Methane-oxygen Flames Research Articles

Optical Investigations on Soot Forming Methane-Oxygen Flames

Abstract The Lorentz-Mie theory of the extinction and scattering of light by small spherical particles is applied to soot particles in flames. Extinction and scattering coefficients and angular scattering patterns are calculated, in the visible range, for different values of the complex refractive index of soot. Theoretical results for the emissivity and dispersion of the extinction coefficients are presented for monodisperse and polydisperse systems. An experimental apparatus for optical measurements and a flat flame burner are described, and results of measurements on rich fuel premixed methane-oxygen flames are given.

Temperature distribution in an open air methane-oxygen flame

Measurements of the distributions of population temperatures of OH and Na, obtained by emission-absorption spectroscopy at 3064 A and 5890 A respectively, are presented for a stoichiometrically premixed, open air and turbulent methane-oxygen flame. The distributions were obtained from the emission and absorption measurements by inversion procedures for the local emission and absorption coefficients in the case of axisymmetry. The population temperature distributions show large spatial gradients inside the inner cone of the flame. In the hottest region of the flame the adiabatic flame temperature of about 3054°K is almost reached. Negligible differences between the two population temperatures are observed at and around the maxima of the radial temperature distribution. Larger differences are obtained in the outer flame regions and near the axis of the inner cone. The temperatures calculated directly from the measured emission and absorption along the flame axis without taking into account temperature gradients outside the inner cone of the flame are about 120 and 200°K, respectively, smaller than the local temperatures at the flame axis.

Carbon formation in premixed methane-oxygen flames under constant-volume conditions

An experimental study of the combustion of rich methane-oxygen mixtures under constant-volume conditions in the pressure range 30–70 atm is described. The experiments were made in a calorimeter bomb, which permitted the heat of reaction to be measured directly. The combustion products were analyzed gravimetrically. Measured maximum flame temperatures and product compositions are compared with equilibrium values. It is shown that the threshold mixture strength for the onset of carbon formation is weaker than that required for chemical equilibrium and that at this threshold condition the heat release per atom of oxygen burned has a minimum value.

Chlorine dioxide and perchloric acid flames

A detailed comparison is presented of the properties of premixed flames of methane with chlorine dioxide and with perchloric acid. Overall activation energies are calculated for several flames with these oxidizers using the van Tiggelen expression. The values so obtained are compared to those for methane-oxygen flames. It is concluded that chlorine dioxide and perchloric acid flames involve basically the same reaction steps, and a unified mechanism is proposed and discussed.

Destruction of methane in water gas by reaction of CH 3 with OH radicals

The decay of residual methane in the burned gas from fuel-rich methane oxygen flames can be expressed − d [CH 4 ]/ dt =9×10 14 ([CH 4 ][H 2 O]/[H 2 ]) exp (−117 kcal/ RT ) moles cm −3 sec −1 in the ranges [H 2 O]/[H 2 ]=0.6 to 1.1, and T =1970° to 2190° K. The equilibria CH 4 =CH 3 +0.5 H 2 H 2 O=OH+0.5 H 2 are both satisfied, and the probable interpretation of the experimental expression is that methane decays at the rate of reaction of CH 3 with OH radicals. If so, the rate constant for this radical-radical reaction is 4(±2)×10 12 cm 3 mole −1 sec −1 , and is insensitive to temperature in the range covered.

Acetylene Formation from Methane Using a Perpendicular Flow Burner

The authors have devised a small scale, new type burner and made an experiment on acetylene formation from methane by partial combustion.The feature of the burner lies in enclosing the premixed methane-oxygen flame with hydrogen-oxygen or oxygen diluted by nitrogen diffusion flame on porous wall through which the oxydant is injected, thereby minimizing the heat loss from the premixed flame to be reduced and, at the same time, obtaining an efficient heat recovery and cooling of the wall material.The dimensions of the burners used in this study are 14mm in diameter and 110, 160 and 260mm in length. Through measurement of gaseous product at the exit of the burner, it is found that 30 to 39% of carbon in off gas is distributed to acetylene. Therefore, at least the same order of carbon distribution to acetylene as in commercial burner is obtained in a very small burner. Although dilution of oxygen injected with nitrogen reduces the acetylene concentration in off gas, the carbon distributed to acetylene remains essentially constant so far as N2/O2 volume ratio ranges between 0 to 2. This means an efficient cooling of the wall material is possible by replacement of a part of oxygen with air without reducing the yield of acetylene.

