We need these two ways of expressing the heating value of fuels because the combustion of some hydrogen-rich fuels releases water that is subsequently evaporated in the combustion chamber. In other words, the process of evaporating water “soaks up” some of the heat released by fuel combustion. That heat, known as the “latent heat of vaporization,” is temporarily lost and therefore does not contribute to the work done by the combustion process. As a result, the formation and vaporization of water in the combustion chamber reduce the amount of thermal energy available to do work, whether it be driving a piston, spinning a turbine, or superheating steam.
If the water vapor released by fuel combustion simply passes out of the chamber into the environment via the exhaust stream, the latent heat of vaporization is irreversibly and irretrievably lost. That is the case, for example, with most internal-combustion engines, such as diesel and gasoline engines. On the other hand, some advanced boilers have a secondary condensation process, downstream of the combustion step, which condenses the water vapor in the exhaust stream and recovers most of the latent heat being carried with it. The recovered heat can then be used productively.
So, in summary:
1. The numerical difference between the LHV and HHV of a fuel is roughly equivalent to the amount of latent heat of vaporization that can be practically recovered in a secondary condenser per unit of fuel burned.
2. When internal-combustion engines or boilers with no secondary condenser are designed, the appropriate fuel value to use in the design process is the LHV, which assumes that the water vapor generated when the fuel is burned goes out in the exhaust stream.
3. When advanced combustion units having secondary or tertiary condensers are designed, the appropriate fuel value to use in the design process is the HHV.
4. The numerical value of HHV is always greater than or equal to the LHV.