Combustion calculations are starting point for Boiler or any Thermal equipment design. Here is a sample calcualtion done for Coal with a calorific value of approximately 7200 Kcal/kg (12960 BTU/Lb). 

Coal composition in % is as follows :

Carbon , C - 76.0 ; Hydrogen, H2 - 4.1 ; Nitrogen , N2 - 1.0  ;   Oxygen, O2 - 7.6 ;   Suphur, S - 1.3 ;   Moisture, H2O - 3.0 ;   Ash - 7.0 ;   

Excess Air : 30 %