The present paper represents the first effort to date in which a combined experimental and theoretical approach has been used to study the effects of several inhibitors on hydrocarbon-air flames. This work is part of an attempt to build a consistent picture of chemical kinetic flame inhibition, beginning with a simple halogen molecule such as HBr and progressing sequentially towards more complex and more practical inhibitors such as CF/sub 3/Br. Inhibition efficiency can be defined as the rate of flame speed reduction, the amount of flame speed change per unit inhibitor added. Both the numerical model and the flame tube measurements found that the inhibition efficiency gradually decreases as the amount of inhibitor is increased. The present experimental and modeling results are shown, together with earlier data for CF/sub 3/Br-CH/sub 4/-air and CF/sub 3/Br-C/sub 3/H/sub 8/-air as well as HBr-CH/sub 4/-air, CH/sub 3/Br-CH/sub 4/-air and CF/sub 3/Br-CH/sub 4/-air. In the numerical study it was found that a stoichiometric methane-air mixture with up to 8% methyl bromide could support a flame, propagating at a speed of about 5 cm/sec, even though the addition of the first 1% of CH/sub 3/Br had reduced the flame speed from 38 cm/sec to about 26 cm/sec. Extensions of the model to include CF/sub 3/Br are currently under development. The available experimental data suggest that CF/sub 3/Br is somewhat more efficient as an inhibitor than HBr or CH/sub 3/Br

Alvares, N. J.

Beason, D. G.

Westbrook, C. K.

