Lean premix flames

Emissions Control. From the combustion chemistry standpoint, lean mixtures produce the least amount of emissions. Hence, one pollution prevention alternative would be to use lean premixed flames. However, lean mixtures are difficult to ignite and form unstable flames. Furthermore, thek combustion rates are very low and can seldom be appHed dkectly without additional measures being taken. Consequently the use of lean mixtures is not practical. 

Nonpremixed edge flames (a) 2D mixing layer (From Kioni, P.N., Rogg, B., Bray, K.N.C., and Linan, A., Combust. Flame, 95, 276, 1993. With permission.), (b) laminar jet (From Chung, S.H. and Lee, B.J., Combust. Flame, 86, 62,1991.), (c) flame spread (From Miller, F.J., Easton, J.W., Marchese, A.J., and Ross, H.D., Proc. Combust. Inst., 29, 2561, 2002. With permission.), (d) autoignition front (From Vervisch, L. and Poinsot, T., Annu. Rev. Fluid Mech., 30, 655, 1998. With permission.), and (e) spiral flame in von Karman swirling flow (From Nayagam, V. and Williams, F.A., Combust. Sci. Tech., 176, 2125, 2004. With permission.). (LPF lean premixed flame, RPF rich premixed flame, DF diffusion flame). 

Figure 9 Simplified representation of different catalytic combustor concepts. A Multimonolith combustor B partial catalytic combustor C hybrid combustor. LPF = lean premixed flame.

NO regulations have become more severe each year throughout the world. Therefore, low NO combustion technologies for gas turbines, such as water/steam injection, lean diffusion flame combustion, and lean premixed flame combustion, have been developed and used for commercial gas turbines. However, NO levels are still not low enough to meet some of the more severe regulations in some areas. In such cases, catalytic combustion is considered to be the best method to achieve the reqired NO levels. Catalysts support stable combustion with a lower flame temperature than the flammable limit, and NO is not produced at such a flame temperature. 

Among the various selection considerations are specific combustion characteristics of different fuels. One of the combustion characteristics of gaseous fuels is their flammability limit. The flammability limit refers to the mixture proportions of fuel and air that will sustain a premixed flame when there is either limited or excess air available. If there is a large amount of fuel mixed with a small amount of air, then there is a limiting ratio of fuel to air at which the mixture will no longer sustain a flame. This limit is called the rich flammability limit. If there is a small amount of fuel mixed with excess air, then there is a limiting ratio of the two at which the flame will not propagate.This limit is called the lean flammability limit. Different fuels have different flammability limits and these must be identified for each fuel. 

Y. Huang and V. Yang. Bifurcation of flame structure in a lean-premixed swirl-stabilized combustor Transition from stable to unstable flame. Combust. Flame, 136(3) 383-389, 2004. 

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... 

It is also well known that there exist different extinction modes in the presence of radiative heat loss (RHL) from the stretched premixed flame (e.g.. Refs. [8-13]). When RHL is included, the radiative flames can behave differently from the adiabatic ones, both qualitatively and quantitatively. Figure 6.3.1 shows the computed maximum flame temperature as a function of the stretch rate xfor lean counterflow methane/air flames of equivalence ratio (j) = 0.455, with and without RHL. The stretch rate in this case is defined as the negative maximum of the local axial-velocity gradient ahead of the thermal mixing layer. For the lean methane/air flames,... 

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. 

Cannon, S. M., B. S. Brewster, and L. D. Smoot (1999). PDF modeling of lean premixed combustion using in situ tabulated chemistry. Combustion and Flame 119, 233-252. 

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 tendency of premixed flames to detach from the flame holder to stabilize further downstream has also been reported close to the flammability limit in a two-dimensional sudden expansion flow [27]. The change in flame position in the present annular flow arrangement was a consequence of flow oscillations associated with rough combustion, and the flame can be particularly susceptible to detachment and possible extinction, especially at values of equivalence ratio close to the lean flammability limit. Measurements of extinction in opposed jet flames subject to pressure oscillations [28] show that a number of cycles of local flame extinction and relight were required before the flame finally blew off. The number of cycles over which the extinction process occurred depended on the frequency and amplitude of the oscillated input and the equivalence ratios in the opposed jets. Thus the onset of large amplitudes of oscillations in the lean combustor is not likely to lead to instantaneous blow-off, and the availability of a control mechanism to respond to the naturally occurring oscillations at their onset can slow down the progress towards total extinction and restore a stable flame. 

The premixed flame model and calculation procedures of Westen-berg (18) were used to estimate Idnetically limited CO oxidation for lean... 

Air and fuel are intimately mixed prior to entenng the combustion zone, and hence the local air-fiiel ratios can be controlled and variations minimized. The homogeneous air-fuel mixture is combusted at a very high air-fuel ratio, and hence involves only a small adiabatic temperature nse. Therefore, the maximum temperature in the combustor is kept at levels at which the thermal NOx concentration is low. Lean-premix systems suffer from stability problems at ultralean conditions, i.e, low temperatures, and may therefore require stabilization by a diffusion flame [28] It was already mentioned previously that optimizing both CO/UHC and NO is difficult. This can be deduced from Fig. 3, showing the NO and CO emissions versus adiabatic flame temperature for a hypothetical lean-premix combustor. At low temperatures, the CO emissions increase rapidly, due to flame instability (the comparable increase in NOx levels at high temperatures has already been discussed in the previous section). It is clear that a temperature zone exists where low levels of both CO and NOx niay be obtained. 

Figure 3 Influence of adiabatic flame temperature on CO and NO emissions for a hypothetical lean premix combustor (From Ref 26)...

Fig. 14 (A) Premixed flames of trichloroethylene under fuel-rich, (B) stoichiometric, and (C) fuel-rich conditions. In contrast to regular hydrocarbons, soot formation can be seen in CHC flames under stoichiometric, and in some cases even under fuel lean conditions.

NO c emissions can also be reduced by using a dilute fuel/air mixture in so-called lean premixed combustion. Air and fuel are premixed at a low fuel air ratio before entering the combustor, resulting in a lower flame temperature and hence lower NOj emissions. Lean premix combustors (sometimes called dry low NO c combustors) based on this concept have generally achieved NO c levels of 25 ppm in commercial operation. To achieve lower NO levels, however, this technology must overcome some significant hurdles. As the fuel air mixture is increasingly diluted, the flame temperature approaches the flammability limit and the flame becomes unstable. The flame instability produces noise and vibration that can reduce the combustor life, increase maintenance costs and adversely impact the operational reliability of the turbine [3]. 

This experimental investigation was motivated by the requirements of lean-premixed methane-air flames in modern gas-turbine combustors and the periodic extinction and relight observed close to the lean limit [1]. The first involves low equivalence ratios with possible dynamic effects, and the second involves a strain rate mechanism that may imply oscillations in bluff-body stabilized flames at all equivalence ratios. Opposed flames are used here to examine the nature of extinction, and to a lesser extent ignition to quantify extinction velocities and times and to determine limitations of this comparatively simple arrangement. The same arrangement was used in investigations of the corresponding isothermal flow [2]. 

At the Imperial College of Science in London, Whitelaw and his researchers (Chapter 6) conducted detailed experiments with turbulent premixed opposed-jet flames relevant to modern gas-turbine technologies based on lean premixed combustion. Using photography and chemiluminescence imaging, they examined the effects of equivalence ratio, bulk flow velocity, and flow separation on... 

Assess and verify the flow, flame eharacteristies and d5mamic stability limits of the lean premixed hydrogen swirl burner. 

Establish a framework for modeling hydrogen-enriched lean premixed combustion in the presence of acoustically active flame processes. 

---