The electric power supplied to the boiler is, boiler loss Qn = mC(T, – T,)+Qiaz Substitute the respective values, 1 kW Qm = (2 kg/s)(4.37 kJ/kgK)(362 K – 293 K)+157.5 W| 1000 W Qm = 603.21 kW The electric power supply to the boiler is 603.21 kW. The mass flow rate of the fuel required to heat water is, Qm = (m )(high heating value) fuel Qm (high heating value (mpa) = 603.21 kJ/s ' fusl 50.02 MJkg ṁ = 0.012 kg/s Step 4 The total entropy change for the system is, dQvailer boiler т ds = For the constant pressure heat transfer, T2 Sz - S, = mC In| T, Substiute the respective values, 362 K S; - S, = (2 kg/s)(4.37 kJ/kgK)In 293 K Sz - S, =1.848 kW/K г1 Step 6 The rate at which exergy loss takes place is, Qaoge =Twrouning AS Qne te = 293 K(1.84 kW/K) xagic gic rgic ЕЗ
1.1 Determine the electrical power supplied to a boiler when the temperature of the entering water is 20 C and the exiting temperature is 89 C. The flow of.the pressured water is 2 Kg/s. There is a negligible pressure drop through this boiler and it operates at a constant pressure of 3 bars. The specific heat is c = 4,370 J/(Kg K). There is a 1.5(105 ) W rate of heat loss from the boiler during this process to a surrounding at 293.2 k. Consider steady state conditions. Calculate the mass flowrate of fuel (natural gas, CH4) required to heat the water flow to the conditions of problem 1.1 if the electrical heating device is replaced with a gas fired boiler. The high heating value (HHV) of the fuel is 50.02 MJ/kg. Calculate the exergy destroyed in the process described by problem 1.4. The exergy of the fuel entering this process is 51.82 MJ/Kg. The dead state temperature is 293.2 K and pressure is 1 bar. The products of combustion leave this process at the dead state.
I already figured out most of it however i'm struggling with the following. Answers to previous parts are shown below
Asnwer:
The utility providing the electricity to the boiler in problem 1.1 uses the same fuel as that used in problem 1.4 and has an efficiency of 0.35. The cost of the electric boiler is, $3,000 with a 15 year lifetime. The cost of the fuel based boiler system is $5000 and has a lifetime of 20 years. The fuel cost is $0.10/kg and the cost of electricity is $0.20/kWh. There is a tax charge of $0.50/kg of CO2 emission that is passed on to the user whether the utility based electrical system or gas fired system is used. The unit is used for 10 hours per day, 280 days per year. Which process would you recommend in terms of fuel consumption, exergy production, carbon dioxide emission and theroeconomic cost? There is no cost associated with the water input. Note the total amount of fuel consumed is directly related to the carbon dioxide emission from each process.
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