Heat of combustion
Not to be confused with Flash point
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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:
- energy/mole of fuel
- energy/mass of fuel
- energy/volume of the fuel
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 H
2O 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⦵
f of the products and reactants (though this approach is somewhat artificial since most heats of formation are calculated from measured heats of combustion). For a fuel of composition CcHhOoNn, the (higher) heat of combustion is 418 kJ/mol (c + 0.3 h – 0.5 o) usually to a good approximation (±3%), though it can be drastically wrong if o + n > c (for instance in the case of nitroglycerine (C
9) this formula would predict a heat of combustion of 0). The value corresponds to an exothermic reaction (a negative change in enthalpy) 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 (in its liquid state), 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:
- CxHyNzOn (std.) + O2 (g, xs.) → xCO2 (g) + y⁄2H2O (l) + z⁄2N2 (g)
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.[obsolete source]
Ways of determination
Higher heating value
The quantity known as higher heating value (HHV) (or gross energy or upper heating value or gross calorific value (GCV) or higher calorific value (HCV)) 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 25 °C (77 °F; 298 K). 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 (e.g., in a gas-fired boiler used for space heat). In other words, HHV assumes all the water component is in liquid state at the end of combustion (in product of combustion) and that heat delivered at temperatures below 150 °C (302 °F) can be put to use.
Lower heating value
The quantity known as lower heating value (LHV) (net calorific value (NCV) or lower calorific value (LCV)) 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 (HHV) (a.k.a. gross calorific value or gross CV) 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 150 °C (302 °F) cannot be put to use.
The above is but one definition of lower heating value adopted by the American Petroleum Institute (API) and uses a reference temperature of 60 °F (16 °C; 289 K).
Another definition, used by Gas Processors Suppliers Association (GPSA) and originally used by API (data collected for API research project 44), is the enthalpy of all combustion products minus the enthalpy of the fuel at the reference temperature (API research project 44 used 25 °C. GPSA currently uses 60 °F), minus the enthalpy of the stoichiometric oxygen (O2) 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 (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.
Measuring heating values
The higher heating value is experimentally determined in a bomb calorimeter. The combustion of a stoichiometric mixture of fuel and oxidizer (e.g. two moles of hydrogen and one mole of oxygen) in a steel container at 25 °C (77 °F) 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.
Note: Higher heating value (HHV) is calculated with the product of water being in liquid form while lower heating value (LHV) 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 (sensible heat exchange causes a change of temperature. In contrast, latent heat is added or subtracted for phase transitions at constant temperature. Examples: heat of vaporization or heat of fusion). 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 (142 MJ/kg vs. 120 MJ/kg). 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.
- Most applications that burn fuel produce water vapor, which is unused and thus wastes its heat content. In such applications, the lower heating value must be used to give a 'benchmark' for the process.
- However, for true energy calculations in some specific cases, the higher heating value is correct. This is particularly relevant for natural gas, whose high hydrogen content produces much water, when it is burned in condensing boilers and power plants with flue-gas condensation that condense the water vapor produced by combustion, recovering heat which would otherwise be wasted.
Usage of terms
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 (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|
|Coal (lignite - USA)||15.00||6,500|
|Aldehydes and ketones|
- 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 during the combustion of those substances.
- BTU/lb values are calculated from MJ/kg (1 MJ/kg = 430 BTU/lb).
Higher heating values of natural gases from various sources
- Algeria: 39.57 MJ/m3
- Bangladesh: 36.00 MJ/m3
- Canada: 39.00 MJ/m3
- China: 38.93 MJ/m3
- Indonesia: 40.60 MJ/m3
- Iran: 39.36MJ/m3
- Netherlands: 33.32 MJ/m3
- Norway: 39.24 MJ/m3
- Pakistan: 34.90 MJ/m3
- Qatar: 41.40 MJ/m3
- Russia: 38.23 MJ/m3
- Saudi Arabia: 38.00 MJ/m3
- Turkmenistan: 37.89 MJ/m3
- United Kingdom: 39.71 MJ/m3
- United States: 38.42 MJ/m3
- Uzbekistan: 37.89 MJ/m3
The lower heating value of natural gas is normally about 90 percent of its higher heating value.
- Adiabatic flame temperature
- Energy density
- Energy value of coal
- Exothermic reaction
- Fuel efficiency#Energy content of fuel
- Food energy
- Internal energy
- Thermal efficiency
- Wobbe index: heat density
- ISO 15971
- Electrical efficiency
- Mechanical efficiency
- Figure of merit
- Relative cost of electricity generated by different sources
- Energy conversion efficiency
- Guibet, J.-C. (1997). Carburants et moteurs. Publication de l'Institut Français du Pétrole. ISBN 978-2-7108-0704-9.
- Schmidt-Rohr, K (2015). "Why Combustions Are Always Exothermic, Yielding About 418 kJ per Mole of O2". J. Chem. Educ. 92 (12): 2094–2099. Bibcode:2015JChEd..92.2094S. doi:10.1021/acs.jchemed.5b00333.
- Note however that a compound like nitroglycerine for which the formula predicts a zero heat of combustion does not actually "combust" in the sense of reacting with air or oxygen. Nitroglycerine will explode, giving off heat, but this is a decomposition requiring no molecular oxygen to react with the nitroglycerine. The formula also gives poor results for (gaseous) formaldehyde and carbon monoxide.
- Kharasch, M.S. (February 1929). "Heats of combustion of organic compounds". Bureau of Standards Journal of Research. 2 (2): 359. doi:10.6028/jres.002.007. ISSN 0091-1801.
- Air Quality Engineering, CE 218A, W. Nazaroff and R. Harley, University of California Berkeley, 2007
- "The difference between LCV and HCV (or Lower and Higher Heating Value, or Net and Gross) is clearly understood by all energy engineers. There is no 'right' or 'wrong' definition. - Claverton Group". www.claverton-energy.com.
- "NIST Chemistry WebBook". webbook.nist.gov.
- "Methanal". webbook.nist.gov.
- "Key World Energy Statistics (2016)" (PDF). iea.org.
- NIST Chemistry WebBook
- "Lower and Higher Heating Values of Gas, Liquid and Solid Fuels" (PDF). Biomass Energy Data Book. U.S. Department of Energy. 2011.