Wood does not have a single, fixed temperature at which it ignites. Its ignition is a complex chemical process influenced by various conditions, explaining why temperatures can vary significantly.
The Chemistry of Wood Ignition
When wood is exposed to heat, it undergoes pyrolysis, a thermal decomposition without oxygen, typically starting around 200 to 300 degrees Celsius (390 to 570 degrees Fahrenheit). During pyrolysis, wood’s organic compounds break down, releasing flammable volatile gases, often called “wood gas” or “syngas,” including methane, carbon monoxide, and various hydrocarbons.
These volatile gases, not the solid wood, primarily ignite and produce visible flames. The solid residue, char, is a carbon-rich material that can burn without flame as embers. Char also acts as an insulating layer, protecting unburnt wood, and begins to form around 300 degrees Celsius.
Defining Key Ignition Temperatures
Wood’s ignition involves several temperature points, each describing a different stage of flammability.
The flash point is the lowest temperature at which wood releases enough volatile vapors to form an ignitable mixture with air, causing a brief flash of flame if an external ignition source is present. For wood, this typically ranges from 250 to 300 degrees Celsius (482 to 572 degrees Fahrenheit).
Slightly higher is the fire point, where pyrolysis generates enough volatile gases to sustain a continuous flame after an external ignition source. This usually occurs around 350 to 360 degrees Celsius.
The autoignition temperature is the lowest temperature at which wood spontaneously ignites and sustains combustion without any external flame or spark. This is often the temperature people refer to when asking about wood catching fire. It generally falls between 260 and 400 degrees Celsius (500 to 752 degrees Fahrenheit). Prolonged exposure to lower temperatures, such as 121 to 149 degrees Celsius (250 to 300 degrees Fahrenheit), can lead to slow charring, which may lower the effective autoignition temperature over time to as low as 93 degrees Celsius (200 degrees Fahrenheit).
Factors Influencing Wood’s Flammability
Several factors influence how readily wood ignites and burns.
Moisture content plays a significant role; wet wood requires more energy to evaporate water before pyrolysis and ignition, increasing its ignition temperature. Dry wood ignites more easily.
Density and wood type also affect flammability. Denser hardwoods like oak or maple require more energy to ignite and burn slower than less dense softwoods like pine. Softwoods often contain more flammable resins and are more porous, contributing to quicker ignition.
The size and shape of wood, specifically its surface area to volume ratio, significantly impact ignition. Smaller pieces like sawdust or kindling have a high surface area, leading to rapid ignition. Larger, solid pieces like logs have a lower surface area, resulting in slower ignition and a prolonged burn.
Oxygen is essential for combustion. A sufficient supply is required for flames to sustain themselves; lower concentrations can lead to smoldering. There is a minimum oxygen concentration, below which combustion cannot occur.
Pre-heating and duration of heat exposure also influence flammability. Prolonged exposure to temperatures below the typical ignition point can cause slow charring, which can reduce the wood’s effective ignition temperature over time. Intentionally pre-charring wood can enhance its fire resistance by creating an insulating layer that delays ignition.
Chemical treatments and coatings can significantly alter wood’s fire properties. Fire retardants work by forming a protective barrier, releasing non-combustible gases, or promoting char formation. These treatments slow flame spread, insulate wood from heat, and reduce heat release and smoke production.