Diffusion flames hydrocarbon

The vapor cloud of evaporated droplets bums like a diffusion flame in the turbulent state rather than as individual droplets. In the core of the spray, where droplets are evaporating, a rich mixture exists and soot formation occurs. Surrounding this core is a rich mixture zone where CO production is high and a flame front exists. Air entrainment completes the combustion, oxidizing CO to CO2 and burning the soot. Soot bumup releases radiant energy and controls flame emissivity. The relatively slow rate of soot burning compared with the rate of oxidation of CO and unbumed hydrocarbons leads to smoke formation. This model of a diffusion-controlled primary flame zone makes it possible to relate fuel chemistry to the behavior of fuels in combustors (7). 

Combustion chemistry in diffusion flames is not as simple as is assumed in most theoretical models. Evidence obtained by adsorption and emission spectroscopy (37) and by sampling (38) shows that hydrocarbon fuels undergo appreciable pyrolysis in the fuel jet before oxidation occurs. Eurther evidence for the existence of pyrolysis is provided by sampling of diffusion flames (39). In general, the preflame pyrolysis reactions may not be very important in terms of the gross features of the flame, particularly flame height, but they may account for the formation of carbon while the presence of OH radicals may provide a path for NO formation, particularly on the oxidant side of the flame (39). 

Smith, S.R. and Gordon, A.S., Precombustion reactions in hydrocarbon diffusion flames The paraffin candle flame, /. Chem. Phys., 22, 1150,1954. 

CNTs can also be produced by diffusion flame synthesis, electrolysis, use of solar energy, heat treatment of a polymer, and low temperature solid pyrolysis. In flame synthesis, combustion of a portion of the hydrocarbon gas provides the elevated temperature required, with the remaining fuel conveniently serving as the required hydrocarbon reagent. Hence, the flame constitutes an efficient source of both energy and hydrocarbon raw material. Combustion synthesis has been shown to be scalable for a high volume commercial production. 

Jones, W. P. andM. Kakhi (1997). Application of the transported PDF approach to hydrocarbon turbulent jet diffusion flames. Combustion Science and Technology 129, 393 130. 

Since diffusion rates vary with pressure and the rate of overall combustion reactions varies approximately with the pressure squared, at very low pressures the flame formed will exhibit premixed combustion characteristics even though the fuel and oxidizer may be separate concentric gaseous streams. Figure 6.1 details how the flame structure varies with pressure for such a configuration where the fuel is a simple higher-order hydrocarbon [1], Normally, the concentric fuel-oxidizer configuration is typical of diffusion flame processes. 

As Kang el al. [3] have reported, counter-flow diffusion flames are located on the oxidizer side when hydrocarbons are the fuel. Appropriate dilution with inert gases of both the fuel and oxidizer streams, frequently used in the co-flow situation, can position the flame on the fuel side. It has been shown [4] that the criterion for the flame to be located on the fuel side is... 

The relative importance of these three mechanisms in NO formation and the total amount of prompt NO formed depend on conditions in the combustor. Acceleration of NO formation by nonequilibrium radical concentrations appears to be more important in non-premixed flames, in stirred reactors for lean conditions, and in low-pressure premixed flames, accounting for up to 80% of the total NO formation. Prompt NO formation by the hydrocarbon radical-molecular nitrogen mechanism is dominant in fuel-rich premixed hydrocarbon combustion and in hydrocarbon diffusion flames, accounting for greater than 50% of the total NO formation. Nitric oxide formation by the N20 mechanism increases in importance as the fuel-air ratio decreases, as the burned gas temperature decreases, or as pressure increases. The N20 mechanism is most important under conditions where the total NO formation rate is relatively low [1],... 

The last point is worth considering in more detail. Most hydrocarbon diffusion flames are luminous, and this luminosity is due to carbon particulates that radiate strongly at the high combustion gas temperatures. As discussed in Chapter 6, most flames appear yellow when there is particulate formation. The solid-phase particulate cloud has a very high emissivity compared to a pure gaseous system thus, soot-laden flames appreciably increase the radiant heat transfer. In fact, some systems can approach black-body conditions. Thus, when the rate of heat transfer from the combustion gases to some surface, such as a melt, is important—as is the case in certain industrial furnaces—it is beneficial to operate the system in a particular diffusion flame mode to ensure formation of carbon particles. Such particles can later be burned off with additional air to meet emission standards. But some flames are not as luminous as others. Under certain conditions the very small particles that form are oxidized in the flame front and do not create a particulate cloud. 

The presence of halogen additives substantially increases the tendency of all fuels to soot under diffusion flame conditions [69], The presence of H atoms increases the soot pyrolysis rate because the abstraction reaction of H + RH is much faster than R + RH, where R is a hydrocarbon radical. Halogenated compounds added to fuels generate halogen atoms (X) at modest temperatures. The important point is that X + RH abstraction is faster than H + RH, so that the halogen functions as a homogeneous catalyst through the system... 

Tolocka, M.P. and Miller, J.H. Production of polycyclic hydrocarbons from underventilated hydrocarbon diffusion flames, in Proceedings Chemical and Physical Processes in Combustion (Worcester, MA Combustion Institute/Eastern States section, October 16-18, 1995), pp. 253-256. 

Hewson, J. C., and M. Bollig. 1996. Reduced mechanisms for NOj, emissions from hydrocarbon diffusion flames. 26th Symposium (International) on Combustion Proceedings. Pittsburgh, PA The Combustion Institute. 2171-79. 

Blevins and Gore [14, 15] found that low-stretch-rate partially premixed flames involve multiple peaks in the profiles of intermediate hydrocarbon species. In particular, the CH species existing between the premixed and the diffusion flame part of the partially premixed flames were observed to react with NO and create an intermediate NO consumption zone. DuPont et al. [16] for low-stretch-rate flames also found the double peaks of intermediate hydrocarbon species and the NO consumption zone. However, Tanoff et al. [17] used the CH peak to characterize the location of the rich premixed flame and the OH peak to characterize the location of the diffusion flame. The NO concentration profiles showed that the peak NO mole fractions first increased and then decreased with increasing levels of partial premixing. However, the emission index of NO was not reported. 

Vander Wal, R. L., K. A. Jensen, and M. Y. Choi, Simultaneous Laser-Induced Emission of Soot and Polycyclic Aromatic Hydrocarbons within a Gas-Jet Diffusion Flame, Combust. Flame, 109, 399-414 (1997). 

Diffusion Flame. When a slow stream of fuel g s flows from a tube into the atmosphere, air diffuses across the boundary of the stream and Brms an envelope of expl mixture around a core of gas. The core decreases in height until it disappears at some distance above the tube. It thus assumes the shape of a cone. On ignition, a flame front spreads thru the mixture and stabilizes itself around the cooe of fuel gas. The hydrocarbons in common fuel gases crack to form free C H. The shell of carbon-bearing gas so formed gives such flames their luminosity Turbulent Jet Flame. When a gas stream issues from an orifice above a certain critical velocity, it breaks up into a turbulent jet that entrains the surrounding air. The flame of such a jet consists of random patches of combustion and no cohesive combustion surface exists... 

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