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Spectra and combustion mechanism of flames supported by the oxides of nitrogen
718
KINETICS OF COMBUSTION REACTIONS
81 SPECTRA AND COMBUSTION MECHANISM OF FLAMES SUPPORTED BY THE OXIDES OF NITROGEN By H. G. WOLFHARD AND W. G. PARKER Introduction In a paper presented at the Fourth Symposium on Combustion z the authors described some premixed gas flames in which various fuels including hydrogen, carbon monoxide, ammonia and hydrocarbons were burned with nitric oxide and with nitrogen dioxide. Burning velocities and quenching diameters were reported which showed that the characteristics of these flames differed widely from those of the more familiar oxygen and air flames. I t was suggested elsewhere2 that one of the reasons for these differences lay in the fact that nitric oxide can react in two ways. I t can react with radicals such as NH or NH2 in steps which require only a small activation energy oz' it can decompose thermally by a process which requires a high activation energy and which will not proceed unless the temperature approaches about 2800~ In many flames both mechanisms are operative, but do not occur at the same temperature level. The combustion then takes place in two or more stages separated in space and the flame exhibits two or even three reaction zones. A detailed spectroscopic examination has been made of these flames and the results which are reported strongly support and amplify the views expressed by Adams et al. 2. The extension of spectroscopy to flames burning with oxidants other than oxygen has also produced a good deal of fresh ev'dence of the existence of non-equilibrium conditions in the combustion zone. This phenomenon is discussed at the end of the chapter.
Experimental For spectroscopic examination of a premixed gas flame the reaction zone should be flat and of sufficient optical depth for the application of absorption spectroscopy. To obtain a flame of this kind which was nonturbulent and did not strike back a rectangular burner was used having a length along the optical path of about 1 cm and a width of around 0.1 cm depending on the quenching distance of the flame. The reaction zone on this burner was nearly fiat as shown in Figure 1 (top right). The optical arrangement for absorp-
tion work is shown diagrammatically in Figure 1 (bottom) and it should be emphasized that quartz fluo 'ite achromatic lenses are essential. The ones used by us had a focal length of 30 cm and a diameter of 2 cm although the actual aperture employed was smaller than this and much less than that shown in the diagram. The light source was either the anode of a carbon arc, a medium pressure mercury vapor lamp or a hydrogen discharge tube and the spectrograph was either a Hilger medium quartz instrument or a HilgerLittrow type instrument with a quartz or glass set. The type of photographic plates used are indicated in the descriptions to F g u r e s 2 and 3.
Experimental Results NITROUS OXIDE FLAMES
The spectra of these flames burning at atmospheric pressure are similar to those obtained previously at reduced pressure 4. Thus the reaction zone of a hydrogen-nitrous oxide flame emits ammonia a-bands, NH, OH and NO bands, and these bands are strongest for mixtures near to stoichiometric ~roport ons. The ~urncd ~ases emit only OH bands and even these are much weaker than in the reaction zone. The reaction zone of an ethylene flame supported by nitrous oxide is typical of all hydrocarbon fuels and emits C2, CH, CN, OH and NO bands with the maximum intensity in shghtly fuel rich mixtures. The burned ~ases again emit only OH bands which are weaker than those in the react on zone (Fig. 2A). Mixtures which are very fuel rich burn with a halo above the reaction zone. This halo, in which CN and C2 bands appear, is also shown by flames supported by NO and NO~ and occurs when there is not sufficient oxygen in the mixture to convert all the carbon to carbon monoxide. Under these extremely reducing conditions C2 and CN molecules formed in the reaction zone have an unusually long lifetime and persist in the burned gases. In an ethylene flame a halo appears in mixtures zicher than C2H4 + 2N20. The absorption spectra of the nitrous oxide flames were examined and revealed several inter-
719
FLAMES SUPPORTED BY OXIDES OF NITROGEN
esting differences. In the hydrogen flame the OH absorption bands unlike the emission bands showed no enhanced strength in the reaction zone. The NO bands behaved similarly although some NO absorption was present in the burned gases whatever the mixture strength, which in the case of fuel rich flames indicates a small degree of incomplete combustion. I n lean flames the Schumann-Runge oxygen bands could be seen in absorption. I n an ethylene flame the NO bands are emitted so strongly in the reaction zone that reversal cannot be achieved even with the continuum of a powerful hydrogen lamp. I n this flame NH absorption bands occur at a position in the reaction zone where the OH absorption bands are still weak. NITRIC OXIDE
second reaction zone above the first. This coincides with an increase in the strength of the OH radiation brought about by an increase in the flame temperature as the excess nitric oxide decomposes. This is accompanied by a greyish continuous radiation in the visible region. Two other flames were examined. The first of these was a moist carbon-monoxide-nitric oxide flame which has a yellowish reaction zone. The emissions from the reaction zone and the burned gases were similar and consisted of the CO-flame bands and weak OH bands. Oxygen bands were prominent in lean flames but there were no NH bands emitted at any mixture strength. I n the
FLAMES
Emission spectra In the NO decomposition flame which was obtained by the authors 1 Schumann-Runge oxygen bands were observed in emission from the visible region down to a wave',ength of 2200~ (Fig. 2B). This was the only emission apart from some OH bands which may be attributed to the presence of moisture. When hydrogen was burned with nitric oxide, OH and 05 bands were the principal emitters and appeared in both the reaction zone and in the burned gas. Weak NO bands may have been present in the reaction zone. Very weak NH bands were also observed but these may have been caused by the presence of N20 as impurity. The NH bands disappeared when the hydrogennitric oxide mixture was preheated to 1000~ presumably because the N~O was decomposed at this temperature. The reaction zone of a methane-nitric oxide flame emits very weak ammonia-~ and C2 bands and strong CH, CN, NH, OH and NO bands. Other hydrocarbon flames give a similar spectrum except that the ammonia s-bands are missing and the C2 bands are much stronger. Oxygen bands are always strongly emitted from the reaction zone and from the burned gas when the mixture is fuel lean. Contrary to their behavior in the nitrous oxide flames the OH bands have no enhanced strength in the reaction zone, and indeed in fuel lean flames OH emission is actually weaker in the reaction zone (Fig. 2C). Acetylene is an exception however. All fuel rich hydrocarbonnitric oxide flames show the halo above the reaction zone which was discussed for the nitrousoxide flames (Fig. 2D). Fuel lean flames possess a
OII$IRYA'/'K~
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FIG. 1. Flat flame supported on rectangular burner, and scheme of optical system. other flame examined, ammonia was burned and NH2, NH, OH and NO bands were obseFved in the flame front. The OH bands in the reaction zone were a little enhanced compared with the emission in the burned gas immediately above but the strength of these bands increased again a few millimeters higher up which seems to indicate a small temperature rise brought about by further reaction in the burned gas.
Absorption spectra I n the hydrogen-nitric oxide flame there are no intermediate radicals except OH which can be detected in absorption but it is possible to follow the NO ~ bands through the flame. It was found that whatever the mixture strength the nitric oxide was decomposed in the reaction zone and
720
KINETICS OF COMBUSTION REACTIONS
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NOTE : All the s p e c t r a shown with the exception of 2B and 3H were t a k e n from fiat flames s u p p o r t e d on the rectangular burner illustrated in Fig. 1. The reaction zone can usually be identified and located in emission and a b s o r p t i o n by the band systems of C~ , CN, NH etc. The radiation above this zone is e m i t t e d by the b u r n t gases. All the spectra were t a k e n with a medium Hilger quartz spectrograph unless otherwise stated.
