The plant was given until Oct. 21 to shut down a boiler at the plant next to the Delaware Memorial Bridge.
The state says the emissions can mix with moisture in the air to form hydrochloric acid aerosol, which can irritate the respiratory system.
The DEP says the company ignored orders to fix the problem for months.
Company spokeswoman M.Q. Riding says Conectiv was surprised by the fine, adding it has been appealing the limits of its emissions permit since the spring. She says the company disputes the DEP's findings.