NISTIR 6360

Inhibition of Premixed Carbon Monoxide-HydrogenOxygen-Nitrogen Flames by Iron Pentacarbonyl

Marc D. Rumminger and Gregory T. Linteris

Building and Fire Research Laboratory

Gaithersburg, Maryland 20899

United States Department of Commerce

Technology Administration

National Institute of Standards and Technology
NISTIR 6360

Inhibition of Premixed Carbon Monoxide-HydrogenOxygen-Nitrogen Flames by Iron Pentacarbonyl

 Marc D. Rumminger and Gregory T. Linteris

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

INHIBITION OF PREMIXED CARBON MONOXIDE-HYDROGEN-OXYGENNITROGEN FLAMES BY IRON PENTACARBONYL†

Marc D. Rumminger‡ and Gregory T. Linteris§

Building and Fire Research Laboratory

National Institute of Standards and Technology, Gaithersburg MD 20899-8651, USA

(marc.rumminger@nist.gov, linteris@nist.gov)

ABSTRACT

This paper presents measurements of the burning velocity of premixed CO-H2-O2-N2

flames with and without the inhibitor Fe(CO)5 over a range of initial H2 and O2 mole

fractions. A numerical model is used to simulate the flame inhibition using a gas-phase

chemical mechanism. For the uninhibited flames, predictions of burning velocity are

excellent and for the inhibited flames, the qualitative agreement is good. The

agreement depends strongly on the rate of the CO+OH↔CO2+H reaction and the rates

of several key iron reactions in catalytic H- and O-atom scavenging cycles. Most of the

chemical inhibition occurs through a catalytic cycle that converts O atoms into O2

molecules. This O-atom cycle is not important in methane flames. The H-atom cycle

that causes most of the radical scavenging in the methane flames is also active in CO-H2

flames, but is of secondary importance. To vary the role of the H- and O-atom radical

pools, the experiments and calculations are performed over a range of oxygen and

hydrogen mole fraction. The degree of inhibition is shown to be related to the fraction

of the net H- and O-atom destruction through the iron species catalytic cycles. The Oatom cycle saturates at a relatively low inhibitor mole fraction (~100 ppm), while the Hatom cycle saturates at a much higher inhibitor mole fraction (~400 ppm). The

calculations reinforce the previously suggested idea that catalytic cycle saturation

effects may limit the achievable degree of chemical inhibition.

INTRODUCTION

The production of the effective and widely-used fire suppressant CF3Br and other

related compounds has been banned due to their contribution to ozone depletion.

Currently, there exists no replacement fire suppressant with all of the desirable

 

† Official contribution of the National Institute of Standards and Technology, not subject to copyright in

the United States.

‡ National Research Council/NIST postdoctoral fellow

§ Corresponding author
2

properties of CF3Br, and the search for alternative compounds has intensified. It is

well-established that some metallic compounds are very powerful flame inhibitors [1-

5]. The compound iron pentacarbonyl (Fe(CO)5) has been found to be up to two orders

of magnitude more effective than bromine-containing compounds at reducing the

burning velocity of premixed hydrocarbon-air flames [1,6]. Although Fe(CO)5 is toxic

and flammable, understanding its inhibition mechanism could provide insight into the

behavior of other highly effective agents and aid in the development of new non-toxic

agents.

In recent years, several detailed studies of Fe(CO)5 have been undertaken.

Reinelt and Linteris [7] studied the flame inhibition effect of iron pentacarbonyl in

methane-oxygen-nitrogen mixtures by measuring the burning velocity of premixed

flames and the extinction strain rate of counterflow diffusion flames. The experiments

showed that for small amounts of Fe(CO)5 the effect was roughly linear, but above a

certain loading the effect was nearly independent of the Fe(CO)5 concentration. The

authors postulated that the decreasing inhibition was due to loss of active gas-phase

species through condensation. Rumminger et al. [8] developed a chemical mechanism

for Fe(CO)5 inhibition of methane-oxygen-nitrogen flames, and through numerical

simulations concluded that the inhibition occurs primarily through gas-phase

chemistry. Flame simulations using the chemical mechanism in Ref. [8] showed

promising agreement between model and experiment. The mechanism successfully

predicted the trends for various equivalence ratios and oxygen mole fractions;

however, the authors had increased the rates of several key reactions** (within

experimental uncertainty) to improve quantitative agreement, and consequently

recommended additional testing and refinement of the mechanism.

In the present paper we examine the effect of Fe(CO)5 in CO-H2-O2-N2 flames,

which have a different radical pool than the methane flames of previous studies.

Calculations of uninhibited premixed flame structure indicate that the peak mole

fraction of O atom is four times higher in stoichiometric CO-H2-air flames (containing

between 0.2 and 1.0% H2) than a stoichiometric CH4-air flame, while the concentrations

of H-atom and OH are between 1.5 and 7 times lower. These differences permit

examination of the effect of radical pool composition on inhibition. Since previous

calculations [8]†† have shown that inhibition in methane flames occurs primarily from

radical recombination cycles involving H atoms, it is of interest to determine if other

mechanisms are important in flames with reduced H-atom mole fraction. In CO flames

the H-atom mole fraction can be adjusted by varying the amount of H2 in the reactant

mixture. Finally, the numerical calculations in methane-air flames indicated that above

a certain loading chemical inhibitors can lose their effectiveness because the radical

 

** The reactions and increased rates were FeO+H2O↔Fe(OH)2 (kf = 1.62E+13, increased by 3X),

FeOH+H↔FeO+H2 (kf = 1.50E+14 exp(-805/T), increased by 5X), Fe(OH)2+H↔FeOH+H2O (kf =

1.98E+14 exp(-302/T), increased by 3X).

†† To avoid excessive repetition, references to previous calculations of Fe(CO)5-inhibited methane flames

refer to Ref. [8] unless otherwise noted.
3

pool has already been lowered to equilibrium levels. By varying the radical pool size,

CO flames can be used to investigate whether the observed saturation in iron

pentacarbonyl’s effectiveness is due to condensation or radical pool depletion.

The oxidation mechanism of CO has fewer reactions than those of hydrocarbon

fuels, allowing easier isolation of the important inhibition reactions. For this reason,

many researchers have used CO flames in studies of halogen inhibition [9-16]. The

simplicity of the CO system has additional significance in the present work. In

previous studies with methane flames, the rates of key inhibition reactions suggested

by Jensen and Jones [17] were increased in order to accurately match the observed

burning velocity reductions. It is of interest here to determine if the faster rates are

necessary for modeling moist CO flames—which are chemically much closer to the rich

H2-O2 flames used by Jensen and Jones (reactions of iron-containing species with

hydrocarbons are not presently included in the mechanism). Finally, carbon monoxide

flames are relevant to fire research since CO is a dominant intermediate species in

hydrocarbon flames and its oxidation is often the rate-limiting step in product

formation.

