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Low Temperature Autoignition of C8
H16
O2 Ethyl and Methyl Esters in a Motored Engine
Yu Zhang and André
BoehmanThe EMS Energy Institute
Penn State University
Motivation
Need for Fundamental Understanding of Unconventional Fuels
2006 BRN Workshop on Basic Research Needs for Clean and Efficient Combustion of 21st Century Transportation Fuels
identified a single over-arching grand challenge:
“the development of a validated, predictive, multi-scale, combustion
modeling capability to optimize the design and operation of evolving fuels in advanced engines for transportation applications.”
Second Generation and Optimal Biodiesel
Howell (DEER 2007) pointed to optimizing the fatty acid ester distribution around mono-unsaturated esters
Knothe
(Energy & Fuels, 2008) recommended genetic modification of the fatty acid profile to fuel property issues simultaneously. Methyl palmitoleate
and esters of decanoic
acid are strong candidates for improving fuel properties besides methyl oleate
Kinney and Clemente (Fuel Processing Technology, 2005) demonstrated that biotechnology can be used to shape the genetics of soybeans to optimize soybean oil for engine performance
Background
Lack of fundamental understanding of the combustion chemistry of
fatty acid esters
Few studies have been focused on the low temperature chemistry of fatty acid esters, which is relevant to application of biodiesel in modern engine designs that employ low temperature combustion strategies
Detailed chemical mechanism of long chain fatty acid esters (e.g. methyl
decanoate) may pose considerable computational challenges for kinetic modeling studies in engine applications
Need to find model compounds for biodiesel that not only exhibit
evident low temperature oxidation behavior, but also have chemical kinetic mechanisms whose sizes are manageable for computational purposes.
Kinetic mechanisms published for methyl butanoate (various authors) and methyl decanoate (Herbinet, Pitz, Westbrook, Combust. Flame,
2008)
Comparisons of ignition characteristics of methyl decanoate with
Fischer-
Tropsch diesel and conventional diesel fuels (DEER 2006 and in Szybist,
Boehman, Haworth, Koga, Combust. Flame,
2007)
Examination of C9 fatty acid esters (Zhang, Yang and Boehman, Combust. Flame, 2009)
Objectives
Investigate the premixed ignition behavior of fatty acid
methyl and ethyl esters in a engine environment
Examine the applicability of the two esters as surrogates for biodiesel in studies of low temperature oxidation
Achieve a fundamental understanding of the major low
temperature oxidation pathways of fatty acid ethyl esters
An important step in the low temperature oxidation of paraffins is the isomerization of RO2
into QOOH through internal H-atom shift by
forming transition state ring-like structures. Six-
and seven-membered rings have the most rapid reaction rate
Experimental Setup
Test Fuels Engine Setup
GDI Injector
Main AirHeater
Heating Tape
Air FlowMeter
VariableCompression
Ratio CFREngine
Exhaust
Up to 260˚
C
Varied Φ
Compression Ratio=4.0-13.75
Combustion Analysis
Optional intake dilution with N2
Test Conditions
GC TCD/FIDAnalysis
GC/MS Analysis
Pin
: 1 atm Tin
: 155 °C
Equivalence Ratio : 0.25
Ignition Progression of Methyl Heptanoate
-0.001
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
300
400
500
600
700
800
900
300 320 340 360 380 400 420App
aren
t Hea
t Rel
ease
Rat
e (k
J/D
eg) B
ulk Cylinder G
as Temperature (K
)
CA (Deg)
LTHR
-0.001
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
300
400
500
600
700
800
900
300 320 340 360 380 400 420App
aren
t Hea
t Rel
ease
Rat
e (k
J/D
eg) B
ulk Cylinder G
as Temperature (K
)
CA (Deg)
LTHR
-0.001
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
300
400
500
600
700
800
900
1000
300 320 340 360 380 400 420App
aren
t Hea
t Rel
ease
Rat
e (k
J/D
eg) B
ulk Cylinder G
as Temperature (K
)
CA (Deg)
LTHR
-0.01
0.01
0.03
0.05
0.07
0.09
0.11
50
250
450
650
850
1050
1250
1450
1650
300 320 340 360 380 400 420App
aren
t Hea
t Rel
ease
Rat
e (k
J/D
eg) B
ulk Cylinder G
as Temperature (K
)
CA (Deg)
LTHR
HTHR
CR=7.05
CR=8.06 CR=9.95
CR=7.69
As CR increases, methyl heptanoate experiences the transition from single
