UPDATE 3-Fire at Exxon refinery sends plume of smoke over Houston

(Adds unit involved, ops appearing normal, other operational details)

HOUSTON, April 7 (Reuters) - A fire erupted at Exxon Mobil Corp’s Baytown, Texas, refinery on Thursday afternoon, sending a large plume of black smoke into the air that was visible 22 miles (35 km) away in downtown Houston.

Exxon said the fire was later extinguished, no workers were injured and output would not be curtailed at the 560,500 barrel per day (bpd) oil refinery, the second-largest in the United States.

The company provided no specific information on the affected unit but said it would monitor air quality around the complex and the community.

However, sources familiar with plant operations said the blaze broke out on a 20,000 bpd lube oil hydrotreating unit, said sources familiar with plant operations. A hydrotreater is a unit that removes sulfur from oil products.

In response to a question from Reuters, Exxon said in a statement “the unit affected processes diesel fuel.”

The sources said the unit, LHU 1, processes diesel.

Energy industry intelligence service Genscape said activity at units it was monitoring at the refinery remained normal after the fire was extinguished.

Television footage showed firefighters battling the blaze on one of the plant’s towers.

People in the vicinity and the U.S. Coast Guard’s Vessel Traffic Service on the Houston Ship Channel had first reported smoke coming from the facility, which includes a chemical plant.

Unrelated to the blaze, the Baytown refinery was boosting production for the first time since crude intake was cut in early February, the sources said.

The refinery’s largest crude distillation unit, the 280,300 bpd Pipestill 8, began taking in a larger amount of crude oil late on Tuesday and was continuing to do so on Thursday.

Exxon cut back the refinery’s run rate in early February amid a glut of crude oil and gasoline in the United States .

Exxon confirmed the increased production saying planned maintenance had been completed.

Additional reporting by Kristen Hays and Liz Hampton; Editing by David Gregorio and Christian Schmollinger