Heat of combustion

The heat of combustion (ΔH_c⁰) is the energy released as heat when a substance undergoes complete combustion with oxygen. The chemical reaction for the combustion is typically that of a hydrocarbon fuel reacting with oxygen derived from atmospheric air to form carbon dioxide, water and heat. It may be quantified with these units:

• energy/mole of fuel (such as kJ/mol or Btu/lb-mol)
• energy/mass of fuel (such as MJ/kg or Btu/lb)
• energy/volume of fuel (such as MJ/m³ or Btu/ft³)

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.

Common expressions for the heat of combustion of fuels

The heat of combustion of a fuel is commonly referred to as the heating value or the caloric value and briefly defined as the amount of heat released when a unit amount of the fuel is completely combusted. The heating value is a characteristic of each specific fuel.

The heating value of a fuel may be categorized as either the higher heating value (HHV) or the lower heating value (LHV). The HHV is also known as the gross heating value (GHV) or the gross caloric value (GCV) and the LHV is also known as the net heating value (NHV) or the net caloric value (NCV).

More completely defined, the HHV is the amount of heat released when a unit amount of fuel at a given initial temperature (usually 20 °C or 25 °C) is completely combusted at stoichiometric conditions and constant pressure with the combustion products being cooled to the initial temperature and any water vapor produced being condensed. Condensing any water vapor produced during determination of the HHV means that the HHV includes the heat of vaporization (ΔH_v⁰ or more simply H_v) of the water produced. Stoichiometric combustion means that the combustion products do not contain any oxygen (i.e., there was no excess of combustion air during the combustion).

The LHV is similarly defined except that any water in the combustion products is not condensed and remains as a vapor. Thus, the LHV does not include the heat of vaporization of the water produced.

The relation between the HHV and the LHV may be simply expressed as:

LHV = HHV – H_v

Fuel gases and fuel liquids usually contain little, if any, water. However, raw solid fuels like coal, wood or peat do contain significant amounts of water. Coal, in particular, also contains significant amounts of non-combustible minerals that form ash when the coal is combusted.

More expressions for fuel heating values

Both the HHV's and LHV's of fuels (especially coal and other solid fuels) can be further sub-categorized and expressed as:

• As Received (AR): Indicates the fuel heating value was measured with all inherent moisture and ash forming minerals present.

• Moisture Free (MF) or Dry: Indicates that the fuel heating value was measured after the fuel has been dried of all inherent moisture but still retained its ash forming minerals.

• Moisture and Ash Free (MAF) or Dry and Ash Free (DAF): Indicates that the fuel heating value has been measured in the absence of both inherent moisture and ash forming minerals.

Heating values of some common fuels

Table 1: Higher Heating Value (HHV) Of Various Common Fuels

Fuel	Phase	Molecular  Weight	kJ/mol	MJ/kg	MJ/m3	Btu/lb	Btu/ft3
Hydrogen	gas	2.016	285.84	141.79	12.75	60,986	324
Methane	gas	16.043	890.31	55.50	39.72	23,870	1,009
Ethane	gas	30.069	1,559.88	51.88	69.59	22,313	1,768
Propane	gas	44.096	2,220.05	50.35	99.05	21,654	2,516
Butane	gas	58.122	2,878.52	49.53	128.43	21,301	3,263
Ethanol	liquid	46.068	1,375.01	29.85		12,837
Gasoline	liquid	110	5,013.47	45.58		19,603
Kerosene	liquid	178	8,084.99	45.42		19,536
Diesel oil	liquid	225	10,124.99	45.00		19,355
Coal	solid			25.58		11,002
Wood (dry)	solid			21.14		9,093
Peat (dry)	solid			22.09		9,500
-- The gas temperature and pressure for the values of MJ/m3 are 0 °C and 101.325 kPa. -- The gas temperature and pressure for the values of Btu/ft3 are 60 °F and 14.696 psia. -- LPG is marketed as propane or butanes or a mixture of propane and butanes.  -- Natural gas, after removal of impurities and natural gas liquids (NGL), is essentially pure methane.

Table 2: Lower Heating Value (HHV) Of Various Common Fuels

Fuel	Phase	Molecular  Weight	kJ/mol	MJ/kg	MJ/m3	Btu/lb	Btu/ft3
Hydrogen	gas	2.016	241.83	119.96	10.79	51,596	274
Methane	gas	16.043	802.32	50.01	35.80	21,511	909
Ethane	gas	30.069	1,427.84	47.49	63.70	20,424	1,618
Propane	gas	44.096	2,044.00	46.35	91.19	19,937	2,317
Butane	gas	58.122	2,658.45	45.74	118.61	19,673	3,013
Ethanol	liquid	46.0684	1,241.66	26.95		11,593
Gasoline	liquid	110	4,675.00	42.50		18,280
Kerosene	liquid	178	7,519.05	42.24		18,169
Diesel oil	liquid	225	9,395.99	41.76		17,961
Coal	solid			24.429		10,507
Wood (dry)	solid			20.09		8,639
Peat (dry)	solid			20.65		8,883
-- The gas temperature and pressure for the values of MJ/m3 are 0 °C and 101.325 kPa. -- The gas temperature and pressure for the values of Btu/ft3 are 60 °F and 14.696 psia. -- LPG is marketed as propane or butanes or a mixture of propane and butanes.  -- Natural gas, after removal of impurities and natural gas liquids (NGL), is essentially pure methane.

Sources of additional heating values

• Robert C. Weast (Editor) (1975). Handbook of Chemistry and Physics, 56th Edition. CRC Press. ISBN 0-87819-455-X. 

• Perry, R.H. and Green, D.W. (Editors) (1997). Perry's Chemical Engineers' Handbook, 7th Edition. McGraw Hill. ISBN ISBN 0-07-049841-5. 

• Selected Values of Heats of Combustion and Formation of Organic Compounds Containing the Elements C, H, N, O, P and S, Eugene S. Domalski, Chemical Thermodynamics Data Center, National Bureau of Standards, Washington, D.C., J. Phys. Chem. Ref. Data, Vol. 1, No. 2, 1972

• Lower and Higher Heating Values of Hydrogen and Fuels Hydrogen Analysis Resource Center, U.S. Department of Energy

• NIST Chemistry WebBook Website of the National Institute of Standards and Technology