When wood burns, it undergoes a complex chemical process known as combustion, transforming the solid material into various new substances. This rapid reaction between the wood’s organic compounds and oxygen releases stored chemical energy. The process alters the wood’s original composition, yielding both solid and gaseous byproducts.
Ash and Residue
The solid material remaining after wood combustion is primarily ash, a fine, powdery substance. Wood ash is largely composed of calcium compounds, such as calcium carbonate and calcium oxide, along with other minerals like potassium, magnesium, manganese, and phosphorus. The exact composition of ash can vary depending on the type of wood burned and the combustion temperature.
When wood combustion is incomplete, charcoal can form. Charcoal is a lightweight, black carbon residue produced when wood is heated with a limited supply of oxygen. It is mainly carbon but also contains small amounts of hydrogen, oxygen, and the wood’s ash. This incomplete burning leaves behind a material that can still combust under different conditions.
Gaseous Emissions
Wood burning releases a variety of gases into the atmosphere, with the specific types and amounts depending on the completeness of the combustion. In complete combustion, the primary gaseous products are carbon dioxide (CO2) and water vapor (H2O). Water vapor represents a significant portion of the emissions.
However, wood combustion is often incomplete, leading to the formation of other gases. Carbon monoxide (CO), a colorless, odorless, and toxic gas, is a common product of incomplete combustion when there is insufficient oxygen for carbon to fully oxidize into carbon dioxide. Incomplete combustion also produces volatile organic compounds (VOCs), carbon-based chemicals that easily become vapors or gases at room temperature. These include various hydrocarbons, such as methane, benzene, and formaldehyde. Wood burning is a significant source of VOCs, particularly in colder months.
Airborne Particulates
Beyond gases, wood burning generates airborne particulates, commonly perceived as smoke. These are tiny solid and liquid particles suspended in the air. Particulate matter (PM) is categorized by size, such as PM2.5 (particles smaller than 2.5 micrometers) and PM10 (particles smaller than 10 micrometers), with PM2.5 being a primary pollutant from wood smoke. These microscopic particles are formed during incomplete combustion of the wood’s organic components.
Soot is a specific type of particulate matter, primarily composed of carbon, that contributes to the black appearance of smoke. Soot particles can form from the condensation of unignited gases and unburnt carbon that fail to fully mix with oxygen during the burning process. The formation of soot is influenced by factors like flame temperature and oxygen availability. Wood smoke, including these particulates, contains a complex mixture of over 200 chemicals and compounds.
Energy Release
When wood burns, energy is released in the form of heat and light. This energy originates from the chemical bonds within the wood’s organic compounds, such as cellulose, hemicellulose, and lignin. The burning process is an exothermic reaction, meaning it releases more energy than it absorbs to break chemical bonds.
As wood combustion occurs, the chemical energy stored in the wood is converted into thermal energy (heat) and electromagnetic energy (light). This heat is utilized for purposes like warming homes and cooking. The light produced creates the visible flames and glow associated with a fire. The amount of energy released is influenced by factors such as the wood’s moisture content and density, with drier, denser wood yielding more heat.