The coal fires are similar to those that smoldered for months beneath the wreckage of the World Trade Center, in that they involve buried fuels and are sustained and intensified by slight drafts of air and heat locked into surrounding rubble or rock. Geologists and engineers who have studied coal fires offered their expertise and specialized equipment -- like firefighting foams -- to emergency officials in Lower Manhattan. But firefighters at the scene stuck mainly with the simplest method: pouring endless streams of water on the wreckage as work crews slowly removed layers of debris. (source)
When coal, exposed at or near the surface by erosion, combines with oxygen, a chemical reaction produces heat. That process can build for years; low-grade, soft coals—crumbly and low in carbon—can spontaneously combust, at temperatures as low as 104 degrees Fahrenheit. (source)
Centralia Pennsylvania - Anthracite Coal Fire (started 1962) (source)
Anthracite coal mine fires are different (and generally more difficult to extinguish) from a bituminous coal mine fire. Bituminous coal typically is closer to the surface of the ground, and lies in level planes relative to the ground. Anthracite coal, however, lies in veins that are very steep, and are often folded over themselves. Because of this difference in nature, different methods must be employed to combat an anthracite fire compared to a bituminous fire, and different methods have varying degrees of success depending on the characteristics of each individual fire. To burn, a fire (coal or otherwise) needs oxygen, fuel to burn, and heat to remain ignited once it has started. Elimination of one of these ingredients will cause a fire to cease.
A popular method of controlling such fires is the total excavation of the area. This amounts to simply digging out the dirt and rock of the area, and has the highest success rate of the main methods in controlling the fire. It is also the only method with a success rate greater than 50%. The down side is that it can also become the most expensive, depending on the size of the fire. It is usually feasible for small fires. A supply of water is also needed to cool the rock and ground as it is being dug, as well as a place to put the dirt. There is also a potential to recover some of the unspent coal as it is dug from the ground.
Another method of extinguishing or controlling a fire is inundation. This refers to using water to surround the fire, and both cool it and cut off the supply of oxygen. One advantage to this method is that the water is applied through small boreholes in the surface, and does not require the destruction of whatever may be built on the land above the affected area. However, there is no guarantee that the water will flow to all areas of the fire. Also, a mammoth volume of water may be required for pumping into the mine for a long time, depending on the size of the fire.
A third method is to flush the mine out with non-flammable material. This is often in the form of a liquid slurry, which can also be injected through boreholes in the surface. The idea is to cut off the air from the fire, but one of the risks is that it may not seal up totally underground, particularly in a hilly region. Settling and drying of the material may also cause it to crack, creating another passageway for air, and defeating the purpose.
Fire barriers, or trenches, can be created to stop the spread of a fire. This is done by digging, and then "backfilling" the trench with a non-flammable material, like clay. The trench must be deep to reach the bottom of the coalbed, and wide to prevent the transfer of heat to the "cold side". This is important, because mine fires need not have a continuous supply of fuel. They can spread across a break in the fuel by transferring the heat, and causing the "cold" side to spontaneously combust. A trench must also be dug far in advance of the fire, for if the fire crosses it before completion, the trench is rendered useless.
Another method of controlling a mine fire is known as surface sealing. As its name suggests, this is accomplished by sealing up the surface of the area below which the fire is located, in an attempt to cut off the supply of oxygen to the fire. This only is effective on fires that are close to the surface. For fires that are deeper into the ground, the seal must be kept over a larger area for a longer period of time, not always a feasible option. "In some cases, a fire control project may have been successful if pursued to completion rather than terminated when allotted funds were expended."
"The fire temperature reaches temperatures of 1,700°C deep beneath the ground." (source)
"coal fires can exceed 1,000 degrees F" (source)
"Coal fires can reach temperatures of 1200 degrees Fahrenheit" (source)
"Temperatures in some of the hundreds of underground fires topped one thousand degrees Fahrenheit" Gasp!: The Swift and Terrible Beauty of Air page 344 (source)
"a mine fire that can reach intensity of 1,900 degrees Fahrenheit" (source)
"We are concerned about that and if there's affordable and safe technology to protect the SCSRs, we would encourage the panel to adopt standards to allow those to be used. In that connection, as well, when miners are unable to escape, there has been mentioned of safe havens. It's been called shelters, refuge chambers. We call it a safe house. We are concerned that with the experiences of MSHA as noted in the Federal Register of the March 9, 2006, hearings concerning the reported underground fires, 56, I believe, in a 10 year period of 30 degrees or greater duration and the studies by MSHA of the various disasters in the last 50 years, indicating that mine fires will sometimes range to 1,952 degrees Fahrenheit, that explosions have had measured intensity up to 75 PSI, we are concerned with some of the comments indicating that an acceptable level for resistance to fire would be 300 degrees Fahrenheit and an acceptable level to explosion would be 26 PSI for any sort of shelter, chamber, or safe house. " (source)
"The heat from the fires reportedly sent temperatures in the mine above 2,500 degrees Fahrenheit." (source)
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