Thermodynamics

C 8 H 18 a th 1O 2 3.76N 2 2 S 8CO 2 9H 2 O 12 3.76a th N 2FIGURE 15–20Chapter 15 | 765Solution The enthalpy of combustion of a fuel is to be determined usingh C = H prod – H reactenthalpy of formation data.C 8 H 18 ()Properties The enthalpy of formation at 25°C and 1 atm is 393,520kJ/kmol for CO 2 , 285,830 kJ/kmol for H 2 O(), and 249,950 kJ/kmol for25°C, 1 atmCO25°C 2C 8 H 18 () (Table A–26).AIR1 atm H 2 O()NAnalysis The combustion of C 8 H 18 is illustrated in Fig. 15–20. The stoichiometricequation for this reaction is225°C, 1 atmBoth the reactants and the products are at the standard reference state of Schematic for Example 15–5.25°C and 1 atm. Also, N 2 and O 2 are stable elements, and thus theirenthalpy of formation is zero. Then the enthalpy of combustion of C 8 H 18becomes (Eq. 15–6)h C H prod H reactSubstituting, a N p h° f,p a N r h° f,r 1Nh° f 2 CO2 1Nh° f 2 H2 O 1Nh° f 2 C8 H 18h C 18 kmol2 1393,520 kJ>kmol2 19 kmol2 1285,830 kJ>kmol2 11 kmol21249,950 kJ>kmol2 5,471,000 kJ>kmol C 8 H 18 47,891 kJ>kg C 8 H 18which is practially identical to the listed value of 47,890 kJ/kg in TableA–27. Since the water in the products is assumed to be in the liquid phase,this h C value corresponds to the HHV of liquid C 8 H 18 .Discussion It can be shown that the result for gaseous octane is5,512,200 kJ/kmol or 48,255 kJ/kg.When the exact composition of the fuel is known, the enthalpy of combustionof that fuel can be determined using enthalpy of formation data asshown above. However, for fuels that exhibit considerable variations incomposition depending on the source, such as coal, natural gas, and fuel oil,it is more practical to determine their enthalpy of combustion experimentallyby burning them directly in a bomb calorimeter at constant volume orin a steady-flow device.15–4 FIRST-LAW ANALYSISOF REACTING SYSTEMSThe energy balance (or the first-law) relations developed in Chaps. 4 and 5are applicable to both reacting and nonreacting systems. However, chemicallyreacting systems involve changes in their chemical energy, and thus itis more convenient to rewrite the energy balance relations so that thechanges in chemical energies are explicitly expressed. We do this first forsteady-flow systems and then for closed systems.Steady-Flow SystemsBefore writing the energy balance relation, we need to express the enthalpyof a component in a form suitable for use for reacting systems. That is, weneed to express the enthalpy such that it is relative to the standard reference