Wood, an organic material, consists primarily of three complex polymers: cellulose, hemicellulose, and lignin. Cellulose forms the structural framework of wood cells, while hemicellulose and lignin act as binders. When wood burns, it undergoes rapid oxidation, a complex chemical transformation that releases energy as heat and light. This process involves a series of stages that alter the wood’s composition.
Preparing for Combustion
Before visible flames appear, wood undergoes physical changes when exposed to heat. The first step involves the drying out of moisture. As the temperature rises to approximately 100°C, this water evaporates, causing the wood to shrink and crack. Efficient burning requires wood to have a low moisture content, ideally less than 20 percent, as excess moisture consumes energy that would otherwise contribute to heating and ignition.
As the wood continues to heat beyond the drying phase, preliminary thermal decomposition begins. This process involves the initial breakdown of the wood’s organic components. While some volatile substances may start to form, there is not yet significant gas release or ignition at this stage. This preparatory phase sets the foundation for the more intense chemical transformations that follow.
The Pyrolysis Process
Following the initial heating, wood enters the pyrolysis phase, a chemical transformation occurring in the absence of oxygen. During pyrolysis, high temperatures, ranging from 300°C to 900°C, break down the complex polymers within the wood. Hemicellulose decomposes first, between 200°C and 315°C, followed by cellulose, which breaks down between 315°C and 400°C. Lignin, being more resistant, decomposes over a broader temperature range, extending from 200°C up to 900°C.
This thermal decomposition yields three primary products: volatile gases, a liquid called tar (often referred to as bio-oil), and a solid residue known as char. The volatile gases, which include substances like carbon monoxide, methane, and hydrogen, are flammable. These gases mix with oxygen to fuel the visible flames of a fire. The char, rich in carbon, remains a solid product.
Sustained Burning and Flame
Once the volatile gases produced during pyrolysis are released, they mix with oxygen from the air. When this mixture reaches its ignition temperature, combustion occurs, resulting in visible flame. The flames seen during wood burning are the result of these gases combusting, releasing both heat and light.
The heat generated by these burning gases sustains the fire. This heat feeds back into the unburnt wood, causing further pyrolysis and the release of more volatile gases, perpetuating the combustion cycle. An adequate supply of oxygen is necessary for this process; insufficient oxygen can smother the flames, while an excessive amount can cause the wood to burn too quickly. After the volatile gases have burned away, the solid char left behind begins to undergo its own slower, flameless combustion, often seen as glowing embers.
The End Products of Combustion
After the burning process concludes, several products remain or are released. The primary solid residues are ash and charcoal. Ash consists of the inorganic mineral components present in the wood, such as oxides of calcium, magnesium, sodium, and potassium, along with some phosphates. Charcoal is the remaining carbonaceous material from the char that did not fully combust.
Beyond the solid remnants, energy is released as heat and light. Gaseous byproducts are also formed, predominantly carbon dioxide and water vapor. Smoke is also produced, a complex mixture. It contains unburnt particles, along with various gases such as carbon monoxide, methane, and other volatile organic compounds. Smoke primarily results from incomplete combustion or the pyrolysis process itself.