NCO Quantitative Measurement in Premixed Low Pressure Flames by

ARTICLE pubs.acs.org/JPCA

NCO Quantitative Measurement in Premixed Low Pressure Flames by Combining LIF and CRDS Techniques Nathalie Lamoureux,* Xavier Mercier, Jean-Franc- ois Pauwels, and Pascale Desgroux Laboratoire PC2A, UMR8522 CNRS/Universite Lille1, Cite Scientifique, 59655 Villeneuve d’Ascq, France ABSTRACT: NCO is a short-lived species involved in NOx formation. It has never been quantitatively measured in flame conditions. In the present study, laser-induced fluorescence (LIF) and cavity ring-down spectroscopy (CRDS) were combined to measure NCO radical concentrations in premixed low-pressure flames (p = 5.3 kPa). NCO LIF excitation spectrum and absorption spectrum (using CRDS) measured in a stoichiometric CH4/O2/N2O/N2 flame were found in good agreement with a simulated spectrum using PGOPHER program that was used to calculate the high-temperature absorption cross section of NCO in the A2ΣþX2Π transition around 440.479 nm. The relative NCOLIF profiles were measured in stoichiometric CH4/O2/N2O/N2 flames where the ratio N2O/O2 was progressively decreased from 0.50 to 0.01 and in rich CH4/ O2/N2 premixed flames. Then, the LIF profiles were converted into NCO mole fraction profiles from the absorption measurements using CRDS in a N2O-doped flame.

’ INTRODUCTION The NCO radical has been largely studied because of its implication in the NO-reburning and the fuel-NO kinetic mechanisms. It is indeed a very important intermediate species in the HCN oxidation modeling.13 NCO radical was detected by laserinduced fluorescence (LIF) in flow reactor,1 in shock tubes,4,5 and in flames.68 NCO is also an important species involved in the prompt-NO mechanism. Indeed, the NCN radical which has been shown to be the product of the CH þ N2 reaction913 is rapidly consumed yielding HCN and CN. Both species are then oxidized yielding the NCO radical. NCO radical has been extensively studied from the spectroscopic point of view mainly because of the existence of a Renner Teller effect in the ground-state X2Πi and of a Fermi resonance in the A2Σþ state.1418 In addition, the B2Π state is not only the subject of both RennerTeller coupling and Fermi resonances, but it is complicated by strong predissociation and mixing with the A2Σþ state.19 Dixon20 identified three main groups in the BX vibrational band among a very complicated spectrum. Elliott et al.21 were able to fit the hyperfine structure of the A2ΣþX2Π and B2ΠX2Π transitions. The molecule is linear in both the ground and the excited electronic state, and the application of the FranckCondon principle predicts maximum intensity in the Δν2 = 0 transitions. Radiative lifetimes were measured to be 350 ( 30 ns in the A2Σþ(0,0,0) state22,23 and 60 ns in the B2Π(0,0,0) state.18,24 Collision quenching rates in the A state are reported at room temperature,2527 and the fluorescence quenching was measured in flames.28 Moreover, the absorption coefficients were determined at 1470 K in shock tube experiments5 at 440.479 and 304.681 nm in the A2Σþ(0,0,0)X2Πi (0,0,0) and B2Π(1,0,0) X2Πi (0,0,0) transitions, respectively. In flames, NCO radical was first measured in a rich atmospheric CH4/N2O flame using intracavity laser excitation by Anderson et al.6 r 2011 American Chemical Society

NCO was pumped at 465.8 nm with an Arþ laser in the A2Σþ(0,0,0)X2Πi(1,0,0) vibrational band, and the fluorescence spectrum was collected in a wide range of wavelengths (430.0 442.5 nm). They estimated a lower limit of the peak density of NCO using LIF calibrated against the Raman N2 signal. However, accuracy was limited by the lack of knowledge of spectroscopic and collisional data. They attempted to measure NCO in a slightly rich CH4/air flame, but the sensitivity limit for NCO density was found to be 100 times lower than in a CH4/N2O flame. Copeland et al.7 performed NCO LIF measurements in atmospheric pressure CH4/N2O flame according to various vibrational bands (A2Σþ(0,0,0)X2Πi(0,0,0) around 440 nm, A2Σþ(0,0,0) X2Πi(1,0,0) around 465 nm, and B2Πi(0,0,0)X2Πi(0,0,0) around 315 nm). They showed a very congested spectrum of NCO after exciting the A(000)X(000) band and collecting the A(000)X(100) band in order to prevent the laser-scattering perturbation. They recommended the excitation of the band A(000)X(100) because of its less congested spectrum. Relative NCO profiles have been recorded by Williams and Fleming29 in stoichiometric low pressure CH4/O2/Ar flame-doped with N2O, NO, or NO2 using LIF diagnostics. NCO relative profiles were obtained by exciting the A2Σ(0,0,0)X2Πi(1,0,0) band and by collecting the fluorescence in the A2Σ(0,0,0)X2Πi(0,0,0) band. For a given mole fraction of additives, nitrogen intermediates such as NCO, CN, or NH are enhanced in the presence of NO and NO2 compared to N2O. Williams and Fleming30 examined the effect of NO2 added in a stoichiometric low pressure CH4/ O2/N2 flame. Using the same excitation-collection scheme for the NCO LIF, they showed that the peak value of the relative Received: February 14, 2011 Revised: April 20, 2011 Published: May 06, 2011 5346