stage LTHR to two-stage ignition
Ignition Progression of Ethyl Hexanoate
Both of the two esters exhibit evident low temperature heat release
-0.001
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
300
400
500
600
700
800
900
300 320 340 360 380 400 420App
aren
t Hea
t Rel
ease
Rat
e (k
J/D
eg) B
ulk Cylinder G
as Temperature (K
)
CA (Deg)
LTHR
-0.001
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
300
400
500
600
700
800
900
300 320 340 360 380 400 420App
aren
t Hea
t Rel
ease
Rat
e (k
J/D
eg) B
ulk Cylinder G
as Temperature (K
)
CA (Deg)
-0.001
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
300
400
500
600
700
800
900
1000
300 320 340 360 380 400 420App
aren
t Hea
t Rel
ease
Rat
e (k
J/D
eg) B
ulk Cylinder G
as Temperature (K
)
CA (Deg)
-0.01
0.01
0.03
0.05
0.07
0.09
0.11
50
250
450
650
850
1050
1250
1450
1650
300 320 340 360 380 400 420App
aren
t Hea
t Rel
ease
Rat
e (k
J/D
eg) B
ulk Cylinder G
as Temperature (K
)
CA (Deg)
LTHR
HTHR
CR=7.05
CR=8.47 CR=9.95
CR=7.69
Ignition Behavior of Test Fuels at CR=7.05
-0.002
0
0.002
0.004
0.006
0.008
0.01
320 330 340 350 360 370 380App
aren
t Hea
t Rel
ease
Rat
e (k
J/D
eg)
CA (Deg)
N-Heptane
Methyl Heptanoate Ethyl
Hexanoate
The presence of ester functionality suppresses low temperature reactivity
Methyl heptanoate is more reactive than ethyl hexanoate in the low temperature
regime due to the higher possible number of 1,5-H and 1,6-H atom transfers available for the isomerization of peroxy alkyl ester radicals during methyl heptanoate oxidation than during ethyl hexanoate oxidation
Gaseous Species Concentrations
Methyl Heptanoate
Ethyl Hexanoate
In the early stage of oxidation, CO is mainly formed through the decomposition of
aldehydes; small olefins are produced through C-C bond
β-scission reactions
The higher concentration of CO for methyl heptanoate in the early stage of oxidation further confirms the higher low-temperature reactivity of methyl heptanoate
More fuel carbon is observed to be converted to ethylene during ethyl hexanoate
oxidation than during methyl heptanoate oxidation despite that ethyl hexanoate is less
reactive than methyl
heptanoate in the low
temperature regime
0
0.5
1
1.5
2
2.5
3
3.5
6.5 7 7.5 8 8.5 9 9.5 10 10.5
CO
and
CO
2 Mol
e Fr
actio
ns (%
)
Compression Ratio
CO2
CO
0
100
200
300
400
500
600
700
6.5 7 7.5 8 8.5 9 9.5 10 10.5
C2H
4 and
C3H
6 Mol
e Fr
actio
ns (p
pm)
Compression Ratio
C2H
4
C3H
6
0
0.5
1
1.5
2
2.5
3
3.5
6.5 7 7.5 8 8.5 9 9.5 10
CO
and
CO
2 Mol
e Fr
actio
ns (%
)
Compression Ratio
CO2
CO
0
100
200
300
400
500
600
700
6.5 7 7.5 8 8.5 9 9.5 10C
2H4 a
nd C
3H6 M
ole
Frac
tions
(ppm
)
Compression Ratio
C3H
6
C2H
4
Condensed Species Collected from the Oxidation of Ethyl Hexanoate at CR= 7.69
Methyl Esters AldehydesEthyl 2-Propenoate Ethyl 3-Butenoate Ethyl 4-Pentenoate Ethyl 3-HexenoateEthyl HexanoateEthyl 2-HexenoateEthyl 2,5-Epoxyhexanoate
Ethyl 3,5-Epoxyhexanoate
Ethyl 2,3-Epoxyhexanoate
2-methyl-5-butyl-1,3-dioxolan-4-one
Acetaldehyde AcroleinPropanalButanal 2-Butenal Pentanal
AcidsFormic AcidAcetic AcidAcrylic AcidHexanoic Acid
Ketone2-Butanone
Cyclic EtherEthylene Oxide
AlkenesPropene1-Butene1,3-Butadiene1-Pentene2-Pentene
Reaction Pathways Analysis
Type Ia. C-C bond scission to form aldehydes, unsaturated esters with a terminal double bond and hydroxyl radicals
Type Ib. C-O bond scission to form unsaturated esters and hydroperoxy radicals
Type Ic. O-O bond scission to form epoxy esters and hydroxyl radicals
1,4-H atom transfer
1,6-H atom transfer
1,5-H atom transfer
1,6-H atom transfer
Type II. reaction pathways leading to low temperature chain branching
The observation of more fuel carbon being converted to C2
H4
during ethyl hexanoate oxidation together with the identification of hexanoic acid among the reaction intermediates from the oxidation of ethyl hexanaoate provide the
evidence for the existence of the six-centered unimolecular elimination reaction during low
temperature oxidation of ethyl esters
Based on the above analysis, major low temperature oxidation pathways of fatty acid ethyl esters can be summarized as follows
Relative Abundance of Unsaturated Ethyl Esters
0
5
10
15
20
25
6.5 7 7.5 8 8.5
