With an Environmental Protection Agency permit now in hand, the Architect of the Capitol just needs the District Department of the Environment’s green light to proceed with its plans to expand the Capitol Power Plant.

Its ambitions, however, could be thwarted by a group of Capitol Hill residents and environmental activists who don’t want the city to sign off on any expansion until the AOC promises to stop burning coal and establish a lower baseline for plant emissions.

They also have the backing of Councilman Tommy Wells, whose ward of jurisdiction includes the plant and who has pledged to introduce legislation that would remove coal as an energy source in the District.

“For the most immediate neighbors of this facility, there simply is no choice but to breathe the air that is polluted by our last remaining coal-fired plant,” Wells said in a statement.

The staunch disapproval being voiced at community meetings has grown so loud it caused the District Department of the Environment to extend the public comment period on the AOC’s permit application until Feb. 18, meaning a final decision won’t be issued until March or April.

The AOC needs the DDOE’s permission, along with the go-ahead it received last week from the EPA, to expand the Capitol Power Plant for “cogeneration.” That refers to the use of a natural gas-powered turbine to produce the energy needed to run the plant itself; the steam from that process would, in turn, help heat the Capitol complex.

Cogeneration would allow the plant to use natural gas 100 percent of the time, AOC spokeswoman Eva Malecki has stressed, improving air quality in D.C. and reducing Congress’ environmental footprint.

Until new technology is installed, the plant will still have to rely on coal about 8 percent of the time. According to official terms agreed to in 2009, coal would be used only when “heating needs exceed the capacity of the natural gas pipeline currently serving the complex,” during “abnormally cold conditions” and in cases where “equipment outages on the gas boilers require a backup.”

But 8 percent of the time is too frequent, critics argue, especially when coupled with the lack of certainty about how long it would actually take the AOC to phase out coal entirely once the cogeneration construction begins. Malecki told CQ Roll Call late last year that coal would continue to be in supply at the plant until all systems are go, which could take years.

They also say the plant’s permit application with DDOE seeks to cap its emissions at 2007-2008 levels — when coal accounted for 50 percent of its fuel source.

“We don’t want the DDOE to accept an application for pollution, that allows for emission levels at or higher than when they were using coal,” said Daryl Kimball, a Capitol Hill resident who has been involved with efforts to fight plant expansions without AOC’s promise to lighten its environmental footprint.

Public outcry against the plant’s use of coal worked in 2009, when Senate Majority Leader Harry Reid, D-Nev., and then-House Speaker Nancy Pelosi, D-Calif., demanded that the AOC reduce its reliance on the fuel source in part because of demands from the community.

But Capitol Hill veterans say that episode was very different from what is happening now.

“They have met the obligation to dramatically reduce the split between coal consumption and natural gas consumption,” said a Democratic aide who was closely involved in the 2009 campaign to push the Capitol Power Plant into being a better environmental steward.

In delaying the DDOE’s ability to rule on the permit, the anti-coal activists “have effectively delayed better air quality for Capitol Hill residents,” the aide continued. “It’s not actually helpful to the long-term health issues and air quality issues that we have with the Capitol Power Plant.”

Wells’ office did not return requests for comment Monday, but his forthcoming legislation might also fall short: It is currently unclear whether the D.C. Council has the authority to dictate certain behavior in a federal facility like that of Congress’ power plant.