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Show 35 30 25 20 15 : ' No t •••"' FU Nor. vflamm >-•''" immabk able Rich ..••"7 Lean . -flammable . . . . I •" _j / r ( ( y i , 21°C 3S0*C 0 10 20 30 40 50 60 70 80 CARBON DIOXIDE IN T H E MIXTURE. % BY VOLUME Fig. 5 Variation in the lean and rich limits of methane-carbon dioxide mixtures for two initial temperatures [7] at atmospheric pressure. Rich E I | J ^ Flammable 2 J .' /•' 1 : f X r / 21*C 350°C Non-flammable Lean 0.00 0.05 0.10 015 0.20 025 0.30 0.35 (FLAMMABILITY LIMITS OF CH^-COj IN AIR. % BY VOLUME)'1 Fig. 6 Plots of the inverse of the flammability limits in air, both lean and rich limits, for methane-carbon dioxide mixtures versus the concentration of methane in the fuel. The laminar burning velocity of methane is known to be lowered significantly as the concentration of the carbon dioxide added to the methane is increased [10]. This is mainly a consequence of the reduction in the reaction rates, flame temperature, diffusivity and transport properties of the mixture. Similarly, the rate of initial flame spread within flowing homogeneous methane-carbon dioxide-air streams following spark ignition will also be reduced significantly from those values obtained with methane, as the concentration of carbon dioxide is increased. A s shown typically in Fig. (8), for a stream within a tubular reactor flowing with a R e of 2000, the flame spread rate is reduced, while the range of equivalence ratio over which the flame can be made to propagate is narrowed very significantly. With flows of higher velocity, the relative reduction in the flame propagation rates on the addition of carbon dioxide to the methane is yet greater. Moreover, the extent of cyclic variation in the flame initiation phase, as well as the subsequent flame propagation within combustible flowing streams increases virtually linearly with the extent of carbon dioxide presence in the methane-carbon dioxide mixture [11]. Thus, the unsteadiness in flame initiation and flame propagation contributes further to the difficulties encountered in the combustion of methane in the presence of carbon dioxide. The stoichiometric mixture range remains to be the most tolerant to the presence of carbon dioxide in the fuel. The increased presence of carbon dioxide with the methane undermines the stability of diffusion jet flames stabilized on top of circular burners. As shown typically in Fig. (9), the jet flame blow- out velocity in co-flowing streams of air are markedly reduced as the concentration of carbon dioxide was increased in the fuel used. This would lead to a much reduced thermal energy release for any burner installation and would also make the flame less tolerant to changes in the surrounding velocity [12]. 7 |