purposes end is not to be

ill any pubhcsiiorh

NATIONAL BUREAU OF STANDARDS REPORT

6588

INHIBITION OF FLAME REACTIONS:

A PRELIMINARY INVESTIGATION OF THE ROLE OF IONS AND ELECTIONS

by

E. C. Creitz

<NBS>

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NATIONAL BUREAU OF STANDARDS REPORT

NIBS PROJECT NBS REPORT

1002-11-10121 November 4, 1959 6588

INHIBITION OF FLAME REACTIONS:

A PRELIMINARY INVESTIGATION OF THE ROLE OF IONS AND ELECTRONS

by

E. C. Creitz

IMPORTANT NOTICE

NATIONAL BUREAU OF ST

intended for use yvithin the

to additional evaluation and i listing of this Report, either the Office of the Director, Nr however, by the Government

to reproduce additional copic

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 201 5.

irogress accounting documents

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prepared if that agency wishes

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

INHIBITION OF FLAME REACTIONS:

A PRELIMINARY INVESTIGSTI ON OF THE ROLE OF IONS AND ELECTRONS

E. C. Creitz

ABSTRACT

It is proposed that activation of oxygen

molecules in flames proceeds by capture of electrons to form O2 ions and that inhibition may be

accomplished by capture of electrons by substances

other than oxygen molecules. The proposal is

supported by evidence that the efficiency of an

inhibitor is related to the cross section for

capture of electrons of the inhibitor, that its

efficiency is related to the oxygen concentration

and is relatively independent of the nature of the

(hydrocarbon) fuel, and, in the case of dry powder

extinguishing agents, it is related to the ability

to capture ions or electrons.

Experimental results are presented which indicate that both the average lifetimes of ions and

their steady-state concentrations in uninhibited

flames depend on the initial oxygen concentration.

1. INTRODUCTION

The modern physical concept of a gas involves particles

of molecular dimensions and smaller, which are free to move

in a random manner. Flame gases are in no way an exception

to this concept. Gaseous particulate matter present in flames

may be classified as electrically neutral, i.e., molecules

or free radicals, or electrically charged, as ions and electrons.

2

Flame reactions must, of necessity, involve interactions

between only these kinds of particles. In general, molecules

serve only as sources of molecular fragments which possess

characteristics of instability, under flame conditions, permitting them to react, chemically, one with another. Reactions

between free radicals are probable in flames because free

radicals need very little additional activation .energy to prepare them for reaction. Free radicals are known to be present

in flames./ l7* Reactions between ions bearing charges of

opposite sign are even more probable. The presence of the

charges is responsible for large, long-range forces and large

effective collision diameters. These effects result in a

probability for reaction between ions of opposite charge that

may be several orders of magnitude larger than that for

reactions between free radicals of the same concentration.

The work reported here is an attempt to deduce whether the

inhibition of hydrocarbon flame reactions is through interference with free radical or ionic flame processes and to suggest

a mechanism of inhibition based on the results of several

experimental approaches.

2. The Mechanism in Normal Flames

Rosser, Inami and Wise /~27have observed that neither

C^Br or CF^Br inhibit the methane-nitrogen dioxide flame.

The same is true of the CH/-NO flame. There are two important

consequences of these observations. The usually advanced

theory that inhibition_takes place through interference with

H, OH and H02 chains /2 ,/7 cannot be applicable in this case

even though all of the chain-forming elements are presumably

present. The equation given by Rosser et al. for the overall

reaction between N02 and methane is:

C% + 3 N02 > CO + 2H2 0 +3N0

It should be noted that the decomposition of N02 is not complete, having stopped after the liberation of one oxygen atom

per molecule. The freed oxygen atom, whether neutral or ionic,

is thermodynamically very active, and one may infer, that since

no inhibition is produced by CH3Br or CF3Br, inhibition is, in

some way, related to the activation of oxygen atoms or molecules.

* Numbers in brackets refer to "References" at the end of the

report.

- 3

-

The fact that interruption of hydrogen chain reactions

is not important in the CHq. N02 flame, casts some doubt on

the applicability to hydrocarbon flames in general. This

doubt is strengthened by the lack of any correlation between

the hydrogen content of the fuel molecule and the efficiency

of the inhibiting agent. For these reasons it seemed reasonable to study further the process of activation of oxygen in

normal and inhibited flames.

Of the possible processes for the activation of oxygen,

the most likely appears to be the capture of one electron

per molecule to form the O2 ion. The ease with which this

process takes place was demonstrated by Crain /~47 who filled

a microwave cavity resonator with various gases, subjected it

to gamma radiations from an atomic reactor and from cobalt-60.

By measurements of the resonant frequency he was able to calculate the concentration of electrons produced by ionization

of the gases. Nitrogen and argon gave electron concentrations

in order of lO'/cmo. When air was used in the cavity, he was

unable to detect any evidence of electrons even at radiation

levels as high as 8 x 10H gammas/cm2 . He attributed lack of

electrons, when oxygen was present, to the capture by oxygen

molecules.

Burch, Smith and Branscomb/-^ Have determined the energy

required to detach an electron from the O2 ion as having a

threshold of 0.l5i 0.05 electron volts. The collision cross

section at this electron energy is quite small, but increases

gradually, reaching a maximum somewhat above 3 electron volts

where the curve was stopped. Chanin, Phelps and Biondi / jS7

have extended the studies of attachment cross section to near

thermal energies (0.04- ev) and have shown that it consists of

two separate processes, two-body and three-body, having

appreciable cross sections at this low value of electron energy

Thus, on the basis of energy and collision cross section,

oxygen molecules are able to capture, with varying probabilities, electrons of almost any energy which may be encountered

in flames. The electron affinity of the O2 ion determines

whether it can retain its extra electron under flame conditions

For atoms, the electron affinity, radiative attachment coefficient, and photodetachment threshhold are identical. For molecules these three energy differences are usually not the same.

