Article pubs.acs.org/EF
Intracavity Laser Absorption Spectroscopy Study of HCO Radicals during Methane to Hydrogen Conversion in Very Rich Flames Alexey Fomin,† Tatyana Zavlev,† Vladimir A. Alekseev,‡ Alexander A. Konnov,‡ Igor Rahinov,§ and Sergey Cheskis*,† †
School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel Division of Combustion Physics, Lund University, P.O. Box 118, SE-221 00 Lund, Sweden § Department of Natural Sciences, The Open University of Israel, Raanana 4353701, Israel ‡
S Supporting Information *
ABSTRACT: Stoichiometric and very rich flames of methane have been investigated using nonintrusive laser diagnostics. Absolute concentration profiles of HCO were measured by intracavity laser absorption spectroscopy, and temperature profiles were obtained with laser-induced fluorescence of OH. Premixed CH4 + O2 + N2 flames were stabilized at a pressure of 30 ± 0.3 Torr. These new experimental data were compared with predictions of two models: GRI-mech. 3.0 and Aramco mech. 1.3. GRImech. performs better in a stoichiometric flame, whereas Aramco mech. is in better agreement with experiments in the rich flames. Detailed analysis of the behavior of these two models revealed that similar performance is essentially fortuitous and explained by balancing of different reactions involved in HCO formation and consumption.
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inert packed beds.1−5 In the most common noncatalytic reforming technique, a rich combustion wave (filtration wave) propagates within a reactor consisting of a porous medium1−4 or in a configuration similar to that of Swiss Roll combustors.5 Optimization of these processes can be achieved using a mathematical simulation. Unfortunately, most known chemical mechanisms related to methane oxidation or combustion are optimized for conditions that differ considerably from those used in the syngas production. The GRI-mech. release 3.06 is a very popular instrument for the kinetic analysis of natural gas combustion (including NO formation and reduction) and usually gives satisfactory agreement between experiment and simulation at temperatures above 1000 K and under conditions close to stoichiometric (O2-to-CH4 ratio is equal to 2.0; the equivalence ratio, ϕ = 1) or fuel lean. The optimal conditions for partial oxidation are expected at ϕ ∼ 4, which corresponds to an extremely fuel-rich combustion. Experimentally, noncatalytic partial oxidation of methane has been studied over a wide temperature range (823−1531 K) using flow reactors.7,8 Konnov et al.8 analyzed these experimental results and suggested several adjustments to the mechanism of hydrocarbon combustion that allow better agreement between experimental results and simulation predictions. However, the possibilities of the mechanism modification were limited since the concentrations of only the final products were measured in the experiment7 using gas chromatography. Moreover, partial oxidation of methane in a flow reactor with long residence times can be affected by the surface reactions.8 Therefore, experimental measurement of absolute radical concentrations and their dependence on the location in a flame, followed by
INTRODUCTION Developing an economic and reliable way for hydrogen production is one of the main problems of “hydrogen energetics”. Hydrogen can be produced from fossil hydrocarbons or organic material (biomass), or from water by supplying energy in order to break H−O chemical bonds. One of the possible ways for hydrogen production from hydrocarbon fuel is reforming of methane to syngas (H2/CO mixture). Methane is the main component of both natural gas and biogas. The traditional technology to produce syngas is steam reforming according to the reaction CH4 + H 2O → CO + 3H 2
ΔH = +206 kJ/mol
(1)
The main disadvantage of this process is its high endotermicity, which demands large amounts of energy. Alternatively, syngas can be produced by the partial methane oxidation according to the following reaction CH4 +
1 O2 → CO + 2H 2 2
ΔH = −36 kJ/mol
(2)
Partial oxidation of methane can be achieved both with and without catalysts. A noncatalytic process has practical advantages, all other factors being equal. The main problem with noncatalytic oxidation is identifying the conditions, where, on one hand, one can make the process occur fast enough to become self-sustaining and provide a high product yield, and on the other hand, maintain the conditions of partial oxidation and prevent full oxidation of methane to CO2 and H2O. In order to reach this goal, it is desirable to exploit very fuel-rich mixtures so that the amount of oxygen will suffice only for methane oxidation to CO but will not be sufficient for further oxidation to CO2. To date, the researches have focused on ultrarich combustion of premixed fuel/air mixtures in inert reticulated ceramic or © 2015 American Chemical Society
Received: July 2, 2015 Revised: August 26, 2015 Published: August 27, 2015 6146
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stainless steel head with a 6 cm diameter flame zone. This burner is placed inside a low-pressure vessel and operated with a premixed CH4/O2/N2 gas flow regulated by calibrated mass flow controllers (MFC model 1259, MKS Instruments) at a pressure of 30 ± 0.3 Torr regulated by an exhaust throttle valve with a feedback valve controller (model 252/253, MKS Instruments). The parameters of the gas flows are shown in Table 1.
