Mechanism of smoke formation in diffusion flames

316

DIFFUSION FLAMES AND CARBON FORMATION

(2) At the yellow-tip limit, yellow appears in the secondary mantle, not in the primary combustion zone or below it. This can be made more apparent in certain instances by inverting the flame with an axially placed rod and a surrounding

flow of nitrogen. Inverted yellow-tipped flames of ethylene-air, toluene-air and acetylene-air clearly showed a blue-green combustion zone upstream of the yellow.

25

MECHANISM OF SMOKE FORMATION IN DIFFUSION FLAMES By ROSE L. SCHALLA AND GLEN E. McDONALD Introduction

A number of theories have been proposed to explain the mechanism b y which smoke is formed in diffusion flames, but at present no definite and completely consistent theory seems to be available. Most of the mechanisms which have been proposed in the past have been partial or generalized mechanisms in that they usually dealt with segments of the over-all process rather than tracing the complete course of the formation of smoke. These generalized mechanisms, such as the polymerization of C2 or build-up of aromatic ring structures to form smoke, have been subjected to various theoretical and experimental investigation in recent years, and, consequently, information which offers a basis for supporting or rejecting these ideas has become available. With the belief that still more experimental work might be helpful in understanding the over-all mechanism, several experimental projects were undertaken. Smoke formation was investigated in this study from the standpoint of the effect of pressure, fuel type, external air-flow rate, oxygen enrichment, argon substitution in external air, and fuel temperature. The experimental results were interpreted to indicate a possible step involved in the early stages leading to smoke formation. The understanding gained from these experiments, together with information from the literature, has been used to postulate a possible and relatively complete mechanism of smoke formation. Pressure

The effect of pressure on smoke formation was investigated by burning nine hydrocarbon fuels

as diffusion flames from a modified wick lamp in an enclosed chamber. A sketch of the apparatus used to study the effect of pressure on smoke formation is shown in Figure 1. The chamber in which the fuels were burned was approximately 300 mm in diameter and 700 mm high. To obtain pressures above atmospheric, compressed air was admitted at the bottom of the chamber through holes in a perforated circular copper manifold. To remove the combustion products a constant air exhaust of 220 cc/sec was maintained at all pressures. The exhaust rate was measured by a flowmeter connected to the exhaust valve on the lid of the chamber. I t was found that appreciable variations could be made in the exhaust rate without changing the smoking tendency of the flame. To obtain pressures below atmospheric, a vacuum line was connected to the exhaust valve and the flowmeter was attached to the air intake line. As a safety precaution and as a means of extinguishing tim flame, gaseous carbon dioxide could be admitted to the chamber through a valve in the center of the lid. The fuels were burned from a wick lamp which consisted of a 15 mm inside diameter glass tube 80 mm high to which was sealed a calibrated side arm. A hydraulic jack, operated from outside the chamber, permitted the raising and lowering of the wick lamp through a supporting collar. The rate of fuel flow needed to attain the smoking point could be controlled by a e r a t i n g the height that the wick extended above this support collar. A chimney 180 mm high and 47 mm in diameter was placed with the bottom edge level with the wick tip to keep the flame erect and stable. The smoking point of the diffusion flame was observed

