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Higher heating value
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 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).
Additional recommended knowledge
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 changes at constant temperature. Examples: heat of vaporization or 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 or in absolute numbers, 142 MJ/kg vs. 120 MJ/kg for the two cases. 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
Fuel HHV(MJ/kg) LHV(MJ/kg) HHV/LHV LHV/HHV Coal 1) 34.1 33.3 1.024 0.977 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
1) Anthracite, average
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Higher_heating_value". A list of authors is available in Wikipedia.|