Effects of CHâNO Interactions on Kinetics of Prompt NO in High

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Effects of CH-NO Interactions on Kinetics of Prompt NO in High-Pressure Counterflow Flames Sameer V. Naik* and Normand M. Laurendeau School of Mechanical Engineering, Room 13, Mechanical Engineering Building, 585 Purdue Mall, Purdue UniVersity, West Lafayette, Indiana 47907-2088 ReceiVed June 7, 2007. ReVised Manuscript ReceiVed October 25, 2007

Laser-induced fluorescence (LIF) measurements of nitric oxide (NO) and methylidyne (CH), previously reported for prompt-dominated laminar, counterflow, methane-air non-premixed flames at a global strain rate of 20 s-1 and pressures from 1 to 6 atm, are analyzed by considering three important aspects of complex CH-NO kinetics. First, net reaction rates are studied as a function of pressure so as to understand the observed peaks in maximum [CH] and [NO] between 2 and 4 atm. Second, key elementary reactions are compared for two detailed chemical kinetic mechanisms, GRI 3.0 and Lindstedt, in order to investigate differences in computed [CH] and [NO]. Third, variations in rate coefficients are studied for those elementary reactions involving CH that strongly influence NO concentrations in these non-premixed flames. Rate coefficients are also considered for reaction steps which directly affect the formation and destruction of NO. On the basis of all three inquiries, rate coefficients for controlling elementary reactions are selected so as to give the best match between measured and predicted [CH] and [NO] over the investigated pressure range. In general, these analyses reveal the unique behavior of controlling elementary reactions in the intermediate pressure range of 2-4 atm. Further work on reaction dynamics is undoubtedly needed to investigate fully the highly complex chemical interactions controlling NO formation in these non-premixed flames.

Introduction The formation of nitric oxide (NO) in combustion systems is a complex phenomenon. Typically, NO is produced via thermal, N2O, NNH, and prompt pathways, depending on temperature and stoichiometry. At temperatures above 1800 K, thermal NO dominates owing to the importance of the Zeldovich reactions. In a previous study, we found that the N2O pathway increasingly controls NO formation with rising pressure for both partially premixed and non-premixed flames.1 Significant contributions from the NNH pathway could exist for lean, premixed conditions. At intermediate temperatures (1500-1700 K), prompt NO is found to be the most influential among the various NO formation pathways. Prompt-NO formation and destruction is closely tied to complex interactions among elementary reactions involving methylidyne (CH) and nitric oxide. The primary reaction, considered as either CH + N2 S HCN + N or CH + N2 S NCN + H by various researchers, with subsequent oxidation of HCN or NCN to NO via O, O2, OH, and HO2, controls the amount of NO produced via the prompt pathway. Furthermore, the CH radical is involved in reburn reactions such as CH + NO S O + HCN CH + NO S H + NCO CH + NO S N + HCO which lead to the removal of nitric oxide. Methylene (CH2) and methyl (CH3) radicals, which are germane to production and * Corresponding author. E-mail: [email protected]. Telephone: 1-765-494-6552. Fax: 1-765-494-0539. (1) Naik, S. V.; Laurendeau, N. M. Combust. Sci. Technol. 2004, 176, 1809–1853.

destruction of CH, also cause the destruction of NO via reactions such as CH2 + NO S H + HNCO CH2 + NO S OH + HCN CH2 + NO S H + HCNO CH3 + NO S HCN + H2O CH3 + NO S H2CN + OH In addition, removal of NO occurs via HCCO + NO S HCNO + CO and HCCO + NO S HCN + CO2. Uncertainties remain concerning the branching ratios and rate coefficients for these HCCO reactions, and such ambiguities influence predictions of [NO] using current chemical kinetic mechanisms. Moskaleva et al.2 and Moskaleva and Lin3 have investigated the products of the reaction between CH and N2. Their work, using a high-level ab initio, molecular orbital calculation along with a multichannel RRKM (Rice-Ramsperger-Kassel-Marcus) analysis, supports the spin-allowed reaction, CH + N2 S NCN +H, rather than the spin-forbidden reaction, CH + N2 S HCN + N. Vasudevan et al.4 recently measured the rate coefficient for the CH + N2 reaction over a range of temperatures and confirmed that the NCN channel is indeed dominant. Smith5 has previously presented evidence favoring NCN as the initial intermediate for prompt NO by using laser-induced fluorescence (2) Moskaleva, L. V.; Xia, W. S.; Lin, M. C. Chem. Phys. Lett. 2000, 331, 269–277. (3) Moskaleva, L. V.; Lin, M. C. Proc. Combust. Inst. 2000, 28, 2393– 2401. (4) Vasudevan, V.; Hanson, R. K.; Bowman, C. T.; Golden, D. M.; Davidson, D. F. J. Phys. Chem. A 2007, 111, 11818–11830. (5) Smith, G. P. Chem. Phys. Lett. 2003, 367, 541–548.

10.1021/ef700327a CCC: $40.75  2008 American Chemical Society Published on Web 12/08/2007

CH-NO Interactions in Counterflow Flames

Figure 1. Variation of peak [CH] and [NO] as a function of pressure at a global strain rate of 20 s-1 in CH4/O2/N2 counterflow non-premixed flames. The error bars for the peak concentrations are at the 95% confidence limit. The measurements have been reported previously by Naik and Laurendeau.1,6

(LIF) detection in low-pressure CH4-N2-O2 and CH4-N2-N2O flames. Because of its importance for prompt-NO formation and destruction, accurate knowledge of rate coefficients for elementary reactions related to the production and destruction of CH is essential. Correct predictions of CH concentrations are required so as to compute NO concentrations reasonably well in hydrocarbon combustion systems. In particular, NO concentrations in prompt-dominated flames are extremely sensitive to elementary reactions such as CH2 + H S CH + H2, CH + O2 S HCO + O, and CH + H2O S CH2O + H. These reactions are also significant for the formation and destruction of CH in flames. Moreover, considerable uncertainty exists regarding these rate coefficients and also those for other elementary reactions affecting CH concentrations in flames. In previous work, we reported a peculiar peaking behavior for maximum [NO] and [CH] in counterflow non-premixed flames at pressures of 2-4 atm, as shown in Figure 1.1,6 In addition to standard predictions from GRI 3.0,7 Figure 1 includes (6) Naik, S. V.; Laurendeau, N. M. Appl. Phys. B: Laser Opt. 2004, 79, 891–905. (7) Smith, G. P.; Golden, D. M.; Frenklach, M.; Moriarty, N. W.; Eiteneer, B.; Goldenberg, M.; Bowman, C. T.; Hanson, R. K.; Song, S.; Gardiner, W. C., Jr.; Lissianski, V.; Qin, Z. http://www.me.berkeley.edu/ gri_mech/, accessed 2007.

