Resource Papers
VII
Prepared under the sponsorship of The Advisory Council on College Chemistry
Robbin C. Anderson University of Texas Austin, 78712
Combustion and Flame
J u s t as fire was one of the earliest examples of a chemical process controlled by man, the investigation of combustion was one of the areas in which chemistry first developed as a science. It was in this area that the first major theoretical concept in the infant science originated. The work on combustion is also distinguished by one of the real classics in the exposition of chemical experimentation and logical thought. I n December, 1860, Michael Faraday gave a series of six Christmas lectures on "The Chemical History of a Candle." This series comprised an admirably lucid and interesting demonstration and exposition of the candle as a chemical system. The nature of the chemical changes occurring was explored in some detail, along with physical problems such as heat effects. The fundamental nature of gaseous systems was discussed, and also the significance of combustion in understanding other phenomena such as respiration. Fortunately these lectures were preserved for us by William Crookes ( 5 ) . They are still worth studying in themselves and they also offer a fine indication of the potential which exists for use of different aspects of work on combustion to
This series of "ltesorme Papers" is being prepared under the sponsorship of the Advisory Council on College Chemistry ( A G ) which is supported by the National Science Foundation. Professor L. Carroll King of Northwestern University is the chairman. Single copy reprints of this paper ere being sent to chemistry department chairmen of every U. S. institution offering college chemistry courses and to others on the mailing list for the ACa Nezusletler. Additional single copies will he sent free to all interested individuals who make request t,o: Advisory Council on College Chemistry Department of Chemistry Stanford University Stanford, California. 94305 For class use, multiple copy orders (in lots of 10) can be filled if accompanied by remittance of $1.50 per unit of 10 copies. No order can be billed. Checks should be drawn to the order of the Advisory Council on College Chemistry. Orders must be addressed to AC., not lo ihe Journal of Chemical Education. This is SerialPublication No. 23 of the Advisory Council.
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help students at various levels to attain a better understanding of modern chemistry. I n addition to its historical interest, combustion continues to have broad scope and significance in chemistry today. Fire is still a major source of warmth, of high temperatures, and of power-as well as danger. The fine techniques of Lavoisier and Faraday have been extended much further-to Bunsen flames, industrial heaters, and internal combustion engines; to jet engines, ramjets and supersonic rockets; and to a wide variety of chemically different combustion systems, including liquid and solid propellants. The field offers,therefore, a wealth of information on various types of chemical systems which are of practical importance today. It also offers numerous opportunities to attain a deeper understmding of chemicals and their reactions-thus of chemistry. These include the study of flow systems as contrasted to static; of adiabatic systems as contrasted to the classic isothermal cases; of the fundamentals of gaseous reactions a t moderately high temperahres (100~3000°K), including the significance of "temperature" itself; and of the interplay of reaction kinetics with other dynamic processes such as molecular diffusion, fluid dynamics, radiation, and ionization. I n addition to the contribution these make to understanding strncture and behavior on the molecular level so extensively studied today, they also help extend the consideration of chemical reactions to systems in which macroscopic parameters must be considered. For beginning students such study of combustion might well involve the extension of Faraday's concept of "chemical history" to the Bunsen flame as well as to the candle-observing the interplay of energy release and flow and chemical reaction, the nature of the reaction steps and int,ermediates, the formation of ions, and the occurrence of chemiluminescence. For more advanced students it may be used to study details of reaction mechanisms, to understand more fully the concept of temperature and the processes of energy interchange among various states, and to explore a variety of time-dependent and flow phenomena in reacting systems. The bibliography which follows bas been chosen to indicate sources of materials which may be useful in courses a t various levels. It is of necessity highly selective and no attempt has been made to make it comprehensive. I n particular it does not do justice to
is likely to be visible because of radiation resulting from reaction; but the really fundamental characteristic of the system is the existence of a reaction zone which can propagat,e through space. On this basis we may recognize certain different types of systems: those in which the flame may propagate through an approximately quiescent gaseous mixture, as in the internal combustion engine, or those in which the flame may be held steady by propagation against a flowing gas stream, as in the classical Bunsen burner. The aerodynamic flow conditions, whether laminar or turbulent, may cause differences. There is also distinction between the flames Figure 1. Burning velocity. The rate at whish o plone Rome fmnt advancer into the cold in pwr~zi~ed gases, which occur in ordinary unburned gor i s colled the burning velocity. The velocity depends primarily on the inlet burners (Figs. 1 and 4), and the d kC m , I n c , SPU. York, 1946.
Another survev of earlier work. which includes chrtn-
hon monoxide, and hydrocarbon flames. AND VON ELBE,G., "Combustion, Flames and Explosions of Gases," 2nd Ed., Academic Press, Inc., New York, 1961. The most recent comprehensive monograph in the field. Comhust.ian is treated in terms of the chemistry and kinetics of reactions between fuel and oxidsnt, flame propagation, and stabe of the burned gas, followed by s. section on technical processes including industrial burners, Otto and Diesel engine cycles, and jet engines.
(3) LEWIS,B.,
A briefer and somewhat more popular survey may be found in the following:
History
Discussion of some of the historical aspects of combustion as already noted may be found in the next four references. (5) FARADAY, M., "The Chemical History of a Candle" (Editor: Crooks. Wm.). Vikine Press. New York. 1960.
ful use of demonstration experiments and its wideranging consideration of tlssociated phenomena, such as behavior of gases, etc. (8s well as its presentation of quantitative results without use of equations). (6) LA~OISIER, A., "The Elements of Chemistry (Trsite elementaire de Chimie)," Dover Publ., New York, 1965. Recently made avsilsble in a facsimile of the original English edition of 1790. (7) CONANT,J. B., editor, "Overthrow of the Phlogiston Theory." Harvard Case Histmies in Ezperimntol Science, Case 2, Val. 1, Harvrtrd Press, Cemhridpe, Mass., 1957. (8) SEMENOV, N. N., "Some Problems Relating to Chain Reactions and to the Theorv of Combustion." Nobel The Nobel prize lecture of 1956, interesting for exposition of basic heat theory of combustion and flame propagation by a. self-accelerating surge of heat. Theoreticol Boris
bumr surtacel
Wondary reaction retlon
Point of iniBd
1emporet~~ risu Trmrprt or prsh8at rccion Primly reaction and lvminous
region
-Secondary reaction IeKlOn
Point initial temp lure rise
F
Primaw reaction 1 luminous ion
SPHERICAL FLAME Figure 4. Flame geometry. A premixed laminar Rome, such or the laboratory Bvnren flame, bums with a unique geometry. The flames are errentiolly o thin reaction sheet that conforms to the incoming gas stream or Rame holder in such o way that the burning velocity component bdoncer the gas velocity component normal to the name front. (From Reference 26, reproduced by premirrion of Chemical and Engineering Newr.)
