Wood is one of humanity’s oldest and most relied-upon fuel sources, yet the process by which a solid piece of wood transforms into heat and light is complex. The flammability of wood is the result of a precise sequence of chemical and thermal reactions. Understanding why wood burns requires looking into its biological structure and the steps that must occur before a visible flame can appear.
The Chemical Components of Wood
Wood is a complex, natural composite material composed primarily of three organic polymers: cellulose, hemicellulose, and lignin. These compounds are essentially chains of carbon, hydrogen, and oxygen atoms that store the solar energy captured by the tree during photosynthesis. This stored chemical energy is the ultimate fuel source released during burning.
Cellulose is the most abundant component, typically making up 40 to 50 percent of the wood’s mass, and functions as the structural backbone of the plant cell walls. Hemicellulose, composing around 20 to 30 percent, is a more diverse and less ordered sugar polymer that acts as a matrix around the cellulose fibers. Lignin, also about 20 to 30 percent of the wood, is a highly complex, three-dimensional polymer that acts as a strong binder, cementing the cellulose and hemicellulose together to give wood its rigidity.
The hydrocarbon-rich composition of these components makes them inherently combustible once sufficient energy is applied. The differing molecular structures of these polymers mean they break down at slightly different temperatures during the heating process. This staggered decomposition contributes to the sustained release of fuel necessary for prolonged burning.
Pyrolysis: The Essential Pre-Combustion Step
The visible flame is the result of a process called pyrolysis, which is the chemical decomposition of the wood by heat in the absence of oxygen. When wood is heated to temperatures generally starting between 200°C and 300°C, the chemical bonds within the cellulose, hemicellulose, and lignin begin to break down.
This thermal breakdown releases a mix of volatile organic compounds, which are hot, flammable gases and vapors that account for a significant portion of the wood’s potential energy. These released gases, not the solid wood, are the actual fuel for the flame.
The pyrolytic process leaves behind a solid residue called char, or charcoal, which is nearly pure carbon and ash. This char does not burn with a flame but glows red as it undergoes a separate, slower process called glowing combustion. For a flame to appear, the volatile gases must be heated to their ignition temperature and mix with oxygen. This process is sustained by the heat radiating from the flame back onto the wood surface to drive further pyrolysis.
The Three Necessary Elements for Combustion
Sustaining the fire requires the specific interaction of three elements, often described by the combustion triangle. If any one of these components is removed, the chemical reaction of fire will stop. For wood, the first element is the fuel, which is not the solid log but the flammable gases and vapors released through pyrolysis.
The second element is oxygen, which acts as the oxidizer and is necessary for the rapid chemical reaction of combustion to occur. Oxygen molecules from the surrounding air combine with the carbon and hydrogen atoms in the fuel gases, releasing energy in the form of heat and light. The third element is heat, which serves two purposes: it acts as the initial activation energy to start pyrolysis, and the heat generated by the flame sustains the reaction by continuously driving more volatile gases out of the unburned wood.
This self-sustaining cycle means the combustion process feeds itself as long as the three elements are present. The heat produced by the burning gases radiates back into the cooler, unpyrolyzed wood, causing it to release more fuel gases. This continuous loop of heating, gas release, and ignition is what allows a fire to persist and grow.
Physical Factors That Modify Flammability
While the chemical composition determines that wood can burn, various physical factors significantly influence how easily and how quickly it will burn. The moisture content is a major determinant of flammability, as any water inside the wood must be heated to 100°C and converted into steam before the wood can begin to pyrolyze. This evaporation process consumes a large amount of thermal energy, effectively delaying and slowing down the start of combustion.
The surface area to volume ratio is another powerful factor, explaining why kindling ignites much faster than a large log. Smaller pieces of wood have more surface exposed relative to their mass, allowing them to absorb heat and reach the necessary pyrolysis temperature much more quickly. A large log, conversely, acts as a heat sink, requiring a longer period for the heat to penetrate to the core and drive off internal moisture and volatile compounds.
The density and grain structure of the wood affect the rate of heat transfer and gas release. Denser woods tend to burn slower than less dense woods because they contain more mass in a given volume and their structure resists rapid heat penetration. The natural porosity and grain alignment also influence how easily the flammable gases can escape the interior of the wood, impacting the overall burn rate.