0784(06)80101-3] F.N. Egolfopoulos; D.X. Du; C.K. Law -- A Study on Ethanol Oxidation Kinetics in Laminar Premixed Flames, Flow Reactors, And Shock Tubes

7/30/2019 0784(06)80101-3] F.N. Egolfopoulos; D.X. Du; C.K. Law -- A Study on Ethanol Oxidation Kinetics in Laminar Premi

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Twenty-Fourth Symposium (International) on Combustion /The Combustion Institute, 1992/pp. 833-841

A ST UDY ON E T HANOL OXI DAT I ON KI NE T I C S I N L AMI NARP R E M I X E D F L A M E S , F L O W R E A C T O R S , A N D S H O C K T U B E S

F. N. EGOLFOPOULOSD e par tm e n t o f M e c han ic a l Eng ine e r ingUnivers i ty o f Souther n Cal i fornia , Los Ange les , C A 90089-1453

ANDD. X. DU AND C. K. LAW

D e p ar tm e n t o f M e c han ic a l and Ae rospac e Eng ine e r ingPrince ton Univers i ty , Pr ince ton , N] 08544-5263

A comprehensive experimental and numerical study on ethanol oxidation kinetics has beenconducted. The laminar flame speeds of ethanol/air mixtures were determined by using thecounterflow twin-flame technique at 1 atm pressure and for initial mixture temperatures be-tween 363 and 453 K. A detailed kinetic scheme was subsequently compiled by grafting thelatest information on ethanol kinetics onto a previously developed methanol scheme, and wasfound to be self-consistent in that it closely predicts not only the experimental laminar flamespeeds of ethanol, but also those of methane, methanol, and all the C2-hydrocarbons. Furtherrecognizing that prediction of the laminar flame speeds is not sufficient for the satisfactoryvalidation of a kinetic mechanism, the present scheme has also been tested against experi-menta l data in the literature on the species and temperature profiles in flow reactors and onthe ignition delay times in shock tubes. Such studies demonstrate the importance of the CH:3and HO2 radical chemistry, and the present results suggest that the rate of CH3 + HO2CHaO + OH may be slower while that of CHa + HO2 ~ CH4 + O~ may be faster thanvalues frequently used in recent literature.

Introduction

Based on economic and environmental consid-erations, in the past decade or so the potential andconcern with the use of alcohols as alternate fuelshave stimulated extensive fundamental studies ontheir combustion behavior. 1-7Methanol, being the simplest of the alcohols, hasbeen extensively studied 1-6 and its oxidation andpyrolysis steps can be considered to be reasonablywell understood. Ethanol, the next simplest alco-hol, has also been extensively studied experimen-tally. 4'5'7 Its kinetics, however, was much lessunderstood until the recent contribution of Norton 4and Norton and Dryer z'7 from the ir turbulent flowreactor studies. Their studies demonstrated the im-portance of the branching ratios for the initial fuelreaction to produce the three C.2H50 isomers, whichsubsequently react through different routes to formdifferent products. It was emphasized, however, thatsince these kinetic understandings were obtainedfrom flow reactor studies, some of the steps in theproposed reaction scheme need to be further testedfor different reaction conditions.Flame studies can contribute towards the com-

prehensive validation of a mechanism because of thelarge variations of temperature and species concen-trations associated with the flame structure. Suchstudies, however, are useful only if the experimen-tal data are obtained from configurations which canbe modelled with confidence. For example, it hasbeen well established that determination of thelaminar flame speed can be significantly and sys-tematically 'affected by the presence of stretch ef-fects, sIn view of the above considerations, and furtherrecognizing that ethanol flame studies are limi-ted9-1z while detailed modeling of the flame prop-agation and structure have not been conducted, thefirst objective of the present investigation is to pro-vide accurate experimental data on the laminar flamespeed by using the counterflow twin-flametechniquesA3'14 which systematically eliminates thestretch effects. These data are useful not only forthe fundamental study of ethanol kinetics, but theirquantitative values are also needed for the simu-lation of practical combustion situations.Our second objective is then to compile an ethanolkinetic mechanism and compare the calculated lam-inar flame speeds with the experimental data.

833


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834 L A M I N A R F L A M E S - - K I N E T I C S T R U C T U R E ST hr ough suc h c om pa r i sons , a nd by u s ing s e ns i t i v i t ya nd spe c i e s c onsum pt ion pa th a na ly se s , t he a c c u -r a c y o f t he m e c ha n i sm w i l l be t e s t e d a nd e nh a nc e dins igh t i n to t he c on t r o l l i ng k ine t ic p r oc e s se s w i l l bega ine d .T he th i r d ob j e c t i ve i s t o f u r the r t e s t t he m e c h -anism aga ins t da ta obta ined f rom f low reac tor s andshock tubes . These compar isons a l low the assess-m e n t o f c e rt a in a spe c t s o f t he m e c ha n i sm w h ic h a r ein se ns i t i ve t o l a m ina r f l a m e spe e d s tud i e s . T he sef u r the r c om pa r i sons i nvo lve t he t e m pe r a tu r e r a ngef rom 1000 K up to 1700 K a nd the s to i c h iom e t r yrange f rom fue l lean to fue l r ich condi t ions .In t he fo l lowing we sha l l fi r s t spec i fy the ex-pe r im e n ta l a nd num e r i c a l a spe c ts o f t he p r e se n t i n -vestigation. This will be followed by comparison andd i sc us sion o f t he r e su l t s f r om the va r ious s tud i e sinvolv ing lamin ar f lame spee ds , f low reac tor s , an dshock tubes .

