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.
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.(Nazaroff, 2007)
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
|Fuel||HHV MJ/kg||HHV BTU/lb||HHV kJ/mol||LHV MJ/kg|
Lower heating value for some organic compounds (at 15.4°C)
|Aldehydes and ketones|
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.
- Air Quality Engineering, CE 218A, W. Nazaroff and R. Harley,
University of California Berkeley, 2007
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