A coal-seam fire is a burning of an outcrop or underground coal seam. Most coal-seam fires exhibit smoldering combustion, particularly underground coal-seam fires, because of limited atmospheric oxygen availability. Coal-seam fire instances on Earth date back several million years. Due to thermal insulation and the avoidance of rain/snow extinguishment by the crust, underground coal-seam fires are the most persistent fires on Earth and can burn for thousands of years, like Burning Mountain in Australia. Coal-seam fires can be ignited by self-heating of low-temperature oxidation, lightning, wildfires and even arson. Coal-seam fires have been slowly shaping the lithosphere and changing atmosphere, but this pace has become fast and extensive in modern times, triggered by enormous mining activities.
Coal fires are a serious health and safety hazard, affecting the environment by releasing toxic fumes, reigniting grass, brush, or forest fires, and causing subsidence of surface infrastructure such as roads, pipelines, electric lines, bridge supports, buildings, and homes. Whether started by humans or by natural causes, coal seam fires continue to burn for decades or even centuries until either the fuel source is exhausted, a permanent groundwater table is encountered, the depth of the burn becomes greater than the ground's capacity to subside and vent, or humans intervene. Because they burn underground, coal seam fires are extremely difficult and costly to extinguish, and are unlikely to be suppressed by rainfall. There are strong similarities between coal fires and peat fires.
Across the world, thousands of underground coal fires are burning at any given moment. The problem is most acute in industrializing, coal-rich nations such as China. Global coal fire emissions are estimated to cause 40 tons of mercury to enter the atmosphere annually, and to represent three percent of the world's annual CO2 emissions.
Coal seam fires can be divided into near-surface fires, in which seams extend to the surface and the oxygen required for their ignition comes from the atmosphere, and fires in deep underground mines, where the oxygen comes from ventilation.
Mine fires may begin as a result of an industrial accident, generally involving a gas explosion. Historically, some mine fires were started when bootleg mining was stopped by authorities, usually by blowing the mine up. Many recent mine fires have started from people burning trash in a landfill that was in proximity to abandoned coal mines, including the much-publicized Centralia, Pennsylvania, fire, which has been burning since 1962. Of the hundreds of mine fires in the United States burning today, most are found in the state of Pennsylvania.
Some fires along coal seams are natural occurrences. Some coals may self-ignite at temperatures as low as 40 °C (104 °F) for brown coal in the right conditions of moisture and grain size. The fire usually begins a foot or two inside the coal at a depth in which the permeability of the coal allows the inflow of air but in which the ventilation does not remove the heat which is generated. Self-ignition was a recognised problem in steamship times and is claimed to have contributed to the sinking of the Titanic. One well known source of fires is mining breaking into a high pressure cavity of methane gas which on release can generate a spark of static electricity to ignite the gas and start a coal explosion and fire. The same gas static is well known in ships and care has to be taken to ensure no such static sparking can occur.
Two basic factors determine whether spontaneous combustion occurs or not, the ambient temperature and the grain size:
Wildfires (lightning-caused or others) can ignite the coal closer to the surface or entrance, and the smouldering fire can spread through the seam, creating subsidence that may open further seams to oxygen and spawn future wildfires when the fire breaks to the surface. Prehistoric clinker outcrops in the American West are the result of prehistoric coal fires that left a residue that resists erosion better than the matrix, leaving buttes and mesa. It is estimated that Australia's Burning Mountain, the oldest known coal fire, has burned for 6,000 years.
Globally, thousands of inextinguishable mine fires are burning, especially in China where poverty, lack of government regulations and runaway development combine to create an environmental disaster. Modern strip mining exposes smoldering coal seams to the air, revitalizing the flames.
Rural Chinese in coal-bearing regions often dig coal for household use, abandoning the pits when they become unworkably deep, leaving highly combustible coal dust exposed to the air. Using satellite imagery to map China's coal fires resulted in the discovery of many previously unknown fires. The oldest coal fire in China is in Baijigou (白芨沟, in Dawukou District of Shizuishan City, Ningxia) and is said to have been burning since the Qing Dynasty (before 1912).
Before attempting to extinguish a near-surface coal seam fire, its location and underground extent should be determined as precisely as possible. Besides studying the geographic, geologic and infrastructural context, information can be gained from direct measurements. These include:
Underground coal mines can be equipped with permanently installed sensor systems. These relay pressure, temperature, airflow and gas composition measurements to the safety monitoring personnel, giving them early warning of any problems.
