Stack gas losses
Before you can calculate the stack gas losses you will need to know what the components are. These calculations can be very detailed using kinetic data for hundreds of reactions. However, such calculation would require an extreme knowledge. For each reaction you need to know  temperature, reaction rate constant, concentration of all species in the reaction and residence time. In contrast by using chemical equilibrium we only need to know one variable i.e. the equilibrium temperature in order to calculate the composition of the stack gas.