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Inhibition of Premixed Monoxide---Nitrogen Flames by Iron Pentacarbonylt

MARC D. RUMMINGER+ AND GREGORY T. LINTERIS* Building and Research Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA

This paper presents measurements of the burning velocity of premixed CO-HrOrNz flames with and without the inhibitor Fe(CO)s over a range of initial Hz and Oz fractions. A numerical model is used to simulate the flame inhibition using a - chemical mechanism. For the uninhibited flames, predictions of burning velocity are excellent and for the inhibited flames, the qualitative agreement is good. The agreement depends strongly on the rate of the CO + OH <-> COz + H reaction and the rates of several key iron reactions in catalytic H- and O- scavenging cycles. Most of the chemical inhibition occurs through a catalytic cycle that converts o into Oz . This O-atom cycle is not important in flames. The H-atom cycle that causes most of the scavenging in the methane flames is also active in CO-Hz flames, but is of secondary importance. To vary the role of the H- and O-atom radical pools, the experiments and calculations are performed over a range of oxygen and hydrogen mole fraction. The degree of inhibition is shown to be related to the fraction of the net H- and O-atom destruction through the iron species catalytic cycles. The O-atom cycle saturates at a relatively low inhibitor mole fraction (-100 ppm), whereas the H-atom cycle saturates at a much higher inhibitor mole fraction (-400 ppm). The calculations reinforce the previously suggested idea that catalytic cycle saturation effects may limit the achievable degree of chemical inhibition. Published by Science Inc.

INTRODUCTION tive agents and aid in the development of new nontoxic agents. The production of the effective and widely used In recent years, several detailed studies of fire suppressant CF3Br and other related com• Fe(CO)s have been undertaken. Reinelt and pounds has been banned due to their contribu• Linteris [7] studied the flame inhibition effect of tion to depletion. Currently, there exists iron pentacarbonyl in methane-oxygen-nitro• no replacement fire suppressant with all of the gen by measuring the burning velocity desirable properties of CF3Br, and the search of premixed flames and the extinction strain for alternative compounds has intensified. It is rate of counterflow diffusion flames. The exper• well established that some metallic compounds iments showed that for small amounts of are very powerful flame inhibitors [1-5]. The Fe(CO)s the effect was roughly linear, but compound iron pentacarbonyl, Fe(CO)s, has above a certain loading the effect was nearly been found to be up to two orders of magnitude independent of the Fe( CO)s concentration. The more effective than -containing com• authors postulated that the decreasing inhibi• pounds at reducing the burning velocity of tion was due to loss of active gas-phase species premixed -air flames [1, 6]. Al• through . Rumminger et al. [8] though Fe(CO)s is toxic and flammable, under• developed a chemical mechanism for Fe(CO)s standing its inhibition mechanism could provide inhibition of methane-oxygen-nitrogen flames, insight into the behavior of other highly effec- and through numerical simulations concluded that the inhibition occurs primarily through gas-phase . Flame simulations using 'Corresponding author. E-mail: [email protected] the chemical mechanism in Ref. 8 showed tOfficial contribution of the National Institute of Standards promising agreement between model and ex• and Technology, not subject to copyright in the . periment. The mechanism successfully pre• 'National Research Council/NIST postdoctoral fellow, dicted the trends for various equivalence ratios 1996-1998. Current address: Sandia National Laboratory, and oxygen mole fractions; however, the au• MS 9052, Livermore, CA 94551. thors had increased the rates of several key

COMBUSTION AND FLAME 120:451-464 (2000) 001O-2180/00/$-see front matter Published by Elsevier Science Inc. PH SOOlO-2180(99)00114-5 452 M. D. RUM MINGER ET AL. reactions1 (within experimental uncertainty) to order to accurately match the observed burning improve quantitative agreement, and conse• velocity reductions. It is of interest here to quently recommended additional testing and determine if the faster rates are necessary for refinement of the mechanism. modeling moist CO flames--which are chemi• In the present paper we examine the effect of cally much closer to the rich Hr02 flames used Fe(CO)s in CO-H2-02-N2 flames, which have by Jensen and Jones (reactions of iron-contain• a different radical pool than the methane flames ing species with are not presently of previous studies. Calculations of uninhibited included in the mechanism). Finally, carbon premixed flame structure indicate that the peak monoxide flames are relevant to fire research mole fraction of 0 atom is four times higher in since CO is a dominant intermediate species in stoichiometric CO-H2-air flames (containing hydrocarbon flames and its oxidation is often between 0.2 and 1.0% H2) than a stoichiometric the rate-limiting step in product formation. CHcair flame, whereas the concentrations of The approach in the present research is to H-atom and OH are between 1.5 and 7 times determine the effect of Fe(COh on the overall lower. These differences permit examination of reaction rate of premixed flames. The laminar the effect of radical pool composition on inhi• burning velocity is a widely used measure of the bition. Since previous calculations [8]2 have effectiveness of chemical inhibitors, and allows shown that inhibition in methane flames occurs straightforward comparison with the numeri• primarily from radical recombination cycles in• cally calculated flame structure, providing use• volving H atoms, it is of interest to determine if ful insight into the mechanism of inhibition. other mechanisms are important in flames with Our study of flame inhibition is intended to reduced H-atom mole fraction. In CO flames provide an understanding of flame suppression. the H-atom mole fraction can be adjusted by Although the processes have different end varying the amount of H2 in the reactant mix• points (weakening the flame vs. extinguishing ture. Finally, the numerical calculations in methane-air flames indicated that above a cer• it), the underlying mechanism is similar: the agent reduces the overall reaction rate of the tain loading chemical inhibitors can lose their fuel-air . Inhibition can be viewed as an effectiveness because the radical pool has al• early stage of suppression in which the inhibitor ready been lowered to equilibrium levels. By weakens the flame, making it more vulnerable varying the radical pool size, CO flames can be to extinction by external factors such as heat loss used to investigate whether the observed satu• ration in the effectiveness of iron pentacarbonyl or fluid-mechanical instability. Inhibition--and is due to condensation or radical pool depletion. eventually suppression--occurs as the agent re• The oxidation mechanism of CO has fewer duces the rate of heat release of the flame. For reactions than those of hydrocarbon fuels, al• chemically acting agents, the agent interferes with the reactions that consume the fuel and lowing easier isolation of the important inhibi• tion reactions. For this reason, many research• intermediates. Hence, we can study the effect of ers have used CO flames in studies of small agent concentrations on flame chemistry and build chemical kinetic models that describe inhibition [9-16]. The simplicity of the CO system has additional significance in the present the effect of the agent on the reac• work. In previous studies with methane flames, tions. By using laboratory-scale premixed and the rates of key inhibition reactions suggested diffusion flames that are amenable to modeling, by Jensen and Jones [17] were increased in we can validate the chemical kinetic model and gain insight into which processes are most im• 'The reactions and increased rates were FeO + H20 • portant. Later, we can continue to construct and Fe(OHh (kf = 1.62E + 13, increased by 3x), FeOH + H• refine the mechanism for higher inhibitor con• FeO + Hz [kf = 1.50E + 14 exp( -805fT), increased by 5x), centrations and for flames and fuels that are Fe(OHh + H - FeOH + HzO [kf = 1.98E + 14 more representative of . Finally, an under• exp(-302fT), increased by 3x). 2To avoid excessive repetition, references to previous calcu• standing of the modes of action of effective lations of Fe(CO)s-inhibited methane flames refer to Ref. 8 agents such as Fe(CO)s can researchers to unless otherwise noted. chemicals that have similar favorable proper-

