The heat of combustion is traditionally measured with a bomb calorimeter. It may also be calculated as the difference between the heat of formation (Δf''H''0) of the products and reactants.
The heat of combustion for fuels is expressed as the HHV, LHV, or GHV.
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).
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 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 cannot be put to use.
The above is but one definition of lower heating value adopted by the American Petroleum Institute (API) and they used a reference temperature of 60 °F (15.56 °C).
Another definition—used by Gas Processors Suppliers Association (GPSA) and originally used by API (data collected for API research project 44)—is that the lower heating value 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 back down to the reference temperature but then the heat content from the condensing vapor is considered to be not useful. This is more easily calculated from the higher heating value than when using the previous definition and will in fact give a slightly different answer.
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 an arbitrary choice.
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.
A common method of relating HHV to LHV is:
::: HHV = LHV + h''v'' x (nH2O,out/nfuel,in)
:where h''v'' 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 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 energy 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.
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 would 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 for a power plant on natural gas between the two methods.
+Higher (HHV) and Lower (LHV) Heating values of some common fuels | Fuel !! HHV megajoule | MJ/kg !! HHV BTU/lb !! HHV kJ/mol !! LHV MJ/kg | |||
141.80 | 61,000| | 286 | 121.00 | ||
align=left | Methane | 55.50| | 23,900 | 889 | 50.00 |
align=left | Ethane | 51.90| | 22,400 | 1,560 | 47.80 |
align=left | Propane | 50.35| | 21,700 | 2,220 | 46.35 |
align=left | Butane | 49.50| | 20,900 | 2,877 | 45.75 |
align=left | Pentane | | | 45.35 | ||
align=left | Gasoline | 47.30| | 20,400 | 44.40 | |
align=left | Paraffin | 46.00| | 19,900 | 41.50 | |
align=left | Kerosene | 46.20| | 19,862 | 43.00 | |
align=left | Diesel | 44.80| | 19,300 | ||
align=left | Coal (Anthracite) | 27.00| | 14,000 | ||
align=left | Coal (Lignite) | 15.00| | 8,000 | ||
align=left | Wood | 15.00| | 6,500 | ||
align=left | Peat (damp) | 6.00| | 2,500 | ||
align=left | Peat (dry) | 15.00| | 6,500 |
+Higher heating value of some less common fuels | Fuel !! HHV megajoule | MJ/kg !! BTU/lb !! kJ/mol | ||
22.7 | 9,800 | |||
align=left | Ethanol | 29.7| | 12,800 | 1,300.0 |
align=left | Propanol | 33.6| | 14,500 | 2,020.0 |
align=left | Acetylene | 49.9| | 21,500 | 1,300.0 |
align=left | Benzene | 41.8| | 18,000 | 3,270.0 |
align=left | Ammonia | 22.5| | 9,690 | 382.0 |
align=left | Hydrazine | 19.4| | 8,370 | 622.0 |
align=left | Hexamine | 30.0| | 12,900 | 4,200.0 |
align=left | Carbon | 32.8| | 14,100 | 393.5 |
+Heat of Combustion for some common fuels (higher value) | |||
! Fuel | kilojoule>kJ/g | kilocalorie>kcal/g | ! BTU/lb |
141.9 | 33.9 | 61,000 | |
47.0 | 11.3 | 20,000 | |
align=left | 45.0 | 10.7 | 19,300 |
29.7 | 7.1 | 12,000 | |
49.9 | 11.9 | 21,000 | |
49.2 | 11.8 | 21,200 | |
15.0 | 3.6 | 6,000 | |
15.0 | 4.4 | 8,000 | |
27.0 | 7.8 | 14,000 | |
54.0 | 13.0 | 23,000 |
! Fuel | megajoule>MJ/kg | ! Joule | ! [[BTU/lb | ! Paraffins |
[[Methane">Joule | ||||
Paraffins | ||||
[[Methane | 50.009 | — | 23,934.6 | 802.34 |
Ethane | 47.794 | — | 22,871.8 | 1437.17 |
Propane | 46.357 | — | 22,182.6 | 2044.21 |
Butane | 45.752 | — | 21,896.1 | 2659.30 |
Pentane | 45.357 | 28.39 | 21,705.9 | 3272.57 |
Hexane | 44.752 | 29.30 | 21,415.6 | 3856.66 |
Heptane | 44.566 | 30.48 | 21,326.0 | 4465.76 |
Octane | 44.427 | 31.23 | 22,748.1 | 5430 |
Nonane | 44.311 | 31.82 | — | — |
Decane | 44.240 | 33.29 | — | — |
Undecane | 44.194 | 32.70 | — | — |
Dodecane | 44.147 | 33.11 | — | — |
Isoparaffins | ||||
Isobutane | 45.613 | — | — | — |
Isopentane | 45.241 | 27.87 | — | — |
2-Methylpentane | 44.682 | — | — | — |
2,3-Dimethylbutane | 44.659 | 29.47 | — | — |
2,3-Dimethylpentane | 44.496 | — | — | — |
2,2,4-Trimethylpentane | 44.310 | 30.49 | — | — |
Naphthenes | ||||
Cyclopentane | 44.636 | 33.52 | — | — |
Methylcyclopentane | 44.636 | 33.43 | — | — |
Cyclohexane | 43.450 | 33.85 | — | — |
Methylcyclohexane | 43.380 | 33.40 | — | — |
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 derivated | ||||
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 | — | — | — |
Propylbenzene | 41.193 | — | — | — |
Cumene | 41.217 | — | — | — |
Alcohols | ||||
Methanol | 19.930 | 15.78 | — | — |
Ethanol | 28.865 | 22.77 | — | — |
n-Propanol | 30.680 | 24.65 | — | — |
Isopropanol | 30.447 | 23.93 | — | — |
n-Butanol | 33.075 | 26.79 | — | — |
Isobutanol | 32.959 | 26.43 | — | — |
Tert-butanol | 32.587 | 25.45 | — | — |
34.727 | 28.28 | — | — | |
Ethers | ||||
Methoxymethane | 28.703 | — | — | — |
Ethoxyethane | 33.867 | — | — | — |
Propoxypropane | 36.355 | — | — | — |
Butoxybutane | 37.798 | — | — | — |
Aldehydes and ketones | ||||
Methanal | 17.259 | — | — | — |
Ethanal | 24.156 | — | — | — |
Propionaldehyde | 28.889 | — | — | — |
Butyraldehyde | 31.610 | — | — | — |
Acetone | 28.548 | 22.62 | — | — |
Other species | ||||
32.808 | — | — | — | |
Hydrogen | 120.971 | — | — | — |
Carbon monoxide | 10.112 | — | — | 283.23712 |
Ammonia | 18.646 | — | — | — |
Sulfur (solid) | 9.163 | — | — | — |
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.
The lower heating values of the above natural gases are about 90 percent of the higher heating values.
Category:Chemical engineering Category:Combustion Category:Fuels Category:Thermodynamics Category:Nuclear physics Category:Thermochemistry Category:Thermodynamic properties
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