WASHINGTON – As Congress tries to clean up the nation’s energy sources and cut gases blamed for global warming, it is struggling to do so in its own backyard.

The Capitol Power Plant, a 99-year-old facility that heats and cools the hallowed halls of Congress, still burns coal and accounts for one-third of the legislative branch’s greenhouse gas emissions. For a decade, lawmakers have attempted to clean it up.

In recent years, Congress has reduced its energy consumption. The steam and chilled-water power plant has become more efficient. It now burns more natural gasand only 35 percent coal, compared with 49 percent in 2007.

But Congress is running out of options to make the plant fully green. Also, there are questions about whether it can afford to keep paying to use the extra natural gas, which burns cleaner than coal.

The plant’s story is one that is likely to play out across the United States as Congress looks to limit greenhouse gases and require more of the country’s energy to come from wind, solar and other renewable sources.