I", ill I Ii 11 INHIBITION OF CO FLAMES BY IRON PENTACARBONYL 453 ties, while avoiding characteristics such as di• stream consists of nitrogen (boil-off from minishing effectiveness at high concentration. Nz) and oxygen (MG Industries, HzO < 50 In addition to the flame inhibition experi• ppm, total hydrocarbons < 5 ppm). The exper• ments and modeling, we present measurements imental arrangement has been described in de• of the burning velocity of uninhibited CO-Hz• tail previously [25] and the optical and image Oz-Nz flames with varying amounts of Hz, and processing systems have been refined as de• compare the measurements with calculations. scribed below. Although there are numerous measurements of Inhibitor is added to the flame by diverting CO flame speed in the literature [9, 18-21], part of the nitrogen stream to a two-stage there is a lack of data for near-stoichiometric saturator maintained in an ice bath at O°c. The CO-Hz-Oz-Nz flames with Hz mole fractions diverted gas (less than 8% of the total flow) between 1.50· 10-4 and 2.0 . lO-z. bubbles through liquid Fe(CO)s before return• In the remainder of the paper, we describe ing to the main nitrogen flow, and is assumed to the experimental technique, the numerical cal• be saturated with Fe(CO)s vapor. Although the culation procedure, and the results for uninhib• saturator is the same as used previously, the ited and inhibited CO-Hz-Oz-Nz flames. We assumption of saturation has been verified by interpret the chemical kinetics of the inhibiting flowing the Fe(CO)s-laden into a conden• iron species in CO flames, comparing them with sation coil immersed in a -60°C bath. results for CH4 flames, and analyze how chang• After a specified elapsed time, the ends of the ing oxygen or hydrogen content affects the coil were capped and the weight gain of the coil flame inhibition. [i.e., the amount of Fe(CO)s] was measured. After performing the condensation experiment EXPERIMENTAL for several carrier-gas flow rates, a linear fit of the data was generated to quantify the depen• dence of Fe(CO)s concentration on carrier-gas Premixed Flame System flow rate. The experimental results and theoret• ical prediction based on carrier-gas saturation A Mache-Hebra nozzle burner (inner diameter were within 5% of each other across a wide 1.02 0.005 cm) [22] with a schlieren imaging ± range of flow rates. system [23] is used to measure the average The schlieren image of the flame is used to burning velocity with the total area method [24]. represent the flame surface. An optical system The burner produces straight-sided schlieren (a white-light source with a vertical slit at its and visible images which are very closely paral• exit, lenses, a vertical band, and filters) gener• lel. Gas flows are measured with digitally con• ates the schlieren image of the flame for capture trolled flow controllers (Sierra Model by a 776 X 512 pixel CID array (Cidtec 860)3 with a quoted repeatability of 0.2% and CID371OD). The image is digitized by a 640 X accuracy of 1% of full-scale flow, which have 480 pixel frame-grabber board (Data Transla• been calibrated with (Gillian Gilibrator) tion 3155) in a Pentium-II computer. The im• and dry (American Meter Co. DTM-200A) flow ages are acquired and written to disk using the meters so that their accuracy is 1% of indicated free UTHSCSA ImageTool program [26]. For flow. The fuel gas is (Mathe• each flame condition, 10 images are collected at son UHP, 99.9% CO, with the sum of CH4 and a rate of one per second. The flame area is HzO 10 ppm) and hydrogen (Matheson < determined (assuming axial symmetry) using UHP, 99.999% Hz, with sum of Nz, Oz COz, custom image-processing software. Finally, the CO, Ar, CH4, and HzO 10 ppm); the oxidizer < burning velocity is calculated by dividing the volumetric flow rate (corrected to 1 3Certain commercial equipment, instruments, or materials and 298 K) by the average flame area for the 10 are identified in this paper to adequately specify the proce• Images. dure. Such identification does not imply recommendation or endorsement by the National Institute of Standards and In these experiments, the low rate of heat loss Technology, nor does it imply that the materials or equip• to the burner, the low strain rate, and the low ment are necessarily the best available for the intended use. curvature facilitate comparisons of the burning 454 M. D. RUMMINGER ET AL.

