Heat of Combustion
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Heat of Combustion

The heating value (or energy 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 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 enthalpy 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 high heat values are conventionally measured with a bomb calorimeter. Low heat values are calculated from high heat value test data. They may also be calculated as the difference between the heat of formation ?H?
f
of the products and reactants (though this approach is somewhat artificial since most heats of formation are typically calculated from measured heats of combustion). For a fuel of composition CcHhOoNn, the (higher) heat of combustion is usually to a good approximation (±3%),[1] though it can be significantly off if (for instance in the case of nitroglycerine, , this formula would predict a heat of combustion of 0[2]). 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.[1]

By convention, the (higher) 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, ?H°comb, is the heat of reaction of the following process:

(std.) + (g, xs.) -> c (g) + h2 (l) + n2 (g)

Chlorine and sulfur are not quite standardized; they are usually assumed to convert to hydrogen chloride gas and or gas, respectively, or to dilute aqueous hydrochloric and sulfuric acids, respectively, when the combustion is conducted in a bomb containing some quantity of water.[3][obsolete source]

## Ways of determination

### Gross and net

Zwolinski and Wilhoit defined, in 1972, "gross" and "net" values for heats of combustion. In the gross definition the products are the most stable compounds e.g. (l), (l), (s) and (l). In the net definition the products are those produced when the compound is burned in an open flame e.g. (g) (g) (g) and (g). In both definitions the products for C, F, Cl and N are (g) (g) (g) and (g) respectively.[4]

### Higher heating value

The higher heating value (HHV; gross energy, upper heating value, gross calorific value GCV, or higher calorific value; HCV) indicates the upper limit of the available thermal energy produced by a complete combustion of fuel. It is measured as a unit of energy per unit mass or volume of substance. The HHV 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 lower heating value (LHV; net calorific value; NCV, or lower calorific value; LCV) is another measure of available thermal energy produced by a combustion of fuel, measured as a unit of energy per unit mass or volume of substance. In contrast to the HHV, the LHV considers energy losses such as the energy used to vaporize water - although its exact definition is not uniformly agreed upon. One definition is simply to subtract 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.

Another definition of the LHV is the amount of heat released when the products are cooled to 150 °C (302 °F). This means that the latent heat of vaporization of water and other 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.

One definition of lower heating value, adopted by the American Petroleum Institute (API), uses a reference temperature of 60 °F (15+59 °C).

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 definition in which the combustion products are all returned to the reference temperature is more easily calculated from the higher heating value than when using other definitions and will in fact give a slightly different answer.

### Gross heating value

Gross heating value accounts for water in the exhaust leaving as vapor, as does LHV, but gross heating value also 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.

When the lower heating value (LHV) 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 (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, while 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 (142MJ/kg vs. 120MJ/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:

${\displaystyle \mathrm {HHV} =\mathrm {LHV} +H_{\mathrm {v} }\left({\frac {n_{\mathrm {H_{2}O,out} }}{n_{\mathrm {fuel,in} }}}\right)}$

where Hv is the heat of vaporization of water, n,out is the number of moles of water vaporized and nfuel,in is the number of moles of fuel combusted.[5]

• 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, and doing this allows them to publish more attractive numbers than are used in conventional power plant terms. The conventional power industry had used HHV (high heat value) exclusively for decades, even though virtually all of these plants did not condense exhaust either. 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.[6] 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

