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Heat of combustion

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The heat of combustion (ΔHc0) 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:

The heat of combustion is traditionally measured with a bomb calorimeter. It may also be calculated as the difference between the heat of formationfH0) 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).[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 (ΔHv0 or more simply Hv) 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 – 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 further sub-categorized and expressed as:[3]

  • 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/ft
Hydrogen [4] gas 2.016 285.84 141.79 12.75 60,986 324
Methane [4][5] gas 16.043 890.31 55.50 39.72 23,870 1,009
Ethane [4][5] gas 30.069 1,559.88 51.88 69.59 22,313 1,768
Propane [4][5] gas 44.096 2,220.05 50.35 99.05 21,654 2,516
Butane [4][5] gas 58.122 2,878.52 49.53 128.43 21,301 3,263
Ethanol [6] liquid 46.068 1,375.01 29.85 12,837
Gasoline [6] liquid 110 5,013.47 45.58 19,603
Kerosene [7] liquid 178 8,084.99 45.42 19,536
Diesel oil [7] liquid 225 10,124.99 45.00 19,355
Coal [8] solid 25.58 11,002
Wood (dry) [9] solid 21.14 9,093
Peat (dry) [10] 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/ft
Hydrogen [4] gas 2.016 241.83 119.96 10.79 51,596 274
Methane [4] gas 16.043 802.32 50.01 35.80 21,511 909
Ethane [4] gas 30.069 1,427.84 47.49 63.70 20,424 1,618
Propane [4] gas 44.096 2,044.00 46.35 91.19 19,937 2,317
Butane [4] gas 58.122 2,658.45 45.74 118.61 19,673 3,013
Ethanol [6] liquid 46.0684 1,241.66 26.95 11,593
Gasoline [6] liquid 110 4,675.00 42.50 18,280
Kerosene [11] liquid 178 7,519.05 42.24 18,169
Diesel oil [11] liquid 225 9,395.99 41.76 17,961
Coal [11] solid 24.429 10,507
Wood (dry) [11] solid 20.09 8,639
Peat (dry) [11] 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. 

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. 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 Perry, R.H. and Green, D.W. (Editors) (1997). Perry's Chemical Engineers' Handbook, 7th Edition. McGraw Hill. ISBN ISBN 0-07-049841-5. 
  5. 5.0 5.1 5.2 5.3 National Institute of Standards and Technology's Chemistry WebBook
  6. 6.0 6.1 6.2 6.3 Heating Values of Hydrogen and Fuels U.S. Department of Energy
  7. 7.0 7.1 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. Thermalochemical 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. 11.0 11.1 11.2 11.3 11.4 Estimated the difference between LHV and HHV