The gas-phase oxidation of n-hexadecane

Article abstract of DOI:10.1002/kin.1053

Since n-hexadecane or cetane is a reference fuel for the estimation of cetane numbers in diesel engines, a detailed chemical model of its gas-phase oxidation and combustion will help to enhance diesel performance and reduce the emission of pollutants at t

Full text of DOI:10.1002/kin.1053

The Gas-Phase Oxidation
Of n-Hexadecane
1
1
1
1
1
1
ˆ
R. FOURNET, F. BATTIN-LECLERC, P. A. GLAUDE, B. JUDENHERC, V. WARTH, G. M. COME,
G. SCACCHI,1 A. RISTORI,2 G. PENGLOAN,2 P. DAGAUT,2 M. CATHONNET2
1De´partement de Chimie-Physique des Re´actions, UMR 7630 CNRS, INPL-ENSIC 1, rue Grandville—BP 451—54001
Nancy Cedex, France
2Laboratoire de Combustion et Syste`mes Re´actifs, CNRS, 1C av. de la Recherche Scientifique, 45071 Orleans Cedex 2,
France
Received 15 November 2000; accepted 18 May 2001
ABSTRACT: Since n-hexadecane or cetane is a reference fuel for the estimation of cetane num-
bers in diesel engines, a detailed chemical model of its gas-phase oxidation and combustion
will help to enhance diesel performance and reduce the emission of pollutants at their outlet.
However, until recently the gas-phase reactions of n-hexadecane had not been experimentally
studied, prohibiting a validation of oxidation models which could be written.
This paper presents a modeling study of the oxidation of n-hexadecane based on ex-
periments performed in a jet-stirred reactor, at temperatures ranging from 1000 to 1250 K,
1-atm pressure, a constant mean residence time of 0.07 s, and high degree of nitrogen dilution
(0.03 mol% of fuel) for equivalence ratios equal to 0.5, 1, and 1.5. A detailed kinetic mechanism
was automatically generated by using the computer package (EXGAS) developed in Nancy. The
long linear chain of this alkane necessitates the use of a detailed secondary mechanism for
the consumption of the alkenes formed as a result of primary parent fuel decomposition. This
high-temperature mechanism includes 1787 reactions and 265 species, featuring satisfactory
C
agreement for both the consumption of reactants and the formation of products. 2001 John
Wiley & Sons, Inc. Int J Chem Kinet 33: 574–586, 2001
a stoichiometric cetane–air mixture at 13.6 bar. How-
ever, because of the lack of experimental data for
n-hexadecane oxidation, they compared their simula-
tions with experimental results obtained for n-heptane.
More generally there are very few studies of the com-
bustion or the oxidation of actual components of Diesel
fuel heavier than n-decane [5].
The manual generation of detailed mechanisms for
compounds as heavy as n-hexadecane becomes diffi-
cult, whereas an automatic procedure is a convenient
and rigorous way to write such large mechanisms.
It is worth noting that the mechanism proposed by
Chevalier et al. [4] was also developed by a compu-
tational technique based on generic rules of oxidation
reactions. However, the mechanism represents only
the formation of primary products in low temperature
INTRODUCTION
Cetane or n-hexadecane is a reference fuel for the esti-
mation of cetane numbers,which characterize the abil-
ity of Diesel fuels to auto-ignite. The cetane number
of n-hexadecane is equal, by definition, to 100. De-
spite the importance of this compound for diesel en-
gines, its gas-phase combustion and oxidation had not
been experimentally studied until recently [1]. How-
ever, the pyrolysis of n-hexadecane and its steam-
cracking have been investigated [2,3]. Chevalier et al.
[4] proposed a mechanism to model the autoignition of
Correspondence to: R. Fournet; e-mail: fournet@ensic.inpl-
nancy.fr.
c
2001 John Wiley & Sons, Inc.