FLAMES SUPPORTED BY OXIDES OF NITROGEN
only very small concentrations corresponding to the equilibrium amounts appeared in the burned gas. I n fuel weak mixtures the excess nitric oxide is decomposed and the oxygen can be detected by the Schumann-Runge bands in absorption (Fig. 3B). Nitric oxide behaves differently in the ammonia-nitric oxide flame, however, and can be readily detected in absorption in the burned gas of nitric oxide rich flames. Decomposition of the excess n~tric oxide does not take place and no oxygen bands can be detected (Figs. 3C and 3D). The behavior of the hydrocarbon-nitric oxide flames is again shown to be more complex and depends on the nature of the hydrocarbon. Acetylene behaves rather like hydrogen and any excess nitric oxide is decomposed in the reaction zone. Contrary to other hydrocarbons acetylene shows an excess radiation in the reaction zone which is especially visible in the (0, 1) band of NO (Fig. 3A). Lower hydrocarbons such as ethane convert all the excess NO of weak mixtures into oxygen but it is noticeable that the resulting oxygen bands only begin a short distance above the reaction zone and this position corresponds to the second reaction zone mentioned above. CN radicals have been detected in absorption in these flames by a line absorption method. The positive column of a carbon are which emits CN at a temperature of about 6000~ was focused into the flame and the CN emission lines of the arc were absorbed by the CN radicals in the flame. This results in an interruption of the CN emission lines at the position of the reaction zone (Fig. 2H). In an acetylene-nitric oxide flame the concentration of CN radicals is sufficiently large
721
for this effect to be very marked, and in an ethylene flame it can just be detected but it was not observed in an ethane flame. Thus the CN radical concentration in the reaction zone seems to depend on either the reaction velocity or on the temperature of the flame. Absorption bands of NH radicals have been observed in a variety of flames. In an ammoniaoxygen flame these bands were very strong in the reaction zone and were not greatly affected by the mixture strength (Fig. 2I). In the ammonianitrous oxide flame the bands were almost as strong as those in the oxygen flame and in both cases the radicals are probably formed by the decomposition of the ammonia. I n the ammonianitric oxide flame, however, the NH absorption is very weak and onty just visible in fuel rich flames. This is an interesting difference, because the burning velocity of the. nitric oxide flame is about the same as that of the nitrous oxide flame and the reaction velocities are presumably similar, also. I n hydrocarbon-nitric oxide flames the NH bands could not be observed in absorption. In the acetylene flame NH bands were in emission against the radiation of the anode of a carbon arc and the same was true for the CH band at 3143A. These observations coupled with the fact that the neighboring bands of OH were obtained in absorption demonstrate the existence of non-equilibrium conditions in the acetylene flame (Fig. 2E). N I T R O G E N DIOXIDE F L A M E S
Emission spectra I n all hydrocarbon flames supported by nitrogen diox'de there are two visible reaction zones
FIG. 2 FIG. 2A. Ethylene-nitrous oxide flame in emission. Zenith plate. OH radiation stronger in r~eaction zone than in burnt gas. FIG. 2B. Nitric oxide decomposition flame in emission. Flame supported in a 3 cm diameter quartz tube in a furnace at approx. 1000~ Schumann-Runge bands of oxygen and OH bands can be seen. FIG. 2C. Ethane-nitric oxideflame (fuel lean) in emission. Zenith plate. OH radiation weaker in reaction zone than in burnt gas. FIG. 2D. Acetylene-nitric oxide flame (very fuel rich) in emission. HP3 plate. C~ and CN bands extend into hole above the reaction zone. Fro. 2E. Acetylene-nitric oxide flame in absorption. Carbon arc light source and Hilger Littrow type spectrograph used with quartz set. Process plate. Bands of NH (3360) and CH (3143) stand out in emission whereas the OH band at 3064 appears only in absorption in the burnt gas. FIG. 2F. Hydrogen burning with a mixture of nitrous and nitric oxides. Absorption spectrum. Carbon arc light source and Littrow type spectrograph. Zenith plate. OH bands alone seen in absorption. FIG. 2G. Hydrogen flame as in 2F seen in emission on the same plate. Two reaction zones can be distinguished. The strength of the OH emission in the first reaction zone is not matched by a corresponding strength in absorption. Fro. 2H. Acetylene-nitric oxide flame in absorption against the positive column of a carbon are. Littrow type spectrograph with glass set. Panchromatic plate. The CN bands of the light source are absorbed in the reaction zone of the flame. Fro. 2I. Ammonia-oxygen flame in absorption. Continuous radiation of the anode of the carbon arc used as light source. Littrow type spectrograph with quartz set. Process plate. NH bands seen in absorption in the reaction zone.
722
KINETICS OF C O M B U S T I O N REACTIONS
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FLAMES SUPPORTED BY OXIDES OF NITROGEN
and although the separation between these zones is small in fuel lean flames it increases to as much as 2 mm for very fuel rich mixtures (Fig. 1). The spectra of the two zones can be separately identified as shown in Figure 3F, but the emission of the second zone is usually much the stronger which makes identification difficult at times. The emission of the second reaction zone is similar to that of a nitric oxide flame burning with the same fuel, and a halo may be observed as with the nitric oxide flame. Furthermore, the OH emission of flames of the lower hydrocarbons is weak in the second reaction zone but increases in strength in the gases above. This effect is particularly marked in fuel lean mixtures, as was the case with the nitric oxide flames, and in very lean flames three reaction zones appear, the last being the decomposition region of the excess nitric oxide formed in the flame. I n fuel rich flames on the other hand, a carbon zone sometimes forms between the first and second zone. Its presence in this region is not surprising, since thermal cracking of the hydrocarbon must occur at the elevated temperature and oxygen is velT deficient until the nitric oxide reacts in the second zone. The emission of the first reaction zone was investigated more easily by quenching the second in a special burner. In this device the flame was allowed to strike back in a small quartz tube and settle on a smaller burner inside. The diameters of the tube and burner were such that only the first reaction zone remained on the burner with the
723
tube acting as a sort of Smithells separator. The intensity of this reaction zone is not great and a Hilger-Raman type spectrograph with a glass prism was used to examine the fame. The spectrum was found to be completely continuous and stretched from the red into the blue region giving the overall impression of yellow light. Absorption bands of nitrogen dioxide were superimposed on the continuum because the reaction zone was conical and part of the light traveled through unburned gases. Hydrogen and carbon monoxide flames supported by nitrogen dioxide have only one reaction zone and the spectrum of this is identical with that of the first reaction zone of the hydrocarbon flames (Fig. 3H). The three minima which appear in the spectrum are caused by the three sensitivity gaps in the Ilford long-range spectrum plates used. If nitrogen dioxide is present in excess, the hydrogen and carbon monoxide flames show an after-glow which is caused by slow reaction in the burned gas. The after-glow has a grey color and its spectrum is again mainly continuous. I n the hydrogen flame strong bands of NO and OH and weak bands of NH are superimposed on the continuum, and in the after-glow of the carbon monoxide flame OH and NO bands are visible. The first reaction zone of a separated hydrocarbon flamo also shows an after-glow when NO2 is in excess. This is not a proper second reaction zone although in a methane flame for example very
FIG. 3 FIG. 3A. Acetylene-nitric oxide flame in absorption. Continuous radiation from medium pressure Hg-vapour lamp used as light source. Littrow type spectrograph with quartz set. Schumann plate. Nitric oxide bands end abruptly in reaction zone where emission occurs. FIG. 3B. Hydrogen-nitric oxideflame (fuel lean) in absorption. Reaction zone not quite flat. Hg vapour lamp used as light source. Littrow type spectrograph with quartz set. Schumann plate. Excess, nitric oxide is decomposed and oxygen bands appear. FIG. 3C. Ammonia-nitric oxide flame (fuel rich) in absorption. Hg-vapour lamp used as light source. Littrow type spectrograph with quartz set. Schumann plate. Absorption bands of ammonia and nitric oxide end in the reaction zone. FIG. 3D. Ammonia-nitric oxide flame (fuel lean) in absorption. Conditions as in Fig. 3C. Ammonia absorption bands end in the reaction zone but those of nitric oxide go partially through. No oxygen bands are observed. FIG. 3E. Methane-nitrogen dioxide flame in absorption. Carbon arc used as light source. Panchromatic plate. The limit of the NO2 absorption marks the beginning of the first reaction zone. FIG. 3F. Methane flame as in 3E seen in emission on the same plate. Position of absorption and emission can be correlated relative to the hair lines. FIG. 3G. Methane-nitrogen dioxide flame in absorption. Hydrogen discharge lamp used as light source. Schumann plate. NO~ absorption can be seen together with the appearance and disappearance of NO bands. FIG. 3H. Carbon monoxide-nitrogen dioxide flame in emission. Raman spectrograph with glass set. Long range spectrum plate. Only one reaction zone is observed corresponding to the first reaction zone of a hydrocarbon-nitrogen dioxide flame. FIG. 3I. Ethane-oxygen flame in emission. Zenith plate. S~lall quantity of iron penta-carbonyl added. Fe lines seen to be highly excited in the reaction zone. FIG. 3K. Ethane-nitric oxide flame in emission. Zenith plate. Small quantity of iron penta carbonyl carbonyl added. Fe lines of equal strength in reaction zone and burnt gas. Reaction zone indicated by the emission of the NO bands.
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KINETICS OF COMBUSTION REACTIONS
weak N H and CN bands are visible together with the bands of NO and OH.