Higher (HHV) and lower (LHV) heating values
of some common fuels[7] at 25 °C
Fuel HHV LHV
MJ/kg BTU/lb kJ/mol MJ/kg
Hydrogen 141.80 61,000 286 119.96
Methane 55.50 23,900 890 50.00
Ethane 51.90 22,400 1,560 47.62
Propane 50.35 21,700 2,220 46.35
Butane 49.50 20,900 2,877 45.75
Pentane 48.60 21,876 3,509 45.35
Paraffin wax 46.00 19,900 41.50
Kerosene 46.20 19,862 43.00
Diesel 44.80 19,300 43.4
Coal (anthracite) 32.50 14,000
Coal (lignite - USA) 15.00 6,500
Wood (MAF) 21.70 8,700
Wood fuel 21.20 9,142 17.0
Peat (dry) 15.00 6,500
Peat (damp) 6.00 2,500
Higher heating value
of some less common fuels[7]
Fuel MJ/kg BTU/lb kJ/mol
Methanol 22.7 9,800 726
Ethanol 29.7 12,800 1,367
1-Propanol 33.6 14,500 2,020
Acetylene 49.9 21,500 1,300
Benzene 41.8 18,000 3,268
Ammonia 22.5 9,690 382.6
Hydrazine 19.4 8,370 622.0
Hexamine 30.0 12,900 4,200.0
Carbon 32.8 14,100 393.5
Lower heating value for some organic compounds
(at 25 °C [77 °F])[]
Fuel MJ/kg MJ/L BTU/lb kJ/mol
Alkanes
Methane 50.009 6.9 21,504 802.34
Ethane 47.794 -- 20,551 1,437.2
Propane 46.357 25.3 19,934 2,044.2
Butane 45.752 -- 19,673 2,659.3
Pentane 45.357 28.39 21,706 3,272.6
Hexane 44.752 29.30 19,504 3,856.7
Heptane 44.566 30.48 19,163 4,465.8
Octane 44.427 -- 19,104 5,074.9
Nonane 44.311 31.82 19,054 5,683.3
Decane 44.240 33.29 19,023 6,294.5
Undecane 44.194 32.70 19,003 6,908.0
Dodecane 44.147 33.11 18,983 7,519.6
Isoparaffins
Isobutane 45.613 -- 19,614 2,651.0
Isopentane 45.241 27.87 19,454 3,264.1
2-Methylpentane 44.682 29.18 19,213 3,850.7
2,3-Dimethylbutane 44.659 29.56 19,203 3,848.7
2,3-Dimethylpentane 44.496 30.92 19,133 4,458.5
2,2,4-Trimethylpentane 44.310 30.49 19,053 5,061.5
Naphthenes
Cyclopentane 44.636 33.52 19,193 3,129.0
Methylcyclopentane 44.636? 33.43? 19,193? 3,756.6?
Cyclohexane 43.450 33.85 18,684 3,656.8
Methylcyclohexane 43.380 33.40 18,653 4,259.5
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 derived
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 -- -- --
n-Propylbenzene 41.193 -- -- --
Cumene 41.217 -- -- --
Alcohols
Methanol 19.930 15.78 8,570 638.6
Ethanol 26.70 22.77 12,412 1,230.1
1-Propanol 30.680 24.65 13,192 1,843.9
Isopropanol 30.447 23.93 13,092 1,829.9
n-Butanol 33.075 26.79 14,222 2,501.6
Isobutanol 32.959 26.43 14,172 2,442.9
tert-Butanol 32.587 25.45 14,012 2,415.3
n-Pentanol 34.727 28.28 14,933 3,061.2
Isoamyl alcohol 31.416? 35.64? 13,509? 2,769.3?
Ethers
Methoxymethane 28.703 -- 12,342 1,322.3
Ethoxyethane 33.867 24.16 14,563 2,510.2
Propoxypropane 36.355 26.76 15,633 3,568.0
Butoxybutane 37.798 28.88 16,253 4,922.4
Aldehydes and ketones
Formaldehyde 17.259 -- -- 570.78 [8]
Acetaldehyde 24.156 -- -- --
Propionaldehyde 28.889 -- -- --
Butyraldehyde 31.610 -- -- --
Acetone 28.548 22.62 -- --
Other species
Carbon (graphite) 32.808 -- -- --
Hydrogen 120.971 1.8 52,017 244
Carbon monoxide 10.112 -- 4,348 283.24
Ammonia 18.646 -- 8,018 317.56
Sulfur (solid) 9.163 -- 3,940 293.82
Note
• 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

The International Energy Agency reports the following typical higher heating values per Standard cubic metre of gas:[9]

The lower heating value of natural gas is normally about 90% of its higher heating value. This table is in Standard cubic metres (1atm, 15°C), to convert to values per Normal cubic metre (1atm, 0°C), multiply above table by 1.0549.

## References

• Guibet, J.-C. (1997). Carburants et moteurs. Publication de l'Institut Français du Pétrole. ISBN 978-2-7108-0704-9.
1. ^ a b 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.
2. ^ 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.
3. ^ 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.
4. ^ Zwolinski, BJ, & Wilhoit, RC (1972) Heats of formation and Heats of Combustion in American Institute of Physics Handbook, 3rd ed., D.E. Gray, ed., McGraw-Hill, New York, pp. 4-316-342. https://web.mit.edu/8.13/8.13c/references-fall/aip/aip-handbook-section4l.pdf
5. ^ Air Quality Engineering, CE 218A, W. Nazaroff and R. Harley, University of California Berkeley, 2007
6. ^
7. ^ a b "NIST Chemistry WebBook". webbook.nist.gov.
8. ^ "Methanal". webbook.nist.gov.
9. ^ "Key World Energy Statistics (2016)" (PDF). iea.org.