COMBUSTION SCIENCE AND TECHNOLOGY
Paper submission
Flammability limits of premixed methane / methanol / air flames
by
D. MARKUS*, H.–P. SCHILDBERG, W. WILDNER, G. KRDZALIC and U. MAAS
(*) Corresponding author Email:
[email protected] Phone: 49 531 592 3556 Fax: 49 531 592 3415
February 2003 Revised May 2003
I hereby affirm, that this manuscript has not been published elsewhere and it has not been submitted for publication elsewhere.
Flammability limits of premixed methane / methanol / air flames
D. MARKUS, Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany H.-P. SCHILDBERG, BASF AG, 67056 Ludwigshafen, Germany W. WILDNER, Degussa AG, 63457 Hanau, Germany G. KRDZALIC, U. MAAS, Institut für technische Verbrennung, Universität Stuttgart, Pfaffenwaldring 12, 70659 Stuttgart, Germany
Abstract - Many devices used in the chemical or petrochemical industry involve hazards because of the existence of flammable gases. Thus, the lean and rich flammability limits are very important characteristics of homogeneous gas mixtures. These data are documented and summarised in various reports, but there are discrepancies between the reported limits. Also, there is a lack of data for mixtures of flammable gases. To complement experimental measurements, the calculation of the flammability limits is of tremendous interest. This paper reports a computational study of premixed CH3OH / air and CH3OH / CH4 / air flames close to the flammability limits using detailed chemical models and transport properties and comparing the results with data from experiments. A comparison between the results and flammability limits calculated by the Le Chatelier principle is also performed. The deviations are stated, and the causes are explained on the basis of detailed reaction flow analyses.
Key words: Flammability limit, Kinetics, Ternary systems, Methanol oxidation
Title (shortened): Flammability limits of methane/methanol/air flames
1
INTRODUCTION Flammability limits are considered the limiting concentrations of fuel / oxidant mixtures supporting flame propagation. They therefore are of vital importance for the assessment of explosion hazards. But the flammability limits depend on the method of determination, so it is not possible to define them as constants. As a result, various arbitrary definitions lead to different flammability limits. For practical purposes, the flammability limits are often measured by placing gas mixture and ignition source into specific flammability tubes. After ignition the propagation and extinction of the flame has to be observed (Coward and Jones 1952, Zabetakis 1965). In another determination method according to the German Standard DIN 51649 (DIN 51649) only the initial development of the flame kernel is observed so that somewhat wider flammability limits are obtained. The problems and efforts in determining flammability limits increase extremely when ternary systems are investigated. A matter of particular interest are the flammability limits of gas mixtures containing two flammable gases in air. Several methods to estimate these limits exist, e.g. Le Chatelier's principle (Le Chatelier 1891) which calculates the flammability limits from those of pure compounds on the basis of a linear interpolation. It is quite accurate at the lower flammability limit (LFL) of many mixtures, especially for mixtures containing different hydrocarbons. At the upper flammability limit (UFL), large discrepancies can often be observed compared to experimental results, because the principle does not take the complexity of the chemical reaction systems into account. The present paper reports numerical flammability studies of CH3OH / CH4 mixtures in air in comparison with experimental data. Especially the lower flammability limit of CH4 in air has been the subject of many experimental (Cashdollar et al. 2000, Ronney 1988) and numerical (Lakshmisha et al. 1990, Sibulkin and Frendi 1990, Sung and Law 1996, Ju et al. 1999) studies. But on the flammability limits of CH3OH in air less data are available. In Table 1 the results of various experimental studies are summarised. Especially the upper flammability limit of this system differs much for the different experimental methods,
2
depending on the limit criterion chosen. The differences show the complexity of determining flammability limits discussed in literature (Cashdollar et al. 2000). The calculations which have been carried out are based on numerical studies using detailed chemical kinetics and complete transport models, which provide insight into the mechanisms of flame extinction in premixed gas mixtures. In the absence of heat losses, there is no chemical limit for a premixed, planar flame (Lakshmisha et al. 1988). It thus is well established that fundamental limits stem from heat loss due to radiation (Lakshmisha et al. 1990, Law and Faeth 1994, Christiansen et al. 1998, Ronney 1998). The application of a suitable radiative model and the observed flame configuration are important factors and have an influence on the calculated results (Ronney 1998). To match the conditions of experimental measurements as closely as possible, the simulation of unsteady, spherical flames using detailed chemistry for unconfined, premixed gas mixtures would be preferable (Maas and Warnatz 1988, Sibulkin and Frendi 1990). Nevertheless, as will be shown later on, the fundamental principles can be well described by the model of a steady, laminar, planar, one-dimensional, premixed flame, enhanced with a heat loss term. The numerically determined flammability limits of CH3OH / CH4 mixtures in air are compared with experimental data. The deviation of numerical and experimental results from the Le Chatelier principle at the upper flammability limit will be discussed on the basis of an analysis of the underlying reaction mechanism.
