ST. LOUIS – The state environmental agency has denied a permit request to build a $2 billion plant to convert coal to synthetic natural gas in southern Illinois, further clouding prospects for the clean-energy project already more than six years in the making.

The Illinois Environmental Protection Agency told developers Power Holdings in a May 29 letter that the group had not provided required details about emissions, air-quality analysis and other matters involving the proposed plant near Waltonville in Jefferson County.

The agency said Power Holdings can reapply for the construction permit, which the developers' first sought in February.