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Contents
  1. 1Definition
  2. 2Gross and net values
  3. 3Typical values
  4. 4Heat per unit of oxygen
  5. 5Heat release rate and combustion efficiency
  6. 6Use in fire safety engineering
  7. 7See also
  8. 8Notes
  9. 9References

The heat of combustion is the amount of heat released when a unit quantity of a fuel is oxidised completely to stable end products, such as carbon dioxide and water.[1] It is usually expressed in kilojoules per gram or megajoules per kilogram of fuel.[2] Typical values range from about 16 MJ/kg for cellulose and 19.5 MJ/kg for beech wood to about 43 MJ/kg for polyethylene and 50 MJ/kg for methane.[2] In fire safety engineering, the value used is the net heat of combustion, in which the water formed remains a vapour.[2]

The heat of combustion links the mass of fuel that burns to the energy that a fire releases. Multiplied by the total mass of combustible material, it gives the fire load of a room or building.[3] Multiplied by the rate at which fuel is consumed, it gives the heat release rate.[4] Real fires never burn completely, so engineers often use a lower, effective heat of combustion that allows for the combustion efficiency.[4][5] A closely related property is the heat released per kilogram of oxygen consumed. It is almost the same for most fuels, about 13.1 MJ/kg, and forms the basis of modern heat release measurements.[2][6]

Definition

Combustion is a set of mainly exothermic chemical reactions between a fuel and an oxidiser, usually oxygen, in which heat and light are produced.[7] The heat of combustion quantifies that heat. Drysdale defines it as the amount of heat released when a unit quantity of fuel is oxidised completely to yield stable end products.[1] The reaction must be specified by a balanced chemical equation, which also fixes the stoichiometry: the exact proportions of fuel and oxygen for complete conversion.[1] The physical states of the fuel and the products must also be stated, because a fuel such as hexane can be burnt as a liquid or as a vapour.[1]

Fires take place at constant pressure, so the heat of combustion is an enthalpy change, written ΔHc. Because combustion releases heat, ΔHc is negative by the sign convention of chemistry.[1] Fire engineering tables, however, usually list it as a positive number.[2] The New Zealand Verification Method C/VM2 (second edition, 2025) defines it as the thermal energy produced by combustion of a unit mass of a given substance, in kJ/g.[8]

The heat of combustion can be expressed per mole of fuel, per gram of fuel, per gram of oxygen consumed or per gram of air consumed.[2] The value per mole is common in chemistry texts. The value per gram of fuel is more useful to fire protection engineers. The values per gram of oxygen and per gram of air have very specific uses in fire problems.[2] Introduction to Fire Safety Engineering calls the property the heat of combustion per unit mass and gives it in MJ/kg.[3]

Gross and net values

Heats of combustion are measured in a bomb calorimeter. A precise amount of fuel is burnt in pure oxygen inside a strong pressure vessel, and the heat released follows from the measured rise in temperature.[1] The apparatus is designed to keep heat losses to a minimum, and small corrections are made for the losses that remain.[1] Because the vessel is closed, the test measures the heat released at constant volume. A small correction converts this to the enthalpy change at constant pressure.[2]

The bomb calorimeter gives the gross heat of combustion, with all products in their standard states, so that the water formed is liquid.[2] The net heat of combustion assumes that the water remains a vapour. It is lower than the gross value by the latent heat of evaporation of the water formed, 2.26 kJ per gram of water.[2] In a fire the water leaves as vapour, so the net value is the one to use in fire calculations.[2] Tewarson uses the term net heat of complete combustion for the same quantity.[9]

Moisture in the fuel reduces the useful heat further. Part of the energy of a burning material is used to evaporate the water that it contains.[3] This must be subtracted when the heat of combustion is given per kilogram of dry material. For wood, the value is usually given at the equilibrium moisture content, so no correction is needed. Values in Dutch sources such as NEN 6060 already allow for this effect.[3]