TABLE 1 The primary sources of uncertainty in the Measured and Calculated Burning Velocity of average burning velocity measurement are (l) Uninhibited Stoichiometric CO-H2-Oz-N2 Flames with accuracy of the flow controllers, (2) measure• Varying Oxygen Content and Varying Hydrogen Content ment of ambient pressure and temperature, (3) determination of the flame area, (4) the effect 32.746.236.225.159.039.60.010.0050.0020.015XH2(cm/s)vo,num(cm/s)±35.841.122.331.248.730.50.60.90.82.41.41.1 V o,exp 0.210.24X(h,ox of flame location on flame area, (5) the 0.183 location of the schlieren image relative to the cold gas boundary. The relative uncertainty for burning velocity measurements ranges between 3% and 6.5%, and between 1% and 4.5% for normalized burning velocity. In general, unCer• tainty increases with increasing burning velocity. The tables of burning velocity assign uncertainty velocity with those predicted by one-dimensional to each measurement of uninhibited burning numerical calculations. Although the burning ve• velocity. The expanded relative uncertainties for locity in Bunsen-type flames is known to vary at parameters related to the accuracy of the flow the tip and base of the flame and is influenced by controllers are as follows: 1.4% for equivalence curvature and stretch [24], these effects are most ratio; and 1.1%, 1.2%, and 6.5% for the inlet important over a small portion of the flame. In reactant mole fractions of Oll Hz, and Fe(CO)s, order to minimize the influence of these effects respectively. For the latter, we use a vapor on interpretation of the action of the chemical pressure correlation from Ref. 30 (which does inhibitor, we present the burning velocity as a not report an uncertainty). Based on the man• normalized parameter: the burning velocity of ufacturer's analysis of the bottled gases, the the inhibited flame divided by the burning ve• estimated background hydrogen in the reactant locity of the uninhibited flame at the same flow stream is less than 75 ppm. conditions (the burning velocities used for the normalizations are listed in Table 1). NUMERICAL MODEL Uncertainty Analysis One-dimensional freely propagating premixed The uncertainty analysis consists of calculation flames are simulated using the Sandia flame of individual uncertainty components and root code Premix [31], the Chemkin subroutines mean square summation of components [27]. [32], and the transport property subroutines Calculation of the individual uncertain compo• [33]. For all of the calculations the absolute nents is accomplished analytically for simple tolerance is 10-14, the relative tolerance is 10-9, expressions (e.g., equivalence ratio), and GRAD is 0.20, and CURV is 0.35. Thermal through use of a "jitter program" [28] for more diffusion (Soret effect) is included in the calcu• complex expressions [e.g., Fe(CO)s mole frac• lations. Solutions typically contain between 85 tion]. The jitter program sequentially varies the and 130 grid points. The initial temperature is input data and computes the resulting contribu• 298 K and the pressure is 1 atmosphere. The tion to the uncertainty for each output variable. kinetic mechanism and thermodynamic data of All uncertainties are reported as expanded un• Yetter et al. [34] (13 species and 37 chemical certainties: X ± U, where U is kuo and is reactions) serve as a basis for describing moist determined from a combined standard uncer• carbon monoxide oxidation. Note that we use tainty (estimated standard deviation) uc' and a the modified Arrhenius rate for CO + OH +-i> coverage factor k = 2 (level of confidence COz + H from Baulch et al. [35] because approximately 95%). Likewise, when reported, numerical calculations indicated that most of the relative uncertainty is U / X . 100%, or the CO oxidation was occurring below 2000 K kuc / X . 100%. A brief summary of the uncer• (temperature above which a different rate ex• tainty results is presented below; an expanded pression was recommended [34]). Iron penta• discussion can be found in Ref. 29. carbonyl is added to the unburned CO-Hz-

I i I ,I I 1,1· 1',1 -I- , ~ I i INHIBITION OF CO FLAMES BY IRON PENTACARBONYL 455

50 TABLE32.] 2:!:0.8 O.O1ll 40.6:!: 1.1 (cm/s)(cm/s)XH2 V o,exp XH2 0.0036 26.6:!: 0.7 0.0070 0.00300.00500.00400.003333.9:!:0.00450.0039 CO-Hz-Air0.924.8:!:28.4:!:25.5:!:28.7:!:Z5.7:!:26.9:!:27.3:!:0.01110.70.8 Flames0.00600.006]0.00810.009140.7:!:0.00800.0070with1.230.9:!:37.3:!:36.1:!:33.2:!:38.0:!:35.4:!:Hz Content0.91.01.1 0.01010.01320.01180.01170.0122Varied39.1:!:43.7:!:41.9:!:42.6:!:42.5:!: 1.11.21.4 0.0024 22.5:!: 0.8 0.0050Measured29.8:!: 0.8Burning0.0101Velocity39.7:!:vofo,exp1.0Uninhibited Stoichiometric 40 :F E .!!. ~ 30 "" Baulch at al. (1973) [35] 'g ~ g' 20 'E

&l

10 -I- \almer & Seery [9]

o o 2000 4000 6000 8000 1??oo 12000 14000

H2 Mole Fraction x 106

Fig. 1. Comparison of measured and calculated burning velocities of stoichiometric CO-Hz-air flames with varying amounts of Hz and four different rates for CO + OH ....,. COZ + H. Also shown are Palmer and Seery's measure• several different sets of rate constants for CO + ments [9]. OH ~ COz + H [35-38]. Table 3 lists calcu• lated forward rate constants for each formula• Oz-Nz mixture at mole fractions of up to 500 tion at several temperatures. The 1992 rate ppm. The chemical mechanism for Fe(CO)5 expression of Baulch et aI. [37] yields a burning inhibition of flames (12 species and 55 reactions) velocity about 20% higher than the experi• and necessary thermodynamic and transport data ments, while the remaining three rate expres• are compiled from a variety of sources as de• sions predict values less than 10% lower than scribed in Ref. 8. It should be emphasized that the the experiments. Since there is a relatively large reaction mechanism used for the present calcu• uncertainty in the rate of CO + OH near 2000 lations should be considered only as a starting K [39], adjustment of this rate to match the point. Numerous changes to both the rates and present data seems unwarranted. Examination of the numerical results shows the reactions incorporated may be made once a variety of experimental and theoretical data are that the CO + OH reaction is important even at available for testing the mechanism. very low hydrogen concentration. At XH2 = 0.001, 70% of the CO oxidation proceeds through CO + OH, while 28% of the CO RESULTS AND DISCUSSION oxidation proceeds through CO + 0 + M. At Uninhibited Flames XH2 = 0.015 the split is 88% through CO + OH and 10% through CO + 0 + M.

