Spectroscopy of Inhibited Counterflow Diffusion Flames

R. G. Daniel, K. L. McNesby, R. R. Skaggs, P. Sagear, and A. W. Miziolek

U.S. Army Research Laboratory

Weapons and Materials Research Directorate

AMSRL - WM - PC

Aberdeen Proving Ground, MD 21005-5066

ABSTRACT

As part of the continuing effort to fmd an effective, yet safe, replacement compound for the commonly

used halon fIre suppression agents, we have begun a study of reduced pressure counterflow diffusion flames

inhibited with iron pentacarbonyl (Fe(CO)s)' Iron pentacarbonyl has been shown to be a very effective fIre

suppression agent in small quantities, but there is little understanding to the mechanism of its activity.

Unfortunately, Fe(CO)s is highly toxic and thus cannot be used in occupied spaces. The goal of this study is to

understand the mechanism by which Fe(CO)s inhibits flames by coupling experimental measurements with

computer modeling of diffusion flames. Presented here are results from the spectroscopic examination of

uninhibited and inhibited counterflow diffusion flame using emission spectroscopy of active flame species at

reduced pressure. The species studied include OH, CH, C2, and iron and iron containing compounds. This project

is part of the Next Generation Fire Suppression Technology Program (NGFSTP).

INTRODUCTION

Halogenated fIre extinguishment agents, such as Halon 1301 (CF3Br) and Halon 1211 (CF2ClBr), continue

to be some of the most effective fIre extinguishing agents known. However, due to their linkage to stratospheric

ozone depletion their use has been greatly restricted by international agreement 1. Many replacement agents are still

highly halogenated, i.e. FM-200 (C3F7H), and while effective, often require greater quantities to be used in fIre

fIghting applications 2. Other replacement compounds, although proven effective, suffer from ozone depletion

potentials which are unacceptable (CF3I) 3. The search for effective, non-toxic, environmentally benign replacement

agents thus continues.

One compound which has been shown, through experimental and modeling studies 4,S, to be extremely

effective in small concentrations is Fe(CO)s, which also suffers from high toxicity 6. This project is designed to

understand the mechanism offrre suppression/extinguishment of this superagent type compound. This

understanding should lead to the design and development of new agents which meet all the criteria for a

replacement compound. The study presented here utilizes a counterflow diffusion burner capable of being operated

at either reduced or elevated pressures. We will spectroscopically map flame species via emission spectroscopy, as

well as map flame temperatures with traditional thermocouple techniques, in flames at several pressures and

inhibited with several different agents.

BACKGROUND

The combustion process of any fuel is the combination of many elementary reactions of many different

chemical species interacting with the fluid dynamics of the system. This complexity can be reduced to a few key

reactions which are primarily responsible for the propagation of the overall chemistry. When a chemical agent is

introduced which interrupts this propagation, the flame can be extinguished or suppressed. The principal radical

reactions for hydrocarbon combustion in air are 7 :

0+ H2 ®OR- + H

H+02®OH' +0

OH· + H2 ®H20 + H

R(l)

R(2)

R(3)

These three reactions form the major chain-branching mechanism for flame propagation, with R(2) being the

JANNAF Combustion Subcommittee, 1997
dominant chain-branching reaction in the flames and the primary channel for oxygen consumption.

When an inhibiting agent is introduced to the flame, chemical inhibition is believed to arise from the

lowering of the concentration of radicals through scavenging reactions and/or physical inhibition arises from an

absorption of heat energy due to the heat capacity of the agent. A scavenging reaction is defmed as a process that

converts reactive radicals to stable species and/or less reactive radicals 9 • Halon 1301 (CF3Br) acts primarily as a

chemical inhibitor via the follwing scavenging reactions 10:

H (0, OH) + HBr -, H2 (OH, HP) + Br R(4)

Br + HBr -, Br2 + H R(5)

Br + H02 -, HBr + O2 R(6)

Br + Br + M -, Br2 + M R(7)

Br + H + M -, HBr + M R(8)

These reactions remove the reactive radicals OH and H02 and regenerate the active species HBr, which then repeats

the cycle. This catalytic activity is why CF3Br is so effective as an inhibiting agent.

