E3.5 Combustion Products

Example 3.5: A gas having the following composition:

CH₄ = 94.44%, C₂H₆ = 4.56%, CO = 0.1%, CO₂ = 0.2%, N₂ = 0.4%, O₂ = 0.3%

leaves a furnace after complete combustion when 125 % excess air is used. Determine the composition of the product.

Solution Basis: 1 mole of gas

Moles of O₂ required, and CO₂ and H₂O:

Stoichiometric O₂ requirement = 0.9444(2) + 0.0456(3.5) + 0.001(0.5) = 2.0489 mole

Available oxygen in the feed = 0.003 mole

Net oxygen required = 2.0489 - 0.003 = 2.0459 mole

Oxygen to be supplied from the air = (1 + 1.25)(2.0459) = 4.603

Nitrogen associated with air = 4.603(3.764) = 17.326

Combustion Products:

CO₂ = 0.9444(1) + 0.0456(2) + 0.001(0.5) + 0.002 = 1.0386 mole

N₂ = 17.326 + 0.004 = 17.33 mole

H₂O = 0.9444(2) + 0.0456(3) = 2.0256 mole

O₂ = 1.25(2.0459) = 2.557 mole

Total number of moles = 22.452

Composition of products:

CO₂ = 1.0386/22.452 = 4.5%; N₂ = 17.33/22.452 = 75.6%

H₂O = 2.0256/22.452 = 8.8%; O₂ = 2.557/22.452 = 11.1%