Heat of Combustion - Heating Value - Lower Heating Value

Lower Heating Value

The quantity known as lower heating value (LHV) (net calorific value (NCV) or lower calorific value (LCV)) is determined by subtracting the heat of vaporization of the water vapor from the higher heating value. This treats any H2O formed as a vapor. The energy required to vaporize the water therefore is not realized as heat.

LHV calculations assume that the water component of a combustion process is in vapor state at the end of combustion, as opposed to the higher heating value (HHV) (a.k.a. gross calorific value or gross CV) which assumes that all of the water in a combustion process is in a liquid state after a combustion process.

The LHV assumes that the latent heat of vaporization of water in the fuel and the reaction products is not recovered. It is useful in comparing fuels where condensation of the combustion products is impractical, or heat at a temperature below 150°C cannot be put to use.

The above is but one definition of lower heating value adopted by the American Petroleum Institute (API) and uses a reference temperature of 60°F (15.56°C).

Another definition, used by Gas Processors Suppliers Association (GPSA) and originally used by API (data collected for API research project 44), is the enthalpy of all combustion products minus the enthalpy of the fuel at the reference temperature (API research project 44 used 25°C. GPSA currently uses 60°F), minus the enthalpy of the stoichiometric oxygen (O2) at the reference temperature, minus the heat of vaporization of the vapor content of the combustion products.

The distinction between the two is that this second definition assumes that the combustion products are all returned to the reference temperature and the heat content from the condensing vapor is considered not to be useful. This is more easily calculated from the higher heating value than when using the preceding definition and will in fact give a slightly different answer.

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