[Table 1]
PHYSICAL MODEL The investigation reported on was performed using two different numerical models describing premixed flames. The solutions of the well known mass, species and energy equations for the description of steady, planar, one-dimensional flames and also for spherically symmetrical, expanding one-dimensional flames in the absence of gravity were obtained using a time integration method (Maas and
3
Warnatz 1988) implementing detailed transport models and chemical kinetics. For the calculation of steady, planar flames the coordinate system was extended well into the cold and hot regions. At the unburnt boundary, constant values for temperature and mass fractions were used, whereas zero gradients were applied at the burnt boundary. The initial conditions for the calculations of unconfined spherically symmetrical flames were those of a quiescent, homogeneous gas mixture. The boundary conditions were zero gradients at the inner and outer boundary, except for p = const. at the outer boundary. The ignition source was represented using a source term in the energy conservation equation (Maas and Warnatz 1988). The source term was large enough for results to be obtained independent of the ignition energy. The radiative heat loss per unit volume qr as part of the energy equation was modelled assuming the optical thin flame limit (Lakshmisha et al. 1990, Sibulkin and Frendi 1990)
(
)
q r = 4αrσ T 4 − Tu4 ,
(1)
where σ is the Stefan-Boltzmann constant, Tu the temperature of the unburnt mixture and T the temperature in the volume respectively, and αr the total Planck mean absorption coefficient of the mixture. In the present work radiative heat loss from CO2, H2, CO and CH4 was considered, and the total mean absorption coefficient is given by
αr = ∑αr ,i ⋅ pi ,
(2)
i where pi denotes the partial pressure and αp,i the Planck mean absorption coefficient of the i–th species. Using the radiative emission from the nonoverlapping bands of the considered species, the absorption coefficients are taken from the literature (Hubbard and Tien 1978) as a function of temperature. The kinetic system is a detailed C2-mechanism of 34 species and 288 elementary reactions, which is part of a C4-mechanism for the simulation of propane and butane flames (Chevalier 1993). In both numerical models the lower (upper) flammability limit corresponds to the minimum (maximum) fuel concentration at which a flame could be observed.
4
EXPERIMENTAL METHODS The experimental data presented in this paper were obtained by two different methods. The results at Tu = 323 K were obtained in a confined spherical vessel of 5 l volume using an experimental procedure according to the ‘bomb method’ as described in the draft European Standard prEN 1839 (prEN 1839). The mixture in the vessel was prepared by the partial pressure method and then mixed inside the vessel for about 5 min using a blade stirrer. After switching off of the stirrer, the mixture was allowed to become quiescent before being ignited. The ignition source was a fusing wire generating an arc discharge with a total energy of 70 J being released. For all experiments the pressure was measured as a function of time. When a pressure rise smaller than 50 mbar (at 1 bar absolute initial pressure) was measured, the ignition test was considered to be unsuccessful, i.e. self-sustaining flame propagation did not take place. At Tu = 373 K, the experimental data according to the German Standard DIN 51649 (DIN 51649) were obtained. A cylindrical vessel 60 mm in diameter and 300 mm in height (volume approx. 0.8 l) was filled with a premixed gas mixture at atmospheric pressure and 373 K. The quiescent gas mixture was ignited using a spark plug with an ignition energy of 10 J close to the bottom of the cylindrical vessel. When a flame kernel was observed moving away from the electrodes, the ignition test was regarded as successful and, thus, the mixtures composition as flammable. Otherwise it was considered inflammable. The accuracy of all given flammability limits experimentally determined by these two methods is 0.5 % by vol. of CH4 for a given value of CH3OH. This accuracy is suitable for the upper flammability limits on which this paper is mainly focussed. In the case of lower flammability limits an accuracy of 0.1 % by vol. would be preferable and will be achieved in further work.