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calculations using the kinetic mechanism developed by Lindstedt and co-workers.8–11 The predicted variation of peak [CH] as a function of pressure is qualitatively similar for both mechanisms; however, there are obvious quantitative differences. Both mechanisms fail to capture the [CH] peak near 2 atm. The predicted variation of peak [NO], on the other hand, is not only quantitatively but also qualitatively different for the two mechanisms; again, neither mechanism correctly predicts the LIF data. This discrepancy clearly reflects substantial differences in the elementary reactions influencing [CH] and [NO] for these two mechanisms. To develop a robust kinetic model describing pollutant formation for gas turbines and internal combustion engines, we must understand these conflicting aspects within the two mechanisms. By considering important rate coefficients germane to both CH and NO kinetics, we should be able to identify those reaction steps necessary to reproduce the measured data most effectively over a range of pressures. In practical combustion systems, flames of all types are encountered, viz., premixed to partially premixed to non-premixed. Premixed combustion typically leads to low NOx emissions; however, flashback and other stability issues become important. Non-premixed systems are usually more stable but give rise to higher pollutant levels. In most combustors, the fuel and oxidizer mix to some degree, thus producing partially premixed zones. In our previous work, we measured CH and NO concentrations using LIF in counterflow partially premixed and non-premixed CH4/air flames from 1 to 15 atm.1,6 The counterflow configuration gives stable and nearly one-dimensional flat flames between the fuel and oxidizer nozzles. Such a geometry is, therefore, ideal for a rigorous test of chemical kinetics in flames. For partially premixed flames, the peak flame temperatures ranged from 2000 to 2150 K; thus, thermal NO dominated throughout the entire 1-15 atm range.1 Since rate coefficients for the thermal NO mechanism are well-known, good agreement was observed between [NO] predictions and measurements for all partially premixed flames. The non-premixed CH4/air flames, on the other hand, were highly diluted with nitrogen (25% CH4 and 75% N2 in the fuel stream), thus dropping the peak flame temperatures to 1700–1800 K, thereby accentuating the promptNO pathway.1 Owing to deficiencies in our understanding of the complex CH-NO interactions affecting NO formation and destruction via the prompt mechanism, current kinetic mechanisms fail to predict measured NO concentrations for high-pressure nonpremixed flames, especially between 2 and 5 atm as seen from Figure 1. Above 6 atm, increasing contributions occur from the N2O and thermal mechanisms; thus, the predicted and measured NO concentrations again match reasonably well, even for nonpremixed flames.1 Therefore, non-premixed flames in the intermediate pressure range (2 to 5 atm) constitute the focus of this study. We first consider net reaction rates for the most important elementary reactions influencing [CH] and [NO] in prompt-NO dominated non-premixed flames. Next, we compare the most significant reaction steps for the GRI 3.0 and Lindstedt mechanisms. An evaluation is then presented of the available rate coefficients for those elementary reactions most influencing [CH] and [NO] in our non-premixed flames. Particular attention is given to the effects of the spin-allowed prompt-NO pathway on computed CH and NO (8) Leung, K. M.; Lindstedt, R. P.; Jones, W. P. Combust. Flame 1991, 87, 289–305. (9) Lindstedt, R. P.; Lockwood, F. C.; Selim, M. A. Combust. Sci. Technol. 1994, 99, 253–276. (10) Leung, K. M.; Lindstedt, R. P. Combust. Flame 1995, 102, 129–160. (11) Lindstedt, R. P.; Lockwood, F. C.; Selim, M. A. Combust. Sci. Technol. 1995, 108, 231–254.

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Naik and Laurendeau

Figure 2. Variation of net reaction rate as a function of pressure in CH4/O2/N2 counterflow non-premixed flames at a global strain rate of 20 s-1 for reactions relevant to formation and destruction of CH when using GRI 3.0.

concentrations. Finally, we modify rate coefficients for important elementary reactions within the GRI 3.0 mechanism in an effort to match the measured CH and NO concentrations. Kinetic Modeling Numerical computations for all counterflow flames were conducted using OPPDIF, a Sandia opposed-flow flame code.12 The mathematical model reduces the two-dimensional, axisymmetric flow field to a one-dimensional formulation by using a similarity transformation. The model predicts the species, temperature, and velocity profiles along the centerline in the core flow between the two counterflow nozzles. A detailed derivation of the governing equations is given by Kee et al.13 The GRI mechanism, version 3.0,7 is used to handle the chemical kinetics. The mechanism developed by Lindstedt and co-workers8–11 is also used in some calculations. Gas-phase radiation is considered by adding a radiation source term in OPPDIF via a subroutine provided by Gore et al.14 The effect of radiative heat loss is calculated in the optically thin limit. (12) Lutz, A. E.; Kee, R. J.; Grcar, J. F. OPPDIF: A Fortran Program for Computing Opposed-flow Diffusion Flames, Report No. SAND96-8243; Sandia National Laboratories: Sandia, NM, 1996. (13) Kee, R. J.; Miller, J. A.; Evans, G. H.; Dixon-Lewis, G. Proc. Combust. Inst. 1988, 22, 1479–1494. (14) Gore, J. P.; Lim, J.; Takeno, T.; Zhu, X. L. A study of the effects of thermal radiation on the structure of methane/air counterflow diffusion flames using detailed chemical kinetics. 5th ASME/JSME Joint Thermal Engineering Conference, San Diego, CA, March 14–19, 1999; Paper AJTE99-6311.

