Many scientists, environmentalists, politicians and business people agree that coal-fired power plants, such as Rochester's Silver Lake plant, should eventually be phased out in favor of cleaner fuel sources to save the environment. The burning of coal not only emits high amounts of sulfur dioxide, but it releases other potentially harmful chemicals, such as mercury, that can end up in lakes and drinking water. And it creates so-called greenhouse gases that many scientists believe are causing a harmful increase in the surface temperature of the earth.

But a phase-out of coal-fired power plants doesn't appear likely anytime soon. While no major coal-fired plants have been built in this country for more than a decade, coal is still by far the nation's largest fuel source, responsible for about 57 percent of the nation's electricity.

Nuclear power plants, which are responsible for about 20 percent of the country's electrical needs, were once seen as coal's eventual successor. But the nuclear industry has fallen into disfavor as a result of major accidents at Three Mile Island in Pennsylvania in 1979 and Chernobyl in the former Soviet Union in 1987. And it's been more than 20 years since construction of a new nuclear plant was begun.

In addition, there's been almost no growth in hydro-electric power production in the last 15 years.

There's a boom going on now in natural gas and methane gas extraction in this country, and small plants are being built or planned all over the country, including the Great River Energy plant proposed near Sargeant in Mower County, to take advantage of it. But gas is more expensive to extract, process and transport than coal, and there is much less of it.

Scientists estimate that at current consumption rates, the world's coal will run out in about 200 years, while natural gas would peter out in about 60 years and oil in 45 years.

The natural gas plants being built today, which supply about 10 percent of the nation's electricity demand, are intended only as "peaking," or supplemental, facilities that are fired up when there's unusually high demand, such as humid, 90-degree days when everyone has their air conditioners humming, said Larry Koshire, manager of Rochester Public Utilities.

"With nuclear out of the picture for the time being, coal is still going to supply the bulk of our power," he said.

All of Rochester's power is supplied by the Southern Minnesota Municipal Power Agency, a cooperative of municipalities that acquires its electricity from a variety of sources, most notably the coal-fired Sherco 3 power plant near Becker, Minn. SMMPA owns a stake in Sherco 3 and relies on it for about 80 percent of the cooperative's power needs.

But the city still owns and operates its own plant near Silver Lake, a 52-year-old facility that discharges warm water as a byproduct, keeping the city's manmade lake open all year.

Coal for the Rochester plant, which generates electricity sold under contract to Chaska-based Minnesota Municipal Power Association, is purchased from mines in Illinois. That's not likely to change anytime soon, Koshire said, because the Silver Lake plant doesn't operate well with low-sulfur coal. Nonetheless, he says the city-owned plant meets clean-air standards, at least for now.

In Minnesota, coal provides about 55 percent of the state's power, according to Marya White, manager of energy planning and advocacy for the state Department of Commerce.

"For the foreseeable future, coal is going to continue to be the backbone of the power grid in Minnesota and the rest of the country," she said.