comparison with computer modeling, provides an unbiased test for chemical mechanism used in the modeling. In this work, we measured concentrations of the HCO radical. HCO radical reactions have a strong influence on the chemistry of the chain reactions of oxidation or combustion, since the concentration of the H atom is strongly dependent on the presence of HCO. This is due to two concurrent processes: (1) fast thermal decomposition of HCO radicals (because of the low C−H bond energy) produces H atoms, and (2) consumption of H atoms via their reaction with HCO, which is also very fast, serves as a chain termination reaction. Competition of these reactions, and the reactions of HCO with O, OH, and O2, makes the chemical mechanism sensitive to their rate constants. Absolute HCO concentration measurements employing conventional flame sampling and mass spectrometry have proved to be a daunting task.9−12 Moreover, recent numerical simulations13 have indicated that radicals playing a key role in fuel oxidation, such as OH and HCO, are affected by the sampling procedure and their concentrations at the sampling location can be substantially altered in comparison to unperturbed one-dimensional flames. Laser-induced fluorescence, which is often the method of choice for measurement of a plethora of key combustion intermediates, cannot be conveniently applied in the case of HCO due to the strong predissociation of the latter. Jeffries et al.14 succeeded in measuring the relative HCO concentration in a low-pressure (6 Torr) flame via the 245 nm BX transition. The challenges associated with using LIF in general and planar LIF (PLIF) in particular for flame measurements have been recently discussed by Zhou et al.15 In the case of HCO, highly sensitive methods of laser absorption spectroscopy techniques, such as intracavity laser absorption spectroscopy (ICLAS) and cavity ring-down spectroscopy (CRDS), can be a useful alternative to LIF.16−21 In practical combustion situations, the sensitivity of ICLAS exceeds that of CRDS,22 which made it the method of choice to monitor HCO in a visible range in quazi 1-D low-pressure flames.
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Table 1. Parameters of the Studied Flamesa equivalence ratio ϕ ϕ ϕ ϕ ϕ
= = = = =
1.0 1.5 1.6 1.7 1.9
CH4
O2
N2
450 1350 1320 1320 1470
900 1800 1650 1550 1550
2670 1600 990 780 0
The mass flow rates are given in standard cubic centimeters per minute (SCCM). The total pressure is constant, at 30 Torr.
a
ICLAS. The arrangement of the ICLAS layout in the flat flame apparatus (see Figure 1) is outlined in detail in our previous works,19,23−26 and only a brief description is given below. In ICLAS, the absorber is placed inside the cavity of a broadband laser. Because of the positive feedback mechanism in lasers, even trace quantities of a narrow-line absorber produce holes in the spectrum, where the laser output is partially quenched. The combustion chamber hosting the flat low-pressure flame supported on the McKenna burner is situated inside the cavity of a home-built quasi-cw dye-jet laser and is isolated from other parts of the cavity by two glass windows placed at Brewster’s angle. The laser consists of the dye (Kiton-620) jet placed inside an astigmatically compensated, three-mirror folded cavity formed by M2, M3, and OM (output mirror). The dye laser is pumped by a second harmonic of a Nd:YAG laser (SPROUT, Lighthouse Photonics). The central wavelength of this broadband laser source can be tuned by introducing a thin pellicle beamsplitter (BP108, Thorlabs) inside the cavity. The laser generation time, tg, in the ICLAS is the time interval between the beginning of the laser generation and the sampling, which is controlled with the aid of two acousto-optical modulators: AOM1 and AOM2 (AOM-40 IntraAction Corp). The spectral output is analyzed with the aid of a highresolution spectrograph (SPEX 1000M) with a spectral resolution of about 0.003 nm. LIF Thermometry. Laser-induced