Exam 2018 07 24 solution

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Department of Energy Politecnico di Milano Author E. Colombo Pag.1 of 10 Date 01/08/2018 Burner Boiler C8H18 Air Flue Gases @420 K @ 70°C, 3 bar Steam @ 200°C, 6 bar Water @ 30°C, 6 bar Flue Gases @1920 K Milan, 1st August 2018 Exam – Advanced Thermodynamic and Thermoeconomics Session of 24-07-2018 Exercise 1. The system depicted in the figure is composed by a Burner and a Boiler. The Burner produces flue gases at 1920 K and 3 bar by burning gaseous p ropane with excess air λ equal to 1.446. Notice that fuel and air enter separately in the burner at the conditions of 70°C and 3 bar. Flue gases exit the Boiler at 420 K (pressure drop is negligible), and the Boiler produces 90 t/h of superheated steam. Properties of inlet/outle t bulk flows in the two components are reported in table 1 and 2. Table 1 Table 2 Isobaric heat capacities of the fuel and the air are respectively equal to 2.401 kJ/kg-K and 29.17 kJ/kmol-K. Considering the data provided in tables 1 and 2, and the e nvironmental reference conditions of 25°C and 1 bar, it is required to: a. Write the chemical combustion reaction. Then write and simplify th e exergy balance for the burner and write the analytical expressions for the exergy of its reactants and products streams, including both the physical and the chemical components. b. Derive numerical results (per unit of molar flow of C8H18) for point a. c. Write the analytical expressions for the exergy dest ruction of the burner and for the rational exergy efficiency of the combustion process. Briefly discuss the possible ways to simplify the calculation of the rational exergy efficiency of the combustion process. d. Derive numerical results (per unit of molar flow of C8H18) for point c. e. Apply the energy balance to the boil er and determine the molar flow…