Perchloric acid flames VIII-methane-rich flames with oxygen

The effect of adding oxygen to a very fuel-rich methane-perchloric acid flame has been examined. At sufficiently large additions of oxygen a second flame zone is established downstream of the first flame. Burnt gas compositions from the first flame have been measured for a range of oxygen additions. Most of the added oxygen reacts in the perchloric acid flame and results probably indirectly, in the consumption of a further amount of methane. The second flame is shown to be a methane-oxygen flame. The results are discussed in the light of the mechanism previously proposed for methane-rich perchloric acid flames.

Energy distribution in electronically excited CH and C2 in low pressure hydrocarbon flames

A study has been made of the rotational energy distribution of electronically excited CH and of the vibrational energy distribution of electronically excited C2 radicals in low pressure (0.2 to 35 Torr) hydrocarbon flames. The rotational energy of the CH radical in the A2Δ (ν = 0) state has been found to be statistically distributed in the levels with quantum numbers 14 to 21. This distribution can be described by a temperature which has, however, no thermodynamic significance. In the methane–oxygen flame this temperature has been found to be independent of both the composition of the gas mixture and the pressure, whereas in the ethylene–oxygen system it varies with these parameters. Hydrogen–methane–oxygen flames behave as ethylene flames. Inert diluents such as nitrogen and carbon dioxide do not affect the temperature at very low pressures. This indicates that the energy distribution is not perturbed measurably by collisions before emission takes place. In order to explain these observations we have to accept that at least two reactions produce CH* in ethylene flames.The cross section for energy transfer out of the high rotational levels of CH(A2Δ) to flame gases is found to be smaller than 0.6 Å2 and that for transfer to CO2 is about 2 Å2.A statistical distribution also has been observed in the vibrational energy distribution of electronically excited C2. The corresponding temperature is about 6 500 °K and is independent of the composition of the gas mixture and of the fuel. It decreases very slowly with increasing pressure above 3 Torr. Inert diluents added to the gas mixture do not alter this temperature.The cross section for de-excitation of C2(A3πg) is found to be smaller than 2.5 Å2.

Perchloric acid flames: Part III. Chemical structure of methane flames

The chemical structure of premixed flames of methane and perchloric acid vapor diluted with argon has been obtained in terms of composition, temperature, and velocity profiles. All flames were at a pressure of 14.5 Torr and mixture ratios ranged from 0.8 to 1.7 (where mixture ratio is the fuel/oxidizer ratio in the mixture divided by that in a stoichiometric mixture). Composition data were measured for hydrogen, methane, carbon monoxide, carbon dioxide, oxygen, argon, and methyl chloride, and were estimated for water and other species by an atom balance. From these profiles, calculations were made of mass-fraction fluxes and net reaction rates for methane and carbon dioxide in all flames, and for other species in the stoichiometric flame. It has been shown that, in comparison with methane-oxygen flames of similar temperature, methane-perchloric acid flames have an increased rate of carbon dioxide formation early in the flame and that they have an enhanced consumption of methane in the reaction zone. The chlorine concentration is a maximum in the reaction zone. A reaction mechanism has been proposed to account for these observations and for other known features of perchloric acid flames. The main step in this mechanism is the consumption of methane by ClO radicals. In a stoichiometric flame, the rate constant for the reaction CO+OH→CO2+H has been estimated to be 1.3×1012 cm3 mole−1 sec−1 at 2200°K.

Peculiarities of laminar- and turbulent-flame flashbacks

This work presents experimental data on the investigation of laminar- and turbulent-flame flashbacks in mixtures of methane and hydrogen with oxygen and air. It is shown that flashback of laminar flames in methane oxygen mixtures, preheated various amounts from 20°C to 400°C, is described by the gradient theory of Lewis and von Elbe. Flashback of laminar flames into nozzles is determined by the shape of velocity profile at the outlet. If the gas velocity at the axis is minimal, the flame has the shape of a meniscus and flashes back through the center. If the gas velocity at the axis is maximal, the flame has the usual shape and flashes back at the wall. With a flat gas-velocity profile, the flash-back velocity is minimal and equal to U L . Experimental data on the flashback of turbulent methane-air and methane-oxygen flames into a tube proved to be in accordance with the suggestion that flashback occurs at the wall and obeys the following law: Re=2.0 Pe f 1.10 The curve of turbulent flashback velocity in hydrogen-air mixtures is characterized by anomalous shifting, with respect to the curve of laminar flashback velocity.