GAS-PHASE OXIDATION OF n-HEXADECANE
575
conditions and it is just able to predict ignition delays
but no product profiles. In this work, we used the com-
puter package (EXGAS) to perform the automatic gen-
eration of a detailed kinetic model to reproduce experi-
mental results obtained in a jet-stirred reactor (JSR) [1].
This package and its application to model the oxidation
of several alkanes (e.g., n-butane [6], n-heptane and
iso-octane [7], n-octane [8], and n-decane [8,9]) have
been described previously [10]. Because of the long
linear chain of n-hexadecane, it was necessary to up-
date EXGAS in order to improve the secondary mech-
anism of alkenes which are primary products formed
in large amounts. Thus, primary alkenes consumption
reactions were detailed and a feedback between pri-
mary and secondary mechanisms was introduced for
alkyl free radicals formed during the consumption of
alkenes. Finally, an analysis of the mechanism permit-
ted to point out the relative importance of the various
reaction pathways.
features of EXGAS is provided for completeness.
Only the part concerning the secondary mechanism of
alkenes, which has been improved during this work,
is more thoroughly described. Examples of mecha-
nisms generated by EXGAS (i.e., for the oxidation of n-
butane, n-heptane, iso-octane, n-octane, and n-decane)
can be found on the web site:
http://www.ensic.inpl-nancy.fr/ENSIC/ DCPR
/cinetique/Chimieinfo/TEXTES/software.htm
Construction of the Mechanism
The system of generation (EXGAS) provides reaction
mechanisms made of three parts:
A C0–C6 reaction base, including all the reac-
tions involving radicals or molecules containing
less than three carbon atoms (C0–C2 reaction
base [13]) and the reactions of C3–C4 unsaturated
hydrocarbons [14], such as propyne, allene, or bu-
tadienes, including reactions leading to the forma-
tion of benzene. The kinetic data used in these re-
action bases were taken from the literature. Values
recommended by Baulch et al. [15] and Tsang
et al. [16] are mainly used in the C0–C2 reaction
base; the pressure-dependent rate constants follow
the formalism proposed by Troe [17].
A comprehensive primary mechanism, where the
only molecular reactants considered are the ini-
tial organic compounds and oxygen. The primary
mechanism includes only elementary steps. The
generation of this mechanism is performed us-
ing an algorithm warranting its comprehensive-
ness [10]. Above 1000 K, primary mechanisms
can be strongly reduced, since the rate of the ad-
ditions of alkyl radicals to an oxygen molecule
becomes negligible compared to that of decompo-
sitions and oxidations [18]. The types of reactions
that are considered are presented in Table I, along
with the corresponding rate constants used.
EXPERIMENTAL SETUP
The JSR setup used for studying n-hexadecane oxida-
tion is similar to that described earlier [11,12]. The re-
actor consisted of a 40-mm diameter (30.5 cm3) sphere
made of fused silica (to prevent wall catalytic reac-
tions), equipped with four nozzles of 1-mm ID for the
admission of the gases, which achieve the stirring. A ni-
trogen flow of 100 l/h was used to dilute the fuel; all the
gases were preheated before injection in order to mini-
mize temperature gradients inside the JSR. High-purity
reactants were used in these experiments: oxygen was
99.995% pure and n-hexadecane >99% pure [1]. The
samples (30 Torr, i.e. 4 kPa) were taken at steady
temperature and residence time via some probe sam-
pling. They were analyzed on-line by GC–MS (low-
vapor pressure species) and off-line after collection in
1-l Pyrex bulbs (high-vapor pressure species and per-
manent gases). These experiments were performed at
steady state, constant mean residence time of 0.07 s,
and high degree of dilution (0.03 mol% of fuel). A
carbon balance of 5% was obtained in the present
experiments [1]. Inlet concentrations of C16H34 are
A secondary mechanism describes the consump-
tion of the products formed in the primary mecha-
nism that do not react in the reaction base (C0–C6).
4
the same for all equivalence ratios (3 10 in mole
fraction) while the inlet concentrations of molecular
In the case of the long linear molecule of
n-hexadecane, the primary mechanism leads to the for-
mation of 1-alkenes by decomposition of alkyl free
radicals. Even if 1-alkenes are very important prod-
ucts and have to be considered in more details than
in our previous work [6–9], the significant number
of 1-alkenes generated prevents the writing of a sec-
ondary mechanism as comprehensive as the primary
2
oxygen are equal to 1.47 10 at 8 = 0.5, 7.35
10 3 at 8 = 1 and 4.9 10 3 at 8 = 1.5.
THE GENERATION OF THE MODEL
The main features of EXGAS has been detailed pre-
viously [6–10]; hence only a summary of the main

576
FOURNET ET AL.
Table I Quantitative Structure–Reactive Relationships Used for the Estimation of the Rate Constanta Used in the
Primary Mechanism for the Oxidation of n-Hexadecane
Primary H
Secondary H
(i.e. R CH3)
(i.e. R1 CH2 R2)
Types of H Abstracted
(per H-atom)
log A
b
E
log A
b
E
Reference
[19]
Initiation with O2
12.62
0
205
13.00
0
201
to form alkyl radicals and
OOH
Oxidation
i.e. the reaction of alkyl
11.37
0
20.9
11.90
0
20.9
Estimated
from
radicals with O2 to form the
[20,21]
conjugated alkene and OOH
H atom abstraction by
O
H
OH
CH3
OOH
CHO
CH2OH
OCH3
OOCH3
C2H5
iC3H7
Rprimary
Rsecondary
13.23
6.98
5.95
1
11.30
4.53
1.52
10.73
12.30
11.00
2.85
11.00
11.00
0
2
2
4
32.8
32.2
1.88
13.11
6.65
6.11
11.0
11.30
6.73
0
2
2
0
0
1.9
2.95
0
0
0
4.2
0
0
21.7
20.9
3.20
[19]
[19]
[22]
[19]
[19]
[23]
[23]
[23]
[19]
34.3
71.1
77.3
58.5
30.5
83.6
56.4
36.4
56.4
60.6
39.8
64.8
71.1
50.2
18.8
73.2
46.0
33.4
46.8
51.0
0
2.5
2.95
0
0
0
4.2
0
0
1.48
10.68
12.18
11.00
2.85
11.00
11.00
Estimated
Estimated
[24]
[24]
Other reactions
Beta-scission
of a free radicals
to CH3+ moleculeb
to R + moleculeb
13.30
13.30
0
0
130.0
120.0
Estimated
from [19,23,25]
by breaking of a C C bond
Isomerizations
involving the internal transfer of
an hydrogen atom
Calculated by using the software
KINGAS according to the method
proposed by S.W. Benson [26]
[26,27]
[26,27]
Unimolecular initiations
by breaking of a C C bond
Calculated by using the software
KINGAS using the modifiedm
collision theory
aRate constants are assumed to follow the modified Arrhenius law, k = AT b exp( E/RT ), with the units cm3, mol, s, kJ.
bValues given for log A, b, and E.
one. Nevertheless, it is necessary to write for the
consumption of 1-alkenes less global reactions than
those usually written and to create for secondary free
radicals for which that is possible, a feedback with the
primary mechanism. By this coupling effect between
the two parts of the mechanism, it is possible to better
describe the combustion of primary 1-alkenes.
The reactions considered in the secondary mecha-
nism for alkene consumption are unimolecular initi-
ations, additions to the double bond, H-abstractions,
retro-ene reactions, and combinations of allylic
free radicals.
Unimolecular Initiations. Because of their relatively
low activation energy (approximately 290 kJ/mol),
only unimolecular initiations leading to the formation
of allyl radical ( C3H5) have been considered. The ki-
netic parameters used have been estimated using the
software KINGAS [26].
Additions. Table II presents the secondary reactions
proposed for 1-alkenes and the related rate constants
used for additions to the double bond. The addition
of H atom and CH3 radicals leads to the formation
of alkylic radicals which react in the detailed primary

