Heat of combustion
The Heat of combustion (ΔcH0) 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 units;
- 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 sometimes expressed as the HHV (Higher Heating value), LHV (Lower Heating Value), or GHV (Gross Heating Value).
Lower heating value accounts for water in the exhaust leaving as vapor. The energy required to vaporize the water therefore is not realized as heat.
Gross heating value accounts for water in the exhaust leaving as vapor, and includes liquid water in the fuel prior to combustion. This value is important for fuels like wood or coal, which will usually contain some amount of water prior to burning.
Higher Heating Value is the same as the 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.
Heat of combustion for common fuels
| Heat of Combustion | |||
|---|---|---|---|
| Fuel | MJ/kg | BTU/lb | kJ/mol |
| Hydrogen | 141.6 | 61,000 | 286 |
| Gasoline | 47.3 | 20,400 | --- |
| Diesel | 44.8 | 19,300 | --- |
| Ethanol | 29.7 | 12,800 | 1,300 |
| Butane | 48.6 | 20,900 | 2,800 |
| Wood | 15 | 6,500 | --- |
| Coal | 15-27 | 8,000 - 14,000 | --- |