Dry Discharge at KEZO, Hinwil

The development of the dry discharge of bottom ash added an additional process variable to the incineration process, the tertiary air. Like the primary and secondary air, tertiary air supports an optimal incineration process and is therefore integrated in the firing power control. The amount of tertiary air is less than 10% of the total combustion air.

To keep the total amount of combustion air equal, the secondary air was reduced by the same amount, without increasing the CO figures of the raw gas. Besides supporting the incineration process the tertiary air fulfills another important task, it cools down the bottom ash; it supports the afterburning of organic parts and the air sifting of the bottom ash.

Process Description:

The bottom ash is discharged continuously via ash drum. By falling down on special baffle plates, the bottom ash is crushed again. Any glowing embers in agglomerates are also torn apart. The vibration conveyor then transports the ash parts to the screening machine. It supports the cooling of the bottom ash, the afterburning, the air sifting and the screening.


  • Implementing the dry bottom ash discharge process overall Switzerland would reduce long distance transportation of 120'000 t bottom ash/y.
  • Dry bottom ash shows no significant combustion loss.
  • Because of the after burning the TOC content in the dry bottom ash is reduced by over 50 %, which equals the incineration of additional 6'000 t of waste.
  • The eluate values differ significantly, too. The wet discharged bottom ash has 5 times higher values for lead, 25 times higher values for cadmium and 250 times higher values for copper.
  • The water in the wet discharger already starts the hardening of the bottom ash, whereas the hardening of the dry ash can later be started on demand.
  • Granulometry: in the dry bottom ash all substances and particles keep their original shape. Therefore the possibilities for the separation of the ash according size, weight, shape, color and conductivity etc. are unlimited. Bigger parts can be removed simply and safely.