GAS-PHASE OXIDATION OF n-HEXADECANE
577
Table II Secondary Mechanism of Alkenes: Detailed Reactiona and Rate Constantsb for the Additions of Radicals to
the Double Bond
Type of Addition
log A
b
E
Addition of an H atom
(A) H + CH2 CH CnH2n+1 ↔ CH3 CH CnH2n+1
12.86
12.86
0
0
2.6
5.8
(B) H +CH2 CH CnH2n+1 ↔ CH2 CH2 CnH2n+1
Addition of CH3
(C) CH3 + CH2 CH CnH2n+1 ↔ CH3 CH2 CH CnH2n+1
(D) CH3 + CH2 CH CnH2n+1 ↔ CH2 CH(CH3) CnH2n+1
CH2 CH(CH3) CnH2n+1 → C3H6 + CnH2n+1
11.0
11.0
13.3
0
0
0
14.7
16.0
57.4
Addition of OH
(E) OH + CH2 CH CnH2n+1 ↔ HCHO + CH2 CnH2n+1
12.10
12.10
0
0
2.1
2.1
(F) OH + CH2 CH CnH2n+1 ↔ CnH2n+1 CHO + CH3
Addition of O -atoms
(G) O + CH2 CH CnH2n+1 ↔ H + CH2CO CnH2n+1
7.53
13.3
1.83
0
1.1
57.4
CH2CO CnH2n+1 → CH2CO + CnH2n+1
Addition of HO2
(H) HO2 + CH2 CH CnH2n+1 ↔ cyclo(C2H3O) CnH2n+1 + OH
12.0
0
28.8
aThe reactions of all the free alkyl radicals formed ( CnH2n+1) are included in the primary detailed mechanism.
bThe rate constants for the additions of H, CH3, O , and OH are derived from those proposed by Tsang [28] for propene. In the case
of the addition of HO2 , the values of Baldwin et al. [29] are used. Rate constants are assumed to follow the modified Arrhenius law,
k = AT b exp( E/RT), with the units cm3, mol, s, kJ.
mechanism (reactions A–C in Table II). A particu-
lar point concerns the addition of CH3 leading to a
branched alkyl free radical. As this last radical does not
exist in the primary mechanism, a ␤-scission is written
in order to give propene and a linear alkyl radical which
reacts in the detailed primary mechanism (reaction D
in Table II).
The additions of O atoms, HO2 , and OH radi-
cals (reactions E–H in Table II) lead to the formation
of alkyl free radicals which decomposed according to
steps already written in the primary mechanism, or of
stable species (aldehydes and cyclic ethers) which react
in the secondary mechanism according to rules previ-
ously defined [6].
2. The abstraction of a vinylic H-atom. This leads to
the formation of acetylene or allene and an alkyl
freeradical(reactionsBandCinTableIII). These
H-abstractions being more difficult than the pre-
vious ones, only those involving H, OH, and
CH3 free radicals have been taken into account.
3. The abstraction of an alkylic H-atom. It is worth
noting that these reactions were never taken into
account in our previous studies because the linear
chain of alkenes formed were sufficiently small,
and so the alkylic H-abstraction were negligible
with respect to the allylic metatheses. In the case
of long linear alkenes, these H-abstractions play
a significant role and explain the formation of
specific products observed experimentally, such
as diolefins (e.g. C5H8). The reactions included
in the kinetic scheme are derived from an analysis
of the detailed reactions of linear 1-alkenes and
are summarized in Table IV.
H-Abstractions. Tables III and IV present the sec-
ondary reactions proposed for 1-alkenes reactions
and their rate constants for the abstraction of allylic
(Table III), vinylic (Table III), and alkylic (Table IV)
hydrogen atoms.
The abstraction of an alkylic H-atom leads
to an alkenyl radical. In the temperature range
1000–1200 K, this alkenyl radical reacts accord-
ing to two main pathways: isomerization and
␤-scission (Table IV).
1. The abstraction of an allylic H-atom. This leads
to the formation of a resonance-stabilized radi-
cal. This radical can react by terminations with
radicals such as H, CH3, and HO2 or by
␤-scissions to give 1,3-butadiene and an alkyl
free radical reacting according to the detailed pri-
mary rules of oxidation (reaction A in Table III).
Alkenyl radical isomerization gives a
resonance-stabilized free radical (allylic
radical). Thefastestisomerizationsarethose