Flame suppression by metal containing agents, such as iron pentacarbonyl (Fe(CO)s), is often assumed to

occur by three competing suppression pathways: metal-oxygen-oxygen monomers form and cause homogeneous

free radical removal, metal containing particles form and undergo heterogeneous free radical recombination on their

surfaces, and heat absorption due to agent decomposition. Reinelt et al. 11 have suggested the following mechanism

for the inhibition activity of Fe(CO)s' Iron pentacarbonyl initially undergoes thermal decomposition and then the

free iron reacts with oxygen and water to produce scavenging iron oxides.

Fe(CO)s @Fe + 5 CO

Fe + 02@FeO+0

Fe + H20@FeOH+H

R(9)

R(lO)

R(ll)

The iron oxides then promote the catalytic recombination ofH atoms to hydrogen (H2).

FeOH + H@FeO+H2

FeO + H20 @Fe(OH)2

Fe(OH)2 + H @FeOH + H20

R(12)

R(13)

R(14)

The removal of the H atoms inhibits the chain-branching reaction R(2) which reduces the overall rate of

combustion. In flames, iron oxides have been observed to form condensed phase particles that are highly

lurninous12. This observation also lends support to the heterogeneous inhibition mechanism mentioned above.

As illustrated above, the proposed inhibition mechanism is quite complex, involving both homogeneous

and heterogeneous catalytic cycles that result in the extinguishment of the flame. Presented here are results from the

fIrst portion of this study, in which the flame temperature is measured with Pt/10%Pt-Rh thermocouples and the

emission of the neat and inhibited flames is recorded. The next statges of the study will include the mapping of

species concentration by laser induced fluorescence (LIF), planar laser induced fluorescence (PLIF), tunable diode

laser (TDL) absorption spectroscopy, near-IR TDL spectroscopy, and a computational modeling study of the

flames.

EXPERIMENTAL

The counterflow diffusion burner apparatus consists principally of three components: a gas handling

manifold, the burner chamber with vacuum system, and the analytic equipment. Each component will be outline

briefly in the following paragraphs. The oxidizer and fuel gases are prepared in the desired proportions, with or

without inhibiting agent, by the gas manifold before entering the burner chamber. The fuel usually enters through

the bottom head and the oxidizer from the top. The oxidizer and fuel mix between the two heads and there the

flame front is supported. Unbumt gases and combustion products are exhausted from the chamber through eight

ports, two by each window, by a high volume vacuum pump.
FLOW CONTROL SYSTEM

The gas handling manifold consists of a network of Tylan General flow controllers originally designed to

produce a flow velocity of 100 cm/s from both heads in the pressure range of 0.5 torr to 2 atm. We do not utilize

this high flow rate due to the excessive amounts of gases required to maintain these flows. Each arm of the

manifold has two flow controllers installed, a high flow and medium flow, and selected via valves. The manifold

also allows for the inhibitor to be introduced to either the fuel flow or the oxidizer flow. The flow controllers are

electronically operated via two mass flow control boxes, with one channel operating a single gas being introduced to

the system. The gases normally introduced are fuel (typically methane), nitrogen (to manufacture air, as a diluent,

as a inhibitant carrier, and as a window buffer), oxygen, and inhibitant. The manifold has solenoid controlled safety

valves, normally closed, as the fmal output valve to the system. No gas is allowed to flow in the event of a power

outage or the pressing of an emergency cut-off switch.

The inhibitant is introduced as a gas when possible. One arm of the gas manifold is used to introduce

argon and Halon 1301, both of which are gaseous at room temperature. Iron pentacarbonyl is a liquid at room

temperature with a vapor pressure of 19.4 mm/Hg at 15°C. To introduce the Fe(CO)s to the system, nitrogen is

bubbled through a flask, immersed in a constant temperature bath, containing the liquid. The nitrogen is introduced

via one of the gas manifold flow controllers. The output of the bubler passes through a rotameter to measure

precisely how much of the saturated vapor is entering the gas stream. We have minimized the distance traveled

between the bubbler and the burner head to reduce the amount of Fe(CO)s which decomposes rapidly in air.

THE BURNER SYSTEM

The combustion chamber is a six armed

stainless steel cross with the four horizontal arms

being ten inches in diameter and the two vertical

arms being twelve inches in diameter. The

opposed jet burners (Figure 1) are mounted in

the vertical arms, with each head having two axis

of motion under computer control. The active

burner area is three inches in diameter. The gas

and cooling water are fed to the head via

stainless tubing. Gases introduced fIrst impinge

on a deflector plate and then expand through the

porous plate to a second chamber also sealed by

a porous plate. This second porous plate opens

to the combustion chamber.

