Inhibition of Premixed Carbon

Monoxide-Hydrogen-Oxygen-Nitrogen Flames by Iron

Pentacarbonylt

MARC D. RUMMINGER+ AND GREGORY T. LINTERIS*

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

Gaithersburg, MD 20899, USA

This paper presents measurements of the burning velocity of premixed CO-HrOrNz flames with and without

the inhibitor Fe(CO)s over a range of initial Hz and Oz 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 <-> COz + 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 Oz 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-Hz flames, but is of secondary

importance. To vary the role of the H- and O-atom radical pools, the experiments and calculations are

performed over a range of oxygen and hydrogen mole fraction. The degree of inhibition is shown to be related

to the fraction of the net H- and O-atom destruction through the iron species catalytic cycles. The O-atom cycle

saturates at a relatively low inhibitor mole fraction (-100 ppm), whereas the H-atom cycle saturates at a much

higher inhibitor mole fraction (-400 ppm). The calculations reinforce the previously suggested idea that

catalytic cycle saturation effects may limit the achievable degree of chemical inhibition. Published by Elsevier

Science Inc.

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 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)s, 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)s is toxic and flammable, understanding its inhibition mechanism could provide

insight into the behavior of other highly effec-

'Corresponding author. E-mail: linteris@nist.gov

tOfficial contribution of the National Institute of Standards

and Technology, not subject to copyright in the United

States.

'National Research Council/NIST postdoctoral fellow,

1996-1998. Current address: Sandia National Laboratory,

MS 9052, Livermore, CA 94551.

COMBUSTION AND FLAME 120:451-464 (2000)

Published by Elsevier Science Inc.

tive agents and aid in the development of new

nontoxic agents.

In recent years, several detailed studies of

Fe(CO)s 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)s the effect was roughly linear, but

above a certain loading the effect was nearly

independent of the Fe(CO)s 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)s

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

001O-2180/00/$-see front matter

PH SOOlO-2180(99)00114-5
452

reactions1 (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)s in CO-H2-02-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 0 atom is four times higher in

stoichiometric CO-H2-air flames (containing

between 0.2 and 1.0% H2) than a stoichiometric

CHcair flame, whereas 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]2 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 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 the effectiveness of iron pentacarbonyl

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

'The reactions and increased rates were FeO + H20 - Fe(OHh (kf = 1.62E + 13, increased by 3x), FeOH + HFeO + Hz [kf = 1.50E + 14 exp( -805fT), increased by 5x),

Fe(OHh + H - FeOH + HzO [kf = 1.98E + 14

exp(-302fT), increased by 3x).

2To avoid excessive repetition, references to previous calculations of Fe(CO)s-inhibited methane flames refer to Ref. 8

unless otherwise noted.

M. D. RUM MINGER ET AL.

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 Hr02 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(COh on the overall

reaction rate of premixed flames. The laminar

burning velocity is a widely used measure of the

effectiveness of chemical inhibitors, and allows

straightforward comparison with the numerically calculated flame structure, providing useful insight into the mechanism of inhibition.

Our study of flame inhibition is intended to

provide an understanding of 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 release of the flame. For

chemically acting agents, the agent interferes

with the reactions that consume the fuel and

intermediates. Hence, we can study the effect of

small agent concentrations on flame chemistry

and build chemical kinetic models that 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 that are

more representative of fires. Finally, an understanding of the modes of action of effective

agents such as Fe(CO)s can lead researchers to

chemicals that have similar favorable properI", ill I Ii 11
INHIBITION OF CO FLAMES BY IRON PENTACARBONYL 453

ties, 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-HzOz-Nz flames with varying amounts of Hz, 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-Hz-Oz-Nz flames with Hz mole fractions

between 1.50· 10-4 and 2.0 . lO-z.

