Heat of combustion
Amount of heat released by combustion of a quantity of substance
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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:
There are two kinds of heating values, called high(er) and low(er), depending on how much the products are allowed to cool and whether compounds like H
2O 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 standard enthalpies/heats of formation ΔH⦵
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).
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, ΔH°comb, is the heat of reaction of the following process:
- C
cH
hN
nO
o (std.) + (c + h⁄4 - o⁄2) O
2 (g) → cCO
2 (g) + h⁄2H
2O (l) + n⁄2N
2 (g)
Chlorine and sulfur are not quite standardized; they are usually assumed to convert to hydrogen chloride gas and SO
2 or SO
3 gas, respectively, or to dilute aqueous hydrochloric and sulfuric acids, respectively, when the combustion is conducted in a bomb calorimeter containing some quantity of water.
Definitions
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, including 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).
The American Petroleum Institute (API) refers to the HHV at 25 °C as the standard heat of combustion, and the HHV at 60 °F as the gross heat of combustion. The two values are approximately equal.
When used as a "heating value", HHV assumes all the water component is in liquid state at the end of combustion (in product of combustion) and that all 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.
- Water-only LHV
- One definition is simply to subtract the heat of vaporization of the produced water from the higher heating value. This treats any H2O formed as a vapor that is released as a waste. The energy required to vaporize the water is therefore lost. It 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.
- GPSA LHV
- This definition, used by Gas Processors Suppliers Association (GPSA), is the enthalpy of all combustion products minus the enthalpy of the fuel at the reference temperature (GPSA currently uses 60 °F (15+5⁄9 °C)), 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.
- It can be alternatively restated as the HHV at reference temperature minus the heat of vaporization of the vapor content of the combustion products. By specifying a reference temperature, all products can be taken into account.
- This work is based on American Petroleum Institute (API) research project 44, which used 25 °C (77 °F) as the reference.
- API LHV
- The API defines a net heat of combustion to correspond to LHV. It uses the reference temperature of 60 °F (15+5⁄9 °C), but with gaseous water. It is calculated as the gross heat of combustion (HHV at reference temperature) subtracted by the heat of vaporization of water at reference temperature (and at the vapor pressure corresponding to reference temperature).
- 150 °C LHV
- This 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 many other potential 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 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. It will in fact give a slightly different answer.
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.
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.
End products for different elements
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. H
2O(l), Br
2(l), I
2(s) and H
2SO
4(l). In the net definition the products are the gases produced when the compound is burned in an open flame, e.g. H
2O(g), Br
2(g), I
2(g) and SO
2(g). In both definitions the products for C, F, Cl and N are CO
2(g), HF(g), Cl
2(g) and N
2(g), respectively.
There are many other definitions of "gross" and "net".
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Estimating heating values
By elemental composition
The heating value of a fuel can be estimated with the results of ultimate analysis of fuel, which provides for the percentages of each element by mass.
By oxygen consumption
For an organic fuel of composition CcHhOoNn, the (higher) heat of combustion is 438kJ/mol × (c + 0.3 h − 0.5 o) usually to a good approximation (±3% for more than 500 organic compounds). This corresponds to 438kJ/mol for each mole of O2 consumed. The accuracy of such a simplistic formula is due to the very high bond enthalpy of O2, which renders other bond enthalpies largely irrelevant.
This estimate does not work well for inorganic fuels such as carbon monoxide. It also works poorly for explosives such as nitroglycerin and other substances containing high-energy functional groups such as nitro and azide groups.
Dulong's Formula
The heating value can be calculated using Dulong's Formula, which treats carbon, hydrogen, sulfur as combustible elements:
- HHV [kJ/g] = 33.87 mC + 122.3 (mH − mO/8) + 9.4 mS
where mC, mH, mO and mS are the mass fractions of carbon, hydrogen, oxygen, and sulfur on any (wet, dry or ash free) basis, respectively.
There are many modifications of the formula using different weighing terms. Some estimate the LHV instead of the HHV. A comparison of these formulas is available in Hokosai et al. (2016).
The API publishes its own Dulong-style formulas for petroleum liquids and synthetic fuels.
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. 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 at the datum temperature (typically 25 °C), nH
2O,out is the number of 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, 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. 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.
Heat of combustion tables
- 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:
- Algeria: 39.57 MJ/Sm3
- Bangladesh: 36.00 MJ/Sm3
- Canada: 39.00 MJ/Sm3
- China: 38.93 MJ/Sm3
- Indonesia: 40.60 MJ/Sm3
- Iran: 39.36 MJ/Sm3
- Netherlands: 33.32 MJ/Sm3
- Norway: 39.24 MJ/Sm3
- Pakistan: 34.90 MJ/Sm3
- Qatar: 41.40 MJ/Sm3
- Russia: 38.23 MJ/Sm3
- Saudi Arabia: 38.00 MJ/Sm3
- Turkmenistan: 37.89 MJ/Sm3
- United Kingdom: 39.71 MJ/Sm3
- United States: 38.42 MJ/Sm3
- Uzbekistan: 37.89 MJ/Sm3
The lower heating value of natural gas is normally about 90% of its higher heating value. This table is in Standard cubic metres (1 atm, 15 °C), to convert to values per Normal cubic metre (1 atm, 0 °C), multiply above table by 1.0549.