Question

five thousand kilograms of coal/h is combusted with 3000 kg mol of air/h. assume the following reaction:

C+O -> CO₂

2H+ 1/2 O₂-> H₂O

S+O₂-> SO₂

C+1/2 O₂->CO

the ultimate analysis of coal reveals it contains 0.75 kg C / kg;0.17 kg H / kg; 0.02 kg S / kg; 0.06 kg ash / kg. Calculate precent excess air supplied?

Answer 1

C+O -> CO2   equation (1),

2H+ 1/2 O2-> H2O equation (2)

S+O2-> SO2 equation (3)

C+1/2 O2->CO equation (4)

C+O₂ -----> CO₂

As per the stoichiometry of equation (1)

32 kg of oxygen combines with 12 kg carbon to form 44 kg of carbon dioxide.

So, 1 kg of carbon requires 2.667 kg of oxygen.

As per the stoichiometry of equation (2)

16 kg of oxygen combines with 2 kg Hydrogen to form 18 kg water

So, 1 kg of hydrogen requires 8 kg of oxygen

As per the stoichiometry of equation (3)

24 kg of oxygen combines with 32 kg carbon to form 56 kg of carbon dioxide

So, 1 kg of carbon requires 1.33 kg of oxygen

As per the stoichiometry of equation (4)

32 kg of oxygen combines with 32 kg Sulphur to form 64 kg of Sulphur dioxide

1 kg of Sulphur requires 1 kg of O2.

Let the ultimate analysis of a coal sample is as follows

The analysis of coal reveals it contains 0.75 kg C / kg; 0.17 kg H / kg; 0.02 kg S / kg; 0.06 kg ash / kg.

Oxygen required for burning of carbon, hydrogen and Sulphur are as follows:

Basis: 1 kg coal burned

C: 0.75 × 2.667 kg = 2.00025 kg of oxygen

H: (0.17 × 8) = 1.36 kg of oxygen

S: 0.02×1 kg = 0.02 kg of oxygen

Total oxygen required to accomplish the complete burning = 2.00 +1.36 + 0.02 = 3.38 kg

Oxygen required to complete burning of 1 Kg coal = 3.38 Kg

Since, the air contains 23% of oxygen on mass basis (the remaining 77% being considered as nitrogen),

1 kg of oxygen is contained in 100/23 kg of air.

Hence, minimum or theoretical mass of air required for complete combustion of

1 kg of fuel = 100/23 (2 67C + 8H + S - O) kg ..(7.1)

1 kg of fuel = 100/23(3.38-0.12 ) kg = 14.17 Kg air

Air required to complete burning of 5000 Kg coal = 14.17 Kg x 5000 kg = 70850Kg

Excess air %( 3000/70850) x 100 = 4.23%