Combustion

Combustion is a set of chemical reactions in which a fuel reacts with an oxidiser, usually oxygen, and releases energy. In a fire, these reactions are predominantly exothermic, meaning that they release heat. They can also produce light as a visible flame or as a glow. Combustion can involve gaseous, liquid or solid fuels, but liquids and solids usually have to produce gaseous fuel before flaming combustion can occur.

The process is more than a single reaction between a fuel and oxygen. It includes heating, decomposition, mixing, chemical reaction and heat transfer. These processes form a feedback system: heat released by the reactions can produce more gaseous fuel, which can then react and release further heat. The balance between heat production, fuel supply, oxygen supply and heat loss determines whether combustion starts, continues, spreads or stops.

For fire safety, combustion matters because it converts stored chemical energy into heat and combustion products. The amount of energy per unit of fuel, the speed of conversion and the conditions around the fuel all influence the resulting fire. Combustion therefore connects material properties with flame behaviour, heat release, smoke production and the effectiveness of extinguishing measures.

Fuel, oxidiser and the combustion system

The conventional description of combustion starts with fuel, oxygen and sufficient heat. These three elements form the Fire triangle. Fuel supplies the material that can be oxidised, oxygen acts as the oxidiser, and heat raises the fuel and reaction zone to conditions at which the reactions can proceed. A combustible material and oxygen can coexist without a fire when insufficient energy is available for ignition.

The Fire tetrahedron adds the uninhibited chemical chain reaction to these three elements. This addition recognises that combustion is not extinguished only by removing fuel, oxygen or heat. Some extinguishing agents interfere with active chemical species in the flame and thereby interrupt the chain reaction. The tetrahedron is therefore a model of four conditions that can be interrupted, rather than a picture of four physical substances.

Oxygen normally reaches a fire as part of ambient air. Van Rossum gives the approximate composition of air as 21% oxygen, 78% nitrogen and 1% other gases by volume. Nitrogen does not have to take part in the principal combustion reaction, but it forms part of the gas mixture that must be heated and moved. When the oxygen concentration falls, fire growth can be restricted and extinction can eventually follow, depending on the fuel and conditions.

The fuel side is not limited to materials commonly called fuels. Wood, paper, plastics, liquids and combustible gases can all supply chemical energy. Their composition and physical form affect how easily gaseous fuel is produced and mixed with oxygen. The surrounding geometry also matters because it influences heat retention, air movement and the transfer of heat to unburnt material.

How liquids and solids become combustible gases

A gaseous fuel can mix directly with oxygen from the air. This makes gas combustion the most direct of the three physical fuel states. A liquid normally has to evaporate, or form a mist of droplets, before its fuel can participate in flaming combustion. During a pool fire, vapour produced at the liquid surface mixes with air and burns above that surface rather than throughout the liquid volume.

Most solids require thermal decomposition before flaming combustion. Pyrolysis breaks large molecules into smaller ones as the material is heated, producing combustible gases and often leaving a solid residue. Pyrolysis itself does not require oxygen. The released gases burn only when they mix with oxygen in suitable proportions and receive enough energy for ignition. They can burn immediately, burn elsewhere or remain unburnt.

Heat feedback links pyrolysis and flaming in a sustained solid-fuel fire. Based on Van Rossum (2027), §3.1 and §4.3.

Polymer decomposition shows why the original material alone does not determine fire behaviour. The rate, mechanism and products of decomposition depend on both chemical composition and physical properties. Some materials melt or soften before decomposition, while thermosetting and cellulosic materials do not pass through the same fluid state. These differences affect how heat and volatile gases move through the material.

A sustained solid-fuel fire depends on an energy feedback loop. A Flame transfers heat back to the fuel, the heated fuel releases volatile gases, and those gases support further flaming. Some released heat is lost to the surroundings, so the feedback must be large enough to maintain decomposition. Surface orientation changes this balance. Upward spread over a vertical surface can preheat material above the burning region and accelerate fire development.