Absorption spectra Absorption experiments were particularly interesting because nitrogen dioxide and the nitric oxide derived from it in the flame could be detected. Figures 3E and 3F show an absorption and an emission spectrum respectively of a flame under the same conditions. The lower hairline seen in the spectra was located lust under the second reaction zone and it m a y be seen that the NO2 absorption ceases at the beginning of the first reaction zone and that the OH absorption bands which are an indication of the temperature level in this part of the flame begin just after the second reaction zone. Absorption by NO bands beTABLE 1
Composition of combustible
Theoretical flame temperature ~
H~ -+- ~O~ H~ ~ NO H~ + 1/~NO2 NH3 + ~O~ NH3 -}- 11~NO C~H2 + 2~O2 C2H2 % 5NO CEH~ -}- 7NO C2H6 -}- 7NO
2810 2840 2660 2560 2675 3055 3090 2855 --
Experimental flame temperature ~
-2820 1550 -2640 -3095 -2865
gins where the NO2 disappears but it does not persist far into the second reaction zone (Fig. 3G). Some indication of the progress of the fuel molecules was obtained by mixing benzene with methane and observing the disappearance of the benzene bands in absorption. These bands disappeared when the NO: bands disappeared at the beginning of the first reaction zone. Hydrogen and carbon monoxide flames show strong nitric oxide bands in absorption in their burned gas above the single reaction zone. This fact holds for fuel rich and lean flames, and shows that NO does not play a major part in the reaction. In hydrocarbon flames the absorption in the burned gas is analogous to that of the NO flame. Flame Temperatures The temperature of a flame is the best indication of the completeness of the combustion and measurements were therefore made using the
line-reversal method. For flames of moderate temperature sodium was introduced locally as sodium chloride; in high temperature flames, however, it was more convenient to use the OH radicals already contained in the flame. The experimental results for various flames are shown in Table 1 in comparison with the theoretical adiabatic flame temperature assuming complete combustion. In most cases the agreement is good and within the experimental error. In a fuel lean ammonianitric oxide flame which is not quoted in Table 1 the flame temperature seemed to depend very much on the flame size because the NO which escaped decomposition in the reaction zone decomposed slowly in the burned gas and the resulting temperature rise was dependent on cooling by the surrounding air. Hydrocarbons supported by nitric oxide burn completely as shown in Table 1 and it is fairly certain that the same is true when these fuels burn with NO2. The only striking exceptions are the hydrogen-NO, flame and probably the carbon monoxide-NO, flame in which the nitric oxide formed does not react, and the combustion does not go to completion. Relative I n t e n s i t i e s of t h e OH Bands The emissions of many bands and especially the OH bands differ very much in strength in the reaction zones of various flames. Comparisons can only be made for flames having the same optical depth and burning at the same pressure. A t a pressure of 20 mm Hg and with a light path of 2 cm the following relative intensities of the OH bands in various flames were estimated assuming the strength of these bands in the hydrogen-oxygen flame to be unity. Nature of flame
NH~-O2 H2-NO2 H~-N20
Relative intensity
of OH bands
5 ~oo 180
These flames were burning stoichiometric mixtures, and theoretically the temperature of the I-I2-NsO flame is close to that of the H2-N02 flame, but the strength of the OH emission in the reaction zone is nearly 20,000 times as great. This indicates that the reaction mechanism is quite different in the two flames. I t may be mentioned here also that the radiation of OH radicals is usually stronger in the reaction zone than in the burned gas which means that either the concen-
FLAMES SUPPORTED BY OXIDES OF NITROGEN
tration is higher in the reaction zone or the mode of excitation is not thermal. F l a m e s w i t h D o u b l e R e a c t i o n Zones
725
this can be readily seen from the Fe lines in the ultraviolet region. We have applied the technique to flames supported by the oxides of nitrogen, and Figure 3K shows the spectra of an ethane nitric oxide flame where the iron lines are if anything a little weaker in the reaction zone and there is evidently no excess exc:tation in this flame. In a faster burning flame such as acetylene and nitric oxide a slight amount of excess excitation is observed, however. NO2 flames behave in all respects like the NO flames as regards the second reaction zone. The first reaction zone is not hot enough to give an Fe emission spectrum. In ammon'a flames there is some excess excitation when oxygen is used and only slight evidence of it when oxygen is replaced by nitric oxide. All flames burning with nitrous oxide show the effect quite strongly.
The formation of more than one reaction zone in a flame is linked with the fact that different reactions begin at different temperature levels in the combustion wave. I n several of the flames studied two principal reactions began at nearly the same temperature level and no clear separation occurred. An example of this is a hydrocarbon burn:ng with a mixture of nitric oxide and air in which the reaction zone was more complex than that of a flame supported by either air or nitric oxide alone but did not separate into two distinct luminous zones. On the other hand a flame of hydrogen burning with a mixture of nitrous and nitric oxides has two separate reaction zones because the H2/N20 reaction and the H2/NO reaction are very different and begin at very different temperature levels. In a single combustion wave reactions of this kind, although separate, must be interrelated and the resulting spectrum depends very much on variables such as the N20/NO ratio and the mixture strength. A complete analysis of this and similar flames has not yet been attempted on account of the experimental difficulties. Evidence of excess excitation of the OH bands in part of the flame has been obtained however and in Figure 2G it may be seen that very strong OH bands are emitted by the H2/N:O reaction in the first zone, while the OH radiation from the H2/NO reaction on the second zone is not excessive compared with that in the burned gas. The second reaction zone in this spectrum can be distinguished by a continuum in the visible region. From the absorption spectrum of the same flame (Fig. 2F) it may be inferred that there is no excess OH concentration in the first reaction zone because the OH absorption is clearly nmch weaker in this part of the flame.
Either NO or NO~ can replace air or oxygen in a diffusion type flame. A nitric oxide-hydrocarbon diffusion flame is similar in appearance to an oxygen flame and with the help 0 f absorption spectroscopy it may be seen that most of the NO decomposes into oxygen (and nitrogen) in the high temperature zone. A little of the NO diffuses into the carbon zone and is responsible for rather weak CN and N H emission bands which occur in the position occupied by the C~ and CH bands of an oxygen flame3. A hydrocarbon-NO2 diffus'on flame is difficult to keep steady hut in this flame also it seems that the oxidizer has sufficient time to decompose before reacting with the fuel. The zone in which the NO2 decomposes to NO radiates a yellow continuum which is presumably identical with that observed in the premixed H~-NO~ flame, and the cause of the emission is probably a reverse reaction such as NO + 0 ~ NO2 rather than a reaction involving the fuel.
Abnormal Excitation in Flames
Discussion of Results
A method of distinguishing between thermal and non-thermal radiation in flames has been described ~ which depends on the relative intensities of lines in the iron emission spectrum, the iron being introduced by adding traces of iron penta-carbonyl to the unburned gases. Figure 3I shows an example of this technique applied to a fuel rich acetylene-oxygen flame. The iron lines are preferentially excited in the reaction zone and
We believe that the results obtained in the spectroscopic study of these flames have helped to clarify our knowledge of the reactions involved and have also provided valuable information on the mode of excitation of molecules in the reaction zone. There can be little doubt now that two mechanisms exist whereby nitric oxide is decomposed in flames. At high temperatures and in the absence
Diffusion F l a m e s
726
KINETICS OF COMBUSTION REACTIONS
of suitable radicals such as NH, NH2 and possibly C~, CH and CN, nitric oxide decomposes thermally. Thus it is decomposed in a hydrogennitric oxide flame irrespective of the mixture strength because the temperature of such flames is always sufficiently high. In an ammonia-nitric oxide flame, on the other hand, the temperature is lower and the rate of thermal decomposition is too small to be significant. Decomposition in this
sions: (1) because the final temperature is probably below the threshold required for thermal decomposition, and (2) because the initial stage in the reaction between hydrogen and nitrogen dioxide has too high a burning velocity for the subsequent slow reaction between hydrogen and nitric oxide to follow. The second zone even if it could exist would not be stationary. This general picture of the role of nitric oxide in
TABLE 2 Flame
Condition of flame
Hydrocarbon-oxygen Hydrocarbon-air
Premixed Premixed
Hydrogen-O2 and carbon monoxide-O2 Hydrogen-air and carbon mouoxide-air Hydrogen-nitrous oxide and carbon monoxide-nitrous oxide Ammonia-oxygen and ammonianitrous oxide Cyanogen-oxygen Carbon disulfide-oxygen Methane, ethane and ethylenenitric oxide Acetylene-nitric oxide
Premixed
Premixed Premixed
High excitation
5
Premixed Premixed Premixed
High excitation Thermal radiation Thermal
5 5
Premixed
Very slight indication of high excitation Moderate amount of high excitation Thermal Very slight high excitation (in second reaction zone) Thermal
Premixed
Premixed
Ethane-nitrogen dioxide Acetylene-nitrogen dioxide
Premixed Premixed
Hydrocarbon-oxygen
Diffusion, high pressure Diffusion, very low pressure Diffusion, any pressure
Hydrocarbon-air or very diluted hydrocarbon with oxygen
Reference
High excitation High excitation (very pronounced) Mainly thermal with slightly high excitation Slight indication of high excitation High excitation (moderate)
Ammonia-nitric oxide
Hydrocarbon-oxygen
Remarks
flame depends on a radical mechanism and is not always complete. In fuel lean flames, for example, the concentration of radicals is limited by the lower concentration of the fuel and the lower temperature so that only a portion of the nitric oxide is decomposed, the rest is present in the burned gas. The hydrogen-nitrogen dioxide flame is interesting and shows the remarkable stability of nitric oxide. In this flame there is no second reaction zone as in a lean hydrocarbon-nitric oxide flame and the nitric oxide formed in the first reaction zone does not seem able to react for two rea-
