DME/O2/He

(2 sec, 1 atm (0.02/0.1/0.88)

H2O2

HO2 ?

2. Flame Chemistry: Kinetic &Transport Interaction

•Interaction of Transport and Chemistry on Flame Extinction

•Low Temperature Ignition and New Flame Regimes

50

150

250

350

450

0.05 0.09 0.13 0.17 0.21 0.25 0.29

Exti

nc

tio

n s

tra

in r

ate

aE

[1/s

]

Fuel mole fraction, Xf

Tf = 500 K, Tox = 298 K

Methyl Formate

Methyl Ethanoate

Methyl Propanoate

Methyl Butanoate

Methyl Pentanoate

Methyl Hexanoate

Methyl Octanoate

Methyl Decanoate

2A. Diffusion Flame Extinction Limits: From Methyl Formate to Methyl Decanoate

ΔHcomb (kcal/mol)

MW (g/mol)

MB -651.6 102.14

MD -1533.3 186.29

How to separate chemistry from

transport and fuel heating value?

i

fp

FF

F

e RTTC

QY

MMa *

)(/

1 ,

A generic correlation for extinction limit:

Transport weighted Enthalpy & radical

index Theoretical analysis of Extinction Damkohler

number

Transport Heat release/heat loss

Fuel chemistry

Radical production

rate

32

, 3

2

,

1 2 1( , , ) ( , ) exp

fO aF F F O F F

E F f a f

TY TLe P Le Le L Le

Da e Y T T T T

Extinction Strain Rate

Won et al. CNF 159 (2012)

Transport weighted Enthalpy *Radical index

0

100

200

300

400

500

0.5 1 1.5 2

Exti

nc

tio

n s

tra

in r

ate

aE

[1/s

]

Transport-Weighted Enthalpy [cal/cm3]

Tf = 500 K, Tox = 298 K,

Methyl Formate

Methyl Ethanoate

Methyl Propanoate

Methyl Butanoate

Methyl Pentanoate

Methyl Hexanoate

Methyl Octanoate

Methyl Decanoate

Reactivity Scaling of Small/Large Methyl Esters: From Methyl Formate (C1) to Methyl Decanoate (C10)

•Uniqueness of small methyl ester

•Similarity of large methyl ester

Extinction limit vs. Transport weighted enthalpy (TWE) flux

CH3OH+

CO

CH2O+

HCO

CH3O+

CO

CH3

+CO2

H + CO HO2 + CO

35% 18%

42%

+R/-RH

+R/-RH

62%38%

81%

9%

88% 12%

+M +O2

Impact of alkyl chain length on methyl ester reactivity

Methyl Formate, R0C

Higher reactivity

CH2OCH3OCH2CO CH3CO

CH3 + CO

+ +

-H

+R/-RH +R/-RH

47% 47%

5%

95%

HCCO

CO + CO

+H35%

56%

+OH+O

Methyl Acetate, R1C

Lower reactivity

Diévart et al, 2012

to presented on Monday at 34th

Symposium

H abstraction reactions,

Fuel, CH3OCO, and CH3OC(O)CH2

decomposition reaction rates

Extinction Limit: n-Alkanes, iso-Alkanes, Aromatics

0

100

200

300

400

500

0 0.05 0.1 0.15 0.2

Ex

tin

cti

on

str

ain

ra

te a

E[1

/s]

Fuel mole fraction Xf

n-decane

n-nonane

n-heptane

JETA POSF 4658

Princeton Surrogate

iso-octane

nPB

toluene

124TMB

135TMB

n-alkanes

aromatics

Tf = 500 K and To = 300 K

How to separate chemistry from transport?

What is the ranking high temperature reactivity?

A General Correlation of Hydrocarbon Fuel Extinction vs. TWE and Radical Index

30

R² = 0.97

0

100

200

300

400

500

0.5 1 1.5 2

Ex

tin

cti

on

str

ain

ra

te a

E[1

/s]

Ri[Fuel]Hc(MWfuel/MWnitrogen)-1/2 [cal/cm3]

n-decane

n-nonane

n-heptane

iso-octane

n-propyl benzene

toluene

1,2,4-trimethly benzene

1,3,5-trimethly benzene

Tf = 500 K and To = 300 K

Radical Index for Screening of Alternative Fuels

• Extinction limits of diffusion flames for pure fuel samples have been completely measured and compared by using TWE – Heat of combustion, Hc has been re-estimated based on H/C ratio correlation. – Re-evaluation of Hc might be necessary.

