Heating value

Heating value (or calorific value) is used to define the amount of heat released during the combustion of a fuel or food. It is measured in units of energy per amount of material. Depending on the context, heating values may be reported as Btu/m³, kcal/kg, kJ/kg, J/mol, or a variety of other combinations of units. 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. The quantity known as lower heating value (or net calorific value) is determined by subtracting the heat of vaporization of the water in the by-product from the higher heating value results. The lower heating value is what is typically used for vehicle engine analysis.

Where gas burning appliances cannot utilize the heat content of the water vapor, gross calorific value is of little interest. Fuel should be compared based on the net calorific value. This is especially true for natural gas, since increased hydrogen content results in high water formation during combustion. However, modern condensing boiler designs incorporate an additional heat exchanger to recover the lost heat content in the exhaust, condensing the vapour as it does so.

Comparison of fuels

Fuel needed to run a lightbulb (100 W) for a year (= 876kWh)

• 400 kg (876 lb) of coal
• 230 kg (508 lb) of oil
• 170 kg (377 lb) which equals 255 m³ of natural gas
• 147 kg (324 lb) equalling 220 m³ of natural gas (using cct)

Gross calorific value of natural gas by source of production

Bangladesh 36000Source: Key World Energy Statistics (2005)^[1], p. 59. To get net calorific value, multiply by a factor of 0.9

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References

See also

• Wobbe index
• energy value of coal
• food energy

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