Optical access to the chamber is

provided on the four vertical arms of the

chamber where four inch diameter windows are

mounted. There are also four additional ports in

the horizontal plane, each located between two

of the window arms. These ports serve as access

for pressure monitoring, thermocouple

manipulation, and electrical access. Also mounted on top and bottom of each window arm are vacuum ports to

remove the combustion gases. These eight ports are joined to a single two inch I.D. tube which feeds the gas to a

scrubber mounted on a high volume vacuum pump. The scrubbed exhaust gases are then vented to the atmosphere.

ANALYTIC EOUIPMENT

The combustion chamber is located between two optical tables which support the majority of the analytical

equipment used in characterizing the flames under study.
The thermocouple is constructed from Pt and lO%Pt!Rh wires which are .008 inch in diameter or smaller. The

thermocouple wires were used with a refractory coating of magnesium oxide (MgO) in both the uninhibited flames and

inhibited flames. This was produced by making a slurry of magnesium perchlorate (Mg(Cl04)2), swabbing the wires

with the slurry, and then passing the coated wires through the flame of a propane torch. The flame drives off the

moisture, producing the MgO coating13•

The emission data is collected using a two dimensional CCD camera attached to a monochromator. A 0.5 meter

focal length lens is use dto image the center of the flame on the input slits of a Spex 500 mm monochromator. Mounted

on the output of the monochromator is a Princeton Instruments two dimensional CCD camera. The camera is operated

with a model ST 138 controller and PG200 Pulse Generator. The active array area is 576 x 384 pixels. This active pixel

area produces an image which is 11.1 mm in height and approximately 20 nm wide. The images obtained thus contain

both spectral information and spatial information. A software package supplied by Princeton Instruments allows the

images to be separated into both of these components, binning the individual pixels according to spatial dimension or

spectral region.

EXPERIMENTAL CONDITIONS

In order to understand the complexities of a flame, inhibited or unihibited, one must reduce it to as simple a

system as possible. In the past we examined low pressure premixed flames14 which simplified the modeling of the

system and also produced a flame which was readily characterized spectroscopically by expanding the luminous zone

of the flame.

In this study we are examining counterflow diffusion flames at reduced pressure conditions. Due to the physical

characteristics of the flames at the pressures examined, the experimental conditions vary flame to flame so that the

flames are stable enough to be examined physically and spectroscopically. The flame pressures at which experiments

were performed are 50 torr and 300 torr. Attempts to study a flame at true atmsopheric pressure (760 torr) with the

present gas flow rates was unsuccessful due to instability in the flame. Table 1 lists the conditions of each of the flames

studied, with flow rates given in standard liters per minute (slm). The buffer flow was the flow rate of nitrogen input

around the windows to prevent condensation of the combustion gases on the windows. This gas was part of the total

gas load to produce the reported pressures.

Table 1. List of experimental conditions for the three flames used in the present study.

Flame Pressure

Fuel Flow

Buffer OxidizerFlowFlow

3.01.51.1 slmCH4 slmN2 slm O2

2

6.03.02.2slmCH4 slmN2 slm O2

1.01.5 20slmslmsl02 CH4N2

6.0 slmN2

RESULTS AND DISCUSSION

We have concentrated on the two reduced pressure flames for the main body of work for ease of operation,

to minimize damage to the experimental apparatus, and to facilitate comparison to the ongoing modeling work.

Most of the results will thus be on the comparison between these two flames and between different inhibitors in the
same flame. Both CF3Br and Fe(CO)s offer challenges as additives. Fe(CO)s forms particulates in the flame, which

while they may aid in the reduction of flame temperature by conductive heat loss, also coat the interior of the

combustion chamber, vacuum lines, and ultimately the high volume vacuum pump. CF3Br generates significant

amounts ofHF. Hydrofluoric acid, while toxic, is also highly corrosive, and it is the corrosivity which complicates

the experimental study of fluorinated additives.

As mentioned above, Fe(CO)s generates particulates in the flames. These particulates then deposit on any

cool surface available to them, which is primarily the cooled burner heads. Samples of these particles have been

collected and analyzed using a scanning electron microscope (SEM) and X-ray fluorescence (XRF) to identify the

molecular composition. The particulate material is a ruddy orange color in appearance, typical of iron containing

compounds. Particles were collected from the upper (oxidizer) burner head and also from a thermocouple which

had been used to record temperatures in a doped flame. The XRF showed the composition of the particles to be iron

and oxygen, although an exact ratio was not determined. This was the expected result, the Fe(CO)s decomposes and

some of the iron combines to form FexOy particles. The SEM photographs showed the size of the particles ranged

from 1 to 10 microns, with most being on the order of 1 - 2 microns. The effectiveness of a 2 micron particle

involved in heat conduction is being explored as a possible component of the overall suppression mechanism of iron

pentacarbonyl, although that determination has not been fmalized.

