The heating value (or energy value, calorific value, heat of combustion) 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 enthalpy of combustion is the same value expressed as an enthalpy, where release of heat is described as negative number.

The calorific value 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:

  • energy/mole of fuel
  • energy/mass of fuel
  • energy/volume of the fuel

There are two kinds of heating values, called high(er) and low(er), depending on how much the products are allowed to cool and whether compounds like are allowed to condense.

The high heat values are conventionally measured with a bomb calorimeter. Low heat values are calculated from high heat value test data. They may also be calculated as the difference between the standard enthalpies/heats of formation ΔH of the products and reactants (though this approach is somewhat artificial since most heats of formation are typically calculated from measured heats of combustion).

By convention, the (higher) heat of combustion is defined to be the heat released for the complete combustion of a compound in its standard state to form stable products in their standard states: hydrogen is converted to water (in its liquid state), carbon is converted to carbon dioxide gas, and nitrogen is converted to nitrogen gas. That is, the heat of combustion, ΔH°<sub>comb</sub>, is the heat of reaction of the following process:

: (std.) + (c + - ) (g) → c (g) + (l) + (g)

Chlorine and sulfur are not quite standardized; they are usually assumed to convert to hydrogen chloride gas and or gas, respectively, or to dilute aqueous hydrochloric and sulfuric acids, respectively, when the combustion is conducted in a bomb calorimeter containing some quantity of water.

Definitions

Higher heating value

The higher heating value (HHV; gross energy, upper heating value, gross calorific value GCV, or higher calorific value; HCV) indicates the upper limit of the available thermal energy produced by a complete combustion of fuel. It is measured as a unit of energy per unit mass or volume of substance. The HHV is determined by bringing all the products of combustion back to the original pre-combustion temperature, including condensing any vapor produced. Such measurements often use a standard temperature of .

; 150 °C LHV

: This LHV is the amount of heat released when the products are cooled to . This means that the latent heat of vaporization of water (and many other potential products) is not recovered. It is useful in comparing fuels where condensation of the combustion products is impractical, or heat at a temperature below cannot be put to use.

The definition in which the combustion products are all returned to the reference temperature is more easily calculated from the higher heating value than when using other definitions. It will in fact give a slightly different answer.

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:

  • AR (as received) indicates that the fuel heating value has been measured with all moisture- and ash-forming minerals present.
  • MF (moisture-free) or dry indicates that the fuel heating value has been measured after the fuel has been dried of all inherent moisture but still retaining its ash-forming minerals.
  • MAF (moisture- and ash-free) or DAF (dry and ash-free) indicates that the fuel heating value has been measured in the absence of inherent moisture- and ash-forming minerals.

Gross heating value

Gross heating value accounts for water in the exhaust leaving as vapor, as does LHV, but gross heating value also 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.

End products for different elements

Zwolinski and Wilhoit defined, in 1972, "gross" and "net" values for heats of combustion. In the gross definition the products are the most stable compounds, e.g. (l), (l), (s) and (l). In the net definition the products are the gases produced when the compound is burned in an open flame, e.g. (g), (g), (g) and (g). In both definitions the products for C, F, Cl and N are (g), (g), (g) and (g), respectively.

There are many other definitions of "gross" and "net".

Estimating heating values

By elemental composition

The heating value of a fuel can be estimated with the results of ultimate analysis of fuel, which provides for the percentages of each element by mass.

By oxygen consumption

For an organic fuel of composition C<sub>c</sub>H<sub>h</sub>O<sub>o</sub>N<sub>n</sub>, the (higher) heat of combustion is usually to a good approximation (±3% for more than 500 organic compounds). This corresponds to 418 kJ/mol for each mole of consumed. The accuracy of such a simplistic formula is due to the very high bond enthalpy of , which renders other bond enthalpies largely irrelevant.

The API publishes its own Dulong-style formulas for petroleum liquids and synthetic fuels.

  • Most applications that burn fuel produce water vapor, which is unused and thus wastes its heat content. In such applications, the lower heating value must be used to give a 'benchmark' for the process.
  • However, for true energy calculations in some specific cases, the higher heating value is correct. This is particularly relevant for natural gas, whose high hydrogen content produces much water, when it is burned in condensing boilers and power plants with flue-gas condensation that condense the water vapor produced by combustion, recovering heat which would otherwise be wasted.

Usage of terms

Engine manufacturers typically rate their engines fuel consumption by the lower heating values since the exhaust is never condensed in the engine, and doing this allows them to publish more attractive numbers than are used in conventional power plant terms. The conventional power industry had used HHV (high heat value) exclusively for decades, even though virtually all of these plants did not condense exhaust either. 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. 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.