In the remainder of the paper, we describe

the experimental technique, the numerical calculation procedure, and the results for uninhibited and inhibited CO-Hz-Oz-Nz 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)3 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

HzO < 10 ppm) and hydrogen (Matheson

UHP, 99.999% Hz, with sum of Nz, Oz COz,

CO, Ar, CH4, and HzO < 10 ppm); the oxidizer

3Certain 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.

stream consists of nitrogen (boil-off from liquid

Nz) and oxygen (MG Industries, HzO < 50

ppm, total hydrocarbons < 5 ppm). The experimental arrangement has been described in detail 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 two-stage

saturator maintained in an ice bath at O°c. The

diverted gas (less than 8% of the total flow)

bubbles through liquid Fe(CO)s before returning to the main nitrogen flow, and is assumed to

be saturated with Fe(CO)s vapor. Although the

saturator is the same as used previously, the

assumption of saturation has been verified by

flowing the Fe(CO)s-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)s] 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)s 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

CID371OD). The image is digitized by a 640 X

480 pixel frame-grabber 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
454

TABLE 1

Measured and Calculated Burning Velocity of

Uninhibited Stoichiometric CO-H2-Oz-N2 Flames with

Varying Oxygen Content and Varying Hydrogen Content

vo,num V o,exp

XH2 (cm/s) (cm/s)

32.7 0.01 ±30.58

39.6 0.01 ±35.8 1.1

25.1 0.002±22.3 06

36.2 0.005±31.2 09

46.2 0.01 ±41.1 1.4

59.0 0.015±48.7 24

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)s 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 kuo and is

determined from a combined standard uncertainty (estimated standard deviation) uc' and a

coverage factor k = 2 (level of confidence

approximately 95%). Likewise, when reported,

the relative uncertainty is U / X . 100%, or

kuc / X . 100%. A brief summary of the uncertainty results is presented below; an expanded

discussion can be found in Ref. 29.

M. D. RUMMINGER ET AL.

The primary sources of uncertainty in the

average burning velocity measurement are (l)

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 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. The expanded relative uncertainties for

parameters related to the accuracy of the flow

controllers are as follows: 1.4% for equivalence

ratio; and 1.1%, 1.2%, and 6.5% for the inlet

reactant mole fractions of Oll Hz, and Fe(CO)s,

respectively. For the latter, we use a vapor

pressure correlation from Ref. 30 (which does

not report an uncertainty). Based on the manufacturer's analysis of the bottled gases, the

estimated background hydrogen in the reactant

stream is less than 75 ppm.

NUMERICAL MODEL

One-dimensional freely propagating premixed

flames are simulated using the Sandia flame

code Premix [31], the Chemkin subroutines

[32], and the transport property subroutines

[33]. 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 1 atmosphere. The

kinetic mechanism and thermodynamic data of

Yetter et al. [34] (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 +-i>

COz + H from Baulch et al. [35] because

numerical calculations indicated that most of

the CO oxidation was occurring below 2000 K

(temperature above which a different rate expression was recommended [34]). Iron pentacarbonyl is added to the unburned CO-HzI i I ,I I 1,1· 1',1 -I- , ~ I i
INHIBITION OF CO FLAMES BY IRON PENTACARBONYL 455

Uninhibited Flames

RESULTS AND DISCUSSION

o

o 2000 4000 6000 8000 1??oo 12000 14000

H2 Mole Fraction x 106

several different sets of rate constants for CO +

OH ~ COz + H [35-38]. Table 3 lists calculated forward rate constants for each formulation at several temperatures. The 1992 rate

expression of Baulch et aI. [37] 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 [39], 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 + 0 + M. At

XH2 = 0.015 the split is 88% through CO + OH

and 10% through CO + 0 + M.

