Carbon formation in premixed flames

CARBON FORMATION IN PREMIXED FLAMES K, !1. IiOMANN lw, l,lm lu~ I'h,,-,~k,d~-,~hc ( ' h c l m c ilct I [lu,~cl,,,li;il (it~ll~lll]~..n

A sped;ill,, m',ttcd r¢~,te~,'. ~-. prcscfllct[ (~1 k'Cl[illll aspCt1", ~)1 ~;Is phase t.;trb,~l] I(~M'11;ttl(ul u] prcllllX,.~t ltaZ]lCS v, ilh file ~t~leC[ ~,1 strflllll:.lllll V (t/',t tlsSlt:~l!l :lll~.J lillur¢ u,~uk Jrl Ih¢ lield, t:tcha~,,~ur of :l fuel u) ttfll'~jsum I]:m~c', :tnd pyml%v',Js J,, iclclrcd h~ ;~s llcte~,s;ir~ }lilllt}Fl~_, lilt c~pcfmlelH;~] I;~ut,, tlJsctjswd tile Ill,.: appearauce ~1 fuel.itch 1]ii11'1¢s. lilllrls o[ Cill-t~l)n t0fl|l;lll.t)ll, tClllpcraliln.' dt!p¢l)dcm,e of tile ht]lll t)fcarb,m I~rrTl;ttrOll. prc,,,,r,., dl.'pcrldcrltc o l lilt' li~rlrl;lliOll illllfl dlld the )~¢[tl ,~l ,_;trh, m. ~))ttr~)t.';llba)ll llllL'llllCdlillt'",, rt'i~t:llc)l| I)l~dllt. l., iI1~I|1 l}tc pyl't)lo)'~r., o] ilct:l.~l¢llt' ;liItl hdll/dn¢, p;IrlleJe l(~/llLlI~C)ll alld ~Jox,,lh. Itlcl¢ I~)lh~; a dlsL!lV,,lllrl t.~f I~1,,: rt~t~/¢ of carb~u~ IorrlI;IIN~II '~ iltl (ulther tdln)llli.'nl~ ~tl Ihc IIl'lHk'llt¢ ,)I d(ldlltI, cs ;lll(J clot.trio tield~,.

Al.lllOt~[ill wc speak of clttboll ;1,4it producl ~)t incomplele combustion or thermal decomposilit)f1 Of a carbon-containing compound we do not mean elementary carbon but rather a variety of citrbon-rich solid material, the properties of which depend slrongly on its history of formation, il can be obtained as a hard and brittle or as a sofl and fatty substance, its colder ranging from brown to bktck and its hydrogen content from more than Iifly per cenl on an alorrfic basis to almosl none. A vitriablc part of its mass is usually exlraclabk by organic SOlVelltS.

in the wide field of thermal processl,~ giving solid carbon or rather carbonaceous material il is the formation of carbon from burning gases or vapours which has attracllxl most interesl both for industrial and scientific reasons. The

subject has also been studied in relation to the pyrolytic decomposition of various fuels. Only recenlly have these studies been extended mo p)rolysis in the liquid phase t and to carbon formation by radiolysis z. There arc several excellent and extensive review papers a list of which has been given by Palmer and Cullis s in their recent article on this subject dealing mainly with soot lormalion in flamc."s and pyrolytic decompositions. This paper will be restricted to the tbrmation of carbon in the gas phase of prcmix'cd flames. The depositiorJ of carbon on hol or cold stlrfaces ,,

which leads to a material having pr(,perlics quite diflcrcnt from those of the gas phase carbon will not be ronsMercd. This flame soot is a ralher fllll'[~, aild soft material consisting of single, rnorc or Its,; spherical parlieles which slick to each olher in a very open network. 1"lie size of Ihe particles is of Ihe order of sorne t',tlntlr~] Angstr6n~ units. The pr.:~perlies of this ,soot have been sludied ex[ensively. They are widely indcpcndeili of the kind of fuel and gener~llly it is nol possible to delermine the properties of soot from the nature of the original fuel unless for empirical reaso~ls :,r th,: manufacturing process. Nor is it possible by investigation of ihe Fnal product to lind out fftuch about the way carbon is formed '|'o learn more aboul Ihe process of carbon formation it is important to study the different tendencies of the fuels to form carbon together with their dillerenl needs of premixed oxygen to suppress it. To decide which species can act its 'buikling bricks" for carbon particles it is t]ecessaiy to know tile composition of the Ililme gas and ils temperature ill Ihe ~tOlle where carbon particles appear arid grow. We do not claim ~o cover all aspects ol this limiltxl stlbjecl of g;Is phase carbon Ibrm'ltion in premixed flames This. however, cannot be conlplctely separated fronl considering Ih,~ behaviour of Ihe fuel in dill'usion flames alltl pyt'~dysi~ Wc shall Jefer to the I:ttter ;Is necessary.

.?n5

2n~

K. II. 11~gMAN".

I'.xpt'rimtrtal I:acls (!) Tire al)lWaram'( , c!tJ, el-ri('h ./horn's

A qualilalive ICsl ['|)r c H f b o , t tormatitul is the yellc, w to or,ngc huznmosily v,.ldch ix elf|tiled by carbon particles. There arc scvcrat w~ys in which this Itnn:il]os;t) can appear m rich Ilarnes depending ~ ~: the libel ;u~d OI1 Ilamc siructure, While the appcaran:,.c of latninar burner flames of most hydroc~;: "~olls being mainlaillcd h,y lean 'IliXlLIrCs is ve~' I|lllch a~ik6 .sh,t~wi~lg a uniform, contir~uous t~,~,~lcflont, c.g hi Ihc shape of a cone or a flal disc. :his is n,t~l so wilh rich mixtures of many ti~cls. Ilerc ~.hc flame front becornes wavy and finally breaks up. Flames of this kind which C~,lll also be oh~ervcd in lean mixlures of methane and hydrog,..'n with air are usnally referred to as polyhedral or cellular flames. The reason for these dislttrbanccs of Ihe flame front is the different rate.,, of diffnsion of various species involved in the flame propagati'on process together with the dependence of the flame velocity on concentrati<>n. This also iniluences the way in which carbon luminosity appears at different parts of the: flame. Together with the different tendencies of the fuels to form carbon at a given carbon/::~xygc.a ratio this gives

c(~ntillilOl, lS

Vial. I I

ri,~¢It~~ll~c di'.dillClivc[)i~C',i~l'y¢]h}W Itl|llinotts

llame,, "I'luI~ proI~:Iga'.llg llames, altlmugh occasion:lIly Itscd as cxph~,,,ion Ilames 4. or in is~l~i~ic IFIICCI" eXlICl'illl¢llln',are less slillablc Ibr lhu ,,lud~ ~I carhops I~rmali~m. I.cl us Iir,,tdeal wilh :~mical llalnes burning with exclusion oI secondary air. I|ehrcns ~' and Street and "lhoma~' de, tribe iwo main lypes, each behmging to cerlahs fuels; Il)'Ihc Iir~L which we ,hallcall (after Behrcns) the "acct)'Ic~e-lype'.Is cl~araclcrized by a yellow lumhmsity emerging more or less equally brighlly i'lOlll lhc oulcr sUlt'acc of the colic showing no dislLlrb~lllCC'., of tile flanle front. The "acetylene-type" is givc:~ by acetylene [Figure l(al] arid ethylene [I.igurc IIb)l. In acelylcnc tlames Ihcrc may be a~ addflional carhon luminosity cxtcl'lting ir~ a bowl-like shape upwards from the rim of the burrlcr [rFigure Ila)l With ethylene the yellow cap above the cone is shifted more to the tip. (2) The secol',d typ.; ('benzene-type' after [3ehrensL which is frcgucntly observed with a disturbance of the flame front, is distinguished by a moru or less n,..rrow streak of carbort luminescense emerging only from Ihe tip of the cone. This behaviour is shc, wn by t',en/cnc

(a) (b) lq
August 1967

267

UAI,tlI(.PN F(IRMATION IN lsRliMlY,,Iil) I.I.AMliS

(a)

(b)

(c)

I:,catto 21.t).Rich benzene air tlame with carbon fi~rm:tlionat the lip: Ib). I,Iichcyclc;hex;m,~:fir flame shc~wivngdark fegiOll I)¢U,~,qeZl COlt,.:;ll]d carh(m ,~cm,r:de). I>,,lyhcdr:d '.... ;,. ,ti, JLmic ¢,Jlls ..,¢~,.:i,,,.,,,,.,tk, ,A t.';trboa f~rfllatiOll. 'lhc Ilam¢ "fonti'; hr, kcn up al ttlc tip

[Figure 21a)]. higher aromatics, paralllns hi~her than prop,'me, cycloparaffins [Figure 2ib)] and also by some oxygen-containing compounds such as alcohols, ketortcs and ethers 7. Under conditions where tile tlame front is broken up. showing a hole at tile lip, Ihe yc]~:,;. ' streak may appear scpar:J!cd from the cone by a space less lunlinOtns lhan boll'| tile cone and the carbon streak [Figure 2(b)]. If thc tlame breaks up developing several tips. one above each rklge of the disturb~xt flame front, then every tip will carry its own carbon streak [Figure 2(c)]. A mmsition between the 'acetylene-type' and tile 'benzene-type' is given by a polyhedral ctharle ;fir Ilame (I;'igurc 3~. The carbon lumiJ~osily is visible above Ihe whole cone. but it is stron~gest ,'tt the tip aml above iJle ridges of tile llam¢ fronl. The latter f::lCt ¢;ttllIOt be clearly seen on Ihe photograph. The admittance of secondary air can prevent the Ibrmation of polyhedral P.ames under cerlain conditions and tim,, change 1tlo type of carbon appearance. A polyhedral buta|le air Ilarne. giving a carbon streak above the disturbed flame cone when burning ill tl I1;lille separator, can change into ;i r~oil-

disturbed flame with a unitbrm carbon regiol~ covering tile upper part of the inner cone when secondary air is admitted. See Figure 4(;0 and 4(b).