The production of a char layer adds another feedback mechanism. Char can insulate underlying material and change the route by which volatile gases escape. An intumescent char can slow further decomposition considerably. Carbonaceous char can itself oxidise at the surface at higher temperatures, so its presence does not simply mean that combustion has ended.

Flaming and non-flaming combustion

Flaming combustion takes place in the gas phase. The flame is the visible part of the combustion region, where gaseous fuel reacts with oxygen. Fuel and air may already be mixed before ignition, as in a premixed flame, or they may mix at the flame through diffusion and turbulence, as in a diffusion flame. These mixing arrangements influence flame shape, temperature distribution and the location of the reaction zone.

In a laminar diffusion flame, molecular diffusion controls much of the oxygen transport. A candle flame is a familiar example. In a turbulent diffusion flame, continually changing motion draws surrounding air into the reaction region. The flow and the flame therefore interact, and the combustion rate cannot be explained by chemical reaction rates alone.

The colour of a flame does not provide a complete measure of its temperature or completeness of combustion. Hot soot particles emit much of the yellow or orange light seen in many diffusion flames. Fuels producing little soot can have flames that are difficult to see. Oxygen availability, fuel vapour and local temperature all vary within the flame, so different regions can have different appearances.

Smouldering is a slow, lower-temperature, flameless form of combustion in which oxygen directly attacks the surface of a condensed-phase fuel. It should not be confused with forced pyrolysis, where an external heat source causes decomposition and smoke with little or no oxidation. Porous, finely divided and thermally insulating fuel beds can retain enough heat and admit enough oxygen to sustain smouldering.

Flaming combustion and smouldering occur in different regions and forms. Based on Van Rossum (2027), §4.1 and Ohlemiller (2002), p. 2-200.

Char-forming materials are particularly relevant to smouldering. Char has a carbon-rich, porous structure that can support surface oxidation. The heat from this oxidation helps the reaction zone move through the fuel. Smouldering can generate a relatively high conversion of fuel to toxic compounds and can later provide a route to flaming, even when the initial heat source was too weak to ignite a flame directly.

Energy release and combustion products

The Heat of combustion is the heat released when a unit quantity of fuel is oxidised completely to stable end products. It is measured with a combustion bomb calorimeter, in which a known amount of fuel burns in pure oxygen and the temperature rise is used to determine the energy release. This is a controlled measurement of energy content, not a direct measurement of how a real room fire develops.

Selected heats of combustion at 25 °C. Based on Drysdale (2002), p. 1-93.

The distinction between gross and net heat of combustion concerns the state of water in the products. Net heat of combustion treats product water as vapour and is lower than the gross value by the energy needed to evaporate that water. Drysdale identifies the net value as the one to use in fire calculations. Data in the SFPE Handbook are given at an initial temperature of 25 °C and are suitable for most fire problems covered there.

Values vary substantially between fuels. In the handbook table, propane has a heat of combustion of 46.45 kJ/g, cellulose 16.09 kJ/g and polyethylene 43.28 kJ/g. These figures describe complete oxidation under the stated measurement basis. They do not mean that equal masses of these materials will produce the same fire, because decomposition, burning rate, oxygen access and combustion efficiency also affect heat release.

A balanced reaction for complete propane combustion produces carbon dioxide and water. It also shows how much oxygen and air the reaction requires. Drysdale calculates that 1 g of propane requires 3.64 g of oxygen, or 15.6 g of air when nitrogen is included. For an assumed empirical composition of wood, the corresponding stoichiometric air requirement is 5.38 g of air per gram of wood. Such values describe complete combustion under specified assumptions, not the variable air supply of every fire.

Incomplete combustion occurs when the available conditions do not convert all fuel to the stable products associated with complete oxidation. Fire conditions can produce carbon monoxide, soot and other compounds in addition to carbon dioxide and water. Wood provides a useful example of separate pathways: its volatile gases can undergo flaming combustion while the residual char oxidises much more slowly at the surface.

The total chemical energy in a fuel and the rate at which that energy is released answer different questions. Energy content is expressed in units such as megajoules, while heat release rate is power expressed in watts, kilowatts or megawatts. A large energy store can therefore coexist with a small current heat release rate. Fire duration and total thermal exposure depend on energy, whereas flame size and short-term fire severity are strongly related to power.