TABLE 2

Measured Burning Velocity of Uninhibited Stoichiometric CO-Hz-Air Flames with Hz Content Varied vo,exp

V o,exp

XH2

(cm/s) (cm/s) XH2

0.0024 22.5:!: 0.8 0.0050 29.8:!: 0.8 0.0101 39.7:!: 1.0

0.0030 0.0033 24.8:!: Z5.7:!: 0.7 0.7 0.0060 0.006] 30.9:!: 0.9 0.0101 39.1:!: 1.1

32.] :!:0.8 O.O1ll 40.6:!: 1.1

0.0036 26.6:!: 0.7 0.0070 0.00504 0.00 33.9:!: 0.004539 0.925.5:!: 28.4:!: 28.7:!: 26.9:!: 27.3:!: 0.0111 0.78 40.7:!: 0.00819 0.00708 0.0091 1.237.3:!: 33.2:!: 36.1:!: 38.0:!: 35.4:!: 0.9 10.1 0.013218 0.01722 43.7:!: 41.9:!: 42.5:!: 42.6:!: 1.241

"" Baulch at al. (1973) [35]

10 -I- \almer & Seery [9]

50

40

:F

E

.!!.

~ 30

'g

~

g' 20

'E

&l

Oz-Nz 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.

Fig. 1. Comparison of measured and calculated burning

velocities of stoichiometric CO-Hz-air flames with varying

amounts of Hz and four different rates for CO + OH ....,.

COZ + H. Also shown are Palmer and Seery's measurements [9].

Figure 1 shows the measured burning velocities

for CO-Hz-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 (XH,) 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 Hz.

Figure 1 also shows the calculated burning

velocity using the mechanism from Ref. 34 with

Inhibited Flames

EtTect of O2 Mole Fraction

Figure 2 presents measurements (symbols) and

calculations (lines) of the burning velocity of

Fe(CO)s-inhibited CO-Hz-Oz-Nz flames with

varying amounts of oxygen. As previously seen

for CHcair flames inhibited by Fe(COh, inhibition is strongest in the flames with the lowest

XQ2,ox (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,
456 M. D. RUMMINGER ET AL.

TABLE 3

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

kf

Ea/R b1000200 15

-385 1.7E2.61E 13 3.56E + 11 + 11 1.50E + 07

2.57E -250 4.34E 641 1.5+1 + 11 3.25E + 10

1.53E 2630 3.67E 0+5.69E11 + 11 2.12E + 121.89E67+1.547 3.56E 58 11 + 11 4.4E + 06

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

are em, mole, s.

additional Fe(CO)s has little effect on the burning velocity. A plausible (but untested) explanation is that particle formation removes the

inhibiting species at high inhibitor loading [7].

The experimental results in Fig. 2 support this

suggestion: as the flame temperature increases,

the leveling-off point shifts to higher Fe(CO)s

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)s' Table 4 lists the peak

temperature and peak mole fraction of 0, R,

and OR in uninhibited flames with X02.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)s, yielding the smaller slope

in Fig. 2. (A detailed analysis of the effect of

X02,ox is undertaken later in this paper.)

The calculated normalized burning velocities

(shown by the lines in Fig. 2) reproduce the

trend of greater inhibition at lower X02,ox for

lowXim 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 X02,ox' as well as

other features. Further, results at higher values

of Xin can be used to investigate the relevant

gas-phase chemistry predicted by the model if

there was not a loss of the active species to a

condensed phase.

Fig. 2. Measured and calculated inhibition effect of

Fe(CO)s for various O2 concentrations in stoichiometric

CO-Hz-Oz-Nz flames with XH, = 0.01.

1.0

~ 0.8 o

;;; >

~ 0.6

.~III

...

~ 0.4

~o

z a'

0.0

a 100

_-+--_~ ---+---J 200 300 400 50(

Fe(CO), (ppm)

TABLE 4

Calculated Maximum Mole Fraction and

Superequilibrium Mole Fraction of 0, H, and OH, and

Maximum Temperature for Three Different Xo"ox for the

Uninhibited Flames of Fig. 2

Xo"ox

0.183

0.24 0.21

Xo,max

0.0148 0.011

XH,max

0.00582 0.0045

XoH,max

0.002935 0.0024

Xo,max -Xo,eq

0.D15 0.013 0.01

XH,max -XH,eq

0.004954 0.0043

XOH.max-XOH,cq

0.00095 0.00084 0.00097

Tmax (K)

237641 2262

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

Fig. 3. Comparison of experimental data with calculated

results using four mechanism variations: two rates for CO +

OH - CO2 + H, and two rates for the catalytic H-atom

scavenging cycle. Dotted lines use the slower rates for the

reactions in the H-atom cycle; the solid lines, the faster

rates. cP = 1.0, X02,ox = 0.21, XH, = 0.01.