I'J~,l m 3. Rich pMyhedral ~.qh,'me:d: flame dlowing re-

~()~

rot. 11

K, II. II(I~IAN~';

(,o)

(b)

I:L(~URI: 41a). Rich polyhedral butane.air flame burning withoul secondary air, Note ¢:|rhon formation above lip: ib). ,~alne Iqalne as ill 4(a) burning with secondary air. The ilalilC i;one is rio Ioltgcr polyhedral; the lip is sartou|ldcd b) carbon hm~ii|osity

The way in which carbon luminosity appears in flal flames is largely governed by the condition whether a cellular flame is formed or not 8. If the reaction zone remains fiat, which it usually does

(a)

tip to (,:-fuels in Ilam~:s on burners having a diameter < 5 cm. a COIItiIIUOLIS carbon region exlends into the buml gas. This is separated from ihe blue combusljon zone by a space of lower luminosity, which is barely recognizable ,tt lower pressures. "Ibis "dark space' must, however. be distinguished from thai which is due If an opening of the flame fronl al the tip of a conical flame. Due to heal losses to the side the carbon region appears as a yellow luminous cone [Figure 5(a)]. If the flame front be,:omes ridged or breaks up inlo cells lcelhtlar flames) the carbon zone is discontinuous, forming yetlew streaks which emerge from the tops of Ihe ridges or from the places where Ihree or four cells come into contact wilh each o~.her [Figure 5(b)]. Qualilalively, this behaviour is widely independenl of the kind of fuel. How can these dJlTerenl fornls of carbon luminosity bz inlerpretcd? The appearance o1' carbon luminosity for Ihe different fuels is strongly relal~ to tl|e appearance of polyhedral or cellular flames and the processes causing these flame slructures. In this process Ih¢ interdiffusion of reactants and producls gives rise to a partial enrichment of fuel molc~~tles and hydrocarbon inlermediates over the oxidizing

{h}

Fl(;t~Rt~ 5(a). Flat rich C2H,--air flame showing cone-shaped carbon lure nosily aho~c Ilamc front. Note dark space: (hi. Cellular rich benzene air fit.me witll carbon ,,,ircaks abow: ridges ol flame front 15a from tqossdorf and Wagner Rc[ :'~1

Attgu'+l 1967

( ,~.RP](Y~ 14 )l,~M,'.}ll{),",i 15; I'I(I:MiXt!D Iq+AM]!S

reactants ill certain r~gions ,)1+ the rcaelii"m zone". This Ilas an effect similar to an mcrca,~e of the C;O ratio in the origi0ai gas nlixturc which generally leads to reinforced soot forrr|alioa. Jost. Krug and Sieg ~'' have measured a considerablc increase in the C ( ) ratio of the btmlt gas above the tip of a benzene air flame ,ts corrtparcd t,) the input C;() ratio. In a sirnilar wily Krischer '~ h,'ts ['ound thai in the ridge of a polyhedral benzene flame a richer mixture is burned than ill a valley. Markstein rneasurcd cornposilion traverses across a ridge and a valley irl curved larninar propane air tliIlllCS 0II slot burners. The profiles across a ridge showed higher concentrations of hydrocarh(m intertactile.lies, such +.t~acetylene and methane, extending markedly tarther into tile httrnt gas at: compared to tile concenlration profiles of these species in a valley t2. This expMns why carbon apper.rs at file tip o1 the flallle c,r ahovc' Ihc ridges of polyhetlrill flames. In ce~luhtr flarnes the same parli,'d sel',+..ratitm o1 fuel ;llrd o,',;ygen occurs. ~lleihcr Ihe yellow earb,m slreak merges v,'ith Ihe I]allle tip as in sligi+tly sootforming Ienzene flarnes [Figtue 211:~] or is separated from it by a less hlminous (~, grey region as wi,h eyclohexanc [l"igure 2:bil depends on the t.ondi:ion of tile flame front at the lip. "rile qttestion whether the rnixtule there is still within the limits of flame propagati,m or not. governs the Ieellaviour of tilell.amc fr(mt :t, the cone tip. If the front is closed the carbm, streak usually merges with the tip. In the latter case. Ilowevcr. the illtme, front tends to break up and part of the unburl+t n]ixttu¢ slrea,llS into the hot l'mrnt gas of the surrotmdings. The cornhustiorl of this gas is now go,'erned by radial transport of heat artd a,:tive particles which might give rise to hollow Ihreads of luminosity whicll appear visually not to be m cot|tact with tile f];llllO front ~+. Since the difl'ttsion rates of the light llydro. carbollS givilig flames of the 'acetylene-type" are not appreciably different from that oI' o~ygen there is no enrichment of il'Je fuel towards the cone tip and the carhtm Itmtinosity is equally distribt, toJ arourld lhc cone. The 'light-howl' tlrOlllld the base of the cone in lhis flame-type is probably caused by ,he quenching effect of the cold burner rim (see ~). ,

,,l

_~69

(2) Limits Of carhop/.j'ormatimt The crilical ratio of fuel to oxidizer in the initial mixlure of unburnt gas. at which carbon formation can jusl be detected in any part of the flame, is called the limit of carbon formation. This limit depends bolll ori the properties of the fuel such ;ts hydrogen COlltenl. molecular strtlclure. aror+latic characler and on flame p;u','lmeters such its strtnctur¢, temperature and fktme quenching, to quote some of the relevant points. The kind and the .size of the burner may also be important in this respect. In most of the experiments performed to determine the limit of carbon formation secondary air was excluded. (,arbon fornlaiiort was said to start when tile first yellow or orange lundnosily could be observed by eye. "Ihis is a relatively simple and repro, lueible proc,~dure. Whether tile ernission of a c,rlain nurn bet tlensity of carbon particle'+ can I'e detected by eye depends strongly on lenlper;tlurc. Oll tile optical 7',~p,'rties of the. carbon pa:ticles, on (lensily ~md tile overa!~ hJnlinusity in the vi:,ible region. Thus the value of the critical C 0 or fuel air ratio ()hi'tined b' this r,let]~(;d do not exactly n'tean tire presence of ;ttt cqtmlly small concentlatltm ol+ c,'trbon it, flimleS of ditferenl fuels twhich may L,tve tli0i~rer.t lemperalures) and fl'eqtJently they do nol give Ihe (.'.() ralin ill that p~u't of Ihc "limit where carbon is really formed. This. however, does not diminish the great irnportatlee of these critical (" () ratios Ior practic, tl l'mrposes. Slreel and Thomas 7 have stttdied it great ntnmber (ffaliph,'tlic and aromatic hydrocarbons. alcohols, aldehytl,+s and ethers along these lines u+ir~g f~itll'leS o n ;I btmsen-typc bttrner at :ltmospheric pressure+ For these liMs they Mve dctvrrnirted Ihe crilical air to fuel ratio Iby weigltt) suflicient to suppress carbori htrrJirlosity+ Th,,.;r data might also lee evaluated in terms of the critical C.() r'ttlt(: or (after l)aniels ~3)+.ts the 'number (71+ Oxygen Atoms per n,olecuIc c,f hydr,~)e;ub,tm rlecessaty t() Suppress C'itrhtm tbrnJmti()n' ff)ASCI. See Table I. The (CO),:,+,. lot n-paraffins vitries very little from 11.475 Ibr ethane to 0"44 for el-eel;me. This comparatively small variation suggests that there might be ahllotlgh far li'om tile eqtdlibrium ctmdititm C?,() = I a kind at stoichiomelri¢ relation lot Ihe c;.lr'bOll limit within this series of

27(i

K, It. IIOMANN

hydrocarbons. Daniels t3 plotted the OASC against the number of C atoms for each nparaffinic and n-olefinic hy.drocarbon, getting "rMJJ.l~ i NO, (!]" 0 tttom.~ per

Ii),drocarl~ott

IC OL,,,.

ttud{'('uh' o j

hydrocurl,,,n Io sl,lpprt'.ss

carbtm IOAS('I .............

