Quantitative measurements of HO2 and other products of n -butane oxidation (H2O2, H2O, CH2O, and C2H4) at elevated temperatures by direct coupling of a jet-stirred reactor with sampling nozzle and cavity ring-down spectroscopy (cw-CRDS)

Article abstract of DOI:10.1021/ja510719k

For the first time quantitative measurements of the hydroperoxyl radical (HO₂) in a jet-stirred reactor were performed thanks to a new experimental setup involving fast sampling and near-infrared cavity ring-down spectroscopy at low pressure. The experiments were performed at atmospheric pressure and over a range of temperatures (550-900 K) with n-butane, the simplest hydrocarbon fuel exhibiting cool flame oxidation chemistry which represents a key process for the auto-ignition in internal combustion engines. The same technique was also used to measure H₂O₂, H₂O, CH₂O, and C₂H₄ under the same conditions. This new setup brings new scientific horizons for characterizing complex reactive systems at elevated temperatures. Measuring HO₂ formation from hydrocarbon oxidation is extremely important in determining the propensity of a fuel to follow chain-termination pathways from R + O₂ compared to chain branching (leading to OH), helping to constrain and better validate detailed chemical kinetics models.

Full text of DOI:10.1021/ja510719k

Article
pubs.acs.org/JACS
Quantitative Measurements of HO2 and Other Products of n‑Butane
Oxidation (H2O2, H2O, CH2O, and C2H4) at Elevated Temperatures by
Direct Coupling of a Jet-Stirred Reactor with Sampling Nozzle and
Cavity Ring-Down Spectroscopy (cw-CRDS)
Mokhtar Djehiche,† Ngoc Linh Le Tan,† Chaithanya D. Jain,† Guillaume Dayma,† Philippe Dagaut,*,†
Christian Chauveau,† Laure Pillier,† and Alexandre Tomas‡
†ICARE, INSIS, CNRS, 1c Avenue de la recherche scientifique, 45071 Orlea
‡Ecole Nationale Super
ieure des Mines de Douai, 941 Rue Charles Bourseul, 59508 Douai cedex, France
́
ns cedex 2, France
́
ABSTRACT: For the first time quantitative measurements of the hydroperoxyl
radical (HO2) in a jet-stirred reactor were performed thanks to a new experimental
setup involving fast sampling and near-infrared cavity ring-down spectroscopy at low
pressure. The experiments were performed at atmospheric pressure and over a range
of temperatures (550−900 K) with n-butane, the simplest hydrocarbon fuel
exhibiting cool flame oxidation chemistry which represents a key process for the
auto-ignition in internal combustion engines. The same technique was also used to
measure H2O2, H2O, CH2O, and C2H4 under the same conditions. This new setup
brings new scientific horizons for characterizing complex reactive systems at elevated
temperatures. Measuring HO2 formation from hydrocarbon oxidation is extremely
important in determining the propensity of a fuel to follow chain-termination
pathways from R + O2 compared to chain branching (leading to OH), helping to
constrain and better validate detailed chemical kinetics models.
stable species were quantified under the same conditions using
1. INTRODUCTION
a simple and affordable technique. These measurements of
stable species are similar to what was reported earlier for
H2O2,12,13 H2O,12 CH2O,12 and C2H4.12
Hydroperoxyl radicals (HO2) play an important role in both
atmospheric and combustion chemistry.1,2 These radicals
contribute to the formation of H2O2 (2 HO2 → H2O2 + O2)
that is considered to play a key role for the ignition in
homogeneous charge compression ignition (HCCI) engines.
