carbon formation in the combustion wave

248 LAMINAR COMBUSTION AND DETONATION WAVES

REFERENCES

1. MURRAY, R. C., AND HALL, A. R.: Trans. Faraday Soc., 47, 743 (1951).

2. SZWARC, M.: Proc. Roy. Soc., A198, 267 (1949). 3. SE~mNOV, N.: Prog. Phys. Sci., U.S.S.R. 24, 433

(1940). Translated in NACA Tech. Memo. No. 1026.

4, HIRSCHFELDER, J. O., BIRD, R. B., AND SPOTZ, E. L.: Chem. Rev., 44, 205 (1949).

5. WILXE, C. R.: J. Chem. Phys., 18, 517 (1950). 6. ELGIN, J. C., A~D TAYLOR, H. S.: J. Amer. Chem.

Soc., 51, 2059 (1929). 7. BIRSE, E. A., AND MELVILLE, A. W.: Proc. Roy.

Soc., A175, 64 (1940).

8. HANRATTy, T. J., PATTERSON, J. W., AND CLEGG, J. W.: Ind. Eng. Chem., 43, 1113 (1951).

9. RICE, F. O., AND HERZFELD, K. F. : J. Amer. Chem. Soc., 56, 284 (1934).

10. AMDUR, I.: J. Amer. Chem. Soc., 60, 2347 (1938). 11. Sco'r'r, D. W., ET ALIA: J. Amer. Chem. Soe., 71,

2293 (1949). 12. GLOCKImR, G.: J. Chem. Phys., 16, 602 (1948). 13. Tables of Selected Values of Chemical Thermo-

dynamic Properties: National Bureau of Stand- ards, Washington (1947).

14. HIRBCHFELDER, J. O., AND CUIITISS, C. F. i J. Chem. Phys., 17, 1076 (1949).

15. HIRSCHFELDER, J. O., AND CURTISS, C. F.: J. Phys. Colloid Chem., 55, 774 (1951).

27

CARBON FORMATION IN THE COMBUSTION WAVE

By GEORGE PORTER

The mechanism by which carbon is formed during the pyrolysis and combustion of hydrocarbons has interested chemists for over a century (1). Recent theories have been summarised by Gaydon (2) whose work is largely responsible for a renewed interest in the problem. As well as being unsatis- factory from other points of view, these theories are not in accord with results recently obtained by the flash photolysis technique (3). A different mechanism is therefore presented briefly here which gives a satisfactory, and rather more detailed explanation of the processes involved.

When large carbon particles are formed from smaller molecules polymerisation must occur at some stage and the two main theories held at present differ widely as to the stage at which this takes place. They may be summarised as follows:

1. THE C2 POLYM:ERISATION THEORY

This supposes a complete breakdown of the hydrocarbon to its elements, and carbon, which is present to a considerable extent as C2 molecules, then polymerises to graphite. I t has been discussed by Smith (4) who supported it on the basis of a correlation between the C2 emission spectrum and carbon formation.

2. THE HYDROCARBON POLYM[ERISATION

THEORY

This has been stated by Gaydon (2) as follows: " In the presence of an excess of fuel molecules,

free radicals initiate chain polymerisation processes which lead to the formation of higher hydrocarbons which decompose thermally to solid carbon and hydrogen. In the presence of sufficient oxygen the radicals are removed by reaction with this and do not cause so much polymerisation. I t is possible that polymerisation and ring closure to form aromatic molecules may lead to the ult imate elimination of graphite."

The latter theory seemed to be the only reasona- ble alternative to the C2 mechanism against which Gaydon found several objections. I t will now be shown that there are equally strong objections against the intermediate formation of higher hydrocarbons under the conditions of combustion pyrolysis.