dx.doi.org/10.1021/jp201453k | J. Phys. Chem. A 2011, 115, 5346–5353

The Journal of Physical Chemistry A

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Table 1. Flame Compositiona

a

flame

CH4 (slpm)

O2 (slpm)

N2O (slpm)

N2 (slpm)

Φ

(108-20)

0.522

0.768

0.393

2.505

1.08

20

51.2

(100-20)

0.480

0.768

0.384

2.312

1.00

20

50.0

(100-10)

0.480

0.864

0.192

2.772

1.00

10

22.2

(100-05)

0.480

0.912

0.096

3.039

1.00

5

10.5

(100-01)

0.480

0.950

0.019

3.340

1.00

1

2.0

(100-005)

0.480

0.955

0.010

3.345

1.00

0.5

1.0

(125-00)

0.600

0.960

3.290

1.25

O-atoms from N2O (vol %)

N2O/O2 (vol %)

Φ represents the equivalence ratio. Volumetric flow rates are given in slpm unit in standard condition of pressure (101.3 kPa) and temperature (273.15 K).

NCO mole fraction profiles increased with the NO2 addition in a much lower way than the peak value of the simulated NCO mole fraction profiles. Later on, Williams and Pasternack31 studied the effect of a small amount of NO added to different fuel/O2/N2 flames. NCO relative profiles were measured using both LIF schemes (excitation at 465 nm and collection at 440 nm and inversely). They switched the NCO LIF excitation/collection scheme with the acetylene flame in order to reduce the flame luminescence interferences. NCO profiles were not corrected for quenching or temperature variations. The BX band of NCO around 315 nm is very attractive because of the vicinity of the CH, OH, NH, and CN vibrational bands and because of the possibility of simultaneously measuring these species using LIF techniques in flames. For this purpose, Jeffries et al.8 observed NCO fluorescence after exciting the B2Πi(0,0,0)X2Πi(0,0,0) band of NCO in an atmospheric CH4/N2O flame. The LIF excitation spectrum is characterized by two bandheads (R1 and R2). The fluorescence spectrum clearly exhibits many separated vibrational levels in the ground state, but the fluorescence signal is extremely weaker than in the AX band. NCO was also measured in low-pressure CH4/N2O/Ar flames by molecular beam mass spectrometry (MBMS).32 However, NCO radicals have never been measured in nondoped flames and have never been quantitatively measured using laser diagnostics in flames (doped or not, at atmospheric or low pressure). The present study aims to quantitatively measure the NCO radical in low-pressure (p = 5.3 kPa) CH4/O2/N2 flames. As previously pointed out, NCO has never been measured in nondoped flames. Thus, the adopted strategy during this work was to assess and optimize the experimental setup in slightly N2O-doped flames using a combination of LIF and cavity ringdown spectroscopy (CRDS) for the absolute quantitative measurement. The LIF measurements in nondoped flames were then converted into NCO mole fractions using the calibrated LIF signal issued from a doped flame.

2. EXPERIMENTAL SECTION 2.1. Low-Pressure Burner. Experiments were performed in laminar, premixed flames stabilized at 5.3 kPa (40 Torr) on a 6 cm diameter bronze water-cooled McKenna burner. The burner is vertically mobile in a stainless steel enclosure connected with a vacuum pump. The pressure was controlled via a 0 13.3 kPa pressure gauge and was automatically adjusted by operating a regulating valve in between the enclosure and the vacuum pump. Details are given elsewhere.13 The mixture composition was regulated using mass flow controllers of 1, 2, and 5 standard liters per minute (slpm) (with a precision of 1% of the full scale, FS) for CH4, O2, and N2,