The approach in the present research is to determine the effect of Fe(CO)5 on the

overall reaction rate of premixed flames. The burning velocity of a premixed laminar

flame is a relative measure of the overall reaction rate. Since this reaction rate is a

fundamental parameter affecting the stabilization and fuel consumption rate of fires,

the extent to which it is influenced by the agent is a first measure of the agent’s

potential as a fire suppressant. Although suppression tests on full-scale fires will be

required to assure the effectiveness of any agent, laboratory burners have several

important benefits. Their simplicity allows rapid assessment of inhibitor performance

in many flame conditions and at various concentrations, and the flames are highly

reproducible and stable. Premixed laminar flames can be nearly adiabatic in the

laboratory, and are easily modeled with existing computer programs, allowing

straightforward interpretation of the effect of the inhibitor. By using detailed chemical

kinetic mechanisms together with full transport calculations, the chemical species

profiles throughout the flames are calculated. These results provide great insight into

both the chemical and physical mechanisms of the inhibitor. Also, in a well-designed

laboratory burner, there is little ambiguity about how much agent reaches the flame.

Although techniques such as detailed flame structure measurements provide valuable

information about the chemical species at each location in the flame for one particular

condition, burning velocity and extinction strain rate measurements have the

advantage of allowing rapid testing of the effect of the inhibitor over a wider range of

conditions.

Our study of flame inhibition is intended to provide insights into flame

suppression. Although the processes have different end points (weakening the flame vs.

extinguishing it), the underlying mechanism is similar: the agent reduces the overall

reaction rate of the fuel-air mixture. Inhibition can be viewed as an early stage of

suppression in which the inhibitor weakens the flame, making it more vulnerable to

extinction by external factors such as heat loss or fluid-mechanical instability.

Inhibition—and eventually suppression—occurs as the agent reduces the rate of heat
4

release of the flame. For chemically-acting agents, the agent interferes with the

reactions which consume the fuel and intermediates. Hence, we can study the effect of

small agent concentrations on flame chemistry and build chemical kinetic models

which describe the effect of the agent on the combustion reactions. By using

laboratory-scale premixed and diffusion flames that are amenable to modeling, we can

validate the chemical kinetic model and gain insight into which processes are most

important. Later, we can continue to construct and refine the mechanism for higher

inhibitor concentrations and for flames and fuels which are more representative of fires.

Finally, an understanding of the modes of action of effective agents such as Fe(CO)5 can

lead researchers to chemicals that have similar favorable properties, while avoiding

characteristics such as diminishing effectiveness at high concentration.

In addition to the flame inhibition experiments and modeling, we present

measurements of the burning velocity of uninhibited CO-H2-O2-N2 flames with varying

amounts of H2, and compare the measurements with calculations. Although there are

numerous measurements of CO flame speed in the literature [9,18-21], there is a lack of

data for near-stoichiometric CO-H2-O2-N2 flames with H2 mole fractions between

1.50·10-4 and 2.0·10-2.

In the remainder of the paper, we describe the experimental technique, the

numerical calculation procedure, and the results for uninhibited and inhibited CO-H2-

O2-N2 flames. We interpret the chemical kinetics of the inhibiting iron species in CO

flames, comparing them with results for CH4 flames, and analyze how changing

oxygen or hydrogen content affects the flame inhibition.

EXPERIMENTAL

Premixed Flame System

A Mache-Hebra nozzle burner (inner diameter 1.02 ± 0.005 cm) [22] with a schlieren

imaging system [23], is used to measure the average burning velocity with the total

area method [24]. 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, with the

sum of CH4 and H2O < 10 ppm) and hydrogen (Matheson UHP, 99.999% H2, with sum

of N2, O2 CO2, CO, Ar, CH4, and H2O < 10 ppm); the oxidizer stream consists of

nitrogen (boil-off from liquid N2) and oxygen (MG Industries, H2O < 50 ppm, total

hydrocarbons < 5 ppm). The experimental arrangement has been described in detail

 

‡‡ 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.
5

previously [25] and the optical and image processing systems have been refined as

described below.

Inhibitor is added to the flame by diverting part of the nitrogen stream to a twostage saturator maintained in an ice bath at 0oC. 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 assumed to be saturated with Fe(CO)5 vapor. Although the saturator is the same

as used previously, the assumption of saturation has been verified by flowing the

Fe(CO)5-laden gases into a condensation coil immersed in a -60°C alcohol bath. After a

specified elapsed time, the ends of the coil were capped and the weight gain of the coil

(i.e., the amount of Fe(CO)5) was measured. After performing the condensation

experiment for several carrier-gas flow rates, a linear fit of the data was generated to

quantify the dependence of Fe(CO)5 concentration on carrier-gas flow rate. The

experimental results and theoretical prediction based on carrier-gas saturation were

within 5% of each other across a wide range of flow rates.

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

CID array (Cidtec CID3710D). The image is digitized by a 640 x 480 pixel framegrabber board (Data Translation 3155) in a Pentium-II computer. The images are

acquired and written to disk using the free UTHSCSA ImageTool program [26]. For

each flame condition, 10 images are collected at a rate of one per second. The flame area

is determined (assuming axial symmetry) using custom image-processing software.

Finally, the burning velocity is calculated by dividing the volumetric flow rate

(corrected to 1 atmosphere and 298 K) by the average flame area for the 10 images.

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 one-dimensional 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 [24], 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 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 (the burning velocities used for the

normalizations are listed in Table 1).

Uncertainty Analysis

The uncertainty analysis consists of calculation of individual uncertainty components

and root mean square summation of components [27]. Calculation of the individual

uncertain components is accomplished analytically for simple expressions (e.g.,

equivalence ratio), and through use of a ‘jitter program’ [28] for more complex

expressions (e.g., Fe(CO)5 mole fraction). The jitter program sequentially varies the

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

variable. All uncertainties are reported as expanded uncertainties: X ± U, where U is kuc,

and is determined from a combined standard uncertainty (estimated standard
6

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 1)

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

3) determination of the flame area, 4) the effect of flame base location on flame area, 5)

the location of the schlieren image relative to the cold gas boundary.

The relative uncertainty in total flow rate is 0.7%, which includes the uncertainty

of the flow controllers and uncertainty in the measurements of ambient temperature

and pressure. The uncertainty in flame area is affected by uncertainty in the pixel to

mm conversion (1%); the uncertainty in the distance between the flame base and the

burner surface; and image-to-image variability (from 0.5% to 6%, with higher

variability at higher flow rates).

