The fires below: burning coal sculpts landscapes worldwide - underground fires in coal seams

Science News,  May 10, 2003  by Sid Perkins

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Consider oxygen. Trying to smother a coal fire with a thick layer of material such as fly as--chosen simply because it's available in large quantities at low cost from coal-fired power plants and other industries--just doesn't work, says Jones. Long-worked mines are typically made up of air-filled corridors with pillars of coal left to support the roof. That coal can smolder even when the air contains only 2 percent oxygen, about one-tenth the concentration in the atmosphere.

When those pillars burn through and collapse, the ground above the mine subsides. Cracks in the ground or any covering then create new routes for fresh air to reach the fire, which flares up again.

Pumping water into a burning coal fire to smother and cool it doesn't often work, either, because mines are voluminous and the rock around them is so fractured that the water doesn't stay in place. Also, most mine fires burn the coal or carbon-rich rocks in the roof of the mine's tunnels, which are the last areas to be inundated with rising water.

The best way to put out a coal fire currently, says Jones, is to deprive it of fuel. Firefighters dig a moat to isolate the burning coal and then fill the trench with noncombustible material, such as rock. Sometimes, as an added precaution, the encircled coal can be spread atop the ground and doused with water or simply left until it cools.

These excavation techniques are currently being used to reopen a portion of Boyce Park in western Pennsylvania's Allegheny County. About 2 hectares of land there reignited in 1989, more than 25 years after officials thought they had smothered the coal fire at a site more than a 500 m away with a thick layer of clay. The $1.2-million reclamation project should be completed this summer, says Jones.

Scientists are also investigating more-advanced firefighting methods, such as the strategy described at the AAAS meeting by Ann G. Kim of the National Energy Technology Laboratory in Pittsburgh. The high-tech approach reduces the fire's heat and oxygen supply.

In a field test at a coal fire that's been burning for 30 years, Kim and her colleagues pumped a slurry of frozen carbon dioxide and liquid nitrogen into a smoldering pile of waste coal. When this -180[degrees]C mixture hit the 350[degrees]C coal, the carbon dioxide and nitrogen vaporized, says Kim. The process absorbed extraordinary amounts of heat. Furthermore, the rapid expansion of cold gases forced hot, oxygenated air from the waste pile and replaced it with chemically inert nitrogen and carbon dioxide. Measurements showed that temperatures in a bore-hole close to the slurry injection dropped to around 100[degrees]C, near the temperature at which a coal fire can't sustain itself. In the test, the fire wasn't completely extinguished, says Kim, because the researchers didn't have enough of the quenching slurry on hand.

HEAT ATTACK The prodigious heat generated by coal fires affects the landscape in small ways and large, and in time frames both short and enduring.