The model utilizes Planck mean absorption coefficients for the major species CO2, H2O, CO, and CH4; the temperature dependence of these coefficients is considered using fourth-order polynomial fits to the results of narrow-band calculations.14 A sensitivity analysis option within OPPDIF permits the computation of sensitivity coefficients for each species with respect to changes in the rate coefficient for each elementary reaction within a given kinetic mechanism. Such an analysis identifies key reactions controlling peak NO concentrations. Net Reaction Rate Analysis A complete sensitivity analysis for prompt-NO dominated counterflow flames has indicated the importance of certain elementary reactions controlling CH and NO concentrations.15 We can divide such elementary reactions into three categories as follows: (a) reactions affecting formation as well as destruction of CH, (b) reactions affecting formation of NO, and (c) reactions related to destruction of NO via different reburn pathways. As a first step, we study variations in net reaction rate with pressure for these three groups of reactions for counterflow non-premixed flames at pressures up to 15 atm. Figure 2a shows that the net reaction rate for CH4 + H S CH3 + H2 increases continuously with rising pressure except (15) Naik, S. V. Laser-induced Fluorescence Measurements and Modeling of Nitric Oxide and Methylidyne in Laminar, Counter-flow Partially Premixed Flames at High Pressure. PhD Dissertation, School of Mechanical Engineering, Purdue University, West Lafayette, IN, 2004.



Figure 3. Variation of net reaction rate as a function of pressure in CH4/O2/N2 counterflow non-premixed flames at a global strain rate of 20 s-1 for reactions relevant to the formation of NO when using GRI 3.0.

between 3 and 4 atm, when the net reaction rate remains almost constant. This behavior implies that the rate of fuel pyrolysis changes significantly in the same pressure range for which peak [NO] and [CH] exhibit maxima (Figure 1). Figure 2b shows the variation of net reaction rate with pressure for the dominant CH formation reaction (CH2 + H S CH + H2) as well as the most important CH destruction reactions (CH + H2O S CH2O + H and CH + O2 S HCO + O). While the net reaction rate decreases with pressure for these three reactions, we again observe a peculiar reaction rate variation between 3 and 4 atm for each case. Figure 2c and d displays net reaction rates for the remaining reactions relevant to CH kinetics. From Figure 2c, we find that the net rates for HCCO + M S CH + CO + M and HCCO + O2 S OH + 2CO remain constant between 3 and 4 atm while otherwise increasing with rising pressure. This trend is significant because the former becomes important for CH formation at higher pressures, as indicated by the pathway analysis of Naik and Laurendeau.6 Furthermore, ketenyl radicals are involved in the destruction of NO via the HCCO + NO channel; thus, reactions involving HCCO tend to have implications for predicted NO. Figure 2d shows net rates as a function of pressure for the reactions CH + CO2 S HCO + CO and CH2 + CH3 S C2H4 + H. We immediately notice a peak in the net reaction rate around 3–4 atm, which closely mirrors the peak observed in both [NO] and [CH] for these non-premixed flames.1,6 Overall, we find that reactions important to CH kinetics show

noticeably different net reaction rates in the intermediate pressure range of 3-4 atm. Figure 3 shows variations in net reaction rate with pressure for the initiation reactions of the thermal (N2 + O S N + NO), N2O (N2O + M S N2 + O + M), NNH (NNH + O S NH + NO), and prompt (CH + N2 S HCN + N or CH + N2 S NCN + H) pathways, as these routes account for most NOx production in flame environments. Figure 3a shows that the net reaction rate for N2 + O S N + NO increases strongly with pressure up to 3 atm and then decreases slowly with increasing pressure. Figure 3b indicates that the net reaction rate for N2O + M S N2 + O + M increases rapidly with rising pressure, except at 3-4 atm. The net reaction rate for the NNH pathway initially increases and then slowly decreases with rising pressure, as seen from Figure 3c. This behavior is not surprising as the pathway analysis of Naik and Laurendeau1 predicts that both the thermal and NNH pathways are relatively insignificant for these nitrogen-diluted, non-premixed flames. In comparison, the net reaction rate for the initiation reaction of the prompt pathway (either HCN or NCN) displays a peak at approximately 3–4 atm and subsequently falls off with rising pressure. Hence, as for the important CH routes, reactions germane to NO formation clearly exhibit peculiar behavior between 3 and 4 atm for these non-premixed flames. Destruction of NO occurs via several reburn paths which involve reactions of nitric oxide with species such as CH, CH2,


Naik and Laurendeau

Figure 4. Variation of net reaction rate as a function of pressure in CH4/O2/N2 counterflow non-premixed flames at a global strain rate of 20 s-1 for reactions relevant to destruction of NO when using GRI 3.0.

CH3, and HCCO. Figure 4a shows that net reaction rates for the various CH + NO reburn paths drop with rising pressure; however, peculiar behavior again occurs between 3 and 4 atm. Figure 4b indicates that net reaction rates for the various CH2 + NO pathways increase with rising pressure up to about 4 atm and then decrease at higher pressures. For the CH3 + NO destruction routes, net reaction rates continuously increase with rising pressure, except again between 3 and 4 atm, where they remain approximately constant, as shown in Figure 4c. Destruction of NO via HCCO displays similar behavior, as indicated in Figure 4d. In summary, we conclude from Figures 2-4 that net reaction rates for those elementary reactions controlling maximum [CH] and [NO] in counterflow non-premixed flames either peak between 3 and 4 atm or remain relatively constant in this same pressure range. Therefore, the peculiar behavior of peak [CH] and [NO] at intermediate pressures should be related to these complex reaction dynamics. Further insight can be provided by considering the variation of other species concentrations germane to NO chemistry with respect to pressure. Figure 5 shows the computed variation in maximum concentration of the radicals HCCO, HCNO, and HNCO with rising pressure. While the concentration of HCCO continuously decreases with increasing pressure, that for HCNO or HNCO displays a peaking behavior consistent with the measured LIF data. This result suggests that the reburn reactions involving HCNO and HNCO play a crucial role at intermediate pressures. Although not shown here, the concentrations of other species participating in the prompt-NO mechanism (such as CN,

Figure 5. Variation of peak [HCCO], [HCNO], [HNCO], and [C2H2] as functions of pressure at a global strain rate of 20 s-1 for CH4/O2/N2 counterflow non-premixed flames when using GRI 3.0.

