# Heat of Combustion

The heat of combustion (ΔHc0) is the energy released as heat when a compound undergoes complete combustion with oxygen under standard conditions. 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.

## Heating Value

The heating 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 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.
• A common method of relating HHV to LHV is:
HHV = LHV + hv x (nH2O,out/nfuel,in)
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.[1]

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 and power plants with flue gas condensation 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.

## Heat of Combustion Tables

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 141.80 61,000 286 121.00
Methane 55.50 23,900 889 50.00
Ethane 51.90 22,400 1,560 47.80
Propane 50.35 21,700 2,220 46.35
Butane 49.50 20,900 2,877 45.75
Pentane 45.35
Gasoline 47.30 20,400 44.40
Paraffin 46.00 19,900 41.50
Kerosene 46.20 43.00
Diesel 44.80 19,300
Coal (Anthracite) 27.00 14,000
Coal (Lignite) 15.00 8,000
Wood 15.00 6,500
Peat (damp) 6.00 2,500
Peat (dry) 15.00 6,500
Higher heating value
of some less common fuels[1]
Fuel HHV MJ/kg BTU/lb kJ/mol
Methanol 22.7 9,800 726.0
Ethanol 29.7 12,800 1,300.0
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 9,690 382.0
Hydrazine 19.4 8,370 622.0
Hexamine 30.0 12,900 4,200.0
Carbon 32.8 14,100 393.5
Blau gas  ???  ???  ???
Pintsch gas ???  ???  ???
Town gas ???  ???  ???
Heat of Combustion for some common fuels (higher value)
Fuel kJ/g]] 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 29.8 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) 27.0 7.8 14,000
Natural Gas 54.0 13.0 23,000

## Lower heating value for some organic compounds (at 15.4°C)

Fuel MJ/kg MJ/L BTU/lb kJ/mol
Paraffins
Methane 50.009 802.34
Ethane 47.794 1437.17
Propane 46.357 2044.21
Butane 45.752 2659.30
Pentane 45.357 - 3272.57
Hexane 44.752 - 3856.66
Heptane 44.566 - 4465.76
Octane 44.427 -
Nonane 44.311 -
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 -
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
Isoprene 44.078 -
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
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.

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