Heat of Combustion - Heating Value - Relation Between Heating Values

Relation Between Heating Values

The difference between the two heating values depends on the chemical composition of the fuel. In the case of pure carbon or carbon monoxide, the two 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 transitions 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 vapour 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, and for natural gas about 11%.

A common method of relating HHV to LHV is:

HHV = LHV + hv x (nH2O,out/nfuel,in)
where hv is the heat of vaporization of water, nH2O,out is the moles of water vaporized and nfuel,in is the number of moles of fuel combusted.

Most applications that burn fuel produce water vapour, which is unused and thus wastes its heat content. In such applications, the lower heating value is generally used to give a 'benchmark' for the process; however, for true energy calculations the higher heating value is correct. This is particularly relevant for natural gas, whose high hydrogen content produces much water. The gross energy value is relevant for gas burned in condensing boilers and power plants with flue-gas condensation that condense the water vapour produced by combustion, recovering heat which would otherwise be wasted.

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