There, the chain-terminating reaction H + O2 + M → HO2 + M
consumes most of the H atoms, and the main chain-branching
reaction (H + O2 → O + OH) is relatively unimportant. The
ignition would be caused by H2O2 decomposition (H2O2 →
OH + OH). Also, when exhaust gas recirculation is used in
internal combustion engines to reduce NOx emissions, HO2
plays a complex role via its reaction with NO: NO + HO2 →
NO2 + OH.3 To better assess the importance of these processes
during the combustion of fuels, quantitative measurements of
HO2 are highly desirable. Whereas such measurements4−6 have
been performed in chemical systems operating around room
temperature, difficulties appear at elevated temperature where
HO2 seems to be too reactive to be quantitatively measured
after sampling. Previous efforts involved the use of the indirect
fluorescence assay by gas expansion (FAGE) technique where
HO2 reacts with NO to yield OH that is measured by laser-
induced fluorescence7 and dual-modulation Faraday spectros-
copy.8,9 In this work, we built a new experimental setup based
on a room temperature demonstrator described earlier10 to
measure HO2 radicals by cavity ring-down spectroscopy with
continuous wave light (cw-CRDS).11 Beside the measurements
of HO2 concentrations during the oxidation of n-butane, more
2. EXPERIMENTAL SECTION
The experimental setup involves two parts: (i) a jet stirred reactor
(JSR)-sampling nozzle assembly and (ii) a low-pressure cw-cavity ring-
down spectrometer.
2.1. The JSR-Sampling Nozzle Assembly. A fused silica
spherical JSR with a volume of 37 cm3 (Figure 1) was used. The
stirring is achieved by four jets exiting 0.5 mm i.d. injectors nozzles.
Details of the development and operation of such rector can be found
in an earlier publication by Dagaut et al.14 The reactor design is based
on the construction rules proposed by David et al.15 and Matras et al.16
The residence time for the gas mixture inside the reactor can be varied
from a few milliseconds to seconds by changing the total inlet gas flow.
Total length of the reactor including the side arms is 75 cm. The
reactor is heated by a 30 cm regulated electrical oven that can reach
1200 K. The oven is surrounded by ceramic wool. The temperature
along the main axis of the reactor is measured by a movable
thermocouple (type K). The reactants were high-purity oxygen
(99.995% pure form Air Liquide) and high-purity n-butane (>99.5%
pure from Air Liquide). The reactants were diluted with nitrogen
(<100 ppm of H2O, <50 ppm of O2, <1000 ppm Ar, <5 ppm of H2
from Air Liquide) and mixed just before entering the injectors. The
fuel−nitrogen mixture flowed through a capillary, whereas the O2/N2
Received: October 18, 2014
Published: November 7, 2014
© 2014 American Chemical Society
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are the ring-down times in the presence and absence of absorber,
respectively. The absorption length (d) was determined by injecting
known quantities of methane and acetylene and measuring the
absorbance at the center of the absorption lines at 6623.18 cm−1
(Figure 2) and 6625.15 cm−1, respectively.19 We used absorption cross
Figure 1. Simplified scheme of the experimental setup.
mixture flowed in the reactor extension tube (Figure 1). Mass flow
controllers (Brooks 5850TR) were used to deliver the gases that were
preheated before injection to minimize temperature gradients inside
the reactor. To determine the concentrations of different species
formed during the combustion process, the gas mixture is sampled
with the CRDS cell through a fused silica nozzle (100 μm tip orifice,
53° angle; the tip is located ca. 5 mm inside the reactor) fixed between
the JSR and the CRDS cell.
Figure 2. Measured absorption coefficient (α) of methane as a
function of CRDS cell pressure at 9 cm of absorption path length.
sections of 1.54 × 10−23 and 7.8 × 10−22 cm2 for CH4 and C2H2,
respectively, at 0.3−10 mbar (we took into account the variation of
cross section with pressure) in the presence of a mirror protection
nitrogen flow of 67 cm3/min. An effective absorption length d = 9
1.5 cm was determined.