Catalytic Inhibition Cycles

In previous research on bromine-containing

species [41, 42], conversion of radicals (e.g., H)

into less reactive species (e.g., Hz) has been

found to be the dominant mode of action for

of CO + OH near 1800 K. Although the rate of

Yu et al. [36] provides better agreement with

our experimental data (Fig. 1), we retain the

1973 Baulch et al. [35] rate since it has been

extensively tested against a variety of experimental data from flames, flow reactors, and

shock tubes [34]. For the inhibition reactions,

the premixed methane flame results suggest use

of the faster rates for the H-atom cycle. In Fig.

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

[40] for the reaction FeO + HzO + He(Nz) ~

Fe(OHh + He(Nz), 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 selected for the H-atom

cycle, the qualitative behavior is not, and the

conclusions of this paper are unaffected by

these rates.

500

... -.

400

Baulch et al.

(1973)[35]

.... -..

200 300

Fe(CO), (ppm)

100

1.0

0.0

o

!l 0.8 ..

o

a;

>

~ 0.6

C

~III

~ 0.4

7ii

E

o

Z 0.2

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 H-atom cycle recommended by Jensen and

Jones [17]) were increased (within experimental

uncertainty) to better predict the experimental

results. In Fig. 2, the model shows slightly more

inhibition for XQ2,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 H-atom cycle reactions for the analyses in the present paper.

As described in a preceding section and Fig.

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)s 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. [35] CO + OH rate recommended by Yetter et al. [34] and the faster rates

for the H-atom cycle reactions from Ref. 8. As

illustrated in the NIST Chemical Kinetics Database [39], there is large uncertainty in the rate
458 M. D. RUMMINGER ET AL.

Fig. 4. Schematic diagram of reaction pathways in CO-HzOz-Nz 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.

Fe(CO)s

1

Saturation of Catalytic Cycles

Returning to the results in Fig. 2 , we examine

why the inhibition decreases as X02,ox increases

using the technique of reaction flux analysis.

Fe ~ / Fe(OH},

WMJ~F~~F1H

O-atom cycle H-atom cycle

Fe02 and then to FeO. A portion of the FeO is

converted back to Fe, thus completing the 0-

atom 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 CH4-02-N2 flames.

Instead, reactions (R1 )-(R2) converted Fe to

one of the species in the main 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 Fig. 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 Xm =

0.01 and various X;n- 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 Hatom superequilibrium also decreases, partially

through catalytic scavenging, and partially as a

result of the fast 0, H, and OH shuffle reactions.

chemical inhibitors and suppressants. A similar

H-atom catalytic cycle involving iron oxide and

hydroxide intermediates has been found to be

important in CHC02-N2 flames inhibited by

Fe(CO)s [8]. Reaction pathway and sensitivity

analyses for the inhibited CO flames reveals

that the H-atom catalytic cycle is active. A new

finding, however, is that there is an additional

catalytic cycle which scavenges 0 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 (Table 4), 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 [43]. O-atom recombination by iron compounds has been reported previously [44] for

flow tube experiments in which Fe(CO)s was

added to the reactant gases. Removal of 0-

atom by CF3Br in a flow tube has also been

reported [45]. These previous studies, however,

did not propose a catalytic cycle for O-atom

recombination.

