Characteristics of premixed trimethylaluminiumoxygen flames

Character&tics of Premixed Trimethylalum&ium-Oxygen Flames M. VANP~E, E. C. HINCK* and T. F. SEAMANS Physics Department, Reaction Motors Division, Thiokol Chemical Corporation, Denville, New Jersey, U.S.A. (Received February 1965) Premixed flames of trimethylaluminium vapours and oxygen have been stabilized at reduced pressures (3 to 25 m m of mercury) and a number o] their characteristics (stability region, burning velocity, emission spectrum) have been investigated. The flame is characterized by a double reaction zone, the first caused by aluminium reactions, and the second by hydrocarbon reactions. The experimental results lead to the conclusion that the overall burning velocity is kinetically controlled by hydrocarbon reactions with the aluminium supplying only additional heat. The usefulness of investigating metal-alkyl flames for the understanding of metal combustion in general is discussed.

Introduction METAL-alkyl flames have been very little investigated. The only extensive work on the subject, of which we are aware, is that of A. C. EGERTON and S. RODRAKANCHANA 1 who investigated the diffusion flames of zinc dimethyl, cadmium dimethyl and lead tetramethyl in a Wolfhard burner using either oxygen or air as the supporter. So far it appears that premixed flames of this type of compounds have not yet been obtained. In addition to their interest for the morphology of flames in general, these flames are of special interest in relation to metal combustion because the metal is introduced in the gas phase, thus eliminating the complex processes of evaporation, Trimethylaluminium (TMA) was chosen for this investigation because of the interest which exists today in the combustion of aluminium, This compound is a liquid which boils at 130°C and which ignites spontaneously in air at ordinary conditions of pressure and temperature, Because of these properties, premixed flames of trimethylaluminium cannot be stabilized at one atmosphere. Use of a low pressure flame technique appeared to be inevitable.

burner. The same kind of housing as that designed earlier by H. G. WOLFHARD2 was used. The TMA was heated to 90°C in the lecture bottle in which it was received. A metal throttle-valve regulated the vapour flow through an orifice. The orifice and its associated manometers were placed in a 90°C oil bath and calibrated for various back pressures by condensing the metered TMA vapours and observing their displaced volume during measured time intervals. All TMA lines were Pyrex tubing and had heating wires spiralled around them to provide a temperature of at least 90°C throughout the system. The TMA was introduced with the oxygen at the bottom of the 1 m heated burner tube; and mixing took olace in a 30 cm section filled with glass beads of 6 mm diameter. A nitrogen shroud surrounding the burner swept all products from the vessel. Ignition was provided by two plate electrodes approximately 20 cm apart fed by a 2 kW transformer. The system was maintained under vacuum by two Heraeus mechanical pumps with a capacity of 750W/min at 3 to 25 mm of mercury. Trimethylaluminium was obtained from the Ethyl Corporation and was used as received. The vendor's analysis indicated 36.7 per cent aluminium and 0"09 per cent chloride. Nitrogen and oxygen were obtained in commercial cylinders at 99"8 per cent purity. Since the flames were quite bright, a spectro-

Experimental Throughout the experiments flames were formed at low pressure on a 28 mm diameter Pyrex *Present address: Ingersoll-RandCompany, Bedminster, New Jersey. 393

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graph of relatively large dispersion could be used for identification of spectra in the visible and ultra-violet regions. A Wadsworth Jarrell Ash 1'5 m grating spectrograph was found very convenient for this work. For the first exploratory work and for the much less bright oxygenrich flames, a Roman Hilger spectrograph with interchangeable ~ u s quartze and dglass oDtics . was In addition, emission spectra in the infra-red were obtained with a Perkin-Elmer Model 12C Special spectrophotometer using a calcium fluoride prism and a lead sulphide detector. This instrument was also used to take axial spectrophotometric traverses of the flames both in the u.v. and visible regions.

