Chlorine inhibition of carbon monoxide flames

Chlorine Inhibition of Carbon Monoxide Flames' H. B. P ~ r ~ . R and D. J. S~RY Oe.pa.tment el Fuel Technology, College el Mineral Industries. Pennsylvania State University

O3e~eiveO April 1939) Chlorine. and brondt~e reduce the burning *:eloc~lie.~t of b!tdro.qen.¢ontaininfl carbon monoxide flam¢.~ ~npported b~J n,i.r, oa~jffen and nit~em~ oxide. ~eith tt~ qualification, that ~ the hlld~ojen content L~ lore. tt,~ ,'fleet of chlorine upon, nit.term o~rlde.~tpported llamcs ca~ become an a,cecler~ttiott eaaa,'rt by eMorlne eatabJ~&~ of n i t ~ m ~ o~ide d,.eompoMtion. Meanarem, enfe of the redaction, q[ S u b~j chlorine o~:er a u'Cde ran qc of carbon ~.,tono~hle.~r eompositlon~ ~h.ow that the rehtliiw~ chan.q¢! itt (,.qu)z urith an ~.ere.meat of chlorine ~s, at most. otd!l te,,akt!l dependent upor~ q~e ¢,".~!,o~,~monoxide-air ratio or upon tl~: h!/dro.qcn content. The effect of carbon tetrachloride reported by/ .D.oztlov a~vl Z~tdcr:iclt ie f o u n d to t.! identical to t/w. t,qult.ule,nt amo,~n t of chlorine. ;1 tower lind/to the ehah~ l..nffth i~t tlu~ abn~nce of inhibitor i.~ ~,.~tnbliehed to be 55, with the probable actaag el~ain len.qth, appearln.q to be morn like 1000. "]'t~,rad.ic~,tl.free' eontributlou to the burnin.cj veloelty a npe,~rs to be 1 em,lse¢ or le~¢~. ,f 'it ea:i~cts rtt ~ll. Cldr~rin¢ inMbgt~on can be ewpre~scd b!l a ~imple eq.,atlon havin.q the ferret f!tp~cal qt" oh,sin in,hlbit~o~,. .,In tzpprowimate anoly,,eb~ of a h.lcpotheMzed 'tn.*'ebaniectn for h~,tdrogen.e#taly.~ed carbo~ .monoxide combustion eTr~the preuenee and absence of inhibitor h.¢ub~ to prediet,'d inhibition eon.~i.~te,tt with the empirical r,'~+alt for ~.mall ehlorin*~ addit, iona. tat:/tirOl, lilt chloride .is ap.Fmre,ntl!l the pritmip,rl iubit}itor in the flttm,e, .im,pedlnff pr6p~zffatgon b!l reaetln 9 with hydroxyl rtt~.iettl~l, Sorn.e iJtforntatioo coneerninff the rate conatant oJ tlw principal in,hlb~tio~t reaction ert~ be ¢.tlean.edfrom the ~tm~t!m/,~,

Introduction ~I0sr investigations of flame inhibition reported in the literature, which have been summarized up lo 1956 by R. FRIEDI~aN and J. B. LEvY j, have mad~ use of organic halides. One supposes that this is dictated by practical considerations t~hen thinking in tm'rns of tire extinguishing; nevertheless, if one wishes to discover the mt'cbar~ism of flame inhibition, an avowed objective of a number of publisheq studies, it appears th~tt one should use the shaple~t possible z,nh:cul,.s as inhibil:ors, via. the pure halogens trr lh,.. hydrogen halides. Indeed this has been d0rle in some work disc~tssed in ref. i. W . A . Rossr~l,:. l I, \VtsE and J. MI'LLER~ have recently r,'p~rl,~,l detailed measurements of the h~hibition rjf tr:rtt,,te--air flames, in which were included the: @,.ct.s of chh.>rinr,, bromine and hydrobr~n. f: :mid as weft as a number of organic Italid~,r~. With the thought that simple molecules

those of hydrocarbons, firmer conclusions regarding inhibitor action might be reached. Particular attention has been given to the effect of chlorine upon the burning velocities of hydrogen-containing carbon monoxide-air flames. So far as we can determine, no quantitative measurements of inhibition of carbon monoxide flames by chlorine have previously been reported in the literature. We also report here st~me qualitative observations which seem to be of interest. The materials used were: 'C.P.' grade carbon monoxide* {two tanks; one with less than 150 p.p.m, hydrogen, ve W dry: the other with 45 p.p.m, hydrogen, very dry); 'Commercial' gr;~dc carbo~J monoxide (3S% CO, 2.04% H~, vt'ry dry); dry air 11! p.p.rn, water max.): 'Extra. d W' grade oxygen (99-6 per cent rain,): 9S-0 per c e n t , ~re nitrous oxide: 99"1:;5 per cent pttre chlorine; and 99-9 per cent bromine?~. 5~ost (,f the ilame observations were made ~sing a bunsen tube of l'28 crn i,d.. approximately I m long. A :¢cm i.d. jacket was available for