E x p e r i m e n t a l M e t h o d o l o g yT he c oun te r flow tw in f l a m e t e c hn ique f o r t he de -t e r m ina t ion o f l a m ina r f l a m e spe e ds i s w e l l doc u -m e n te d . 13A 4 I t i nvo lve s t he e s t a b l i shm e n t o f tw osym metr ica l , p lanar , n ea r ly-ad iaba t ic f lames in anoz z l e - ge ne r a t e d c oun te r f low c on f igu ra t ion , a nd thesubse qu e n t de t e r m in a t ion o f t he a x i al ve loc i ty p r o -f i l e a long the c e n t e r l i ne o f t he f l ow by l a se r D op-p l e r ve loc im e t r y . T he m in im um po in t o f t he ve loc -ity profile is identif ied as a referen ce up stream f lamespe e d , S , , w hi l e t he ve loc i ty g r a d i e n t a he a d o f t h ispoin t cha rac te rizes the imp osed s t re tch ra te , K. T husby plott in g Su versus K, th e s tre tch-free f lame sp eedS~ i s ob t a ine d th r ough l i ne a r e x t r a po la t i on t o z e r os t re tch . Some recen t theo re t ica l s tudies 15'1~ haveind ic a t e d t ha t t he l i ne a r e x t r a po la t i on m a y y i e ldh ighe r va lue s f o r S~ T he se c onc e r ns a ppe a r t o beba se d on h igh e r o r d e r c ons ide r a t i ons w hose e f fe c tsse e m to be l e s s t ha n 5 t o 10% . S inc e t he r e ha s no tbe e n a ny sy s t e m a t i c e xpe r im e n ta l ve r i fi c a ti on o f t he

exten t of these e f fec ts, i t i s be l i eve d tha t the p res-e n t da t a a r e s t i l l t he m os t a c c u r a t e a nd a s such w i l lbe u se d in c om pa r i sons w i th t he num e r i c a l c a l c u -la t ions .T he f low c on t r o l o f e tha no l , w h ic h i s l i qu id un -de r s t a nda r d c ond i t ions , w a s s im i l a r t o t ha t u se d inthe p r e v ious e xpe r im e n ta l w or k on m e tha no l a ndthe de ta i l s can be foun d in Ref . 6 .

Numerical M e t h o d o l o g yN um e r i c a l s im u la ti ons o f a ll t he r e a c t ion sy s t e m s

w e r e c o n d u c t e d b y u s i n g C H E M K I N - b a s e d p r o -g r a m s . 17-21 T he the r m o dyna m ic da t a ba se de ve l -o p e d b y K e e e t a l . ~ z w a s u s e d a n d t h e t h e r m o d y -n a m i c p r o p e r ti e s o f C 2 H s O H a n d t h e t h r e e i s o m e r s

of C2H ~O were taken f rom Burca t. 23 Th e t r ans por tp r o p e r t i e s o f C z H s O H w e r e t a k en f r om M o n c h i cka nd M a son z4 a nd those o f t he t h r e e i som e r s o fC 2H ~ O w e r e c a l c u l a t e d by u s ing the a pp r oa c h o fP r a usn i t z , z 5 i n w h ic h the L e nna r d - Jone s pa r a m e -t e r s f o r t he r a d i c a l c om ple xe s w e r e e s t im a te d byaveraging two s t ruc tura l ly s imi lar molecules wi th thea pp r op r i a t e m ix ing r u l e s . F o r e xa m ple , t he s t r uc -tu r e o f C 2H 50 w a s a s sum e d to be " ' be tw e e n" thoseo f C 2H sO H a nd C 2H 5 a nd i t s po t e n t i a l w e l l de p thw a s de t e r m ine d a s :(e/Kn)c~HsO = [ E K B ) c z H s O H > ( ( e / K B ) c z l a s] 1 / 2

a nd the c o l l i s ion d i a m e te r a s :(O-)czH~o = (O'czHsOH + O'C2H s)/2.

F o r t he l a m ina r f l a m e spe e d de t e r m ina t ion , t hef r e e ly - p r opa ga t ing f l a m e ve r s ion o f t he one - d im e n-s iona l code l sA9 was u sed , and the rm al d i f fus ion wasinc lude d . F o r t he s im u la t i on o f f low r e a c to r s a nds h o c k t u b e s , t h e S E N K I N c o d e 2~ was used whichc a l c u l a t e s t he hom oge ne ous ga s pha se c he m ic a l k i -ne t ic s .

Study Based on Laminar Flame Speed ResultsT h e e x p e r i m e n t a ll y d e t e r m i n e d l a m i n ar f la m espe e ds , S~ o f e tha no l / a i r m ix tu r e s a t a tm osph e r i cp r e s s u r e a n d f o r u n b u r n e d m i x t u r e t e m p e r a t u r e s ,Tu, of 363, 428, an d 453 K ar e shown in Fig . 1 fora w ide r a nge o f s to i c h iom e t r y . T he f l a m e spe e ds a t2 9 8 K w e r e n o t m e a s u r e d b u t w e r e o b t a i n e d b ye x t r a po la t i ng t he da t a ob t a ine d a t h ighe r T u da t a t o298 K . F o r t he t e m pe r a tu r e r a nge inve s t i ga t e d , t her e su l t s show tha t S~ bas ica l ly inc reases l ine a r ly wi th