578
FOURNET ET AL.
Table III Secondary Mechanism of Alkenes: Detailed Reactionsa and Rate Constantsb for Abstractions of Allylic and
Vinylic H Atoms
Type of Abstraction
Abstraction of an allylic H atom
R
log A
4.45
0.2
0.15
6.17
b
E
H
CH3
C2H5
OH
2.5
3.5
3.5
2
0.6
7.1
8.7
(A) R + CH2 CH CnH2n+1 → RH + C4H6 + Cn 2H2(n
2)+1
3.0
O
HO2
10.5
3.5
0.7
2.6
15.3
27.8
Abstraction of an vinylic H atom
H
OH
CH3
5.61
6.04
8.8 10
2.5
2
3.5
40.9
6.1
48.9
(B) R + CnH2n → RH + aC3H4 + C(n 3)H2(n
3)+1
2)+1
3
H
OH
CH3
5.91
6.34
0.13
2.5
2
3.5
51.3
11.6
53.9
(C) R + CnH2n → RH + C2H2 + C(n 2)H2(n
aThe reactions of all the alkyle free radicals formed ( CnH2n+1) are included in the primary detailed mechanism.
involving a cyclic transition state containing
five or six atoms. At 1100 K, the isomer-
ization of an alkenyl radical to give an al-
lylic radical is about 5–10 times faster than
␤-scission. Consequently, if the alkenyl
free radical obtained by H-abstraction can
isomerize via a cyclic transition state con-
taining five or six atoms, the formation of
an allylic radical is written. This allylic rad-
ical is decomposed by ␤-scission in 1,3-
butadiene and an alkyl radical which reacts
in the primary mechanism (reaction A in
Table IV). In the following example, the
alkenyl free radical C7H13 can isomerize
and gives an allylic radical, which decom-
poses in 1,3-butadiene and n-propyl radical
(1)
The reaction written in the secondary mech-
anism is then
C7H14 + R → RH + 1,3-C4H6
+ n- C3H7
(2)
Table IV Secondary Mechanism of Alkenes: Detailed Reactionsa and Rate Constantsb for Abstractions of alkylic
H Atoms
Abstraction of an Alkylic H Atom
Examples (C10H20)
(A) R + CnH2n → RH + 1,3-C4H6 + Ci H2i+1
R + C10H20 → RH + 1,3-C4H6 + 1- C6H13
(4 secondary H atoms involved)
(B) R + CnH2n → RH + CmH2m 2 + Ci H2i+1
R + C10H20 → RH + C5H8 + 1- C5H11
R + C10H20 → RH + C6H10 + C4H9
R + C10H20 → RH + C9H16 + CH3
C3H5
R + C10H20 → RH + 1-C8H16 + C2H3
(C) R + CnH2n → RH + CmH2m
+
C3H2
(D) R + CnH2n → RH + CmH2m+ 1,3-C4H6 + Ci H2i+1
R + C10H20 → RH + 1-C7H14 + C3H5
R + C10H20 → RH + C4H8 + 1,3-C4H6 + C2H5
R + C10H20 → RH + C3H6 + 1,3-C4H6 + 1- C3H7
R + C10H20 → RH + C2H4 + 1,3-C4H6 + 1- C4H9
(3 primary H atoms involved)
n
10
(E) R + CnH2n → RH + CmH2m+ 1,3-C4H6 + C2H4 + Ci H2i+1
a
Ci H2i+1 reactsintheprimarymechanism;Cm H2m reactsinthesecondarymechanismas1-alkene;Cm H2m 2 reactsinthesecondarymechanism.
bThe rate constants for alkylic metatheses derived from those presented in Table I.

GAS-PHASE OXIDATION OF n-HEXADECANE
579
If an isomerization of the alkenyl radical is
not possible, a ␤-scission of a C C bond
occurs giving an alkyl radical and a diolefin
(reaction 3) or an 1-alkene and a smaller
alkenyl radical (reaction 4). For instance,
the alkenyl radical 3-C11H21 can react as fol-
lows:
(7)
(3)
(4)
This reaction corresponds to reaction E in Table IV.
The reaction of H-abstraction written in the
secondary mechanism for reaction (3) is
then
Retro-ene Reactions. Another new type of reaction
considered is retro-ene reaction [30,31]. A retro-ene
reaction is a 1,5-hydrogen shift reaction followed by
dissociation. The detailed mechanism of these reac-
tions has been discussed and structures of the transition
state have been explored [31]. For example in the case
of 1-heptene, the transition state can be represented as
follows:
C11H22 + R → RH + C10H18 + CH3
(5)
This reaction (reaction B in Table III)
formed 1-alkene, which in turns reacts in
the secondary mechanism according to the
rules previously described.
For the reaction 4, the 1-alkenyl radical
formed must still undergo specific reactions.
In fact, several cases can be viewed:
(8)
In the case of linear 1-alkenes, the products formed are
propene and lower 1-alkenes:
If the unsaturated radical formed in reac-
tion 4 is small ( C3H5 and C2H3), it can-
not undergo isomerization or ␤-scission
of a C C bond (reaction C in Table IV).
1-CnH2n ↔ C3H6 + C(n 3)H2(n
(9)
3)
A pre-exponential factor of 8 1012 s 1 and an activa-
tion energy of 236 kJ/mol has been used based on the
results presented in Ref. 30. As shown in the follow-
ing part of this paper, the retro-ene reactions are very
important to explain the formation of C3H6.
The diolefins formed in the secondary mechanism
can react in the same ways as 1-alkenes. We consid-
ered unimolecular initiations, additions of free radicals,
H-abstractions, and retro-ene reactions. The species
formed are smaller 1-alkenes and diolefins, which react
again in the secondary mechanism.
If the alkenyl radical can isomerize, we
considered isomerization yielding an al-
lylic radical, followed by a ␤-scission
producing 1,3-butadiene and an alkyl
radical (reaction D of Table IV). In the
previous example (reaction 4), 1-C7H13
isomerizes and the reaction written in the
secondary mechanism is
Thermochemical data for molecules and radicals are
automatically calculated and stored as 14 polynomial
coefficients, according to the CHEMKIN II formalism
[32]. These data were calculated using the software
THERGAS [33], based on the group and bond additiv-
ity methods proposed by Benson [27].
C11H22 + R → RH + 1,3-C4H6
+1-C4H8 + 1-C3H7
(6)
For larger alkenes (C
10), 1-alkenyl
free radical produced by reaction 4
cannot isomerize because of the too large
cyclic transition state. In this case, it can
only react by successive ␤-scissions un-
til the new alkenyl free radical produced
can isomerize. For instance, the alkenyl
free radical 1- C12H23 reacts according
to the following scheme:
COMPARISON BETWEEN COMPUTED
AND EXPERIMENTAL RESULTS
Simulations have been performed using the software
PSR of CHEMKIN II [32]. In order to keep this

