Methane-based flames

The crystallinity of the same material composition varies depending on the type of flame that is used. The intrinsically high temperature of the H2/O2 flame leads to more crystalline materials compared to colder methane-based flames [71]. 

Figure 6.2.5 also shows the effect of RHL, which has an influence on the global quenching of lean methane/ air flames based on the behaviors between N2- and CO2-diluted flames of the same Sl lOcm/s. The larger the RHL is, the smaller is the value of Ka, . For example, Ka, = 26.1 for N2-diluted flames (small RHL), while Ka, = 20.4 for C02-diluted flames (large RHL) when (j> 0.64. It is found that for lean mefhane/air flames of constant Sl, the values of Ka, increased with for both N2- and C02-diluted flames, and fhe difference in the values of Ka, befween these two different diluted flames also increased with (f>, as shown in Figure 6.2.5. On the other hand, the effects of RHL did not have influence on the global quenching of rich methane/air flames, because Ka, 8.4 for both N2- and C02-diluted flames (values of Ka are in a log plof in Figure 6.2.5).

A number of theoretical (5), (19-23). experimental (24-28) and computational (2), (23), (29-32). studies of premixed flames in a stagnation point flow have appeared recently in the literature. In many of these papers it was found that the Lewis number of the deficient reactant played an important role in the behavior of the flames near extinction. In particular, in the absence of downstream heat loss, it was shown that extinction of strained premixed laminar flames can be accomplished via one of the following two mechanisms. If the Lewis number (the ratio of the thermal diffusivity to the mass diffusivity) of the deficient reactant is greater than a critical value, Lee > 1 then extinction can be achieved by flame stretch alone. In such flames (e.g., rich methane-air and lean propane-air flames) extinction occurs at a finite distance from the plane of symmetry. However, if the Lewis number of the deficient reactant is less than this value (e.g., lean hydrogen-air and lean methane-air flames), then extinction occurs from a combination of flame stretch and incomplete chemical reaction. Based upon these results we anticipate that the Lewis number of hydrogen will play an important role in the extinction process. 

To illustrate the behavior of a stagnation flame impinging into a wall, consider the following example based on an atmospheric-pressure, stoichiometric, premixed, methane-air flame [271]. Geometrically the situation is similar to that shown in Fig. 17.1. The manifold-to-surface separation distance is one centimeter, the inlet mixture is at 300 K, and the surface temperature is maintained at Ts = 800 K. Figure 17.4 shows the flow field and flame structure for two inlet velocities. The flow is from right to left, with the inlet manifold on the right-hand side and the surface on the left. 

Oxygen-deficient cool flame partial oxidation of methane to methanol and/or formaldehyde has long been known,30 but methane-based selectivities of 71 percent methanol and 14 percent formaldehyde at 2 percent... 

DOC analysers on the market at the time of purchase, operate based on two different principles A) high temperature oxidation over a catalyst followed by measurement of the evolved CO 2 by infra-red photometry, or after conversion to methane, by flame ionisation detection, and B) wet oxidation of organic carbon by the action of UV light in the presence of potassium persulphate. The CO2 produced is stripped from solution and reduced to methane which is measured quantitatively by flame ionisation detection. 

Let us note the specific character of laminar combustion of a binary fuel H2 + CH4 (Hytane) in air. Assume that the volume fractions of H2 and CH4 are Xh2 and Xch4 so that Xch4 = 1 — Xu2. The dependencies of the laminar flame velocity on this binary fuel composition and the fuel/air ratio are illustrated in Fig. 2.38 based on data obtained in [87, 88]. The solid curve with the solid circles correspond to experimental results, and the dashed line shows the following relations 5hy = > h2 Xn2 + (1 — Xh2) 5 ch4- Here Suy, 5h2 and 5ch4 - hytane, hydrogen and methane laminar flame velocities. 

A device based on flame ionization measures the total concentration of hydrocarbons. By using a catalyst, such as a heated platinum wire, hydrocarbons other than methane can be removed from the sample gas. With a platinum catalyst, these hydrocarbons are oxidized at a lower temperature than methane. Hence, the total concentration of hydrocarbons, methane, and hydrocarbons other than methane can be determined. 

It is presumed that the global-quenching criteria of premixed flames can be characterized by turbulent shaining (effect of Ka), equivalence ratio (effect of 4>), and heat-loss effects. Based on these aforemenhoned data, it is obvious that the lean methane flames (Le < 1) are much more difficult to be quenched globally by turbulence than the rich methane flames (Le > 1). This may be explained by the premixed flame shucture proposed by Peters [13], for which the premixed flame consisted of a chemically inert preheat zone, a chemically reacting inner layer, and an oxidation layer. Rich methane flames have only the inert preheat layer and the inner layer without the oxidation layers, while the lean methane flames have all the three layers. Since the behavior of the inner layer is responsible for the fuel consumption that... 

The gaseous atmosphere was then vented through a trap at -78° (to remove most of the benzene vapor) into an evacuated vessel. Samples were removed by gas-tight syringe and injected into a Hewlett-Packard 5790 gas chromatograph, equipped with a U ft, 1/8 in Porapak P column and a flame ionization detector. Use of known samples of hydrocarbons (methane and ethane) established that the minimum detectable amounts of product by this procedure were about 0.5-1 0 % (based on starting Nb complex). Several of the reactions (Mo(CO)g, W(C0)g and Ru (CO) p) gave small amounts (around 1-2 %) of these alkanes only with Cr(C0)g was a substantial yield of hydrocarbon product consistently observed (see below). 

Catalyst base site densities inj,) were measured by Temperature-Programmed Desorption (TPD) of CO2 pre-adsorbed at room temperature. Samples were pretreated in situ in a N2 flow at 773 K, eooled down to room temperature and then exposed to a flowing mixture of 3 % of CO2 in N2 until surface saturation. Weakly adsorbed CO2 was removed by flushing in N2. Finally, temperature was raised up to 773 K at 10 K/min. The desorbed CO2 was converted to CH4 over a methanation catalyst (Ni/Kieselghur) and then analyzed with a flame ionization detector. 

A burner is designed to allow gas and air to mix in a controlled manner. The gas often used is natural gas, mostly the highly flammable and odorless hydrocarbon methane, CH4. When ignited, the flame s temperature can be adjusted by altering the various proportions of gas and air. The gas flow can be controlled either at the main gas valve or at the gas control valve at the base of the burner. Manipulation of the air vents at the bottom of the barrel allows air to enter and mix with the gas. The hottest flame has a violet outer cone, a pale-blue middle cone, and a dark-blue inner cone the air vents, in this case, are opened sufficiently to assure complete combustion of the gas. Lack of air produces a cooler, luminous yellow flame. This flame lacks the inner cone and most likely is smoky, and often deposits soot on objects it contacts. Too much air blows out the flame. 

The premixed methanol flame [11, 12] does not show the Swan bands of C2, which are prominent in a methane flame [13]. The base of the flame shows strong emission from excited formaldehyde and further up the flame emission from OH and CH occurs. The burning velocity of a stoichiometric methanol—air flame [12] is about 45 cm. sec", and the global activation energy and global order are 43—47 kcal. mole" and unity, respectively [14(a)]. 

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