fluorescence of OH radicals is a well-established and commonly accepted nonintrusive method of flame thermometry, verified and characterized for a variety of combustion systems, including low-pressure flat flames.27,28 In this work, the OH spectra (R-branch of the (0,0) band of the A2Σ+ ← X2Πi system system centered around 307 nm) were measured by LIF and used for the evaluation of temperature profiles of the flames under study according to protocol described elsewhere.26 Briefly, the broadband fluorescence is collected at a right angle to the laser beam by an intensified CCD (ICCD) camera (PI-MAX, Princeton Instruments). The laser intensity is monitored by a postflame photodiode, whose signal is digitized by an oscilloscope (LeCroy Waverunner, LT 262, 350 MHz, 1 Gs/s). The gated ICCD camera allowed us to use a detector gate, which is short (10 ns) compared to the OH fluorescence lifetime under our experimental conditions. Using such a short detector gate promptly after the excitation laser pulse allows one to neglect the rotational-level-dependent quantum yield, since only negligible quenching occurs during the short temporal window of detection.29 The laser intensity is recorded for each wavelength during the excitation scan and is used to normalize the spectra on a point-by-point basis. A homemade LabView code was employed to evaluate the observed intensity of each rotational line in the spectrum. The evaluation of the rotational lines intensities was conducted using a nonlinear least-squares fitting, where Gaussian line width and polynomial approximated baseline were also considered as fitting parameters. The signal of the fluorescence belonging to a specific transition can be expressed as
EXPERIMENTAL SECTION
Low-Pressure Flat Flame Burner. The experimental setup is shown in Figure 1. The flat flame McKenna burner has a porous
Figure 1. Experimental setup: M1, M2, M3, and OM are mirrors, W1 and W2 are windows, BS is a beamsplitter, and AOM1 and AOM2 are acousto-optical modulators. 6147
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Energy & Fuels S = A exp(− Erot /kbT )
GRAD 0.02 and CURV = 0.5, resulting in typical number of grid points around 500. The simulations of the flame structure were performed with experimental temperature profiles approximated with a spline function. Two detailed kinetic mechanisms were tested in the present work: GRI-mech. 3.06and Aramco mech. 1.3.32 GRI-mech. 3.0 is, since 1999, an established standard for methane combustion modeling. Aramco 1.3 is a recently developed mechanism for oxidation of small hydrocarbons and oxygenates. This mechanism includes 1542 reactions between 253 species.
(3)
where S is the fluorescence signal and A=
CIlasηB12 N (2J ″ + 1) Q (T )
(4)
where Ilas is the energy of the laser pulse, η is the fluorescence quantum yield, B12 is the absorption coefficient of the excited transition, Erot is the rotational energy of the ground level, Q(T) is the partition function, J″ is the rotational quantum number of the ground level, N is the total concentration, and C is the experimental constant, which depends on the solid angle of light collection, the transmission efficiency of the optics, and the photoelectric efficiency of the light detection. The temperatures are obtained from Boltzmann plots, based on the rotational line intensities extracted from the experimental spectra and LIFBASE30-tabulated Einstein absorption coefficients of individual rovibronic transitions in the R-branch of the (0,0) band of the A2Σ+ ← X2Πi system system of OH (see Figure 2). The extracted temperatures were used for conversion of the measured HCO absorption to absolute concentration values and as an input for kinetic modeling.
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RESULTS HCO Spectrum and Absorption Cross Section. Figure 3 depicts the HCO absorption spectrum obtained by dividing the
Figure 3. Fragment of the HCO ICLAS spectrum recorded in the ϕ = 1.7 flame. The C2 lines are marked by asterisks.
