Chemical Engineering

Heat of combustion

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Woodburning fireplace generating heat. Source: Francisco Belard

The heat of combustion (ΔH c0) is the energy released as heat when a substance undergoes complete combustion with oxygen. The chemical reaction for combustion is typically that of a hydrocarbon fuel reacting with oxygen derived from atmospheric air to form gaseous carbon dioxide, water vapor and heat. For example:

methane + oxygen ⇒ carbon dioxide + water vapor
CH4 + 2O2 ⇒ CO2 + 2H2O

The heat of combustion may be quantified with these units:

●  energy per mole of fuel, such as kilojoule per mol (kJ/mol) or British thermal unit per pound-mol (Btu/lb-mol)

●  per mass of fuel, such as megajoule per kilogram (MJ/kg) or British thermal unit per pound (Btu/lb)

●  energy per volume of fuel, such as megajoule per cubic meter (MJ/m3) or British thermal unit per cubic foot (Btu/ft3)

The heat of combustion is usually measured with a bomb calorimeter. It may also be calculated as the difference between the heat of formation (ΔH f0) 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 unique 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).[1][2] 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 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 – Hv

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 be further sub-categorized and expressed as:[3]

●  As Received (AR): Indicates the 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



Sources of additional heating values

References

  1. William D. McCain (1990). The Properties of Petroleum Fluids, 2nd Edition. Pennwell Publishing. ISBN 0-8714-335-1.
  2. Heating values, Princeton University website.
  3. Christopher Higman and Maarten van der Burgt (2008). Gasification, 2nd Edition. Gulf Professional Publishing, pages 53-54. ISBN 0-7506-8528-X
  4. R.H. Perry and D.W. Green (Editors) (1997). Perry's Chemical Engineers' Handbook, 7th Edition. McGraw Hill. ISBN 0-07-049841-5
  5. National Institute of Standards and Technology's Chemistry WebBook.
  6. Lower and Higher Heating Values of Hydrogen and Fuels, Hydrogen Analysis Resource Center, U.S. Department of Energy.
  7. Average of various sources.
  8. There are a great many different coals. The values given here are of a single, specific bituminous coal on an "as received" basis which includes the ash and inherent moisture content of the coal.
  9. How To Estimate Forest Recoverable Heat Products, Peter J. Ince, 1979. The values in Table 1 are the average of oven-dried woods from various different species of trees, as listed in this publication by the U.S. Forest Service's laboratory.
  10. Thermochemical and Catalytic Upgrading in a Fuel Context: Peat, Biomass and Alkenes, Thesis by Christina Hornell, Chemical Engineering Dept., Royal Institute of Technology, Stockholm, Sweden, 2001.
  11. Estimated the difference between LHV and HHV.
Glossary

Citation

Beychok, M. (2012). Heat of combustion. Retrieved from http://www.eoearth.org/view/article/51cbf3167896bb431f6abc66