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For all the various flame systems, basic understanding involves an interplay of chemical kinetics, thermodynamics, and fluid dynamics. I n addition to the three monographs already listed (1-S), the following volume has good coverage of these. (9) LEWIS, B., PEASE, R. N.,
A N D TAYLOR, H. S., editors, "Combustion Processes," in "High Speed Aerodynamics and Jet Propulsion," Vol. 11, Princeton University Press, New Jersey, 1961. This deals somewhat more with combustion theory than phenomena. I t is part of a. series written to give a comprehensive treatment of fundamental aspects of aerodynamic and propulsion problems of high-speed flight, including the aspects of the basic sciences cognate to such problems. A number of different aulhors contribute to sections on thermodynamics and chemical kinetics of combustion, flame propagation in gases, combustion of liquids and solids, detonation processes, and an unususl section on energy from nuclear reactions.
Modern Combustion Problems
The next group of references is useful in outlining the field of combustion and flames as a branch of modem chemistry and technology. The first five in the group are articles for the reader interested in the general nature and scope of the field. The later listings show annual reviews, symposia, and research journals in which one may find a cross-section of current research
activity. For the non-specialist,, brief cousideration of one of these (e.g., the Teuth Symposiunl, ref. 17) can be very helpful in simply showing typcs of problems under investigation. (10) PENNER,S. S., "Chemical Problems hl Jet Propnlsion," Pergamon Press, Inc., New York, 1957. Part of the International Series on Aerouaut,ics and Space Flight,. Written to supply engineers and others in a modem rocket research team with background in physiexl chemistry, etc. Inelrtdes discussion of atomic and molecular struel.ure, thermodynamics, kinetics, and transport properties as well as flames and combnstion. S. S., "Combu~tionand Prop~~lsion Research," (11) PENNER, Chem. and Eng. News, 41 (Jan. 14, 1963), p. 74. Brief discussion of diffusian flames, droplet and spray burning, supersonic flames, etc. (12) "The US. Effort in Space. The Politics and Science Involved,'' Special stxif report in t,wo pxrt,s. Chem. and Eng. News, p. $18 (Sept,. 28, 1!)6'3) and p. 70 (Sept. 30.1963). Includes discussion of magnitde of propellant needs and industrial effort involved, different types of chemical systems used, and names of companies and their specific research efforts plns an interesting comparison of liqoid and solid rocket engines with ion, plasma, and arc-jet ongincs. (13) GLASSTONE, S., "Pt.op111~ionand Powel. fur Space," Ch. 3, Sourcehook on lhe Space Sciences, (NASA), D. Van Nastrand Co., Inc., New York, 1965. Has rel~tivelylittle coverage of scientific chemistry; but is good for those who wish information on the design, hardware, and fuels wed in various rocket systems-bat,h those in use and possible future types. (14) WEGENER,P. P., "GRS Dynamics-Impact on Chemistry," Chem. and Eng. News, 44, (July 18, 10GG), q.76. IXseussion of the modern intw-relxlions of chem~stry and gas dynamics, which slnrled with flames and rockets and has now pmgressed into the study of shock waves, supersonic nosales, elc. Reviews and Reseorch Journols
Articles will he found a t fairly frequent intervals in journals such as Industrial and Engineering Chemistry and Journal of Chemical Physics. However, the two chief journals for flame and comhustioll research are: (15) Fuel. A jon~~nal of fuel ~cienccpublishcd bimonthly by Butterworth & Ca., London. This has more emphasis on the natore, stability, etc. of bhe fuel it,self than on its bondrrg. I t inclodes much work on coal and ~.eldedmnt,erinls. (16) Cnnlbuslion and Flame. Quarterly jow~rwl of the Comhmtion Inst,it,ute, published by Buttsrwort,h & Co., London. A good soorce for art,icles o n cul.rent research in the areas indicat,edby its W e .
The most exteusive source for reports on conlbustion research is the series of symposia ou combustion. These are held biennially under spousorship of the Combustion Institute, and papers prcseuted are published in volumes of proceedings. The latest of these is: (17) Tenlh Symposium (Internalional) on Combustia, 1488 pages, published by The CombnstLm Institnte, 1965. Report of symposium held at. Cambridge University (England) in 1964. Includes some 131 papers (plus discussion) typifying modern wol.k on flames and eornbust,ian. Also includes enmiilat,ive index of the nine earlier symposia.
Reviews on various aspects of combustion research am readily available. There is a fine series of annual reviews published in Industvial and Engineering
Che.mistry which is particularly useful in coverage of industrial processes. Others which may be noted are: (18) BAWN,C. E. H., A N D TIPPER,C. F.H., "Comhustion and Flames," Ann. Rev. o j Phgs. Chem., 7 , 231 (1956). Primarily gaseous combustion, flames and flame propagation, spectroscopy, etc., with some coverage of propellnnt burning. IT. G., A N D BURGE~S, D. S., "Combustion and (19) WOLFHARD, Flames," Ann. Rev. ofPhys. Chem., 8 , 389 (1057). Flame propagation and structure, rocket and ramjet eomhustion, and industrial engines, furnaces, fire hazards. n (20) PENNEB,S. S., AND JACOBS,T. A., i ' C ~ m h u s t i ~and Flsmes," Snn. Rev. of Phys. Chem., 11, 391 (1960). Includes basic reseal.eh and rocket-engine combustion problems. (21) WEENEE,J . F., "Flame Processes-Theory and Experiment," .4duanees in Chcm. Eng. 5, 1, Acadomic Press, Inc., 1964. Relatively brief but thorough survey of flame and combustion phenomena-both expel.imental results and iheory-detonation, solid propellants, etc. Includes an int,erestirrg discussion of the potentialities of the use of flame systems for processes of synthesis. (22) TIPPER, C. F. H., editor, "Oxidation and Combustion 12eviews," Tol. 1, American Elsevier Publishing Co., Ine., 1965. First volume of planned annual series. This volume has much more coverage of oxidation reactions than of combustion.