T u a t a g ive n s to i c h iom e t r y , a s e xpe c t e d .T h e m a x i m u m S~ f o r e tha no l / a i r w a s f ound tooccur a roun d ~b = i . 15 , which i s h igh er than thec o r r e spond ing va lue s o f a bou t ~b = 1 .05 f o r a lka n e /a i r f la m e s s bu t i s s im i l a r t o t he va lue f o r m e tha -no l / a i r f l am e s . 6 T he r e a son f o r suc h a d i f f e re nc ew i th t he a lka ne f l a m e s i s t he p r e se nc e o f t he e x t r aO a tom in t he e tha no l m o le c u le w h ic h p r ov ide s ana dd i t i ona l a m ou n t o f " ox id i z e r" a s t he f ue l c onc e n -t r a t i on be c om e s r i c he r .F igu r e 1 show s the num e r i c a l l y c a l c u l a t e d r e su l t sa n d t h e i r c l o s e a g r e e m e n t w i t h t h e e x p e r i m e n t a lda t a . T he k ine t i c s c he m e inc lude s 35 spe c i e s a nd196 revers ib le reac t ions for the C1, C2, CH3OH,a nd C 2H sO H subm e c ha n i sm s . I t s c om pi l a t i on w a sc onduc te d i t e r a t ive ly s inc e t he va l id i t y o f c e r t a inr e a c t ions c ou ld no t be a s se s se d by u s ing the f l a m espe e d da t a a lone . T he C j , C 2 , a nd C H 3O H sub -mech anisms, involv ing 30 spec ies and 171 revers-ib l e r e a c t ions , w e r e i n i t i a ll y c om pi l e d i n E go l fo -pou los e t a l . 6 T hi s m e c ha n i sm w a s f u r the r r e v i se d


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T o f o l 1 ~ ' 1 1 2 ' , i , ' 1 ~ l l sEquivalence Ratio, 2.0

F IG . 1 . E xp e r im e n ta l l y a nd num e r i c a l l y de t e r -m ine d l a m ina r f l a m e spe e ds , S ~ T , , ) , a t p = 1a r m , u s ing the p r e se n t k ine t i c s c he m e .

i n o r d e r t o p r o v i d e b e t t e r a g r e e m e n t s o n m e t h a -no l / a i r l a m ina r fl a m e spe e ds a nd invo lve s 31 spe -c i e s a nd 175 r e ve r s ib l e r e a c t ions . I n t he ne wC H 3O H subm e c ha n i sm low e r r a t e s f o r t he r e a c -tions CH 3 + OH~------CH30 + H 7 and HO e +H - - O H + O H z6 w e r e u sed . F u r the r m or e , s t r onge rcol l i s ion e f f ic iency for H2 0 in the H + O2 +M = H O e + M re ac ti one7 was used as we l l a s de -ta i led ICH e an d 3CHe subm echanism s, e s Suf fice tono te t ha t t h i s m e c ha n i sm ha s be e n show n to s a t -i s f ac to ri l y p r e d i c t a n um be r o f ox ida t ion p r ope r t i e so f C 1- a nd C 2 - hyd r oc ar bons a nd m e tha n o l m ix tu r esw i th oxyge n a nd ine r t . T he on ly m od i f i c at i ons o fthe s c he m e im p le m e n te d he r e in a r e r e a c t ions ( R 1)a nd ( - R 2) w h ic h w i l l be d i s c us se d l a t e r . F o r t heC z H sO H subm e c ha n i sm the l a t e s t f low r e a c to r ox -ida t ion r e su l t o f N or ton a nd D r y e r 7 was used . Fa l l -o f f c o r r e c t i ons a nd a pp r op r i a t e t h i r d body e f fi c ie n -c i e s ha ve be e n u se d f o r a l l p r e s su r e de pe nde n t r e -ac t ions .I n o r de r t o u t i l i z e t he f l a m e spe e d c om pa r i sonsf o r va lida t ion o f t he k ine t i c s c he m e , i t i s im por t a n tto i de n t i f y t he m a in spe c i e s c onsum pt ion pa th s i n

18 19t he se f l am e s a nd c onduc t s e ns i t i v i ty a na ly si s ' byde t e r m in ing the i n f lue nc e o f a l l r e a c t ion r a t e s onthe m a ss bu r n ing r a t e m ~ = p ~ , S ~ w he r e pu i s t hed e n s i t y o f t h e u n b u r n e d m i x t ur e .T he spe c i e s c onsum pt ion pa th s f o r s t o i c h iom e t r i c