Branscomb ITiJ gives a value of 0.9 electron volt for the

electron affinity of O2 , derived from thermochemical data, as

probably the most reliable value. Branscomb and Smith

give a value of 1.4-5- 0.15 electron volts for the photodetachment threshold of 0“and, since the atomic form is involved,

- If -

this is also the electron affinity. In the case of molecular

oxygen, the energy acquired in the capture of an electron is

either redistributed or lost, resulting in a more stable configuration which requires additional energy for removal of the

electron over that acquired in the capture process.

With an electron affinity of 1.^-5 volts, it would appear

that there is a small, but finite probability of a neutral

oxygen atom capturing an electron in a flame to become an

0" ion. It seems more likely that N02 would be able to capture an electron and, in the process of redistribution of

energy, liberate 0

-

. Fox /~2/ gives a value of 1.35- 0.05 ev

for the onset of capture of electrons for NO2 with a maximum

cross section at I.m-5 ev at room temperature. There seems to

be no information available which would permit the estimation

of the electron energy corresponding to the maximum at flame

temperatures, but one would estimate that it might be less

than at room temperature because of the instability of the

molecule.

The above properties of oxygen along with Rosser's

observations on the NC^-CHi^. flame imply that the mechanism

of normal flame propagation, is, in part, the formation, from

the fuel, of positive ions and electrons, followed by the

capture of the electrons by oxygen molecules to form O2 ions,

which, in turn, is followed by reactions between the positive

and negative ions. It is probable that the first products of

the ionic reactions will be unstable at flame temperatures

and may decompose to give positive ions and electrons or 0”

atoms which continue the reactions until stable products are

formed. It is of interest to see hew some known facts of

hydrocarbon flame propagation substantiate this mechanism.

3. The Rate Controlling Process

If one assumes that the combination of positive and negative

ions is rapid, the rate of ionic reactions in flames would appear

to be controlled by one of two processes: the rate of production of electrons (and positive ions) or the rate of activation

of C>2 molecules. Calculation of the maximum rate in terms of

the activation of oxygen molecules is not consistent with the

known facts with respect to flame velocity. The detonation

velocity studies of Breton /107 show that the maximum reaction

rate between propane and oxygen is attained at about 30% propane and 70% oxygen, which is considerably above the stoichiometric 16,7% propane. The continuous increase in reaction rate

with fuel concentration suggests that below the limiting value

of 30% fuel, the rate controlling factor is the rate of fuel supply.
.
- 5

-

Flame velocities for fuels in air are also known to increase

as a function of the rate of fuel addition until some other

effect becomes limiting. The results given in Table 1 show

that the rate of reaction in lean flames at a constant rate

of fuel supply is independent of the rate of oxygen supplied

to the flame. An Egerton-Powling burner /II7 was supplied

with propane at vqry nearly the same rate for each determination shown in the table, while the oxygen and nitrogen were

proportioned to give various concentrations of oxygen, all

at very nearly the same total mixture velocity. Measurement

of the flame velocity with this type of burner involves a

measurement of the flame area, which was not done in this

experiment. However, there was little apparent change in

flame area with change in oxygen concentration so that the

mixture velocity is, for practical purposes, equivalent to

flame velocity. Column 4, therefore seems to show that the

reaction rate is very nearly constant at a constant fuel

supply rate, and is independent of oxygen concentration.

One explanation of this might be that the decomposition of

the fuel into positive ions and electrons is the rate-limiting

reaction.

h. The Lifetime of Positive Ions as a Function

of Oxygen Concentration

Deflection of a flame by a high potential electrostatic

field is in such a direction that the flame follows the movement of the positive ions. At the instant of formation of

the ion, the direction of the motion of the freed electron,

with respect to the field, is completely random (unless the

molecule, under the influence of the field, forms a dipole

of sufficient strength to cause orientation prior to dissociation). A certain fraction of the electrons will have a

component in the direction of the movement of the positive

ions already present, immediately after being freed. After

a time, depending on the field strength and its initial

energy, such an electron will acquire a net motion opposite

to that of the positive ions. In weak or moderate fields,

this will take place only after the electron has made a

number of collisions with molecules. If some of them are

oxygen molecules, there is a good probability that the

electrons will be absorbed to form oxygen ions. If the distance between 0^ and the positive ions is not too great and

the external field is not larger than that between the ions

they will eventually react. If the applied field is so

great that the electrons and positive ions are carried in

opposite directions with sufficient energy, the flame will

be extinguished /l27. Statistically, the net distance of

- 6 -

travel in the external field by a positive ion is determined

by the field strength and the time it exists as an ion, i.e.,

the time interval between its formation and its destruction

by reaction. On the average, this time is that required for

an oxygen molecule to capture an electron, form a negative

ion and make a reactive collision with a positive ion. For a

given rate of positive ion production, the overall time interval should be reduced by any process which increases the

number of oxygen ions. The proposed activation mechanism relates the ionic and molecular oxygen concentrations with the

result that the amount of deflection of a flame should be

inversely related to the oxygen concentration, provided the

fuel concentration is held constant. Table 2 shows the

results of measurements of this relationship.