580
FOURNET ET AL.
paper reasonably short, the kinetic mechanism used
is not presented, but it is available on request. The
primary mechanism involves 87 species and includes
346 reactions and the secondary mechanism involves
91 species and includes 815 reactions. Overall, the re-
action scheme includes 1787 reactions involving 265
species.
Figures 1–7 present a comparison of the computed
profiles and the experimental results [1]. The agree-
ment obtained for the conversions of n-hexadecane
(Fig. 1a) and oxygen (Fig. 1b) is satisfactory. Com-
putations capture the conversion of n-hexadecane
without significant consumption of oxygen below
1100 K. However, above 1050 K, the experiments show
a stronger dependence of fuel conversion with increas-
ing temperature than the calculations. A sharp decrease
in oxygen is observed at a higher temperature and for
equivalence ratios equal to 0.5 or 1. Simulations also
show that the temperature at which the concentration
of oxygen drops, increases with the equivalence ratio.
However, a difference of 20 K is observed between
experimental data and modeling results for an equiv-
alence ratio of 1. It is worth noting that we have used
the same reaction bases and the same set of kinetic
parameters (see Tables I–IV) in the primary and sec-
ondary mechanisms generated by EXGAS as in our
Figure 2 Oxidation of n-hexadecane in a jet-stirred reactor
at 1 atm and a residence time of 70 ms for 8 = 0.5. Evo-
lution of the concentrations of products with temperature.
The points refer to experimental observations and the curves
come from simulations.
previous studies [6–9]. However, some kinetic param-
eters of the C0–C2 reaction base have been updated and
are presented in Table V.
Itisworthnotingthattherateconstantofthereaction
CH3 + HO2 → CH3O + OH
(10)
is relatively sensitive and the uncertainty factor given
by Baulch et al. [15] is about 10.
The formation of alkenes such as 1-pentene,
1-heptene, 1-octene, and 1-nonene (Figs. 2c, 3a, 4c,
5a, 6c, and 7a) are adequately reproduced. Hence, the
primary production and the secondary consumption of
these products are mostly correct. However, we observe
a lack of conversion of these intermediate products be-
tween 1050 and 1150 K. This problem is more signifi-
cant at low equivalence ratios. The formation of other
importantalkenessuchaspropene(Figs. 2b, 4b, and6b)
and 1-butene (Figs. 2c, 4c, and 6c) is satisfactory and
thus validates the different ways of production and con-
sumption of these species. On the other hand, a problem
arises for the formation of ethylene which is always un-
derestimated by a factor up to 2 at high temperature.
Figure 1 Oxidation of n-hexadecane in a jet-stirred reactor
at 1 atm and a residence time of 70 ms. Evolution of the
conversions of alkane (a) and oxygen (b) with temperature.
The points refer to experimental observations and the curves
come from simulations.

GAS-PHASE OXIDATION OF n-HEXADECANE
581
Table V Up-dated Reactions of the C0–C2 Reaction Base
Reactions
log A
b
E
References
CH3 + HO2 ↔ CH3O + OH
C2H4 + H ↔ C2H3 + H2
13.17
7.7
11.8
6.1
0
1.93
0
1.61
1.39
0.0
54.0
16.0
1.7
[34]
[35]
[36]
[37]
[37]
C2H4 + CH3 ↔ C2H3 + CH4
O2 + C2H3 ↔ C2H2 + HO2
O2 + C2H3 ↔ HCHO + HCO
16.6
4.2
aRate constants are assumed to follow the modified Arrhenius law, k = AT b exp( E/RT ), with the units cm3, mol, s, kJ.
This problem will be discussed in the following part of
this paper. The formation of 1,3-butadiene (Figs. 2b,
4b, and 6b) is well predicted, showing that the sec-
ondary reactions proposed for its formation are reason-
able. The formation of other unsaturated species such
as C5H8, acrolein, C3H4 (allene and propyne), C2H2, or
benzene (Figs. 3(b,c), 5(b,c), and 7(b,c)) are rather well
reproduced. The reactions of formation and consump-
tion of these last three species are essentially included
in the C0–C6 reaction base.
The shape of the oxygenated compounds (i.e., CO,
CO2, and formaldehyde) are correctly reproduced by
the simulations (Figs. 2a, 4a, and 6a), even if the yield
of final products (i.e., CO and CO2) is overestimated
for equivalence ratios of 1 and 1.5 at high temperatures
(T > 1100 K).
ANALYSIS OF THE MECHANISM
OF n-HEXADECANE OXIDATION
In order to better understand the results obtained and
to point out the main reaction pathways, a sensitivity
Figure 3 Oxidation of n-hexadecane in a jet-stirred reactor
at 1 atm and a residence time of 70 ms for 8 = 0.5. Evo-
lution of the concentrations of products with temperature.
The points refer to experimental observations and the curves
come from simulations.
Figure 4 Oxidation of n-hexadecane in a jet-stirred reactor
at 1 atm and a residence time of 70 ms for 8 = 1. Evolution of
the concentrations of products with temperature. The points
refer to experimental observations and the curves come from
simulations.

