ALBANY — The New York Public Service Commission Thursday reiterated its approval of NRG Energy’s request to convert the Dunkirk coal-fired generator to natural gas.

The commission unanimously rejected a rehearing request by the Sierra Club and a community group, which contended the state’s environmental review was deficient and that ratepayers would be unfairly subsidizing the plant’s owner (12-E-0577).

The Sierra Club and the community group also filed suit in the state Supreme Court last month to block the $140 million conversion. The PSC approved the repowering in June.

Just one of the four coal units at the 635-MW plant is currently in use. NRG plans to convert three of the units to natural gas, to bring its total generating capacity to 475 MW.

State and NYISO officials say Dunkirk, 47 miles southeast of Buffalo, is needed to maintain the reliability of the transmission system in western New York.