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 I. BABUSHOK

Building and Fire Research Laboratory, National Institute of Standards and Technology,

Gaithersburg, MD 20899

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 f from 0.6 to 3.0, with

added nitrogen up to a mole fraction of XN2 5 0.25, and with hydrogen added up to XH2 5 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 1 M 5 N2 1 MO and CO 1 MO 5 CO2 1 M, where M is Fe, FeO, or FeOH. Published by

Elsevier Science Inc.

INTRODUCTION

The present investigation was conducted for the

dual purpose of studying the effectiveness of the

flame inhibitor iron pentacarbonyl in nonbranching chain 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 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. In

addition, we are also responding to the need

described by Allen et al. [1] for burning velocity

measurements in the CO–N2O system to assess

the role of the direct reaction under dry conditions at flame temperatures.

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 [2–4], and they are also important for

understanding the combustion emission characteristics of stationary and mobile power plants.

The direct reaction of CO with N2O is of

fundamental interest because it is one of the

simplest examples of an exchange reaction between saturated molecules. For inhibition studies, the reactant mixture provides a nonchain

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.

The CO–N2O reaction has been studied in

shock tubes [5–7] and flow reactors [1]. 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 low-pressure

premixed flames using gas chromatography, and

Vandooren et al. [10] recently used mass spectrometry to measure the structure of CO–

N2O–H2 flames. Cor and Branch [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

*Corresponding author: Gregory T. Linteris, National Institute of Standards and Technology, Bldg. 224, Room B356,

Stop 8651, Gaithersburg, MD 20899-8651, USA. E-mail:

linteris@nist.gov

† National Research Council/NIST Postdoctoral Fellow,

1996–1999; current address: Ceryx, Inc., Santa Paula, CA

93060.

‡ Official contribution of the National Institute of Standards

and Technology, not subject to copyright in the United

States.

0010-2180/00/$–see front matter COMBUSTION AND FLAME 122:58–75 (2000)

PII S0010-2180(00)00103-6 Published by Elsevier Science Inc.
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 ppm1 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 (f 5 2.0), diluted by nitrogen, with unquantified, but low,

levels of hydrogen impurities. The burning velocity of premixed CO–N2O flames has been

calculated [1, 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 3.0, 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]. Although Fe(CO)5 is flammable and highly toxic and could never be used

as a fire suppressant itself, other iron-containing

compounds have been identified which are far

less toxic and flammable, and which also demonstrate strong flame inhibition [22]. If means

can be identified to safely introduce metal species into a fire, they may be beneficial components in a fire suppressant blend, particularly for

unoccupied spaces. Further, the fundamental

understanding gained from study of these nearideal inhibitors, which catalytically recombine

radicals, provides a theoretical basis for development of new inhibitors.

A detailed chemical kinetic mechanism for

iron-species inhibition of flames has been introduced [23], and modeling with the mechanism

supports the premise that the inhibition is primarily a gas-phase phenomenon. 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 Oatom 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 [24]. Since systems using nitrogen oxides instead of O2 undergo non–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.

1

All references to ppm in this paper are on a volume basis

and refer to ml/L.

CO–N2O FLAME SPEEDS WITH IRON PENTACARBONYL 59
Recent research with Fe(CO)5 inhibition of

CO–O2–N2–H2 flames has shown that in some

systems a new O-atom radical recombination

cycle may dominate radical destruction [25].

Also, Kaufman [26] 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 Oatom radical recombination cycle. Because gasphase 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 that 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. The results

may have implications for the practical problem

of the suppression of propellant fires.

EXPERIMENTAL

A Mache-Hebra nozzle burner (inner diameter

1.02 cm 6 0.005 cm) [27] with a schlieren

imaging system [28] provides the average burning velocity of these Bunsen-type flames using

the total area method [29]. The experimental

system has been described in detail previously

[30]. Calibrated mass flow controllers meter the

gas flows, and a two-stage saturator adds

Fe(CO)5 as a vapor. The burner produces

straight-sided schlieren images that are captured by a video frame-grabber board in a

computer. The fuel gas is carbon monoxide

(Matheson2 Ultra High Purity (UHP), 99.97%

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 in-house Fourier transform infrared radiation [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 according to an in-house FTIR

analysis had 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.

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;

however, these effects are most important over

a small portion of the flame. In order to minimize the influence of curvature and stretch 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],

and has been described in detail in a previous

publication [32]. All uncertainties are reported

as expanded uncertainties: X 6 U (level of

confidence approximately 95%), or relative uncertainties: U/X z 100%. The primary sources of

uncertainty in the average burning velocity measurement are the (1) accuracy of the flow controllers, (2) determination of the flame area, (3)

effect of flame base location on flame area, and

(4) location of the schlieren image relative to

the cold gas boundary. In general, uncertainty

increases with burning velocity. A major source

of uncertainty arises from selection of the flame

location. 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 [29], use of the

schlieren image as the flame area can lead to an

underestimation of the burning velocity [33], as

compared to the area at the cold-gas boundary.

2

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.

60 G. T. LINTERIS ET AL.
Previous researchers [29, 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. Further, some researchers argue for the use of the visible flame area

rather than the schlieren area (which would

tend to reduce the burning velocities in the

present work) [35]. We retain the traditional

approach of using the schlieren area, but note

that differences on the order of 10% can result

from the choice of image to use in the analyses.

Inaccuracies in the flow controllers result in

uncertainties of 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. Neglecting the uncertainty (unspecified) in the vapor pressure correlation of

Gilbert and Sulzmann [36], the uncertainties in

the bath temperature, ambient pressure, and

carrier gas flow rate yield an Fe(CO)5 mole

fraction uncertainty of 6.5%.

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 10214, the relative tolerance is 1029

,

GRAD is 0.15, and CURV is 0.35. The computational domain is 0 to 50 cm (except when

noted). The initial temperature is 298 K and the

pressure is one atmosphere. The moist CO–

N2O flames are modeled using the mechanism

and thermodynamic data set of Allen et al. [1],

from which the species NCN, NCO, CNO, CH,

CH2, CH3, C2H, HCCO, HNCO, HOCN,

HCNO, and 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. For all

calculations (except where noted) we use the

rate for CO 1 OH 7 CO2 1 H from Yu et al.

[40], and for CO 1 N2O 3 CO2 1 N2 from

Milks and Matula [6] since they provide reasonable agreement with our data. 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. [23].

RESULTS AND DISCUSSION

General Features

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. A plot of the major species

profiles and temperature of a stoichiometric

CO–N2O flame is shown in Fig. 1. 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 2 meters, it is of interest to determine how

the features of the calculated results over the

Fig. 1. Calculated major species mole fraction and temperature profiles in a stoichiometric premixed dry CO–N2O

flame (note log distance scale).

CO–N2O FLAME SPEEDS WITH IRON PENTACARBONYL 61
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 f 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 rate of the CO 1 N2O reaction is also

listed. For pure CO–N2O flames with f 5 1.0

and hydrogen addition up to 6800 ppm, or dry

flames with 0.6 # f # 1.3 (the first six lines of

Table 1), the A.F.T. is 2830 K 6 40 K, whereas

for the richest flames (f 5 3.2) and stoichiometric flames with nitrogen dilution, the A.F.T.

is significantly lower, 2378 K and 2559 K respectively. As Table 1 shows, however, the calculated peak temperature at the point of 99.5%

N2O consumption is 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 temperature in the main

reaction zone is closer to the final temperature

far downstream, since the faster overall reaction

rate allows more of the CO to be consumed

before the N2O is gone. Similarly, at the peak of

the CO 1 N2O reaction, the temperature is

about 1800 K; for flames with 6800 ppm of

hydrogen it is about 100 K higher, for rich or

lean flames it is about 100 K lower, and with

25% dilution nitrogen is about 40 K lower.

Hence, the temperature at the peak of the

CO 1 N2O reaction is less sensitive to changes

in f, XN2 or XH2 than is the adiabatic flame

temperature (which is not nearly achieved in the

experimental flames). In the present experiment

which has a flame thickness of about 1 mm, the

calculated temperatures in the main reaction

zone, where most of the fuel and oxidizer

consumption occur, are more relevant than the

adiabatic flame temperatures (which would only

be reached nearly 3 m downstream).

The significant but gradual temperature rise

far downstream of the main reaction zone is

caused by the slow reaction of the remaining

TABLE 1

Calculated CO–N2O Flame Properties for Various Reactant Streamsa

Reactant Conditions Temperature (K)

Peak Radical Mole

Fraction within Flame

(ppm)

f

Mole

% H2

Mole

% N2

vo, num

(cm/s) A.F.T.

At Point of

99.5% N2O

Consumption

At peak of

CO1N2O

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 25 16.6 2559 2155 1737 1590 0 0

CH4–air flame

1 — — 40.0 2230 — — 3150 7660 6740

CO–air–H2 flame

1 1.0 — 35.8 2376 — — 14,000 5200 2900

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

62 G. T. LINTERIS ET AL.
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 a high rate of heat feedback, and the

flow field in the experiment does not support

such a large, one-dimensional downstream region. Numerical experiments for 0.6 # f # 3.2

show that turning off the CO 1 O and CO 1 O2

reactions, and the even slower NO consumption

reactions, provides a burning velocity within 2%

of those using the entire domain with all reactions included. Calculations were also performed over the entire range of f and XH2

; for

each condition, domains of 2 mm, 3 mm, or

3000 mm all yield calculated burning velocities

within 1% of each other, providing further

evidence that the slow reactions in the very

large domain do not affect the measured or

calculated burning velocities of smaller, practical flames.

Unlike CO or CH4 flames, these CO–N2O

flames do not demonstrate the characteristic

radical super-equilibrium. The O, H, and OH

mole fractions, XO, XH, and XOH, rise monotonically throughout the computational domain

up to the point where the N2O is 99.5% gone.

The peak in-flame XO is about the same as in

stoichiometric CH4–air flames, and about a

factor of 4 lower than in CO–air flames with 1%

H2. Even with about 0.7% of added H2, the

peak in-flame 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.

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 6 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 1

N2O 3 CO2 1 N2. The bottom curve does not

Fig. 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 1 N2O from the reference

indicated in the figure. 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.

TABLE 2

Measured Burning Velocity of CO–N2O Flames.a

f

vo, exp

(cm/s)

Mole

% H2

(f 5 1.0)

vo, exp

(cm/s)

Mole

% N2

(f 5 1.0)

vo, exp

(cm/s)

0.60 16.3 6 0.4 0.00 23.4 6 0.6 0.0 23.7 6 0.6

0.75 19.9 6 0.5 0.10 31.9 6 0.8 2.5 22.2 6 0.6

0.80 20.8 6 0.5 0.15 35.9 6 1.1 5.3 22.8 6 0.6

0.85 21.6 6 0.5 0.20 38.7 6 1.2 8.1 22.2 6 0.6

0.90 22.2 6 0.5 0.25 41.8 6 1.6 11.1 21.7 6 0.6

0.95 22.7 6 0.6 0.30 44.8 6 1.5 14.3 20.9 6 0.6

1.0 23.4 6 0.6 0.32 44.6 6 1.7 17.7 20.2 6 0.6

1.1 24.4 6 0.6 0.35 46.9 6 2.0 21.3 19.3 6 0.5

1.2 25.5 6 0.6 0.40 48.2 6 2.5 25.1 17.9 6 0.6

1.3 26.3 6 0.7 0.45 49.3 6 2.3

1.5 27.6 6 0.7 0.50 51.8 6 3.4

1.8 28.4 6 0.7

2.1 28.8 6 0.7

2.5 28.4 6 0.7

3.0 27.3 6 0.7

a Data are presented for dry flames at varying equivalence

ratio f and for stoichiometric flames with varying % H2 and

% N2.

CO–N2O FLAME SPEEDS WITH IRON PENTACARBONYL 63
include the direct reaction in the mechanism,

and indicates a burning velocity of 2.9 cm/s with

no added hydrogen, clearly illustrating that this

reaction is required to reproduce our data. The

remaining curves in the figure show the calculated result using the rate expressions of Dindi

et al. [9], Milks and Matula [6], Loirat et al. [41],

Fujii et al. [7], and Loirat et al. [8]. The value

recommended by Dindi et al. overpredicts the

burning velocity, with too weak of a dependence

on XH2

, while the other expressions underpredict it for all values of XH2

. The expression of

Milks and Matula [6] provides a burning velocity which is very close to the present data for low

values of XH2

, but which deviates for XH2 $

0.2%. 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. Based on calculations with

either added H2 or H2O, we 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 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.

Because the CO 1 OH reaction becomes

increasingly important as the hydrogen content

increases, the rate of the CO 1 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 [6], but using the

CO 1 OH rate expression of Baulch et al. 1973

[42], Yu et al. [40], Baulch et al. 1992 [43], and

Wooldridge et al. [44]. The recommendation of

Baulch et al. 1992 [43] significantly overpredicts

the burning velocity for our data, while that of

Baulch et al. 1973 is slightly lower. The rates of

Yu et al. and Wooldridge et al. provide results

that are nearly identical for the present conditions, and that are very close to the measured

burning velocities. 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 1 OH.

The suggested rate expressions for the direct

reaction include those with a high activation

energy (e.g. [5, 8, 41]) and those with a low

value (e.g. [6, 7, 45]). An Arrhenius plot for

some of the literature values of the direct

reaction is shown in Fig. 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 Fig. 2 (comparing for

example the curves for Loirat et al. [41] and

Fujii [7], which are fairly close at XH2 5 0). In

order to obtain some temperature variation

while avoiding the complications from uncertainty in the CO 1 OH rate, we examine dry

(5–15 ppm H2) flames with inert additive or

over a range of f.

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

Fig. 3. Calculated burning velocity (lines) of CO–N2O

flames with values of the CO 1 OH 3 CO2 1 H rate from

Baulch et al. [42], Yu et al., [40], Wooldridge et al. [44], and

Baulch et al. [43], together with data from the present study

(symbols).

64 G. T. LINTERIS ET AL.
[6]. Also shown in the figure are the predictions

when the preexponential factors of the direct

reaction rates have been adjusted to provide

agreement at XN2 5 0 (dotted lines). The higher

activation 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.

Van Wonterghem and Van Tiggelen [12] also

presented data on the flame speed as a function

of XN2

. 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. (Even with 1000

ppm of H2, half the CO consumption occurs via

the direct reaction, and the burning velocity is

very sensitive to the rate of the direct reaction.)

We estimate the background level of H2 in their

reactant mixtures to be that value of XH2 (1560

ppm) which provides a calculated burning velocity which matches their experimental burning

velocity for f 5 1.0 and XN2 5 0. (In the

calculation, we use the CO 1 N2O rate of Milks

and Matula with a 10% lower preexponential,

which gives the best fit to our data.) We also

show the calculated result using the rate of

Loirat et al., again matched for agreement at

XN2 5 0. The triangles and the dashed lines in

Fig. 5 show the data of Van Wonterghem and

Van Tiggelen and our calculations. The lower

activation energy again provides a closer agreement.

The dependence of the burning velocity on

the fuel–oxidizer equivalence ratio f is shown

in Fig. 6. The experimental results (symbols) for

Fig. 4. Arrhenius plots of the rate of the direct exchange reaction CO 1 N2O 3 CO2 1 N2 from various investigators (k in

cm, K, mole, s).

Fig. 5. Measured burning velocity (squares) and calculated

results (lines) of CO–N2O flames with added N2. The solid

lines correspond to the as given CO 1 N2O rate of Milks

and Matula [6] or Loirat et al., 1987 [41]; the dotted lines

have the preexponential adjusted for agreement at XN2 5 0.

Triangles are data from Ref. 12 and dashed line has 1560

ppm H2 and adjusted preexponential factor.

CO–N2O FLAME SPEEDS WITH IRON PENTACARBONYL 65
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 [6]. As shown previously

in Fig. 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 preexponential factor in the direct exchange reaction

rate is adjusted to give agreement at f 5 1.0

(dotted lines in the figures), the high activation

energy rate of Loirat et al. underpredicts the

burning velocity for richer flames (which have

lower temperature) more than does the lowactivation energy rate of Milks and Matula

(note from Table 1 that at f 5 3.2, the peak

flame temperature within the primary reaction

zone drops by about 200 K compared to its peak

at f 5 1.0). Nonetheless, the rate expression of

Milks and Matula overpredicts the burning velocity by about 18% at f 5 0.6, and underpredicts it by about 25% at f 5 3.0. The data of

Van Wonterghem and Van Tiggelen [12] for

flames with XH2 # 2000 ppm are also shown in

Fig. 6. The dashed line shows the calculated

burning velocity for 1560 ppm (as described

above). As with our data, the calculations

slightly overpredict the flame speed for lean

flames and underpredict it for rich flames.

For the given mechanism, over the range of f

in Fig. 6, the burning velocity is only significantly sensitive to the rate of the exchange

reaction (as described below). We have attempted to modify the rate of the next most

sensitive reactions (including the rates of the

reactions: N2O 1 M, N2O 1 O, and CO 1 O)

to improve agreement over f; however, adjustment of these rates within their uncertainty does

not improve agreement in Fig. 6. Also, addition

of H2 impurity at 100 ppm, while raising the

burning velocity 10% to 15% at all values of f,

does not affect the qualitative variation with f.

Finally, Fig. 7 presents the experimental

burning velocity data of Simpson and Linnett

[14] for rich ( f 5 2.0), slightly preheated (Tin 5

323 K) CO–N2O flames with 25% dilution

nitrogen and added water vapor up to about 5%

(error bars estimated here based on Ref. 29).

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 Allen

et al. [1] provides reasonable agreement with

the flame data, while use of the CO 1 OH rate

Fig. 6. Measured burning velocity (squares) and calculated

results (lines) of CO–N2O flames as a function of fuel–

oxidizer equivalence ratio f. The solid lines correspond to

the CO 1 N2O rate of Milks and Matula [6] or Loirat et al.

[41] as given; the dotted lines have the preexponential

adjusted for agreement at f 5 1.0. Triangles are data from

Ref. [12] and dashed line has 1560 ppm H2 and adjusted

preexponential factor.

Fig. 7. Experimental burning velocity of CO–N2O flames as

a function of the mole percent H2O in the reactants, from

[14] for f 5 2.0 and XN2 5 0.25, together with numerically

calculated prediction.

66 G. T. LINTERIS ET AL.
of Yu et al. [40] or Wooldridge et al. [44] and

the CO 1 N2O rate of Milks and Matula [6]

provide improved agreement. Lowering the preexponential factor of the CO 1 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 (f 5 0.5) is overpredicted by about 18%, and that of rich flames

(f 5 3.0) is underpredicted by about 25%. 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. Because the direct reaction dominates the reaction system even more

so under rich conditions (as described below), it

is perhaps more reasonable to infer the direct

reaction rate from the burning velocity at f 5

3.0. Doing this implies a reaction rate of 109.3

cm3

/mol s at 1800 K, or about 30% higher than

the Milks and Matula rate.

The important reactions in the CO–N2O

flames with and without added hydrogen can

now be examined using the kinetic mechanism

described above. These calculations use the

recommendation of Milks and Matula [6] for

the direct reaction of CO with N2O. In Fig. 8,

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 occurring due to 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 Fig. 8. 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, whereas

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 1 OH 3 CO2 1 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 51000 ppm.

In Fig. 9, a similar plot is shown for CO and

N2O consumption as a function of f. 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,

Fig. 8. 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.

CO–N2O FLAME SPEEDS WITH IRON PENTACARBONYL 67
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. [46] found the nitrous oxide

flames to be inhibited much more weakly than

hydrogen–air flames. Similarly, Rosser et al.

[24] 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 by HBr. In studies of moist

CO–O2 and CO–N2O flames with chlorine,

Palmer and Seery [47] 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 nonbranching 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 described, may

be modified for Fe(CO)5.

Figure 10 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 hydrogenfree 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 [23, 25]

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 [48] observed that

nitrous oxide destruction is catalyzed by Fe,

producing chemiluminescent FeO. Rates for the

reaction Fe 1 N2O 3 FeO 1 N2 have been

measured by Campbell and Metzger [49] and

Plane and Rollason [50]. This reaction alone

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

Kappes and Staley [51] proposed a catalytic

Fig. 9. Calculated flux of important CO (dotted lines) and

N2O reactions (solid lines) in a dry, stoichiometric CO–N2O

flame as a function of f.

Fig. 10. Normalized burning velocity of stoichiometric CO–

N2O flames with XH2 5 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.

68 G. T. LINTERIS ET AL.
oxidation cycle involving ions: Fe 1

1 N2O 3

FeO 1

1 N2 and FeO 1 1 CO 3 Fe 1 1 CO2,

but reaction of neutral FeO with CO is also

possible, and would provide a gas-phase catalytic cycle, with the net reaction: CO 1 N2O 3

CO2 1 N2

Previous experimental work suggests such

catalytic effects. In a shock tube study, Matsuda

[52] 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 [53] 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 Plane

and Rollason [50], while the others are our

estimates. These reactions are added to the

iron-species inhibition mechanism described in

Ref. 23. The thermodynamic data for FeOOH

(and the related rate expressions) have been

updated based on the recent calculations of

Kellogg and Irikura [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 10 also 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 5

0.001 or 0.002, but the variation is greater in the

experiments. The calculated burning velocity in

the moist flames with Fe(CO)5 is very sensitive

to the rates of the catalytic steps involving

FeOOH. While changes in these rates do affect

the separation of the curves for XH2 5 0.001 and

0.002, they cannot provide agreement between

experiment and calculation, and further research is required.

Careful investigation of Fig. 10 reveals that

for all values of XH2

, the slope of the experimental curve decreases as XFe(CO)5 increases,

but that the calculated curve is linear. The

curvature is mild, and ordinarily might not be of

interest given the other discrepancies between

the measured and predicted flame speeds with

added Fe(CO)5. This reduction in the marginal

influence of the Fe(CO)5 (reaction promotion)

as XFe(CO)5 increases, however, is much like the

loss of effectiveness of Fe(CO)5 observed in

flames of CH4 or CO with air, 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 iron-containing inhibiting species

[21]. Through the use of classical laser scattering and extinction measurements with phasesensitive detection, it was later 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 Refs. 55 and 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 that clearly increased as the mole fracTABLE 3

Reactions in the Iron Catalytic Cycle for the CO–N2O

System, and Their Estimated Reaction Rates*

Reaction A b Ea/R

Fe1 N2O 5 FeO 1 N2 1.40 E114 0 5940

FeO 1 N2O 5 FeO2 1 N2 3.00 E113 0 5033

FeOH 1 N2O 5 FeOOH 1 N2 1.30 E114 0 4530

FeO 1 CO 5 Fe 1 CO2 1.80 E112 0 3522

FeO2 1 CO 5 FeO 1 CO2 1.18 E113 0 4530

FeOOH 1 CO 5 FeOH 1 CO2 6.00 E113 0 4026

* kf 5 A Tb exp(2Ea/RT), and units are cm, K, mole, s.

CO–N2O FLAME SPEEDS WITH IRON PENTACARBONYL 69
tion of Fe(CO)5 increased. Hence, we cannot

conclude that the reduction in the promotion

effect near XFe(CO)5 5 60 ppm for XH2 5 0.002

is due to formation of particulates in the flame.

Although agreement between the measured

and calculated normalized burning velocity in

Fig. 10 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)u

max]. In both of these

tables, calculated results are provided for XH2 5

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, 94% 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 Oatoms. 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 Fig. 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. 25, 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

to be the important inhibition reaction). These

dry N2O flames appear to be similar to the

CO–H2–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 1 OH and

N2O 1 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

70 G. T. LINTERIS ET AL.
TABLE 4

Calculated Fractional Flux of the Total Reaction of Each Species Proceeding Through

the Indicated Reaction for Stoichiometric CO–N2O Flamesa

Species

XH2

:

Fractional Flux (%)

0.000 0.002

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

CO Destruction

CO 1 OH 7 CO2 1 H — — 60 48

CO 1 N2O 7 CO2 1 N2 94 68 38 30

CO 1 O(1M) 7 CO2 (1M) 3010

CO 1 NO2 7 CO2 1 NO 2———

FeO 1 CO 7 Fe 1 CO2 — 9— 3

FeO2 1 CO 7 FeO 1 CO2 — 23 — 6

FeOOH 1 CO 7 FeOH 1 CO2 — — — 12

N2O Destruction

CO 1 N2O 7 CO2 1 N2 40 30 19 16

N2O (1M) 7 N2 1 O(1M) 31 30 26 26

N2O 1 H 7 N2 1 OH — — 28 20

N2O 1 O 7 O2 1 N2 13 10 11 10

N2O 1 O 7 2NO 13 10 11 10

NH 1 NO 7 N2O 1 H 0033

NO 1 N2O 7 NO2 1 N2 2111

FeO 1 N2O 7 FeO2 1 N2 — 12 — 5

Fe 1 N2O 7 FeO 1 N2 — 6— 4

FeOH 1 N2O 7 FeOOH 1 N2 ——— 6

O Creation

N2O(M1) 7 N2 1 O(1M) 99 99 95 88

H 1 O2 7 O 1 OH — — 4 11

Destruction

N2O 1 O 7 O2 1 N2 44 36 42 36

N2O 1 O 7 2NO 44 36 42 36

H 1 O2 7 O 1 OH —— 6 2

NH 1 O 7 NO 1 H —— 4 4

CO 1 O(1M) 7 CO2(1M) 4222

NO2 1 O 7 O2 1 NO 5311

NO 1 O(1M) 7 NO2(1M) 2———

O 1 H2 7 H 1 OH —— 1 1

Fe 1 O2 7 FeO 1 O — 12 — 9

FeO2 1 O 7 FeO 1 O2 — 11 — 7

H Creation

CO 1 OH 7 CO2 1 H — — 87 89

H 1 O2 7 O 1 OH —— 4 2

NH 1 O 7 NO 1 H —— 3 3

NH 1 NO 7 N2O 1 H —— 3 3

O 1 H2 7 H 1 OH —— 1 1

Destruction

N2O 1 H 7 N2 1 OH — — 83 73

NH 1 NO 7 N2O 1 H — — 10 10

H 1 O2 7 O 1 OH — — 3 12

NO2 1 H 7 NO 1 OH — — 3 3

OH Creation

N2O 1 H 7 N2 1 OH — — 87 77

H 1 O2 7 O 1 OH — — 3 13

NH 1 NO 7 N2 1 OH —— 3 3

NO2 1 H 7 NO 1 OH —— 3 3

H2O 1 O 7 2OH —— 1 2

O 1 H2 7 H 1 OH —— 1 1

CO–N2O FLAME SPEEDS WITH IRON PENTACARBONYL 71
ppm of Fe(CO)5 [25], 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 reactions. Hydrogen and

hydroxyl radicals are important for consumption of N2O and CO, but the system is straightchain 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 1 H2 is also low,

ranking as the twelfth most sensitive reaction, as

compared to methane–air or moist CO–air

flames, where the burning velocity is very sensitive to the rate of the branching reaction H 1

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 1 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) levels of

hydrogen-containing impurities have been obtained, for 0.6 # f # 3.0 and with added

nitrogen up to XN2 5 0.25; data have also been

collected for flames with added hydrogen up to

XH2 5 0.005. The measured burning velocity of

pure stoichiometric flames is 23.1 cm/s 6 0.6

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

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

those of earlier investigations were numerically

TABLE 4

Continued

Species

XH2

:

Fractional Flux (%)

0.000 0.002

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

OH Destruction

CO 1 OH 7 CO2 1 H — — 93 95

H 1 O2 7 O 1 OH —— 5 2

H2O 1 O 7 2OH —— 1—

Fe Creation

FeO 1 CO 7 Fe 1 CO2 — 53 — 35

Fe 1 O2 7 FeO 1 O — 45 — 54

Fe 1 O2(1M) 7 FeO2(1M) ——— 6

FeO 1 H 7 Fe 1 OH — — — 3

Destruction

Fe 1 N2O 7 FeO 1 N2 — 85 — 88

Fe 1 O2(1M) 7 FeO2(1M) — 14 — 11

FeO Creation

Fe 1 N2O 7 FeO 1 N2 — 33 — 42

FeO2 1 O 7 FeO 1 O2 — 16 — 20

FeO2 1 CO 7 FeO 1 CO2 — 51 — 35

Destruction

FeO 1 CO 7 Fe 1 CO2 — 21 — 17

FeO 1 N2O 7 FeO2 1 N2 62 53

Fe 1 O2 7 FeO 1 O — 18 — 26

FeO 1 H 7 Fe 1 OH — — — 2

FeO 1 H2O 7 Fe(OH)2 ——— 1

a Results are given for XH2 5 0.0 and 0.002, and for XFe(CO)5 5 0 and 213 ppm.

72 G. T. LINTERIS ET AL.
modeled using a mechanism based on Allen et

al. [1] with a CO 1 OH rate from Yu et al. [40],

and the CO 1 N2O direct reaction rate of Milks

and Matula [6]. 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

preexponential factor of the Milks and Matula

rate. Experiments with nitrogen dilution and

over a range of f suggest an activation energy

near 71 kJ/mole. For dry flames with 0.6 # f #

3.0, the mechanism overpredicts the burning

velocity by about 18% for lean flames and

underpredicts it by about 25% for rich flames.

For the moist flames, the CO 1 OH rate also

has a strong effect on the predicted burning

velocity, and the rates of Yu et al. [40] or

Wooldridge et al. [44] provide 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 1 M 5 N2 1 MO and CO 1

MO 5 CO2 1 M, where M is Fe, FeO, or

FeOH. The rate expression of Plane and Rollason [50] for the former reaction with M 5 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 extraordiTABLE 5

Sensitivity of Burning Velocity to the Specific Reaction Rate Constant for

Stoichiometric CO–N2O Flames with XH2 5 0 and 0.002, and for XFe(CO)5 5 0 ppm

and 213 ppm.a

d (ln v)/d (ln k) d (ln v)/d (ln k)umax

XH2

: 0 0.002

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

Reaction

Dry Reactions

CO 1 N2O 7 CO2 1 N2 1.00 1.00 1.00 1.00

N2O(1M) 7 N2 1 O(1M) 20.13 20.13 0.19 20.10

N2O 1 O 7 2NO 20.07 20.07 20.34 20.23

N2O 1 O 7 O2 1 N2 0.00 0.04 0.04 0.06

Moist Reactions

CO 1 OH 7 CO2 1 H 0.83 0.63

N2O 1 H 7 N2 1 OH 0.32 0.31

H2O 1 O 7 2OH 20.10 20.06

O 1 H2 7 H 1 OH 0.09 0.09

Iron Reactions

FeO2 1 CO 7 FeO 1 CO2 0.30 0.28

FeO 1 CO 7 Fe 1 CO2 0.09 0.04

FeO 1 N2O 7 FeO2 1 N2 0.14 20.05

Fe 1 N2O 7 FeO 1 N2 0.02 0.04

FeOH 1 N2O 7 FeOOH 1 N2 0.44

FeOOH 1 CO 7 FeOH 1 CO2 0.25

FeO 1 H2O 7 FE(OH)2 0.21

FeOH 1 H 7 FeO 1 H2 0.20

Fe(OH)2 1 H 7 FeOH 1 H2O 0.13

FeOH 1 O 7 FeO 1 OH 20.08

FeOOH 1 OH 7 FeO2 1 H2O 20.04

a Sensitivities are normalized by the value for the maximum sensitivity, which is the direct CO 1 N2O reaction.

CO–N2O FLAME SPEEDS WITH IRON PENTACARBONYL 73
nary 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 that

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.

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 Prive ́

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.

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Received 17 August 1999; revised 11 January 2000; accepted

23 January 2000

CO–N2O FLAME SPEEDS WITH IRON PENTACARBONYL 75