Figure 1 shows the measured burning velocities Inhibited Flames for CO-Hz-air flames with varying amounts of hydrogen (for the reader's convenience, the EtTect of O2 Mole Fraction data are also presented in tabular form in Table 2). We were not able to obtain a steady flame Figure 2 presents measurements (symbols) and for hydrogen mole fractions (XH,) below 0.0024 calculations (lines) of the burning velocity of or above 0.014. If extrapolated to hydrogen-free Fe(CO)s-inhibited CO-Hz-Oz-Nz flames with conditions, our measurements would agree rea• varying amounts of oxygen. As previously seen sonably well with the measurements of Palmer for CHcair flames inhibited by Fe(COh, inhi• and Seery [9], who used the cone frustum bition is strongest in the flames with the lowest method (reported measurement uncertainty of XQ2,ox (the oxygen mole fraction in the oxidizer ± 10%) to measure the burning velocity of prior to mixing with the fuel). The experimental CO-air flames with 45-150 ppm of Hz. results show that inhibition is proportional to Figure 1 also shows the calculated burning inhibitor concentration at low inhibitor mole velocity using the mechanism from Ref. 34 with fraction (Xin), but after a certain concentration, 456 M. D. RUMMINGER ET AL.

TABLE 3

Comparison of the Rates for CO + OR 4 CO2 + R Used in Figure 1a

1.89EEa/R1.53E2.57E-3851.75E-250263067b+10001.5473.56E2.61E3.67E20001.35.58E+05.69E15004.34E6.41E1.511+ 1111 + 11 2.12E3.25E1.50E4.4E + 07061210 kf WooldridgeBaulchYu et al.et [36]al.et(1992)al. [38][37] Baulch et al. (1973) [35] A

a The rate expression and the forward rate kf = A Tb exp( -(Ea)/RT) at several temperatures are presented. Units for k are em, mole, s.

additional Fe(CO)s has little effect on the burn• in Fig. 2. (A detailed analysis of the effect of ing velocity. A plausible (but untested) explana• X02,ox is undertaken later in this paper.) tion is that particle formation removes the The calculated normalized burning velocities inhibiting species at high inhibitor loading [7]. (shown by the lines in Fig. 2) reproduce the The experimental results in Fig. 2 support this trend of greater inhibition at lower X02,ox for suggestion: as the flame temperature increases, low Xim and predict a reduced marginal effect as the leveling-off point shifts to higher Fe(CO)s more inhibitor is added. At low values of Xin, mole fraction. the slopes of the predicted normalized burning Changing the oxygen mole fraction of the velocity curves are reasonably close to the mea• uninhibited CO flames changes several features sured slopes, but above about 100 ppm the which may affect the burning velocity reduction measured and calculated normalized burning caused by added Fe(CO)s' Table 4 lists the peak velocities diverge, with the model failing to temperature and peak mole fraction of 0, R, reproduce the leveling off and overpredicting and OR in uninhibited flames with X02.ox equal the inhibition effect. Nonetheless, we can use to 0.183, 0.21, and 0.24. Raising the oxygen the numerical results at low inhibitor mole mole fraction over this range increases the final fraction to understand the reasons for the temperature by 209 K and the peak radical mole greater inhibition at lower X02,ox' as well as fractions by 30% to 60%. Although several other features. Further, results at higher values variables are changing simultaneously, it is rea• of Xin can be used to investigate the relevant sonable to expect the hotter flames, with their gas-phase chemistry predicted by the model if larger radical pool, to be affected less by a fixed there was not a loss of the active species to a quantity of Fe(CO)s, yielding the smaller slope condensed phase.

1.0 TABLE 4

Calculated Maximum Mole Fraction and

~ 0.8 o Superequilibrium Mole Fraction of 0, H, and OH, and >;;; Maximum Temperature for Three Different Xo"ox for the ~ 0.6 Uninhibited Flames of Fig. 2 .~ III ... 0.00580.D150.00520.00490.00540.0140.0180.00290.00350.000950.000840.240.013237624710.21 22620.0110.00240.000970.00450.00430.01 ~ 0.4 Xo"ox ~ XH,maxXo,maxTmaxXOH.max-XOH,cqXoH,max(K)-Xo,eqXH,eq 0.183 zo Xo,max a'

0.0 _-+--_~ ---+---J a 100 200 300 400 50( Fe(CO), (ppm)

Fig. 2. Measured and calculated inhibition effect of Fe(CO)s for various O2 concentrations in stoichiometric CO-Hz-Oz-Nz flames with XH, = 0.01.

