Higher heating value
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2008-07-04T18:15:59Z
Wogone
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/* Relation to lower heating value */
{{Mergeto|Heat of combustion|date=March 2008}}
{{Refimprove|date=April 2008}}
The '''higher [[heating value]]''' (HHV;also known as the gross calorific value or gross energy) of a [[fuel]] is defined as the amount of [[heat]] released by a specified quantity (initially at 25 °C) once it is [[combustion|combusted]] and the products have returned to a temperature of 25 °C.
The higher heating value takes into account the [[latent heat of vaporization]] of [[water]] in the combustion products, and is useful in calculating heating values for fuels where [[condensation]] of the reaction products is practical (e.g., in a gas-fired boiler used for space heat).
== Measuring heating values ==
The higher heating value is experimentally determined in a [[bomb calorimeter]] by concealing a [[stoichiometric]] mixture of fuel and oxidizer (e.g., two moles of hydrogen and one mole of oxygen) in a steel container at 25 °C. Then the [[exothermic reaction]] is initiated by an ignition device and the combustion reactions completed. When hydrogen and oxygen react during combustion, water vapor emerges. Subsequently, the vessel and its content are cooled down to the original 25 °C and the higher heating value is determined as the heat released between identical initial and final temperatures.
When the [[lower heating value]] (LHV) is determined, cooling is stopped at 150 °C and the reaction heat is only partially recovered. The limit of 150 °C is an arbitrary choice.
== Relation to lower heating value ==
The difference between the two heating values depends on the chemical composition of the fuel. In the case of pure carbon or carbon dioxide, both heating values are almost identical, the difference being the sensible heat content of carbon dioxide between 150°C and 25°C ([[sensible heat]] exchange causes a change of temperature. In contrast, [[latent heat]] is added or subtracted for [[phase change]]s at constant temperature. Examples: [[Enthalpy of vaporization|heat of vaporization]] or [[Enthalpy of fusion|heat of fusion]]). For [[hydrogen]] the difference is much more significant as it includes the sensible heat of water vapor between 150°C and 100°C, the latent heat of condensation at 100°C and the sensible heat of the condensed water between 100°C and 25°C. All in all, the higher heating value of hydrogen is 18.2% above its lower heating value (142 MJ/kg vs. 120 MJ/kg). For [[hydrocarbons]] the difference depends on the hydrogen content of the fuel. For [[gasoline]] and [[diesel]] the higher heating value exceeds the lower heating value by about 10% and 7%, respectively, for natural gas about 11%.
Higher (HHV) and lower (LHV) heating values for some fuels are shown in the following table.
Table A.
Heating values for selected fuels<ref>Ulf Bossel: [http://www.efcf.com/reports/E10.pdf Well-to-Wheel Studies, Heating Values, and the Energy Conservation Principle], Proceedings of Fuel Cell Forum 2003</ref>
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Fuel HHV(MJ/kg) [[lower heating value|LHV]](MJ/kg) HHV/LHV LHV/HHV
[[Coal]] 1) 34.1 33.3 1.024 0.977
[[carbon monoxide|CO]] 10.9 10.9 1.000 1.000
[[Methane]] 55.5 50.1 1.108 0.903
[[Natural gas]] 2) 42.5 38.1 1.115 0.896
[[Propane]] 48.9 45.8 1.068 0.937
[[Gasoline]] 3) 46.7 42.5 1.099 0.910
[[Diesel]] 3) 45.9 43.0 1.067 0.937
[[Hydrogen]] 141.9 120.1 1.182 0.846
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1) [[Anthracite]], average<br>
2) [[Groningen (province)|Groningen]] ([[The Netherlands]])<br>
3) Average gas station fuels
== References ==
<references/>
==See also==
*[[Energy conversion efficiency]]
*[[Thermal efficiency]]
*[[Lower heating value]]
[[Category:Heat]]
[[Category:HVAC]]
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