Heat of combustion

The heat of combustion is the total energy released as heat when a substance undergoes complete combustion with oxygen under standard conditions. The chemical reaction is typically a hydrocarbon or other organic molecule reacting with oxygen to form carbon dioxide and water and release heat. It may be expressed with the quantities:

The heat of combustion is conventionally measured with a bomb calorimeter. It may also be calculated as the difference between the heat of formation ΔHo
of the products and reactants (though this approach is purely empirical since most heats of formation are calculated from measured heats of combustion). For a fuel of composition CcHhOoNn, the magnitude of the heat of combustion is 418 kJ/mol (c + 0.3 h – 0.5 o) to a good approximation (±3%).[1] The heat of combustion of all organic compounds has the sign corresponding to an exothermic reaction (negative in the standard chemical convention) because the double bond in molecular oxygen is much weaker than other double bonds or pairs of single bonds, particularly those in the combustion products carbon dioxide and water; conversion of the weak bonds in O2 to the stronger bonds in CO2 and H2O releases energy as heat.[1]

The heating value (or energy value or calorific value) of a substance, usually a fuel or food (see food energy), is the amount of heat released during the combustion of a specified amount of it. The energy value is a characteristic for each substance. It is measured in units of energy per unit of the substance, usually mass, such as: kJ/kg, kJ/mol, kcal/kg, Btu/lb. Heating value is commonly determined by use of a bomb calorimeter.

Heating value unit conversions:

The heat of combustion for fuels is expressed as the HHV, LHV, or GHV.

Higher heating value

The quantity known as higher heating value (HHV) (or gross energy or upper heating value or gross calorific value (GCV) or higher calorific value (HCV)) is determined by bringing all the products of combustion back to the original pre-combustion temperature, and in particular condensing any vapor produced. Such measurements often use a standard temperature of 15 °C (59 °F; 288 K). This 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 condensed to a liquid, hence yielding its latent heat of vaporization. Mechanical systems such as gas-fired boilers used for space heat are suited for the purpose of capturing the HHV as the heat delivered is at temperatures below 150 °C (302 °F; 423 K) yet usable in space heating.

Lower heating value

The quantity known as lower heating value (LHV) (net calorific value (NCV) or lower calorific value (LCV)) is determined by subtracting the heat of vaporization of the water vapor from the higher heating value. This treats any H2O formed as a vapor. The energy required to vaporize the water therefore is not released as heat.

LHV calculations assume that the water component of a combustion process is in vapor state at the end of combustion, as opposed to the higher heating value (HHV) (a.k.a. gross calorific value or gross CV) which assumes that all of the water in a combustion process is in a liquid state after a combustion process.

The LHV assumes that the latent heat of vaporization of water in the fuel and the reaction products is not recovered. It is useful in comparing fuels where condensation of the combustion products is impractical, or heat at a temperature below 150 °C (302 °F) cannot be put to use.

The above is but one definition of lower heating value adopted by the American Petroleum Institute (API) and uses a reference temperature of 60 °F (16 °C; 289 K).

Another definition, used by Gas Processors Suppliers Association (GPSA) and originally used by API (data collected for API research project 44), is the enthalpy of all combustion products minus the enthalpy of the fuel at the reference temperature (API research project 44 used 25 °C. GPSA currently uses 60 °F), minus the enthalpy of the stoichiometric oxygen (O2) at the reference temperature, minus the heat of vaporization of the vapor content of the combustion products.

The distinction between the two is that this second definition assumes that the combustion products are all returned to the reference temperature and the heat content from the condensing vapor is considered not to be useful. This is more easily calculated from the higher heating value than when using the preceding definition and will in fact give a slightly different answer.

Gross heating value

Measuring heating values

The higher heating value is experimentally determined in a bomb calorimeter. The combustion of a stoichiometric mixture of fuel and oxidizer (e.g. two moles of hydrogen and one mole of oxygen) in a steel container at 25 °C (77 °F) is initiated by an ignition device and the reactions allowed to complete. When hydrogen and oxygen react during combustion, water vapor is produced. The vessel and its contents are then cooled to the original 25 °C and the higher heating value is determined as the heat released between identical initial and final temperatures.

