My suspicion is that the 3rd kiln takes the petcok

My suspicion is that the 3rd kiln takes the petcoke residue either in liquid or semi solid form and bakes out the rest of the hydrocarbons. This recovers a bit of fuel but more importantly makes the residue suitable for sale or disposal as non-hazardous waste.

With 3 machines running at 4000lbs /hr that generates 2.9 tons of residue per day or thereabouts.