Smolder

Smoldering is a slow, flameless form of combustion that occurs in porous solid fuels such as wood, charcoal, coal, or peat. It is characterized by a glowing, heat-producing reaction at the surface or within the interior of the fuel, driven by oxidation at a rate limited by diffusion of oxygen rather than by a visible flame.

During smoldering, oxygen from the surrounding air diffuses into the porous fuel and reacts with exposed fuel at a reaction zone. The heat released sustains the process and may advance gradually through the material. The process typically produces substantial heat with little visible flame and generates smoke and gases, including carbon monoxide and carbon dioxide, along with various organic compounds, depending on the fuel and temperature.

Smoldering occurs in many contexts, including charcoal embers, wood embers in a fire, peat fires, and coal or lignite seams. It can persist long after flames have ceased and can reignite into flaming combustion if fresh oxygen or heat is supplied.

Preventing and controlling smoldering can be challenging, as heat can be retained within the fuel mass. Extinguishing often requires cooling the fuel, removing heat sources, and limiting oxygen or removing fuel pockets. In environmental and industrial contexts, smoldering can contribute to air pollutants and prolonged smoke events.