ICLAS spectrum at the distance from the burner (DFB) of 5.2 mm by the “reference” spectrum recorded at DFB = 74 mm. The concentration of HCO at the former position is assumed to be negligible, and therefore, this procedure produces correction for absorption by nonradical molecules in the laser cavity (mainly water lines) and partially compensates deviations of the baseline. However, the intracavity laser intensity may vary between the vicinity of the flame front, where the HCO radicals are located, and the large distances from the burner, where the reference spectrum is recorded. Therefore, baseline transmittances can be >100%. The absorbance deduced from the ICLAS spectra is defined as a natural logarithm of the ratio of the baseline in the vicnity of the absorption deep to the value of the absorption deep minimum. This part of the spectrum involves the R, Q, and part of the P branches of the (0,9,0)− (0,0,0) vibrational band of the A ̃ ← X̃ system. The spectrum of this transition has been observed in many works,18,19,33−37 and the lines could easily be assigned. The spectral lines in this transition are broadened due to predissociation, and their line widths are substantially wider than the Doppler or collisional broadening. Therefore, the line shape does not depend on the temperature, and concentration can be evaluated basing on the absorbance in the line center, taking into account changes of the partition function with the temperature. The ICLAS spectrum in this spectral range under fuel-rich flame conditions contains also lines of the C2 radical,17 which are marked in Figure 3 by asterisks. Therefore, in the present work, the P(9) line used in ref 19, where its absorption cross
Figure 2. Experimentally observed and simulated LIF spectrum of the OH radical used for temperature determination (Panel B). Panel A depicts a Boltzmann plot. Y is a normalized intensity of the various rotational lines Y = S/A; see eqs 3 and 4. The OH spectrum was recorded in the flame with equivalence ratio ϕ = 1.5 at 5.3 mm from the burner surface. The Boltzmann plot yields temperature value of 1930 K.
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MODELING DETAILS The modeling was performed using Chemkin Software Release 10112.31 For the simulation of the flame structure, the PREMIX code of Chemkin was used. The modeling was performed with multicomponent transport coefficients and thermal diffusion option. Adaptive mesh parameters were 6148
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predictions. Earlier analysis of methane flames, for example,38 indicated that flame chemistry at equivalence ratios above ϕ = 1.4 is dramatically different from other conditions. A stoichiometric flame was studied for comparison; one may expect that kinetic models tested have been extensively validated for near-stoichiometric mixtures. The rich flames with the largest equivalence ratios (ϕ > 1.8) can only be stabilized under our conditions with a methane/ oxygen mixture without any dilution by nitrogen. On the other hand, the flame velocity of the similar flame with lower ϕ is much higher, and these flames stabilize closer to the burner, which makes the accurate measurements in these flames more difficult. Therefore, we increased the dilution of the flames with lower ϕ in order to keep the observed flame front (maximum of chemiluminescence) at an approximately constant location from the burner with the HCO absorption peak position at a DFB of 4−7 mm.
section was inferred from the critical analysis of the literature, could not be invoked for absolute mole fraction evaluation, since it overlaps with the R(12) line of the d3Πg ← a3Πu (0-2) transition of the C2 radical at 16235.15 cm−1. The HCO concentration measurements were, therefore, performed using the P(10) line which is not overlapped with C2 lines, using the absorption cross section value of 1.05 × 10−18 cm2. The absorption cross section of the P(10) line was evaluated previously17 by comparison of its intensity with that of the P(9) line under identical flame conditions, where no C2 radical formation was observed. Temperature Measurements. The knowledge of temperature profile is needed both for correct concentration evaluation from the measured absorbance and for computer modeling. The ICLAS ability to observe many rotational lines simultaneously in a single recording (single pulse of the laser) makes, in principle, possible the temperature determination using the HCO spectrum itself. Unfortunately, the narrow spatial profile of HCO in the flame and weak dependence of the relative intensity of rotational lines with low J number result in relatively large uncertainties in the temperature evaluation.19 In this work, we used the LIF spectrum of OH for the temperature determination. The temperatures can be also evaluated using kinetic modeling of the flames by solving the energy equations. In this case, only heat losses to the burner are taken into account. In a real situation, the temperatures must be lower than predicted due to additional heat losses to the surrounding gas and the walls of the combustion chamber. Figure 4 shows a
Figure 5. Experimental HCO profile (filled circles) and those calculated using the GRI mechanism (dashed magenta line) and Aramco mechanism (solid blue line). Equivalence ratio is ϕ = 1.0. Red diamond: temperatures measured by LIF.