The distance between the flame base and the burner surface is one of the inputs

to the image-processing software. For a given fuel and oxidizer, a single separation

distance is acceptable since extensive examination of the flame images shows it to be

constant even for different inhibitor loading or mixture stoichiometry. Additionally, it

has been shown that the distance between the flame base and burner base is nearly

constant for Bunsen-type hydrocarbon flames with a uniform velocity profile at the

burner exit for φ between 0.8 and 1.3 [29]. When the Fe(CO)5 loading is high, though,

the location of the flame base can change as iron oxide powder collects on the burner

rim. We have measured the buildup and accounted for it when specifying the flameto-burner separation distance. (Note that the measurements of absolute burning

velocity of uninhibited flames that we report here, Table 1 & Table 2 below, were made

using clean, deposit-free burner rims.)

From analysis of multiple flame images and calculation of flame area using

varying flame base locations, we estimate that the uncertainty of the flame base location

is 0.1 mm, and the resulting uncertainty in the flame area is about 2%. The selection of

flame base location has a negligible effect on normalized burning velocity, as the

normalization eliminates the error in flame base selection. Some uncertainty results

from variations in heat loss at each test condition, but since we maintain a constant

flame height of 13 mm by adjusting the total flow rate, this term is small for absolute

flame speed measurements, and negligible when the burning velocity is normalized. In

summation, without consideration of the schlieren image position, the relative

uncertainty for burning velocity measurements ranges between 3% and 6.5%, and

between 1% and 4.5% for normalized burning velocity. In general, uncertainty

increases with increasing burning velocity. The tables of burning velocity assign

uncertainty to each measurement of uninhibited burning velocity.

Uncertainty about 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 [24], use of the schlieren image as the flame area can lead to an

underestimation of the burning velocity [30]. Previous researchers [24,30,31] have

attempted to adjust the flame area to the 298 K isotherm (T298), although this generally
7

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) data from a flame.

The uncertainties in the equivalence ratio, oxygen mole fraction and hydrogen

content are related to the accuracy of the flow controllers. The expanded relative

uncertainties for the these quantities are as follows: 1.4% for equivalence ratio; 1.1%

for oxygen mole fraction measurement; and 1.2% for hydrogen mole fraction in the

reactants.The amount of hydrogen contamination of the bottled gases, based on the

manufacturer’s analysis, is estimated to be less than 75 ppm.

The uncertainty of Fe(CO)5 concentration is calculated assuming that the

concentration depends on the vapor pressure of Fe(CO)5, laboratory temperature,

laboratory pressure, and gas flow rate. The correlation for the vapor pressure of

Fe(CO)5 is from Gilbert and Sulzmann [32], but the paper does not provide uncertainty

for the correlation. Nonetheless, uncertainty in the bath temperature, the ambient

pressure and the carrier gas flow rate results in an uncertainty of Fe(CO)5 mole fraction

of approximately ±6.5%.

NUMERICAL MODEL

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

flame code Premix [33], the Chemkin subroutines [34], and the transport property

subroutines [35]. For all of the calculations the absolute tolerance is 10-14, the relative

tolerance is 10-9, GRAD is 0.20, and CURV is 0.35. Thermal diffusion (Soret effect) is

included in the calculations. Solutions typically contain between 85 and 130 grid

points. The initial temperature is 298 K and the pressure is one atmosphere. The kinetic

mechanism and thermodynamic data of Yetter et al. [36] (13 species and 37 chemical

reactions) serve as a basis for describing moist carbon monoxide oxidation. Note that

we use the modified Arrhenius rate for CO+OH↔CO2+H from Baulch et al. [37]

because numerical calculations indicated that most of the CO oxidation was occurring

below 2000 K (temperature above which a different rate expression was recommended

[36]). Iron pentacarbonyl is added to the unburned CO-H2-O2-N2 mixture at mole

fractions of up to 500 ppm. The chemical mechanism for Fe(CO)5 inhibition of flames

(12 species and 55 reactions) and necessary thermodynamic and transport data are

compiled from a variety of sources as described in Ref. [8]. It should be emphasized

that the reaction mechanism used for the present calculations should be considered

only as a starting point. Numerous changes to both the rates and the reactions

incorporated may be made once a variety of experimental and theoretical data are

available for testing the mechanism.
8

RESULTS AND DISCUSSION

Uninhibited flames

Figure 1 shows the measured burning velocities for CO-H2-air flames with varying

amounts of hydrogen (for the reader’s convenience, the data are also presented in

tabular form in Table 2). We were not able to obtain a steady flame for hydrogen mole

fractions (XH2) below 0.0024 or above 0.014. If extrapolated to hydrogen-free

conditions, our measurements would agree reasonably well with the measurements of

Palmer and Seery [9], who used the cone frustum method (reported measurement

uncertainty of ±10%) to measure the burning velocity of CO-air flames with 45-150 ppm

of H2.

Figure 1 also shows the calculated burning velocity using the mechanism from Ref.

[36] with several different sets of rate constants for CO+OH→CO2+H [37-40].
9

Table 3 lists calculated forward rate constants for each formulation at several

temperatures. The 1992 rate expression of Baulch et al. [39] yields a burning velocity

about 20% higher than the experiments, while the remaining three rate expressions

predict values less than 10% lower than the experiments. Since there is a relatively

large uncertainty in the rate of CO+OH near 2000 K [41], adjustment of this rate to

match the present data seems unwarranted.

Examination of the numerical results shows that the CO+OH reaction is

important even at very low hydrogen concentration. At XH2 = 0.001, 70% of the CO

oxidation proceeds through CO+OH, while 28% of the CO oxidation proceeds through

CO+O+M. At XH2 = 0.015 the split is 88% through CO+OH and 10% through CO+O+M.

Inhibited Flames

Effect of O2 Mole Fraction

Figure 2 presents measurements (symbols) and calculations (lines) of the burning

velocity of Fe(CO)5-inhibited CO-H2-O2-N2 flames with varying amounts of oxygen. As

previously seen for CH4-air flames inhibited by Fe(CO)5, inhibition is strongest in the

flames with the lowest XO2

,ox (XO2,ox refers to the oxygen mole fraction in the oxidizer

prior to mixing with the fuel). The experimental results show that inhibition is

proportional to inhibitor concentration at low inhibitor mole fraction (Xin), but after a

certain concentration, additional Fe(CO)5 has little effect on the burning velocity. A

plausible (but untested) explanation is that particle formation removes the inhibiting

species at high inhibitor loading [7]. The experimental results in Figure 2 support this

suggestion: as the flame temperature increases, the leveling-off point shifts to higher

Fe(CO)5 mole fraction.

Changing the oxygen mole fraction of the uninhibited CO flames changes

several features which may affect the burning velocity reduction caused by added

Fe(CO)5. Table 4 lists the peak temperature and peak mole fraction of O, H, and OH in

uninhibited flames with XO2

,ox equal to 0.183, 0.21, and 0.24. Raising the oxygen mole

fraction over this range increases the final temperature by 209 K and the peak radical

mole fractions by 30% to 60%. Although several variables are changing

simultaneously, it is reasonable to expect the hotter flames, with their larger radical

pool, to be affected less by a fixed quantity of Fe(CO)5, yielding the smaller slope in

Figure 2. (A detailed analysis of the effect of XO2

,ox is undertaken later in this paper.)