RESULTS The high sensitivity of combustion processes close to the flammability limits requires critical verification of the numerical model applied for the
5
computational determination of flammability limits in ternary mixtures like CH3OH / CH4 / air. Two different sets of one-dimensional conservation equations are therefore considered, the first one representing an unsteady, spherical flame, and the second one a steady, planar flame. Typical results of calculations using the unsteady, spherical model are shown in Figure 1 where the peak temperatures inside three different gas mixtures close to an upper flammability limit are plotted during the ignition period and the following flame propagation as a function of time. The energy input causing the ignition leads to the peak temperature in all mixtures during the ignition period of 5 ms being increased, from 323 K, the initial temperature, to 2100 K. At the end of the ignition period the peak temperature drops due to heat conduction to the surroundings of the ignition volume. The ignition of a mixture containing 32.6% of CH3OH and 4.0% of CH4 in air leads to stable flame propagation with a constant peak temperature of 1070 K. Increasing the amount of CH3OH leads to extinction, a limit which has been observed previously (Ronney 1998, Law and Faeth 1994). In the mixture with 32.8% of CH3OH and 4.0% of CH4 in air, there is flame propagation in the unburnt region for about 1.2 s. Then the heat loss, mainly due to radiation, causes the flame to extinguish. The numerically determined flammability limit of a CH3OH / CH4 / air mixture with 4% of CH4 is 32.6±0.2% CH3OH at 323 K, the flammability limit obtained with the steady, planar model is 33.6±0.1% CH3OH. This slight difference results from the effect of curvature (Sibulkin and Frendi 1990). The difference is however small compared to the scatter of different experimental results as e.g. shown in Table 1 for pure CH3OH/air mixtures. Thus the model of a steady, planar flame was chosen for the following calculations to reduce the computer time as desired.
[Figure 1]
METHANOL / AIR Calculated flame velocities for CH3OH / air flames with an initial temperature of 373 K using the steady, planar model are plotted in Figure 2 with and without
6
considering the heat loss due to radiation. As shown in detail in the range of the upper flammability limit, the radiative heat loss has a significant effect in mixtures close to this limit. The flame velocity is reduced, and compared to the non-radiating flame, the radiating flame extinguishes at a specific concentration. The upper flammability limit corresponds to the maximum mole fraction at which a flame could be observed. The numerically determined limits are 6.6±0.1% and 36.3±0.1% for 298 K, 6.3±0.1% and 39.9±0.1% for 373 K, respectively. The results agree very well with experimental results obtained by using upward flame propagation in tubes (National 1997, Sax 1989) as shown in Table 1.
[Figure 2]
METHANE / AIR The numerically determined flammability limits for CH4 in air using the steady, planar flame model are 5.2±0.1% and 17.2±0.1% for 298 K, 4.8±0.1% and 18.3±0.1% for 373 K, respectively. The lower flammability limit for 298 K agrees well with results from literature, 4.93-5.18%, using similar numerical models (Law and Egolfopoulos 1992, Lakshmisha et al. 1990) and 4.31-5.25% from experiments with different criteria (Lakshmisha et al. 1990, CHEMSAFE). At the upper flammability limit there is a larger discrepancy between our result and experimental data given in the literature for 298 K (15.0 to 16.28% (Cashdollar et al. 2000, CHEMSAFE)). The reaction mechanism used in this report does not allow for possible soot formation in CH4 / air flames near the upper flammability limit, which may cause this discrepancy (Vanderstraeten et. al 1997). As will be shown in the following section, this deviation gets smaller when CH3OH is added.