When the lower heating value (LHV) is determined, cooling is stopped at 150 °C and the reaction heat is only partially recovered. The limit of 150 °C is based on acid gas dew-point.

Note: Higher heating value (HHV) is calculated with the product of water being in liquid form while lower heating value (LHV) is calculated with the product of water being in vapor form.

Relation between heating values

The difference between the two heating values depends on the chemical composition of the fuel. In the case of pure carbon or carbon monoxide, the two heating values are almost identical, the difference being the sensible heat content of carbon dioxide between 150 °C and 25 °C (sensible heat exchange causes a change of temperature. In contrast, latent heat is added or subtracted for phase transitions at constant temperature. Examples: heat of vaporization or heat of fusion). For hydrogen the difference is much more significant as it includes the sensible heat of water vapor between 150 °C and 100 °C, the latent heat of condensation at 100 °C, and the sensible heat of the condensed water between 100 °C and 25 °C. All in all, the higher heating value of hydrogen is 18.2% above its lower heating value (142 MJ/kg vs. 120 MJ/kg). For hydrocarbons the difference depends on the hydrogen content of the fuel. For gasoline and diesel the higher heating value exceeds the lower heating value by about 10% and 7% respectively, and for natural gas about 11%.

A common method of relating HHV to LHV is:

where Hv is the heat of vaporization of water, nH2O,out is the moles of water vaporized and nfuel,in is the number of moles of fuel combusted.[2]

Usage of terms

Many engine manufacturers rate their engine fuel consumption by the lower heating values. American consumers should be aware that the corresponding fuel-consumption figure based on the higher heating value will be somewhat higher.

The difference between HHV and LHV definitions causes endless confusion when quoters do not bother to state the convention being used.[3] since there is typically a 10% difference between the two methods for a power plant burning natural gas. For simply benchmarking part of a reaction the LHV may be appropriate, but HHV should be used for overall energy efficiency calculations, if only to avoid confusion, and in any case the value or convention should be clearly stated.

Accounting for moisture

Both HHV and LHV can be expressed in terms of AR (all moisture counted), MF and MAF (only water from combustion of hydrogen). AR, MF, and MAF are commonly used for indicating the heating values of coal:

Heat of combustion tables

Higher (HHV) and lower (LHV) heating values
of some common fuels[4]
Fuel HHV MJ/kg HHV BTU/lb HHV kJ/mol LHV MJ/kg
Hydrogen 141.80 61,000 286 119.96
Methane 55.50 23,900 889 50.00
Ethane 51.90 22,400 1,560 47.622
Propane 50.35 21,700 2,220 46.35
Butane 49.50 20,900 2,877 45.75
Pentane 48.60 21,876 3,507 45.35
Paraffin wax 46.00 19,900 41.50
Kerosene 46.20 19,862 43.00
Diesel 44.80 19,300 43.4
Coal (anthracite) 32.50 14,000
Coal (lignite - USA) 15.00 6,500
Wood (MAF) 21.70 8,700
Wood fuel 21.20 9,142 17.0
Peat (dry) 15.00 6,500
Peat (damp) 6.00 2,500
Higher heating value
of some less common fuels[4]
Fuel HHV MJ/kg BTU/lb kJ/mol
Methanol 22.7 9,800 726.0
Ethanol 29.7 12,800 1,300.0
1-Propanol 33.6 14,500 2,020.0
Acetylene 49.9 21,500 1,300.0
Benzene 41.8 18,000 3,270.0
Ammonia 22.5 9,690 382.6
Hydrazine 19.4 8,370 622.0
Hexamine 30.0 12,900 4,200.0
Carbon 32.8 14,100 393.5
Heat of combustion for some common fuels (higher value)
Fuel HHV MJ/kg kcal/g BTU/lb
Hydrogen 141.9 33.9 61,000
Gasoline 47.0 11.3 20,000
Diesel 45.0 10.7 19,300
Ethanol 29.7 7.1 12,000
Propane 49.9 11.9 21,000
Butane 49.2 11.8 21,200
Wood 15.0 3.6 6,000
Coal (lignite) 15.0 4.4 8,000
Coal (anthracite) 36 7.8 14,000
Natural gas 54.0 13.0 23,000
Lower heating value for some organic compounds (at 25 °C [77 °F])
Fuel MJ/kg MJ/L BTU/lb kJ/mol
Methane 50.009 6.9 21,504 802.34
Ethane 47.794 20,551 1,437.2
Propane 46.357 25.3 19,934 2,044.2
Butane 45.752 19,673 2,659.3
Pentane 45.357 28.39 21,706 3,272.6
Hexane 44.752 29.30 19,504 3,856.7
Heptane 44.566 30.48 19,163 4,465.8
Octane 44.427 19,104 5,074.9
Nonane 44.311 31.82 19,054 5,683.3
Decane 44.240 33.29 19,023 6,294.5
Undecane 44.194 32.70 19,003 6,908.0
Dodecane 44.147 33.11 18,983 7,519.6
Isobutane 45.613 19,614 2,651.0
Isopentane 45.241 27.87 19,454 3,264.1
2-Methylpentane 44.682 29.18 19,213 6,850.7
2,3-Dimethylbutane 44.659 29.56 19,203 3,848.7
2,3-Dimethylpentane 44.496 30.92 19,133 4,458.5
2,2,4-Trimethylpentane 44.310 30.49 19,053 5,061.5
Cyclopentane 44.636 33.52 19,193 3,129.0
Methylcyclopentane 44.636? 33.43? 19,193? 3,756.6?
Cyclohexane 43.450 33.85 18,684 3,656.8
Methylcyclohexane 43.380 33.40 18,653 4,259.5
Ethylene 47.195
Propylene 45.799
1-Butene 45.334
cis-2-Butene 45.194
trans-2-Butene 45.124
Isobutene 45.055
1-Pentene 45.031
2-Methyl-1-pentene 44.799
1-Hexene 44.426
1,3-Butadiene 44.613
Isoprene 44.078 -
Nitrous derived
Nitromethane 10.513
Nitropropane 20.693
Acetylene 48.241
Methylacetylene 46.194
1-Butyne 45.590
1-Pentyne 45.217
Benzene 40.170
Toluene 40.589
o-Xylene 40.961
m-Xylene 40.961
p-Xylene 40.798
Ethylbenzene 40.938
1,2,4-Trimethylbenzene 40.984
n-Propylbenzene 41.193
Cumene 41.217
Methanol 19.930 15.78 8,570 638.55
Ethanol 28.865 22.77 12,412 1,329.8
1-Propanol 30.680 24.65 13,192 1,843.9
Isopropanol 30.447 23.93 13,092 1,829.9
n-Butanol 33.075 26.79 14,222 2,501.6
Isobutanol 32.959 26.43 14,172 2,442.9
tert-Butanol 32.587 25.45 14,012 2,415.3
n-Pentanol 34.727 28.28 14,933 3,061.2
Isoamyl alcohol 31.416? 35.64? 13,509? 2,769.3?
Methoxymethane 28.703 12,342 1,322.3
Ethoxyethane 33.867 24.16 14,563 2,510.2
Propoxypropane 36.355 26.76 15,633 3,568.0
Butoxybutane 37.798 28.88 16,253 4,922.4
Aldehydes and ketones
Methanal 17.259 570.78 [5]
Ethanal 24.156
Propionaldehyde 28.889
Butyraldehyde 31.610
Acetone 28.548 22.62
Other species
Carbon (graphite) 32.808
Hydrogen 120.971 1.8 52,017 244
Carbon monoxide 10.112 4,348 283.24
Ammonia 18.646 8,018 317.56
Sulfur (solid) 9.163 3,940 293.82

Higher heating values of natural gases from various sources

The International Energy Agency reports the following typical higher heating values:[6]

The lower heating value of natural gas is normally about 90 percent of its higher heating value.

See also


External links

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