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Unformatted text preview: .30) i Using the internal energy relations, we can re-express the heat release at constant volume in terms of enthalpies as ÀQ0 ;v ¼ UR À UP rxn ¼ X ^ Ni;R Dho;R i À i ¼ X ^ Ni;R Ru T0 À " X i X ^ Ni;R Dho;R À i i X ^ Ni;P Dho;P i X À i ^ Ni;P Dho;P þ i i X i Ni;P À i X # ^ Ni;P Ru T0 ! ^ Ni;R Ru T0 i (2.31) Therefore, HHV for combustion processes is calculated as HHV ¼ ÀQ0 ;v rxn À  P Ni;P À P i Nfuel Mfuel i  ^ Ni;R Ru T0 ; (2.32) 28 2 Thermodynamics of Combustion where Nfuel is the number of moles of fuel burned and Mfuel is the molecular mass of the fuel. The negative sign in front of Q0 ;v is to make sure that HHV is rxn positive. For a general fuel, CaHbOg, the difference between –Qrxn,v and –Qrxn,p is !   X X ^ ^u T0 ¼ DN Ru T0 ¼ b þ g À 1 Ru T0 ^ Ni;P À Ni;R R (2.33) 42 i i and is usually small in comparison to HHV; therefore normally no distinction is made between the heat of reaction at constant pressure or constant volume. Experimental Determination of HHV: The Bomb Calorimeter To experimentally measure the heating value of a fuel, the fuel and air are often enclosed in an explosive-proof steel container (see Fig. 2.2), whose volume does not change during a reaction. The vessel is then submerged in water or another liquid that absorbs the heat of combustion. The heat capacitance of the vessel plus the liquid is then measured using the same technique as other calorimeters. Such an instrument is called a bomb calorimeter. A constant-volume analysis of the bomb calorimeter data is used to determine the heating value of a particular fuel. The fuel is burned with sufficient oxidizer in a closed system. The closed system is cooled by heat transfer to the surroundings such that the final temperature is the same as the initial temperature. The standard conditions are set for evaluation of heating values. Conservation of energy gives UP À UR ¼ Q0 ;v rxn (2.34) Thermocouple Stirrer Igniter Insulated container filled with water Reaction chamber (bomb) Sample cup Fig. 2.2...
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This document was uploaded on 01/20/2014.

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