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The general appearance of the stoichiometric flame (F/O=0-167) is depicted in Figure 2(a). The flame is characterized by multiple reaction zones. A conically shaped, very bright, whiteyellow reaction zone approximately 2 mm thick is observed at 7 mm of mercury pressure. This is succeeded by an 8 mm zone of intense blue light (purple close to the burner rim). An orange column extends above these zones surrounded in turn by a faint pink-orange region. With fuel enrichment the white-yellow reaction zone turns to white-green and the blue zone becomes more diffuse and finally disappears into the overall plume. At F / O = 0 . 2 8 the carbon point is reached near the burner rim. Upon

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The first experiments were performed on a concentric tube burner which had only a small premixing section to prevent any explosions. At the low pressures concerned, 3 to 10 mm of mercury, the stoichiometric mixture TMAoxygen did not ignite spontaneously and premixed flames could be obtained which burned smoothly without incident. The inner tube was then removed and full premixing over the 1 m burner length was allowed. The stability region for the stoichiometric flame, (F/O)mo]e= 0"167, is shown in the velocity/ pressure diagram of Figure I. For comparison the stability region of the stoiehiometric ethylene-oxygen flame is also shown.

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Burning velocity Burning velocities were obtained at pressures from 4 to 18 m m of mercury. T h e y were calculated as the volumetric gas flow (corrected for a burner temperature of 90°C) divided b y the area of the reaction zone. The latter was taken as being the bright inner cone and was determined b y tracing the flame-burner image on a ground glass plate at the focal point of a camera. The burning velocity of the stoichiometric flame was found to be 7"5 m / s e c . The change in burning velocity with mixture ratio is shown in Figure 3.

Spectroscopic observations (I) Stoichiometric flames--A spectrogram of the stoichiometric flame using the Jarrell Ash 1-5 m grating spectrograph is shown in Figure4. The prominent features of the spectrum are the aluminium doublets, and OH bands in the u.v.,

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Figure 5. Spectra of a stoichiometric premixed TMA-oxygen flame, small dispersion The stoichiometric flame also emits v e r y weak C 2 b a n d s . These b a n d s were difficult to observe because t h e y occurred on t o p of a strong continuous b a c k g r o u n d a n d also because the prom i n e n t b a n d h e a d at 5 105"1• (Swan band) is o v e r s h a d o w e d b y A10 bands. T h e C., b a n d s are seen in Figure 5(b) t a k e n with the Hilger R a m a n spectrograph. Spectra t a k e n in the b u r n t gases show again a p r o m i n e n t line a n d b a n d emission due to A1, A10, O H a n d C H . However, C, b a n d s were absent. T h e overall intensity is m u c h lower t h a n in the reaction zone b u t there is a strengthening

of the c o n t i n u u m b a c k g r o u n d as c o m p a r e d to the line and b a n d intensities. R a d i a t i o n intensity m e a s u r e m e n t s were also m a d e using the P e r k i n - E l m e r spectrophotometer. Figure 7 shows axial spectrophotometric traverses t a k e n in a stoichiometric flame. The m e a s u r e m e n t s were m a d e for the OH, C H , A10 a n d A1 r a d i a t i o n a n d for the c o n t i n u u m at 4 470A, a wavelength free of other radiation. All radiations show a s h a r p intensity m a x i m u m in the reaction zone. The A10 b a n d s a n d the c o n t i n u u m reach their m a x i m u m somewhat earlier t h a n the CO, O H and A1 radiations indi-

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Figure 6. Microphotometric record of the 2 712.~ and 2 782~ bands oJ a TMA--oxygen flame cating t h a t the combustion of a l u m i n i u m precedes t h a t of the h y d r o c a r b o n , Some i.r. s o e c t r a were also t a k e n using the P e r k i n - E l m e r with a calcium fluoride prism and a lead sulphide detector. The spectra (Figure 8) show the H_oO b a n d s at 1-25, 1-8 a n d 2-5a on top of a strong continuum. T h e c o n t i n u u m is strongest in the reaction zone a n d occurs at an earlier stage t h a n the water bands. Spectral scans t a k e n just a b o v e the b u r n e r rim do in fact show only a continuous radiation. (2) L e a n f l a m e s - - L e a n flames were similar in a p p e a r a n c e to stoichiometric flames with the exception t h a t at v e r y low fuel concentrations (below F / O = 0 - 0 2 ) the flame s e p a r a t e d into two zones with a d a r k space between them. Spectra showing the double reaction zones are given in Figure 9. The first reaction zone is again characterized b y the A1, OH, C H a n d A10 emission on t o p of a grey b o d y t y p e continuum. I n the s e p a r a t i o n zone only O H b a n d s

are present. A1 lines a n d A10 bands r e a p p e a r in the second zone. The m a i n feature of the second zone is, however, its strong continuous emission which shows, in a d d i t i o n to a grey b o d y t y p e radiation, a second continuum with a well defined m a x i m u m in the blue. I t is likely t h a t this second c o n t i n u u m is the well known C O + O continuum which a p p e a r s c o m m o n l y in h y d r o c a r b o n and c a r b o n m o n o x i d e - o x y g e n flames. Discussion