The reactions that comprise the catalytic 0-

atom scavenging cycle are

Fe + O2 + M ~ Fe02 + M (R1)

Fe02 + 0 ~ FeO + O2 (R2)

FeO + 0 ~ Fe + O2 (R3)

o + 0~02'

The first and third reactions in the cycle have

been studied in the laboratory. The rate for

(R1) has been measured by several researchers

[46, 47], and the rate for (R3) has been measured by Fontijn et al. [48]. 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 Fig. 4, where both the H- and

O-atom catalytic cycles are illustrated. Fe(CO)s

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

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

Fig. 6. Calculated fractional H-atom and O-atom flux

through key iron reactions at XHz = 0.01 and varying O2

concentrations.

500 600

0.24

0.21

xo~ox = 0.24

0.21

300 400

Fe(CO), (ppm)

100 200

C

o

'"u:s

~"

21 f! E 0 .s 'il 0.6 ., " :>:&!

oaf

E""

.s '" 0.4 •• :s • 11

~5

C

o

'"u

E

... "--300 o -J-..llLL-t-·---+---.,--+---+----+---.,~- ---+-- +--- o ~1 0.2 0.3 0.4 Q5 0.6 0.7 Q8 0.9

x (mm)

0.002

--

C 0012 ::::~t fAA \

:8 0.01 u

E

...

lD 0.008 '0

~

E 0.006

~

o 0.004

Fig. 5. Profiles of O-atom mole fraction at different

Fe(CO)5 concentrations. cp = 1.0,X02,ox = 0.21,XHz = 0.01.

The equilibrium value of Xo is 0.0014 for the uninhibited

flame and 0.0011 for the inhibited flames.

The flux of a species though a particular reaction is defined as the integral of the reaction

rate over the domain of interest (here taken to

be the cold boundary to the time of peak radical

mole fraction). The total flux of a species is the

sum of fluxes from each reaction, and the

fractional flux is the flux of interest divided by

the total. 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. For

the H-atom, the following reactions are considered: Fe(OHh + H ~ FeOH + HzO, FeOH +

H ~ FeO + Hz, and FeO + HzO ~ Fe(OH)z.

For the O-atom, the following reactions are

considered: Fe + Oz(+ M) ~ FeOz( + M),

FeOz + 0 ~ FeO + Oz, FeO + 0 ~ Fe + Oz,

FeOH + 0 + M ~ FeOOH + M. These are

the reactions in the cycles of Fig. 4; previous

modeling results and analysis using the graphical

postprocessor Xsenkplot [49]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 X02,ox' At low X;m the

fractional flux is inversely related to X02,oX' As

X02,ox decreases, stronger inhibition may be

caused by the larger fraction of H atoms (and to

a lesser extent, 0 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 0 atoms, the reaction

flux results imply that the difference in behavior

at low and high X02,ox is due to the increasing

importance of the H-atom cycle at low X02.ox'

The diminished effectiveness of Fe(CO)s at

high X;n may be explained, in part, by the

behavior illustrated in Fig. 6. Above 100-200

ppm of Fe(CO)s, the O-atom cycle saturates.

That is, increasing X;n 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 Fig. 2 have

decreasing slopes as X;n increases. It is of interest to note that inhibition caused by Fe(CO)s

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 X;n' 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 (501;a similar saturation was shown for

fluoromethanes, which trap H-atoms noncata-
460 M. D. RUM MINGER ET AL.

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

Normalized Sensitivity Coefficient

FeOH+O=FeO+OH

FeOOH+O=Fe02+0H FeOOH+OH=Fe02+H20 Fe(OHh+H=FeOH+Hp FeOH+O+M=FeOOH+M Fe+O+M=FeO+M FeOH+H=FeO+H2 FeO+H2=Fe+H2O

IH-atom

cycle

FeO+H20=Fe(OHh

Fe02+0=FeO+02 Fe+02=FeO+O

lo-atom

cycle Fe+02(+M)=Fe02(+M)

I

I

0.020

0.040

"0,. y: ox = 0.183

Xo,.ox = 0.21

''0,. )( ox = 0.24

•

•

o

Fig. 7. First-order sensitivity coefficients of the burning velocity with respect to reaction rate for selected reactions at Xin =

50 ppm, normalized by the peak value for all reactions at all flame locations (which corresponds to CO + OH - CO2 + H).

