![]() These aggregates facilitate the surface reaction with oxygen by allowing gas flow through the fuel and providing a large surface area per unit volume. Smouldering fuels are generally porous, permeable to flow and formed by aggregates (particulates, grains, fibres or of cellular structure). Many materials can sustain a smouldering reaction, including coal, tobacco, decaying wood and sawdust, biomass fuels on the forest surface ( duff) and subsurface ( peat), cotton clothing and string, and polymeric foams (e.g., upholstery and bedding materials). Smouldering materials Polyurethane foam sample from the NASA smouldering experiments. The emitted gases are flammable and could later be ignited in the gas phase, triggering the transition to flaming combustion. Smouldering emits toxic gases (e.g., carbon monoxide) at a higher yield than flaming fires and leaves behind a significant amount of solid residue. In spite of its weak combustion characteristics, smouldering is a significant fire hazard. ![]() Smouldering propagates in a creeping fashion, around 0.1 mm/s (0.0039 in/s), which is about ten times slower than flames spread over a solid. The characteristic temperature and heat released during smouldering are low compared to those in the flaming combustion. Smouldering is a surface phenomenon but can propagate to the interior of a porous fuel if it is permeable to flow. The fundamental difference between smouldering and flaming combustion is that smouldering occurs on the surface of the solid rather than in the gas phase. Common examples of smouldering phenomena are the initiation of residential fires on upholstered furniture by weak heat sources (e.g., a cigarette, a short-circuited wire), and the persistent combustion of biomass behind the flaming front of wildfires. ![]() Many solid materials can sustain a smouldering reaction, including coal, cellulose, wood, cotton, tobacco, cannabis, peat, plant litter, humus, synthetic foams, charring polymers including polyurethane foam and some types of dust. Smouldering ( British English) or smoldering ( American English see spelling differences) is the slow, flameless form of combustion, sustained by the heat evolved when oxygen directly attacks the surface of a condensed-phase fuel. Smouldering combustion in glowing embers of barbecue coal briquettes For other uses, see Smoulder (disambiguation).
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