Summary
The heat of combustion is the energy liberated when a substance undergoes complete combustion, at constant pressure usually in an environment with excess Oxygen. The heat of combustion is utilised to quantify the performance of a fuel in a combustion system such as furnaces, power generation turbines and motors. This article describes the heat of combustion and provides a list of heats of combustion for commons fuels and fuel components.
Definitions
| $ c_{p}$ | : | Specific heat capacity |
| $ h_{vap,water}$ | : | Heat of vaporisation of water $ (h_{vap,water} = 40.68 kJ/mol) $ |
| $ m $ | : | Mass |
| $ n $ | : | Stoichiometric coefficient |
| $Q$ | : | Heat transfer duty |
| $ T $ | : | Temperature |
| $ \Delta H_{c}$ | : | Heat of combustion |
Heating Value
The heat of combustion is typically presented in the form of a heating value. The heating value is the amount of energy released during combustion and can be referenced as a higher or lower heating value.
Higher Heating Value
The higher heating value (HHV) accounts for the heat of combustion and any energy released to bring the combustion products back their pre-combustion temperatures (typically 25°C). By bringing the combustion products back to pre-combustion temperatures, the water component of the combustion products condenses and therefore the latent heat of vaporisation of water is incorporated in the higher heating value. The higher heating value is most useful in circumstances where condensation of combustion products is practical.
Lower Heating Value
The lower heating value (LHV) assumes that the combustion products are not brought back to pre-combustion temperatures and is therefore essentially the higher heating value minus the latent heat of vaporisation of the water product. The lower heating value may be approximated from the higher heating value as follows:
$$ \displaystyle LHV = HHV - h_{vap,water} \times \frac{n_{H_2O}}{n_{fuel}} $$
The accuracy of the conversion between LHV and HHV may be improved by accounting for the change in sensible heat of the combustion products.
Heats of Combustion for Alkanes
The molar heat of combustion (HHV) for a selection of alkanes is presented below.
| Fuel | Fuel Molecular Weight | Formula | Heat of Combustion (ΔHc) kJ/mol (@25°C) |
|---|---|---|---|
| Hydrogen (H2) | 2.0 | 2H2(g) + O2(g) → 2H2O(l) | -286 |
| Methane (CH4) | 16.0 | CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) | -890 |
| Ethane (C2H6) | 30.1 | 2C2H6(g) + 7O2(g) → 4CO2(g) + 6H2O(l) | -1560 |
| Propane (C3H8) | 44.1 | C3H8(g) + 10O2(g) → 3CO2(g) + 4H2O(l) | -2220 |
| Butane (C4H10) | 58.1 | 2C4H10(g) + 13O2(g) → 8CO2(g) + 10H2O(l) | -2874 |
| Pentane (C5H12) | 72.1 | C5H12(l) + 8 O2(g) → 5CO2(g) + 6H2O(l) | -3509 |
| Hexane (C6H14) | 86.2 | 2C6H14(l) + 19O2(g) → 12CO2(g) + 14H2O(l) | -4163 |
| Heptane (C7H16) | 100.2 | C7H16(l) + 11O2(g) → 7CO2(g) + 8H2O(l) | -4817 |
| Octane (C8H18) | 114.2 | 2C8H18(l) + 25O2(g) → 16CO2(g) + 18H2O(l) | -5470 |
| Nonane (C9H20) | 128.3 | C9H20(l) + 14O2(g) → 9CO2(g) + 10H2O(l) | -6125 |
| Decane (C10H22) | 142.3 | 2C10H22(l) + 31O2(g) → 20CO2(g) + 22H2O(l) | -6778 |
| Undecane (C11H24) | 156.3 | C11H24(l) + 16O2(g) → 11CO2(g) + 12H2O(l) | -7431 |
| Dodecane (C12H26) | 170.3 | 2C12H26(l) + 37O2(g) → 24CO2(g) + 26H2O(l) | -8087 |
| Hexadecane (CH3(CH | 226.4 | 2CH3(CH | -10699 |
The heat of combustion is exothermic, that is, energy is liberated through the combustion reaction. To calculate the total heat generation for the fuel listed above simply multiply the number of moles of fuel burnt by the molar heat of combustion listed above.
