Kinetics in Flames

A very schematic representation of a system for following kinetics in flames is shown in Fig. 22. The arrangement used in a given case will depend to a large extent on the nature of the reaction. The type of equipment available has been discussed very thoroughly by Fristrom and Westenberg it will be considered here briefly under four headings. 

The action of the walls is both to cool the flame gases and to terminate the propagation chains. The theories of extinction distance have suggested that the extinction distance to a first approximation is inversely proportional to the burning velocity, and also depends somewhat on burner geometry. The major importance of extinction distance to kinetics in flames is, however, indirect its magnitude dictates the design of burner to be used in flame studies. 

The distinction between major and minor species is still valid and has proved to be important in the analysis of kinetics in flame gases. At the present juncture it should be noted that, as well as the major species (partial pressure 0-1 atm) and the residues of the additives which are usually fairly inactive kinetically (partial pressures on 0-01 atm), there is now a group of free radicals and changed species whose partial pressures are 10 atm or less, but which are active and observable participants in flame kinetics. 

Either the Na/Li or the Na/Cl method may be used to determine the hydrogen atom concentration in a flame, as well as the variants discussed below. However these techniques are not very easy to apply particularly in the study of kinetics in flames, there is a call for a method which will rapidly trace the relative H atom concentration in one flame. Such a method is provided by the copper flame band method developed by Bulewicz and Sugden. They showed that the intensity of the band system around 428 nm, due to CuH (A S+-X S+) and the broad system in the green, ascribed by them to CuOH, depends upon height in the flame in the same way as the concentration of H atoms however the intensity of the Cu resonance doublet at 325 nm was not affected by the composition of the flame. Sugden had demonstrated mathematically that if a compound AB can be balanced against free A by the two processes... 

It is apparent that the situation is ripe for a further step, and that the experimental tools are available to gather the necessary information whereon to base an understanding of kinetics in turbulent flames, and even if it is unlikely that a change in classical kinetic thought will result, the extension of ideas which will be needed to establish physical and chemical processes going on in a statistically variable medium as a normal and proper vehicle for the application of kinetic principles will ensure the keen interest and attention of all who study kinetics in flames. 

An upper limit for the rate constant of the reaction NH2 + HNC0 NH3 + NC0, which is of importance for the NH2 kinetics in flames, has been determined by following the NH2 concentration profiles in shock-heated HNCO-Ar mixtures in the temperature range 2340 to 2680 K, k<5 X 10 cm - mol" s [184]. 

Palmer, H. 1974. Equilibria and Chemical Kinetics in Flames, in Combustion Technology Some Modern Developments (H. Palmer and J. Beer, eds). Academic Press, New York. 

Hewson, J. and A. R. Kerstein (2001). Stochastic simulation of transport and chemical kinetics in turbulent CO/H2/N2 flames. Combustion Theory and Modelling 5, 669-697. 

Mobus, H., P. Gerlinger, and D. Briiggemann (1999). Monte Carlo PDF simulation of compressible turbulent diffusion flames using detailed chemical kinetics. In Paper 99-0198, AIAA. 

In kinetics studies, as in mass spectrometry, data reduction can be helpful before starting a detailed analysis. A typical application in which data reduction is of value is high-temperature kinetics. Reactions in flames are complex, so study of these reactions is challenging not just experimentally but also... 

Although Bowman and Seery s results would, at first, seem to refute the suggestion by Fenimore that prompt NO forms by reactions other than the Zeldovich mechanism, one must remember that flames and shock tube-initiated reacting systems are distinctively different processes. In a flame there is a temperature profile that begins at the ambient temperature and proceeds to the flame temperature. Thus, although flame temperatures may be simulated in shock tubes, the reactions in flames are initiated at much lower temperatures than those in shock tubes. As stressed many times before, the temperature history frequently determines the kinetic route and the products. Therefore shock tube results do not prove that the Zeldovich mechanism alone determines prompt NO formation. The prompt NO could arise from other reactions in flames, as suggested by Fenimore. 

Unfortunately, OH and O concentrations in flames are determined by detailed chemical kinetics and cannot be accurately predicted from simple equilibrium at the local temperature and stoichiometry. This is particularly true when active soot oxidation is occurring and the local temperature is decreasing with flame residence time [59], As a consequence, most attempts to model soot oxidation in flames have by necessity used a relation based on oxidation by 02 and then applied a correction factor to augment the rate to approximate the effect of oxidation by radicals. The two most commonly applied rate equations for soot oxidation by 02 are those developed by Lee el al. [61] and Nagle and Strickland-Constable [62],... 