e tha no l / a i r f l a m e i s show n in F ig . 2 . T he se pa th swere de te rm ined b y in tegra t ing a l l r eac tions throug hthe f l a m e a nd de t e r m in ing the f r a ct i on o f e a c h spe -c ies consumed by a spec i f ic r eac t ion; th is f rac t ionis indica ted next to each species. F rom such a s tudythe f o l low ing obse r va t ions ca n be m a de :( 1 ) C z H sO H : I t i s m a in ly a t t a c ke d by H , O , a ndO H w h ic h a bs t r a c t one H a nd c r e a t e the t h r e eC z H 50 i som e r s . T he p r e se n c e o f t he O a tom in t hem ole c u le c l e a r ly a l t e r s t he i n i t i a l p r oduc t s a ndthe r e by the subse que n t k ine t i c s t e ps a s c om pa r e dto t hose i nvo lv ing the c o r r e spond ing a lka ne, C 2H 6 .S ign i f i c a n t d i f f e r e nc e s a r e a l so obse r ve d be tw e e nC z H 5O H a nd C H 3O H in t ha t the m a in i n i ti a l p r od -uc t s a re C H z O H a nd C H 30 f or m e tha no l w h il e t he ya r e no t a s im por t a n t i n e tha no l ox ida t ion . F o r f ue lr i c h c onc e n t r a t i ons i t i s f ound tha t t he r e a c t ion o ff ue l w i th H inc r e a se s due t o r e duc e d c onc e n t r a -t i ons o f O H a nd O .(2 ) C H 3C H O H : T h i s is t he m a jo r p r odu c t o f t heinit ia l fuel reactions. Fo r a ll stoichiom etr ies i t reactson ly w i th O z to p r oduc e a c e t a lde hyde ( C H 3H C O )a nd H O e . T he r e f o r e , s im i l a r t o m e tha n o l ox ida tion ,a l a r ge a m oun t o f H O e i s p r oduc e d du r ing e tha no lox ida t ion by m e a ns o the r t ha n the t e r m ina t ion r e -ac t ion H + 02 + M ~ HO e + M. As such , theH O e c he m is t r y i s e xpe c t e d t o be im por t a n t . F u r -the r m o r e , t he subs t a n t ia l p r oduc t ion o f C H 3H C Oc ons t i t u t e s a po t e n t i a l e m i s s ion p r ob le m , s im i l a r t othe t ha t o f f o r m a lde hy de in m e tha no l ox ida tion .C H 3 H C O i s su b s e q u e n t l y c o m p l e t e l y c o n v e r t e d t othe methyl r ad ica l (CH3) .(3 ) C e H 4O H : I t t he r m a l ly de c om pose s t o C 2H 4w hic h i s t he p r im a r y p r oduc t ion s t e p o f e the ne ine tha no l ox ida t ion . S ubse que n t r e a c t ions o f e the nelead to 60% convers ion to C2 spec ies through Ha nd O H r e a c t ions a nd 40% c onve r s ion to C H 3th r ough O a nd O H r e a c tions . T he p r oduc t ion o f C espe c i e s be c om e s a bou t 85% f o r r i c h f l a m e s due t othe a bun da nc e o f H r a d i ca l s a nd the p r oduc t ion o fC H 3 be c o m e s 65% f o r l e a n fl a m e s due t o t he a bun -dance o f O radica ls .(4) CH 3CH eO: I t ex is ts in sma l le r conce ntra t ions asc om pa r e d to t he o the r i som e r s . I t t he r m a l ly de -c om pose s t o C H 3 w i th t he s im u l t a ne ous p r oduc t iono f C H e O w h ic h , how e ve r , e x i s ts i n m uc h low e rc onc e n t r a t i on l e ve l s a s c om pa r e d to t hose i n m e th -anol f lames .(5) CH3: I t i s the major prod uc t f rom a l l thre eC 2H 50 i som e r r e a c t ion c ha nne l s a nd i t s subse qu e n tr e a c t ions a r e e xpe c t e d t o be im por t a n t i n e tha no loxida t ion . This i s s imi la r to m ethan e oxida t ion inw h ic h C H 3 is p r odu c e d d i r e c t l y fr om th e f ue l. T hemain d i f fe rence f rom the methane mechanism is tha ts inc e C H 3 a nd H O e bo th e x i s t i n l a r ge c onc e n t r a -t i ons i n e tha no l ox ida t ion , t he i n t e r a c t i on o f t he setw o r a d i c a l s i s e xpe c t e d t o be im por t a n t . I t m a ya l so be no t e d t ha t r e a c t ions be tw e e n C H 3 a nd H O e


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K and p = 1 a tm.

a r c o f i n t e r e s t i n t he h igh p r e s su r e c om bu s t ion o fm e tha ne a nd e tha ne , a l t hough r e pu r t e d r a t e s ha vehigh uncer ta in ty .The f i r st orde r norm al ized sens i t iv i ty coeff i-c ients lS .t9 o f se lec ted reac t ions on the mass burn ing

rate, m", of lean, stoichiometric, and rich flames areshown in F ig . 3 . S im i la r to the hvdrocarb on andm e tha no l f la m e s , t he m a in b r a nc h ing re a c t ion H +O2 -- -> OH + O and the C O o xida t ion reac t ion CO+ OH ~ CO2 + H have dom inant e ffect on m" .F u r the r m or e , s im i l a r t o t he m e tha ne f l a m e s a bove5 a tm , a nd m e tha no l f l a m e s a t a tm osphe r i c p r e s -su r e , t he H O z c he m is t r y a ppe a r s t o be im por t a n t ,wi th n ot iceable sens i t iv i ty for the C I t3 + HO2 ---->C I | 3 0 + O H r e a c t i o n . T h e o n l y e t h a n o l - s p e c i f i cr e a c tion a ppe a ri l ag t o be im por t a n t i s C 2H sO H +O il - -- ) C21t401t + H2 0, which favors prop aga t ionof lean and s to ich iom etr ic f lames but r e ta rds thepropag a t ion of rich f lames . The reason for such aqua l i ta t ive d i f fe rence i s tha t C2H4OH i s subse -que n t ly c onve r t e d t o C z H 4 w hic h i s c onsum e d byO r a d i c a l s , i n a dd i t i on t o t t a nd O H , a nd O r a d -icals ex is t in ve ry smal l quan t i t ie s for fue l r ich m ix-tu r e s. The m a in in t e r m e d ia t e s C H 3 H C O a nd

C H 3C H z O o f t he o the r tw o poss ib l e pa th s o f t hein i ti a l f ue l c onsum pt ion r e a c tions do no t r e qu i r e Ofor the i r oxida t ion . Th e sen s i t iv i t ie s of HC O + M---) H + C O + M a n d H C O + O 2 ~ C O + H O 2arc s imi la r to those in meth anol f lames . Refe renc e6 g ive s a de t a i l e d e xp la na t ion o f t he r u l e o f t he sereac t ions and the i r sens i t iv i ty s ign reversa l a s s to i -c h iom e t r y c ha nge s .

Study Based on Flow Reactor Resu l tsTur bu le n t f low r e a c to r s t yp i c a l l y ope r a t e i n t e m -pe r a tu r e r a nge s o f a bou t 1000 to 12 00 K , a nd thesystem is near-adiab atic before an y significant amo unto f he a t i s r e l e a se d . I n t he p r e se n t i nve s t i ga ti on t heda t a o f N or ton a nd D r ye rs ' r unde r ox ida t ion c on -d i t i ons w e r e s im u la t e d by u s ing S E N K I N 24 f o r ac ons t a n t p r e s su r e , a d i a ba t i c sy s t e m w h ic h c lo se lyresembles the exp er imenta l configura t ion . W hen the

o r ig ina l ( ;1 , C 2 , a nd C H 3 O H subm e c ha n i sm s w e r euse d in t he s im u la t i ons , t he p r e d i c t e d e tha no l p r o -f i l e w a s t oo " s t e e p" w h i l e t he t e m pe r a tu r e a l so i n -c r e a se s t oo f a s t a s c om pa r e d to t he e xpe r im e n ta l