The CRDS cell is kept at low pressure (0.3 to 10 10% mbar),
whereas the reactor operates at atmospheric pressure. A rotary vane
pump was used to withdraw samples from the reactor. The flow
through the sampling cone was ca. 70 cm3/min, whereas the total flow
rate entering the reactor ranged from ca. 1400 to 4500 cm3/min. A
high-precision gauge was used to measure the cell pressure
(THYRACONT vd85, 1200 to 5 × 10−4 ( 0.3−10%) mbar). The
expansion of the gas sample into the CRDS cell causes a pressure drop
and cooling which both slow chemical reactions. This helps detecting
highly reactive species such as free radicals. The volume (approx-
imately 3 L) of the detection cell was kept relatively high for such
instruments in order to limit wall reactions. The connection of the
nozzle to the CRDS cell is cooled by circulating a water−ethanol
mixture (80:20) at 0−5 °C. This limits heat transfer from the reactor
to the CRDS cell and prevents O-ring degradation (Figure 1).
2.2. The cw-Cavity Ring-Down Spectrometer. CRDS has been
well adopted by the scientific community in the past decades to carry
out a variety of measurements.17,18 Only a brief description of the
technique is given here. Two ultrareflective mirrors (AT Films, 1 m
radius of curvature, 99.999% reflectivity) are mounted on a rectangular
cuboid separated by 74 cm to form an optical resonator (Figure 1). A
continuous laser light source emitted by a DFB laser emitting at
around 1510 nm (Fitel, 40 mW) was used as a light source. The laser
diode is tunable over a 3 nm interval using a temperature and current
controller (Newport, model 6100). A key point in cw-CRDS is the
frequency matching between the laser emission and a cavity mode: this
condition was achieved by mounting one of the mirrors onto a ring-
shaped piezoelectric transducer fed with a triangular voltage and
modulating the length of the cavity over a free spectral range.7 The
light escaping from the cavity through the rear mirror is detected by an
avalanche photodiode, and once the light exceeds a user-determined
threshold, the laser beam is switched off using a fibered acousto-optic
modulator (Optoelectronic). The light intensity is recorded by a data
acquisition card (National Instruments PCI-6111E) with 200 ns time
resolution, and the ring down time is determined through an
exponential fitting using the Labview 2010 software (National
Instruments Corp.). Ring-down times are then converted to absorbing
species concentrations [A] using the following equation:
The absorption cross section of HO2 has been determined
previously in air and helium (Table 1). Using an average HO2 air-
Table 1. Line Strength, Cross Sections, and Pressure
Broadening Coefficient of HO2 at 6625.79 cm−1
Pref, mbar
80 (air)20
65 (He)21
40 (air)20
γ in cm−1 atm−1
S × 1021 cm−1
σ × 1019 cm2 at Pref
0.1
0.057
4.2
0.106
4.8
3.5
1.04
1.68
1.97
broadening coefficient of 0.106 cm−1/atm,20 it is possible to calculate
the HO2 absorption cross section at 6625.79 cm−1 and 0.3 mbar based
on literature data. Using the most recent determination of Tang et al.20
(30 Torr air), we calculated a value of 3.16 × 10−19 cm2 for a line
strength of 4.8 × 10−21. Using the data from Johnson22 (60 Torr air,
́
see footnote c in Table 2 of Tang et al.)20 and Thiebaud21 (50 Torr
He) enables to determine Doppler-limited cross sections of 2.45 ×
10−19 and 2.58 × 10−19 cm2, respectively. On the other hand, using the
upper limit value of γair = 0.14 cm−1/atm obtained by Ibrahim et al.,23
yields a cross section of 3.56 × 10−19 cm2 based on the determination
of Tang et al.20 Thus, we decided to use an average HO2 absorption
cross section of 3 × 10−19 cm2 at 0.3 mbar air associated with an
uncertainty of 0.6 × 10−19 cm2 (i.e., 20%). A global uncertainty of
about 40% may be expected taking into account the uncertainties on
the literature data (about 20%).
At low fuel conversion and at 0.3 mbar in the CRDS cell, some
stable species are not detectable. For that reason, we measured them at
10 mbar.
