Pyrolysis

Pyrolysis is the chemical decomposition of a material caused by heating. In many organic materials, it breaks large molecules into smaller fragments. The process can produce combustible gases, liquids and a solid residue. In fire safety, pyrolysis is especially relevant because it supplies gaseous fuel for the flaming combustion of many solids, including wood and plastics.

Pyrolysis is not itself combustion. It does not require oxygen, whereas combustion requires an oxidiser. The gases produced can burn immediately, burn after travelling elsewhere or remain unburnt, depending on mixing, temperature and ignition conditions. A solid can therefore emit fuel gases and smoke without yet supporting a flame.

The process links heat transfer with fire growth. External heat or flame feedback raises the material temperature, decomposition releases volatile fuel, and combustion of those volatiles can return more heat to the material. Whether this loop sustains itself depends on heat losses, material properties and oxygen availability. Fuel production, ignition and heat release therefore cannot be described by one material constant.

Thermal decomposition and fuel production

Heating does not affect every material in the same way. Thermoplastics can soften and melt before substantial decomposition, while thermosetting and cellulosic materials do not pass through the same fluid state. These physical changes alter heat transport, exposed surfaces and the routes through which volatile products escape.

Pyrolysis produces volatiles and residue within heated material. Based on Van Rossum (2027), §3.1 and Beyler and Hirschler (2002), pp. 1-110–1-112.

Chemical composition also affects decomposition. Beyler and Hirschler state that its rate, mechanism and product composition depend on both the original material's chemistry and its physical properties. Natural and synthetic polymers can therefore behave differently even when they are exposed to the same nominal heating conditions.

Breaking a polymer chain is only one possible reaction. Decomposition can remove groups attached to the chain or create new bonds between chains, a process called cross-linking. Several mechanisms may occur in the same material. Oxygen can change these reactions and their rates when it reaches the material, even though heating can also produce decomposition without oxygen.

Cellulosic materials release adsorbed water during heating before or alongside decomposition. The desorption process starts below the boiling point of water and varies with the material. This moisture movement changes the energy needed for heating and can affect the onset and rate of later decomposition.

Some materials form carbonaceous char. Volatile gases may have to travel through this layer before reaching the surface, where hot char can cause secondary reactions. Intumescent chars can slow further thermal decomposition considerably, while other chars can continue to oxidise at higher temperatures. The residue is consequently an active part of the fire process rather than a uniform inert layer.

Inorganic residues behave differently again. They can form glassy barriers that restrict volatile escape and protect underlying material. Where such barriers do not form, carbonaceous char can remain susceptible to surface oxidation. The type and integrity of the residue therefore influence continuing fuel production.

Pyrolysis and combustion

Combustion involves chemical reaction with an oxidiser and release of energy, while pyrolysis is decomposition caused by heating. In a typical flaming solid-fuel fire, the two occur in different regions. Pyrolysis takes place in or at the heated material, and the volatile products burn after mixing with oxygen in the gas phase.

This separation explains why flaming can occur away from the original solid. Van Rossum describes oxygen-limited compartments in which pyrolysis gases burn mainly in external flames after encountering fresh air. The fuel then originates inside the compartment, but part of its combustion and heat release occurs outside.

Smouldering must also be distinguished from pyrolysis. Ohlemiller defines smouldering as flameless combustion sustained by oxidation at the surface of a condensed-phase fuel. Forced pyrolysis caused by an external heat flux can produce gas and smoke with little or no oxidation. Smoke production alone therefore does not prove that smouldering combustion is taking place.

Char links the two processes. Thermal decomposition can leave a carbon-rich, porous residue that supports direct oxygen attack. Heat released by char oxidation can then sustain smouldering and drive further decomposition nearby. Pyrolysis helps create the fuel structure in which this later surface combustion occurs.

Ignition and heat exposure

Ignition of a solid-fuel fire commonly requires pyrolysis gases to mix with air and reach ignitable conditions. Piloted ignition uses a local flame, spark or hot object. Non-piloted ignition occurs when the gas mixture is hot enough to ignite without such a pilot. Self-induced ignition starts with oxidation inside a solid that supplies energy for further heating and decomposition.

Heat flux is the rate of heat transfer per unit area and is a useful way to describe exposure of a material surface. A higher imposed flux can raise temperature and accelerate decomposition, but the response also depends on heat losses and material properties. The surface temperature alone does not capture all of these processes.

The 2005 International Fire Engineering Guidelines note that wood ignition is complex and not fully quantified. Their tabulated ignition temperatures for wood, paper and cotton range from 250 to 365 °C for piloted ignition and 250 to 400 °C for autoignition. These are grouped experimental values from older guidance, not universal thresholds for every product or exposure.