;m, the,
c;Irbor~ monoxide flames. The hope was that ,~ir}eq~ the kinetics of carbon monoxide comt,usti.n are somewhat belter ,mdcrstood t h ~

~Ait cyllrlder l~ascn were obtaJncd from the Mrttlle.son Co, ~'Tbe $. T. Baker Co. 213

214

H, 13. Palmer and D. J. Seery

experiments in control of the secondary atmosphere. Standard mixing and metering devices were employed. The materials were used without further purification except for removal of iron pentacarbonyl from the carbon mono:vdde by means of a glass chip-packed col,alan held at 320°C, Results and DLscussion Some of the qualitative observations may be mentioned before giving the numerical dat~ on burning velocities. They may be summarized as follows:

Commercial CO(2'04% H=) (t) Flames with air were inhibited by both chlorine and bronline to a roughly similar degce. There was a hint that quite rich flames were slightly accelerated by very small chlorine additions. Effects of smaU halogen additions on the flame spectra were to weaken slightly both the OH radiation and the continuum. With Iar~er amount~---several per c e n t ~ a 'tail" appeared on the outer cone that had a colour (no spectra were taken here) appropriate to halogen recombination continua, viz. yellowred, and also appropriate to halogen band spectra,~. (2) Addition of oxygen accelerated the flames, as expected, (3) Nitrous oxide addition had at first very tittle effect; with larger additions, acceleration was produced. Ct~aracteris~ic N O + O radiation was obset~-ed in the outer cone. Flames supported entirely by nitrous oxide had a higher burning velocity than air-supported flames. They were inhibited by chlorine, but not strongly. C.P. CO(45-150 p,p.m, H:) !I) Flames with air were inhibited by both chlorine and bromine. Air-supported flames burned stably only in a moist sec(:mda~, attar,sphere (we discuss the atrnospheric effects t'l.wwhcre~). Halog~.n addition appeared to weaken somewhat bofla the OH radiation, which was not strer.g to begh. with, and also the continuum. Wi.h bromine addition, three weak emission baud:., of BrO were identifiable

vcI. 4

in the outer cone. The characteristic halogen radiation appeared in the outer cone with larger halogen additions. (2} Oxygen addition accelerated the burning. Flumes supported by oxygen alone could be burned successfully in a dry atmosphere. (3) Nitrous oxide addition weakly accelerated the ,flames at first; with large additions, inhibition finally resulted. Flames with .nitrous oxide alone burned faster than flames with air and could be burned in a dry atmosphere. Radiation from N 0 + O was strong in the outer cone. Addition of ch!orlne to the~c flames had the following c~trious effect: small amounts inhibited the flames very slightly; at a critical amount of chlorine, the outer eerie faded from sight, including the NO + 0 radiation, and there was a virtually discontinuous increase in burning velocity when this occurrcd. Further chlorine addition accelerated the combustion and turboed the flame to a brilliant white. With very large amounts of chlorine, inhibition finally sopcared. We did not succeed in adding enough chiorine to bring the flame to blowoff, alttlough a great deal was added (several per cent). These qualitative observations are largely what one would expect. The catalysis of carbon monoxide flames by hydrogen or water vapeur is a wall&hewn fact. l-tal9gens can react in various ways with hydrogen-containing species and remove them, at least momentarily, from a reaction scquence in wl~iela they may have been involved. Partictflarly if the reaction sequence is a chain, as is accepted here, such momentary removal breaks tim chain and reduces the reaction rate, The halogens are largely ¢lissociatt:d or converted to HX molecules by the time t]:ey reach the radiation zone. It is [tenet not t. be expecteo that the diatomic halogen molecules act as inhlbitot's, but rather the au,ll~ or else l.lX molecules, nr both. In the fonnaliot~ of HX molecules, t[ atoms ere generated so that it is conceivable thal with suflichmt hydror~en and a sm;OI r":~'''',::'t ~f ird:dbil;or, t|mre could be slight acccler'tlion, as observed in rich flarnes of commercial e~rbon monoxldc. Wh.h larger amount~ of hal<~+~,,:;n,the chain-breaking comes rapidly to dominance and inhil)ition results. Weakening of both OH radiation and

~pteraber I~SO

Chlorine inhibitloa of carbon monoxide flames

the CO + 0 continuum is to be expected if H or OH or both are removed by halogens because of the coupled effects of the reaction pair H + 0.., -->- ,OH + 0

215

hydrogen available, as in commercial caxbon monoxide, there is continuous but rather weak inhibition by halogens because of the plentiful supply of hydrogen for the reaction

H + N,~O ---+- N. + OH

and OH+CO

~

COz+I'{

Halogen recombination radiation is to be expected in the eater cone becausc in the equilibrium gases, a sizeable fraction of the halogen is present as atoms. Oxygen accelerates the flame because of the H + t)2 chain-branching reaction. Nitrous 0xkle addition provides a new source of O atoms through Ihe decomposi.tion reaction N~,O ~ >

