Thermodynamics

15–42 Determine the enthalpy of combustion of methane(CH 4 ) at 25°C and 1 atm, using the enthalpy of formationdata from Table A–26. Assume that the water in the productsis in the liquid form. Compare your result to the value listedin Table A–27. Answer: –890,330 kJ/kmolAIR12°CCombustionchamberChapter 15 | 785air–fuel ratio and (b) the fraction of water vapor that would 15–43 Reconsider Prob. 15–42. Using EES (or other)15–40C The h° f of N 2 is listed as zero. Does this mean that with 150 percent excess air that enters the combustionN 2 contains no chemical energy at the standard reference chamber at 12°C. If the combustion is complete and the exitcondense if the product gases were cooled to 20°C at 1 atm.Answers: (a) 18.6 kg air/kg fuel, (b) 88 percentsoftware, study the effect of temperature on theenthalpy of combustion. Plot the enthalpy of combustion as a15–30 Reconsider Prob. 15–29. Using EES (or other)function of temperature over the range 25 to 600°C.software, study the effects of varying the percentagesof CH 4 ,H 2 , and N 2 making up the fuel and theproduct gas temperature in the range 5 to 150°C.15–4415–45Repeat Prob. 15–42 for gaseous ethane (C 2 H 6 ).Repeat Prob. 15–42 for liquid octane (C 8 H 18 ).15–31 A certain coal has the following analysis on a mass First-Law Analysis of Reacting Systemsbasis: 82 percent C, 5 percent H 2 O, 2 percent H 2 , 1 percent 15–46C Derive an energy balance relation for a reactingO 2 , and 10 percent ash. The coal is burned with 50 percent closed system undergoing a quasi-equilibrium constant pressureexcess air. Determine the air–fuel ratio. Answer: 15.1 kgexpansion or compression process.air/kg coal15–47C Consider a complete combustion process during15–32 Octane (C 8 H 18 ) is burned with dry air. The volumetricwhich both the reactants and the products are maintained atanalysis of the products on a dry basis is 9.21 percentCO 2 , 0.61 percent CO, 7.06 percent O 2 , and 83.12 percentN 2 . Determine (a) the air–fuel ratio and (b) the percentage oftheoretical air used.the same state. Combustion is achieved with (a) 100 percenttheoretical air, (b) 200 percent theoretical air, and (c) thechemically correct amount of pure oxygen. For which casewill the amount of heat transfer be the highest? Explain.15–33 Carbon (C) is burned with dry air. The volumetric 15–48C Consider a complete combustion process duringanalysis of the products is 10.06 percent CO 2 , 0.42 percentCO, 10.69 percent O 2 , and 78.83 percent N 2 . Determine (a)the air–fuel ratio and (b) the percentage of theoretical airused.which the reactants enter the combustion chamber at 20°Cand the products leave at 700°C. Combustion is achievedwith (a) 100 percent theoretical air, (b) 200 percent theoreticalair, and (c) the chemically correct amount of pure oxygen.15–34 Methane (CH 4 ) is burned with dry air. The volumetricFor which case will the amount of heat transfer be the lowest?Explain.analysis of the products on a dry basis is 5.20 percent CO 2 ,0.33 percent CO, 11.24 percent O 2 , and 83.23 percent N 2 . 15–49 Methane (CH 4 ) is burned completely with the stoichiometricDetermine (a) the air–fuel ratio and (b) the percentage of theoreticalamount of air during a steady-flow combustionair used. Answers: (a) 34.5 kg air/kg fuel, (b) 200 percent process. If both the reactants and the products are maintainedEnthalpy of Formation and Enthalpy of Combustionat 25°C and 1 atm and the water in the products exists in theliquid form, determine the heat transfer from the combustion15–35C What is enthalpy of combustion? How does it differchamber during this process. What would your answer be iffrom the enthalpy of reaction?combustion were achieved with 100 percent excess air?15–36C What is enthalpy of formation? How does it differAnswer: 890,330 kJ/kmolfrom the enthalpy of combustion?15–50 Hydrogen (H 2 ) is burned completely with the stoichiometricamount of air during a steady-flow combustion15–37C What are the higher and the lower heating valuesof a fuel? How do they differ? How is the heating value of aprocess. If both the reactants and the products are maintainedfuel related to the enthalpy of combustion of that fuel?at 25°C and 1 atm and the water in the products exists in theliquid form, determine the heat transfer from the combustion15–38C When are the enthalpy of formation and the chamber during this process. What would your answer be ifenthalpy of combustion identical?combustion were achieved with 50 percent excess air?15–39C Does the enthalpy of formation of a substance 15–51 Liquid propane (C 3 H 8 ) enters a combustion chamberchange with temperature?at 25°C at a rate of 1.2 kg/min where it is mixed and burnedstate?·Q out15–41C Which contains more chemical energy, 1 kmol ofH 2 or 1 kmol of H 2 O?C 3 H 825°CFIGURE P15–51Products1200 K