Fuel Heating Value
Knowing the heating value of a fuel tells you what fuel rate your fired heater requires. All fuels containing hydrogen produce water vapor during combustion. Such fuels have two “heating values” depending on the final state of the water.
The higher or gross heating value (HHV) is that obtained with the product water present as a liquid.
The lower or net heating value (LHV) assumes the water remains in the vapor state. The difference is the water’s “latent heat of vaporization.” In actual practice, the lower heating value is used in designing and rating process heaters because the product water normally is not condensed on heat transfer surfaces and leaves the stack as a vapor.
For hydrocarbon gases and mixtures of hydrocarbon gases, the lower heating value on a volume basis (Btu/scf) is linearly related to the specific gravity of the fuel (air=1) as shown in Figure 200-6. But on a mass basis, the net heating value (LHV) of all hydrocarbon gases is close to 20,000 Btu/lb.
Fuel oil always contains some noncombustible materials like water, ash or metals. So the heating value of fuel oil is generally a little lower, typically around 18,000 Btu/lb. (Energy is often expressed in Btu per “equivalent fuel oil” barrels (EFO). Note that an EFO is approximately equivalent to 6MM BTU’s.)