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Dulong's formula estimates gross calorific value from ultimate analysis: 8080C + 34,500(H - O/8) + 2240S in kcal/kg, with percentages by mass. Oxygen already combined with hydrogen contributes nothing, hence the O/8 deduction. Net calorific value subtracts the latent heat of the water formed from hydrogen.
Dulong formula
GCV (kcal/kg) = 8080 C% + 34,500 (H% - O%/8) + 2240 S%, per 100
Net value
NCV = GCV - 2442 x (9 x H% / 100) kJ/kg
Within about 2-3% for bituminous coals but poorer for lignites and high-oxygen fuels. A bomb calorimeter measurement should be used for contractual purposes.
Both are used, but they must not be mixed. European practice generally uses net; North American practice uses gross. Efficiency quoted on net is several points higher for the same boiler.