High excitation very pronounced High excitation
5 s Unpublished results Unpublished results
3 8 Unpublished results
flames is confirmed by a number of the detailed spectroscopic observations. I t is in accord with the temperature measurements and the absorption experiments which show the presence or absence of nitric oxide and oxygen in the burned gases. The purely thermal mode of decomposition does not seem to involve radicals such as N H or NH~ and these are not detected in the decomposition flame or in the hydrogen-nitric oxide flame. I n the ammonia flame, however, reaction occurs at a lower temperature and it seems fairly certain that NH and NH2 radicals known to be present in the flame provide an alternative mechanism for
FLAMES SUPPORTED BY OXIDES OF NITROGEN
the decomposition of the nitric oxide. I t is probably significant that the N H concentration in the reaction zone of the ammonia-nitric oxide flame is smaller than that in the ammonia-oxygen or the ammonia-nitrous oxide flames since the NH is rapidly removed by reaction with nitric oxide. Whether radicals such as C2, CH and CN play a similar role in the hydrocarbon-nitric oxide flame is uncertain, but it seems probable, although the concentration of CN is found to be high, indicating a low rate of removal. All the spectroscopic evidence shows that the second reaction zone of hydrocarbon-nitrogen dioxide flames is identical with a hydrocarbonnitric oxide flame. The hydrocarbon is not completely decomposed into hydrogen and carbon monoxide in the first reaction zone, however, because subsequent reaction of these gases with nitric oxide would not yield the C : , CH, CN and N H radicals wh+ch are the principal characteristics of the second reaction zone. I t is, however, likely that complicated hydrocarbons such as benzene break down to simpler ones which cannot be detected by absorption. The first reaction zone of the hydrocarbon-nitrogen dioxide flame is difficult to understand since the radiation is identical with that of a pure NO2 decomposition into NO and 05 in a diffusion flame, and does not show any characteristics which help to unravel the reactions concerned. I t is strange that the oxygen produced in the decomposition of the nitrogen dioxide does not react with the hydrocarbon in the usual way to give rise to radiation of C2, CH and OH. The explanation may be that the reactions in the first reaction zone go at very low temperatures ~ and are analogous to those in a cool flame, where the emiss on by these radicals is also missing. In most flames OH radiation is much stronger in the reaction zone than in the burned gas but there is no corresponding enhancement of the absorption of the OH bands in the reaction zone which clearly indicates nonthermal excitation. This phenomenon is absent, however, in hydrocarbon-nitric oxide and nitrogen dioxide flames, except for acetylene. In a similar way, iron atoms show excess radiation in the reaction zone of many flames, but appear to be thermally excited in the very hot nitric oxide and nitrogen dioxide. The behavior of iron atoms introduced as traces of iron carbonyl has been used as a basis of assessing the existence of abnormal radiation in various flames and the results of the tests are summarized in Table 2.
727
I t is clear from this table, that the excess excitation in the reaction zones of flames is a matter of degree rather than kind. Two variables seem to influence the effect, viz., flame-temperature and reaction velocity. It is proposed here that reactions in hot gases always lead to a disturbance of the Maxwell-Boltmann distribution, because a number of collisions are involved before statistical equilibrium is installed. Whether or not this disturbance is observable will depend on the velocity of the reaction and on temperature. At high temperatures fast particles are present in equilibrium and a small disturbance will not be detected. These two parameters cannot explain all the facts, but provide a basis for a general understanding of the phenomena in both premixed gas and diffusion flames. A hot diffusion flame has only a small reaction velocity as diffusion is the rate determining factor, the temperature is very high and nonthermal conditions are not observed. If such a flame is highly diluted with inert gas the reaetion veloeity is not much affeeted, but the temperature is greatly reduced and nonthermal radiation may be detected by the iron speetrum technique already described. Premixed hydrocarbon-oxygen flames have both a high reaction velocity and a high temperature. The disturbanee seems to be so strong that despite the high temperature the effect shows up by a strengthening of the iron lines in the ultraviolet relative to the burned gas. Hydroearbon-air flames have a lower temperature and a smaller reaction velocity, and the effect is very strong. I t seems that at low tempel'atures iron lines resulting from higher eleetronie states are thermally excited so little that the method is very sensitive and a small disturbance can be readily detected. The behavior of nitrie oxide and nitrogen dioxide flames fits in with this general picture. These flames are even hotter than oxygen flames; despite the greater heat, the burning velocity is only about one-tenth and thus the reaetion velocity is only one-hundredth. The effect is clearly not observable as not enough fast particles out of equilibrium are present. Aeetylene flames are slightly different beeause they are quite fast, and therefore the non-thermal radiation becomes observable. I t seems to be possible to explain the thermal or nonthermal nature of most premixed and diffusion flames by considering only the temperature and reaction veloeity. There is no doubt that a more quantitative treatment will make it necessary to consider the type of eollision and the exact way in which energy is dissipated in the primary act
728
KINETICS OF COMBUSTION REACTIONS
of reaction. Hydrogen and carbon monoxide flames which show only slight nonthermal behavior will certainly have to be considered in such detail, since they do not easily fit into the simplified picture presented above. Acknowledgments Acknowledgment is made to the Chief Scientist, British Ministry of Supply, for permission to publish this paper. Crown copyright Reserved Reproduced with the permission of the Controller of Her Brittanic Majesty's Stationery Office.
REFERENCES 1. PARKER, W. G., AND WOLFHARD, H. G.: Fourth Symposium (International) on Combustion, p.
420. Baltimore, The Williams & Wilkins Co., 1953. 2. ADAMS,G. K., PARKER, W. G., AND WOLFHARD, H. G.: Faraday Soc., t3, 97 (1935). 3. WOLFHARD,H. G., AND PARKER, W. g.: Proc. Phys. Soc., A65, 2 (1952). 4. GAYDON,A. G., AND WOLFHARD, IX. G.: Third Symposium on Combustion Flame and Explosion Phenomena, p. 504. Baltimore, The
Williams & Wilkins Co., 1949. 5. GAYDON, A. G., AND WOLFHARD,H. G.: Proc. Roy. Soc., A205, 118 (1951).
82
PREDICTION OF THE QUENCHING EFFECT OF VARIOUS SURFACE GEOMETRIES By A. L. BERLAD AND A. E. POTTER, JR.
Introduction Recent flame quenching research I has indicated that there should be a set of simple relations among the various channel geometries which are capable of just quenching a given flame at a given pressure. The diffusional quenching mechanism proposed by Simon et al. is used there to predict that the quenching effect of a cylindrical tube of a given diameter dc is equivalent to that displayed by infinitely long plane parallel plates of separation dp when dp = ~/i2/32 dc An adequate check of this relation was somewhat hindered at that time by the facts that (1) in practice, quenching data associated with infinitely long plane parallel plates can be at best only approximated to by rectangular channels of large length to width ratio, and (2) rectangular slot quenching data ~ are usually obtained with a downward propagating flame, whereas cylindrical tube quenching data ~ are usually obtained with an upward propagating flame. The objectives of this investigation were essentially twofold: (1) To derive, on the basis of the average active particle chain length criterion of Simon et aI. ~, or on an equivalent thermal basis, a set of equations
which predict the relations among the dimension of a number of simple geometries which are capable of just quenching a given flame at a given pressure, and (2) To test several of these relations by determining experimentally the wall quenching of downward propagating propane-air flames as a function of fuel-air ratio and pressure for rectangular slots, cylinders, and cylindrical annuli.
Nomenclature The following symbols are used in this paper: fraction of molecules present in gas phase which must react for flame to continue to propagate a inside annulus diameter BI an arbitrary constant B2 an arbitrary constant be ellipse major axis b, rectangular slot length Cr total number of active particles c numerical concentration of active particles c average numerical concentration of active particles Co number of active particles created per unit time per unit volume Di diffusion coefficient for active particles of one kind A
KINETICS OF COMBUSTION REACTIONS
81 SPECTRA AND COMBUSTION MECHANISM OF FLAMES SUPPORTED BY THE OXIDES OF NITROGEN By H. G. WOLFHARD AND W. G. PARKER Introduction In a paper presented at the Fourth Symposium on Combustion z the authors described some premixed gas flames in which various fuels including hydrogen, carbon monoxide, ammonia and hydrocarbons were burned with nitric oxide and with nitrogen dioxide. Burning velocities and quenching diameters were reported which showed that the characteristics of these flames differed widely from those of the more familiar oxygen and air flames. I t was suggested elsewhere2 that one of the reasons for these differences lay in the fact that nitric oxide can react in two ways. I t can react with radicals such as NH or NH2 in steps which require only a small activation energy oz' it can decompose thermally by a process which requires a high activation energy and which will not proceed unless the temperature approaches about 2800~ In many flames both mechanisms are operative, but do not occur at the same temperature level. The combustion then takes place in two or more stages separated in space and the flame exhibits two or even three reaction zones. A detailed spectroscopic examination has been made of these flames and the results which are reported strongly support and amplify the views expressed by Adams et al. 2. The extension of spectroscopy to flames burning with oxidants other than oxygen has also produced a good deal of fresh ev'dence of the existence of non-equilibrium conditions in the combustion zone. This phenomenon is discussed at the end of the chapter.