The net heat of complete combustion can also be calculated from the standard heats of formation of the fuel, carbon dioxide and water.[9] Both routes give similar results. For PMMA, the bomb calorimeter gave 25.3 kJ/g and the calculation from heats of formation 24.9 kJ/g; for polystyrene, the values were 39.2 and 39.8 kJ/g.[9] Heats of combustion say nothing about chemical reactivity. Drysdale notes that the heats of combustion of endothermic compounds, such as acetylene, give no indication of their reactivity.[10][4]

Tabulated values normally refer to an initial temperature of 25 °C. Drysdale notes that such values are satisfactory for virtually all fire problems.[2] When the reaction takes place at a higher temperature, the heat of combustion can be corrected with the heat capacities of the reactants and products, following the first law of thermodynamics.[10]

Typical values

The heat of combustion depends mainly on the chemical composition of the fuel. Hydrocarbons such as methane, propane and polyethylene release 43 to 50 kJ/g. Materials that already contain oxygen in their molecules, such as alcohols, cellulose, wood and polymethylmethacrylate (PMMA), release much less.[2] Polyvinyl chloride, which contains chlorine, releases about 16 kJ/g. The table below lists net values at 25 °C.[2]

Heats of combustion of selected fuels at 25 °C[2]
Fuel ΔHc (kJ/g) ΔHc per gram of oxygen (kJ/g)
Carbon monoxide 10.10 17.69
Methane 50.00 12.54
Propane 46.45 12.80
Benzene 40.00 13.06
Methanol 19.83 13.22
Ethanol 26.78 12.88
Cellulose 16.09 13.59
Polyethylene 43.28 12.65
Polystyrene 39.85 12.97
Polyvinyl chloride 16.43 12.84
Polymethylmethacrylate 24.89 12.98
Nylon 6,6 29.58 12.67
Wood (European beech) 19.5 –

Wood shows how one material can give different values for its parts. Beech wood as a whole releases 19.5 kJ/g. Its volatiles release only 16.6 kJ/g, while the remaining char releases 34.3 kJ/g.[2] In flaming combustion the volatiles burn in the flame, while the char burns much more slowly by surface oxidation.[1]

Introduction to Fire Safety Engineering gives practical values for fire load surveys. Diesel oil releases 42 MJ/kg, beech wood 18 MJ/kg, leather 20 MJ/kg and PVC roofing 18 MJ/kg.[3] For whole objects, the book quotes about 1,884 MJ for a three-seat sofa and 6,000 to 12,000 MJ for a car.[3] The two sources give slightly different values for beech wood, 18 MJ/kg in the book and 19.5 kJ/g in the SFPE Handbook.[3][2] A value of 1 kJ/g is the same as 1 MJ/kg.

Heat per unit of oxygen

Expressed per gram of fuel, heats of combustion vary widely. Expressed per gram of oxygen consumed, they are nearly constant for most fuels, with an average of 13.1 kJ/g.[2] Per gram of air consumed, the average is about 3 kJ/g.[2] In 1917, Thornton showed this for many organic liquids and gases. Huggett later found the same for organic solids and obtained the average value of 13.1 kJ per gram of oxygen, accurate to within about 5 % with very few exceptions.[6]

This regularity is the basis of the oxygen consumption principle. It is sufficient to measure the oxygen consumed by a fire in order to determine the heat released.[6] The cone calorimeter and large-scale furniture calorimeters use this method, with a small correction for incomplete combustion based on the carbon monoxide produced.[11] NUREG-1805 describes the constant as 13,100 kJ per kilogram of oxygen, accurate within 5 % for most hydrocarbon fuels.[5] Tewarson's own average over many fuels is slightly lower, 12.8 kJ/g with a spread of 7 %.[12]

The same regularity explains why fuels need very different amounts of air per kilogram. One gram of propane needs 3.64 g of oxygen, or 15.6 g of air, for complete combustion.[1] Wood, with the approximate formula CH1.5O0.75, needs only 5.38 g of air per gram, because it already contains oxygen.[1] Per gram of air consumed, however, propane and cellulose release similar amounts of heat, 2.97 and 3.15 kJ/g.[2]

The constant also gives a quick estimate of the air needed by a fire. At 13.1 kJ/g of oxygen and 3 kJ/g of air, a fire of 1 MW consumes about 0.076 kg of oxygen, or about 0.33 kg of air, every second.[2] When a compartment fire is ventilation-controlled, Drysdale estimates the heat release rate inside the room from the inflow of air multiplied by the heat of combustion per unit mass of air, 3 kJ/g.[4]

