Heat of combustion

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The heat of combustion (ΔHc0) is the energy released as heat when a compound undergoes complete combustion with oxygen. The chemical reaction is typically a hydrocarbon reacting with oxygen to form carbon dioxide, water and heat. It may be expressed with the quantities:

  • energy/mole of fuel (J/mol)
  • energy/mass of fuel
  • energy/volume of fuel

The heat of combustion is traditionally measured with a bomb calorimeter. It may also be calculated as the difference between the heat of formation (ΔfH0) of the products and reactants.

Contents

[edit] Heating value

The heating value or calorific value of a substance, usually a fuel or food, is the amount of heat released during the combustion of a specified amount of it. The calorific value is a characteristic for each substance. It is measured in units of energy per unit of the substance, usually mass, such as: kcal/kg, kJ/kg, J/mol, Btu/m³. Heating value is commonly determined by use of a bomb calorimeter.

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

  • The quantity known as higher heating value (HHV) (or gross calorific value or gross energy or upper heating value) is determined by bringing all the products of combustion back to the original pre-combustion temperature, and in particular condensing any vapor produced. 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 liquid.
  • The quantity known as lower heating value (LHV) (or net calorific value) 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 realized as heat.
  • Gross heating value (see AR) accounts for water in the exhaust leaving as vapor, and 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.

Most applications which burn fuel produce water vapor which is not used, and thus wasting its heat content. In such applications, the lower heating value is the applicable measure. This is particularly relevant for natural gas, whose high hydrogen content produces much water. The gross calorific value is relevant for gas burnt in condensing boilers which condense the water vapor produced by combustion, recovering heat which would otherwise be wasted.

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.

[edit] Heat of combustion for common fuels (higher value)

Heat of Combustion
Fuel MJ/kg Mcal/kg BTU/lb
Hydrogen 141.9 33.9 61,000
Gasoline 47 11.3 20,400
Diesel 45 10.7 19,300
Ethanol 29.8 7.1 12,800
Propane 49.9 11.9 21,500
Butane 49.2 11.8 21,200
Wood 15 3.6 6,500
Coal 15–27 4.4–7.8 8,000–14,000
Natural Gas ~54 ~13 ~23,000

[edit] Lower heating value for some organic compounds (at 25°C)

Fuel MJ/kg kJ/L BTU/lb kJ/mol
Paraffins
Methane 50.009 — — —
Ethane 47.794 — — —
Propane 46.357 — — —
Butane 45.752 — — —
Pentane 45.357 72.455 — —
Hexane 44.752 68.34 — —
Heptane 44.566 64.68 — —
Octane 44.427 63.19 — —
Nonane 44.311 61.71 — —
Decane 44.240 — — —
Undecane 44.194 — — —
Dodecane 44.147 — — —
Isoparaffins
Isobutane 45.613 — — —
Isopentane 45.241 — — —
2-Methylpentane 44.682 — — —
2,3-Dimethylbutane 44.659 — — —
2,3-Dimethylpentane 44.496 — — —
2,2,4-Trimethylpentane 44.310 64.40 — —
Naphthenes
Cyclopentane 44.636 — — —
Methylcyclopentane 44.636 — — —
Cyclohexane 43.450 — — —
Methylcyclohexane 43.380 — — —
Monoolefins
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 — — —
Diolefins
1,3-Butadiene 44.613 — — —
Isoprene 44.078 64.73 — —
Nitrous derivated
Nitromethane 10.513 — — —
Nitropropane 20.693 — — —
Acetylenes
Acetylene 48.241 — — —
Methylacetylene 46.194 — — —
1-Butyne 45.590 — — —
1-Pentyne 45.217 — — —
Aromatics
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 — — —
Propylbenzene 41.193 — — —
Cumene 41.217 — — —
Alcohols
Methanol 19.937 — — —
Ethanol 28.865 — — —
n-propanol 30.680 — — —
Isopropanol 30.447 — — —
n-Butanol 33.075 — — —
Isobutanol 32.959 — — —
Tertiobutanol 32.587 — — —
n-Pentanol 34.727 — — —
Ethers
Methoxymethane 28.703 — — —
Ethoxyethane 33.867 — — —
Propoxypropane 36.355 — — —
Butoxybutane 37.798 — — —
Aldehydes and ketones
Methanal 17.259 — — —
Ethanal 24.156 — — —
Propionaldehyde 28.889 — — —
Butyraldehyde 31.610 — — —
Acetone 28.548 — — —
Other species
Carbon (graphite) 32.808 — — —
Hydrogen 120.971 — — —
Carbon monoxide 10.112 — — —
Ammonia 18.646 — — —
Sulfur (solid) 9.163 — — —

Note that 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 in combusting those substances.

[edit] Heating values of some fuels

Higher (HHV) and Lower (LHV) Heating values
of some common fuels[1]
Fuel HHV MJ/kg HHV BTU/lb HHV kJ/mol LHV MJ/kg
Hydrogen 141.8 61,100 286 121
Methane 55.5 23,900 889 50.0
Ethane 51.9 22,400 1570 47.8
Propane 50.35 21,700 2220 46.35
Butane 49.5 20,900 2875 45.75
Pentane 45.35
Gasoline 47.3 20,400 44.4
Paraffin 46 19,900 16,300
Kerosene 46.2 43.0
Diesel 44.8 19,300
Coal 15–27 8000–14,000   200–350
Wood 15 6500 300
Peat 6–15 2500–6500
Higher heating value
of less common fuels[1]
Fuel HHV MJ/kg BTU/lb kJ/mol
Methanol 22.7 9800 726
Ethanol 29.7 12,800 1300
Propanol 33.6 14,500 2020
Acetylene 49.9 21,500 1300
Benzene 41.8 18,000 3270
Ammonia 22.5 9690 382
Hydrazine 19.4 8370 622
Hexamine 30.0 12,900 4200
Carbon 32.8 14,100 393.5

[edit] Higher heating values of natural gases from various sources

These data on higher heating values were obtained from the International Energy Agency:[2]

The lower heating values of the above natural gases are about 90 percent of the higher heating values.

[edit] Fuel needed to run a 100 W lightbulb for a year (876 kWh, or 3153.6 MJ)

(The fuel quantities below assume 100% conversion efficiency. As most power generation/distribution systems only achieve 30% - 35% efficiency, the actual quantity of fuel used to power a 100 W light bulb in your home will be about three times the quantity shown.)

  • 117 to 210 kg (257 to 462 lb) of coal (using the heating values in the table above)
  • 73.34 kg (161.6 lb) of kerosene
  • 78.8m³, of natural gas, using an average value of 40000 kJ/m³.
  • .006 kg (.014 lb) of uranium
  • 17.5 µg (0.000000039 lb) of antimatter

[edit] References

  1. ^ a b NIST Chemistry WebBook
  2. ^ Key World Energy Statistics (2005), page 59
  • "Carburants et moteurs", J-C Guibet, Publication de l'Institut Français du Pétrole, ISBN 2-7108-0704-1

[edit] See also

[edit] External links