N._,+ 0

N~ + OH

and it carl react directly with carbon monoxide in the e×(}thermic reaction (:~).t- NzO ---+ CO: + N= I-Iowevc.r, this last is not a chain reaction, and the r,~,',w+t;or,with 1-[ is not a chain-branching r~"aetioa (in contrast to H+O=); furthermore, Jlitrou+ .xide decomposition has the elfect of ditulin~ the: flame somewhat with nitrogen. Hc:,,:~, '~ '=,:t rff nitrous oxide addition is not pr<.motmced. The inhibition observed with Iarge nitrous oxide additions to C.P, carbon +no:mxidt++oairIlame+ must hnve represented an appma.:h t. the lean limit. Nitri,: .xide is produced from nitrous oxide in the ~
Kivin:~ri:,r t,, ttm N O + O contitmum. wdl ;tl:., bc tmMuced :' via

CI+N=O --9- N._.+CIO In the presence of carbon monoxide, the CIO may be expected to react in the very exotitermic reaction ClO + CO --->- CO= ÷ CI

it can participate in the chain via

1~ + N=O ~

a very exothermic reaction ~md surely a very fast one. With C.P. carbon monoxide, however, the initial inhibition is ~oon overwhelmed by the catalysis of the nitrous oxide decomposition. There is some controversy over the kinetics of this halogen-catalysed reaction~'L but fortunately there is agreement <)n tL,-.- ~r,.i+,b_l step, which is ait that need concern us. It is

Oxygen

N~O+ () --~- N~ + t).. tl)'dr%!+:h,c.ataining, nitruus oxktc-snpport,:d fal'b¢+l~ I!l,+lloXit~(r fl::tl, It!S l'Ilay be e x p e c t e d to be iTd+Lbitq,df+y hak;gens, ltowt:ver, tile situation h,r~ i. +:,:replicated by thtr halogcn-catalysed eh'<::mq).sili~m of nitrous oxiN~, which is in r+mq~+,tlthm with If-species renmval as a sink I++r ha]~Gen atoms. When there is much

regenerating the chlorine so that an excel]¢nt chain results. There appears to be no multiplicity problem in the reaction of CtO with carbon mon,.)xlde and it should be very fast. The ultimate inhibition of this system by sufficient halogen may be attributed to a combination of the effects of dilution and of heat extraction because of the necessity of having a considerabh:' concentration of halogen atoms in the burnt gases. It may be expected that the arrival at domirtance of the chlorine-eataiysed nitrous oxide decomposition will be rather ;tbrupt, ~o Hint a uearly disc,mthmous change in tlanle characteristics occurs at this point. Tim nitr.lls oxide also nt? longer acts as a

s.urce of N()+ O, so that the N()+ 0 radiation disappears. The brilliarrt white colour, which certainly mcrlt~, more detailed examination, may wNl bc the ha.h>gen r~.c~.mLblnation spectrum, spread
Effect ~/ chMrine on burning +.,elocitv 'F[l~' rc~.tl[lS of <~llr II'IC:.ISnt't'HI(!IltS. rllLld'k.~ with the vigil}k' c.tte ft'llSttln! vn¢'th,,dL ttl'e gi'¢erx in l'ablds I a n d 2 arid settle t>f the data have been

plotted in Figure 1. Tbc prccMoa of the data lr~r C.f'. carbon monoxide-air is trot as good as that for commercial carbon monoxide, partly t}t'C:LI|Sc' <}f the thicker reacliot: zc::~.'s a:',d h)wer

14. B. Palmer and D. J. Seem,-

216

Vol. 4

Table I. Burning velocities o[ co,i,merzial carbon monoxide-.aiv mixtures with a~dcJd chlorine ,Mole %

Mole ~

Cl=

S= cm I se~

MOte %

CO

CO

C]=

St= c,n / sea

25".5

0 0'08 0'14 0'22 0"34 0'49 0'63 0"81

24"7 ~Z2'8 23'8 2Z'7 22"4 19"9 19"1 18"2

37'6

0 0"07 0.1.5 0.26 0-51 0.79 ;-28 2'05

36,1 32.9 33-6 ~0.9 28.3 26.6 26.6 24..5

0 0"09 0'85 1"00 1"48 2"04

32"2 32"0 2&9 22"4 18'2 15"5

41'2

0 0"08 0'29 0'60 1"12

36'1 33"4

0 0"10 0'22 0"68 1.13 1"97

38'0 32".5 30'9 28"6 27'3 t9"2

29:0

87"4

Moh~ %

burning velocities, and Fartly because of the pronounced effects of the secondary atmospheric moisturet For this reason, further examination of the data will be confined principally to the commercial carbon monoxide results. It can fi~t be remarked that the effect of chlorine addition is in genera] a continuous reduction in burning vdocky, with the strongest relative effect occurring at small additions, a characteristic oI m~st inhibitions. The inhibition is comparable to that observed by Rosser, Wise and Miller= in methane-air. One per cent chlorine reduces the burning velocity of carbon monoxide-air by qO to 40 pe~"cent. The rate of change of S,, of methane-air with chlorine addition reported by Rosser et al. predicts a lowering of about 20 per cent by 1 per cent chlorine, slightly dependent upon the composition of the combustible mixture. These authors found bromine a much more effective inhibitor than chlorine in methane-air, whereas our (qualitative) observations indicate that the two are comparable in their effects on carbon monoxide-air.