HCN or NCN, H2CN, and HCNN) also decrease monotonically with rising pressure. Shimizu and Williams16 have reported that the GRI 3.0 mechanism overpredicts acetylene concentrations by a factor of two in flames for which NO concentrations are overpredicted by approximately the same factor. They suggest that C2H2 is the primary precursor for CH, which would thus directly affect predicted NO concentrations in such flames. From Figure 5, (16) Shimizu, T.; Williams, F. A. J. Propul. Power 2005, 21, 1019– 1028.



Figure 6. Comparisons between CH, CH2, CH3, and HCCO concentrations calculated using the GRI 3.0 and Lindstedt mechanisms in a CH4/O2/N2 counterflow non-premixed flame for a global strain rate of 20 s-1 at 1 atm.

we notice that the predicted peak acetylene concentration exhibits a minimum between 2 and 4 atm, for which the measured peak NO concentration is a maximum. If a direct correlation exists between [C2H2] and [NO], we should observe a maximum rather than a minimum in predicted [C2H2]. For our flame conditions, the production of CH is most sensitive to CH2 + H S CH + H2. Therefore, any errors in predicting [C2H2] may exercise only a minor impact on predicted [NO] for our flame conditions. Differences between the GRI 3.0 and Lindstedt Mechanisms As shown in Figure 1, the variation of peak [NO] with respect to pressure differs qualitatively between the GRI 3.0 and Lindstedt mechanisms. Such disparities between the two mechanisms can be understood by studying differences between key elementary reactions and by comparing predicted profiles of important species involved in the prompt-NO pathway. Sensitivity coefficients related to peak [NO] and [CH] in the two mechanisms for reactions CH + N2 S HCN + N, CH2 + NO S OH + HCN, CH3 + NO S HCN + H2O, and CH3 + NO S H2CN + OH show rather minor differences with respect to pressure. For NO reburn via the ketenyl radical, GRI 3.0 focuses on the HCCO + NO S HCNO + CO pathway. The Lindstedt mechanism, on the other hand, considers HNCO and CO as the primary products of the reaction between HCCO and NO. An additional channel, HCCO + NO S HCN + CO2, is also included in the Lindstedt mechanism. Again, sensitivity

coefficients for the various HCCO + NO paths do not show significant differences between the two mechanisms. Figures 6 and 7 display comparisons between computed concentration profiles for CH, CH2, CH3, and HCCO using the GRI 3.0 and Lindstedt mechanisms at 1 and 4 atm, respectively. Quantitative differences are clearly noticeable between the two kinetic mechanisms. With increasing pressure, the HCCO peak moves toward the fuel-lean region while the CH, CH2, and CH3 profiles shift toward the fuel-rich side. In addition, for both pressures, the shape of the CH3 profile differs substantially from the remaining profiles in the fuel-rich region, where poor agreement has been observed between NO computations and measurements.1 The most significant quantitative difference between the two mechanisms is observed with respect to HCCO, with GRI 3.0 predicting about four times higher [HCCO] as compared to the Lindstedt mechanism. From the above comparisons, we conclude that kinetic changes related to the formation and destruction paths alone may not be sufficient to match the observed pressure dependence of [CH] and [NO], as their sensitivity coefficients do not change drastically with respect to pressure. Nevertheless, destruction of NO via HCCO clearly differs between the two mechanisms; consequently, HCCO chemistry should have a substantial effect on the net NO produced from such flames. Evaluation of Rate Coefficients We now evaluate the effect of varying rate coefficients within GRI 3.0, as available in the literature, for important elementary


Naik and Laurendeau

Figure 7. Comparisons between CH, CH2, CH3, and HCCO concentrations calculated using the GRI 3.0 and Lindstedt mechanisms in a CH4/O2/N2 counterflow non-premixed flame for a global strain rate of 20 s-1 at 4 atm.

reactions relevant to the formation and destruction of both CH and NO. We note that changing the rate coefficient of one elementary reaction within an optimized mechanism, such as GRI 3.0, is not completely scientific since GRI 3.0 has been originally optimized and validated based on other specific targets for CH and NO. However, this simplified approach does provide limits within which computed CH and NO concentrations vary when employing different rate coefficients for specific elementary reactions influencing CH and NO. Rate Coefficients for CH Kinetics. For the reaction, CH4 + H S CH3 + H2, a modest variation in rate coefficient17–19 does not affect either predicted [CH] or [NO] in the flames of this investigation. Destruction of CH for our flame conditions is strongly influenced by CH + O2 S HCO + O and CH + H2O S CH2O + H. Considering the available rate coefficients for the CH + O2 reaction,19–21 we find little effect on predicted CH and NO concentrations. For the CH + H2O S CH2O + H reaction, we examined rate coefficients provided by Baulch et

al.,19 Bosnali and Perner,22 and Blitz et al.23 Since the rate coefficient suggested by Bosnali and Perner22 is higher as compared to the other two, a severe underprediction occurs in both CH and NO, while the other two rate coefficients lead to about 40% higher predictions for both [CH] and [NO] as compared to GRI 3.0. Baulch et al.,19 Berman and Lin,24 and Brownsword et al.25 have provided rate coefficients for the reaction, CH2 + H S CH + H2, which controls the production of CH for our flame conditions. Using their recommendations, CH and NO concentrations rise by about 25% as compared to GRI 3.0. Considering the two rate coefficients for the three-body reaction, HCCO + M S CH + CO + M,19,26 the predicted CH and NO concentrations are affected only slightly. While rate coefficients for the CH + CO2 S HCO + CO reaction show considerable differences,19,20,27 surprisingly little change occurs in the predicted CH and NO concentrations. The rate coefficient for another reaction influencing CH at flame conditions, CH2 +