Iii. ill ,.11 i 1·1, I ·1· III INHIBITION OF CO FLAMES BY IRON PENTACARBONYL 457

The information obtained from flame speed 1.0 measurements is global, and many reactions in the mechanism can affect the results. Nonethe• !l 0.8 o.. less, a comparison of the measured and pre• >a; .... -.. ... -. dicted burning velocities together with investi• ~ 0.6 C~ gation of the numerical results can be used to III identify key reactions for further investigation. ~ 0.4 7ii Baulch et al. E In previous work with methane flames, the rates o (1973)[35] of the dominant inhibition reactions (those in Z 0.2 the H-atom cycle recommended by Jensen and 0.0 Jones [17]) were increased (within experimental o 100 200 300 400 500 uncertainty) to better predict the experimental Fe(CO), (ppm) results. In Fig. 2, the model shows slightly more Fig. 3. Comparison of experimental data with calculated inhibition for XQ2,ox= 0.24 than the experi• results using four mechanism variations: two rates for CO + ments. While it is tempting to use the present OH - CO2 + H, and two rates for the catalytic H-atom data to move the rates of the H-atom cycle back scavenging cycle. Dotted lines use the slower rates for the in the direction of the original recommenda• reactions in the H-atom cycle; the lines, the faster rates. cP = 1.0, X02,ox = 0.21, XH, = 0.01. tions [17], it is premature to do so. The excessive inhibition only occurs for one case; also, the effect of reaction rates in the fuel oxidation of CO + OH near 1800 K. Although the rate of mechanism (exclusive of the inhibition mecha• Yu et al. [36] provides better agreement with nism) must be considered, as will be described below. Based on these considerations, we use our experimental data (Fig. 1), we retain the 1973 Baulch et al. [35] rate since it has been the increased rates for the H-atom cycle reac• extensively tested against a variety of experi• tions for the analyses in the present paper. mental data from flames, flow reactors, and As described in a preceding section and Fig. shock tubes [34]. For the inhibition reactions, 1, for uninhibited CO flames, the rate of the the premixed methane flame results suggest use reaction CO + OH has a large effect on the of the faster rates for the H-atom cycle. In Fig. burning velocity predicted by the numerical 3 , considering only the region of low Xin, the calculation. For inhibited flames, the rates of the results are ambiguous, although with the two key inhibition steps obviously have a large effect rates for CO + OH, the faster rates for the cycle on model . However, the rate se• lected for the CO + OH reaction also affects (solid lines) bracket the experimental data, while the slower rates for the cycle (dotted the degree of predicted inhibition by Fe(CO)5' lines) lie predominantly outside the data. Also, Figure 3 shows the combined effect of differing the recent measurements of Rollason and Plane rates for the CO + OH reaction and for the [40] for the reaction FeO + HzO + He(Nz) ~ reactions in the H-atom cycle [8]. The measured Fe(OHh + He(Nz), which is part of the H• and predicted normalized burning velocities as atom cycle, argue for a rate even higher than a function of the mole fraction of Fe(CO)s are shown for two values of the CO + OH rate and that suggested in Ref. 8. Note that while the for the fast and slow rates for the H-atom absolute magnitude of the predicted inhibition is affected by the rates selected for the H-atom recombination cycle. As the figure shows, the cycle, the qualitative behavior is not, and the selection of the appropriate rates for the inhi• conclusions of this paper are unaffected by bition reactions depends upon selection of the these rates. CO + OH rate. For the remainder of this paper, we use the Catalytic Inhibition Cycles 1973 Baulch et al. [35] CO + OH rate recom• mended by Yetter et al. [34] and the faster rates In previous research on bromine-containing for the H-atom cycle reactions from Ref. 8. As species [41, 42], conversion of radicals (e.g., H) illustrated in the NIST Chemical Kinetics Da• into less reactive species (e.g., Hz) has been tabase [39], there is large uncertainty in the rate found to be the dominant mode of action for 458 M. D. RUMMINGER ET AL. chemical inhibitors and suppressants. A similar Fe(CO)s H-atom catalytic cycle involving iron and hydroxide intermediates has been found to be important in CHC02-N2 flames inhibited by 1 Fe(CO)s [8]. Reaction pathway and sensitivity analyses for the inhibited CO flames reveals that the H-atom catalytic cycle is active. A new Fe ~ / Fe(OH}, finding, however, is that there is an additional catalytic cycle which scavenges 0 atoms and which is much more important than the H-atom WMJ~F~~F1H cycle for CO flames. The appearance of O-atom scavenging is consistent with the high peak O-atom mole fraction of the present flames as O-atom cycle H-atom cycle compared to H or OH (Table 4), and the finding Fig. 4. Schematic diagram of reaction pathways in CO-Hz• that O-atom trapping is important for fluorinat• Oz-Nz flames based on the gas-phase mechanism of Ref. 8. ed-species inhibition of CO flames [16]. Recent Thicker arrows correspond to higher reaction flux. Reaction partners are listed next to each arrow. computations of the thermochemistry of iron compounds at flame conditions support the possibility of multiple radical recombination Fe02 and then to FeO. A portion of the FeO is cycles [43]. O-atom recombination by iron com• converted back to Fe, thus completing the 0• pounds has been reported previously [44] for atom cycle; the remainder of the FeO partici• flow tube experiments in which Fe(CO)s was pates in the H-atom cycle. added to the reactant gases. Removal of 0• Although the reactions (R1 )-(R3) were atom by CF3Br in a flow tube has also been present in the inhibition mechanism, a signifi• reported [45]. These previous studies, however, cant contribution from the O-atom cycle was did not propose a catalytic cycle for O-atom not found in the studies of CH4-02-N2 flames. recombination. Instead, reactions (R1 )-(R2) converted Fe to The reactions that comprise the catalytic 0• one of the species in the main inhibition cycle atom scavenging cycle are (FeO), and (R3) was a minor reaction. The authors found that most of the radical scaveng• Fe + O2 + M ~ Fe02 + M (R1) ing for methane flames occurs through the Fe02 + 0 ~ FeO + O2 (R2) H-atom catalytic cycle. Part of the effect of the catalytic cycles illus• FeO + 0 ~ Fe + O2 (R3) trated in Fig. 4 is a suppression of radical o + 0~02' superequilibrium. Figure 5 shows the calculated The first and third reactions in the cycle have O-atom mole fraction as a function of position been studied in the laboratory. The rate for in stoichiometric CO-H2-air flames with Xm = 0.01 and various X;n- As the inhibitor mole (R1) has been measured by several researchers fraction increases, the peak mole fraction de• [46, 47], and the rate for (R3) has been mea• creases; and perhaps more important, the amount sured by Fontijn et al. [48]. The rate for second of superequilibrium decreases, providing fewer reaction was estimated [8]; lowering its rate radicals to support flame propagation. The H• reduces the calculated inhibition and improves atom superequilibrium also decreases, partially agreement with the present experiments. through catalytic scavenging, and partially as a The primary reaction pathways for iron spe• result of the fast 0, H, and OH shuffle reactions. cies are shown in Fig. 4, where both the H- and O-atom catalytic cycles are illustrated. Fe(CO)s Saturation of Catalytic Cycles occurs through several steps in which the CO are sequentially removed Returning to the results in Fig. 2 , we examine (for simplicity, these steps are not shown in the why the inhibition decreases as X02,ox increases figure), leaving Fe. The Fe is converted first to using the technique of reaction flux analysis.

