In which type of combustion does a material burn on its surface without a visible flame, generating smoke and glowing embers?

The type of combustion where a material burns on its surface without a visible flame, generating smoke and glowing embers, is called smoldering combustion. Smoldering combustion is a slow, flameless form of combustion sustained by the heat evolved when oxygen directly attacks the surface of a porous solid fuel. This process involves a heterogeneous reaction, meaning the oxygen from the surrounding air reacts directly with the solid material itself, rather than with volatile gases released from the material. The heat generated by this surface oxidation causes the solid material to become incandescent, producing the characteristic glowing embers. Simultaneously, incomplete combustion of the solid fuel releases a significant amount of smoke, which is composed of unburnt particulate matter and various gaseous products, including carbon monoxide. This type of combustion typically occurs in porous materials such as wood, cotton, foam, upholstered furniture, or peat, where oxygen can penetrate the material's structure. For instance, a burning cigarette or a slowly charring piece of wood are common examples of smoldering. Although flameless, smoldering combustion is hazardous because it can persist for long periods undetected, generating large quantities of toxic smoke and potentially leading to flaming combustion under altered conditions.