2"OEt

SO'O 29"0 24.3. 20'0

Mole %

CO ,~6"2

Mol~ %

S=

el=

em / sec

0 0.06

42'9 40'4 37"8 3S'7 34"~Z 29'8 26,~ 23"1

O'lS 0.20 0,41 0.75

1.21 1.81

54'8

0 0.05 0-10 049 0,27 0.58 0"86 1"26

37.8 40,1

40.2 34-0 32.2 27.0 25' 3 22" I

In an effort to reach more general conclusions about the inhibition, we have plotted the data in various ways. One of thc more successful of these is shown in Figure 2, which is a plot of the square of the relative burning velocity versus the square root of the mole per cent chlorJne in the unburnt gas. In this figure are included the results of N. P. DROZDOV and Y, B. ZELDOViCtP' [or addition of carbon tetrachloride to carbon monoxide-oxygen (.~toichiometric) containing 1"8 per cent water vaponr. The agreement with our data is strikingly gc,od and seems to imply that addition e f a CCt, molecule is essentially equivalent to adding two C1...s. This equivalence has been assumed in the p~otting. Tlae line indicating the average behaviour was drawn by eye, The l square of the relative burning velocity was1 chosen as the ordi'aate because this should! correspond to the relative rate of reaction in th¢~ ltamC u, assuming only slight effects upon trans~ port phenomena. For additions of chlorine n~ to ! ~.~iole per cem, equilibrium calculations'~ show Lhat the change in flame temperature is

,September t~60

Chlorine inhibition of carbon monoxide flames

217

Table 2. Bttenlng velocities o[ C.P. aarbon monoxlde--air m/xtures witlt added ¢ltlorine" Mole % CO fll'O

Mole % CI~ 0 0"08

0'21 0'26

...... : i ; ~ . . . . . . . . . i;" . . . . . . . . . . . 0.08 0"19 0"40 0"89 I'09

--~74

............... i; . . . . . . . . . . . . . .

0'09 0'16 0"31 O'fi6

5,, emlse~

Mole % CO

Mole % Clz

Su cm I sec

34'5

0 0"08 0"14 0",21 0"30 0'42 0"51 0"68

12'8 Ii$,7 11'8 11 '4 1 !'7 11'6 1 !'7 10"7

0 0"08 0"20 0'42 0"66 1 '35

15'4 13'5 13'0 I 1'3 10'7 9'0

1 "70

6'6

14'9 12'6 12"6 I 1'9

7¢.6 ..................

s8 i;"

16.1 16"7 15'4 9"9 9"5

i ~ : ~ - .........

17'2 18.3 17'3 16-4

41'9

0

13'6

0'07 0'11 0'13 0'20

12'4

11"7 12~2 11.7

0"28

9'8

0"58

9'3

0'$9

7.~

Mole % CO , 45'7

Mole % CI~

0'49

15-0 t3'9 13' 1 11'7

I '16 1'60

8"4 5'8

0

0'10 0"24

........

S= on~see

4 s : 7 ............. i; ................... ; i : i

0'06 0'12 0'20 0'33 0"54 0"77 1.24

.........

! 1"fi I0'9 10'4 11 '7 10'S

9"I 6'i

]

Wrhcr;t:v~21O¢tlie,:~,ate [tta:tlons.0[ 'tile a,trtto~#rd'~cHc huilx[dtly(Selltcxt). slight, the principal effect showing up in rich (50.M';(] per cent) mixtures, where phosgene formation raises the final flame t e m p e r a t u r e b y as much a~ 30"K, E v e n this c h a n g e should be small in its effect on reaction rates relative to the effect of h a l o g e n il~ stopping chains. T h e l'~@lt is that to a reasonable a p p r o x i m a t i o n , @~ dr¢rct ,d halogen :'dd/tioa can be rvgarded il.~ ~lrt isothermal effect o a the reaction kinetics. lrt this light, the results in Figures I ~md 2 fma'idc some i n f o r m a t i o n a b o u t the chain length ill lilt, al~selx:e Of inhibitor. Making use of the coil,:idem.~. ~Jl the c a r b o n tetra.chloride results ;tt~i! rlllr owll, ili..-; se{.'fi [hat 9 per ecrtt chlerlne will wducc (S,,/S';) ~ to about i x 10 ..... of its ilfitial v~duc. C)n the face of it this wouh:l hnply ,m uIJinhil::,ited chain lcnRlh of a t least l 000: bu( a '-ystem (,ontainirt g 9 per cent chlorine is 15, m~ means isothermal with the u n i n h i b i t e d sy~ter,, and at,solute concentrations of reactants arc also appreci:tMy decreased by such a

