Lower heating value
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The lower heating value (also known as net calorific value 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.
The lower heating value assumes the latent heat of vaporization of water in 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.
Some engine manufacturers rate their engines using lower heating values (LHV) in their product literature which can be both confusing and potentially misleading to the casual or domestic user. Lower heating values neglect the energy in the water vapor formed by the combustion of hydrogen in the fuel. This water vapor typically represents about 10% of the energy content. Therefore the lower heating values for natural gas are typically 900–950 Btu per cubic foot (34–35 MJ/m³.
The error can occur when a manufacturer says their engine uses 900,000 Btu/h (260 kW) but it was expressed on a LHV basis. The engine would actually use about 1,000,000 Btu/h (290 kW or 1050 MJ/h) as purchased from a gas supplier. Therefore, always check the fuel rating method when natural gas is the fuel for the system. This potential confusion almost never exists with liquid fueled systems. Compare to higher heating value.
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.
