TOPEKA, Kan. — A federal judge in Washington has sided with the Sierra Club in a legal dispute over a proposed coal-fired power plant in western Kansas, and an attorney for the environmental group predicted Thursday that the decision will at least delay the project.

U.S. District Judge Emmett Sullivan ruled in a lawsuit filed in October 2007 by the Sierra Club against the federal Rural Utilities Service. The lawsuit alleged that the RUS is required by federal law to do an environmental study of the project and have public hearings before allowing Sunflower Electric Power Corp., based in Hays, to build additional coal-fired generating capacity.

Sullivan said in a three-page order filed Tuesday that he decided in favor of the Sierra Club, but most of his decision remains under seal to protect the utility’s confidential financial information. Attorneys have seen a longer, 54-page version from the judge, who plans to consider how much more information to make public next month.

Sunflower plans to build its new coal-fired plant outside Holcomb, in southwest Kansas. The RUS, part of the U.S. Department of Agriculture, must sign off on decisions related to the project because it provided past financial support to Sunflower and oversaw corporate reorganizations.

Sullivan’s order doesn’t say what RUS will be required to do to comply with his order and asks for more arguments from attorneys on proposed remedies.

Jan Hasselman, an attorney for Earthjustice, which is representing the Sierra Club, said the decision means the federal agency is likely to be required to conduct an environmental study and consider alternatives to the coal plant. He said that process could put the project on hold for 18 months to two years and could ultimately block it.

“He essentially agreed with all our arguments,” Hasselman said. “What we’re saying is that the government needs to take a time-out on further approval of this project before we’ve thought it through.”

Sunflower declined to comment because most of Sullivan’s decision remains under seal. The RUS also declined comment because the case is pending.

The company supplies power for about 400,000 Kansans and plans to build a plant with a capacity of 895 megawatts, enough to meet the peak demands of 448,000 households, according to one state estimate. Three-quarters of the new capacity, or 695 megawatts, would be reserved for a Sunflower partner, Tri-State Generation and Transmission Association Inc., of Westminster, Colo.

That’s a sore point for many critics of Sunflower’s push to add coal-fired generating capacity, but the utility’s supporters say exporting electricity is as beneficial as exporting beef, wheat and other agricultural commodities.

The lawsuit in Washington is among several challenges by the Sierra Club aimed at slowing down or stopping Sunflower’s $2.8 billion project.

In a case before the Kansas Supreme Court, the group is trying to overturn an air-quality permit issued for the project by the state Department of Health and Environment in December, arguing that it’s too lax in regulating potential pollutants.

Last month, the group demanded aggressive action by the U.S. Environmental Protection Agency, telling its regional administrator in Kansas City, Mo., in writing that he’s legally obligated to object to the state permit. The regional EPA office has said parts of the permit aren’t stringent enough and has asked for a “dialogue” with state officials.

Hasselman said the court ruling in Washington will force the RUS to examine options for Sunflower, such as investing in conservation efforts and renewable energy.

“This project is bad for human health. It’s bad for the environment,” Hasselman said. “It’s financially risky.”

But Sunflower has said it will need extra generating capacity, and its supporters, including many legislators, argue that the project will bring economic development to western Kansas.

Sunflower previously wanted to build two 700-megawatt plants. But in October 2007, KDHE rejected an air quality permit, citing the plants’ potential emissions of greenhouse gases linked by scientists to global warming.

Then-Democratic Gov. Kathleen Sebelius stood behind the decision, and her dispute with the utility spilled into the Republican-controlled Legislature. It blocked legislation promoting renewable energy and tried unsuccessfully to override the permit denial.

Sebelius stepped down as governor in April 2009 to become U.S. health and human services secretary. Almost immediately, new Democratic Gov. Mark Parkinson brokered a deal with Sunflower to allow one coal-fired plant, clearing the way for passage of “green” legislation he favored.

Parkinson left office in January, but Republican Gov. Sam Brownback is a strong supporter of the utility’s plans.