q u a n t i t y of additive, so that the result should be accepted with caution. An a p p r o x i m a t e lower limit to the c h a i n lcngth m a y bc found b y m a k i n g use of the C.P. c a r b o n m o n o x i d e results. A n S. o[ 3"8 c m / ~ e c wa,~ r e a c h e d with chlorine addition to the 45'7 per cent carbon mcmoxlde flarlle, in which the effects oi1 composition a n d t e m p e r a t m e were not great. If this is c o m p a r e d to the value, 4 2 9 c m / s e c . found for u n i n h i b i t e d 4/1,2 per cent commercial e ~ r b o n monoxide, the ratio of reaction rates is ( 4 2 9 / 5 " 8 ) ' - ' = 5 5 . TI,e chains in the c,nnn.~ercial c a r b o n m o n o x i d e shotlld hot,c(' h;.u.'e ;t ]ength ~,f more t h a n 55, at least at 46 per cent c a r b o n mfmoxldc. Tiw previous result leads us to believe t h a t the m d n h i b i t e d chain length is m u c h greater t h a n 55, b a t this is at a n y rate a r a t h e r firm v;.|[tlt* as it lower bound. Thc.re is n,~ indication in Figttre 2 that extrapotatlo~a will yield S,, --, 0 at a n y chlorine concen-. trafion, no m a t t e r how large. One m i g h t say

Vol. 4

H. B. Palmer and D. J. Seery

2.I8

~

,

l

O 46'2 @ 4t,2

chemical inhibitor, one might hope to find a relation of the type .

Commercial grad .

(s. IS:r" = (x ÷

(z-o4 ~o H z)

~[clx)

-~

which has a form typical of chain inhibition,~. In Figure 3 are shown the experimental (S.] S~) ~" values plotted against mole per cent chlorine, where we have included Zeldovich's carbon, tetraehloride points. Within the uncertainty 0£ the data, a single curve expresses the behaviouri up to 2 per cent chlorine addition. It is given; by the above expression with .x---l, ~=1"2. oo

%C0 • 2S 5

1.Q

~ S7 4 "~o'

0'7-~

• 61..~

~ "t~. r ~

~ ~4.e ~From C ~ r~U]l~ ~I ~,h~

0

05

1,o

r>

.slolchiomelric

CO-Oz

1.5

I~ole per cent CIz Figure t. Effect a] ~hlOrJmt add}t~o~t ~m carbon ,,.o~w.ride-,ie burning veiocitie~~ a~ several Voacentra. lions el carbon monoxide. The C.P, carbon mOnoxide-values are no~ tru~ b~trning velocities ]or C.P. carbon .monoxide-dry air (see re]. 4)

that the~e must then be & 'radical-fre.e' ¢~ntrb bution t~ to the burning mechanism, but if so, it is apparently I era/see or less, rather than the 17era/see value chosen by C. T~Or~D and R, N. P~As~. There is of course the possibility that chlorine might inhibit both the radlcal-free meehanis.m and the r.~dical--chain mechanism, bet it is difficult to imagine how such an inhibition could Junction, Or it might be supposed t:hat there i~ a residual contribution at large el'define contents from an 'H-free' mechanism that would not be affected by chlorine. If it exists, it appears again that the $., produced by s~eh a mechanism cannot exceed abort I cm/sec. Although the plot: of ~?,,/S','~)" against [CL,]~ is reasonably satisfaeto:T ~or examiaitTg trends, it does not provide a satisfying m~thematk:al expression for the behaviour, i.e. satisfying from the chemical standpoint. If chIor.;ne is a

02~

0"5

1'C,

~'5

2,0

2'5

(blole per cent C[2)llz t"t~are 2. Dependence o/ square el relative tmrnleg ~:el,eity ttpolJ 5qttare root o/ t~zbuwltt g~,ls chlorine con:cut. The rrstdts t~[ Drozdov ~rttl gchlx~vlch included in the plot are not lrom their rar,'~ data but were picl¢~d off the.lv curve

::.:1'3 is almost equally satisfactory, but == 1,1 arid ~ = i . 4 are less so. There may be a sllght trend with composition over the raogc studied (25 to 55 per cent earl)on monoxide); it is most, evident in the tw~ richest mixtures (46 and 55 w r cent), where the points consistently lle sr~mewhat below the line except for the two points representing acceleration at iow chlorine addition in 55 per cent carbon mot, oxide-air. The C.P. carbon monoxide result~ conform reasonably well to the plot in Figure 3, but" with much larger scatter. This conformity can actually be seen quite well from Figure I, so the C.P. result.~ an, not .shown in Figltz¢ 3. Alfl~ough moisture entered the C.P. flames from the secondary atmosphere during rite inhibition measurements, these flames still contained much