Flame Structure
I n earlier days, con~bustionhad to be considered simply in terms of a flame region as a reaction zone into which fuel and oxidant mere fed and from which the combustion products eveutually emerged. Row a
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occurring within the flame zone such as the temperature variations and different stages in reaction. The next group of refcrences exemplify this concept of flame structure and its application to the study of combustion. The first two, in particular, are excellent monographs on the topic. (23) GAYDON, A. G., A N D WOLFHARD, H. G., "Flames. Their Structure, Radiation, and Temperature," 2nd Ed., Chapman and Hall, Ltd., London, 1960. Covers in particular the topics indicated in the titleboth for premixed and for diffusion flames. Also includes the interesting case oi flame vihmtion (singing flames), along with discussion of ionization and carbon formation and a. cogent final chapter on "Recent
physical and chemical processes in flames; analysis of experimental data; burner systems; aerodynamic measurements and determination oi temperatures, coneent,rations, distances, and tramport propert,ies in flames; and physical and chemical interpretation of structure data. Also provides a good "Selected Bibliography on Combwt,ion" as well as bibliographies for ench chapter. (25) GAYDON, A. G., "Fl~mes: Theil. Structure and Radiaiion," Endeavor, Vol. X, No. 37, p. 17, Jan. 1951.
.
available: but i t does have two Dazes of fine color photogmphs of one and two-stage flames, low-presswe flames, and flame spectra. Volume
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(26) FEISTROM,R. M.', "The Mechanism of Combustion in Flames," Cham. and Eng. News, 41 (Oct 14, 1963), p. 150. Brief hot effective discussion of thermodynamics, chemistry, and physics of flames andof flame structure. Notable for figures showing basic flmne geometry and the spatial profiles across a. typical flame for temperature, concentration, and flux. ( 2 i ) WEINBERG, F. J., "The Optics of Flames and Methods for the Study of Refractive Index Fields in Gases," Butterworth & Co., Ltd., London, 1963. More general in coverage than the title indicates. Has good survey of schlieren and shadow photography and interferometry. Also discussion of optical p r o p erties and flame processes including ignition, quenching, flammability limits, and steady-state prapagation in premixed flames; diffusion flames; and highintensity combustion.
The effectiveness of control in the Bunsen burner has made i t possible to recognize and study the flame "cone" and then to investigate a section of the surface of that cone (Fig. 1) (cf. 86, 27, Ch. I1 of ref. 23, Ch VI of ref. 24). The Smithells separator showed clearly that burning occurred in stages (Ch. 11. ref. 25). More recently the development of "flat flame" burners (26, Ch. I1 of ref. 23, Ch. VI of ref. 24) and of "spherical' flames has helped to avoid some of the complications even of the conical shape (cf. Fig. 4) (also see ref. 65, 66, 67 for recent designs). Use of low pressures can increase the length of the flame zone and make further observation possible (Ch. 11, ref. 23). Flat-flame and spherical burners have also been designed for diffusion flames (26, Ch. VI of 24). Today, therefore, we may approach the basic and fundamental study of combustion in its simplest form by considering the flame as a reaction zone in a simple laminar flow system, with a mixture of fuel and oxidant being introduced from one side and the combustion products flowing out the other (Fig. 5 ) . A steady state is maintained so that the time sequence of changes occurring during passage through the flame may be studied and shown as a spatial profile of changes occurring along a certain segment of the path of flow through the flame. The system becomes essentially a
TEMPERATURE
,I
fl -
width flame of Effectwe front the
Ti DISTANCE
-
Figurs 5. One-Dimensional Flome Propagotion. A one-dimensional, laminar Rome, propagating without convection currents, heat loner, or other wall effects, reprerents an idealized process which illustroter the essential features of the physical phenomena. A very small pressure drop taker place across the Rome front, accompanied b y a large tsmperatwe rire. The curve represents the temperature rire ocrarr o laminar Rome "front." The temperature, Tb, is opprooched asymptotically, and the width of the Rome front is therefore not strictly defined. The adiabatic, one-dimensional problem of name propogotion has a solution only if it is orrvmed that the chemicol reaction rater ore zero below o well-denned ignition temperature, or that o rmoll ornovnt of heat is lost at a region where the Rome is anchored Ithot is, at the Rame holder). The mothematicai problem can b e described quontitotively b y solving the coupled aquotions of Raw with chemical reactions. (From Reference I I, reproduced b y permislion of Chemicol and Engineering New..)