The calculated normalized burning velocities (shown by the lines in Figure 2)

reproduce the trend of greater inhibition at lower XO2

,ox for low Xin, and predict a

reduced marginal effect as more inhibitor is added. At low values of Xin, the slopes of

the predicted normalized burning velocity curves are reasonably close to the measured

slopes, but above about 100 ppm the measured and calculated normalized burning

velocities diverge, with the model failing to reproduce the leveling off and

overpredicting the inhibition effect. Nonetheless, we can use the numerical results at

low inhibitor mole fraction to understand the reasons for the greater inhibition at lower

XO2

,ox, as well as other features. Further, results at higher values of Xin can be used to
10

investigate the relevant gas-phase chemistry predicted by the model if there was not a

loss of the active species to a condensed phase.

The information obtained from flame speed measurements is global, and many

reactions in the mechanism can affect the results. Nonetheless, a comparison of the

measured and predicted burning velocities together with investigation of the numerical

results can be used to identify key reactions for further investigation. In previous work

with methane flames, the rates of the dominant inhibition reactions (those in the Hatom cycle recommended by Jensen and Jones [17]) were increased (within

experimental uncertainty) to better predict the experimental results. In Figure 2, the

model shows slightly more inhibition for XO2

,ox=0.24 than the experiments. While it is

tempting to use the present data to move the rates of the H-atom cycle back in the

direction of the original recommendations [17], it is premature to do so. The excessive

inhibition only occurs for one case; also, the effect of reaction rates in the fuel oxidation

mechanism (exclusive of the inhibition mechanism) must be considered, as will be

described below. Based on these considerations, we use the increased rates for the Hatom cycle reactions for the analyses in the present paper.

As described in a preceding section and Figure 1, for uninhibited CO flames, the

rate of the reaction CO+OH has a large effect on the burning velocity predicted by the

numerical calculation. For inhibited flames, the rates of the key inhibition steps

obviously have a large effect on model performance. However, the rate selected for the

CO+OH reaction also affects the degree of predicted inhibition by Fe(CO)5. Figure 3

shows the combined effect of differing rates for the CO+OH reaction and for the

reactions in the H-atom cycle [8]. The measured and predicted normalized burning

velocities as a function of the mole fraction of Fe(CO)5 are shown for two values of the

CO+OH rate and for the fast and slow rates for the H-atom recombination cycle. As the

figure shows, the selection of the appropriate rates for the inhibition reactions depends

upon selection of the CO+OH rate.

For the remainder of this paper, we use the 1973 Baulch et al. [37] CO+OH rate

recommended by Yetter et al. [36] and the faster rates for the H-atom cycle reactions

from Ref. [8]. As illustrated in the NIST Chemical Kinetics Database [41], there is large

uncertainty in the rate of CO+OH near 1800 K. Although the rate of Yu et al. [38]

provides better agreement with our experimental data (Figure 1), we retain the 1973

Baulch et al. [37] rate since it has been extensively tested against a variety of

experimental data from flames, flow reactors, and shock tubes [36]. For the inhibition

reactions, the premixed methane flame results suggest use of the faster rates for the Hatom cycle. In Figure 3, considering only the region of low Xin, the results are

ambiguous, although with the two rates for CO+OH, the faster rates for the cycle (solid

lines) bracket the experimental data, while the slower rates for the cycle (dotted lines)

lie predominantly outside the data. Also, the recent measurements of Rollason and

Plane [42] for the reaction FeO+H2O+He(N2)↔Fe(OH)2+He(N2), which is part of the Hatom cycle, argue for a rate even higher than that suggested in Ref. [8]. Note that

while the absolute magnitude of the predicted inhibition is affected by the rates
11

selected for the H-atom cycle, the qualitative behavior is not, and the conclusions of this

paper are unaffected by these rates.

Catalytic Inhibition Cycles

In previous research on bromine-containing species [43,44], conversion of

radicals (e.g., H) into less reactive species (e.g., H2) has been found to be the dominant

mode of action for chemical inhibitors and suppressants. A similar H-atom catalytic

cycle involving iron oxide and hydroxide intermediates has been found to be important

in CH4-O2-N2 flames inhibited by Fe(CO)5 [8]. Reaction pathway and sensitivity

analyses for the inhibited CO flames reveals that the H-atom catalytic cycle is active. A

new finding, however, is that there is an additional catalytic cycle which scavenges O

atoms and which is much more important than the H-atom cycle for CO flames. The

appearance of O-atom scavenging is consistent with the high peak O-atom mole

fraction of the present flames as compared to H or OH (see Table 5), and the finding

that O-atom trapping is important for fluorinated-species inhibition of CO flames [16].

Recent computations of the thermochemistry of iron compounds at flame conditions

support the possibility of multiple radical recombination cycles [45]. O-atom

recombination by iron compounds has been reported previously [46] for flow tube

experiments in which Fe(CO)5 was added to the reactant gases. Removal of O-atom by

CF3Br in a flow tube has also been reported [47]. These previous studies, however, did

not propose a catalytic cycle for O-atom recombination.

The reactions that comprise the catalytic O-atom scavenging cycle are

Fe + O2 +M ↔ FeO2 +M (R1)

 FeO2 + O ↔ FeO + O2 (R2)

 FeO + O ↔ Fe + O2 (R3)

 O + O ↔ O2 .

The first and third reactions in the cycle have been studied in the laboratory. The rate

for (R1) has been measured by several researchers [48,49], and the rate for (R3) has been

measured by Fontijn et al. [50]. The rate for second reaction was estimated [8];

lowering its rate reduces the calculated inhibition and improves agreement with the

present experiments.

The primary reaction pathways for iron species are shown in Figure 4, where

both the H- and O-atom catalytic cycles are illustrated. Fe(CO)5 decomposition occurs

through several steps in which the CO ligands are sequentially removed (for simplicity,

these steps are not shown in the figure), leaving Fe. The Fe is converted first to FeO2

and then to FeO. A portion of the FeO is converted back to Fe, thus completing the Oatom cycle; the remainder of the FeO participates in the H-atom cycle.

Although the reactions (R1)-(R3) were present in the inhibition mechanism, a

significant contribution from the O-atom cycle was not found in the studies of CH4-O2-

N2 flames. Instead, reactions (R1)-(R2) converted Fe to one of the species in the main
12

inhibition cycle (FeO), and (R3) was a minor reaction. The authors found that most of

the radical scavenging for methane flames occurs through the H-atom catalytic cycle.

