Heating Values

• The thermal properties of a pure substance are defined using internal energy, u, enthalpy, h, specific heat, Cp, etc.
• Heating values of a fuel ( kJ/kg or MJ/kg) are generally used to define the maximum amount of heat that is generated by combustion with air at standard conditions ( STP). The temperature and pressure at standard conditions are  398 k (25°C) and 101.3 kPa.
• The amount of heat released during the combustion of the fuel depends on the phase of water in the products. If water is found in the gas phase in the products, the value of total heat release is called as the lower heating value (LHV).
• For water vapor in the condensed form, additional heat energy (which is equal to the latent heat of vaporization) is extracted. Then the total energy release is called higher heating value (HHV).
• The value of the LHV is calculated from the  HHV and latent heat of  phase change for water to vapor

In above equation N_H2O,P is the number of moles of water in the products. Latent heat for water at STP is h_fg= 2.44 MJ/kg = 43.92 MJ/kmol. In combustion literature, the LHV is normally called the enthalpy or heat of combustion (Q_c) and is a positive quantity.