PIERRE, S.D. - The South Dakota Board of Minerals and Environment voted unanimously Thursday to approve an air quality permit for the proposed Hyperion Center, which would be the first new oil refinery built in the United States since 1976.

The long-awaited decision moves the Dallas, Texas-based Hyperion a major step closer to starting construction in rural Union County.

The location is in extreme southeast South Dakota, across the Missouri River from Nebraska and near the border with Iowa. The nearest Nebraska towns are Newcastle and Ponca.

Hyperion executive Preston Phillips said the permit allows the firm to proceed with contingency plans that include securing a source of crude oil from Alberta, Canada, lining up construction contracts and solidifying commitments from investors for the $10 billion project, which would be one of the largest private investments in U.S. history.

Opponents are expected to fight the decision in court.

Late last year, the South Dakota Department of Environment and Natural Resources recommended, with some modifications, approval of the permit, which would allow the discharge of specified levels of pollutants.

During a contested hearing, supporters and opponents presented evidence and expert witnesses during eight days of testimony that spanned more than three months.

The nine-member board, appointed by the governor, heard closing arguments from both sides Thursday afternoon

Hyperion's lead attorney highlighted the state-of-the-art pollution control technology for the refinery, which would process 400,000 barrels of Canadian tar sands crude per day into low-sulfur gas, diesel and jet fuel.

The project includes a power plant that would turn petroleum coke, a refinery by-product, into electricity and hydrogen to run the refinery. The gasification process is a unique, cutting-edge technology that significantly reduces emissions, attorney Rick Addison said.

"It captures more pollutants in a gaseous stream before they reach the stack," Dallas attorney Rick Addison said. "That is different than any other type of technology, where pollutants are captured only when they are leaving the stack."

Opponents argued the proposed energy center would emit thousands of pounds of dangerous pollutants into the atmosphere each year. Lead attorney Bob Graham said Hyperion failed to meet the burden of proof that the refinery and power plant would meet federal air quality standards or uses the best technology available.

'"It's our position that the operation of the HEC will be in violation of the applicable clean air act and standards under the law," said Graham, who represents the South Dakota Sierra Club and two local groups opposed to the project, Save Union County and Citizens Against Oil Pollution.

Hyperion still must receive more than a dozen additional permits and consents from federal, state and local governments. But company leaders viewed the air permit at the same level of importance as the rezoning of more than 3,000 acres of land just north of Elk Point, which Union County voters approved in June 2008.

Phillips said litigation over the air permit would not stop the company from moving ahead with construction.

The massive project would take more than four years to construct, requiring an average of 4,500 construction workers each year during that time, with a peak of 10,000.

When finished, the energy center would employ 1,800 workers, pumping tens of millions of dollars into the local economy.