NISTIR 6374

Premixed Carbon Monoxide-Nitrous OxideHydrogen Flames: Measured and Calculated Burning

Velocities with and without Fe(CO)5

Gregory T. Linteris, Marc D. Rumminger and Valeri Babushok

Building and Fire Research Laboratory

Gaithersburg, Maryland 20899

United States Department of Commerce

Technology Administration

National Institute of Standards and Technology
NISTIR 6374

Premixed Carbon Monoxide-Nitrous OxideHydrogen Flames: Measured and Calculated Burning

Velocities with and without Fe(CO)5

Gregory T. Linteris, Marc D. Rumminger and Valeri Babushok

October, 1999

Building and Fire Research Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899 

U.S. Department of Commerce 

William M. Daley, Secretary 

Technology Administration

Gary Bachula, Acting Under Secretary for Technology

National Institute of Standards and Technology

Raymond G. Kammer, Director
1

Premixed Carbon Monoxide-Nitrous Oxide-Hydrogen Flames:

Measured and Calculated Burning Velocities with and without Fe(CO)5

Gregory T. Linteris† , Marc D. Rumminger‡, and Valeri Babushok

Building and Fire Research Laboratory

National Institute of Standards and Technology

 Gaithersburg MD 20899, USA

Submitted for publication in Combustion and Flame

ABSTRACT

The burning velocity of premixed carbon monoxide-nitrous oxide flames (background water

levels of 5 to 15 ppm) has been determined experimentally for a range of fuel-oxidizer

equivalence ratio φ from 0.6 to 3.2, with added nitrogen up to a mole fraction of XN2 = 0.25,

and with hydrogen added up to XH2 = 0.005. Numerical modeling of the flames based on a

recently developed kinetic mechanism predicts the burning velocity reasonably well, and

indicates that the direct reaction of CO with N2O is the most important reaction for CO and N2O

consumption for values of XH2≤ 0.0014. The calculations show that a background H2 level of

10 ppm increases the burning velocity by only about 1 % compared to the bone-dry case.

Addition of iron pentacarbonyl, Fe(CO)5, a powerful flame inhibitor in hydrocarbon-air flames,

increases the burning velocity of the CO-N2O flames significantly. The promotion is believed to

be due to the iron-catalyzed gas-phase reaction of N2O with CO, via N2O + M = N2 + MO and

CO+ MO = CO2 + M, where M is Fe, FeO, or FeOH.

INTRODUCTION

The most effective chemical flame inhibitors are believed to act through catalytic cycles that

recombine radicals in the flame. These inhibitors, however, have not been tested in systems

without chain branching. The present investigation was conducted for the dual purposes of

studying the effectiveness of the flame inhibitor iron pentacarbonyl in non-branching flames of

CO and N2O, and to investigate the role of the direct reaction of CO with N2O at flame

temperatures with controlled amounts of hydrogen. The high-temperature gas-phase reactions

of carbon monoxide and nitrous oxide are an important and well-studied system. These

reactions occur in the gas-phase region during combustion of nitramine-based solid rocket

propellants [1-3], and they are also important for understanding the combustion emissions

characteristics of stationary and mobile power plants. The direct reaction of CO with N2O is of

fundamental interest since it is one of the simplest examples of an exchange reaction between

saturated molecules. For inhibition studies, the reactant mixture provides a non-chain

mechanism involving oxygen atoms, so that the significance of catalytic O-atom recombination

cycles of iron species from the inhibitor can be tested, as well as those of H and OH when

trace hydrogen is added as a reactant.

 

†

 Corresponding author, linteris@nist.gov

‡

 National Research Council/NIST postdoctoral fellow, 1996-1999; current address: Sandia National Lab, MS9052,

Livermore CA, 94551
2

The CO-N2O reaction has been studied in shock tubes [4-6] and flow reactors [7].

Loirat et al. [8] measured the critical ignition pressure of CO-N2O mixtures in a cylindrical

reactor. In flame studies, Dindi et al. [9] measured the stable species mole fractions in lowpressure premixed flames using gas chromatography, and Vandooren et al. [10] recently used

mass spectrometry to measure the structure of CO-N2O-H2 flames. Cor et al. [11] measured

the stable species profiles in low-pressure counterflow CO-N2O-N2 diffusion flames. These

studies have provided data for determining elementary rates and for testing comprehensive

mechanisms. In many of the studies, the possible interference of H-atom reactions from

impurities has been described, but not always quantified.

The burning velocity of premixed CO-N2O flames has been measured previously in

three investigations. Van Wonterghem and Van Tiggelen [12] measured the flame speed of

lean, stoichiometric, and rich flames, some with nitrogen dilution, having estimated hydrogen

impurities of less than 2000 ppm§ in the CO (but not noted for the N2O). Kalff and Alkemade

[13] provided data on stoichiometric and rich flames with up to 10 % added water vapor, and a

minimum hydrogen content estimated to be less than 500 ppm. Simpson and Linnett [14]

investigated quite rich systems (φ = 2.0), diluted by nitrogen, with unquantified, but low, levels

of hydrogen impurities. The burning velocity of premixed CO-N2O flames has been calculated

[7,15], but the absence of data for flames with low hydrogen content was noted in both

studies.

Since the levels of hydrogen are somewhat high or unquantified in previous

experiments, additional experiments are required to understand the importance of the direct

reaction at flame temperatures. We report burning velocity measurements for stoichiometric

CO-N2O flames with added H2 mole fractions from 0 ppm to 6800 ppm. For the driest

conditions (5 to 15 ppm H2O), we also report flame speeds for equivalence ratios from 0.6 to

3.2, and for stoichiometric flames with nitrogen dilution up to 25 % of the total volumetric flow.

For all conditions, the flame structure is numerically calculated using a detailed chemical

kinetic mechanism, providing an estimate for the rate of the direct reaction at the flame

temperature, and allowing assessment of the relative importance of the different reaction

routes for consumption of N2O and CO.

The CO-N2O flames were also used to study the inhibition mechanism of Fe(CO)5.

There is an urgent need to find replacements for the effective and widely used fire suppressant

CF3Br and related compounds [16]; however, a replacement with all of the desirable properties

of CF3Br is proving difficult to find and research has intensified [17]. Certain metallic

compounds have been found to be substantially more effective flame inhibitors than halogencontaining compounds [18-20]. Iron pentacarbonyl is among the most effective flame

inhibitors ever identified [18], up to two orders of magnitude more effective than CF3Br, and

recent progress has been made in understanding its mechanism [21]. A detailed chemical

kinetic mechanism for iron-species inhibition of flames has been introduced [22], and modeling

with the mechanism supports the premise that the inhibition is primarily a gas-phase

phenomena. Numerical calculations using the mechanism predict many of the properties of

the flames examined; nonetheless, some of the features of the flames are not well-described,

and much work remains to be done to test and validate the mechanism. In particular, inhibition

in lean flames (where O-atom reactions are much more important) is not accurately modeled

by the mechanism.

In previous research, oxides of nitrogen have been used as the oxidizer in studies of

the effectiveness of flame inhibitors [23]. Since systems using nitrogen oxides instead of O2

 

§

 all references to ppm in this paper are on a mole basis and refer to μl/l.
3

undergo non-chain (rather than chain-branching) radical reaction sequences, the relative

effectiveness of agents believed to act through catalytic radical recombination cycles may be

very different, affording new opportunities to understand the inhibition by Fe(CO)5.

In the absence of hydrogen, the oxidation mechanism of CO-N2O flames is believed to

proceed through either the direct reaction or through the thermal decomposition of N2O

followed by O-atom reaction with N2O or CO. Recent research with Fe(CO)5 inhibition of COO2-N2-H2 flames has shown that in some systems a new O-atom radical recombination cycle

may dominate radical destruction [24]. Also, Kaufman [25] has shown that Fe(CO)5 addition to

a flow tube causes very strong reduction in O-atom concentration. The extent to which

Fe(CO)5 affects dry CO-N2O flames will depend upon the predominance of the O-atom route

for N2O and CO consumption and the effectiveness of the iron-catalyzed O-atom radical

recombination cycle. Since gas-phase iron is also believed to catalyze the decomposition of

N2O (as discussed below), the relative efficiency of this reaction will also affect the influence of

iron in this flame. In summary, in the flame inhibition experiments and modeling, we seek to

determine the effectiveness of Fe(CO)5 in a system which is non-chain branching, to test the

postulate that for these systems, radical recombination—even by a very powerful catalytic

agent—is not as effective.

EXPERIMENTAL

A Mache-Hebra nozzle burner (inner diameter 1.02 cm ± 0.005 cm) [26] with a schlieren

imaging system [27] provides the average burning velocity of these Bunsen-type flames using

the total area method [28]. The experimental system has been described in detail previously

[29]. The burner produces straight-sided schlieren and visible images which are very closely

parallel. Gas flows are measured with digitally-controlled mass flow controllers (Sierra Model

860**) with a quoted repeatability of 0.2 % and accuracy of 1 % of full-scale flow which have

been calibrated with bubble (Gillian Gilibrator) and dry (American Meter Co. DTM-200A) flow

meters so that their accuracy is 1 % of indicated flow. The fuel gas is carbon monoxide

(Matheson UHP, 99.9 % CO, for which a batch analysis by the manufacturer shows the

concentration of H2O and CH4 to be less than 0.3 ppm and 1.0 ppm, respectively, and an inhouse FTIR analysis showed H2O and HCs to be present at less than 1 ppm each). For some

tests, the fuel stream contains added hydrogen (Matheson UHP, 99.999 % H2, with sum of N2,

O2, CO2, CO, Ar, CH4, and H2O < 10 ppm). The oxidizer stream consists of nitrous oxide

(Matheson UHP, 99.99 % N2O, with sum of N2, O2, CO, CO2, and CH4 less than 100 ppm,

which an in-house FTIR analysis showed to have 25 ppm of H2O, and less than 4 ppm of the

sum of hydrocarbons up to C4). Added nitrogen is boil-off from liquid N2.

Inhibitor is added to the flame by diverting part of the nitrogen stream to a two-stage

saturator maintained in an ice bath at 0o C. The diverted gas (less than 8 % of the total flow)

bubbles through liquid Fe(CO)5 before returning to the main nitrogen flow, and is saturated with

Fe(CO)5 vapor. The schlieren image of the flame is used to represent the flame surface. An

optical system (a white-light source with a vertical slit at its exit, lenses, a vertical band, and

filters) generates the schlieren image of the flame for capture by a 776 x 512 pixel Charged

Injection Device (CID) array (Cidtec CID3710D). The image is digitized by a 640 x 480 pixel

frame-grabber board (Data Translation 3155) in a Pentium-II computer. The images are

 

** Certain commercial equipment, instruments, or materials are identified in this paper to adequately

specify the procedure. Such identification does not imply recommendation or endorsement by the

National Institute of Standards and Technology, nor does it imply that the materials or equipment are

necessarily the best available for the intended use.
4

acquired and written to disk using the free University of Texas Health Science Center of San

Antonio (UTHSCSA) ImageTool program [30]. The flame area is determined (assuming axial

symmetry) using custom image-processing software, and the burning velocity is calculated by

dividing the volumetric flow rate (corrected to 1 atmosphere and 298 K) by the flame area.

In these experiments, the low rate of heat loss to the burner, the low strain rate, and the

low curvature facilitate comparisons of the burning velocity with those predicted by onedimensional numerical calculations. Although the burning velocity in Bunsen-type flames is

known to vary at the tip and base of the flame and is influenced by curvature and stretch [28],

these effects are most important over a small portion of the flame. In order to minimize the

influence of these effects on interpretation of the action of the chemical inhibitor, we present

the burning velocity of inhibited flames as a normalized parameter: the burning velocity of the

inhibited flame divided by the burning velocity of the uninhibited flame at the same flow

conditions.

Uncertainty Analysis

The uncertainty analysis consists of calculation of individual uncertainty components and root

mean square summation of components [31]. All uncertainties are reported as expanded

uncertainties: X ± U, where U is kuc, and is determined from a combined standard uncertainty

(estimated standard deviation) uc, and a coverage factor k = 2 (level of confidence

approximately 95 %). Likewise, when reported, the relative uncertainty is U / X · 100 %, or kuc

/ X · 100 %. The primary sources of uncertainty in the average burning velocity measurement

are the 1) accuracy of the flow controllers, 2) measurement of ambient pressure and

temperature, 3) determination of the flame area, 4) effect of flame base location on flame area,

and 5) location of the schlieren image relative to the cold gas boundary (as discussed in more

detail in ref. [32]. The relative uncertainty for average burning velocity measurements ranges

between 2.7 % and 6.5 %, and between 1.3 % and 3.1 % for normalized burning velocity. In

general, uncertainty increases with increasing burning velocity. Imprecision in the location of

the schlieren image in the flame contributes additional uncertainty to the absolute burning

velocity measurement. Traditionally, the burning velocity of the flame is the flow velocity of the

unburned reactants; however, since the schlieren image corresponds to a surface with a

temperature above that of the unburned gas [28], use of the schlieren image as the flame area

can lead to an underestimation of the burning velocity [33]. Previous researchers [28,33,34]

have attempted to adjust the flame area to the 298 K isotherm (T298), although this generally

requires knowledge of the 2-D temperature field in the flame. We do not make such

corrections in this paper, and note that it has not yet been done using experimental (rather

than calculated) temperature profiles in a flame.

The uncertainties in the equivalence ratio, nitrogen mole fraction and hydrogen content

are determined by the accuracy of the flow controllers. The uncertainties are 1.4 % for the

equivalence ratio, 1.1 % for the nitrogen mole fraction, and 1.2 % for the hydrogen mole

fraction. For stoichiometric mixtures of N2O and CO, trace hydrogen (as H2O) is present in the

reactant gases at about 13 ppm.

The uncertainty in the Fe(CO)5 concentration is calculated assuming that it depends on

the vapor pressure of Fe(CO)5, laboratory temperature and pressure, and carrier gas flow rate;

these uncertainties are combined using a “jitter” program [35], which sequentially varies the

input data and computes the resulting contribution to the uncertainty for each output variable.

The correlation for the vapor pressure of Fe(CO)5 is from Gilbert and Sulzmann [36], but the

paper does not provide uncertainty for the correlation. Neglecting uncertainties in the vapor

pressure correlation, the uncertainties in the bath temperature, ambient pressure and carrier

gas flow rate yield an Fe(CO)5 mole fraction uncertainty of 6.5 %.
5

NUMERICAL

One-dimensional freely-propagating premixed flames are simulated using the Sandia flame

code Premix [37], the Chemkin subroutines [38], and the transport property subroutines [39].

For all of the calculations the absolute tolerance is 10-14, the relative tolerance is 10-9, GRAD is

0.15, and CURV is 0.35. The solution grids have about one hundred points and the

computational domain is 0 to 50 cm (except for data in Figure 1, for which the domain is 0 to

200 cm). The initial temperature is 298 K and the pressure is one atmosphere. The moist CON2O flames are modeled using the mechanism and thermodynamic data set of Allen et al. [7],

from which the species NCN, NCO, CNO, CH, CH2, CH3, C2H, HCCO, HNCO, HOCN, HCNO,

H2CN have been removed (they have been found to be unimportant for the present flames).

The resulting reduced mechanism has 20 species and 92 reactions. We use the rate for

CO+OH↔CO2+H from Yu et al. [40] since it provides better agreement with our data. Iron

pentacarbonyl is added to the unburned CO-N2O-H2-N2 mixture at mole fractions of up to 180

ppm. The chemical mechanism for Fe(CO)5 inhibition of flames (12 species and 55 reactions)

and necessary thermodynamic data are compiled from a variety of sources as described in

Rumminger et al. [22].

RESULTS AND DISCUSSION

Prior to discussing the inhibited flames, we first present findings for the uninhibited flames.

Calculations show that the pure CO-N2O flame has a very large domain due to the slow

reaction of CO and NO downstream of the main heat release region. The significance of this

behavior with regard to comparisons between the calculated results and the modeling are

presented first, with subsequent discussion of the burning velocity and properties of the

uninhibited flames. Finally, the results for the inhibited flames are presented.

General Features

A plot of the major species profiles and temperature of a stoichiometric CO-N2O flame is

shown in Figure 1. These results were obtained from the numerical calculations using the

rates of Milks and Matula [5] and Yu et al. [40], as described below. There are three distinct

regions in the calculated results. In the first, which extends a few millimeters from the start of

the temperature rise, there is very fast reaction of CO with N2O, and thermal decomposition of

N2O followed by reaction of N2O with O atoms. In the second region, [NO] is constant, but CO

is consumed through its slow reaction with O and O2. In the third region, NO is consumed.

Although no practical flame can support such weak reaction over a domain of two meters, it is

of interest to determine how the features of the calculated results over the entire domain

influence the comparisons with experimental results from a Bunsen-type flame which remains

quasi-one-dimensional only for a few millimeters.

Table 1 presents some calculated burning velocities vo,num , temperatures and peak

radical mole fractions for dry, undiluted CO-N2O flames over a range of φ and for stoichiometric

flames, over a range of added N2 and H2. The adiabatic flame temperature (A.F.T.) is given,

as well as the temperature at the location of 99.5 % consumption of N2O, which roughly

corresponds to the end of the region of rapid heat release. The temperature at the location of

the peak of the CO+N2O reaction rate is also listed. For pure CO-N2O flames with φ = 1.0 and

hydrogen addition up to 6800 ppm, or dry flames with 0.6 ≤ φ ≤ 1.3 (the first six lines of Table

1), the A.F.T. is 2830 K ± 40 K, while for the richest flames (φ = 3.2) and stoichiometric flames

with nitrogen dilution, the A.F.T. is significantly cooler, 2378 K and 2317 K respectively.
6

However, as the table shows, the peak temperature in the region of rapid reaction rarely

comes close to the A.F.T.

The significant but gradual temperature rise far downstream of the main reaction zone

is caused by the slow reaction of the remaining CO with O or O2, and from NO consumption.

Nonetheless, this additional temperature rise does not have much consequence for the main

reaction zone; the thickness is too great to provide much heat feedback, and the flow field in

the experiment does not support such a large, one-dimensional downstream region.

Numerical experiments for 0.6 ≤ φ ≤ 3.2 show that turning off the CO+O and CO+O2 reactions,

and the even slower NO consumption reactions, or limiting the computational domain to 3 mm,

both provide the burning velocity within 2 % of those using the entire domain with all reactions

included. As Table 1 shows, the peak temperature just past the region of rapid heat release

(the point of 99.5 % N2O consumption) is estimated to be 240 K to 550 K lower than the

adiabatic flame temperature. For flames rich in CO or with added N2, the adiabatic flame

temperature is much lower because of dilution, but the temperature at the point of N2O

consumption is only slightly lower. For flames with added hydrogen, the faster overall reaction

rate allows more of the CO to be consumed before the N2O is gone, making the temperature in

the main reaction zone closer to the final temperature far downstream.

The temperature at the peak of the CO+N2O is about 1800 K; it is about 100 K higher

with 6800 ppm of hydrogen, drops by about 100 K for rich or lean flames, and is about 150 K

lower with 25 % dilution nitrogen. Hence, the temperature at the peak of the CO+N2O reaction

is less sensitive to changes in φ, XN2 or XH2 than is the adiabatic flame temperature (which is

not nearly achieved in the experimental flames). For the purpose of model validation, the

temperatures in the main reaction zone are more relevant than the adiabatic flame

temperatures.

These CO-N2O flames have radical pools completely unlike either CH4-air flames or

moist CO flames. The O, H, and OH mole fractions, XO, XH, and XOH, (H and OH for moist

conditions only) rise monotonically throughout the computational domain up to the point where

the N2O is 99.5 % gone, so that values at this point are estimates of the peak values in the

flame. Unlike CO or CH4 flames, these flames do not demonstrate the characteristic radical

super-equilibrium. The peak XO is about the same as in stoichiometric CH4-air flames, and

about a factor of four lower than in CO-air flames with 1 % H2. With even about 0.7 % of

added H2, the peak XH in the CO-N2O flames is still more than two orders of magnitude lower

than that for the CH4 or CO flames, clearly demonstrating the straight-chain character of these

flames, and suggesting their utility for testing the effectiveness of chemical inhibitors in nonbranching systems. These results are of significance for the suppression of flames that do not

use oxygen as the oxidizer, such as some propellant flames.

Burning Velocity of Uninhibited Flames

Figure 2 presents the burning velocity of the premixed CO-N2O flames from the experiments

(symbols) as a function of the hydrogen concentration in the reactants. The 'dry' burning

velocity is measured to be 23.1 cm/s ±0.6 cm/s and this value increases to about twice that

value with 0.5 % of hydrogen. The marginal effect of added H2 is smaller at higher values of

the hydrogen mole fraction. (For the reader’s convenience, Table 2 contains the experimental

data and uncertainties).

 The figure also shows the calculated burning velocity (lines) for various values of the

specific reaction rate constant for the reaction CO+ N2O -> CO2 + N2. The bottom curve does

not include the direct reaction in the mechanism, and indicates a burning velocity of 2.9 cm/s
7

with no added hydrogen, clearly illustrating that this reaction is required to reproduce our

burning velocity data. The remaining curves in the figure show the calculated result using the

rate expressions of Dindi et al. [9], Milks and Matula [5], Loirat et al. [41], Fujii et al. [6], and

Loirat et al. [8]. The value recommended by Dindi et al. yields a burning rate about three times

too high, with too weak of a dependence on XH2. The expression of Milks and Matula [5]

provides a burning velocity which is very close to the present data for low values of XH2, but

which deviates for XH2 above about 0.2 %. The other expressions provide too low of a burning

velocity for all values of XH2.

Also shown in the figure are the data of Van Wonterghem and Van Tiggelen [12] (<

2000 ppm of H2), and the data of Kalff and Alkemade [13] (<500 ppm H2O). The present data

are in good agreement with the low-hydrogen data of both previous investigations, and provide

the additional advantage of a very low background level of hydrogen with a controlled level of

added hydrogen. Note that although the data of Kalff and Alkemade are presented on the

same figure, the experiments contained H2O rather than H2. To estimate the magnitude of the

difference between H2 and H2O in these flames, calculations were performed for either added

H2 or H2O. From these calculations we can estimate that H2 addition, as compared to H2O

addition, lowers the burning velocity about 10 % at 250 ppm and raises it about 10 % at 7200

ppm. Thus, after estimating the difference between H2 and H2O addition for these conditions,

we find that the data of Kalff and Alkemade are in excellent agreement with the present data at

low hydrogen content, and approximately within the uncertainty of both experiments if our data

are extrapolated to their conditions at 7200 ppm H2O.

Since the CO+OH reaction becomes increasingly important as the hydrogen content

increases, the rate of the CO+OH reaction has a large influence on the ability of the model to

predict the experimental data at higher XH2. Figure 3 presents the calculated burning velocity

of the CO-N2O flames with the direct reaction rate of Milks and Matula [5], but using the CO +

OH rate expression of Baulch et al. 1973 [42], Yu et al. [40], and Baulch et al. 1992 [43]. The

expression of Yu at al. is the closest to the present data, while the recommendation of Baulch

et al. 1992 [43] significantly over-predicts the burning velocity for our data. Although this rate

is important for predicting the dependence of the burning velocity on the added hydrogen, it is

not important for predicting the burning velocity of the driest flames here (less than 15 ppm

H2O). For the remainder of the calculations in the present paper, we retain the rate of Yu et al.

for CO + OH.

The suggested rate expressions for the direct reaction include those with a high

activation energy (e.g. [4,8,41]) and those with a low value (e.g. [5,6,44]). An Arrhenius plot

for some of the literature values of the direct reaction is shown in Figure 4. Since the flame

temperature in the main reaction zone of the flames varies by about 260 K, the different

activation energies can affect the shape of the burning velocity curves in Figure 2 (comparing

for example the curves for Loirat et al. 1987 and Fujii, which are fairly close at XH2=0). Thus it

is of interest to examine flames where uncertainty in the CO+OH rate does not complicate

interpretation of the rate parameters for the direct reaction. Experiments with dry (~ 13 ppm

H2) flames with added dilution nitrogen or over a range of φ can provide some temperature

variation so that the influence of the activation energy of the direct reaction can be

investigated.

Figure 5 presents the burning velocity of the dry flames with added nitrogen up to about

25 % together with calculations using the rate expressions of Loirat et al. [41] and Milks and

Matula [5]. Also shown in the figure are the predictions when the pre-exponential factors of the

direct reaction rates have been adjusted to provide agreement at XN2

=0. The higher activation
8

energy expression appears to yield too low of a burning velocity with added nitrogen, whereas

the lower activation energy expression provides a closer temperature dependence.

The dependence of the burning velocity on the fuel-oxidizer equivalence ratio φ is

shown in Figure 6. The experimental results (symbols) for dry flames are presented along with

the calculated results (solid lines), again using the direct reaction rate expressions of Loirat et

al. [41] and Milks and Matula [5]. As shown previously in Figure 2, the expression of Loirat et

al. [41] (along with most of the others) provides a burning velocity significantly lower than the

present data for dry flames. When the pre-exponential factor in the direct exchange reaction

rate is adjusted to give agreement at φ =1.0 (dotted lines in the figures), the high activation

energy rate of Loirat et al. under-predicts the burning velocity for richer flames (which have

lower temperature) more so than does the low-activation energy rate of Milks and Matula (note

from Table 1 that at φ = 3.2, the peak flame temperature within the primary reaction zone

drops by about 200 K compared to its peak at φ = 1.0). Nonetheless, the rate expression of

Milks and Matula over-predicts the burning velocity by about 25 % at φ = 0.6, and underpredicts it by the same amount at φ =3.0. For the given mechanism, over the range of φ in

Figure 6, the burning velocity is only significantly sensitive to the rate of the exchange reaction

(as described below); adjustment of the other rates within their uncertainty does not improve

agreement in Figure 6. Also, addition of H2 impurity at 100 ppm, while raising the burning

velocity 10 % to 15 % at all values of φ, does not affect the qualitative variation with φ.

Van Wonterghem and Van Tiggelen [12] also presented data on the flame speed as a

function of φ and XN2, and it is of interest to see if the present model and suggested rates for

the CO+N2O direct reaction can account for their results. Although the background hydrogen

levels in their experiment are somewhat high (and not measured), their data can still be used

to test the present mechanism since even with 1000 ppm of background H2, about half of the

CO reaction occurs via the direct reaction, and the burning velocity is sensitive to the rate of

this reaction. To proceed further, we estimate the actual background level of H2 in their

reactant mixtures to be that value of XH2 which provides a calculated burning velocity which

matches their experimental burning velocity for φ =1.0 and XN2=0 (we use the present

mechanism with the Milks and Matula CO+N2O rate but a 10 % lower pre-exponential, which

gives the best fit to our data). This constant value of 1560 ppm for the background hydrogen

is then used for all subsequent calculations over a range of φ and XN2, allowing a comparison

of model predictions with their experimental results. Figure 7 shows that the calculations

predict very closely the variation of burning velocity with nitrogen addition as measured by Van

Wonterghem and Van Tiggelen. In Figure 8, their data for burning velocity as a function of φ

are plotted with the present experimental data from Figure 6 (5 ppm to 15 ppm H2O), along

with calculated burning velocities for 0 ppm and 1560 ppm of background hydrogen. As Figure

8 shows, in both cases the model somewhat over-predicts the burning velocity for the lean

flames and under-predicts it for rich flames.

Finally, Figure 9 presents the experimental burning velocity data of Simpson and

Linnett [14] for rich ( φ =2.0), slightly pre-heated (Tin=323 K) CO-N2O flames with 25 % dilution

nitrogen and added water vapor up to about 5 % (error bars estimated here based on ref. [28]).

The calculated results for the same initial conditions are shown by the solid line. The

agreement is reasonable considering possible differences in the straight tube/flame angle

method of determining the burning velocity relative to the nozzle burner/total area method

used here.

The numerical modeling of our experimental flame requires the use of the direct

reaction for prediction of burning velocity, and the flame results of earlier researchers are also

reasonably modeled using the rates suggested in the present work. The kinetic mechanism of
9

Allen et al. [7] provides reasonable agreement with the flame data, while use of the CO + OH

rate of Yu et al. [40] and the CO + N2O rate of Milks and Matula [5] provide improved

agreement. Lowering the pre-exponential factor of the CO + N2O reaction rate constant of

Milks and Matula by about 10% provides the best agreement with our data for dry conditions.

Using this rate, the temperature dependence is reasonably modeled, the burning velocity of

lean flames (φ=0.5) is overpredicted by about 18%, and that of rich flames (φ=3.0) is

underpredicted by the same amount. The flame results imply a rate of 109.2 cm3 / mol s at

1800 K for the direct reaction, with an activation energy near 71 kJ/mol.

The important reactions in the CO-N2O flames with and without added hydrogen can

now be examined using the kinetic mechanism described above. The rate selected for the

CO+OH reaction is that of Yu et al [40], and that for the direct CO-N2O reaction is that of Milks

and Matula [5]. In Figure 10, the fractional flux of CO (dotted lines) is shown for each

important reaction as a function of the mole percent hydrogen. The fractional consumption flux

is the fraction of the total consumption of a species which occurs from a given reaction. The

total flux for a species is determined by integrating each consumption reaction over the domain

of interest (the cold boundary to the point of 99.5 % N2O consumption), and summing the

contribution from each reaction. The fractional flux is the flux for a specific reaction divided by

the total flux. As indicated, the direct exchange reaction accounts for about 96 % of CO

consumption for the dry condition, decreasing rapidly to about 50 % at 1000 ppm, with a more

gradual decrease for greater hydrogen concentrations. Correspondingly, the fraction of CO

consumption due to reaction with OH increases. For the reasonably small domain of these

calculations (about 2 mm), reactions of CO with O or O2 (not shown) are at most a few percent

of the total consumption flux for CO. The reaction of N2O is more varied, and the fractional

consumption flux of N2O is shown by the solid lines in Figure 10. Thermal decomposition

accounts for about 30 %, and is weakly dependent on the hydrogen concentration. Likewise,

reactions of N2O with O atom to form 2NO or O2 and N2 account for about 13 % each, while

reactions with NO (not shown) account for a few percent; all of these reactions vary little with

XH2. The contribution of the reaction of N2O with hydrogen radical (produced from the CO+OH-

>CO2+H reaction) increases rapidly as XH2 approaches 1000 ppm and more slowly above that

value, with the reaction of N2O with CO again decreasing correspondingly. A main feature of

these flames is that while hydrogen has a large effect on the reaction mechanism, the direct

reaction still accounts for half of the CO consumption and a quarter of the N2O consumption at

XH2 =1000 ppm.

In Figure 11, a similar plot is shown for CO and N2O consumption as a function of φ. In

these dry flames (5 to 15 ppm H2O) the direct reaction of CO and N2O is always responsible

for about 96 % of the CO consumption; however, for N2O consumption, its contribution varies

from about 33 % for lean flames to 80 % for the rich flames. Similarly, the contributions of

other reactions for N2O consumption decrease when CO is present in abundance.

Effect of Fe(CO)5 on CO - N2O Flames

In order to study the action of chemical inhibitors in systems without radical chain branching,

previous researchers have employed nitrogen oxides as the oxidizers. In studies of H2–N2–

N2O flames inhibited by CF3Br and HBr, Dixon-Lewis et al. [45] found the nitrous oxide flames

to be inhibited much more weakly than hydrogen-air flames. Similarly, Rosser et al. [23] found

that neither CH4 – NO2 nor C2H4 – NO2 flames were significantly inhibited by HBr. They further

postulated that any flames in which NO2 is present in significant quantities will not be inhibited
10

by HBr. In studies of moist CO - O2 and CO - N2O flames with chlorine, Palmer and Seery [46]

predicted that after initial inhibition, further addition of Cl2 would accelerate the N2O

decomposition, and that CO would then react with ClO; however, they provided no burning

velocity data for such flames, and stated that the flames are still ultimately inhibited by

chlorine. To further understand the effect of Fe(CO)5 on radical recombination, particularly in

non-branching systems, it is of interest to study its effect in dry and moist CO - N2O flames.

Since Fe(CO)5 is about 200 times more effective as a flame inhibitor than is Cl, it is of interest

to determine if the region of inhibition that Palmer and Seery suggested would occur for low Cl2

mole fraction, as well as the ultimate inhibition that they suggested, may be modified for

Fe(CO)5.

Figure 12 shows the experimental data (symbols) for the burning velocity of N2O flames

with added Fe(CO)5, normalized by the burning velocity with no inhibitor. Data are provided for

added H2 mole fractions XH2 of 0, 0.001 and 0.002 (note that the background water is 15 ppm).

Unlike flames of CH4-air, and CO-air which are inhibited by Fe(CO)5, the overall reaction rates

of these CO–N2O flames are increased. As the figure shows, for hydrogen-free flames with

171 ppm of Fe(CO)5, the burning velocity is increased by 25 %; as the amount of hydrogen is

increased, the rate of burning velocity increase is smaller. In order to understand the reasons

for the promotion of the reactions, and also the reduction in this promotion, the flames were

modeled as described below.

A mechanism for Fe(CO)5-inhibition of hydrocarbon and carbon monoxide flames

[22,24] indicates that for those systems, the inhibition occurs due to gas-phase catalytic cycles

in which H- or O-atoms are recombined by iron oxide and hydroxide species. Using this

mechanism, calculations for the present flames do not show promotion. Recent research in

atmospheric chemistry, however, suggests that interactions between Fe-species, N2O, and CO

may be important. West and Broida [47] observed that nitrous oxide destruction is catalyzed by

Fe, producing chemiluminescent FeO. Rates for the reaction Fe+ N2O→FeO+N2 have been

measured by Campbell and Metzger [48] and Plane and Rollason [49]. This reaction alone

does not provide promotion; however, with subsequent reaction of FeO and CO, it does.

Kappes and Staley [50] proposed a catalytic oxidation cycle involving ions: Fe+ + N2O → FeO+

+ N2 and FeO+ + CO → Fe+ + CO2, but reaction of neutral FeO with CO is also possible, and

would provide a gas-phase catalytic cycle, with the net reaction: CO + N2O → CO2 + N2

Previous experimental work suggests such catalytic effects. In a shock tube study,

Matsuda [51] reports that addition of Fe(CO)5 to CO-O2-Ar mixtures promotes the oxidation of

CO. Also, in a fluidized-bed study of the interactions of compounds of iron with those of

nitrogen, Hayhurst and Lawrence [52] argue that iron acts as a catalyst to convert N2O to N2

and CO to CO2 (but this appears to be a heterogeneous effect). In addition to the cycle above

with FeO, estimates of the heats of reaction indicate that cycles with FeO2 and FeOOH are

possible. The rates for the reactions in the iron-catalyzed CO-N2O system are listed in Table

3. The rate of the first reaction in the table is from by Rollason and Plane [53], while the others

are our estimates. These reactions are added to the iron-species inhibition mechanism

described in Ref. [22]. The thermodynamic data for FeOOH (and the related rate expressions)

have been updated based on the recent calculations in Ref. [54]. Calculations of the flame

structure using this mechanism have been performed for the N2O-H2-Fe(CO)5 flames and are

described below.

Figure 12 shows the calculated burning velocity (normalized by the calculated burning

velocity in the absence of iron pentacarbonyl) as a function of the initial Fe(CO)5 mole fraction

XFe(CO)5

 for added hydrogen mole fractions of 0, 0.1 % and 0.2 %. The calculated results show

the correct qualitative behavior for Fe(CO)5 addition to the dry N2O flames. For the two cases

with added hydrogen, a slight difference in the promotion effect is predicted for XH2

 = 0.001 or
11

0.002, but the variation is greater in the experiments. Also, for XH2 = 0.002 in particular, the

slope of the experimental curve decreases as XFe(CO)5 increases. That is, the marginal

influence of the Fe(CO)5 (reaction promotion) decreases as XFe(CO)5 increases. This behavior is

similar to the loss of effectiveness in CH4 and CO flames, in which the marginal inhibition is

reduced at higher Fe(CO)5 mole fractions.

In previous work, the loss of inhibition effectiveness was argued to be due to the

formation of condensed-phase particulates which serve as a sink for the active gas-phase ironcontaining inhibiting species [21]. Through the use of classical laser scattering and extinction

measurements with phase-sensitive detection, it was shown that a large increase in the

particle scattering signal was well correlated with the point where the inhibitor lost its marginal

effect [55,56]. It is possible that the reduction in the promotion effect in the present flames is

also due to loss of the active iron intermediates through condensation. In order to test this

hypothesis, we performed laser scattering measurements on premixed CO-N2O flames with 0

% and 0.2 % added hydrogen and for 0 and 180 ppm of added Fe(CO)5 using the apparatus

described in detail in references [55,56]. The experiments show that with addition of iron

pentacarbonyl, there was no significant particle scattering signal in the main reaction region for

either level of H2 in the reactants, unlike the hydrocarbon flames in which a significant

scattering signal was detected which clearly increased as the mole fraction of Fe(CO)5

increased. Hence, we cannot conclude that the reduction in the promotion effect near XFe(CO)5

= 60 ppm for XH2 = 0.002 is due to formation of particulates in the flame.

Although agreement between the measured and calculated normalized burning velocity

in Figure 12 is not perfect, it is still of interest to investigate the numerical results to determine

the reasons for the promotion of the reaction (and the lower promotion when hydrogen is

present). Reaction flux and sensitivity analyses are used to provide insight. For each

important species in the mechanism, Table 4 shows the fractional flux through the reactions

contributing more than 1 % to their consumption or production; Table 5 shows the first-order

sensitivity coefficient of the burning velocity with respect to the reaction rate constant (d(ln

v)/d(ln k) / d(ln v)/d(ln k)|max). In both of these tables, calculated results are provided for XH2= 0

and 0.002, and for added Fe(CO)5 of 0 ppm and 213 ppm.

Dry Flames

The properties of the dry flames without Fe(CO)5 are as described above. As Table 4 shows,

ninety-four percent of the CO is consumed by the direct reaction, while N2O consumption is

roughly equally portioned between thermal decomposition, the direct reaction, and reaction

with O atoms. In Table 5, the burning velocity of the iron-free dry flames is most sensitive to

the rate of the direct reaction, followed by the decomposition of N2O, and to a lesser extent its

reaction with O. Upon addition of iron species, 32 % of the CO is consumed through reaction

with FeO or FeO2, which increases the burning velocity; likewise, 18 % of the N2O is consumed

through the related reactions of Fe or FeO with N2O, reducing the N2O consumption through

both the direct reaction and reaction with O-atoms. While reaction of N2O through the catalytic

route with iron species or through the direct reaction with CO proceeds at roughly the same

rate, reaction of N2O with O-atoms is less exothermic than the direct reaction (because of the

slow consumption of NO as shown in Figure 1), so reducing the importance of the O-atom

reactions with N2O increases the burning velocity. Nonetheless, the effect of iron as a catalytic

agent to reduce radical mole fractions is secondary in these flames.

As described in Ref. [24], Fe(CO)5 in moist CO-O2–N2 flames acts as a catalyst to

recombine O atoms (as opposed to methane-air flames, where H-atom recombination appears
12

to be the important inhibition reaction). These dry N2O flames appear to be similar to the COH2-O2-N2 flames in that iron species do serve to recombine O atoms. For example, with 213

ppm of Fe(CO)5, about 23% of the O-atom consumption occurs through reactions with iron

intermediates. In this flame, however, a reduction in O-atom mole fraction leads to a slightly

higher overall reaction rate. Very little of the CO consumption occurs via reaction with O

atoms, but a reduced mole fraction of O reduces its reaction with N2O. Nonetheless, the effect

of O-atom recombination is minor, and the main influence of adding iron pentacarbonyl to the

dry flames is to promote the N2O reaction through the iron-catalyzed reaction sequence

described above.

Moist Flames

The moist N2O flames without Fe(CO)5 are described above and in Tables 4 and 5. Carbon

monoxide is consumed by reaction with OH (60 %) and N2O (38 %), and N2O is consumed

roughly equally by reaction with CO, H atom, O atom, and by thermal decomposition. With

addition of iron species, the catalytic route described above again contributes to CO and N2O

consumption, but it is only about two-thirds as important as in the dry case (primarily because

of the dominant role of the CO+OH and N2O+H reactions). The iron species do enter into

catalytic recombination reactions for H and O atoms, but these cycles are not significant. For

H and OH, less than 1 % of the flux of each involves iron-species reactions. For O atom,

although iron-species reactions account for about 8 % of its consumption, the sensitivities (not

shown in Table 5) indicate that consumption of O atoms by reaction with iron species

increases the burning velocity as described above. (For reference, Fe reactions in CO flames

with 1 % hydrogen cause 30 % and 70 % of the H- and O-atom destruction at about 200 ppm

of Fe(CO)5 [24], and about a 30 % reduction in the flame speed.)

In the present moist CO-N2O flames, oxygen atom is not a significant species for CO

consumption or in chain-branching reactions. Hydrogen and hydroxyl radicals are important

for consumption of N2O and CO, but the system is straight-chain rather than chain-branching,

and neither radical reaches very high concentrations (their estimated peak mole fractions in

CH4, O2, and the present flames are listed in Table 1). The sensitivity of the burning velocity to

the rate of the branching reaction O+H2 is also low, ranking as the twelfth most sensitive

reaction, as compared to methane- or moist CO-air flames, where the burning velocity is very

sensitive to the rate of the branching reaction H+O2 (which is also not important in the present

flames).

Hence, while there is some inhibition, the effect of the iron species is mostly to promote

the overall reaction through the iron-catalyzed reactions of CO and N2O described above.

With H2 addition the promotion is less pronounced, not because of significant hydrogen radical

recombination by the iron species, but because the moist system is dominated by the fast

OH+CO reaction, so the iron-catalyzed reactions account for less of the CO and N2O

consumption.

CONCLUSIONS

The first measurements of the burning velocity of CO-N2O flames with low (13 ppm) quantities

of hydrogen-containing impurities have been obtained, for 0.6 ≤ φ ≤ 3.2 and with added

nitrogen up to XN2

 = 0.25; data have also been collected for flames with added hydrogen up to

XH2

 = 0.005. The measured burning velocity of pure stoichiometric flames is 23.5 cm/s +/- 0.6

cm/s, and the measurements with added hydrogen are in good agreement with those of other
13

researchers which were obtained at higher hydrogen mole fractions. The present data and

those of earlier investigations were numerically modeled using a mechanism based on Allen et

al. [7] with a CO+OH rate from Yu et al. [40], and the CO + N2O direct reaction rate of Milks

and Matula [5]. Modeling of the flames requires the use of the direct reaction, and the present

results imply a rate of 109.2 cm3 / mol s at 1800 K, which corresponds to a 10 % decrease in

the pre-exponential factor of the Milks and Matula rate. Experiments with nitrogen dilution and

over a range of φ suggest an activation energy near 71 kJ/mole. For the moist flames, the

CO+OH rate also has a strong effect on the predicted burning velocity, and the rate of Yu et al.

[40] provides good agreement with our data.

Iron pentacarbonyl, which is the most effective flame inhibitor identified for

hydrocarbon-air flames, is not effective in N2O flames; in fact, 213 ppm of Fe(CO)5 actually

increases the burning velocity of the dry flames by about 25 %. The promotion is believed to

be due to the iron-catalyzed gas-phase reaction of N2O with CO, via N2O + M = N2 + MO and

CO+ MO = CO2 + M, where M is Fe, FeO, or FeOH. The rate expression of Plane and

Rollason [49] for the former reaction with M=Fe, together with estimates of the rates of other

reactions provide reasonable agreement with the present data. For moist CO – N2O flames,

the promotion provided by the iron pentacarbonyl is less pronounced, not because of radical

recombination by the inhibitor, but because the iron-catalyzed reaction of CO and N2O is of

lesser importance relative to CO and N2O reaction with OH and H, respectively.

The present results show that the extraordinary effectiveness of iron pentacarbonyl

may be limited to systems in which the oxidizer is O2. The findings emphasize that unlike

thermal diluents, the effect a chemical “inhibitor” will have on the overall reaction rate is highly

dependent upon the chemical system involved. For example, for some propellant flames

which release CO and N2O in the gas phase, the most effective inhibitor found for

hydrocarbon-air flames (Fe(CO)5) would likely accelerate the burning, as may halogen-based

inhibitors.

ACKNOWLEDGEMENTS

We are grateful to Dr. Richard Yetter for helpful discussions of N2O kinetics and for providing

his N2O mechanism, to Dr. Pamela Chu for the FTIR analysis of N2O and CO, and to Ms. Nikki

Privé for assistance with the data acquisition and uncertainty analysis programs. The helpful

conversations and encouragement of Dr. Wing Tsang of NIST contributed much to this

research.
14

REFERENCES

[1] Cor, J.J. and Branch, M.C., J. Propul. Power 11:704 (1995).

[2] Litzinger, T.A., Fetherolf, B.L., Lee, Y.J., and Tang, C.J., J. Propul. Power 11:698

(1995).