UNT Digital Library

CTRL- 85409 PREPRINT * » An Experimental and Theoretical Study of Flame Inh'bition by Bromine-Containing Compounds Charles K. Westnnvk Donald C. Boas or Norman J . Alvarez This paper was prepared fi>r Mibmii l . i l i r F i m Internat ional Spci- i n 1 i«t ^ Meeting of the Combustion I n i t i t u t o 2P-25 J i i lv 198 I Bordeaux, Franrr January 20, |»8o T»il is i pntriul of i Mptr nhnM fi» naikHiM in • jtwul « Mwwtln> Sine* dumps nil) br rr-^t btfarc niMialbii. tiK •rrsrinl n mtt itiiltbk »lth itir «• dtrstlMlHR Alt it Mill not bt cited or rrf roAmd without thr permiwinn of Ihr author. AN EXPERIMENTAL AND THEORETICAL STUDY OF FLAME INHIBITION BY BROMINE-CONTAINING COMPOUNDS Charles K. Westbrook Donald G, Beason Norman J. Alvares Lawrence Livertnore National Laboratory Livermore, California 94550 Introduction Flame inhibition by halogenated compounds is an important problem having great practical and theoretical interest. Experimentally, inhibition and retardance of flames has been observed for many years, and the properties of many different inhibitors have been catalogued. With halogenated species, it is well known that bromine and iodine are substantially more effective inhibitors than are chlorine or fluorine. In addition, the rate of inhibition is roughly proportional to the number of halogen jtoms present, so that CHBr,, C^-Br*. and CH,Br reduce the 'lame speed in methane-air approximately in the ratio 3:2:1 [1]. Relatively little detailed kinetic modeling work on flame inhibition has been carried out. Dixon-Lewis and co-workers [2-4] studied H--air flames inhibited by HBr, and Westbrook has examined methane-air and methanol-air flames inhibited by HBr [5] and by CH,Br [6]. The present paper represents the first effort to date in which a combined experimental and theoretical approach has been used to study the effects of several inhibitors on hydrocarbon-air f 1 a.nes. This work is part of an attempt to build a consistent picture of chemical kinetic flame inhibition, beginning with a simple halogen molecule such as HBr and progressing sequentially towards more complex and more practical inhibitors such as CF 38r. Measurement technique and instrumentation Acoustically stabilized flat propagating flames are used to measure laminar flame speed in premixed fuel-air gases with and without halogenated gas phase inhibitors. The technique [7] involves measuring the downward propagation iate of a flame in a quartz tube with a polaroid camera, shuttered by a 300 rpm sectored disc. Flat flame fronts result when a flame-driven resonance is established between the bottom of the tube and a screened orifice at an empirically determined location above the igniting spark gap [8], Combustible gas mixtures are prepared, assuming that all mixture gases are in temperature equilibrium. Thus the ratio of partial pressure of each gas to the pressure of the total mixture is also the ratio of its volume to the volume of the total mixture. Mixing is accomplished by successively adding the desired gases to an insulated mixture tank until its r Y - U , i. Figure 1 absolute pressure increases to a final mixture pressure of nearly 2 atm. We insulate the mixture tank to ensure its temperature remains constant during the mixing process. Before addinr ?ach gas, we evacuate and purge the manifold and lines leading to the mixture tank. Once each gas is added to the tank, we assure mixture uniformity by repeatedly inverting the tank and by allowing sufficient time before testing for thorough mixing by diffusion. After the mixture is tstabHshed in the mixture tank, we introduce it (via the manifold) into the evacuated flame tube. When the manometer indicates atmos­pheric pressure in the flame tube, we activate the diagnostic circuits, remove the top cap of the flame tube and turn on the spark source. The test apparatus is shown schematically in Figure 1. In the present series of experiments methane-air mixtures were studied, with HBr and CH,Br inhibitors in varying amounts. Results with CF,Br inhibitor in the same apparatus have been reported previously [8], Numerical flame modeling Tho numerical model used in this work was developed by Lund [S] and has been used in previous studies of flime propa­gation and Inhibition [5,6,10,11], The equations of conserva­tion of mass, momentum, and energy are solved simultaneously with the chemical species conservation equations including the kinetics terms, in a planar one-dimensional configuration. All transport coefficients and thermodynamic data nave been validated in earlier studies and are used here without modification. The reaction mechanism combines an extensive model for the oxidation of simple hydrocarbons (CH,, C O H , C.,H., C.H,) with a mechanism for the reactions of species contaVning Br atoms. The specific reactions and their rates are not repro­duced here due to limitations in space but are reviewed in detail in the references [5,10]. Although other reactions of Br, HBr, and B r 2 are included, the most important reactions involving these species are: 3. H + HBr = H E + Br H + Br2 = HBr + Br r + Br + M = Br2 + M H + Br + M = HBr + M (1) (2) (3) (4) Species such as BrO and BrOH are not included. As a first approximation it is assumed that the principal reactions of CH 3Br involve abstraction of the Br atom only, through CH 3Br + H = HBr + C H 3 (5) CH 3Br + Br = B r 2 > C H 3 (6) followed by conventional reactions of the methyl radicals. Reactions abstracting H atoms from methyl bromide and subse­quent reactions involving CH-Br, CHBr and CBr are not considered in this mechanism, This mechanism, combined with the hydrocarbon oxidation mechanism, has been shown L 5.6] to reproduce available flame propagation and inhibition data with HBr and CH 3Br inhibitors. Future additions will include elementary reactions dealing with CF^Br and its intermediates. Discussion The general picture of kinetic flame inhibition which is predicted by the numerical model consists of a catalyzed recombination of hydrogen atoms, removing them from the induc­tion and reaction zones of the flame. Hydrogen atoms removed in this manner are then unavailable for the key chain branching reaction H + D ? = 0 + OH , leading to a reduced rate of fuel consunption, heat release, and flame propagation. The H atom recombination is catalyzed by Br through reactions 1-4. Methyl bromide acts primarily as a source of Br atoms which then inhibit the flame through the same HBr mechanism. HBr and CH 3Br do not act in exactly the same way; the C-Br and H-Br bond energies are different, -JO the temperature Dependence of reactions 1, 2, 5, and 6 a're not the same. Furthermore, the addition of CH,Br to a given methano-air mixture produces a slight shift in the overall fuel-air equivalence ratio in the rich direction which can also affect the flame speed. Still, computed flame speeds with HBr inhibitor are found to be very nearly equal to those computed with CH,Br inhibito-. Experimental flame speed measurements were generally con­sistent with the computed results, although several problems made it difficult, to obtain precise values for the flame speed in some cases. With HBr-CH.