The Manitoba fire commissioner's office has ruled that the Oct. 1 blaze at Speedway International fuel plant in Winnipeg was accidental.
The office said in a report Friday that the fire is believed to have started in an oily substance in the plant's filter press area.
The report did not specify which liquid started the blaze, but said it began in a part of the plant where biodiesel is manufactured. Methanol fuel for racing cars and windshield washer fluid are also produced at the sprawling industrial complex.
Spontaneous combustion is not uncommon, but it takes time for heat to build.
"In industrial processes, the storage or disposal of oily rags in piles can allow them to self heat, or the combustion process could have been accelerated due to heat created from industrial processes like equipment operation or friction," reads the report.
"Piles of straw, coal and even large manure piles can spontaneously combust."
The findings raise questions about whether the fire might have been prevented had the plant not been empty when it started.
Evan Basarowich, one of the plant's nine employees, told The Canadian Press the day after the fire that the plant would normally operate throughout the day and into the evening. But the day shift had been sent home early on Oct. 1 because work had stopped while the plant waited for a shipment of magnesol — a powder used to purify biodiesel. Basarowich and others on a later shift were told to come in late, after the shipment was scheduled to arrive.
The fire ignited in the empty plant around 5 p.m. It set off alarms that prompted the company to call in Basarowich and other workers. When the fire reached a large methanol tank, a huge fireball erupted that could be seen for kilometres.
Firefighters spent hours dousing rail cars, trucks and large storage barrels to try to prevent them from exploding.
Businesses and homes in a one-kilometre radius were evacuated. As the evening wore on, firefighters retreated twice to keep out of the way of barrels that exploded from the heat.
Crews eventually gained the upper hand and started beating back the flames. By the time the fire was extinguished the following day, damage was estimated at $15 million. There were no injuries.
The fire commissioner's office said it will review what happened to determine "whether there should be any changes, or mandated time frames, for inspections of these, or similar types of industrial facilities."
Speedway International, in a written statement two days after the fire, said it was federally licensed and had complied with all regulatory safety standards and codes.
The company said Friday it wanted to reassure residents.
"We want to assure the public the fire they witnessed was burning Canola M.E. (mono alkyl ester), which is derived from long chain fatty acids of canola oil and not dangerous goods or hazardous materials,” owner Royce Rostecki said in a written statement.
“This confirms what we were told by an independent investigator — this was an exothermal reaction of Canola M.E. inside a filter press of the biodiesel facility.
"The investigator also determined that no human error, equipment failure or negligence on behalf of Speedway International contributed to the fire."