The fundamental difference between smouldering and flaming combustion is that smouldering occurs on the surface of the solid rather than in the gas phase. The characteristic temperature and heat released during smouldering are low compared to those in the flaming combustion (i.e., ~600 °C vs. ~1500 °C). Smoulder propagates in a creeping fashion, around 0.1 mm/s, which is about ten times slower than flame spread over a solid. In spite of its weak combustion characteristics, smouldering is a significant fire hazard. Smouldering emits toxic gases (e.g. carbon monoxide) at a higher yield than flaming fires and leaves behind a significant amount of solid residue. The emitted gases are flammable and could later on be ignited in the gas-phase, triggering the transition to flaming combustion [1].

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). The general features that characterize smouldering fuels are that they are porous, permeable to flow and formed by aggregates (particulates, grains, fibres or of cellular structure). These aggregates facilitate the surface reaction with oxygen by allowing gas flow through the fuel and providing large surface areas per volume. They also act as thermal insulation, reducing heat losses. The most studied materials to date are cellulose and polyurethane foam.

The characteristics of smouldering fires make them a threat of new dimensions, taking the form of colossal underground fires or silent fire safety risks, as summarized below.

* Fire safety. The main hazard pose by smouldering generates from its ease to be initiated (by heat sources too weak to ignite flames), and its difficulty to be detected. Fire statistics draw attention on the magnitude of smouldering combustion as the leading cause of fire deaths in residential areas. I.e., more than 25% of the fire deaths in the United States are attributed to smoulder-initiated fires (with similar figures in other developed countries). A fire scenario that is particularly common is when a cigarette ignites a piece of upholstered furniture. This ignition leads to a smouldering fire that lasts for a long period of time (in the order of hours), spreading slowly and silently until critical conditions are attained and flames suddenly erupt [2]. Smouldering combustion is also a fire-safety concern aboard space facilities (e.g. International Space Station), because the absence of gravity is thought to promote smouldering ignition and propagation.

* Wildland fires. Smouldering combustion has a great impact in forest fires, being responsible for a large amount of the fuel consumed and the pollutants emitted. Smouldering combustion of the forest ground does not have the visually dramatic impact of flaming combustion. However it is an important component of forest fires and causes the killing of roots, seeds and plant stems at the ground. Smouldering of forest biomass can linger for days or weeks after flaming has ceased, resulting in large quantities of fuels consumed and becoming a global source of emissions to the atmosphere [3].

* Subsurface fires. Fires occurring many meters below the surface are a type of smouldering events of colossal magnitude. Subsurface fires in coal mines, peat lands and landfills are rare events but when active they can smoulder for very long periods of time (months or years), emitting enormous quantities of combustion gases into the atmosphere and causing deterioration of air quality and health problems. The oldest and largest fires in the world are of a smouldering nature burn for centuries. These fires are fed by small but continuous quantities of air flowing through fractured strata, cracks, openings or abandoned mine shafts, which permits oxidizer to circulate into the subsurface. The reduced heat losses and the high thermal inertia of the underground, together with the high fuel availability promote long-term smouldering combustion and allows for creeping but extensive propagation. These fires prove difficult to be detected and frustrate most efforts to be extinguished. The dramatic 1997 peat-land fires in Borneo made subsurface smouldering fires emerge as a global threat with significant economic, social and ecological impacts [4] [5]. In the summer of 2006, the Borneo peat fires came back [6].

* World Trade Center Debris. After the attack, fire and subsequent collapse of the Twin Towers on September 11th, 2001, the colossal pile of debris left on the site smouldered for more than five months. It resisted attempts by the fire fighters to extinguish it until most of the rubble was removed. The effects of the gaseous and aerosol products of smouldering on the health of the emergency workers were patent but the details are still a matter of debate.

Not all are bad news in smouldering combustion, and it also has a few beneficial applications.
* In wildland fire management, controlled smouldering fires aiming to reduce the load of ground fuels are useful due to the ease to control their propagation and their lower severity to the ecosystem.
* Smouldering of tires produces tar and energy at the same time, fostering the recycling of tires.
* In-situ combustion of petroleum sites is becoming more often used for oil recovery when traditional-extraction methods become inefficient or too costly.
* In-situ smouldering combustion as a novel remediation technology for land contaminants[7].

My papers on smouldering: