Premcor (formerly NYSE symbol PCO) was a Fortune 500 oil refinery group based in Greenwich, Connecticut. It operated five refineries, which are located in Port Arthur, Texas; Memphis, Tennessee; Lima, Ohio; Hartford, Illinois; and Delaware City, Delaware with a combined crude oil volume processing capacity of approximately 768,120 barrels per day (122,121 m3/d) (July, 2007).
Premcor was acquired by Valero in 2005. Valero would later sell the Lima facility to Husky Energy in July 2007 and close the Delaware City refinery in November 2009.
Researchers at the University of Massachusetts Amherst previously identified Premcor as the 39th-largest corporate producer of air pollution in the United States, with roughly 1 million pounds of toxic chemicals released annually into the air. Major pollutants indicated by the study included sulfuric acid and chlorine. 
- Political Economy Research Institute Toxic 100 (Study released May 11, 2006) retrieved 15 Aug 2007
- Toxics Release Inventory courtesy rtknet.org
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