Glossary — Fire Science
Pyrolysis
Pyrolysis is the thermal decomposition of organic material into flammable gases and char — it occurs before ignition, not during it. Understanding pyrolysis explains why a fire can start before any open flame is present, and why cooling the fuel surface is so effective at stopping fires.
Definition
Pyrolysis — the thermal decomposition of organic material in the absence of oxygen (or with insufficient oxygen for combustion), producing flammable gases (including carbon monoxide and hydrocarbons), carbon dioxide, water vapour, and a solid carbon-rich residue known as char. It is the essential precursor to flaming combustion in solid and liquid fuels.
Stages of solid fuel heating and pyrolysis
Using wood as a typical solid fuel example. Temperatures are approximate and vary significantly by species, moisture content, and heating rate.
| Stage | Temperature | What is happening | Fire status |
|---|---|---|---|
| Initial heating | ~100–200 °C | Moisture driven off; material begins to dry and weaken | No combustion yet |
| Early pyrolysis | ~200–300 °C | Chemical bonds begin breaking; early volatile gases released; material discolours | Smouldering possible; no open flame |
| Active pyrolysis | ~300–500 °C | Rapid gas release — CO, CO2, and flammable hydrocarbons; char forms | Flammable gas-air mixture can ignite if ignition source present |
| Ignition / flaming combustion | >~500 °C (varies by material) | Flammable gases reach ignition temperature; sustained flame begins | Flaming fire; heat release rate accelerates rapidly |
Why pyrolysis matters in building fires
In a building fire, pyrolysis occurs in all exposed combustible surfaces — wall linings, furniture, flooring, timber elements — as the room heats up. As the temperature in the upper gas layer rises above 300–400 °C, more and more surfaces begin releasing flammable gases. When sufficient gas has accumulated and the temperatures are high enough, these gases ignite simultaneously — the phenomenon known as flashover.
This is why the upper gas layer temperature is the primary indicator firefighters use to predict flashover: it reflects the rate of pyrolysis occurring across all surfaces in the room simultaneously.
Pyrolysis and creosote formation
When wood burns, the pyrolysis gases that are not fully combusted travel up the flue. In a cool flue (below about 250 °C), these gases condense on the inner surface to form creosote — a tarry, highly flammable deposit. Creosote is a direct pyrolysis product, and its ignition is the cause of almost all chimney fires. Burning dry, seasoned wood at adequate temperatures produces less creosote because the pyrolysis gases complete combustion before reaching the cool flue. See our chimney fire prevention guide for practical implications.
Pyrolysis and the fire tetrahedron
In the fire tetrahedron model, fuel, heat, oxygen, and a chain reaction are the four elements required for sustained combustion. Pyrolysis converts solid fuel into gaseous fuel — making it available for the chain reaction. Extinguishing agents that work by cooling (water) or removing oxygen (CO2, inert gas) interrupt pyrolysis and remove the fuel supply from the chain reaction. This is why these agents are effective on solid fuel fires.
For more on fire science fundamentals, see our fire science section and the fire triangle glossary entry.
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