Besides destruction of the affected areas, coal fires often emit toxic gases, including carbon monoxide and sulfur dioxide. China's coal fires, which consume an estimated 20 – 200 million tons of coal a year, make up as much as 1 percent of the global carbon dioxide emissions from fossil fuels.
One of the most visible changes will be the effect of subsidence upon the landscape. Another local environmental effect can include the presence of plants or animals that are aided by the coal fire. The prevalence of otherwise non-native plants can depend upon the fire's duration and the size of the affected area. For example, near a coal fire in Germany, many Mediterranean insects and spiders were identified in a region with cold winters, and it is believed that elevated ground temperatures above the fires permitted their survival.
In order to thrive, a fire requires fuel, oxygen, and heat. As underground fires are very difficult to reach directly, firefighting involves finding an appropriate methodology which addresses the interaction of fuel and oxygen for the specific fire in question. A fire can be isolated from its fuel source, for example through firebreaks or fireproof barriers. Many fires, particularly those on steep slopes, can be completely excavated. In the case of near-surface coal seam fires, the influx of oxygen in the air can be interrupted by covering the area or installing gas-tight barriers. Another possibility is to hinder the outflow of combustion gases so that the fire is quenched by its own exhaust fumes. Energy can be removed by cooling, usually by injecting large amounts of water. However, if any remaining dry coal absorbs water, the resulting heat of absorption can lead to re-ignition of a once-quenched fire as the area dries. Accordingly, more energy must be removed than the fire generates. In practice these methods are combined, and each case depends on the resources available. This is especially true for water, for example in arid regions, and for covering material, such as loess or clay, to prevent contact with the atmosphere.
Extinguishing underground coal fires, which sometimes exceed temperatures of 540 °C (1,000 °F), is both highly dangerous and very expensive.
Near-surface coal seam fires are routinely extinguished in China following a standard method basically consisting of the following phases:
Efforts are underway to refine this method, for example with additives to the quenching water or with alternative extinguishing agents.
Underground coal seam fires are customarily quenched by inertization through mine rescue personnel. Toward this end the affected area is isolated by dam constructions in the galleries. Then an inert gas, usually nitrogen, is introduced for a period of time, usually making use of available pipelines.
In 2004, the Chinese government claimed success in extinguishing a mine fire at a colliery near Urumqi in China's Xinjiang province that had been burning since 1874. However, a March 2008 Time magazine article quotes researcher Steven Q. Andrews as saying, "I decided to go to see how it was extinguished, and flames were visible and the entire thing was still burning.... They said it was put out, and who is to say otherwise?"
A jet engine unit, known as Gorniczy Agregat Gasniczy (GAG), was developed in Poland and successfully used for fighting coal fires and displacing firedamp in mines.
Time magazine reported in July 2010 that less expensive alternatives for extinguishing coal seam fires were beginning to reach the market, including special heat-resistant grouts and a fire-smothering nitrogen foam, with other innovative solutions on the way.
Some of the more notable mine fires around the world are listed below.
In China, the world's largest coal producer with an annual output around 2.5 billion tons, coal fires are a serious problem. It has been estimated that some 10–200 million tons of coal uselessly burn annually, and that the same amount again is made inaccessible to mining. Coal fires extend over a belt across the entire north China, whereby over one hundred major fire areas are listed, each of which contains many individual fire zones. They are concentrated in the provinces of Xinjiang, Inner Mongolia and Ningxia. Beside losses from burned and inaccessible coal, these fires contribute to air pollution and considerably increased levels of greenhouse gas emissions and have thereby become a problem which has gained international attention.
In Planitz, now a part of the city of Zwickau, a coal seam that had been burning since 1476 could only be quenched in 1860. In Dudweiler (Saarland) a coal seam fire ignited around 1668 and is still burning today. This so-called Burning Mountain ("Brennender Berg") soon became a tourist attraction and was even visited by Johann Wolfgang von Goethe. Also well-known is the so-called Stinksteinwand (stinking stone wall) in Schwalbenthal on the eastern slope of the Hoher Meißner, where several seams caught fire centuries ago after lignite coal mining ceased; combustion gas continues to reach the surface today.
