Higher heating value

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The higher heating value (HHV;also known 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).

The higher heating value is experimentally determined 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 chemical reaction of the components is completed. This test device is known as a bomb calorimeter. If hydrogen and oxygen are combined, water vapor emerges at high temperatures. Subsequently, the vessel and its content are cooled down to the original 25°C and the “Heat of Formation” (or the "Higher Heating Value" HHV) is determined by measuring the heat released between identical initial and final temperatures.

In contrast, when the lower heating value (LHV) is determined, the cooling is stopped at 150°C and the reaction heat is only partially recovered. The limit of 150°C, although a practical number on our temperature scale, is an arbitrary choice. The LHV is intended to be used as a practical number rather than an intrinsic property of the material.

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 CO2 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 HHV of hydrogen is 18.2% above its Lower Heating Value LHV 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 HHV exceeds the LHV by about 10% and 7%, respectively, for natural gas about 11%.

The dependence of efficiencies on the choice of heating values is illustrated in the following tables.

Table A. Heating values for selected fuels [8]

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 4)       45.9       43.0       1.067   0.937 
Hydrogen       141.9      120.1       1.182   0.846

1) Anthracite, average 2) Groningen (The Netherlands) 3) Average gas station fuels 4) Average gas station fuels