The heating value is the amount of ideal heat released by a unit of fuel, mass or standard volume, during complete combustion, at constant pressure, with the stoichiometric amount of dry air. The fuel humidity must be stated, as well as whether the gas volume is real or ideal. This involves a heat balance at standard conditions of temperature and pressure, (usually 60°F [15°C] and 760 mm Hg).
Hydrogen in the fuel burns to water and when the flue gases are cooled to 60°F, the physical state — either vapor or liquid — of this water must be assumed. So the latent heat of vaporization of the water may or may not be considered to be part of the heating value. The result is two definitions for the heating value. The higher or gross heating value, HHV, includes the heat of condensation and the lower or net heating value, LHV, assumes the water remains in the vapor state.
Fig. 23-2 in the “Physical Properties” section gives the net and gross heating values of most pure hydrocarbons. For mixtures calculate the molar, or volume, average.
The higher, ideal, dry heating value of sweet natural gas at 60°F and 760 mm Hg may be calculated with the following equation:
Where HHV is in Btu/scf, SG is the ideal relative density, and X is the fraction of carbon dioxide and nitrogen if present.