Ignition, persistence and extinction

Ignition is the transition into self-sustaining combustion, not simply the appearance of heat or smoke. The International Fire Engineering Guidelines distinguish three modes. Piloted ignition occurs when a flame, spark or hot object ignites pyrolysis gases. Non-piloted ignition occurs when the gas mixture reaches a temperature at which it ignites without such a local pilot. Self-induced ignition begins with oxidation inside a solid, which supplies energy for decomposition and further heating.

Persistence requires the energy released by the reactions to exceed the energy lost from the reaction zone. Near a lower flammability limit, radiation and other heat losses can exceed heat production, preventing the flame from sustaining itself. This heat balance explains why a fuel and oxidiser can react in one configuration but fail to maintain combustion in another. Chemical composition alone does not settle the outcome.

A change from smouldering to flaming requires two conditions at the same place and time: a mixture of gases and air within its flammability range, and sufficient heat to ignite it. Increased oxygen supply, a larger fuel bed and reduced heat losses can all raise local temperature and pyrolysis-gas production. The transition is therefore not an automatic stage that follows every period of smouldering.

Experimental results show the variability of this transition. In a survey of upholstered chairs and beds cited by Ohlemiller, about two thirds underwent a transition to flaming. Reported times ranged from 22 to 306 minutes after smouldering began. These values belong to the tested items and conditions, and should not be treated as a universal waiting time for furniture fires.

Extinction also depends on the combustion configuration. Removing oxygen from a deep porous fuel may suppress the reaction temporarily, but readmitting oxygen can cause reignition while the fuel remains hot. Ohlemiller notes that oxygen removal may have to continue until the fuel has cooled. Cooling such a fuel bed can be difficult because the same insulating structure that supports smouldering restricts heat removal.

In an enclosure, incomplete combustion within the room does not necessarily end the combustion process. Fuel volatiles can leave through an opening, mix with fresh air and burn outside. This behaviour affects both the location of heat release and the interpretation of measurements taken inside the enclosure. It also illustrates why a description limited to the original fuel package can miss part of the fire.

Measuring combustion behaviour

Different measurements describe different parts of combustion. Bomb calorimetry establishes chemical energy under controlled, complete-oxidation conditions. Oxygen-consumption calorimetry instead estimates heat release during a fire test from the oxygen consumed. The first method is associated with energy content, while the second can follow changing heat release rate as a specimen burns.

Oxygen-consumption calorimetry uses the approximately consistent heat released per unit mass of oxygen consumed across many fuels. Drysdale gives a conversion factor of 13.1 kJ/g of oxygen, with a correction for incomplete combustion based on carbon monoxide yield. The technique can be used for a small specimen in a cone calorimeter and at larger scales for furniture and wall linings.

The cone calorimeter exposes a small sample to an imposed radiant heat flux. This controlled exposure makes it possible to compare material responses, but the result remains tied to the test conditions. Specimen shape, applied heat flux and the actual burning environment influence the measurements. A test heat-release curve is therefore not automatically the heat-release curve of every object made from that material.

Combustion efficiency expresses the difference between theoretical complete oxidation and actual energy release. Even with unrestricted ventilation, natural fires produce partially oxidised species such as carbon monoxide, aldehydes, ketones and particulate matter. Drysdale reports likely efficiencies from approximately 0.3 to 0.4 for heavily fire-retarded materials to 0.9 or higher for some oxygen-containing products. These ranges are material-dependent and do not remove the need to consider changing fire conditions.

Temperature estimates also require care. An adiabatic flame temperature is a theoretical maximum calculated with no heat transfer to the surroundings. Real flames lose energy by radiation, and high temperatures cause dissociation, in which combustion products split into other species through heat-absorbing processes. Drysdale's simplified propane example therefore predicts a higher temperature than the real flame can reach. The example demonstrates why energy conservation must be combined with heat transfer and chemical behaviour.

