| OCR Text |
Show Mixtures of methane and air corresponding to the lean flammability limits at various initial temperatures tend to be associated with an approximately constant flame temperature [5]. For lean limit mixtures the addition of a diluent to the fuel or the mixture can be viewed to replace some of the excess air. The maximum amount of diluent that can be tolerated will be approximately that which will produce a stoichiometric fuel air ratio in the total mixture [6]. O n the other hand, the increasing presence of a diluent in rich limit mixtures can be viewed as a gradual replacement of some of the excess fuel with the diluent while providing approximately the same energy release levels. Accordingly, the flammable mixture region will narrow gradually with the addition of increasing amounts of the diluent to the fuel. Preheating of mixtures using energy from sources other than the combustion of the fuel itself or reducing heat losses from the combustion zone to the surroundings would permit combustion with higher concentrations of diluents in the fuel. Fig. (5) shows the flammability limits for methane - carbon dioxide for two initial temperatures [7]. A plot of such data as suggested by Wierzba et al. [6 ] in the form of the inverse of the flammability limits versus the concentration of methane in the fuel mixture will yield, as shown in Fig. (6) straight lines that will provide convenient means for predicting the limit in accordance with the following relationships: for the lean limit: 1001^ = YF-LL l * aL(l00 - YF) and for the rich limit: 1001^ = YF-LR l + afi(100 - YF) whenL^L and L^ R are the lean and rich limits of the methane-carbon dioxide mixture respectively (% by volume) and L L and L R are the corresponding lean and rich limits of pure methane (% by volume) while Y F is the concentration of the methane in the methane - carbon dioxide mixture (% by volume); aLand aR constants that were found to be - 0.01 and + 0.0225 respectively over the temperature range of 21°C to 350°C. The presence of diluents in the methane will also affect significantly flame propagation within flowing streams of homogeneous fuel-air mixtures. The flammable range, established on the basis whether an ignition source of adequate energy can initiate a propagating flame or not, narrows significantly with the increase of the stream velocity [8], as shown typically in Fig.(7). For sufficiently fast streams, flame propagation cannot be achieved even with stoichiometric mixtures since the rate of dissipation of the energy release by the flame is much too fast to permit adequate time for the chemical energy to be released and propagate the reaction further [9]. As can be seen in Fig (7), for any stream velocity, the flammable mixture range narrows rapidly as the concentration of carbon dioxide in the methane-carbon dioxide fuel mixture increases. Very fast streams cannot tolerate the excessive presence of carbon dioxide. Throughout, the stoichiometric mixture as expected remains to be the most capable of supporting flame propagation for the highest of carbon dioxide and/or turbulent velocities. Changes in the scale of turbulence can modify these limits somewhat. Larger scales tend to widen the limits. Thus, the presence of carbon dioxide with the methane will not only narrow the combustible mixture range but will make it more sensitive to the velocity level and turbulent characteristics of the mixture to be burnt. 6 |