respectively. The N2O flow was supplied in the premixed gases via a 1 slpm or a 0.1 slpm mass flow controller (with a precision of 1% FS). The CH4/O2/N2 flame was doped with N2O by substituting O2 and by keeping the equivalence ratio equal to 1 (considering that 4 mols of N2O are needed for 1 mol of CH4). It was not possible to maintain constant the overall volumetric flow rate except with the lowest amounts of N2O added. Volumetric flow rates are listed in Table 1. The ratio between the O-atoms provided from N2O and the total of O-atoms in the mixture is comprised between 0 and 0.2. 2.2. Laser Diagnostics for NCO Detection at 440 nm. The laser system consists of a Quantel Nd:YAG laser pumping a dye laser. A wavelength around 440 nm was provided from the fundamental dye radiation (Coumarin 440 diluted in ethanol) pumped with the third-harmonic frequency at 355 nm. The 6 ns duration pulse had a bandwidth of around 0.17 cm1 at 440 nm. The dye laser wavelength was calibrated using the LIF excitation spectrum of CH in the BX(01) vibrational band around 440 nm using the LIFBASE software.33 The laser energy fluctuations were monitored by a postflame photodiode located after the burner enclosure. The photodiode signal was used as the trigger source for the data acquisition. The laser beam was introduced parallel to the burner surface. To limit laser reflection inside the low-pressure vessel, the exit window was tilted at a Brewster angle. The LIF signal was collected at f/4 by a two-lens system and was focused on the entrance slit of a 0.275 m monochromator providing a high degree of rejection of background radiation. The fluorescence signals were collected with a 1.5 ns rise-time Philips XP2020Q photomultiplier tube (PMT). The fluorescence and laser intensity signals were simultaneously averaged and stored by a digital oscilloscope (LECROY 9354A, 8-bit, 500 MHz bandwidth, 1 GS/s sampling rate). The collected traces were processed using Labview 7.1 software. CRDS technique was used to determine the absolute concentration of NCO in the doped flames. For the CRD measurements, the laser beam was shaped with an optical system consisting of a 100 μm pinhole aperture and two lenses of 100 mm and 300 mm focal length in order to match approximately the TEM00 transverse mode of the cavity.34,35 The 420 mm length ring-down cavity consisted of two highly reflective mirrors (25 mm diameter, 250 mm radius of curvature, R > 99.85%). The CRD signal was collected with an XP2020Q PMT. The signal was recorded using the digital oscilloscope triggered on the laser pulse averaged over 127 pulses and processed in order to measure the cavity decay time, τ(λ). The fitting procedure was performed over 2000 points which represent a duration of 4 μs (i.e., around 4τ(λ)). Assuming that the species concentration is uniform along the flame diameter, the net losses per pass because of the absorbing species can be determined using the ON and 5347

dx.doi.org/10.1021/jp201453k |J. Phys. Chem. A 2011, 115, 5346–5353


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Table 2. Absolute NCO Mole Fraction (ppm) at the Peak Location (pk in mm) Determined from the LIF Signal Calibrated Using CRDS Measurements in Flame (100-20)a flame

pk (mm)

T (K)

σ(440.479 nm,T) (1018 cm2 molecule1)

LIF (au)

losses/pass (ppm)

NCO (ppm) CRDS

NCO (ppm) LIF

(100-20)

5.75

2100

4.7

100.00

101 ( 5

19.45

19.45

(100-10) (100-05)

6.00 5.75

1940 1787

6.2 8.3

54.54 24.86

67 ( 5 26 ( 5

8.98 2.41

7.38 2.33

(100-01)

6.50

1700

9.8

(100-005)

6.75

1700c

(125-00)

7.75

1710

b

9.6

5.42

0.392

2.88

0.215c

1.11

0.086

Measurements of temperature (T), NCO relative mole fraction normalized to the flame (100-20), net losses/pass were all performed at the NCO peak location (pk). Absorption cross sections, σ(440.479 nm, T), have been calculated using PGOPHER procedure (see text) knowing the temperature. b Flame in which the CRD measurement has been used for the LIF calibration. c Temperature is estimated. a

OFF resonance wavelength procedure previously detailed.13 These losses are equal to σ(λ, T)nls where σ(λ, T) is the absorption cross section of the sample (depending on the temperature T and the wavelength λ) whose concentration is n, and ls is the flame diameter (ls = 6 cm). However, according to previous works combining CRD and LIF techniques, it has been shown that for CH radicals, the line-of-sight measurements of the species density were about 20% lower than in the flame center because of spatial inhomogeneities.3638 Very recently, Nau et al.39 measured radial distribution of NH2 in morpholine/ oxygen/argon flame using CRD techniques. The NH2 mole fraction profile was determined by knowing the actual flame diameter and the radial distribution determined using an Abel inversion procedure. The procedure of correction yielded a decrease of the maximum mole fraction of NH2 on the burner axis of around 20%. They observed a spatial shifting between the corrected and the noncorrected profiles. None of these measurements could be performed during the present work with our experimental setup. Therefore, it is assumed here that NCO concentration is homogeneous along the flame diameter. 2.3. Temperature Measurements. Wavelengths around 225 nm (with a bandwidth of 1 cm1) (for NO-LIF thermometry) were obtained by mixing the residual infrared radiation of the YAG laser with the doubling of the fundamental dye radiation pumped with the second-harmonic frequency at 532 nm. Temperature measurements using NO-LIF have been performed. The LIF excitation spectrum was recorded scanning the wavelength from 225.3 to 225.75 nm following Hartlieb et al.40 by probing the P1(34), P1(33), S21(13), P2 þ P12(35), R1 þ R21(17), S21(12), R2(21), R2(20), P2 þ P12(33), S21(10), P1(29), R1 þ R21(14) lines of the A2ΣþrX2Π(00) band. The LIF signal was measured at the temporal peak. The temperature was determined by fitting the Boltzmann lines after correcting the LIF signal from the laser fluctuations, the absorption Einstein coefficient, and the level degeneracy. With the N2O-doped flames, the native NO allows the direct determination of the temperature at the NCO peak location in the flames. These temperature values are reported in Table 2. The accuracy of the temperature measurements is estimated to be equal to 5%. Lower N2 dilution in the N2O-doped flames led to higher flame temperature than in nondoped flames. The temperature profile was already measured in the rich CH4/O2/N2 flame.13