Cherian, M. A., P. Rhodes, R. J. Simpson, and G. Dixon-Lewis. 1981. Kinetic modelling of the oxidation of carbon monoxide in flames. 18th Symposium (International) on Combustion Proceedings. Pittsburgh, PA The Combustion Institute. 385-96. 

Fig. 14.10 Reaction path diagram [149] illustrating major steps in volatile-N conversion in flames for different nitrogen species hydrogen cyanide (HCN), ammonia (NH3), cya-nuric acid (HNCO), acetonitrile (CH3CN), and pyridine (C5H5N). The diagram is based on chemical kinetic modeling at moderate fuel-N concentrations. Solid lines denote elementary reaction pathways, while dashed arrows denote routes that involve intermediates and reactions not shown.

S.G Davis, C.K. Law, and H. Wang. Propene Pyrolysis and Oxidation Kinetics in a Flow Reactor and Laminar Flames. Combust. Flame, 119 375-399,1999. 

J.A. Miller and C.F Melius. Kinetic and Thermodynamic Issues in the Formation of Aromatic Compounds in Flames of Aliphatic Fuels. Combust. Flame, 91 21-39, 1992. 

Two very distinguishing features of flames are the emission of visible radiation and the presence of an abnormal number of ions in the reaction zone (Cl, Gl). Both of these features appear to be kinetic by-products, which are completely unnecessary for flame propagation.5 The radical CH is responsible for much radiation and now also appears to be the precursor of many of the ions observed in flames (C2). Enhancement of CH or other radical or ion concentration in flames could result in any number of applications if an understanding of their formation made such control possible. 

The simplest example of a flame-supporting medium is a pure chemical compound which decomposes exothermically. The widespread interest in such flames is due to their possibilities as monopropellants. Many studies are motivated by purely fundamental considerations, since a decomposition flame can be a kinetically simple flame. The most widely used and studied combustion reactions are those between hydrocarbons or hydrocarbon derivatives and air or oxygen. However, many other chemical substances may be substituted for the common fuels and/or oxidizers. Flames of uncommon fuels and oxidizers are most important because of their possibility of surpassing ordinary hydrocarbon oxidation as a source of energy. Some unusual flames are discussed in reference (PI). 

Combustion mechanism as described here is limited to proposed theories relating to burning rates. No effort is made to describe the complete kinetics in the combustion process, the detailed structure of the flame, or the flame species... 

Zeldovich Ya. B., Semenov N. N. Kinetika khimicheskikh reaktsii v plamenakh [Kinetics of Chemical Reactions in Flames].—ZhETF 10, 1116 (1940). 

Thus, the study of chemical kinetics in a jet in the MCD sector of the curve in Fig. 4 will be extremely significant for the theory of flame propagation. 

This work is part of an on-going program. Analysis of the effects of sulfur on radical decay, further examination of the stoichiometric H2/02/N2 data, and analysis of sulfur chemistry in rich C2H2/02/N2 flames are underway. The laboratory program is continuing with fluorescence measurements of NO, NO2, NH, NH2 and CN in an effort to develop a unified kinetic model for fuel nitrogen chemistry in flames. 

In recent years numerous experiments have been reported on the fluorescence and energy transfer processes of electronically excited atoms. However, for flame studies the rates of many possible collision processes are not well known, and so the fate of these excited atoms is unclear. An interesting example concerns the ionization of alkali metals in flames. When the measured ionization rates are interpreted using simple kinetic theory, the derived ionization cross sections are orders of magnitude larger than gas kinetic (1,2,3). More detailed analyses (4,5) have yielded much lower ionization cross sections by invoking participation of highly excited electronic states. Evaluation of these models has been hampered by the lack of data on the ionization rate as a function of initial state for the alkali metals. 

The kinetics and mechanisms of radical reactions important in combustion chemistry are best studied under conditions in which single reactions can be isolated rather than in flames where there are multiple pathways for formation and disappearance of the radicals. Reactions of C2 are of particular importance since recent laser saturation measurements in our laboratory (1) have shown that C2 a3IIu is present in oxyacetylene flames at concentrations on the order of 1016 molecules/cm3 (approximately 0.1 torr). Although concentrations of ground state C2 in flames are unknown and cannot be measured by the same technique due to spectroscopic constraints, we expect that C2 X3 g populations are at least comparable. Because of these relatively large concentrations the reactions of both species are of considerable importance in combustion chemistry. However, until recently very little was known about these reactions due to the difficulty of producing a clean source of C2 radicals. 

There are a great many published reports describing studies of carbon black formation in flames. Many of these deal with gaseous fuels in either pre-mixed flames or diffusion flames. The principal objectives are to develop a better knowledge of combustion through an understanding of the kinetics and mechanism of carbon formation,... 

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