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In a furnace, 76 lbs propane C3H8 are burned with 17.2% excess air for complete combustion. What is the percent CO2 in combustion products? Round your answer to the one decimal place.
Complete combustion of propane
C3H8 + 5(O2 + 3.76N2) = 3CO2 + 4 H2O + 18.8 N2
Moles of propane = mass/molecular weight
= 76lb / 44.1lb/lbmol
= 1.723 lbmol
Theoretical moles of O2 required = 1.723 x 5 = 8.617 lbmoles
Theoretical air required = 8.617/0.21 = 41.033 lbmol
Actual air supplied = 41.033 + 41.033*17.2/100 = 48.1 lbmol
Moles of O2 in air = 0.21 x 48.1 = 10.101 lbmol
Moles of N2 in air = 37.9 lbmol
In the product
Moles of CO2 formed = 1.723*3 = 5.169 lbmol
Mass of CO2 = moles x molecular weight
= 5.169 lbmol x 44 lb/lbmol = 227.436 lb
Moles of H2O = 1.723*4 = 6.892 lbmol
Mass of H2O = 6.892 lbmol x 18 lb/lbmol = 124.056 lb
Moles of N2 = 37.9 lbmol
Mass of N2 = 37.9 lbmol x 28 lb/lbmol = 1061.2 lb
Moles of O2 = actual - theoretical = 10.101 - 8.617
= 1.484 lbmol
Mass of O2 = 1.484 lbmol x 32 lb/lbmol = 47.488 lb
Total mass of products
= 227.436 + 124.056 + 1061.2 + 47.488
= 1460.18 lb
Mass % of CO2 = mass of CO2 x 100 / total mass
= 227.436*100/1460.18
= 15.6 %