Experimental For spectroscopic examination of a premixed gas flame the reaction zone should be flat and of sufficient optical depth for the application of absorption spectroscopy. To obtain a flame of this kind which was nonturbulent and did not strike back a rectangular burner was used having a length along the optical path of about 1 cm and a width of around 0.1 cm depending on the quenching distance of the flame. The reaction zone on this burner was nearly fiat as shown in Figure 1 (top right). The optical arrangement for absorp-
tion work is shown diagrammatically in Figure 1 (bottom) and it should be emphasized that quartz fluo 'ite achromatic lenses are essential. The ones used by us had a focal length of 30 cm and a diameter of 2 cm although the actual aperture employed was smaller than this and much less than that shown in the diagram. The light source was either the anode of a carbon arc, a medium pressure mercury vapor lamp or a hydrogen discharge tube and the spectrograph was either a Hilger medium quartz instrument or a HilgerLittrow type instrument with a quartz or glass set. The type of photographic plates used are indicated in the descriptions to F g u r e s 2 and 3.
Experimental Results NITROUS OXIDE FLAMES
The spectra of these flames burning at atmospheric pressure are similar to those obtained previously at reduced pressure 4. Thus the reaction zone of a hydrogen-nitrous oxide flame emits ammonia a-bands, NH, OH and NO bands, and these bands are strongest for mixtures near to stoichiometric ~roport ons. The ~urncd ~ases emit only OH bands and even these are much weaker than in the reaction zone. The reaction zone of an ethylene flame supported by nitrous oxide is typical of all hydrocarbon fuels and emits C2, CH, CN, OH and NO bands with the maximum intensity in shghtly fuel rich mixtures. The burned ~ases again emit only OH bands which are weaker than those in the react on zone (Fig. 2A). Mixtures which are very fuel rich burn with a halo above the reaction zone. This halo, in which CN and C2 bands appear, is also shown by flames supported by NO and NO~ and occurs when there is not sufficient oxygen in the mixture to convert all the carbon to carbon monoxide. Under these extremely reducing conditions C2 and CN molecules formed in the reaction zone have an unusually long lifetime and persist in the burned gases. In an ethylene flame a halo appears in mixtures zicher than C2H4 + 2N20. The absorption spectra of the nitrous oxide flames were examined and revealed several inter-
719
FLAMES SUPPORTED BY OXIDES OF NITROGEN
esting differences. In the hydrogen flame the OH absorption bands unlike the emission bands showed no enhanced strength in the reaction zone. The NO bands behaved similarly although some NO absorption was present in the burned gases whatever the mixture strength, which in the case of fuel rich flames indicates a small degree of incomplete combustion. I n lean flames the Schumann-Runge oxygen bands could be seen in absorption. I n an ethylene flame the NO bands are emitted so strongly in the reaction zone that reversal cannot be achieved even with the continuum of a powerful hydrogen lamp. I n this flame NH absorption bands occur at a position in the reaction zone where the OH absorption bands are still weak. NITRIC OXIDE
second reaction zone above the first. This coincides with an increase in the strength of the OH radiation brought about by an increase in the flame temperature as the excess nitric oxide decomposes. This is accompanied by a greyish continuous radiation in the visible region. Two other flames were examined. The first of these was a moist carbon-monoxide-nitric oxide flame which has a yellowish reaction zone. The emissions from the reaction zone and the burned gases were similar and consisted of the CO-flame bands and weak OH bands. Oxygen bands were prominent in lean flames but there were no NH bands emitted at any mixture strength. I n the
FLAMES
Emission spectra In the NO decomposition flame which was obtained by the authors 1 Schumann-Runge oxygen bands were observed in emission from the visible region down to a wave',ength of 2200~ (Fig. 2B). This was the only emission apart from some OH bands which may be attributed to the presence of moisture. When hydrogen was burned with nitric oxide, OH and 05 bands were the principal emitters and appeared in both the reaction zone and in the burned gas. Weak NO bands may have been present in the reaction zone. Very weak NH bands were also observed but these may have been caused by the presence of N20 as impurity. The NH bands disappeared when the hydrogennitric oxide mixture was preheated to 1000~ presumably because the N~O was decomposed at this temperature. The reaction zone of a methane-nitric oxide flame emits very weak ammonia-~ and C2 bands and strong CH, CN, NH, OH and NO bands. Other hydrocarbon flames give a similar spectrum except that the ammonia s-bands are missing and the C2 bands are much stronger. Oxygen bands are always strongly emitted from the reaction zone and from the burned gas when the mixture is fuel lean. Contrary to their behavior in the nitrous oxide flames the OH bands have no enhanced strength in the reaction zone, and indeed in fuel lean flames OH emission is actually weaker in the reaction zone (Fig. 2C). Acetylene is an exception however. All fuel rich hydrocarbonnitric oxide flames show the halo above the reaction zone which was discussed for the nitrousoxide flames (Fig. 2D). Fuel lean flames possess a
OII$IRYA'/'K~
\ ~uR~
'/ ~OP
I SF~mO~I~N
FIG. 1. Flat flame supported on rectangular burner, and scheme of optical system. other flame examined, ammonia was burned and NH2, NH, OH and NO bands were obseFved in the flame front. The OH bands in the reaction zone were a little enhanced compared with the emission in the burned gas immediately above but the strength of these bands increased again a few millimeters higher up which seems to indicate a small temperature rise brought about by further reaction in the burned gas.
Absorption spectra I n the hydrogen-nitric oxide flame there are no intermediate radicals except OH which can be detected in absorption but it is possible to follow the NO ~ bands through the flame. It was found that whatever the mixture strength the nitric oxide was decomposed in the reaction zone and
720
KINETICS OF COMBUSTION REACTIONS
Cz
5/G5 ~~
Cz
C~ 4312 ~~
CN o 3883 ~
C~
OH 3064
~ . 33~0
,
OH o 28//R B
as'oa n"
0// 3 0 6 4
cH 4.7/2 A ~
C'er o 3g$3 fl
CN
.
2;;00 a ~
2s'oa a -
,#
0/-I . N X 3428H
o/t . 3064//
D
9 CNredCz
o Cz
.
CaC~
5635a 5/(:,5R 4737P"
CHC/V 42/[,,,q"
cN 3883~
Nlt ~ 336O/?
~" C N o 3590 n
NH 3360 ,q=
CH o 3/43 R
Et/ ~ 3143 R
OH
F
OH o 3428 R
NH . 3360 R
0// 3064 R~
H
388317
3871~
NH 3 3 6 0 R~ Fz~. 2
NOTE : All the s p e c t r a shown with the exception of 2B and 3H were t a k e n from fiat flames s u p p o r t e d on the rectangular burner illustrated in Fig. 1. The reaction zone can usually be identified and located in emission and a b s o r p t i o n by the band systems of C~ , CN, NH etc. The radiation above this zone is e m i t t e d by the b u r n t gases. All the spectra were t a k e n with a medium Hilger quartz spectrograph unless otherwise stated.