3. NCO SPECTROSCOPY 3.1. NCO Spectra. The spectroscopy of NCO is well documented at ambient and flame temperatures. Using the spectroscopic

Figure 1. LIF excitation spectrum in the B2ΠX2Π transition of NCO in CH4/O2/N2O/N2 flame (108-20) at 5.3 kPa collected at 365 nm with a bandpass of 4 nm and comparison with an absorption spectrum simulated using PGOPHER program.41 The peak (arrow) in the LIF spectrum at 314.84 nm is due to the P2(12) transition in the AX(00) band of OH.

constants recommended by Elliot et al.,21 the A2ΣþX2Π and B2ΠX2Π transitions of NCO have been simulated in this work at high temperature (T = 2100 K) using PGOPHER program41 and have been compared to experimental spectra. For this purpose, CRD absorption spectrum and excitation LIF spectra (obtained in the linear regime of fluorescence with the laser beam being attenuated and not focused) have been measured in N2Odoped flames at the NCO peak location. 3.1.1. B2ΠX2Π Transition. Attempts to detect NCO LIF using the B2Πi(0,0,0)X2Πi(0,0,0) vibrational band around 315 nm have been performed in a slightly rich N2O-doped flame (108-20) at the NCO peak location where the temperature has been estimated to be around 2100 K. This band is attractive, but as previously pointed out,7 both R1 and R2 bandheads are perturbed by OH and CH transitions hampering any possibility of absorption measurements of NCO. However, the spectrum around 315 nm, first simulated using PGOPHER, shows a very good agreement with the experimental excitation LIF spectrum collected at 364.5 nm with a bandpass of 4 nm obtained at 2100 K as shown in Figure 1. The experimental LIF spectrum is consistent with the excitation LIF spectrum obtained in atmospheric CH4/N2O flame by Jeffries et al.8 5348



Figure 2. NCO spectra measured in N2O-doped flame (100-20) at the NCO peak location (HAB = 5.75 mm, T = 2100 K) using LIF and CRD techniques compared to a simulated absorption spectrum using PGOPHER (with a Gaussian line width of 0.2 cm1) showing the P2 þ PQ12 and OP12 bandheads of the A2Σþ(0,0,0)rX2Πi(0,0,0) transition.

The B2Πi(1,0,0)X2Πi(0,0,0) band around 305 nm offers a good compromise without any perturbations inherent to CH or OH. However, the LIF intensity was found to be very weak in agreement with observations previously made.20,42 Because of the very weak LIF signal collected after exciting the B(1,0,0)X(0,0,0) band, it has not been possible to detect NCO in nondoped flames. Consequently, our efforts have been focused on the AX vibrational band. 3.1.2. A2ΣþX2Π Transition. NCO excitation LIF spectrum (collected at 465 nm with a bandpass of 10 nm) has been recorded in the N2O-doped flame (100-20) at the NCO peak location (height above the burner, HAB = 5.75 mm) where the temperature has been determined to be equal to 2100 K. Figure 2 shows a portion of the excitation LIF spectrum between 440 and 441.2 nm. NCO LIF spectra previously reported6,7 in atmospheric CH4/N2O flame conditions show a very congested spectrum probably because of a lower spectral resolution. The absorption spectrum of NCO using CRD technique was obtained in the same flame conditions as shown in Figure 2. The AX transition being not affected by the B state, the NCO spectrum has been simulated using the PGOPHER program41 after limiting the spectroscopic data to both the A2Σþ(0,0,0) and the X2Πi(0,0,0) states. The simulated absorption spectrum at T = 2100 K is reported in Figure 2 showing both P2 þ PQ12 and OP12 bandheads at around 440.479 and 441.02 nm, respectively. To fit the experimental absorption