(17) Tsang, W.; Hampson, R. F. J. Phys. Chem. Ref. Data 1986, 15, 1087–1279. (18) Sutherland, J. W.; Su, M. C.; Michael, J. V. Int. J. Chem. Kinetic. 2001, 33, 669–684. (19) Baulch, D. L.; Bowman, C. T.; Cobos, C. J.; Cox, R. A.; Just, Th.; Kerr, J. A.; Pilling, M. J.; Stocker, D.; Troe, J.; Tsang, W.; Walker, R. W.; Warnatz, J. J. Phys. Chem. Ref. Data 2005, 34, 757–1397. (20) Berman, M. R.; Fleming, J. W.; Harvey, A. B.; Lin, M. C. Proc. Combust. Inst. 1982, 19, 73–79. (21) Rohrig, M.; Petersen, E. L.; Davidson, D. F.; Hanson, R. K.; Bowman, C. T. Int. J. Chem. Kinetic. 1997, 29, 781–789.

(22) Bosnali, M. W.; Prener, D. Z. Naturforsch. 1971, A26, 1768. (23) Blitz, M. A.; Pesa, M.; Pilling, M. J.; Seakins, P. W. J. Phys. Chem. A 1999, 103, 5699–5704. (24) Berman, M. R.; Lin, M. C. J. Chem. Phys. 1984, 81, 5743–5752. (25) Brownsword, R. A.; Canosa, A.; Rowe, B. R.; Sims, I. R.; Smith, I. W. M.; Stewart, D. W. A.; Symonds, A. C.; Travers, D. J. Chem. Phys. 1997, 106, 7662–7677. (26) Taatjes, C. A. J. Chem. Phys. 1997, 106, 1786–1795. (27) Butler, J. E.; Fleming, J. W.; Goss, L. P.; Lin, M. C. Chem. Phys. 1981, 56, 355–365.


CH3 S C2H4 + H, also shows a rather large variation;19,28,29 however, the variation again causes little change in computed [CH] and [NO]. Finally, we considered rate coefficients provided by Baulch et al.,19 Murray et al.,30 and Temps et al.31 for the reaction HCCO + O2 S OH + 2CO. We again found negligible effects on computed CH concentrations and only minor effects on the shape of the predicted NO profile. We also investigated other steps involving CxHy (x ) 1, 2; y ) 1-6) as well as HCCO. On the basis of our detailed sensitivity analysis,15 such reactions were found to influence both [CH] and [NO] but not to the same extent of the most important CH formation and destruction reactions. In particular, the rate coefficients for these reactions were varied by two orders of magnitude to study their effects on predicted CH and NO concentrations. We found that, among the selected reactions, CH and NO were most sensitive to CH3 + H + M S CH4 + M, CH3 + OH S CH2 + H2O, CH4 + OH S CH3 + H2O, and 2CH3 + M S C2H6 + M. Therefore, reliable kinetic data for these reactions may be important, especially at higher pressures for the above three-body reactions. Rate Coefficients for NO Formation. Among the NO formation pathways, the kinetics related to thermal NO are wellknown in the literature.32 We note that contributions from the thermal mechanism are small when compared with the prompt pathway in these non-premixed flames owing to the fact that peak temperatures are below 1800 K.1 Nevertheless, we considered rate coefficients available from Baulch et al.,19,33 Mick et al.,34 and Wennberg et al.35 for the initiation reaction, N2 + O S N + NO. As expected, little change occurs in either the computed CH or NO concentrations when using these various rate coefficients. In comparison, the contribution to total NO from the N2O pathway increases with rising pressure for the non-premixed flames of this investigation. However, employing rate coefficients reported by Baulch et al.,19 Johnsson et al.,36 and Rohrig et al.37 for the initiation reaction, N2O + M S N2 + O + M, we find that little change occurs in either predicted [CH] or [NO] for our flame conditions. Apart from the thermal and N2O pathways, a small contribution from the NNH pathway can also occur at higher pressures. Konnov and De Ruyck38 have proposed a temperature-dependent rate coefficient for the NNH initiation reaction, NNH + O S NH + NO, rather than the temperature-independent coefficient used in current kinetic mechanisms. They claim improved agreement between measured and predicted [NO] for low-pressure, H2-air flames when using this new rate coefficient. Nevertheless, we find that the alternate NNH rate coefficient has negligible effect on predicted (28) Hidaka, Y.; Sato, K.; Henmi, Y.; Tanaka, H.; Inami, K. Combust. Flame 2000, 118, 340–358. (29) Wang, B.; Fockenberg, C. J. Phys. Chem. A 2001, 105, 8449–8455. (30) Murray, K. K.; Unfried, K. G.; Glass, G. P.; Curl, R. F. Chem. Phys. Lett. 1992, 192, 512–516. (31) Temps, F.; Wagner, H. Gg.; Wolf, M. Z. Phys. Chem. 1992, 176, 27–39. (32) Naik, S. V.; Laurendeau, N. M. Combust. Flame 2002, 129, 112– 119. (33) Baulch, D. L.; Cobos, C. J.; Cox, R. A.; Esser, C.; Frank, P.; Just, Th.; Kerr, J. A.; Pilling, M. J.; Troe, J.; Walker, R. W.; Warnatz, J. J. Phys. Chem. Ref. Data 1992, 21, 411–734. (34) Mick, H. J.; Matsui, H.; Roth, P. J. Phys. Chem. 1993, 97, 6839– 6842. (35) Wennberg, P. O.; Anderson, J. G.; Weisenstein, D. K. J. Geophys. Res. 1994, 99, 18839–18846. (36) Johnsson, J. E.; Glarborg, P.; Dam-Johansen, K. Proc. Combust. Inst. 1992, 24, 917–923. (37) Rohrig, M.; Petersen, E. L.; Davidson, D. F.; Hanson, R. K. Int. J. Chem. Kinet. 1996, 28, 599–608. (38) Konnov, A. A.; De Ruyck, J. Combust. Flame 2001, 125, 1258– 1264.


