26 i usually excess air is used in such a test to

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Unformatted text preview: ^ ^ ^ ÀQrxn:p ¼ HR À Hp ¼ Ni;R Dho;R þ hsi;R À Ni;P Dho;P þ hsi;P i i " ¼ i X i ^ Ni;R Dho;R À i X # ^ Ni;P Dho;P þ i i X i ^ Ni;R hsi;R À i X ^ Ni;P hsi;P ; (2.25) i where Ni is the number of moles of species i. The sensible enthalpies of common reactants and products can be found in Appendix 1. When the products are cooled to the same conditions as the reactants, the amount of heat transfer from the constant-pressure reactor to the surroundings is defined as the heating value. At STP the sensible enthalpy terms drop out for both reactants and products and the heat release is À Q0 ;p ¼ rxn X ^ Ni; R Dho;R À i i X ^ Ni;P Dho;P i (2.26) i Usually excess air is used in such a test to ensure complete combustion. The amount of excess air used will not affect À Q0 ;p at STP. Unless the reactant rxn mixtures are heavily diluted, the water in the products at STP normally will be liquid.3 Assuming that water in the products is liquid, HHV is determined: HHV ¼ ÀQ0 ; p rxn : Nfuel Mfuel The negative sign in front of Q0 ;p ensures that HHV is positive. rxn (2.27) 2.3 Heating Values 2.3.2 27 Determination of HHV for Combustion Processes from a Constant-Volume Reactor A constant-volume reactor is more convenient than the constant-pressure reactor to experimentally determine the HHV of a particular fuel. For a closed system, conservation of energy yields À Qrxn;v ¼ UR À Up (2.28) Because of the combustion process, the same type of accounting must be used to include the change in chemical bond energies. The internal energy will be evaluated by using its relation to enthalpy. Note that relation h ¼ u + pv is mass based and ^^^ the corresponding molar base relation is h ¼ u þ Ru T . At STP (T ¼ T0 ¼ 25 C), the total internal energy of the reactants, UR, inside the closed system is UR ¼ HR À PV X ^ Ni;R Ru T0 ¼ HR À ¼ X i ^ Ni;R Dho;R À i X i (2.29) ^ Ni;R Ru T0 i The total internal energy of products is evaluated in a similar manner: UP ¼ X ^ Ni;P Dho;P À i X i ^ Ni;P Ru T0 (2...
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This document was uploaded on 01/20/2014.

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