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HOUSTON, June 21 (Reuters) - A fire was extinguished in a furnace at the co-generation plant providing steam and power to Marathon Petroleum's MPC.N 631,000-barrel-per-day Galveston Bay Refinery in Texas City, Texas, on Sunday, according to two people familiar with plant operations.

Marathon spokesman Jamal Kheiry did not identify the unit where the fire broke out at the refinery, which is the nation's second-largest. Refinery firefighters "extinguished a small fire," Kheiry said. No injuries were reported.

The fire broke out in a furnace of the Green Power 2 generating station, the sources said. The sources did not know if production at the refinery was affected by the fire on Green Power 2.