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"one-dimensional" flame (11, Ch. V of ref. 24, Ch. I of ref. 3) without complications of convection currents or wall effectssuch as heat losses. The experimental results show clearly that significant changes occur well outside the visible region one usually associates with the term "flame." Fristrom (24, 26) points out for example four "zones" for hydrocarbon flames (Fig. 6): an initial heating stage, or region of transport and concentration effects; a primary reaction zone in which the hydrocarbon reactions start; a secondary reaction zone involving mainly conversion of carbon monoxide formed in the primary zone to carbon dioxide; and a recombination zone in which concentrations of free radicals, etc., return to thermal equilibrium. The luminous zone ordinarily called the flame is roughly coincident with the primary reaction zone only. The overall boundaries of these zones cannot be sharply and clearly defined (27, Ch. VIII and Ch. X I of ref. 24). Nonetheless the concept of the flame remains very useful. Luminescence and Flame Spectroscopy
For a long period in man's history, flames were of major importance as a source of illumination. For practical purposes this usage has decreased steadily in recent times because the actual radiation emitted from most flames is relatively weak. Among the energy changes involved in flames, radiation is a minor one and most of the energy released is in other form-so that better lighting ran he obtained, for example, indirectly by using the flame to heat a mantle to incandescence. (A notable exception to this is, of course, the combustion of magnesium for flares or in flash bulbs; but this can have only sperialized usage berarrse of its major smoke problem.) As spectroscopic techniques have developed, however, it has become possible to observe in detail the behavior of various emitters-the OH bands, the SchumannRunge bands of Oz, and the NO and NH bands which may be observed in hydrogen flames; the COzbands of carbon monoxide flames; the CH bands, the Swan bands of Cz,and the Vaidya "hydrocarbon flame bands" (HCO) which may be observed in Bunsen flames, and many others. The techniques and theory involved, as well as many results, are clearly presented by Gaydon: (28) GAYDON,A. G., "The Spectroscopy of Flames," John Wiley & Sons, Inc., New York, 1957. Discussion of theory of light emission from flames and experimental methods for observing this, including especially the infrared region; also spplicrttions to measurement of temperature and flame structure. Data given on hydrogen, carbon monoxide and hydrocarbon flames, ss well as some systems containing nitrogen, halogens, sulfur, or inorganic materials.
These methods offer unusually good opportunities to study the specific nature of the excited states which occur in flames and the fundamental nature of energy transfer processes in the molecules. The study of the 'Very recently another article has been published by FRI* "Flame Chemistry," Survey of Progress in Chemistry, (SCOTT,F., editor), Vol. 3, p. 55, Academic Press, Inc., New York, 1966. I t contains an excellent survey of the history and scope of combustion, character of flames, theory and experimental methods, and specific examples of flame systems. TROM:
actual nature of the emission processes-the chemiluminescence involved-has proved rewarding. The energy processes are also of major significance in our consideration of the temperature of the flames as well as the specific steps in reactions occurring. A brief summary of some of these aspects of combustion and light has been given by Norrish: (29) NORRISH,R. G . W., "The Study of Combustion by Pbotoehemiml Methods," p. 1. Ref. (17). A review which has some discussion of catalysis of combustion by light and of flash photolysis, but also coven kinetic spectroscopy, including some consideration of knocking in engines.
Some recent results are given later in the listings for Current Research (68 through 73). Temperofures
Earlier concepts of flames and combustion tended to emphasize the mazinzum temperatures observed in flames; but it is now possible to follow in detail the variations which occur across the flame front, and thus to determine a temperature "profile" (Figs. 5, 6) (Ch. VIII of ref. 84, Ch. X of ref. 23). The classical method of using thermocouples has been much improved by the construction of very fine thermocouples; but a number of other methods have also been developed, using spectroscopic techniques in particular. These include measurements of radiation emitted from additives such as sodium vapor, of line broadening, or of vibration-rotation or electronic bands. The use of low-pressure flames, which are thicker than ordinary flames, helps in getting further details of the temperature variations. The actual measurement of temperature has been discussed in a series of reviews, one of which is of particular interest for study of flames:
(30) BROIDA, H. P., "Experimental Temperature Measurement in Flames and Hot Gases," in "Temperature, Its
Measurement and Control in Science and Industry," Val. 11, p. 265, Reinhold Publishing Carp., New York, ,net
Covers especially the range of 15004000°K. Techniques discussed include thermocouples (usually best below 1500°K), radiance (blaek-body), line broadening (translational temperature); and intensity distribution (rotational, vibrational, or electronic "temperature").
A feature of particular interest in recent times has been the discrepancies which are often found between values determined by thermocouple measurements and those calculated for translational states, as indicated by line broadening in spectra, or for rotational or vibrational states, as indicated by appropriate lines in the spectra of OH, CO, CH, etc. (28). (For recent results see 74, 75, 76.) These may show variations within the same flame--e.g., 3Z0O0K for the rotational "temperature" for the 3900 A line of CH in an oxyacetylene flame a t atmospheric pressure and 3600°K for that for the 4315 A line. Marked variations may also appear a t different pressures-eg., 4950°K for the rotational temperature of Cz in an oxy-acetylene flame ~t one atmosphere and 3800°K a t 2.1 m a s well as variations for different types of measurements. These variations cause difficulties in establishing "temperatures"; but they also supply interesting evidence of systems in which equilibrium does not necessarily exist between various energy states. They offer excellent opportunities for study and discussion to develop better understanding of the concept of temperature itself and of the Maxwell-Boltzman distribution. Reodion Mechanisms
Both the detailed data on temperature variations and Tempentun (degnes Kehin)
2000"
+-
.
0.07
PRIMARY REACTION REGION ZONE I
-
% .
s 0.06 -
-B
RADICAL RECOMBINATION REGION
. TRANSPORi ::0.04 - REGION
ZONE Ill
0.05
0.03 -
ZONE 0
---------------
'ATOMIC OXYGEN
--d' 0.5
1.0 1.5 Distance from the burner surface (in centimeters)
2.0
Figure 6. Spatiol Separation. Mort complex flames involve a sequence of reactions If these reactions have different rates, the Rome reporoter into two or more regions. The typicol regions in a 0.1-atmosphere methane-oxygen flame ore delineated by plotting the flux of a rpesies whore reaction ir chorocterirtic of that region: mclhonc for Zone 1, carbon dioxide for Zone 11, and oxygen atoms for Zone Ill. The transport region, where no reaction occurs, i l Zone 0. [From Reference 26, reproduced b y permission of Chemical and Engineering News.)