582
FOURNET ET AL.
Figure 5 Oxidation of n-hexadecane in a jet-stirred reactor
at 1 atm and a residence time of 70 ms for 8 = 1. Evolution of
the concentrations of products with temperature. The points
refer to experimental observations and the curves come from
simulations.
Figure 6 Oxidation of n-hexadecane in a jet-stirred reactor
at 1 atm and a residence time of 70 ms for 8 = 1.5. Evo-
lution of the concentrations of products with temperature.
The points refer to experimental observations and the curves
come from simulations.
analysis of the mechanism of n-hexadecane oxidation
was performed at 1100 K in the conditions of Fig. 4
(Fig. 8).
Alkenes
Primary 1-Alkenes (C > 3). Two flow rate analyses
have been carried out for temperatures of 1050 K
(Fig. 9) and 1200 K, an equivalence ratio of 1, and
a conversion of C16H34 of 85% for each case.
n-Hexadecane
The primary alkenes formed are all 1-alkenes and
are mainly produced by ␤-scissions of primary alkyl
free radicals. The flux of formation by these reactions
can reach 90% of their total production, as shown in
Fig. 9. A second way of formation of these species
is through retro-ene reactions. As previously described,
these reactions yield propene and 1-alkenes. However,
the flux of production is relatively small compared
to the flux of ␤-scissions. On the other hand, retro-
ene reactions are also important consumption reaction
for 1-alkenes.
Other important reactions of consumption are uni-
molecular initiations (UI) leading to the formation of
the allyl radical ( C3H5) and alkyl radicals reacting in
the primary mechanism. The importance of this chan-
nel increases with temperature and decreases with the
number of carbon atoms in the molecule, e.g., U.I.
The flow rate analysis shows that the consumption
of n-hexadecane is essentially due to the reactions of
H-abstraction by the free radicals H and OH. The
metathesis by methyl radical is only important at low
temperature (11% at 1050 K). An interesting point is
related to the role of unimolecular initiations which be-
come significant at high temperature (35% at 1200 K
against 14% at 1050 K). In fact, the sensitivity analysis
(Fig. 8) shows that the major reactions governing the
conversion of reactant are included in the C0–C2 reac-
tion base. The branching step between H atoms and
oxygen molecules to give O atoms and OH radicals
is especially important. As the concentrations of the
rather unreactive CH3, HO2 , and a-.C3H5 radicals are
relatively high, the reactions involving these species are
also important for the overall rate of reaction.

GAS-PHASE OXIDATION OF n-HEXADECANE
583
H-abstraction reactions must be differentiated accord-
ing to the type of H-atom (allylic or alkylic) which can
be abstracted. For large alkenes such as C12H24, the
allylic H-abstractions, represent about 14% of the total
consumption against 35% for alkylic H-abstractions,
while for C5H10, alkylic H-abstractions represent only
8% of its consumption against 17% for allylic metathe-
ses. So, even if the activation energies of alkylic H-
abstraction are greater than those involved in the al-
lylic metatheses, the high number of alkylic H-atoms
in large alkenes do not allow to neglect them.
The last type of reaction playing a role in the con-
sumption of alkenes is addition. The flux of consump-
tion of alkenes by these reactions is relatively small,
especially at high temperature. Thus, at 1050 K, the
reactions of addition consume approximately 9% of
C12H24 (addition of H and OH) against 5% at 1200 K
(addition of H and O ).
Propene. Propene is a significant product of the reac-
tion of oxidation of n-hexadecane and its formation dif-
fers from that of the other alkenes. Indeed, it is mainly
formed by retro-ene reactions according to the follow-
ing iterative process:
CnH2n → Cn 3H2(n 3) + C3H6
(11)
Figure 7 Oxidation of n-hexadecane in a jet-stirred reactor
at 1 atm and a residence time of 70 ms for 8 = 1.5. Evo-
lution of the concentrations of products with temperature.
The points refer to experimental observations and the curves
come from simulations.
This result can be explained by the fact that all linear
primary alkenes are 1-alkene and can react by retro-ene
reactions to give propene and another linear 1-alkene
which can react, in its turn, in the same way (iterative
process). Therefore, about 50% of propene is formed
by this type of reaction at 1050 K; the other way of
formation occurs by ␤-scissions of primary alkyl free
radicals.
Propene is mainly consumed by two ways: additions
and H-abstractions. At 1050 K, additions of H, O , and
OH free radicals represent about 50% of the global
consumption of C3H6:
represent 8% of the total consumption of C12H24 at
1050 K against 40% for C5H10 at 1200 K. The vari-
ation with the number of carbon atom is due to a
competition with H-abstraction reactions for which
the number increases with the size of alkene. In fact
H + C3H6 → C2H4 + CH3
O + C3H6 → H + CH3 + CH2CO
(17%)
(12)
(12%)
(13)
OH + C3H6 → HCHO + C2H5
(17%)
(14)
The H-abstraction reactions by H and OH free
radicals, leading to the formation of allyl radical
( C3H5), represent about 43% of the global flux of
consumption.
Ethylene. The formation of ethylene is mainly due to
␤-scissions of primary and secondary alkyl free radi-
cals (85%). These reactions involved C C and C H
Figure 8 Sensitivity analysis for the mole fraction of
n-hexadecane at 1100 K, in the conditions of Fig. 4.

