http://www.nytimes.com/2010/04/07/science/07methane.html
Though there is no definitive proof that it was a methane explosion that killed 25 miners in West Virginia on Monday, miners have said the gas has been a constant problem, leading to several evacuations in recent months and a history of violations.
The gas, like coal, is a molecule made of hydrogen and carbon, and it is produced from the same raw material as coal, ancient piles of biological material, by the same processes. Much of the natural gas sold in the United States is drawn from coal seams.
In undisturbed coal deposits, the methane is loosely attached to the coal molecules when the deposit is under pressure; when the area is opened up by miners, the pressure is reduced and the methane bubbles out.
“Methane is ubiquitous on the coal mines,” said Neville A.H. Holt, a chemical engineer at the Electric Power Research Institute, in Palo Alto, Calif.
Often, the deeper the mine, the more pressure released in mining and the more methane freed. The Upper Big Branch mine, where the explosion occurred on Monday, is more than 1,000 feet deep.
Methane has no smell. It is the main ingredient of natural gas, but the smell that consumers associate with natural gas is actually a chemical added by the gas company to make leaks obvious. Humans cannot sense methane itself, although at times it is present with another gas, hydrogen sulfide, that has an evident stink. But because the methane is often pure, miners long ago began carrying canaries to work with them, knowing that when the birds showed signs of distress it was time to get out.
The main method for getting methane out of mines is to pump fresh air in, so much that visitors can often feel a breeze.
But the release of methane is not uniform over time; it can appear in puffs, creating a potentially explosive concentration.
Dr. Edward Kavazanjian Jr., a professor of civil engineering at Arizona State University, said, “I don’t think it’s a big secret about how to deal with the problem; it’s time and money.”
Ventilating tunnels properly, Dr. Kavazanjian said, was not a technical challenge, but was cumbersome. He said that while he did not know what caused the most recent disaster, both mine operators and miners generally want to mine as much coal as possible and are sometimes prone to cutting corners.
Raymond C. Pilcher, a mining consultant in Grand Junction, Colo., who is the chairman of a United Nations committee on methane risk in mines, said that one frequent source of methane reaching the working areas of mines, where workers could accidentally create a spark and ignite it, are the “gob areas” of the mine, which have been mined out and shut down. These areas are supposed to be sealed off, but sometimes the seal is ineffective.
Some coal mine operators drill wells to pump methane out of the gob areas, or sometimes out of coal seams that they have not yet mined. They use the methane as fuel to run engines and generators to make electricity.
Mr. Pilcher said that most machinery used in coal mines has methane sensors that shut everything down if the concentration in the air gets too high. A key to safety, he said, is whether miners inform managers when equipment shuts itself down. The incentives to do so are mixed, because some miners are paid bonuses for higher production.