The Heat of combustion (Δ_cH⁰) 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 (Δ_fH⁰) 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 (Gross Value)

Heat of Combustion
Fuel	MJ/kg	BTU/lb	kJ/mol
Hydrogen	141.79	61,000	286
Gasoline	47.3	20,400	---
Diesel	44.8	19,300	---
Ethanol	29.7	12,800	1,300
Propane	50.35	---	2,219
Butane	49.51	20,900	2,800
Wood	15	6,500	---
Coal	15-27	8,000 - 14,000	---

Liquid fuels

Combustion of a liquid fuel in an oxidizing atmosphere actually happens in the gas phase. It is the vapour that burns, not the liquid. Therefore, a liquid will normally catch fire only above a certain temperature, its flash point. The flash point of a liquid fuel is the lowest temperature at which it can form an ignitable mix with air. It is also the minimum temperature at which there is enough evaporated fuel in the air to start combustion.

Assuming perfect combustion conditions, such as an adiabatic (no heat loss and no heat gain) and complete combustion, the adiabatic combustion temperature can be determined. The formula that yields this temperature is based on the first law of thermodynamics and takes note of the fact that the heat of combustion is used entirely for heating the fuel, the combustion air or oxygen, and the combustion product gases (commonly referred to as the flue gas).

In the case of fossil fuels burnt in air, the combustion temperature depends on

• the heating value
• the stoichiometric air to fuel ratio λ
• the heat capacity of fuel and air
• the air and fuel inlet temperatures

The adiabatic combustion temperature (also known as the adiabatic flame temperature) increases for higher heating values and inlet air and fuel temperatures and for stoichiometric air ratios approaching one.

Typically, the adiabatic combustion temperatures for coals are around 1500 °C (for inlet air and fuel at ambient temperatures and for λ = 1.0), around 2000 °C for oil and 2200 °C for natural gas.

In industrial fired heaters, power plant steam generators, and large gas-fired turbines, the more common way of expressing the usage of more than the stoichiometric combustion air is percent excess combustion air. For example, excess combustion air of 15 percent means that 15 percent more than the required stoichiometric air is being used