Your confusing two separate issues. A fowling problem; petcoke sticking to the walls of the kiln and the actual bulk removal of petcoke from the system. The system still makes the same percentage of petcoke when run, it just doesn't stick to the walls anymore fowling the system and requiring extensive cleaning.

The system is closed loop so the reactor and premelt would have to be idled to remove the residue. The third kiln solves the Idling issue of the reactor and premelt as only the third kiln would have to be idled to remove the petcoke, allowing the premelt and reactor to continue to run Uninterrupted for much longer periods of time between cleanings.