-air mixtures, rapid deterioration of the HBr through reaction with the walls of the storage tank made it impossible to carry out a second chemical analysis of the reactive mixture composition subsequent to the flame speed measurements. Compressibility of CHjBr in the manifold and lines produced uncertainties in the composition of the CH 3Sr-CH.-air mixtures. In both cases the flame speed measurements themselves are accurate and highly repeatable, with the uncertainties resting primarily on the possible errors in 4 specifying the exact mixture composition. Improved equipment and techniques involving heated lines and mixing tank may reduce these uncertainties, but the trends which are observed in the numerical results were confirmed by the experiments. The Inhibitors used, HBr and CH,Br, were found to be almost equally effective in their ability to reduce the laminar flame speed. With both inhibitors the numerical model predicts an increased inhibition efficiency for fuel-lean mixtures. This is a result of the lower H atom concentration in the front portion of the t'lame ( 1 0 0 0 K < T < 1500K) for lean flames. There­fore the addition of a given quantity of Inhibitor has a proportionally greater effect in these lean flames. Stoichio­metric ;nd rich flames have an ample supply of H atoms and are not as sensitive to a partial depletion of them due to inhibitor addition, This effect could not be resolved in the latest experiments with HBr or CH,Br, but earlier results with CF,Br [8] as inhibitor in CH 4-a1r and C,Hg-air flames clearly demonstrated the increased effect.veness of inhibition for fuel-lean flames. The same trend should be observed in •rhe efficiency of inhibition for all fuel-air mixtures in which the reaction H + 0* = 0 + OH plays a central role, including flames of other more complex hydrocarbons and of ammonia. Inhibition efficiency can be defined as the rate of flame speed reduction, the amount or flam, speed change per unit inhibitor added (-dS /dX. , with S the flame speed and X. the inhibitor mole fraction). Both the numerical model and the flame tube measurements found that the inhibition efficiency gradually decreases as the amount of inhibitor is increased, as shown in Figure 2, The present experimental and modeling results are shown, together with earlier data for CF,Br-CH 4«air and CF,Br-C 3H 8-air [8] as well as HBr-CH.-air, CH 3Br-CH.-air and CF,Br-CH.-air [1J. In the numerical'study it was found that a stoichiometric methane-air mixture with up to 8* methyl bromide could support a flame, pripagating at a speed of about 5 cm/sec, even though the addition of the first It of CHoBr had reduced the flame speed from 38 cm/sec to about 26 cm/sec. Extensions of the model to include CF^Br are currently ur.de" development, 'The available experimental data suggest that CF^Br is somewhat more efficient as an inhibitor than HBr or CH.Br. This is due primarily to the fact that the fluorine atoir,sJ permanently remove H atoms from the flame throuqh the formation of HF, which is much less reactive than its analog HBr. Since these H atoms never are returned to the flame, the rate of chain initiation and branching throughout the flame is lower and the Br atoms have a larger proportionate effect, similar to the situation prevailing in fuel-lean flames as discussed above. 5. t o x I/) t i D x CHjBr • HBr a CH 3Br A HBr o CF 3Br-CH 4 • C F 3 B r - C 3 H £ + KDr • CH 3Br 2 3 Percent inhibitor added present experiments present model reference 8 reference 1 Figure 2 Acknowledgements We wish to acknowledge with pleasure the contributions of Mr. Steven Stover who carried out many of the experimental measurements and of Or. R. M. Fnstrom for valuable discussions on the subject of flame inhibition. This work was performed under the auspices of the U. S, Department of Energy by the Lawrence Livermore National Laboratory under contract number W-7405-ENG-48. 6. nisei \mm I tiK dwuniMii »as pripmil a*, an acvinini nf »<irk spnnwrcd h\ an aHonn «t thr I Hind Mati'i (piiutnmonl. Nvilhcr lhr I ciilwl Main l.inrrnmi'ni nnr ihr I nitiTMU itf (alifnrnia nnr un> nf rhcir i'mplnu'i>*. niuko am uikrrtini>. ,"i-prvs*. or implied, nr iivMimi's an> legal llahilih nr inpnnsiHiliU fnr Iht- av> curacv, cnmpk'ieiu'ss. <ir uscfulncvi nFam infiirmutifin. apparatus, prnducl,«» prnci'ss dhi'lrivt-d, i>r represents thai its use VVCIUIU not infringe prisuiih imnvd rights. Reference herein warn speeMfyeiimmt'ri'iBlpnHlij.ivprniTss.nr senile hi Irndt1 name, trademark, manuhvliuet. IIT ntheT»ise, d»e\ mil neccvsarih cimsiituir nt inipli ils rndimi.niiil. nriimmendali'in. er raxirtni! I» ihe I nluit Main Cusernmenl m ihr I niicrsilj nr (ullfnrnia. I lit sie»s and npininns nf authnrs expressed herein dn nni neeessiirils slate I T reflect thnsi nf Ihc I nilid Mali1*! (iiiirrnmrni Ihcrenf. and shall nm hi used '»t advertisine nr prodix! en-diirsenuni purposes. References 1. Rosser, W.A.. Wise, H., and Miller, J., Seventh Symposium ( I n t e r n a t i o n a l ) on Combustion, Bu t t e rwo r ths . London, p. 175 (1958). 2. Day, M.J,, Stamp, C.V., Thompson, K., and Dixon-Lewis, G.. Th i r t een th Symposium ( I n t e r n a t i o n a l ) on Combustion, The Combustion Institute, Pittsburgh, p, 705 (1971). 3. Dixon-Lewis, G., and Simpson, R.J.. Sixteenth Symposium (international) on Combustion, The Combustion Institute, Pittsburgh, p. 1111 (1977) 4. Dixon-Lewis, G. , Combust, and Flame 36, 1(1979). 5. Westbrook, C.K., Combust. Sci. and Technol. 23, 191 (1930). 6. Westbrook, C.K., to be published. 7. Fuller, L.E., Parks, D.J., and Fletcher, E.A., Combust, and Flame 1_3, 459(1969). 8. Parks, D.J7," Alvares, N.J., and Beason, D.G.. Fire Safety Journal 2, 237(1979). 9. Lund, C M . , HCT-A genera! computer program for calculating time-dependent phenomena involving one-dimensional hydro­dynamics, transport, and detailed chemical kinetics. Univ­ersity of California Lawrence Livermore National Laboratory report UCRL-52504, 1978. 10. Westbrook, C,K., and Dryer . F .L . , Combust, and Flame 37, 171(1980). 11. Westbrook, C.K., Adamczyk, A.A., and Lavoie, G.A., Combust, and Flame, in press (1981). 

Experimental and theoretical study of flame inhibition by bromine-containing compounds

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Experimental and theoretical study of flame inhibition by bromine-containing compounds

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