[3] Yetter, R.A., Dryer, F.L., Allen, M.T., and Gatto, J.L., J. Propul. Power 11:683 (1995).

[4] Zaslonko, I.S., Losev, A.S., Mozzhukhin, E.V., and Mukoseev, Yu.K., Kinetics and

Catalysis 20:1144 (1979).

[5] Milks, D. and Matula, R., Fourteenth Symposium (International) on Combustion, The

Combustion Institute, Pittsburgh, 1973, pp. 84.

[6] Fujii, N., Kakuda, T., Takeishi, N., and Miyama, H., J. Phys. Chem. 91:2144 (1987).

[7] Allen, M.T., Yetter, R.A., and Dryer, F.L., Combust. Flame 109:449 (1997).

[8] Loirat, H., Caralp, F., and Destriau, M., J. Phys. Chem. 87:2455 (1983).

[9] Dindi, H., Tsai, H.M., and Branch, M.C., Combust. Flame 87:13 (1991).

[10] Vandooren, J., Van Tiggelen, P.J., and Pauwels, J.F., Combust. Flame 109:647 (1997).

[11] Cor, J.J. and Branch, M.C., Combust. Sci. Technol. 127:71 (1997).

[12] Van Wonterghem, J. and Van Tiggelen, A., Bull. Soc. Chim. Belg. 64:780 (1955).

[13] Kalff, P.J. and Alkemade, C.Th.J., Combust. Flame 19:257 (1972).

[14] Simpson, C.J.S.M. and Linnett, J.W., Sixth Symposium (International) on Combustion,

Reinhold, New York, 1957, pp. 257-265.

[15] Linteris, G.T. and Williams, F.A., Twenty-Fourth Symposium (International) on

Combustion, The Combustion Institute, Pittsburgh, 1992, pp. 803-811.

[16] Andersen, S.O., Fire Journal 81:56 (1987).

[17] Gann, R. G. Ed., Fire Suppression System Performance of Alternative Agents in

Aircraft Engines and Dry Bay Laboratory Simulations, National Institute of Standards

and Technology, NIST SP 890, 1995.

[18] Lask, G. and Wagner, H.G., Eighth Symposium (International) on Combustion, Williams

and Wilkins Co., Baltimore, 1962, pp. 432-438.

[19] Vanpee, M. and Shirodkar, P., Seventeenth Symposium (International) on Combustion,
15

The Combustion Institute, Pittsburgh, 1979, pp. 787-795.

[20] Miller, D.R., Evers, R.L., and Skinner, G.B., Combust. Flame 7:137 (1963).

[21] Reinelt, D. and Linteris, G.T., Twenty-Sixth Symposium (International) on Combustion,

The Combustion Institute, Pittsburgh, 1996, pp. 1421-1428.

[22] Rumminger, M.D., Reinelt, D., Babushok, V., and Linteris, G.T., Combust. Flame

116:207 (1999).

[23] Rosser, W. A, Inami, S. H., and Wise, H., Study of the Mechanisms of Fire

Extinguishment of Liquid Rocket Propellants, WADC Technical Report 59-206, 1959.

[24] Rumminger M.D. and Linteris, G.T., "Inhibition of Premixed Carbon MonoxideHydrogen-Oxygen-Nitrogen Flames by Iron Pentacarbonyl", accepted for publication in

Combust. Flame, August, 1999.

[25] Kaufman, F., Proc. R. Soc. London, A A247:123 (1958).

[26] Mache, H. and Hebra, A., Sitzungsber. Österreich. Akad. Wiss. IIa, 150:157 (1941).

[27] Van Wonterghem, J. and Van Tiggelen, A., Bull. Soc. Chim. Belg. 63:235 (1954).

[28] Andrews, G.E. and Bradley, D., Combust. Flame 18:133 (1972).

[29] Linteris, G.T. and Truett, L., Combust. Flame 105:15 (1996).

[30] Image Tool is a free Windows95-based program developed at the University of Texas

Health Science Center at San Antonio, Texas and available from the Internet by

anonymous FTP from ftp://maxrad6.uthscsa.edu or http://ddsdx.uthscsa.edu

[31] Taylor, B. N. and Kuyatt, C. E., Guidelines for Evaluating and Expressing the

Uncertainty of NIST Measurement Results, National Institute of Standards and

Technology, NIST Technical Note 1297, 1994.

[32] Rumminger, M. D. and Linteris, G. T., Inhibition of Premixed Carbon MonoxideHydrogen-Oxygen-Nitrogen Flames by Iron Pentacarbonyl, National Institute of

Standards and Technology, NIST Internal Report, 1999.

[33] Weinberg, F. J., Optics of Flames, Butterworth, London, 1963.

[34] Dunn-Rankin, D. and Weinberg, F., Combust. Flame 113:303 (1998).

[35] Moffat, R.J., Transactions of the ASME 104:250 (1982).

[36] Gilbert, A.G. and Sulzmann, K.G.P., J. Electrochem. Soc. 121:832 (1974).

[37] Kee, R. J., Grcar, J. F., Smooke, M. D., and Miller, J. A., A Fortran Computer Program

for Modeling Steady Laminar One-dimensional Premixed Flames, Sandia National

Laboratories Report, SAND85-8240, 1991.
16

[38] Kee, R. J., Rupley, F. M., and Miller, J. A., CHEMKIN-II: A Fortran Chemical Kinetics

Package for the Analysis of Gas Phase Chemical Kinetics, Sandia National Laboratory,

SAND89-8009B, 1989.

[39] Kee, R. J., Dixon-Lewis, G., Warnatz, J., Coltrin, R. E., and Miller, J. A., A Fortran

Computer Package for the Evaluation of Gas-Phase, Multicomponent Transport

Properties, Sandia National Laboratory, SAND86-8246, 1986.

[40] Yu, C.-L., Wang, C., and Frenklach, M., Eastern States Section of the Combustion

Institute, The Combustion Institute, 1990, pp. 20-1 to 20-4.

[41] Loirat, H., Caralp, F., Destriau, M., and Lesclaux, R., J. Phys. Chem. 91:6538 (1987).

[42] Baulch, D. L., Drysdale, D. D., Horne, D. G., and Lloyd, A. C., Evaluated Kinetic Data

for High Temperature Reactions, vol. 1 and 2, Butterworths, London, 1973.

[43] Baulch, D.L., Cobos, C.J., Cox, R.A., Esser, C., Frank, P., Just, T., Kerr, J.A., Pilling,

M.J., Troe, J., Walker, R.W., and Warnatz, J., J. Phys. Chem. Ref. Data 21:411 (1992).

[44] Lin, M.C. and Bauer, S.H., J. Phys. Chem. 50:3377 (1969).

[45] Day, M.J., Stamp, D.V., Thompson, K., and Dixon-Lewis, G., Thirteenth Symposium

(International) on Combustion, The Combustion Institute, Pittsburgh, 1971, pp. 705-

712.

[46] Palmer, H.B. and Seery, D.J., Combust. Flame 4:213 (1960).

[47] West, J.B. and Broida, H.P., J. Chem. Phys. 62:2566 (1975).

[48] Campbell, M.L. and Metzger, J.R., Chem. Phys. Lett. 253:158 (1996).

[49] Plane, J.M.C. and Rollason, R.J., J. Chem. Soc., Faraday Trans. 92:4371 (1996).

[50] Kappes, M.M. and Staley, R.H., Journal of the Americal Chemical Society 103:1286

(1981).

[51] Matsuda, S., J. Phys. Chem. 57:807 (1972).

[52] Hayhurst, A.N. and Lawrence, A.D., Combust. Flame 110:351 (1997).

[53] Rollason, R.J. and Plane, J.M.C., 15th International Symposium on Gas Kinetics ,

Royal Society of Chemistry, 1998, pp.

[54] Kellogg, C.B. and Irikura, K.K., Journal of Physical Chemistry a 103:1150 (1999).

[55] Rumminger, M.D. and Linteris, G.T., Halon Options Technical Working Conference,

Albuquerque, NM, 1999, pp. ***-***.

[56] Rumminger, M.D. and Linteris, G.T., "An Experimental Study Of The Role Of Particles

In Flame Inhibition By Iron Pentacarbonyl", in preparation for submission to Combustion
17

and Flame, 1999.
18

TABLE CAPTIONS

Table 1 - Calculated CO-N2O flame properties for various reactant streams. Values for

stoichiometric CH4 – air and CO-air-H2 flames are provided for comparison.

Table 2 - Measured burning velocity of CO-N2O flames. Data are presented for dry flames at

varying equivalence ratio φ and for stoichiometric flames with varying % H2 and % N2

.

Table 3 - Reactions in the iron catalytic cycle for the CO-N2O system, and their estimated

reaction rates ( kf =A T

b

 exp(-Ea/RT), and units are cm, K, mole, s).

Table 4 - Calculated fractional flux of the total reaction of each species proceeding through the

indicated reaction for stoichiometric CO-N2O flames. Results are given for XH2 = 0.0

and 0.002, and for XFe(CO)5 = 0 and 213 ppm.

Table 5 - Sensitivity of burning velocity to the specific reaction rate constant for stoichiometric

CO-N2O flames with XH2 = 0 and 0.002, and for XFe(CO)5 = 0 ppm and 213 ppm.

