COMBUSTION BERNARD LEWIS and GUENTHER von ELBE U. S. BUREAU OF MINES, PITTSBURGH 13, P A .
slow but steady progress is continuing in fundamental research, comprising chemical kinetics of combustion reactions; ignition, propagation, stabilization and quenching of combustion waves; effect of turbulence on combustion waves; simultaneous mixing and combustion of fuel jets; and other subjects. N e w practical problems arise continually from changing demands on piston engines, gas turbines, jet and rocket engines, high explosives and propellants, gas burners, furnaces, smoke abatement, and control of explosion and fire hazards.
1
T
HIS review covers the period from late 1950 to late 1952; the authors were unable to write a review article for 1951 because of other commitments. Some earlier publications which have since come t o attention are also included; on the other hand, preprints of papers presented at meetings have been omitted this time, because it was deemed advisable to limit these reviews t o fully published papers. During the report period several meetings and symposia were held, reflecting the need for contact and communication between the numerous groups of chemists, physicists, and engineers engaged in combustion studies. Mention is made of the Symposium on Combustion Chemistry in Cleveland, April 1951, sponsored by the Petroleum Division of the AMERICAN CHEMICAL SOCIETY, and particularly of the Fourth Symposium on Combustion organized by the Standing Committee on Combustion Symposia and held at Massachusetts Institute of Technology in September 1952. The papers of the latter meeting, however, could not be included in the present review because they are in the process of publication. They will be included in the next review.
FUNDAMENTALS OF COMBUSTION PROCESSES KINETICS OF G A S - P H A S E REACTIONS
I
The thermal reaction between hydrogen and oxygen has received continued attention. Broida and Oldenberg (46-4)have made observations of the explosion region at temperatures down t o 350’ C. The results are in agreement with the scheme of chain branching and chain breaking proposed by Lewis and von Elbe, but the mode of chain initiation at these low temperatures requires additional consideration. Egerton and Warren ( 7 I A , l 7 9 A ) have presented evidence which seems t o show t h a t the second-order chain process H HOz = 2 0 H is also involved in the scheme. From studies on hydrogen peroxide, Kruglyakova and Emanuel(1ISA) obtained evidence that HOz radicals combine in the liquid phase to Hz04, which is fairly stable and seems t o be present in the condensation products obtained b y passing a hydrogen flame through a trap cooled with liquid nitrogen. Baldwin, Corney, and Precious (W1A)observed several new facts concerning the inhibition of the second-limit explosion by hydrocarbons: Thus, they found that with propane and other higher hydrocarbons the limit is depressed proportionately to the hydrocarbon concentration, whereas methane has almost no effect up to 2.5% and thereafter completely suppresses the explosion. The authors believe that, in the case of methane, inhibition proceeds through the intermediate formation of formaldehyde. Baldwin and Precious (WSA) have also made further studies of the self-inhibition of the second-limit explosion by water vapor, previously reported by other authors. McLane (lW6A)has published additional material on chain initiation in the hydrogen-oxygen reaction by hydrogen peroxide. The effect of powerful light flashes on hydrogen-oxygen mixtures containing nitrogen dioxide, suf-
+
ficient to cause explosion, has been investigated by Norrish and Porter
(I4SA). As usual, the largest number of
papers concerns the reaction of oxygen with hydrocarbons. Vanpee and others ( I W A ,1?SA,l 7 4 A ) have continued their studies of methane, the simplest member of the series; this work is noteworthy, particularly for clarifying the role of carbon monoxide as an intermediate reaction product. Egerton and others (.@A, 69A) have shown t h a t adiabatic compression of methane-air mixtures may produce different types of ignition characterized by either blue or white flame; however, in the judgment of these investigators, the phenomenon is not akin to the low-temperature ignition region of higher hydrocarbons. Abbott and Miller (Id)made similar observations on rich ethylene-oxygen mixtures strongly diluted with nitrogen; a t 300’ C. and pressures up t o 40 atmospheres (depending on the mixture composition) they noted pale blue flames traveling a t less than 1 cm./sec. and yielding peroxides, formaldehyde, and acetaldehyde a s products; a t higher pressures bright “normal” flames traveling faster than 50 cm./sec. were observed. For ethylene, at least, the effect thus bears a distinct resemblance t o the cool flames of higher aliphatics. As might be expected, these cool flames and associated effects of low-temperature reactivity and explosibility have received much attention during the report period. Malherbe