584
FOURNET ET AL.
Figure 9 Rate analysis performed at 1050 K, an equivalence ratio of 1, and a conversion of n-hexadecane of 85% for C12H24
(a) and C5H10 (b). The numbers represent the percentage of the normalized flux of formation or consumption of the 1-alkene.
bonds and an important reaction is
C2H5 + (M) → C2H4 + H + (M)
The other problem is closely linked in a too fast
consumption of C2H4, at high temperature, and it
is well connected with the too high concentration
of final oxygenated products observed (CO, CO2)
(Figs. 2a, 4b, and 6a). At 1100 K, the flow rate
analysis shows clearly that CO is formed espe-
cially from formaldehyde via the radical HCO :
(15)
This reaction represents more than 30% of the total
production of C2H4. All the important reactions of con-
sumption of ethylene are contained in the C0–C2 reac-
tion base:
HCO + M → CO + H + M
O2 + HCO → CO + HO2
(41%) (21)
(33%) (22)
H + C2H4 → C2H3 + H2
O + C2H4 → CH3 + CHO
O + C2H4 → CH2CHO + H
OH + C2H4 → C2H3 + H2O
OH + C2H4 → CH3 + HCHO
(12%)
(22%)
(16)
(17)
(13%) (18)
(24%) (19)
(19%) (20)
HCO isformedbyH-abstractionofHCHO(about
50% of its global consumption) but also directly
from reactions involving C2H4 or C2H3:
A problem remains for the concentration of C2H4,
which is underestimated by the model. In fact, two
problems can be pointed out:
C2H4 + O → CH3 + HCO
O2 + C2H3 → HCHO + HCO
(14%)
(23)
(21%)
(24)
A lack of conversion of the reactant or large pri-
mary alkenes can lead to a deficit in the production
of ethylene. This problem arises in a temperature
ranging from 1050 K to 1150 K and can be ex-
plained by a lack of reactivity in the primary and
secondary mechanisms.
This last reaction is very important because it
forms two species directly related to the forma-
tion of CO. Moreover, a significant part of the
formaldehyde formed is due to a reaction directly

GAS-PHASE OXIDATION OF n-HEXADECANE
585
involving C2H4:
allows simulating properly the benzene formation in
the temperature range studied, i.e. 1000–1200 K.
OH + C2H4 → CH3 + HCHO
(20%) (25)
Acrolein. CH2CHCHO is formed mainly by the reac-
tion between allyl radical and HO2 :
Thus, it seems that the high rate of C2H4 con-
sumption observed in the simulations, and leading
to a too high formation of final products (CO, CO2,
CH4, . . .), is due to a too high reactivity of small
species involved in the C0–C2 base. This result is
corroborated by the sensitivity analysis; the reac-
tions with the highest sensitivity coefficients for
the consumption of ethylene are those involving
OH radicals, the metathesis giving vinyl radicals
(Eq. (19)) and the addition leading to formalde-
hyde and methyl radicals (Eq. (25)). In order to
improve the level of C2H4, we have increased, by
a factor 2, the rate of unimolecular initiations of
n-hexadecane. This factor is reasonable because
of the uncertainties in Benson’s methods used in
the software KINGAS. In addition, rate constants
of terminations between H atom and important
free radicals such as allyl ( C3H5) have been taken
to be equal to 2.0 1014 cm3/(mol s), according to
Ref. 28. These modifications have led to a better
agreement between experimental and simulated
C2H4 concentrations.
C3H5 + HO2 → CH2CHCHO + OH + H (76%)
(27)
This reaction is rather sensitive (see Fig. 8) because this
step acts as a branching step in the mechanism by con-
verting two unreactive free radicals into two reactive
ones.
The other way of production is the reaction between
1,3-butadiene and OH radicals:
1,3-C4H6 + OH → CH2CHCHO + CH3 (20%)
(28)
CONCLUSION
A kinetic detailed mechanism was built by auto-
matic generation in order to simulate the oxidation of
n-hexadecane in a JSR operating at 1 atm and for
0.5
8
1.5. The importance of large alkenes as ma-
jor primary products required significant refinements
of their consumption mechanisms in comparison with
our previous studies on alkane oxidation. Generic re-
actions were added, such as unimolecular initiations,
abstractions of alkylic hydrogen atoms, and retro-ene
reactions. To our knowledge, it is the first time that
retro-ene reactions are considered in a detailed com-
bustion mechanism. A feedback was carried out be-
tween the primary and secondary mechanisms, in or-
der to consume the alkyl free radicals formed during
the reaction of alkenes. Simulations performed with
the software CHEMKIN II reproduced, with a rea-
sonable agreement, experimental profiles for major
species (CO, CO2, CH4, . . .) as well as minor products
(C3H6, 1,3-C4H6, acrolein, benzene, . . .). However, a
too important reactivity of small species involved in the
C0–C2 reaction base was observed. A large part of this
too high reactivity results in a too important consump-
tion of ethylene and in concentrations of CO and CO2
higher than those observed experimentally.
Toxic pollutants
Model presented reproduces the reactivity of n-
hexadecane as well as the formation of pollutants like
CO, CO2 or formaldehyde. In addition, the model is
able to reproduce correctly the formation of some toxic
species observed experimentally.
1,3-C4H6. About 50% of the 1,3-butadiene formed
is produced by allylic H-abstraction from 1-alkenes
against 14% for alkylic metatheses. Another way
of production of 1,3-C4H6 is the ␤-scission of the
resonance-stabilized free radical C4H7. This radical
comes from H-abstraction from 1-butene and produces
20% of the amount of 1,3-C4H6 at 1050 K. On the other
hand, the consumption of 1,3-C4H6 is mainly due to re-
actions of additions (55%) and metatheses (28%).
Benzene. In our conditions, the formation of benzene
is essentially due to the reaction between allyl and
propargyl radicals according to the reaction proposed
by Marinov et al. [38]
The analysis of the mechanism showed that cetane
reacts mostly by H-abstractions followed by decom-
position of alkyl free radicals into 1-alkenes. The
importance of the different reaction channels for the
consumption of alkenes was studied. Retro-ene reac-
tions represent an important way that permits one to
reproduce correctly the formation of propene whereas
alkylic H-abstractions are very important consumption
C3H5 + C3H3 → C6H6 + H + H
(26)
It is worth noting that this reaction has been proposed
for flame conditions, i.e. for high temperature, but also