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spectrum, a Gaussian line width of 0.2 cm1 was assumed consistently with the laser and NCO Doppler bandwidths. Globally spectra are in very good agreement. The ratio of the CRD intensities of the bandheads OP12 and P2 þ PQ12 is equal to the simulated one, while the LIF intensities ratio is twice as much as the simulated one. This is probably due to a variation of the fluorescence quantum yield. From the fluorescence decay, the fluorescence lifetime was determined by adjusting an exponential fit between 90% and 10% of the signal peak value. Thus, the total fluorescence lifetime was measured to be around 45 and 65 ns in the P2 þ PQ12 and in the OP12 bandheads, respectively. 3.2. Determination of NCO Absorption Cross Section at Around 440.5 nm. The simulated spectrum using PGOPHER program41 (Figure 2) shows that the spectroscopic constants21 fit perfectly the experimental spectrum at flame temperatures. Then, the parameter (q00)1/2Re (where q00 is the FranckCondon factor of the transition A2Σþ(0,0,0)rX2Πi(0,0,0) and Re is the electronic transition moment) is required by the program for the determination the absorption cross section. It has been calculated according to the following expression:43 q00Re2 = 3.292 105 (1/ω00) [(2 δ0,Λ0 0 )/(2 δ0,Λ0 þΛ00 )] f00, where ω00 is the wavenumber of the vibrational band origin (cm1), δ0,Λ corresponds the Kronecker delta, which equals 1 for Σ states and 0 for all other electronic states, and f00 is the oscillator strength for which the value of 0.0026 has been considered.5 In this expression, the electronic moment is obtained in atomic units, that is, in units of (ea0) where e is the electron charge and a0 is the Bohr radius. Hence, this leads to the A2Σþ(0,0,0)rX2Πi(0,0,0) transition of NCO to a parameter (q00)1/2Re equal to 0.4928 D. The absorption cross section at the peak of the P2 þ PQ12 band, around 440.479 nm, is then directly calculated by PGOPHER according to the experimental spectral resolution. This value has to be scaled down by the value of Qv(T) (vibrational partition function of NCO) as PGOPHER only accounts for the input vibrational frequencies (i.e., only the lowest vibrational state X2Πi(0,0,0) here) in its calculation. In the present work, the wavenumber values of the vibrational bands of NCO ω1, ω2, and ω3 have been considered equal to 1286.7, 534.2, and 1972 cm1, respectively.17 To validate this procedure, we first tried to recalculate the absorption cross sections of NCO from the work of Louge et al.,5 which is the only quantitative approach we found in the literature. To do this, we used their reported experimental spectrum (defined by a Voigt parameter a = 0.1) and fitted it with PGOPHER for a temperature of 1450 K. Hence, the simulated spectrum enables the determination of the absorption cross section at the peak of the band, σ(440.479 nm, 1450 K), with their experimental spectral resolution. This way, we obtained an absorption cross section σ(440.479 nm, 1450 K) equal to 2.13 1017 cm2 molecule1 which corresponds to an absorption coefficient β(440.479 nm, 1450 K) equal to 108 cm1 atm1 that is in excellent agreement with the value of 110 cm1 atm1 previously determined.5 Moreover, the variation of the ratio β/f00 as a function of temperature has been calculated (with a Voigt parameter a = 0) and has been compared to the values of Louge et al.5 in Figure 3. The obtained excellent agreement confirms the reliability of the PGOPHER procedure implemented in this work for the absorption cross section calculation. Therefore, by assuming a constant Gaussian line width of 0.2 cm1 corresponding to our experimental spectral resolution, we get the theoretical absorption cross sections σ at the peak of the P2 þ PQ12 bandhead of the A2Σþ(0,0,0)rX2Πi(0,0,0) transition of NCO as a function of the temperature as reported 5349



Figure 3. Theoretical absorption coefficient of NCO (A2Σþ(0,0,0)r X2Πi(0,0,0) transition at around 440.479 nm) as a function of temperature considering f00 = 0.0026 and a Voigt parameter a = 0 (line). Comparison between the present calculations (line) and previous values5 ()).

Figure 4. Theoretical absorption cross section of the A2Σþ(0,0,0)rX2Πi(0,0,0) transition of NCO at the peak of the P2 þ PQ12 bandhead (around 440.479 nm) as a function of temperature considering a Gaussian line width of 0.2 cm1.

in Figure 4. The absorption cross section depends strongly on the temperature. There is indeed 1 order of magnitude of variation of σ between 1000 and 2000 K. The absorption cross section at 2100 K has been calculated to be 4.7 1018 cm2 molecule1. Considering the temperature measured at the NCO peak location in each flame, the corresponding absorption cross sections are reported in Table 2.