Sensitivities are normalized by the value for the maximum sensitivity, which is the

direct CO+N2O reaction.
19

FIGURE CAPTIONS

Figure 1 - Calculated major species mole fraction and temperature profiles in a stoichiometric

premixed dry CO-N2O flame (note log distance scale).

Figure 2 - Burning velocity of stoichiometric CO-N2O flames as a function of hydrogen mole

percent. Points (squares) are experimental data, and the solid lines are the modeling

results using the rate of the CO+N2O from the reference indicated in the figure, and

the CO+OH rate of Yu et al. [39]. The data of Kalff and Alkemade [13] with H2O (not

H2) are indicated by ‘K(H2O)’ and diamonds, and the datum of Van Wonterghem and

Van Tiggelen [12] by ‘V’ and a triangle.

Figure 3 - Calculated burning velocity (lines) of CO-N2O flames with values of the CO+OH-

>CO2+H rate from Baulch et al. [41], Yu et al. [39], and Baulch et al. [42], together

with data from the present study (symbols).

Figure 4 - Arrhenius plot of the rate of the direct exchange reaction CO+N2O → CO2+N2 from

various investigators (k in cm, K, mole, s).

Figure 5 - Measured burning velocity (symbols) of CO-N2O flames with added N2. The solid

lines are calculated results using the direct exchange reaction rate of Milks and

Matula [5] or Loirat et al. [40], while the dotted lines are those calculated with the preexponential factor of each rate modified to provide agreement at XN2

=0.

Figure 6 - Measured burning velocity (symbols) of CO-N2O flames as a function of fuel-oxidizer

equivalence ratio φ. The solid lines are calculated results using the direct exchange

reaction rate of Milks and Matula [5] or Loirat et al. [40], while the dotted lines are

those calculated with the pre-exponential factor of each rate modified to provide

agreement at φ = 1.0.

Figure 7 - Burning velocity of CO-N2O flames (<2000 ppm H2) as a function of mole percent N2

from [12], together with numerically calculated results with 1560 ppm H2.

Figure 8 - Burning velocity of CO-N2O flames (<2000 ppm H2) as a function of equivalence

ratio from [12] (labeled ‘V’), and the present investigation, together with numerically

calculated results using 0 ppm and 1560 ppm H2.

Figure 9 - Experimental burning velocity of CO-N2O flames as a function of the mole percent

H2O in the reactants, from [14] for φ =2.0 and XN2 = 0.25, together with numerically

calculated prediction.

Figure 10 - Calculated flux of important CO (dotted lines) and N2O reactions (solid lines) in a

stoichiometric CO-N2O flame as a function of mole percent of hydrogen.

Figure 11 - Calculated flux of important CO reactions (dotted lines) and N2O reactions (solid

lines) in a dry, stoichiometric CO-N2O flame as a function of φ.
20

Figure 12 - Normalized burning velocity of stoichiometric CO-N2O flames with XH2 = 0.0, 0.001,

and 0.002 for increasing quantities of Fe(CO)5. The symbols are the experimental

data; the lines are the calculated results.
21

TABLES

Table 1 - Calculated CO-N2O flame properties for various reactant streams. Values for

stoichiometric CH4 – air and CO-air-H2 flames are provided for comparison.

Reactant

Conditions

Temperature (K)

Peak Radical Mole

Fraction within Flame

(ppm)

 φ 

Mole

 % H2

Mole

% N2

vo, num

(cm/s)

A.F.T.

At Point of

99.5 % N2O

Consumption

At peak

of

CO+N2O

Reaction

O OH

(ppm)

H

CO – N2O Flame

1 0 0 24.5 2870 2323 1770 2833 0 0

1 0.012 0 25.9 2872 2377 1773 3524 79 2

1 0.68 0 45.0 2866 2589 1896 4524 1811 38

0.6 0 0 20.1 2789 2303 1693 2690 0 0

1 0 0 24.5 2860 2324 1770 2833 0 0

1.3 0 0 25.7 2867 2319 1789 2710 0 0

3.2 0 0 22.1 2378 2139 1658 1109 0 0

1 0 0 24.5 2860 2323 1770 2833 0 0

1 0 50 9.3 2317 1928 1494 643 0 0

CH4 - air Flame

1 - - 40.0 2230 - - 3150 7660 6740

CO–air-H2 Flame

1 1.0 - 35.8 2376 - - 14000 5200 2900
22

Table 2 - Measured burning velocity of CO-N2O flames. Data are presented for dry flames

at varying equivalence ratio φ and for stoichiometric flames with varying % H2 and % N2 .

φ vo,exp

(cm/s)

Mole % H2

(φ =1.0)

vo,exp

(cm/s)

Mole % N2

(φ =1.0)

vo,exp

(cm/s)

0.60 16.3 ± 0.4 0.00 23.4 ± 0.6 0.0 23.7 ± 0.6

0.75 19.9 ± 0.5 0.10 31.9 ± 0.8 2.5 22.2 ± 0.6

0.80 20.8 ± 0.5 0.15 35.9 ± 1.1 5.3 22.8 ± 0.6

0.85 21.6 ± 0.5 0.20 38.7 ± 1.2 8.1 22.2 ± 0.6

0.90 22.2 ± 0.5 0.25 41.8 ± 1.6 11.1 21.7 ± 0.6

0.95 22.7 ± 0.6 0.30 44.8 ± 1.5 14.3 20.9 ± 0.6

1.0 23.4 ± 0.6 0.32 44.6 ± 1.7 17.7 20.2 ± 0.6

1.1 24.4 ± 0.6 0.35 46.9 ± 2.0 21.3 19.3 ± 0.5

1.2 25.5 ± 0.6 0.40 48.2 ± 2.5 25.1 17.9 ± 0.6

1.3 26.3 ± 0.7 0.45 49.3 ± 2.3

1.5 27.6 ± 0.7 0.50 51.8 ± 3.4

1.8 28.4 ± 0.7

2.1 28.8 ± 0.7

2.5 28.4 ± 0.7

3.0 27.3 ± 0.7
23

Table 3 - Reactions in the iron catalytic cycle for the CO-N2O system, and their estimated

reaction rates ( kf =A T

b

 exp(-Ea/RT), and units are cm, K, mole, s).

Reaction A b Ea/R

Fe + N2O = FeO + N2 1.40 E+14 0 5940

FeO + N2O = FeO2 + N2 3.00 E+13 0 5033

FeOH + N2O = FeOOH + N2 1.30 E+14 0 4530

FeO + CO = Fe + CO2 1.80 E+12 0 3522

FeO2 + CO = FeO + CO2 1.18 E+13 0 4530

FeOOH + CO = FeOH + CO2 6.00 E+13 0 4026
24

Table 4 - Calculated fractional flux of the total reaction of each species proceeding

through the indicated reaction for stoichiometric CO-N2O flames. Results are given for

XH2 = 0.0 and 0.002, and for XFe(CO)5 = 0 and 213 ppm.

 Fractional Flux (%) _

XH2 : 0.000 _ 0.002 _

XFe(CO)5 (ppm): 0 213 0 213

Species Reaction

CO Destruction

CO + OH <=> CO2 + H - - 60 48

CO + N2O <=> CO2 + N2 94 68 38 30

CO + O(+M) <=> CO2 (+M) 3 0 1 0

CO + NO2 <=> CO2 + NO 2 - - -

FeO + CO <=> Fe + CO2 - 9 - 3

FeO2 + CO <=> FeO + CO2 - 23 - 6

FeOOH + CO <=> FeOH + CO2 - - - 12

N2O Destruction

CO + N2O <=> CO2 + N2 40 30 19 16

N2O (+M) <=> N2 + O(+M) 31 30 26 26

N2O + H <=> N2 + OH - - 28 20

N2O + O <=> O2 + N2 13 10 11 10

N2O + O <=> 2NO 13 10 11 10

NH + NO <=> N2O + H 0 0 3 3

NO + N2O <=> NO2 + N2 2 1 1 1

FeO + N2O <=> FeO2 + N2 - 12 - 5

Fe + N2O <=> FeO + N2 - 6 - 4

FeOH + N2O <=> FeOOH + N2 - - - 6

O Creation

N2O(+M) <=> N2 + O(+M) 99 99 95 88

H + O2 <=> O + OH - - 4 11

Destruction

N2O + O <=> O2 + N2 44 36 42 36

N2O + O <=> 2NO 44 36 42 36

H + O2 <=> O + OH - - 6 2

NH + O <=> NO + H - - 4 4

CO + O(+M) <=> CO2 (+M) 4 2 2 2

NO2 + O <=> O2 + NO 5 3 1 1

NO + O (+M) <=> NO2 (+M) 2 - - -

O + H2 <=> H + OH - - 1 1
25

Fe + O

2 <=> FeO + O

- 12-9

FeO

2 + O <=> FeO + O2

- 11-7

H Creation

CO + OH <=> CO

2 + H

-- 87 89

H + O

2 <=> O + OH

--42

NH + O <=> NO + H--33

NH + NO <=> N

2O + H

--33

O + H

2 <=> H + OH

--11

Destruction N2O + H <=> N

2 + OH

-- 83 73

NH + NO <=> N

2O + H

-- 10 10

H + O

2 <=> O + OH

--3 12

NO2 + H <=> NO + OH--33

OH Creation N2O + H <=> N

2 + OH

-- 87 77

H + O

2 <=> O + OH

--3 13

NH + NO <=> N

2 + OH

--33

NO

2 + H <=> NO + OH

--33

H

2O + O <=> 2OH

--12

O + H

2 <=> H + OH

--11

Destruction

CO + OH <=> CO

2 + H

-- 93 95

H + O

2 <=> O + OH

--52

H

2O + O <=> 2OH

--1-

Fe Creation

FeO + CO <=> Fe + CO

2

- 53- 35

Fe + O

2 <=> FeO + O

- 45- 54

Fe + O

2 (+M) <=> FeO2 (+M)