[CH] and [NO] for our non-premixed methane-air flames, except for a small increase in [NO] at 4 atm. Rate Coefficients for NO Destruction. Reburning is a complex chemical process for which NO reacts with hydrocarbon radicals, especially under fuel-rich conditions, to generate intermediate nitrogenous species which are either converted back to N2 or recycled to NO. On the basis of our analysis of net reaction rates, we find that the effectiveness of all reburn pathways depends on pressure. For reburn reactions related to methylidyne, i.e., CH + NO S O + HCN, CH + NO S H + NCO, and CH + NO S N + HCO, we implemented branching ratios of 0.78, 0.16, and 0.06, respectively, based on the work of Bergeat et al.39 Employing total rate coefficients suggested by Baulch et al.,19,33 we found no significant effects on predicted CH and NO concentrations. For reburn reactions of methylene, CH2 + NO S H + HNCO, CH2 + NO S OH + HCN, and CH2 + NO S H + HCNO, we employed branching ratios of 0.02, 0.1, and 0.88, respectively.19 Invoking the total rate coefficient suggested by Baulch et al.,19 we found only a small effect on the shape of the NO profile in the fuel-rich region of our non-premixed flames, with almost no change in the predicted CH concentrations. Accurate information regarding branching ratios and hightemperature rate coefficients is unfortunately scarce for two possible methyl reburn paths, CH3 + NO S HCN + H2O and CH3 + NO S H2CN + OH. We utilized branching ratios of 0.14 and 0.86 based on the work of Hennig and Wagner40 and total rate coefficients suggested by Baulch et al.19 as well as Bergeat et al.39 On this basis, we again found little change in predicted CH and NO concentrations, except for a small shift in the shape of the NO profile for fuel-rich regions of our flames. The elementary reaction, HCCO + NO S HCNO + CO, is an important reburn process, along with HCCO + NO S HCN + CO2. Tokmakov et al.41 have indicated that other channels, such as HCCO + NO S HNCO + CO, HCCO + NO S HOCN + CO, and HCCO + NO S HNC + CO2, are unimportant in hydrocarbon flames. We implemented the total rate coefficients suggested by Temps et al.31 and Boullart et al.,42 with branching ratios of 0.8 and 0.2 for the HCNO + CO and HCN + CO2 channels, respectively.19 As for the other reburn paths, we found no effect on predicted CH concentrations; however, a significant change occurred in the shape of the NO profiles, which generally compromised agreement between measured and predicted NO concentrations. Table 1 summarizes the above discussion regarding the influence of variations in rate coefficient on [CH] and [NO] for critical elementary reactions within the GRI 3.0 mechanism. Those reactions strongly affecting either [CH] or [NO] are selected as targets for revisions of the GRI 3.0 mechanism, as discussed in the next section. Alternate Prompt NO Pathway: Evaluation and Discussion We next consider the spin-allowed, prompt-NO pathway, CH + N2 S NCN + H, suggested by Moskaleva and Lin.3 The oxidation of NCN to NO utilizes slightly different rate coef(39) Bergeat, A.; Calvo, T.; Daugey, N.; Loison, J. C.; Dorthe, G. J. Phys. Chem. A 1998, 102, 8124–8130. (40) Hennig, G.; Wagner, H. Ber. Bunsen-Ges. Phys. Chem. 1994, 98, 749–753. (41) Tokmakov, I. V.; Moskaleva, L. V.; Paschenko, D. V.; Lin, M. C. J. Phys. Chem. A 2003, 107, 1066–1076. (42) Boullart, W.; Nguyen, M. T.; Peeters, J. J. Phys. Chem. 1994, 98, 8036–8043.


Naik and Laurendeau

Table 1. Influence of Rate Coefficient Variation on Computed CH and NO Concentrations using the GRI 3.0 Mechanism influence on [CH] influence on [NO] elementary reaction

strong

CH Kinetics CH4 + H S CH3 + H2 CH4 + OH S CH3 + H2O CH3 + OH S CH2 + H2O CH3 + H + M S CH4 + M 2CH3 + M S C2H6 + M CH2 + CH3 S C2H4 + H CH2 + H S CH + H2 × × CH + O2 S HCO + O CH + H2O S CH2O + H × CH + CO2 S HCO + CO HCCO + O2 S OH + 2CO HCCO + M S CH + CO + M

weak

strong

× × × × × ×

× × ×

× × ×

weak × × × × × ×

× × ×

NO Formation × × × ×

N2 + O S N + NO N 2O + M S N 2 + O + M NNH + O S NH + NO CH + N2 S NCN + H

×

× × ×

NO Destruction × × × × × × × × × ×

CH3 + NO S HCN + H2O CH3 + NO S H2CN + OH CH2 + NO S H + HNCO CH2 + NO S OH + HCN CH2 + NO S H + HCNO CH + NO S O + HCN CH + NO S H + NCO CH + NO S N + HCO HCCO + NO S HCNO + CO HCCO + NO S HCN + CO2

× ×

× × × × × × × ×

Table 2. Rate Coefficients of Reactions Suggested by Zhu and Lin43,44 in the Alternate CH + N2 S NCN + H Pathway for Prompt-NO Formation

elementary reaction CH + N2 S NCN + H NCN + O S CN + NO CN + NH S NCN + H NCN + NO S N2O + CN NCN + NO S NCO + N2 NCN + O2 S NCO + NO NCN + OH S HCN + NO NCN + H S HCN + N

activation pre-exponential factor (A; cm3 temperature energy (Ea; exponent (n) cal mol-1) mol-1 s-1) 2.22 × 107 2.50 × 1013 4.00 × 1013 2.00 × 1012 2.00 × 1012 4.39 × 109 2.24 × 1011 1.89 × 1014

1.48 0.20 0 0 0 0.50 0.30 0

Table 3. Rate Ceofficients of Reactions Suggested by Smith45 in the Alternate CH + N2 S NCN + H Pathway for IAB Prompt-NO Formation

