Jul. 26--Running all out, AmerenUE's Meramec plant in south St. Louis County burns 380 tons of coal an hour.

Built in 1953, it's the oldest piece of the utility's coal-fired fleet, and among the cheapest to operate. These days, however, it's less a symbol of cheap electricity than a potential casualty if Congress passes a law to slash greenhouse gas emissions.

The House of Representatives passed the sweeping, 1,500-page American Clean Energy and Security Act last month. The House bill, sponsored by Henry Waxman, D-Calif., and Edward Markey, D-Mass., aims to cut emissions of heat-trapping gases 17 percent from 2005 levels by 2020 and 83 percent by 2050. Debate is now under way in the Senate.

The bill's centerpiece would require major sources of greenhouse gases such as power plants and oil refineries to reduce emissions of carbon dioxide and methane or pay a price.

The so-called "cap and trade" plan -- because businesses would be allowed to buy and sell emissions credits-- is causing high anxiety in states such as Missouri, which gets 80 percent of its electricity from coal.

AmerenUE has said rates for Missouri electric customers would rise 10 to 20 percent by 2015 if Congress imposes carbon limits, and more in future years. The increase doesn't include other rate increases.

St. Louis-based Ameren said electric customers in Illinois would see the same kind of increases even though the state is less dependent on coal and the electricity market was deregulated in 2007.

Under the House cap-and-trade plan, the federal government would initially provide free credits, or allowances, to help Ameren compensate for some of the 70 million tons of carbon dioxide, or CO2, produced by its power plants each year. But it would have to slash emissions in subsequent years or pay a penalty.

Options for complying with the proposed regulations include building wind farms and other renewable generation to cut CO2, and investing in energy efficiency -- both steps Ameren is already taking. It could also buy emissions allowances or so-called carbon offsets -- permits to pollute beyond allowed levels by investing in projects that reduce CO2 output somewhere else.

Ameren has said carbon caps could also force it to mothball the Meramec plant.

The same fate could await other coal-fired power plants in the state and region, said Warren Wood, president of the Missouri Energy Development Association, the Jefferson City-based lobby for investor-owned utilities.

"They may not be justifiable to continue to operate, and you'll frankly be retiring a resource that still probably has decades of useful service in it," Wood said.