22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Kinetic study of low temperature methane oxidation in a nanosecond
repetitively pulsed dielectric barrier discharge
J.K. Lefkowitz, P. Gou, A. Rousso and Y. Ju
Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ, U.S.A.
Abstract: Methane oxidation was investigated in a nanosecond repetitively pulsed
dielectric barrier discharge. Quantitative measurements of temperature and product species
were performed using tunable diode laser absorption spectroscopy and gas chromatograph
sampling. Discrepancies with modelling work revealed uncertainties in the low
temperature oxidation reactions, particularly for methyl radical oxidation pathways.
Keywords: plasma-aided combustion, methane, oxidation, dielectric barrier discharge
1. Introduction
The initiation of hydrocarbon oxidation by plasma
discharges is a topic relevant for both plasma-assisted
combustion (PAC) and plasma gas conversion. For PAC,
the active species and gas temperature rise produced by
the plasma determines the following oxidation rate, which
may be vastly different depending on the temperature
regime and radical concentration [1, 2]. Thus, detailed
understanding of both the reaction rates and energy
coupling mechanism of electron collision reactions and
the following ionized and excited species reactions is
necessary when modelling PAC.
For plasma gas
conversion, the yield of a desired product species is also a
function of temperature, and thus plasma properties [3, 4].
Again, understanding the plasma kinetics is necessary for
to model the initial condition for the following oxidation
process.
In addition to plasma kinetics, the correct reaction rates
for ground state species must be utilized. In the case of
high temperatures (> 800 K) the rates of oxidation for
small hydrocarbons have been extensively studied by the
combustion community. However, at lower temperatures
(300 - 800 K), small hydrocarbons do not generally react
with oxygen at an appreciable rate, mainly due to a lack
of available radical species to initiate the oxidation
process, and thus have not received the same degree of
attention by the combustion kinetics community.
However, the low temperature kinetics become important
in the case of PAC when the radical species concentration
is large and can initiate the oxidation process. In this
case, the degree of oxidation is completely dependent
upon the supply of active species from the plasma.
The aim of the current study is to explore the oxidation
process unique to low temperature PAC and assess the
current degree of understanding of the reaction pathways.
For this purpose, in-situ temperature and formaldehyde
concentration measurements were taken in a nanosecond
repetitively pulsed (NRP) dielectric barrier discharge
(DBD) using tunable diode laser absorption spectroscopy
(TDLAS) during a 300 pulse burst. Formaldehyde is
known to be an important intermediate species in methane
O-4-3
PAC [5-7] indicative of the degree of oxidation. In
addition, gas chromatograph sampling downstream of the
plasma was utilized to quantify all major product species
in a continuously pulsed discharge.
2. Experiment
The experimental apparatus has been described in detail
elsewhere [2, 8, 9], and is thus only briefly discussed
here. The plasma is produced using a nanosecond
(~12 ns) pulsed power supply (FID GmbH FPG
30-50MC4), which supplies repetitive pulses at up to
30 kHz frequency and up to 22 kV peak voltage. The
dielectric barriers are 1.6 mm quartz sheets covering both
stainless steel electrodes, and the discharge gap is 14 mm.
The current during the nanosecond discharge is measured
using a Pearson Coil current probe (Model 6585), and the
voltage externally applied to the discharge is measured
using a Lecroy high voltage probe (PPE20KV). All
experiments were performed with average peak voltage of
8.76 kV. The per pulse energy input is 1.5 ± 0.2 mJ.
Uniform discharge was confirmed using single shot ICCD
imaging (100 ns gate width).
Fig. 1 shows a schematic of the experimental setup.
The fuel, oxygen, and helium are metered using mass
flow controllers (Brooks instruments), and are premixed
prior to entering the plasma reactor. The flow velocity
inside the quartz section is maintained at 0.2 m/s. The
quartz cell is contained inside a stainless steel vacuum
chamber, and nitrogen is supplied around the quartz
section to ensure exhaust gases do not affect the
absorption measurements.
For continuously pulsed
experiments, reactant and product species are sampled at
the exit of the discharge cell and analyzed using a gas
chromatograph coupled with a thermal conductivity
detector (GC-TCD, Inficon MicroGC 3000).
A Continuous Wave External Cavity Mode Hop Free
Quantum Cascade Laser (CW-EC-MHF-QCL, Daylight
Solutions) is used to scan two methane absorption lines at
1343.56 cm-1 and 1343.63 cm-1. The ratio of these lines
can be used to extract the gas temperature using the
two line absorption method [2, 10]. In addition, a
1
Reactor
Ge Etalon
Beam Splitter
Flip Mirror
Quartz
Wall
Macor
Wall
Collimating Mirror
Lenses
Fig. 1. Diagram of experimental apparatus.