In India, as of 2010, 68 fires were burning beneath a 58-square-mile (150 km2) region of the Jharia coalfield in Dhanbad, Jharkhand. Mine fires started in this region in 1916 and are rapidly destroying the only source of prime coking coal in the country.
Coal and peat fires in Indonesia are often ignited by forest fires near outcrop deposits at the surface. It is difficult to determine when a forest fire is started by a coal seam fire, or vice versa, in the absence of eyewitnesses. The most common cause of forest fires and haze in Indonesia is intentional burning of forest to clear land for plantation crops of pulp wood, rubber and palm oil.
No accurate count of coal seam fires has been completed in Indonesia. Only a minuscule fraction of the country has been surveyed for coal fires. The best data available come from a study based on systematic, on-the-ground observation. In 1998, a total of 125 coal fires were located and mapped within a 2-kilometer strip either side of a 100-kilometer stretch of road north of Balikpapan to Samarinda in East Kalimantan, using hand-held Global Positioning System (GPS) equipment. Extrapolating this data to areas on Borneo and Sumatra underlain by known coal deposits, it was estimated that more than 250,000 coal seam fires may have been burning in Indonesia in 1998.
Land clearing practices which use fire, often starting forest fires, may be the cause of coal seam fires in Indonesia. In 1982–83 one of the largest forest fires in this century raged for several months through an estimated 5 million hectares of Borneo's tropical rainforests. Goldammer and Seibert however concluded that there are indications that coal seam fires already occurred between 13,200 and 15,000 BP.
A fire season usually occurs every 3–5 years, when the climate in parts of Indonesia becomes exceptionally dry from June to November due to the El Nino Southern Oscillation off the west coast of South America. Since 1982, fire has been a recurring feature on the Islands of Borneo and Sumatra, burning large areas in 1987, 1991, 1994, 1997–98, 2001 and 2004.
In October 2004 smoke from land clearing again covered substantial portions of Borneo and Sumatra, disrupting air travel, increasing hospital admissions, and extending to portions of Brunei, Singapore and Malaysia. Coal outcrops are so common in Indonesia it is virtually certain these fires ignited new coal seam fires.
In 1944, Longyearbyen Mine #2 on Svalbard was set alight by sailors from the Tirpitz on its final sortie outside of Norwegian coastal waters. The mine continued to burn for 20 years, while some of the areas were subsequently mined from the reconstructed Mine #2b.
Many coalfields in the US are subject to spontaneous ignition. The federal Office of Surface Mining (OSM) maintains a database (AMLIS), which in 1999 listed 150 fire zones. In mid-2010, according to OSM, more than 100 fires were burning beneath nine states, most of them in Colorado, Kentucky, Pennsylvania, Utah and West Virginia. Some geologists say that many fires go unreported, so that the actual number of them may be nearer to 200, across 21 states.
In Pennsylvania, 45 fire zones are known, the most famous being the Centralia mine fire in the Centralia mine in the hard coal region of Columbia County, which has been burning since 1962. Burning Mine, near Summit Hill, caught fire in 1859.
In Colorado, coal fires have arisen as a consequence of fluctuations in the groundwater level, which can increase the temperature of the coal up to 300 °C, enough to cause it to spontaneously ignite.
The Powder River Basin in Wyoming and Montana contains some 800 billion tons of brown coal, and the Lewis and Clark Expedition (1804 to 1806) reported fires there. Fires have been a natural occurrence in this area for about three million years and have shaped the landscape. For example, an area about 4,000 square kilometers in size is covered with coal clinker, some of it in Theodore Roosevelt National Park, where there is a spectacular view of fiery red coal clinker from Scoria Point.
The 1991 film Nothing but Trouble, directed and co-written by Dan Aykroyd, features a town, Valkenvania, that has an underground coal fire that has been burning for decades. The judge of the town references the constantly-burning coal-mine fire as the source of his hatred of financiers.
In the TV show Scorpion, Season 3, Episode 23, the Scorpion team extinguishes an underground coal fire in Wyoming.
An der Stinksteinwand in der Nähe des Gasthauses Schwalbenthal strömen im Übrigen seit 300 Jahren durch die Klüfte des Basaltes die Gase eines schwelenden Kohleflözes aus.