I t will first be of interest to c o m p a r e the b u r n i n g characteristics of the m e t a l - a l k y l flame with those of o r d i n a r y h y d r o c a r b o n flames. This comparison is m a d e in Table I a n d reveals that the b u r n i n g velocity of the TMA flame is intermediate between that of acetylene a n d ethylene flames. This is w h a t one would h a v e expected n o r m a l l y in view of the a d i a b a t i c flame temperatures which are seen to v a r y in the same

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order as the burning velocities. It appears, therefore, that the presence of aluminium has not affected the propagation mechanism appreciably and that the TMA flame is kinetically controlled b y the hydrocarbon reactions with the aluminium combustion adding only supplemental heat to the system. It is of interest to note that an analogous situation has been found for the diborane-oxgen system which in this case is rate-controlled by the hydrogen reactions*, Except for the additional features due to aluminium and its oxides, the TMA flame spec-

399

trum is very little different from a hydrocarbon flame spectrum although the overall intensity is much higher, due very probably to the higher temperature. The relative intensities of the OH, CH and C 2 bands are very much the same as in the ethylene f a m e of equivalent fuel/oxygen ratio. Also very high rotational temperatures of the order 5 000°K were found for OH as has been observed earlier in low pressure hydrocarbon flames't Evidence of high rotational temperatures existed also in the (0,0) and (0,1) bands of the 3 900/%, system of CH. The predissociation usually observed in these bands was apparent in the vicinity of K = 2 0 in the (0,0) band; that is, at relatively high quantum numbers as was observed by R. A. DURIE6 in hydrogen flames to which had been added small amounts of ethylene. Before discussing the combustion of aluminium, a word should be said about the reactions which cause TMA and other organometallic compounds to ignite spontaneously with air. These reactions have been investigated by various authors 7, ~ and have been shown to consist of the formation of peroxide which can, upon decomposition, initiate a chain mechanism leading to ignition. The present investigation has shown that these low temperature reactions m a y be avoided by operating at reduced pressUre. In view of the hydrocarbon character of the TMA flame, there is doubt that the low temperature reaction plays any significant role in the flame mechanism. The region where the combustion of aluminium takes place is undoubtedly the bright yellow zone which appeared to be not more than 2 m m thick to the naked eye. The flame spectrum in this zone showed the A1 lines and the A10 bands strongly in emission against a continuous background indicating the presence of incandescent particles. The fact that the A10 bands vary similarly in intensity as the continuum indicates that A10 is an intermediate of reaction rather than a product of dissociation of aluminium oxide. The mechanism b v which the condensed metal~ oxide is formed is still a matter of controversy 9-u. G. n . MARKSTEIN12, who conducted experiments on dilute diffusion flames of magnesium in an oxygen atmosphere, has found that

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the flame spectrum consisted only of continua with m a x i m a at 3900, 4500 and 6500.K and has concluded that at the relatively low temperature of his experiments (1 000°K) the combustion of the metal took place essentially on the surface of the growing oxide particles. However, W . G . C O U ' R T N E Y ' S 11 recent results on magnesium diffusion flames produced b y concentric flows of oxygen and magnesium v a p o u r do show the MgO bands and the magnesium lines at the higher temperatures. The author points out the inconclusiveness of arguments based on the presence or absence of atomic or molecular spectra in regard to the combustion mechanism. The strong incandescence observed in the reaction zone of the TMA flame and its subsequent rapid decay seem at least to favour a mechanism in which highly exothermic surface reactions must play an essential role. As noted above, lean flames show a very interesting and striking feature. At the v e r y low fuel concentrations (below F / O = 0 . 0 2 ) the flame separates into two zones. The spectroscopic data indicate that the first zone is characteristic of aluminium and h y d r o c a r b o n

combustion and that the second is probably due to the combustion of carbon monoxide. It is very likely that the condensation reactions of AI.,Q from species such as A10, A1 and atomic oxygen are the cause of this delayed combustion of carbon monoxide. In the stoichiometrie flame there is a large concentration of atomic oxygen (see T a b l e 2) and such condensation reactions will have little effect on the main chain reaction involving atomic oxygen. Therefore, the tombustion region will emerge in a single zone with only minute separations between the various types of combustion. F o r lean flames, close to Table 2. Calculated temperature and composition o[ a TMA-oxygen flame, (F/O)voz.=O.166, P=9"2 m m Hg. Flame temperature, 2 633°K Composilion