FLAMES SUPPORTED BY OXIDES OF NITROGEN
only very small concentrations corresponding to the equilibrium amounts appeared in the burned gas. I n fuel weak mixtures the excess nitric oxide is decomposed and the oxygen can be detected by the Schumann-Runge bands in absorption (Fig. 3B). Nitric oxide behaves differently in the ammonia-nitric oxide flame, however, and can be readily detected in absorption in the burned gas of nitric oxide rich flames. Decomposition of the excess n~tric oxide does not take place and no oxygen bands can be detected (Figs. 3C and 3D). The behavior of the hydrocarbon-nitric oxide flames is again shown to be more complex and depends on the nature of the hydrocarbon. Acetylene behaves rather like hydrogen and any excess nitric oxide is decomposed in the reaction zone. Contrary to other hydrocarbons acetylene shows an excess radiation in the reaction zone which is especially visible in the (0, 1) band of NO (Fig. 3A). Lower hydrocarbons such as ethane convert all the excess NO of weak mixtures into oxygen but it is noticeable that the resulting oxygen bands only begin a short distance above the reaction zone and this position corresponds to the second reaction zone mentioned above. CN radicals have been detected in absorption in these flames by a line absorption method. The positive column of a carbon are which emits CN at a temperature of about 6000~ was focused into the flame and the CN emission lines of the arc were absorbed by the CN radicals in the flame. This results in an interruption of the CN emission lines at the position of the reaction zone (Fig. 2H). In an acetylene-nitric oxide flame the concentration of CN radicals is sufficiently large
721
for this effect to be very marked, and in an ethylene flame it can just be detected but it was not observed in an ethane flame. Thus the CN radical concentration in the reaction zone seems to depend on either the reaction velocity or on the temperature of the flame. Absorption bands of NH radicals have been observed in a variety of flames. In an ammoniaoxygen flame these bands were very strong in the reaction zone and were not greatly affected by the mixture strength (Fig. 2I). In the ammonianitrous oxide flame the bands were almost as strong as those in the oxygen flame and in both cases the radicals are probably formed by the decomposition of the ammonia. I n the ammonianitric oxide flame, however, the NH absorption is very weak and onty just visible in fuel rich flames. This is an interesting difference, because the burning velocity of the. nitric oxide flame is about the same as that of the nitrous oxide flame and the reaction velocities are presumably similar, also. I n hydrocarbon-nitric oxide flames the NH bands could not be observed in absorption. In the acetylene flame NH bands were in emission against the radiation of the anode of a carbon arc and the same was true for the CH band at 3143A. These observations coupled with the fact that the neighboring bands of OH were obtained in absorption demonstrate the existence of non-equilibrium conditions in the acetylene flame (Fig. 2E). N I T R O G E N DIOXIDE F L A M E S
Emission spectra I n all hydrocarbon flames supported by nitrogen diox'de there are two visible reaction zones
FIG. 2 FIG. 2A. Ethylene-nitrous oxide flame in emission. Zenith plate. OH radiation stronger in r~eaction zone than in burnt gas. FIG. 2B. Nitric oxide decomposition flame in emission. Flame supported in a 3 cm diameter quartz tube in a furnace at approx. 1000~ Schumann-Runge bands of oxygen and OH bands can be seen. FIG. 2C. Ethane-nitric oxideflame (fuel lean) in emission. Zenith plate. OH radiation weaker in reaction zone than in burnt gas. FIG. 2D. Acetylene-nitric oxide flame (very fuel rich) in emission. HP3 plate. C~ and CN bands extend into hole above the reaction zone. Fro. 2E. Acetylene-nitric oxide flame in absorption. Carbon arc light source and Hilger Littrow type spectrograph used with quartz set. Process plate. Bands of NH (3360) and CH (3143) stand out in emission whereas the OH band at 3064 appears only in absorption in the burnt gas. FIG. 2F. Hydrogen burning with a mixture of nitrous and nitric oxides. Absorption spectrum. Carbon arc light source and Littrow type spectrograph. Zenith plate. OH bands alone seen in absorption. FIG. 2G. Hydrogen flame as in 2F seen in emission on the same plate. Two reaction zones can be distinguished. The strength of the OH emission in the first reaction zone is not matched by a corresponding strength in absorption. Fro. 2H. Acetylene-nitric oxide flame in absorption against the positive column of a carbon are. Littrow type spectrograph with glass set. Panchromatic plate. The CN bands of the light source are absorbed in the reaction zone of the flame. Fro. 2I. Ammonia-oxygen flame in absorption. Continuous radiation of the anode of the carbon arc used as light source. Littrow type spectrograph with quartz set. Process plate. NH bands seen in absorption in the reaction zone.
722
KINETICS OF C O M B U S T I O N REACTIONS
k NO
NO o ZZ69 A
z~to ;
B Oz
NO 2.2.6q
/40 I.Z69 ~4
NO
z,..~
NO ~. 215.5" A
NH~,
N#
0
NO ZZ6q
NH$.
N lt~.,
zz./.,
2,6716~.
;,
NO
J30@ I~
6000
OH
! I 84
A
GX 1~tl A
4ooo
9.~--
rG.
NO
...................
NO FIG.
3
( S e e n o t e o n p . 720)
9 NO
a '
'J.I$ 5 A
rC
NO
FLAMES SUPPORTED BY OXIDES OF NITROGEN
and although the separation between these zones is small in fuel lean flames it increases to as much as 2 mm for very fuel rich mixtures (Fig. 1). The spectra of the two zones can be separately identified as shown in Figure 3F, but the emission of the second zone is usually much the stronger which makes identification difficult at times. The emission of the second reaction zone is similar to that of a nitric oxide flame burning with the same fuel, and a halo may be observed as with the nitric oxide flame. Furthermore, the OH emission of flames of the lower hydrocarbons is weak in the second reaction zone but increases in strength in the gases above. This effect is particularly marked in fuel lean mixtures, as was the case with the nitric oxide flames, and in very lean flames three reaction zones appear, the last being the decomposition region of the excess nitric oxide formed in the flame. I n fuel rich flames on the other hand, a carbon zone sometimes forms between the first and second zone. Its presence in this region is not surprising, since thermal cracking of the hydrocarbon must occur at the elevated temperature and oxygen is velT deficient until the nitric oxide reacts in the second zone. The emission of the first reaction zone was investigated more easily by quenching the second in a special burner. In this device the flame was allowed to strike back in a small quartz tube and settle on a smaller burner inside. The diameters of the tube and burner were such that only the first reaction zone remained on the burner with the
723
tube acting as a sort of Smithells separator. The intensity of this reaction zone is not great and a Hilger-Raman type spectrograph with a glass prism was used to examine the fame. The spectrum was found to be completely continuous and stretched from the red into the blue region giving the overall impression of yellow light. Absorption bands of nitrogen dioxide were superimposed on the continuum because the reaction zone was conical and part of the light traveled through unburned gases. Hydrogen and carbon monoxide flames supported by nitrogen dioxide have only one reaction zone and the spectrum of this is identical with that of the first reaction zone of the hydrocarbon flames (Fig. 3H). The three minima which appear in the spectrum are caused by the three sensitivity gaps in the Ilford long-range spectrum plates used. If nitrogen dioxide is present in excess, the hydrogen and carbon monoxide flames show an after-glow which is caused by slow reaction in the burned gas. The after-glow has a grey color and its spectrum is again mainly continuous. I n the hydrogen flame strong bands of NO and OH and weak bands of NH are superimposed on the continuum, and in the after-glow of the carbon monoxide flame OH and NO bands are visible. The first reaction zone of a separated hydrocarbon flamo also shows an after-glow when NO2 is in excess. This is not a proper second reaction zone although in a methane flame for example very
FIG. 3 FIG. 3A. Acetylene-nitric oxide flame in absorption. Continuous radiation from medium pressure Hg-vapour lamp used as light source. Littrow type spectrograph with quartz set. Schumann plate. Nitric oxide bands end abruptly in reaction zone where emission occurs. FIG. 3B. Hydrogen-nitric oxideflame (fuel lean) in absorption. Reaction zone not quite flat. Hg vapour lamp used as light source. Littrow type spectrograph with quartz set. Schumann plate. Excess, nitric oxide is decomposed and oxygen bands appear. FIG. 3C. Ammonia-nitric oxide flame (fuel rich) in absorption. Hg-vapour lamp used as light source. Littrow type spectrograph with quartz set. Schumann plate. Absorption bands of ammonia and nitric oxide end in the reaction zone. FIG. 3D. Ammonia-nitric oxide flame (fuel lean) in absorption. Conditions as in Fig. 3C. Ammonia absorption bands end in the reaction zone but those of nitric oxide go partially through. No oxygen bands are observed. FIG. 3E. Methane-nitrogen dioxide flame in absorption. Carbon arc used as light source. Panchromatic plate. The limit of the NO2 absorption marks the beginning of the first reaction zone. FIG. 3F. Methane flame as in 3E seen in emission on the same plate. Position of absorption and emission can be correlated relative to the hair lines. FIG. 3G. Methane-nitrogen dioxide flame in absorption. Hydrogen discharge lamp used as light source. Schumann plate. NO~ absorption can be seen together with the appearance and disappearance of NO bands. FIG. 3H. Carbon monoxide-nitrogen dioxide flame in emission. Raman spectrograph with glass set. Long range spectrum plate. Only one reaction zone is observed corresponding to the first reaction zone of a hydrocarbon-nitrogen dioxide flame. FIG. 3I. Ethane-oxygen flame in emission. Zenith plate. S~lall quantity of iron penta-carbonyl added. Fe lines seen to be highly excited in the reaction zone. FIG. 3K. Ethane-nitric oxide flame in emission. Zenith plate. Small quantity of iron penta carbonyl carbonyl added. Fe lines of equal strength in reaction zone and burnt gas. Reaction zone indicated by the emission of the NO bands.
724
KINETICS OF COMBUSTION REACTIONS
weak N H and CN bands are visible together with the bands of NO and OH.