JI i 1,1· 1 ,I, I' INHIBITION OF CO FLAMES BY IRON PENTACARBONYL 459

--

C o '" xo~ox = 0.24 :su ~" 0.21 C 0012 t fAA \ 21 f! ::::~0.01 E 0 :8 u .s 'il 0.6 E ... ., " :>:&! lD 0.008 '0 oaf ~ E"" E 0.006 .s '" 0.4 0.21 •• :s ~ • 11 o 0.004 ~5 C 0.24 o '" 0.002 u E ... "--300 o -J-..llLL-t-·---+---.,--+---+----+---.,~- ---+-- +--- o ~1 0.2 0.3 0.4 Q5 0.6 0.7 Q8 0.9 100 200 300 400 500 600 x (mm) Fe(CO), (ppm) Fig. 5. Profiles of O-atom mole fraction at different Fig. 6. Calculated fractional H-atom and O-atom flux Fe(CO)5 concentrations. cp = 1.0,X02,ox = 0.21,XHz = 0.01. The equilibrium value of Xo is 0.0014 for the uninhibited through key iron reactions at XHz = 0.01 and varying O2 flame and 0.0011 for the inhibited flames. concentrations.

The flux of a species though a particular reac• from recombination of 0 atoms, the reaction tion is defined as the integral of the reaction flux results imply that the difference in behavior rate over the domain of interest (here taken to at low and high X02,ox is due to the increasing be the cold boundary to the time of peak radical importance of the H-atom cycle at low X02.ox' mole fraction). The total flux of a species is the The diminished effectiveness of Fe(CO)s at sum of fluxes from each reaction, and the high X;n may be explained, in part, by the fractional flux is the flux of interest divided by behavior illustrated in Fig. 6. Above 100-200 the total. We measure the relative importance ppm of Fe(CO)s, the O-atom cycle saturates. of the iron reactions by calculating the fractions That is, increasing X;n does not lead to a greater of H-atom and O-atom consumption that occur fraction of the radicals being recombined by the through the most important iron reactions. For iron species. The H-atom cycle saturates at the H-atom, the following reactions are consid• values of Xin only slightly higher. The saturation ered: Fe(OHh + H ~ FeOH + HzO, FeOH + explains why the calculated curves in Fig. 2 have H ~ FeO + Hz, and FeO + HzO ~ Fe(OH)z. decreasing slopes as X;n increases. It is of inter• For the O-atom, the following reactions are est to note that inhibition caused by Fe(CO)s considered: Fe + Oz(+ M) ~ FeOz( + M), diminishes more rapidly in CO flames than CH4 FeOz + 0 ~ FeO + Oz, FeO + 0 ~ Fe + Oz, flames. Part of the reason may be due to the FeOH + 0 + M ~ FeOOH + M. These are importance of the O-atom cycle for CO flames the reactions in the cycles of Fig. 4; previous and the rapid saturation of this cycle relative to modeling results and analysis using the graphical the H-atom cycle. These explanations are ob• postprocessor Xsenkplot [49]have shown them to tained from the calculated results; while satura• be the most important inhibition reactions. tion might conceivably occur in the actual Figure 6 shows the fraction of H-atom and flames, it should be emphasized that the exper• O-atom flux through the catalytic cycles for iments show an even greater loss of effective• three different values of X02,ox' At low X;m the ness, and the modeling and experimental results fractional flux is inversely related to X02,oX' As do not agree well at higher values of X;n' The X02,ox decreases, stronger inhibition may be discrepancy has been attributed to loss of inhib• caused by the larger fraction of H atoms (and to iting species through condensation [7]. a lesser extent, 0 atoms) scavenged through the The saturation of catalytic cycles again sug• catalytic cycles. The inhibition cycles essentially gests the existence of limits to chemical inhibi• compete with the chain branching reactions, tion, as was previously shown for an idealized and hence reduce the overall reaction rate. inhibitor (501;a similar saturation was shown for While most of the inhibition appears to result fluoromethanes, which trap H-atoms noncata- 460 M. D. RUM MINGER ET AL.

0.040I FeOH+O=FeO+OH I FeO+H20=Fe(OHh0.020 lo-atomIH-atom • "0,.y: ox = 0.183 Fe+02(+M)=Fe02(+M) Fe(OHh+H=FeOH+HpFeOH+H=FeO+H2FeOH+O+M=FeOOH+MFe+O+M=FeO+MFeO+H2=Fe+H2OFe+02=FeO+Ocycle FeOOH+O=Fe02+0HFeOOH+OH=Fe02+H20Fe02+0=FeO+02cycle • Xo,.ox = 0.21

o ''0,.)( ox = 0.24

-0.120 -0.100 -0.080 -0.060 -0.040 -0.020 0.000 Normalized Sensitivity Coefficient

Fig. 7. First-order sensitivity coefficients of the burning velocity with respect to reaction rate for selected reactions at Xin = 50 ppm, normalized by the peak value for all reactions at all flame locations (which corresponds to CO + OH - CO2 + H).

Iytically [51]. Although Fe(CO)s may have the Effect of 92 Mole Fraction confounding effect of condensation, the finding of catalytic cycle saturation has practical impli• When XQ2,ox is changed, the radical pool and cations for fire suppressant design. For example, flame temperature are affected. Since the inhib• a blend of non condensing chemical fire suppres• itor behavior is affected by both of these vari• sants might be used to rapidly drop the radical ables, it is desirable to change the radical pool mole fractions, but the performance of such a without changing the temperature. In CO blend could be limited by catalytic cycle satura• flames, this can be accomplished by varyingXm. tion. To produce effective blends, it may be Within the limits set by our experimental appa• advantageous to combine catalytic agents with ratus, we were able to stabilize CO flames with those that lower the equilibrium mole fraction Xm of 0.002,0.005,0.01, and 0.015. An elevated of radicals, either thermally or chemically [52, oxygen content relative to air (24% Oz/76% Nz) 53]. was necessary to achieve this range. Below Xm To further explore the importance of the = 0.002 the flame would not stabilize at a height various reactions, we calculate first-order sensi• of 13 mm; above Xm = 0.015 the flame oscil• tivity coefficients of the burning velocity to the lated too much for accurate measurement. Ta• reaction rates [31, 54]. Figure 7 shows the ble 5 lists calculated maximum mole fraction sensitivity coefficients for the three different and amount of superequilibrium for 0, H, and values of XQ2,OX with 50 ppm of Fe(COk The OH for CH4-air and CO-Hz-Oz-Nz flames calculations show that among the reactions in with varying Xm. As the table shows, the vari• the iron mechanism, the reactions in the 0• ation of Xm results in a change in maximum atom and H-atom catalytic cycles are most H-atom and OH mole fraction of about a factor important for reducing burning velocity. As the of 3.5 (approximately linearly with XH), and oxygen concentration increases, the sensitivity little change in maximum a-atom mole fraction. coefficients of the reactions in the catalytic Note also that the a-atom superequilibrium is cycles decrease. approximately constant with XHz' but that H-