Heats of Combustion for Alcohols
The molar heat of combustion (HHV) for a selection of alcohols is presented below.
| Fuel | Fuel Molecular Weight | Formula | Heat of Combustion (ΔHc) kJ/mol (@25°C) |
|---|---|---|---|
| Methanol (CH3OH) | 32.0 | 2CH3OH(l) + 3O2(g) → 2CO2(g) + 2H2O(l) | -726 |
| Ethanol (CH3CH2OH) | 46.1 | CH3CH2OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l) | -1367 |
| 1-Propanol (CH3(CH2)2OH) | 60.1 | 2CH3(CH2)2OH(l) + 9O2(g) → 6CO2(g) + 8H2O(l) | -2021 |
| 2-Propanol (CH3CH(OH)CH3) | 60.1 | 2CH3CH(OH)CH3(l) + 9O2(g) → 6CO2(g) + 8H2O(l) | -2006 |
| 1-Butanol (CH3(CH2)3OH) | 74.1 | CH3(CH2)3OH + 6O2(g) → 4CO2(g) + 5H2O(l) | 2676 |
| 1-Pentanol (CH3(CH2)4OH) | 88.2 | 2CH3(CH2)4OH + 15O2(g) → 10CO2(g) + 12H2O(l) | 3331 |
| 1-Hexanol (CH3(CH2)5OH) | 102.2 | CH3(CH2)5OH+ 9O2(g) → 6CO2(g) + 7H2O(l) | 3984 |
| 1-Heptanol (CH3(CH2)6OH) | 116.2 | 2CH3(CH2)6OH+ 21O2(g) → 14CO2(g) + 16H2O(l) | 4638 |
| 1-Octanol (CH3(CH2)7OH) | 130.2 | CH3(CH2)7OH+ 12O2(g) → 8CO2(g) + 9H2O(l) | 5294 |
Heats of Combustion for Commercial Fuels
Below the heat of combustion (HHV) for several common commercial fuels is presented in order of decreasing heating value.
| Commercial Fuels | Typical Heat of Combustion (ΔHc) MJ/kg |
|---|---|
| Natural Gas | -54 |
| Gasoline | -47.3 |
| Kerosene | -46.2 |
| Diesel Fuel | -44.8 |
| Ethanol | -29.7 |
| Coal (Anthracite) | -27.0 |
| Methanol | -22.7 |
| Wood | -15.0 |
| Coal (Lignite) | -15.0 |
Experimental Calculation of Heat of Combustion
In the absence of published data the heat of combustion (LHV) can be experimentally determined using the following procedure:
- Measure a known quantity of water into a flask and stand this on top of a tripod situated above a fuel source.
- Measure the initial temperature of the water and keep a thermometer submersed in the water to measure any temperature changes.
- Measure the initial mass of the fuel being used. For example methanol in a controlled burning manner e.g. through a wick.
- When the temperature of the water has increase by at least 10 C then the experiment can be stopped by extinguishing the flame.
- Record the final mass of the fuel.
- Record the final temperature of the water and calculate the temperature change.
- Calculate the heat of combustion using the methodology demonstrated below.
An example of the experimental calculation process is shown as follows. Please note that these numbers are not obtained from an actual experiment, they are purely for demonstrating the experimental and calculation method to apply if performing the heat of combustion determination experiment please refer to literature for actual heat of combustion values for diff erent fuels and do not undertake any experiments without completely understanding and controlling the hazards associated with the experiment.
| Initial temperature of water | 20°C |
| Final temperature of water | 30°C |
| Change in temperature of water | 10°C |
| Mass of water | 100g |
| Initial mass of methanol | 20.00g |
| Final mass of methanol | 19.79g |
| Fuel consumed | 0.21g |
Calculate mols of fuel consumed: MW of methanol = 32 g/mol mols (n) = 0.21 g / 32 g/mol = 6.55x10-3 mol of methanol
Calculate enthalpy increase of water using $ Q = m c_{p} \Delta T$ : Mass of water = 100 g Specific heat = 4.18 J/g.K Temperature change = 10 °C (K) Enthalpy = 100g x 4.18 J/g.K x 10 K = 4180J = 4.18 kJ
Calculate the heat of combustion assuming heat not lost to surroundings: 6.55x10-3 mol of methanol produces 4.18kJ of heat.
4.18 kJ / 6.55x10-3 mol = 638.6 kJ/mol = experimental ΔHC kJ/mol methanol.
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