Heat of combustion


Not to be confused with Flash point

The heating 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 the total energy released as heat when a substance undergoes complete combustion with oxygen under standard conditions. The chemical reaction is typically a hydrocarbon or other organic molecule reacting with oxygen to form carbon dioxide and water and release heat. It may be expressed with the quantities:
There are two kinds of heat of combustion, called higher and lower heating value, depending on how much the products are allowed to cool and whether compounds like are allowed to condense.
The values are conventionally measured with a bomb calorimeter. They may also be calculated as the difference between the heat of formation ΔH of the products and reactants. For a fuel of composition CcHhOoNn, the heat of combustion is usually to a good approximation, though it can be drastically wrong if . The value corresponds to an exothermic reaction because the double bond in molecular oxygen is much weaker than other double bonds or pairs of single bonds, particularly those in the combustion products carbon dioxide and water; conversion of the weak bonds in oxygen to the stronger bonds in carbon dioxide and water releases energy as heat.
By convention, the heat of combustion is defined to be the heat released for the complete combustion of a compound in its standard state to form stable products in their standard states: hydrogen is converted to water, carbon is converted to carbon dioxide gas, and nitrogen is converted to nitrogen gas. That is, the heat of combustion, ΔH°comb, is the heat of reaction of the following process:
Chlorine and sulfur are not quite standardized; they are usually assumed to convert to hydrogen chloride gas and SO2 or SO3 gas, respectively, or to dilute aqueous hydrochloric and sulfuric acids, respectively, when the combustion is conducted in a bomb containing some quantity of water.

Ways of determination

Higher heating value

The quantity known as higher heating value or higher calorific value ) is determined by bringing all the products of combustion back to the original pre-combustion temperature, and in particular condensing any vapor produced. Such measurements often use a standard temperature of. 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 condensed to a liquid. The higher heating value takes into account the latent heat of vaporization of water in the combustion products, and is useful in calculating heating values for fuels where condensation of the reaction products is practical. In other words, HHV assumes all the water component is in liquid state at the end of combustion and that heat delivered at temperatures below can be put to use.

Lower heating value

The quantity known as lower heating value or lower calorific value ) is determined by subtracting the heat of vaporization of the water from the higher heating value. This treats any H2O formed as a vapor. The energy required to vaporize the water therefore is not released as heat.
LHV calculations assume that the water component of a combustion process is in vapor state at the end of combustion, as opposed to the higher heating value which assumes that all of the water in a combustion process is in a liquid state after a combustion process.
The LHV assumes that the latent heat of vaporization of water in the fuel and the reaction products is not recovered. It is useful in comparing fuels where condensation of the combustion products is impractical, or heat at a temperature below cannot be put to use.
The above is but one definition of lower heating value adopted by the American Petroleum Institute and uses a reference temperature of.
Another definition, used by Gas Processors Suppliers Association and originally used by API, is the enthalpy of all combustion products minus the enthalpy of the fuel at the reference temperature, minus the enthalpy of the stoichiometric oxygen at the reference temperature, minus the heat of vaporization of the vapor content of the combustion products.
The distinction between the two is that this second definition assumes that the combustion products are all returned to the reference temperature and the heat content from the condensing vapor is considered not to be useful.
This is more easily calculated from the higher heating value than when using the preceding definition and will in fact give a slightly different answer.

Gross heating value

Gross heating value 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.

Measuring heating values

The higher heating value is experimentally determined in a bomb calorimeter. The combustion of a stoichiometric mixture of fuel and oxidizer in a steel container at is initiated by an ignition device and the reactions allowed to complete. When hydrogen and oxygen react during combustion, water vapor is produced. The vessel and its contents are then cooled to the original 25 °C and the higher heating value is determined as the heat released between identical initial and final temperatures.
When the lower heating value is determined, cooling is stopped at 150 °C and the reaction heat is only partially recovered. The limit of 150 °C is based on acid gas dew-point.
Note: Higher heating value is calculated with the product of water being in liquid form while lower heating value is calculated with the product of water being in vapor form.

Relation between heating values

The difference between the two heating values depends on the chemical composition of the fuel. In the case of pure carbon or carbon monoxide, the two heating values are almost identical, the difference being the sensible heat content of carbon dioxide between 150 °C and 25 °C. For hydrogen the difference is much more significant as it includes the sensible heat of water vapor between 150 °C and 100 °C, the latent heat of condensation at 100 °C, and the sensible heat of the condensed water between 100 °C and 25 °C. All in all, the higher heating value of hydrogen is 18.2% above its lower heating value. For hydrocarbons the difference depends on the hydrogen content of the fuel. For gasoline and diesel the higher heating value exceeds the lower heating value by about 10% and 7% respectively, and for natural gas about 11%.
A common method of relating HHV to LHV is:
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.
Engine manufacturers typically rate their engines fuel consumption by the lower heating values since the exhaust is never condensed in the engine. American consumers should be aware that the corresponding fuel-consumption figure based on the higher heating value will be somewhat higher.
The difference between HHV and LHV definitions causes endless confusion when quoters do not bother to state the convention being used. since there is typically a 10% difference between the two methods for a power plant burning natural gas. For simply benchmarking part of a reaction the LHV may be appropriate, but HHV should be used for overall energy efficiency calculations if only to avoid confusion, and in any case, the value or convention should be clearly stated.

Accounting for moisture

Both HHV and LHV can be expressed in terms of [|AR], MF and MAF. AR, MF, and MAF are commonly used for indicating the heating values of coal:
FuelHHV MJ/kgHHV BTU/lbHHV kJ/molLHV MJ/kg
Hydrogen141.8061,000286119.96
Methane55.5023,90088950.00
Ethane51.9022,4001,56047.62
Propane50.3521,7002,22046.35
Butane49.5020,9002,87745.75
Pentane48.6021,8763,50745.35
Paraffin wax46.0019,90041.50
Kerosene46.2019,86243.00
Diesel44.8019,30043.4
Coal 32.5014,000
Coal 15.006,500
Wood 21.708,700
Wood fuel21.209,14217.0
Peat 15.006,500
Peat 6.002,500

FuelMJ/kgBTU/lbkJ/mol
Methanol22.79,800726.0
Ethanol29.712,8001,300.0
1-Propanol33.614,5002,020.0
Acetylene49.921,5001,300.0
Benzene41.818,0003,270.0
Ammonia22.59,690382.6
Hydrazine19.48,370622.0
Hexamine30.012,9004,200.0
Carbon32.814,100393.5

;Note
The International Energy Agency reports the following typical higher heating values:
The lower heating value of natural gas is normally about 90 percent of its higher heating value.