These data were obtained with an Egerton-Powling burner

with a 15-mesh screen over the recessed section of the

burner to provide a reasonably flat lower, negative electrode.

The heat-radiating upper screen was used as the positive

electrode and was adjusted so that the flat flame assumed a

position 1 cm above the lower electrode when no voltage was

applied. It was assumed, that because of the conductivity

of the flame gases, the reaction zone of the flame is effectively the positive electrode. This is probably an acceptable assumption because the concentration of ions and

electrons down-stream from the reaction zone is several orders

of magnitude larger than it is up-stream in the unburned gases,

so that the voltage drop is almost entirely across the space

between the reaction zone and the lower screen. A constant

potential of 500 volts was applied and the deflection measured

by sighting across a scale attached to the outer side of the

burner jacket. The displacement given in Table 2 for the

lowest oxygen concentration is marked with because portions

of the flame were deflected downward to a definite flat surface 2.5 nun above the lower screen. This seems to correspond

to a quenching distance and it is not known how much farther

the flame would have been deflected in the absence of this

effect. The movement of the reaction zone under the influence

of the electric field changes the effective electrode spacing

and, therefore the potential gradient, at constant applied

potential. The fact that the potential gradient does not increase without limit is an indication of strong forces

connected with the positioning of the flame in the burner.

In addition to clarifying somewhat the mechanism of deflection of flames in electrostatic fields, these results give

further evidence that the rate of reaction in lean flames is

controlled by the rate of formation of positive ions and electrons from the fuel, and give a qualitative explanation of the

statement by Knewstubb and Sugden /13.7 that ionization persists

longer in rich than in lean flames.

- 7

-

5. The Relationship Between Electron and

Oxygen Concentration

The presence of large concentrations of electrons and

positive ions and low concentrations of negative atomic and

molecular ions in normal flames /l47indicates that negative

ions react as rapidly as they are formed. Since the rate of

capture of electrons by oxygen molecules is proportional to

the number of collisions occurring between the respective

species, it is also proportional to their concentrations.

In lean flames the rate of supply of electrons appears to

be fixed by the rate of fuel supply as shown in Section 3?

so that electron concentration should change as the oxygen

concentration is varied. This is shown in Figure 1, which

is a plot of some of the results shown in Table 1. These

data were obtained on propane, supplied at very nearly the

same rate for each determination, to an Egerton-Powling

burner, equipped with a Langmuir probe /l^/mounted axially

thru the burner and extending into the reaction zone through

the unburned fuel-air mixture. The heat-reflecting screen

above the free-floating flame was used as the second electrode.

With this method one may obtain ion concentrations and,

on the basis that the flame is a plasma, being neutral in

the macroscopic sense, one may infer electron concentration

is zero, or at least low /l¥/. The ordinate of Figure 1 is

thus approximately equal to the electron, as well as the

positive ion concentration. The disappearance of electrons

as a linear function of oxygen concentration seems to provide

direct evidence of the electron attachment process.

6. The Relationship Between Electron and

Inhibitor

Conductivities of flames, measured with Langmuir probes,

give ambiguous information on electron concentrations when

negative ions are present. This is because there is no way

of determining what fraction of the negative charges is

carried by ions and what fraction is carried by electrons.

When methyl bromide was added to a flame burning in the

Egerton-Powling burner, even in very small quantities, the

conductivity increased inordinately. There are two possible

explanations of this effect. Since the flame was not perfectly

stable, it was necessary to extend the probe a short distance

beyond the reaction zone into the burned gases. It is probable

that bromide ions formed in the reaction zone were carried into

- 8 -

the area above it where the concentration of negative charge

carriers is normally low. This would have the effect of

increasing the probe area. However, the increase in area

seemed too small to account completely for the magnitude of

the effect. It seems more fikely that bromine ions are

formed by electron attachment, but unlike oxygen ions, are

unable to react with positive ions until the cooler parts of

the flame are reached, persisting for longer times so that

the total concentration of carriers of negative charges is

increased. Because of their high electron affinity, halogen

ions cannot readily take part in charge exchange reactions.

The increased lifetime of positive ions was demonstrated

by adding C^Br to a mixture of propane and air which was

burned in the Egerton-Powling burner. Propane was supplied

at the same rate as for column 2 of Table 2, 267 cm-vmin, so that the rate of production of positive ions and electrons

would be the same. The final concentration of oxygen was

20.5$ as before, the C^Br concentration was 0.08$. Methyl

bromide was supplied at a rate of 10 cm3/min and the air

adjusted to give a stable flame. The velocity of the final

mixture indicated that the flame velocity had been reduced

by 7.8$ so that the rate of production of positive ions

was correspondingly less. However, application of a

500 volt/cm electrostatic field produced a deflection of

6.5 mm, more than twice that of the former case. This increased deflection of the flame under the same conditions is

taken as evidence of an increase in positive ion concentration and thus for the capture of electrons in the formation

of Br _ ions.

Additional evidence for the capture of electrons by

halogens is provided by the work of Sugden /l 67who used

microwave techniques. His method was insensitive to the

presence of negative ions, measuring only the concentrations

of electrons. However, because of the unfavorable lower

limit for the detection of electrons, his measurements were

made on flames salted with alkali metal ions from alkali

halides. He reports that when additional halogen was added

to such flames the electron concentration decreased. The

effect was attributed to the formation of unionized salt or

to the capture of electrons to form negative halogen ions.

The latter seems more likely.