:]p~ember

Chlorine inhibition of carbon monoxide flames

19.60

)S h y ~ o g e u , whether as H~, HaO, H or OH, S n d i d the commercial carbon monoxide Iraes, Hence the similarity in behaviour of ~P carbon monoxide is interesting in its ~-':¢ation that the relative inhibition effect of a , eta total chlorine concentration is independent h f hydrogen content. It has previously been ioted that dependence upon the carbon ~onoxide and oxygen concentrations in the unburnt gas is at most quite weak, !If an interpretation of the experimental results .n terms of the flame kinetics is to be attempted, .he empirical expressiota ~or inhibition must be ,'elated to conditions within the reaction zone. iAs a first step it is noted that the hal~-life of CI~ with respect to dissociation into atoms at i. "O00°K should be of the order of i00 microi ';~nds, b y analog] to Br, dissociation ~a, The 't .adtion o£ Cl with H= is very fast:', so CI,a %;ntrodnced into the unburnt gas can be expected ! et9 be converted almost completely to a mixture : C1 and HCI before it reaches the flame ~reaction zone. Thus ! m o l e of CI= is replaced by 2 moles of (CI+ HCI) in the reaction zone. Converting the inhibition eqnat{on to units of ,. concentration in moles/era a at the flame 'temperature ..

(STlS.)'-'~ 1 + o.e × lo, {[¢11+ [ H C t ] }

where 2000*K has been taken as a typical

219

temperature, This equation should have some meaning in terms of the kinetics in the flame. In the hydrogen.containing flame, we presume the most important reactions to be: O H ÷ C O --+. CO.:+H. propagation . . . [i] H + O , --~- O H + O , branching O+H i ~

H + OH + M ~

....

1"2]

O H + H , branching . . . .

[3]

H:O + M, ending

[4]

....

Selection of reaction 4 as the only important ending reaction is based upon the recetat 'work of E. M. Bo'L~wicz and T, M. SUGDEt~"L', who found the rate constant o£ tbis reaction to be some 100 'times that of the H + H + M reaction at typical flame temperatures. This was in the presence of water as the third body, so it is possible that the rate constant ratio might differ in the carbon monoxide flame sys.tem: however, it seems likely that reaction 4 should still be strongly favoured over H atom rec,ombination. The rate of burning of carbon monoxide will be the rate of reaction 1; i.e, R = ~, [CO] [OH1 If the concentration of OH is assumed to be steady in the burning velocity-determining region of the flame, ttlc rate of reaction 1, to wbictl the square of the burning velocity is assumed to be proportional, is

R =- k, [ c o t { ~., [~ [o..t t.0, J

0,75 tO

+ r~:~[o1 [[4..,1} i {J,= l.CO] ÷ ~, [H] [~!.] }

% CO • 25.5 *a 37'*, ~, 7.9'0 "~ *.I,2 %

~6'2 O ~t,.B ~ From Cel~ ro'~u~Ir~ ol tl I'P.[~lfOZl'~)V ,'t rld V i.:'~ZI I aal'1611

'~-,,. ~i,UNI..~+ ~ '-.,...

,.;~0.50 0"25

0

(SJSu0)2=(1,1.2 x Cl~)-I '

-~

~. . . .

*, L____.A~

0,5 l,O 1'5 2,0 2,5 Mole per cerTt CI2{unbumt gas)

.....

~'%,tl.O' 3 l;,@e~*c.letoic ~t] ¢1o: aqaare ,~f the *~'lalJv+" , f,.at.i~tg ':,~l,,city rtf, e,n l h : uM,.~'M. Ray .'hl,,r*ne' '

i'¢a?t~1"11a'

An order-ot'-magnitude calculafiQn shows .that a,:[co]>tq[a]t.x[]. This is to be expected since, in the abst.nce o~ inhibition, tile ratio of the rates of eeactio~a I ~trid :1 will be the chahl length. Reacti,m I is cxothermic, with a, low activation energy (llirscl'd:elder's rule ~' predicts t', '.i 8.5 kcal), and should haw, a rate ct, ustant l,'~" ; 1o 'a cma/m.le sec at 2 fl0tP'l{. Thl? total ,~as co~tccr!tr:ttitin is [M~ ':~ 6 x 10 -a m~le/crn :~ at I alrt, and 2 i)00~1,:. For a typical starting concc.ntratiem ..)f carbcm monoxide, its concentrafi,m iu the rt'acti~m zone should be of tile fw&'r t~[ H } " ' ; t o 1 0 ' r m i l l e / e r r 1 a. / ( , ' . . . ; 2 * 1 0 ~: c;n:~Imolc~ see at 2000°K"~; and [H] in the flame zone, which is of course dependent upon the overall hydrogen content, should be typically

220

H. B, Palmer and D. J. Seery'

•between 10-~° and 10-s mole/cm ~ for commercial carbon monoxide, making allowance for the factu,'~ that its concentration should exceed considerably its cquilibrium concentration. Then the typical range for kt[CO] is from 10* to i0 r, and that for k,[I-1][M-j is from about 10= to 10L Thus the chain length in the absence of inhibition should be somewhere between 10= and l0 s. We previously co,mhded experimentally that it was certainly greater than 65 and probably greater than 1 000. From the equation for the propagation rate expressed in terms of [OH] and [CO], it appears that the likely effect of an inhibitor will be to reduce the steady-state concentration of OH. This may occur by direct reaction with OH, or, as seen from the above equation for the steadystate propagation rate, by reaction with H, O, O.. or H~ as well. The most probable reactions seem to be the following: CI+OH ----> HCI+O

....