Heat release rate and combustion efficiency

Thermochemistry gives the total energy that a fuel can release, but not how fast it is released. Drysdale argues that the rate of heat release is the more important quantity in fires.[4] When a single item burns in isolation, its burning rate and its heat release rate are coupled.[4] It is then convenient to express the heat release rate in terms of the burning rate, the rate at which the fuel loses mass:[4]

Q˙=m˙ΔHc

Symbols in the heat release rate relation
Symbol Description Unit
Q˙ heat release rate kW
m˙ burning rate (mass loss rate) of the fuel kg/s
ΔHc net heat of combustion, or the effective heat of combustion for real fires kJ/kg

With the net heat of combustion, this relation assumes complete combustion, which never happens in natural fires.[4] Even with unrestricted ventilation, the smoke contains partly oxidised products such as carbon monoxide, aldehydes, ketones and soot. Their presence shows that not all of the available energy has been released.[4] NUREG-1805 therefore writes the same relation with the effective heat of combustion, which is needed to estimate the heat release rate from a measured mass loss rate.[5] The Australian FSVM data sheets use it in the opposite direction, to convert a design fire curve into a mass burning rate.[13]

Introduction to Fire Safety Engineering gives a worked example for a fire in a shop. The fire burns at a constant 1 MW for 120 s, and the effective heat of combustion is 20 MJ/kg.[14] The relation gives a burning rate of 1 MW / 20 MJ/kg = 0.05 kg/s. Over 120 s the fire releases 120 MJ and therefore burns 6 kg of fuel.[14] With a soot yield of 0.1 kg/kg, the smoke layer then contains 0.6 kg of soot, from which the book calculates the visibility.[14]

Combustion efficiency is the ratio between the heat actually released and the net heat of complete combustion.[15] The heat released per unit mass in a real fire is called the chemical heat of combustion.[9] For PMMA, it falls from 24.9 kJ/g for complete combustion to 22.1, 18.2 and 11.5 kJ/g as more carbon monoxide, soot and ethylene are formed. The corresponding combustion efficiencies are 0.89, 0.73 and 0.46.[9] Below about 0.40, combustion becomes unstable and the flame goes out.[9]

For liquid pool fires, the effective values are close to the complete ones. NUREG-1805 lists 20,000 kJ/kg for methanol, 26,800 kJ/kg for ethanol and 44,600 kJ/kg for heptane in large pool fires.[16] These are within about 1 % of the complete values for methanol and ethanol in the SFPE Handbook.[2]

Measured combustion efficiencies vary widely. They are close to one for methanol and heptane pool fires, but about 45 % for polystyrene and 63 % for a fully involved stack of wood pallets.[17] Drysdale gives a range from about 0.3 to 0.4 for heavily fire-retarded materials to 0.9 or more for oxygen-containing products.[4]

Use in fire safety engineering

The fire load of a building is the total energy that would be released if all materials in it burned completely. Related to the floor area, it becomes the fire load density, in MJ/m².[3] The book calculates the energy in a stored volume as the volume times the density times the heat of combustion per unit mass.[3] For a warehouse with 3,500 m³ of goods at 135 kg/m³ and 23 MJ/kg, this gives 3,500 × 135 × 23 = 10,867,500 MJ.[3] Divided by 19 MJ/kg for spruce wood, the fire load is equivalent to about 571,973 kg of wood.[3]

In practice a fire seldom releases all of this energy, because of its limited combustion efficiency.[3] Engineers use the fire load mainly to estimate the duration of a fire or the total thermal load on a structure.[3] A fire load survey cited in the Australian FSVM data sheets, for example, converted masses into energy assuming 18 MJ/kg for timber.[18] The book gives example values of about 950 MJ/m² for residential use and 1,800 MJ/m² for a library.[3]