Heat of combustion tables

{| class="wikitable sortable" style="text-align: right;" align="left"

|+ Higher (HHV) and lower (LHV) heating values <br/>of some common fuels at 25 °C

! rowspan=2 | Fuel

! colspan=3 | HHV

! LHV

|-

! MJ/kg

! BTU/lb

! kJ/mol

! MJ/kg

|-

|align=left| Hydrogen || 141.79 || 60969 || 285.83 || 119.96

|-

|align=left| Methane || 55.52 || 23874 || 890.7 || 50.00

|-

|align=left| Ethane || 51.90 || 22319 || 1560.7 || 47.62

|-

|align=left| Propane || 50.33 || 21641 || 2219.2 || 46.35

|-

|align=left| Butane || 49.51 || 21288 || 2877.5 || 45.75

|-

|align=left| Pentane || 48.64 || 20913 || 3509 || 45.35

|-

|align=left| Jet kerosene || 46.42 || 19960 || || 44.1

|-

|align=left| Kerosene || 46.20 || 19866 || || 43.00

|-

|align=left| Paraffin wax || 46.00 || 19780 || || 41.50

|-

|align=left| Diesel || 44.80 || 19264 || || 43.4

|-

|align=left| Coal (anthracite) || 32.50 || 13975 || ||

|-

|align=left|Wood (MAF)|| 21.70 || 9331 || ||

|-

|align=left|Wood fuel || 16.0 || 6880 || || 17.0

|-

|align=left|Coal (ligniteUSA)|| 15.00 || 6450 || ||

|-

|align=left|Peat (dry)|| 15.00 || 6450 || ||

|-

|align=left|Peat (damp)|| 6.00 || 2580 || ||

|}

{| class="wikitable sortable" style="text-align: right;" align="left"

|+Higher heating value <br/>of some less common fuels

|-

| Acetaldehyde

| 24.156

| —

| —

| —

|-

| Propionaldehyde

| 28.889

| —

| —

| —

|-

| Butyraldehyde

| 31.610

| —

| —

| —

|-

| Acetone

| 28.548

| 22.62

| —

| —

|-

! colspan="5" | 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 (<small>solid</small>)

| 9.163

| —

| 3,940

| 293.82

|-

|}

; Note

  • There is no difference between the lower and higher heating values for the combustion of carbon, carbon monoxide and sulfur since no water is formed during the combustion of those substances.
  • BTU/lb values are calculated from MJ/kg (1&nbsp;MJ/kg = 430&nbsp;BTU/lb).

Higher heating values of natural gases from various sources

The International Energy Agency reports the following typical higher heating values per Standard cubic metre of gas:

  • Algeria: 39.57MJ/Sm<sup>3</sup>
  • Bangladesh: 36.00MJ/Sm<sup>3</sup>
  • Canada: 39.00MJ/Sm<sup>3</sup>
  • China: 38.93MJ/Sm<sup>3</sup>
  • Indonesia: 40.60MJ/Sm<sup>3</sup>
  • Iran: 39.36MJ/Sm<sup>3</sup>
  • Netherlands: 33.32MJ/Sm<sup>3</sup>
  • Norway: 39.24MJ/Sm<sup>3</sup>
  • Pakistan: 34.90MJ/Sm<sup>3</sup>
  • Qatar: 41.40MJ/Sm<sup>3</sup>
  • Russia: 38.23MJ/Sm<sup>3</sup>
  • Saudi Arabia: 38.00MJ/Sm<sup>3</sup>
  • Turkmenistan: 37.89MJ/Sm<sup>3</sup>
  • United Kingdom: 39.71MJ/Sm<sup>3</sup>
  • United States: 38.42MJ/Sm<sup>3</sup>
  • Uzbekistan: 37.89MJ/Sm<sup>3</sup>

The lower heating value of natural gas is normally about 90% of its higher heating value. This table is in Standard cubic metres (1atm, 15°C), to convert to values per Normal cubic metre (1atm, 0°C), multiply above table by 1.0549.

See also

  • Adiabatic flame temperature
  • Cost of electricity by source
  • Electrical efficiency
  • Energy content of fuel
  • Energy conversion efficiency
  • Energy density
  • Energy value of coal
  • Exothermic reaction
  • Figure of merit
  • Fire
  • Food energy
  • Internal energy
  • ISO 15971
  • Mechanical efficiency
  • Thermal efficiency
  • Wobbe index: heat density

References

Further reading

  • NIST Chemistry WebBook