- 9

-

7. The Relationship Between Extinguishing

Efficiency and Oxygen Concentration

For lean flames, in the absence of inhibition, the proposed mechanism and the results of Table 1, indicate that the

overall rate controlling process is the rate of production of

positive ions and electrons. In the case of fuel-rich mixtures, the burning rate depends on the rate at which active

oxygen molecules can be supplied to the reaction zone and

thus, depends on the rate of oxygen supply and the rate of

oxygen activation. A diffusion flame, involving a gaseous

fuel and having a high concentration of electrons, should be

able to activate almost every oxygen molecule coming within

range of the electron cloud in the flame, so that the overall

rate controlling process is the rate of oxygen supply. If

inhibition is produced by the proposed mechanism, the activation process must become rate controlling. This implies that

inhibition should be related to the oxygen content of the

oxygen-nitrogen mixture supplied to the flame. The addition

of an amount of inhibitor sufficient to extinguish a diffusion

flame should be counteracted by an increase in oxygen content.

The relationship between extinguishment efficiency of

methyl bromide and oxygen content was determined using the

simple jacketed burner shown in Figure 2. In order that

diffusion effects remain as nearly constant as possible,

flames were set to a constant height (2 cm) rather than to a

constant volume rate of fuel supply. Mixtures of nitrogen,

oxygen and methyl bromide were supplied to the annular space

between the burner tube and jacket at rates producing linear

velocities of the order of 7 cm per second which did not

appreciably disturb the flames and were sufficient to remove

combustion products and prevent back diffusion of air into the

burner. The composition of the atmosphere surrounding the

flames was produced by means of calibrated flow meters. In

each measurement the methyl bromide content was slowly increased until extinguishment resulted. Since the precision

was not high, a series of four determinations was averaged for

each point on the curve.

10

Each of the fuel gases requires a certain concentration

of oxygen in the O^-N^ mixture in order to maintain a flame

in the absence of inhibitor. Since this concentration is a

characteristic of the particular fuel, subtraction of this

value from the total oxygen concentration permits comparison

of the fuels on an equal basis. In Figure 3, th<? concentration

of methyl bromide required to extinguish a diffusion flame is

plotted against the oxygen concentration in excess of that required for the fuel alone for methane, ethane, propane, butane,

isobutane and ethylene. The curves for the individual gases

vary somewhat, as might be expected, and are subject to undetermined, but probably sizable, errors. Nevertheless, it

appears that the amount of inhibitor required for extinction

is more closely related to the oxygen content than to the kind

of fuel, as required by the proposed mechanism.

It should be pointed out that the ratio of hydrogen to

carbon for these gases ranges from 4.0 to 2.0, yet the efficiency of extinguishment is approximately the same. This

seems to provide further evidence that the mechanism of

inhibition does not involve chain reactions of the hydrogen

atom. The above results correspond to the results of Rosser,

et al/f~2_7 on ethyl alcohol inhibited by H Br.

The curvature of the relationship of Figure 3 is partially

explained by the fact that methyl bromide itself is capable of

supporting combustion over a limited range of oxygen concentrations.

The total fuel consumed, therefore, is the sum' of the hydrocarbon, supplied at a constant rate, and the methyl bromide,

the concentration of which was varied. As the concentration of

methyl bromide is increased, the amount of oxygen required is,

therefore, increased. The closer to the point of combustion of

methyl bromide, the less effective it is as an inhibitor. At

some oxygen concentration it is capable of not only capturing

electrons for the formation of Br~ but also of supplying them

for the activation of 0;? molecules. This implies that there may

be a shift from a reaction of the type

CH3 Br + ^ >

CH3 + Br“

in the cooler parts of the flame to one of the type

CH3 Br >XH3 + +Br

as more molecules are able to penetrate into the hotter flame zones.

This experiment should be repeated using a non-combustible inhibitor.
s
11

The Interior of a diffusion flame, being surrounded by

the reaction zone, should have a higher concentration of

electrons than the region just outside of it. The increased

field should result in electrons being repelled toward the

outside of the flame, where they are more useful, until an

equilibrium is established. The addition of methyl bromide

to the fuel, resulting in the replacement of electrons with

Br" ions inside the reaction zone, should have little effect

on the equilibrium other than to reduce the mobility of the

carriers of negative charges and, as a result, should have

less effect as an inhibitor than when added to the oxygen

supply outside the flame. It is easy to show that concentrations approximately 5 times as great are required for the

extinguishment of a propane diffusion flame when the methyl

bromide is added to fuel as when it is added to the air supply.

Since the rate of diffusion into the reaction zone should be

the same from either the fuel side or the oxygen side and the

inhibitor is more effective when added from the oxygen side of

the reaction zone, it must be concluded that inhibition is connected with the utilization of oxygen.

8. The Efficiency of an Inhibitor

The proposed mechanism requires that an inhibitor be

capable of capturing electrons or of removing them from Op

ions under flame conditions and, in addition, disposing of

them in such a manner that they are no longer available for

flame reactions. Because the concentration of electrons is

considerably larger than that of Op ions in flames, it is

more probable that the mechanism involves capture of free

electrons rather than removal of electrons from Op ions,

although the latter is not completely ruled outo If the

capture of free electrons is involved, the most efficient

inhibitor would have the largest possible cross section

for capture over an energy range comparable to that found

for electrons in flameso From the standpoint of high efficiency of inhibition, the electron would have to be disposed

of by the formation of a highly stable negative ion of a kind

which does not form compounds with the positive ions found in

flames capable of remaining stable at the temperatures involved.