317

MECHANISM OF SMOKE FORMATION IN DIFFUSION FLAMES

visually through a 25.4 mm Lucite window on the front of the chamber. The rate at which the fuel could be burned at its incipient smoking point was determined b y measuring the time required for the level of the fuel in the side arm of the wick lamp to drop a unit distance. The side arm scale was calibrated against water; the fuel rates in gram/sec could be obtained from this calibration and the specific gravity of the fuel. Reproducibility of the order of =i=3 per cent was obtained by this procedure. In Figure 2a the maximum rate (gm/sec) at which 6 pure hydrocarbon compounds, n-octane, 2-methyloctane, octene-1, heptene-1, iso-octane, and cyclohexene, could be burned smoke free is plotted against pressure over a range of about 1/~ to 4 atm. The pressure range was extended for 2 of the compounds and these results are shown in Figure 2b. In figure 2c the maximum rate for 100 per cent n-octane, 2 blends of n-octane and toluene, and a JP-4 fuel are plotted against pressure. As can be observed from figures 2a, b, and c, the rate at which the fuels can be burned smoke free consistently decreases with increasing pressure. The JP-4 fuel and the 2 blends of n-octane and toluene show the same variation in smoke formation with pressure as do the pure hydrocarbon compounds. A plot of the smoke-fi'ee fuel flow against 1/p is shown in Figure 3. The fairly straight lines obtained in these plots clearly indicates that the smoke-free fuel flow is inversely proportional to the pressure. Fuel Type A systematic survey of the smoking tendency of various hydrocarbon fuel types at one atmosphere pressure and room temperature was conducted at the Lewis laboratoryL The fuels were burned in still air as diffusion flames fi'om wicks and burner tubes, and the maximum quantity of hydrocarbon which could be bm'ned smoke-free in a given time was used as the criterion of smoking tendency. The maximum smoke-free fuel flow in gm/sec is plotted against the number of carbon atoms in the molecule in Figure 4. The results indicate that the rates at which the hydrocarbons could be burned smoke-free varied as follows: nparaffins > isoparaffins > mono-olefins > alkynes > aromatics.

External Air-Flow Rate The effects of external air-flow rate on the smoking tendencies of the laminar diffusion flames of eight of the hydrocarbons were investigated. The apparatus used for this purpose is shown in Figure 5. The eight hydrocarbons studied were burned from a stainless steel burner tube 9 mm in diameter (see Fig. 5a). The burner tube was enclosed by a Pyrex glass tube 40 mm in diameter and 760 mm in height. All of the air supplied to the flame was admitted through holes in a perforated cylindrical chamber sealed into the base of the Pyrex tube. Both the fuel and air were metered through flowmeters before entering the tubes. At each selected air flow a CARBON DIOXIDE iNLET. . . . .

AIR EXHAUST

; - "TO FLOWMETER I[

FIG. 1. Diagram of apparatus for determining smoking tendency. determination was made of the maximum rate of fuel which could be burned without causing the flame to smoke. A visual observation was used to detect the first smoke which issued from the flame. Since the vapor pressure of pentene-l was not sufficiently high at room temperature to give the needed fuel flow, a heated bomb was used to hold the vaporized pentene-1 which was then burned in the gas phase in the same manner as the gaseous compounds (Fig. 5b). To determine the fuel flow rate in this case, the bomb plus the sampie were weighed before burning the fuel and then reweighed after the fuel had burned at its incipient smoking point for a given interval of time. From the loss in weight over this time interval th~ maximum smoke-free fuel rate could be obtained. F i g u r e 6 shows the variation in smoke-free fuel flow with increasing air flow for butene-1,

318

DIFFUSION

FLAMES

AND

cyclopropane, propene, pentene-1, neopentene, isobutane, ethylene, and n-butane. As the rate of air flow past the flame is increased the rate of flow of fuel which will burn without smoking is at first increased proportionally; however, a limiting fuel flow (FL) is shortly reached, and additional increases in the air flow do not permit further increase in the fuel flow for smoke-free burning. Of course all of these fuel flows are still in the laminar region and a different maximum

CARBON

Figure 7a is representative of cyclopropane and propene and Figure 7b is representative of the other five hydrocarbons. With the initial increase in the flow rate of a given oxygen enriched mixture, the maximum smoke-free fuel flow also increased. The fuel flow, however, reached a limiting value which is usually greater than that for air although for some fuels the limit fuel flow with oxjrgen enrichment is actually less than for air. To show the variations of the maximum limit fuel 5.6

5.4 ~0"3 0 E] O A V

s ~'4.0

3.2

HYDROCARBON n-OCTANE 2- METHYLOCTANE OCTENE"I HEPTENE'I ISOOCTANE CYCLOHEXENE

.j

3.2

u.

