JOURNAL
DE
PHYSIQUE
IV
Colloque C4, supplCment au Journal de Physique
ID,
Volume
5,
mai 1995
Thermochemistry and Reaction Mechanisms of Nitromethane Ignition
C.F. Melius
Combustion Research Facility, Sandia National Laboratories, Livemore, CA
94551-0969,
U.S.A.
Abstract:
The thermochemistry and reaction mechanisms of nitromethane initiation are
modeled using detailed chemical kinetics.
Initial conditions correspond to gaseous
nitromethane at atmospheric and liquid-like densities and initial temperatures between
1100 and 2000
K.
Global reactions as well as elementary reactions are identified for each
of the two stages of ignition.
The chemical steps to convert the nitro group to
N2
involve
a
complex set of elementary reactions. The time-dependence of the ignition steps (ignition
delay times) as a function of temperature and pressure is used to determine effective
activation energies and pressure dependencies to ignition. The ignition delay times range
from several nanoseconds to tens of microseconds. At atmospheric conditions, the delay
times for both ignition stages are in excellent agreement with observed experimental data.
At the high densities, the ignition times at these elevated temperatures appear to be
dominated by the same reaction mechanism that occurs for atmospheric gaseous
nitromethane initiation. This is to be contrasted with lower temperature, condensed-
phase ignition studies where it appears that solvent-assisted reactions dominate.
1.
INTRODUCTION
The ability to identify the chemical reactions involved in the initiation of energetic materials is
challenging due to the extremely short time scales involved as well as the high temperatures and
pressures that are generated. Furthermore, the chemical intermediates of ignition are highly
reactive. The chemistry of nitro compounds, such as nitramines and nitroaromatics, is further
complicated by the fact that they may undergo two-stage ignition. We therefore have investigated
the initiation of nitromethane as a prototype of an energetic material containing the nitro moiety.
The ignition of nitromethane has been studied both experimentally
11-9
]
and computationally
[1,10-15
1.
In
particular, the ignition delay time of gas-phase ignition of nitromethane has been
studied by Guirguis
et
al.
[I] in which they were able to observe two stages of ignition. Several
groups have modeled the ignition process using detailed chemical kinetics
[1,9], including previous
work by us
[13,14]. The modeling was able to treat the first-stage ignition reasonably well, but
the second-stage ignition delay times were off by several orders of magnitude. Recent work by
Lin and coworkers
[16-181 have improved our understanding of the elementary reactions involving
the NO,
HNO,
NO2, and HCN species. This has enabled us to generate an improved detailed
reaction mechanism for nitromethane. We use this improved mechanism to study the initiation of
gaseous nitromethane at atmospheric conditions as well as at densities corresponding to that of
*Work supported by the
U.
S.
Department of Energy.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1995443
C4-536
JOURNAL
DE
PHYSIQUE
IV
liquid nitromethane. We present the thermodynamics and reaction pathways of the ignition steps.
We then use the ignition delay times to determine effective activation energies and pressure
dependencies.
2.
METHOD
The study of ignition in a uniformly heated system, such as in a shock tube, allows us to study the
combustion chemistry of energetic materials in a environment that can provide the initial
decomposition and initiation reaction mechanisms. The energetic material can be treated as a
homogeneous gas mixture in a closed system, in which we follow the time-dependent evolution of
the chemical species. For instance, we can treat an adiabatic system with constant pressure or
volume, or we can follow the chemistry under shock detonation conditions. We use the
SENKIN
program [19], which includes sensitivity analysis, for the integration of the energy and mass
conservation equations.
In this section, we study the chemistry of gaseous nitromethane initiation. The ignition of
nitromethane is treated as being under constant volume. The governing equations are
[14,19]
-
dYk
=
V
ik
~k
(k
=
1,
...,
K).
dt
and
K
dT
cv-
=
-
V
C
ek
&k
~k,
dt
k=
1
In the above equations, T is the temperature, Yk the mass fraction of the kth species,
Wk
is the
molecular weight of the
kth species, t is time,
v
=
V/m
is the specific volume, ek is the internal
energy of the kth species,
c,
is the constant volume heat capacity of the mixture, and
&
is the
molar rate of production of the
kth species. The chemical reaction mechanism included
48
species
and 312 elementary reactions. The thermochemical dynamic properties and chemical production
rates are evaluated using the
Chemkin package [19]. The 48 species and their heats of formation
are presented in Table
I.
Table
I.
The chemical species used in the detailed chemical kinetic modeling of nitromethane.
Also given are the heats of formation at 298
K
(AH&*),
where energies are given in kcal-mol-1.
CH3N02
NO
HNO
N20
H2
02
H20
N
NH
NH3
HNNO
H2NO
CO
HCO
CH3O
CH30NO
NO2
HONO
H
0
OH
H02
N2
NH2
NNH
HNOH
N2H2
co2
CH2O
CH20H
Table
I.
(Cont.)
31
CH30H
33 CH4
35 HCN
37
CH3NH
39 HOCN
41 NCO
43
CH3NO
45
H2CNO
47
H2CNOH
CH3
CN
HzCN
HCNO
HNCO
NCN
H2CN02
CH3NHO
CH2N02H
3.
