If we know the ultimate analysis of fuel, we can easily calculate its GCV.
The basic principle is that there are only 3 components in a fuel which generate heat. These are:
Carbon, Hydrogen and Sulphur.
According to Dulong's formula gross calorific value of a fuel is;
GCV = ((35.5 x C + 114.8 x H + 9.5 x S – 14.5 x O) x 1000) / (100 x 4.1868)
Each multiple of carbon, hydrogen and sulphur represents heat generated by its one mole.
The formula gives GCV in kcal/kg. Simple isnt it !