Ethane Propane Butagas {mainly butane) n-Pent,me i.Pentane n-Hex~ne i-Hexane n-Octane i-Octane i-Dodecane n-Celane

(1475 0.47 (}.46 0.46 0.47 0.46 0,46 0.45 0'46 0,46 0,44

4,2 64 8,7 109 1(I,7 13.I 13 1 t ~0 17.3 26.3 36,3

Acetylene Ethylene Propylene Butylene Amylene Cyclohexane n-Heptene Benzene :~oluene Xylene Cumene Tetralin Decalin I-Methylnaphthalene

(I.83 0,60 0,56 0,48 0.51 0,51 0.48 0,57 0.52 0,4t4 0,48 0.,i4 0"435 0.42

2.42 3,3 5,4 7,7 9'8 I 19 14.5 10.5 13.5 16.6 18.7 24,7 22.9 26.2

straight lines for both series, Their slopes are nearly the same, corresponding to 2.27 O atoms per additional CH2 group for n-paraffins and 2.24 O-atoms for n..olefins.This is not valid, however, for the CH2 groups of ethylene itself or those of cyclohexane either. Considering the OASC data for the three C2 hydrocarbons it is evident that ethane, because of its hydrogen content, needs more oxygen to suppress carbon than ethylene or acetylene ifa certain amount of the hydrogen reacts to water. This difference happens to be 09 O atoms per two hydrogen atoms. The assumption, however, that up to 90 per cent of the additional hydrogen in ethylene and ethane

Vol. it

must be converted into water before soot separates is erroneous, as analyses of the burnt gas of these flames show. The CO2/CO ratio increases simultaneously from acetylene through ethylene to propane, which might be assumed to behave somewhat similarly to ethane t'*. To compare this empirically found stoichiometric relation for a CH2 group in the paraffins and olefins with that for a CH group in benzene. Daniels argues further that the OASC per CH in paraffins i~ 2,27- 0,5 = 1.77 (0,5 for one H atom). The coincidence with 1.75 {= t0.5/6) for CH in benzene must be considered accidental since the additional hydrogen in paraffins is not turned completely into water compared to the burnt gas conditions in benzene flames. The OASC for CH in acetylene would be 1,2 which does not agree with 1.77 for CH in paraffins, The OASC evaluation also reveals that methyl substitution in benzene is much more effective for carbon iormation than the lengthening of the paraffin chain by a CH 2 group; in the series benzene-toluene-xylene the OASC increases by 3~) and 3.1. respectively, as against ca. 2.25 in the ascending paraffin and olefin series. This will be interpreted in a later discussion, This analysis shows that in a few cases only is it possible to compare the critical C/O ratios of different hydrocarbons on a stoichiometric basis. It is not the initial fuel to oxygen ratio which bears a close relation to the onset of carbon formation but the flame product concentrations and the temperature at the place where soot separates. Moreover. for some fuels the C/O ratio in the carbon-forming zone is not the same as that in the unburnt gas. Thus with fuel-oxidant mixtures in which diffusion processes play a role, the critical C/O ratio will be different for flat, conical or cellular flames8. A better criterion for the onset of carbon formation from different fuels is the composition and temperature of the burnt gas. Fenimore. Jones and Moore ts studied the exhaust P.~,: composition in carbon.forming flat flames :.~ cooled ~burners. They observed that the concentration of hydrocarbons such as acetylene and methane and those of molecular hydrogen and water bear a closer relation to the onset of carbon formation than the initial C/O ratio. At the limit of carbon formation which

August 1967

CARIION FORMATION IN PREMIXED FLAMI'2$

has been obtained by visual estimation and by extrapolation of soot yields at different C/O ratios to zero. the authors found the empirical relationship

(2Pc2,, + Pea,)P~l~_ R = 0.6 [cm Hg] ~ Ptl,o

This holds fairly well for the paraffins from methane to butane, isobutane and neopentanc and for ethylene and acetylene, The flames burning with pure oxygen are reported all to have a maximum temperature of about ! 600°K. Their formula which is fairly independent of burning pressure between 50 and 300 mm Hg suggests that carbon formation is promoted by hydrocarbons in the burnt gas and inhibited by OH since the OH concentration is proportional to Pn2o/P~h under equilibrium conditions between the species H. H2, OH and H20. However. in the richer mixtures when more soot is formed no such unique relation exists for an equal percentage of carbon yield from the different fuels. Measurements on acetylene flames by Mochizuki. Homann and Wagner"* have shown that under conditions where more carbon is formed these equilibria and also the water gas equilibrium are not well established. Moreover, there are not only methane and acetylene in the post-flame gases but a great variety of higher hydrocarbons such as polyacetylenes and polycyclic compounds. *,heconcentrations of which are relevant to the separation of carbon. The concentrations of these species are not proportional to the acetylene and methane concentration at all C/O ratios and one cannot expect a simple relation such as that for R to hotd for a wide r6gime of fuel/oxidant ratios. A variation of the expression R with pressure results from the measurements of Homann et al. ~* who studied flat acetylene-oxygen flames at the limit of carbon formation (C/O = 091) burning at pressures between !0 and 30 mm Hg with an initial linear flow velocity of 50 cm/sec. A re-interpretation of their results gives a variation of R = 2Pc~a~ x Pi~,/Pn,o from 0.43 I'cm Hg] t at 10 mm Hg to 0.78 [cm Hg] t at 30 mm Hg. The concentration of methane was negligibly small. Also, R seems somewhat dependent on the initial linear flow velocity and

271

thus on temperature: 0.78 at 50 cm/sec (30 mm Hg and 0.89 at 25 cm/sec. It is not only the concentration ratios of stable flame products which are relevant to commencement of carbon formation but also those of radicals such as OH and all kinds of hydrocarbon radicals, which are usually not known and difficult to measure. Millikan 16 found that soot did not form behind the oxidation zone of flat ethylene-air flames until the OH concentration had decreased to somewhere near its equilibrium value. He found that the 'dark space' between the oxidation zone and the luminous burnt gas coincides with Ihis zone of OH decrease. (3) Temperature dependence of the limit of

carbonformation The depepdence of the critical C/O ratio on the flame temperature in the carbon region has been investigated both with bur, sen-type and with flat laminar and turbulent flames. Street and Thomas 7 have preheated the gas mixture by about 400 deg. C to obtain a change in the burnt gas temperature of about 160 dee. C due to the increase in heat capacity of the gases. in flat flames the temperature can be influenced most easily by varying the linear gas velocity which governs the flow of heat to the cooled burner surface 8' 16. In both experiments a higher temperature allows a richer mixture to be burned without producing carbon, independent of the fuel. i.e. the critical C/O ratio is shifted to higher values, This is shown for flat ethylene-air flames burning at atmospheric pressure in Figure 6, It is a plot of absolute concentration of solid carbon normalized to a post-flame gas of 2000°K and I aim against the maximum flame temperature. Parameter of the curves is the initial C/O ratio s. The broken line l corresponds to the formation limit as determined by Millikan t6 through visual estimation of the continuous emission. With increasing temperatures it crosses the lines of richer mixtures, the region above curve 1 being that of visual soot formation. Line 2 means the same but has been obtained by Flossdorfand Wagner ° studying the same flame. This demonstrates clearly that the formation limits determined visually are arbitrary to a certain degree and do

272

Vol. II

K. IL IIOMANN lx10"9

IxI0 "~

5

] .........

[

i

..........

2

......

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P',,,,~ '',a,.d~...I i '"~}6112/12

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i

1650 T, oK

1900

lq(iix, 7. Absolute carbon concentration in flal and cclhJl:zr L:tllltl)¢ air flan1¢s a~ain,~t maximum Name tcmpcralurc. normalix~ a5 in lq(;z'm(,~6: parameter: C/O ratio ( + t).474. .... 0.47x. A 0.4~5L (a.~:.l no vi,qble c¢II formation" numerals

in parcnth¢sc.~ give from icfl to right: nv.P.,bt~rof cells on

2

Ixi0 9

1800

flame, height (mmL mean di~,',:ncc ~mml and diam~lcr of ¢cll tram}. (At'zcr Ho.,:,dorl' and Wagner Rcl: 8}

1600

1850 T, OK

1900

1950

ROURE 6. Absolute carbon concentrationsand visual limits of carbon formation in burnt gas of fiat ethylene air flame.~ against maximum flame temperature. Carbon is normalized to I aim and 20OO'K. Parameter is the C/O ratio: + 0.62, [-]0"646. 00-66. A 0"678 (After Flossdorfand Wagner Rcf, 81

not represent states of equal carbon concentrations. At a higher temperature a smaller concentration of soot can be detected. Figure 7 shows a similar plot for ethane flames. The carbon concentration only decreases with temperature as long as the flame front stays flat. At higher temperatures (and higher linear flow rates) the flame becomes cellular and carbon formation is reinforced above the ridges°. A temperature increase brought about by the use of argon-oxygen mixtures instead of air has a similar effect upon the critical C/O ratio. In Figure 8 the percentage of fuel carbon recovered in the soot (soot formation ratio) is plotted against the mixture strength for an n-hexaneoxygen-nitrogen mixture (A) and for the equiva. lent but hotter flame in which nitrogen is replaced by argon (B)t~. Up to a C/O ratio of

0.665 an increase in temperature suppresses soot formation but in richer flames a far greater fraction of the fuel is turned into soot. The calculated maximum flame temperatures with argon are higher by 400 dog. C at C/O = 0.47.

