An experiment was performed to determine the efficiency of a boiler using natura

Question

An experiment was performed to determine the efficiency of a boiler using natural gas and air. The volume flow rates of the air and natural gas were measured as 296,760 ft^3/hr and 12,576 ft^3/hr , respectively, with the gases at STP. A flue gas analyzer measured two of the product?s concentrations to be 5.496% 02 and S.769% CO2 (Volume o/e) on a dry basis and 4.672% O2 and 7.454% CO2 (Volume %) on a wet basis. The composition of the natural gas on a volume basis is 93.2% CH4, 3.7% C2H6, 1.9% N2, and 1.2% CO2. If the natural gas flow rate is considered trustworthy, is the flow rate measurement for the air correct?

Explanation / Answer

CH4 + 2 O2 CO2 + 2 H2O

For gases, volume is proportional to moles:

a.
Supposing air to be 21% oxygen:
(200 m^3 CH4/min) x (2 mol O2 / 1 mol CH4) / (0.21) x (1.10) = 2095 m^3/min air

b.
(2095 m^3/min air) / (60 sec/min) / (4 m/sec) = 8.729 m^2
V = pi x r^2 = 8.729 m^2
r^2 = 8.729 m^2 / (3.14159) = 2.7785 m^2
Take the square root:
r = 1.67 m
diameter = 2 x r = 3.33 m

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