Dopant 300 50 Torr Torr

Argon

400 3.0 cc/mins1m

CF3Br

120120cc/min cc/min

Fe(CO)s

1515cc/min cc/min

For each of the flames studied, a scan of the full spectrum was taken from 250 to 1100 urn. Portions of this

spectrum were recorded with the CCD for further analysis. The position of the flame relative to the oxidizer burner

head, and intensity of the molecular spectra varied with dopant identity. Table 2lists the dopant level for the 50 and

300 torr flames. The spectra recorded contains both wavelength dispersion and physical height. The spatial

distibution of the OH emission between the burner heads is

shown in Figure 2 for both the 50 and 300 torr flames. In

the 50 torr flame, the addition of argon, which acts as an

inert diluent, shifts the flame position away from the oxidizer head. This is indicative of a change in the position of

the stoichiometric region supporting the flame, caused by a change in the momentum balance of the gas flows. The

CF3Br also appears to shift the OH emission peak away from the oxidizer head by a small amount. There is

virtually no change in the position of the emission intensity upon the addition of the Fe(CO)s. This is not the case in

the 300 torr flame. The argon shifting the flame front away from the oxidizer head can be atributed to the dilution

of the air, as we increase the total flow from the oxidizer head by 30%. Both the Fe(CO)s and CF3Br move the

flame front closer to the oxidizer head. This may be an indication of their radical scavenging ability. As the H and

OH population is depleted early in the oxidizer flow, the flame must reposition itself to remain supported on the

remaining radical population. Similar effects have been observed for both the C2 emission at 473.7 urn and the CH

emission at 431.4 urn.

The addition of an iron containing compound to the flame does change the molecular emission spectra

through the addition of atomic iron lines and of iron species such as FeO and FeOH. Iron has a very rich line

spectrum which spans from the near ultraviolet to the near infrared regions. The most intense lines lie toward the

blue end of the spectrum. These lines greatly alter the spatial distribution of the emission intensity, but also lend

key insight into the area of activity of the Fe(CO)s within the flame. Figure 3 illustrates this effect. The cross

section (Figure 3a) of the emission of CH has an additional region of emission intensity closer to the oxidizer head

which is missing in the neat and other two doped flames. Some of this has been identified as iron lines not present

in the neat flame, shown in Figure 3b. This spatial mapping shows that the iron pentacarbonyl has at least started to

decompose prior to the luminous flame region. This change in intensity is also observed visually, as a second bright
luminous zone appears nearer the oxidizer head in the 300 torr flame upon the addition of the iron pentacarbonyl.

In Figure 4 is shown both the spectrum of FeOH at 352 nm and the spatial distribution of the FeOH in the two

flames studied. The spatial distribution of the FeOH for the two flames is very similar, with just an off-set due to

the flame position relative to the oxidizer burner head. This suggests a similarity in the chemical activity at the two

reduced pressures.

We have also examined the response of flame temperature to the addition ofCF3Br and Fe(CO)s, and the

effect of flame temperature on the position and intensity of the emission of OH. Shown in Figure 5 are plots of

flame temperature for the 50 and 300 torr flame when undoped and when doped with Fe(CO)s. The Fe(CO)s

lowered the peak temperature by over 300°C at both pressures. This temperature change was not observed with the

addition of CF3Br, but the flame temperature remained essentially superimposable upon the neat flame temperature,

independent of the amount of agent added.

CONCLUSIONS

Weare continuing to study the effects of iron pentacarbonyl on counterflow diffusion flames at

atmospheric and reduced pressure to elucidate the suppression mechanism of this prototypical superagent. The next

phase of this study, currently underway, is to examine species concentrations using laser induced fluorescence. The

species to be studied are 0, H, and OH. We will also examine 0, CH4, H20, and HF (when produced) using near

infrared absorption spectroscopy. The study will also address different fuels. All of the results will be compared

with the modeling study of diffusion flames performed in a collaborative effort.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the Next Generation Fire Suppression Technology Program for

fmancial support.

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