23367 -34 0 0 0 24493 4645 8425

ficients, as suggested by Zhu and Lin,43,44 Smith,45 and Glarborg et al.46 and listed in Tables 2-4, respectively. Although [CH] and [NO] are well-predicted at atmospheric pressure when using GRI 3.0, we must nevertheless investigate potential ramifications on [CH] and [NO] predictions from the alternate prompt-NO route. In general, we find that the alternate prompt-NO mechanism does not change predicted CH concentrations in any of our atmospheric flames. However, as expected, computed NO concentrations are affected when using different NCN routes, as shown in Figure 8a. For the atmospheric-pressure flame, the mechanisms for oxidation of NCN to NO proposed by Zhu and Lin43,44 and Smith45 are too slow and result in severe underprediction of (43) Zhu, R. S.; Lin, M. C. Int. J. Chem. Kinet. 2005, 37, 593–598. (44) Zhu, R. S.; Lin, M. C. Ab initio study on the oxidation of NCN by O and OH radicals: Prediction of the total rate constant and product branching ratios. 6th International Conference on Chemical Kinetics, Gaithersburg, MD, July 25–29, 2005. (45) Smith, G. P. Personal Communication, SRI International, Menlo Park, CA, 2005. (46) Glarborg, P.; Alzueta, M. U.; Dam-Johansen, K.; Miller, J. A. Combust. Flame 1998, 115, 1–27.

elementary reaction CH + N2 S NCN + H NCN + O S CN + NO CN + NH S NCN + H NCN + NO S N2O + CN NCN + NO S NCO + N2 NCN + O2 S NCO + NO NCN + OH S HCN + NO HCN + N S NCN + H


1.48 0 0 0 0 0 0 0

23367 0 0 0 0 20000 0 20000

Table 4. Rate Coefficients of Reactions Suggested by Glarborg et al.46 in the Alternate CH + N2 S NCN + H Pathway for Prompt-NO Formation

elementary reaction CH + N2 S NCN + H NCN + O S CN + NO CN + NH S NCN + H NCN + NO S N2O + CN NCN + NO S NCO + N2 NCN + O2 S NCO + NO NCN + OH S HCN + NO HCN + N S NCN + H


1.48 0 0 0 0 0 0 0

23367 0 0 0 0 0 0 20000

measured NO concentrations. The faster oxidation route proposed by Glarborg et al.,46 on the other hand, predicts NO concentrations similar to those from GRI 3.0. In fact, the LIF data are well-represented by predictions from Glarborg et al.46 Hence, little difference occurs in predicted [NO] when using either prompt-NO route (HCN or NCN) at atmospheric pressure if appropriate rate coefficients for the NCN pathway are implemented within GRI 3.0. Recently, El Bakali et al.47 investigated the two routes for prompt-NO formation in order to predict NO concentrations in natural gas flames. They concluded that production of NO was dominated by the oxidation sequence; thus, the actual initiation step (CH + N2 S HCN + N or CH + N2 S NCN + H) did not influence predicted NO concentrations. From Figure 8a, we find that our results are consistent with their observations when appropriate rate coefficients are implemented for the oxidation of NCN to NO. In the remainder of this paper, we have implemented the NCN pathway to reflect recent support for the CH + N2 S NCN + H channel.2–5 Figure 8b displays predicted NO concentrations for the nonpremixed flame at 3 atm when using the alternate prompt-NO pathway. We find the same trends as at 1 atm; however, in this case, peak [NO] is still severely underpredicted when using the alternate prompt pathway. Nevertheless, a slight improvement occurs as compared to GRI 3.0 at higher pressures, based on computations for 4-6 atm. We also find somewhat better agreement between predicted and measured NO concentrations for both fuel-lean and fuel-rich regions at higher pressures. Although not shown in Figure 8b, the alternate prompt-NO pathway again has little effect on computed CH concentrations, similar to that for the atmospheric-pressure flame. In Tables 5-7, we suggest three possible revisions based on our discussion of the most significant reaction steps for CH and NO kinetics. Figures 9-11 show comparisons between measured and predicted CH and NO profiles when employing the (47) El Bakali, A.; Pillier, L.; Desgroux, P.; Lefort, B.; Gasnot, L.; Pauwels, J. F.; da Costa, I. Fuel 2006, 85, 896–909.



Table 5. Summary of Rate Coefficients in Revision 1 Modified within the GRI 3.0 Kinetic Mechanism to Match Measured CH and NO Data in Counterflow Non-premixed Flames at a Global Strain Rate of 20 s-1 when using the CH + N2 S NCN + H Prompt Pathwaya elementary reaction

pre-exponential Factor (A; cm3 mol-1 s-1)

temperature exponent (n)

activation energy (Ea; cal mol-1)

ref

CH + O2 S HCO + O CH2 + H S CH + H2 CH+ H2O S CH2O + H CH + CO2 S HCO + CO CH2 + CH3 S C2H4 + H CH + N2 S HCN + N CH + N2 S NCN + H

3.30 × 9.05 × 1010 3.08 × 1016 3.43 × 1012 7.23 × 1013 6.02 × 1011 2.22 × 107

0 1.02 -1.42 0 0 0 1.48

0 2495 0 683 0 13860 23367

20 24 23 27 19 42 3

a

1013

The remaining reactions related to the NCN mechanism are listed in Table 4.

Table 6. Summary of Rate Coefficients in Revision 2 Modified within the GRI 3.0 Kinetic Mechanism to Match Measured CH and NO Data in Counterflow Non-premixed Flames at a Global Strain Rate of 20 s-1 when using the CH + N2 S NCN + H Prompt Pathwaya elementary reaction


temperature exponent (n)


ref


3.30 × 9.05 × 1010 2.75 × 1013 3.43 × 1012 7.23 × 1013 6.02 × 1011 2.22 × 107

0 1.02 0 0 0 0 1.48

0 2495 0 683 0 13860 23367

20 24 22 27 19 42 3

a

1013


rate coefficients listed in Tables 5-7. The rate coefficients for the NCN pathway follow those suggested by Glarborg et al.46

Figure 8. Effect of alternate prompt-NO pathway (CH + N2 S NCN + H) on predicted NO concentrations (base GRI 3.0) in CH4/O2/N2 counterflow non-premixed flames for a global strain rate of 20 s-1 at a pressure of (a) 1 and (b) 3 atm.

