Heat of combustion - Revision history

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2026-11-02T17:12:37Z

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	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.<ref name="VR-4.4">Van Rossum (2027), §4.4.</ref> Multiplied by the rate at which fuel is consumed, it gives the [[heat release rate]].<ref name="DR-1-95">Drysdale (2002), p. 1-95.</ref> Real fires never burn completely, so engineers often use a lower, [[effective heat of combustion]] that allows for the combustion efficiency.<ref name="DR-1-95" /><ref name="IS-3-3">Iqbal and Salley (2004), p. 3-3.</ref> 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.<ref name="DR-1-93" /><ref name="JA-3-45">Janssens (2002), p. 3-45.</ref>		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.<ref name="VR-4.4">Van Rossum (2027), §4.4.</ref> Multiplied by the rate at which fuel is consumed, it gives the [[heat release rate]].<ref name="DR-1-95">Drysdale (2002), p. 1-95.</ref> Real fires never burn completely, so engineers often use a lower, [[effective heat of combustion]] that allows for the combustion efficiency.<ref name="DR-1-95" /><ref name="IS-3-3">Iqbal and Salley (2004), p. 3-3.</ref> 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.<ref name="DR-1-93" /><ref name="JA-3-45">Janssens (2002), p. 3-45.</ref>

			[[File:fse-heat-of-combustion-v1.svg\|thumb\|upright=1.5\|alt=Flow scheme: fuel mass gives fire load, burning rate gives heat release rate, oxygen consumed times 13.1 MJ/kg gives heat release rate.\|Three uses of the heat of combustion: fire load from the fuel mass, heat release rate from the burning rate, and heat release rate from the oxygen consumed. Based on Van Rossum (2027), §4.4, and Drysdale (2002), pp. 1-93 and 1-95.]]

	== Definition ==		== Definition ==
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	== Gross and net values ==		== 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.<ref name="DR-1-92" /> The apparatus is designed to keep heat losses to a minimum, and small corrections are made for the losses that remain.<ref name="DR-1-92" /> 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.<ref name="DR-1-93" />		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.<ref name="DR-1-92" /> The apparatus is designed to keep heat losses to a minimum, and small corrections are made for the losses that remain.<ref name="DR-1-92" /> 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.<ref name="DR-1-93" />

			[[File:fse-heat-of-combustion-v2.svg\|thumb\|upright=1.5\|alt=Steel bomb with a fuel sample in oxygen inside a water bath; beside it a bar splitting gross heat into net heat and latent heat of water.\|Bomb calorimeter (left) and the difference between gross and net heat of combustion (right): the net value excludes the latent heat of the water formed (2.26 kJ per gram of water). Based on Drysdale (2002), pp. 1-92 to 1-93.]]

	The bomb calorimeter gives the gross heat of combustion, with all products in their standard states, so that the water formed is liquid.<ref name="DR-1-93" /> 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.<ref name="DR-1-93" /> In a fire the water leaves as vapour, so the net value is the one to use in fire calculations.<ref name="DR-1-93" /> Tewarson uses the term net heat of complete combustion for the same quantity.<ref name="TE-3-103">Tewarson (2002), p. 3-103.</ref>		The bomb calorimeter gives the gross heat of combustion, with all products in their standard states, so that the water formed is liquid.<ref name="DR-1-93" /> 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.<ref name="DR-1-93" /> In a fire the water leaves as vapour, so the net value is the one to use in fire calculations.<ref name="DR-1-93" /> Tewarson uses the term net heat of complete combustion for the same quantity.<ref name="TE-3-103">Tewarson (2002), p. 3-103.</ref>
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	== Heat per unit of oxygen ==		== 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.<ref name="DR-1-93" /> Per gram of air consumed, the average is about 3 kJ/g.<ref name="DR-1-93" /> 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.<ref name="JA-3-45" />		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.<ref name="DR-1-93" /> Per gram of air consumed, the average is about 3 kJ/g.<ref name="DR-1-93" /> 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.<ref name="JA-3-45" />

			[[File:fse-heat-of-combustion-v3.svg\|thumb\|upright=1.5\|alt=Two bar charts for twelve fuels: heat per gram of fuel varies from 10 to 50 kJ/g, heat per gram of oxygen stays near 13 kJ/g.\|Heat of combustion of twelve fuels per gram of fuel (left) and per gram of oxygen consumed (right); the dashed line marks 13.1 kJ/g. Based on Drysdale (2002), p. 1-93.]]