METHANOL / METHANE / AIR To our knowledge the flammability limits for the ternary mixture CH3OH / CH4 / air have not yet been determined on the basis of a detailed model. We used the model of a steady, planar flame to numerically determine the flammability limits of these gases for different temperatures. In the range of rich CH3OH mixtures,
7
we used a specified amount of CH4, and the concentration of CH3OH was varied until the limit was detected with an accuracy of 0.1% of CH3OH. In the range of rich CH4 mixtures, the amount of CH3OH was specified, and the concentration of CH4 was varied until the limit was detected with an accuracy of 0.1% of CH4. In Figure 3 the numerically determined flammability limits for an initial temperature Tu = 323 K using the steady, planar model are compared with experimental results obtained by the measurement of the pressure rise in the 5 l vessel. Interpolations of the experimental flammability limits for Tu = 323 K based on the Le Chatelier principle (Le Chatelier 1891) are also plotted in Figure 3. Starting from the upper flammability limits of pure CH4 in air and CH3OH in air, respectively, the interpolation yields a straight line. Each point given in Figure 3 determines the mole fraction of CH4 and CH3OH for a particular flammability limit; the mole fraction of air for this mixture is equal to the remaining amount. Mixture compositions between the flammability limits are explosive. The experimental and numerical results are nearly identical for the lower flammability limits. At the upper flammability limits both methods show a large deviation from the Le Chatelier interpolation. The agreement between experimental and numerical results up to a mole fraction of 0.12 CH3OH and in the range of 0.26 to 0.36 is quite good, but there is a large deviation in the transition region between these two extreme values. The numerical results in this region show a significantly greater CH4 mole fraction at the flammability limits for a given CH3OH mole fraction. The reason for this deviation possibly is the pressure criterion used in the ‘bomb method’ of prEN 1839 to distinguish between self–sustaining flame propagation and no flame propagation. Due to the fairly slow propagation velocities of flames in mixtures close to the flammability limits as shown in Figure 2, heat conduction to the surrounding walls influences the total pressure rise in the vessel. This influence is not represented in the numerical model. By visual inspection of the flame propagation, which is the basis of the flammability criterion of the German Standard DIN 51649 (DIN 51649), a flame in a mixture close to the explosion limits might be regarded as a self-sustaining propagating flame front, although the pressure rise in the vessel might still remain below the threshold of
8
50 mbar. Because of this effect inherent in the ‘bomb method’, the upper flammability limits were also determined by the experimental procedure according to the German Standard DIN 51649. Results obtained by these methods with an initial temperature of Tu = 373 K are plotted in Figure 4. This figure also shows the numerically determined flammability limits and the interpolation by the Le Chatelier principle. Again, the experimental and the numerical results show a large deviation from the Le Chatelier principle. The agreement of the experimental and numerically determined upper flammability limits is quite good for rich CH4 mixtures with a small amount of CH3OH. For rich CH3OH mixtures with a small amount of CH4, the numerically determined limits are lower than the experimental limits, but both curves show a linear dependence of the limits in this mixture region. In the transient mixture region between these two extreme values from a mole fraction of CH3OH of 0.05 up to 0.18, the discrepancy between the results is larger. Both curves have however a non-linear trend in this range. The determined limits shown in Table 1 are very sensitive to the limit criterion chosen, which explains the deviation between experimental and numerically determined flammability limits. Nevertheless, all the results above have shown that the Le Chatelier principle does not hold for the ternary mixture CH3OH / CH4 / air.
[Figure 3] [Figure 4]
The Le Chatelier principle is based on the assumption that different flames behave similarly in mixtures representing different flammability limits, i.e. nearly identical flame temperature and flame velocity. But this explanation is not valid for the CH3OH / CH4 / air system. This is mainly because the underlying chemical reaction pathways in the combustion process are neglected by the Le Chatelier principle. To examine the influence of the governing elementary reactions, detailed investigations into some typical flames were performed. Figure 5 shows the formaldehyde mass fraction as a function of the axial distance from the inlet for different mixture compositions, obtained by the
9
steady, planar model. The five mixtures have selected compositions close to numerically determined upper flammability limits for Tu = 373 K as shown in Figure 4. In rich CH4 flames with low CH3OH concentrations up to 8%, the formaldehyde, which is formed directly in the flame front, immediately reacts in the zone behind the flame front. The mass fraction of formaldehyde in these flames thus decays very fast to zero, which is typical of lean CH3OH / air flames, but also of rich CH4 / air flames. The formaldehyde production in stoichiometric CH3OH / air flames is up to 15 times higher than in stoichiometric CH4 / air flames (Egolfopoulos et al. 1992). Thus, there is an increase in the overall formaldehyde production with the increase on CH3OH at the inlet. Beginning with a CH3OH concentration of 12%, some formaldehyde does not further react in the post flame zone. This is typical for rich CH3OH / air flames.
