Citgo notified U.S. EPA and IEPA in mid-February that it needed to take its CO Boiler on the FCCU down for unplanned maintenance due to an unexpected failure of its refractory brick. Because of the configuration of the control equipment on the FCCU, this meant that the ESP on the FCCU had to be shut down as well. Therefore, uncontrolled particulate emissions from the FCCU were emitted directly to the atmosphere during the 18 days that the CO Boiler was being repaired.