Smouldering (or smoldering in American spelling) is a flameless form of combustion, deriving its heat from oxidations occurring on the surface of a solid fuel. A flame is often defined as the visible (light-emitting part of a Fire. Combustion or burning is a complex sequence of Exothermic chemical reactions between a Fuel and an Oxidant accompanied by the production of Fuel is any material that is burned or altered in order to obtain energy Common examples are the initiation of 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 wildland fires. A cigarette ( French "small Cigar " from cigar + -ette) is a product consumed through Smoking and manufactured Short Circuit is a 1986 comedy Science fiction film starring Ally Sheedy and Steve Guttenberg and directed by Biomass refers to living and recently dead Biological material that can be used as fuel or for industrial production A wildfire, also known as a wildland fire, forest fire, brush fire, vegetation fire, grass fire, Peat fire,

Fundamentals

A lighted cigarette is the most common smouldering example

Smouldering combustion in glowing embers of BBQ coal briquettes

The fundamental difference between smouldering and flaming combustion is that smouldering occurs on the surface of the solid rather than in the gas phase. A flame is often defined as the visible (light-emitting part of a Fire. The characteristic temperature and heat released during smouldering are low compared to those in the flaming combustion (i. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature In Physics, heat, symbolized by Q, is Energy transferred from one body or system to another due to a difference in Temperature e. , ~600°C vs. ~1500°C). Smouldering propagates in a creeping fashion, around 0. 1 mm/s, which is about ten times slower than flames 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. Carbon monoxide, with the chemical formula CO is a colorless odorless tasteless yet highly toxic Gas. The emitted gases are flammable and could later be ignited in the gas phase, triggering the transition to flaming combustion. ^[1]

Smouldering materials

Polyurethane foam sample from the NASA smouldering experiments.

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. A coal seam fire or mine fire is the underground Smouldering of a Coal deposit often a Coal mine. Tobacco is an Agricultural product recognized as an addictive drug processed from the fresh Leaves of plants in the genus Nicotiana. Wood is hard fibrous lignified structural tissue produced as secondary Xylem in the stems of Woody plants notably trees but also shrubs Biomass refers to living and recently dead Biological material that can be used as fuel or for industrial production Peat is an accumulation of partially Decayed Vegetation matter. Cotton is a soft staple Fibre that grows around the seeds of the cotton plant ( Gossypium sp g. , upholstery and bedding materials). Upholstery is the work of providing Furniture, especially seats with Padding, springs Webbing, and fabric or Leather Bedding refers to the materials laid above the mattress of a bed for warmth Smouldering fuels are generally 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 a large surface area per unit volume. They also act as thermal insulation, reducing heat losses. The most studied materials to date are cellulose and polyurethane foams. Cellulose is an Organic compound with the formula, a Polysaccharide consisting of a linear chain of several hundred to over ten thousand β(1→4 A polyurethane, commonly abbreviated PU, is any Polymer consisting of a chain of organic units joined by urethane links

Some smouldering cases

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 hazards posed by smouldering arise from the fact that it can be easily initiated (by heat sources too weak to ignite flames) and is difficult to detect. Fire safety refers to precautions that are taken to prevent or reduce the likelihood of a Fire that may result in death injury or property damage alert those in a structure Fire statistics draw attention to 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). The United States of America —commonly referred to as the A particularly common fire scenario is a cigarette igniting a piece of upholstered furniture. A cigarette ( French "small Cigar " from cigar + -ette) is a product consumed through Smoking and manufactured Upholstery is the work of providing Furniture, especially seats with Padding, springs Webbing, and fabric or Leather 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. A wildfire, also known as a wildland fire, forest fire, brush fire, vegetation fire, grass fire, Peat fire, A forest is an area with a high density of Trees There are many definitions of a forest based on various criteria Pollution is the introduction of contaminants into an environment that causes instability disorder harm or discomfort to the physical systems or living organisms they are in Smouldering combustion of the forest ground does not have the visually dramatic impact of flaming combustion; however, as the cause of the killing of roots, seeds, and plant stems at the ground level, it is an important component of forest fires. ROOT is an object-oriented program and library developed by CERN. A seed (in some plants referred to as a kernel) is a small embryonic Plant enclosed in a covering called the seed coat usually with some stored A stem is one of two main structural axes of a Vascular plant. 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]. Biomass refers to living and recently dead Biological material that can be used as fuel or for industrial production

Smoke and pollution from fires in Borneo, 1997.

• Subsurface fires: Fires occurring many meters below the surface are a type of smouldering event 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, causing deterioration of air quality and subsequent health problems. A coal seam fire or mine fire is the underground Smouldering of a Coal deposit often a Coal mine. Peat is an accumulation of partially Decayed Vegetation matter. The Air Quality Index ( AQI) is a standardized indicator of the Air Quality in a given location The oldest and largest fires in the world, burning for centuries, are smouldering fires. These fires are fed by the oxygen in the small but continuous flow of air through natural pipe networks, fractured strata, cracks, openings or abandoned mine shafts which permit the air to circulate into the subsurface. In Geology and related fields a stratum (plural strata) is a layer of rock or Soil with internally consistent characteristics that distinguishes Mining is the extraction of valuable Minerals or other geological materials from the earth usually (but not always from an Ore body The reduced heat losses and high thermal inertia of the underground together with high fuel availability promote long-term smouldering combustion and allow for creeping but extensive propagation. These fires prove difficult to detect, and frustrate most efforts to extinguish them. The dramatic 1997 peatland fires in Borneo caused the recognition of subsurface smouldering fires as a global threat with significant economic, social and ecological impacts ^{[4] [5]}. Peat is an accumulation of partially Decayed Vegetation matter. The summer of 2006 saw the resurgence of the Borneo peat fires ^[6].

The smouldering pile of debris, Manhattan, NY (USA).

• World Trade Center Debris: After the attack, fire and subsequent collapse of the Twin Towers on September 11 2001, the colossal pile of debris left on the site smouldered for more than five months. The World Trade Center site sits on 16 Acres (65000 m² in Lower Manhattan in New York City. The collapse of the World Trade Center occurred after the September 11 attacks. It resisted attempts by fire fighters to extinguish it until most of the rubble was removed. The effects of the gaseous and aerosolized products of smouldering on the health of the emergency workers were significant but the details are still a matter of debate.

Beneficial applications

Smouldering combustion also has a few beneficial applications.

• In wildland fire management, controlled smouldering fires aiming at reducing the load of ground fuels are useful because they are easy to control and propagate, and because of their reduced effect on the ecosystem.
• Smouldering of tires produces tar and energy at the same time, fostering the recycling of tires. This article is about tires used on road Vehicles including pneumatic tires and solid tires.
• In-situ combustion of petroleum sites is increasingly used for oil recovery when traditional extraction methods prove inefficient or too costly. Petroleum ( L petroleum, from Greek πετρέλαιον, lit Petroleum ( L petroleum, from Greek πετρέλαιον, lit
• In-situ smouldering combustion is being explored as a novel remediation technology for land contaminants^[7].

References and external links

• Microgravity smouldering (NASA-funded) research facility at the University of California at Berkeley.