[Figure 5]
The dominance of the oxidation of CH3OH can be seen in Figure 6 which shows a phase diagram of CH4 / CO2 mass fractions for two different CH4 / CH3OH / air flames of mixture compositions close to different flammability limits. In the rich CH4 / air flame with an added amount of 2% of CH3OH, the oxidation of CH4 is scarcely influenced by the CH3OH. As plotted in Figure 6, the CH4 mass fraction in this flame subsequently decreases with increasing CO2 mass fraction up to a nearly complete reaction to the product (note that the mass fraction of CH4 is greater than 0.02 because this is a rich mixture). In contrast to this rich CH4 / air flame the consumption of CH4 in the rich CH3OH / air flame with an added amount of 6% H4 is very weak. The CO2 is almost exclusively formed through oxidation of CH3OH.
[Figure 6]
Figures 5 and 6 show that CH3OH / CH4 / air flames with different mixture compositions differ fundamentally. In order to explain these observations, the
10
main species consumption paths are determined for selected CH3OH / CH4 ratios based on a chemical flow analysis (Warnatz et al. 2001). The important reaction paths are shown in Figure 7 and Figure 8 for a 4.0% CH3OH / 16.4% CH4 / 79.6% air flame and for a 33.7% CH3OH / 6.0% CH4 / 60.3% air flame, respectively. These mixture compositions represent specific upper flammability limits. Due to the recombination of CH3 in rich CH4 / air flames, the formation of C2–hydrocarbons and the following oxidation of these species to CO2 is the dominant reaction path in these flames (Warnatz 1981, Warnatz et al. 2001). The addition of a small amount of CH3OH is only of minor influence, as can be seen in Figure 7 for the 4.0% CH3OH / 16.4% CH4 / 79.6% air flame, where the reaction path via CH2O to CHO and the further oxidation to CO2 is less important. In CH3OH / air flames the formation of CH2O mainly via CH2OH with a subsequent oxidation via CHO to CO2 is very important. In these flames the formation of CH3 from CH3O and CH2O can also be observed. Especially in rich CH3OH / air flames the reactions of CH3 with H and CH3 respectively, yield CH4 and C2–hydrocarbons (Egolfopoulos et al. 1992). This can also be seen in Figure 8 where the chemical flow analysis for a 33.7% CH3OH / 6.0% CH4 / 60.3% air flame is shown. The dominant reaction in this flame is the oxidation of CH3OH via CH2O. CH4 is consumed nearly at the same rate at which it is produced, which can also be seen in Figure 6.
[Figure 7] [Figure 8]
The flammability limits are governed by the competing processes of heat loss from the flame and heat release due to chemical reactions. The underlying elementary reactions are generally of the Arrhenius type and very sensitive to temperature changes. A relatively small heat loss can therefore extinguish the flame. The ratio of heat loss to heat release is not, however, a constant value for various upper flammability limits (Lakshmisha et al. 1990). The two different dominant reaction paths in rich CH3OH / air and in rich CH4 / air flames, respectively, are thus responsible for the deviations of the upper
11
flammability limits both numerically and experimentally determined compared to the interpolations according to the Le Chatelier principle. In Figure 9 the calculated flame temperatures of mixtures along the curve of the upper flammability limits of CH3OH / CH4 in air using the steady, planar model with an initial temperature of Tu = 323 K are plotted as a function of the CH3OH concentration. The flame temperature at the upper flammability limit of 37.6% of CH3OH in air is nearly 1100 K. Decreasing the amount of CH3OH, i.e. increasing the amount of CH4, does not significantly influence the flame temperature up to a mixture composition of 18% of CH3OH and 16% of CH4 in air. In this region only the oxidation of CH3OH via CH2O to CO2 is important, and the oxidation of CH4 has only a minor influence on these flames. Starting with 16% of CH3OH and 15.9% of CH4 there is a sudden increase in the necessary flame temperature at the flammability limits. The reaction paths have changed to the type of rich CH4 / air flames, the recombination of CH3 and the subsequent oxidation of C2– hydrocarbons gets more and more important, leading to flame temperatures at the upper flammability limit up to nearly 1600 K in the 17.6% CH4 / 82.4% air flame without any CH3OH.