This calls for an atomic ion having a high electron affinity and

points directly toward the halogens as the best known examples

of these characteristics. The cross section for the reactions

12

Ip + 6 ^ I + I" as a function of electron energy, as given

by Buchdahl fy]7 is shown in Figure 4, along with similar

results on O2 + f > C>2 taken from Burch, Smith and Branscomb /~"^7and for 0 + f > 0“ from Branscomb and Smith /f"87.

It seems obvious from these curves that Ig is a better electron

sink for low energy electrons than O2

. Hickam and Berg /l87

and Buchel 'nikova 71^7 give results on the electron energy for

maximum capture cross section and the range of energies of the

electrons capable of capture for a number of compounds, some

of which have been rated as extinguishing agents by Malcolm /207.

These results are shown in Table 3? along with results on

HC1 from Fox /217 and on 1 2 from Frost and McDowell /227.

The last column is taken from Malcolm's report and is the concentration of extinguishing agent above which a flame of

n-heptane will not burn in air. The smaller the value the

more effective the inhibitor. The table shows the combined

effects of the position of the maximum cross section for capture

and the range on the efficiency as an extinguishing agent.

The information given by Hickam and Berg and by Fox does not

permit the estimation of the cross section for capture so that

no inferences can be drawn on this basis. However, the table

shows that poor extinguishing efficiency results if the maximum cross section for capture occurs at energies which are

either too high or too low, or if the range of energies covered

is too small.

Rosser Inami and Wise's results on the CHy_ -NOp flame

indicate that, compared to CH3 Br and CF3 Br NO2 is a very efficient electron sink. The end product of the capture of an

electron, the 0

“ ion, should take part actively in flame

reactions, and evidently has a high enough electron affinity

so that it cannot lose its electron to CH

3 Br or CF 3 Br molecules under flame conditions. It is possible that its lifetime

is too short for such, a'- transfer to occur with any reasonable

probability.

Hickam and Berg point out that there may be strong temperature coefficients with respect to both cross section and

the position of the maximum on the electron energy scale.

They cite carbon tetrachloride as an example. It has two

maxima, one of which decreases with an increase in temperature,

the other increases, but at a slower rate. The net result is

a shift of the range to higher electron energies. The temperature coefficient must undoubtedly be considered in predicting

extinguishing efficiency.

- 13 -

9. Dry Powders

Several mechanisms have been proposed to account for the

effectiveness of dry powders as extinguishing agents. Dolan /2^7

has shown that for a given chemical compound, the effectiveness

is a function of surface area and chemical composition and

bears no relationship to particle weight or volume. Because

absorption of thermal enefgy is a function of surface area, he

proposes that extinguishment is produced by a thermal mechanism.

However, since the heat capacity is a function of the weight of

the particle, and there appears to be no correlation of effectiveness with weight, this mechanism does not appear to be too

promising. McCamy, Shoub, and Lee /2f±7 investigated the effects

of infrared shielding by dry powder clouds and concluded that,

while the effectiveness of dry powders could not be attributed

wholly to infrared shielding, the effect should be considered in

any proposed mechanism.

Surface effects in general correlate with adsorption phenomena.

Free radicals, being very reactive, should be readily adsorbed

on surfaces, including those of dry powder clouds. While the

main flame reactions appear to be ionic, it is possible that preignition reactions between free radicals and oxygen lead to

oxygenated compounds which may have low ionization temperatures.

Adsorption of free radicals in these early reactions would reduce

the concentration of compounds producing positive ions and

electrons in the reaction zone and thus produce inhibition. This

possibility has not yet been investigated.

Surfaces may also be active in removal of electrons from

the reaction zone. The detailed process is somewhat as follows:

Because of their high energy and small mass, electrons can

rapidly leave the region in which they are freed and collect on

surfaces, charging them negatively with respect to the plasma.

The charge would have two effects. It would repel electrons

and it would attract positive ions, which would fall freely to

the surface as a result of the electric fields in which they

would happen to be on formation. The electrons make so many

collisions that they would rapidly establish a Maxwellian distribution among themselves. A steady state would finally be

reached in which positive ions would be attracted to the surfaces

while only electrons having sufficient energy to scale the sheath

could reach the surface. Under these conditions, surfaces would

develop a finite negative charge relative to the space surrounding

them. Positive ions would thus be reunited with electrons instead

of oxygen molecules and the excess of electrons present on the

surfaces would be removed from the reaction zone.

- lb -

It should be noted that, because the energy of the electrons

increases with temperature, the electron sheath formed by this

process will become thicker and more electrons will be removed

at higher temperatures. This is quite a different effect from

that of wall -quenching of flames which has a temperature coefficient in the opposite direction /25.7* The two effects differ

in another respect. If the surfaces are made of materials which

can conduct electrons away as rapidly as collected, no electron

sheath can form. Wall-quenching effects are independent of the

material of which the wall is made and appear to be purely

thermal in nature.

Dry powders of different chemical composition have different efficiencies as extinguishing agents. It is to be expected

that the electrical properties of different chemical entities

would be different and particularly so at flame temperatures

where such differences may very well be exaggerated. For a

single chemical compound inhibition by absorption of electrons

should be a function of oxygen concentration for dry powders,

just as it is with halogen compounds. In addition, particles

of dry powder which have passed near a diffusion flame should

bear negative charges and should be readily precipitated on a

positively charged electrode. Only qualitative results are

available, at present, on these two effects, but they substantiate the proposed mechanism.