586
FOURNET ET AL.
paths for larger alkenes. This analysis also pointed
out the importance of unimolecular initiations at high
temperature (1200 K).
An important point in the modeling of diesel fuel is
the low-temperature chemistry and autoignition phe-
nomenon. Further experimental work should focus on
these conditions in order to validate low-temperature
models.
Pilling, M. J.; Troe, J.; Walker, R. W.; Warnatz, J. Com-
bust Flame 1994, 98, 59.
16. Tsang, W.; Hampson, R. F. J Phys Chem Ref Data 1986,
15(3).
17. Troe, J. Ber Buns Phys Chem 1974, 78, 478.
18. Glaude, P. A.; Battin-Leclerc, F.; Fournet, R.; Warth, V.;
Coˆme, G. M.; Scacchi, G. Combust Flame 2000, 122,
451.
19. Chevalier, C.; Warnatz, J.; Melenk, H. Bes Buns Phys
Chem 1990, 94, 1362.
20. Stocker, D. W.; Pilling, M. J. Communication at the 14th
Int Symp of Gas Kinetics, 1996.
BIBLIOGRAPHY
21. Walker, R. W. Reaction Kinetics, 1, Specialist Periodical
Report, The Royal Chemical Society, London, 1975.
22. Atkinson, R. Int J Chem Kin 1986, 18, 555.
23. Tsang, W. J Phys Chem Ref Data 1988, 19(1), 1.
24. Ranzi, E.; Sogaro, A.; Gaffuri, P.; Pennati, G.;
Westbrook, C. K; Pitz, W. J Comb Sci Technol 1994,
100, 299.
25. Allara, L. R.; Shaw, R. J Phys Chem Ref Data 1980, 9(3),
523.
26. Bloch-Michel, V. Logiciel d’estimation de parame`tres
cine´tiques de processus e´le´mentaires, Thesis, Nancy,
INPL, 1995.
27. Benson, S. W. ThermochemicalKinetics, 2nded.;Wiley:
New York, 1976.
28. Tsang, W. J Phys Chem Ref Data 1991, 20, 221.
29. Baldwin, R. R.; Stout, D. R.; Walker, R. W. J Chem Soc
Faraday Trans 1991, 87(4), 3147.
30. Richard, C.; Scacchi, G.; Back, M. H. Int J Chem Kin
1978, 10, 307.
31. O’Neal, H. E.; Benson, S. W. J Phys Chem 1967, 71,
2903.
32. Kee, R. J.; Rupley, F. M.; Miller, J. A. Sandia Laborato-
ries Report, SAND 89—8009B, Livermore, 1993.
33. Muller, C.; Michel, V.; Scacchi, G.; Coˆme, G. M. J Chim
Phys 1995, 92, 1154.
34. Scire, J. J.; Yetter, R. A.; Dryer, F. L. Int J Chem Kin
2001, 33, 75.
35. Knyasev, V.; Bencsura, A.; Stoliarov, S. I.; Slagle, I. Pro-
ceedingsofthe14thIntGasKineticsSymposium, Leeds,
1996.
36. Ahonkai, S. I.; Lin, X-H; Back, M. H. Int J Chem Kin
1989, 21, 1.
37. Mebel, A. M.; Diau, E. W. G.; Lin, M. C.; Morokuna,
K. J Am Chem Soc 1996, 118, 9759.
38. Marinov, N. M.; Pitz, W. J.; Westbrook, C. K.; Castaldi,
M. J.; Senkan, S. M. Combust Sci Tech 1996, 116/117,
211.
1. Ristori, A. Mode´lisation de la Combustion du gazole.
Etude des aspects chimiques, The`se de l’Universite´
d’Orle´ans (2000).
2. Doue´, F.; Guiochon, G. J Chim Phys 1968, 64, 396.
3. Depeyre, D.; Flicoteaux, C., Ind Eng Chem Res 1991,
30, 1116.
4. Chevalier, C.; Pitz, W. J.; Warnatz, J.; Westbrook,
C. K.; Melenk, H. Proc Combust Inst 1992, 24, 93.
5. Cavaliere, A.; Ciajolo, A.; D’Anna, A.; Mercogliano, R.;
Raguci, R. Combust Flame 1993, 93, 279.
6. Warth, V.; Stef, N.; Glaude, P. A.; Battin-Leclerc, F.;
Scacchi, G.; Coˆme, G. M. Combust Flame 1998, 114,
81.
7. Glaude, P. A.; Warth, V.; Fournet, R.; Battin-Leclerc, F.;
Coˆme, G. M.; Scacchi, G. Bull Soc Chim Belg 1997,
106(6), 343.
8. Glaude, P. A.; Warth, V.; Fournet, R.; Battin-Leclerc,
Scacchi, G.; Coˆme, G. M. Int J Chem Kin 1997, 30, 949.
9. Battin-Leclerc, F.; Fournet, R.; Glaude, P. A.; Judenherc,
B.; Warth, V.; Coˆme, G. M.; Scacchi, G. Proc Combust
Inst 2000, 28, 1597–1605 .
10. Warth, V.; Battin-Leclerc, F.; Fournet, R.; Glaude, P. A.;
Coˆme, G. M.; Scacchi G. Comput Chem 2000, 24(5),
541.
11. Dagaut, P.; Cathonnet, M.; Rouan, J. P.; Foulatier, A.;
Quilgars, A.; Boettner, J.-C.; Gaillard, F.; James, H.
J Phys E: Sci Instrum 1986, 19, 207.
12. Dagaut, P.; Reuillon, M.; Cathonnet, M. J Chim Phys
1995, 92, 47.
13. Barbe´, P.; Battin-Leclerc, F.; Coˆme, G. M. J Chim Phys
1995, 92, 1666.
14. Fournet, R.; Bauge, J. C.; Battin-Leclerc, F. Int J Chem
Kin 1999, 31, 361.
15. Baulch, D. L.; Cobos, C. J.; Cox, R. A.; Franck, P.;
Hayman, G. D.; Just, Th.; Kerr, J. A.; Murrells, T. P.;