	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.<ref name="JA-3-45" /> The cone calorimeter and large-scale furniture calorimeters use this method, with a small correction for incomplete combustion based on the carbon monoxide produced.<ref name="DR-1-96">Drysdale (2002), p. 1-96.</ref> NUREG-1805 describes the constant as 13,100 kJ per kilogram of oxygen, accurate within 5 % for most hydrocarbon fuels.<ref name="IS-3-3" /> Tewarson's own average over many fuels is slightly lower, 12.8 kJ/g with a spread of 7 %.<ref name="TE-3-104">Tewarson (2002), p. 3-104.</ref>		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.<ref name="JA-3-45" /> The cone calorimeter and large-scale furniture calorimeters use this method, with a small correction for incomplete combustion based on the carbon monoxide produced.<ref name="DR-1-96">Drysdale (2002), p. 1-96.</ref> NUREG-1805 describes the constant as 13,100 kJ per kilogram of oxygen, accurate within 5 % for most hydrocarbon fuels.<ref name="IS-3-3" /> Tewarson's own average over many fuels is slightly lower, 12.8 kJ/g with a spread of 7 %.<ref name="TE-3-104">Tewarson (2002), p. 3-104.</ref>
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	Combustion efficiency is the ratio between the heat actually released and the net heat of complete combustion.<ref name="TE-3-102">Tewarson (2002), p. 3-102.</ref> The heat released per unit mass in a real fire is called the chemical heat of combustion.<ref name="TE-3-103" /> 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.<ref name="TE-3-103" /> Below about 0.40, combustion becomes unstable and the flame goes out.<ref name="TE-3-103" />		Combustion efficiency is the ratio between the heat actually released and the net heat of complete combustion.<ref name="TE-3-102">Tewarson (2002), p. 3-102.</ref> The heat released per unit mass in a real fire is called the chemical heat of combustion.<ref name="TE-3-103" /> 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.<ref name="TE-3-103" /> Below about 0.40, combustion becomes unstable and the flame goes out.<ref name="TE-3-103" />

			[[File:fse-heat-of-combustion-v4.svg\|thumb\|upright=1.5\|alt=Four bars for PMMA: 24.9 kJ/g at complete combustion, then 22.1, 18.2 and 11.5 kJ/g as more carbon monoxide, soot and ethylene form.\|Chemical heat of combustion of PMMA for four reaction schemes, from complete combustion (efficiency 1.00) to strongly incomplete combustion (efficiency 0.46). Based on Tewarson (2002), p. 3-103.]]

	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.<ref name="IS-3-4">Iqbal and Salley (2004), p. 3-4.</ref> These are within about 1 % of the complete values for methanol and ethanol in the SFPE Handbook.<ref name="DR-1-93" />		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.<ref name="IS-3-4">Iqbal and Salley (2004), p. 3-4.</ref> These are within about 1 % of the complete values for methanol and ethanol in the SFPE Handbook.<ref name="DR-1-93" />

Daan: New FSE wiki article: Heat of combustion (written by Claude Opus 5.5)

2026-11-01T22:42:47Z

New FSE wiki article: Heat of combustion (written by Claude Opus 5.5)

New page

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.<ref name="DR-1-92">Drysdale (2002), p. 1-92.</ref> It is usually expressed in kilojoules per gram or megajoules per kilogram of fuel.<ref name="DR-1-93">Drysdale (2002), p. 1-93.</ref> 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.<ref name="DR-1-93" /> In fire safety engineering, the value used is the net heat of combustion, in which the water formed remains a vapour.<ref name="DR-1-93" />

