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The heating value or calorific value of a substance, usually a fuel or food, is the amount of heat released during the combustion of a specified amount of it. The calorific value is a characteristic for each substance. It is measured in units of energy per unit of the substance, usually mass, such as: kcal/kg, kJ/kg, J/mol, Btu/m³. Heating value is commonly determined by use of a bomb calorimeter.
The quantity known as higher heating value (or gross calorific value or gross energy) is determined by bringing all the products of combustion back to the original pre-combustion temperature, and in particular condensing any vapour produced. The quantity known as lower heating value (or net calorific value) is determined by subtracting the heat of vaporization of the water produced by combustion from the higher heating value.
Most applications which burn fuel produce water vapour which is not used, and thus wasting its heat content. In such applications, the lower heating value is the applicable measure. This is particularly relevant for natural gas, whose high hydrogen content produces much water. The gross calorific value is relevant for gas burnt in condensing boilers which condense the water vapour produced by combustion, recovering heat which would otherwise be wasted.
Additional recommended knowledge
Higher heating values of some fuels
Higher heating values of natural gases from various sources
These data on higher heating values were obtained from the International Energy Agency:
The lower heating values of the above natural gases are about 90 percent of the higher heating values.
Fuel needed to run a 100W lightbulb for a year (876 kWh)
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Heating_value". A list of authors is available in Wikipedia.|