The apparatus shown in Figure 5 was used to disperse dry

powder into a stream of air to which had been added oxygen or

nitrogen. Air was supplied through the fritted disk and powder

bed at various rates and it was assumed that the rate of powder

application was proportional to the rate of air supply. The

powder bed was agitated, to prevent channeling, by means of a

buzzer attached to the tube. It was further assumed that mixing of the air-oxygen or air-nitrogen mixture supplied through

the side tube would be reasonably complete by the time the

flame was reached. The flame position was approximately 10 diameters above the side tube and the powder streamlines indicated that reasonably good mixing occurred. Propane was

supplied at approximately 50 cnrVmin. Linear air flow rates

of 20 to 50 cm/sec in the area surrounding the flame did not

appreciably change the flame characteristics. The relative

rates of powder application were indicated by the rate of air

supplied through the powder bed and a dilution factor based on

the volume of mixture entering from the side tube. The powder

used was of 4 micron average diameter, potassium bicarbonate

base, and fresh powder was used for each run. Each determination

was made by setting the desired mixture composition supplied

through the side tube, lighting the flame, starting the buzzer

and slowly increasing the air rate through the powder bed until

the flame was extinguished. The results are shown in Figure 6.
.

-
- 15 -

The curvature shown in the figure is undoubtedly caused

by the lack of a linear relationship between the velocity of

the airflow through the powder bed and the amount of powder

it is capable of carrying. In spite of this difficulty, the

curve shows that it is necessary to apply more powder to produce extinguishment as the oxygen concentration is increased,

and thus supports the proposed mechanism.

The same apparatus was used to show that powder particles

which have passed near the flame bear negative charges. Two

electrodes 1.4 x 3.5 cm were supported vertically above the

top of the tube with a separation of 1.4 cm. A d-c potential

of 500 volts was applied between them. With the flame burning

in air and a rather low rate of powder application, about 80-90%

of the particles collected by the electrodes, as estimated

visually, were collected by the positive electrode. Streamers

of particles 1 -2 mm long formed at the upper (downstream) edge

of the electrode. When the flame was extinguished only a very

small fraction of the powder was collected by either electrode

and about equal amounts were collected by each. With the flame

burning and oxygen added to the air stream, no powder was

collected by either electrode until rather high rates of application were used, when rather small amounts of powder were

collected, a slightly larger deposit appearing on the positive

electrode. When nitrogen was added to the air stream, the

collection differed from that for unmodified air only in that

the deposition appeared to be a little more rapid.

Some of the powder which collected at the positive electrode

was analyzed and found to be predominantly potassium hydroxide,

indicating that decomposition takes place along with acquisition

of negative charges. Since the powder particles passed near

the diffusion flame where a relatively higher density of electrons would be expected, the sign of the charge mighthave been

predicted. Because of the difficulty of producing a suspension

of dry powder in a slowly moving gas stream, no attempt was

made to determine the sign of the charge acquired when the

powder was passed through a diffusion flame. No information is

available on the average number of charges per particle, in

the experiment reported here.

The above facts seem to provide rather conclusive evidence,

not only of the mechanism of inhibition by dry powders, but

also they are evidence of the ionic nature of one step in the

combustion process.
.
- 16 -

10. Discussion

Several arguments have been presented in apposition to

the ionic mechanism. They resolve themselves into three

general classes. It is argued that ions are produced only

incidentally as a by-product of some other, non-ionic,

process and serve as an indicator of the other process /267

If this were the case, it would be possible to deflect the

ionic indicator by means of an electric field without

disturbing the reaction zone itself. That this is not the

case can be shown by probing a flame with a small thermocouple under conditions of deflection and non-deflection.

Secondly, it is argued that ions may be catalytic.

Catalysis in a homogeneous mixture of reacting molecules

A and B by a catalyst C, can take place by one of two

processes

A + B + C —VABC* >AB + C (1)

A + C —-*AC* )

) ( 2 ) AC* + B -—->AB + C ) Reaction (1) involves a 3-body collision whereas reactions (2)

require two 2-body collisions in rapid succession. The

activated complex, which can exist only momentarily because

of its excess energy, is designated by the asterisk. If the

concentration of catalyst is low enough that each catalyst

molecule is in constant circulation, the reaction rate must

be a function of catalyst concentration. The ionic concentration data of Table 1 is of the order of 1(P ions per cm2 in a molecular concentration of approximately 10^9 which would

seem to fulfill the requirement of constant circulation of

catalyst, yet a 5-fold change in ionic concentration produced

negligible change in reaction rate. On this basis it seems

improbable that ions can be catalysts for reactions between

neutral particles in a flame.

The third objection to an ionic mechanism in flames is

that the concentration of ions is too low to provide the

number of collisions required by the observed reaction rate.
-
- 17 -

Rates for both free radical and ionic reactions are

dependent not only on concentrations but also on the reaction cross sections. Cross sections for reacting neutral

molecules are functions of the relative velocities of the

molecules and are therefore related to temperature. They

vary between wide limits, depending on the activation

energy required by the particular reacting species. In

the case of species requiring a low but finite activation

energy, as is the case with some free radicals, the cross

section is zero at zero velocity, goes through a maximum

at some particular velocity and returns to zero asymtotically

at larger velocities. Because of the strong forces between

ions of opposite charge the cross section approaches infinity

as the velocity approaches zero and decreases as the relative

velocities increase, approaching zero at higher velocity.

The situation is further complicated by the fact that some

molecules, notably the O2 molecule, can readily exchange

charges, so that charges which started at some distance

apart can migrate, through charge exchange reactions, until

they are close enough together that neutralizing reactions

can take place. It therefore appears that large effective

reaction cross sections can compensate for low concentrations.

