Exercise 2 Solution

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Prof. Gianpiero Groppi – Exercises – Fundamentals of Chemical Processes – A.A. 2023-2024 1 EXERCISE 2 Analysis of a combustion boiler supplied by natural gas In a power plant, a boiler is fueled by natural gas (NG), with the following composition (molar basis): CH4 80% C2H6 10% C3H8 5% N2 5% The natural gas enters the boiler at 25°C and 1 atm and is burned with air, at 50°C , 1 atm and relative humidity of 25%. The air is fed with 30% excess. The exhaust gases exit the boiler at 150°C, and do not contain any hydrocarbon species (i.e., the combustion is complete). In the steam production section, the incoming liquid water is pumped at 28 bar and 90°C (the effect of pressure on the enthalpy content of water can be neglected) and is converted to saturated steam at 28 bar and 238°C. Assuming that the heat losses amount to 3% of the LHV of the natural gas, calculate the amount of steam produced per Nm3 of NG and the efficiency of the boiler. DATA: Natural Gas: TIN = 25°C, P = 1 atm ∆𝐻𝐶,𝐶𝐻4 0 = −802 𝑘𝐽 𝑚𝑜𝑙 ∆𝐻𝐶,𝐶2𝐻6 0 = −1429 𝑘𝐽 𝑚𝑜𝑙 ∆𝐻𝐶,𝐶3𝐻8 0 = −2043 𝑘𝐽 𝑚𝑜𝑙 Air: TIN = 50°C, P = 1 atm, R.H. = 25% a 50°C, PSATH2O(50°C) = 12.349 kPa Water/Steam: hINH2O (90°C) = 376.92 kJ/kgH2O hOUTH2O (238°C) = 2804 kJ/kgH2O Exhaust Gases: TOUT = 150°C, P = 1atm Prof. Gianpiero Groppi – Exercises – Fundamentals of Chemical Processes – A.A. 2023-2024 2 Reference state: ideal gas, 1 bar, 298 K 𝐶̃𝑃,𝑖 = 𝑎𝑖 + 𝑏𝑖 ∙ 𝑇 + 𝑐𝑖 ∙ 𝑇2 + 𝑑𝑖 ∙ 𝑇3 [J/mol/K] T in [K] Species a b c d O2 28.11 -3.680 x 10-6 -17.46 x 10-6 -10.65 x 10-9 N2 31.15 -13.57 x 10-3 26.80 x 10-6 -11.68 x 10-9 H2O 32.24 1.924 x 10-3 10.55 x 10-6 -3.596 x 10-9 CO2 19.80 73.44 x 10-3 -56.02 x 10-6 17.15 x 10-9 SOLUTION As we are not given extensive information on the inlet flowrates, we have to a ssume a base of calculus. It is convenient in…