II" ill II i 1,1 I I INHIBITION OF CO FLAMES BY IRON PENTACARBONYL 461

TABLE 5

Calculated0.00350.005441.10.00580.0150.000840.01824710.0050.0150.002431.20.00340.0160.0150.00050.0180.0170.0037CH40.01247524680.00680.00440.001110.00740.01148.755.60.00490.00760.00370.00852353 247122.3Maximum0.0180.0020.0160.000240.00140.0018 0, H, and OH Mole Fraction and Superequilibrium; Maximum XOH.max-XOH.eqXO.maxXH.maxXOH,maxXH,maxTmaxvo•num(K)-Xo.eqXH.eq(cm/s) 0.002 Temperature; for Calculated Burning Velocity for Four Different Values of XH: Xo.max XHz

a Also shown for comparison are the same parameters for a CHcOz-Nz flame. (Xo,.ox = 0.24 and

~tom and OH superequilibrium increases asXHz mentally determined normalized burning veloc• mcreases. ities for CHcair flames [7] are also shown. (The Comparing the CO flames with a CH4 flame, slight discontinuity in the XHz = 0.015 data ("x" several features stand out: (1) the maximum symbols) between X;n = 150 and 250 ppm O-atom mole fraction is about four times higher occurs because mass flow controllers with in the CO flames, (2) the H-atom mole fraction slightly different calibrations were used for the is lower in the CO flames (but is approaching low Xin and high Xin measurements.) Compared the CH4 value as XHz increases), (3) the maxi• to methane flames, the effect of small amounts mum OH concentration is 3 to 8 times lower in of Fe(CO)s in CO-Hz flames is weaker for the CO flames. These differences allow testing XHz = 0.01 and 0.015, about equal for XH, = and study of the inhibition mechanism under 0.005, and stronger for XHz = 0.002 (note that significantly different flame conditions. the XHz = 0.002 flame was not stable for Xin > Figure 8 shows the effect of Fe(CO)s on 50 ppm). A significant finding is that the CH4 flames with different levels of Hz in the reac• curve shows linear behavior longer than the CO tants and X02,ox = 0.24. For reference, experi- curves, resulting in a lower normalized burning velocity at high Fe(CO)s. This is surprising since the reduced effect at high Xin is believed to be due primarily to condensation of iron species.

0.8 One would expect less condensation in the 120 ~ ~ 0.01 K hotter CO flames, and therefore a strong 1: g' 0.6 ~;;:0.Q15 inhibition effect to higher Xin' The opposite CHor-air. +'" 1.0 al~ behavior indicated in Fig. 8 implies that satura•

J 0.4 tion of the catalytic cycles may be the dominant

~o cause for reduced effectiveness, or perhaps that z 0.2 the degree of iron-species condensation is in• creased in the slower CO flames. 0.0 o 100 200 300 400 800 700 Numerical calculations have been performed Fo(CO), (ppm) for the moist CO flames with varying XH2, and Fig. 8. Measured inhibition effect of Fe(CO)s for various Fig. 9 compares the calculations with measure• Hz mole fractions in CO-Hz-Oz-Nz flames with

fraction than the H-atom cycle. The importance of the a-atom cycle in CO flames and its more complete saturation may be one of the reasons that the normalized burning velocity levels off at a higher value in the CO flames than the CH4 flame. Sensitivity coefficients of the burning velocity to selected reaction rates were also calculated for flames with 50 ppm of Fe(CO)s. and varying XHz' At low hydrogen content the O-atom cycle 100 200 300 400 500

Fe(CO), (ppm) reactions have the highest sensitivities, but as Xm increases, their sensitivity coefficients de• Fig. 9. Calculated and measured normalized burning veloc• XHz ities for flames inhibited by Fe(CO)s for various Hz concen• crease. As increases, the regeneration step (FeO + H20) in the H-atom cycle becomes trations. cp = 1.0 and XOz,ox = 0.24. more important, while the other two reactions become less important. predicted inhibition is too strong for higher We also measured normalized burning veloc• values of XH). In addition, the leveling off is not ity for rich (cP = 1.2) and lean (cP = 0.8) CO-H2 predicted and there is little difference between flames with XHz = 0.01, and found that inhibi• the calculations at different XHz' other than the tion is slightly stronger for lean flames than for varying initial slopes; that is, the point where rich flames. As in the case of CH4 flames, the diminishing effectiveness starts is not predicted dependence on cP is weak. by the mechanism. Figure 10 presents the fractional H- and O-atom destruction from the iron species cata• lytic cycles. The fractional amount of O-atom CONCLUSIONS scavenging is highest in flames with low XHz' explaining why the hydrogen-deficient flames In this paper we have presented measurements are inhibited more strongly at low Xin" For Xin of the burning velocity of CO-H2-02-N2 flames above 100 ppm, the fraction of O-atoms con• with and without the inhibitor Fe(CO)s over a sumed through the iron reactions is around 0.85 range of initial H2 and O2 mole fractions. The inhibited flames had four different H2 concen• for XHz = 0.002, but only 0.55 for XHz = 0.015. Figure 10 shows (as does Fig. 6) that the trations, and three different O2 concentrations. O-atom cycle saturates at a lower Fe(CO)s mole A numerical model was used to simulate the flame inhibition using a gas-phase chemical mechanism. Based on the experiments and cal• 1.0 ...•o c '"0",u 0.8 0.60.4 E.c E:8c~tic0 ~!l'~:J:d!o~~~.fIIe 0.2 culations, we reach the following conclusions: XHz = 0.002 0.005 0.01 For uninhibited flames, experimental burning . velocities are within 10% of the results of 0.015 calculations using the mechanism from Yetter et al. [34] for several different rates of the CO + OH ~ CO2 + H reaction. Calculated burning 0.015 velocities using the rate of Yu et al. [36] agree