The absorption cross section of CH2O at 6624.78 cm−1 has been
24
calculated using S = (9.1 1.8) × 10−24 cm−1 and γ = 0.1 cm−1/
atm.25 We obtained a value of σ = (5.1 0.5) × 10−22 cm2 at 10 mbar.
The cross section of water at 6621.5 cm−1 was calculated using a
19
line strength of 6.69 × 10−24 cm−1 and a broadening coefficient of
⎛
⎜
⎞
⎟
⎠
L
1
1
τ0
[A]t =
−
0.1 cm−1/atm.12 At 10 mbar this yields a water vapor cross section of
(2.9 0.1) × 10−22 cm2.
d × c × σ τ
⎝
(1)
t
where σ is the absorption cross section at the absorbing wavelength, L
the distance between the two cavity mirrors, d the length, over which
the absorbing species is present, c is the speed of light, and τt and τ0
The cross sections of H2O2 and C2H4 have been determined
previously by Bahrini et al.12 For H2O2 at 1 mbar we considered that
the cross section is the same in air and in He (γHe = 0.068 cm−1/
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atm),26 which allowed calculating the concentrations at 1 mbar. By
changing the pressure to 10 mbar in air, the calculated absorption
cross sections of H2O2 at 6624.1 cm−1 is σ = (6.8 1.3) × 10−23. Due
to the absence of data for C2H4 (γair and S), we used the cross section
obtained at 13 mbar published by Bahrini et al.12 (σ6623.37 = 3.6 × 10−23
cm2).
3. RESULTS AND DISCUSSION
Recently, Bahrini et al.12 have studied the combustion of n-
butane in similar conditions with a JSR connected to a CRDS
cell via a microprobe and have reported the detection of stable
Figure 4. Concentrations profiles of HO2 measured as a function of
temperature during the oxidation of n-butane. Results from Blocquet
et al.7 obtained in similar conditions are shown for comparison. The
results of a kinetic modeling using the PSR code30 and our updated
kinetic scheme31 are also presented (lines).
species H2O2, CH2O, C2H4, and H2O. However, they failed to
detect HO2 due to probable losses in the sampling probe,
which extended inside to the centerline of the reactor. This
motivated us to take up this study again and try to measure
HO2 concentrations.
To confirm our detection of HO2 we carried out two series
of experiments: (1) The oxidation of n-butane at 3% was
performed at 650 K, in a jet-stirred reactor at 1 atm (φ = 0.5
and at mean residence time τR = 3s). Figure 3 shows a spectrum
in the near IR region recorded by cw-CRDS (3%, φ = 0.5, 3s
and 650 K). The absorption lines of HO2, CH2O, C2H4, H2O2,
and H2O in the range 6623 to 6626 cm−1 are identified (Figure
3a). This allows the detection of all these species using CRDS
in the same conditions just by tuning the appropriate
absorption wavelength. The hydroperoxyl radical is a very
reactive species that is difficult to detect.27 Generally when the
total pressure is increased the signal corresponding to stable
species increases, but that of radicals decreases because of their
loss through increasingly fast chemical reactions.
Figure 3b,c shows the absorption peaks of CH2O (6623.52,
6623.62 and 6625.74) and HO2 at (6623.57 and 6625.79
cm−1). We increased the pressure from 1 to 10 mbar to see the
effect on absorption. It was observed that the formaldehyde
absorption strongly increases with increasing pressure, while
that was not the case for the signal attributed to the HO2
radical. Since increasing the pressure in the CRDS cell has an
adverse effect on radical detection, these results confirm that
HO2 radical is the species detected at 6623.57 and 6625.795
cm−1. It was observed (Figure 3c) that stable species absorb at
6623.57 cm−1, whereas that is not the case at 6625.795 cm−1
(Figure 3b). Therefore, we selected the absorption line at
6625.795 cm−1 for our measurements.