Distributed Feedback Quantum Cascade Laser (DFBQCL, Alpes Lasers) is used to quantify formaldehyde
concentration, using the absorption line centered at
The absorption profile at different
1725.97 cm-1.
conditions of temperature and pressure can be modeled
using data from HITRAN [11]. A least squares nonlinear
fitting algorithm for calculating the Voigt function [12] is
used to fit the calculated absorption profiles to the
measured ones. Species measurements are estimated to
have a maximum uncertainty of 5%, while temperature
measurements have an uncertainty of ±10 K in the range
of 300 - 500 K.
3. Numerical Modelling
Predictions of the measurements were computed
assuming a homogeneous and adiabatic discharge system.
Square pulses are assumed for the reduced electric field
profile, with 3-4 ns in duration and 180 Td peak height.
The pulse duration was varied to match the measured
temperature profile. The assumption of fixed height,
square waves is necessary because the 1-D solution for
charge separation in an electric field has not been solved
in this study, thus, it is not possible to calculate the
reduced electric field a priori from the applied voltage
waveform. The plasma discharge reaction kinetics are
predicted using the ZDPlasKin code [13]. Combustion
chemistry is calculated using the CHEMKIN based
SENKIN code[14].
Electron collision reactions are limited to the plasma
model, while the combustion chemistry involving neutral
ground state species are mostly limited to the combustion
model. Excited species quenching reactions, electron-ion
recombination reactions, and charge exchange reactions
are used in both models. The combustion kinetic model
employed here is HP-Mech, previously described and
available in [2], which is specifically designed for low
temperature (< 800 K) chemistry. The plasma model has
been assembled primarily from an air plasma model
[13, 15], with helium reactions from [16, 17] and methane
2
4. Results and Discussion
All experiments were performed in a stoichiometric
methane/oxygen mixture with 75% helium dilution, at
293 K initial temperature and 60 Torr pressure. The
measurement of temperature by two-line absorption
spectroscopy in a 300 pulse burst discharge at 30 kHz
repetition frequency is presented in Fig. 2.
The
temperature increases steadily throughout the pulse burst
until it peaks at ~ 350 K on the last pulse (at 10 ms), at
which point heat losses bring the gas temperature back to
its original value. The accurate fitting of modeled
temperature with the measurement ensures that the
predicted chemical reactions are in the correct
temperature regime, thus any discrepancies in species
concentrations will be solely due to the reaction pathways
and rates used in the model.
360
Model
350
Temperature (K)
Vacuum
Chamber
reactions from [7, 17]. Included excited and ionized
species are: O2 (𝑐1 Σu− ), O2 (C 3 Δu ), O2 (𝐴3 Σu+ ) (all grouped
as O 2 (4.5eV) by analogy with [7]); O2 �𝑎1 Δg �; O2 �𝑏1 Σg+ �;
1
1
+
1
+
+
O+
2 ; O( D); O( S); O ; He(2s S); CH4 ; CH3 .
Vibrationally excited species, negative ions, and complex
positive ions are neglected. The electron-collision crosssections for O 2 and He are downloaded from the LXCat
online database [18, 19], and cross-sections for CH 4 are
computed in the method described by Janev and Reiter
[20]. The complete plasma model can be found in [9].
Experiment
340
330
320
310
300
290
0
5
10
15
20
Time (ms)
Fig 2. Temperature measurements and modelling during
and after a 300 pulse burst at 30 kHz repetition rate
(10 ms duration) in a 0.083 CH 4 /0.167 O 2 /0.75 He
mixture.
The
measured
and
predicted
formaldehyde
concentration, at the same conditions as in Fig. 2, is
presented in Fig. 3. At the temperatures reached in the
reactor, formaldehyde is unable to be consumed after the
plasma, and thus builds up throughout the pulse burst
before reaching a final concentration. The model underpredicts the measured concentration by a factor of four.
To explore which part of the mechanism is responsible for
this discrepancy, the plasma is switched from a pulse
O-4-3
250
CH2O Experiment
Mole Fraction (ppm)
CH2O Model
200
150
x 1000
200
O2 Experiment
CH4 Experiment
H2O Experiment
175
O2 Model
CH4 Model
H2O Model
150
Mole Fraction (ppm)
burst mode to a continuously pulsed mode, and
temperature measurements are taken again, as presented
in Fig. 4, as well as GC-TCD sampling measurements, as
presented in Fig. 5.
125
100
75
50
25
100
0
100
1000
10000
100000
Frequency (Hz)
50
14000
CO Experiment
0
CO Model
12000
10
5
15
Time (ms)
Fig. 3. Formaldehyde measurements and modelling
during and after a 300 pulse burst at 30 kHz repetition
rate and 8.76 kV peak voltage in a 0.083 CH 4 /0.167
O 2 /0.75 He mixture.
500
CO2 Model
H2 Experiment
10000
Temperature Model
475
CO2 Experiment
20
Mole Fraction (ppm)
0
H2 Model
8000
6000
4000
2000
Temperature Experiment
0
100
425
1000
10000
100000
Frequency (Hz)
400
1000
CH2O Experiment
375
CH2O Model
CH3OH Experiment
350
800
325
300
275
100
1000
10000
100000
Frequency (Hz)
Fig. 4. Temperature measurements and modelling in
continuously pulsed plasma at 8.76 kV peak voltage in a
0.083 CH 4 /0.167 O 2 /0.75 He mixture.
At frequencies > 10 kHz there is some disagreement in
predicting the gas temperature of the discharge. This is
due to heating of the reactor walls, which changes the
heat loss rate from the bulk gas, thus the heat loss rate in
the model is no longer as accurate as the time-dependent
measurements, in which the low duty cycle of the
discharge burst prevents significant wall heating. The
disagreement is not large enough to significantly change
the oxidation regime, staying within 25 K of the measured
value up until the 30 kHz case, in which the discrepancy
is ~ 50 K.
Fig. 5 presents the species concentrations at the exit of
the discharge cell for the same conditions as in Fig 4. The
major product species are H 2 O, CO, CO 2 , and H 2 , and
the minor products are CH 2 O, CH 3 OH, C 2 H 6 , C 2 H 4 , and
C 2 H 2 . Excellent agreement between the model and
O-4-3
Mole Fraction (ppm)
Temperature (K)
450
CH3OH Model
C2H2 Experiment
C2H2 Model
C2H4 Experiment
600
C2H4 Model
C2H6 Experiment
C2H6 Model
400
200
0
100
1000
10000
100000
Frequency (Hz)
Fig. 5.
Species measurements and modelling in
continuously pulsed plasma at 8.76 kV peak voltage in a
0.083 CH 4 /0.167 O 2 /0.75 He mixture.
experiment is found for fuel and oxidizer consumption, as
well as the production of all major species. This indicates
that, in general, the model is capturing the rate of
oxidation properly. A path flux analysis, presented in
Fig. 6, reveals that 60% of the fuel is consumed in
electron collision reactions, while 23% is consumed in
reactions with O(1D) and the remaining 15% is consumed
in reactions with OH radical. The two most significant
pathways progress through the formation of methylene
3
radical (17%) or methyl radical (56%).
+ O 6%
CH3 + H2O
+ OH 15%
+ e- 23%
CH4
+ O2 + M 94%
+ e- 100%
CH3+ + H
+ O2 9%
CH3OH + O2
CH2O + HO2
CH3OH + CH
CH2OH + H
CH2O + HO2
CH4+
+ O2 100%
CH3 + OH
+ O2 100%
CH3O2 + M
CH3O + O2/OH
CH3 + H
CH4 + O2+
CH2 + H/H2
+ O2 100%
CH2O + H
CO + OH + H
CO2 + H + H
CO2 + H2
CH2O + O
Fig. 6. Path flux analysis of fuel consumption integrated
over a single pulse period during continuous discharge at
30 kHz repetition frequency. Bold species represent those
which are measured, red arrows are for combustion
reactions, and blue arrows are for plasma reactions.
Agreement for the major species is excellent, however,
disagreement for the minor species, particularly CH 3 OH
and CH 2 O, indicates there are some parts of the oxidation
mechanism which require closer attention. Investigation
of the production rates for these two species reveals that
all of the CH 3 OH and 20% of the CH 2 O is formed after
the reaction of CH 3 + O 2 → CH 3 O 2 . The methyl peroxy
radical (CH 3 O 2 ) consumption pathways lead to either
CH 3 OH or CH 2 O. It is expected that improvements in
the prediction of the CH 3 O 2 reactions will lead to better
prediction of the low temperature oxidation pathways,
especially considering reactions of this radical species
have very few published rate constants [21].
5. Conclusions
Measurement and modelling of temperature and major
product species in and after a NRP DBD of CH 4 /O 2 /He
have been carried out in a low temperature flow reactor.
The major product species are H 2 O, CO, CO 2 , and H 2 ,
while the minor products include CH 2 O, CH 3 OH, C 2 H 6 ,
C 2 H 4 , and C 2 H 2 . Modelling predictions of the gas
temperature, oxidation rate, and major product species are
in agreement with the measurements. However, the
minor product species require further effort to bring into
agreement with the measured concentrations, particularly
for CH 3 OH and CH 2 O. One suggested pathway requiring
further investigation is the reactions of the radical species
CH 3 O 2 . Further work on low temperature oxidation
pathways is required for improved prediction of the
product species.
6. Acknowledgements
The authors would like to acknowledge the AFOSR
Plasma MURI project (grant FA9550-07-1-0136) and the
AFOSR MILD Combustion project (grant FA9550-13-10119) for supporting this work.
4
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