Measurements become harder to interpret in compartments containing several fuels. Each fuel can have a different air requirement and burning rate, while radiation within the compartment changes their decomposition. Some volatiles may burn outside rather than inside the measured region. No single instrument consequently captures the full combustion system without assumptions about boundaries, products and airflow.

Combustion in fire safety engineering

Combustion knowledge supports the description of a design fire, the selection of material data and the assessment of fire spread. An engineering analysis distinguishes the energy potentially available from all fuel from the energy actually released during a selected period. Van Rossum notes that real fires commonly release much less than the total theoretical energy because combustion efficiency is not complete.

For a solid or liquid fuel, an assessment has to account for the processes that supply gaseous fuel. For a smouldering hazard, it also has to account for oxygen transport through a porous fuel and heat retention around the reaction zone. For a gas flame, mixing and chemical reaction require particular attention. These different controlling processes prevent one universal burning rate or combustion efficiency from being applied to every scenario.

Extinguishing strategies follow from the same system view. Fuel removal, oxygen limitation and cooling act on the three sides of the fire triangle. Chemical inhibition acts on the chain reaction represented by the fire tetrahedron. The appropriate mechanism depends on the fuel and the form of combustion; for example, a deep-seated smouldering region can behave differently from an open gas-phase flame.

Enclosure conditions add further feedback. Heat that cannot escape readily warms the compartment and its gases, and radiation from this hotter environment can heat other combustible objects. Ventilation can simultaneously affect oxygen supply, heat loss and the movement of unburnt gases. A description of combustion therefore has to include the surroundings as well as the fuel itself.

A schematic design fire converts this understanding into a simplified description for analysis. The 2005 International Fire Engineering Guidelines distinguish smouldering, non-flashover flaming and flashover fire types. For a flaming design fire, they describe a growth phase, a fully developed phase and a decay phase. This is a deliberate simplification for engineering evaluation, not a claim that every real fire follows the same curve.

The combustion mode changes which uncertainties matter. A smouldering design fire may remain flameless or undergo a transition to flaming, and the timing of that transition is difficult to predict. The 2005 guidelines use a notional maximum heat release rate below 5 kW for their illustrated smouldering design fire. That figure belongs to this older modelling guidance and is not a general upper limit for every smouldering configuration.

A fire-load assessment starts from the quantity of combustible material and its calorific value. It can include contents as well as combustible construction, provided these contributions are identified clearly. Multiplying an inventory by an energy value estimates the chemical energy present, but does not establish that all material will burn. The analysis still needs a combustion scenario and a defensible burning period.

Each of these applications relies on a stated boundary in space and time. Heat released inside a compartment can differ from heat released by all gases originating there, and potential energy can differ from measured energy release. Clear identification of fuel, oxidiser, enclosure, combustion mode and measurement basis prevents these quantities from being mixed.

Limits of simple descriptions

The fire triangle and fire tetrahedron are useful conceptual models, but neither predicts the speed or size of a fire. They do not describe fuel geometry, heat transfer, ignition delay, mixing or changing ventilation. Two arrangements with the same fuel, oxygen and initial heat source can develop differently when their heat losses and exposed surfaces differ.

Complete combustion reactions are similarly limited as descriptions of real fires. They provide clear accounting for reactants, products and energy under stated assumptions, but thermodynamics does not determine the reaction rate or mechanism. Engineering calculations therefore combine thermochemical data with burning-rate information and a representation of the actual fire conditions.

Finally, visible flame is not a reliable indicator of all combustion. Smouldering can persist without flame and may remain difficult to detect within a fuel layer. Some flames are hard to see when little soot is present. Assessment based only on appearance can consequently miss both hidden combustion and the energy and products associated with weakly luminous flames.

See also

• Carbon dioxide
• Fire classes
• Stoichiometry

References

• Australian Building Codes Board, National Research Council of Canada, International Code Council, & Department of Building and Housing New Zealand. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
• Beyler, C. L., & Hirschler, M. M. (2002). Thermal decomposition of polymers. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Drysdale, D. D. (2002). Thermochemistry. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Grant, C. C. (2002). Halon design calculations. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Ohlemiller, T. J. (2002). Smoldering combustion. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.