4. NCO PROFILES 4.1. Foreword about the LIF Calibration. In this work, LIF and CRD techniques have been used to determine the mole fraction profiles of NCO. Both techniques were applied by tuning the laser wavelength ON resonance with the P2 þ PQ12 bandhead of the A2Σþ(0,0,0)rX2Πi(0,0,0) transition hence exciting a large number of rotational levels. The possible contribution of absorbing species which could interfere with NCO in

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Figure 5. NCO net losses/pass profiles in the N2O-doped flame (100-20) obtained by CRD (þ) by exciting the P2 þ PQ12 bandhead. Comparison with the profile of the LIF signal (O) after its peak value has been calibrated to the peak NCO losses from CRD measurements.

the gases surrounding the flame has been estimated as follows. Considering the species present in the flame and the investigated spectral range, we have only identified NO2 as a potential interfering stable species. However, the kinetic species simulation shows that NO2 is formed in the preheated reaction zone and is consumed very quickly in the flame front. This has been comforted by scrutinizing the LIF excitation, dispersed fluorescence, and CRD spectra recorded in the N2O-doped flame (100-20), which did not reveal interferences. The LIF signal, SLIF, and the CRD net losses/pass, L, are both related to the population of the laser-excited levels. The profiles determined from LIF and CRD signals in the N2O-doped flame (100-20) are compared and are found in good agreement as shown in Figure 5. The thinner LIF profile can be explained by the fact that the LIF technique provides a local NCO measurement on the burner axis in contrast to the spatially integrated CRD measurements. The nonperfect homogeneity of premixed flames was shown to cause such enlargement.36 The peak location of both the CRD and the LIF profiles is located at the same position. In this work, the conversion of the losses into absolute mole fractions was performed by considering that the NCO concentration is homogeneous along the flame diameter (equal to 6 cm). A convenient way to express SLIF and L consists in replacing the involved rotational population by the mole fraction of NCO, χNCO. The fluorescence signal, SLIF, can be expressed as the following:44 SLIF µ χNCO

p σðλ, TÞφðλ, TÞ kT

ð1Þ

where p and T are the pressure and temperature. σ(λ, T) is the temperature-dependent absorption cross section of NCO which includes the Boltzmann ratio, that is, the portion of NCO molecules in the rotational levels being excited by the laser at the wavelength λ. φ(λ, T) is the fluorescence quantum yield. The CRD losses, L, may be expressed as follows:35 L ¼ χNCO

p σðλ, TÞls kT

ð2Þ

where k is the Boltzmann constant, and ls is the flame diameter. Because of the consistency between CRD and LIF profiles, the 5350



Figure 6. Fluorescence lifetimes as function of the height above the burner in flames stabilized at p = 5.3 kPa at 440.479 nm. Symbols (), 4, O, and 0) refer to flames labeled (100-20), (100-10), (100-05), and (100-01), respectively (see Table 1).

Figure 7. NCO mole fractions (dotted line) and LIF signal (line) profiles and σ(λ, T)/T (dashed line) normalized at the NCO peak location in a rich CH4/O2/N2 flame (125-00).

procedure to calibrate SLIF into mole fraction of NCO is straightforward. SLIF is converted into χNCO in arbitrary units from eq 1, which is then calibrated in absolute mole fractions using the CRD signal from eq 2 at the peak location of the NCO profile. The contribution of the correction of LIF profiles because of T and φ(λ, T) variation in the flames has been analyzed. In lowpressure flames, the fluorescence quantum yield may vary because of the local variation of the collider concentration because of the chemistry and of the temperature along the burner axis. The fluorescence lifetime profiles measured in the N2Odoped flames were not found significantly affected by the flame conditions as shown in Figure 6. Thus, the variation of the fluorescence quantum yield φ(λ, T) is considered negligible. Considering the temperature profile previously measured in the rich CH4/O2/N2 flame,13 the LIF signal, SLIF, and the NCO mole fraction profiles, SLIF/(σ(λ, T)/T), are compared in Figure 7 after being normalized at the NCO peak location. It is shown that the variations of σ(λ, T)/T (reported in Figure 7)

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Figure 8. Ring-down cavity losses along the burner axis ON () at 440.479 nm) and OFF (þ at 440.486 nm) resonance with NCO radical in the N2O-doped flame (100-20). The net losses profile (star) is determined by subtracting the OFF from the ON profiles.