.~ 12-

o_" 1

g

B

6 O

N-

4 2 0

i

0'4 $1oich.

06

08

1"0

12

14

C/O Ratio ,n unburnt gas Fl~uel~ 8. Dependence of carbon formation ,:n inilial C/O ratio and temperature in turbulent n-hcxanc oxygen nitrogen (A) and n-hexane oxygen argon flames (B) (Redrawn from Maclarlane et ul. Rcf. 17)

August 1967

273 diameter of 19 cm the carbon limit near the axis. as determined by optical absorption of carbon particles, was independent of pressure if the linear velocity was kept constant. Measurements by Gaydon et al.19 on conical acetylene flames for which the burner diameter was kept inversely proportional to the pressure showed that there is. little variation of the critical C/O ratio over a wide range of pressure. Whereas the critical fuel/oxygen ratio is more or i,:~s independent of pressure the amount of soot formed in more fuel-rich flames is strongly influenced by the ambient pressure. This fact is important in the use of acre gas turbines where a great part of the fuel burns under prcmixed conditions. An operation at higher cycle pressures (about 10 to 15 atm) has been accompanied by a marked increase in soot density of the exhaust gas as is demonstrated by every modem jet plane. Macfarlane, Holdemess and Whitcher w have measured the amount of carbon formed from Cs- and C6-hydrocarbons premixed with air for different mixture strengths and pressures up to 20 atm. They used stationary turbulent and laminar flames which were isolated from their surroundings by means of a quartz .~leeve. Some of their results are shown inFigure 9(a) to (c) for n-hexane, n-hexene-I and benzene, Parameter of the curves is the carbon formation ratio expressed as the percentage of fuel carbon recovered in soot. These experiments again demonstrate that the threshold mixture is very little affected by pressure variation. Although benzene gives soot copiously even at low mixture strengths, the dependence on pressure is weak between C/O ratios of 0.5 and 0.75. Soot formation with hexan¢ and hexene starts at richer mixtures and is reinforced much more by pressure increase, These dam refer to turbulent flames. In another series of experiments the authors lowered the injection velocity of the unbumt gas, obtaining flat laminar flames. The result was that cet. par. up to ten times more soot was produced. This is probably caused by the longer contact time of soot particles with the hot burnt gas above the burner. The influence of turbulence on soot reduction is not yet fully clarified. Tar pro.. duction was favoured by low pressure, rich mixtures (i.e. low temperatures) and flame

CARBON FORMATION IN PREMIXED FLAMES

260 deg.C at 0.63 and 1.50 deg.C at 0.79. The positive influence of temperature on carbon formation in very rich flames which normally can only be observed at elevated pressures (because of the extension of the rich limit of flame stability t') and with the burnt gas region thermally isolated is due to reinforced pyrolysis in the burnt gas. The general decrease of flame temperature with richer mixtures finally leads to a fall in the carbon yield. The influence of temperature on soot formation at different C/O ratios will be discussed more extensively later. (4) Pressure dependence of the formation limit

and the y&kl of carbon The questiov whether the critical C/O ratio depends on total pressure or not has been studied on burner flames over the range from l0 mm Hg to 20 arm and in explosion flames up to 100 atm, Most of these experiments demonstrafe that there is no, or only a small, variation of the visually estimated limit of carbon formation with pressure for many fuels. There are some contradictory results which might, however, be due to temperature variations and to an effect on diffusion.controlled processes in conical and disturbed flames. Fenimore et al. ~s. when investigating flat flam~ of light aliphatic fuels up to neopentane on cooled porous plate burners, found that at constant mass flow the critical C/O ratio decreased with increasing pressure and larger burner diameter. The variation of pressure and burner diameter at constant mass flow alters the linear velocity and has thus an appreciable effect on the temperature in the burnt gas, which in turn alters the limit of carbon formation. If, however, the linear velocity of the unbumt gas was kept constant the dependence on pressure and burner diameter almost vanished. A small dependence of the formation limit on pressure still remained but this may be due to non-unidimensional behaviour of the relatively small (1.6 and 3.3 cm diameter) flat flames. In the region above the burner rim carbon formation is strongly reinforced due to heat losses to the side and radial diffusion processes, as Bonne and Wagner Is showed. They found that in flat flames stabilized on a large burner having a

274

K. ii. IrI(D,t,IANN

n-Hexane 10

0,5 10 15

'25 E

08

I I

c

/ I

E

9 ¢1 "

I

04

I

Sto-i:Ech

I t

o

S

(.)

5

10

15

20

Pressure, arm

(a) Hexene-t 1.2

2 ~68

0"51

! = JD

0.8

g C

II

0

Q:

0.4

T

|

" .

.

.

.

.

.

.

~ Stoieh ,,S

5

10

15

20

Pr~'ssure, arm • b)

/ L

~

t _'-~~ T

o.4

-6

"..s

........

L,I

10 Pressure, arm

(c)

number of carbon atoms. Fristrom. Avery and Grunfelder z° have sampled a 4 per cent ethane-96 per cent oxygen flame, which forms Call`*

0.25, CzH z 0"03 and CH,, 0.02 per cent in the

0'5

o 0

(5) Hydrocarbon intermediates The analysis of intermediate species, more or less stable under normal conditions, in the reaction zone of premixed flames has been widely improved during the last ten years. As it is difficult to resolve the reaction zone at atmospheric pressure, measurements of this sort have often been carried out under reduced pressure. Most attention has been focused on oxidation processes which are preferably studied in lean hydrocarbon flames• Thus quantitative information about hydrocarbon intermediate; in carbon-forming flames is comparatively In ,,,toichiometric and lean flames of lower aliphatie hydrocarbons a small amount of the furl loses hydrogen and forms unsaturate~ hydrocarbons having the same or a smaller

//

•C;

turbulence. The threshold mixture was unaffected by turbulent flow. Little information is available on the pressure dependence of carbon formation by other ft.:Is. Earlier experiments made by Bone and Townend on explosion flames'* indicated a complicated behaviour of carbon yield with pressure variation for methane and ethane. While 20 to 30 per cent of the fuel carbon was recovered in the soot for the mixtures 2CH'* + Oz and CzH 6 4Oz at i to 10 atm, only about ! to 3 per cent of .carbon was found at 100 arm. The shape of the explosion vessel had a considerable influence, spherical vessels being more favourable for soot Iormation than long tubes. The interpretation of these results is difficult since the 'temperature history' of the non-isentropically compressed unburnt gas may have a great influence on the formation of carbon. More information aboul lhese effects is desirable.

scarce.

Benzene t'2

r"

Vol. I I

20

Fmual~9(a),lb).(c). Pressuredependenceof carbon formation in turbulent n-hexane-air lag hexene-l--air (b) and benzeneair (c) flames. Parameter is the soot formationratio expressed as the percentage of fuel carbon converted to soot. I" is the threshold for carbon formation and S the flame stability limit under experimental conditions. (Redrawn from Mac. farlane et al. Ref. 171

August 1967

CARl]ON FORMATION IN PREMIXED FL.A.MES

oxidation zone. With increasing fuel content the concentrations of these intermediates rise absolutely and relatively to the input of fuel. in addition there appear hydrocarbons with a greater number of carbon atoms than the original fuel. Singer and Grumer 2t reported acetylene 1.4, diacetylene 0.02 per cent and a trace of benzene when sampling from 2 mm above the blue zone of a flat 8 per cent propane-air flame burning at I atm pressure. Using a mass spectrometer with a molecular beam inlet system Mochizuki, Homann and Wagner t'*~2 measured concentration profiles in carbon-forming flat flames of acetylene, ethylene, propane and benzene burning at pressures between 20 and 60 mm Hg. The limit between the blue-green zone and the yellow luminous par1 of the flame coincides for aliphatic fuels with the zone where the oxygen

10x,t0"3

_

-

2

g

I1~/'~,,~8H2

7

_,

C+H

o :g

03

CO

02

" ~ " ~ " - ' ~ - - ' - " - H2

LP/%

F 0

- ""-r---e-- o--. + - - - ; _ _ ,

~02

i I'~ 10

I 20

i 30

I co

__T¢_.,

C2.2 I 5o

Height above the burner, mm FIoUnE 10. Concentration profil,~ in a flat CzHz-O2 flame at Ihe visual limit of carbon formation. C O = 0'95:20 mm Hg burning pressure. IAfier Homann and Wagner Ref. 22)

275

and, in the cases of C2H4 and C3H8, the original fuel is just completely consumed, in the flames of these aliphatic hydrocarbons unsaturated C3 intermediates such as propylene and methylacetylene reach their maximum concentration within the oxidation zone and disappear almost completely before the oxygen is consumed. C3H6 is formed prior to Call,,. The same is true for dimethylacetylene and vinylacetylene, the maximum concentrations of which precede that of diacetylene at least in ethylene and propane flames. In acetylene flames the simultaneous formation of C4H4 and C4H2 cannot b¢ excluded. Figure 10 shows concentration profiles of some of these unsaturated intermediates in an acetylene-oxygen flame. The simultaneous lbrmation of Ca- and C4hydrocarbons from C2-fuel is strong evidence for the splitting of the C--C bond in the fuel molecule followed by the reaction C + C2 --* C3 (hydrogen omitted) and C2 + C2-* C4, one partner of the reaction probably being a radical. Ferguson's experiments 5 with t3C-propane-2 explosion flames gave evidence that carbon was not formed directly by polymerization of intact t 3C~2C molecules (acetylene) but resulted in a random distribution of ~aC in the soot corresponding to its concentration in the fuel. The concentration profiles in these stationary flames illustrate that the randomization process by C--C bond breaking starts in the oxidation zone before any carbon is formed. A tract of Cs-hydrocarbons but no aliphatic compounds of higher odd carbon numbers could be detected. Benzene is formed in rather low concentrations in the oxidation zone of CzHz-. Cell+- and C3Hs-flames and disappears again before carbon is formed ~4. Methylacetylene and vinylacetylene also disappear within the oxidation zone, where carbon is not yet formed. In contrast to this behaviour, polyacetylenes (up to CtzHz in very fuel-rich flames) reach their maximum concentrations at the limit between the blue and the yellow flame zones and decrease in concentration behind the oxidation zone where carbon particles start to be form~. Polyacetylenes constitute the greatest part of the unsaturated hydrocarbons in these flames. This is a common feature of all lower aliphatic hydrocarbons.