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STUDY ON ETHANOL OXIDATION KINETICS 837] O 2 + H = O H + O H f " ~ 1 .2l lI~H+ HO 2 = H2 + O2 [ C 2 HsOH/ a ir / I- - ' 1 / p = l a tm / [ 1.oI l l l l ~ H+O2+M=HO2+M [ Tu=36 3K / Il l ~ H+OH+M=H20+M " - - " lH + O 2 _ _ _ O H ~ l ~ I ~ 0.8

. ~ C2H3+H=C2H2+H2 I '~'~ 0.6CH3+HO2=CH30+OH I -~ 0.4

co+oHio2 ~ I 0 ~HCO+O2=CO+HO I * = 1.8 IHCO+M=H+CO+M ll ~ = 1.0 ]C2H5OH+OH H4OH+H20 9 l~ = 0.6 0.02HsOH+OH H4OH+H20 9 I~ = 0.6 1.2

-0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 1.0Normalized S e n s i t iv i t y o n M a s s B u r n i n g R a t er 0.8O*~0.6-~o.4

FIG. 3. Normalized first order sensit ivity coeffi-cients of the most important reactions on the massburning rate of lean, stoichiometric, and richethanol/air flames at T, = 363 K and p = 1 atm.

data. This behavior was observed for both lean (~b= 0.81) and rich (~b = 1.24) conditions.In order to resolve this disagreement, the sen-sitivities of the reactions involving CzHsOH,CeH4OH, CH3CHOH, CH3CHeO, CH3HCO,C2H4, C2H3, CH4, CH3, CHeO, HCO, HOz, andHeOe to the fuel decay profile were examined indetail. These species were chosen because they arethe important intermediates subsequent to fuel de-cay. It was consequently found that the reactionshaving the greatest effect on the slope of the ethanolprofile were those between CH3 and HO2:

CH3 + HOe ~ CH30 + OH (R1)CH3 + HOe--> CH4 + O2 (-R2)

This result is not surprising because CH3 and HOeare key radicals in ethanol oxidation with CH3 beingthe main product of all reaction channels. Morespecifically, it was found that a slower rate for (R1)and a faster rate for (-R2) would lead to milderethanol profiles because (R1) produces less stablespecies as compared to (-R2). In the initial scheme 6the rates of (R1) and (R2) were those recommended2 9by Tsaug and Hampson with uncertainty factorsof 3 and 5 respectively, as well as the rate of Nor-ton and Dryer ~ for (R1). Consequently we adopted3 0the rate of Dagaut et al. for (R1) which is 6 and3 times slower at 1000 K as compared to the ratesof Tsang and Hampsone9 and Norton and Dryer7respectively. Furthermore, we also used a rate whichis 5 times faster for (R2) than the rate of Tsang andHampson. Specifically, these reaction rates are ex-pressed as:CH3 + HOe--> CH30 + OH

[k = 4.00E13 T~176 (R1)CH4 + O2---> CH3 + HO2

[k = 20.3E13 T ~ 1 7 6 (a2)

0.2

f~ ! | A Experunentely Norton ndDryer/. .~.~.. NumericalwithModifiedRates or- - CH3+HO2__>products! NuraeticalwithLiterature ates forC H _ 3 _ 3 + H ~ - ~ l ~ r o d u c t s

\ N ~ / -FlowReactor-Oxidation'~. ; , , ~ 4 l C2H5O HProfi le /" ' - . . ' ~ . . . " , L ~=0.81 ]

['FlowReactor-Oxidation)\ ' , . ~ . / C 2 H 'O HPr~ /

20 40 60 80 100Time, msecFIG. 4. Comparison between the experimental(symbols) profiles for C2HsOH at various q~'s as de-termined by Norton and Dryer7 in a flow reactor,and numerical calculations (lines) using the present

scheme with the rates for reactions CH3 + HO2 "--->Products taken from Tsang and Hampson29 as wellas the present scheme with the modified rates.

where k is the specific reaction rate, T the tem-perature, R the universal gas constant, and the unitsare moles, cubic centimeters, seconds, Ke|vins, andcalories/mole. Results of the calculations based onboth the original and modified mechanisms areshown in Fig. 4 together with the experimental data.Improved agreement is obtained for both lean andrich conditions. Variation of the branching ratioleading to the three C2H~O isomers results in dif-ferent induction times in some cases, but the slopeof the ethanol profile is influenced minimally. Theinitial temperature of the mixture was also reducedby 10 K, which is the experimental uncertainty, andno significant change in the ethanol profile wasfound.In Fig. 5 we compare the experimental data of5 7Norton and Dryer 9 for lean conditions (~b = 0.81)with the calculated results obtained by using themodified scheme. Due to the uncer tainty in the ex-perimental induction time, the numerical results hadto be "time shifted "'2'3'6 in order to match the ex-perimental data at the 50% fuel decay point. Theamount of shift is reported in the figure caption.The results show that there is good agreement for


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838 LAMINAR FLAMES--KINETIC STRUCTURES1 .6 [ f ~ . . . . . . . . . . . . C O 2 ~1 .4 - - - I 1 C O - - - O H

