Fire lit up the sky over the Suncor Oil Refinery in Commerce City overnight Saturday.
South Adams County Fire says at 10 p.m., a total power failure from Xcel led to an "operational upset."
That upset caused the flare-up from one of the refinery's stacks.
No one was hurt, but Brighton Boulevard was closed temporarily near the refinery as a precaution.
Suncor says its emergency response team monitored the air around the refinery and found readings were normal.
Back in October, another power failure there sent 75,000 pounds of sulfur dioxide into the air in the middle of the day.
Back then, Suncor and the state health department said that accident did not pose a health risk.