276

Vol. I I

K, il. HO~,IANN

'

f

f

C4H

~,,f

J~Q

gg P

O ",4

I

i

001

1,5 50 55 60 Mote % of C2H2 in the u~x~'nt gas

Fa3t,a~ I I. Acetyleneand polyacetylenesin the burnt gas of flames ~ith ditl'ercra C_,H~ O., ralio,~ 20rara Hg burning pressure. (After Bonne taxi Wagner Rcf. 23~

In suflicientl) rich flames polyacelylenes are not completely consumed in the burnt gas. After a rapid initial decay their concentrations decrease so slowly that they become practically constant in low pressure flames. Figure I I gives the concentration of acetylene and polyacetylenes in the burnt gas of acetylene flam~.~ of different fuel.:oxygcn ratios. They increase steadily towards richer mixtures with a sort of equilibrium between each other and hydrogen z'~, Higher hydrocarbons with molecular masses of more than about 125 mass units which are formed behind the oxidation zone are not polyacetylenic in character but polycyclic. A broad variety of compounds having five- and six-membered rings have been identified, it has been known for a long time that polycyclic aromatics can be extracted from the soot of nearly all hydrocarbon fuels'4. Recently coacentration profiles of these components ha~e been measured in premixed Ilames by means of

9"0 cm above the burner

,.

100

150

2oo

250 300 Mass u~its 1-5 cm above the burner

100

ISO

250

300

350

400

450

500

Miss units FKIU~E IZ Mass spectra of volatile components from acet)'lene-sootsampled at I~o heights above the burner. C:H: 0 : -- 1-31:20 ram HR burning prL~,~ure.Ik.~ond 180 ma~ unit~

only ih¢ e~en.num~ttd masspeaksare shown.IFromHomannand WagnerRe[ 251

277 mass spectrometric analysis2s, Nearly all of to allow for the background mass peaks which these speciessampled at some distancefrom the inevitably occur when sampling from very rich flames, The end of the oxidation zone is located oxidation zone are polycyclicaromatics with no at about !0 nun above the burner. Groups of sidechains.The range of theirmasses isbetween 128 and ~ 350 mass units,The upper part of mass peaks up to 550 mass units lend of the Figure 12 shows a mass spectrum of the volatile spectrometer range) appear closely behind the oxidation zone, reach their maximum intensity substances of soot sampled from 90 mm above the burner at a pressure of 20 mm Hg giving at about 14 mm and have disappeared =piq at a mass peaks of polycydic aromatic hydro- height of 35 mm above the burner. The groups carbons. The energy of the ionizingelectronsis differ in mass by twelve units, one carbon atom. Their individual concentrations are of the order 10 eV so that fragmentation isexduded. of 10-'t mole fractions zs. Closely behind the oxidation zone, however, The hydrocarbon intermediates in flames of a far greater varietyof hydrocarbons is present ranging from ~ 100 to more than 500 mass lower aliphatics up to CsHs are qualitatively units as given by the lower part of Figu~ 12. very similaP'. The order of appearance of higher hydrocarbons is the ~ m e Although The gaps between the peaks of aromatics are acetylene and polyacetylenes form the largest filled in with hydrocarbons containing side chains and more hydrogen than aromatics. part of intermediates with any of these furls, the, These compounds do not survive in the burnt formation of intermediates c~mtaining more than two hydrogen atoms is more pronounced gases, They can also be sampled directly from the flame gases by a molecular beam inlet with increasing saturation of the fuel. For rich system, Mass spectra obtained by these means mixtures of higher aliphatic fuels no such detailed information on intermediates is availare shown in Figure 13. Mass spectra from two different heights in the carbon-forming flame able. are compared to that of a lean non-sooting flame The formation of carbon and hydrocarbons from aromatic fuels has mostly been investigated in flames or pyrolysis experiments in which these substances served as additives z~zs. ~ lO'7Mole fraction 1-2 x lOt°Molecules crn-3 It is only for a benzene flame premixed with oxygen and nitrogen that concentration profiles have been measured t'*. In contrast to lower : Lean flame aliphati¢ fuels the zone of yellou luminosity fa~ve the ......... 14mm I b u r n e r ,n il overlaps the oxidation zone. Acetylene and ~a c a r b o n " polyacetylenes (no higher than CsHz could be 35ram [ f o r m i n g !! identified) are formed in the main rutction [flame :~i zone and are partly consumed in the bumt gas as in aliphatic flames, However, whereas aromatic hydrocarbon intermediates with the exception of benzene can only be found I~___i_nd the oxidation zone in flames of aiiplmtic fueL these are formed early together with oxidation ",.,'~'.. ~.a. ..~.~ ~" "~: ~ ..~" " : •* . ~ t ~ " "~.:~"~ ""~" . " X . . . ' ' ' ~ ' , ~" i " products in benzene flames. The concentrations o[ for example, phenylacetykne, indene` methylnaphthalene and diphenyl, pass through maxima and decrease again in concentration in the t : ~,' :'' ~ ~ i '- : : i I I I I I burnt gas. The concentrations of others such as 550 500 450 400 350 tmphthalene, acenaphthen¢, uthraceue and M a s s number pyrene increase steadily with height in the F~i.'t~e13. Mass spectra of hear)'componemssampk'd flame. Together with the aromatics there are directlyfroma carbon-formingC:H.,-O: flame.CzH:,O., = also hydrogen.rich hydrocarbons of grmter 1.7;20mm Hg.~FromHomannand WagnerRef.25)

August 1967

CARBON FORMATIONIN PRF-IMIXEDFLAMES

Vol. I I K. LI, lit)MANN 278 and 1000 C acetylene tends to polymerize in mass, which have their maximum concentrations the sense that no hydrocarbons having an odd at t.he end of the oxidation zone and, as with aliphatic fuel, do not survive in the burnt gas, number of carbon atoms are formed in the early This is demonstrated by two mass spectra stag~ of the reaction, Benzene and higher aromatic compounds contained in tar are the tFigure 14t of volatile material of soot ,sampled major primary products. The large aromatic near the oxidation zone and from the burnt gas,

9.0 cm aoove the burner

i !

,

L..:LLi:], 0.9 c m above tt~ bu'ner

ii

i

i[,,,, '

i

,

' [I i v

100

150

b .

i'~ ; '!

~ i

I' [: !! I', ~ II !. I 11 .1': :

i ~:I .~

t.!:.,:Liil,,l,-',,~ .::,::,.:~:!,:: ::~,~:.~:..,..., ~..;::..:. ,

200

250

300

350

400

Mass u m t s FIGUI~E 14, Mass s~'ctta of ~'olatile components of [x'ne,me-.~oo~. CbHt, O , = ~25; 3Omra H g IFrom I.'omatm and %%:a:.,nerRel. 251

~6) Reactim products frost pyrolysis of acetylene ~i ~ese The reaction products and intermediates of pyrolytic decompositions of hydrocarbons have re(~ved more attention than those in fuel-rich hydrocarbon flames. This is not the place to quote the results of these experiments in detail, It seems,,however, useful to compare the hydro. carb~ products of acetylene and benzene pyrolysis to those in the corresponding rk:h The pyrolysis of acetyknc has been studied in the temperature range from 400o to 6O0°C ia static vessels2t alld betweeri 600° and 12.q)~ in flow systems 26"3o-ss. Between 400 °

molecules ever:tually 'carbonize" and ethylene, hydrogen and methane are formed during this process. Ethylene might also be formed directly from acetylene by hydrogenation; it undergoes decomposition at temperatures above 750"C, The 'dimer' of acetylene--vinylacetylene--is a very unstable intermediate, it has been found 33 in low concentration as a primary product in the temperature range 500° to 1000°12. The formation of diacetylene is only detectable at temperatures above 900°C At about 1100°C and higher temperatures its concentration as an intermediate becomes more prominent than that of vinylacetylene, There is no evidence