~ I l . . . . . 5 O H x 2 " '" " " T , I . . ..1.2 k / ........ 9 M~o~0s J .:'_':'-'-'-'-u" -~ ~ . - ' ? " A A A0 / / A. ~ ~ 9 9 ' ,. 9 - -

o l. ..........."6" 4,0. 2 O 9 9 9

1 4 0 01 3 5 01 3 0 01 2 5 01201) ~I 1 5 0 [ ~

1 0 5 0

- 1 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0T i m e , m s e c

FIG. 5. Comparison between experimental (sym-bols) flow reactor oxidation data for ~b = 0.81 asdetermined by Norton and Dryer5'7 and numericalcalculations (lines) using the present scheme. Thenumerical results have been "shifted" by - 9 msec.(0.588% CzHsOH, 2.185% O~, 97.227% N2, T~, =1090 K, p~, = 1.0 atm, adiabatic, isobaric; all con-centrations on per mole basis)

most of the species until the fuel is consumed andbefore the CO oxidation is initiated. In this regimeheat loss to the wall (maintained at -1000 K) an d/or molecular diffusion become significant and theadiabatic, homogeneous model is probably not a goodrepresentation of the experimental conditions andthus discrepancies in the CO oxidation region im-mediately following the fuel consumption zone werenot further assessed. The calculated profiles for theminor species such as CgH4, CH4, CgH0, and CgH2agree reasonably with the experimental data. Thecalculated profiles for CHaHCO, while agreeingclosely with the experiments during the productionstage, the subsequent consumption rates are foundto be higher than the experimental ones. Similarresults for the major and minor species are ob-tained for the rich conditions (ok = 1.24).The modification in the rate of the reaction CH3+ HOg ~ Products has a small effect (0.5-1 .0cm/s) on the calculated laminar flame speeds.

Study Based on Shock Tube ResultsWhile reactor-type experiments provide infor-mation for the low- and intermediate-temperatureregimes, shock tube studies operate at the highertemperature range of 1300 K to 2500 K and therebycomplete the temperature range of relevance incombustion.We have thus tested the present ethanol mech-anism against the high temperature ignition delay

data of Natarajan and Bhaskaran l and Cooke e t a l . 32Since in both investigations the measurements wereconducted behind reflected shock waves, this con-figuration was numerically simulated as an adi-

~40o

*i00I S h o c k T u b e - O x i d a t i o n( N a t a ra j a n a n d B h a s k a r a n ) )

"r'

~ o1 2 o o

f n n nu

a an Experim ental o a u n- - N u m e r i c a l a t T [ ] n a o [ ]

1 3 0 0 1 4 0 0 1 5 0 0 1 6 0 0 1 7 0 0InitialMixture Temperature(Ti), KFic. 6. Comparison between experimental (sym-bols) shock tube ignition delay data as determined

by Natarajan and Bhaskaran 31 and numerical cal-culations (lines) using the present scheme. (2.5%C2HsOH, 7.5% 02, 90.0% Ar, P,n = 1.0 atm, adi-abatic, isochoric; all concentrations on per mole ba-sis)

abatic, constant volume system, which allows forboth temperature and pressure increases during thecourse of the reaction.In Fig. 6 the experimental data of Natarajan andBhaskaran31 for a stoichiometric mixture of C2H3OH/O2/Ar at 1 atm are compared with the numericalcalculations. The experimental data were deter-mined by monitoring the OH concentration and ig-nition was assumed to occur when the OH concen-tration reached 2 10 -9 mole/cc, The comparisonin Fig. 6 can be considered to be quite favorable,especially in view of the +-50 K uncertainty in thetemperature determination33'34 in shock tube ex-periments. Similar agreements were obtained bymodeling mixtures at different stoichiometries andpressures. It may be noted that previous modelingefforts31'35'36 resulted in significant disagreementswith the data of Natarajan and Bhaskaran, 31 evenallowing for the uncertainty in the temperature. Theexperimental data of Cooke e t a l . 32 were also com-pared favorably with the numerical calculations.Sensitivity analysis showed that among the fuelconsumption reactions the ignition delay time ismostly sensitive to the decomposition reactionC2HsOH ---> CH3 + CH2OH. Noticeable sensitiv-ity was also found for reactions (R1) and (-R2).

Concluding RemarksIn the present investigation, we first determined

the laminar flame speeds of atmospheric ethanol/air mixtures by using the counterflow technique.These data were obtained for various mixture initialtemperatures and stoichiometries ranging from very


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S T U D Y O N E T H A N O L O X I D A T I O N K I N E T I C S 839l e a n t o v e r y r i c h , a n d c a n b e u s e d w i t h r e a s o n a b l ec o n f i d e n c e f o r t h e s t u d y o f e t h a n o l k i n e t i c s a s w e l la s fo r p rac t i c a l app l i ca t i ons . An e thano l ox ida t i onk i n e t i c s c h e m e w a s t h e n c o m p i l e d b y s i m u l t a -neous ly t e s t i ng i t aga ins t t he expe r imen ta l da t a f romf l ame speed mea suremen t s , f l ow reac to r s , and shockt u b e s .M o d e l i n g o f t h e v a r i o u s e x p e r i m e n t a l c o n f i g u r a-t i o n s p r o v i d e d c o m p l e m e n t a r y i n f o r m a t i o n o n t h ek i n e t i c s , w h i c h a r e e s s e n t i a l f o r t h e c o m p r e h e n s i v ev a l i d a t io n o f t h e m e c h a n i s m . T h e c o m p a r i s o n i n -d i c a t e s a s l o w e r r a t e fo r t h e r e a c t i o n C H 3 + H O zC H 3 0 + O H a n d a f a s t er r a t e f o r t h e r e a c ti o nC H 3 + H O 2 -- -) C H 4 + O z t h a n t h o s e f r e q u e n t l yused i n r ecen t l i t e r a tu re . I t i s c l e a r t ha t such ano b s e r v a t i o n , w h i l e s t i l l t e n t a t i v e , c o u l d n o t b e o b -t a i n e d f r o m s t u d i e s o n l a m i n a r f l a m e s s p e e d s a l o n e .F u r t h e r m o r e , b y u s i n g t h e p r e s e n t k i n e t i c s c h e m e ,sens i t i v i t i e s and spec i e s consumpt ion pa ths i n e thano lf l a m e s w e r e i d e n t i f i e d a n d t h e i m p o r t a n c e o f t h eC H a - c h e m i s t r y d e m o n s t r a t e d . T h e s h o c k t u b e c a l-cu l a t i ons a l so y i e lded sa t i s f ac to ry compar i sons .I t mus t be emphas i zed t ha t c au t i on i s s t i l l neededw h e n a p p l y i n g t h e p r e s e n t s c h e m e t o b u r n i n g a n dr e a c t i n g c o n d i t i o n s w h i c h d e v i a t e s u b s t a n t i a ll y f r o mt h e r a n g e o f p a r a m e t e r s a n d c o n d i t i o n s i n v e s t i g a t e dh e r e i n . F u r t h e r r e s e a r c h i s a l so n e e d e d i n t h e v a r -i o u s k i n e t i c a s p e c t s i n c l u d i n g b r a n c h i n g r a t i o s a n dpyro lys i s s t eps .

