Heat of combustion

The Heat of combustion (ΔHc0) is the energy released as heat when a compound undergoes complete combustion with oxygen. The chemical reaction is typically a hydrocarbon reacting with oxygen to form carbon dioxide, water and heat. It may be expressed with the quantities:
 * energy/mole of fuel
 * energy/mass of fuel
 * energy/volume of fuel

The heat of combustion is traditionally measured with a bomb calorimeter. It may also be calculated as the difference between the heat of formation (ΔfH0) of the products and reactants.

Heating value
The heat of combustion for fuels is expressed as the HHV (Higher Heating value) or LHV (Lower Heating Value).

Lower heating value treats any H2O formed as a vapor. The energy required to vaporize the water therefore is not realized as heat.

Higher Heating Value is the same as the thermodynamic heat of combustion since the enthalpy change for the reaction assumes a common temperature of the compounds before and after combustion, in which case the water produced by combustion is liquid.

Lower heating value for some organic compounds (at 25°C)
Note that there is no difference between the lower and higher heating values for the combustion of carbon, carbon monoxide and sulfur since no water is formed in combusting those substances.