HCI + OH -+- H._O+ CI CI.,+0H ---> HC] +CI,0

....

is] [s]

....

[71

....

[s]

CI+H+M ~>

HCI+M

H C l + H ---> H,.,.CI

....

9]

H + CI= .,--> HCI + Cl

....

lie]

H= + CI ~ > HCI + }T

....

[,l]

C I + O + M ---> CIO+N'

....

lie]

HCI+O ---->- O H + C l

....

[is]

Such an array of reactions presents a discouraging prospect for any success in finding a theoretical expression that may conform to the experimental results. Largely in the hope of stimulating further work on the matter, the following attempt at a kinetfc ",.~a!y~,_, is presented. . Let us make the arbil:rary assumption that rcaction 8 is the only inhibition reaction of imporlasme: i.e, hydrogen chloride is the dominant inhibitor and its prime function is to remove hydroxyl trom the system, terming water*.

Actually, as one considers the reactions suggested, there seems to be some m e r i t i n t h e assumption. The reverse of reaction 6 is difficult, whereas the reverse of 5 is easy, and is indeed listed as reaction 13; and reactions 9: and 11 form a similar pair. Reaction 6 ha.,~. the advantage over 7 and 10, which involve+ CI=, that there will be very little chlorine in the,I reaction zone; and, being a 2-body reaction, 6 may be favoured over 8 and 12, both of which are 3-body reactions, provided its collision, efficiency is fairly high and its activation e n e r g y low. According to the Hirschfelder rule ~, E~ - 5.5 kcal, so the low activation energy requirement seems m be fulfilled. (Note that E_,:~20 kcal by this rule.)i The steric requirements for reaction 6 may, however, be rather stringent. Of course it is not permissible to ignore the effect of a sizeable chlorine addition upon [H=] since hydrogen chloride formatlon must consume, hydrogen: Therefore we shall restrict m a r t e n to the case of small chlorine addition, where it should be a good approximation to treat [Hz] as unchanged by the addition, and assume all ~he chlorine to be converted to hydrochloric acid prior to entering the reaction zone. It L,, assumed, then, that the on]y major change i n . the steady-state concentration of hydroxyl is produced by the removal reaction 6. Inemding this reaction in the steady<~tatc expression, we have

R' = k, [CO] { #~=[H$ ~,0,,] + /,~ [0] [tL]} / { k, I'CO] + ~., [Hi [~!] + t¢,, [~ZCl]} '

Comparing rates R

which has the same form as the experimentM equal:i.n. Recalling that the second ternl in the de,~ominator is much smaller than ~he tirsl,.. lhe equallon becomes s , / R ' - . - i -~. {k,,

;4g;/g;2Ui,iD2"g,h~i ';7~g ;T;,~;d; i;gi;ii~.~ .+,.~i;;i,;;7 ii~ ]; ;o,

intcndclt. ~tli it. a e t s * e e l eertalu t~at t~e refni-minl of O H ltom H:~O bY rcacthm With ~1. O" cr H will be rather SlOW {~a~'l, ~211 g=OI) r d a r k ¢ m other rvil~'lif,ns t~f i/llO,,rtMl¢:l ",, lh;ll ill Cff~t reactton 6 remcvt's OH Irorn the system,

k,, [~-~0!

R, ,,; I + l/~;i {(:~] ":-/< tliJ[~ij}

lk,

[COIl [HCI] •.~ i +.0,¢g z. 1 0 ' [ I l C I ]

Jt fol!ow,~ t h a t

k,/k~ [CO] =0.6 × 10"

