Heating value refers to the amount of heat from the combustion of fuel gas with pure oxygen is measured in BTU's per pound. May use the formula of dulong's.
BTU / lb = 145 C + 620 [H – (O /8)] + 41 S
By that C is Carbon, percent by weight
H is Hydrogen, percent by weight.
O is Oxygen, percent by weight.
S is Sulphur, percent by weight.
N is Nitrogen, percent by weight.