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the spectroscopic measurements themselves have given much evidence on the time sequence of changes in chemical constitution as a reaction mixture undergoes combustion in passing through the flame front (24, 26). I n recent years the development of very fine quartz microprobes which can obtain samples of flame gases for analysis in a mass-spectrometer (Ch. IX, ref. 24) has given even more data on reagents a t various stages. These include reactive intermediates such as free atoms and radicals as well as stable molecular species (Fig. 7) : so some of the combustion systems are among our best examples of knowledge of the sequence of steps in chain reactions. The following- references summarize results of such studies. (31) FENIMORE, C. P., "Chemi~tryin Premixed Flames," The Macmillan Co., New York, 1964. H a s good brief discussion of laminar premixed flames as reacting system and of correlation of properties such as burning velocity and temperature with chemical reactions. Discusses hydrogen and hydracarbon flames in particular, including ionization and electronic excitation, soot formation, flame inhibit,ion, and decomposition of nitric oxide. (32) INKOF OFF, G. J., AND TIPPER, C. F. A., "Chemistry of Combustion React,ions," Butterworth and Cu., London, 1962. With a n interesting Foreword by J. H. Bmgoyne on Lhe role and place of chemistry in eombustion. A t,horoogh discussion of chemistry of flames, including ignition snd cool flames, pyrolysis, and recombination ~reaetions. Concentrates on eombustion of hydrogen, carbon monoxide, and organic compounds. Inchtdes discussion of energy relations and of non-equilibrium dist,~.ibut,ians a t high temperatures. (33) MULI.INS,B. P., A N D FIBRI, J., editors, "Comhust,ion Researches and Review~1957,'"AGARD), Butter-
w o r t h Sci. Publ., London, 1957. Includes s review of hydrogen-halogen flames. E. S., AND FRISTROM, R. .\I., "Reaction Ki(34) CAMPBELL, netics, The~modynamics,and Transport in the Hydrogen-Bromine System," Chem. Rev., 58, l i 3 (1958). A survey of properties for flame studies including reaction mechanisms and rates of individual steps, binary diffusion coefficients far atoms and molecules, thelmal conductivities, ete. One of the most, comprehensive compilations of kinetic data available for any known system. Both molecular and atomic reaction mechanisms are mrveyed. (35) GILBERT,I f . , A N D ALTMAN, D., ''Effect of Isotopic Substitution in HvBh Flames," J. Chea,. Phw., 25, 3i7 (1956). St,udy of flame velocities by isotopic comparisons indicates steady-state asmlmption is valid, and ronBr2 step. t,rolling factor is k for H (36) ASHMORE,P. G., "Elementary Comhuetion ReacbionsNeutral Species", p. 377, Ref. (17). A review written to summarize (subject. to the space limitation of seven pages) the elementary reaction steps of atoms and free radicals which are involved in the chain reaction mechanisms of hydrogen and carbon compounds.
+
I n addition to the oxidation of hydrogen, carbon monoxide. and hvdrocarbons and the hvdrozen-halozen " flames noted ab&e (see also ref. 5.9 and, for recent results, ref. 77 through 81), detailed analyses have also been made of certain decomposition flames (see ref. 55, 82, 83). The development of rocliet,~has now expanded extensively the range of types of chemical reactions under study in flame systems to include fuels such as ammonia, hydrazine, dihorane, asphalt and metals; and oxidants such as hydrogen peroxide, ozone, nitric acid, fluorine, and perchlorates. For most of these the knowledge of intermediates and reaction kinetics is still quite limited, hut they do offer interesting examples of the wide range of chemicals involved in
-
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Figure 7. Flame Profile. A one-dimensional flame, such as a 0.1 -atmosphere spherical methane-oxygen Rome, may be described on the boris of the inlet conditions, including reartants, and the temperature m d composition, including products intermedioter, and radical specie% Major Conrtitventr Methone, a reactant, is consumed completely; oxygen, a reactant, and water and carbon dioxide, product$, reach a steady state within the Rome. (From Reference 26, reproduced by permission of Chemical and Engineering New.) On the right, the free rmdicol%(0;. H and OH .) are the driving forceof the reociion. The intermediate species ore carbon monoxide, molecular hydrogen, and formaldehyde.
.,
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modern flames (cf. p. 29 ref. 24 and also ref. 41, 42, 43 and, for recent studies, ref. 84 through 90). Calculations of equilibrium thermodynamic properties of flames, particularly the nature of the burned gas and the adiabatic flame temperatures are still made by classical methods (9,see also ref. 58),but much more is known about various reactions occurring so that much bet,ter corrections can be made for dissociation, etc. Some attention can even be given to equilibrium among various energy states. Following are the common sources of thermochemical and thermodynamic properties. The available data on thermodynamic properties in the older compilations has been increasing steadily and the interest in new types of combustion systems for rockets has led to the compilation of the extensive JANAF tables which cover all elements and their compounds, not just the more traditional organic compounds. (37) "Selected Yalues of Physical and Thermodynamic Properties of Hvdrocarhons and Related Comuounds." Amer. petroi. Inst. Res. Praj. 44, came& press, Pittsburgh, Penna., 1953. (38) "Selected T-dues of Chemical Thermodynamio Properties," S a t . Bur. Stds. Circular 500, Feb. 1, 1052, US. Govt. Printing Office, Washington, D. C. A continuing- compilation which is being.supplemented ~. steadily. (39) "JANAF Thermochemical Tables," Dow Chemical Co., Midland, Mich. A project supported hy the Adv. Res. Proj. Agency a t the Thermal Res. Lab, Dow Chem. Co. Covers a range of substances-atomic, molecular, and free radical-from aluminum to zirconi~imand electron gar, going well beyond the classical organic compounds. ~
Thermodynamic data are also used commonly now for calculation of rocket performance parameters such as the specific impulse (see also ref. 12, 41,42). (40) DAFLER, J . R., "Rocket Propulsion: The Chemical Challenge," J. CHEM.EDUC.,41, 68 (1964). Survey of relation of chemical factors to specific impdae of rockets, including role of ionization, dissociation, and equilibrium among internal degrees of freedom, with comparative data on some new propellant systems.