• High temperature reactivity based on Radical index – SPK HRJ camelina HRJ Tallow > JP8 IPK (~iso-octane) – Similar order to DCN measurements, IPK must be heavily isomerized.

Fuel Radical

Index DCN

JP8 POSF 6169 0.78 47.3

SHELL SPK POSF 5729 0.85 58.4

HRJ Camelina POSF 7720 0.82 58.9

HRJ Tallow POSF 6308 0.8 58.1

SASOL IPK POSF 7629 0.76 31.3 50

100

150

200

250

300

350

400

450

0.5 1 1.5 2 2.5

Ex

tin

cti

on

str

ain

ra

te [

s-1

]

Transport-weighted enthalpy [cal/cm3]

[fuel]Hc(MWf/MWn)-0.5

JP8 POSF 6169

SHELL SPK POSF 5729

HRJ Camelina POSF 7720

HRJ Tallow POSF 6308

SASOL IPK POSF 7629

n-alkane

iso-octane

Extinction of diffusion flame in counterflow configuration

Tf = 500 K and Tair = 300 K @ 1 atm

Won et al. CNF 159 (2012)

2B. Effects of Transport on Low Temperature Ignition in

Non-premixed Counterflow Flames

Law’s group

• NTC behavior extensively observed for homogeneous systems

• Corresponding non-monotonic behavior signaling NTC chemistry in steady state strained has not been well studied in flows (e.g. counterflow),

Seshadri et al., CF 2009.

• Reason: Reduced residence time => higher ignition temperature => shifting away from NTC temperature regime

• Explore possible existence of NTC behavior for flows – with low strain rates – at high pressures

Heptane/air

flames

No NTC at 1 atm, 200/s

NTC temp. ↑ as pressure ↑

tNTC > tconv

tNTC ~ tconv ?

Decrease k

Increase P Decrease P

Increase k

n-Heptane vs. Air in Counterflow Ignition

Single

ignition

1st ignition, Low-T

chemistry

2nd ignition,

High-T chemistry

Single ignition

Low-T chemistry

1st ign, low-T

chemistry

2nd ign,

high-T

High-T

Chemistry

Unsteady Flow Perturbation on Low Temperature Ignition in Diffusion Flame

6.17 ms at 74 Hz

Rise

from 72 to

73 Hz

• No effect on initial RO2 formation,

• H2O2 decomposition is delayed by

heat loss at high strain rate. Reaction 2: RO2 = R’O2H

Reaction 3: H2O2 + M = 2OH + M

850 K

30 atm

100 s-1

Shan et al., 2012

0.000 0.005 0.010 0.0150.0

0.2

0.4

0.6

0.8

1.0

Hot ignition

LTI

at wall

Low temperature flame dominated

double flame (decoupled)

Single high temperature

flame front

Lo

ca

tio

n o

f m

axim

um

he

at

rele

ase

(cm

)

Time (s)

High temperature flame

dominated double flame (coupled)

Low temperature ignition

Transition

Multi Flame Regimes in HCCI Ignition n-Heptane: Flame Initiation by a Spark at 40 atm, T=700 K

Sf=15.3 cm/s

Sf=27.5 cm/s Sf=25.6 m/s

Movie

Ju et al., 33rd symposium on Comb., 2011

Combustion properties, species, and kinetic data methyl

esters are experimentally measured by a collective effort.

An updated methyl ester (C2-C11) kinetic mechanism is

developed and partially validated.

Large uncertainties in elementary rate constant and

species time history.

Conclusions

Flame theory to correlate flame extinction with TWE and

radical index. Uniqueness and similarity of high

temperature reactivity of methyl esters are demonstrated.

Significant impacts of low temperature ignition on ignition

and flame propagation are demonstrated. New flame

regimes are identified.

Acknowledgement:

Pascale Dievart

Sanghee Won

Xueliang Yang

Funding support: DOE-BES CEFRC