2.4

== ,.=

I I 2 3 PRESSURE, otto

-3

4.8 4.0

(a) ~ ~.4

I I

FORMATION

l

HYDROCARBON

9 ~

(b)

,.s

I 2

I 4

5.6 ~4.8 '4.0

j3.2

o .-OCTANE

.3

~

I 4

I 6 PRESSURE, r

I 8

I I0

I 12

FUEL 0 tO0 PER CENT n-OCTANE 90 PER CENT n-OCTANE **. I0 PER CENT TOLUENE 80 PER CENT n-OCTANE 4. 20 PER CENT TOLUENE dP-4

(c) 2.4 ~

1.6

~ .e I I

I I 2 :5 PRESSURE, otto

I 4

FIG. 2. Variation in smoke-free fuel flow with pressure. (a) Six hydrocarbons at pressures to 4 atmospheres; (b) two hydrocarbons at pressures to 12 atmospheres; (c) various fuels and blends at pressures to 4 atmospheres. limit would be expected if turbulent flames were studied. Oxygen E n r i c h m e n t o f External Air

While increasing the rate of air to a diffusion flame will increase the chances for diffusion of oxygen into the flame, a greater quantity of oxygen could enter the flame if enriched oxygennitrogen mixtures were used. Consequently oxygen-nitrogen mixtures with oxygen concentrations ranging from 25 to 45 per cent were substituted for air. The effect on the smoke-free fuel flow obtained by substituting oxygen enriched mixtures of oxygen and nitrogen for air with the same eight hydrocarbons is shown by Figures 7a and b.

flow with oxygen enrichment a crossplot of the data is shown in Figure 8. For propene, cyclopropane, and butene-1 (Fig. 8a) the limit fuel flow is the same for 21 per cent 02-79 per cent N2 and 25 per cent 0~-75 per cent N : , b u t decreases for 30 and 35 per cent O3 mixtures. An increase is again observed for the 45 per cent concentration. For isobutane, neopentane, and pentene-1 (Fig. 8b) the limit fuel flow continually increases with increasing oxygen enrichment. This was also true for ethylene and n-butene. Argon S u b s t i t i o n i n External Air

The purpose of using argon in place of nitrogen in the external air was to increase the flame tem-

319

MECHANISM OF SMOKE FORMATION IN DIFFUSION FLAMES

perature without changing the oxygen concentration. An increase in temperature of 250 ~ to 400~ is probably obtained by this method. Substituting argon for nitrogen in the oxygen enriched mixtures caused the smoke-free filel flow to increase except for butene-1, cyclopropane, and propene, where initial rates were lower than with the corresponding oxygen-nitrogen mixtures. These results are shown in Figures 7 and 8.

Fuel Temperature To study the effect of preheating a fuel, the apparatus shown in Figure 5c was used. The 9 mm burner tube 700 mm in length, was jacketed by an electric heating coil controlled by a variac transformer. To stabilize the flame a chimney 47 mm in diameter and 300 mm high was placed

smoke formation. While the results did not yield a complete mechanism, they were interpreted to indicate a possible step involved in the early stages leading to smoke formation. The most consistent information which is currently available in the literature was then combined with these findings to complete t m mechanism. The following explanations are proposed to explain the preceding experimental results. The effect that pressure exerts on smoke formation was attributed to the rate of diffusion and mixing of the fuel and air. This explanation is proposed since the smoke-free fuel flow was found to be inversely proportional to pressure and the diffusion coefficient is known to be inversely proportional to pressure. The effects of the other variables studied are FUEL .tO. 3

5.6:

iiiii!:=:

4 8:tO_3

d.

3.2

~

2.4

~

1.6o .8

0 I .2

P 4

I I I I I I .6 .B I O 12 1.4 16 RECIPROCAL OF PRESSURE, otto "1

~' ISOOCTANE b" CYCLOHEXENE i i 18 2.0

o I00 PER CENT n-OCTANE

4.8 jp_240 PER CENT TOLUENE o

4.0

~

2.4

d,

~ .B I .2

'

I

i

I

i

I

I

I

.4 .6 ,8 I0 I2 4 1.6 RECIPROCAL OF PRESSURE, otm"

1.8

(a) (b) FIG. 3. Smoke-free fuel flow against reciprocal pressure. (a) Five hydrocarbon fuels; (b) various fuels and blends.

with the bottom edge level with the port of the burner tube. After the burner tube temperature had reached a constant value with a small air flow passing through it, the maximum rate of fuel which could be burned without smoking was measured. The temperature was then raised until a higher equilibrium value was reached and the smoke-free fuel flow again determined. The results showed that increasing the flame temperature b y preheating the fuels over a range of 30 ~ to 190~ had no effect on smoke formation. The change in flame temperature obtained by this method is probably small.

