The Architect of the Capitol wants to expand the Capitol Power Plant to run entirely on natural gas, but its history of burning coal on the premises could derail the well-intended mission.

And with the District Department of the Environment expected to rule as early as next month on whether to allow the conversion to take place, local officials and activists are increasing the heat on the AOC to come clean about its practices.

On Thursday, Del. Eleanor Holmes Norton, D-D.C., became the latest and highest-profile voice to join this chorus of calls for Architect of the Capitol Stephen Ayers to provide “clarification concerning the apparent inconsistencies between prior AOC commitments and the pending permit.”

In 2009, partly in response to public outcry, the AOC pledged to move away from using coal as the primary fuel source at the plant that heats the Capitol complex.

According to the official terms of the agreement, the plant would only burn coal 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.”

Since that time, the plant has remained true to these terms, AOC spokeswoman Eva Malecki said. She said the plant only uses coal for 8 percent of its current needs.

But in her letter, Norton pointed out that the 2009 agreement to phase out coal also promised upgrades to an existing coal boiler that would “provide the additional capacity needed to operate the Plant using only natural gas even in the event of equipment outages.

“Even though the November 2010 to October 2011 predicted completion date for this project has long passed, the [plant] continues to burn coal and issued on January 8, 2013, a solicitation for 20,000 tons of new coal,” Norton wrote to Ayers. “These actions have been viewed by many in the community as a broken promise, at best, and have led to a great deal of suspicion and consternation about your current cogeneration permit applications.”

Norton asked Ayers to clarify why the upgrade was never completed; how much coal has been purchased and burned at the plant over the previous three fiscal years; and whether emissions would increase as a result of the planned expansions.

Malecki told CQ Roll Call that the AOC had received the letter and would be providing a response, but did not offer a time frame.

Other activists and Capitol Hill residents are using the permitting process as leverage. They are demanding the DDOE withhold permission for the AOC to begin construction until it promises to immediately stop burning coal once and for all.

These critics are unmoved by the AOC’s insistence that the ability to install new technology at the Capitol Power Plant would ultimately achieve everybody’s desire to have a 100 percent natural gas-powered operation. It doesn’t change what it’s done in the past, they say, and the AOC hasn’t offered any timeline for the project’s completion. Coal would still have to be burned until all systems are “go,” Malecki said recently, which could take years.

Their champion on the ground is Tommy Wells, the D.C. councilmember whose ward includes the Capitol Hill neighborhood.

On Tuesday, the day after launching an exploratory committee for a 2014 mayoral run, Wells made good on his promise to introduce legislation that would ban the use of coal as a fuel source in D.C. by Jan. 1, 2016.

Though it’s still unclear whether it would have the necessary enforcement teeth to compel Congress to comply, the bill’s purpose would be “to protect public health and hold the Architect of the Capitol to previous commitments to end the use of coal at the Capitol Power Plant.”