AcknowledgmentsT h i s r e s e a r c h w a s s u p p o r t e d i n p a r t b y t h e A r m yR e s e a r c h O f f i c e a n d t h e N a t i o n a l S c i e n c e F o u n d a -

t i o n . I t i s a p l e a s u r e t o a c k n o w l e d g e t h e h e l p f u ld i s c u s s i o n s w i t h a n d c o n s t r u c t i v e c o m m e n t s b y D r .K . B r e z i n s k y , P r o f e ss o r F . L . D r y e r , P r o f e s s o r W .C . G a r d i n e r , a n d D r . T . S . N o r t o n . W e a ls o w i s hto spec i a l l y t hank P ro fe sso r F . L . D rye r fo r an ad -v a n c e c o p y o f R e f . 7 a n d D r s . H - H . G r o t h e e r a n dT h . J u s t f o r t h e i r c o m m e n t s o n t h e k i n e t i c s c h e m e .

R E F E R E N C E S1. WESTBROOK, C. K. AND DRYER , F. L.: C om -

bust . Fl am e 37, 171 (1980) .2 . NORTON , T . S . AND DRYER, F . L . : Combus t . Sc i .

Tech. 63, 107 (1989) .3 . NORTON, T. S. AND DRYER, F. L . : Int . J . C he m .Kinet . 22, 219 (1990) .4 . N O RT ON , T . S . : T h e C o m b u s t i o n C h e m i s t r y o fS i m p l e A l c o h o l F u e l s , P h . D . T h e s i s , P r i n c e t o nUnive r s i t y , Pr ince ton , N. J . (1990) .

5 . N O R T O N , T . S . AND DRYER, F . L . : T w e n t y - T h i r dS y m p o s i u m ( I n t e r n a t i o n a l ) o n C o m b u s t i o n , p .179 , The Combus t ion Ins t i t u t e , 1991 .

6. EGOLFOPOULOS,F. N. , DU , D . X . AND LAW, C.K. : Combus t . Sc i . Tech . 83 , 33 (1992) .

7 . N O R T O N , T . S . AND DRYER, F . L . : A n E x p e r i -m e n t a l a n d M o d e l i n g S t u d y o f E t h a n o l O x i -da t i on Kine t i c s i n an Atmosphe r i c Pre ssu re F lowR e a c t o r , t o a p p e a r i n I n t. J . C h e m . K i n e t .8 . L A W , C . K . : T w e n t y - S e c o n d S y m p o s i u m ( In -t e r n a ti o n a l ) on C o m b u s t i o n , p . 1 38 1, T h e C o m -bus t i on Ins t i t u t e , 1989 .

9. SMITH,S. R . AND GORDON, A. S.: J. Phys. C he m .60, 1059 (1956).10. LIEB , D. F. AND ROBLEE, L. H. S. JR. : Corn -bust . Flame 14, 285 (1970) .

11. PANDYA,T. P. AND SRIVASTAVA,N . K . : C o m b u s t .Sci. Tech. 11, 165 (1975).12 . G(3LDER, (~. L . : Nin e t ee n th Sym pos iu m ( In t e r -n a t i o n a l ) o n C o m b u s t i o n , p . 2 7 5 , T h e C o m -bus t i on Ins t i t u t e , 1982 .

13. EGOLFOPOULOS, F. N . , CH o, P. AND LAW, C.K. : Combus t . F l ame 76 , 375 (1989) .14. Z H U , D . L., ECOLFOPOULOS, F. N. aND LAW,C . K . : T w e n t y - S e c o n d S y m p o s i u m ( I n t e rn a -t i ona l) on Com bus t ion , p . 1537 , The Co mb us-t i on Ins t i t u t e , 1989 .

15 . DIXON-LEW IS, G. : Tw en ty -Th i rd Sym pos ium(In t e rna t i ona l ) on Co mb us t ion , p . 305 , Th eC o m b u s t i o n I n s t i t u t e , 1 9 9 1 .

16. TIEN, J . H. AND MATALON, M .: C om bust . Fla me84, 238 (1991).

17. K EE , R. J. , WARNATZ, J. AND MILLER, J. A.: AF O R T R A N C o m p u t e r C o d e P a c k ag e f o r t h eE v a l u a t i o n o f G a s - P h a se V i s c o s it ie s , C o n d u c t i v -i t i e s , and Di f fus ion Coe f f i c i en t s . Sand ia Repor tSAND83-8209 , 1983 .

18. K EE , R. J. , GRCAR, J. F ., SMOOKE, M. D . ANDMILLER, J . A . : A For t r an Prog ram fo r Mo de l ingS t e a d y L a m i n a r O n e - D i m e n s i o n a l P r e m i x e dF l a m e s . S a n d i a R e p o r t S A N D 8 5 - 8 2 4 0 , 1 98 5.

19. GREAR, J. F ., K EE , R. J. , SMOOr(E, M. D. ANDMILLER, J . A . : Tw en ty -Fi r s t Sy mp os ium ( In t e r -na t i ona l ) on Combus t ion , p . 1773 , The Com-bus t i on Ins t i t u t e , 1986 .