Postulated Mechanisms of Smoke Formation An attempt to explain the preceding experimental results was undertaken with the hope that an understanding of the factors studied herein would be helpful in postulating a mechanism of

16.5]

e~ i I . O 1 ~10.O 9.O1

...I 8. C ~- 7 . 0 ~

6.C

O ISOPAR~I~

,.', ~.o ISO

4.o

--

2.0 1.0 0

f

ANE

FINS*I

YLG'YGLOPEiMTAIME GYGLOOLEPINS

SUTADI~iE" le37 2

3 4 5 6 T NUMBER OF CARBON

MATIG~.

B 9 ATOMS

I0

F]G. 4. Variation of maximum smoke-free fuel flow with number of carbon atoms. too complex to be satisfactorily explained by any one physical process such as diffusion and consequently can have only a chemical interpretation.

320

DIFFUSION FLAMES AND CARBON FORMATION

(,,.)

@)

(c)

mrn

m

I.I ~EATEO

( r~ ( BURNERTUBE I U I (9 mm diam.)

d

I. l~

~ 700 mm L E N G T H J ~

I ~ I

I

BOMB~ AIR FUEL FUEL ~

BATH-

HEATING

FIG. 5. Diagram of apparatus: (a) burner tube, (b) apparatus modified for vaporized liquid fuels, (c) apparatus modified for preheating fuels. 3,60 I LIMITING FUEL ! FLOW II :> 6 . 4 = 1 0 " 3 I ! I I

500 -

240 -

~

9

r

./ PJ

120 ~

HYDROCARBON

i

o o 0 v t~ & J~ z~

~

60 -

01

I

I

I

8UTENE-I CYCLOPROPANE PROPYLENE FENTENE~ NEOPENTANE ISOBUTANE ETHYLENE n-BUTANE [

1.6 3.2 4.8 6.4 8.0x I0" 3 MAXIMUM SMOKE-FREE FUEL FLOW, g/sec

FIG. 6. Variation in smoke-fuel fuel flow with air flow.

--~

160

OXYGEN, NITROGEN, OXYGEN,ARGON, % % % % 21 79 -4>-- 21.5 78.5 25 75 - t ~ - - 24. i 75.9 Z9.6 70.4 -47--- 34.2 65.8 34.9 65. I 44.5 55.5

From the results of the study on fuel type a correlation was found to exist between the tendency of a fuel to smoke and the stability of the carbon chain or skeleton of the fueU (Fig. 9). Values of the bond strengths are approximately C - - C , 80; C - - H , 100; C ~ C , 150; and C ~ C , 200 kcal/mole. As the stability of the carbon skeleton is increased the chances of dehydrogenation are increased and it is proposed that this increased ease of removal of hydrogen atoms as compared to the breaking of carbon bonds is responsible for the variations in smoking among different fuel types. These findings would indicate that thermal and oxidative dehydrogenation processes, which would occur very early in the burning process, influence the further course of smoke formation. The thermal dehydrogenation is probably occurring as soon as the fuel enters a laminar diffusion flame because the fuel is heated to temperatures of the order of 800 ~ to 1000~ before coming into contact with oxygen. The importance of dehydrogenation processes is supported by the results of the oxygen enrichment and argon substitution studies which showed that when fuels of high smoking tendency (butene-1, propene, and cyclopropane) were subjected to oxygen enrichment and consequently increased flame temperatures, or to increased flame temperatures directly through the use of argon, the smoking tendency increased up to a certain point. The higher flame temperatures probably were able to promote dehydrogenation reactions in the case of these compounds possess-

9,-.o---.c--

180 160

140

-

140

(a) ~

120 120 ,? o

,00-

-J tL

80-

g

40-

25

200

-

u

OXYGEN, NITROGEN, OXYGEN, ARGON, % % % % 2t 79 --..r 21.5 78.5 75 - - l d - - 24. t 75.9 54.9 65.1 - ~ ' - - 54.2 65.8

r

) io I

F

,,?

