Smouldering

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Smouldering (British English) or smoldering (American English) is a slow, flameless form of burning. It happens when heat from oxygen attacking the surface of a solid fuel keeps the reaction going. Many materials can smoulder, including coal, wood, cotton, tobacco, peat, plant litter, certain foams, and some dust. In homes, smouldering often starts from a weak heat source on upholstered furniture (like a cigarette or a faulty wire). It also occurs when biomass burns beneath the visible flames of a wildfire.

The main difference from flames is that smouldering is a surface reaction on the solid, not a gas-phase flame. It can move into the interior of a porous fuel if gas can flow through it. Smouldering produces less heat and at a lower temperature than flaming. It spreads slowly, about 0.1 millimeters per second, roughly ten times slower than flames.

Smouldering is a serious fire hazard. It releases toxic gases, such as carbon monoxide, in large amounts and leaves behind a lot of solid residue. The emitted gases can be flammable and may ignite the surrounding air later, triggering a transition to flaming combustion.

Common smouldering fuels include coal, tobacco, decaying wood and sawdust, forest litter and peat, cotton clothing and string, and polymer foams used in upholstery and bedding. These fuels are usually porous and allow gas to flow through, which helps the surface reaction and provides a large surface area. They also trap heat, reducing heat loss. The most studied materials are cellulose and polyurethane foams.

Smouldering fires pose threats not only on the surface but also as large underground fires or hidden safety risks. There are also some beneficial uses of smouldering in various applications.