Referencing the boiler efficiency to the GCV of the fuel rather than the NCV gives a more accurate picture and draws our attention to the enthalpy losses.
While measuring the energy content of the fuel we assume that it is limited to the heat energy released directly. However a part of the heat energy in the fuel is also used to convert the moisture in fuel to vapor. This part of the energy of the fuel is not taken into account while measuring the NCV but is captured in the GCV. Thus GCV is also called the Higher Heating Value (HHV) and NCV is called the Lower Heating Value (LHV).
The Gross Calorific Value (GCV) of a fuel is defined on the basis that all of the water vapor produced by the complete combustion of the fuel is subsequently converted into the liquid state. The Net Calorific Value (NCV) of a fuel is defined on the basis that all of the water vapor produced by combustion of fuel remains in the vapor state.
Enthalpy loss – i.e. the loss in boiler efficiency due to converting the inherent moisture in fuel to vapor is not captured when the NCV is used calculate the boiler efficiency. Referencing the boiler efficiency to GCV gives a more accurate picture of the efficiency as it takes into account all the energy contained in fuel and draws our attention to the enthalpy loss. We can thus take corrective action.
Consider a 10TPH, 10.5 Kg/cm2 boiler with feedwater temperature of 90degC and S: F ratio of 14. Then the efficiencies of the boiler with reference to GCV would be 80% and with NCV would be 89%. However remember in both cases the quantity of fuel required will be the same!