ThermoSolutions-CHAPTER15

ThermoSolutions-CHAPTER15 - 15-1 Chapter 15 CHEMICAL...

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15-1 Chapter 15 CHEMICAL REACTIONS Fuels and Combustion 15-1C Gasoline is C 8 H 18 , diesel fuel is C 12 H 26 , and natural gas is CH 4 . 15-2C Nitrogen, in general, does not react with other chemical species during a combustion process but its presence affects the outcome of the process because nitrogen absorbs a large proportion of the heat released during the chemical process. 15-3C Moisture, in general, does not react chemically with any of the species present in the combustion chamber, but it absorbs some of the energy released during combustion, and it raises the dew point temperature of the combustion gases. 15-4C The dew-point temperature of the product gases is the temperature at which the water vapor in the product gases starts to condense as the gases are cooled at constant pressure. It is the saturation temperature corresponding to the vapor pressure of the product gases. 15-5C The number of atoms are preserved during a chemical reaction, but the total mole numbers are not. 15-6C Air-fuel ratio is the ratio of the mass of air to the mass of fuel during a combustion process. Fuel- air ratio is the inverse of the air-fuel ratio. 15-7C No. Because the molar mass of the fuel and the molar mass of the air, in general, are different. Theoretical and Actual Combustion Processes 15-8C The causes of incomplete combustion are insufficient time, insufficient oxygen, insufficient mixing, and dissociation. 15-9C CO. Because oxygen is more strongly attracted to hydrogen than it is to carbon, and hydrogen is usually burned to completion even when there is a deficiency of oxygen. 15-10C It represent the amount of air that contains the exact amount of oxygen needed for complete combustion. 15-11C No. The theoretical combustion is also complete, but the products of theoretical combustion does not contain any uncombined oxygen. 15-12C Case (b).
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15-2 15-13 Methane is burned with the stoichiometric amount of air during a combustion process. The AF and FA ratios are to be determined. Assumptions 1 Combustion is complete. 2 The combustion products contain CO 2 , H 2 O, and N 2 only. Properties The molar masses of C, H 2 , and air are 12 kg/kmol, 2 kg/kmol, and 29 kg/kmol, respectively (Table A-1). Analysis This is a theoretical combustion process since methane is burned completely with stoichiometric amount of air. The stoichiometric combustion equation of CH 4 is Air stoichiometric CH 4 Products >@ 2 th 2 2 2 2 th 4 N 3.76 O 2H CO 3.76N O CH a a . . ¡o ¡ . . O 2 balance: aa th th 11 2 . ¡ Substituting, 2 2 2 2 2 4 7.52N O 2H CO 3.76N O 2 CH . . ¡ . . The air-fuel ratio is determined by taking the ratio of the mass of the air to the mass of the fuel, +, + , + , + , + , fuel air/kg kg 17.3 . u kg/kmol 2 kmol 2 kg/kmol 12 kmol 1 kg/kmol 29 kmol 4.76 2 AF fuel air m m The fuel-air ratio is the inverse of the air-fuel ratio, air fuel/kg kg 0.0578 fuel air/kg kg 17.3 1 AF 1 FA 15-14 Propane is burned with 75 percent excess air during a combustion process. The AF ratio is to be determined.
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ThermoSolutions-CHAPTER15 - 15-1 Chapter 15 CHEMICAL...

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