THE PYROLYSIS OF HYDROCARBONS

I t is convenient first to consider a purely py- rolytic mechanism and then to enquire what differences are introduced by the presence of oxygen. Unsaturated hydrocarbons undergo two types of reaction at elevated temperatures, thermal decom-

CARBON FORMATION IN COMBUSTION WAVE 249

position and polymerisation, which may be represented by the equations

AB = A + B (1)

2AB = (AB)~ (2)

For each of these reactions the specific rate constant k is given by k = kT/h exp - A H ~ / R T exp AS/R~. where AH:~ and AS~C refer to the enthalpy and entropy differences between the activated complex and the reactants in their standard states (5). For reactions (1) and (2) we therefore have

AH~ - - z~S~ AS~ -- AS~ kl/k2 = exp exp

R T R

For the simple decompositions and polymerisation of hydrocarbons one can state fairly generally that AHfi< AH1 t and zXSfi<A&~: and therefore, above a certain temperature, which depends on these values and on the concentration, reaction (1) will proceed at a greater rate than reaction (2) and will continue to do so at all higher temperatures. If reaction (2) is the reverse of reaction (1) this is equivalent to the statement that de- polymerisation then becomes more rapid than polymerisation, a special case discussed by Dainton and Ivin (6).

The temperatures at which the velocity con- stants kt become greater than ks can be calculated in a few simple cases by making assumptions about the transition state complex but in general they must be obtained from experimental data. For the present purpose this data is summarised in the statement that, for all hydrocarbons at pressures near to atmospheric, decomposition is the only reaction of importance above 1000~ (7, 8) with the single exception that the carbon polymer, graphite, is formed. The formation of carbon is, however, associated with the simultaneous or preliminary decomposition of the hydrocarbon to simpler molecules, and not with polymerisation to higher hydrocarbons. In spite of the diversity of method and the often conflicting results the primary reactions occurring in homogeneous systems at high temperatures, mostly between 700~ and 1100~ are fairly well represented by the following scheme:

Methane ~ Ethane, hydrogen (9).

Ethane ~ Ethylene, hydrogen (10).

Higher paraffins ~ Lower paraffins, olefins (11).

Ethylene --, Acetylene, hydrogen (12, 13).

Higher olefins ~ Lower olefins, paraffins, acetylene (8).

Acetylene --~ Carbon, hydrogen (8, 13).

Aromatics --~ Carbon, hydrogen, acetylene, methane (8).

Except in the case of aromatics the course of the reactions suggests that acetylene is the last product to appear before carbon formation. It is not surprising that only small amounts of acetylene are isolated in some cases as it decomposes very readily to carbon and hydrogen. This partly explains the difficulty in interpreting the reactions of aromatics where a high temperature is required for decomposition, resulting in an apparent direct dissociation to carbon and hydrogen. There seems to be no doubt, however, that the condensation of aromatics occurs to a decreasing extent at temperatures above 750~ (14). It is interesting to note that the importance of acetylene in thermal decomposition reactions was emphasised by Berthelot (15) and then by Lewes (16) but their ideas were later dis- counted (1).

In extending these considerations to carbon formation in the combustion wave it follows that, unless polymers of very high molecular weight are formed in the preheating zone, before temperatures of about 1000~ are reached, complete thermal decomposition will occur. I t is easy to show that, for most flames, the times available in the preheating zone are quite insufficient for appreciable amounts of reaction to have taken place. The reaction times for 50 per cent polymerisation of hydrocarbons at one atmosphere and at temperatures such that polymerisation proceeds more rapidly than decomposition are usually much greater than 10 -2 see (8). These are to be compared with the times of passage through the preheating zone in a premixed flame at one atmosphere, which for temperatures between 500 ~ and 1000~ are 10 .4 to 10 -7 seconds (17). Similarly, for explosions in closed vessels carbon formation is complete in less than 10 -7 sec (18). In diffusion flames longer times are available, but unless very high molecular weight polymers are formed we shall expect de- polymerisation to occur when the region of higher temperatures is reached. If times of the order of seconds are available at lower temperatures polymerisation to particles of microscopic dimensions


may occur and these will undergo an entirely different, solid phase, decomposition to carbon. If this mechanism proceeds, the following experimental observations should be possible:

1. The high polymers must be very stable and easily isolated from the preheating zone.

2. This process will compete with the usual acetylene mechanism, and owing to the fact that only the large particles will survive the thermal decomposition to lower hydrocarbons at the higher temperatures, two types of carbon should be formed. The first will consist of particles formed by solid phase pyrolysis of large polymer particles, and the second will be ordinary acetylene carbon, formed from the smaller particles which subse- quently decompose and form carbon by the acetylene mechanism. The two mechanisms will in general give a different particle size and structure and particles of intermediate dimensions will be absent.

The writer is not aware of any such observations on diffusion flames but the "droplet" formation observed by Parker and Wolfard (19) in hydrocarbons which were deliberately preheated was probably an example of the polymerisation mechanism. I t seems unlikely that this mechanism contributes to any significant extent in ordinary diffusion flames.

We conclude that carbon formation in the combustion wave occurs via a preliminary decomposition of the hydrocarbon as far as acetylene with the possible exception of very slow diffusion flames. The mechanism of carbon formation from acetylene now assumes especial importance.

CARBON FORMATION FROM ACETYLENE

The results of experiments on the products of thermal decomposition of acetylene show only that, at high temperatures, it appears to decompose directly to carbon and hydrogen. By means of the flash technique, however, it is possible to show that neither C2 nor higher hydrocarbons are essential intermediates.

The first experiments test the possibility of intermediate hydrocarbon formation between acetylene and carbon. The method used is to liberate, photochemically, a very high radical concentration and a great amount of heat internally in the gas and then to quench the products by rapid cooling at the wall, which remains at room temperature. The whole process is complete in a few millisec- onds and although large quantities of carbon are formed, a measurable amount of the original sub-

stance and of each stable intermediate remains, owing to the high temperature gradient. The most detailed analyses were carried out on ketene (13) which, when photolysed at 300~ yields only ethylene and CO. They are summarised as follows:

1. As the intensity and temperature are in- creased ethylene is replaced by an equivalent amount of acetylene and hydrogen until these become the major products.

2. Further increase in intensity results in re- placement of acetylene by carbon and hydrogen.

3. Apart from very small quantities of ethane, methane and propylene, no other hydrocarbons are present. This was shown by the C and H bal- ances and by U.V. and I.R. spectographic investigations. Less than 0.1 per cent benzene would have been detected if present.

4. Spectroscopic investigations during the carbon deposition showed that aromatics such as benzene are not present

5. Electron micrographs of the carbon showed it to be similar to that obtained from smoky acetylene flames, with a mean particle size of about 500 A.

6. Similar though less detailed results have been obtained with acetone and diacetyl where the original hydrocarbon formed is ethane (3).

These results firstly provide confirmation that, under conditions similar to those in flames, in- volving high radical concentrations and temperatures, the only important reaction of ethylene is dehydrogenation to acetylene. More important, they show that carbon formation occurs directly from acetylene without the formation of any stable intermediate, for there is no doubt that, if significant amounts of higher hydrocarbons, espe- cially aromatic ones, had been formed, a detectable quantity would have been condensed out.

We must now consider the C2 polymerisation theory. Gaydon and Wolfard (20) have produced evidence against this, based on measurements of the intensities of the C2 emission from flames, though their arguments are somewhat less convincing in view of the rapid change in C~ concentration which is now known to occur at the critical mixture ratio corresponding to carbon formation (18). The problem has been investigated by absorption measurements of relative C2 concentrations throughout flash photolysis reactions and explosions, the results of which are as follows:

I. C2 is not observed during the carbon deposition which occurs in the flash photolysis of ketene, acetone, etc.

CARBON FORMATION IN COMBUSTION WAVE 251

2. C2 is not observed in flash initiated explosions of methane with chlorine and of acetylene with chlorine, during which large quantities of carbon are deposited.

