Two key components at the Valero refinery in Benicia experienced problems earlier this month, leading to weeks of releases of petroleum coke dust that intensified on Sunday, culminating in the shutdown of a large portion of the facility.
The new details on the refinery's malfunctions are laid out by Valero in a preliminary report filed with Benicia city officials this week.
The malfunctions are the focus of at least one part of an investigation by Solano County inspectors into an incident that led to health concerns for people living near the refinery and to a statewide spike in gasoline prices.
Valero's report says that the recent problems at the refinery began March 11 when a crude oil processing unit called a fluid coker experienced "operating difficulty."
Normally, a fluid coker heats up and "cracks" the thickest, heaviest components of crude oil processed at a refinery, breaking them down into material that can be used in diesel or other petroleum products. One of the byproducts of the process is solid carbon residue called petroleum coke.