2 types of rotary kiln

There are two types of rotary kiln, named after the sense of solids- compared to gas flow in the kiln.

• TYPE 1: Counter current rotary kiln
• TYPE 2: Co-current rotary kiln

Type 1: Counter current rotary kiln

Flue gases flow in the opposite direction of the waste, against the inclination of the kiln. Solids move by the rotary motion and by gravity from the high end to the low end of the kiln.

Fig. 1: Counter current rotary kiln (TYPE 1)

Type 2 : Co-current rotary kiln

Flue gases flow in the same direction of the waste, with the inclination of the kiln. Solids move by the rotary motion and by gravity from the high end to the low end of the kiln.

Fig. 2: Co-current rotary kiln (TYPE 2)

Example: counter current rotary kiln and co-current rotary kiln

Fig 3: Counter-current rotary kiln	Fig 4: Co-current rotary kiln

Those 2 rotary kilns have the same thermal capacity. The kiln of the counter current is shorter and has a larger diameter. The co-current kiln is longer and smaller.

Manufacturing of counter- and co-current rotary kilns

The main difficulty with the counter-current kiln is the temperature gradient across the fire door which seals off the waste hopper (ambient temp.) from the kiln entry (1000 deg. C).This requires specially engineered designs and a lot of experience to make the fire door reliable and long-lasting.

This is the main reason why the counter-current kiln is not more wide-spread.

Comparision of counter current and co-current rotary kiln

	Type 1	Type 2
	Counter current rotary kiln	Co-current rotary kiln
Amount of waste IN	1000 kg/h	1000 kg/h
Waste inlet temperature	1000 C	200 C
Oxygen % at waste inlet	6 %	20 %
Waste residence time	> 2 h	>2 h
Ashes
Unburned Carbon in Ash	< 0,5 %	> 2 %
Ash Temp.	200 C	1000 C
Mass reduction (%)	> 85 %	> 75 %
Post cumustion additional support fuel consumption	0 kg/h	> 100 kg/h
Fly ashes	750 mg/Nm3	1500 mg/Nm3