An $10 billion oil refinery in Union County is one step closer to becoming reality. The South Dakota Board of Minerals and Environment unanimously approved the air permit for Hyperion Energy Center.

Hyperion says Thursday is a milestone for bringing its refinery to Union County. But the decision came in front of a room full of opponents from Union County who've been voicing concerns about the project's environmental impact since it was announced more than two years ago.

More than 60 supporters and opponents of a Union County refinery witnessed a historic step for the Hyperion Energy Center.

"I think that the preponderance of the evidence favors the permit and I vote yes," board member Dick Sweetman said.

The Board of Minerals and Environment granted the refinery its air permit after hearing from both sides.

"South Dakota has a facility in this instance that it should be proud of in issuing this permit," Hyperion attorney Rick Addison said.

But opponents testified that Union County and the State of South Dakota should not be proud of the refinery

"This production will come at a significant and heavy price. Because the HEC will emit several tons a year of significant and serious air pollutants," the attorney for opponents, Robert Graham, said.

Now, opponents plan to appeal the decision to the circuit court.

"We will review all of our options and obviously avail ourselves of the various rights of appeal we have in the next 30 days," Ed Cable with Save Union County said.

But Hyperion officials say they're prepared to move forward with the project and are pleased the Board trusts the refinery will be as clean as they say.

"We hope that those activities will help make the project stronger and we look forward to pushing the project down the road," Hyperion Project Executive Preston Philips said.

Hyperion says the next step for the project is to get a water permit and oil contracts it needs for the refinery. They expect to break ground on the project in 2011.