Among the methods the results from the detailed ame

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Unformatted text preview: ibbs free energy, even without knowledge of the chemical kinetics. Computer programs (such as STANJAN, Chemkin, Cantera) are preferred for this task, as hand calculations are time consuming. 2.4.2 Comparison of Adiabatic Flame Temperature Calculation Methods The presented methods of estimating adiabatic flame temperature will produce different values from each other. Predicted adiabatic flame temperatures of a methane/air mixture at ambient pressure using these methods are compared in Fig. 2.4 for a range of equivalence ratios. Also included are the results from a flame calculation using a detailed, non-equilibrium flame model. On the lean side, the results agree reasonably well among all methods, as the major products are CO2, H2O, unburned O2, and N2. Visible deviations arise near stoichiometric conditions and become larger in richer mixtures. One reason for the deviation is the assumptions made about product species in the rich mixtures. For rich mixtures at the equilibrium state, CO is preferred over CO2 due to the deficiency in O2. Because the conversion of CO into CO2 releases a large amount of energy, the rich mixture equilibrium temperatures are lower than those from the flame calculation, which has a residence time of less than 1 s. Among the methods, the results from the detailed flame model calculations are closest to reality, as real flames have finite residence times and generally do not reach equilibrium. Example 2.6. Estimate the adiabatic flame temperature of a constant-pressure reactor burning a stoichiometric mixture of H2 and air at 101.3 kPa and 25 C at the inlet. Solution: The combustion stoichiometry is H2(g) þ 0.5 H2O (g) þ 1.88 N2(g) X X ^ ^ ÀQ0 ;p ¼ Ni;R Dho;R À Ni;P Dho;P rxn i i i (O2(g) +3.76 i ^ ^ ^ ^ ¼ Dho 2 þ 0:5Dho 2 þ 1:88Dho 2 À 1 Á Dho 2O H O N H ¼ 0 þ 0 þ 0 À 1 mol Á ðÀ241:88 kJ/molÞ ¼ 241:88 kJ N2(g)) ! 2.4 Adiabatic Flame Temperature 37 Method 1: Assuming a constant ^ cp; H2 Oð1; 500 KÞ ¼ 0:0467 kJ/mol À K and ^ cp (average) at 1,500 K, ^ cp;N2 ð1;500 KÞ ¼ 0:0350 kJ/mol À K : P ^ ÀQ0 ;p þ Ni;R hsi;R ðTR...
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