combustionheat

Combustion heat, or heat of combustion, is the energy released when a substance reacts completely with an oxidizer, typically oxygen, under specified conditions. In thermodynamics this is the enthalpy change of combustion (ΔHc) reported at constant pressure. Two common standards are used to account for water condensation: higher heating value (HHV) and lower heating value (LHV).

HHV includes the latent heat of condensation of water produced, while LHV assumes water leaves as vapor. The difference between HHV and LHV depends on whether the condensable water is recovered as liquid or remains as vapor.

Heat of combustion is usually expressed as energy per mass (MJ/kg), per volume for gases (MJ/m^3), or per mole (kJ/mol). It can be measured by bomb calorimetry, where a sample is burned in excess oxygen in a sealed bomb and the resultant temperature rise is used to compute the released heat. Corrections may be applied for water content and calorimeter heat capacity.

Applications include assessing the energy content of fuels for boilers and engines and guiding energy balance, efficiency, and emissions analyses. In practice, the usable energy from a fuel can be lower than its thermodynamic heat of combustion due to incomplete combustion, heat losses, and formation of partially oxidized products.