[Figure 9]
CONCLUSIONS Numerical and experimental flammability studies on CH3OH in air and of CH3OH / CH4 mixtures in air have been performed. Two different numerical models using detailed chemistry and flame radiation have been applied. Compared to the scatter of different experimental results, the discrepancy between the results obtained with a steady, planar, premixed flame model and an unsteady, onedimensional, spherically symmetric flame model is small. It is therefore reasonable to apply the simple planar flame model to determine the flammability limits for all mixtures investigated in this paper. This in turn results in the computational effort being considerably reduced.
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The Le Chatelier principle is not applicable for determining the upper flammability limits of CH3OH / CH4 mixtures in air. This principle is based on the assumption that flames in different mixture compositions, which represent various flammability limits, show the same behaviour. As was shown by the analyses of the reaction paths in different CH3OH / CH4 / air flames, the underlying different chemical pathways in these flames are very important. In rich CH4 flames, the recombination of CH3 and the oxidation of the resulting C2– hydrocarbons are the dominant reactions. But when the oxygen in the mixture is consumed during the oxidation of CH3OH and the resulting product CH2O, which is predominant in flames with high CH3OH content, the oxidation of CH4 has scarcely any influence on the combustion processes close to the flammability limits. Due to this change of the main reaction path, the required heat release in rich CH3OH flames to stabilise flame propagation is smaller than in rich CH4 flames. The measured and calculated flammability limits are thus wider than from the interpolation following the Le Chatelier principle. The calculations have shown that especially in rich CH3OH flames the dependence of total heat release and thus of the resulting flame temperature on the amount of fuel is very weak. As a consequence, minor differences of various experimental flame propagation criteria may result in large deviations of the flammability limit as shown in Table 1. Nevertheless, the numerical studies on the flammability limits of CH3OH / CH4 / air flames have shown good agreement with experimental results.
ACKNOWLEDGMENTS The authors are grateful for support of this work from the BG Chemie, BASF AG, Bayer AG, Degussa AG and the Schweizer Institut zur Förderung der Sicherheit.
REFERENCES CHEMSAFE, Database for safety characteristics, host STN International / FIZ Karlsruhe (online). Edited by: Bundesanstalt für Materialforschung und -prüfung (BAM), Berlin, Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, and
13
Deutsche Gesellschaft für Chemisches Apparatewesen, Chemische Technik und Biotechnologie (DECHEMA) e.V., Frankfurt/Main. Inhouse version database: DECHEMA e.V., P.O.Box 150104, D-60061 Frankfurt/Main, Germany. Cashdollar, K.L., Zlochower, I.A. and Green, G.M., Thomas, R.A. and Hertzberg, M. (2000) Flammability of methane, propane, and hydrogen gases. J. Loss. Prev. Process. Ind. 13, 327. Chevalier, C. (1993) Entwicklung eines detaillierten Reaktionsmechanismus zur Modellierung der Verbrennungsprozesse von Kohlenwasserstoffen bei Hoch- und Niedertemperaturbedingungen. PhD Thesis, Stuttgart University. Christiansen, E.W., Sung, C.J. and Law, C.K. (1998) Pulsating instability in near-limit propagation of rich hydrogen/air flames. Proc. Combust. Instit. 