Part of the effect of the catalytic cycles illustrated in Figure 4 is a suppression of

radical superequilibrium. Figure 5 shows the calculated O-atom mole fraction as a

function of position in stoichiometric CO-H2-air flames with XH2=0.01 and various Xin.

As the inhibitor mole fraction increases, the peak mole fraction decreases; and perhaps

more important, the amount of superequilibrium decreases, providing fewer radicals to

support flame propagation. The H-atom superequilibrium also decreases, partially

through catalytic scavenging, and partially as a result of the fast O, H, and OH shuffle

reactions.

Returning to the results in Figure 2, we examine why the inhibition decreases as

XO2

,ox increases using the technique of reaction flux analysis. From the numerical

results, we calculate the reaction flux ( )

∫

=

2

1

,

t

t

i k ik i F n q RT P dt where Fi,k is the flux of

species k through reaction i, νik is the stoichiometric coefficient for species k in reaction i,

qi is the rate of progress of reaction i, R is the universal gas constant, T is the local gas

temperature, P is the pressure, and t1 and t2 are the time bounds of integration. The

distance coordinate at the j-th grid point (xj) is converted to time via

tj = tj-1 + (xj - xj-1) ρ / m& ′′ , where ρ is the gas density, m& ′′ is the mass flow rate per unit

area. The rate of progress qi is defined as ∏ [ ] ∏ [ ]

= =

−

K

k

r k

K

k

f k

ik

i

ik

i

k X k X

1 1

' ''

n n

, where kf

i

 and kr

i

are the forward and reverse rate constants of reaction i; [Xk] is the molar concentration

of species k; nik′ and nik′′ are the forward and reverse stoichiometric coefficients for

species k in reaction i; and K is the total number of species. The start of the integration

(t1) is the cold boundary. The choice of t2 is somewhat arbitrary, and we use the time of

maximum mole fraction of species k. Such a choice for analysis of radical consumption

may seem counterintuitive given that most of the net consumption occurs after the peak

mole fraction. However, the bulk of the chain-branching and inhibition reactions occur

before the point of peak mole fraction, and thus we integrate over the region where the

important flame propagation chemistry occurs. Calculations show that the qualitative

relationships of the fluxes at different flame conditions are not sensitive to the values of

t2 as long as t2 is at or after the peak mole fraction.

We measure the relative importance of the iron reactions by calculating the

fractions of H-atom and O-atom consumption that occur through the most important

iron reactions (the “fractional flux”). For the i-th reaction and the k-th species the

fractional flux fi,k is calculated as f

i k Fi k n

Fn k

N

, , , = / ∑ =1

, where N is the number of

reactions in the chemical mechanism. For the H-atom, the following reactions are

considered: Fe(OH)2+H↔FeOH+H2O, FeOH+H↔FeO+H2, and FeO+H2O↔Fe(OH)2.

For the O-atom, the following reactions are considered: Fe+O2(+M)↔FeO2(+M),

FeO2+O↔FeO+O2, FeO+O↔Fe+O2, FeOH+O+M↔FeOOH+M. These are the reactions

in the cycles of Figure 4; previous modeling results and analysis using the graphical
13

post-processor Xsenkplot [51] have shown them to be the most important inhibition

reactions.

Figure 6 shows the fraction of H-atom and O-atom flux through the catalytic

cycles for three different values of XO2

,ox. At low Xin, the fractional flux is inversely

related to XO2

,ox. As XO2,ox decreases, stronger inhibition may be caused by the larger

fraction of H atoms (and to a lesser extent, O atoms) scavenged through the catalytic

cycles. The inhibition cycles essentially compete with the chain branching reactions,

and hence reduce the overall reaction rate. While most of the inhibition appears to

result from recombination of O atoms, the reaction flux results imply that the difference

in behavior at low and high XO2

,ox is due to the increasing importance of the H-atom

cycle at low XO2

,ox.

The diminished effectiveness of Fe(CO)5 at high Xin may be explained, in part, by

the behavior illustrated in Figure 6. Above 100-200 ppm of Fe(CO)5, the O-atom cycle

saturates. That is, increasing Xin does not lead to a greater fraction of the radicals being

recombined by the iron species. The H-atom cycle saturates at values of Xin only

slightly higher. The saturation explains why the calculated curves in Figure 2 have

decreasing slopes as Xin increases. It is of interest to note that inhibition caused by

Fe(CO)5 diminishes more rapidly in CO flames than CH4 flames. Part of the reason

may be due to the importance of the O-atom cycle for CO flames and the rapid

saturation of this cycle relative to the H-atom cycle. These explanations are obtained

from the calculated results; while saturation might conceivably occur in the actual

flames, it should be emphasized that the experiments show an even greater loss of

effectiveness, and the modeling and experimental results do not agree well at higher

values of Xin. The discrepancy has been attributed to loss of inhibiting species through

condensation [7].

The saturation of catalytic cycles again suggests the existence of limits to

chemical inhibition, as was previously shown for an idealized inhibitor [52]; a similar

saturation was shown for fluoromethanes, which trap H-atoms non-catalytically [53].

Although Fe(CO)5 may have the confounding effect of condensation, the finding of

catalytic cycle saturation has practical implications for fire suppressant design. For

example, a blend of non-condensing chemical fire suppressants might be used to

rapidly drop the radical mole fractions, but the performance of such a blend could be

limited by catalytic cycle saturation. To produce effective blends, it may be

advantageous to combine catalytic agents with those that lower the equilibrium mole

fraction of radicals, either thermally or chemically [54].

To further explore the importance of the various reactions, we calculate firstorder sensitivity coefficients of the burning velocity to the reaction rates [33,55]. Figure

7 shows the sensitivity coefficients for the three different values of XO2

,ox with 50 ppm of

Fe(CO)5. The calculations show that among the reactions in the iron mechanism, the

reactions in the O-atom and H-atom catalytic cycles are most important for reducing

burning velocity. As the oxygen concentration increases, the sensitivity coefficients of

the reactions in the catalytic cycles decrease.
14

Effect of H2 mole fraction

When XO2

,ox is changed, the radical pool and flame temperature are affected. Since the

inhibitor behavior is affected by both of these variables, it is desirable to change the

radical pool without changing the temperature. In CO flames, this can be accomplished

by varying XH2

. Within the limits set by our experimental apparatus, we were able to

stabilize CO flames with XH2

 of 0.002, 0.005, 0.01, and 0.015. An elevated oxygen

content relative to air (24% O2/76% N2) was necessary to achieve this range. Below

XH2=0.002 the flame would not stabilize at a height of 13 mm; above XH2=0.015 the flame

oscillated too much for accurate measurement. Table 5 lists calculated maximum mole

fraction and amount of superequilibrium for O, H, and OH for CH4-air and CO-H2-O2-

N2 flames with varying XH2

. As the table shows, the variation of XH2

 results in a change

in maximum H-atom and OH mole fraction of about a factor of 3.5 (approximately

linearly with XH2), and little change in maximum O-atom mole fraction. Note also that

the O-atom superequilibrium is approximately constant with XH2, but that H-atom and

OH superequilibrium increases as XH2 increases.