RESULTS
In Figs. 1 and 2, we present the temperature and species profiles as a function of time for the
initiation of 100 per cent gaseous
CH3N02 at 7.25 atmospheres and 1131 K. These conditions are
representative of the experimental conditions of Guirguis
et
al.
[I]. The initiation undergoes two
ignition stages, at 13.3 psec and 25.8
pet,
respectively. The final temperature is 3598 K.
The chemistry can be subdivided by time into several stages:
(I)
a preliminary stage of slow
decomposition followed by
(11) a first-stage ignition that undergoes a rapid temperature rise
followed by (III) an intermediate stage in which the temperature remains on a plateau followed by
(IV) a second-stage ignition where the temperature again rises rapidly to form the final products.
The reaction mechanism flow diagrams for the first-stage ignition (11), the intermediate stage (111),
and the second-stage ignition (IV) are shown in Figs. 3-5. These reaction pathways show the
dominating elementary reactions occurring in each of these stages, with thick arrows being the
major pathways while thin arrows are the minor pathways. We have separately shown the
reaction chemistry for the carbon atom and the nitrogen atom of nitromethane. The
carban atom
must be oxidized, to form CO and, to a lesser extent,
C02 (note that there is not enough oxygen in
nitromethane to oxidize the carbon to C02 and the hydrogen to
H2O). Meanwhile, the nitrogen,
which is in
an
oxidized state as part of the nitro group moiety, must be reduced to N2. The reaction
flow diagram for the preliminary decomposition stage (I) is similar to that of the first-stage ignition
(11) and thus is not shown. The major difference is that the preliminary stage (I) occurs at the low
end of the temperature range, giving rise to slower rates and resulting in a greater variety of minor
intermediate species being formed.
In Figs. 6-8, we show the enthalpies and free energies of possible species corresponding to the
composition
ClH3N102. This represents the composition of nitromethane and includes tautomers
of nitromethane as well as various product species. Besides the AH at 300K (see Table I for
individual species heats of formation), we present the
AG's at 300K and 2000K.
Fig.
1
showed the time profile for nitromethane initiation at 7.25 atmospheres and 113 1
K,
indicating two rapid temperature rises at 13.6 psec and 26.1 psec, designated 71 and 72
respectively. In Table 11, we present these ignition delay times,
71
and 72, for various initial
temperatures and pressures. These results are used to determine activation energies of ignition,
defined by the relation
In
('Ta/7b)
=
(AEI
R)
(1/T,
-
1ITb). The resulting AE's for the first-stage
and second-stage ignitions are presented in Table
111. In a similar manner, we can define pressure
dependencies to the ignition delay time by the relation
In
(za/7b) n
In
(Pa
/
Pb), which corresponds
to
T
being proportional to P". The resulting pressure exponents are given in Table IV.
(3-538
JOURNAL
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Time
(microsec)
4000'
3000,
E
V
E'
7
+,
!!
2000
al
E"
8'
1000.
0'
Fig. 1. Temperature vs. time profile for ignition of nitromethane. Initial conditions are 1131
K
and
7.25
atm.
Nitromethane Ignition
I
Temperature
1
4
Species
Concentration
0 10 20 30
Time (microsec)
1
.o
0.8
Fig.
2.
Species vs. time profile for ignition of nitromethane. Intial conditions are 1131
K
and
7.25
atm.
CH3N02
...........
NO
-.-.-.-,
H2
...........
C
.P
0.6
+,
0
E
'C
al
-
0
0.4'
0.2.
......'........*.
*.-
0.0
0
10
2
0
30
H20
N2
-.-,-,-,
CO
...........
CH20
...........
CH4
HCN
......
.mL-,-,-,-
.
.
/s-'---2-
,
,,&.,.f:
............
.-.em-
-
,:r
.........
---
.---.-.
Nitromethane First Stage Ignition Chemistry
Carbon
Mechanism
CbONO
+
M
+
H
+
NO
+
CH3
+
M
+
CH3
+
NOp
CH3N02- CH3x
CH30
HCO CO
+
H
CH4 CH30H CH20H
Nitrogen Mechanism
+
M
+
HCO
+
HNO
+
M
CH3N02+ NO
HNO
-+
+
M
0
-
Nz
Fig.
3.
Reaction mechanism flow diagrams for the first stage-ignition of nitromethane. Thick arrows
indicate major pathways. Dominant collision partners for
bimolecular reactions are indicated next to
arrows. Unimolecular decomposition reactions are indicated by the third-body notation
+M.
The
chemistry of carbon- and nitrogen-containing species are presented separately.
Nitromethane Intermediate Stage Chemistry
Carbon Mechanism
+M
+
H
+M
CH4
*
CH3
----t
H2CN
-
HCN
+
OH
CH30H
=
CH,OH dCH20
4
HCO
4
CO
+
CO,
+H
+H
+
M
+
OH
Nitrogen
Mechanis
rr
+
CH3
NO
-
HCN
Fig.
4.
Reaction mechanism flow diagrams for the intermediate stage (between first- and second-
stage) ignition of nitromethane. See Fig.
3
for further caption descriptions.