Absorption spectra Absorption experiments were particularly interesting because nitrogen dioxide and the nitric oxide derived from it in the flame could be detected. Figures 3E and 3F show an absorption and an emission spectrum respectively of a flame under the same conditions. The lower hairline seen in the spectra was located lust under the second reaction zone and it m a y be seen that the NO2 absorption ceases at the beginning of the first reaction zone and that the OH absorption bands which are an indication of the temperature level in this part of the flame begin just after the second reaction zone. Absorption by NO bands beTABLE 1
Composition of combustible
Theoretical flame temperature ~
H~ -+- ~O~ H~ ~ NO H~ + 1/~NO2 NH3 + ~O~ NH3 -}- 11~NO C~H2 + 2~O2 C2H2 % 5NO CEH~ -}- 7NO C2H6 -}- 7NO
2810 2840 2660 2560 2675 3055 3090 2855 --
Experimental flame temperature ~
-2820 1550 -2640 -3095 -2865
gins where the NO2 disappears but it does not persist far into the second reaction zone (Fig. 3G). Some indication of the progress of the fuel molecules was obtained by mixing benzene with methane and observing the disappearance of the benzene bands in absorption. These bands disappeared when the NO: bands disappeared at the beginning of the first reaction zone. Hydrogen and carbon monoxide flames show strong nitric oxide bands in absorption in their burned gas above the single reaction zone. This fact holds for fuel rich and lean flames, and shows that NO does not play a major part in the reaction. In hydrocarbon flames the absorption in the burned gas is analogous to that of the NO flame. Flame Temperatures The temperature of a flame is the best indication of the completeness of the combustion and measurements were therefore made using the
line-reversal method. For flames of moderate temperature sodium was introduced locally as sodium chloride; in high temperature flames, however, it was more convenient to use the OH radicals already contained in the flame. The experimental results for various flames are shown in Table 1 in comparison with the theoretical adiabatic flame temperature assuming complete combustion. In most cases the agreement is good and within the experimental error. In a fuel lean ammonianitric oxide flame which is not quoted in Table 1 the flame temperature seemed to depend very much on the flame size because the NO which escaped decomposition in the reaction zone decomposed slowly in the burned gas and the resulting temperature rise was dependent on cooling by the surrounding air. Hydrocarbons supported by nitric oxide burn completely as shown in Table 1 and it is fairly certain that the same is true when these fuels burn with NO2. The only striking exceptions are the hydrogen-NO, flame and probably the carbon monoxide-NO, flame in which the nitric oxide formed does not react, and the combustion does not go to completion. Relative I n t e n s i t i e s of t h e OH Bands The emissions of many bands and especially the OH bands differ very much in strength in the reaction zones of various flames. Comparisons can only be made for flames having the same optical depth and burning at the same pressure. A t a pressure of 20 mm Hg and with a light path of 2 cm the following relative intensities of the OH bands in various flames were estimated assuming the strength of these bands in the hydrogen-oxygen flame to be unity. Nature of flame
NH~-O2 H2-NO2 H~-N20
Relative intensity
of OH bands
5 ~oo 180
These flames were burning stoichiometric mixtures, and theoretically the temperature of the I-I2-NsO flame is close to that of the H2-N02 flame, but the strength of the OH emission in the reaction zone is nearly 20,000 times as great. This indicates that the reaction mechanism is quite different in the two flames. I t may be mentioned here also that the radiation of OH radicals is usually stronger in the reaction zone than in the burned gas which means that either the concen-
FLAMES SUPPORTED BY OXIDES OF NITROGEN
tration is higher in the reaction zone or the mode of excitation is not thermal. F l a m e s w i t h D o u b l e R e a c t i o n Zones
725
this can be readily seen from the Fe lines in the ultraviolet region. We have applied the technique to flames supported by the oxides of nitrogen, and Figure 3K shows the spectra of an ethane nitric oxide flame where the iron lines are if anything a little weaker in the reaction zone and there is evidently no excess exc:tation in this flame. In a faster burning flame such as acetylene and nitric oxide a slight amount of excess excitation is observed, however. NO2 flames behave in all respects like the NO flames as regards the second reaction zone. The first reaction zone is not hot enough to give an Fe emission spectrum. In ammon'a flames there is some excess excitation when oxygen is used and only slight evidence of it when oxygen is replaced by nitric oxide. All flames burning with nitrous oxide show the effect quite strongly.
The formation of more than one reaction zone in a flame is linked with the fact that different reactions begin at different temperature levels in the combustion wave. I n several of the flames studied two principal reactions began at nearly the same temperature level and no clear separation occurred. An example of this is a hydrocarbon burn:ng with a mixture of nitric oxide and air in which the reaction zone was more complex than that of a flame supported by either air or nitric oxide alone but did not separate into two distinct luminous zones. On the other hand a flame of hydrogen burning with a mixture of nitrous and nitric oxides has two separate reaction zones because the H2/N20 reaction and the H2/NO reaction are very different and begin at very different temperature levels. In a single combustion wave reactions of this kind, although separate, must be interrelated and the resulting spectrum depends very much on variables such as the N20/NO ratio and the mixture strength. A complete analysis of this and similar flames has not yet been attempted on account of the experimental difficulties. Evidence of excess excitation of the OH bands in part of the flame has been obtained however and in Figure 2G it may be seen that very strong OH bands are emitted by the H2/N:O reaction in the first zone, while the OH radiation from the H2/NO reaction on the second zone is not excessive compared with that in the burned gas. The second reaction zone in this spectrum can be distinguished by a continuum in the visible region. From the absorption spectrum of the same flame (Fig. 2F) it may be inferred that there is no excess OH concentration in the first reaction zone because the OH absorption is clearly nmch weaker in this part of the flame.
Either NO or NO~ can replace air or oxygen in a diffusion type flame. A nitric oxide-hydrocarbon diffusion flame is similar in appearance to an oxygen flame and with the help 0 f absorption spectroscopy it may be seen that most of the NO decomposes into oxygen (and nitrogen) in the high temperature zone. A little of the NO diffuses into the carbon zone and is responsible for rather weak CN and N H emission bands which occur in the position occupied by the C~ and CH bands of an oxygen flame3. A hydrocarbon-NO2 diffus'on flame is difficult to keep steady hut in this flame also it seems that the oxidizer has sufficient time to decompose before reacting with the fuel. The zone in which the NO2 decomposes to NO radiates a yellow continuum which is presumably identical with that observed in the premixed H~-NO~ flame, and the cause of the emission is probably a reverse reaction such as NO + 0 ~ NO2 rather than a reaction involving the fuel.
