Top Democratic lawmakers Thursday urged Congress architect Stephen Ayers to switch the Capitol's century-old power plant from burning coal to using natural gas, in keeping with an initiative launched in 2007.

House of Representative speaker Nancy Pelosi and Senate Majority Leader Harry Reid wrote Ayers asking him to prepare the Capitol Power Plant (CPP) for a conversion to cleaner burning natural gas.

The letter comes only days before a March 2 protest by environmental organizations against the preponderance of coal-fired power plants in the United States that will converge precisely on the CPP, on the outskirts of the Capitol Complex.

The CPP was built in 1904 and supplied the complex with electricity until 1952. Since then, its three boilers have provided steam for heating and cooled water for air conditioning to congressional buildings.

With cost-effectiveness in mind, the Democrats told Ayers, "it is our desire that your approach focus on retrofitting at least one of the coal boilers as early as this summer, and the remaining boiler by the end of the year."

"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," they said.

"Taking this major step toward cleaning up the Capitol Power Plant's emissions would be an important demonstration of Congress' willingness to deal with the enormous challenges of global warming," they added.

The CPP's retrofitting is part of the 2007 "Green the Capitol" initiative by the House and Senate that Pelosi spearheaded to make the Capitol carbon neutral.

Some 2,500 people are expected at Monday's anti-coal demonstration outside the CPP, said a spokesman for Greenpeace, one of the groups organizing the event.