N2 210 21 where air consisting of 21 o2 and 79 n2 is

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Unformatted text preview: stion   79 CH4 þ ? O2 þ N2 ! ?CO2 þ ?H2 O þ ?N2 ; (2.10) 21 where air consisting of 21% O2 and 79% N2 is assumed.1 The coefficients associated with each species in the above equation are unknown. By balancing the atomic 18 2 Thermodynamics of Combustion abundance on both the reactant and product sides, one can find the coefficient for each species. For instance, let’s determine the coefficient for CO2: on the reactant side, we have 1 mol of C atoms; hence the product side should also have 1 mol of C atoms. The coefficient of CO2 is therefore unity. Using this procedure we can determine all the coefficients. These coefficients are called the reaction stoichiometric coefficients. For stoichiometric methane combustion with air, the balanced reaction equation reads: CH4 þ 2ðO2 þ 3:76N2 Þ ! 1CO2 þ 2H2 O þ 7:52N2 : (2.11) Note that on the reactant side there are 2·(1 + 3.76) or 9.52 mol of air and its molecular mass is 28.96 kg/kmol. In this text, the reactions are balanced using 1 mol of fuel. This is done here to simplify the calculations of the heat of reaction and flame temperature later in the chapter. Combustion stoichiometry for a general hydrocarbon fuel, Ca Hb Og , with air can be expressed as     bg b bg Ca Hb Og þ a þ À ðO2 þ 3:76N2 Þ!aCO2 þ H2 O þ 3:76 a þ À N2 : (2.12) 42 2 42 The amount of air required for combusting a stoichiometric mixture is called stoichiometric or theoretical air. The above formula is for a single-component fuel and cannot be applied to a fuel consisting of multiple components. There are two typical approaches for systems with multiple fuels. Examples are given here for a fuel mixture containing 95% methane and 5% hydrogen. The first method develops the stoichiometry of combustion using the general principle of atomic balance, making sure that the total number of each type of atom (C, H, N, O) is the same in the products and the reactants. 0:95CH4 þ 0:05H2 þ 1:925ðO2 þ 3:76N2 Þ ! 0:95CO2 þ 1:95H2 O þ 7:238N2 : The...
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This document was uploaded on 01/20/2014.

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