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).

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 (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

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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

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1) Anthracite, average
2) Groningen (The Netherlands)
3) Average gas station fuels

References

See also

• Energy conversion efficiency
• Thermal efficiency
• Lower heating value