Figures 5 and 6 show acquired HCO profiles in all flames studied. Figure 5 depicts the results in the reference stoichiometric flame, and Figure 6 shows the rich flames. Also presented in these figures are experimental temperature profiles, which have been used for determination of the absolute HCO concentrations. All experimental data in numerical format are available in the Supporting Information. Predictions of the GRI mechanism and Aramco mechanism obtained using the same experimental temperature profiles are shown in Figures 5 and 6 as well. Both models are in good agreement with the experimental data with respect to profile shape and in the absolute concentration values of HCO. Closer look shows that GRImech. performs better in a stoichiometric flame, whereas Aramco mech. is in better agreement with experiment in rich flames. This is further illustrated in summarizing Figure 7 presenting peak values of [HCO] for all flames studied. Strictly speaking, even the flat flame configuration allowing for direct comparison of species concentrations and kinetic models output is only approximately one-dimensional. For a thorough comparison of the experimentally measured absolute mole fractions and the model predictions, one has, in principle, to conduct 2-D computational fluid dynamics (CFD) simulations and compare them with radially resolved temperature and
Figure 4. Measured temperature profiles (symbols) and those calculated from energy conservation equation using GRI-3.0 mechanism (dashed magenta line) and Aramco mechanism (solid blue line).
typical situation using as an example two flames with ϕ = 1.0 and 1.5. The experimentally measured temperatures are somewhat lower than those predicted by the simulation. Interestingly, both the predicted and the experimentally evaluated temperatures are lower for the stoichiometric flame than for the rich one. It is due to the use of higher dilution of this flame by nitrogen (see Table 1) in order to control the distance of the flame front from the burner, as will be explained below. Concentration Profiles. The goal of the present study was to investigate very rich flames of methane and to compare experimental data on HCO concentrations with model 6149
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Figure 6. Experimental HCO profile (filled circles) and those calculated using the GRI mechanism (dashed magenta line) and Aramco mechanism (solid blue line). Equivalence ratio is (A) ϕ = 1.5, (B) ϕ = 1.6, (C) ϕ = 1.7, (D) ϕ = 1.9. Note the different scales in A,B and C,D graphs. Red diamond: temperatures measured by LIF.
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DISCUSSION A convenient approach to elucidate model behavior is performing sensitivity analysis of the predicted HCO concentrations with respect to the rate constants implemented. Figures 8 and 9 show sensitivity spectra for maximum [HCO] in stoichiometric and rich, ϕ = 1.7, flames, respectively, for both mechanisms. Reaction numbering in each panel of Figures 8 and 9 corresponds to that of individual models. The sensitivity spectra in both flames are dramatically different not only in absolute values of the normalized sensitivities but also in the ranking of specific reactions. Moreover, some reactions are manifested among the 20 most sensitive only in one of the models. Table 2 summarizes the most important reactions in two models together with their rate constants and sensitivity ranking in stoichiometric and rich flames. In most cases, the rate constant expressions are different except for reactions
Figure 7. Experimental and calculated concentration of HCO in the maximum of concentration profiles.
H + HCO ↔ H 2 + CO
(5)
and
concentration fields retrieved by imaging and/or tomographic techniques. However, our previous observations26 indicate that, for maxima of radical profiles (around 5−7 mm from the burner at our flame conditions), the tomographically recovered mole fraction at the central radial position is very similar to that retrieved by standard path-integrated measurements. This is due to two counterbalancing effects: the presence of radicals outside of the nominal 6 cm burner diameter on one hand and the concentration drop at the burner edges on the other. Therefore, it is concievable that, with the path integrated measurement approach undertaken in the present work, the main conclusion is still valid: both GRI 3.0 and Aramco mech. capture well the experimental findings. However, detailed analysis of the behavior of these two models described below revealed that this similar performance is essentially fortuitous and explained by balancing of different reactions involved in HCO formation and consumption.
CH3 + HCO ↔ CH4 + CO
(6)
To facilitate comparison of the rate constants, the values at 1700 K, which is close to the position of the maximum concentration of HCO in stoichiometric and rich flames, are also presented. The most sensitive reaction in both mechanisms is decomposition of the HCO radical. In the GRI-mech., it is presented as two reactions: one for H2O as a third body and another for all other species. The rate constant of the reaction HCO + H 2O ↔ H + CO + H 2O
(7)
served as an optimization parameter for GRI-mech. adjustment. After optimization, the collisional efficiency of H2O with respect to other colliders was found to be 8. This value could be compared with collisional efficiency of H2O implemented in the Aramco mech. set to 12. 6150
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Figure 8. Normalized sensitivity coefficients of the maximum [HCO] in stoichiometric flame. Top: GRI-mech.; bottom: Aramco mech.
