U.S. exports of coal hit their highest levels in two decades last year, according to the Department of Energy.

While demand for coal is strong in other countries, it's falling in the United States.

"There's no question that our supplies of coal are adequate. The question is, how do we find new markets for coal to keep the share of electricity generation strong?" said Luke Popovich with the National Mining Association.

Coal faces competition from natural gas and expensive new government regulations for coal-fired power plants. That means companies are looking for new customers internationally.
    
Many of them are in Asia, which is growing quickly and needs new energy sources.
    
"High demand by China has rippled through the markets. It's really an issue of how long this demand is going to last," Bill Watson, an analyst with the Energy Information Administration, told The Associated Press.

"The prices have been very high, so anybody who can mine and ship coal certainly has a lot of incentive to do that," he said.