Structure, kinetics, and mechanism of a methane-oxygen flame inhibited with methyl bromide

Experimental profiles of temperature and composition were obtained for a 0.05-atm, lean, methane-oxygen flame inhibited with methyl bromide. The net rates of reaction were calculated and a hydroxyl-radical concentration derived from the measured methane rate and the known rate constant. The hydroxyl-radical concentration is about equal to the thermal equilibrium value in contrast to the excess radical concentration usually found in hydrocarbon flames indicating a reduction of radical concentration in the presence of inhibitor. The primary reaction of methyl bromide is probably CH 3 Br+OH→CH 2 Br+H 2 O A reaction rate constant of k ≈1.5×10 13 cm 3 /mole/sec is calculated for the temperature range 1800°–2000°K. The calculated rates of hydrogen bromide, methyl bromide, and methane show that the reaction of methane does not begin until most of the hydrogen bromide and part of the methyl bromide have reacted. A comparison of methane rates indicates that the primary reaction zone in the inhibited flame is shifted to a higher temperature, is narrower, and has a greater absolute value than in an uninhibited flame. These observations are used to support a flame-inhibition mechanism in which a major effect of the inhibitor is to prolong the preignition zone and shift the primary reaction to a higher temperature. The inhibition reactions have a lower activation energy so that radicals which diffuse into the preignition zone react preferentially with the inhibitor. These reactions do not initiate chains so that the rate of radical build-up is decreased and ignition is prolonged until a higher temperature is reached.

Radical concentrations and reactions in a methane-oxygen flame

A complete set of characteristic profles is presented for a spherical methane-oxygen flame [(CH4)=0.078; (O2)=0.92; P=0.05 atm]. The internsive properties which are presented as a function of radial distance include measured temperature, derived velocity, measured compositions of all of the stable species and measured and derived concentrations of the major free radical species. The new type of information presented is the radical concentrations. Oxygen atoms were measured by a combination of microprobe, sampling and chemical scavening. Hydrogen atom concentrations were derived from the observed rate of oxygen reaction and confirmed by a preliminary scavenger sampling study. Hydroxyl radical concentrations were derived from the measured rate of carbon dioxide appearance and confirmed by direct measurement using the UV absoption. An upper limit has been set for methyl radical concentrations. These data have been used to test various proposed mechanisms in this flame. Two alternative mechanisms are suggested. For the reaction OH+CH4→H2O+CH3 the data suggests an activation energy of 6.5 kilocalories/mole and a frequency factor of 1.4×10−14 cc/moles/sec.

Experimental chemical kinetics from methane-oxygen laminar flame structure

Experimental chemical kinetics from methane-oxygen laminar flame structure

The Detection of Atoms and Free Radicals in Flames by Mass Spectrometric Techniques

A mass spectrometric method employing a molecular beam gas sampling system has been developed for the detection of atoms and radicals in chemical reactions. Background signals have been virtually eliminated by mechanically modulating the molecular beam and applying phase detection to the ion signal. The application of the method is illustrated by examples of low-pressure flames. In the hydrogen-oxygen flame H, O, and OH have been positively identified. The mass spectrum of a simple hydrocarbon flame, such as the methane-oxygen flame, is complicated by the presence of a large number of stable components generated in the flame. In the methane-oxygen flame the stable intermediates include C2H2, CO, CH2O or C2H6, CH4O, and C4H2. The methyl radical has been clearly identified in the methane-oxygen flame. A search for the HO2 radical in the hydrogen-oxygen flame was made without obtaining positive results. The HO2 detection problem is discussed in detail.

Rotational Temperatures of OH in Methane-Air Flames

Rotational line intensity distributions in emission and absorption of the 0↔0 band of the 2Σ↔2Π transition of OH have been measured in methane-air flames. The number of electronically excited OH radicals in the inner cone greatly exceeds the equilibrium concentration, and the intensity distribution of the high quantum numbers gives a temperature of 5200°K. However, OH radicals in the ground state were in thermal equilibrium at 2000°K. A break in the intensity distribution indicates a two stage combustion reaction. It is suggested that the excessive amount of excited OH is due to its formation in the intermediate reaction steps. Line reversal temperatures using Na, Hg, and OH have been measured in the same flame for purposes of comparison. Also the rotational temperature of excited OH in a methane-oxygen flame was measured.