Held group

The heat of combustion ($\Delta H_{c}^{\circ }$) is the energy released as heat when a compound undergoes complete combustion with oxygen under standard conditions. The chemical reaction is typically a hydrocarbon reacting with oxygen to form carbon dioxide, water and heat. It may be expressed with the quantities:

• energy/mole of fuel (kJ/mol)
• energy/mass of fuel
• energy/volume of fuel

The heat of combustion is conventionally measured with a bomb calorimeter. It may also be calculated as the difference between the heat of formation ${\big (}\Delta H_{f}^{\circ }{\big )}$ of the products and reactants.

Heating value

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 energy value is a characteristic for each substance. It is measured in units of energy per unit of the substance, usually mass, such as: kJ/kg, kJ/mol, kcal/kg, Btu/lb. Heating value is commonly determined by use of a bomb calorimeter.

Heating value unit conversions (for more visit Wolfram Alpha):

• kcal/kg = MJ/kg * 238.846
• Btu/lb = MJ/kg * 429.923
• Btu/lb = kcals * 1.8

The heat of combustion for fuels is expressed as the HHV, LHV, or GHV.

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. 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 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 below 150°C 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 vapor 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 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 (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 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.

• Note that GPSA 12th Edition states that the Gross Heating Value of a gas is equivalent to Higher Heating Value. This suggests that there may be different standards in play. The use of the term Gross normally describes a larger value than the Net, which usually describes a smaller value. GPSA is consistent with this, and equates the Gross Heating Value to the higher heating value (for a gas - so probably with no liquid water present), and the Net Heating Value to the lower heating value.

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° 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 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.

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:

HHV = LHV + h_v x (n_H2O,out/n_fuel,in)

where h_v is the heat of vaporization of water, n_H2O,out is the moles of water vaporized and n_fuel,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 is generally used to give a 'benchmark' for the process; however, for true energy calculations the higher heating value is correct. This is particularly relevant for natural gas, whose high hydrogen content produces much water. The gross energy value is relevant for gas 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

For historical reasons, the efficiency of power plants and combined heat and power plants in Europe may have once been calculated based on the LHV. However, this does not seem to be the case nowadays and most countries are tending to correctly use HHV for true efficiency figures. This is becoming noticeable in modern UK energy publications with the increase of energy awareness and based on the simple fact that it is correct. While in the US, values may have been reported to be generally based on the HHV, although any initial investigation may reveal that the US is still tending to use LHV in some circumstances, whether technically correct or not. This has the peculiar result that contemporary combined heat and power plants, where flue-gas condensation is implemented, may report efficiencies exceeding 100%. Using LHV in other energy calculations brings similar errors, especially when pulled (incorrectly) into electrolysis calculations etc.

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. 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

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	47.80
Propane	50.35	21,700	2,220	46.35
Butane	49.50	20,900	2,877	45.75
Pentane				45.35
Gasoline	47.30	20,400		44.4
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)	15.00	8,000
Wood (MAF)	21.7	8,700
Peat (damp)	6.00	2,500
Peat (dry)	15.00	6,500

Higher heating value
of some less common fuels

Fuel	HHV 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	382.0
Hydrazine	19.4	8,370	622.0
Hexamine	30.0	12,900	4,200.0
Carbon	32.8	14,100	393.5

50 year old Petroleum Engineer Kull from Dawson Creek, spends time with interests such as house brewing, property developers in singapore condo launch and camping. Discovers the beauty in planing a trip to places around the entire world, recently only coming back from .

Heat of Combustion for some common fuels (higher value)Template:Cn

Fuel	kJ/g	kcal/g	BTU/lb
Hydrogen	141.9	33.9	61,000
Gasoline	47.0	11.3	20,000
Diesel	45.0	10.7	19,300
Ethanol	29.7	7.1	12,000
Propane	49.9	11.9	21,000
Butane	49.2	11.8	21,200
Wood	15.0	3.6	6,000
Coal (Lignite)	15.0	4.4	8,000
Coal (Anthracite)Template:Cn	27.0	7.8	14,000
Natural Gas	54.0	13.0	23,000

50 year old Petroleum Engineer Kull from Dawson Creek, spends time with interests such as house brewing, property developers in singapore condo launch and camping. Discovers the beauty in planing a trip to places around the entire world, recently only coming back from .

Lower heating value for some organic compounds (at 15.4°C)Potter or Ceramic Artist Truman Bedell from Rexton, has interests which include ceramics, best property developers in singapore developers in singapore and scrabble. Was especially enthused after visiting Alejandro de Humboldt National Park.

Fuel	MJ/kg	MJ/L	BTU/lb	kJ/mol
Alkanes
Methane	50.009	—	21,504	802.34
Ethane	47.794	—	20,551	1,437.2
Propane	46.357	—	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 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	—	—	—
PropylbenzeneTemplate:Dn	41.193	—	—	—
Cumene	41.217	—	—	—
Alcohols
Methanol	19.930	15.78	8,570	638.55
Ethanol	28.865	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
Methanal	17.259	—	—	—
Ethanal	24.156	—	—	—
Propionaldehyde	28.889	—	—	—
Butyraldehyde	31.610	—	—	—
Acetone	28.548	22.62	—	—
Other species
Carbon (graphite)	32.808	—	—	—
Hydrogen	120.971	—	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 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. 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:

• Algeria: 42.00 MJ/m³
• Bangladesh: 36.00 MJ/m³
• Canada: 38.20 MJ/m³
• Indonesia: 40.60 MJ/m³
• Netherlands: 33.32 MJ/m³
• Norway: 39.88 MJ/m³
• Pakistan: 34.90 MJ/m³
• Russia: 38.23 MJ/m³
• Saudi Arabia: 38.00 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 val.

See also

• Adiabatic flame temperature
• Combustion
• Energy density
• Energy value of coal
• Exothermic reaction
• Fire
• 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

References

43 year old Petroleum Engineer Harry from Deep River, usually spends time with hobbies and interests like renting movies, property developers in singapore new condominium and vehicle racing. Constantly enjoys going to destinations like Camino Real de Tierra Adentro.

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

External links

• NIST Chemistry WebBook
• ASTM Standard Testing