27, 555. Christner, H.K. (1974) Experimentelle und theoretische Bestimmung der Druck- und Temperaturabhängigkeit von Zündgrenzen, dargestellt am Beispiel von Alkohol/Luft-Gemischen. PhD Thesis, Erlangen-Nürnberg University. Coward, H.F. and Jones G.W. (1952) Limits of flammability of gases and vapors. Bureau of Mines Bulletin 503. DIN 51649 (1986) Bestimmung der Explosionsgrenzen von Gasen und Gasgemischen in Luft, Beuth Verlag, Berlin. Egolfopoulos, F.N., Du, D.X. and C.K. Law (1992) A comprehensive study on methanol kinetics in freely-propagating and burner-stabilized flames, flow and static reactors, and shock tubes. Combust. Sci. and Tech. 83, 33. Lakshmisha, K.N., Paul, P.J., Rajan, N.K.S., Goyal, G. and Mukunda, H.S. (1988) Behavior of methane-oxygen-nitrogen mixtures near flammability limits. Proc. Combust. Instit. 22, 1573. Hubbard, G.L. and Tien, C.L. (1978) Infrared mean absorption coefficients of luminous flames and smoke. ASME J. Heat Transfer 100, 235. Ju, Y., Matsumi, H., Takita, K. and Masuya, G. (1999) Combined effects of radiation, flame curvature and stretch on the extinction and bifurcations of cylindrical CH4 / air premixed flames. Combust. Flame 116, 580. Lakshmisha, K.N., Paul, P.J. and Mukunda, H.S. (1990) On the flammability limit and heat loss in flames with detailed chemistry. Proc. Combust. Instit. 23, 443.
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Law, C.K. and Egolfopoulos, F.N. (1992) A unified chain-thermal theory of fundamental flammability limits. Proc. Combust. Instit. 24, 173. Law, C.K. and Faeth, G.M. (1994) Opportunities and challenges of combustion in microgravity. Prog. Energy Combust. Sci. 20, 65. Le Chatelier, H.L. (1891) Note sur le dosage du grisou par les limites d'inflammabilité. Annales des mines 19, 388. Maas, U. and Warnatz, J. (1988) Ignition processes in hydrogen-oxygen mixtures. Combust. Flame 74, 53. National Fire Protection Association (1997) Fire Protection Guide to Hazardous Materials. National Fire Protection Association, Quincy, Mass., U.S., 12th Edn. Nabert, K. and Schön, G. (1973) Sicherheitstechnische Kennzahlen brennbarer Gase und Dämpfe. Deutscher Eichverlag GmbH, Braunschweig, 2nd Edn. prEN 1839 (under approval) Determination of explosion limits of gases, vapours and their mixtures. Ronney, P.D. (1988) On the mechanisms of flame propagation limits and extinguishment processes at microgravity. Proc. Combust. Instit. 22, 1615. Ronney, P.D. (1998) Understanding combustion processes through microgravity research. Proc. Combust. Instit. 27, 2485. Sax, N.I. and Lewis, R.J. (1989) Dangerous Properties of Industrial Materials. Van Nostrand, New York, 7th Edn. Sibulkin, M. and Frendi, A. (1990) Prediction of flammability limit of an unconfined premixed gas in the absence of gravity. Combust. Flame 82, 334. Sung, C.J. and Law, C.K. (1996) Extinction mechanisms of near-limit premixed flames and extended limits of flammability. Proc. Combust. Instit. 27, 865. Vanderstraeten, B., Tuerlinckx, D., Berghmans, J., Vliegen, S., van’t Oost, E. and Smit, B. (1997) Experimental study of the pressure and temperature dependence on the upper flammability limit of methane/air mixtures. J. Hazard. Mater. 56, 237. Warnatz, J. (1981) The structure of laminar alkane-, alkene-, and acetylene flames. Proc. Combust. Instit. 18, 369. Warnatz, J., Maas, U. and Dibble, R.W. (2001) Combustion. Springer Verlag, Berlin, 3rd Edn.
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Zabetakis, M.G. (1965) Flammability characteristics of combustible gases and vapors. Bureau of Mines Bulletin 627.
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TABLE TABLE 1 Experimental results on the limits of methanol / air flames at 298 K No.
