Sept. 23 (Bloomberg) -- ConocoPhillips, the second- largest U.S. refiner, said a malfunction necessitated flaring yesterday at its plant in Borger, Texas.

The refinery burned off about 500 pounds (227 kilograms) of sulfur dioxide for one hour, ending at 4:38 p.m. local time, according to a company report on the Texas Commission on Environmental Quality Web site. The incident marks the sixth malfunction at the plant this month.

Terry Hunt, a ConocoPhillips spokeswoman, didn't immediately respond to messages seeking comment on the malfunction and its impact, if any, on production.

The Borger refinery is about 50 miles (80 kilometers) north of Amarillo, Texas, and is capable of processing 146,000 barrels of oil a day. The plant serves markets in West Texas, New Mexico, Colorado and the Mid-Continent region.

Flares burn off chemicals into the air, keeping pressure from building up in refining units. Sulfur dioxide is a waste product of hydrogen sulfide, a poisonous gas.

Valero Energy Corp. is the largest U.S. refiner.

To contact the reporter on this story: Jim Kennett in Houston at jkennett@bloomberg.net.

Last Updated: September 23, 2007 12:18 EDT