When the water vapor is condensed to liquid

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Unformatted text preview: ons given in Table 2.1 to convert %EA to f and l f¼ l¼ 2.3 100 100 ¼ ¼ 0: 6 %EA þ 100 66:7 þ 100 1 ¼ 1:67 f Heating Values Heating values of a fuel (units of kJ/kg or MJ/kg) are traditionally used to quantify the maximum amount of heat that can be generated by combustion with air at standard conditions (STP) (25 C and 101.3 kPa). The amount of heat release from combustion of the fuel will depend on the phase of water in the products. If water is in the gas phase in the products, the value of total heat release is denoted as the lower heating value (LHV). When the water vapor is condensed to liquid, additional energy (equal to the latent heat of vaporization) can be extracted and the total energy release is called the higher heating value (HHV). The value of the LHV can be calculated from the HHV by subtracting the amount of energy released during the phase change of water from vapor to liquid as LHV ¼ HHV À NH2O;P MH2O hfg Nfuel Mfuel (MJ/kg), (2.22) 24 2 Thermodynamics of Combustion where NH2O,P is the number of moles of water in the products. Latent heat for water at STP is hfg ¼ 2.44 MJ/kg ¼ 43.92 MJ/kmol. In combustion literature, the LHV is normally called the enthalpy or heat of combustion (QC) and is a positive quantity. 2.3.1 Determination of HHV for Combustion Processes at Constant Pressure A control volume analysis at constant pressure with no work exchanged can be used to theoretically determine the heating values of a particular fuel. Suppose reactants with 1 kmol of fuel enter the inlet of a control volume at standard conditions and products leave at the exit. A maximum amount of heat is extracted when the products are cooled to the inlet temperature and the water is condensed. Conservation of energy for a constant pressure reactor, with HP and HR denoting the respective total enthalpies of products and reactants, gives À Qrxn;p ¼ HR À Hp : (2.23) The negative value of Qrxn,p indicates heat transfer out of the system to the surroundings. It follows from above that the heating value of the fuel is the difference in the enthalpies of the reactants a...
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This document was uploaded on 01/20/2014.

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