Question

A high grade fuel oil which consists of only hydrocarbon, CxHy is burnt in a combustion chamber at 25 °C. The standard heat of combustion is -43,515 J/g with CO2 (g) and H2O (l) as products. The temperature of the fuel and air entering the combustion chamber is 25 °C. The air is assumed dry. The flue gases leave at 300 °C and their average composition is 11.2 % CO2, 0.4 % CO, 6.2 % O2 and 82.2 % N2 based on dry basis.
Solve the following problems using a basis of 100 mol dry flue gases.

(a) Determine the molecular composition and mass of the fuel oil, CxHy.

(b) Determine the heat of combustion that is absorbed/released from the combustion chamber.

Answer 1

a) Basic information : Air contains 79% N₂ and 21% O₂.

General equation for complete combustion :

General equation for incomplete combustion :

Now, basis is given : 100 mol of dry flue gas

from mol balance we can see,

Also,

So, x and y are in ratio 5.8 : 26.3. Thus, molecular composition is . And molecular mass is 95.9 g/mol.

b) Heat of combustion for complete combustion is given

Now, in reaction mixture, there would also be water and composition of wet gas would be 11.2 mol CO2, 0.4 mol CO, 6.2 mol O2, 82.2 mol N2 and 26.3 mol H2O.

Now, because flue gas is leaving at 300 deg C. That means heat released by the reaction will be used for raising temperature of these gases.

Also, because amount of CO is very less, let us assume major amount of heat is only being released by complete combustion. For specific heats of different substances, the values that have been used contain only the first term and have been taken by the book Smith Van Ness Thermodynamics. One can also use further terms and can reach to more exact answers

So,

This amount of heat will be absorbed by the combustion chamber.

.