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.<ref name="VR-4.4">Van Rossum (2027), §4.4.</ref> Multiplied by the rate at which fuel is consumed, it gives the [[heat release rate]].<ref name="DR-1-95">Drysdale (2002), p. 1-95.</ref> Real fires never burn completely, so engineers often use a lower, [[effective heat of combustion]] that allows for the combustion efficiency.<ref name="DR-1-95" /><ref name="IS-3-3">Iqbal and Salley (2004), p. 3-3.</ref> 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.<ref name="DR-1-93" /><ref name="JA-3-45">Janssens (2002), p. 3-45.</ref>

== 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.<ref name="VR-4.1">Van Rossum (2027), §4.1.</ref> 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.<ref name="DR-1-92" /> 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.<ref name="DR-1-92" /> 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.<ref name="DR-1-92" />

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.<ref name="DR-1-92" /> Fire engineering tables, however, usually list it as a positive number.<ref name="DR-1-93" /> 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.<ref name="NZ-65">Ministry of Business, Innovation and Employment (2025b), p. 65.</ref>

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.<ref name="DR-1-93" /> 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.<ref name="DR-1-93" /> ''Introduction to Fire Safety Engineering'' calls the property the heat of combustion per unit mass and gives it in MJ/kg.<ref name="VR-4.4" />

== 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.<ref name="DR-1-92" /> The apparatus is designed to keep heat losses to a minimum, and small corrections are made for the losses that remain.<ref name="DR-1-92" /> 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.<ref name="DR-1-93" />

The bomb calorimeter gives the gross heat of combustion, with all products in their standard states, so that the water formed is liquid.<ref name="DR-1-93" /> 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.<ref name="DR-1-93" /> In a fire the water leaves as vapour, so the net value is the one to use in fire calculations.<ref name="DR-1-93" /> Tewarson uses the term net heat of complete combustion for the same quantity.<ref name="TE-3-103">Tewarson (2002), p. 3-103.</ref>

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.<ref name="VR-4.4" /> 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.<ref name="VR-4.4" />

The net heat of complete combustion can also be calculated from the standard heats of formation of the fuel, carbon dioxide and water.<ref name="TE-3-103" /> 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.<ref name="TE-3-103" /> 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.<ref name="DR-1-94" /><ref name="DR-1-95" />

Tabulated values normally refer to an initial temperature of 25 °C. Drysdale notes that such values are satisfactory for virtually all fire problems.<ref name="DR-1-93" /> 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.<ref name="DR-1-94">Drysdale (2002), p. 1-94.</ref>

== 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.<ref name="DR-1-93" /> Polyvinyl chloride, which contains chlorine, releases about 16 kJ/g. The table below lists net values at 25 °C.<ref name="DR-1-93" />

{| class="wikitable"
|+ Heats of combustion of selected fuels at 25 °C<ref name="DR-1-93" />
! 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.<ref name="DR-1-93" /> In flaming combustion the volatiles burn in the flame, while the char burns much more slowly by surface oxidation.<ref name="DR-1-92" />

''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.<ref name="VR-4.4" /> 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.<ref name="VR-4.4" /> 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.<ref name="VR-4.4" /><ref name="DR-1-93" /> 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.<ref name="DR-1-93" /> Per gram of air consumed, the average is about 3 kJ/g.<ref name="DR-1-93" /> 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.<ref name="JA-3-45" />

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.<ref name="JA-3-45" /> The cone calorimeter and large-scale furniture calorimeters use this method, with a small correction for incomplete combustion based on the carbon monoxide produced.<ref name="DR-1-96">Drysdale (2002), p. 1-96.</ref> NUREG-1805 describes the constant as 13,100 kJ per kilogram of oxygen, accurate within 5 % for most hydrocarbon fuels.<ref name="IS-3-3" /> Tewarson's own average over many fuels is slightly lower, 12.8 kJ/g with a spread of 7 %.<ref name="TE-3-104">Tewarson (2002), p. 3-104.</ref>

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.<ref name="DR-1-92" /> Wood, with the approximate formula CH1.5O0.75, needs only 5.38 g of air per gram, because it already contains oxygen.<ref name="DR-1-92" /> Per gram of air consumed, however, propane and cellulose release similar amounts of heat, 2.97 and 3.15 kJ/g.<ref name="DR-1-93" />

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.<ref name="DR-1-93" /> 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.<ref name="DR-1-95" />