In a hydrocarbon gas-oxygen flame, positive ions can come

only from the fragmentation of fuel molecules. When air is

substituted for oxygen it seems probable that the fuel molecules are still the only source of positive ions. Because

of this, it has been postulated that the rate of formation

of positive ions depends upon the rate of fuel supply.

Under steady-state conditions the rate of recombination is

equal to the rate of formation of ions and steady-state

concentration of ions must be that required to produce the

observed rate of recombination. The low steady-state concentration observed is an indication of very large effective

reaction cross section.

The fact that a 5-fold increase in oxygen concentration

produced an almost identical decrease in the concentration

of positive ions seems to be evidence for the applicability

of the mass-action law to the capture of electrons by neutral

oxygen molecules. It should also provide a correspondingly

larger number of possible charge exchange chains and a greater

chance for neutralization of the positive ions, and explain

the resultant decrease in their steady-state concentration.

- 18 -

In order that a dynamic reaction like a flame may

continue to go, it is necessary that energy be fed back from

the reacting particles to those about to react. This activation energy may be of two kinds, kinetic or radiant. In the

case of the Egerton-Powling burner some of the feed-back is

by way of radiant energy as can be demonstrated by changing

the position of the upper screen and noting that the flame

follows the movement of the screen.

When a flame is deflected by means of an electric field,

the movement is in the direction that positive ions would

travel under the influence of the field. This seems to indicate that positive ions are important in the kinetic feedback of activation energy. One may envisage several mechanisms

by which such energy might be transported, some of which do not

appear to be very probable.

The presence of the field produces a motion of the positive ion which gives it energy in addition to its thermal

energy. However, the drift velocity is so small compared to

its thermal velocity that it seems unlikely that enough energy

could be acquired from the field to explain the observed

deflections.

If activation energy is transported from product molecules

to reactant molecules by a purely thermal process and positive

ions are postulated to carry only thermal energies, one may

consider that a certain amount of thermal energy could be

directed because of the charges on the ions and the presence

of the field. However, because of a small number of particles

capable of being directed in this way, it seems unlikely that

deflection of flames can be explained on this basis.

It is possible that an ion can carry more vibrational

energy than a neutral molecule of the same species because

it may have more internal degrees of freedom caused by the

removal of an electron. This is particularly true if the

ion is excited above its ground state. However, the additional

energy above thermal and exclusive of that associated with the

charge is likely to be small and cannot explain the observed

effect.

The largest energy carried by a positive ion is associated

with its charge. However, if this energy is to be imparted to

a molecule in the process of activating it, the charge must

also be transferred to the molecule being activated, since the
•

.

‘

f
- 19 -

energy goes along with the charge. Since positive ions

disappear continuously by recombination, it is implied

that every positive ion must produce more than one positive

ion in the activation process. This, in turn, implies that

energy in excess of that associated with the charge must be

transferred at the time the charge is transferred.

So little information is available on cross sections

for charge transfer between positive ions that it is useless

to speculate further. We can only conclude that the motion

of a reaction zone in an electric field indicates that positive ions are implicated in the activation process.

11. Summary

It must be concluded, on the basis of deflection experiments, that ionic reactions are important in the mechanism

of combustion. The work of Rosser, Inami and Wise /y£7 interpreted on the basis of ionic reactions, indicates that

inhibitions interferes with the activation of oxygen molecules.

The work of Branscomb et al^ /j>, 87, and Crain £ V7, and

Chanin, Phelps and Biondi / 67 indicates that a possible

activation process for oxygen molecules is through the formation of the C>2 ion. This proposal is supported by additional

evidence which relates the oxygen concentration to the positive

ion concentration, as measured with Longmuir probes, and the

lifetime of positive ions, as measured by deflection of the

flame by an electric field. Additional evidence for an ionic

mechanism is provided by the proposed mechanism of inhibition

in which electrons, O2 ions and possibly positive ions are

captured by the inhibitor and permanently removed from the

system. Sugden /l67 has shown that halogenated inhibitors

reduce the electron_concent rat ions. Hickam and Berg /I87

and Buchel 'nikova /1^7 have shown that halogenated inhibitors

have captured cross sections appropriate for the removal of

thermal electrons from flames. Evidence is presented in this

report that dry powder extinguishing agents acquire negative

charges when mixed with the air supplied to a diffusion flame,

that a halogenated inhibitor is more effective when mixed with

the air supplied to a diffusion flame than when mixed with fuel,

and that inhibition efficiency is more intimately connected

with oxygen concentration than with the nature of the fuel

supplied to a flame.

Because a reaction zone is displaced in an electric field

in the direction that positive ions would be displaced, it is

concluded that positive ions are connected with the transport

of activation energy between products and reactants.

r17 S. N. Foner and R. L. Hudson, J. Chem. Phys, 21,

137^-1382 (1953).

W. A. Rosser, Jr., S. H. Inami, and H. Wise, Study

of the Mechanisms of Fire Extinguishment of Liquid

Rocket Propellants, WADC Tech. Report, 59-206.

C3 F. E. Belles, Nat. Advisory Committee for Aeronautics,

Tech. Note 3565* (1955)*

£\7 C. M. Crain, J. Applied Phys. £2? 1605 (1958)*

r$ D. S. Burch, S. J. Smith and L. M. Branscomb, Phys. Rev 0 171-5 (1958).

eg l. m. Chanin, A. V. Phelps, and M. A. Biondi, Bull Am.