affect the NCO mole fraction profiles only close to the burner surface. Since the whole temperature profiles have not been measured in the N2O-doped flames, the variations of σ(λ, T)/T along the burner axis have been neglected hereafter. 4.2. LIF Calibration by CRD. In the flames doped with a significant amount of N2O, NCO profiles could be measured using the CRDS technique. The ON resonance wavelength was tuned at the peak of the bandhead of the P2 þ PQ12 at 440.479 nm while the OFF resonance wavelength was tuned immediately after the bandhead (around 7 pm from the peak of the bandhead). Figure 8 shows the cavity losses per pass determined at wavelengths ON and OFF resonance with the NCO radicals along the height above the burner in the N2O-doped flame (100-20). At the NCO peak location (T = 2100 K), the net losses per pass have been determined to be equal to 100 ( 5 ppm. Close to the burner surface, the negative values of the losses come from the ON resonance minus the OFF resonance losses. This may be explained by light scattering and by a small deviation of the beam waist in the vicinity of the burner surface. Net losses per pass in the three N2Odoped flames ((100-20), (100-10), and (100-05)) have been measured and have been converted into absolute NCO mole fraction knowing T and σ(λ, T) at the NCO peak location as shown in Table 2. 4.3. NCO Mole Fraction Profiles. The LIF technique has been applied to measure NCO profiles after exciting the P2 þ PQ12 bandhead at 440.479 nm. We worked intentionally in the nonlinear regime of fluorescence to improve the signal-to-noise ratio (SNR) as required to detect NCO in nondoped flames. To increase the collection volume for improving the SNR, the laser beam was focused in the flame with a cylindrical lens of 200 mm focal length whose axis was set parallel to the burner surface. The resulting beam was carefully oriented parallel to the burner surface. The laser arrangement and the optical collection alignment were optimized in the N2O-doped flame (100-20). The entrance slit of the monochromator was adjusted to 300 μm 15 mm parallel to the laser beam, and the exit slit was adjusted in order to provide a spectral bandwidth of 10 nm in order to collect the fluorescence signal in the entire A2Σþ(0,0,0)X2Πi(1,0,0) band at 465 nm. Because of the very low signal-to-noise ratio, especially in the nondoped flames, and to minimize the 5351



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Figure 9. NCO mole fraction profiles along the burner axis in flames stabilized at p = 5.3 kPa from LIF measurements calibrated by CRD measurements in the flame (100-20). Symbols (), 4, O, 0, and star) refer to flames labeled (100-20), (100-10), (100-05), (100-01), and (100-005), respectively (see Table 1).

ratio N2O/O2 varies gradually between 0.01 and 0.50. Even a small amount of N2O added tremendously affects the peak NCO mole fraction showing the very sensitive effect of N2O addition on NO formation. The overall uncertainties of the absolute NCO measurements are estimated to be around 40% in the N2Odoped flames (5% for the CRD net losses, 5% for the temperature, 25% for the absorption cross section, 5% for the linking procedure between the flames, assuming a uniform radial distribution of NCO). In the CH4/O2/N2 flame of richness 1.25, NCO mole fraction at the peak location was determined to be equal to about 85 ppb with an uncertainty estimated to 50% as shown in Figure 10. NCO mole fraction profile has been measured in a stoichiometric CH4/O2/N2 flame for which the measurements are close to limit of detection and for which a rough estimation of the peak NCO mole fraction value yields less than 50 ppb. Figure 10. NCO mole fraction profile along the burner axis in the rich CH4/O2/N2 flame stabilized at 5.3 kPa.

background noise relative to the weak LIF signal, the LIF signal was measured by integrating the signal over 100 ns, 1 ns after the laser pulse. This procedure is appropriate without further correction because in our flames the fluorescence lifetime was relatively constant (Figure 6). The SLIF peak values obtained in the different flames have been linked to each other by comparing their LIF intensity to the one obtained in the N2O-doped flame (100-20) and are reported in Table 2. Following a procedure previously described,13 the linking procedure was repeated several times with a standard deviation lower than 5%. Then, knowing the temperature at the NCO peak location and taking into account the ratio σ(λ, T)/T, the NCO LIF signals were converted into χNCO (in arbitrary units, eq 1.). Final calibration was achieved using the absolute mole fraction of NCO measured by CRD in the N2Odoped flame (100-20). NCO mole fraction profiles obtained in the N2O-doped flames referenced in Table 2 are reported in Figure 9. The NCO mole fractions determined at the peak location are given in Table 2. Figure 9 shows the NCO mole fraction profiles measured in five stoichiometric N2O-doped flames where the

5. CONCLUSIONS A combination of LIF and CRDS techniques has been implemented in premixed low-pressure flames in order to quantitatively measure profiles of NCO radical that is involved in the NOx formation. NCO spectroscopic data21 have been used to fit experimental spectra obtained in N2O-doped flames using PGOPHER program.41 Then, the simulated spectrum has been converted into absorption spectrum in order to calculate the absorption cross section of NCO in the P2 þ PQ12 band of the A2Σþ(0,0,0)rX2Πi(0,0,0) transition at around 440.48 nm. The theoretical absorption cross section at 2100 K is equal to 4.7 1018 cm2 molecule1 considering a Gaussian line width of 0.2 cm1. NCO mole fraction profiles have been measured for the first time in low-pressure (p = 5.3 kPa) premixed CH4/O2/ N2O/N2 and CH4/O2/N2 flames. First, relative NCO profiles have been obtained using LIF diagnostics in six flames. Second, relative profiles have been converted in mole fraction of NCO from CRD measurements performed in a N2Odoped flame. The determined value of NCO mole fraction at their peak location is very low in the rich CH4/O2 /N2 flame (around 85 ppb). 5352


The Journal of Physical Chemistry A These data will be useful for the improvement of the prompt NO mechanism currently developed at the laboratory.13

’ AUTHOR INFORMATION Corresponding Author

*Phone: (33) 320 434930; fax: (33) 320 436977; e-mail: nathalie. [email protected].