August 1967

CAR~ FO~M~T~O~tN PR~Xm FLA~ES

that vinylacetylene is a direct p~cursor of diacetylene at higher temperatures. The appearance of methylacetylene together with diacetyiene as an early product of the thermal decomposition suggests that carbon-carbon bonds and carbon-hydrogen bonds are broken in the primary steps of the reaction. That mcans radical reactions become more important in this temperature r~me. The intermediate concentrations of benzene and higher aromatics decrease relative to the consumption of acetylene. Stehling, Frazee and Anderson 26 investigated the pyrolysis of benzene between 700° and 900°C: hydrogen, diphenyl, tar and carbon were the main products. At these temperatures benzene is more resistant to pyrolysis than acetylene. Slysh and Kinney 3x have studied this decomposition at 1200°C in a flow system. Under these conditions benzene decomposes more rapidly than acetylene. The formation of 'polymers' such as diphenyl and higher aromatic~ however, is still prominent in addition to the cracking of the benzene ring to form diacetylene and acetylene, This rapid formation of higher aromatic hydrocarbons and 'polymers' is also observable in the oxidation zone of benzene flames t'*, the formation of diacetylene and higher polyacetylenes being not as pronounced as in acetylene flames, There is good evidence that the many large hydrocarbon molecules together with comparatively few solid particles are responsible for the yellow continuous emission in the oxidation zone of benzene flames. (7) Pa~'ticlefor~tioa and growth Hitherto the stepwise growth of carbon particles from a large hydrocarbon molecule to an aggregatiou of soa~ hundred thousand carbon atoms could not he measured in detail. The heaviest chemical species which have been detected by mass spectrometry in carbonforming flames have m ~ of some 500 mass units while the smallest carbon particles measurable by electron micrography have diameters of ~40 A corresponding to about 40000 mass units, This gap can only be bridged by extrapo]ating from both sides, using the measured rates of carbon pardcle growth and variation

279

of number density both of carbo~ p~fides and large hydrocarbon molecules. The conception of a nucleus for a carbon particle remains somewhat vague. It is not possible to think of it in the same way as in the process of condensation of a vapour since there is no critical size of a carbon particle which can be correlated with a supersaturated *carbonparticle-vapour'. There is no evidence for any soot panicles which have the tendency to vaporize again rather than to grow. ,Johnson and Anderson 3° have shown that in acetylene pyrolysis soot particles are not formed by 'carbonization' of liquid polymer droplets. These arc only formed if the reacting gases are cooled below a certain temperature. In acetylene pyrolysis in the r~,gime 600° to 900°C at 1 arm this is about 300°C A direct condensation of supersaturated carbon vapour is imaginable at a very high temperature. But this does not eves happen in the very hot cyanogen-oxygen Ibmes and it is definitely not the mechanism in ordinary hydrocarbon flames. In somewhat cooler flames of cyanogen at lower im:ssure carbon is deposited only on solid surfaces ~* brought into the burnt gas. The outer appearance of solid carbon particles --the term soot particles would be better since they are not pure carbon--formed in the gas phase is remarkably similar, no matter whether they are produced by pyrolysis, in flames or in premixed flames. When exsmined under the electron microscope soot comists of a network of crosslinked chains of approximately spherical particles. The single partkles are made up of a large number, about I03 to I0', of crystallites 3s. Measurements of crystallite growth in soot particles have not yet been made. It is probable that the array of carbon atoms to small crystallites is due to a kind of mnpenag process and that small hydrocarbon molecules are--at least m the first plmse of particle

growth--not added in an ordered way. As experiments on carbonization d tars and aromatic hydrocarbonshave simwn, IpapMdmtion is not fully obtained even at temlgratun~ of 3000°K if the sampk has not passed a mettm state, which is not the case for carbon formation in the gas phase 36. X.ltay measmengats imlimte

2St)

K. II. IIOMANN

that the final crystallites consist of 5 to 20 sheets of carbon atoms thus having a length and breadth of 20 to 30 A3s 3~.3a. The layer planes a,'¢ parallel to e~ch other, but they are randomly stacked relative to one another and the interlayer spacing is about 3 to 5 per cent larger than that in graphite.

/ I "75

/ 0

5O

~

250

350

Diameter, F~t:KE iS. Distribution of particle diameters at different heights in flat acetylene oxygen flame, CzH ., O, = I-6: _20mm Hg, ffrom Bonne and Wagner Rcf. 231

The formation of chainlike asgregates is a rather hie stage in the growth of soot particles, They may grow as individuals to sizes I:etwcen 100 and 2 500 A depending on fuel, temperature during their growth and contact time with the hot ~ The particle size distribution in the buret las of an acetylene-oxysen flame is shown in Figure 15, The diameters show a normal distribution; particles with more than

Vol. I !

twice or less than half the mean diameter are relatively fewz3. Small particles (below 100 A) have almos: disappeared at a height of 50 mm above the burner. Parker and Wolfhard s9 collected soot particles from a non-smoky acetylene-air diffusion flame, They report particles with diameters of about 100 A, very uniform in size. The authors could not find any further growth of these particles in the luminous zone, On the other hand Johnson and Anderson ~° found that particles formed by pyrolysis of acetylene continue to grow if mixed with fresh acetylene and heated in a second reactor tube placed directly behind the first one, The further growth of particles is governed by the concentration ol fuel or decomposition products of fuel, by temperature and by contact time. Since these parameters arc not known in the experiments of Parker and Wolfhard, it is difficult to interpret their results, Burning of the particles might have played a role in the luminous zone of the non-smoky diffusion flames. The largest single particles have been found in pyrolysis experiment (6000 A) while the final size of flame soot particles seldom exceeds 500A. Kinetic measurements of particle growth are rather scarce and can hardly be compar, r~l with each other because of very different experimental conditions. However, all measurements along these lines show that the first phase of particle growth up to about 100 A diameter is extremely rapid. The particle size distfibv,tions of Figure 15 demonstrate that the total number of particl~ decreases while their diameters grow, The measured rate of decrease h~ number density is compatible with the mechanism that every collision between two small particles results in the formation of a bigger one ~s-'5, This process toge!her with the fact that "young' carbon particles are extraordinarily active for addition of small hydrocarbon molecules is an explanation of the rapid first phase of particle growth, The assumption that hydrocarbons are added in this first phase and not decomposed to carbon and hydrogen at the surface is supported by the H/C ratio of the young soot particles 2s, This is plotted in Figure 16 against the height in an acetylene flame. The first carbon is

August 1967

CARBON FORMATION IN PREMIXEDFLAMES

comparatively rich in hydrogen corresponding to a compound, say Call2. Moreover, the increase in the total amount of carbon in this first phase agrees almost quantitatively with the addition of the polyacetylenes CsH., and C t olin assuming a reaction with no activation energy -'5,

-CsH2

co R

o3,

\

U

-CsHz o

:- 02 g.

_t2 E

C~H 2

o

I

°,

2o

3'0

281

time equal to the time the soot particle needs to move one diameter. That means two colliding particles do not feel a~ all their opposing hydrogen streams, it is most probable that Tesner's measurements do not refer to the rapid first phase of particle growth so that the number of soot particles seems to remain practically constant in his experiments. The loss of activity for addition of hydrocarbon molecules in a later phase of growth parallels the decrease in .radical character of the particles as indicated by a decrease of ESR signals of soot sampled from increasing heights in the flame where particles no longer grow 2s, The following slow growth is mainly caused by heterogeneous decomposition of unsaturated hydrocarbons like acetylene, polyacetylenes and probably also aromatics. The kinds of forces which hold the larger particles together in chainlike aggregates is not clear. Electrical forces may play a role4t 4, The soot aggregates are reported to be separable into single particles by ultrasonic waves"*3,

Height above the burner, rnm |:lta'Rt 16. H C atomic ratio of soot at different heights in Ilal acct~l¢,e ox.~gen Ilamc. C,tI_, (1: -- 1.7: _~) mm tlg. IFrom |lomann and ~3;agner Rcf 25)

The initial decrease in number density has so far only Ix~n observed in low pressure flames. since the spatial resolution for measurements at atmospheric pressure is not attained. Tesner"*° states that the coagulation of carbon black particles is impossible in the region of their intensive formation and growth because hydrogen streaming away from their surfaces acts as a shield so that diffusion of particles towards each other is impeded. Calculating with a decrease in hydrogen content as shown in Figure 16, however, and a mean particle diameter of 50 A a rough estimate shows that the momentum transferred by the hydrogen molecules to an approaching particle would be much less than 10-6 of the linear momentum of the soot particle. This estimate assumes thermal velocities for both the particles and the hydrogen molecules, and an effective repulsion

Discusshm of the Route of Carbon Fomatkm Hypotheses for the mechanism of carbon formation are many; not all will be discussed here. Good summaries of the main conceptions arc to be found, for instance, in the book by Gaydon and Wolthard =° and in the review article by Palmer and Culliss {cf. for further review literature). Nearly all hypotheses suffer from the lack of qualitative information about the species in the gap between the hydrocarbons containing a few carbon atoms and the smallest soot particles with some ten thousand carbon atoms and the reaction rates of these intermediates, To recognize all details of the reactions causing rapid particle growth seems almost impossible, since there are too many parallel steps leading to hundreds of intermtxliates, most of which are not known as chemical individuals. It would take a tremendous analytical effort to separate all species which can be sampled and get more information on their properties than a mass spectrum of a mixture of them can render. Naturally, up to now most of the intermediate products which have been idendtied are comparatively unreactive and therefore have longer

~. ii.