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December 1965

Characteristics of premixed t r i m e t h y l a l u m i n i u m - o x y g e n flames

the limit of flame propagation, the conditions will be more critical, and the consumption of oxygen atoms in the condensation region may prove to be completely detrimental to the propagation of the carbon monoxide chain reactions. The combustion of carbon monoxide will, therefore, be inhibited in this zone and will start only at a later stage, While the data developed in these experiments are not sufficient to define completely a mechanism for the combustion of aluminium and the subsequent formation of aluminium oxide, they do suggest a convenient new approach for the study of metal combustion in general, namely, the introduction of the metal into the combustion system in the form of a metal-alkyl, A well defined zone, characteristic of the metal combustion itself, has been isolated. This zone precedes the main hydrocarbon combustion

region and appears to be kinetically independent of it. The reason for this independency is very likely due to the fundamental differences existing between the two types of combustion mechanisms. In the one case, the essence of the combustion is the condensation reactions leading to the formation of Al.o03; in the other, hydrocarbon combustion proceeds essentially through a chain mechanism in the gas phase. By operating at low pressure the metal combustion zone can be spread out considerably and is susceptible to being investigated spectroscopically in detail by the use of a flat flame technique. The AIO bands are emitted with great intensity and quantitative measurement of the intensities of the vibrational bands and rotational lines is possible which should give insight into the mode of excitation of the molecule. In this respect it is worthwhile to note

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that a definite weakening of the (3,4) vibrational band of A10 was observed in the burnt gases. This band showed all the characteristics of an accidental predissociation as observed by B. ROSEN13 with the tellurium molecule (Te2). See F i g u r e 10.

One academic point of interest in metal cornbustion is to know whether the combustion takes place in the gas phase or whether it is a heterogeneous process taking place on the surface of growing metal oxide particles. It is crucial in this respect to know whether the metal oxide can exist in the gas phase or not. So far,

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The authors are grateful to Dr H. G. Wolfhard this work. The work described herein was sponsored by the A d v a n c e d Research Projects Agency through the O ~ c e of N a v a l Research under Contract NOnr 3543(00).

for helpful discussion of

Ref~*ence$ 1 EGERTON, A. C. a n d RUDRAKANCHANA, S. Proc. Roy. Soc. A, 1954, 225~ 427 *- WOLFHARD, H. G. Z. tech. Phys. 1943, 9, 206 COHEUR, F. P. a n d ROSEN, B. Bull. So¢. Sci. Liege, 1941, 1O, 405

403

, X~'OLFHARD, t{. G., CLARK, A. H. a n d VANPEE, M.

Heterogeneous Combustion, Progress in Astronautics and Aeronautics, Vol. 15, p 327. Academic

Press: New York a n d London, 1964 .~ GAYDON, A. G. a n d ~.VOLFHARD, H. G. Third Symposium (International) on Combustion, p 505. \Villiams a n d \Vilkins: Baltimore, 1949 (; DURIE, R. A. Proc. phys. Soc. Lond., A, 1952, 7 65, 125 ABRAHAM, M. H. J. chem. Soc. 1960, 4130 s BAMFORD, C. H. and NEWlTT, D. M. J. chem. Soc. 1946, 695 BRZUSTOWSKI, T. A. and GLASSMAN, I. Hetero-

geneous Combustion, Progress in Astronautics and Aeronautics, Vol. 15, p 75. Academic Press: New

10 York and London, 1964 MARKSTEIN, G. H. AIAA ]nl, 1963, I~ 550 11 COURTNEY, W. G. Heterogeneous Combustion, Progress in Astronautics and Aeronautics, Vol. 15, p 677. Academic Press: New York and London, 1964 12 MARKSTEIN, G. H. Ninth Symposium (International) on Combustion, p 137. Academic Press: New York, 1963 ~3 ROSEN, B. Phys. Key. 1945, 6~, 124 1J, DROWART, J., DEMARIA, G., BURNS, R. P. and ][NGHRA.'.i, M. G. ]. chem. Phys. 1960, 3 ~ 1366