Figure 9. Normalized sensitivity coefficients of the maximum [HCO] in rich (ϕ = 1.7) flame. Top: GRI-mech.; bottom: Aramco mech.
The ranking of other reactions in the two mechanisms is largely different. Some reactions, e.g.
partitioning between (a) and (b) is not settled yet; Baulch et al.50 stated that channels (a) and (b) are dominating with branching ka/k = 0.8 and kb/k = 0.2 at 298 K. Hack et al.52 determined the branching ka/k = 0.55 ± 0.05 and kb/k = 0.45 ± 0.05, also at 298 K. According to Balucani et al.,53 the main products, channel (a), are formed via methoxy radicals of high internal energy. The theoretical work by Harding et al.49predicts a slight temperature dependence. In the GRI-mech., both channels (a) and (b) are implemented with the overall rate from Slagle et al.41 and the branching ratio suggested by Seakins and Leone.40 The channel (b) is written in the mechanism as a forward reaction only In the Aramco mech., the overall rate constant expression by Harding et al.49 is accepted and attributed to channel (a) only. This choice leads to a dramatic difference in the sensitivity spectra shown in Figures 8 and 9. Channels (a) and (b) implemented in the GRI-mech. are among the most sensitive and, even more important, have opposite signs. Thus, the branching ratio between channels (a) and (b) defines the impact of the overall rate constant of the reaction between atomic oxygen and methyl radical on the HCO concentration. In Aramco mech., this reaction is less sensitive since its impact was compensated by other reactions adjusted in the mechanism. Both GRI-mech. and Aramco mech. are “tuned”
CH3 + OH ↔ CH 2OH + H
(8)
and HCO + OH ↔ CO + H 2O
(9)
are in the top 10 for Aramco mech. and almost insensitive (the ranking below 20) for GRI-mech. The most dramatic difference between the two models is in the presentation of the reaction between atomic oxygen and methyl radical: ↔
CH 2O + H
(a)
CH3 + O → CO + H 2 + H (b)
(10)
The overall rate of this reaction is well-determined; the recommendation by Baulch et al.50 was the mean of eight studies that agree within error limits. The later works by Hack et al.52 and Harding et al.49 also fall in the same range. The product branching has shown to be an experimental challenge, and the results differ. There is an agreement that channel (a) is dominating, whereas the channel (b) less important. The 6151
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Energy & Fuels Table 2. Most Sensitive Reactions for Maximum [HCO] in Stoichiometric and Rich (ϕ = 1.7) Flamesa A
GRI reactions 166. HCO + H2O ↔ H + CO + H2O 167. HCO + M ↔ H + CO + M 168. HCO + O2 ↔ HO2 + CO 38. H + O2 ↔ O + OH 10. O + CH3 ↔ H + H2CO 284. O + CH3 → H + H2 + CO 55. H + HCO ↔ H2 + CO 61. H + CH2OH ↔ OH + CH3 53. H + CH4 ↔ CH3 + H2 100. OH + HCO ↔ H2O + CO 160. CH3 + HCO ↔ CH4 + CO 98. OH + CH4 ↔ CH3 + H2O 3. O + H2 ↔ H + OH Aramco reactions 30. HCO + M ↔ H + CO + M 31. HCO + O2 ↔ HO2 + CO 1. H + O2 ↔ O + OH 146. CH3 + O ↔ CH2O + H 32. HCO + H ↔ CO + H2 135. CH3 + OH ↔ CH2OH + H 128. CH4 + H ↔ CH3 + H2 35. HCO + OH ↔ CO + H2O 38. HCO + CH3 ↔ CH4 + CO 129. CH4 + OH↔ CH3 + H2O 2. O + H2 ↔ H + OH a
1.5 1.87 1.34 2.65 5.06 3.37 7.34 1.65 6.60 5.00 2.65 1.00 3.87 A 5.7 7.58 1.04 5.54 7.34 4.69 6.14 1.02 2.65 5.83 5.08
× × × × × × × × × × ×
× × × × × × × × × × × × ×
1018 1017 1013 1016 1013 1013 1013 1011 108 1013 1013 108 104
1011 1012 1014 1013 1013 1010 105 1014 1013 104 104
n
E
−1 −1 0 −0.7 0 0 0 0.7 1.6 0 0 1.6 2.7
17000 17000 400 17041 0 0 0 −284 10840 0 0 3120 6260
n
E
0.7 0 0 0.1 0 0.8 2.5 0 0 2.6 2.7
14870 410 15286 −136 0 3566 9587 0 0 2190 6292
k at 1700 K
source
5.76 × 1012 7.18 × 1011 1.19 × 1013 9.36 × 1011 5.06 × 1013 3.37 × 1013 7.34 × 1013 3.28 × 1013 3.93 × 1012 5.00 × 1013 2.65 × 1013 5.86 × 1012 3.2 × 1012 k at 1700 K
GRI 39 GRI GRI 40, 41 40, 41 42 GRI GRI 43 44 45 46 source