Reported ignition-temperature ranges for wood, paper and cotton; these are not pyrolysis thresholds. Based on Australian Building Codes Board et al. (2005), p. 3.3-4.

Exposure duration changes the problem. The same guidelines report that many wood ignition studies used exposures of 20 minutes or less, while a day-long experiment produced a much lower heat-flux value. Prolonged heating can also involve self-heating, where material size and heat retention influence safe storage conditions. Pyrolysis must therefore be considered over the actual exposure time.

Burning rate and char formation

Burning rate describes how quickly fuel is consumed or converted during a fire. For a decomposing solid, it is connected to the rate at which heat reaches the material and volatile products are generated. Heat of combustion provides energy per unit mass, but it does not by itself determine this rate.

A sustained flame supplies part of the required heat through radiation and convection. Some energy is lost to the surroundings, so fuel production continues only when feedback is sufficient. Van Rossum uses this balance to explain why materials that release substantial energy and need little energy for decomposition can burn more rapidly.

A simplified steady-state heat balance relates this feedback to the mass of material converted to vapour each second. The heat of gasification is the net energy needed to convert a unit mass of material into volatiles. Dividing net heat flux by that energy gives mass loss per unit area and time. Incoming heat includes both external exposure and flame feedback, while surface reradiation reduces the net input:

$${\dot{m}}^{″}=\frac{{\dot{q}}^{\prime }{\text{'}}_{\mathrm{i}\mathrm{n}}-{\dot{q}}^{\prime }{\text{'}}_{\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}}{L_g}$$

Symbol	Description	Unit
${\dot{m}}^{″}$	Mass loss rate per unit exposed area	kg/(m² s)
${\dot{q}}^{\prime }{\text{'}}_{\mathrm{i}\mathrm{n}}$	Combined external and flame heat flux reaching the surface	kW/m²
${\dot{q}}^{\prime }{\text{'}}_{\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}$	Heat flux lost by surface reradiation	kW/m²
$L_g$	Heat of gasification	kJ/kg

Tewarson's worked example uses an incoming heat flux of 70 kW/m² and a surface loss of 10 kW/m². The assumed material has a heat of gasification of 1.8 kJ/g, equivalent to 1,800 kJ/kg. The calculated mass-loss flux is about 0.033 kg/(m² s):

$${\dot{m}}^{″}=\frac{70-10}{1,800}\approx 0.033\mathrm{k}\mathrm{g}/\mathrm{(}{\mathrm{m}}^{\mathrm{2}}\mathrm{s}\mathrm{)}$$

For the example's exposed area of 2 m², the total mass loss is about 0.067 kg/s, or 67 g/s. This result uses the unrounded mass-loss flux before multiplying by area. It estimates fuel generation under the stated heat-balance assumptions; it does not calculate ignition time or identify the chemical products.

The steady approximation becomes less direct when a char layer grows or exposure varies with time. Janssens notes that charring materials may take a long time to approach steady burning, or may never do so. Their effective heat of gasification and heat fluxes can change throughout the test. A single fitted average therefore needs evidence that it represents the intended material and exposure.

For timber, Charring rate describes how quickly wood is converted to char. Its value depends on fire exposure, density and moisture, so it is not a universal conversion factor for gas production. The remaining char changes heat penetration and the movement of volatile products. These effects connect structural loss of uncharred wood with the continuing supply of gaseous fuel.

Material orientation changes heat feedback. During upward flame spread on a vertical surface, heat reaches material above the burning zone and preheats it. This can accelerate decomposition and spread. A small horizontal specimen result therefore does not automatically describe a large vertical installation.

Plastics and other polymeric materials

Plastics in fire encompass thermoplastics, thermosets and elastomers with different physical and chemical responses. Thermoplastics can soften reversibly below decomposition temperature, whereas thermosets undergo irreversible changes when formed and do not melt in the same way. These distinctions affect dripping, shape retention and exposure of fresh material during heating.

Material type changes physical behaviour during thermal decomposition. Based on Beyler and Hirschler (2002), pp. 1-111–1-112.

Polymer composition influences both volatile products and residue. Oxygen-containing, nitrogen-containing and halogen-containing polymers have different chemical structures and decomposition pathways. The SFPE Handbook consequently treats thermal decomposition as varied and complex rather than as one universal reaction.

Wood and paper are natural polymeric materials with substantial cellulosic content. Their thermal response includes moisture desorption, volatile production and char formation. The residual char is generally richer in carbon and has greater surface area per unit mass than the original fuel, making it susceptible to oxidation under suitable conditions.

These differences prevent a single pyrolysis temperature from representing all materials. Decomposition occurs over conditions influenced by heating rate, composition and physical transformations. A material name should therefore be accompanied by the test or scenario conditions whenever its pyrolysis behaviour is used in an assessment.