9

tOO

,,

V

-;

(b)

o

o ^ ~,

80

eo-

60

2

20

_ I I I LO 1 'q" 0 2. .4 .6 .S 1,2x10"3 MAXIMUM SMOKE-FREE FUEL FLOW, g/see

m

40

T I I l [ I T I .4 .8 1.2 1.6 2.0 2.4 2.8 3.2 0 MAXIMUM SMOKE-FREE FUEL FLOW, g/sec

g~ g ~

FIG. 7. Variations in maximumsmoke-free fuel flow with oxygen-enriched mixtures. (a) butene-1; (b) neopentane.

MECHANISM OF SMOKE FORMATION IN DIFFUSION FLAMES

ing a very stable carbon skeleton. I t would appear that once the dehydrogenation process has advanced sufficiently far the increased chances for diffusion of oxygen resulting from oxygen enrichment are no longer effective in reducing smoke formation. However, at the highest oxygen concentration, 44.5 per cent, the temperature may have become so severe that the thermal and oxidative dehydrogenation processes were no longer favored over the breaking of carbon bonds, or at this high oxygen concentration the direct reaction of oxygen with carbon forming intermediates is favored.

hydrogen removal by existing hydrogen atoms or other active particles. These two types of dehydrogenation are of course occurring simultaneously. Other investigators have considered the importance of dehydrogenation reactions and tend to support these findings. For example, the energy requirements for removal of a hydrogen atom from an acetylene molecule by an existing hydrogen atom, I-I + C~H~ ~ H~ + C~H was considered by Frazee and AndersonE ThL

,/

3"550" 3 (AT437% 02, FL>gXJO"3) l 3.0 [ 2.5

(a)

//'~

!1141

2.01.5

/(AT

1.0

~ o- a

.5 l,

I

(AT 437% % FL>9,1dS)

4 44.S~ 02, FL> 35x;0 "31

I

I

I

I

, ~ o~.~r

"

_+s

3V

MIXTURE ,,--o.,,,- OXYGEN-NITROGEN - .-0- - OXYGEN-ARGON I

321

~

S~.t.O.3

(AT 437~ 02, FL'9'IO-3)

I

" 1.51-3

(d)

MIXTURE

'''"

(e)

4~T437% 02, FL~9'.O'3)

(b) :E 9

l

,.~0 "5

l

I

1

I

I

I

}

I

I

1,

I

I

I

I

4

/,,T ,,.,. 02, ~:,.,0-3

-3

I~flAT 43.7% 02, FL>gXIO'3)

..~

3

(f)

(c) .fi

2;

I I I I I T I 20 24 2B 32 36 40 44 OXYGEN, PER CENT

24 28 32 36 40 44 418 OXYGEN,PER CENT

FZG. 8. Variation limiting fuel flow with oxygen enrichment. (a) propane, (c) butene-1, (d) isobutane, (e) neopentane, (f) pentene-1. For compounds of lower smoking tendency the smoke-free fuel flow increased with increased oxygen enrichment or increased flame temperature. While these compounds of lower smoking tendency were also undergoing increased thermal decomposition at higher temperatures, the decomposition may have become more evenly distributed among C - - C and C - - H bond breaking reactions. Actually, the breaking of C - - C bonds probably exceeded that of the C - - H bonds since, as mentioned above, the bond strengths for C - - C and C - - H bonds are about 80 and 100 kcal/mole, respectively. Since the carbon bonds in unsaturated molecules are appreciably stronger, such molecules could be more readily dehydrogenated. Compounds possessing very stable carbon skeletons would not only contribute a large concentration of H atoms to the flame as the result of thermal dehydrogenation, but would also be most subject to additional

2O tO-3

~,

16

o,.

