Coal fires

Coal fires occur in operating coal mines, abandoned coal mines and waste coal piles. They sometimes start because of a nearby blaze, but they can also ignite through spontaneous combustion: certain minerals in the coal, such as sulfides and pyrites, can oxidize and in the process generate enough heat to cause a fire.

Coal fire hazards to health and the environment include toxic fumes, reigniting fires, and sinkholes. 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.

Internationally, thousands of underground coal fires are burning now. Global coal fire emissions are estimated to include 40 tons of mercury going into the atmosphere annually, and three percent of the world's annual carbon dioxide emissions.

Number of coal fires
According to a 2010 article in Discover Magazine, the United States has 112 documented underground fires, along with many more not yet known or counted. In addition to underground fires, there are also 93 known surface coal fires, some of them in huge coal waste piles created during the process of coal mining, such as a 100-foot-high burning fire near Birmingham, Alabama.

Eastern India has the densest concentration of known coal fires in the world: sixty-eight of them burn within a 174-square-mile region in the Jharia Coalfield in the state of Jharkand.

In China, estimates of the amount of coal consumed or made inaccessible by uncontrolled fires runs as high as 200 million metric tons per year, 10 percent of the country’s total coal production.

Indonesia, a major exporter of coal to the Pacific Rim, has many thousands of coal fires. A 2004 paper "Coal fires in Indonesia" estimates the number of coal fires in eastern Borneo might be as high as 3,000, a number that its authors now believe is too low.

In January 2011 an Xstrata Coal subsidiary reported that a fire occurred at its recently commissioned Blakefield South mine, a part of Beltana mine complex in the Hunter Valley in New South Wales, Australia. The fire burnt for several days.

History
According to Discover: "Coal fires are as ancient and as widely distributed as coal itself. People have reported fires in coal beds close to the earth’s surface for thousands of years—in fact, Australia’s Burning Mountain, once thought to be a volcano, sits atop a coal seam that has been on fire for some six millennia. But ever since the Industrial Revolution, the number of coal fires has grown dramatically. There are now thousands of such fires around the world, in every country—from France to South Africa to Borneo to China—where mining exposes coal deposits."

1962 Centralia fire still burns today
In 1962 residents of the small mining town of Centralia, Pennsylvania burned trash in an abandoned coal strip mine used as a dump near the Odd Fellows Cemetery, not realizing that the mine had not been properly sealed. The fire continued to burn a month later, and bulldozers arrived for a more concerted effort to put it out. The citizens then discovered that the dump contained a 15-foot-long opening that connected to a maze of underground mine tunnels. These passages allowed the fire to spread to the coal seam underneath the town and expand along four fronts, eventually affecting a surface area about two miles long and three-quarters of a mile wide. Since then, around $4 million has been spent to put the Centralia fire out, yet it continues to burn today. No one knows how extensive these empty spaces are, and the effort to quell the blaze has come to an end. “It’s too expensive to tackle, and we’re not sure we can do it anyway,” said Alfred Whitehouse, chief of the Reclamation Support Division of the federal Office of Surface Mining.

According to Discover Magazine: "The town of Centralia is almost completely deserted today. After some residents passed out from carbon monoxide inhalation and another fell into the earth in 1981, when the ground suddenly collapsed—as the coal burns away, the ground above it often subsides into the resulting cavity —Pennsylvania received $42 million from Congress to relocate Centralia’s residents. Folks accepted the buyout one by one, and their homes were demolished to discourage squatters. (Nine holdouts are still fighting eviction today.)"

Health and environmental effects
Beyond the immediate effects of leaking carbon monoxide, sulfur dioxide, and mercury and creating sinkholes, coal fires have other negative long-term effects. In 1995, a chemist reported that some of the sulfur in Centralia crystallized and stayed on the ground, potentially tainting the local water, and some of it floated away as a gas, polluting the air - findings he published in the 2004 International Journal of Coal Geology paper, “Coal Fires Burning Out of Control Around the World: Thermodynamic Recipe for Environmental Catastrophe”.

Coal fires also release more noxious pollutants. When coal is burned in a power plant, operators supply oxygen so that the coal burns hot enough to burn less by-products. Coal burning in an abandoned mine, however, typically gets far less oxygen. As a result, the coal smolders and releases a wide range of partially oxidized compounds. Testing at Centralia has revealed 45 organic and inorganic chemicals, including toxins like benzene, toluene, and xylene. Fifty-six compounds have been identified in the gases from one of China’s coal fires.