Reducing the pressure in the CRDS cell favors the
measurement of HO2 concentrations whereas a compromise
with good sensitivity must be found. Therefore, the
quantification of HO2 was performed at a CRDS cell pressure
of 0.3 mbar.
(2) A second series of experiments was performed for n-
butane oxidation where we operated at (a) variable temperature
and fixed equivalence ratio and (b) variable equivalence ratio
and constant temperature. (a) First, we carried out the
oxidation of n-butane under the same conditions as in the
work of Bahrini et al.,12 except they worked in helium, whereas
Figure 3. cw-CRDS spectra obtained during the oxidation of n-butane
at 650 K. (a) Spectrum from 6623 to 6626 cm−1 (average of 50 ring-
down events recorded at 3 mbar in the CRDS cell). (b) Effect of
pressure in the CRDS cell on the absorption peaks of CH2O and HO2
at 6625.75 and 6625.79 cm−1, respectively. (c) Effect of pressure on
the absorption peaks of CH2O (6623.52 and 6623.62 cm−1) and of
HO2 at 6623.57 cm−1 at 1−10 mbar. Red line: formaldehyde
absorption;28 black symbol: absorption of combustion products
(circles: at low pressure, squares: at higher pressure). The vertical
lines indicate HO2 absorption lines.
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Figure 6. Comparison between experimental data (symbols) and
computations31 (lines) for (a) HO2, (b)H2O, (c) H2O2, and (d)
CH2O produced during the combustion of 3% n-butane (black squares
and line) and 1% n-butane (red circles and line); φ = 0.3−1, τR = 3s,
and T = 650 K.
Figure 5. Concentrations profiles of stable intermediate species and
products measured as a function of temperature during the oxidation
of n-butane. Results from our FTIR measurements (see ref 29 for
details) and those of Bahrini et al.12 obtained in similar conditions are
shown for comparison. The results of a kinetic modeling using the
PSR code30 and our updated kinetic scheme31 are also presented
(lines).
the present experiments were performed in nitrogen.
Quantification of oxidation products was performed as a
function of temperature in the JSR. Figure 4 shows the
measurements of different species sampled from the JSR
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operating at 1 atm, over the temperature range 600−900 K, and
at a mean residence time of 6 s. The reacting mixture molar
composition was 2.3% n-butane, 14.95% oxygen, and 82.75%
nitrogen. To confirm the reliability of the measurements by our
CRDS technique the present results were compared with our
FTIR measurements and the CRDS and gas chromatography
(GC) results reported by Bahrini et al.12 and the FAGE data of
Blocquet et al.7 Our measurements by CRDS were limited to
900 K for practical reasons. In the present experimental setup
there are two potential sources of errors. The first one is the
uncertainty in the reported absorption cross section for the
given species (generally 10−20%),12 and the second one is the
uncertainty in the estimated effective absorption path length
about 15% due to the fluctuation of total gas flow. Therefore, a
maximum total uncertainty of 35% was assigned to all the
measurements except HO2, for which an uncertainty of 50%
was estimated. In addition signal-to-noise ratio of the
instrument also becomes important because of the low product
yields during the combustion. To overcome this problem up to
150 ring-down events were averaged for each wavelength
during the absorption line measurements. The noise in the
baseline signal was around 0.2 μs, indicating a minimum
measurable absorption coefficient <7 × 10−9 cm−1.
measurements. A good agreement with the model predictions
and other measurements was observed.
(b) The oxidation of n-butane (1% and 3% mole) was
performed at a fixed temperature of 650 K, corresponding to
the maximum of n-butane cool flame in the jet-stirred reactor.
We operated at 1 atm, at a mean residence time τR = 3s, and the
equivalence ratio was varied (φ = 0.3−1). Figure 6 shows the
experimentally measured concentrations of HO2, H2O, H2O2,
and CH2O during the oxidation of n-butane (1% and 3% mole)
in the JSR. For the CRDS measurements, the pressure in the
cell was 10 mbar for measuring the stable species and 0.3 mbar
to measure HO2.