Properties of some of the newer chemical systems being used or investigated as power sources are available in the following articles: (41) GLASSMAN, I., "The Chemistry of Pmpellants," Am. Scienti% 53,508 (1965). Discussion of performance factors, especially specific impulse, of propellant svstems for rockets and comparison of a number of new systems-using for example H201, HNOa, N20., Fs, C103F, or solid perehlorates as oxidizers, with fuels such as ammonia, aniline, hydrazine, or asphalt and metals, as well as the ordinary ones. (42) "High Energy Chemicals-High Performance," Staff report, Chem. and Eng. News, 36, (May 27,1957), p. 18. Comparison of ordinary systems with some of the more "exotic" ones, involving oxidizers such as ozone, nitric acid, and fluorine, and fuels such as ammonia, hydrazine, diborane, and even free radicals. Has good set of bar graphe showing comparative values of energy release. (43) "Advanced Propellant Chemistry," Advances in Chemistry Series, No. 54, Amer. Chem. Sac., Washington, D. C., 1066. Twenty-six papers of a special symposium.
A relatively recent and growing development in the quantitative study of flames has been that of the electrical properties of flames. Langmuir probes have been used to determine charge distributions and "plasma" properties of flame gases, and the use of sampling probes connected with mass-spectroscopic analyzers makes i t possible to determine the nature of specific ions present. Numerous interesting examples of positive and, more recently, negative ions have been detected in flames-for example C3H2+,H30+,CzH30+, CHsO+, CH30+, and CHO+ plus CI-, C-, and COI. H,O- in oxyacetylene flames. Mass profiles are now known for a number of these. (44) CILCOTE. H. F.. '>Ion and Electron Profiles in Flames." Ninth ~gm~os'(1nlernat.) on Combus., p. 622, ~cademie Press, New York, 1963. Good discussion of Lengmuir probe as an experimental technique, and of the mass spectrometer. Data on "plasma" properties of propane, ethylene, and acetylene and on ion profiles across an acetvleneoxygen flame. (45) SPOKES,G. N., AND Ev.4~6,B. E., "Ion Sampling from Chemical Plasmas." o. 639. Ref. 1171. , , Study of various facton influencing act,ual sampling with probes. Data on nitrogen afterglow and on a low-pressure atomic oxygen-acetylene diffusion flame.
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The occurrence of these ions-as even a brief consideration of formulas such as those above will bring out-raises many interesting questions about the fundamental nature of the processes by which they may be formed and the role which they may play in the flame processes. The study of chemi-ionization has therefore, become a rapidly growing, though not extensive, branch of the science of combustion in the last ten years or so. This is discussed in the following (see also ref. 44 and for recent results, ref. 73,91,92). (46) MUKAERTEE, N. R., FUENO,T., EYAING,H., AND PIE, T., "Ions in Flames," Eighth Sgmpos. (Internat.) n Combustion, p. 1, Williams and Wilkins Co., Balbimore, 1962.
Presents brief historical background, then a critical analysis of suggested mechanisms for ionization in flames-more particularly, in hydrocarbon flames. (47) SUGDEN,T. M., "Elementary Combust,ion Reactions. Charged Species," p. 539, Ref. (17). A brief but effective summary of types of ionization processes in flames and of examples of each iype if known. Also gives citations of work on natural ionization in flames and on related processes such as eleet,ron attachment or detachment, recornhination of positive ions with electrons, and ion-molecule reactions. (48) CALCOTE,H. F., Kuaztus, S. C., AND MILLER,W. J., "Negative and Secondary Ion Formation in LowPressure Flames." , D. 60.5. , Ref. (171. ,-~, Not,able for diioussion of negative ions and also of formation of CaHaC.
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Solid Formofion
Still another aspect of combustion processes which is of fundamental scientific importance, but which receives relatively little attention because it is not a major reaction is that of the formation of solid particles. Soot has long been a familiar component of many combustion systems, especially those with limited air supply. It was recognized that the incandescence of these particles is responsible for the light of candle and lamp flames (cf. ref. 5 ) ; and, in the form of lampVolume 44, Number 5, May 1967
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black, gas carbon, carbon black, etc., this by-product forms thebasis for some large-scale industrial operations. Scientifically, however, the question of major interest is that of the mechanisms by which the actual formation of solid occurs. The condensation process-first the nucleation and then growth of solid particles-is not well understood a t best. Yet in flames-in environments of high temperatures, etc.,which favor fragmentation of molecules and the oxidation of hydrogen and carbon-it becomes possible on occasion to build up articles involvine thousands of atoms of carbon and other elements. These intriguing processes are discussed in the following articles (as well as in ref. 51 and 95 and Ch. VIII, ref. 25).
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(49) TESNER,P. A., ROBINOVITCH, H. J., AND RAFALKES, I. S., "The Formation of Dispersed Carbon in Hydroearhan Diffusion Flames," Eighth Sympos. (Internot.) an Combus., Williams and Wilkins Ca., Baltimore, 1962. Experimental studies on particle formation and growth in methane flames. R. C., (50) *EHLING, F. C., FRAZEE,J. D., AND ANDERSON, "Mechanisms of Nucleation in Carhon Formation," Eighth Sympos. (Internat.) on Comhs., p. 774, Williams and Wilkins Co.. Baltimore. 1962. Experimental studies on pyrolysis of various hydrocarbons. Discussion of basic mechanisms for formation of first nuclei of carbon particles. (51) PALMER,H. B., A N D CULLIS,C. F., "The Formation of Carbon from Gases," Chemi9try and Physica of Carbon, (Walker, P. L., editor), Marcel Dekker, Inc., New York, 1965. An excellent recent review of experimental results and kinetics and mechanism of particle formation, and of some unsolved problems.
In modern rockets, there are a number of other possibilities for solid' formation, particularly when metallic fuels are used, but relatively little study has been made of solid formation in these. (52) KASKAN, W. E., "Light-scattering Measurements on Particles Condensed from Boron-containing Flames," Combustion and F h e 5, 93 (1961). An interesting example of systems involving formation of a solid other than carhon. Experimental data, on boric oxide condensation in post-flame gas from trimethyl borate-air flame.
