American Electric Power, parent of Appalachian Power Co., reports that it is "the largest purchaser of coal in the Western Hemisphere."

For Appalachian, coal-fired power plants generate about 98 percent of the electricity it delivers to customers in a territory that includes portions of Virginia, West Virginia and Tennessee. For AEP companywide, coal fuels about 70 percent of power generation.

And all that coal, an increasingly controversial fuel, helps explain the upward spiral of Appalachian's costs for complying with environmental regulations.

Appalachian and AEP report the companies have spent billions of dollars to improve air quality under the Environmental Protection Agency's Clean Air Interstate and Clean Air Mercury Rules adopted in 2005. It has installed scrubbers and selective catalytic reduction systems.

The company says it has cut and continues to reduce emissions of gases tied to acid rain.

But its coal-fired power plants spew into the atmosphere huge amounts of carbon dioxide, a gas closely linked to global warming.

According to CFO.com, which provides information for chief financial officers, the five corporations in the Standard & Poor's 500 with the largest carbon dioxide emissions are ExxonMobil, Chevron, AEP, The Southern Company (another electric utility) and ConocoPhillips.

AEP responded, "This appears correct."

The company said it "is committed to cutting carbon dioxide emissions through carbon capture and storage" and has a project under way at its Mountaineer plant to test the technology.

AEP said it supports the Waxman-Markey energy and climate change bill that passed the U.S. House of Representatives. Among a host of other provisions, the bill provides for a cap-and-trade program to reduce greenhouse gases. The program would allow companies that can easily reduce emissions to sell credits to other companies for which such reduction would be difficult. The cap ensures that emissions would not exceed a desired amount.

Facing mounting pressures to both rein in emissions and increase its use of renewable energy sources Appalachian's costs seem destined to keep climbing, as do customers' bills.

"We assume that any increase in environmental requirements -- mandated or otherwise -- will increase the costs necessary to meet them. Recovery of those costs would ultimately be requested," said Appalachian Power spokesman John Shepelwich.