Design fires for performance-based design also need a heat of combustion. In New Zealand, C/VM2 (second edition, 2025) lists the heat of combustion among the parameters that define a design fire.[19] For pre-flashover design fires in all building uses it specifies 20 MJ/kg, together with a soot yield of 0.07 kg/kg.[20] The Australian FSVM data sheets (2020) suggest the same values as an indication when the make-up of the fire load is unknown.[21]

Fire models use the heat of combustion to convert a specified heat release rate into a flow of fuel gas. In the Fire Dynamics Simulator, the user sets a heat release rate per unit area, and the program divides it by the gas-phase heat of combustion to find the mass flux of fuel.[22] The chosen value therefore determines how much fuel, and hence how much smoke and soot, a model fire produces for a given heat release rate.[23]

The heat of combustion has clear limits as a design input. Drysdale stresses that heats of combustion can rarely, if ever, be used directly to calculate the heat released in real fires.[4] In compartments, some fuel vapours may also escape and burn outside, and this share cannot easily be estimated.[4] The value per unit of oxygen is more general, but it also has exceptions of more than 5 %.[6]

See also

Notes

  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 Drysdale (2002), p. 1-92.
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 2.18 2.19 2.20 2.21 Drysdale (2002), p. 1-93.
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 3.13 Van Rossum (2027), §4.4.
  4. ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 Drysdale (2002), p. 1-95.
  5. ↑ 5.0 5.1 5.2 Iqbal and Salley (2004), p. 3-3.
  6. ↑ 6.0 6.1 6.2 6.3 Janssens (2002), p. 3-45.
  7. ↑ Van Rossum (2027), §4.1.
  8. ↑ Ministry of Business, Innovation and Employment (2025b), p. 65.
  9. ↑ 9.0 9.1 9.2 9.3 9.4 9.5 Tewarson (2002), p. 3-103.
  10. ↑ 10.0 10.1 Drysdale (2002), p. 1-94.
  11. ↑ Drysdale (2002), p. 1-96.
  12. ↑ Tewarson (2002), p. 3-104.
  13. ↑ Australian Building Codes Board (2020a), Data Sheet B2, p. 19.
  14. ↑ 14.0 14.1 14.2 Van Rossum (2027), §6.5.
  15. ↑ Tewarson (2002), p. 3-102.
  16. ↑ Iqbal and Salley (2004), p. 3-4.
  17. ↑ Heskestad (2002), p. 2-2.
  18. ↑ Australian Building Codes Board (2020a), Data Sheet B2, p. 15.
  19. ↑ Ministry of Business, Innovation and Employment (2025b), p. 19.
  20. ↑ Ministry of Business, Innovation and Employment (2025b), p. 20.
  21. ↑ Australian Building Codes Board (2020a), Data Sheet B2, p. 20.
  22. ↑ McGrattan et al. (2026), p. 82.
  23. ↑ Van Rossum (2027), §6.3.

References

  • Australian Building Codes Board. (2020a). Data sheets: Fire Safety Verification Method (Handbook annex, Print version 1.1).
  • Drysdale, D. D. (2002). Thermochemistry. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 1-90–1-98). National Fire Protection Association; Society of Fire Protection Engineers.
  • Heskestad, G. (2002). Fire plumes, flame height, and air entrainment. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-1–2-17). National Fire Protection Association; Society of Fire Protection Engineers.
  • Iqbal, N., & Salley, M. H. (2004). Fire Dynamics Tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission Fire Protection Inspection Program (NUREG-1805). U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation.
  • Janssens, M. (2002). Calorimetry. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-38–3-62). National Fire Protection Association; Society of Fire Protection Engineers.
  • McGrattan, K., Hostikka, S., Floyd, J., McDermott, R., Vanella, M., Mueller, E., & Paul, C. (2026). Fire Dynamics Simulator technical reference guide: Vol. 1. Mathematical model (6th ed., NIST Special Publication 1018-1). National Institute of Standards and Technology. https://doi.org/10.6028/NIST.SP.1018
  • Ministry of Business, Innovation and Employment. (2025b). Verification Method C/VM2: Framework for fire safety design (2nd ed.).
  • Tewarson, A. (2002). Generation of heat and chemical compounds in fires. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-82–3-161). National Fire Protection Association; Society of Fire Protection Engineers.
  • Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
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