20. LUTZ, A. E. , K EE , R. J . AND MILLER, J. A.:S E N K I N : A F o r t r a n P r o g r a m f o r P r e d i c t i n gHomogeneous Gas Phase Chemica l Kine t i c s wi thS e n s i t i v i t y A n a l ys i s. S a n d i a R e p o r t S A N D 8 7 -8248, 1987.

21. K EE , R. J . , RUPLEY, F. M. AND MILLER, J. A.:C h e m k i n - I I : A F o r t r a n C h e m i c a l K i n e t i c sP a c k a g e f o r t h e A n a l y s is o f G a s - P h a s e C h e m i c a lKine t i c s . Sand ia Repor t SAND89-8009 , 1989 .

22. K EE , R. J . , RUPLEY, F. M. AND MILLER, J. A .:T h e C H E M K I N T h e r m o d y n a m i c D a t a B a s e .S a n d i a R e p o r t S A N D 8 7 - 8 2 1 5 , 1 9 8 7 .

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840 LAMINAR FLA MES --K INET IC STRUCTURESof Fluid Phase Equilibria, Prentice-Hall, NJ,1969.26. KEYSER, F. L.: J. Phys. Chem. 90, 2994 (1986).

27. BAULCH, D. L. , DRYSDALE, D. D. , HORNE, D.G . AND LLOYD, A . C . : Evaluated Kinetic Datafor High Temperature Reactions, vol. 1 and 2,Butterworths, London (1973).

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K . M . AND NATAaAJAN, K.: 1st Specia li sts'Meeting (International), Vol. 2, p. 278, TheCombustion Institute, 1981.

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COMMENTSR. Ramaprabhu, Anna University, India. In your

paper, as I see, you have drawn on results fromboth shock tube and flow reactor experiments, be-sides your own experime nts, to validate your m ech-anism for ethanol oxidation. I want to know whyyou have not thought of arriving at global or quasiglobal schemes, and validate the same against yourexperimental results. This would help practical de-signers to arrive at heat release much more easilyand confidentally. Besides, it would have been bet-ter to study flame structures for these type of fuelsto gain more understanding.

Author's Reply. It is not the objective of thepres ent study to propose any global and semi-globalschemes. We feel that since the detailed mecha-nism of ethanol oxidation has yet to be established,it is premature to attempt for simplified schemes,even if such attempts are themselves meaningful.

A meaningful flame structure study would needfor comparison and guidance a comprehensive setof experimental data, which however, does not ex-ist.

P. Van Tiggelen, Universite Catholique de Lou-vain, Belgium. Although the flame burning velocityis not very sensitive to the detailed mechanism, itshould be pointed out that flame structure studiescan cast some light on specific elementary stepsprovided that the experimental conditions are cho-sen accordingly (equivalence ratio, fuel, and so on).

Author's Reply. We completely agree with theusefulness and essential importance of flame struc-ture studies.

Horst-Henning Grotheer, DLR Stuttgart, Ger-many. With regard to the kinetic data a lot of un-knowns remain. For the reactions of primary ethanolattack, branching fractions into the three isomericradicals have never been measured directly and eventhe total rate coefficients are not precisely enoughknown, in particular for CzH~OH + H. F or theradicals produced and their reactions, rate coeffi-cients are even more unclear. For instance,CH3CH OH + M, or C2H4OH + 02 have not beenmeasured directly.

How can you know that CH3CHOH reacts exclu-sively with O3 and that C~H4OH or C2H~O are onlylost by their decomposition?

Author's Reply. In the paper we have mentionedthe need for further research on the branching ra-tios and pyrolysis kinetics, as you have also em-phasized. We have also cautioned on the extent ofpotential applicability of the present kinetic model.More fundamental kinetic studies are obviouslyneeded, and a study of the present nature can pro-vide useful guidelines for such further investiga-tions.

Although space limitations does not permit us toprovide a complete listing of the rate coefficients,the pape r is self-contained in that all thes e infor-mation are either stated or referenced.

Michael Tanoff, Chalmers University of Technol-ogy, Sweden. The Combustion Chemistry Group atChalmers has been probing the structure of low-pressure, laminar premixed, fiat ethanol/oxygen/argon flames, using continuous microprobe sam-piing mass spectrometry. We have observed dis-crepancies, identical to those that you report from


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STUDY ON ETHA NOL OXIDATI ON KINETICS 841your flow reactor studies, between measured andcomputed (using detailed chemical mechanisms ap-plied to fiat flame burner simulations) ethanol con-sumption rates. Specifically, the modeling predictsa faster rate of ethanol consumption than observedin the flame reactor. Although the disagreement maybe expla ined, partially, by uncertain ties in the flame'stemperature profile, we are anxious to include yoursuggestions for the CH3 + HOz rates, and to ob-serve their effect on the computed ethanol concen-tration profiles.

D a v i d S m i t h , B r i t is h G a s p l c , U . K . Various peo-ple have speculated that your burning velocities maybe high, possibly by 10%. This would affect the

pre sen t data and all your earlier results. If correct,what are the implications for modelling?

A u t h o r ' s R e p l y . We are well aware of some re-cent theoretical studies which indicate that thecounterflow technique could yield 5-10% overes-timates of the laminar burning velocity. In fact, wehave specifically mentioned this point in the paper,and have been trying to verify it experimentally.On the other hand, one should recognize that a 5-10% inaccuracy amounts to only a few cm/s in themeasured burning velocity, which is frequentlywithin the experimental uncertainty anyway. Assuch, it is not appropriate to attempt to extract ki-netic information based on differences which arewithin the range of these inaccuracies and/or un-certainties. We have followed this guideline in ourstudies.

Documents

0784(06)80101-3] F.N. Egolfopoulos; D.X. Du; C.K. Law -- A Study on Ethanol Oxidation Kinetics in Laminar Premixed Flames, Flow Reactors, And Shock Tubes