Pyrolysis in compartment fires

A compartment can strengthen the feedback between decomposition and combustion. Heat retained by the enclosure warms gases and surfaces, and radiation can then reach fuel beyond the original burning item. This additional exposure can produce volatile gases from several objects at once. The fire's fuel supply therefore changes as the compartment heats.

Multiple materials make the response more complex. Drysdale notes that fuels in a compartment can have different stoichiometric air requirements and burning rates, while radiation changes their decomposition. The total rate of heat release cannot be inferred by treating every item as an identical fuel package with one fixed response.

Ventilation influences where the products burn. Pyrolysis can continue inside fuel even when oxygen within the compartment is insufficient to oxidise all volatile gases. Some products leave through openings and burn after mixing with external air. An assessment limited to combustion inside the enclosure can then miss part of the energy released by the pyrolysed fuel.

Schematic design fires simplify these interacting processes into selected growth, developed and decay behaviour. The 2005 guidelines describe such curves as conservative engineering representations rather than exact reproductions of natural fires. Pyrolysis data and test results provide inputs to this simplification, but do not remove the assumptions made when constructing it.

Engineering use and measurement

Thermogravimetric analysis measures how a small sample's mass changes during a defined temperature programme. The sample can be held at a selected temperature or heated at a specified rate. Differentiating the mass record gives the mass-loss rate and helps identify separate decomposition stages. The test atmosphere is also part of the result because oxygen can alter decomposition.

A mass-loss curve is not automatically a fire-growth curve. Its shape depends on heating rate and the decomposition that has already occurred. A small thermal-analysis specimen also lacks the changing feedback from a large flame. These limitations explain why decomposition measurements are combined with fire tests and heat-transfer information rather than used alone to predict installed-product behaviour.

Material tests impose a defined heat flux and measure responses such as mass loss and heat release. Oxygen-consumption calorimetry estimates heat release from oxygen used, with correction associated with incomplete combustion. It measures the outcome of combustion rather than pyrolysis alone, so interpretation must separate decomposition, mixing and oxidation.

A test specimen is usually much simpler than the installed product. Thickness, joints, composite layers and orientation can alter heat penetration and volatile movement. Char formation can add further time-dependent changes. Test data consequently need to be applied to comparable configurations or supported by an appropriate model.

For fire-load estimates, calorific values describe potential energy in the material inventory. They do not establish how much material will decompose and burn during a given scenario. Moisture, container protection and incomplete combustion can all reduce effective energy release. Pyrolysis is one of the processes between potential inventory and observed fire severity.

Limits and common misunderstandings

Pyrolysis is not synonymous with melting. Melting is a physical phase change, while pyrolysis changes chemical structure and produces new substances. Some thermoplastics melt before decomposing, but other materials decompose without entering a fluid state.

It is also not synonymous with evaporation. A liquid can vaporise without the molecular breakdown involved in pyrolysis, although both processes can supply fuel vapour to a flame. Solids that sublime or melt and evaporate require a description suited to their actual behaviour.

The absence of oxygen does not make pyrolysis products safe. Combustible gases can travel, accumulate and ignite after finding oxygen. The location where decomposition occurs can therefore differ from the location of rapid heat release or flame.

Charring does not necessarily mean that fuel consumption has ended. Char can protect underlying material, but it can also oxidise and sustain smouldering. Assessment needs the condition of both the residue and the remaining unburnt material.

Controlled pyrolysis outside fire safety helps clarify the distinction between decomposition and burning. Charcoal production heats wood with little or no oxygen, leaving a carbon-rich porous residue after volatile products escape. The desired process is decomposition rather than immediate combustion of all released gases.

Wood-gas production uses related decomposition and subsequent reactions to generate a combustible gas mixture. Van Rossum describes cleaning and cooling this gas before use as engine fuel. The gas is produced in one device and burned elsewhere after oxygen is supplied, illustrating the separation that can also occur between pyrolysis and flaming in a fire.

See also

• Timber in fire

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., pp. 1-110–1-131). National Fire Protection Association.
• Drysdale, D. D. (2002). Thermochemistry. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 1-90–1-98). National Fire Protection Association.
• Janssens, M. (2002). Calorimetry. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-38–3-62). National Fire Protection Association.
• Kanury, A. M. (2002). Ignition of liquid fuels. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-188–2-199). National Fire Protection Association.
• Kodur, V. K. R., & Harmathy, T. Z. (2002). Properties of building materials. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 1-155–1-181). National Fire Protection Association.
• Ohlemiller, T. J. (2002). Smoldering combustion. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-200–2-210). National Fire Protection Association.
• Tewarson, A. (2002). Generation of heat and chemical compounds in fires. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-82–3-161). National Fire Protection Association.
• Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.