Comparing the CO flames with a CH4 flame, several features stand out: 1) the

maximum O-atom mole fraction is about four times higher in the CO flames, 2) the Hatom mole fraction is lower in the CO flames (but is approaching the CH4 value as XH2

increases), 3) the maximum OH concentration is 3 to 8 times lower in the CO flames.

These differences allow testing and study of the inhibition mechanism under

significantly different flame conditions.

Figure 8 shows the effect of Fe(CO)5 on flames with different levels of H2 in the

reactants and XO2

,ox=0.24. For reference, experimentally determined normalized

burning velocities for CH4-air flames [7] are also shown. (The slight discontinuity in the

XH2 = 0.015 data (‘x’ symbols) between Xin = 150 and 250 ppm occurs because mass flow

controllers with slightly different calibrations were used for the low Xin and high Xin

measurements.) Compared to methane flames, the effect of small amounts of Fe(CO)5

in CO-H2 flames is weaker for XH2=0.01 and 0.015, about equal for XH2 = 0.005, and

stronger for XH2 = 0.002 (note that the XH2 = 0.002 flame was not stable for Xin > 50 ppm).

A significant finding is that the CH4 curve shows linear behavior longer than the CO

curves, resulting in a lower normalized burning velocity at high Fe(CO)5. This is

surprising since the reduced effect at high Xin is believed to be due primarily to

condensation of iron species. One would expect less condensation in the 120 K hotter

CO flames, and therefore a strong inhibition effect to higher Xin. The opposite behavior

indicated in Figure 8 implies that saturation of the catalytic cycles may be the dominant

cause for reduced effectiveness, or perhaps that the degree of iron-species condensation

is increased in the slower CO flames.

Numerical calculations have been performed for the moist CO flames with

varying XH2, and Figure 9 compares the calculations with measurements of normalized

burning velocity. Similar to the predictions for varying XO2

,ox (Figure 2), at low Fe(CO)5

mole fraction (< 100 ppm), the mechanism reasonably predicts the magnitude of
15

inhibition and dependence on XH2

(although the predicted inhibition is too strong for

higher values of XH2). In addition, the leveling off is not predicted and there is little

difference between the calculations at different XH2

, other than the varying initial slopes;

that is, the point where diminishing effectiveness starts is not predicted by the

mechanism.

Figure 10 presents the fractional H- and O-atom destruction from the iron

species catalytic cycles. The fractional amount of O-atom scavenging is highest in

flames with low XH2, explaining why the hydrogen deficient flames are inhibited more

strongly at low Xin. For Xin above 100 ppm, the fraction of O-atoms consumed through

the iron reactions is around 0.85 for XH2=0.002, but only 0.55 for XH2

=0.015. Figure 10

shows (as does Figure 6), that the O-atom cycle saturates at a lower Fe(CO)5 mole

fraction than the H-atom cycle. The importance of the O-atom cycle in CO flames and

its more complete saturation may be one of the reasons that the normalized burning

velocity levels off at a higher value in the CO flames than the CH4 flame.

Sensitivity coefficients of the burning velocity to selected reaction rates were also

calculated for flames with 50 ppm of Fe(CO)5 and varying XH2. At low hydrogen

content the O-atom cycle reactions have the highest sensitivities, but as XH2 increases,

their sensitivity coefficients decrease. As XH2 increases, the regeneration step

(FeO+H2O) in the H-atom cycle becomes more important, while the other two reactions

become less important.

We also measured normalized burning velocity for rich (φ=1.2) and lean (φ=0.8)

CO-H2 flames with XH2 = 0.01, and found that inhibition is slightly stronger for lean

flames than for rich flames. As in the case of CH4 flames, the dependence on φ is weak.

CONCLUSIONS

In this paper we have presented measurements of the burning velocity of CO-H2-O2-N2

flames with and without the inhibitor Fe(CO)5 over a range of initial H2 and O2 mole

fractions. The inhibited flames had four different H2 concentrations, and three different

O2 concentrations. A numerical model was used to simulate the flame inhibition using

a gas-phase chemical mechanism. Based on the experiments and calculations, we find

1. For uninhibited flames, experimental burning velocities are within 10% of the results

of calculations using the mechanism from Yetter et al. [36] for several different rates

of the CO+OH↔CO2+H reaction. Use of the rate of Yu et al. [38] results in excellent

agreement with our data, whereas use of the 1992 recommendation of Baulch et al.

[39] results in predicted burning velocities that are significantly higher than the

measurements.

2. For inhibited flames, the calculations and experiments show good qualitative

agreement. The degree of quantitative agreement depends strongly on the rates of

the CO+OH↔CO2+H reaction, the reactions in the H-atom cycle, and the

FeO2+O↔FeO+O2 reaction.
16

3. Most of the chemical inhibition occurs through a catalytic recombination cycle that

converts O atoms into O2 molecules. This cycle was not found to be important in the

previous numerical study of methane flames. The H-atom cycle that caused most of

the radical scavenging in the methane flames is also active in the CO flames

described here, but is of secondary importance.

4. Reaction flux calculations show that when XH2 is varied, the fractional flux of O

atom through key iron reactions is proportional to the experimentally measured

degree of inhibition, while the fractional flux of H atom does not change

appreciably. When XO2

,ox is varied, the fractional flux of O atom does not change

appreciably, while the fractional flux of H atom is proportional to the

experimentally measured degree of inhibition.

5. In the calculated results, the O-atom cycle saturates at a relatively low inhibitor

mole fraction (~100 ppm), while the H-atom cycle saturates at a much higher

inhibitor mole fraction (~400 ppm). The earlier saturation of O-atom cycle

compared to the H-atom cycle could be a reason why the CO flames have higher

normalized burning velocity than CH4 flames at high Fe(CO)5 mole fraction.

6. Catalytic cycle saturation has practical implications for fire suppressant design. For

example, catalytic cycle saturation could limit the performance of a blend of

chemical fire suppressants.

There are several practical implications for the suppression of hydrocarbon

flames from the present work. Depending upon the hydrogen content of the fuel, the

catalytic recombination of O atom may be as important as catalytic recombination of H

atom and should be considered. Catalytically-acting agents may be useful only up to a

certain mole fraction; above that value, additional agent may prove far less effective.

Finally, if elimination of radical superequilibrium is readily achieved by a catalyticallyacting agent, further reduction in overall reaction rate may require additives which

lower the equilibrium values of radicals, either thermally or chemically.

The inhibition mechanism qualitatively reproduces the dependence of flame

inhibition on hydrogen and oxygen concentration at low inhibitor loading, but several

questions remain unanswered. The most critical one is why the normalized burning

velocity levels off as the Fe(CO)5 concentration increases. Results in the present paper

imply that saturation of the catalytic cycles may be important. This suggestion is in

addition to the strong arguments for the importance of condensation of active iron

compounds in the flame presented in previous work. In light of the present results,

there is an even greater need for measurements of particles in flames inhibited by

Fe(CO)5 in order to understand the reason for the loss of effectiveness of this very

powerful flame inhibitor. Of course, measurement of the rates of key iron reactions,

especially the reactions in the catalytic cycles, is also an important area for future work.

We are grateful to undergraduate student researcher Nikki Prive for assistance with data
17

acquisition and uncertainty analysis programs, and Dr. Rich Yetter for discussion of the

chemical kinetics of CO oxidation.

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21

FIGURE AND TABLE CAPTIONS

Figure 1: Comparison of measured and calculated burning velocities of stoichiometric

CO-H2-air flames with varying amounts of H2 and four different rates for

CO+OH↔CO2+H. Also shown are Palmer and Seery’s measurements [9].

Figure 2: Measured and calculated inhibition effect of Fe(CO)5 for various O2

concentrations in stoichiometric CO-H2-O2-N2 flames with XH2=0.01.

Figure 3: Comparison of experimental data with calculated results using 4 mechanism

variations: 2 rates for CO+OH↔CO2+H and 2 rates for the catalytic H-atom

scavenging cycle. Dotted lines use the slower rates for the reactions in the Hatom cycle; the solid lines, the faster rates. φ=1.0, XO2

,ox=0.21, XH2

=0.01.

Figure 4: Schematic diagram of reaction pathways in CO-H2-O2-N2 flames based on the

gas-phase mechanism of Ref. [8]. Thicker arrows correspond to higher reaction

flux. Reaction partners are listed next to each arrow.

Figure 5: Profiles of O-atom mole fraction at different Fe(CO)5 concentrations. φ=1.0,

XO2

,ox=0.21, XH2

=0.01. The equilibrium value of XO is 0.0014 for the uninhibited

flame and 0.0011 for the inhibited flames.

Figure 6: Calculated fractional H-atom and O-atom flux through key iron reactions at

XH2 = 0.01 and varying O2 concentrations.

Figure 7: First-order sensitivity coefficients of the burning velocity with respect to

reaction rate for selected reactions at Xin= 50ppm, normalized by the peak value

for all reactions at all flame locations (which corresponds to CO+OH↔CO2+H ).

Figure 8: Measured inhibition effect of Fe(CO)5 for various H2 mole fractions in CO-H2-

O2-N2 flames with φ=1.0 and XO2

,ox = 0.24. For comparison, the normalized

burning velocity profile for a φ=1.0, XO2

,ox=0.24 CH4-air flame [7] is also shown.

The uninhibited burning velocities used for the normalizations are listed in the

Table 3. Polynomial curve fits of the data are provided for clarity.

Figure 9: Calculated and measured normalized burning velocities for flames inhibited

by Fe(CO)5 for various H2 concentrations. φ=1.0 and XO2

,ox = 0.24.

Figure 10: Calculated fractional H-atom and O-atom flux through iron reactions at

XO2

,ox = 0.24 and varying H2 concentrations.

Table 1: Measured burning velocity of uninhibited stoichiometric CO-H2-air flames

with H2 content varied.

Table 2: Measured and calculated burning velocity of uninhibited stoichiometric COH2-O2-N2 flames with varying oxygen content and varying hydrogen content.

Table 3: Comparison of the rates for CO+OH↔CO2+H used in Figure 1. The rate

expression and the forward rate kf =A Tb

 exp(-(Ea)/RT) at several temperatures

are presented. Units for k are cm, mole, s.
22

Table 4: Calculated maximum mole fraction and superequilibrium mole fraction of O,

H, and OH, and maximum temperature for three different XO2

,ox for the

uninhibited flames of Figure 2.

Table 5: Calculated maximum O, H, and OH mole fraction and superequilibrium;

maximum temperature; and calculated burning velocity for four different

values of XH2. Also shown for comparison are the same parameters for a CH4-

O2-N2 flame. (XO2

,ox = 0.24 and φ=1 for all flames shown.)
23

Table 1: Measured burning velocity of uninhibited stoichiometric CO-H2-air flames

with H2 content varied.

XH2vo,exp

(cm/s)

XH2vo,exp

(cm/s)

XH2vo,exp

(cm/s)

0.0024 22.5 ± 0.8 0.0050 29.8 ± 0.8 0.0091 38.0 ± 1.1

0.0030 24.8 ± 0.7 0.0060 30.9 ± 0.9 0.0101 39.7 ± 1.0

0.0033 25.7 ± 0.7 0.0061 32.1 ± 0.8 0.0101 39.1 ± 1.1

0.0036 26.6 ± 0.7 0.0070 33.9 ± 0.9 0.0111 40.6 ± 1.1

0.0039 27.3 ± 0.7 0.0091 38.0 ± 1.1 0.0111 40.7 ± 1.2

0.0040 25.5 ± 0.7 0.0070 33.2 ± 0.9 0.0117 41.9 ± 1.1

0.0040 26.9 ± 0.7 0.0080 36.1 ± 1.0 0.0118 42.6 ± 1.2

0.0045 28.7 ± 0.7 0.0081 35.4 ± 0.9 0.0122 42.5 ± 1.4

0.0050 28.4 ± 0.8 0.0091 37.3 ± 1.0 0.0132 43.7 ± 1.4

Table 2: Measured and calculated burning velocity of uninhibited stoichiometric

CO-H2-O2-N2 flames with varying oxygen content and varying hydrogen content.

XO2

,ox XH2

vo,exp

(cm/s)

vo,num

(cm/s)

0.183 0.01 32.7 ± 0.8 30.5

0.21 0.01 39.6 ± 1.1 35.8

0.24 0.002 25.1 ± 0.6 22.3

0.24 0.005 36.2 ± 0.9 31.2

0.24 0.01 46.2 ± 1.4 41.1

0.24 0.015 59.0 ± 2.4 48.7
24

Table 3: Comparison of the rates for CO+OH↔CO2+H used in Figure 1. The rate

expression and the forward rate kf =A Tb exp(-(Ea)/RT) at several temperatures are

presented. Units for k are cm, mole, s.

kf

A b Ea/R 1000 1500 2000

Baulch et al. (1973) [37] 1.50E+07 1.3 -385 1.75E+11 2.61E+11 3.56E+11

Baulch et al. (1992) [39] 3.25E+10 1.5 -250 2.57E+11 4.34E+11 6.41E+11

Wooldridge et al. [40] 2.12E+12 0 2630 1.53E+11 3.67E+11 5.69E+11

Yu et al. [38] 4.4E+06 1.547 67 1.89E+11 3.56E+11 5.58E+11

Table 4: Calculated maximum mole fraction and superequilibrium mole fraction of

O, H, and OH, and maximum temperature for three different XO2

,ox for the

uninhibited flames of Figure 2.

XO2

,ox

0.183 0.21 0.24

XO,max 0.011 0.014 0.018

XH,max 0.0045 0.0052 0.0058

XOH,max 0.0024 0.0029 0.0035

XO,max-XO,eq 0.01 0.013 0.015

XH,max-XH,eq 0.0043 0.0049 0.0054

XOH,max-XOH,eq 0.00097 0.00095 0.00084

Tmax (K) 2262 2376 2471
25

Table 5: Calculated maximum O, H, and OH mole fraction and superequilibrium;

maximum temperature; and calculated burning velocity for four different values of

XH2. Also shown for comparison are the same parameters for a CH4-O2-N2 flame.

(XO2

,ox = 0.24 and φ=1 for all flames shown.)

XH2

0.002 0.005 0.01 0.015 CH4

XO,max 0.018 0.018 0.018 0.017 0.0044

XH,max 0.002 0.0037 0.0058 0.0074 0.0085

XOH,max 0.0014 0.0024 0.0035 0.0044 0.011

XO,max-XO,eq 0.016 0.016 0.015 0.015 0.0037

XH,max-XH,eq 0.0018 0.0034 0.0054 0.0068 0.0076

XOH,max-XOH,eq 0.00024 0.0005 0.00084 0.00111 0.0049

Tmax (K) 2471 2468 2471 2475 2353

vo,num (cm/s) 22.3 31.2 41.1 48.7 55.6
26

0

10

20

30

40

50

0 2000 4000 6000 8000 10000 12000 14000

H2 Mole Fraction x 106

Burning Velocity (cm/s)

Palmer & Seery [9] 

Baulch et al. (1973) [37]

Baulch et al. (1992) [39]

Wooldridge et al. [40]

Yu et al. [38]

Figure 1: Comparison of measured and calculated burning velocities of

stoichiometric CO-H2-air flames with varying amounts of H2 and four different rates

for CO+OH↔CO2+H. Also shown are Palmer and Seery’s measurements [9].
27

0.0

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400 500

Fe(CO)5 (ppm)

0.21

X = 0.183 O2,ox

0.24

Normalized Burning Velocity

Figure 2: Measured and calculated inhibition effect of Fe(CO)5 for various O2

concentrations in stoichiometric CO-H2-O2-N2 flames with XH2=0.01.
28

0.0

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400 500

Fe(CO)5 (ppm)

Baulch et al.

(1973) [37]

Yu et al. [38]

Normalized Burning Velocity

Figure 3: Comparison of experimental data with calculated results using 4

mechanism variations: 2 rates for CO+OH↔CO2+H and 2 rates for the catalytic Hatom scavenging cycle. Dotted lines use the slower rates for the reactions in the Hatom cycle; the solid lines, the faster rates. φ=1.0, XO2

,ox=0.21, XH2

=0.01.
29

Fe(CO)5

Fe

FeO2

FeO

Fe(OH)2

FeOH +O

+H

+H2O

+H

+O2 + M

+O

O-atom cycle H-atom cycle

Figure 4: Schematic diagram of reaction pathways in CO-H2-O2-N2 flames based on

the gas-phase mechanism of Ref. [8]. Thicker arrows correspond to higher reaction

flux. Reaction partners are listed next to each arrow.
30

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

x (mm)

0 ppm

150

50

300

O-Atom Mole Fraction

100

Figure 5: Profiles of O-atom mole fraction at different Fe(CO)5 concentrations. φ=1.0,

XO2

,ox=0.21, XH2

=0.01. The equilibrium value of XO is 0.0014 for the uninhibited

flame and 0.0011 for the inhibited flames.
31

0.0

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400 500 600

Fe(CO)5 (ppm)

Fraction of O-atom or H-atom Destruction 

through Fe Reactions

0.24

0.183

0.21

0.21

0.183

H atom

O atom

X = 0.24 O2

,ox

Figure 6: Calculated fractional H-atom and O-atom flux through key iron reactions at

XH2 = 0.01 and varying O2 concentrations.
32

-0.120 -0.100 -0.080 -0.060 -0.040 -0.020 0.000 0.020 0.040

Normalized Sensitivity Coefficient

H-atom 

cycle

O-atom

cycle

Fe+O2(+M)=FeO2(+M)

FeO2+O=FeO+O2

Fe+O2=FeO+O

FeO+H2O=Fe(OH)2

FeOH+H=FeO+H2

Fe(OH)2+H=FeOH+H2O

FeOH+O+M=FeOOH+M

Fe+O+M=FeO+M

FeOOH+OH=FeO2+H2O

FeO+H2=Fe+H2O

FeOOH+O=FeO2+OH

FeOH+O=FeO+OH

X = 0.183 O2

,ox

X = 0.21 O2

,ox

X = 0.24 O2

,ox

Figure 7: First-order sensitivity coefficients of the burning velocity with respect to

reaction rate for selected reactions at Xin= 50ppm, normalized by the peak value for

all reactions at all flame locations (which corresponds to CO+OH↔CO2+H ).
33

0.0

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400 500 600 700

Fe(CO)5 (ppm)

CH4-air, φ = 1.0

X = 0.015 H2

0.01

0.005

0.002

Normalized Burning Velocity

Figure 8: Measured inhibition effect of Fe(CO)5 for various H2 mole fractions in COH2-O2-N2 flames with φ=1.0 and XO2

,ox = 0.24. For comparison, the normalized

burning velocity profile for a φ=1.0, XO2

,ox=0.24 CH4-air flame [7] is also shown. The

uninhibited burning velocities used for the normalizations are listed in the Table 2.

Polynomial curve fits of the data are provided for clarity.
34

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 100 200 300 400 500

Fe(CO)5 (ppm)

0.01 

0.005

0.002

X = 0.015 H2

0.01 

0.005

0.015

Normalized Burning Velocity

Figure 9: Calculated and measured normalized burning velocities for flames

inhibited by Fe(CO)5 for various H2 concentrations. φ=1.0 and XO2

,ox = 0.24.
35

0.0

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400 500 600

Fe(CO)5 (ppm)

0.01

0.005

0.015

H atom

O atom

X = 0.002 H 2

0.01

0.005

0.015

0.002

Fraction of O-atom or H-atom 

Destruction through Fe Reactions

Figure 10: Calculated fractional H-atom and O-atom flux through iron reactions at

XO2

,ox = 0.24 and varying H2 concentrations.