Theories of Flame Propagation
Since the ability of a flame to propagate through space is a basic characteristic distinguishing the flame systems from ordinary chemical reactions, the approach to better theoretical understanding of flames has involved primarily attempts to calculate the flame prop agation velocity (or burning velocity, Fig. 1). This has required detailed consideration of various rate processes such as the reaction rates-both in the use of experimental data from flames to estimate reaction rate constants and in calculations of rates for complex reactions. Reliable rate constants are by no means easy to obtain, but in some cases detailed calculations of reaction kinetics for chain reactions have been made. An interesting aspect of these is the question whether residence times and radical concentrations justify making the usual steady-state simplification (see also Ch. XIV. ref. 24) : (53) BENSON,S. W., "Found&tions of Chemical Kinetics," McGraw-Hill Book Co., New Yark, 1960. A comprehensive text on chemical kinetics which includes a chapter on the kinetic behavior of nonstationary-state systems. This has brief discussion
256 / Journol o f Chemicol Educofion
of ignition and combustion, stationary flames, thermal explosions, and shock waves and detonation. (54) KONDRATIET, V. N., "Chemical Kinetics of Gas RescJ. M., AXD C.LRRUTAtions" (translation: CRABTREE, ERS, S. N.),Addison-Wesley Pnbl. Co., Ine., Reading, Xass., 1964.
.4 Russian monograph, but now available in translation. Includes discussion of cool flames; rarefied flames; role of OH, CS, and 0 ; and the ignition peninsula; also sections on H2 combustion as s. model type of reaction process, on diffusion flames and laminar flames, on flame propagation, and an spontaneous combustion and explosion. J. O., CTIRTISS, C. F., .ANDBIRD, R. B., (55) HIRSCRFELDER, "Molecular Theory of Gases and Liquids," John Wiley & Sons, Inc., New York, 1954. A comprehensive treatise an equilibrium and nonequilibrium properties of gases, transport phenomena, and intermolecular forces. Includes discussion of theories of flame propagation, detonations, and flow of propellant gases in rockets; qualitative examination of the Bunsen flame: and com~arisonof rx~erimental and ozone.
Much work has also been devoted to developing better knowledge of molecular transport properties (55). Flame profiles have been used to correlate data on reaction rates and the rate of heat release with temperature and concentration changes. The earliest theories of flames were thermal concepts, attempting to explain propagation on the basis of heat flow from the high-temoerature zone. Later theories were based on ldiffusion of certain highly reactive species such as hydrogen atoms. Current theories are based on comprehensive treatment of chemical reactions in flow systems, taking into account mass transoort bv convection and diiusion as well as thermal cond;ction" (64). (Other processes such as thermal diffusion and radiation are usually considered negligible.) Exact treatment is fairly well limited to the one-dimensional laminar flames, with reaction kinetics treated on the basis of simple collision concepts; but progress is being made. (For recent discussions see ref. 94-97.) (56) HIRSCHFELDRR, J. O., Yhme Remarks on the Theory of Flame Propagation," Ninth Sympos. (Intemt.) a n Combus., p. 553, Academic Press, Inc., 1963. A survey of the basic problems of flame theory and a brief summary of work for some fifteen years in one of the key research centers. (57) PENNER,S. S., rTntr~ductionto the Study of Chemical Reactions in Flow Systems," Butterworth Publications Ltd. London, 1955. A publication of the Advisory Group for Aeronautical Research and Development, North Atlantic Treaty Oreanization (AGARD). P r e s u ~ ~ o s ebackeround s in ;hemistry a i d in calkus. A slim but volume with chapters on classical chemical kinetics in stationery, isothermal systems; on conservation laws and transport coefficients; on chemical reactions during adiabatic expansion through a. nozzle; and on heterogeneous reactions. (58) WILLIAMS,F., 'Combustion Theory-The Fundamental Theory of Chemically Reacting Flow Systems,'' AddisonWesley Publ. Co., Inc., Reading, Mass., 1965. For advanced and graduate stndents. Has good "review" sections on thermodynamics and statistical mechanics, chemical kinetics, conservation equations, and transport properties as appendices. Includes quantitative relations for theory of laminar and turbulent flames; diffusion flames, droplet burning and spray combustion; chemical reactions in boundary layers.
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Non-Stationary-State Phenomena
There are numerous phases of combustion phenomena which are much more complex in theoretical treatment because they are time dependent and no simplifying assumption of a stationary-state condition or one dimensional flow can be applied. The available literature on these is voluminuous and extensive. The following few examples may help a reader to make a start but they are definitely not comprehensive or even necessarily "typical" of the work which has actually been done. (59) HAWTHORNE, W. R., AND FABRI, J., editors, "Selected Combustion Problems" (AGARD), Butterworths Sci. Public., London, 1954. Papers from a combustion colloquium held at Cambidge Univenity (England). Has a good section on "Turbulent Combustion" with papers by J e a n J . Bernard and Arch Scurlock and John Grover on experimental investigations and by Bela. Karlovits on turbulent flame theory. (60) "Explosions, Detonation, Flammability and Ignition" (AGARD), Pergamon Press, The Macmillan Co., New York, 1959. Part I, by S. S. Penner, deals with experimental and t,heoretical studies of ignition, flammability, and explosion limits nnd detonation. Part 11, by B. P. Mullins, deals with flash-points, spark ignition, spontaneous ienition. and flammebilitv limits. D C., " ~ h e i r i e sof Detonation," (61) EVANS,M., ~ N MABLOW, Chem. Rev., 61, 129 (1961). (62) SHCHELKIN, K. I., AND TRASHIN,YA. K., '%,8 Dynilmics of Combustion," (translation: KUVSHINOFF, B. W., AND HOLTSCHLAQ, L.), Mono Book Corp., Baltimore, Md., 1965. A Russian work now made available in an American translittion. Has chapters on detonation, deflagration, nonstationary double discontinuities, accelerating flames, and high-frequency vibrations in highthroughput combustion chambers (ramjets and rockets). (63) MCCLURE,F. T., "Rockets, Resonance, and Physical Chemistry," Science 135, 771 (1962). Interesting discussion of problems encountered in solid-fuel rockets-the mass transport and heat transpo1.t involved in the very thin boundary layer in which combustion occurs and the possibility of acoustic resonance.