3. The experiments of Norrish, Porter and Thrush on explosions of acetylene with oxygen, initiated by the flash photolysis of a small per- centage of nitrogen dioxide, are particularly significant (18). If carbon is formed exclusively from C2 the rate of formation at any time will be propor- tional to some power of the C2 concentration, and the total carbon formed will be given by an expres- sion of the form

ao

where all the exponents n are positive. The area under the (C2)/time curve should therefore increase with the total carbon formed. In fact the opposite effect is observed in rich mixtures and eventually, when much carbon is formed, the C2 absorption is absent throflghout the reaction. This is convincing evidence that C2 is not an essential intermediate between acetylene and carbon in explosive combustion.

The formation of carbon from acetylene therefore involves neither the formation of higher hydrocarbons nor a dissociation to C2 and only one alternative remains--a simultaneous polymerisation and dehydrogenation. The process may pos- sibly be represented as follows:

j I I J l -r ..... f I H H H H

l H~ H

The formation of acetylene from graphite and hydrogen is endothermic by 54kcal/gm mole and this probably results in a very low activation energy for hydrogen elimination, and explains why carbon formation directly from other molecules, even from ethylene and benzene, is much less probable. If hydrogen is not eliminated in a particular case, owing to an unfavourable energy transfer, then a single carbon-carbon bond will remain with a high probability of depolymerisa- tion. Ring closure and chain branching will result in further dehydrogenation, and as acetylene carbon usually contains about 10% atomic propor-

tion of hydrogen, the mechanism seems entirely probable.

T H E E F F E C T OF OXYGEN ON CARBON"

FORMATION

If oxygen is present, oxidation reactions will compete at every step with the thermal decomposition mechanism, and oxygen will also facilitate chain initiation. Even in the low temperature oxidation of hydrocarbons, however, decomposition predominates over polymerisation, unless high pressures and very low temperatures are used and the primary products isolated are lower hydrocarbons, aldehydes, alcohols, ethers, peroxides, etc. (2i). Formaldehyde and methyI alcohol decompose mainly to CO and H2 and consequently no carbon is observed either in their thermal decomposition or their flames (2). All other com- pounds of this type, in the absence of further oxidation, decompose thermally to hydrocarbons and CO (7) and therefore no difference in the mechanism of the subsequent carbon formation is to be expected.

I t is possible that the reaction scheme leading to the eventual elimination of carbon plays a not insignificant part even when oxygen is in excess of stoichiometrie proportions, for the rate considerations discussed earlier may also apply to transition complexes formed with oxygen. The following examples show that, in some cases, part of the decomposition reactions proceed as far as carbon before oxidation takes place.

1. In certain diffusion flames decomposition to carbon is complete before the oxygen zone is reached (22).

2. In premixed flames, emission from carbon particles is sometimes Observed before the main reaction zone (23).

3. In premixed flames, containing sufficient oxygen for complete combustion, the C~ bands still appear in emission (2). Recent work supports the idea that the occurrence of C2 is associated with the oxidation of carbon (18, 22), and in any case it cannot be accounted for by any of the low temperature oxidation mechanisms.

In formulating the acetylene theory of carbon formation, it has not been possible to discuss the explosion experiments of Bone (1), the "pyrolysis continuum" (19), the effect of additives (2) and much other important work which bears on the problem, but does not affect our conclusions. The object has been to present a mechanism, which is believed to be in accord with the observations, in


sufficient detail to allow criticism and to suggest fur ther experiment.

ACKNOWLEDGMENT

I t is with pleasure tha t I express my great in- debtedness to Dr . A. G. Gaydon for making available to me his account of carbon formation, prior to publicat ion, and also to Professor R. G. W. Norr ish for m a n y s t imulat ing discussions.

REFERENCES

1. BONE, W. A., AND TOWNEND, D. T. A.: Flame and Combustion in Gases, Chap. 32. New York, Longmans, Green and Co. (1927).

