Lower heating value

The lower heating value (also known as net calorific value, net CV, or LHV) of a fuel is defined as the amount of heat released by combusting a specified quantity (initially at 25 °C or another reference state) and returning the temperature of the combustion products to 150 °C.

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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.

By contrast, the higher heating value (HHV) (a.k.a. gross calorific value or gross CV) includes the heat of condensation of water in the combustion products.

Usage

For historical reasons, the efficiency of power plants and combined heat and power plants in Europe is calculated based on the LHV, while in e.g. the US, it is generally based on the HHV. This has the peculiar result that contemporary combined heat and power plants, where flue gas condensation is implemented, may report efficiencies exceeding 100 % in Europe.

Many engine manufacturers rate their engine fuel consumption by the lower heating values. American consumers should be aware that the corresponding fuel consumption figure based on the higher heating value would be somewhat higher.

Alternative definitions

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

Another definition [used by GPSA - Gas Processors Suppliers Association and originally used by API (data collected for API research project 44)] is that the lower heating value 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 vapour content of the combustion products.

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