Phys. Soc. 1+, 316 (1959).

ri7 L. M. Branscomb, Advances in Electron Physics IX, 4-3-94

(1957).

Z”87 L. M. Bi*anscomb and S. J. Smith, Phys. Rev. £8, 1127-8

(X955)

C$7 E. Fox, Bui. Am. Phys. Soc. 4, 318 (1959).

/Io7 M. J. Breton, Ann. office combustibles liquides, 11, - 487-54-8 .v (1936 ) . ~

^Tl7 J. Powling, Fuel, 28, 25-8 (1949).

Z127 B. Lewis, J. Am. Chem. Soc., 2£, 1304- (1931).

ZT17 P. F. Knewstubb and T. M. Sugden, Seventh Symposium

(International) on Combustion, 24-7-253 (1959).

/T4-7 T. Kimbara and H. Ikegami, Combustion and Flame, 1, 199

(1957).

eg l. b. Loeb, Basic Processes of Gaseous Electronics,

Univ. of California Press, Berkeley, California (1955).

eg t. m. Sugden, Fifth Symposium (International) on

Combustion, 4-06-11 (1955).

eg R. Buchdahl, J. Chem. Phys., 2, 14*6-52 (194-1).
-

•*

.
21

/l87 W. M. Hlckam and D. Berg, J. Chem. Phys. 2%, 517-23 (1958).

/XS7 !• S. Buchel ' nikova, J. Ejtptl. Theoret. Phys. (USSR) 35 . 783 (1959).

/207 J. E. Malcolm, Report 1177? Interim Report on Vaporizing

Fire Extinguishing Agents, Engineer Research and

Development Laboratories, Ft. Belvoir, Va.

/2l7 R. E. Fox, J. Chem. Phys. 26, 1281-4-, (1957).

Z227 D.C. Frost and C. A. McDowell, J. Chem. Phys. £2? 964-

(1958).

/2.2J J. E. Dolan, Sixth Symposium (International) on

Combustion, 4-87-91+ (1956).

7247 C. S. McCamy, H. Shoub and T. G. Lee, Sixth Symposium

(International) on Combustion, 795-801 (1956).

J2 57 R. Friedman and W. C. Johnson, J. Applied Phys. 21,

791-5 (1950).

/_267 J. 0. Hir schfelder , Private Communication.

USC OMM-NBS-DC

Table 1. Positive ion concentration and calculated flame

i • temperature as a function of oxygen concentration

at constant rate of fuel supply and mixture velocity.

%

Propane

%

°2 CVJ Mixture

Velocity

cm/ sec

Positive ion

Concentration

Arbitrary Units

Calc

Temp

°C

1.97 12.9 85.2 7. 98 5.6 1012

2.06 16.2 81.8 -0 -vU CG 6.2 1067

2.05 18.6 79.4 7.88 6.0 1055

1.93 20.5 77.6 7.88 5.8 982

1.96 23.6 74.5 8.05 4.9 997

1.97 27.8 70.3 8.01 4.2 929

1.95 32.1 66.0 8.06 4.2 1002

1.94 35.1 63.0 7.98 3.8 969

1.96 39.7 58.5 8.03 2.96 1007

1.96 4^.7 53.4 8.04 2.15 989

1.86 54.7 43.4 8.01 1.26 962
5

‘
Table 2. Deflection of a flame as a function of oxygen

concentration at constant mixture velocity.

1 2 3

Propane concentration in per cent 1.96 2.00 2.01

Mixture velocity in cm/sefc 7.88 7.85 7.84

Oxygen concentration in per cent 12.07 20.52 51.5

Deflection in mm for a 500 v field 8+ 3 <*
•:
Table 3. Cross section and width of electron capture band

for selected compounds;

Compound Ion

Onset

of

Capture

ev

Width of

Capture

Band

ev

% of Peak

FI ammability

sf6 sf6

" 0 0 20 .5

sf6 SF^ 0.05 1.4 20.5

c ci2f2 Cl" 0.5 0.4 14.9

C CI4Cl" 0.2 1.0 11.5

, ci 2 cf2 ci 2 Cl" 0.2 2.8 9.0

HCJ Cl" 0.66 1.0 25.5

J 2 I" 0.34 1.0 —
•V
PERCENT

0

2

Figure

1.

Ion

concentration

as

a

function

of

oxygen

concentration.

Figure 2

51 mm l.D. X 25 cm

GLASS BURNER JACKET

Burner for supplying atmospheres of various

compositions to a diffusion flame.

CH.Br,

PERCENT

OF

TOTAL

o2 concentration,in PERCENT, ABOVE THAT REQUIRED

BY THE FUEL ALONE

Figure 3» Extinguishing efficiency of methyl bromide

as a function of oxygen concentration for

diffusion flames of various gases.

ELECTRON

ENERGY

IN

VOLTS

-

Figure

4.

Absorption

cross

section_as

a

function

of

electron

energy

for

the

reactions

12+f

“

>1

+

I,

0

+£

—

^CTand

O

2

+6

^-O 2

AIR TO DISPERSE POWDER

Figure 5« Apparatus for applying dry powder, to a diffusion

flame

240

200

160

120

80

40

0

20 30 40 50 60

02 CONCENTRATION IN PERCENT

6. Effect of changing oxygen concentration on

the concentration of dry powder required

for extinction of a diffusion flame.

U.S. DEPARTMENT OK COMMERCE

Frederick II. M iicllrr, Secretary NATIONAL HUH EAU OF STANDAH OS

A. V. Amin, Director THE NATIONAL BUREAU OF STANDARDS

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