12

~

o (3 v z, ~. 'q O

. O~o~

0

>

O~ 5L

0

40

(b) cyclopropane,

n - PAR~CFINS I SOPARAFFINS OLEFINS - I CYCLOPARAFFINS ALKYNES-I AROMATICS CYCLOOLEFINS

0 o

SO e 0 70 80 9 0 IOO i10 12:0 130 TOTAL CARBON-CARBON BOND STRENGTH (kcol/mole) NO. OF CARBON ATOMS

14o

FIG. 9. Variation of maximum smoke-free fuel flow with total carbon-carbon bond strength per number of carbon atoms. reaction has an activation energy of only 5 kcal/mole which indicates that it could easily be occurring. Activation energies for the reaction of hydrogen atoms with various other hydrocarbon compounds are of similar magnitude ~. I t was observed by Arthur 4 that suppression of the hydrogen atom concentration in flames ac-

322

DIFFUSION FLAMES AND CARBON FORMATION

companied the suppression of smoke formation. Arthur concluded t h a t smoke formation could be prevented by reactions which consume hydrogen atoms or render them ineffective. I t was observed ~that smoke formation could be suppressed by CO and N~ which serves as nuclei for the recombination and removal of H atoms. In addition to H atoms Sachs and Ziebell6 report that other atoms and radicals such as those found in the pyrolysis of halogen compounds increases smoke formation. All of these active species probably promote polymerization reactions leading to smoke by stripping hydrogen from the fuel molecules. The halogen atoms might be expected to react with and thereby remove existing hydrogen atoms, but the concentration of fuel molecules is probably so much greater than the concentration of the shortqived hydrogen atoms that the halogen probably tends to dehydrogenate the fuel molecules. I t is therefore proposed that the removal of H atoms from the fuel molecules both by thermal processes and by the presence of other active atoms is probably the initial process involved in the formation of smoke. The steps that might occur after total or partial dehydrogenation are very controversial. Since the aromatics are by far the class of compounds having the greatest smoking tendencies and since the graphite structure of carbon particles resembles the molecular structure of benzene's multi-ringed homologues, the formation of smoke by a build-up of aromatic ring structures has been suggested. This theory is not in opposition to the initial step of hydrogen removal since the aromatic fuels would have to lose some hydrogen atoms before a polymerization of the rings might begin. For nonaromatic fuels the hydrogen atoms would have to be removed to permit the formation of unsaturated ring structures. However, absorption spectroscopic examinations in the ultraviolet of a benzene flame by Parker and Wolfhard 7 indicated that benzene is consumed in the lowest portion of the flame and that an appreciable gap exists between the disappearance of benzene and the appearance of smoke particles. No intermediate aromatic products, which absorb very strongly in the ultraviolet, are found in this region. The absence of aromatic intermediates in this gap is evidence against the ring building theory. Thorp, Long, and Garner s analyzed the smoke from a fiat benzene flame and found diphenyl to

be present, but in later work 9 they did not find diphenyl present in the products of the soots from paraffinic or naphthenic fuels. Consequently Garner, Long and Thorp 9 now consider it unlikely that carbon formation in hydrocarbon diffusion flames occurs through diphenyl, the polyphenyls, or polycyclic aromatics as intermediate compounds. Parker and Wolfhard have proposed as mechanisms for smoke formation (1) the formation of large molecules which may graphitize from within; or (2) an increase in the concentration of moderately high hydrocarbons until the saturation vapor pressure is reached, at which time condensation occurs to form a mist of droplets that form nuclei and graphitize. Observations on the physical nature of smoke by Grisdale '~ show that the first mechanism of these two proposals is not likely to be occurring. Frazee and Anderson: also object to the first mechanism since a prohibitive energy requirement would be expected if large molecules build up and then are dehydrogenated directly to solid carbon. They also propose that similar difficulties arise for a mechanism involving formation of liquid droplets of polymeric hydrocarbons followed by pyrolysis. Porter u concludes from considerations based on the time available for polymerization and on the nature of the smoke formed that the droplet formation mechanism or polymerization mechanisms in general do not contribute to any significant extent in ordinary diffusion flames. After the initial dehydrogenation process it would appear that neither aromatic type compounds nor large polymers are immediately built up, but rather that fuel molecules continually decompose to yield relatively small molecules or fragments of molecules. The findings of Ported' would also lead to this conclusion. By rapid adiabatic photolysis Porter was able to simulate in a reaction vessel the rates of liberation of heat and free radicals which occur in flames. A rapid quenching resulted in the retention of a fraction of all stable intermediates. Complete analysis of the products clearly indicated that the series of reactions leading to smoke did not involve the formation of higher hydrocarbons. Porter's work indicates that fuels in combustion waves decompose thermally and also via intermediate oxidation products to acetylene. A series of reactions proposed by Porter are as follows: C2H6 ~ C~H4 ~