One group of phenomena includes ignition of flames, quenching and flammability limits, and "flash-back" of the flame along a burner tube or "blow-off" as a flame lifts off the burner (3, 68, 60). I n these, interaction of the combustion system with surroundingsthe burner walls, the outside air, a spark discharge, or a hot wire-may be important. In other such systems, interaction of flames with one another may occur, as in spray combustion (68). I n the high-speed combustion of ram-jets and rockets, turbulent mixing and turbulent flow behind bluff bodies may become critical (3, 58, 59). I n other cases detonation may occur (3, 58, 61) or high-frequency vibrations (62). Problems such as those of flame stabilization or of reactions of solids, etc., may require detailed treatment of chemical and other processes in their boundary layers (58, 63). Empirical observations of such effects are of interest because they show the variety and scope of combustion phenomena. However, they involve complex physical and structural or design factors superimposed upon chemical comhustion systems. Quantitative study and interpretation are therefore fields for specialists, thus for students a t advanced or graduate levels. (For r e cent results, see ref. 98 through 106.)
Current Developments Some Unusual Applications
The follouring items may be of interest because they represent types of combustion or flame effects which are quite different from ordinary ones. They are not given references in the regular bibliography because most of these are simply "spot news" items, brief paragraphs which may suggest an area for further exploration. A signscant development in recent times has been the growing use of flame spectroscopy or flame photometry as an analytical tool. Essentially this method just uses the flame as an environment to atomize the elements, but the problems of control and interpretation have required much attention to the characteristics of the burners used and to the details of the flame processes involved in the vaporization and detection of various constituents. (Interested readers might start with the brief report in Chem. & Eng. News, Sept. 23, 1963, p. 57 and then go to the extensive analytical and instrumental literature of the field.) A corollary field of development is the use of the flame as a chemical reactor for synthetic processes. Controlled comhustion has been used on a large scale to produce carbon black and acetylene. Other processes may include such possibilities as the chlorination of hydrocarbons, and production of boron nitride and titania @ I ) , as well as the separation of some reactive intermediates. Some other unusual developments which might he noted are the possible use of atomic flame reactions and chemical explosions to energize chemical lasers (Chem & Eng. News, Jan. 25,1965, p. 23); work on propellant combinations of materials such as azides and perchlorates which can produce air as exhaust gas (Chem. & Eng. News, April 5,1965, p. 41); and progress on engines using ammonia as fuel (Chem & Eng. News, Jan. 18, 1965, p. 37) which includes the attractive logistic possibility of a fuel which might be produced locally from air and water. Those interested in the space age should find intriguing the proposed "scramjet" (Time, Nov. 26, 1965, p. 46)-a supersonic plane which will have its fuselage encircled by its ram-jet engine and use the liquid hydrogen fuel as a coolant to avoid searing by the heat of friction. Davy's safety lamp is a well-known example in chemical history of attempts to cope with fire hazards. This has its modern counterpart in a new radiation detector and automatic quencher for mine explosions (Chem & Eng. News, Aug. 22,1966, p. 31). A stillmore sobering, hut nonetheless important aspect of modern comhustion problems is reflected in reports a t a recent meeting (Western States Section, The Combustion Institute, April, 1965) on aircraft and spacecraft fire hazards and crash fire protection. The most extensive hazard, however-which must be of concern to all combustion workers and experts today-involves the broad and growing problems of air pollution [see, for example, the editorial by ARELSON, P. H., Science 147, 1527 (March 26, 1965)l. The formation of nitrogen oxides is one phase of these problems causing particular concern a t present (Chem. & Eng. News, Nov. 5, 1965, p. 33, also ref. 107). Volume 44, Number 5, May 1967
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Current Research
The Elevcnth Combustion Symposium was held a t the University of California in August, 1966. This meeting alone comprised a plenary lecture, a roundtable discussion and two svecialist,s' discussion eroutx. and 130 research papers. Wit,h meetings such as this plus the regular output of papers in journals it is apparent that any bibliography of research in the last several years which is within the scope of this article must be just arbitrarily select,ive. The list below may, however, serve as examples to show something of the scope of current research on flames. u
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(64) VIDEEN,TOM,ChenzGtv 39, No. 8, p. 26 (1966). On burning rates of candles. A Westinghonse Science
Project on the inthence of candle diameter and wick diameter on rate of burning. (Even today Faraday's candle flames are still subjects of research.) (65) YUMLU, V. S., Comb. and Flame 10, 147 (1966). Temperatures of flat flames on pomas burners in Hr02 and Hrair flames, with correction for atmospheric diffusion and the smount of unburned Hz. (66) DAY,M. J., DIXON-LEWIS,G., SUTTON, M. M., AND WAP TON, M. T., Comb and Flame 10, 200 (1966). EBeet of NO on ~tability, burning velocity, and luminescence of some HrOrNz flame on a flat flame burner. (67) ROSSER,W. A., JR.,A N D PESRIN,R. L., Comb. and Flame 10,152 (1966).
Decomposition burning of liquid hydrttzine on small, spherical, alumina burner. (68) MILLER,W. J., AND PALMER, HOWLRD, B., J. Chem. Phys. 40,3701 (1964).
On chemiluminescence and radical reactions in diffnsion flames of alkali metals and organic halides. (69) BELLES,F. E., A N D LANVER,M. R., J. Chem. Phys. 40, 415 (1964).
On origin of OH chemilrminescence dnring indnct,ion period of Ha-0%reaction behind shock waves. (70) FORTIIN,A,, J. Chem. Phys. 44,1702 (1966). On the Vaidya hydrocarbon flame band emitter (CHO*). (71) GOODFRIEND, P . L., A N D WOODS,H. P., Comb. and Flame 9,421 (1965).
Search for N F spectrum in NFz-He and NIR-CSz flames. (72) JONATHAN, N., MARMO,F. F., AND PADUR,J. P., J . Chem. P h p . 42,1463 (1965). Shock-tube studies of luminescence from CO in reaction of atomic 0 and CzH1. (73) GLASS,G. P., KISTIAKOWSKY, G. B., MICHAEL,J . V., A N D NIKI, H., J . Chem. Phys. 42, 608 (1965). Studies of mechanism of HZ-O2reactions in shock waves, including bhsis for chemiluminescence and ionization steps. (74) ZINMAN, W. G., AND BOGCL.