0.01 closely with our data, whereas those using the

0.005 1992 recommendation of rate from Baulch et al. [37] are significantly higher than the measure• ments. 100 200 300 400 500 600 Fe(CO), (ppm) For inhibited flames, the calculations and

Fig. 10. Calculated fractional H-atom and O-atom flux experiments show good qualitative agreement. through iron reactions at X02,ox = 0.24 and varying Hz The degree of quantitative agreement depends concentrations. strongly on the rates of the CO + OH ~ CO2 +

III i II I II I INHIBITION OF CO FLAMES BY IRON PENTACARBONYL 463

H reaction, the reactions in the H-atom cycle, tant. While both radical saturation and iron and the FeOz + 0 ~ FeO + Oz reaction. species condensation are likely operative, it is Most of the chemical inhibition occurs not possible with the present data to separate through a catalytic recombination cycle that the effects. Hence, there is an even greater need converts 0 atoms into Oz molecules. This cycle for measurements of particles in flames inhib• was not found to be important in the previous ited by Fe(CO)s in order to understand the numerical study of methane flames. The H• reason for the loss of effectiveness of this very atom cycle that caused most of the radical powerful flame inhibitor. Of course, measure• scavenging in the methane flames is also active ment of the rates of key iron reactions, espe• in the CO flames described here, but is of cially the reactions in the catalytic cycles, is also secondary importance. an important area for future work. Based on reaction flux analyses, the degree of inhibition by Fe(CO)s is shown to be related to We are grateful to undergraduate student re• the fraction of the total 0- and H-atom destruc• searcher Nikki Prive for assistance with data tion which is attributable to iron species reactions. acquisition and uncertainty analysis programs, In the calculated results, the O-atom cycle and Dr. Rich Yetter for discussion of the chemical saturates at a relatively low inhibitor mole frac• kinetics of co oxidation. tion (~100 ppm), whereas the H -atom cycle saturates at a much higher inhibitor mole frac• tion (~400 ppm). The earlier saturation of REFERENCES O-atom cycle compared to the H-atom cycle 1. Lask, G., and Wagner, H. G., Eighth Symposium could be a reason why the CO flames have (International) on Combustion, Williams and Wilkins, Baltimore, 1962, pp. 432-438. higher normalized burning velocity than CH4 2. Miller, D. R., Evers, R. L., and Skinner, G. B., flames at high Fe(CO)s mole fraction. Combust. Flame 7:137 (1963). There are several practical implications for 3. Bulewicz, E. M., and Padley, P. J., Thirteenth Sympo• the suppression of hydrocarbon flames from the sium (International) on Combustion, The Combustion present work. Depending upon the hydrogen Institute, Pittsburgh, 1971, pp. 73-80. content of the fuel, the catalytic recombination 4. Bulewicz, E. M., and Padley, P. J., Chem. Phys. Left. 9:467 (1971). of 0 atom may be as important as catalytic 5. Vanpee, M., and Shirodkar, P., Seventeenth Symposium recombination of H atom and should be consid• (International) on Combustion, The Combustion Insti• ered. Catalytically acting agents (singly or in tute, Pittsburgh, 1979, pp. 787-795. blends) may be useful only up to a certain mole 6. Bonne, V., Jost, W., and Wagner, H. G., Fire Res. fraction; above that value, additional agent may Abstracts Rev. 4:6 (1962). 7. Reinelt, D., and Linteris, G. T., Twenty-Sixth Sympo• prove far less effective. Finally, if elimination of sium (International) on Combustion, The Combustion radical superequilibrium is readily achieved by Institute, Pittsburgh, 1996, pp. 1421-1428. catalytically acting agents, further reduction in 8. Rumminger, M. D., ReineIt, D., Babushok, V., and overall reaction rate may require additives that Linteris, G. T., Combust. Flame 116:207 (1999). lower the equilibrium values of radicals, either 9. Palmer, H. B., and Seery, D. J., Combust. Flame 4:213 (1960). thermally or chemically. 10. Safieh, H. Y., Vandooren, J., and Van Tiggelen, P. J., The inhibition mechanism qualitatively re• Nineteenth Symposium (International) on Combustion, produces the dependence of flame inhibition on The Combustion Institute, Pittsburgh, 1982, pp. 117. hydrogen and oxygen concentration at low in• 11. Safieh, H. Y., and Van Tiggelen, P. J., 1. Chimie hibitor loading, but several questions remain Physique 81:461 (1984). 12. da Cruz, F. N., Vandooren, J., and Van Tiggelen, P., unanswered. The most critical one is why the Bull. Soc. Chim. Belg. 97:1011 (1988). normalized burning velocity levels off as the 13. Vandooren, J., da Cruz, F. N., and Van Tiggelen, P. J., Fe(CO)s concentration increases. Because the Twenty-Second Symposium (International) on Combus• leveling-off point changes with varying Xm, but tion, The Combustion Institute, Pittsburgh, 1988, pp. 1587-1595. the flame temperature does not change, there is less reason to believe that condensation is the 14. Masri, A. R., Dally, B. B., Barlow, R. S., and Carter, C. D., Combust. Sci. Technol. 114:17 (1996). cause; results in the present paper imply that 15. Fallon, G. S., Chelliah, H. K., and Linteris, G. T., saturation of the catalytic cycles may be impor- Twenty-Sixth Symposium (International) on Combus- 464 M. D. RUMMINGER ET AL.

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