A gaseous fuel containing methane and ethane is burned with excess air. The fuel

Question

A gaseous fuel containing methane and ethane is burned with excess air. The fuel enters the furnace at 25°C and 1 atm, and the air enters at 200°C and 1 atm. The stack gas leaves the furnace at 800°C and 1 atm and contains 5.32 mole% CO2, 1.60% CO, 7.32% O2, 12.24% H2O, and the balance N2.

(a) Calculate the molar percentages of methane and ethane in the fuel gas and the percentage excess air fed to the reactor.

(b) Calculate the heat (kJ) transferred from the reactor per cubic meter of fuel gas fed.

(c) A proposal has been made to lower the feed rate of air to the furnace. State advantages and a drawback of doing so.

Felder, Richard M.; Rousseau, Ronald W.; Bullard, Lisa G.. Elementary Principles of Chemical Processes, 4th Edition (Page 560). Wiley. Kindle Edition.

Explanation / Answer

Basis: 100moles of stack gas

Assume that x = moles of methane y= moles of ethane

Carbon balance x + 2y = 5.321 + 1.596

x+ 2y= 6.917 ...... (1)

Hydrogen balance 4x + 6y = 2(12.239)

4x + 6y = 24.478 ...... (2)

Solving the equation (1) and (2), we get x = 3.727

y = 1.595

the molar % of methane present in the fuel gas = 70.03 %

the molar % of ethane present in the fuel gas = 29.97%

then, Oxygen entering :

(100.000 — 5.321 —1.596 — 7.317 — 12.239)(21 / 79) = 19.545

Oxygen required : 2(3.727) + 3.5(1.595) = 13.037

Therefore , % Excess air feed to the reactor = 100(19.545 — 13.037)/ 13.037 = 49.92%

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