Question

Determine the chemical exergy of diesel fuel at 25 ◦C, 1 atm, in kJ/kg . Model diesel fuel as dodecane.

Answer 1

Dodecane: C₁₂H₂₆

Molar Mass: 12 x 12 + 26 x 1 = 170 g/mol = 170 Kg/kmol

Chemical energy of diesel fuel is the total energy released during its combustion.

The combustion reaction is:

C₁₂H₂₆(l) + 18.5 O₂(g) → 12 CO₂(g) + 13 H₂O(g)

The standard enthalpy of combustion is -7901.74 KJ/mol. ( -ve sign indicates that enegy is released)

Note: ( Std enthalpy is calculated at 1 atm and 25^oC)

We have been told to find the chemical energy of dodecane.

1 Kg of Dodecane = 1/ molar mass of dodecane

= 1/170

= 5.882x10^-3 kmoles = 5.882 moles

Therefore chemical energy= Std enthalpy of combustion x number of moles of dodecane

(per Kg) = -7901.74 KJ/mol x 5.882 moles

= 46478.03 KJ

So the chemical energy of Diesel fuel = 46478.03 KJ/Kg