2018 Husky Energy Refinery explosion

The oil refinery in Superior, Wisconsin, which processes 50,000 barrels of oil a day, was acquired by Husky Energy from Calumet Specialty Products Partners in November 2017, retaining 180 Calumet employees. The company processes heavy oil from oil sands and conventional operations in western Canada. The refinery has a partnership with the Superior Fire Department, which provides its personnel training and resources when fighting industrial fires.[3] It was eventually acquired by Canadian company Cenovus Energy in March 2021.[4]

On the morning of the explosion, at 5:30 a.m., refinery workers were planning to shut down the Fluid Catalytic Cracking Unit (FCCU) of the refinery for routine maintenance. The FCCU is used to break down hydrocarbons from crude oil into smaller hydrocarbons, which can blend into products such as gasoline. When the FCCU is shut down, slide valves in the middle of the unit are used as a barrier between a reactor, containing flammable hydrocarbons, and a regenerator, containing air. If the hydrocarbons and air mix, an explosion can occur. On the morning of the incident, workers attempted to stop the flow of hydrocarbons to the regenerating unit, however, one of the valves was eroded, causing air to flow upward into the reactor, into equipment that contained flammable hydrocarbons.[1]

The mix of the air and hydrocarbons caused a massive explosion in the FCCU at approximately 10:00 a.m. that morning. The explosion sent debris flying 200 feet, one piece puncturing a hole in an above-ground storage tank that contained around 50,000 barrels of asphalt, causing the asphalt to spill along the ground into the refinery's main units. The asphalt ignited near additional storage tanks, eventually traveling to the FCCU of the refinery, causing a fire producing thick black smoke to travel into the air.[1] The smoke went so far into the sky, it was picked up on weather radar.[5]

The black smoke traveled southeast as far as Solon Springs in Douglas County. An evacuation of the plant was ordered at 1:00 p.m., however at 2:41 p.m., an evacuation for residents 10 miles south of the plant was ordered due to concerns of the toxicity of the huge plumes of black smoke. Residents who lived 3 miles north, east, and south of the refinery were also ordered to evacuate due to concerns about the change in the direction of the wind forcing the toxic smoke above more homes, and due to concerns about the refinery's hydrofluoric acid tank causing potential further explosions and damage. Schools in the Superior School District as well as the Essentia Health-St. Mary's Medical Center in Superior was also urged to evacuate. Students were evacuated to an AMSOIL building in northwestern Superior. Schools in the Maple School District in Douglas County were also closed as a precaution. Those injured were taken to hospitals in Duluth, Minnesota.[6][7]

The fire was initially expected to burn for days. The refinery's emergency response team, as well as the Superior Fire Department, initially went defensive against the fire due to concerns of safety due to the intense heat as well as other units of the refinery which were a cause of concern. A plan was developed at around 3 p.m. The large asphalt fire was extinguished with foam, and the fires around the crude unit of the refinery were extinguished with dry chemical fire extinguisher and water. The fire was fully extinguished at 7 p.m., however the evacuation order remained in place until 6 am the next day due to concerns regarding the stability of the plant's hydrogen fluoride tank.[8]