House and Senate leaders have long struggled with how to eliminate the embarrassing cloud of pollution hanging over the nearby Capitol Power Plant.

But fixing the problem is expensive — and politically difficult.

The plant is a century old, built for times when coal was the main fuel source. Renovating it costs millions of dollars, while eradicating coal could alienate powerful Members from coal-heavy states such as Sens. Robert Byrd (D-W.Va.) and Mitch McConnell (R-Ky.).

But it’s an issue that won’t go away. In just the past few years, Members have commissioned a half-dozen reports on the issue from the Government Accountability Office and the Congressional Research Service.

CRS issued the most recent report on March 13, just two weeks after Speaker Nancy Pelosi (D-Calif.) and Senate Majority Leader Harry Reid (D-Nev.) announced their intention to switch the power plant to 100 percent natural gas. Right now, the plant still burns almost 19,000 tons of coal per year.

Pelosi and Reid called the plant a “shadow that hangs— over Congress and its efforts to green the Capitol. But all the options for making it more environmentally friendly carry significant costs and need to be researched further, according to the CRS report.

The report lays out three possibilities: switching the plant to 100 percent natural gas, outsourcing the plant’s management or converting the plant to a combined heat and power facility.

“These options may be costly, however, both in terms of fuel expenses and capital requirements — and may involve price and operational risk,— the report reads. “Consequently, careful comparison of all the options for this aspect of Greening the Capitol’ may be required to ensure that the most cost-effective and environmentally beneficial investments are made while ensuring the continued supply of utility services to the Capitol.—

Congressional leaders have already moved toward the first option of switching to natural gas, increasing the proportion of natural gas by about 40 percent since 2007. Outsourcing the plant’s management is an idea that died down years ago, and the CRS report called its environmental benefits “unclear.—

But the report found that converting the power plant to a combined heat and energy facility could have “large— environmental benefits. And in the past, acting Architect of the Capitol Stephen Ayers has supported that option.

At a Senate Rules and Administration Committee hearing in June, Ayers said a $200 million investment could transform the plant into a “cogeneration— facility, or a plant that creates both electricity and energy. By generating electricity, the plant could use the byproduct to heat the Capitol complex — resulting in fewer emissions.

Right now, the plant burns fuel to produce steam and purchases Pepco electricity to chill water. It only heats and cools Congressional buildings; Pepco powers them.

For Ayers, making the plant a cogeneration facility seemed like the “most cost-efficient thing we can do.— It would mean a plant that operates with a conversion efficiency of 80 percent, according to the CRS report — or more than 40 percent more than a coal-powered plant.

But the idea has never caught on. At the June hearing, Rules ranking member Bob Bennett (R-Utah) called Ayers’ $200 million plan a “tough sell.—

Instead, Members have been receptive to Ayers’ $7 million price tag for converting the plant to 100 percent natural gas. But the hidden costs would probably be much higher, owing to the fluctuating price of natural gas.

In their February letter, Pelosi and Reid directed Ayers to provide a budget estimate for doing just that.

“While the costs associated with purchasing additional natural gas will certainly be higher, the investment will far outweigh its cost,— they wrote. “The switch to natural gas will allow the CPP to dramatically reduce carbon and criteria pollutant emissions, eliminating more than 95 percent of sulfur oxides and at least 50 percent of carbon monoxide.—