Discover magazine reports that coal fires in the United States may leak as much as 11.5 metric tons of mercury annually, nearly a quarter as much as all the nation’s coal-fired power plants. Unlike power plant emissions, coal fire emissions cannot be regulated or controlled.

Greenhouse gas emissions
The 2010 Discover article goes on to say that the fires "are an insidious, persistent, and often nearly invisible threat to local health and to the natural and built environment." In September 2009, the United States Geological Survey (USGS) issued a report, "Emissions from Coal Fires and Their Impact on the Environment" saying the agency was developing techniques to measure emissions of greenhouse gases and other pollutants from coal fires around the United States, starting with three in the Powder River Basin of Wyoming. According to the report: "The contribution of coal fires to the global pool of atmospheric carbon dioxide is little known but potentially significant." The CO2 leaks out through both vents and the soil, and the long-term burning can add up to significant GHG emissions.

Detection
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:
 * Temperature measurements of the land surface, in fissures and boreholes, for example using pyrometers
 * Gas measurements to characterize the fire ventilation system (amount and velocity) and the gas composition, so that the combustion reactions can be described
 * Geophysical measurements on the ground and from airplanes and helicopters to establish the extent of conductivity or other underground parameters. For example, conductivity measurements map humidity changes near the fire; measuring the magnetism can determine changes in the magnetic characteristics of the adjacent rock caused by heat
 * Remote sensing from aircraft and satellites. High resolution optical mapping, thermal imaging and hyperspectral data play a role. Underground coal fires of several hundred to over a thousand degrees Celsius may raise the surface temperature by only a few degrees. This order of magnitude is similar to the temperature difference between the sunlit and shadowed slopes of a slag heap or sand dune. Infrared detecting equipment is able to track the fire's location as the fire heats the ground on all sides of it. However, remote sensing techniques are unable to distinguish individual fires burning near one another and often lead to undercounting of actual fires. They may also have some difficulties distinguishing coal seam fires from forest fires.

Extinguishing the fires
Extinguishing a coal fire requires cooling the coal and isolating it from both the heat and the oxygen that feeds combustion. Surface fires are the easiest to put out, with firefighters creating moats or breaks to keep the fire from spreading and then smothering it in a nonflammable material, most often clay. Slightly deeper fires can sometimes be quenched by digging out the burning coal—in Indonesia, crews have done this by hand—and then burying the entire area.

But fires that rage deep underground, fed by oxygen coming from cracks in the earth, are extremely difficult to deal with. Most solutions involve pumping some combination of mud or fly ash combined with an inert gas or water, but the mixture does not always flow thoroughly enough to cover the burning coal, and it can crack when dried, allowing oxygen to get back in.

In the 1980s, a Texas firefighter named Mark Cummins developed an improved system to produce fire-fighting foam. In 2003, he used the nitrogen foam (made by condensing a laundry-detergent-like material) to act as a blanket, separating fire from the oxygen that fed it to help extinguish a blaze in West Virginia’s Pinnacle Mine. Since then, Cummins has developed a foam that is loaded with microbes - after he puts out a fire with the nitrogen foam, he shoots the second foam into the mine, where the microbes will consume oxygen and replace it with carbon dioxide: “At that point, you couldn’t burn that mine if you set off a bomb down there,” he said.

Coal fires linked to mass extinction
In January 2011, researchers reported in Nature Geoscience that they found charred particles in the Arctic. The particles may be coal fly ash, created from burnt coal. The researchers say the ash supports speculations that, 250 million years ago, volcanic eruptions sparked coal fires that may have led to the extinction of over 90 percent of marine species. The end of the Permian period culminated in a mass extinction, know as the "Great Dying."

Stephen Grasby, the lead author, and his colleagues believe that huge volcanic eruptions east of Siberia could have set coal seams on fire. This would have sent huge amounts of highly toxic fly ash into the atmosphere. After dispersal, the ash would have settled in the water, limiting light penetration and creating poisonous conditions for aquatic life. As further evidence of this theory, it appears that a large amount of char was deposited right before the mass extinction, which could have come from the combustion of coal. According to Gransby, "It's the first literal smoking gun to show that coal combustion was occurring."

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