We observed that the concentrations of the measured species
decrease when equivalence ratio increases, which corresponds
to a reduced fuel conversion due to a reduction of the initial
oxygen concentration. Also, we observed higher concentrations
(∼3 times) of products formed when the initial fuel
concentration is increased from 1% to 3% in mole. These
observations follow both expectations and predictions obtained
through detailed chemical kinetic modeling, although the
computations do not match closely the data.
4. CONCLUSIONS AND PERSPECTIVES
For the first time HO2 concentrations were measured during
the oxidation of n-butane by cw-CRDS. Recently Bloquet et al.7
have reported an indirect quantification of HO2 during low
temperature oxidation of n-butane using the FAGE technique
which involves the conversion of HO2 into OH by reaction
with NO and detection of OH by laser-induced fluorescence.
The comparison of these two sets of data (Figure 4) indicates
that our measured HO2 concentrations are ca. 50% higher than
previously reported.7 This difference might be due to losses of
HO2 during their probe sampling which differs from ours.
Finally, one should also note that the present data are in
reasonably good agreement with our model predictions. The
kinetic model indicated that in helium the computed HO2 mole
fraction profiles are the same as in nitrogen up to 700 K. Above
that temperature a shift of +10 K of the computed HO2 profiles
was observed, due to a reduction of H2O2 decomposition
caused by the lower chaperon efficiency of helium versus
nitrogen.
A new experimental setup was built for measuring unstable
species at elevated temperatures. The quantitative measurement
of HO2 at 550−900 K, by coupling of cw-CRDS and a jet-
stirred reactor-sampling nozzle assembly, was performed for the
first time thanks to the use of small tip orifice/wide angle
sampling nozzle and very low pressure in the CRDS cell (0.3
mbar). The concentrations of H2O2, H2O, CH2O, and C2H4
were also measured using the same system (CRDS cell at 10
mbar) and found in good agreement with previously published
measurements.
This new experimental setup will allow the measurement of
concentrations of labile and stable species over a wide range of
conditions (temperature, initial fuel concentration, equivalence
ratio) and for a variety of fuels. These data will be useful for
assessing the validity of combustion chemical kinetic models.
Such studies are underway and the results should be presented
in the near future.
The measured concentrations of stable species formed during
the same experiment are presented in Figure 5.
AUTHOR INFORMATION
Corresponding Author
dagaut@cnrs-orleans.fr
■
Water measurements performed at a CRDS cell pressure of
10 mbar are in good agreement with our FTIR data and
previous CRDS measurements.12 The model predictions are in
good agreement with the measurements. The presently
measured hydrogen peroxide concentrations (CRDS cell
pressure = 10 mbar) were higher (ca. 30%) than those
previously reported.12 This difference might be due to losses of
hydrogen peroxide during their microprobe sampling. Form-
aldehyde measurements performed at a CRDS cell pressure of
10 mbar are in good agreement with FTIR and GC
measurements.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The research leading to these results has received funding from
the European Research Council under the European
Community’s Seventh Framework Programme (FP7/2007-
2013)/ERC grant agreement no. 291049-2G-CSafe. The
authors thank Dr. C. Fittschen for useful discussions and Mr.
The model predictions are in good agreement with the
present measurements in the cool flame region, whereas it
slightly overestimates the formation of formaldehyde between
750 and 870 K. This results from an overestimation of the
overall reactivity in this temperature range.
Finally, the ethylene concentration (measured at a CRDS cell
pressure of 10 mbar) in the lower temperature region (<750 K)
was lower than our detection limit, and hence only results
obtained at higher temperatures could be compared to other
P. Demaux, Dr. M. Idir, Mr. S. Thion, and Dr. C. Togbe
technical help.
́
for
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