---6

FeO + H <=> Fe + OH---3

Destruction

Fe + N

2O <=> FeO + N2

- 85- 88

Fe + O

2 (+M) <=> FeO2 (+M)

- 14- 11

FeO Creation

Fe + N

2O <=> FeO + N2

- 33- 42

FeO

2 + O <=> FeO + O2

- 16- 20

FeO

2 + CO <=> FeO + CO2

- 51- 35

Destruction

FeO + CO <=> Fe + CO

2

- 21- 17

FeO + N

2O <=> FeO2 + N2 62 53

Fe + O

2 <=> FeO + O

- 18- 26

FeO + H <=> Fe + OH---2

FeO + H

2O <=> Fe(OH)2

---1
26

Table 5 - Sensitivity of burning velocity to the specific reaction rate constant for

stoichiometric CO-N2O flames with XH2 = 0 and 0.002, and for XFe(CO)5 = 0 ppm and 213

ppm. Sensitivities are normalized by the value for the maximum sensitivity, which is the

direct CO+N2O reaction.

d (ln v)/d (ln k)

 d (ln v)/d (ln k)|max

XH2 : 0 _ 0.002 _

XFe(CO)5 (ppm): 0 213 0 213

Reaction

 Dry Reactants

CO + N2O<=>CO2 + N2 1.00 1.00 1.00 1.00

N2O (+M) <=> N2 + O (+M) -0.13 -0.13 0.19 -0.10

N2O + O<=>2NO -0.07 -0.07 -0.34 -0.23

N2O + O<=> O2 + N2 0.00 0.04 0.04 0.06

 Moist Reactions

CO + OH <=> CO2 + H 0.83 0.63

N2O + H <=> N2 + OH 0.32 0.31

H2O + O<=>2OH -0.10 -0.06

O+ H2 <=> H + OH 0.09 0.09

 Iron Reactions

FeO2 + CO <=> FeO + CO2 0.30 0.28

FeO + CO <=> Fe+CO2 0.09 0.04

FeO + N2O <=> FeO2 + N2 0.14 -0.05

Fe + N2O <=> FeO+N2 0.02 0.04

FeOH + N2O <=> FeOOH + N2 0.44

FeOOH + CO <=> FeOH + CO2 0.25

FeO + H2O <=> Fe(OH)2 0.21

FeOH + H <=> FeO + H2 0.20

Fe(OH)2 + H <=> FeOH + H2O 0.13

FeOH + O <=> FeO + OH -0.08

FeOOH + OH <=> FeO2 + H2O -0.04
27

FIGURES

0

0.1

0.2

0.3

0.4

0.5

0.01 0.10 1.00 10.00 100.00 1000.00 10000.00

Distance /mm

Mole Fraction

0

500

1000

1500

2000

2500

3000

Temperature /K

N2O

N2

T

CO

CO2

O2

NO

O

Main Reaction Zone CO Cons. NO Consumption

Figure 1 - Calculated major species mole fraction and temperature profiles in a

stoichiometric premixed dry CO-N2O flame (note log distance scale).
28

0

20

40

60

80

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Mole % H2

Burning Velocity (cm/s)

Dindi et al.

Fujii et al., 1987

Loirat et al., 1983

no CO+N2O

 reaction

Milks and Matula

Loirat et al., 1987

K (H2O) V

K (H2O)

Figure 2 - Burning velocity of stoichiometric CO-N2O flames as a function of hydrogen

mole percent. Points (squares) are experimental data, and the solid lines are the

modeling results using the rate of the CO+N2O from the reference indicated in the figure,

and the CO+OH rate of Yu et al. [40]. The data of Kalff and Alkemade [13] with H2O (not

H2) are indicated by ‘K(H2O)’ and diamonds, and the datum of Van Wonterghem and Van

Tiggelen [12] by ‘V’ and a triangle.
29

0

20

40

60

80

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Mole % H2

Burning Velocity (cm/s)

Baulch et al., 1973

Baulch et al., 1992

Yu et al., 1990 

Figure 3 - Calculated burning velocity (lines) of CO-N2O flames with values of the CO+OH-

>CO2+H rate from Baulch et al. [42], Yu et al. [40], and Baulch et al. [43], together with

data from the present study (symbols).
30

4

6

8

10

12

0.4 0.6 0.8 1

1/Temperature (1000/K)

log (k)

Zaslonko et al., 1979

Dindi et al., 1993

Fujii et al., 1987

Lin and Bauer, 1969

Milks and Matula, 1973

Loirat et al., 1983

Loirat et al., 1987

Zaslonko et al., 1979

Tsang and Herron, 1991 

This work

Allen et al., 1997

Figure 4 - Arrhenius plot of the rate of the direct exchange reaction CO+N2O → CO2+N2

from various investigators (k in cm, K, mole, s).
31

0

5

10

15

20

25

0 10 20 30 40 50 60

Mole % N2

Burning Velocity (cm/s)

Loirat et al., 1987

Milks and Matula, 1973

Figure 5 - Measured burning velocity (symbols) of CO-N2O flames with added N2. The

solid lines are calculated results using the direct exchange reaction rate of Milks and

Matula [5] or Loirat et al. [41], while the dotted lines are those calculated with the preexponential factor of each rate modified to provide agreement at XN2

=0.
32

0

10

20

30

0.5 1.0 1.5 2.0 2.5 3.0 3.5

φ

Burning Velocity (cm/s)

Loirat et al., 1987

Milks and Matula, 1973

 Figure 6 - Measured burning velocity (symbols) of CO-N2O flames as a function of fueloxidizer equivalence ratio φ. The solid lines are calculated results using the direct

exchange reaction rate of Milks and Matula [5] or Loirat et al. [41], while the dotted lines

are those calculated with the pre-exponential factor of each rate modified to provide

agreement at φ = 1.0.
33

0

5

10

15

20

25

30

35

0 10 20 30 40 50 60

Mole % N2

Burning Velocity (cm/s)

Data of Van Wonterghem and Van Tiggelen (1955)

 Calculation (with 1560 ppm of hydrogen)

Figure 7 - Burning velocity of CO-N2O flames (<2000 ppm H2) as a function of mole

percent N2 from [12], together with numerically calculated results with 1560 ppm H2.
34

0

10

20

30

40

50

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

φ

Burning Velocity (cm/s)

this work

V

0 ppm H2

1560 

Figure 8 - Burning velocity of CO-N2O flames (<2000 ppm H2) as a function of equivalence

ratio from [12] (labeled ‘V’), and the present investigation, together with numerically

calculated results using 0 ppm and 1560 ppm H2.
35

0

5

10

15

20

0.0 1.0 2.0 3.0 4.0 5.0

Mole % H2O

Burning Velocity (cm/s)

Figure 9 - Experimental burning velocity of CO-N2O flames as a function of the mole

percent H2O in the reactants, from [14] for φ =2.0 and XN2 = 0.25, together with

numerically calculated prediction.
36

0

0.2

0.4

0.6

0.8

1

0.0 0.2 0.4 0.6 0.8

Mole % H2

Fraction of CO or N O Consumption Flux 2

CO+N2O=CO2+N2

N2O+M=N2+ O

 N2O+O=N2+O2 

N2O+O=2NO 

N2O+H=N2+ OH

CO+N2O=CO2+N2

CO+OH=CO2+H

CO flux

N2O flux

Figure 10 - Calculated flux of important CO (dotted lines) and N2O reactions (solid lines)

in a stoichiometric CO-N2O flame as a function of mole percent of hydrogen.
37

0

0.2

0.4

0.6

0.8

1

0.0 1.0 2.0 3.0 4.0

φ

Fraction of CO or N O Consumption Flux 2

CO flux

N2O flux

CO+N2O=CO2+N2

N2O+M=N2+ O

 N2O+O=N2+O2

N2O+O=2NO 

CO+O+M=CO2

 Figure 11 - Calculated flux of important CO reactions (dotted lines) and N2O reactions

(solid lines) in a dry, stoichiometric CO-N2O flame as a function of φ.
38

0.9

1

1.1

1.2

1.3

0 50 100 150 200

Fe(CO)5 Mole Fraction (ppm)

Normalized Burning Velocity

X = H2

0

0.001

0.002

Figure 12 - Normalized burning velocity of stoichiometric CO-N2O flames with XH2 = 0.0,

0.001, and 0.002 for increasing quantities of Fe(CO)5. The symbols are the experimental

data; the lines are the calculated results.