Iytically [51]. Although Fe(CO)s may have the

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 [52,

53].

To further explore the importance of the

various reactions, we calculate first-order sensitivity coefficients of the burning velocity to the

reaction rates [31, 54]. Figure 7 shows the

sensitivity coefficients for the three different

values of XQ2,OX with 50 ppm of Fe(COk The

calculations show that among the reactions in

the iron mechanism, the reactions in the 0-

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.

Effect of 92 Mole Fraction

When XQ2,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 varyingXm.

Within the limits set by our experimental apparatus, we were able to stabilize CO flames with

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

oxygen content relative to air (24% Oz/76% Nz)

was necessary to achieve this range. Below Xm

= 0.002 the flame would not stabilize at a height

of 13 mm; above Xm = 0.015 the flame oscillated too much for accurate measurement. Table 5 lists calculated maximum mole fraction

and amount of superequilibrium for 0, H, and

OH for CH4-air and CO-Hz-Oz-Nz flames

with varying Xm. As the table shows, the variation of Xm results in a change in maximum

H-atom and OH mole fraction of about a factor

of 3.5 (approximately linearly with XH), and

little change in maximum a-atom mole fraction.

Note also that the a-atom superequilibrium is

approximately constant with XHz' but that HII" ill II i 1,1 I I
INHIBITION OF CO FLAMES BY IRON PENTACARBONYL

TABLE 5

461

Calculated Maximum 0, H, and OH Mole Fraction and Superequilibrium; Maximum

Temperature; for Calculated Burning Velocity for Four Different Values of XH:

XHz 

02

0.0051 0.01 CH4

0.018 0.0187 0.0044 0.018

0.0058 0.003774 0.0085 0.002

0.0035 0.00244 0.011 0.0014

0.015 0.0165 0.0037 0.016

0.0054 0.003468 0.0076 0.0018

0.00084 0.0500111 0.0049 0.00024

2471 246875 2353 247141.1 31.2 487 55.6 22.3

a Also shown for comparison are the same parameters for a CHcOz-Nz flame. (Xo,.ox = 0.24 and <P = 1 for all flames

shown.)

Fig. 8. Measured inhibition effect of Fe(CO)s for various

Hz mole fractions in CO-Hz-Oz-Nz flames with <P = 1.0

andX02•ox = 0.24. For comparison, the normalized burning

velocity profile for a <p = 1.0, X02.ox = 0.24 CHcair flame

[7J is also shown. The uninhibited burning velocities used

for the normalizations are listed in Table 1. Polynomial

curve fits of the data are provided for clarity.

~tom and OH superequilibrium increases asXHz

mcreases.

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 H-atom mole fraction

is lower in the CO flames (but is approaching

the CH4 value as XHz 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)s on

flames with different levels of Hz in the reactants and X02,ox = 0.24. For reference, experi0.8

~

~

1:

g' 0.6

~al

J 0.4

~o

z 0.2

0.0

o 100 200

CHor-air. +'" 1.0

300 400

Fo(CO), (ppm)

0.01

~;;:0.Q15

800 700

mentally determined normalized burning velocities for CHcair flames [7] are also shown. (The

slight discontinuity in the XHz = 0.015 data ("x"

symbols) between X;n = 150 and 250 ppm

occurs because mass flow controllers with

slightly different calibrations were used for the

lowXin and high Xin measurements.) Compared

to methane flames, the effect of small amounts

of Fe(CO)s in CO-Hz flames is weaker for

XHz = 0.01 and 0.015, about equal for XH, =

0.005, and stronger for XHz = 0.002 (note that

the XHz = 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)s. 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 Fig. 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

Fig. 9 compares the calculations with measurements of normalized burning velocity. Similar to

the predictions for varyingXo2,ox (Fig. 2), at low

Fe(CO)s mole fraction « 100 ppm), the mechanism reasonably predicts the magnitude of

inhibition and dependence onXH2 (although the
462 M. D. RUMMINGER ET AL.