5., 2~ml0cr 19.60

Chloritae inhibition of carbon monoxide flames

:ur, recalling that k,[CO] was estimated to be betwee~ 10 ~ and I0 ¢, one finds ~ , = 6 × l0 w to 6 x 10 z:~ cma/mole sec at 2000"K gith the previous estimate for g~ = 5 . 5 kcal, the ollision factor is A , = 2 . 5 x I0 '~ to 2"5 x tO ~" cm'~/mole see ~his range is such as to indicate a collision .~fficiency of at least 0"1. The result of this crude kinetic analysis seems .together radaer reasonable. It is tempting to ~nclude that hydrochloric acid is in fact the "xincipal inhibitor in the flame and that its naior effect is rapid reaction with hydroxyl. But we have avoided the s e r i o u s ' p r o b l e m ,(among others) of accounting for the fact that the empirical inhibition equation holds rather well over a m u c h wider range of chlorine addih~on than is implied by the theoretical analy~s. Tim final comment m a y b,? made that thc inhibitii~:,n eqraation ::~ derived requires, ostensibly, tl~at k,,/,~:,[CO] be a constant;. This can bo appni~ximately so only if k~ is a stronger function of lemperature than k~. As [CO] increases from stoichiometric to rich, the temperature falls, assuming the reaction zone temperatmt,: to parallel the equilibrium temperaAura. If i~t~is as:mined to equal the equilibrium temperatur.':, and if it is assumed that ~he carbon monoxide q,t~nceutration in tile rcacth)n zone !s proporliona,~ to the unburot carbon monoxide concentratio~ times the appropriate temperature ratio, then z~i computation shows that the acti. ration energl,t E t must be son.~e 9 kcal greater than E¢,, .r'Iid)out 1,I kcal. N. N. SEStENOV~r qu,:)tes a v , l @ of i.0+:~ kcal for,this reaction. Au E , = - I 4 l.~(~al nt,,:essitale.~ a rcvi.;ion downtw~rd by a fa,vt¢~r of about ~0 c)f the Incvious ,'~tim.'ltcs ftJr~k,[(;()], the uM,,hii.)itt.;d chltin I,.~l~lh, 'a~.t k,~t :tll ~)f whie]~ rem;tlrl seemingly rt':s¢)ttable in n!!agnitude.

221

This work was supported in part b y a grant [rom the E x p e r i m e n t Station, College o[ Mineral industries, The Pennsylvania State University. References t Fr~II~o,~mr,, R. and LI~vY, J. I3, W.,4.D.C. tech. Rap. No, 56-568, Wright Air Deveiopmcn.': Center, Wright-Patterson Air Force Base, Ohio, 28 November I956. Regrettably, this excellent rvview has not, t~ our knowledge, aplx~tred in fine open Iiterature " Rossr~u. W. A,, Wrs~E, H. and Mint, r-R, J. Seuenth Symposiz,m (International) on Comb~ztion, p 175. Butterworths: London. I959 a GarDen-, A. G. The Spectroscopy o[ Flames, Chapmau & Ha|l : London, i957 t>,~.t,.~tEr~, H. B. artd .GERRY. D. J, Paper submitted to Combustion C~ Flame For a discussion oI nitrous oxide decomposition, #~

TI¢O'I'MAN-IJIcI~.FrN~ON,

.'X,

]:,

(~O,~: ]~irtL'!.h.:,!,,

Buttc~vo~ths : Londou, 195S

'~ Bl¢,~sox, b, %V, and Buss, J. fi[. 1. chem, t"q:ys. 19,57, 27, 1382 r t(Auvv~t,x~. F,, Gr;Rv~U,N', J, an([ PASCALE,D. A, f . chant, Phys. 1956, 24, 32 Dt:vv, R. J, Unpublished work; as discussed by HAid':rs, M, b;., 4;r~t~.~wte, J.. vox ]'~LI?,I':, ('~, ;lilt] L):~vts, B. in the Third Svolpo.quur on Co)ul);isti,m, Flame und Explo.q~;n I"henontenu, p. 8o %Villi,tins awl kVilklns: ]~;dtiln~m~. 1949 DROZDOV', N. P. and gzt, Dov~cFL Y, B, Zh. tic Khim., Mosk. (J, phys. chalet., Moscow). t943. 17. 134 IG F~,:,','.~. J. B, rind CAt.CO:OF;,It. F. in the Fourth .b'ymposium (lntt.'rm~tionaf) I,m Co,n.bustwn, i ) 231. Williams and WJlkins: ]'3MLimt.'~re.1953 II g~

"[',~,NV(~r;l'.). C, ;trial PEASV., l~, 19.17, 15~ 861 ; FI¢OsT, A. A. ,,'~,..: PI¢~r¢~*~N, Mechanisnt. ~,Viluiy: Ncw York.

~', f . CL'e~)l~. Phys, :, l~. G. K'.fnetics and 1/98:1

l~a) ,~tt.'.l~, H. I:L n:nd t[cmxt{3.D.tF. ]. them, Phys. 1957, 26) 98 ' t Ia

17

I:hJLl:,WlCZ, 1:. ~1 itm'[ SL)~;~JJ:N, l:., .~[, 7"ra)tL Faradlty Soc. 19c$, $4!, 1835 Fv.m~r,,m~, C. i) ,and JoNt,;~, (;. '~, ], phys. Chem

I.q38. 62, 178 I ]':,~K~', XV. E., Combustion d;-,t'rl,,~v, N. N. I L'sp, ~:, .\'a,k 5"S.5"R. (.qdv phy:. ~,'ci...ll,:)s:,;w), 19.}II, 24. No. 4, 433: I:.r.'gli~h tr:tm~Llri, m : l~,cilt, 3It,re(IF, .Vat. ,'tell', Ccowl .,tern., IV.,~h , .Vo 1026. S e p t e m b e r i9.1"

[[tl,~scn!.'l~r.~.q.'t(. ji l O ./ ,:hew Phvs, 1941. 9, 645