Abnormal Excitation in Flames
Discussion of Results
A method of distinguishing between thermal and non-thermal radiation in flames has been described ~ which depends on the relative intensities of lines in the iron emission spectrum, the iron being introduced by adding traces of iron penta-carbonyl to the unburned gases. Figure 3I shows an example of this technique applied to a fuel rich acetylene-oxygen flame. The iron lines are preferentially excited in the reaction zone and
We believe that the results obtained in the spectroscopic study of these flames have helped to clarify our knowledge of the reactions involved and have also provided valuable information on the mode of excitation of molecules in the reaction zone. There can be little doubt now that two mechanisms exist whereby nitric oxide is decomposed in flames. At high temperatures and in the absence
Diffusion F l a m e s
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KINETICS OF COMBUSTION REACTIONS
of suitable radicals such as NH, NH2 and possibly C~, CH and CN, nitric oxide decomposes thermally. Thus it is decomposed in a hydrogennitric oxide flame irrespective of the mixture strength because the temperature of such flames is always sufficiently high. In an ammonia-nitric oxide flame, on the other hand, the temperature is lower and the rate of thermal decomposition is too small to be significant. Decomposition in this
sions: (1) because the final temperature is probably below the threshold required for thermal decomposition, and (2) because the initial stage in the reaction between hydrogen and nitrogen dioxide has too high a burning velocity for the subsequent slow reaction between hydrogen and nitric oxide to follow. The second zone even if it could exist would not be stationary. This general picture of the role of nitric oxide in
TABLE 2 Flame
Condition of flame
Hydrocarbon-oxygen Hydrocarbon-air
Premixed Premixed
Hydrogen-O2 and carbon monoxide-O2 Hydrogen-air and carbon mouoxide-air Hydrogen-nitrous oxide and carbon monoxide-nitrous oxide Ammonia-oxygen and ammonianitrous oxide Cyanogen-oxygen Carbon disulfide-oxygen Methane, ethane and ethylenenitric oxide Acetylene-nitric oxide
Premixed
Premixed Premixed
High excitation
5
Premixed Premixed Premixed
High excitation Thermal radiation Thermal
5 5
Premixed
Very slight indication of high excitation Moderate amount of high excitation Thermal Very slight high excitation (in second reaction zone) Thermal
Premixed
Premixed
Ethane-nitrogen dioxide Acetylene-nitrogen dioxide
Premixed Premixed
Hydrocarbon-oxygen
Diffusion, high pressure Diffusion, very low pressure Diffusion, any pressure
Hydrocarbon-air or very diluted hydrocarbon with oxygen
Reference
High excitation High excitation (very pronounced) Mainly thermal with slightly high excitation Slight indication of high excitation High excitation (moderate)
Ammonia-nitric oxide
Hydrocarbon-oxygen
Remarks
flame depends on a radical mechanism and is not always complete. In fuel lean flames, for example, the concentration of radicals is limited by the lower concentration of the fuel and the lower temperature so that only a portion of the nitric oxide is decomposed, the rest is present in the burned gas. The hydrogen-nitrogen dioxide flame is interesting and shows the remarkable stability of nitric oxide. In this flame there is no second reaction zone as in a lean hydrocarbon-nitric oxide flame and the nitric oxide formed in the first reaction zone does not seem able to react for two rea-
High excitation very pronounced High excitation
5 s Unpublished results Unpublished results
3 8 Unpublished results
flames is confirmed by a number of the detailed spectroscopic observations. I t is in accord with the temperature measurements and the absorption experiments which show the presence or absence of nitric oxide and oxygen in the burned gases. The purely thermal mode of decomposition does not seem to involve radicals such as N H or NH~ and these are not detected in the decomposition flame or in the hydrogen-nitric oxide flame. I n the ammonia flame, however, reaction occurs at a lower temperature and it seems fairly certain that NH and NH2 radicals known to be present in the flame provide an alternative mechanism for
FLAMES SUPPORTED BY OXIDES OF NITROGEN
the decomposition of the nitric oxide. I t is probably significant that the N H concentration in the reaction zone of the ammonia-nitric oxide flame is smaller than that in the ammonia-oxygen or the ammonia-nitrous oxide flames since the NH is rapidly removed by reaction with nitric oxide. Whether radicals such as C2, CH and CN play a similar role in the hydrocarbon-nitric oxide flame is uncertain, but it seems probable, although the concentration of CN is found to be high, indicating a low rate of removal. All the spectroscopic evidence shows that the second reaction zone of hydrocarbon-nitrogen dioxide flames is identical with a hydrocarbonnitric oxide flame. The hydrocarbon is not completely decomposed into hydrogen and carbon monoxide in the first reaction zone, however, because subsequent reaction of these gases with nitric oxide would not yield the C : , CH, CN and N H radicals wh+ch are the principal characteristics of the second reaction zone. I t is, however, likely that complicated hydrocarbons such as benzene break down to simpler ones which cannot be detected by absorption. The first reaction zone of the hydrocarbon-nitrogen dioxide flame is difficult to understand since the radiation is identical with that of a pure NO2 decomposition into NO and 05 in a diffusion flame, and does not show any characteristics which help to unravel the reactions concerned. I t is strange that the oxygen produced in the decomposition of the nitrogen dioxide does not react with the hydrocarbon in the usual way to give rise to radiation of C2, CH and OH. The explanation may be that the reactions in the first reaction zone go at very low temperatures ~ and are analogous to those in a cool flame, where the emiss on by these radicals is also missing. In most flames OH radiation is much stronger in the reaction zone than in the burned gas but there is no corresponding enhancement of the absorption of the OH bands in the reaction zone which clearly indicates nonthermal excitation. This phenomenon is absent, however, in hydrocarbon-nitric oxide and nitrogen dioxide flames, except for acetylene. In a similar way, iron atoms show excess radiation in the reaction zone of many flames, but appear to be thermally excited in the very hot nitric oxide and nitrogen dioxide. The behavior of iron atoms introduced as traces of iron carbonyl has been used as a basis of assessing the existence of abnormal radiation in various flames and the results of the tests are summarized in Table 2.
727
I t is clear from this table, that the excess excitation in the reaction zones of flames is a matter of degree rather than kind. Two variables seem to influence the effect, viz., flame-temperature and reaction velocity. It is proposed here that reactions in hot gases always lead to a disturbance of the Maxwell-Boltmann distribution, because a number of collisions are involved before statistical equilibrium is installed. Whether or not this disturbance is observable will depend on the velocity of the reaction and on temperature. At high temperatures fast particles are present in equilibrium and a small disturbance will not be detected. These two parameters cannot explain all the facts, but provide a basis for a general understanding of the phenomena in both premixed gas and diffusion flames. A hot diffusion flame has only a small reaction velocity as diffusion is the rate determining factor, the temperature is very high and nonthermal conditions are not observed. If such a flame is highly diluted with inert gas the reaetion veloeity is not much affeeted, but the temperature is greatly reduced and nonthermal radiation may be detected by the iron speetrum technique already described. Premixed hydrocarbon-oxygen flames have both a high reaction velocity and a high temperature. The disturbanee seems to be so strong that despite the high temperature the effect shows up by a strengthening of the iron lines in the ultraviolet relative to the burned gas. Hydroearbon-air flames have a lower temperature and a smaller reaction velocity, and the effect is very strong. I t seems that at low tempel'atures iron lines resulting from higher eleetronie states are thermally excited so little that the method is very sensitive and a small disturbance can be readily detected. The behavior of nitrie oxide and nitrogen dioxide flames fits in with this general picture. These flames are even hotter than oxygen flames; despite the greater heat, the burning velocity is only about one-tenth and thus the reaetion velocity is only one-hundredth. The effect is clearly not observable as not enough fast particles out of equilibrium are present. Aeetylene flames are slightly different beeause they are quite fast, and therefore the non-thermal radiation becomes observable. I t seems to be possible to explain the thermal or nonthermal nature of most premixed and diffusion flames by considering only the temperature and reaction veloeity. There is no doubt that a more quantitative treatment will make it necessary to consider the type of eollision and the exact way in which energy is dissipated in the primary act
728
KINETICS OF COMBUSTION REACTIONS
of reaction. Hydrogen and carbon monoxide flames which show only slight nonthermal behavior will certainly have to be considered in such detail, since they do not easily fit into the simplified picture presented above. Acknowledgments Acknowledgment is made to the Chief Scientist, British Ministry of Supply, for permission to publish this paper. Crown copyright Reserved Reproduced with the permission of the Controller of Her Brittanic Majesty's Stationery Office.
REFERENCES 1. PARKER, W. G., AND WOLFHARD, H. G.: Fourth Symposium (International) on Combustion, p.
420. Baltimore, The Williams & Wilkins Co., 1953. 2. ADAMS,G. K., PARKER, W. G., AND WOLFHARD, H. G.: Faraday Soc., t3, 97 (1935). 3. WOLFHARD,H. G., AND PARKER, W. g.: Proc. Phys. Soc., A65, 2 (1952). 4. GAYDON,A. G., AND WOLFHARD, IX. G.: Third Symposium on Combustion Flame and Explosion Phenomena, p. 504. Baltimore, The
Williams & Wilkins Co., 1949. 5. GAYDON, A. G., AND WOLFHARD,H. G.: Proc. Roy. Soc., A205, 118 (1951).
82
PREDICTION OF THE QUENCHING EFFECT OF VARIOUS SURFACE GEOMETRIES By A. L. BERLAD AND A. E. POTTER, JR.
Introduction Recent flame quenching research I has indicated that there should be a set of simple relations among the various channel geometries which are capable of just quenching a given flame at a given pressure. The diffusional quenching mechanism proposed by Simon et al. is used there to predict that the quenching effect of a cylindrical tube of a given diameter dc is equivalent to that displayed by infinitely long plane parallel plates of separation dp when dp = ~/i2/32 dc An adequate check of this relation was somewhat hindered at that time by the facts that (1) in practice, quenching data associated with infinitely long plane parallel plates can be at best only approximated to by rectangular channels of large length to width ratio, and (2) rectangular slot quenching data ~ are usually obtained with a downward propagating flame, whereas cylindrical tube quenching data ~ are usually obtained with an upward propagating flame. The objectives of this investigation were essentially twofold: (1) To derive, on the basis of the average active particle chain length criterion of Simon et aI. ~, or on an equivalent thermal basis, a set of equations
which predict the relations among the dimension of a number of simple geometries which are capable of just quenching a given flame at a given pressure, and (2) To test several of these relations by determining experimentally the wall quenching of downward propagating propane-air flames as a function of fuel-air ratio and pressure for rectangular slots, cylinders, and cylindrical annuli.
Nomenclature The following symbols are used in this paper: fraction of molecules present in gas phase which must react for flame to continue to propagate a inside annulus diameter BI an arbitrary constant B2 an arbitrary constant be ellipse major axis b, rectangular slot length Cr total number of active particles c numerical concentration of active particles c average numerical concentration of active particles Co number of active particles created per unit time per unit volume Di diffusion coefficient for active particles of one kind A