(see Table 4) and appear to be consistent with our flame data; no other changes were made to the GRI 3.0 mechanism. The rate coefficients for the proposed revisions are deliberately chosen so as to increase [CH] and [NO] in an effort to match the measured LIF data in the intermediate pressure range (2–4 atm), for which GRI 3.0 significantly underpredicts measured CH and NO concentrations. This strategy results in an overprediction of [CH] and [NO] at atmospheric pressure, as shown in Figure 9; however, significantly improved agreement is observed for the non-premixed flame at 3 atm (see Figure 10). Unfortunately, the alternate rate coefficients predict even higher [CH] and [NO] at pressures beyond 3 atm, as shown in Figure 11; thus, the agreement between predictions and measurements degrades at higher pressures as compared to that for GRI 3.0. We summarize the variation of peak CH and NO concentrations as a function of pressure in Figure 12 when using the modified rate coefficients. For peak [CH] and [NO], the agreement deteriorates toward both 1 and 6 atm; nevertheless, somewhat improved predictions occur in the intermediate pressure range (2–4 atm) for which the GRI 3.0 and Lindstedt mechanisms display substantial difficulties. The match between predictions and measurements clearly remains imperfect over the entire pressure range, as seen from Figure 12. We thus conclude that reasonable modifications to the rate coefficients for key elementary reactions, as described in Tables 5-7, cannot replicate the pressure dependence of [CH] and [NO] for these non-premixed flames. As noted previously, we did not implement a systematic optimization strategy for the controlling chemical kinetics via approaches such as solution mapping; that was not the purpose of this study. We did make sure, however, that the adjusted kinetics did not significantly alter flame structure, thus causing departures from chemical reality. In general, two aspects of this study are particularly useful. First, we found that net reaction rates of important elementary reactions controlling [CH] and [NO] display peculiar behavior for the pressure range corresponding to substantial disagreement between measured and predicted [CH] and [NO]. This observation clearly demands further measurements and better computations of potential energy surfaces, transition states, and minimal energy paths for


Naik and Laurendeau

Table 7. Summary of Rate Coefficients in Revision 3 Modified within the GRI 3.0 Kinetic Mechanism to Match Measured CH and NO Data in Counterflow Non-premixed Flames at a Global Strain Rate of 20 s-1 when using the CH + N2 S NCN + H Prompt Pathwaya

a

elementary reaction




ref


1.70 × 1013 2.00 × 1014 4.58 × 1016 6.38 × 1007 7.23 × 1013 3.62 × 1012 2.22 × 107

0 0 -1.42 1.51 0 0 1.48

0 0 0 -712.8 0 21899 23367

19 19 19 19 19 19 3


Figure 9. Effect of revising rate coefficients for critical elementary reactions of GRI 3.0 on the predicted (a) CH and (b) NO concentrations in a CH4/O2/N2 counterflow non-premixed flame for a global strain rate of 20 s-1 at 1 atm.


these elementary reactions. Second, we have established that shifting from the spin-forbidden to the spin-allowed promptNO pathway using fast NCN kinetics begins to replicate more successfully the measured data. In fact, Vasudevan et al.4 have recently measured the rate coefficient for the CH + N2 reaction and concluded that the NCN channel is indeed the only significant pathway. Any remaining discrepancies must be resolved by careful measurements of rate coefficients and branching ratios over a wide range of pressure and temperature conditions. Quantitative measurements of radical species such as CH2 and HCCO would also be helpful in validating any postulated kinetics. These challenges should be addressed by researchers in the chemical kinetics community. Their efforts augur better predictions of NOx levels from future combustors

and engines, an important goal as society continues to depend on fossil fuels for power generation and transportation. Conclusions In this paper, we have investigated prompt-NO formation and destruction in counterflow non-premixed flames at a global strain rate of 20 s-1 for pressures of 1-6 atm. A net reaction rate analysis reveals that elementary reactions important in the production and consumption of either CH or NO display a peculiar behavior in the intermediate pressure range between 2 and 4 atm; in particular, the reaction rates either remain constant or display a local maximum. This behavior implies that the peaking trend observed in measured [CH] and [NO] over the same pressure range could arise from the cumulative effect of



characteristic kinetics embodied by these controlling elementary reactions. A sensitivity analysis indicates small differences for reburn reactions involving CH2 + NO, CH3 + NO, and HCCO + NO between the GRI and Lindstedt mechanisms. To investigate further the prompt-NO mechanism, the spinallowed prompt-NO pathway, CH + N2 S NCN + H, was implemented along with various routes for subsequent oxidation of NCN to NO. For the flame conditions of this investigation, only minor differences were observed between the HCN (spinforbidden) and NCN (spin-allowed) pathways. Finally, we combined various rate coefficients for critical elementary reactions so as to match measured CH and NO profiles for the non-premixed flames of this study. Predicted CH and NO concentrations show a mixed degree of agreement with measured LIF concentrations, even when employing the spinconserved, prompt-NO initiation reaction. On the basis of our analysis, we identify five specific reactions in Table 1 that


Figure 12. Variation of peak [CH] and peak [NO] as a function of pressure at a global strain rate of 20 s-1 in CH4/O2/N2 counterflow non-premixed flames. The revised predictions are obtained using the rate coefficients listed in Tables 4-6 (base GRI 3.0). The error bars for the peak concentrations are at the 95% confidence limit. The measurements have been reported previously by Naik and Laurendeau.1,6

strongly affect [CH] or [NO] for non-premixed flames. Uncertainties regarding these influential reactions should be resolved via careful measurements and modeling efforts. In that direction, a comprehensive database is being generated by a group of eminent chemists (www.primekinetics.org) so that our CH and NO measurements, along with those of others, will be available to the kinetics community for further evaluation. Acknowledgment. We wish to thank James Cooke from the Department of Mechanical Engineering at Yale University for his help in implementing the Lindstedt mechanism within CHEMKIN. EF700327A

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