Ethyl 2-PropenoateEthyl 3-ButenoateEthyl 4-PentenoateEthyl 3-HexenoateEthyl 2-Hexenoate
Compression Ratio
Mol
e Fr
actio
ns o
f Uns
atur
ated
Est
ers
(% o
f unr
eact
ed fu
el)
The appreciably higher abundance of ethyl 2-propenoate than the rest of the unsaturated esters suggests the abstraction of H-atoms on the α-
carbon of the ester carbonyl
group plays an important role in
the oxidation of ethyl hexanoate
The preferred H-atom abstraction
on the α-
carbon of the ester carbonyl group among the C-H
bond breakings is primarily due to the electron-withdrawing nature of the oxygen atom in the ester carbonyl group
Ethyl 2-propenoate can be formed mainly through the reaction
pathways shown on the left
Ignition Studies of Unsaturated C9 Fatty Acid Methyl Esters
CR=6.08, Tin
= 250 °C, Equivalence Ratio=0.25
Low temperature reactivity follows the
order: Methyl Nonanoate > Methyl 2-
Nonenoate >Methyl 3-Nonenoate
The presence of a double bond in the
aliphatic chain of fatty acid esters inhibits low temperature reactivity; the inhibition effects becomes more pronounced as the double bond moves toward the center of the aliphatic chain
Unsaturated fatty esters are less reactive in the low temperature region due to the fact that the presence of double bonds inhibits the formation of six-
and seven-
membered transition state rings for
internal H-atom transfers that are
important in low temperature oxidation
-0.002
0
0.002
0.004
0.006
0.008
0.01
320 330 340 350 360 370 380 390 400App
aren
t Hea
t Rel
ease
Rat
e (k
J/D
eg)
CA (Deg)
-0.001
-0.0005
0
0.0005
0.001
0.0015
330 340 350 360 370 380 390 400App
aren
t Hea
t rel
ease
Rat
e (k
J/D
eg)
CA (Deg)
-0.001
-0.0005
0
0.0005
0.001
0.0015
330 340 350 360 370 380 390 400
App
aren
t Hea
t Rel
ease
Rat
e (k
J/D
eg)
CA (Deg)
Methyl Nonanoate
Methyl 3-Nonenoate
Methyl 2-Nonenoate
Selected Condensed Species Collected from the Oxidation of Methyl 3-Nonenoate at CR=5.54
The presence and high abundance of hexanal and methyl 2-propenoate implies methyl 3-nonenoate experiences the following olefin-like reaction sequence
Species Relative Peak AreaMethyl 2-Propenoate 0.16Butanal 0.3Pentanal 0.2Hexanal 1
LTHR magnitude is dependent on CN and LTHR magnitude is dependent on CN and equivalence ratioequivalence ratio
0
5
10
15
0.0 0.5 1.0 1.5 2.0
LTH
R (%
of t
otal
)
Equivalence ratio
○
#2 diesel-light cut◊
FT diesel-light cut■
methyl decanoate
••
LTHR magnitude trends with derived cetane numberLTHR magnitude trends with derived cetane number
FT diesel-light cut > methyl decanoate > #2 diesel-light cut
••
Methyl decanoate LTHR likely overMethyl decanoate LTHR likely over--predicts LTHR of biodieselpredicts LTHR of biodiesel–
Aliphatic chain is responsible for LTHR, not methyl ester–
Over 50% of soy-based biodiesel is comprised of species with multiple unsaturations
–
Unsaturated species exhibit less LTHR than saturated species
CH3
O
O
CH3
methyl decanoate
Szybist and Boehman, DEER 2006
2
4
6
8
10
12
14
5.5 6 6.5 7 7.5 8 8.5
LTH
R%
Compression Ratio
Ethyl Nonanoate
Methyl Nonanoate
Methyl 2-Nonenoate
Conclusions
The alkyl chain of fatty acid esters experience the typical paraffin-
like low temperature oxidation sequence; the alkyl chain length of fatty acid esters has a crucial impact on the ignition behavior of fatty acid esters
The six-centered unimolecular
elimination reaction occurs during the early stage oxidation of fatty acid ethyl esters
The abstraction of H-atoms on the α-carbon of ester carbonyl
group plays an important role in the oxidation of fatty acid esters
The presence of a double bond in the aliphatic chain of fatty acid esters suppresses the low temperature reactivity of fatty acid
esters; the inhibition effect becomes more pronounced as the
double bond moves toward the center of the aliphatic chain of fatty acid esters
For unsaturated esters, the autoignition
can undergo olefin ignition pathways
Acknowledgements
National Biodiesel Board for support of this study
Infineum
USA L.P. for support of the C9 methyl ester study
Dr. Yi Yang (now at Sandia National Laboratory) and Dr. Dania Fonseca of Penn State University
Prof. Rodolphe
Minetti
of Université
des Science et Technologies de Lille
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