Heating value

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.

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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:^[2]

• Russia: 38,231 kJ/m³
• United States: 38,416 kJ/m³
• Canada: 38,200 kJ/m³
• Netherlands: 33,320 kJ/m³
• United Kingdom: 39,710 kJ/m³
• Indonesia: 40,600 kJ/m³
• Algeria: 42,000 kJ/m³
• Uzbekistan: 37,889 kJ/m³
• Saudi Arabia: 38,000 kJ/m³
• Norway: 39,877 kJ/m³
• Bangladesh: 36,000 kJ/m³

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)

• 400 kg (876 lb) of coal
• 230 kg (508 lb) of oil
• 170 kg (377 lb), 255 m³, of natural gas
• .006 kg (.014 lb) of uranium
• .0000075 kg (.000016 lb) of the Sun

See also

• Heat of combustion
• Fuel efficiency#Energy content of fuel
• Energy value of coal
• Food energy
• Wobbe index: heat density
• Energy density
• Thermal efficiency

References