1.28 6.71 1.13 1.21 7.34 6.26 4.28 1.02 2.65 7.64 4.16
× × × × × × × × × × ×
1012 1012 1012 1014 1013 1012 1012 1014 1013 1012 1012
Aramco 47 48 49 42 Aramco 50 47 44 Aramco 51
ϕ=1
ϕ = 1.7
1 1 1 9 2 3 5 >20 7 10 19 8 >20 ϕ=1
1 5 4 2 3 6 7 11 9 >20 8 17 13 ϕ = 1.7
1 2 3 6 4 9 7 5 7 8 15
1 5 2 6 4 3 8 14 9 >20 9
Units are cm3, mole, s, cal, and K. k = ATn × exp(−Ea/RT).
models; rate constants adjusted in these models are explicitly designated in Table 2 in the “source” column. To further illustrate the differences between the models, a rate-of-production (ROP) analysis of HCO at the maximum of the spatial profile has been performed in rich (ϕ = 1.7) flame, as shown in Figure 10. At these conditions, the overall rate is zero; that is, the sum of positive ROP of HCO is equal to the sum of negative ROP, as seen in Figure 10. In both models, the major source of HCO is from formaldehyde reacting with H atoms, while the major sink is due to the HCO decomposition. Since the channel of reaction 10b is not implemented in Aramco mech., the formation of HCO is augmented in this model, and it is compensated by the higher rate of reaction HCO + M ↔ H + CO + M
(11)
at 1700 K; see Table 2. The overall good agreement with the experimental HCO profiles for each of the two models occurred due to different reasons; therefore, additional experimental studies are required to determine which of the two models is better. A priori, Aramco mech. seems to be an obvious choice due to the more extensive range of validation cases used for its development. At the present conditions, the high importance of HCO decomposition channel via collisions with H2O was identified; therefore, the systems with a higher amount of H2O can be studied in the future, e.g., steam-diluted CH4 + O2 mixtures. Additional experiments could be also aimed at investigating formation of HCO in flames in a wider temperature range.
Figure 10. Rate of HCO production in rich (ϕ = 1.7) flame. Top: GRI-mech.; bottom: Aramco mech.
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measured by intracavity laser absorption spectroscopy, and temperature profiles were obtained with laser-induced fluorescence of OH. These new experimental data were compared with predictions of two models: GRI-mech. 3.0 and Aramco mech. 1.3. GRI-mech. performs better in the stoichiometric
CONCLUSIONS In the present study, stoichiometric and very rich flames of methane have been investigated using nonintrusive laser diagnostics. Absolute concentration profiles of HCO were 6152
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Energy & Fuels flame, whereas Aramco mech. is in better agreement with experiments in the rich flames. Detailed analysis of the behavior of these two models revealed that similar performance is essentially fortuitous and explained by balancing of different reactions involved in HCO formation and consumption. To scrutinize the validity of the models, further investigation of other species during methane to hydrogen conversion in very rich flames should be performed, which is an objective of the authors. A potential candidate could be C2 radical registered in these flames, yet it is left beyond the scope of the present study, since neither GRI-mech. nor Aramco mech. include reactions of this species.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.5b01497. Experimental data of temperature and HCO concentrations in methane flames with various equivalence ratios (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported in part by the Israel Science Foundation (Grant No. 1149/12), Israel Ministry of Energy and Water Resources (Grant No. 211-11-007/2011-7-10), and the Research Authority of The Open University of Israel (Grant No. 47324).). V.A.A. is grateful to the COST Action CM0901 for financial support.
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