2. GAYDON, A. G.: Spectroscopy and Combustion Theory. London, Chapman a.nd Hall (1948).

3. PORTER, G.: Proc. Roy. Soc., A200, 284 (1950). 4. S~ITH, E. C. W.: Proc. Roy. Soc., A174, 110 (1940). 5. GLASSTONE, S., LAIDLER, K. J., AND EYRING, H.:

Theory of Rate Processes. New York McGraw- Hill Book Co. (1941).

6. DAINTON, F. S., AND IVIN, K. J.: Nature, 162, 705 (1948).

7. STEAClE, E. W. R.: Atomic and Free Radical Reac- tions. A. C. S. Monograph (1946).

8. EGLOFF, G.: Reactions of Pure Hydrocarbons. A. C. S. Monograph (1937).

9. STOI~Cn, H. H.: J. Am. Chem. Soc., 54, 4185 (1932). 10. TROPSCH AND EGLOFF, G.: Ind. Eng. Chem., 27,

1063 (1935). 11. ST~BBS, F. J., AND HINSnELWOOD, C. N.: Trans.

Faraday Soc., 10, 129 (1951). 12. STORCU, H. H.: Ind. Eng. Chem., g6, 56 (1934). 13. NORRISH, R. G. W., KNox, K., AND PORTER, G.: J.

Chem. Soc., (1952). 14. ZANEW'rI, J. E., AND EGLOFF, G.: Ind. Eng. Chem.,

9, 350 (1917). 15. BERTHELOT, M. P. E.: Ann. Chim. Phys., 9, 471

(1866).

16. LEWES, V. B.: Proc. Roy. Soc., 55, 90 (1894). 17. GAYDON, A. G., AND WoLFnAm~, H. G.: Proc. Roy.

Soc., 196, 105 (1949). 18. NORRISH, R. G. W., PORTER, G., AND THRUSH, B.

A.: Nature, 169, 582 (1952). 19. PARKER, W. G., AND WOLFHARD, H. G.: J. Chem.

Soc., 2038 (1950). 20. GAYDON, A. G., AND WOLFHARD, H. G.: Proc. Roy.

Soc., 201, 570 (1950). 21. LEWJS, B., AND VON ELBE, G.: Combustion,

Flames and Explosions. New York, Academic Press (1951).

22. PARKER, W. G., AND WOLFHARD, H. G.: Proc. Phys. Soc., 65, 2 (1952).

23. GAYDON, A. G.: Private communication.

oDIscusSION BY HANS BEHRENS*

The mechanism of soot formation proposed by Dr. Porter differs considerably from the chain reaction proposed by me (see p. 538), whose overall course is that of Boudouard reaction. However, my considerations are confined to the development of the soot particles, as we are dealing with a limiting surface reaction that can only proceed by way of existing nuclei of soot particles. I have not dealt with the formation of these nuclei and I believe that Porter's investigation can be applied precisely to this initial stage 6f soot formation. To assume that soot formation in flames generally proceeds by way of acetylene leads to diffi- culties in interpreting the structure of luminous flames. According to my own observations, soot formation does not occur in the acetylene flames until the end of the combustion zone, whereas for higher hydrocarbon flames (e.g. benzene, octane) soot formation occurs at the beginning of the combustion zone. This would be surprising, if in these latter flames soot formed by way of acetylene. However, there appears to be no con- tradiction if the proposed mechanisms are considered to apply to various stages of soot formation.

* Weil/Rhein, Germany

28

QUANTITATIVE SPECTROGRAPHIC INVESTIGATION OF FLAMES

By P. AUSLOOS AND A. VAN TIGGELEN

(Translated by RUT~ F. BRINKLEY)

INTRODUCTION cats or atoms formed as intermediates in vir tual ly

The extreme rapidi ty of active combust ion re- all chemical reactions, makes the s tudy of the mech- actions in flammable gas mixtures, together with anism of these reactions extremely difficult and the general react ivi ty and instabi l i ty of free radi- raises a very special problem of chemical kinetics.

carbon formation in the combustion wave

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