2~2

lifetimes and are present it) larger concentrations under burnt gas conditions than the really rapidly growing species. In premixed flames carbon is formed at relatively high temperatures and it is most probable that the routes leading to carbonatx',ous material are different from lho~ important in pyrolysis at lower temperature; Ibelow I000 CI where 'polymerization" predominates over degradation of hydrocarbons. In the following we do not want to set up a t~e~ hypothesis for a "mechanism" of carbon formal.ion but only to put in ~rspective things which hax'e been measured quantitatively in carbon-forming flames so that a reasonable )oute for carbon formation, which explains most of the phent.mena observed in connection with this proces ;, can he recognized. For this purpose it is usehd to regard flames of aliphatic furls su¢~) as methane, ethylene, acetylene, ethane and propane separately from the aromatic~ for ~hich we shall take benzene as a model suhst~nta.~. if .the burnt gases were in equilibrium with each other and with solid carbon, and we take a final flame temperature greater than 1500K, then practically any overall reaction era hydrocarbon with oxygen should be described by the equation C . H , + ~O 2 = 2)CO + ~nH; + Im - 2y~

for m > 2). Note that H,,O, CO_, and CH.~ are only formed in negligibly small concentrations. Solid carbon would separate if m > 2y, that is for an atomic ratio C/O > I. In pracuce, however, ~he amounts of HzO and CO., formed in carbon-producing flames and in flames at the 'carbon limit' are t~ot at all negligible, The concentration profiles of water and carbon dioxide in an acctylene--oxyge~ flame at the 'carbon limit" as shown in Figure I0 give an inerca,~ o1' both components at the beginning of the oxidation z~ne and a decrease at the end of it, which is stronger for H20 than for CO.,. in the ¢oo~ pan of the oxidation zone HzO and CO2 are mainly formed by the reactions: H 2 + OH ~ H20 + H

[I]

Cx.q,, + OH -,, H20 + C~,H,._,

[2]

CO + OH --. CO_, + a

[3]

iiOMA~

VOI, I I

binding enough oxygen so that the rate of formation of larger hydrocarbons tpolyacetylenes~ is greater than their oxidation rate. It is not before the end of the oxidation zone where the temperature reaches its maximum thai the b:,ck reactions of I and 3 play a role and O1-1is re-formed. This. together with the now smaller fuel and hydrocarbon radical concentrations makes the rate of oxidation of t he polyacetylenes or other large hydrocarbons by OH greater than that of their formation and their concentrations decrease at the end of the oxidation zone ~"'~~. This process slows down soon because of the rapid decrease of H atom and OH concentration to the equilibrium values behind the main reaction zone~a. The u'onccntrations of acetylene and polyacelylenes continue to decrease more slowly due to the fall of tempe-alure and arc practically constant in the low pressure flame of Figure 10. Since large hydrocarbons which lead to soot formation must be formed in successive steps from smaller hydrocarbon~ their concentrations cannot become large enough-., before the lower hydrocarbons and hydrocarbon radicals disappear again by oxidation and recombination` respectively--so that no sel~lra|ion of soot becomes observable, This is the situation at the limit of carbon formation tinder the conditions of Figure 10. Polyacetylenes do not suddenly appear at the limit carbon/oxygen rat~ but are also formed in flames of leaner mixtures. Their concentration increases steadily with the fuel content -':--'3. The decrease in water concentration at the end of the main reaction zone can also be observed both in leaner and richer mixtures. There is no property of the flame which shows a sudden change when the "carbon limit" is exceeded. The qualitative influence on carbon formation by increasing the mixture strength is clear. A greater fuel and a smaller OH concentration TM favours the formation of higher hydrocarbons. A lowering of the flame temperature shifts the critical C O ratio to smaller values, If the C/O ratio is kept constant the absolute carbon concentration increasess. This can also be understood by the reactions cited above, A decrease in temperature brings about higher concentrations of water, ~rbon dioxide and

August 1967

('AR~)N H)RMATION IN PRI!MIXEDFLAMES

polyacetylenes in the burn! gag since the rate of consumption of H,O and CO., is reduced at the end of the oxidation zone t4. Less OH is supplk.xt by the kick reactions of I and 3 so that the maximum of the polyacetylenes is shifted more into the burnt gas and the decrea~ of their concentrations by oxidations occurs r.'~ore slowly-'-'. The concentration of higher hydrocarbons at the end of the oxidation zone now becomes sufficientlyhigh so that soot separation is observable. This can only be compensat~ if the C O ratio is lowered. In most studies the variation of the carbo~ limit with pressure has been determined by visual estimation of the appearance and disappearance of the continuous emission of carbon particles. However, a rise. in pressure is usually accempanied by an increase in flame temperature. This fact and the higher total density allow a relatively smaller concentration of carbon to be detected visually, and thus seems to shift the critical C O ratio to smaller values. To decide whether there is a real pressure effect on carbon formation in a C O r~gime near the threshold, measurements of the absolute carbon concentration at constant temperature are desirable. The fact that the lermolecular recombi.nation of active species such as OH, O and H which promote the combustion is enhanced by pressure increase, while the polyatomic hydrocarbon radicals which favour carbon formation can recombine in binary collisions. ".s proba!)ly relevant for the relative reaction rates in the recombination zone of the flame. The strong increase in carbon concentration with pressure in fuel-rich flames as in the experiments cf Macfarlane et al.t7 can reasonably be explained by a more complete pyrolysis of the hydrocarbons in the thermally isolated burnt gas. For the route ofcarbon formation in premixed flames of lower aliphatic fuels up to propane, the concentration profiles~'~~a.... 's and profiles of particle density and size as well as their radiation give the following scheme. In the first part of the oxidation zone unsaturated hydrocarbons, viz. C.~H6, C,LH4,C3H6and C3H,~ are formed, the maximum concentrations of the H6..compounds preceding those of the H.c compounds. As the temperature rises these hydrocarbons decrease again and acetylene

2S3

lin propane qames after cthylenel and polyacetylenes appear, reaching much higher concentrations than the more hydrogen-rich hydrocarbons mentioned abo~e. The rate of the polyacelylene formation can be explained by radical reactions of the following type: C.~H_,

•C , H

-F C 2 H 2 ---* oC4H 3 - c : l t : . I~!

l

C.~H.~ C~,H,

T,,

C~H_~

T,,

•CoH3I + H,) "C:u:' 'CsH ~l+ H.,)-'c:t: ' The reaction" C2H3 + C2H2 giving C4H5 which then decomposes to C.~H.~ and C.,H2 is less probable. In the reaction system H + CzH 2 at room temperature in which C,H~ must be an in|crmedia~.e no C.,-hydrocarbons could be detected444~. Concentration profiles of C.,H.~ and C.~H., in acetylene flames indicate that C.,H~ which has been found in the oxidalion zone gives C.~H4 at lower and C4H2 at higher temperatures ."". . .'*. . The radical C,,Ha further reacts with acetylene lit higher temperatures to form the hi,gher polyacetylenes. This pro.,~-~s of polyacet)lene formation is only rapid enough in the radicalrich oxidation zone but is slowed down considerably in the burnt gas. in times not forming carbon polyacetylenes decrease at the end of the main reaction zone due to oxidation while an increasing part of them is consumed by carbon formation in richer mixtures-'s, A continuation of this reaction series, however, canno: lead to carbon particles since they are not giant chain molecules. There arc other possibilities for further reactions of the polyacetylcaes, if a C:H or any other radical attacks a pola~lylcn¢ molecule, the probability that it will hit one end decreases with increasing cl~in length. A to,action at any other carbon atom without breaking a carbon-carbon bond cannot had to

2S4

~, n. no~t.,,xx

another polyacetylene since there are no branched polyacetylenes. For example: "C-,H -,, C.,H, --, H~__C.__C=CH • ,I ~ ltC-=C tl --C----CH .

I

i

C

C

lil

I,t ,',

C

C

H

H

{I)

The radical I might be stabilized at a lower temperature by forming a methylene group but such a substance is--likevinylact.tylene--probably very unstable at the maximum flame temperature, !I is. therefore, reasoc.able to assume that such branched radicals add further polya~tylenes and also acetylene without losing their radical character. Ring closures are easily imaginable which lead to the great number of cyclic and polycyclic hydrocarbons containing side chains and more hydrogen than aromatics. The maximum concentrations of these substances having masses of ~ 150 to more than 600 mass units occm closely behind the oxidation zone, a flame region where thepolyacetylenes are decreasing and the formation of polycydic aromatics and soot particles is just beginning, These very reactive hydrocarbons, probably still having radical character at these temperatures 46, add further polyacetylenes and agglomet'ate with each other growing to small soot panicles. The prevailing addition of higher polyacetylenes in this stage of growth is sup. ported by the fact that the hydrogen content of soot decreases while the particles are growing, The concentrations of most of these heavy hydrocarbons fall again very rapidly, while those of pure polycydic aromatics continue to increase in the burnt gas. The concentration profiles of these aromatics show that they must be considered byproducts rather than highly reactive intermediates in the process of carbon formation 2 s. ,7.

