net calorific value

(net heat of combustion at constant pressure), n

the heat produced by combustion of unit quantity of a solid or liquid fuel when burned, at a constant pressure of 1 atm (0.1 MPa), under conditions such that all the water in the products remains in the forms of vapor.

DISCUSSION — The net calorific value is a lower heating value that can be calculated from the gross calorific value by making a correction for the difference between a constant volume process and a constant pressure process, and a deduction for the vaporization of the water in the combustion products, including both the water initially present as moisture and that formed by combustion. At 20°C (68°F), the total deduction is 1030 Btu/lb of water (2.395 MJ/kg) including 1055 Btu/lb of water (2.454 MJ/kg) for the enthalpy of vaporization of water.

The net calorific value is closely related to the net enthalpy change of combustion at standard temperature and pressure, ∆H°C(t°C). It is of opposite sign and differs in magnitude by a small amount due to enthalpy effects resulting from compression, temperature differences from t, and solution effects, that vary with the combustion conditions.