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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

The calorific value is conventionally measured with a bomb calorimeter. It may also be calculated as the difference between the heat of formation ΔHo f of the products and reactants (though this approach is purely empirical since most heats of formation are calculated from measured heats of combustion). For a fuel of composition CcHhOoNn, the magnitude of the heat of combustion is 418 kJ/mol (c + 0.3 h – 0.5 o) usually to a good approximation (±3%),[1] though it can be drastically wrong if o + n > c (for instance in the case of nitroglycerine this formula would predict a heat of combustion of 0). The calorific value of all organic compounds has the sign corresponding to an exothermic reaction (negative in the standard chemical convention) 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]

Contents

1 Higher heating value 2 Lower heating value 3 Gross heating value 4 Measuring heating values 5 Relation between heating values 6 Usage of terms 7 Accounting for moisture 8 Heat
Heat
of combustion tables 9 Higher heating values of natural gases from various sources 10 See also 11 References 12 External links

Higher heating value[edit] 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 15 °C (59 °F; 288 K)[citation needed]. 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, hence yielding its latent heat of vaporization. Mechanical systems such as gas-fired boilers used for space heat are suited for the purpose of capturing the HHV as the heat delivered is at temperatures below 150 °C (302 °F; 423 K) yet usable in space heating. Lower heating value[edit] 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
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[edit] 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[edit] 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[edit] 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:

H H V

=

L H V

+

H

v

(

n

H

2

O , o u t

n

f u e l , i n

)

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, nH2O,out is the moles of water vaporized and nfuel,in is the number of moles of fuel combusted.[2]

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[edit] Many engine manufacturers rate their engine fuel consumption by the lower heating values. 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.[3] 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[edit] 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
Heat
of combustion tables[edit]

Higher (HHV) and lower (LHV) heating values of some common fuels[4]

Fuel HHV MJ/kg HHV BTU/lb HHV kJ/mol LHV MJ/kg

Hydrogen 141.80 61,000 286 119.96

Methane 55.50 23,900 889 50.00

Ethane 51.90 22,400 1,560 47.622

Propane 50.35 21,700 2,220 46.35

Butane 49.50 20,900 2,877 45.75

Pentane 48.60 21,876 3,507 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
Coal
(anthracite) 32.50 14,000

Coal
Coal
(lignite - USA) 15.00 6,500

Wood
Wood
(MAF) 21.70 8,700

Wood
Wood
fuel 21.20 9,142

17.0

Peat
Peat
(dry) 15.00 6,500

Peat
Peat
(damp) 6.00 2,500

Higher heating value of some less common fuels[4]

Fuel MJ/kg BTU/lb kJ/mol

Methanol 22.7 9,800 726.0

Ethanol 29.7 12,800 1,300.0

1-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 9,690 382.6

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])[citation needed]

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 6,850.7

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

1,3-Butadiene 44.613 — — —

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 8,570 638.55

Ethanol 26.70 22.77 12,412 1,329.8

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 [5]

Acetaldehyde 24.156 — — —

Propionaldehyde 28.889 — — —

Butyraldehyde 31.610 — — —

Acetone 28.548 22.62 — —

Other species

Carbon
Carbon
(graphite) 32.808 — — —

Hydrogen 120.971 1.8 52,017 244

Carbon
Carbon
monoxide 10.112 — 4,348 283.24

Ammonia 18.646 — 8,018 317.56

Sulfur
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[edit] The International Energy
Energy
Agency reports the following typical higher heating values:[6]

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. See also[edit]

Adiabatic flame temperature Energy
Energy
density Energy
Energy
value of coal Exothermic
Exothermic
reaction Fire Fuel
Fuel
efficiency# Energy
Energy
content of fuel Food
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
Energy
conversion efficiency

References[edit]

Guibet, J.-C. Carburants et moteurs. Publication de l'Institut Français du Pétrole. ISBN 2-7108-0704-1. 

^ a b Schmidt-Rohr, K (2015). "Why Combustions Are Always Exothermic, Yielding About 418 kJ per Mole of O2". J. Chem. Educ. 92: 2094–2099. Bibcode:2015JChEd..92.2094S. doi:10.1021/acs.jchemed.5b00333.  ^ 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.  ^ a b "NIST Chemistry WebBook". webbook.nist.gov.  ^ "Methanal". webbook.nist.gov.  ^ "Key World Energy
Energy
Statistics (2016)" (PDF). iea.org. 

External links[edit]

NIST Chemistry WebBook ASTM Standard Testing "Lower and Higher Heating Values of Gas, Liquid and Solid Fuels" (PDF). Biomass Energy
Energy
Data Book. U.S. Department of Energy. 2011. 

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