== 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.<ref name="DR-1-95" /> When a single item burns in isolation, its burning rate and its heat release rate are coupled.<ref name="DR-1-95" /> It is then convenient to express the heat release rate in terms of the [[burning rate]], the rate at which the fuel loses mass:<ref name="DR-1-95" />

<math display="block">\dot{Q} = \dot{m} \, \Delta H_c</math>

{| class="wikitable"
|+ Symbols in the heat release rate relation
! Symbol !! Description !! Unit
|-
| <math>\dot{Q}</math> || heat release rate || kW
|-
| <math>\dot{m}</math> || burning rate (mass loss rate) of the fuel || kg/s
|-
| <math>\Delta H_c</math> || 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.<ref name="DR-1-95" /> 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.<ref name="DR-1-95" /> 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.<ref name="IS-3-3" /> The Australian FSVM data sheets use it in the opposite direction, to convert a design fire curve into a mass burning rate.<ref name="AB-B2-19">Australian Building Codes Board (2020a), Data Sheet B2, p. 19.</ref>

''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.<ref name="VR-6.5">Van Rossum (2027), §6.5.</ref> 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.<ref name="VR-6.5" /> 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.<ref name="VR-6.5" />

Combustion efficiency is the ratio between the heat actually released and the net heat of complete combustion.<ref name="TE-3-102">Tewarson (2002), p. 3-102.</ref> The heat released per unit mass in a real fire is called the chemical heat of combustion.<ref name="TE-3-103" /> 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.<ref name="TE-3-103" /> Below about 0.40, combustion becomes unstable and the flame goes out.<ref name="TE-3-103" />

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.<ref name="IS-3-4">Iqbal and Salley (2004), p. 3-4.</ref> These are within about 1 % of the complete values for methanol and ethanol in the SFPE Handbook.<ref name="DR-1-93" />

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.<ref name="HE-2-2">Heskestad (2002), p. 2-2.</ref> 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.<ref name="DR-1-95" />

== 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.<ref name="VR-4.4" /> The book calculates the energy in a stored volume as the volume times the density times the heat of combustion per unit mass.<ref name="VR-4.4" /> 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.<ref name="VR-4.4" /> Divided by 19 MJ/kg for spruce wood, the fire load is equivalent to about 571,973 kg of wood.<ref name="VR-4.4" />

In practice a fire seldom releases all of this energy, because of its limited combustion efficiency.<ref name="VR-4.4" /> Engineers use the fire load mainly to estimate the duration of a fire or the total thermal load on a structure.<ref name="VR-4.4" /> A fire load survey cited in the Australian FSVM data sheets, for example, converted masses into energy assuming 18 MJ/kg for timber.<ref name="AB-B2-15">Australian Building Codes Board (2020a), Data Sheet B2, p. 15.</ref> The book gives example values of about 950 MJ/m² for residential use and 1,800 MJ/m² for a library.<ref name="VR-4.4" />

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.<ref name="NZ-19">Ministry of Business, Innovation and Employment (2025b), p. 19.</ref> For pre-flashover design fires in all building uses it specifies 20 MJ/kg, together with a soot yield of 0.07 kg/kg.<ref name="NZ-20">Ministry of Business, Innovation and Employment (2025b), p. 20.</ref> The Australian FSVM data sheets (2020) suggest the same values as an indication when the make-up of the fire load is unknown.<ref name="AB-B2-20">Australian Building Codes Board (2020a), Data Sheet B2, p. 20.</ref>

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.<ref name="MG-82">McGrattan et al. (2026), p. 82.</ref> 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.<ref name="VR-6.3">Van Rossum (2027), §6.3.</ref>

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.<ref name="DR-1-95" /> In compartments, some fuel vapours may also escape and burn outside, and this share cannot easily be estimated.<ref name="DR-1-95" /> The value per unit of oxygen is more general, but it also has exceptions of more than 5 %.<ref name="JA-3-45" />

== See also ==
* [[Plastics in fire]]
* [[Adiabatic flame temperature]]
* [[Incomplete combustion]]
* [[Stoichiometry]]

== Notes ==
<references />

== 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.
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[[Category:Combustion]]