Fig. 10. Calculated fractional H-atom and O-atom flux

through iron reactions at X02,ox = 0.24 and varying Hz

concentrations.

Fig. 9. Calculated and measured normalized burning velocities for flames inhibited by Fe(CO)s for various Hz concentrations. cp = 1.0 and XOz,ox = 0.24.

fraction than the H-atom cycle. The importance

of the a-atom cycle in CO flames and its more

complete saturation may be one of the reasons

that the normalized burning velocity levels off at

a higher value in the CO flames than the CH4

flame.

Sensitivity coefficients of the burning velocity

to selected reaction rates were also calculated

for flames with 50 ppm of Fe(CO)s. and varying

XHz' At low hydrogen content the O-atom cycle

reactions have the highest sensitivities, but as

Xm increases, their sensitivity coefficients decrease. As XHz increases, the regeneration step

(FeO + H20) in the H-atom cycle becomes

more important, while the other two reactions

become less important.

We also measured normalized burning velocity for rich (cP = 1.2) and lean (cP = 0.8) CO-H2

flames with XHz = 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 cP is weak.

In this paper we have presented measurements

of the burning velocity of CO-H2-02-N2 flames

with and without the inhibitor Fe(CO)s 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 reach the following conclusions:

For uninhibited flames, experimental burning

velocities are within 10% of the results of

calculations using the mechanism from Yetter

et al. [34]for several different rates of the CO +

OH ~ CO2 + H reaction. Calculated burning

velocities using the rate of Yu et al. [36] agree

closely with our data, whereas those using the

1992 recommendation of rate from Baulch et al.

[37] are significantly higher than the measurements.

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 +

CONCLUSIONS

500

600

0.015

0.01

500

400

0.015

0.005

0.01

0.005

200 300

Fe(CO), (ppm)

200 300 400

Fe(CO), (ppm)

XHz = 0.002

100

100

1.0

.

c 0.8 E:8~:J:d! ~.fII

0.6 E.c ~!l' o e ~ti o c 0.4 '"c

0",0u

~~ 0.2

...• c

predicted inhibition is too strong for higher

values of XH). In addition, the leveling off is not

predicted and there is little difference between

the calculations at different XHz' 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 XHz'

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 XHz = 0.002, but only 0.55 for XHz = 0.015.

Figure 10 shows (as does Fig. 6) that the

O-atom cycle saturates at a lower Fe(CO)s mole

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

H reaction, the reactions in the H-atom cycle,

and the FeOz + 0 ~ FeO + Oz reaction.

Most of the chemical inhibition occurs

through a catalytic recombination cycle that

converts 0 atoms into Oz molecules. This cycle

was not found to be important in the previous

numerical study of methane flames. The Hatom 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.

Based on reaction flux analyses, the degree of

inhibition by Fe(CO)s is shown to be related to

the fraction of the total 0- and H-atom destruction which is attributable to iron species reactions.

In the calculated results, the O-atom cycle

saturates at a relatively low inhibitor mole fraction (~100 ppm), whereas the H -atom cycle

saturates at a much higher inhibitor mole fraction (~400 ppm). The 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)s mole fraction.

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 0 atom may be as important as catalytic

recombination of H atom and should be considered. Catalytically acting agents (singly or in

blends) 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

catalytically acting agents, further reduction in

overall reaction rate may require additives that

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)s concentration increases. Because the

leveling-off point changes with varying Xm, but

the flame temperature does not change, there is

less reason to believe that condensation is the

cause; results in the present paper imply that

saturation of the catalytic cycles may be important. While both radical saturation and iron

species condensation are likely operative, it is

not possible with the present data to separate

the effects. Hence, there is an even greater need

for measurements of particles in flames inhibited by Fe(CO)s 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

acquisition and uncertainty analysis programs,

and Dr. Rich Yetter for discussion of the chemical

kinetics of co oxidation.

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Received 30 March 1999; revised 28 July 1999; accepted I1

August 1999

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