Question

A sample of coal was found to contain the following constituents. C = 81%, O = 8%, S = 1%, H = 5%, N = 1% and Ash = 4%. Calculate the minimum weight and volume of an required for the complete combustion of 1kg of coal.

Answer 1

Let us calculate the 02 required for 1kg of coal first
Weight of carbon = 81 / 100 * 1 = 0.81 kg
Weight of hydrogen = 5 / 100 * 1 = 0.05 kg
Weight of sulphur = 1 / 100 * 1 =0.01 kg
Weight of oxygen = 8 / 100 * 1 = 0.08 kg
Calculation of O2 needed for 1 kg of coal
CO2 = 0.81 * 32 /12 = 2.16 kg
2H2O = 0.05 * 32 / 12 = 0.4 kg
SO2 = 0.01 * 32 / 12 =0.01 kg
Total O2 required = 2.57 kg
Less O2 available = -0.08
Net O2 required = 2.49 kg 

Weight of air required = weight of O2 / 23 *100
= 2.49 / 23 *100
= 10.82 kg of air
Volume of air:
∴28.94 kg of air = 22400 ml volume at NTP
∴ 10.82 kg of air = 22400 * 10.82 / 28.94
= 8374.84 ml
air = 8.375 litres of air
Weight of air requires = 10.28 kg
Volume of air required = 8.375 Litres.