and Walsh (127A) have studied induction periods and pressure-temperature limits of cool flames in mixtures of oxygen with butane, pentane, and hexane. Bailey and Norrish ( S O A ) measured relative light intensities of cool flames in various hexane-oxygen mixtures and correlated these data with analyses of the complex mixture of reaction products. Kahler, Bearse, and Stoner (106A) found that under precool-flame conditions n-hexane and oxygen form relatively stable organic peroxides with little hydrogen peroxide, whereas under cool-flame conditions unstable mixtures of hydrogen peroxide and formaldehyde are formed. Fallah, Long, and Garner (‘76A),who studied n-heptane oxidation by a flow method, obtained two distinct maxima and minima in plotting yields of peroxides or total aldehydes plus ketones versus temperature in the cool-flame region below 450’. They observed further t h a t the reaction in the region of the first peak of the peroxide-temperature curve depends on the nature of the surface, being inhibited by nickel foil in the reaction vessel, whereas the second peak is independent of the surface; and the peroxide yield in the vicinity of the first peak depends on mixture composition in a manner entirely different from the second peak. A study of pentane oxidation by the California Research Corp. ( M A , 166A) showed that 2,Ppentanedione appears as a n important intermediate in the cool-flame reaction. Using methyl ethyl ketone a8 fuel gas, Bardwell and Hinshelwood (B3A) found that the concentration of peroxides increases rapidly before the cool flame passes and then suffers a “catastrophic” decline; the aldehyde concentration does not follow a parallel course. Additional data and theories on the cool-flame oxidation of propane ( 4 A )and butane (6BA) have been published b y Russian authors. Further experiments on the ignition of higher hydrocarbons simulating engine knock have been reported b y Livengood and Leary ( I W B A )
1921
INDUSTRIAL AND ENGINEERING CHEMISTRY
1922
and Levedahl and Howard (118A)and on cool flames of ether by Ouellet and Ouellet (I44A). Chirkov (5OA) reported on the oxidation of ethylene a t fairly high temperature and low pressure; and Burgoyne and Kapur ( 4 7 A ) on the oxidation of ethylene oxide in temperature-pressure regions above and below the optimum for cool flames. McDonell and Thomas (IB4A) demonstrated that the chain reaction between acetaldehyde and oxygen is inhibited by nitrogen peioxide, prebumably due to nitric oxide (NO) combining with the acetyl radical that normally enters into the oxidation chain. The problems of hydrocarbonoxygen reactions have been revien-ed by Jost (102A). Aldehydes in combustion products may be determined, according t o Bailey and Knox (IQA), by oxidation with silver oxide, which forms separable and identifiable silver salts of fatty acids. Ashmore and Korrish (15A, 16A) reported on the sensitizing effect of chloropicrin on hydrogen and oxygen, hydrogen and chlorine, and carbon monoxide and oxygen. Whittingham (I82i1, 183A) studied the oxidation of sulfur dioxide to trioxide and the reduction to sulfur in carbon monoxide flames. Wolfhard and Parker (186A)studied the influence of sulfur on carbon formation in diffusion flames. Thorp, Long, and Garner (168A) studied the formation of carbon in benzene-oxygen diffusion flames. A kinetic study of diboiane-oxygen mixtures, which exhibit the phenomenon of a second explosion limit, was made by Price (147A). Data and theories on the burning of liquid inntures of organic compounds with tetraiiitroniethane h a w been published by Behrens (S2A). F L A M E SPECTRA, FREE RADICALS, A T O M S , AND
IONS
Gaydon and Wolfhard (86A) observed that in many flames of premixed gases, such as hydrocarbon-oxygen flames, electronic excitation of admixed metal vapors (iron, lead, sodium, thallium) is abnormally high in the reaction zone, though not in the burned gas. They endeavored to study mechanisms of excitation by reacting various organic compounds with atoms and free radicals obtained from electric discharges through oxygen, hydrogen, or water vapor ( H A ) . Durie ( 6 7 A ) investigated emission spectra of flames of fluorine with hydrocarbons and other organic compounds, and Fogarty and Wolfhard ( 7 7 A ) investigated flames with oxides of nitrogen. Laidler and Shuler (IZ6A) discussed some relations betm-een chemical mechanism and emission spectia, particularly for carbon monoxide flames; Shuler (158A) and Broida and Shuler (46A) made similar studies, both experimental and theoretical, on hydroxyl bands emitted from hydrogen-oxygen flames. Hornbeck and Herman ( 9 7 8 ) investigated hydrocarbon-flame spectra a t high dispersion, particularly with a view to identifying the controversial radical HCO as an emitter, and Thomas (167A) studied the effect of preheating of hydrocarbon-air mixtures on the intensity of the Cz- and CH- bands in the flame. Arthur and Littlejohn ( I d A ) studied the role of atomic hydrogen in sodium-line emission from flames. Intense luminescence of various phosphors in contact with a hydrogen flame is ascribed by Bonhoeffer (142A) t o recombination of hydrogen atoms a t the phosphor surface, instead of electron emission b y the flame suggested by Neunhoeffer (241A). The equilibrium between ions, electrons, and alkali compounds in hydrogen-air flames containing alkali metal salts has been studied by Smith and Sugden (162A). COMBUSTION W A V E S
