Thermodynamics

Chapter 15 | 759Assumptions 1 The fuel is burned completely and thus all the carbon in thefuel burns to CO 2 and all the hydrogen to H 2 O. 2 The fuel is burned with thestoichiometric amount of air and thus there is no free O 2 in the productgases. 3 Combustion gases are ideal gases.Properties The saturation pressure of water at 20°C is 2.3392 kPa (Table A–4).Analysis We note that the moisture in the air does not react with anything;it simply shows up as additional H 2 O in the products. Therefore, for simplicity,we balance the combustion equation by using dry air and then add themoisture later to both sides of the equation.Considering 1 kmol of fuel,fuel1555555555555552555555555555553dry air10.72CH 4 0.09H 2 0.14N 2 0.02O 2 0.03CO 2 2 a th 1O 2 3.76N 2 2 SxCO 2 yH 2 O zN 2The unknown coefficients in the above equation are determined from massbalances on various elements,C:H:0.72 0.03 xSx 0.750.72 4 0.09 2 2ySy 1.53155525553O 2 :0.02 0.03 a th x y 2 Sa th 1.465N 2 :0.14 3.76a th zS z 5.648Next we determine the amount of moisture that accompanies 4.76a th (4.76)(1.465) 6.97 kmol of dry air. The partial pressure of the moisturein the air isAssuming ideal-gas behavior, the number of moles of the moisture in theair iswhich yieldsP v,air f air P sat @ 20°C 10.80212.3392 kPa2 1.871 kPaN v,air a P v,air1.871 kPab NP total atotal 101.325 kPa b16.97 N v,air2N v,air 0.131 kmolThe balanced combustion equation is obtained by substituting the coefficientsdetermined earlier and adding 0.131 kmol of H 2 O to both sides of theequation:fueldry air15555555555555255555555555555310.72CH 4 0.09H 2 0.14N 2 0.02O 2 0.03CO 2 2 1.465 1O 2 3.76N 2 2moisture142431555255553includes moisture14243 0.131H 2 O S 0.75CO 2 1.661H 2 O 5.648N 2The dew-point temperature of the products is the temperature at whichthe water vapor in the products starts to condense as the products are cooled.Again, assuming ideal-gas behavior, the partial pressure of the water vapor inthe combustion gases isP v,prod a N v,prod 1.661 kmolb PN prod a b1101.325 kPa2 20.88 kPaprod 8.059 kmol