Slmrkey, 5chultz and Friedei t when studying the thermal stability of various polynudear aromatic hydrocarbons found that species such as naphthalene, phenanthrene~ chrysene, pyrene,

Yol. I I

fluorene and anthracene are comparatively unreactive. These are just the kinds of hydrocarbons which survive in the burnt gas. in contrast to this, hydrocarbons with side chains such as methylnaphthalen¢s, methylphenanthrenes, dimethylnaphthalenes and hydrogenrich compounds like dihydroanthracene, indene and indane dt~'~compo~ much faster and give products with four times the molecular mass of the reactant. The heaviest products with the unreactive species have only twice the man of the original molecule. At this point we recall the remarkably higher efficiency for carbon formation of methyl groups attached to an aromatic nucleus, than that of CH., units prolonging an aliphatic chain. "~~3 Scully and Davies -'r-'s injected aromatic hydrocarbons and derivatives thereof such as chlorobenzene, phenol, pyridine. aniline, tohtidines and others inte the burnt gas of a rich town gas flame and measured the relative amount of soot formed by these additives` Under their experimental conditions chlorobenzene gi~,~ about twice as much soot as benzene. This is prolxlbly due to the fact that a hydrocarbon radical is more easily formed by rupture of a C-- CI than of a C - H bond. The reduction and inhibition of carbon formation b]," hereto atoms such as N, S, O contained in the ring or adjacent to it in a side group is explained by ~Se authors through ring rupture which is favoured by the thermal stability of CO and CN IHCNL It would be interesting to know if ring rupture in these t~ast.~prevents condensation to any larger molecule or if it bec,-mes efficient in a later stage of condens.: .m, ho~,ever, keeping lhe aggregates so small that they are again decomposed in the burnt gas of the main flame. There is no doubt that partial ring rupture does occur in soot formation from benzene also. The small soot particles which have radical character grow by adding hydrot~arbons and by agglomeration with each other, This agglomeration, which must not be confused with that in a later phase ofgrowth giving chainlike aggregates, causes the particle number to decrease rapidly. The rate of this process indicates that every collision of two particles gives one bigger particle. The formation of n~' particles decreases with the fall in hydrocarbon radical

August 1967

('ARIM~N FORMATIONIN PREMIXEDFLAMES

concentration in the burnt gas. Optit.~almeasurements of the amount of carbon have shown that during their growth the particles become inactive for further addition of hydrocarbons such as acetylene and polyacetylenes, probably by a kind of tempering process. This is supported by the fact that an electron paramagnetic resonance (e.p,r.) signal from 'young' soot is very much stronger than that of 'old' particles from farther downstream in the burnt gas2s. The next phase of growth is governed by the heterogeneous decomposition of various unsaturated hydrocarbons, a reaction requiring considerable activation energy. Accordingly further growth is relatively slow. A difference in flames of benzene is that besides acetylene and polyacetylenes, a great number of polynuclear aromatics are already formed in the oxidation zone. Their overall concentration is very much larger than that in the burnt gas of aliphatic fuel. Again the unreactive species of these compounds survive in the burnt gas while those with side chains and more hydrogen are consumed, Since all these species are formed in the radical-rich oxidation zone, carbon is formed earlier and the yellow continuous emission from soot or large mole.. cules sets in before all molecular oxygen is consumed. Since the growing hydrocarbon radicals find larger 'building bricks', soot formation in benzene flames is therefore so much stronger than that in acetylene flames of the same C O ratio. The lower temperature in benzene flames also favours carbon formation. lalbenc¢ of A~litives md Elcctrk Fields We do not want to go through the results which have been ob,ained through study of all kinds of additives in carbon-forming flames. Their influence on c~rbon formation in certain systems is described in the book by Gaydon and Wollhard ~ and in the paper of Street and Thomas "r, Compared to the effect of sulphur trioxide which shows the most outstanding promotion of soot formation, other additives such as halogen compounds, peroxides, nitrogen oxides, sulphides and others, have a minor effect on soot formation in premixed flames, In nearly all of these studies only the variation of the amount of soot or of the intensity of

2s5

carbon luminosity has been measured without knowing the influence of the additive on temperature, concentration of radicals and intermediates, selective diffusion, flame structure and other factors governing carbon formation ",rod particle growth. Thus it seems too uncertain to decide in which way an additive is involved. There is a wide field for further studies. The role of ions in carbon-forming flames has been studied and discussed since Bartholom6 and Sachse4t found that nickel and alkaline earth salts reduce soot formation. They gave a plausible explanation of their results assuming that through an attachment of positive ions to small carbon particles the agglomeration of equally charged particles is hindered. This keeps the particles small and facilitates their oxidation in the burnt gas. by water and carbon dioxide. In more recent times much effort has been devoted to the study of ion concentrations in seeded and unseeded hydrocarbon flames by means of mass spectrometry. A great variety of positive and. in smaller concentrations, negative hydrocarbon ions has been measured within a mass range extending to more than 100 mass units. Theseexperiments were mainly concerned with fuel-lean and slightly rich flames, because the formation of carbon would have disturbed the process of sampling of the ions. Up to now there is no experimenta~ evidence whether hydrocarbon ions influence carbon formation or not. Their natural concentrations in flames is low as compared to, that of large unsaturated hydrocarbons and some hydrocarbon radicals. Place and Weinberg'*:-as studied the influence of electric fiel~ on the process of carbon formation and deposition on charged surfaces. They found that under their experimental conditions an appreciable fraction of the carbon particles is positively charged. A comparison hetwem the carbon mass which deposited on a negative electrode and the load transported by it sug,gest~ that about 104 atomic carbon masses are associated with one positive charge. Further experiments using a counterflow diffusion flame. to which electric fields of the order of some thousand volts per centimetre were applied, showed that it was possible to draw the charged carbon particles rapidly through the pyrolysis

286

~. H, )r)maxx

zone. Their residence time in the hot hydrocarbon atmosphere and their growth could thus be controlled by the applied voltage. If an opposite voltage was appli~, such that the forte exerted on the particle's by the streaming gas was just counterbalanced by the electrical force, the particles could be held in place, no',,, growing to aggregates which could easily be .seen by eye. It wou!d be desirable to have more information on the influence of electric charges upon th;'.f6na,:doa, gro~rth and agglomeration of carbon particles.

Conclusion This review is mainly conccrn~ with carbon formation by lower aliphatic fuels and benzene as a representative of aromatic compounds. The reason for this is that these ate the most thoroughly studied systems, The differences that can be seen for the process of carbon formation with different types of fuels are more quantitative than qualitative ones, Both highly unsaturated chain molecules such as polyacetylenes and large cyclic hydrocarbons can be found in the flames of each of these fuels, While the polyacetylenes predominate in flames of aliphatic fuels the cyclic compounds form a large part in benzene flames. From both groups of intermediates a large variety of cyclic compounds in the mass range from 100 to more than 600 mass units is formed; these are barely distinguishable in acetylene and benzene flames, in both cases the more hydrogen-rich species of these compounds and those with side chains further react to form soot while the purely ~romatic species, being comparatively unreactive, survive in the burnt gas. in both flames hydrocarbon radicals are needed to initiate the growth of molecules, We bavt: seen that it is not only flame chemistry that governs carbon formation but thal effects such as selective diffusion are equally important for the separation of solid carbon, There is a difference whether the unburnt gas enters the radical-rich flame front or passes directly into the burnt gas if the flame front is broken up, Measurements of the rate of carbon formation in some well-defined systems together with quantitative information on intermediate species have given us an insight into some details of

Vol. I I

this complicated proce: .... The) have, however, given rise to new complex Problems. There is still nothing known except the mass and in

some cases the !1 C ratio of the very reactive intermediates in the mass range of ltX) to about 0~,) mass units, it is probable that some of these ha,,e radical character, There is more need for quantitative correlation of the increase of c~)rbon concentration across the flame front to the decrease of hydrocarbon intemaediates due to additi,::i to carbon particles. The knovdedge of the relative reaction rates of hydrocarbon radicals such its C,H. CH,. CH. C.,H3 with molecular oxygen or water to give oxidation products and with acetylene, diacetylene or vinylaeetylene to form larger hydro~trbons or hydrocarbon radicals is most relevant to the limit of carbon fonnation. The role of OH radicals in preventing carbon formation see,us to be a major one. Yet little is known, for example, about the relative rate of polyacetylene fommtion and the oxidation by OH. and the temperature dependenc'e of these processes. It would further be interesting to find out why cyanogen hardly forms any carbon in the gas phase although it gives polycyans CAN.,, C6N, and probably also CgN., and it forms carbon on surfaces, Thus we bgpe that (hN paper not only gives a review of developments in this fiekl but also contains some suggestions for future work,

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August 1967

CARBON FORMATION IN PREMIXED FLAMES

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