’ ACKNOWLEDGMENT This work was supported by the ANR program BLAN-080130 (NO-Mecha) and by the Air Quality Program of IRENI (Institut de Recherche en ENvironnement Industriel). The authors thank the Nord-Pas de Calais Region and the European Funds for Regional Economic Development for their financial support. ’ REFERENCES (1) Perry, R. A.; Melius, C. F. Proc. Combust. Inst. 1984, 20, 639–646. (2) Miller, J. A.; Branch, M. C.; McLean, W. J.; Chandler, D. W.; Smooke, M. D.; Kee, R. J. Proc. Combust. Inst. 1984, 20, 673–684. (3) Dagaut, P.; Glarborg, P.; Alzueta, M. U. Prog. Energy Combust. Sci. 2008, 34 (1), 1–46. (4) Louge, M. Y.; Hanson, R. K. Combust. Flame 1984, 58 (3), 291–300. (5) Louge, M. Y.; Hanson, R. K.; Rea, E. C.; Booman, R. A. J. Quant. Spectrosc. Radiat. Transfer 1984, 32 (4), 353–362. (6) Anderson, W. R.; Vanderhaff, J. A.; Kotlar, A. J.; Dewilde, M. A.; Beyer, R. A. J. Chem. Phys. 1982, 77 (4), 1677–1685. (7) Copeland, R. A.; Crosley, D. R.; Smith, G. P. Proc. Combust. Inst. 1984, 20, 1195–1203. (8) Jeffries, J. B.; Copeland, R. A.; Smith, G. P.; Crosley, D. R. Proc. Combust. Inst. 1986, 21, 1709–1718. (9) Moskaleva, L. V.; Lin, M. C. Proc. Combust. Inst. 2000, 28, 2393–2401. (10) Vasudevan, V.; Hanson, R. K.; Bowman, C. T.; Golden, D. M.; Davidson, D. F. J. Phys. Chem. A 2007, 111 (46), 11818–11830. (11) Sutton, J. A.; Williams, B. A.; Fleming, J. W. Combust. Flame 2008, 153 (3), 465–478. (12) Lamoureux, N.; Mercier, X.; Western, C.; Pauwels, J. F.; Desgroux, P. Proc. Combust. Inst. 2008, 32, 937–944. (13) Lamoureux, N.; Desgroux, P.; El Bakali, A.; Pauwels, J. F. Combust. Flame 2010, 157 (10), 1929–1941. (14) Dixon, R. N. Philos. Trans. R. Soc. London 1960, A252, 165–192. (15) Bolman, P. S. H.; Brown, J. M.; Carrington, A.; Kopp, I.; Ramsay, D. A. Proc. R. Soc. London, Series A: Math. Phys. Eng. Sci. 1975, 343, 17–44. (16) Bondybey, V. E.; English, J. H. J. Chem. Phys. 1977, 67 (6), 2868–2873. (17) Copeland, R. A.; Crosley, D. R. Can. J. Phys. 1984, 62 (12), 1488–1501. (18) Wright, S. A.; Dagdigian, P. J. J. Chem. Phys. 1996, 104 (21), 8279–8291. (19) Yao, J.; Fernandez, J. A.; Bernstein, E. R. J. Chem. Phys. 1997, 107 (21), 8813–8822. (20) Dixon, R. N. Can. J. Phys. 1960, 38 (1), 10–16. (21) Elliot, N. L.; Fitzpatrick, J. A. J.; Western, C. M. J. Chem. Phys. 2008, 129 (16), 164301. (22) Charlton, T. R.; Okamura, T.; Thrush, B. A. Chem. Phys. Lett. 1982, 89 (2), 98–100. (23) Reisler, H.; Mangir, M.; Wittig, C. Chem. Phys. 1980, 47 (1), 49–58. (24) Sullivan, B. J.; Smith, G. P.; Crosley, D. R. Chem. Phys. Lett. 1983, 96 (3), 307–310. (25) Fernandez, J. A.; Puyuelo, P.; Husain, D.; Sanchez Rayo, M. N.; Casta~no, F. J. Chem. Phys. 1997, 106 (17), 7090–7101.

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