IN
EXPLOSIVE GASES
Ignition, Propagation, Quenching. Wit,h regard to measurcments of minimum ignition energies by electric sparks, some uncertainty had existed concerning the fraction of the electric clischarge energv actually imparted to the explosive gas betn-een the electrodes. A study by Roth, Guest, von Elbe, and Lewis (150A) has shown that most of the discharge energy is instantaneously imparted to the gas and is available for the ignition process if the electrodes are separated beyond a critical distance, the quenching
VOl. 45, No. 9
distance. Some data on spark ignition energy and theoretical discussions have also been published by Jost and Sieg (IO&d)and Venn (76A). On the theory of combustiori waves, comprising diffusion, i hermal conduction, and chemical kinetics, a furt'her contribution by Hirschfelder and Curtiss (94A) has appeared. Other coiitributions include papers by Jost (lO3A) and by Friedman and Burke (8OA) on general combusLion wave -theory; a correlation of data on burning velocity from considerations of active particle diffusion by Simon and Wong (169A, 1 6 1 8 ) ; other such correlations by Walker and ght ( l 7 8 A )tending to show that alteriiaCive t,hermel and diffusional mechanisms are equally consis1,ent with the data; a treatment by Blanquet and Hoare (39A)making use of certain reaction-kinetic equations of Seinenoff; an attempt by Behrens (33il)at, analyzing reaction kinetics in combustion waves from burning velocity data; a similar analysis by BarskiT and Zeldovich ( 2 4 4 ) from data on carbon monoxide--oxygen flames; a chain-branching theory of burning velocity by van Tiggelen (17OA); and a review of flame-propagation theories by Egerton (68A). Hibbard and Pinkel (92A) have present,ed an empirical equation correlating burning velocities of hydrocarboiiair mixtures with hydrocarbon structure. Kumerous papers have appeared on burning-velocity measurement. Liiinett and coworkers ( 5 5 4 ZBOA, I b l A , 146.4, 1 8 0 8 ) have used both burner flames and soap-bubble flames to tletermine thc burning velocities of mixtures of ethylene or acetylene r i t h oxygen and inert gases. Ashforth (14'4) discussed errors involved in the use of the outer edge of the luminous cone of a burner flame; Garner, Long, and Ashforth obtained data on pressure dependence of burning velocity for several hydrocarbons (82A) but noted discrepancies between data of various workers (83A). Lewis and von Elbe (see reference below) have pointed out that discrepancies in burning velocities measured on burners, which have been observed by various investigators, may arisc from the contribution to the burning velocity of the curvature of the combustion wave introduced by the use of cylindrical burncr tubes. Dixon-Lewis (5911) and Dugger (63il)studied the effect' of initial mixture temperature on burning velocities by means of simple burner methods, and Dugger and coworkers (64A, 6611) reported additional data on hydrocarbon-oxygen-nitrogen mistures. Gerstein, Levine, and Wong ( 8 7 8 ) developed a met'hod For measuring flame areas in a tube and reported values of burning velocity for various hydrocarbon flames. Other measurements, mostly by burner methods, were made on flames of various organic vapors (189A), acetaldehyde (5A), ammonia mixturrs ( 1 7 A ) ,hydrazine-ammonia-oxygen mixtures (139A), and hydrogen-bromine mixtures ( 6 4 8 ) . A comparative study of burning velocities of acetylene and dideuteroacetylene was made by Friedman and Burke ( 7 9 A ) . Optical density determinations across the widt,h of the conibustion wave in a stoichiometric met'hane-air mixture were made by Dixon-Lewis and Wilson (CIA). The subject of the pressure differential over the combustion wave in Bunsen-burner flames and also between the interior and exterior of a diffusion flame has been discussed by Stampfer (164A). Studies of the decrease of burning velocity on approaching the limits of flammability were made by Mache, Kozak, and Zappc (125A) and Egerton and Thabet (7OA). Delbourgo and Laffittc ( 5 7 A ) , Elston (7SA), and Elston and Laffitte ( 7 4 4 ) determined boundaries of regions of flammabilihy for various hydrocarbons and hydrogen as a function of various paramet,ers besides gas composition. Schumacher and Baum (157A) and Dixon-Lewis and Liiinett (Soil) observed the effect 01various additives on the limits of flammability of hydrogen-air and hydrogen-carbon monoxide.& mixtures, respectively. Propagation of hydrogenbromine flames in vertical tubes and limits of flammability were Rtudied by I