Reference
LFL / % CH3OH
UFL / % CH3OH
1
National 1997
6.0
36.0
2
Sax 1989
6.0
36.5
3
Nabert and Schön 1973
5.5
44.0
4
Christner 1974 (323 K)
6.3
36.7
5
CHEMSAFE (373 K)
5.4
49.8
17
FIGURE CAPTIONS FIGURE 1 Calculated peak temperatures as a function of time in different CH3OH / CH4 / air mixtures close to the UFL at Tu = 323 K with 4.0% CH4, using the unsteady, spherical model. FIGURE 2 Calculated flame velocities of CH3OH / air flames at Tu = 373 K plotted versus CH3OH mole fraction, using the steady, planar model with and without considering radiation. FIGURE 3 Numerically determined flammability limits at Tu = 323 K using the steady, planar model for the CH3OH / CH4 / air system compared with experimental results and the interpolation according to the Le Chatelier principle. The experimental values were determined measuring the pressure rise in a 5 l vessel in accordance with prEN 1839 (prEN 1839). FIGURE 4 Numerically determined flammability limits at Tu = 373 K using the steady, planar model for the CH3OH / CH4 / air system compared with experimentally determined upper flammability limits and the interpolation according to the Le Chatelier principle. The experimental values were determined in accordance with the German Standard DIN 51649 (DIN 51649). FIGURE 5 Calculated formaldehyde mass fractions using the steady, planar model plotted versus the distance for several UFL mixtures, Tu = 373 K. FIGURE 6 Phase diagram for CH4 consumption and CO2 formation in two different calculated UFL mixtures using the steady, planar model, Tu = 373 K. FIGURE 7 Reaction flow analysis for a calculated CH3OH / CH4 flame at the UFL with 4% of CH3OH and 16% of CH4 using the steady, planar model, Tu = 373 K.
18
FIGURE 8 Reaction flow analysis for a calculated CH3OH / CH4 flame using the steady, planar model at the UFL with 33.7% of CH3OH and 6% of CH4, Tu = 373 K. FIGURE 9 Calculated flame temperatures using the steady, planar model in selected UFL mixtures at Tu = 323 K (for detailed mixture composition, see Figure 3).
19
2400 K 2100
4.0 % CH4 32.6 % CH3OH 32.8 % CH3OH 33.0 % CH3OH
Peak temperature
1800 1500
Ignition
1200 900 600
No ignition
300 0.0
0.2
0.4
0.6
0.8 1.0 Time
1.2
1.4
1.6
1.8 s 2.0
FIGURE 1
80
Flame velocity (373 K) no rad. incl. rad.
6
cm/s 4
60
Flame velocity
2
40
UFL
0 0.3
0.4
0.5
0.6
20
0 0.0
0.1
0.2
0.3 CH3OH
FIGURE 2
20
0.4
0.5 mole 0.6 fraction
FIGURE 3
FIGURE 4
21
CH3OH / CH4 2 % / 17.3 % 8 % / 14.9 % 12 % / 16.6 % 18 % / 18.2 % 33.7 % / 6 %
0.025 mass 0.020 fraction
CH2O
0.015 0.010 0.005 0.000 0.20
0.22
0.24 Distance
FIGURE 5
FIGURE 6
22
0.26
0.28
m
0.30
CH3OH
CH3O
CH2OH
CH4
C2H6
CH3
C2H5
CH2O
C2H4
CHO
C2H3
CO
C2H2
CH3
CH4 : 16.4 % CH3OH : 4.0 % CO2
FIGURE 7
CH2O
CH3OH
CH3O
CH2OH
CH4
C2H6
CH3
C2H5
CH2O
C2H4
CHO
C2H3
CO
C2H2
CH4 : 6.0 % CH3OH : 33.7 % CHO
CO2
FIGURE 8
23
CHO
1600 K Tu = 323 K Flame temperatures in CH3OH / CH4 / air flames
1500
Flame temperature
1400 1300 1200 1100 1000 0.00
0.05
0.10
0.15 0.20 CH3OH
FIGURE 9
24
0.25
0.30
0.35 mole fraction
0.40