C~H2 ~

carbon

323

MECHANISM OF SMOKE FORMATION IN DIFFUSION FLAMES

Tropsch and Egloff~2passed pure hydrocarbons rapidly through a heated tube for various contact times and found in the case of ethane that ethylene, acetylene, and carbon were formed. For methane the results were similar with acetylene again being formed. This experiment simulates the reactions that might occur in the lowest portion of a diffusion flame where the fuel is heated previous to coming into contact with oxygen. Since acetylene may be the last stable product to appear before smoke formation, the final step in the mechanism of smoke formation would be that leading from acetylene to smoke. If all hydrogen atoms were removed from the C~H2 molecule, the mechanism for the final step in the formation of smoke would be that of the polymerization of C~ radicals. The polymerization of C~ to smoke was at one time considered a plausible and promising explanation of smoke formation. The idea initially resulted from limited spectrographic observations 13 which showed increasing C2 concentration in flames capable of intense smoke formations. The theory of smoke formation via C~ has now been rejected by numerous authors 2' 7.11 and will be given no further consideration. A mechanism based on simultaneous polymerization and dehydrogenation is proposed by Porter to account for the steps between acetylene and smoke. The reaction is exemplified by the following equation: C - - + C2H~ --) ~ C - - C ~ C --~

I H

I l l ---ff H ~---C~-C-- § H2

I

H

Since no aromatic or high molecular weight hydrocarbons were isolated in Porter's study, it would appear that such compounds do not need to be involved in these final steps leading to smoke. Garner, Long, and Thorp 9 conclude from the work previously discussedthat the formation of smoke probably occurs through the production and further reaction of a diene which would only have a transient existence. Porter's mechanism indicates the formation of such transient diene structures. Analogous to the work of Porter is the indication of Frazee and Anderson that a mechanism involving free radicals or atoms is important in the formation of smoke. They considered the energy requirements for such free

radical reactions and report the following values: E, kcal/mole

C2H -F C2H~ -~ C4H~ --~ C4H 4- H~ C2H + C2H2 -* C4H~ + H C2H2 + C2H~ ~ C,H~ + H

~58 ~29 ~60

(1) (2) (3)

Frazee and Anderson indicate that the first equation is of the type suggested by Porter and propose that, while the reaction is feasible and may occur to some extent, its activation energy is much higher than that for the reaction shown in the second equation. The third equation shows a possibility for the use of the energy of combination in initiating reactions perhaps more effectively than by purely thermal means. At particularly high temperatures, more than one reactive radical or H atom could result from a step such as the second equation, and chain branching might result. They also indicate that steps such as those shown above can occur with molecules containing larger and larger numbers of carbon atoms, leading eventually to formation of carbon nuclei and even to growth of a particle.

Summary In final summary the general mechanism of smoke formation, based on the information currently available, probably proceeds as follows: (1) Some hydrogen atoms are removed from the fuel molecule by thermal processes. These hydrogen atoms in turn cause further dehydrogenation of the molecule. The more readily the hydrogen atoms are removed as compared to the breaking of carbon bonds the greater is the probability of smoke formation. (2) After these initial dehydrogenation steps the fuel molecules probably continue to decompose to smaller molecules and fragments of molecules. Acetylene is reported to be the last stable product to appear before smoke formation. Various authors have shown that a breakdown to smaller products must occur rather than an immediate growth to polymers or aromatic ring structures. (3) Although a breakdown to small fragments and relatively small molecules takes place, the formation of smoke through polymerization of C2 radicals has been rejected. It has been proposed that in the final stages the small molecules such as acetylene and hydrocarbon fragments undergo a simultaneous polymerization and dehydrogenation to form smoke.

324

DIFFUSION FLAMES AND CARBON FORMATION

REFERENCES

7. PARKER, W. G., AND WOLFHARD, H. G.: J.

1. SCHALLA,R. L., AND McDoNALD, G. E.: Ind. & Eng. Chem., 45, 1497 (1953). 2. FRAZEE, J. D., AND ANDERSON, R. C.: Presented at 124th Meeting of Am. Chem. Soc., Chicago, Ill., Sept. 1953. 3. STEACIE, E. W. R.: Atomic and Free-Radical Reactions. Reinhold Pub. Corp., New York, 1946. 4. ARTHUR, J. R.: Nature, 165, 557 (1950). 5. IYENGAR, M. S., VAIDYESWARAN, R., AND DATAR, D, S.: J. Sci. Ind. Res., llB, 455 (1952). 6. SACKS, W,, AND ZIEBELL, M. T. I.: National Aeronautical Establishment, Lab. Report LR-30, June 1952.

Chem. Soc. (London), 1950, 2038 (1950). 8. THORP, N., LONG, R., AND GARNER, F. H.: Fuel, $0, 266 (1951). 9. GARNER, F. H., LONG, R., AND THORP, N.: Fuel, $2,116 (1953). 10. GRISDALE, R. O.: J. Appl. Physics, 24, 1082 (1953). 11. PORTER, G. : Fourth Symposium (International) on Combustion, p. 248. The Williams & Wilkins Co., Baltimore, 1953. 12. TROPSCH, H., AND EGLOFF, G.: Ind. & Eng. Chem., 27, 1063 (1935). 13. SMITH,E. C. W.: Proe. Roy. Soc. (London) 174A, 110 (1940).

26

CARBON FORMATION FROM C~H2 AND CO IN DISCHARGE TUBES By A. G. GAYDON AND A. R. FAIRBAIRN Introduction

Processes of soot formation are far from understood; reviews of possible mechanisms have been given by Gaydon and Wolfhard ~ and Porter 2. The mechanism may depend on the temperature in the region of the flame in which tbe formation of free carbon occurs, on whether or not oxygen is present, and on the nature of the fuel. There is good reason to believe that in pyrolysis of pure hydrocarbons and in some diffusion flames there is condensation or polymerization to relatively large molecules of low vapor pressure which may actually condense to liquid droplets, which then graphitize from within. For hot premixed flames, however, there is little actual evidence for the formation of such large molecules. Porter has pointed out that temperature conditions are likely to lead to breaking down rather than to the building up of large molecules, and he has suggested the formation of acetylene as an intermediary. There is certainly plenty of recent evidence for the formation of acetylene from other hydrocarbons, but the mechanism by which relatively large carbon particles are formed from acetylene remains uncertain; Porter suggests reaction of a free radical with acetylene, giving simultaneous polymerization and dehydrogena-

tion. The theory suggested by Smith 3 of polymerization of C~ radicals (known spectroscopically to be present) has received little support from recent work and it is probably safe to discard it, but it remains possible t h a t the C~ radicals might act as nuclei for the polymerization-dehydrogenation of acetylene. In flames of burning hydrocarbons there will be some competition between processes of oxidation and of carbon formation by pyrolysis. In some cases even when there is sufficient oxygen for fairly complete oxidation, sufficient carbon is formed to render the flame luminous. For ethylene flames carbon should not~ on equilibrium e~nsiderations, be formed, provided oxygen is in excess of 34 per cent of the stoichiometric amount, but actually luminosity may be observed up to 52 per cent ~. Carbon luminosity may even be observed near the weak limit for methane in closed-vessel explosions at high pressure. During some experiments on atomic flames and afterglows, in collaboration with Broida 4. 5, ~ it was found that carbon monoxide gave a type of bluish afterglow and that when acetylene was introduced into this afterglow a carbonaceous deposit was formed on the walls of the afterglow tube. Since the spectrum of the afterglow showed